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Retention of calcium carbonate in mechanical pulp suspensions Modgi, Sivamurthy B. 2001

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Retention Of Calcium Carbonate In Mechanical Pulp Suspensions by Shivamurthy B. Modgi B.A.Sc (in pulp and paper), Karnatak University, Dharwad, India, 1996 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF M A S T E R OF APPLIED SCIENCE In THE FACULTY OF GRADUATE STUDIES (Department of Chemical and Biological Engineering) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October 2001 © Shivamurthy B. Modgi, 2001 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. (Shivamurthy B. Modgi) Department of Chemical and Biological engineering The University of British Columbia Vancouver, Canada Date : October 12, 2001 ABSTRACT This work investigates the retention of precipitated calcium carbonate (PCC) in thermo-mechanical pulp suspensions using polyethylene oxide (PEO), phenol formaldehyde resin (PFR) system in conjunction with Raifix 120, Raifix 2515 and Raisapac. Raifix is a starch-based polymer produced by Raisio chemicals of Finland. Raisapac is poly hydroxy aluminum chloride and is also manufactured by Raisio chemicals. The PCC used for this work was obtained from Specialty Minerals Inc. and its commercially known as Albafil. To maintain the pH of the PCC loaded pulp suspension to neutral the PCC was treated with phosphoric acid (0.2g/g of PCC). A 10 wt. % PCC suspension was pretreated with phosphoric acid for 24 hours according to the method proposed by Pang (Pang, 2001)and used in the first pass retention experiments. First pass retention experiments were performed using a Britt dynamic drainage jar. A 0.5% pulp suspension was loaded with 20% of pretreated PCC (10 wt.%). At first the retention experiments were done with only PEO and PFR at a PEO dosage of 0.15 mg/g of OD pulp. The retention of fines and pretreated PCC increased by 27% and 62% respectively at a PEO, PFR ratio of 1. Retention experiments were then performed with Raifix (120 or 2515) in conjunction with the PEO, PFR system. The best sequence of addition using Raifix was found to be Raifix-PCC-PFR-PEO. A 3 2 statistical design of experiments showed that there is a significant interaction between PEO, PFR and Raifix. This interaction was manifested as an increase in retention at a dosage of 0.05 mg/g of PEO, PFR, but decreased at a PEO, PFR dosage of 0.15 mg/g of OD pulp. It is believed that Raifix scavenges PFR and therefore interferes with retention. The sequence used for first pass retention with Raisapac was Raisapac, PCC, PFR and PEO. The retention obtained using Raisapac was greater than that using Raifix but similar to that obtained by PEO, PFR. The effect of DDJ stirrer speed at various rpm was performed with the best retention conditions for PEO, PFR system, Raifix-PEO-PFR and Raisapac-PEO-PFR system. ii Table O f Contents Abstract i i List Of Tables vii List of Figures... ix Acknowledgements xv CHAPTER-1 Introduction and literature survey 1 1.1 Mineral fillers used in papermaking 1 1.1.1 Clay 2 1.1.1.1 Talc 3 1.1.3 Titanium dioxide 6 1.1.4 Calcium carbonate 6 1.1.4.1 Manufacturing process for various grades of calcium carbonate 8 1.1.4.2 The surface chemistry of calcium carbonate 9 1.2 Retention , 14 1.2.1 Retention Aids 18 1.3 Retention of fines and fillers in mechanical pulp suspensions 19 1.3.1 Retention of precipitated calcium carbonate 22 1.3.1.1 Review of previous work done on PCC in our Laboratory 25 1.3.2 Pretreatment of Calcium carbonate 26 1.4 Microparticle retention aid systems 27 1.5 Research obj ectives 29 CHAPTER-2 Experimental methods and apparatus 31 2.1 Materials 31 2.1.1 Anionic Trash Collector (ATC) 31 2.1.2 Ethylenediaminetetraacetic acid (EDTA) 31 2.1.3 Phenol Formaldehyde Resin (PFR) Cofactor 31 2.1.4 Phosphoric acid 32 2.1.5 Poly(diallyl dimethyl ammonium chloride) (PDADMAC) 32 iii 2.1.6 Poly(ethene sodium sulfonate) (Pes-Na) 32 2.1.7 Polyethylene oxide) (PEO) 32 2.1.8 Precipitated Calcium Carbonate (PCC) 32 2.1.9 Pulp 33 2.1.10 Sodium Hydroxide 33 2.1.11 Water 33 2.2 Instrumentation and Analysis 33 2.2.1 Dynamic Drainage Jar (DDJ) 33 2.2.2 pH and Conductivity Meters 34 2.2.3 Malvern Masterisizer 2000 with Hydro M U Cell 34 2.2.4 Mutek PCD 03 Particle Charge Detector 35 2.2.5 Millipore Pressure Filter 36 2.2.6 Malvern Zetasizer 2000 36 2.3 Experimental Procedures 37 2.3.1 Fines Fractionation 37 2.3.2 pH and Conductivity Analysis 38 2.3.3 Aggregation of Filler 38 2.3.4 Zeta Potential Measurements 38 2.3.5 Pretreatment of PCC 39 2.3.6 First-Pass Retention (FPR) 39 2.3.7 Cationic Demand Analysis 39 2.3.8 Experimental Design 40 C H A P T E R - 3 Results and discussion 42 3.1 Stability of pH of PCC containing mechanical pulp suspensions 42 3.2 PEO, PFR system 43 3.2.1 First pass retention of pretreated PCC, PEO, PFR system 43 3.2.2 Aggregation experiments with pretreated PCC and PEO, PFR system 44 3.2.2.1 Zeta potential measurements of aggregates of pretreated PCC with PEO, PFR 46 3.3 Retention with Raifix 2515 47 iv 3.3.1 Determination of best sequence of addition for Raifix 2515 and PEO, PFR system for pretreated PCC and fibre fines retention 48 3.3.2 Statistical design for Raifix2515 and PEO, PFR system 49 3.3.3 First pass retention with PEO-PFR at different dosages of Raifix 2515 54 3.3.4 Cationic Demand Analysis on DDJ Filtrate 56 3.3.5 Aggregation of pretreated PCC with Raifix 2515 58 3.3.5.1 Zeta potential of aggregates of pretreated PCC with Raifix 2515 59 3.3.6 Aggregation of pretreated PCC with Raifix 2515 and PEO, PFR 61 3.3.6.1 Zeta Potential of Aggregates formed with Raifix 2515 and PEO, PFR.... 62 3.3.7 Proposed Mechanism for Aggregation Of Pretreated PCC with PEO, PFR and Raifix 2515 64 3.3.8 Proposed Mechanism For Flocculation Of Fibre fines and Pretreated PCC with PEO, PFR and Raifix 2515 66 3.4 Retention with Raifix 120 68 3.4.1 Statistical design for Raifix 120 and PEO, PFR system 68 3.4.2 Cationic Demand Analysis on DDJ Filtrate 74 3.4.3 Aggregation of Pretreated PCC with Raifix 120 and PEO, PFR 75 3.4.3.1 Zeta Potential Of Aggregates of pretreated PCC with Raifix 120 and PEO, PFR system 76 3.5 Retention with Raisapac 78 3.5.1 Determination Of Best Sequence Of Addition For Raisapac And PEO, PFR System For Pretreated PCC And Fibre Fines Retention .". 78 3.5.2 Statistical design for Raisapac and PEO, PFR system 79 3.5.3 First Pass Retention Of Pretreated PCC And Fibre Fines With Raisapac In Conjunction With PEO, PFR System 84 3.5.4 Cationic Demand Analysis of DDJ Filtrate 84 3.5.5 Aggregation of Pretreated PCC with Raisapac 86 3.5.6 Aggregation of Pretreated PCC with Raisapac in conjunction with PEO, PFR system 88 3.5.6.1 Zeta Potential of Aggregates of Pretreated PCC formed by Raisapac 88 3.5.6.2 Zeta Potential of Aggregates of Pretreated PCC with Raisapac and PEO, PFR System 90 3.6 Effect Of Shear On Fibre Fines And Pretreated PCC Retention 91 3.7 Comparison Of Retention Of Fibre Fines, Pretreated PCC And Total Solids Retention At Best Conditions For A l l Polymers Used 93 CHAPTER -4 Conclusions and Recommendations for future Work 96 4.1 Conclusions 96 4.2 Recommendations For Future Work 98 References 99 Appendix-1 First Pass Retention Calculations 103 Appendix-2 Statistical Analysis 104 vi LIST O F T A B L E S Table 1.1 7 Mineral rocks containing calcium carbonate Table 1.2 11 Calcium Carbonate thermodynamic equilibrium reactions Table 2.1 : 32 Filler properties Table 2.2 40 A 3 2 factorial design matrix for first pass retention of pretreated PCC, PFR, PEO system together with Raifix 120. Table 2.3 41 A 3 2 factorial design matrix for first pass retention of pretreated PCC, PFR, PEO system together with Raifix 2515. Table 2.4 41 A 3 factorial design matrix for first pass retention of pretreated PCC, PFR, PEO system together with Raisapac Table 3.1 : 51 Analysis of variance for fines retention using Raifix 2515 Table 3.2 ..51 Analysis of variance for pretreated PCC retention using Raifix 2515 Table 3.3 52 Analysis of variance for total solids retention using Raifix 2515 Table 3.4 70 Analysis of variance for fines retention using Raifix 120 Table 3.5 71 Analysis of variance for pretreated PCC retention using Raifix 120 Table 3.6 71 Analysis of variance for total solids retention using Raifix 120 Table 3.7 80 Analysis of variance for fines retention using Raisapac vii Table 3.8 81 Analysis of variance for pretreated PCC retention using Raisapac Table 3.9 82 Analysis of variance for total solids retention using Raisapac viii LIST OF FIGURES Figure 1.1 2 Idealized structure for the aluminum-silica layers of kaolinite Figure 1.2 4 The structure of talc Figure 1.3 5 Talc zeta potential vs. pH variation Figure 1.4 9 Production of PCC Figure 1.5 10 Solubility of calcium carbonate Figure 1.6 12 Electrophoretic mobility as a function of CaC03 concentration Figure 1.7 15 Typical wet end of a paper machine Figure 1.8 22 Schematics of association-induced polymer bridging Figure 1.9 27 Colloidal silica particle Figure 1.10 28 Small microflocs formed between the furnish components and the small colloidal particles in the system Figure 2.1 34 Britt Dynamic Drainage Jar Figure 2.2 35 Malvern Mastersizer for particle charge analysis Figure 2.3 36 Mutek particle charge detector for cationic demand analysis Figure 2.4 37 Zetasizer for measuring zeta potential ix Figure 3.1 42 pH changes vs time for different methods of pretreatment of PCC Figure 3.2 44 First-pass retention of pretreated PCC with PFR, PEO system Figure 3.3 45 Effect of PEO, PFR dosage on rate of aggregation of pretreated PCC Figure 3.4 46 Effect of PEO, PFR dosage on rate of aggregate size of pretreated PCC Figure 3.5 47 Change in zeta potential of pretreated PCC aggregates for different PEO, PFR dosage Figure 3.6 48 Retention with different sequences of addition of Polymers Figure 3.7 49 First pass retention with PEO, PFR and Raifix 2515 Figure 3.8 53 Effect of Raifix 2515 dosage on Fines retention Figure 3.9 53 Effect of Raifix 2515 dosage on pretreated PCC retention Figure 3.10 54 Effect of Raifix 2515 dosage on total solids retention Figure 3.11 55 Retention of pretreated PCC for different dosage of Raifix 2515 Figure 3.12 56 Change in cationic demand of DDJ filtrate with increasing dosage of Raifix 2515 Figure 3.13 57 Change in cationic demand of DDJ filtrate with PEO, PFR and Raifix 2515 dosage Figure 3.14 58 Change in cationic demand of DDJ filtrate with addition of polymers Figure 3.15 59 Effect of Raifix 2515 dosage on rate of aggregation of pretreated PCC Figure 3.16 60 Effect of Raifix 2515 dosage on aggregate size of pretreated PCC Figure 3.17 60 Change in zeta potential of aggregates of pretreated PCC for different dosage of Raifix 2515 Figure 3.18 61 Effect of Raifix 2515 and PEO-PFR dosage on rate of aggregation of pretreated PCC Figure 3.19 62 Effect of Raifix 2515 and PEO, PFR dosage on aggregate size of pretreated PCC Figure 3.20 63 Change in zeta potential of pretreated PCC aggregates with Raifix 2515 and PEO, PFR Figure 3.21 64 Change in zeta potential for different sequence of addition of polymers with pretreated PCC Figure 3.22 65 Schematics of aggregation Mechanism of pretreated PCC with PEO, PFR and Raifix 2515 Figure 3.23 67 Schematics of flocculation Mechanism of pretreated PCC and fibre fines with PEO, PFR and Raifix 2515 Figure 3.24 69 First-pass retention with PEO, PFR and Raifix 120 Figure 3.25 72 First pass fibre fines retention for different dosages of PEO, PFR and Raifix 120 for statistical design of experiments xi Figure 3.26 73 First pass pretreated PCC retention for different dosages of PEO, PFR and Raifix 120 for statistical design of experiments Figure 3.27 74 First pass total solids retention for different dosages of PEO, PFR and Raifix 120 for statistical design of experiments Figure 3.28 75 Change in cationic demand for different dosages of Raifix 120 and PEO,PFR Figure 3.29 76 Rate of aggregation of pretreated PCC with PEO, PFR and Raifix 120 Figure 3.30 77 Maximum aggregate size of pretreated PCC with PEO, PFR and Raifix 120 Figure 3.31 , 77 Change in zeta potential of aggregates of pretreated PCC formed with PEO, PFR and Raifix 120 Figure 3.32 78 First pass retention of pretreated PCC with PEO, PFR and Raisapac for different sequence Figure 3.33 80 First pass retention with PEO, PFR and Raisapac Figure 3.34 82 First pass retention for fibre fines with PEO, PFR and Raisapac for statistical design of experiments Figure 3.35 83 First pass retention for pretreated PCC with PEO, PFR and Raisapac for statistical design of experiments Figure 3.36 84 First pass retention of total solids with PEO, PFR and Raisapac for statistical design of experiments xii Figure 3.37 85 First pass retention with pretreated PCC,PEO, PFR for different dosages of Raisapac Figure 3.38 86 Change in cationic demand of DDJ filtrate with PEO, PFR for different dosages of Raisapac Figure 3.39 87 Change in aggregation of pretreated PCC with Raisapac only Figure 3.40 87 Maximum aggregate size of pretreated PCC aggregates formed with Raisapac only Figure 3.41 88 Change in rate of aggregation of pretreated PCC with Raisapac and PEO, PFR system Figure 3.42 89 Maximum aggregate size of pretreated PCC aggregates formed with Raisapac only Figure 3.43 89 Change in zeta potential of pretreated PCC aggregates formed with Raisapac Figure 3.44 90 Change in zeta potential of pretreated PCC aggregates formed with Raisapac and PEO, PFR system Figure 3.45 91 Effect of DDJ stirrer speed on fibre fines and pretreated PCC retention with PEO, PFR system Figure 3.46 92 Effect of DDJ stirrer speed on fibre fines and pretreated PCC retention with Raifix 2515 and PEO, PFR system Figure 3.47 93 Effect of DDJ stirrer speed on fibre fines and pretreated PCC retention with Raisapac and PEO, PFR system xiii Figure 3.48 94 Retention of fines, pretreated PCC and total solids with PEO, PFR system, Raifix 2515 and PEO, PFR system, Raifix 120 and PEO, PFR system and Raisapac and PEO, PFR system xiv ACKNOWLEDGEMENTS I extend my sincere gratitude to my supervisor Dr. Peter Englezos, for giving me this research opportunity and for providing me with invaluable guidance, excellent suggestions, encouragement, care and attention throughout the study. I would like to thank the Network of Centers of Excellence for Mechanical Pulps for providing financial support for this project. I would like to thank Dr. Ian Thorburn of Raisio Chemicals for his valuable suggestions at the beginning of my work. I would also like to thank Dr. Peter Pang for invaluable discussions, help and suggestions. I would like to thank the excellent staff members of Pulp and Paper center and Chemical Engineering department for providing necessary help whenever required. I would also like to thank Raisio Chemicals for supplying me the polymers for my work. I extend my thanks to Specialty Minerals Inc. for supplying me with PCC. M y deepest love and gratitude is felt for my parents and family members for their never ending encouragement and support. This would have been impossible without them. Last but not the least, I would like to thank all my friends who are directly or indirectly involved in making this dream come true. X V CHAPTER-1 Introduction and literature survey A wide range of chemicals are utilized in a papermaking stock furnish to impart or enhance certain sheet properties or to facilitate the papermaking process. Additives such as sizing agents, optical brightening agents, mineral fillers, starches and dyes are commonly used. Also chemicals such as drainage aids, retention aids, defoamers, pitch dispersants, slimicides and corrosion inhibitors are added as required. The chemicals and minerals contribute to 10% of the cost of papermaking (Smook, 1992). The main aim of a papermaker is to produce paper that meets specific market quality requirements with minimum costs with the available equipment. One way to achieve this is by reducing the amount of fibre required to produce one ton of paper. This can be done by adding cheap mineral fillers to the papermaking stock and retaining them in the paper. The addition of finely divided white mineral fillers not only reduces the amount of fibre required to produce a ton of paper, but also helps in filling the spaces and crevices between the fibres, thus producing a denser, softer, brighter, smoother and more opaque sheet. In fact, some paper qualities cannot be achieved without fillers or they would be too expensive to achieve (Casey, 1981). Fillers are highly desirable for printing papers and generally improve the printing properties. The application of fillers is especially important when opacity is needed at a low basis weight and they are invaluable in packaging grades where low permeability should be combined with opacity for the protection of food-stuff (Smook, 1992). 1.1 Mineral fillers used in papermaking A large number of fillers are available to the papermaker ranging from cheap low grades to the expensive ones. From a practical standpoint both have their place in the industry. The various types of fillers used are clays, calcium carbonate, talc, titanium dioxide, barium oxide, zinc sulfide, diatomaceous silica etc. Clay and calcium carbonate are the most widely used, but titanium dioxide is also regarded as a filler of great importance because of its very special properties. But its cost limits its use to specific grades only. In recent years new synthetic pigments have been developed to fill papers (Casey, 1981). 1 In order to be of practical importance, fillers must meet certain requirements. They should have a high degree of brightness, a high index of refraction, small particle size, very low solubility in water and low specific gravity. It is also desirable that the filler be chemically inert to avoid unfavorable reactions with other papermaking additives. The filler should contain a minimum amount of impurities and it is of special importance that the grit content be low to avoid excessive wear of the wire and other processing equipment. Finally, another important factor is that the filler must be relatively cheap. 1.1.1 Clay Kaolin clay is an aluminum silicate mineral with a layered structure of alternating alumina and silica sheets as represented in the figure 1.1. Chemically it is represented by the formula (OH)s Si4 AI4O10. This formula is frequently written as AI2O3 2SiC>2 2H2O. (Hagemeyer, 1995). Clay meets most of the requirements and hence is admirably suited for filling papers. Clay has a cation exchange capacity that varies between 1-10 meq/lOOg depending on the origin of the mineral. It also has an anion exchange capacity which however, is smaller than the cation exchange capacity. Three different reasons have been suggested for the cation exchange capacity and one reason for the anion exchange capacity. JSI <® jg) OH "-GROUPS ALUMINIUM OCTAHEDRAL N \ ± / ' LAYER " — V f c A A Al « \ / l \ s V ** 1 ^ I V ' .1 A -X _ 1 / X , \ ' , / \ I x? g ^ V is 1 i j ' SILICON T E T R A H E D R A L I L 1 ! LAYER ' v • , T ' N , 4 V Si crt) vcr at) ^0—0 Figure 1.1 Idealized structure for the aluminum-silica layers of kaolinite (Hagemeyer, 1997). 2 The oxide surface consists not only of oxygen atoms but also of OH-groups. These OH-groups gives rise to charges according to the following reactions: (Hagemeyer, 1997). M O H ^ M O " + H + (1) M O H + H + » M O H 2 + (2) The first reaction makes the surface of the oxide mineral negative, and the second gives a positive charge. The charge is dependent on the pH. The first reaction dominates at high pH and the second at low pH. The iso-electric point varies considerably from oxide to oxide. The silanol groups on the surface of the platelets are thus negatively charged at normal pH-values (5-8), while aluminol groups located at the edges of the platelets, are positively charged at pH-values below 9. Isomorphous substitution takes place in which aluminum atom takes place of silicon atom in the silica layer or magnesium atom takes place of aluminum atom in the aluminum layer. The charge neutrality is destroyed and is compensated for by bonding of counterions, which exists between the layers in the structure. A third reason for the cation exchange capacity of kaolin can be contamination by swelling clays (e.g., montmorillonite) which can have cation exchange capacities of the order of magnitude of 100 meq/ lOOg (Eklund and Lindstrom, 1991). When clay is used as filler, the smoothness, gloss and printability of the paper are improved. The refractive index of clay is approximately equal to that of cellulose which is 1.53, an improvement in the opacity of the paper is obtained as a rule when clay is used, depending on the fact that clay strongly increases the light scattering power of the paper ( Eklund and Lindstrom, 1991). The brightness of clay limits its use in fine papers produced from bleached pulp, but for the paper with mechanical pulp an increase is usually obtained. The strength properties of the paper decreases. In large quantities it can give rise to sizing problems. The quantity of clay added varies depending on the use of the final product and the properties desired. The main application of clay is for pigment coating of paper (Eklund and Lindstrom, 1991). 1.1.1.1 Talc Pure talc is hydrated magnesium silicate having a theoretical molecular composition of 3MgO.4SiO2.H2O. (Hagemeyer, 1997). Figure 1.2 shows the structure of 3 IN FIN-I TE C MA IN OR L AY€B Figure 1.2 The structure of talc (Hagemeyer, 1997). a talc. It contains 31.7% MgO, 63.5% Si0 2 , and 4.8% H 2 0 . Talc has a specific gravity of 2.75 g/cm3 and refractive index of 1.58. Talc suspension has a pH of 8.5 in water. Talc is anionic with zeta potential between -40mv to -50mv. The iso-electric point of talc is around pH 3 which is very close to the iso-electric point of fibres which is around pH 3.5 (Gess, 1998). This means that talc addition in the furnish does not affect the final isoelectric point. The cationic exchange capacity is very low, in the order of 2.4meq/100g; thus its effect on the overall system pH is very small. The flat surface of the talc is hydrophobic and organophilic whereas the edge surface is hydrophilic. The particle size varies from 1-1 Op and the brightness is between 90-91%. Because of its good brightness, softness, particle shape, inert properties and price, talc is considered as a good filler for papermaking. . Talc used as a papermaking additive must be of fairly high purity ; it contains less than 1% of quartz. A higher quartz level is extremely abrasive to good filler for papermaking. Ordinarily talc slurry solids would be kept low in solids and no dispersants used, to keep the first-pass retention as high as possible. 4 20 " u u i 1 1 1 1 1 1 1 0 2 4 6 8 10 12 14 p H Figure 1.3 Zeta potential of talc vs pH (Gess, 1998). Due to the large particle size there is less interference with hydrogen bonding of the cellulose , allowing higher filler addition than with clay. The strength properties of talc loaded papers are higher than with other fillers because it does not interfere with the hydrogen bonding of the cellulose. The linting tendency during printing is, however higher with talc than with clay which has reduced its use slightly. Because of its oleophilic character talc is also very good for pitch control. The resin particles are adsorbed by the talc and the formation of agglomerates is prevented. The ability of talc to adsorb pitch and other organic compounds keeps the wet end system of the papermachine clean (Casey, 1981). The retention of talc is high and it disturbs the sizing of the paper relatively little (Gess, 1998). The softness and the good retention of talc brings about economic advantages in the form of a prolonged life span of the wire. The most commonly used retention aid system for talc retention is cationic polyacrylamide, colloidal silica, aluminum polychloride and cationic starch. 5 1.1.3 Titanium dioxide Titanium dioxide exists in two crystal forms that are commercially important: anatase and the rutile. The refractive index of rutile (2.7) is higher than that for anatase (2.55). The brightness of titanium pigments is 98 or more, and the particle size is 0.3 to 0.35 micron (Eklund and Lindstrom, 1991). The high refractive index and the fine particle size gives titanium dioxide exceptionally high opacifying effect (Casey, 1981). The amount of titanium required to produce the same opacity, as different fillers are low mostly in low basis weight paper. It reduces showthrough after printing, but its high price limits its use. Hence it is used only in special grades to produce high-quality and high priced papers. Titanium pigments also have a very high brightening effect. This is valuable in colored papers ( Eklund and Lindstrom, 1991). 1.1.4 Calcium carbonate Calcium at 4.8% is the fifth most common elemental constituent of the earth's crust after oxygen, silicon, aluminum and iron (Wypych, 1999). It is found in rocks and minerals which have a very high concentration of calcium carbonate. Calcium carbonate is the most common deposit found in sedimentary rocks. The majority of calcium carbonate deposits are formed from skeletal fragments of organisms living in the marine environment. Most of these deposits are composed of the skeletal remains of tiny sea creatures (cocolithopore algae) that were deposited on the ocean floor over 100 million years ago. A typical cocolith is the order of 3-5 microns diameter and made up of calcite crystals of around 1 micron in length (Chapnerkar, 1995). Calcium carbonate in its most common form limestone is found throughout the world. Most limestones were formed in the Palezoic era 220-500 million years ago. The initial purity of the deposit was dependent upon the conditions under which it was accumulated. Localized geologic events caused various alterations varying from numerous cycles of solution and reprecipitation to thermal modification by high temperature and intense pressure causing some melting and recrystalization to form marble. In regions where pressures were not great to form marble, the material is known as chalk and the presence of shells and skeletal remains are evident. There are other 6 mineral rocks associated with calcium carbonate (Wypych, 1999). Table 1.1 shows the different type of mineral rocks and their basic characteristics. Table 1.1 Mineral rocks containing calcium carbonate (Wypych, 1999). R O C K T Y P E B A S I C C H A R A C T E R I S T I C S C H A L K :- a sedimentary rock of soft texture formed from nanofossils. CALC1TE :- mineral composed of more than 95% of calcium carbonate. DOLOMITE :- mineral composed of 30% of magnesium carbonate and 70% of calcium carbonate. LIMESTONE :- consolidated sedimentary rock. M A R B L E : - a m etam orphi c rock originally compos ed of either calcite, aragonite or dolomite which was recrystalized to a dense rock Under the influence of high temperature and pressure. Its color depends on admixtures. T R A V E R T I N E :- deposits from spring water in a form of calcite or aragonite which form in caves dripstones. VATERITE - a hexagonal modification of calcium carbonate which is very unstable and it is readily converted to calcite. 7 1.1.4.1 Manufacturing process for various grades of calcium carbonate Calcium carbonate falls in two general classifications, the natural products made directly by physically grinding limestone and the precipitated products manufactured by chemically reacting various raw materials (Kocurek, 1992). Within the two major groups there are several categories based on the differences in particle size, shape and size distribution. The natural products normally have a larger particle size and a broader size range than precipitated materials. Lime quality has considerable importance in the satisfactory production of calcium carbonate. It is therefore necessary to obtain a lime which would slake at lower consistencies and which would produce a carbonate of satisfactory properties for filling (Eklund and Lindstrom, 1991). 1.1.4.1.1 The production of ground calcium carbonate (GCC) Dry and wet grinding are the two basic methods for the production of natural ground calcium carbonate fillers. In the dry method, the feed rock after coarse screening may be dried in a rotary dryer, milled with either a ball or hammer mill and air classified to the desired particle size. This method is usually constrained to chalk fillers that are friable and produces coarse grained fillers only (Chapnerkar, 1995). Wet grinding, after crushing and ball milling, is more typical for the production of ground calcium carbonate from limestone and marble. This permits the removal of contaminants early on in the process by flotation, conferring high brightness to the finished product. In order to form a high solids slurry of the ground calcium carbonate, dispersants can be added during the grinding process. These organic chemicals may be anionic or cationic in nature, imparting a net negative or positive charge to the surface of the particles respectively (Kocurek, 1992). Usually commercial GCC is treated with a dispersant (e.g. polyacrylate) which provides the particles with negative charge. Also cationically charged GCC is available in the market. Natural ground calcium carbonate does not exhibit any internal porosity due to the rhombohedral particle shape of calcite and compaction over geological time frames of the various deposits. The specific surface area of ground calcium carbonate is low and is directly proportional to the mean particle size of the pigment expressed in microns. 8 1.1.4.1.2 Manufacture of precipitated calcium carbonate (PCC) Precipitated calcium carbonate grades are also termed synthetic calcium carbonate since several chemical operations are performed in a kiln at 900 °C. At this stage calcium carbonate is decomposed to calcium oxide and carbon dioxide, which is used in further steps (Chapnerkar, 1995). In the next step, calcium oxide is mixed with water in a process called. slaking. This converts calcium oxide to lime and permits a material purification operation to be performed which results in a product of improved purity. In the final operation, the milk of lime is saturated by carbon dioxide which precipitates calcium carbonate. Depending on process parameters such as temperature, solution concentration and speed of agitation different dimensions and performance characteristics of the precipitates can be altered. By control of precipitation conditions scalenohedral, orthorhombhic aragonite or mixtures may be obtained (Kocurek, 1992). Figure 1.4 shows the different processes used to manufacture precipitated calcium carbonate. • m e s l a n f i — — | K i l n C a r b o n ..- d o x i c i o y a s 1 Sludge M i l k > . C a i t x m a t c p r o c e s s P r e c i p i t a t e d c a l c i u m ; o f l i m e c a r b o n a t e * •_ - ' S o d i u m ' c a r b o n a t e . .. L i m e - s o d a M i l k \ C o u s l i c i z e r : - - - p r o ce s s ; P r e c i p i t a t e d c a l c i u m c a r b o n a l e "*• dl l i m e AmcMiHim M i l k o l limn ' A m m o n i u m . " c h l o r i d e r e a c t o r C l a r l l i e r C a l c i u m S o d i u m , . c h l o r i d e c a r b o n a t e . s o l u t i o n . s o l u t i o n UL R e a c t o r Precipitated c a l c ' u i i c a r b o n a t e Figure 1.4. Production of PCC (Kocurek, 1992). 1.1.4.2 The surface chemistry of calcium carbonate Calcium carbonate is partially soluble in water to the extent of 10"4 to 10"5 moles/litre (depending on the pH and carbon dioxide saturation level) and the crystals undergo dissolution and recrystalization altering the surface chemistry of the particles Figure 1.6 shows the solubility of calcium carbonate at different pH. At acidic pH calcium carbonate solubility is very high. As the pH increases the solubility decreases and at a pH of 9 it is negligible. This behaviour of calcium carbonate limits its application 9 to acid papermaking systems. Because of the solubility equilibria involved, the solubility of calcium carbonate is determined by the partial pressure of carbon dioxide in air. An increase in the partial pressure increases the solubility slightly. Calcium carbonate has a pH of 8.4 when in equilibrium with the carbon dioxide content in the air. In carbonic acid-free water, calcium carbonate has a pH of 9.9. The pH of the solution containing calcium carbonate depends on the electrokinetic potential of the system in which it is introduced and calcium ions activity (Pierre et al., 1990 Kamiti and van de Ven, 1994). 0.12 5 6 7 8 9 10 pH Figure 1.5. Solubility of calcium carbonate at different pH (Pang et al., 1998) Calcium carbonate pigments, used as fillers in papermaking can be divided in to three types: precipitated calcium carbonate (PCC), ground calcium carbonate (GCC) and chalk. Of these PCC and GCC are the most widely used. Commercial GCC is usually treated with a dispersant (e.g. polyacrylate) which provides the particles with a negative charge. The electrostatic repulsion is strong enough to prevent aggregation of the particles and thus commercial GCC suspensions are reasonably colloidally stable. Because of their negative charge, GCC particles do not deposit on negatively charged 10 pulp fibres, therefore the use of retention aids is necessary to incorporate them into the paper (Vanerek et a l , 2000). PCC is usually not treated and in pure form (in distilled water) could have a positive charge that promotes the deposition of PCC particles on the fibers. The charge however is not strong enough to provide sufficient repulsion between the particles to stabilize the system and, consequently when dispersed in water, PCC particles aggregate. When untreated GCC and PCC are suspended in a solution containing anionic trash (anionic dissolved and colloidal substances), these can adsorb onto the GCC or PCC and make the pigments behave much like the commercial GCC (Vanerek et al., 2000). In an aqueous suspension of calcium carbonate the equilibria involved can be written in terms of the following series of equations as shown in Table 1.2 (Eklund and Lindstrom, 1991). Table 1.2. Calcium carbonate thermodynamic equilibrium reactions. CaCC3 ( S ) <^>CaC03 ( a i ) log K i = -5.09 CaCQxaqX^Ca2* + CO3 2 logK2 =-3.25 CO32" + H20<=>HC03" + OH logK3 = -3.67 H C O 3 - + H20<^>H2CQ3+OH" log K 4 = -7.65 H 2 C Q 3 ^ C 0 2 ( g ) + H 2 0 l o g K 5 = 1.47 Ca2* + HCO3" CaHCQ3 + logK* =0.82 C a H C C ^ ^ H * + CaCC^aq) logK? =-7.90 Ca2* + OH"<^>CaOH+ logKs = 1.40 CaOH* + OH"<^Ca(OH)2(aq) logKs = 1.3 Ca(OH)2(aq) <^>Ca(OH)2c» logKio= 2.45 11 The solubility of calcium carbonate decreases with increasing pH and decreasing ionic strength. The surface chemical properties of calcium carbonate are determined by the adsorption of cations and anions on the interfaces of the crystals. When calcium carbonate is dispersed in water Ca + and CO3" ions are released. If the concentration of the two types of ions are electrically equivalent, the solution is at the iso-electric point. This does not mean, however that the particle surface is at the point of zero charge, because the charge neutrality depends on the concentration of the potential-determining ions in the boundary layer closest to the surface. In the case of calcium carbonate, it is the concentrations of calcium and carbonate ions adsorbed onto the crystal surface, which determines the surface charge. > r * ~E CO ' o ; ui 0 200 400 600 800: 1000 GaGG 3CGFiGi,ppm Figure 1.6 Electrophoretic mobility as a function of CaCC>3 concentration measured in distilled, tap and white water, (a) PCC (b) GCC (Alince et a l , 2000). Ions other than calcium ions and carbonate ions can also be adsorbed on the surface of calcium carbonate. The adsorption susceptibility for cations partly forms a series Ca 2 +> B a 2 + > Pb 2 + >Mg 2 + . The anions Cl", N0 3 " , CNS", C r 0 4 2 + and S0 4 2 " are not so different from each other while OH", HCO3", and CO3 2" are adsorbed according to the series Cl" <OH"<HC032"<P043". The solution becomes alkaline because of the release of hydroxyl ions (Garrels and Christ, 1992). Calcium carbonate fillers normally buffers wet end pH in the range of 7.5-8.2. Excessive fibre darkening has precluded their use in most 12 wood containing grades (Evans, 1991). A new acid tolerant form of PCC has been developed that readily allows PCC to be used at pH 5.0 to 7.0 (Evans, 1991). The surface charge of PCC depends on its concentration, varying from negative at low concentrations to positive at higher concentrations (Siffert and Fimbel, 1985). This observation was confirmed for both untreated GCC and PCC dispersed in distilled water by Allince et al. (2000) as shown in figure 1.5. The positive charge is believed to _i_2 originate from the fact that calcium carbonate is partially soluble in water and the Ca ions tend to reabsorb preferentially on the crystal surface. The concentration of impurities in distilled water which is about 10" mol/lit is below the detection limit of conductivity measurements can adsorb on PCC giving it a negative charge. But when the PCC concentration is large, the total surface area of PCC is large as well and small amounts of impurities per particle will affect the charge very little. When the concentration and the total surface area are low, the amount of impurities per particle is much larger and thus when PCC is used in papermaking and added to process water which contains dissolved colloidal and anionic substances it will quickly change its charge from positive to negative. Hence when positively charged PCC is required in papermaking in order to promote the deposition on negatively charged fibers it must be dispersed in clean water and added as late as possible to the papermaking suspension to minimize the adsorption of anionic substances (Vanerek et al., 2000). When calcium carbonate is used as a filler in papermaking, it can be assumed that the pH value is about 8.0. It could then be expected that the zeta potential of the calcium carbonate is positive. This is indeed the case for precipitated calcium carbonates. Ground chalks are often negatively charged, which depends on the silica and phosphate ions coming from silicon algae and skeleton residues in the chalk. The surface of the calcium carbonate is amphoteric, which means that both anionic and cationic polyelectrolytes are expected to function as retention aids. Carbonate fillers are attacked by acids, alum and other chemicals, as a result of which carbon dioxide is released and the corresponding salt is formed. This reactivity with acid material constitutes the main obstacle for the use of carbonate filler in papermaking. Calcium carbonate is very suitable in alkaline systems where it contributes towards high whiteness and ageing resistance of the paper. Because of its irregular 13 particle structure it gives low paper gloss. The scattering coefficient of a particular calcium carbonate is dependent on the morphology (Passaretti, 1993). High opacity PCC has an isotropic morphology that is very similar in appearance to titanium dioxide. The morphology of high opacity PCC is rhombohedral, resembling a cube that is twisted such that it contains no 90° angles. This unique morphology and average particle size of 0.25 to 0.45 microns results in an improved scattering efficiency (Passaretti, 1993) which is half of titanium dioxide. A brightness level of 90-95% is one of the most desired properties of calcium carbonate and the main reason for its use.(Evans, 1991). Calcium carbonate is the whitest filler available in the same price as clay. The particle size varies within wide limits but is usually between 0.3 and 3 micrometers. The transition to neutral systems for papermaking is a condition for the increasing use of calcium carbonate as filler. Most alkaline paper systems operate at a pH of 7-8, where calcium carbonate is having a weak positive charge. The presence of trace organic tends to reverse the zeta potential, making it negative at pH values less than iso-electric point. The pH is a log scale measurement, so one unit change in the pH is a ten fold change in the concentration of the H + ions. Hence when a system pH changes from 5.0 to 8.0, the concentration of the H + ions in the system is 1000 times lower. The system is now rich in OH" ions. Since all retention mechanisms rely on charge and most functional additives are ionically charged, their functionality is significantly impacted by a change in pH of 5.0 to 8.0 (Gess, 1998). 1.2 Retention Figure 1.7 shows a typical wet end of a paper machine with various equipment like screens, cleaners and head box. The stock enters on the paper machine through the head box and the water drained on the wire is collected and used for dilution of the incoming thick stock. Some of the water is sent to the saveall where the fibres and fillers are recovered and used and the clarified water is used in different operations for cleaning purposes. The need to retain fibre fines and fillers draining from the white water system became compulsory for economics and the cost of utilization of various additives. The final product and water are the output of the system. If all the fibres are not converted 14 Head box Stock proportioner Figure 1.7 Typical wet end of a paper machine (Gess, 1998). into the final product, these fibres would flow with the water into the sewer and when this water is recirculated, an accumulation of fines in the system would rise leading to process problems and quality problems. If the retention of the system is low it would not be cost effective. Hence special chemicals known as retention aids are added to prevent the loss of additives and fines. This also helped in reducing the load on the effluent treatment plant (Smook, 1992). The term "retention" is defined as the relative ability to hold the various components of the furnish in the sheet as it consolidates on the forming fabric. Two parameters are used to measure the retention of fibres and additives during papermaking. They are overall retention and single-pass retention (Kocurek, 1992). (1) "Overall retention" is defined as the ratio of the total amount of material that is going in to the paper machine wet end compared to the total amount coming off the reel at the dry end(final production). The overall retention should be in the range of 90-95% for the process to be economical(Kocurek, 1992; Smook, 1992). 15 overall retention % amount retained in sheet X 100 amount added with stock (3) (2) "First-pass retention" compares how efficiently a given component in the head box stock is retained on one pass through the forming fabric. This can be obtained for fibre fines or any other component of the head box (Kocurek, 1992;.Smook, 1992). amount retained with sheet single-pass retention % = X 100 (4) amount from head box Typical first-pass retention is in the order of 75-80% for sheet grammages ranging from 60-80 gsm. (3) " Ash retention" is the ratio of the amount of ash (inorganic material like fillers) going with white water to the amount of ash present in the head box. ash in white water ash retention % = X 100 (5) ash in head box Typical ash retention is in the order of 55-70% for sheet grammages ranging from 60-80 gsm (Kocurek, 1992). The mesh of the forming fabric is defined as the number of machine direction strands present per centimeter to the cross direction strands present per centimeter. The selection of the mesh of the wire is based on the type of paper to be manufactured and the drainage capacity of the paper machine. Most commonly used wire mesh are 60 x 40 mesh and 60 x 60 mesh (Kocurek, 1992) As the modern papermaking systems have a high degree of closure to reduce the volume of effluent, losses of certain constituents can still be substantial. Paper quality 16 and papermachine operations are more affected by single-pass retention (Smook, 1992). A low level of retention indicates a high recycle rate of furnish materials with the recirculating white water, which gives rise to non-uniform distribution in the cross direction of the sheet and may contribute to the two sidedness (Kocurek, 1992). The accumulation of fines and additives in the head box loop retards drainage, and the fines absorb a disproportionate amount of certain additives by virtue of their high specific surface. Also, pitch and slime have a greater propensity for buildup and agglomeration, and are generally more difficult to control. The major factors that affect retention are pH, stock temperature, fibre characteristics and the degree of closure of the system (Smook, 1992). The production parameters like sheet grammage, sheet formation, fabric characteristics, type of dewatering elements, machine speed and shake affect retention. The type of additives added like mineral fillers, order of addition, ionic balance and level of anionic trash determines the retention (Smook, 1992). In order to achieve effective retention, it is necessary that the filler particles, fibre fines, size and starch be flocculated and/or adsorbed on to the large fibres with minimal flocculation between the large fibres themselves. There are three major groups involved in the wet-end chemistry : solids, colloids and solubles. Most attention is focused on the solids and their retention. It is important to cause the fines and fillers to approach each other and form bonds or aggregates which are stable, to shear forces, in order to maximize retention. The smaller molecules present in the stock are a nuisance and consume chemicals. Their natural retention is zero and they have a negative effect on process control and runnability. Conductivity is becoming a problem as mills are closing the white water system in order to use less water. The conductivity of a system is related to the presence of monovalent and divalent ions like sodium, chloride and sulfate ions. The higher the conductivity of a papermaking furnish, the less will be the ionic attraction forces between a polymer and the components of a papermaking furnish. 17 1.2.1 Retention Aids A number of retention aids are available to the papermaker. Since these chemicals act mainly through flocculation and entanglement care should be taken in their utilization (Kocurek, 1992). These chemicals affect stock drainage and sheet formation. The dispersive action of the head box system should be adequate to avoid overflocculated stock that would be detrimental to the sheet formation. Papermakers alum is still commonly used retention aid for acidic papermaking. The aluminum polymer has a significant flocculating effect by bridging from particle to particle and thereby forming large ionically-attracted floes (Smook, 1992). But as the trend is shifting towards alkaline/neutral papermaking, the use of polymers like > polyethylene oxide, polyacrylamide, polyethylene imine, colloidal silica, bentonite clay, etc. is becoming popular. These are used as single polymer or in conjunction with one or more polymers which is the modern trend. These systems not only increase the first pass retention but also increase the ash retention. The first- pass retention is in the order of 75-80% for low sheet grammages. The ash retention is desired in the range of 60-70%. In addition to this they give good drainage and sheet formation. As the papermachine speed is increasing above 1000 m/min the need for faster drainage with maximum retention is becoming necessary without sacrificing the sheet formation. The use of retention aids in the machines producing paper from mechanical pulps is also increasing as they are changing towards the alkaline/neutral papermaking. Acid papermaking operates at a pH of 4.2-5.0. Rosin is used as an internal sizing agent in acid papermaking. Internal sizing is the treatment of the fibers that will be formed into a sheet of paper that will make them resistant to the sorption of water. It contains a complex mixture of carboxylic acids often referred to as rosin acids. A l l rosin acid derivatives are in a tricyclic acid form, because they are made with fumaric acid. Additional acidic groups are introduced into rosin with maleic anhydride. At a pH of 4.2-5.5 these carboxylic anhydrides are relatively stable to hydrolysis. At an alkaline pH, the hydroxyl ions attack the anhydride, saponifying it and thus adding an anionic functionality in the process. As a result rosin's ability to function as a hydrophobic molecule begins to decrease at a pH above 6.5. for this reason rosin cannot be used in alkaline/neutral papermaking as a internal sizing agent. Synthetic sizing agents were 18 developed for alkaline/neutral papermaking system. The most widely used are alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) (Gess, 1998). Papermaking technology is continuously advancing with development of faster and more sophisticated paper machines and various chemical additives. Since the amount of fines in the wet end of the paper machine influences retention, drainage, system closure, etc., promoting proper flocculation of fines and fillers for economic benefits and process improvements is desirable (Shin et al., 1997). Since mechanical means and process modifications have limits in achieving this goal, resorting to chemical means is necessary. The role of retention is critical in papermaking, especially alkaline papermaking. Most colloidal particles in the papermaking system are negatively charged. Because of this, cationic polyelectrolytes that adsorb readily on to anionic surfaces by electrostatic interactions have been the primary choice of flocculants for papermaking. 1.3 Retention of fines and fillers in mechanical pulp suspensions Mechanical pulp furnishes contain a large amount of fines typically about 1/3 of the mass of pulp (Tay and Canley, 1982). Fines are particles of the furnish which will pass through a 200 mesh screen (75 micrometer holes). The fines which are found in all wood pulp systems are produced primarily by mechanical and hydrodynamic shearing of cellulosic material from the fiber surface. Small fiber fragments, vessels and ray cells are also considered as fines (Walkush, 1970). Dissolved and colloidal substances (DCS) are carbohydrates, lignin residues, resin and fatty acids originating from the wood species, fibers, additives, broke or from the bleaching process (Pelton et al. , 1980). The need for higher brightness and sheet opacity has also led to the addition of fillers to these pulps. In order to retain these, retention aid addition has become compulsory for mechanical grade papermaking. Most widely used polymeric flocculants are polyacrylamides (PAM), polyethylene imine, polyamines, polyamidoamines, polyethylene oxide(PEO), polydiallyl dimethyl-ammonium chloride(poly-DADMAC), modified starches, bentonite clay, colloidal silica,etc. have found extensive use (Shin et al., 1997). Polyethylene oxide(PEO) is used for retention of mechanical pulps because it is highly soluble in water and non-ionic in nature. It is used in conjunction with a cofactor usually a phenolic resin. PEO is a high molecular weight resin consisting of repeating ethylene oxide units (-CH2-CH2-0-)n (Bailey and Koleskee, 1990; Braun and DeLong, 19 1999). They are produced by heterogeneous catalytic polymerization of ethylene oxide (Braun and DeLong, 1999). Molecular weight ranges from 0.1 to 8 x 106 Da. Polyethylene glycols (PEG) are low molecular weight ethylene oxide polymers (<100 000) (Gaudreault, 1997). PEG and PEO vary in properties due to the increased effect of reactive hydroxyl end groups on the shorter PEG chains(Headrick and Bollinger, 1998). Near the boiling point of water (approximately 95 °C) the PEO-water system separates into a PEO-rich and PEO-lean phase (Florin et al., 1984; Khoultchaev et al., 1997a, b; Englezos et al.,1998). The temperature at which phase separation occurs is referred to as Cloud Point Temperature(CPT). The presence of inorganic salts in aqueous PEO solutions can reduce the CPT. Above the CPT the structure of PEO is modified and it becomes thermodynamically advantageous for it to leave the water phase and adsorb onto a surface (Khoultchaev et al., 1997). Ever since PEO was introduced as a retention aid in newsprint and groundwood specialty papermaking, controversy has surrounded the mechanism by which it acts. Initially proposed as a neutral bridging macromolecule, shown to be less sensitive to anionic trash than cationic polyelectrolytes, it was found that its efficiency varied from mill to mill and that a second component was often required for it to work properly. Cofactors containing phenolic groups, such as Kraft lignin and phenolic resins were found to be effective. The PEO/Kraft lignin system was originally proposed by Pelton and his co-workers, as a means of overcoming the anionic trash. Since PEO is neutral, no electrostatic interactions between PEO and anionic trash were expected. PEO alone was found to be ineffective in clean water but performed well in white water (Pelton et al., 1980). The role of a cofactor is to trigger or to enhance the PEO adsorption and to preserve the established bridge formed between fibre and filler particle (van de Ven and Allince, 1996). Subsequent laboratory studies showed that the dual component retention aid system works for systems even in which neither PEO nor the cofactor adsorbs on to the fibres and fillers, thus apparently refuting the polymer bridging mechanism. Instead it was proposed that PEO and phenolics form a network in water in which fillers are captured and subsequently swept by the fibers (Lindstrom et al., 1990). The fact that smaller particles were retained less effectively than larger ones was explained by the size 20 of the holes in the network. The network hypothesis is incorrect and shows evidence that fillers are retained instead by a mechanism of association-induced polymer bridging (van de Ven etal., 1996). Phenolformaldehyde resin (PFR) and tannic acid were investigated by Lindstrom and coworkers and were found to be effective (Lindstrom et al., 1990). Neither PEO nor PFR adsorbs on fibres and hence they concluded that polymer bridging was impossible and proposed instead a network mechanism. The association of PEO and Kraft lignin was studied by Picaro and van de Ven who found no evidence of network in PEO/sulphonated Kraft lignin (SKL) solutions (van de Ven and Picaro, 1994). In any system containing colloidal particles that can deposit on fibres and which is subjected to flow, large particles are deposited preferentially. This effect was predicted theoretically by Petticki and van de Ven and confirmed by Alince et al (1994) who showed aggregates of fillers deposit faster on fibres than well-dispersed particles (Alince et al., 1996). The collision mechanism explains why filler retention is enhanced at higher fibre concentrations (Lindstrom and Glad, 1984): the deposition frequency is simply increased. Another argument against the network mechanism is that at certain PEO/SKL concentrations for which efficient retention of clay occurs, the retention of larger calcium carbonate particles is absent (van de Ven and Allince, 1996). When PEO and PFR are used together the PFR appears to adsorb on the microcrystallme cellulose. Similarly, the adsorption of PEO on pulp fibre may take place in the presence of phenolic resins (van de Ven et al., 1998). PEO can by itself cause flocculation by a process called asymmetric polymer bridging . Here the PEO acquires the ability to adsorb on the fibres after first having adsorbed on fillers a process known to occur between Kraft fibres and kaolin fillers. Thus recent studies suggest that PEO/cofactor retention aid systems function by the mechanism of filler-fibre hetroflocculation caused by PEO/cofactor complexes(van de Ven and Allince, 1996). 21 /1 /1 /1 /1) I U.U.J t.t/u / 11 / 7 / I f TI a) b) c) Figure 1.8 Schematics of association-induced polymer bridging.(a) A flexible chain does not adsorb onto the fibre or filler.(b) When the chain is stiffened with the cofactor, polymer adsorption and bridging can occur.(c) Bridging also can occur when the cofactor can link the PEO molecule to a surface. (Alince and van de Ven, 1996). 1.3.1 Retention of precipitated calcium carbonate The use of calcium carbonate in mechanical pulps is increasing in order to attain higher brightness and sheet opacity. Also the on site availability of precipitated calcium carbonate (PCC) has facilitated its increased use as filler. The charge of PCC particles in distilled water is positive and near the iso-electric point, and it readily deposits on fibres suspended in distilled water (van de Ven et al., 1997). The reason for the deposition is electrostatic and van der waals attraction between negatively charged fiber, or uncharged PCC particles. However this mechanism of mutual attraction operates only in distilled water. In white water the PCC acquires a negative charge and does not therefore deposit on negatively charged fibers (van de Ven et al., 1997). The surface charge of PCC depends on its concentration, varying from negative at low concentrations to positive at higher concentrations. This observation was confirmed for both untreated GCC and PCC dispersed in distilled water (Siffert and Fimbel, 1985). The positive charge is believed to originate from the fact that calcium carbonate is partially soluble in water and the C a + 2 ions tend to readsorb preferentially on the crystal surface. The concentration of impurities in distilled water which is about 10"7 mol/lit is 22 below the detection limit of conductivity measurements can adsorb on PCC giving it a negative charge. But when the PCC concentration is large, the total surface area of PCC is large as well and small amounts of impurities per particle will affect the charge very little. When the concentration and the total surface area are low, the amount of impurities per particle is much larger and thus when PCC is used in papermaking and added to process water which contains dissolved colloidal and anionic substances it will quickly change its charge from positive to negative. Hence when positively charged PCC is required in papermaking in order to promote the deposition on negatively charged fibers it must be dispersed in clean water and added as late as possible to the papermaking suspension to minimize the adsorption of anionic substances (Vanerek et al., 2000). PCC spontaneously deposits on the bleached Kraft wood pulp as studied by Kamiti and van de Ven (1994). However in mechanical pulps there are sufficient DCS to adsorb on to PCC particles to render them negatively charged (Sanders and Schaefer, 1995) . Thus a flocculant system that is not neutralized by the DCS is required for good PCC retention (Pelton, 1996). When PEO is added to a mechanical pulp containing PCC there is no adsorption of PEO on the pulp and probably little or no adsorption on the pigment and consequently no deposition takes place in the presence of PEO only(van de Ven, 1997). Low concentrations of S K L in combination with PEO have no effect, yet at high S K L addition very effective deposition can be achieved(van de Ven and Allince, 1996). When S K L is added it adsorbs on the pigment, but no adsorption on the fibre was detected (van de Ven et al., 1997). Stiffening of PEO chains by a cofactor can induce polymer adsorption and thus also polymer bridging. Another way of inducing polymer bridging is for the cofactor to adsorb on to both surfaces (fibre and filler) followed by adsorption of the polymer onto the adsorbed cofactor. In such a case the cofactor itself serves as a bridge linking PEO to a surface (van de Ven and Allince, 1996) . The operational mechanism described as association-induced bridging is applicable. But this happens at a relatively large excess of S K L up to lOOmg/g of PCC. PCC carries a slightly positive charge, is unstable and deposits readily on negatively charged pulp fibres. In the presence of S K L however the rate of deposition is slowed down, as is the steady-state coverage of PCC on the fibres. S K L being negatively charged adsorbs on to PCC and reverses the charge from positive to negative thus causing 23 electrostatic repulsion between fibres and PCC fillers (van de Ven and Allince, 1996). The more S K L is added and adsorbed, the more stable suspension i.e. larger the repulsion between PCC fillers (van de Ven et al., 1997). These negatively charged particles deposit with a lower efficiency on pulp fibres because of the electrostatic repulsion between the negatively charged pulp fibres and negatively charged fillers, (thus causing reduction in the rate of deposition) and they are easier to remove from the fibre for the same reason (thus causing a lower steady-state coverage). In the presence of PEO the flocculation and deposition of PCC proceeds in the same way as that with S K L alone. This pattern is apparent at low S K L additions and implies that under these conditions PEO does not adsorb on PCC, despite the fact that S K L molecules are adsorbed on PCC surface. It appears, therefore, that adsorbed S K L molecules do not form an association with PEO. It is the S K L which appears to dominate the PEO performance. Hence the choice of a proper cofactor wil l determine whether the dual system becomes an effective retention aid for PCC. It is possible to retain PCC with PEO, but S K L cofactor is not an efficient cofactor (van de Ven et al., 1997). Two commercial cationic polymers were compared with an experimental polymer (Pelton et al., 1996). The PCC retention without polymer was less than 15% in a newsprint furnish consisting of 15% semi-bleached Kraft pulp and 85% mechanical pulp ( 60% CTMP and 40% SGW). The maximum retention was obtained with cationic P A M and bentonite. The second commercial polymer poly(DADMAC) and acrylamide/sodium acrylate gave better retention but not as bentonite. The experimental flocculant polyacrylamide 3-methacryloamino propyl trimethyl ammonium chloride (poly A M / M A P T A C ) and methoxy poly(ethylene glycol) methacrylate (MPEGMA) gave good PCC retention at an optimum polymer dosage. PCC retention of 50% was obtained. Usually flocculants based on PEO require a phenolic resin cofactor to be effective but the use of po ly (AM/MPATAC/MPEGMA) gives PCC retention without a phenolic resin. The presence of the phenolic resin reduced the effectiveness of this terpoplymer (Pelton et al., 1996). PFR together with high molecular weight PEO constitutes an effective flocculant for pulp fines in newsprint pulp but is not an effective retention aid for PCC because there is no adsorption of PEO on the pulp and probably little or none on the pigment (van de Ven and Allince, 1996). 24 The PCC retention was increased further when sodium carbonate was used in conjunction with either PEO or the co-polymer. In fact PFR co-polymer in the presence of sodium carbonate was the most effective combination of additives. Addition of sodium carbonate lowered the free calcium ion concentration from 3mM to 0.3mM (Pelton et al., 1996). The retention was not dependent on the order of addition of sodium carbonate and PCC to pulp. Cationic terpopolymer poly(AM/MAPTAC/MPEGMA) addition increased the electrophoretic mobility of PCC suggesting that the cationic polyelectrolyte adsorbed on to the cationic particles. Regardless of the type of polymeric retention system used the scalenohedral PCC was retained to a greater extent compared to the two hexagonal prismatic forms of PCC (Gill, 1990). M i l l experiences with acid tolerant(AT) PCC were studied in papers containing mechanical pulps (Ain and Laleg, 1997). PCC as found effective in increasing the optical properties and it was also effective in raising the coefficient of friction of paper one of the few fillers which are capable of accomplishing this a benefit for good roll winding and web guiding during printing. The high specific area and the slight cationic charge of the PCC surface adsorb colloidal pitch, preventing deposition (Whiting, 1996). A comparative study was performed with cationic retention aids and PEO/PFR system. At high degrees of system closure the first pass retention of the cationic polymers was negatively affected. Conversely the non-ionic retention aid system of PEO/PFR excelled and gave maximum retention. It also gave a good drainage (Allen et al., 1999). Finally ,temperature was found to have no effect on PCC retention (Pelton, 1996). 1.3.1.1 Review of previous work done on P C C in our Laboratory Previous work done by Trigylidas (1999) used a starch based fixative as a retention-enhancing agent for PCC-filled mechanical groundwood pulp suspensions. The starch based fixative was Raifix 120 which was used in conjunction with Polyethylene oxide and Phenol formaldehyde resin. Raifix products are highly cationic, low molecular weight polymers. Raifix polymers are manufactured from high molecular weight starch polymers consisting of linear (amylose) and branched (amylopectin) polymers of glucose units. The source of the cationic charge is a quaternary amine, which can retain its charge over a wide pH 25 range. Raifix polymers have the ability to hydrogen bond due to the abundance of OH-groups on the glucose monomers. Raifix polymers are highly water-soluble and can be supplied as solutions. Aggregation experiments were performed with PCC, PEO, PFR and Raifix 120. It was found that PEO was capable of aggregating PCC on its own at a dosage of 0.5 mg/g. But when PEO dosage was increased to 1.0 mg/g the rate of aggregation and aggregate size decreased. Ratio of PFR to PEO of 1.0 was found to be optimum for the higher rate of aggregation and maximum aggregate size. Raifix had a detrimental effect on the aggregation of PCC. Raifix, PFR and PEO when used together, aggregation of PCC was adversely affected. First pass retention experiments were performed in a dynamic drainage jar using Raifix, PFR and PEO. The PCC loading was 20% on oven dry pulp. At 0.1 mg/g of PEO and 0.05 mg/g of PFR the fines and PCC retention were 67% and 60% respectively. As Raifix dosage was increased from 0 to 5mg/g, the retention gradually decreased. The fines and PCC retention were 59% and 45% respectively. The conclusion was that Raifix over- charged the aggregates and interfered with PEO-PFR flocculation mechanism (Trigylidas, 1999). 1.3.2 Pretreatment of Calcium carbonate Pretreatment of calcium carbonate is necessary i f it is used in mechanical pulp suspensions for neutral/alkaline papermaking system. This is because mechanical pulps have a pH of around 5.50 and we know that calcium carbonate solubility is high at acidic pH. This limits the use of calcium carbonate for acid papermaking systems. Another problem encountered is the alkaline darkening at higher pH (8.5 and above). Many methods are used for pretreatment. One of this methods was proposed by Pang et al. (2000). Phosphoric acid was used for pretreatment (0.2g/g of GCC) and mixed for 24 hours. The time factor is again important so that the inhibition of calcium carbonate is accomplished by growing stable species of calcium and phosphate on the surface which inhibit dissolution. Many methods of pretreatment have been developed with weak acids like citric and acetic acid. 26 1.4 Microparticle retention aid systems In today's changing environment, a retention and drainage program utilizing a microparticle component, such as, bentonite clay or colloidal silica with a conventional single polymer or dual polymer program are finding increasing use in paper industry. These microparticle programs are superior to conventional polymer programs. Increased filler retention and drainage rate with no sacrifice in formation have been the primary reasons for a mill's transition to a higher performance microparticle program. Such a program is specifically designed to promote high fibre and ash retention without affecting the formation and drainage. Colloidal silica in conjunction with a high molecular weight polymer is used as microparticle retention aid system. It may also be used with the combination of cationic starch, alum and/or highly charged, low molecular weight cationic coagulant (Duffy, 1993). Colloidal silica is a stable dispersion with anionic particles approximately four millimicrons size. It is usually added after the high molecular weight flocculant. Due to its size it has an extremely high anionic surface charge density. These sub-micron sized spheres provide very large surface areas, up to 750 sq.metres per gram of silica. Figure 1.9. Colloidal silica particle.(Duffy, 1993). A colloidal silica particle is shown in figure 1.9. The surface of the particle is composed of silica groups that are partially ionized. A counter ion is present as a stabilizer and the resulting system is in equilibrium. The benefit of using small particle is large surface area. Highly charged colloidal silica particles significantly enhance small 27 particle retention by forming ionic bonds with any cationic additive adsorbed on the surface of the furnish components. The most efficient source of this cationic donor is the synthetic coagulant. The cationic source will be attracted and will adsorb onto the anionic surfaces of the fibres, fillers and fines. These adsorbed coagulants form "cationic patches" which are available for bonding. By adding colloidal silica, small microflocs are formed between the furnish components and the small colloidal particles in the system. Figure 1.10. Small microflocs formed between the furnish components and the small colloidal particles in the system (Duffy, 1993). The microparticle flocculation mechanism differs from conventional flocculation in its ability to reflocculate after shear. Even as the web is " travelling " down the wire, shear forces are exerted as it passes over each foil. The bonds formed during conventional flocculation with high molecular weight polymers are not easily reformed. They are essentially lost forever and as a result fibre and filler retention suffers. With microparticle retention, these bonds can reform immediately after the shear is removed, forming tighter, denser floes (Juntai. 1995). Another microparticle system uses three components of polydiallyl dimethyl aluminum chloride(DADMAC), a cationic polyacrylamide and a modified bentonite clay. Poly(DADMAC) a cationic coagulant with higher molecular weight is added first to the stock. The purpose of this polymer is to shift the charge of the system to make the stock FRESH CATIONIC CHARGE 28 less anionic and therefore, bring it closer to the iso-electric point. By doing this, the efficiency of the polymer component of the microparticle system would be significantly enhanced. A relatively low cationically charged high molecular weight polyacryl amide is added at the inlet of the vertical screen. By adding in this position, sufficient mixing and shear is available to deflocculate the stock and condition it to react with bentonite. The modified bentonite (0.5% concentration) is added at the outlet of the vertical screen. The microparticle component of the system reacts with the "coagulated" filler, fines and fibre surfaces to give the microparticle flocculation characteristic of the process (Ford et al., 1990). First pass retention percentages have been increased from the 70's to 90's without adversely affecting formation after using microparticle retention system. Ash retention have also been increased thereby reducing the cost of furnish. Due to the increased drainage afforded by the microparticle program, machine speeds have increased by 15% (Duffy, 1993). The microparticle retention program improves the porosity of the sheet which is responsible for reduction in the steam energy required to dry the web (Lindstrom et al., 1989). It is also believed that the addition of colloidal silica centres around the replacement of the hydrogen bonds with stronger ionic bonds. When this occurs, the floe collapses under its own weight and releases water more easily. The benefit is improved drainage on the forming table (Duffy, 1993). 1.5 Research objectives The scope of this work is to evaluate the retention of precipitated calcium carbonate (PCC) in papers made from mechanical pulps. Polyethylene oxide (PEO)-phenol formaldehyde resin (PFR) system will be used as a retention aid in conjunction with Raifix and Raissapac as potential enhancers. The ratio of PFR/PEO wil l be 1.0 based on the previous work (Trigylidas, 1999). A l l the first pass retention experiments will be performed with a 70 mesh screen. The retention of calcium carbonate achieved by the PFR-PEO dual retention aid system is low (Trigylidas, 1999; a van de Ven, 1996). Raifix was found to enhance clay retention, but this was not the case with calcium carbonate in particular PCC (Trigylidas, 1999). When PCC is introduced in mechanical pulp suspensions the pH rises to 8.5 or higher causing darkening of pulp. Hence, the pH should be kept in the operating range of 6.9 to 29 7.2 for neutral/alkaline papermaking. Pang (2001) proposed a pretreatment process for calcium carbonate using phosphoric acid. This method was able to control the pH in the operating range. The specific objectives are as follows:-1. First pass retention of pretreated PCC experiments will be conducted with PFR-PEO and subsequently a third component (anionic trash collector) will be added. The third component is Raifix 120 or Raifix 2515. The retention experiments with ATC will allow the selection of the best A T C . 2. The optimum sequence of addition of pretreated PCC, A T C (Raifix 120, Raifix 2515), PFR, PEO will be determined. 3. First pass retention with pretreated PCC and PEO-PFR system along with an inorganic polymer Raisapac will be done. The best sequence of addition will also be determined . 4. Retention experiments will be carried out at 1000 rpm. In addition, we will carry out experiments at 1200 and 1400 rpm at best chosen conditions. Finally the elucidation of mechanics will be aided by the zeta potential, aggregation and cationic demand experiments. 30 CHAPTER 2 Experimental methods and apparatus 2.1 Materials 2.1.1 Anionic Trash Collector (ATC) Two types of cationic starches A T C was supplied by Raisio Chemicals (Vancouver, British Columbia). The commercial name for the A T C is Raifix. Two types of Raifix were used for the experiments. Raifix 120 has a charge density of 2.8 meq/g and a molecular weight of 0.5 to 1.5 million. It has a cationic charge. Raifix 2515 is a higher molecular weight and has a cationic charge approximately 25% than that of Raifix 120. It was received in solution and stored in a sealed container at 5°C. The solution was obtained at 20% concentration and used at 0.01% for the experiments. An inorganic polymer Raisapac was also supplied by Raisio chemicals. It's chemical name is poly hydroxy aluminum chloride. The basicity was around 30 %. This was obtained in a liquid form and stored. It has a very low pH of 1.0. The solution was supplied at 100 % strength. It was prepared at a low concentration of 0.1 % and was used within 3 hours of preparation to avoid change in basicity. 2.1.2 Ethylenediaminetetraacetic acid (EDTA) Analytical grade EDTA was supplied by V W R scientific (Products of West Chester Pennsylvania). IT was received as a 0.01M solution and stored in a sealed container at room temperature. EDTA was used for the detection of calcium ion concentration. 2.1.3 Phenol Formaldehyde Resin (PFR) Cofactor A phenol formaldehyde resin was supplied by Raisio Chemicals of (Vancouver, British Columbia). The commercial name of the cofactor is Netbond FRB. The cofactor was received at 36% concentration and was stored at 5°C with no direct contact with sunlight. The solution was used at a concentration of 0.01 % in the experiments. 31 2.1.4 Phosphoric acid Phosphoric acid was obtained in a solution form at a concentration of 85% from Aldrich Chemical Company (Milwaukee, Wisconsin). 2.1.5 Poly(diallyl dimethyl ammonium chloride) ( P D A D M A C ) Analytical grade P D A D M A C was supplied by Muetek Analytic GmbH (Hamilton, Ontario). It was received as a 0.001 N solution and stored in a sealed container. This was used as a titrant for cationic demand analysis for negatively charged systems. 2.1.6 Poly(ethene sodium sulfonate) (Pes-Na) Analytical grade Pes-Na was supplied by Mutek Analytic GmbH (Hamilton, Ontario). It was supplied as a 0.001 N solution and stored in a sealed container. This was used as a titrant for cationic demand analysis for positively charged systems. 2.1.7 Polyethylene oxide) (PEO) PEO with an average molecular weight of 7 million was supplied by Raisio Chemicals (Vancouver, British Columbia). The commercial name of the PEO is Netbond FRA. PEO was received in fine powder form. PEO stock solutions were prepared at 0.01% wt. stock solution and was gently mixed for 24 hours at room temperature with minimal exposure to sunlight. 2.1.8 Precipitated Calcium Carbonate (PCC) Two types of PCC filler was obtained from Specialty Minerals (Adams, M A , USA) with commercial name Albafil and Albacar HO. The PCC was received in a powder form and stored in a sealed bottle. Various properties of the filler are given in Table 2.1. Table 2.1 Filler Properties Albafil Albacar HO Moisture content (%) 0.15 0.2 Dry Brightness 98.1 97.4 Percent +325 Mesh (%) 0.018 0.20 32 2.1.9 Pulp 100% TMP pulp was used which was supplied by an integrated pulp and paper industry in British Columbia. The TMP pulp was collected from the TMP storage chest and was shipped at a consistency of 8%. The pulp was preserved in a sealed container at 5°C. The pH of the pulp was 5.5. 2.1.10 Sodium Hydroxide Sodium Hydroxide was supplied by Fischer Scientific (Nepean, Ontario). It was received as a 0.1 M solution and stored in a sealed container. 2.1.11 Water A l l the water used in the experiments was distilled and de-ionized using an Elgastat UHQ (Ultra High quality) unit. The Elgastat UHQ was supplied by Fischer Scientific (Nepean, Ontario). 2.2 Instrumentation and Analysis 2.2.1 Dynamic Drainage Jar (DDJ) The DDJ is a standard instrument used to perform first pass retention and fines fractionation experiments (refer to Figure 2.1). The DDJ is a single screen classifier which is used to stimulate the wet end of a paper machine. The DDJ is made up of a plastic cylinder with a detachable base. The base contains an air chamber, on top of which a wire mesh is placed. The DDJ is drained from a flow controlled plastic tube connected to the base. A glass Col variable speed stirrer simulates the shear in a paper machine. The shaft of the stirrer is 150 mm long and carries a three-blade propeller. The blades are circular, with a diameter of 17.5 mm and have a pitch of 30°. The distance between the propeller and the screen is approximately 3 mm. The stirrer is connected to an integrally-wound motor generator with servo feedback amplifier type motor drive. The stirrer motor contains 99% of set speed within the range of 50 rpm to 3000 rpm. The direction is counterclockwise, so that the material is pushed towards the screen. The DDJ was supplied by Paper Research Materials Inc. (Gig Harbour, Washington, USA). 33 Figure 2.1 Britt Dynamic Drainage Jar 2.2.2 p H and Conductivity Meters Conductivity measurements were performed using the Orion Model 160 Conductivity Meter. pH measurements were performed using the Metrohm 691 pH Meter. The instruments were supplied by Fischer Scientific Ltd. (Edmonton, Alberta). 2.2.3 Malvern Masterisizer 2000 with Hydro M U Cell The Malvern Mastersizer 2000 is a laser diffraction particle size analyzer. The Mastersizer 2000 is equipped with the Hydro M U cell. The Hydro M U cell is used to analyze particle in liquid suspension. The cell is connected to a re-circulation unit. The unit can accommodate a beaker. It also contains a variable speed stirrer (0 to 2500 rpm) 34 Figure 2.2 Malvern Masetersizer for particle size analysis. and an ultrasonic mixer. The Malvern Mastersizer 2000 and Hydro M U Cell was supplied by Malvern Instruments (Southborough, Massachusetts, USA). The instrument is shown in figure 2. The Mastersizer was used to study the aggregation of fillers and characterizing the fillers. 2.2.4 Mutek P C D 03 Particle Charge Detector The Muetek PCD 03 is a standard instrument used to measure cationic demand of pulp suspensions and white water in the paper industry. It was used to measure the cationic demand of the DDJ filtrate during a first-pass retention experiment. The Muetek PCD 03 cell consists of a plastic vessel with a piston. The piston connects to an oscillating motor, which makes it move up and down in the cell orifice. The cell can hold up to 10 mL of sample. This is also equipped with an automatic titrator which titrates to the filtrate to zero charge. The instrument was supplied by Muetek Analytic GmbH (Hamilton, Ontario). 35 Figure 2.3 Mutek particle charge detector for cationic demand analysis. 2.2.5 Mill ipore Pressure Filter The Millipore pressure filter column was used to filter the DDJ filtrate. It is a stainless steel cylinder with a removable head at each end. The bottom head contains a wire mesh upon which a disposable membrane is placed. The top head contains a pressure valve. When air pressure is applied, the suspension is filtered through the membrane and the filtrate is collected from an outlet valve in the bottom head. The pressure filter and the membranes were supplied by Millipore Corporation (Nepean, Ontario). 2.2.6 Malvern Zetasizer 2000 The Zetasizer 2000 was used to measure the zeta potential of the particles suspended in liquid water. The Zetasizer 2000 was supplied by Malvern Instruments Inc ( Malvern, UK) . When an electric field is applied across the solution, charged particles suspended in the electrolyte are attracted towards the electrode of opposite charge. 36 Figure 2.4 Zetasizer for measuring zeta potential. Viscous forces tend to oppose this movement of the particles. When an equilibrium is reached these two opposing forces, the particles move with constant velocity which is dependent on the strength of the electric field, the dielectric constant, the viscosity of the medium and the zeta potential. The velocity of a particle in a unit electric field is referred to as its electrophoretic mobility. Zeta potential is related to the electrophoretic mobility by Henry's equation. The unit consists of a dip cell in which the solution is pumped. A laser beam shines through this dip cell which measures the velocity of the particles. It also contains a pH electrode that measures the pH and conductivity. 2.3 Experimental Procedures 2.3.1 Fines Fractionation Fines fractionation was performed using a DDJ. The fines fraction for the experiments was defined as the fibres passing through the 70 mesh screen instead of the standard of 200 mesh screen. A 500 g of the pulp sample at a consistency of 0.5 % was taken and filtered through the DDJ. The stirrer speed was set to 1000 rpm. Once the level in the DDJ was about 5mm, 500 ml of de-ionized water water was added to the DDJ and filtered again. This was continued until the DDJ filtrate was clear. Around 6 L of water 37 was used to get a clear DDJ filtrate. The retained fibres were removed from the DDJ and transferred into a glass dish. The retained fibres were dried at 105°C for 4 hours and weighed. Once the weight of the retained fibres were known the fines percentage was calculated. The fines fractionation was repeated three times and the average was reported. The fines fractionation was carried out for 100% TMP pulp. 2.3.2 pH and Conductivity Analysis Conductivity and pH measurements were performed on pulp samples at room temperature. A study of the change in the pH and conductivity with time was done by taking a known weight of the pulp sample and adding PCC to it. The pH electrode and the conductivity probe was immersed into the beaker and the change in the pH and the conductivity was noted with time. 2.3.3 Aggregation of Filler Aggregation experiments was performed using Malvern Mastersizer 2000. A filler solution of 0.01 % was made. 500 g of this sample was weighed in a beaker and placed in the Hydro M U cell. The particle size of the filler was measured initially. Then the polymers were introduced at different dosages and sequences. The change in the particle size with time was recorded. A linear regression was performed and the rate of aggregation was determined. The maximum particle size reached was also noted. 2.3.4 Zeta Potential Measurements Zeta potential of the particles in suspension was measured using the Zetasizer 2000. A dilute solution (0.001%) of the particles to be measured was prepared and pumped into the dip cell of the zetasizer. The pH electrode was immersed into the sample . Three readings were taken for each sample and the average was reported. 2.3.5 Pretreatment of PCC PCC obtained from Specialty Minerals was treated with 0.2 g/g of phosphoric acid for 24 hours. A 10% solution of treated PCC was prepared and used for the experiments. The solution was constantly stirred using a magnetic stirrer. 38 2.3.6 First-Pass Retention (FPR) A 0.5 % pulp suspension and 10% treated PCC solution was prepared. Approximately 500 g of the pulp suspension was taken in a beaker and stirred at a fixed speed using a magnetic stirrer. Then 5 g of 10% treated PCC which corresponds to 20% filler loading was added to the pulp suspension and allowed to mix for one minute. The suspension was then added to the DDJ and the stirrer speed was set to 1000 rpm. A 70 mesh screen was used for all the FPR experiments. Then the polymers were added to the DDJ directly within a difference of 10 seconds between each polymer for PEO and PFR. When Raifix was used the contact time for Raifix with pulp was 20 seconds and then PCC was added and allowed to mix for one minute before adding PFR and PEO. For Raisapac the contact time with pulp was 1 minute. After the addition of the final polymer the suspension was allowed to mix for another 10 seconds and then filtered. The first 40 ml of the filtrate collected was discarded. Then approximately 100 g of the DDJ filtrate was collected in a pre-weighed beaker. This DDJ filtrate was filtered through a 0.22 p pore size filter paper using the Millipore filter column at a pressure of 40 psi. The filter cake and the membrane was removed from the column and dried in the oven for 3 hours at 105°C. The cake was then peeled from the filter paper and transferred in a pre-weighed silica crucible. The filter cake in the crucible was then ashed at 525°C in a muffle furnace for 6 hours. Care was taken that the temperature did not go above 600°C because at this temperature calcium carbonate decomposes into calcium oxide and carbon dioxide. Once ashing was completed, the crucible was placed in a desicator for 30 minutes before weighing. First pass retention of fines, fillers and total solids retention were calculated. 3 sets of experiments were performed for each condition and the average values were reported. 2.3.7 Cationic Demand Analysis Cationic demand analysis was performed on the DDJ filtrate obtained from the FPR experiments. Approximately 10 g of the DDJ filtrate was weighed and taken into the plastic vessel of the Mutek PCD cell. The piston was inserted into the cell and connected to the oscillating motor. The motor was turned on and the piston was allowed to oscillate for one minute. Then the filtrate was automatically titrated to the iso-electric point with 39 0.001 N poly-DADMAC for negatively charged particles and with 0.001 N Pes-Na for positively charged particles. The cationic demand is reported as peq/Kg of water and fines. 2.3.8 Experimental Design A 3 factorial experimental design was used to study the effect of PEO-PFR (P), Raifix (R) and Raisapac on the fines and PCC retention. The design matrix is shown below. Each run was repeated four time Table 2-2. A 3 2 factorial design matrix for first pass retention with Raifix 120. Run PEO=PFR(mg/g ofOD pulp) PEOPFR(mg/g of treated PCC) Raifix 120(mg/g ofOD pulp) Raifix 120(mg/g of treated PCC) 1 0.0 0.0 0.0 0.0 2 0.0 0.0 0.1 0.5 3 0.0 0.0 0.5 2.5 4 0.05 0.25 0.0 0.0 5 0.05 0.25 0.1 0.5 6 0.05 0.25 0.5 2.5 7 0.15 0.75 0.0 0.0 8 0.15 0.75 0.1 0.5 9 0.15 0.75 0.5 2.5 Treated PCC loading = 20% on OD pulp. 40 \ Table 2-3. A 3 2 factorial design matrix for first pass retention with Raifix 2515. Run PEOPFR(mg/g PEO=PFR(mg/g Raifix 2515 Raifix 2515 ofOD pulp) of treated PCC) (mg/g ofOD (mg/g of treated pulp) PCC) 1 0.0 0.0 0.0 0.0 2 0.0 0.0 0.1 0.5 3 0.0 0.0 0.5 2.5 4 0.05 0.25 0.0 0.0 5 0.05 0.25 0.1 0.5 6 0.05 0.25 0.5 2.5 7 0.15 0.75 0.0 0.0 8 0.15 0.75 0.1 0.5 9 0.15 0.75 0.5 2.5 Treated PCC loading =20% on OD pulp Table 2-4. A 3 2 factorial design matrix for first pass retention with Raisapac. Run PEO=PFR(mg/g ofOD pulp) PEO=PFR(mg/g of treated PCC) Raisapac (mg/g ofOD pulp) Raisapac (mg/g of treated PCC) 1 0.0 0.0 2.0 10.0 2 0.0 0.0 3.0 15.0 3 0.0 0.0 4.0 20.0 4 0.05 0.25 2.0 10.0 5 0.05 0.25 3.0 15.0 6 0.05 0.25 4.0 20.0 7 0.15 0.75 2.0 10.0 8 0.15 0.75 3.0 15.0 9 0.15 0.75 4.0 20.0 Treatec 1 PCC loading = 20% on OD pulp 41 CHAPTER-3 Results and discussion 3.1 Stability of pH of PCC containing mechanical pulp suspensions Mechanical pulps have a pH of 5.50 after bleaching. When PCC is introduced into the mechanical pulp suspensions the pH rises to 9.0. This causes alkaline darkening of pulp. The dissolution of PCC at acidic pH is also very high. Two techniques were tried to stabilize the pH in a TMP pulp suspension having a consistency of 0.5% and an initial pH of 5.50. When untreated PCC (20% loading) was introduced into the pulp suspension the pH changed rapidly and reached to an equilibrium after 10 minutes. The pH of the suspension after 15 minutes was 8.95. 10 5. 6 (BSKD O O O 0 O O O • TMP pulp + H3PCM+<untreated)PCC 0 TMP pulp + pretreated PCC V TMP pulp + untreated PCC 20 40 TTME(min) 60 80 Figure 3.1 pH changes vs time for different methods of pretreatment of PCC. To control the pH of the system two methods were tried. One was by pretreating the pulp initially with phosphoric acid (0.2g/g of PCC) and then introducing PCC into the pulp suspension. Here the pH remained stable after 5 minutes and the pH was 6.50 after 15 minutes. Another method used was by pretreating the PCC with phosphoric acid 42 (0.2g/g of PCC) for 24 hours and then introducing PCC into the pulp suspension (Pang, 2001). The pH changes rapidly at the beginning and remains constant at 7.50 after 3.5 minutes. pH after 15 minutes was 7.68. This method of pretreating PCC was adopted for the first pass retention experiments which were done for neutral/alkaline papermaking conditions. Though the pretreatment of pulp with phosphoric acid was able to maintain the pH the dissolution of PCC is high. Therefore the pretreatment of PCC with phosphoric acid was used. Figure 3.1 shows the change in pH with time for the various techniques carried out for a period of 60 minutes. 3.2 PEO, PFR system Approximately 1/3 of the mechanical pulp furnish contains anionic DCS and fibre fines. This consumes high amount of cationic polymers i f used as a retention aid. Therefore, PEO which is non-ionic is used along with PFR as a cofactor to flocculate fibre fines and pretreated PCC. 3.2.1 First pass retention of pretreated PCC, PEO, PFR system First pass retention experiments were performed with pretreated PCC, PFR and PEO. The ratio of PFR to PEO used was 1. This was decided after viewing the results from the PCC aggregation experiments performed by Trigylidas (Trigylidas, 1999). The sequence of addition of the chemicals was chosen to be PCC, PFR and PEO. The dosage of PEO was varied from 0.05 mg/g to 0.2mg/g. As seen from figure 3.2 as the dosage of PEO was increased (as was PFR) the retention of fines and pretreated PCC increased. First pass retention of fines increased from 41.6% to 68.1% while the filler retention increased from 20.9% to 83.5% and the total solids retention increased from 67.4% to 86.6%. Therefore we can conclude that PEO and PFR together act as a good retention aid system for fibre fines and pretreated PCC retention. 43 z o NH H H r/3 PH I H 100 90 80 70 60 50 40 30 1 20 10 0 • o • FIBRE FINES O PRETREATED PCC T TOTAL SOLIDS 0.00 0.05 0.10 0.15 0.20 PEO DOSAGE (mg/g of OD pulp) 0.25 Figure 3.2. First pass retention of pretreated PCC with PFR, PEO system. Ratio of PFR/PEO = 1.0. Sequence of addition is PCC-PFR-PEO, filler loading of 0.2g/g of OD pulp, 295 K , 1000 rpm, conductivity=165 pS/cm, pH= 7.2 . 3.2.2 Aggregation experiments with pretreated PCC and PEO, PFR system Aggregation experiments were performed on pretreated PCC using PEO, PFR. The sequence of addition was pretreated PCC in water (0.01%) then add PFR and lastly PEO. Figure 3.3 shows the rate of aggregation for different dosage of PEO, PFR and figure 3.4 shows the maximum aggregate size reached for different dosage of PEO, PFR. As we increase PEO, PFR dosage from 0.20 mg/g of pretreated PCC to 0.75 mg/g of pretreated PCC the rate of aggregation increases and for a dosage of 1.0 mg/g of pretreated PCC the rate of aggregation decreases. The optimum dosage to aggregate pretreated PCC from this experiments was 0.75 mg/g of pretreated PCC. Above this' dosage there is a decrease in the aggregation rate. Therefore PEO, PFR dosage of 0.75 mg/g of pretreated PCC was fixed as the optimum dosage for further first-pass retention experiments. The maximum aggregate size reached also decreased above a dosage of 0.75 mg/g of pretreated PCC. As seen in figure 3.3 the pH is 8.85 but in retention 44 experiments it was 7.20 (figure 3.2). This is because a pH of 8.85 is the natural pH of pretreated PCC in a water system whereas pH 7.20 refers to a system with the pulp (figure 3.1) for the first 5 minutes. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 PEO, PFRDOSAGE(rr^g of treated PCQ Figure 3.3 Effect of PEO, PFR dosage on rate of aggregation of pretreated PCC. Sequence = pretreated PCC-PFR-PEO, Ratio of PEO/PFR = 1.0, Temperature 295 K, 1200 rpm, Conductivity = 48.5 pS/cm, pH = 8.85. 45 a o >-u W S3 CO W H < X 0.00 0.25 0.50 0.75 1.00 1.25 PEO, PFR DOSAGE (mg/g of treated PCC) Figure 3.4 Effect of PEO, PFR dosage on aggregate size of pretreated PCC. Sequence = pretreated PCC-PFR-PEO, Ratio of PEO/PFR = 1.0, Temperature 295 K, 1200 rpm, Conductivity = 48.5 pS/cm, pH = 8.85. 3.2.2.1 Zeta potential measurements of aggregates of pretreated P C C with P E O , P F R Figure 3.5 shows the change in zeta potential of aggregates of pretreated PCC formed at different dosages of PEO, PFR. To start with pretreated PCC has a zeta potential of -17.03 mV. As PEO, PFR dosage is increased the zeta potential decreases and at a dosage of 1.0 mg/g of pretreated PCC it starts increasing. At this dosage there is an excess of PEO, PFR in the system. But PEO is non-ionic and PFR is negatively charged. Therefore there is an increase in the zeta potential. At this point we can see that there is a decrease in the rate of aggregation and maximum aggregate size reached. The viscosity of the solution is similar to that of water as very dilute solutions of polymers were used. 46 -8 -10 -12 -14 -16 •18 0.00 0.25 0.50 0.75 1.00 P E O , PF R D O S A G E (mg/g of PCC) 1.25 Figure 3.5 Change in zeta potential of pretreated PCC aggregates for different PEO, PFR dosage. PFR/PEO = 1.0 Pretreated PCC = 0.001 % wt, 295 K , PEO/PFR = 1.0, Temperature 295 K , Conductivity = 48.5 p.S/cm, pH = 8.85. To summarize, PEO, PFR flocculates fibre fines and pretreated PCC. From the aggregation experiments we can conclude that a PEO dosage of 0.15 mg/g of OD pulp, and PFR dosage of 0.15 mg/g of OD pulp is the best dosage for pretreated PCC retention. PEO, PFR is a good retention aid system for mechanical pulps loaded with pretreated PCC. 3.3 Retention with Raifix 2515 Raifix 2515 is a cationic polymer used as an anionic trash collector (ATC) in mechanical pulp suspensions. It is capable of hydrogen bonding. 47 3.3.1 Determination of best sequence of addition for Raif ix 2515 and P E O , P F R system for pretreated P C C and fibre fines retention Figure 3.6 indicates the first pass retention for different sequence of Raifix 2515 in conjunction with PEO, PFR. Raifix 2515 dosage was fixed to 0.5 mg/g of OD pulp and 80 w 70 o H W H (Z3 60 H 50 % 40 30 • FIBRE FINES O PRETREATED PCC T TOTAL SOLIDS O o S E Q U E N C E Figure 3.6. Retention with different sequences of addition of polymers. Sequence 1 is PCC-Raifix 2515-PFR-PEO, sequence 2 Raifix 2515-PCC-PFR-PEO, sequence 3 is PCC-PFR-PEO-Raifix-2515. PFR/PEO = 1.0, Dosage is PFR=PEO=0.05 mg/g of OD pulp, Raifix 2515= 0.5 mg/g of OD pulp, filler loading is 0.2 g/g of OD pulp, 295K, 1000 rpm, conductivity = 165 mS/cm , pH = 7.2 PEO, PFR dosage was fixed to 0.05 mg/g of OD pulp. The best sequence was by adding Raifix 2515 first to the pulp suspension and then adding pretreated PCC followed by PFR and lastly adding PEO. This sequence was used for further first-pass retention experiments. 48 3.3.2 Statistical design for Raifix 2515 and PEO, PFR system A 3 2 factorial design was done to investigate the use of Raifix 2515 as an anionic trash collector in conjunction with PEO, PFR for mechanical pulp suspensions loaded with pretreated PCC. In this design each complete trial or replication of the experiment and all possible combination of the levels of factors are investigated. The two factors here o H H H z w H W 70 60 FIBRE FINES H E E I P R E T R E A T E D P C C T O T A L S O L I D S 0.0 0.0 0.0 0.1 0.0 0.5 0.05 0.0 0.05 0.1 0.05 0.5 0.15 0 0 0 15 0.1 0 15 0.5 PEO, PFR AND RAIFIX 2515 DOSAGE (mg/g of OD pulp) Figure 3.7 First-pass retention with PEO, PFR and Raifix 2515. The first number on the X-axis indicates PEO, PFR dosage and the second number indicates Raifix 2515 dosage. Sequence without Raifix 2515 was pretreated PCC-PFR-PEO and with Raifix 2515 it was Raifix 2515-pretreated PCC-PFR-PEO. Sequence without PEO, PFR was Raifix 2515-pretreated PCC, pretreated PCC loading of 0.2 g/g of OD pulp, PFR/PEO = 1.0, 1000 rpm, 295 K Conductivity =165 pS/cm. are Raifix 2515 and PEO, PFR. The three levels for Raifix 2515 are the dosage (mg/g of OD pulp) 0, 0.1 and 0.5. Similarly the factors for PEO, PFR (mg/g of OD pulp) are 0, 0.05 and 0.15. The average first pass retention results are tabulated below. The sequence 49 used in this experiments were Raifix 2515 first followed by pretreated PCC, then PFR and lastly PEO. Figure 3.7 shows the retention with all the combinations of Raifix 2515 and PEO, PFR system together as well as with Raifix 2515 only and PEO, PFR only. The X-axis gives the dosage of PEO, PFR and Raifix 2515. The first number indicates the dosage of PEO, PFR and the second number indicates the dosage of Raifix 2515. At a Raifix dosage of O.lmg/g of OD pulp and no PEO, PFR the retention of fines and pretreated PCC was not so significant. However, as we increase Raifix dosage to 0.5mg/g of OD pulp the retention of fines and pretreated PCC decreases slightly. Retention with just PEO, PFR on fines and pretreated PCC increased as we increased PEO, PFR dosage from 0.05 to 0.15 mg/g of OD pulp. For a PEO, PFR dosage of 0.05 mg/g of OD pulp as Raifix 2515 dosage was increased from 0.1 to 0.5 mg/g of OD pulp the retention of fines and pretreated PCC increased by 4% and 8% respectively as seen in figure 3.8. However, for a PEO, PFR dosage of 0.15 mg/g of OD pulp as we increase Raifix 2515 dosage the retention deteriorates and is even less than that obtained by just PEO, PFR system. So adding Raifix 2515 improves retention of fines and pretreated PCC only when PEO, PFR dosages are equal to 0.05 mg/g of OD pulp. It is noted that a significant improvement is obtained only with Raifix 2515 dosage of 0.5 mg/g when PEO, PFR dosage is 0.05 mg/g. However, one may obtain a better retention only at a PEO, PFR dosage of 0.15 mg/g and without Raifix 2515. One should also examine the economics of these two alternatives. The analysis of variance is shown in Table 3.1 for fines retention . Since the value from the F 0 .5 distribution table for Fo.5,4,i8=2.93 which is less than the Fo value for the interaction which is 8.01, we can conclude that there is a significant interaction between Raifix 2515 and PEO, PFR on fines retention. Further more from the distribution table Fo.5,2,18 =3.55 for PEO, PFR which is less than the F 0 value for PEO, PFR which is 31.59. We can conclude that PEO, PFR affect fines retention. But for Raifix 2515 the Fo.5,2,18 =3.55 which is greater than the Fo value which is 0.06. Therefore we can conclude that Raifix 2515 alone has no significant affect on fines retention. 50 Table 3.1 Analysis of variance for Fines retention using Raifix 2515. Source of Variation Sum of squares Degrees of freedom Mean Square F 0 PEO, PFR 1094.44 2 547.22 31.59 Raifix 2515 2.04 2 1.02 0.06 Interaction 555.02 4 138.75 8.01 Standard Error 311.80 18 17.32 Total 1963.31 26 Fo= Mean square/Mean square of standard error. The analysis of variance is shown in Table 3.2 for pretreated PCC retention. Since the value from the F0.5 distribution table for Fo.5,4,i8=2.93 which is less than the Fo value for the interaction which is 17.07, we can conclude that there is a significant interaction between Raifix 2515 and PEO, PFR on pretreated PCC retention. Further more from the distribution table Fo.5,2,18 =3.55 for PEO, PFR which is less than the Fo value for PEO, PFR which is 219.80. We can conclude that PEO, PFR affect pretreated PCC retention, but, for Raifix 2515 the Fo.5,2,18 value is 3.55 which is greater than the Fo value which is 2.14. Therefore, we can conclude that Raifix 2515 alone has no significant affect on pretreated PCC retention. Table 3.2 Analysis of variance for pretreated PCC retention using Raifix 2515. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 5878.60 2 2939.30 219.80 Raifix 2515 57.25 2 28.63 2.14 Interaction 912.90 4 228.23 17.07 Standard Error 240.71 18 13.37 Total 7089.47 26 51 The analysis of variance is shown in Table 3.3 for total solids retention . Since the value from the F0.5 distribution table for Fo.5,4,18=2.93 which is less than the Fo value for the interaction which is 10.77, we can conclude that there is a significant interaction between Raifix 2515 and PEO, PFR on total solids retention. Further more from the distribution table Fo.5,2,18 =3.55 for PEO, PFR which is less than the Fo value for PEO, PFR which is 76.64. We can conclude that PEO, PFR affect total solids retention. But for Raifix 2515 the Fo.5,2,18 =3.55 which is greater than the Fo value which is 0.31. Therefore we can conclude that Raifix 2515 alone has no significant affect on total solids retention. Table 3.3 Analysis of variance for total solids retention using Raifix 2515. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 569.05 2 284.53 76.64 Raifix 2515 2.31 2 1.156 0.31 Interaction 159.89 4 39.97 10.77 Standard Error 66.82 18 3.71 Total 798.08 26 To assist in interpreting the results graphs are plotted separately for fibre fines (figure 3.8), pretreated PCC (figure 3.9) and total solids (figure 3.10) average retention versus Raifix 2515 dosage. 52 Figure 3.8 Effect of Raifix 2515 dosage on Fines retention. Sequence = Raifix 2515-pretreated PCC-PFR-PEO, PFR/PEO = 1.0, Temperature 295 K , 1000 rpm, pH = 7.20 10 -> , , , , , , 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 RAIFIX 2515 DOSAGE (mg/g of OD pulp) Figure 3.9 Effect of Raifix 2515 dosage on pretreated PCC retention. Sequence = Raifix 2515-pretreated PCC-PFR-PEO, PFR/PEO = 1.0, Temperature 295 K , 1000 rpm, pH = 7.20. 53 The significant interaction is indicated by the lack of parallelism of the lines in the figures. At a PEO, PFR dosage of 0.15 mg/g of OD pulp, the maximum fines retention is obtained when there is no Raifix 2515. As Raifix 2515 dosage is increased the retention decreases. However, for a PEO, PFR dosage of 0.05 mg/g of OD pulp the fines retention increases as we increase Raifix 2515 dosage. When Raifix 2515 is added in the absence of PEO, PFR the fines retention increases initially but decreases beyond a Raifix 2515 dosage of 0.1 mg/g of OD pulp. O M w H W s 03 Q P-N o H o 84 82 80 78 76 74 72 70 68 66 64 PEO,PFR=0(mg/g of OD pulp) PEO,PFR=0.05(mg/g of OD pulp) PEO,PFR=0.15(mg/g of OD pulp) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 R A I F I X 2515 D O S A G E (mg/g of OD pulp) Figure 3.10 Effect of Raifix 2515 dosage on total solids retention. Sequence = Raifix 2515-pretreated PCC-PFR-PEO, PFR/PEO = 1.0, Temperature 295 K , 1000 rpm, pH = 7.20. For pretreated PCC retention and total solids retention the trend is similar to that obtained for fines retention. From the statistical analysis, even though there is a significant interaction between Raifix 2515 and PEO, PFR it does not improve retention, instead it decreases retention. 3.3.3 First pass retention with P E O , P F R at different dosages of Raifix 2515 Figure 3.11 shows the retention of fines and pretreated PCC for a fixed dosage of PEO, PFR at 0.15 mg/g of OD pulp and varying the dosages of Raifix 2515. Since filler loading 54 is 0.2g/g of OD pulp, the optimum dosage of PEO, PFR from the aggregation experiments (figure 3.2 and 3.3) is of 0.75 mg/g of treated PCC, we chose 0.75*0.2= 0.15 mg/g of OD pulp for further experiments. As we increase Raifix 2515 dosage from 0.05 to 1.5 mg/g of OD pulp the retention of fines and fillers decreases. This shows that Raifix 2515 does not improve retention in presence of PEO, PFR system. It also hinders the flocculation of fines and pretreated PCC by PEO, PFR. This decrease in retention is explained after doing some cationic demand, aggregation and zeta potential measurements. o HH H H < CM i H 90 80 ] 70 60 50 40 0.0 0.1 0.2 0.3 0.4 0.5 1.0 RAIFIX 2515 DOSAGE (mg/g) Figure 3.11. Retention of pretreated PCC for different dosage of Raifix 2515. Sequence is Raifix-PCC-PFR-PEO. Dosage is PFR=PEO=0.15 mg/g of OD pulp, Raifix 2515= 0.05 mg/g of OD pulp, filler loading is 0.2 g/g of OD pulp, 295K, 1000 rpm, conductivity = 165 pS/cm , pH.= 7.2. Retention of fibre fines, pretreated PCC and total solids without any chemicals were 41.6%, 20.91% and 67.36 % respectively. 55 3.3.4 Cationic Demand Analysis on DDJ Filtrate A cationic demand study was done using a particle charge detector. The filtrate collected from the first pass retention experiments was used for the determination of the cationic charge demand of the system. The filtrate was titrated with poly D A D M A C to neutral point. Figure 3.12 shows the change in cationic demand of DDJ filtrate as we increase Raifix 2515 dosage. The sequence of addition is Raifix 2515 and then pretreated PCC. Increasing Raifix 2515 dosage decreases the cationic demand. This is because Raifix 2515 is cationic and therefore decreases the anionicty of the system. 0.00 0.25 0.50 0.75 1.00 R a i f i x 2515 dosage (mg/g o f O D pu lp) 1.25 Figure 3.12 Change in Cationic demand of DDJ filtrate with increasing dosage of Raifix 2515. Point zero indicates the cationic demand for pulp and pretreated PCC. Sequence = Raifix 2515-pretreated PCC-PFR-PEO, PFR/PEO = 1.0, PEO, PFR dosage = 0.15mg/g of OD pulp, pretreated PCC loading of 0.2 g/g of OD pulp, pH = 7.60, Conductivity = 170 pS/cm. Figure 3.13 gives an account of the change in cationic demand when PEO, PFR is used either in conjunction with Raifix 2515 or without it. The cationic demand of DDJ 56 filtrate in which PEO, PFR is used as a retention aid is very less compared to that when Raifix 2515 is used in conjunction with PEO, PFR. In fact the cationic demand of DDJ filtrate was more than what we get when Raifix 2515 is used alone or when PEO, PFR is used alone. Adding Raifix 2515 with PEO, PFR system increases cationic demand. The cationic demand refers to water and fines, hence when using PEO, PFR since more fines are retained compared to using Raifix 2515 we expect less fines in the filtrate. Since fines are highly anionic we will have smaller cationic demand when using PEO, PFR. DX) cr zL — z w a u H H z o H H H < puip+PCC(T) o.05,0 0.15,0 0,0.1 0,0.5 0.05,0.1 0.05,0.5 0.15,0.1 0.15, 0.5 PEO,PFR AND RAIFIX DOSAGE (mg/g of OD pulp) Figure 3.13 Change in Cationic demand of DDJ filtrate with PEO, PFR and Raifix 2515 dosage. The first number indicates PEO, PFR dosage and the second number Raifix 2515 dosage. Sequence = Raifix 2515-pretreated PCC-PFR-PEO, pretreated PCC loading of 0.2 g/g of OD pulp, PFR/PEO = 1.0, pH = 7.60, Conductivity = 170 pS/cm, Temperature 295 K. The changes in the cationic demand of the system was studied with the introduction of each polymer into the drainage jar. The DDJ filtrate was collected and titrated with poly D A D M A C to neutral point. At first the pulp was introduced into the drainage jar and the filtrate collected was subjected to cationic demand analysis. Then the 57 same procedure was repeated with the addition of Raifix 2515 along with the pulp. Subsequently the study was performed with pulp, Raifix 2515 and PCC, and again with pulp, Raifix 2515,PCC and PFR. Lastly the cationic demand for pulp, Raifix 2515, PCC, PFR and PEO was done. Figure 3.14 shows the change in cationic demand with addition of each polymer. A n important result to be noted is that when we add PFR to the system containing Raifix 2515 and pretreated PCC the cationic demand of the system remains the same. cr Q U o I—I H < U 0 1 2 3 4 5 6 7 SEQUENCE OF ADDITION Figure 3.14. Change in cationic demand of DDJ filtrate with addition of polymers. 1 is pulp only, 2 is pulp-Raifix 2515, 3 is pulp-PCC, 4 is pulp-Raifix 2515-PCC, 5 is pulp-Raifix 2515-PCC-PFR, 6 is pulp-Raifix 2515-PCC-PFR-PEO. Dosage is PFR=PEO=0.15 mg/g of OD pulp, Raifix 2515 =0.05 mg/g of OD pulp, filler loading is 0.2g/g of OD pulp, 295K, 1000 rpm, conductivity = 165 pS/cm , pH.= 7.60. 3.3.5 Aggregation of pretreated P C C with Raifix 2515 Aggregation experiments were performed using Malvern Mastersizer particle size analyzer. At first the aggregation of pretreated PCC with Raifix 2515 on its own was studied. Figure 3.15 shows the change in rate of aggregation with increasing Raifix 2515 58 dosage. The sequence used was pretreated PCC in water (0.01 %) and then adding Raifix 2515. The rate of aggregation increases as Raifix 2515 dosage is increased. However, beyond a Raifix 2515 dosage of 1.0 mg/g of pretreated PCC the rate of aggregation decreases. This is because Raifix 2515 overcharged the pretreated PCC. Figure 3.16 shows the maximum aggregate size reached in a span of 28 minutes for different Raifix 2515 dosage. From these results we can conclude that Raifix 2515 aggregates pretreated PCC. a | a © H Z O H < o o < 0.0 0.5 1.0 1.5 2.0 2.5 RAIFIX 2515 DOSAGE (mg/g of treated PCC) Figure 3.15 Effect of Raifix 2515 dosage on rate of aggregation of pretreated PCC. Sequence = pretreated PCC-Raifix 2515, Temperature 295 K , 1200 rpm, Conductivity = 48.5 pS/cm, pH = 8.85. 3.3.5.1 Zeta potential of aggregates of pretreated P C C with Raifix 2515 A 0.001% concentration of the aggregates was used to measure the zeta potential using a Malvern Zetasizer. Pretreated PCC has a zeta potential of -17.3 mV. As we add Raifix 2515 the zeta potential decreases as shown in figure 3.17 and beyond a Raifix 2515 dosage of 1.0 mg/g of pretreated PCC the zeta potential is positive. Therefore we see a decrease in the rate of aggregation in figure 3.15. 59 0.0 0.5 1.0 1.5 2.0 2.5 RAIFIX 2515 DOSAGE (mg/g of Treated PCC) Figure 3.16 Effect of Raifix 2515 dosage on aggregate size of pretreated PCC. Point zero indicates the initial size of PCC particles. Sequence = pretreated PCC-Raifix 2515, Temperature 295 K , 1200 rpm, Conductivity = 48.5 pS/cm, pH = 8.85. a, - J < H Z o OH < H H SI 0.0 0.5 1.0 1.5 2.0 2.5 3.0 RAIFIX 2515 DOSAGE (mg/g of treated PCC) Figure 3.17 Change in zeta potential of aggregates of pretreated PCC for different dosage of Raifix 2515. Point zero indicates the initial zeta potential of pretreated PCC particles, pretreated PCC concentration = 0.001 %, Conductivity = 44.5 pS/cm, pH= 8.85, Temperature = 295 K 60 3.3.6 Aggregation of pretreated P C C with Raifix 2515 and P E O , P F R Malvern Mastersizer was used for particle size analysis. The sequence of addition was pretreated PCC (0.01 %) followed by Raifix 2515 and then adding PFR and lastly PEO. For these experiments the PEO, PFR dosage was fixed to 0.75 mg/g of pretreated PCC. Figure 3.18 shows the change in rate of aggregation with increasing dosage of Raifix 2515. The point zero on the X-axis indicates rate of aggregation with PEO, PFR (without Raifix 2515). As we increase Raifix 2515 dosage the rate of aggregation decreases initially and at a Raifix dosage of 1.0 mg/g of pretreated PCC increases and beyond a dosage of 1.25 mg/g of pretreated PCC it again decreases. The initial decrease can be attributed to the interaction between Raifix 2515 and PEO, PFR system. But as we increase Raifix 2515 dosage there is an increase in the rate of aggregation. This is because there is no PEO, PFR to interfere with the excess amount of Raifix 2515 present. a o H O HH H < O O < 0.0 0.5 1.0 1.5 2.0 2.5 RAIFIX 2515 DOSAGE (mg/g of treated PCC) Figure 3.18 Effect of Raifix 2515 and PEO, PFR dosage on rate of aggregation of pretreated PCC. Point zero indicates the aggregation rate at PEO, PFR dosage of 0.75 mg/g of treated PCC, Sequence = pretreated PCC-Raifix 2515-PEO-PFR, PFR/PEO = 1.0, PEO, PFR=0.75 mg/g of pretreated PCC, Temperature 295 K , 1200 rpm, Conductivity = 42.5 pS/cm, pH = 8.85. 61 We know that Raifix 2515 aggregates pretreated PCC and therefore there is an increase in the rate of aggregation. Beyond a Raifix dosage of 1.25 mg/g of pretreated PCC the rate of aggregation decreases as it overcharges the pretreated PCC particles. But the maximum aggregate size reached is less than what we get with PEO, PFR alone or with Raifix 2515 alone. Figure 3.19 shows maximum aggregate size reached at different dosage of Raifix 2515 for a fixed PEO, PFR dosage of 0.75 mg/g of pretreated PCC. a © u u W hH 3 o hH X 0.0 0.5 1.0 1.5 2.0 2.5 RAIFIX 2515 DOSAGE (mg/g of Treated PCC) Figure 3.19 Effect of Raifix 2515 and PEO, PFR dosage on maximum aggregate size of pretreated PCC. Point zero indicates the pretreated PCC size at PEO, PFR dosage of 0.75 mg/g of treated PCC, Sequence = pretreated PCC-Raifix 2515-PEO-PFR, PFR/PEO = 1.0, PEO,PFR=0.75 mg/g of pretreated PCC, Temperature 295 K , 1200 rpm, Conductivity = 42.5 pS/cm, pH = 8.85. 3.3.6.1 Zeta Potential of Aggregates formed with Raifix 2515 and P E O , P F R Figure 3.20 shows the change in zeta potential of aggregates formed at a fixed PEO, PFR dosage of 0.75 mg/g of pretreated PCC and increasing Raifix 2515 dosage. As we increase Raifix 2515 dosage the zeta potential of aggregates decreases and becomes 62 positive at a Raifix 2515 dosage of 1.25 mg/g of pretreated PCC. Beyond this point it overcharges the system and therefore we can see a decrease in the rate of aggregation and maximum aggregate size reached. Figure 3.21 shows how the zeta potential of the pretreated PCC aggregates change as we add each polymer. The PEO, PFR dosage used was 0.75 mg/g of pretreated PCC and Raifix 2515 dosage was fixed at 1.0 mg/g of pretreated PCC. There is no change in the zeta potential of aggregates of pretreated PCC when we add PFR to a system containing Raifix 2515 and pretreated PCC. a, -H Z, H O P H < H W N 0 . 0 0 . 5 1.0 1 . 5 2 . 0 2 . 5 3 . 0 RAIFIX 2515 DOSAGE (mg/g of treated PCC) Figure 3.20 Change in zeta potential of PCC aggregates with Raifix 2515 and PEO, PFR. Point zero indicates the zeta potential of pretreated PCC at PEO,PFR dosage of 0.75 mg/g of treated PCC, PFR/PEO = 1.0, Sequence = pretreated PCC-Raifix 2515-PEO-PFR, PEO, PFR=0.75 mg/g of pretreated PCC, Temperature 295 K, 1200 rpm, Conductivity = 42.5 pS/cm, pH = 8.85. 63 0 3 4 SEQUENCE 7 Figure 3.21 Change in zeta potential for different sequence of addition of polymers with pretreated PCC. Point 1 is pretreated PCC (pH = 8.75), Point2 is pretreated PCC-PFR (pH = 8.80), Point 3 is pretreated PCC-PFR-PEO (pH = 8.7), Point 4 is pretreated PCC -Raifix 2515 (pH = 8.6), Point 5 is pretreated PCC-Raifix 2515-PFR (pH = 8.4), Point 6 is pretreated PCC-Raifix 2515-PFR-PEO (pH = 8.65). Raifix 2515 dosage = 01.0 mg/g of pretreated PCC, PFR/PEO = 1.0, PEO, PFR dosage = 0.75 mg/g of pretreated PCC, pretreated PCC concentration = 0.001 %, Conductivity = 38.5 pS/cm, Temperature 295 K. 3.3.7 Proposed Mechanism for Aggregation Of Pretreated PCC with PEO, PFR and Raifix 2515 Figure 3.22 shows a schematic of the proposed mechanism for aggregation of pretreated PCC with PEO, PFR and Raifix 2515. When PFR is added to pretreated PCC it can adsorb on the surface of PCC and when we add PEO flocculation can occur through PEO, PFR association either by association induced polymer bridging (Alince and van de 64 Ven , 1996) or through a network mechanism (Lindstrom et al.,1990). Therefore we get compact floes of pretreated PCC with PEO, PFR as seen in figures 3.3 and 3.4. Figure 3.22 Proposed Mechanism for aggregation of PCC. Boxl;pretreated PCC, 2; Raifix 2515 addition, 3; Floes of Raifix 2515-pretreated PCC, 4; PFR addition , 5; Raifix 2515-PFR association, 6; PEO addition, 7; Suspensions of floes of Raifix 2515-pretreated PCC, Raifix 2515-PFR association, unreacted PEO and pretreated PCC suspended, 8; PFR addition adsorbs on PCC, 9; PEO addition flocculates pretreated PCC with adsorbed PFR. When we add Raifix 2515 (positively charged) to pretreated PCC (negatively charged) it aggregates PCC and forms larger floes as seen in figure 3.15 and 3.16. However, when we add PEO, PFR to the system containing pretreated PCC and Raifix 2515 we see that there is a decrease in the aggregate size and rate of aggregation as shown in figures 3.18 and 3.19 respectively. We attribute this to the scavenging of PFR by Raifix 2515. Points 4 and 5 in figure 3.21 indicates that there is no change in zeta potential when adding PFR to pretreated PCC and Raifix 2515. This can be explained by the scavenging of PFR by Raifix 2515. This association may be either of chemical nature or electrostatic attraction. Raifix 2515 can hydrogen bond with PFR thus rendering it 65 unavailable for flocculation with PEO. It is noted that PEO is not capable of forming floes on it's own. Therefore the pretreated PCC particles remain suspended and there is no aggregation. The association can also be electrostatic attraction, as Raifix 2515 is cationic and PFR is anionic. It is noted that when we add sufficient amount of Raifix 2515 there is some aggregation of pretreated PCC as shown in figure 3.18 and 3.19. This confirms that there is some association between Raifix 2515 and PFR. Therefore based on these findings Raifix 2515 should be used with caution when the system contains PEO, PFR. 3.3.8 Proposed Mechanism For Flocculation Of Fibre fines and Pretreated P C C with P E O , P F R and Raifix 2515 Figure 3.23 gives a schematic representation of the proposed mechanism for flocculation of fibre fines and pretreated PCC (anionic) with PEO, PFR and Raifix 2515. Mechanical pulps contain anionic dissolved and colloidal substances (DCS) which are negatively charged substances. If pretreated PCC is added to mechanical pulp suspensions it remains suspended. However, when we add PFR and then PEO we see that there is substantial amount of flocculation which can be confirmed by the increase in fibre fines and pretreated PCC retention as shown in figure 3.2. Thus PEO, PFR also flocculates pretreated PCC (figure 3.3 and 3.4). PEO, PFR system together acts as a good retention aid system for pretreated PCC loaded mechanical pulp suspensions. When we add Raifix 2515 to a mechanical pulp containing anionic DCS then Raifix 2515 patch flocculates the DCS. Raifix 2515 is cationic and has an affinity for DCS which are negatively charged particles with a smaller size compared to Raifix 2515 and therefore forms floes with DCS and fibre fines. When we add pretreated PCC, in addition to Raifix 2515 and DCS floes some floes of Raifix 2515 and pretreated PCC can be formed. Since we know that Raifix 2515 aggregates pretreated PCC (figure 3.14 and 3.15) and the extent of aggregation depends on the availability of free Raifix 2515 which has not been consumed to form floes with DCS. As seen in figure 3.7, 3.8, 3.9 and 3.10, in the absence of PEO and PFR addition of 0.1 mg/g of OD pulp of Raifix 2515 increases retention. Further addition to 0.5 mg/g of OD pulp decreases it. This is attribute to initial patch flocculation due to Raifix 2515. However further addition of the cationic coagulant 66 Figure 3.23 Mechanism for flocculation of pretreated PCC and fibre fines with PEO, PFR and Raifix 2515. Box 1; fibre fines and DCS, 2; Raifix 2515, 3; Raifix 2515 patch flocculates DCS, 4; pretreated PCC addition, 5; some pretreated PCC floes along with Raifix 2515-DCS floes, 6; addition of PFR, 7; Raifix-PFR association along with some floes of Raifix-DCS and Raifix 2515-pretreated PCC, 8; PEO addition, 9; Some flocculation of fines and pretreated PCC, Raifix 2515-PFR complex, 10; pretreated PCC suspended with fibre fines and DCS, 11; addition of PFR which adsorbs on pretreated PCC, 12; Flocculation of pretreated PCC and fibre fines in the presence of PEO. possibly overcharges the particles and causes the retention to decrease. When we add PFR followed by PEO at a dosage of 0.15 mg/g of OD pulp the retention of fibre fines and pretreated PCC decreases compared to what we get with PEO, PFR system without Raifix 2515 as seen from figure 3.7. This can be attributed to the formation of Raifix 2515 and PFR complex. It appears that the interaction between PFR and Raifix 2515 which we called scavenging of PFR is responsible for the retention 6 7 results. The more Raifix 2515 is added the more PFR it interacts with and less PFR is available to assist PEO as a flocculant. As seen from figure 3.7, 3.8, 3.9 and 3.10 when the dosage of PEO, PFR is 0.05 mg/g of OD pulp the retention is improved with the addition Raifix 2515 at a dosage of 0.1 mg/g of OD pulp and further addition of Raifix 2515 decreases retention. It appears in this case that the limiting reactant is PFR and not Raifix 2515 as was before. Hence when Raifix 2515 is added all PFR is probably consumed and the retention is due to the coagulant (Raifix 2515). In this case the presence of PEO is irrelevant and we have the scenario with Raifix 2515 alone. As we saw previously addition of Riafix 2515 further, overcharges the system and reduces retention. 3.4 Retention with Raifix 120 Raifix 120 is another starch polymer with a low molecular weight but very high cationicity compared to Raifix 2515. Retention experiments were performed to compare Raifix 2515 and Raifix 120 for fibre fines and pretreated PCC retention. 3.4.1 Statistical design for Raifix 120 and P E O , P F R system A 3 factorial design was done to investigate the use of Raifix 120 as an anionic trash collector in conjunction with PEO, PFR for mechanical pulp suspensions loaded with pretreated PCC. In this design each complete trial or replication of the experiment and all possible combination of the levels of factors are investigated. The two factors here are Raifix 120 and PEO, PFR. The three levels for Raifix 120 are the dosage (mg/g of OD pulp) 0, 0.1 and 0.5. Similarly the factors for PEO, PFR (mg/g of OD pulp) are 0, 0.05 and 0.15. The average first pass retention results are tabulated below. The sequence used in this experiments were Raifix 120 first followed by pretreated PCC, then PFR and lastly PEO. 68 80 70 60 50 40 S 30 20 10 0 F I N E S T R E A T E D P C C TOTAL SOLIDS 0.0,0.0 0.0,0.1 0.0,0.5 0.05,0.0 0.05,0.1 0.05,0.5 0.15,0.0 0.15,0.1 0.15,0.5 PEO,PFR AND RAIFIX 1 2 0 DOSAGE (mg/g o f OD pulp) Figure 3.24 First-pass retention with PEO, PFR and Raifix 120. The first number on the X-axis indicates PEO, PFR dosage and the second number indicates Raifix 120 dosage. Sequence without Raifix 120 was pretreated PCC-PFR-PEO and with Raifix 120 it was Raifix 120-pretreated PCC-PFR-PEO. Sequence without PEO, PFR was Raifix 120-pretreated PCC, pretreated PCC loading of 0.2 g/g of OD pulp, PFR/PEO = 1.0, 1000 rpm, 295 K Conductivity =165 pS/cm. Figure 3.24 shows the retention of fibre fines and pretreated PCC with PEO, PFR and Raifix 120. We see that as we increase Raifix 120 dosage in the absence of PEO, PFR there is an increase in fibre fines and pretreated PCC retention. This increase is not very significant. When we use Raifix 120 along with PEO, PFR at 0.05 mg/g of OD pulp there is an increase in retention of fibre fines and pretreated PCC. However at a dosage of 0.15 mg/g of OD pulp of PEO, PFR retention decreases. The analysis of variance is shown in Table 3.4 for fines retention. Since the value from the F0.5 distribution table for F0.5,4,i8=2.93 which is less than the F 0 value for the interaction which is 9.23, we can conclude that there is a significant interaction between Raifix 120 and PEO, PFR on fines retention. Further more from the distribution table 69 Fo.5,2,18 =3.55 for PEO, PFR which is less than the F 0 value for PEO, PFR which is 34.10. We can conclude that PEO, PFR affects fines retention. But for Raifix 120 the Fo.5,2,18 =3.55 which is greater than the Fo value which is 1.32. Therefore we can conclude that Raifix 120 alone has no significant effect on fines retention in the presence of PEO, PFR. Table 3.4 Analysis of variance for Fines retention using Raifix 120. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 811.00 2 405.50 34.10 Raifix 120 31.39 2 15.70 1.32 Interaction 439.27 4 109.82 9.23 Standard Error 214.04 18 11.89 Total 1495.70 26 Fo= Mean square/Mean square of standard error. The analysis of variance is shown in Table 3.5 for pretreated PCC retention . Since the value from the F0.5 distribution table for Fo.5,4,i8=2.93 which is less than the Fo value for the interaction which is 41.55, we can conclude that there is a significant interaction between Raifix 120 and PEO, PFR with respect to pretreated PCC retention. Further more from the distribution table Fo.5,2,18 value is 3.55 for PEO, PFR which is less than the F 0 value for PEO, PFR which is 243.72. We can conclude that PEO, PFR affects pretreated PCC retention. The Fo.5,2,18 value for Raifix 120 is 3.55 which is less than the F 0 value for Raifix 120 which is 13.20. This indicates that Raifix 120 enhances pretreated PCC retention as seen in figure 3.24. The analysis of variance is shown in Table 3 for total solids retention . Since the value from the F0.5 distribution table for Fo.5,4,18=2.93 which is less than the Fo value for the interaction which is 17.83, we can conclude that there is a significant interaction between Raifix 120 and PEO, PFR on total solids retention. Further more from the distribution table Fo.5,2,18 =3.55 for PEO, PFR which is less than the Fo value for PEO, PFR which is 72.41. We can conclude that PEO, PFR affect total solids retention. But for 70 Raifix 120 the Fo.5,2,18 =3.55 which is greater than the Fo value which is 1.63. Therefore we can conclude that Raifix 120 alone has no significant affect on total solids retention. Table 3.5 Analysis of variance for pretreated PCC retention using Raifix 120. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 3810.08 2 1905.04 243.72 Raifix 120 206.36 2 103.18 13.20 Interaction 1299.14 4 324.78 41.55 Standard Error 140.70 18 7.82 Total 5456.27 26 Table 3.6 Analysis of variance for total solids retention using Raifix 120. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 367.19 2 183.60 72.41 Raifix 120 8.25 2 4.12 1.63 Interaction 180.86 4 45.21 17.83 Standard Error 45.64 18 2.54 Total 601.93 26 71 70 PEO, PFR=0.0(mg/g of OD pulp) •9— PEO,PFR=0.05 (mg/g of OD pulp) «— PEO, PFR= 0.15 (mg/g of OD pulp) 35 0.0 0.1 0.2 0.3 0.4 0.5 R A I F I X 120 D O S A G E (mg /g o f O D p u l p ) 0.6 Figure 3.25 First-pass fibre fines retention for different dosages of PEO, PFR and Raifix 120 for statistical design of experiments. PFR/PEO = 1.0. To assist in interpreting the results graphs are plotted separately for fines (figure 3.25), treated PCC (figure 3.26) and total solids (figure 3.27) average retention versus Raifix 120 dosage. The significant interaction is indicated by the lack of parallelism of the lines in the figures. At a PEO, PFR dosage of 0.15 mg/g of OD pulp the maximum retention is obtained when there is no Raifix 120. As Raifix 120 dosage is increased the retention decreases and then slightly increases. But for a PEO, PFR dosage of 0.05 mg/g of OD pulp the retention increases as we increase Raifix 120 dosage. But the increase is not significant. When Raifix 120 is added in the absence of PEO, PFR the retention increases slightly, but the increment is not significant. Maximum pretreated PCC retention is obtained at the PEO, PFR dosage of 0.15mg/g of OD pulp and no Raifix 120. As Raifix 120 dosage is increased the retention deteriorates. But for a PEO, PFR dosage of 0.05 mg/g as the Raifix dosage is increased the pretreated PCC retention increases. But the increment is not significant. Raifix 120 alone also improves pretreated PCC retention on it's own. 72 10 1 I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 R A I F I X 120 D O S A G E (mg/g o f O D p u l p ) Figure 3.26 First-pass pretreated PCC retention for different dosages of PEO, PFR and Raifix 120 for statistical design of experiments. PFR/PEO = 1.0,. For total solids retention the trend is similar to the fines retention except that Raifix 120 in the absence of PEO, PFR increases total retention up to a dosage of O.lmg/g of OD pulp and then decreases i f we increase Raifix 120 dosage. Compared to Raifix 2515 the retention of pretreated PCC obtained by Raifix 120 is less. Raifix 120 is more positively charged than Raifix 2515. 7 3 8 72 ; ^ ^ ^ ^ Q 70 \ Jt^/l —~~— _____ A 6 8 ; y/ ~~~~~~~~~~—_T g 6 6 ^ y ^ H 64 ^ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 R A I F I X 120 D O S A G E (mg/g o f O D pu l p ) Figure 3.27 First-pass total solids retention for different dosages of PEO, PFR and Raifix 120 for statistical design of experiments.PFR/PEO = 1.0. 3.4.2 Cationic Demand Analysis on D D J Filtrate A cationic demand analysis was performed on the DDJ filtrate obtained from the dosages used for statistical design of experiments for Raifix 120. Figure 3.28 shows the change in cationic demand for polymers added. We see that when PEO, PFR system is used the cationic demand is less as PEO, PFR flocculates anionic DCS and pretreated PCC. Also the retention of fibre fines and pretreated PCC is high which show that there is less fines in the DDJ filtrate and therefore the cationic demand with PEO, PFR is less. When we add increasing amount of Raifix 120 to pulp suspension the cationic demand decreases, because Raifix 120 is cationic. Raifix 120 when used in conjunction with PEO, PFR the cationic demand is more than what we get for either Raifix 120 alone or with PEO, PFR alone. This shows that there is some interaction between Raifix 120 and PEO, PFR. However this interaction increases the cationic demand of the system. 74 Q U HH o HH H - 1 0 0 pulp+PCC(T) 0.05 0 0.15 0 0 0.1 0 0.5 0.05 0.1 0.05 0.5 0.15 0.1 0.15 0.5 P F R , P E O A N D R A I F I X 120 D O S A G E ( m g / g o f O D p u l p ) Figure 3.28 Change in cationic demand for different dosage of Raifix 120 and PEO, PFR. The first number on the X-axis indicates the PEO, PFR dosage and the second number indicates Raifix 120 dosage, Sequence used in absence of Raifix 120 was pretreated PCC-PFR-PEO, and with Raifix 120 it was Raifix 120-pretreated PCC-PFR-PEO and in the absence of PEO, PFR the sequence was Raifix 120-pretreated PCC. Conductivity = 170 pS/cm, Temperature = 295 K, pH = 7.65, PFR/PEO = 1.0. 3 . 4 . 3 Aggregation of Pretreated P C C with Raifix 1 2 0 and P E O , P F R Aggregation of pretreated PCC with PEO, PFR was performed with Raifix 120 in conjunction with PEO, PFR. The sequence used was pretreated PCC solution (0.01 %) followed by Raifix 120 and then adding PFR and lastly PEO. Figure 3.29 shows the rate of aggregation with Raifix 120 and PEO, PFR increases as we increase Raifix 120 dosage keeping PEO, PFR dosage constant. We can conclude that PEO, PFR along with Raifix 120 aggregates pretreated PCC and the maximum aggregate size reached was at a Raifix 120 dosage of 1.25 mg/g of pretreated PCC and PEO, PFR dosage of 0.75 mg/g of PCC. 75 Figure 3.30 shows the maximum aggregate size reached after 28 minutes for the dosages indicated. 1.5 0.5 i 0.0 J . . . . 1 0.5 0.25 0.5 0.75 1.25 0.25 1.25 0.75 R A I F I X 120 , P F R , P E O D O S A G E (mg /g o f t r e a t e d P C C ) Figure 3.29 Rate of aggregation of pretreated PCC with PEO, PFR and Raifix 120. The first number of the values on the X-axis indicates Raifix 120 dosage and the second number indicates PEO, PFR dosage, pretreated PCC concentration = 0.01 %, Conductivity = 39.7 pS/cm, PFR/PEO = 1.0, Temperature = 295 K , pH = 8.93. 3.4.3.1 Zeta Potential Of Aggregates of pretreated P C C with Raifix 120 and P E O , P F R system Figure 3.31 shows the change in zeta potential for different dosage of Raifix 120 and PEO, PFR. At a PEO, PFR dosage of 0.25 mg/g of pretreated PCC increasing Raifix dosage from 0.5 to 1.25 mg/g of pretreated PCC the zeta potential increases and becomes positive. At a PEO, PFR dosage of 0.75 mg/g of pretreated PCC and Raifix 120 dosage of 1.25 mg/g of pretreated PCC the rate of aggregation and maximum aggregate size reached is high and at this point the zeta potential of the aggregates is near neutral point. 76 </5 a o u 40 -. 35 : S3 1—1 Xfl 30 ; W H «C 25 : O 20 : AG 15 : 10 : 5 : i RAIFIX 120 , PFR-PEO DOSAGE(mg/g of treated PCC) Figure 3.30 Maximum aggregate size of pretreated PCC with PEO, PFR and Raifix 120. Sequence used was pretreated PCC (0.01 % wt)-Raifix 120-PFR-PEO, PFR/PEO = 1.0, Temperature 295 K , Conductivity = 45.6 uS/cm, pH = 8.85. 9 -< H W H O PH < H W S3 0.5 0.25 0.5 0.75 1.25 0.25 1.25 0.75 RAIFIX 120, PEO-PFR DOSAGE(mg/g of (T)PCC) Figure 3.31 Change in zeta potential of aggregates of pretreated PCC formed with PEO, PFR and Raifix 120.The first number of the values on the X-axis indicates Raifix 120 dosage and the second number indicates PEO, PFR dosage, pretreated PCC concentration = 0.001 % , Conductivity = 39.7 pS/cm, Temperature = 295 K , pH = 8.93. 77 This is the best condition for pretreated PCC aggregation with Raifix 120 and PEO, PFR system. 3.5 Retention with Raisapac A n inorganic polymer Raisapac was used in conjunction with PEO, PFR system as an anioinc trash collector. The chemical name for Raisapac is poly(hydroxy) aluminum chloride. 3.5.1 Determination Of Best Sequence Of Addition For Raisapac A n d P E O , P F R System For Pretreated P C C A n d Fibre Fines Retention First a particular sequence of addition of polymers was determined. Figure 3.32 gives the first-pass retention of fines and pretreated PCC for various sequences tried. The Z o H Z W H 90 80 1 70 60 50 40 30 FINES RETENTION FILLER RETENTION T O T A L SOLIDS RETENTION 0 1 2 3 4 SEQUENCE Figure 3.32. First-pass retention of Pretreated PCC with PEO, PFR and Raisapac for different sequence. Sequence 1 is PCC-PAC-PFR-PEO, sequence 2 is PAC-PCC-PFR-PEO, sequence 3 is PCC-PFR-PEO-PAC. Dosage of PFR=PEO=0.15 mg/g of OD pulp, Raisapac =2.0 mg/g of OD pulp, filler loading is 0.2 g/g of OD pulp, 295K, 1000 rpm, conductivity =168 pS/cm , pH.= 7.2. 78 sequence which gave a maximum retention was used for further experiments. For these experiments the dosage of PFR and PEO was fixed to 0.15 mg/g and the Raisapac dosage was fixed at 2.0 mg/g. As seen in figure 3.26 the sequence of Raisapac-PCC-PFR-PEO gave the maximum retention values. Hence, this sequence was used for further first pass retention experiments with Raisapac. 3.5.2 Statistical design for Raisapac and PEO, P F R system A 3 2 factorial design was done to investigate the use of Raisapac as an anionic trash collector in conjunction with PEO, PFR for mechanical pulp suspensions loaded with pretreated PCC. In this design each complete trial or replication of the experiment and all possible combination of the levels of factors are investigated. The two factors here are Raisapac and PEO, PFR. The three levels for Raisapac are the dosage (mg/g of OD pulp) 2.0, 3.0 and 4.0. Similarly the factors for PEO, PFR (mg/g of OD pulp) are 0, 0.05 and 0.15. The average first pass retention results are tabulated below. The sequence used in this experiments were Raisapac first followed by pretreated PCC, then PFR and lastly PEO. Figure 3.33 shows the retention with Raisapac and PEO, PFR system. Raisapac on it's own does not increase fibre fines and pretreated PCC retention. When we use PEO, PFR in conjunction with Raisapac the fibre fines retention slightly increases but pretreated PCC retention decreases. At a PEO, PFR dosage of 0.05 mg/g of OD pulp, the fines retention slightly increases and pretreated PCC retention decreases as we increase Raisapac dosage. Finally for a PEO, PFR dosage of 0.15 mg/g of OD pulp as we increase Raisapac dosage there is no significant change in fines retention. Therefore these results indicate that there is no particular trend in retention. The analysis of variance is shown in Table 3.7 for fines retention . Since the value from the F0.5 distribution table for Fo.5,4,i8=2.93 which is greater than the Fo value for the interaction which is 2.78, we can conclude that there is no significant interaction between Raisapac and PEO, PFR on fines retention. Further more from the distribution table Fo.5,2,18 -3.55 for PEO, PFR which is less than the F 0 value for PEO, PFR which is 79 80 \ 70 = 60 § 50 H 40 w H 30 FINES TREATED PCC TOTAL SOLIDS * 20 10 0 0,0 0,2 0,3 0,4 0.05,2 0.05,3 0.05,4 0.15,2 0.15,3 0.15,4 P E O , P F R A N D R A I S A P A C D O S A G E (mg/g of O D pulp) Figure 3.33 First-pass retention with PEO, PFR and Raisapac. The first number on the X-axis indicates PEO, PFR dosage and the second number indicates Raisapac dosage. Sequence without Raisapac was pretreated PCC-PFR-PEO and with Raisapac it was Raisapac-pretreated PCC-PFR-PEO. Sequence without PEO, PFR was Raisapac -pretreated PCC, pretreated PCC loading of 0.2 g/g of OD pulp, PFR/PEO = 1.0, 1000 rpm, 295 K Conductivity =165 pS/cm. Table 3.7 Analysis of variance for Fines retention using Raisapac. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 4037.805 2 2018.90 173.76 Raisapac 10.87 2 5.44 0.47 Interaction 129.25 4 32.31 2.78 Standard Error 209.14 18 11.62 Total 4387.06 26 Fo= Mean square/Mean square of standard error. 80 173.76. We can conclude that PEO, PFR affects fines retention. But for Raisapac the Fo.5,2,18 =3.55 which is less than the Fo value which is 0.47. Therefore we can conclude that Raisapac alone has no significant effect on fines retention in the presence of PEO, PFR. The analysis of variance is shown in Table 3.8 for pretreated PCC retention. Since the value from the F0.5 distribution table for Fo.5,4,18=2.93 which is greater than the Fo value for the interaction which is 1.63, we can conclude that there is no significant interaction between Raisapac and PEO, PFR on pretreated PCC retention. Further more from the distribution table Fo.5,2,18 -3.55 for PEO, PFR which is less than the Fo value for PEO, PFR which is 186.99. We can conclude that PEO, PFR affects pretreated PCC retention. However for Raisapac the Fo.5,2,18 =3.55 which is greater than the Fo value which is 3.20. Therefore we can conclude that Raisapac does not affect pretreated PCC retention. Table 3.8 Analysis of variance for pretreated PCC retention using Raisapac. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 5906.72 2 2953.36 186.99 Raisapac 100.94 2 50.47 3.20 Interaction 103.13 4 25.78 1.63 Standard Error 284.30 18 15.79 Total 6395.09 26 The analysis of variance is shown in Table 3.9 for total solids retention . Since the value from the F0.5 distribution table for Fo.5,4,18=2.93 which is less than the Fo value for the interaction which is 0.92, we can conclude that there is no significant interaction between Raisapac and PEO, PFR on total solids retention. Further more from the distribution table Fo.5,2,18 =3.55 for PEO, PFR which is less than the Fo value for PEO, PFR which is 274.17. We can conclude that PEO, PFR affect total solids retention. However for Raisapac the Fo.5,2,18 value is 3.55 which is greater than the Fo value which is 81 0.22. Therefore we can conclude that Raisapac alone has no significant affect on total solids retention in the presence of PEO, PFR. Table 3.9 Analysis of variance for total solids retention using Raisapac. Source of Variation Sum of squares Degrees of freedom Mean Square Fo PEO, PFR 917.47 2 458.74 274.17 Raisapac 0.73 2 0.36 0.22 Interaction 26.25 4 6.56 0.92 Standard Error 30.12 18 1.67 Total 974.57 26 z O H Z w H ifi W z 70 60 50 40 B 30 20 -•— PEO-PFR=0.0(mg/gofODpulp) -T— PEO-PFR=0.05(mg/gofODpulp) -m— PEO-PFR=0.15(mg/g of OD pulp) II _ ,. 2 3 4 R A I S A P A C D O S A G E (mg/g of O D pulp) Figure 3.34 First-pass retention for fibre fines with PEO, PFR and Raisapac for statistical design of experiments. PFR/PEO = 1.0, 8 2 To assist in interpreting the results graphs are plotted separately for fines (figure 3.34), treated PCC (figure 3.35) and total solids (figure 3.36) average retention versus Raisapac dosage. —•— PEO-PFR=0.0(mg/gofODpulp) PEO-PFR=0.05(mg/g of OD pulp) —M— PEO-PFR=0.15(mg/g of O D pulp) 20 \—•—•—•—•—'—•—•—•—•—,—•—•—•—•—•-1 2 3 4 RAISAPAC DOSAGE (mg/g of OD pulp) Figure 3.35 First-pass retention for pretreated PCC with PEO, PFR and Raisapac for statistical design of experiments, PFR/PEO = 1.0. 83 +— PEO, PFR=0.0(mg/g of OD pulp) 1 2 3 4 RAISAPAC DOSAGE (mg/g of OD pulp) Figure 3.36 First-pass retention of total solids with PEO, PFR and Raisapac for statistical design of experiments. PFR/PEO = 1.0. 3.5.3 First Pass Retention Of Pretreated P C C And Fibre Fines With Raisapac In Conjunction With P E O , P F R System First-pass retention experiments at different dosages of Raisapac were conducted. As seen in figure 3.37, above a Raisapac dosage of 2 mg/g there is no change in fines and filler retention. The retention of fibre fines, pretreated PCC and total solids increased to 60%, 70 % and 82 % respectively when Raisapac dosage was increases from 2 to 4 mg/g ofOD pulp. 3.5.4 Cationic Demand Analysis of D D J Filtrate A cationic demand analysis of the system using Raisapac was studied using Mutek particle charge detector. The dosages used were same as that used for statistical design of experiments. Figure 3.38 shows that as we increase Raisapac dosage the cationic demand decreases in absence of PEO, PFR. The first number of the X-axis shows PEO, PFR dosage and the second number shows Raisapac dosage. But at a PEO, 84 PFR dosage of 0.05 mg/g of OD pulp increasing Raisapac dosage increases cationic demand. Similarly at a PEO, PFR dosage of 0.15 mg/g of OD pulp increasing Raisapac dosage increased cationic demand of the DDJ filtrate. To further investigate the use of Raisapac aggregation experiments were performed. 90 • FIBRE FINES • pretreated PCC • TOTAL SOLIDS 80 £ 70 O H W 60 H 50 t 4 • * 40 J , , , , , , , , , , 1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 R A I S A P A C D O S A G E (mg/g of O D pulp) Figure 3.37 First-pass retention with pretreated PCC, PEO, PFR for different dosage of Raisapac. Sequence of addition is Raisapac-PCC-PFR-PEO, Dosage is PFR=PEO=0.15 mg/g of OD pulp, filler loading is 0.2g/g of OD pulp, PFR/PEO = 1.0, 295 K , 1000 rpm, conductivity =168 pS/cm , pH.= 7.20. Point zero indicates the retention at a PEO, PFR dosage of 0.15 mg/g of OD pulp. 85 BX) "cr-O z o H • < C J -40 -50 -60 -70 -80 -90 -100 # • i pulp+PCC(T) 0.05 0 0.15 0 0 2 0 3 0 4 0.05 2 0.05 3 0.05 4 0.15 2 0.15 3 0.15 4 PEO, PFR AND RAISAPAC DOSAGE (mg/g of OD pulp) Figure 3.38 Change in cationic demand of DDJ filtrate with PEO, PFR for different dosage of Raisapac. Sequence of addition is Raisapac-PCC-PFR-PEO, filler loading is 0.2g/g of OD pulp, PFR/PEO = 1.0, 295 K , 1000 rpm, conductivity = 168 pS/cm , pH.= 7.60 3.5.5 Aggregation of Pretreated P C C with Raisapac Aggregation experiments were performed on a Malvern Mastersizer. Figure 3.39 shows the change in aggregation rate with increasing amount of Raisapac dosage. At a Raisapac dosage of 10 mg/g of pretreated PCC the rate of aggregation is 0.12 microns/min and decreases as we increase the Raisapac dosage. Figure 3.40 shows the maximum aggregate size reached for different dosages of Raisapac for a period of 28 minutes. The aggregates formed were compact and had a maximum aggregate size of 8.76 microns at a Raisapac dosage of 14.5 mg/g of pretreated PCC 86 a o !-U W H <! BJ Z O t-H H < a 8 10 12 14 16 18 20 22 , R A I S A P A C D O S A G E (mg/g of Treated PCC) Figure 3.39 Change in aggregation of pretreated PCC with Raisapac only. Sequence used was pretreated PCC (0.01 %) and Raisapac. Temperature 295 K , 1200 rpm, Conductivity = 44.7 pS/cm, pH = 8.85. a © u S3 H (3 O < 8.9 8.6 8.5 8.4 .3 8.2 « 8 10 12 14 16 18 20 22 RAISAPAC DOSAGE (mg/g of Treated PCC) Figure 3.40 Maximum aggregate size of pretreated PCC aggregates formed with Raisapac only. Pretreated PCC concentration 0.001 %. Temperature 295 K , 1200 rpm, Conductivity = 44.7 pS/cm, pH = 8.85. 87 3.5.6 Aggregation of Pretreated P C C with Raisapac in conjunction with P E O , P F R system Figure 3.41 shows the change in rate of aggregation of pretreated PCC with Raisapac and PEO, PFR system. PEO, PFR dosage was fixed at 0.75 mg/g of pretreated PCC. Figure 3.42 shows the maximum aggregate size reached. As we increase Raisapac dosage there is a decrease in the rate of aggregation and maximum aggregate size reached. The rate of aggregation and the maximum aggregate size are higher compared to those obtained with Raisapac alone in the absence of PEO, PFR. In addition these rates are also higher than those obtained by PEO, PFR system as shown in figure 3.3. Raisapac and PEO, PFR together enhance aggregation of pretreated PCC. 9 10 11 12 13 14 15 16 17 18 19 20 21 RAISAPAC DOSAGE (mg/g of treated PCC) Figure 3.41 Change in rate of aggregation of pretreated PCC with Raisapac and PEO, PFR system. Sequence used was Raisapac-pretreated PCC-PFR-PEO. Pretreated PCC concentration = 0.01 %, PEO, PFR dosage = 0.75 mg/g of pretreated PCC, Temperature 295 K , 1200 rpm, Conductivity = 38.7 pS/cm, pH = 9.13. 3.5.6.1 Zeta Potential of Aggregates of Pretreated P C C formed by Raisapac Figure 3.43 shows the change in zeta potential of the aggregates of pretreated PCC at different dosages of Raisapac. 88 10 11 12 13 14 15 16 17 18 19 20 21 22 PAC DOSAGE (mg/g of Treated PCC) Figure 3.42 Maximum aggregate size of pretreated PCC aggregates formed with Raisapac only. Pretreated PCC concentration 0.001 %, PFR/PEO = 1.0, PEO, PFR dosage = 0.75 mg/g of pretreated PCC, Temperature 295 K , 1200 rpm, Conductivity = 44.7 pS/cm, pH = 8.85. a H fc W H O PH << H W N 10 12 14 16 18 20 RAISAPAC DOSAGE (mg/g of (T) PCC) 22 Figure 3.43 Change in zeta potential of pretreated PCC aggregates formed with Raisapac. Pretreated PCC concentration = 0.001 %, Temperature 295 K , Conductivity = 48.3 pS/cm, pH = 8.85 89 3.5.6.2 Zeta Potential of Aggregates of Pretreated P C C with Raisapac and P E O , P F R System Figure 3.44 shows the change in zeta potential of aggregates formed with Raisapac and PEO, PFR system. PEO, PFR dosage was fixed at 0.75 mg/g of pretreated PCC. Beyond a Raisapac dosage of 12.5 mg/g of pretreated PCC the zeta potential of the aggregates becomes more negative. As seen in figure 3.31 at dosages above 12.5 mg/g of pretreated PCC the rate of aggregation decreased. 9 10 11 12 13 14 15 16 17 18 19 20 21 RAISAPAC DOSAGE (mg/g of treated PCC) Figure 3.44 Change in zeta potential of pretreated PCC aggregates formed with Raisapac and PEO, PFR system. Pretreated PCC concentration = 0.001 %, PFR/PEO=1.0, Temperature 295 K Conductivity = 41.7pS/cm, pH = 9.12 90 3.6 Effect Of Shear On Fibre Fines A n d Pretreated P C C Retention 90 s c cu CU 80 A 70 60 50 40 30 20 10 • Fibre Fines T pretreated PCC • Total solids 1000 1200 DDJ stirrer speed (rpm) 1400 Figure 3.45 Effect of DDJ stirrer speed on fibre fines and pretreated PCC retention with PEO, PFR system. Sequence = pretreated PCC-PFR-PEO, PFR/PEO = 1.0, PEO, PFR dosage = 0.15 mg/g of OD pulp, pretreated PCC loading = 0.2 g/g of OD pulp, Temperature 295 K . Retention experiments were carried out at different DDJ stirrer speed for the best conditions obtained at 1000 rpm. Figure 3.45 shows the retention with PEO, PFR at different stirrer speeds. PEO, PFR dosage is 0.15 mg/g of OD pulp. We see that as we increase the DDJ stirrer speed the retention decreases. This is because the floes formed by PEO, PFR are not strong enough to withstand the shear created by the increasing the speed of the stirrer. The retention of fibre fines and pretreated PCC decreases as we increase the DDJ stirrer speed. Figure 3.46 shows the retention of fibre fines and pretreated PCC for Raifix 2515 and PEO, PFR system at different DDJ stirrer speeds. Raifix 2515 dosage was 0.05 mg/g of OD pulp and PEO, PFR dosage was 0.15 mg/g of OD pulp. The sequence used was Raifix 2515-pretreated PCC-PFR-PEO. At 1200 rpm the retention of pretreated PCC 91 improved but as the stirrer speed was increased further to 1400 rpm the retention of both fines and fillers decreased. It appears that the floes formed in the presence of Raifix 2515 and PEO, PFR is stronger than that formed by PEO, PFR alone. 80 75 2 70 a .2 65 c § 60 55 50 i * • * • Fibre fines • Pretreated PCC • Total Solids • • \ 1000 1200 1400 D D J Stirrer Speed (rpm) Figure 3.46 Effect of DDJ stirrer speed on fibre fines and pretreated PCC retention with Raifix 2515 and PEO, PFR system. Sequence = Raifix 2515-pretreated PCC-PFR-PEO, PFR/PEO = 1.0, PEO, PFR dosage = 0.15 mg/g of OD pulp, Raifix 2515 dosage = 0.05 mg/g of OD pulp, pretreated PCC loading = 0.2 g/g of OD pulp, Temperature 295 K . Figure 3.47 shows the retention of fibre fines and pretreated PCC for Raisapac and PEO, PFR system at different DDJ stirrer speeds. PEO, PFR dosage was fixed at 0.15 mg/g of OD pulp and Raisapac dosage was 2 mg/g of OD pulp. As the stirrer speed was increased to 1200 rpm the retention of fines and pretreated PCC increased slightly but as the DDJ speed was increased to 1400 rpm the retention decreased. This can be again attributed to the floe strength as interpreted before. 92 s c QJ rt 85 80 75 70 65 60 55 50 45 i e * * • Fibre fines T Pretreated PCC • Total Solids • • \ 1000 1200 i 1400 DDJ Stirrer Speed (rpm) Figure 3.47 Effect of DDJ stirrer speed on fibre fines and pretreated PCC retention with Raisapac and PEO, PFR system. Sequence = Raisapac-pretreated PCC-PFR-PEO, PFR/PEO = 1.0, PFR/PEO=1.0, PEO, PFR dosage = 0.15 mg/g of OD pulp, Raisapac dosage = 2.0 mg/g of OD pulp, pretreated PCC loading = 0.2 g/g of OD pulp, Temperature 295 K . 3.7 Comparison Of Retention Of Fibre Fines, Pretreated P C C A n d Total Solids Retention At Best Conditions For A l l Polymers Used Figure 3.48 shows the retention of fibre fines, pretreated PCC and total solids retention for the different systems we studied at a DDJ stirrer speed of 1000 rpm. The condition at which we get maximum retention for PEO, PFR system is at a dosage of 0.15 mg/g of OD pulp and the sequence is pretreated PCC added to pulp and then PFR and lastly PEO. Maximum retention obtained with Raifix 2515 when it was used in conjunction with PEO, PFR system was at a Raifix 2515 dosage of 0.05 mg/g of OD pulp and PEO, PFR dosage of 0.15 mg/g of OD pulp as seen in figure 3.11. The sequence in this case was Raifix 2515-pretreated PCC-PFR-PEO. 93 s H H a CU ai 90 80 70 60 50 40 30 20 : 10 0 Fibre Fines pretreated PCC Total Solids -U-PEO-PFR Raifix 2515/PEO-PFR Raifix 120/PEO-PFR Raisapac/PEO-PFR Polymer (mg/g of O D pulp) Figure 3.48 Retention of fines, pretreated PCC and total solids with PEO, PFR system, Raifix 2515 and PEO, PFR system, Raifix 120 and PEO, PFR system and Raisapac and PEO, PFR system. PFR/PEO = 1.0, PEO, PFR =0.15 mg/g of OD pulp. The condition with Raifix 120 to get maximum retention of fibre fines and pretreated PCC retention was at a Raifix 120 dosage of 0.1 mg/g of OD pulp and a PEO, PFR dosage of 0.15 mg/g of OD pulp as seen in figure 3.24. The sequence was Raifix 120-pretreated PCC-PFR-PEO. Retention was maximum when Raisapac dosage was 2.0 mg/g of OD pulp and PEO, PFR dosage was 0.15 mg/g of OD pulp. The sequence used was Raisapac-pretreated PCC-PFR-PEO. The above results indicate that the PEO, PFR system on it's own gives a maximum retention of fines and pretreated PCC compared to that when used in conjunction with Raifix 2515 or Raifix 120 or Raisapac when the PEO, PFR dosage is 94 0.15 mg/g of OD pulp. It can also be seen that Raifix 2515 is better than Raifix 120 for TMP pulp loaded with pretreated PCC and Raisapac is better than both. 95 CHAPTER-4 Conclusions and Recommendations For Future Work 4.1 Conclusions This work investigates the retention of pretreated PCC in mechanical pulp suspensions using a series of polymers with PEO, PFR system for neutral/alkaline papermaking. First-pass retention experiments were performed with PEO, PFR system on 100 % TMP pulp loaded with 20 % wt. of pretreated PCC using Britt Dynamic Drainage Jar (DDJ). The screen used was of 70 mesh and the fines fraction for pulp was defined for this mesh. The sequence used was pretreated PCC added to Pulp and then adding PFR and lastly PEO. The ratio of PEO/PFR used was 1.0 in all the experiments. The DDJ stirrer was fixed at 1000 rpm. It is found that as we increase PEO, PFR dosage the retention of fibre fines and pretreated PCC increased. In order to study the effect of PEO, PFR on pretreated PCC aggregation experiments were done using a Malvern particle size analyzer. The maximum aggregate size and higher aggregation rate was reached at a PEO, PFR dosages of 0.75 mg/g of pretreated PCC. So this dosage was used for further experiments with PEO, PFR. A study of the change in zeta potential of the aggregates of PCC formed with PEO, PFR shows that there is a charge reversal beyond a dosage of 0.75 mg/g of pretreated PCC due to which the rate of aggregation and maximum aggregate size reached decreases. The investigation of Raifix 2515 as an anionic trash collector (ATC) in conjunction with PEO, PFR system was studied. The best sequence of addition of polymers was Raifix 2515 first and then adding pretreated PCC and then PFR and finally PEO. The statistical design of experiments shows that there is an interaction between PEO, PFR, but this interaction did not improve first-pass retention of fines and pretreated PCC. The first pass retention experiments performed at various dosages of Raifix 2515 keeping PEO, PFR dosage fixed at 0.15 mg/g of OD pulp show that by increasing Raifix 2515 dosage decreased retention of fines and pretreated PCC. Aggregation experiments performed with Raifix 2515 and PEO, PFR show that Raifix 2515 on it's own forms aggregates with pretreated PCC. However, when aggregation experiments were 96 performed with Raifix 2515 in conjunction with PEO, PFR we see a decrease in the rate of aggregation and the maximum aggregate size reached. This also confirms the interaction between Raifix 2515 and PEO, PFR. It is believed that Raifix 2515 scavenges PFR and therefore PEO is not affective in flocculating fines and pretreated PCC. However at a PEO, PFR dosage of 0.05 mg/g of OD pulp the retention of fibre fines and filler improved because there is not enough PFR to be scavenged and Raifix acts as a retention aid. Retention of pretreated PCC and fibre fines was investigated with another ATC Raifix 120, which has a higher cationic charge compared to Raifix 2515. Statistical design results show that there is an interaction between Raifix 120 and PEO, PFR. Raifix 120 increases fibre fines and pretreated PCC retention in the absence of PEO, PFR. However this increase is not significant. Raifix 120 along with a PEO, PFR dosage of 0.05 mg/g of OD pulp show an increase in retention. However at a PEO, PFR dosage of 0.15 mg/g of OD pulp the retention decreases in the presence of Raifix 120. Raisapac on its own does not increase retention. In fact the retention decreases as we increase Raisapac dosage. When Raisapac is used in conjunction with PEO, PFR the fines retention increases slightly but pretreated PCC retention decreases at a PEO, PFR dosage of 0.05 mg/g of OD pulp. The results indicate that there is no particular trend for retention with Raisapac. Aggregation experiments performed with Raisapc on pretreated PCC shows that there is very little aggregation of pretreated PCC. However when Raisapac was used in conjunction with PEO, PFR the rate of aggregation and maximum aggregate size reached increases substantially. This shows that PEO, PFR and Raisapac together aggregates PCC. However when Raisapac was used in conjunction with PEO, PFR at PEO, PFR dosages of 0.15 mg/g of OD pulp, the pretreated PCC retention increased. The effect of DDJ stirrer speed with PEO, PFR system, Raifix 2515-PEO-PFR system and Raisapac-PEO-PFR system was studied. The various speeds used were 1000, 1200 and 1400 rpm. It was seen that as we increase the speed the retention of fibre fines and pretreated PCC decreases in a PEO, PFR system. However in a Raifix 2515-PEO-PFR system by increasing the speed from 1000 rpm to 1200 rpm increased retention and further increasing the speed to 1400 rpm decreased retention. This trend was also 97 observed with Raisapac-PEO, PFR system. These results are co-related with the floe strength. 4.2 Recommendations For Future Work The pretreatment of PCC used for this work required 24 hours. This method will be difficult to use when put in practice. Also phosphoric acid concentrations in the waste water affects the environment. ^  There is a scope of improving this method to make it suitable to use and environment friendly. There is a necessity to study the physical and optical properties of pretreated PCC filled mechanical pulp suspension hand sheets with all the combination of polymers used for this work. This wil l also give an in depth knowledge of the affects of these polymers on strength and optical properties of paper. It will be interesting to see the strength of hand sheets with PEO, PFR, Raifix-PEO-PFR system and Raisapac-PEO-PFR system. The drainage of the pulp flocculated with these polymers is also important as it affects the steam consumption on a paper machine. Formation is another aspect that is important to a papermaker as we know that over flocculation affects formation of paper. When Raifix is used as an A T C it will be interesting to see the consumption of sizing chemicals like Alkenyl succinic anhydride and Alkyl ketene dimer. The first pass retention of pretreated PCC can be studied with different polymers like polyacrylamide in conjunction with Raifix 2515, Raifix 120 and Raisapac. Since PEO needs a cofactor to enhance it's retention capability adding PFR interferes the way the ATC's work. Therefore investigating the use of A T C with other polymers is recommended. 98 R E F E R E N C E S 1. • Ain, R. N . and M . Laleg, " M i l l Experiences With AT-PCC In Papers Containing Mechanical Pulps", Pulp and Paper Canada ; 98(12): T495-T499, 1997. 2. Alince, B. , Porubska, J. and T. G. M . van de Ven, "Ground and Precipitated Calcium 3. Carbonate Deposition On Fibres In The Presence of PEO and Kraft Lignin", Paper Technology, 38(2) ; 51-54, 1997. 4. Andrea Gibbs, Huining Xiua, Yulin Deng and Robert Pelton, "Flocculants For PCC in Newsprint Pulp", TAPPI Journal vol.80;No. 4: 163-170, 1996. 5. Anne M.McCourt, Philip A. Ford and Thomas A. Cauley, " A Practical View Of A Microparticle System In Super Calendar Paper",Tappi Journal, vol.76 No. 10:165-182, April 1990. 6. Bailey jr., F. E. and J. V . Koleskee, "Polyethylene Oxide" , Academic Press New York, USA. 7. Braun, D.B. and D. J. DeLong, "Ethylene Oxide Polymers", Kirkothmer Concise Encyclopedia of Chemical Technology, 4 t h edition, 928-929, John Wiley and sons, New York, U S A 1999. 8. Brian P. Duffy, " A Microparticle Retention Approach To Papermaking, Tappi Proceedings", Papermakers Conference; 171-175, 1993. 9. Bruce Evans, "PCC fillers for groundwood papers", Papermakers conference 1991. 10. Carignan, A. , Gamier, G. and T.G.M. van de Ven, "The Flocculation of Fines By PEO/COFACTOR Retention Aid Systems", JPPS 24 (3); 94-99, 1998. 11. Chapnerkar, V . D., "Precipitated calcium carbonate", Conference ; 1995 Tappi Dyes, Fillers and Pigments Short Course: 163-184, April 1995. 12. Dan Eklund and Tom Lindstrom, "Paperchemistry An Introduction", DT Paper Science publication, Grankulla, Finland ; 1991. 13. Erkki Huusari and Ahti Syrjanen, "TMP and Talc-An Ideal Combination For Uncoated High Quality Magazine Paper", Progress In Paper Recycling, 231-236, August 1992. 99 14. Gaudreault, R. , "Polyethylene Oxide", Dept. of Chemistry, Mc Gil l University, Montreal, Canada 1997. 15. George Wypych, "Handbook of fillers",2n d edition, Toronto-New York, 1999. 16. Gess, J. M . , "Retention of Fines and Fillers During Papermaking", Tappi Press, Atlanta, GA: 1998. 17. Gi l l , R. A . , "The Retention, Drainage and Optical Performance of On-Site Synthesized PCC Fillers", Pulp and Paper Canada ; 91(9): 342-346, 1990. 18. Headrick, G. A . and A. M . Bollinger, "The Use of PEO in Papermaking in Retention of Fines And Fillers During Papermaking" , C h . l l , 246-258, TAPPI Press, Atlanta, USA, 1998. 19. James P. Casey, "Pulp and Paper Chemistry and Chemical Technology", 3 r d edition, vol.3, A Wiley-interscience publication ; 1981. 20. John-Ho Shin, Sin Ho Han, Changman Sohn, Say Kyoun Ow and Soukil Mah, "Highly Branched Cationic Poly electrolytes: Filler Flocculation", TAPPI Jr. ;179-185, Nov. 1997. 21. Kamiti, M . and T. G. M . Van De Ven, "Kinetics of deposition of calcium carbonate particles on to pulp fibers", Journal of Pulp and Paper science vol.20 No. 7, J199 July, 1994. 22. Kocurek, M . J., "Pulp and Paper Manufacture", vol.6, Stock Preparation, published by The Joint Text Book Committee of the Paper Industry, Tappi, CPPA;1992. 23. Lawrence Allen, Marco Polverari, Brigikte Levesque and William Francis, "Effects of System Closure On Retention and Drainage-Aid Performance in TMP Newsprint Manufacture", TAPPI Journal; 188-195, April, 1999. 24. Lindstrom, T., Hans Hallgren and Fritz Hedborg, "Aluminum Based Microparticulate Retention A i d System", Pulp and Paper International, April 1990. 25. Lindstrom, T. and G.Glad-Nordmark, "Flocculation of Latex and Cellulose Dispersion By Means of Transient Polymer Networks" , Colloids and Surfaces 8(4): 337, 1984.. 26. Liu Juntai, "Cationic Polyacrylamide As A Drainage Aid In Mechanical Pulps", TAPPI Journal vol.78, No.4: 149-154, 1995. 100 27. Montgomery, Douglas C , " Design and Analysis of Experiments", John Wiley & Sons, 4 t h Edition, New York, 1997. 28. Pang, P., Ph.D Thesis, University of British Columbia, 2001. 29. Pang, P., K . K . Khoulatchaeu and P. Englezos, 'Inhibition of The Dissolution of Papermaking Grade Precipitated Calcium Carbonate Filler", Tappi J.81(4),188-192, 1998. 30. Passaretti, J. D., "Application of high opacity PCC" , Papermakers conference, 1993, 415. 31. Pelton, R. H. , Allen, L . H . and H. M . Nugent, Sevensk Papperstid. 83(9):251 ,1980. 32. Pelton, R. H. , Allen, L . H . and H . M . Nugent, "Factors Affecting The Effectiveness Of Some Retention Aids In Newsprint Pulp", Sevensk Papperstidn. 83(9) :25 1980. 33. Persello, J., Pierre, A. , Lamarche, J. M . , Mercier, R., Foissy, A. , "Calcium as potential determining ion in aqueous calcite suspensions", Journal of Dispersion Science and Technology, 11(6), 611-635 ; 1990., Tappi, CPPA ; 1992.0 34. Robert Garrels, Charles L. Christ, "Solutions, Minerals and Equilibria", Harper and Row Publishers New York : 1992. 35. Robert W.Hagemeyer, "Pigments For Paper", Tappi Press, Atlanta,GA: 1997. 36. Sanders, N . D. and J. H . Schaefer, "Comparing Wet-end Charge-measuring Techniques in Kraft And Groundwood Systems", TAPPI Journal, 78(11): 142, 1995. 37. Smook, G. A . , "Handbook for Pulp and Paper Technologists", 2 n d edition, Vancouver; 1992. 38. Tay, C. H . and T .A. Canley, "Studies On Polyethylene Oxide As A Retention Aid In Wood Fibre Stock Systems", TAPPI Papermakers Conference 1982. 39. Trigylidas, D., " The Use of Raifix To Enhance Fines and Filler Retention In Mechanical Groundwood Pulps", M.A.Sc. Thesis, Aug. 1999. 40. van de Ven, T. G. M and B. Allince, "Association- Induced Polymer Bridging: New Insights in to Retention of Fillers With PEO", JPPS, 22(7); J257, 1996. 41. Vanerek, A . , Alince, B. , and T.G.M. Van De ven, " Colloidal Behavior of GCC and PCC fillers: Effects of Cationic Polyelectrolytes and Water Quality", JPPS, 26(4); 135-139,2000. 101 42. Whiting, P. L . , "Contamination Control On A High Speed Paper Machine", Proceedings of TAPPI Pulping Conference; Nashville, 1996. 102 APPENDIX-1 First Pass Retention Calculations A B C 1 Experiment No. 2 Date 3 Pulp Consistency (%) 0.5 0.5 0.5 4 Filler Consistency (%) 0.1 0.1 0.1 5 Amount of filler present (g) 0.5 0.5 0.5 6 Fines (%) 40 40 40 7 Temperature (°C) 25 25 25 8 pH 7 7.0-7.20 7.0-7.21 9 DDJ Stirring Speed (rpm) 1000 1000 1000 10 11 Wt. Pulp (g) 499.86 500.01 499.36 12 Wt. 150mL Beaker (g) 109.42 103.57 106.32 13 Wt. Beaker + Filtrate (g) 215.12 205.77 207.08 14 Wt. Filtrate (g) =C13-C12 =D13-D12 =E13-E12 15 16 Wt. Crucible (g) 18.8865 19.3061 18.7521 17 Wt. Cruc. + Filter Paper (g) 19.0173 19.4022 18.8544 18 Wt. Filter Cake (g) =C17-C16 =D17-D16 =E17-E16 19 Wt. Cruc. + Ash (g) 18.9274 19.3356 18.7833 20 Wt. of Ash =C19-C16 =D19-D16 =E19-E16 21 22 Amount of fines present =C11*C6/100*C3/100 =D11*D6/100*D3/100 =E11*E6/100*E3/100 23 Unretained fines =(C18-C20)*C11/C14 =(D18-D20)*D11/D14 =(E18-E20)*E11/E14 24 Unretained Ash =C20*C11/C14 =D20*D11/D14 =E20*E11/E14 25 Unretained solids =C18*C11/C14 =D18*D11/D14 =E18*E11/E14 26 Total solids present =(C11*C3/100)+C5 =(D11*D3/100)+D5 =(E11*E3/100)+E5 27 28 Fines Retention (%) =(C22-C23)/C22*100 =(D22-D23)/D22*100 =(E22-E23)/E22*100 29 Ash Retention (%) =(C5-C24)/C5*100 =(D5-D24)/D5*100 =(E5-E24)/E5*100 30 Total solids retention(%) =(C26-C25)/C26*100 =(D26-D25)/D26*100 =(E26-E25)/E26*100 First Pass Retention = ((arbO/ai) * 100 103 Appendix-2 Statistical Analysis of PEO, PFR and Raifix 2515 on Fines retention Raifix 2515 Raifix 2515 Low (0.0) Medium (0.1) High (0.5) Lew (0.0^2 Medium (0.1)^ 2 High (0.5)^ 2 42.99 54.12 39.37 1848.1401 2928.9744 1549.9969 Low(O) 34.74 40.95 38.95 1206.8676 1676.9025 1517.1025 40.55 43.83 39.18 1644.3025 1921.0689 1535.0724 PEO/PFR 45.25 39.46 5235 2047.5625 1557.0916 2740.5225 Medium (0.05) 36.85 42.27 50.54 1357.9225 1786.7529 2554.2916 38.39 46.68 53.17 1473.7921 2179.0224 2827.0489 64.72 54.36 45.26 4188.6784 2955.0096 2048.4676 Ugh (0.15) 57.47 54.22 53.61 3302.8009 2939.8084 2874.0321 67.42 56.9 54.55 4545.4564 3237.61 2975.7025 Factor 'A' is Raifix 2515 and factor 'B' is PEO/PFR Factor A Summation of Factor A Low Medium High Factor B Low 118.28 138.9 117.5 374.68 Medium 120.49 128.41 156.06 404.96 High 189.61 165.48 153.42 508.51 Summation of Factor B 428.38 432.79 426.98 1288.15 Analysis of Variance The interaction between PEO/PFR and Raifix 2515 is statistically significant for Fines retention. Degrees of Freedom Mean of Squares Fo Sum of Squares of PEO=PFR 1094.442585 2 547.2212926 31.59027061 Sum of Squares of Raifix 2515 2.043118519 2 1.021559259 0.058973095 Sum of Squares of Interaction AxB 555.0283481 4 138.757087 8.010240076 Sum of Squares of Error j 311.8043333 18 17.32246296 Sum of Squares of Total 1963.318385 26 104 

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