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The electrochemical mediation of oxygen delignification of pulp with a manganese polyol complex Todd, Rory 1993

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The Electrochemical Mediation of Oxygen Delignification of Pulp with a ManganesePolyol Complex.byRory Denis ToddB.Eng. (Chemical Engineering) Heriot-Watt University, 1990A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMaster of Applied ScienceinTHE FACULTY OF GRADUATE STUDIESDepartment of Chemical EngineeringWe accept this thesis as conformingt>3^:-.uired standardTHE UNIVERSITY OF BRITISH COLUMBIAApril 1993© Rory Denis Todd, 1993In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely av ailable for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature) Department of C41)2/44-f The University of British ColumbiaVancouver, CanadaDate 22* C1/ DE-6 (2/88)Abstract. A redox cycle involving a manganese gluconate complex and oxygen in an aqueous alkalinemedia was investigated for its ability to delignify chemical pulp. Current applied at a cathodewas used to reduce the higher oxidation states of the manganese complex. The reaction of themanganese (II) and (III) complex forms with oxygen is believed to produce hydrogenperoxide. Evidence presented in this study suggests a redox cycle was operating involving themanganese III and IV oxidation states of the manganese complex, oxygen and sufficientcurrent. It is thought hydrogen peroxide produced in this redox cycle was responsible for thelarger Kappa number and viscosity reductions observed in experimental runs employing themanganese gluconate complex/oxygen redox cycle, than in corresponding runs in the absenceof the manganese complex/oxygen redox cycle.For a pulp consistency of 1 %, in 1 M sodium hydroxide, with oxygen purge, for 3 hours at 50°C and atmospheric pressure, the presence of the manganese gluconate complex and sufficientcurrent on a platinised titanium cathode reduced the pulp Kappa number from 29 --> 17 andthe viscosity from 33 --> 20 cP. In the absence of the complex and current the pulp Kappanumber was reduced from 29 -- > 20 while the viscosity was reduced from 33 -- > 29 cP.The manganese complex/oxygen redox cycle did not significantly enhance the delignificationor cellulose degradation reactions in factorial experiments at 20 °C or 90 °C. At 20 °C it isbelieved that increased peroxide stability prevented the delignification and cellulosedegradation reactions. The formation of free radical species being necessary in peroxidedelignification. At 90 °C it is believed the increased activity of the hydroxide ions and oxygenmask any effect of peroxide produced in the manganese complex/oxygen redox cycle.iiAbstractThe presence of a contaminant, likely copper or iron ions, in the reactor is believed to causethe larger viscosity drop in experimental runs where the contaminant was present. It is thoughtthat the copper and iron ions increased the production of free radical species in the bleachingmixture, thus promoting cellulose degradation.iiiTable of Contents. Abstract^Table of Contents^ ivList of Figures viiList of Tables^ ixNomenclature xiiAcknowledgment^ xivChapter 1.Introduction^ 1Chapter 2.Background and Literature Review2.1 Structure and Chemical Composition of Wood Fibre andUnbleached Pulp2.1.1 Chemical Components in Wood Fiber and Pulp^ 3CelluloseHemicelluloseLignin2.1.2 Structure and Chemical Composition of Wood Fiber^ 62.1.3 Chemical Composition of Unbleached Pulp^ 82.2 Objectives in Pulp Bleaching^ 92.3 Oxygen and Hydrogen Peroxide Delignification2.3.1 Basic Chemical Reactions^  112.3.2 Oxygen Delignification  132.3.3 Metal Ions in Oxygen Delignification^  142.3.4 Hydrogen Peroxide Delignification  152.3.5 Metal Ions in Hydrogen Peroxide Delignification^  162.4 Manganese Gluconate Complexes^  182.5 Electrochemistry^ 212.6 Cyclic Voltammetry 24ivTable of Contents2.7 Spectrophotometry^ 262.8 Factorial Design of Experiments^ 27Chapter 3.Proposed System and Research Objectives^ 32Chapter 4.Experimental Apparatus and Procedure4.1 Cyclic Voltammetry^ 344.2 Spectrophotometry 364.3 Hydrogen Peroxide Analysis^  384.4 Pulp Delignification Trials 39Chapter 5.Experimental Results and Discussion5.1 Cyclic Voltammetry5.1.0 Introduction^ 435.1.1 Experimental Results and Discussion^ 445.2 Spectrophotometry5.2.0 Introduction^ 515.2.1 Experimental Results and Discussion^ 515.3 Hydrogen Peroxide Analysis5.3.0 Introduction^ 595.3.1 Experimental Results and Discussion^ 595.4 Pulp Delignification Trials5.4.0 Introduction^ 635.4.1 Experimental Results and DiscussionMetal Ion Effect 64Evidence of Catalytic Delignification^ 70Effect of Cathode Material^ 72Effect of Oxygen^ 75Effect of DTPA Wash 77Effect of Catalyst Concentration  81Effect of Hydrogen Peroxide Stabilisers^  83- Effect of Chemical Stabilisers  83- Effect of pH^ 85vTable of Contents- Effect of Temperature^  86Effect of Current 90Time Dependance 97Effect of Manganese and Ligand Only^ 99Use of Chemical Reductants^ 101Chapter 6.General Discussion^ 104Chapter 7.Conclusions and Recommendations^ 106Bibliography^ 108Appendices.Appendix 1 Analysis of Factorial Designs ^  111Appendix 2 Calculation of Response Error for Factorial Designs^ 117Appendix 3 Electrode Studies^ 119Appendix 4 Hydroxide Ion Production at 10.0 A^ 121Appendix 5 Hydrogen Peroxide Titration Calculation 122Glossary^ 123viList of Figures. 1. Structure of Cellulose^ 42. Structure of Lignin 53. Gluconate Ion^  184. Diagram of Typical CV Result^ 245. Proposed Redox Cycle for the Continuous Generation of^ 33Hydrogen Peroxide From Manganese Gluconate and Oxygen6. Cyclic Voltammetry Set-up^ 347. Diagram of Equipment Used in Electrochemical Bleaching^ 39Trials8. Cyclic Voltammogram for the Manganese Gluconate Complex 44Formed in the Presence of Oxygen and Deaerated withNitrogen - platinum cathode9. Cyclic Voltammogram for 1 M NaOH Deaerated with^ 45Nitrogen10. Cyclic Voltammogram for the Manganese Gluconate ' 47Complex Formed in the Presence of Oxygen and Deaeratedwith Nitrogen - copper/mercury cathode11. Cyclic Voltammogram for the Manganese Gluconate^ 47Complex Formed in the Presence of Oxygen and Deaeratedwith Nitrogen - graphite cathode12. Cyclic Voltammogram for the Manganese Gluconate^ 49Complex Formed in the Presence of Oxygen and Deaeratedwith Nitrogen - platinum cathode13. Absorption Spectrograph for Manganese Gluconate^ 52Complex14. Absorption Profile for Manganese (II) Gluconate 54Complex in the Presence of Oxygen15. Absorption Profile for Manganese (Ill) Gluconate^ 55Complex in the Presence of CurrentviiList of Figures16. Absorption Profile for Manganese (IV) Gluconate^ 56Complex in the Presence of Current17. Absorption Profile for Manganese Gluconate Complex in  57the Presence of Oxygen and Current18. Effect of Catalyst Concentration on Kappa Number^ 8119. Effect of Catalyst Concentration on Viscosity  8220. Effect of Current on Kappa Number^ 9021. Effect of Current on Viscosity  9122. Time Dependance^ 9823. Electrode Potential vs. Applied Current^ 120viiiList of Tables. 1. Chemical Cost of Oxidants^ 22. Chemical Composition of Wood 63. Chemical Composition of Unbleached Pulp^ 84. Factorial Design Matrices for Two and Three Variables^ 285. Stoichiometry for Manganese Complex Oxidations with 36K3Fe(CN)66. Description of Spectrophotometric Experiments^ 377. Absorbance Values for the Manganese Gluconate Complex^ 538. Hydrogen Peroxide Concentration in NaOH Solution 609. Hydrogen Peroxide Concentration in a Manganese Gluconate^ 60Complex Under Bleaching Conditions10. Hydrogen Peroxide Concentration in a Manganese^ 61Gluconate Complex Under Bleaching Conditions11. Results Prior to and After the Use of a Lead cathode 6412. Results of Factorial Experiment^ 6513. Results of Factorial Experiment 6614. Effect of Metal Ion Addition on Bleaching Result^ 6815. Results of Factorial Experiment^ 7016. Effect Cathode Material on Bleaching Result^ 7217. Effect Cathode Material on Bleaching Result 7318. Effect of Oxygen on Bleaching Result^  7519. Effect of Oxygen on Bleaching Result 7620. Effect of DTPA Wash On Metal Content of Pulp^ 7721. Effect of DTPA Wash on Bleaching Result 7822. Effect of DTPA Wash on Bleaching Result^ 78ixList of Tables23. Effect of DTPA Wash on Bleaching Result^ 7924. Effect of Chemical Stabilisers^  8325. Results of Factorial Experiment at pH 11^ 8526. Results of Factorial Experiment at 20 °C 8627. Results of Factorial Experiment at 90 °C^ 8728. Results of Factorial Experiment at 90 °C 8829. Effect of Hydroxide Addition on Bleaching Result^ 9230. Effect of Acid Addition on Bleaching Result 9431. Results of Factorial Experiment^ 9532. Effect of Hydrogen Peroxide Addition  ^ 9633. Effect of Metal and Ligand on Bleaching Result 9934. Results of Factorial Experiment ^ 10135. Results of Factorial Experiment 10236. Effect of Nitrogen on Bleaching Result^ 10337. Factorial Analysis of Data from Table 11 11138. Factorial Analysis of Data from Table 12^ 11239. Factorial Analysis of Data from Table 15 11240. Factorial Analysis of Data from Table 25^ 11341. Factorial Analysis of Data from Table 26 11342. Factorial Analysis of Data from Table 27^ 11443. Factorial Analysis of Data from Table 28 11444. Factorial Analysis of Data from Table 31^ 11545. Factorial Analysis of Data from Table 34 11546. Factorial Analysis of Data from Table 35^ 116xList of Tables47. Table for Response Error Calculation^ 117xiNomenclature.a : Tafel equation constantA : Absorbanceb : Tafel equation constantc : ConcentrationCINT : Confidence intervalE : Electrode potentialE° : Standard reduction potentialEr : Reversible electrode potentialF : Faraday constanti : Current densityilim : Limiting current densityI : Current: Interaction of variables in factorial design10 : Intensity of incident lightIt : Intensity of transmitted lightK : Mass transfer coefficient1 : Thickness of absorbing mediumM : Main effect estimateN : Number of runs in the factorial design experimentQ : Activity quotient: Replicate runsR : Universal gas constantxiiNomenclatures : Response errort : Time: Transport numberT : Temperaturev : Degrees of freedomX : Independent variables in factorial design experimentsY : Response in factorial design experimentsz : Electron stoichiometric coefficientE : Absorption coefficientA : OverpotentialAcknowledgment. My sincere thanks to Professor Colin Oloman for his advice and assistance in this work.Also I thank the staff of the Department of Chemical Engineering and the Pulp and PaperCentre at U.B.C. for all their help.xivChapter 1. Introduction.One of the main problems facing the chemical pulp bleaching industry is the need to find aneffective delignifying agent that meets the increasingly stringent environmental standards beingimposed by the regulatory bodies. Oxygen delignification, while effective and environmentallyfriendly is expensive in comparison to chlorine based bleaching methods largely due to the costof the high pressure equipment required. Oxygen bleaching also suffers from low selectivitywhen compared to chlorine based bleaching methods. The pulp treated by the more selectivesystem has a higher cellulose content for the same reduction in lignin content.Hydrogen peroxide is also an environmentally friendly bleaching chemical. While its use inboth chemical and mechanical pulp brightening is well documented, its use in chemical pulpdelignification has not been as extensively studied. However recent research employing 1 %H202 and 3 % NaOH, on oven-dry pulp, at 15 % pulp consistency and 90 °C for 90 mins,indicated the ability of hydrogen peroxide to reduce the lignin content of Kraft pulp by 28 %[ 1].At present peroxide delignification is unlikely to rival oxygen delignification due to the highchemical cost of the oxidant as indicated in Table 1.1Chapter 1. IntroductionOxidant Price$/kg02 0.11H202 1.43Table 1. Chemical Cost of Oxidants [2].However, peroxide delignification has the advantage of the use of lower temperature whencompared to conventional oxygen bleaching [1]. Also peroxide delignification has not been asextensively studied as oxygen delignification and thus there is the potential of improving itseffectiveness and selectivity.If the cost of hydrogen peroxide could be decreased, the use of peroxide in chemical pulpdelignification would become more attractive. The objective of the following study is toexamine the effectiveness and selectivity of delignification of chemical pulp by hydrogenperoxide generated through a cathodically mediated redox cycle involving oxygen and amanganese complex in an alkaline environment. The in-situ generation of the relativelyexpensive oxidant hydrogen peroxide from the cheaper oxidant oxygen, via the redox cycle,may make peroxide delignification more attractive (competitive).2Chapter 2. Background and Literature Review.2.1 Structure and Chemical Composition of Wood Fiber andUnbleached Pulp.2.1.1 Chemical Components in Wood Fiber and PulpWood fiber consists of three principle chemical components. They are cellulose, hemicelluloseand lignin. These three materials are all polymeric in structure.Cellulose. Cellulose, a polysaccharide, is a high molecular weight stereo regular linear polymer.Cellulose is the chief structural element and major constituent of the cell walls of trees [3].The fibre bundles it forms impart a high tensile strength to wood. Cellulose is a homogeneouspolysaccharide composed of beta-D-glucopyranose units linked by (1-- > 4)-glycosidic bonds.The structure is as shown below in Figure 1 [4].3Chapter 2. Background and Literature ReviewFigure 1. Structure of Cellulose.Hemicellulose. Hemicellulose functions as a supporting material in wood fibre. Hemicellulose is a short chainpolymer composed of various different sugar monomers. It is relatively easily hydrolysed byacids to these monomeric components when compared to cellulose.Lignin. In chemical pulping and bleaching processes, lignin is the unwanted component of wood in thefinal product It is an important mechanical reinforcement agent for the entire tree. Thestructure of lignin is extremely complex and not fully understood. Lignin is an amorphoussubstance that is partly aromatic in nature. It contains methoxy, aliphatic and phenolic groups4Chapter 2. Background and Literature Reviewin a three dimensional polymer linked by C-O-C and C-C bonds. It is responsible for thecoloured impurities in pulp [3]. The structure of lignin is indicated in Figure 2 [4].Figure 2. Structure of Lignin.5Chapter 2. Background and Literature Review2.1.2 Structure and Chemical Composition of Wood Fiber 13.5]. Wood is of plant origin and therefore variability in structure is common. Differences instructure can be between species, between trees within a species and between wood fromdifferent locations within one tree.The most characteristic differences are between softwoods and hardwoods. Hardwoodstypically contain less lignin and more cellulose as indicated in Table 2.Component Hardwood SoftwoodCellulose 40-50 40-45Lignin 20-25 25-35Hemicellulose 25-35 25-30Table 2. Chemical Composition of Wood [3].Fibres from softwood and hardwood also differ. Generally fibres from softwoods are hollow,3-6 mm in length, with a diameter 25-60 um and a wall thickness of around 2-7 urn.Hardwood fibres however tend to be smaller, the fibres are 0.8-1.9 mm in length and 10-40urn in diameter.A typical fibre wall contains four main layers. The primary wall is a thin outer layersurrounding the fibre. The secondary wall consists of 3 layers, the outer (S 1), middle (S2) andinner (S3) layers. The secondary wall layers (S i ,S2,S3) are built up by lamellae formed by6Chapter 2. Background and Literature Reviewalmost parallel microfibrils (cellulose bundles) between which the lignin and hemicellulose arelocated.7Chapter 2. Background and Literature Review2.1.3 Chemical Composition of Unbleached Pulp [3.5.6]. Before the bleaching stage the wood chips undergo pulping, the purpose of pulping is theselective removal of the chemical fibre bonded lignin with a minimum dissolution of thehemicellulose and cellulose. Pulping converts the wood chips into separate fibres via reactionof chemicals in the pulping liquor (in the case of Kraft pulping NaOH and Na2S) with thelignin. Unfortunately the chemicals employed in pulping also degrade the cellulose andhemicellulose components. Kraft pulping is normally stopped at 4-5 wt% lignin for softwoodpulps as indicated in Table 3. Continuing the pulping further results in excessive cellulosedegradation. The lignin remaining after pulping is highly coloured and is difficult to remove.It is located mainly in the secondary wall.Component SoftwoodCellulose 72Lignin 4Hemicellulose 24Table 3. Chemical Composition of Unbleached Pulp [6].8Chapter 2. Background and Literature Review2.2 Objectives in Chemical Pulp Bleaching.The main objective of chemical pulp bleaching is to make the pulp whiter and brighter. Thepulp can be considered to be made up of lignin and cellulosic components [7]. The maincoloured constituents in the pulp are due to the lignin. It is therefore desirable to remove thelignin from the pulp. The cellulosic component of the pulp is important with respect to thestrength properties of the final bleached product [5]. Chemical pulp bleaching can beconsidered as the process of removing the lignin from the pulp while minimising thedegradation of the cellulosic component of the pulp [8].Bleaching agents react with pulp in two major ways, 1) reaction with lignin and other non-cellulosic materials (desirable), or 2) reacting to degrade the cellulosic component of the pulp(undesirable) [7]. Since lignin can be oxidised it can be removed through oxidation andsolution in alkali [4] the conditions being manipulated to minimise the cellulose degradation.A standard pulp industry method (T230) involving determination of the solution viscosity [9] isthe most commonly used method for measuring the average degree of polymerisation ofcellulose. This method is based upon the relationship of solution viscosity to the length of thesolute molecules [3]. Therefore the pulp viscosity, as measured by the standard method, can berelated to the strength properties of the pulp [5].The lignin content of pulp is commonly measured in terms of the Kappa number (T236) of thepulp [5,9]. The relationship between the lignin content and Kappa number of pulp is very goodfor bleachable pulps.9Chapter 2. Background and Literature ReviewFor Kraft pulp,Lignin % = 0.147 x Kappa number^(1)Conventional oxygen bleaching reduces the Kappa number of pulp from around 30 to 15 andreduces the viscosity from around 40 to 25 cP.10Chapter 2. Background and Literature Review2.3 Oxygen and Hydrogen Peroxide Delignification.2.3.1 Basic chemical reactions. In aqueous solution hydrogen peroxide is weakly acidic. Under alkaline conditions it forms theperhydroxyl ion by Reaction 1 and decomposes to oxygen and water by Reaction 2 [10].H202 + H2O <--> H30+ + H02-^Rxn 1H202 --> H2O + 1/2 02^Rxn 2Oxygen on the other hand is a diradical which reacts with unsaturated compounds, metal ionsand free radicals [8]. In alkaline solutions oxygen reacts with phenolate anions to producesuperoxide radicals via Reaction 3 [8].02 + e (from substrate) --> -Of Rxn 3Reaction 3 is endothermic and requires activation of the substrate and/or higher temperaturesto proceed at a reasonable rate. For this reason oxygen bleaching is usually carried out in analkaline environment (pH > 10) and at a temperature > 80 °C [11].The free radical reactions of oxygen and hydrogen peroxide in bleaching environments areintertwined and many of the same free radical species are produced from both.11Chapter 2. Background and Literature ReviewIn the case of oxygen the superoxide radical can lead to the formation of hydrogen peroxide(Reaction 4) [11].2 -02- + H2O --> 02 +^+ H02- Rxn 4In practice this reaction is not favoured and the production of hydrogen peroxide is generallycatalysed by protons (Reaction 5) or metal ions (Reactions 6 and 7) [11].-02-+ -02- + 2H+ --> 11202 + 02^Rxn 5Mn+ + •0f <_), m(:02.)(n-1)+^Rxn 6H2O + - 02- + M(:02-)( 1-1) + --> 02 + Mn+ +^+ H02- Rxn 7It has been recognised that hydroxyl radicals play an important role in both oxygen andhydrogen peroxide bleaching of pulp [8,11,12]. These radicals can arise through metalcatalysed decomposition of hydrogen peroxide (Reaction 8).Mn+ + 11202 --> Mn+1 +^+ OH-^Rxn 8To be catalytic in its action the metal ion (Mn+1) must return to its Mn+ form. In oxygenbleaching this is may be achieved through the action of the superoxide radical (Reaction 9)[11] .12Chapter 2. Background and Literature Review. 02- mn+1^mn+ 02^Rxn 9The net reaction being+ H202 --> 02 + OW + OH- Rxn 10Reaction 10 is often called the superoxide driven Fenton reaction (M = Fe). According to thismechanism the formation of the hydroxyl radicals requires the presence of both the superoxideradicals and hydrogen peroxide.2.3.2 Oxygen Delignification. During oxygen delignification in alkaline systems reactions of oxygen with lignin initiate thedelignification process through the formation of phenoxy free radicals and superoxide anions(Reaction 11) [12].^RC6H40- + 02 > RC6H40 - +^Rxn 11Degradation of carbohydrates occurs through the direct reaction of oxygen as well as throughthe reactions of hydroxyl radicals produced from the hydrogen peroxide formed [8]. In thebleaching environment hydroxyl radicals can oxidise both the lignin and carbohydratecomponents of the pulp [11]. The reaction with cellulose yielding carbonyl structures openingthe way for alkaline degradation of cellulose through beta-elimination (peeling) [11].Gustaysson and Swan [13] suggested the carbohydrate damage during oxygen delignification to13Chapter 2. Background and Literature Reviewbe due to the formation of hydrogen peroxide and other peroxides and their reactions with themetal ions present.2.3.3 Metal Ions in Oxygen Delignification. Lindeburg and Walding [14] reported that pretreatment of Kraft pulp with nitrogendioxide/oxygen under acidic conditions allowed for extended delignification in the followingoxygen bleaching stage without undue loss of pulp viscosity. The major portion of this resultseems to be due to the removal of the metal ions (particularly iron, copper and manganese) viaboth nitrogen dioxide and acid treatment. This suggests that metal ions may not be necessaryfor oxygen delignification and that their presence is probably harmful to the cellulose in pulp.The observed protection effect against cellulose degradation by magnesium ions [13] duringoxygen delignification supports the view that the reaction of metal ions with peroxides toproduce free radicals (reaction 8) is harmful during oxygen delignification. The protectioneffect of magnesium ions is attributed to their ability to form complexes capable of stabilisingperoxides or through the adsorption of transition metal ions on an insoluble magnesiumcompound preventing their free radical generating reactions with peroxides [15].In contrast Landucci [16] reported that the ions of manganese, copper and iron are of greatimportance in oxygen delignification and that their effective removal (via a chelating agent,EDTA) caused the delignification rate to fall dramatically. In Landucci's study the role of thethree metal ions varied. Manganese appeared to preferentially catalyse the oxidation of lignin,copper appeared to catalyse the oxidation of both the lignin and carbohydrate, and ironappeared to preferentially promote carbohydrate oxidation.14Chapter 2. Background and Literature ReviewObviously further work is required to clarify the role of metal ions in pulp delignification.2.3.4 Hydrogen Peroxide Delignification. When applied under relatively mild conditions hydrogen peroxide is a very effectivebrightening agent used for bleaching mechanical pulps without significant yield loss. Undermore severe conditions hydrogen peroxide may be used for the delignification of chemicalpulps [17]. The lignin preserving bleaching effect of hydrogen peroxide in mechanical pulpbleaching is attributed to the oxidative action of the perhydroxyl ion (H02 -) [17].As shown in Reaction 8 hydrogen peroxide is susceptible to catalytic decomposition by certainmetal ions [10,17,18] and its stability tends to decrease with increasing alkalinity andincreasing temperature. For this reason the bleaching of mechanical pulps is usually carried outat pH 10-12 and temperature 35 - 50 °C [15]. Also the hydrogen peroxide used in mechanicalpulp bleaching must be buffered and stabilised before application. The most common buffer issodium silicate which can also act as a stabiliser. Magnesium ion is usually added to preventthe decomposition of hydrogen peroxide. Some of the silicate reacts with the magnesium ion toform a colloidal precipitate which is believed to protect against decomposition through theadsorption and thus deactivation of trace metal catalysts responsible for the generation of freeradicals from hydrogen peroxide. While the use of these additives in mechanical pulpbleaching is required their benefit in chemical pulp delignification is not so clear.Chemical pulp delignification is carried out at pH 10-12 and temperature 80-90 °C, in theabsence of peroxide stabilising agents. Under these conditions, two types of reaction of15Chapter 2. Background and Literature Reviewhydrogen peroxide with lignin seem to predominate [19]. 1) Nucleophilic attack of thehydroperoxide anion (H02 -) on the carbonyl group of the side chain, leading to lignindepolymerisation, and 2) The oxidation of the phenols to quinones which seems to be broughtabout by free radical reactions followed by ring opening and further oxidation to carboxylgroups. The hydroxyl radicals formed in the reaction of hydrogen peroxide with metal ionswould also react with carbohydrates causing their degradation.2.3.5 Metal Ions in Hydrogen Peroxide Delignification. Decomposition of hydrogen peroxide into free radical species is important with respect todelignification [10]. Metal ions which catalyse that decomposition are likely to be ofimportance.In their work on the oxidation of a model lignin compound with hydrogen peroxide Smith andMcDonough [20] found that the addition of iron, copper or manganese salts catalyzed peroxidedecomposition. The addition of copper, manganese and iron ions also increased the rate ofoxidation of the model lignin compound by hydrogen peroxide. The catalytic activity of themetal, with respect to the lignin model oxidation, decreasing from copper to manganese toiron. Landucci [21] found the same ranking for the above metal ions with respect to theirability to catalyse phenoxy radical formation. Smith and McDonough concluded that peroxidedecomposition to free radical species was important with respect to peroxide delignificationand that metal ions played an important role in that decomposition.16Chapter 2. Background and Literature ReviewA further study by McDonough and Kirk et. al. [18] on Kraft pulp delignification withhydrogen peroxide indicated that the delignification rate of the pulp increases with increasinghydrogen peroxide decomposition, paralleling the model compound work. The hydrogenperoxide decomposition was altered through the manipulation of the metal ion concentrationsin the pulp. The metal ions of importance were shown to be copper, manganese and iron. Intheir study metal ion removal was achieved through the use of chelating agents such as DTPA(di-ethylene triamine pentaacetic acid) in prewashing the pulp. It was found however that theaddition of manganese or copper had a negligible effect on delignification and that the additionof iron actually retarded delignification. Also the addition of peroxide stabilisers whichvirtually stopped peroxide decomposition slowed, but did not stop, delignification. Thisindicated the delignification ability of uncatalysed peroxide.The metal ions of manganese and iron both retarded cellulose degradation while copper hadlittle effect. The protective effect of manganese may have been due to the adsorption oftransition metal ions on the surface of precipitated manganese oxides [18].17Chapter 2. Background and Literature Review2.4 Manganese Gluconate Complexes.Research related to biological oxygen transfer systems investigated polyhydroxy ligandscapable of stabilising the higher oxidation states of manganese. It revealed the ability of thegluconate ion to effectively stabilise the +2, +3 and +4 oxidation states of manganese andform soluble complexes in alkaline media [23-25]. The gluconate ion is obtained through theaddition of D-glucono-lactone to sodium hydroxide solutions [24].HHHHOH/HOOHOGluconate ion (G1.14-)H H H^ [26]Figure 3. Gluconate Ion.It was found that molecular oxygen was capable of oxidising the manganese(II) complex(colourless) producing the manganese(111) complex (orange-brown) and hydrogen peroxide(Reaction 12) [23]. GH3 2- represents the dianion of the gluconate ion.n% 4—.1:111^2 k 1.74.s.3 4 taa2v, 2 /nu I /run2 k^4 k vlai 2 + H02 -+ H2ORxn 12Oxygen also oxidised the manganese(III) complex to produce the manganese(IV) complex(cherry red) and a further peroxide ion (Reaction 13) [23].18Chapter 2. Background and Literature Reviewfnu^(nul—"^2%.--3/4%--124- + 02 + 30H --> mnIV2( GH3)4(OH)2 6- + H02 -+ H2ORxn 13In the reaction scheme proposed by Richens et. al. [22], the manganese (II) gluconate complexreacts rapidly in a binuclear process with kobs = 2.8E4 M-1 s-1 (Rxn 12 in 0.5 M NaOH, 25°C), the manganese(11I) gluconate reacting in a slower process with kob s = 3.9E1 M-1 s-1(Rxn 13 in 0.5 M NaOH, 25 °C) [22]. The reaction rate is dependent on pH, temperature,ligand and manganese concentrations [22-25].The studies also indicated the ability of the lower oxidation states of the manganese complex tobe electrochemically regenerated via controlled potential electrolysis on a mercury-poolworking electrode [22]. The electrochemical studies indicated the reversibility of themanganese 11/111 and the manganese 111/IV couple [23,24]. The reduction potentials were -0.29and -0.03 Volts vs SHE respectively for the conditions employed [24].Studies by Lim et al indicated the ability of these manganese complexes to catalyse high pHoxidation reactions [2]. The peroxide concentration profile during the high pH oxidation ofhydrazine catalysed by the manganese complex indicates a rapid formation of hydrogenperoxide at the start of the reaction followed by a steady generation and consumption ofperoxide until the reaction is completed [2]. The authors suggest the catalytic activity of themanganese complex is related to its ability to participate in an oxidation-reduction cycle inwhich hydrogen peroxide is continuously generated from the reaction with oxygen [2].19Chapter 2. Background and Literature ReviewThe steady generation of hydrogen peroxide requires the continuous regeneration of manganese(11) or manganese (111) following its oxidation to the higher valence states. The authors suggestthe regeneration occurs through the reaction of the manganese(111) complex and a hydroxy freeradical or between the manganese (III) complex and the peroxide sensitive substrate as in thecase of hydrazine (Reactions 14,15) [2].2HnIII,L -k^)(OH) + OH'^HnII(L2)^H202MnIII (L2 )(OH) + MnIII (L2-)N2H4 4- --> 2HnII(L2)Rxn 14Rxn 15+ N2H2 + H2O + H+L - denotes gluconate ligand.N2H4 - denotes hydrazineN2H2 - denotes diimine (hydrazine oxidation product [22])The results of studies on peroxide sensitive substrates appear to indicate the ability of thealkaline manganese complex to promote peroxidation reactions via non-free radical pathways[2]. This result is ascribed to the ability of alkaline manganese to tie up and consume freeradical species [2].20Chapter 2. Background and Literature Review2.5 Electrochemistry.Some chemical reactions can be made to occur via ionic mechanisms by the addition ofelectrical energy. Electrochemical reactions involve mass transfer, adsorption/desorption, andelectron transfer processes at the electrode/electrolyte interface. The rate is dependent on theelectrode material, reactant concentration, temperature and electrode potential.A simple electrochemical system consists of two electrodes and an electrolyte. The electrode atwhich an electron producing ionic reaction is occurring (eg M --> M + + e) is the anode,the electrode at which an electron consuming reaction occurs (eg N + + e- --> N) is thecathode.In most systems multiple reactions at a given electrode are occurring. The electrochemicalreactions of interest in the present study are the cathode reactions below (Reactions16,17,18,19).MnIV-L + e- --> MnIII-L E = -0.04 V vs SHE *^Rxn 16MnIII-L + e --> MnII-L E = -0.30 V vs SHE *^Rxn 172H20 + 2e --> H2 + 20H E0= -0.83 V vs SHE^Rxn 1802 + H2O + 2e- --> OH + H02 el= -0.08 V vs SHE^Rxn 1921Chapter 2. Background and Literature Review(L- denotes gluconate ligand)(*- measurement for 1 mM Mn, 0.1 M Gluconate, 0.3 M NaOH, 1 atm, 25 °C [25].)The Nernst equation describes the reversible reduction potential (E r) of an electrochemicalreaction.Er = E° - RT1nQ/zF^(2)Where,E° - Standard reduction potential (V vs SHE)R - Universal gas constant (8.314 J/mole-K)T - Absolute temperature (K)z - Electron stoichiometric coefficientF - Faraday constant (96500 coulombs/g equivalent)Q - Activity quotientIf the working electrode is a cathode a reaction will occur if the electrode potential (E) is heldbelow the reversible electrode potential (Er). The overpotential (A) is described by= E - Er^(3)The overpotential is related to the current density (i) through the Tafel equation (Equation 4).= a + blogi^(4)Where, a and b are constantsThe limiting current (iLim) is a mass transfer constraint on the system dependant on a masstransfer coefficient (K), the concentration of the reactant (c) and the transport number (t) asdescribed by Equation (5).22Chapter 2. Background and Literature Review1Lim = zFKc/(1-t)^(5)The transport number is the fraction of the total current carried by the ionic species of interest.23Chapter 2. Background and Literature Review2.6 Cyclic Voltammetry [27].In cyclic voltammetry the potential applied to an electrode (by means of a potentiostat) ischanged linearly with time in a repetitive manner. The current is measured as a function ofpotential. The basic feature of a voltammogram (ie. a plot of current vs potential during cyclicvoltammetry) is the appearance of a current peak at a potential characteristic of an electrodereaction taking place (Figure 4).-0 3^-0.2^-0.1^0^0.1^0.2^03POTENTIAL (E - Eo . Volts)Figure 4. Diagram of Typical CV ResultThe position and shape of a given peak depends on such factors as sweep rate, electrodematerial, solution composition and the concentration of reactants.In a typical voltammogram a substance being reduced during the cathodic scan (left to right)appears as a positive peak. When the scan is reversed the reduced species which is still at thesurface of the electrode is re-oxidised and an anodic peak occurs. For an electrochemically24Chapter 2. Background and Literature Reviewreversible couple these two peaks should be clear and occur at approximately the samepotential.Cyclic voltammetry is suited to the identification of steps in the overall reaction and of newspecies which appear in solution during electrolysis. It also gives an indication of the ability ofthe electrochemical system to promote oxidation or reduction reactions.25Chapter 2. Background and Literature Review2.7 Spectrophotometry [28].When light falls upon a homogeneous medium a portion of the incident light is reflected, aportion is absorbed within the medium and the remainder is transmitted. The absorbance of asolution is related to the ratio of the transmitted light (It) to the incident light (10) by Equation6.A = log 10/It^(6)The absorbance is related to the concentration (c) of a component as described in Equation 7.A = Ecl^(7)Where,1- thickness of the absorbing mediumE - absorption coefficientc = concentration of absorbing componentThrough absorbance measurements the predominant ion in the solution can be determinedproviding the different ions vary in their absorbance at a given wavelength. The wavelength tobe used is determined by the construction of a plot of absorption vs wavelength for thedifferent ionic species.26Chapter 2. Background and Literature Review2.8 Factorial Design of Experiments.Many of the experiments performed in the pulp delignification trials (Section 5.4) utilisedfactorial design. Factorial design of experiments is a proven technique of obtaining fastaccurate results. Its benefits include that it can give more information per experiment than anunplanned approach: it is a simple, organised approach and the reliability of the informationcan be assessed in the light of experimental and analytical variation [29]. Also such anapproach is able to highlight the interaction of experimental variables.Two-level factorial designs are utilised in this study. A two level design experiment isperformed by choosing two fixed levels ( denoted -1 for low level and +1 for high level) foreach of the variables (factors) to be studied. The arrangement of 2 n experimental runs for nvariables is called a "2n factorial design". The two level factorial design matrices for two andthree variables which are utilised in the following experiments are shown in Table 4 below.27Chapter 2. Background and Literature Review2 Variable^3 VariableX1 X2 R+1 +1 Y1+1 -1 Y2-1 +1 Y3-1 -1 Y4X1 X2 X3 R-1 -1 -1 Y1+1 -1 -1 Y2-1 +1 -1 Y3+1 +1 -1 Y4-1 -1 +1 Y5+1 -1 +1 Y6-1 +1 +1 Y7+1 +1 +1 Y8Table 4. Factorial Design Matrices for Two and Three Variables.The responses (R, dependent variables) are the Kappa number and viscosity of the pulp, theindependent variables (X) are current, catalyst concentration, gas purge type etc.Main Effects.The main effects (M) of each variable are estimated using the difference between the averagehigh (+1) and low (-1) factor level responses.ie. for the 22 design,M(X1) = ((Yl + Y2)-(Y3 + Y4))/2^(8)28Chapter 2. Background and Literature ReviewM(X2) = ((Yl +Y3)-(Y2+Y4))/2^(9)Interaction EffectsIf the variables are not additive in their effect an interaction occurs. The interaction effect iscalculated using the difference between the average response of the high level factorial runsand the low level runs.ie. for the 22 design,I(Xl, X2) = ((Yl + Y4)-(Y2 +Y3))/2^(10)Significance of the Effects.To gauge the significance of an effect confidence intervals are used. The confidence levelchosen was the 95% level. The confidence interval (CINT) is calculated from Equation 11below,CINT = (Effect Estimate) + ts/(N/4) 112 (11)29Chapter 2. Background and Literature Reviewwhere,s = response error estimate with v degrees of freedomN = number of experimental runs in the factorial designt = students t statistic with v degrees of freedom at thestated confidence intervalResponse error is estimated using replicate runs. The error associated with the Kappa numberand viscosity measurements are included in this estimate. For duplicates where the responsesare denoted V and W,s = ((V..W)2/2)1/2^(12)Error estimates from several sources may be pooled for a firmer estimate of error. The pooledvariance for k separate estimates of error si each with ri replicates is,s2 = E(ri-l)si2/E(ri-1)^(13)with degrees of freedom (v),v = E(ri-1)^(14)In this study the response error (s) for Kappa No and viscosity was calculated to be 0.21 and1.21 respectively. The degrees of freedom in both cases was 4 (see Appendix 2).For a 95% confidence interval the students t statistic with 4 degrees of freedom is 2.776 [30].30Chapter 2. Background and Literature ReviewTherefore for all the factorial design experiments the 95% confidence intervals were calculatedas below,For Kappa number,CINT = (Effect Estimate) ± 0.58x(N/4) 1/2 (15)For viscosity,CINT = (Effect Estimate) ± 3.64x(N/4) 1/2 (16)The confidence intervals for interaction effects were calculated in the same way.The effects are considered significant if the confidence interval does not contain zero.For a more complete overview of factorial design a paper by Murphy [29] is recommended.31Chapter 3. The Proposed System and Research Objectives.In peroxide and oxygen bleaching of chemical pulps the objective is to lower the lignin contentof the pulp whilst minimising the degradation of the cellulose component [15]. An importantcellulose degradation reaction pathway involves free radical species [8]. The results of studieson peroxide-sensitive substrates appears to indicate the ability of the alkaline manganesecomplex/oxygen redox cycle to promote peroxidation reactions via non-free radical pathways[2]. This suggests the possibility of substituting the oxygen and manganese complex system asdescribed above in place of the relatively expensive hydrogen peroxide in the bleaching ofchemical pulps.The proposed system involves the use of the manganese gluconate/oxygen redox cycle todelignify chemical pulp through the in-situ production of hydrogen peroxide in an alkalineenvironment. It is proposed to promote the reduction of the higher oxidation states ofmanganese via the application of current at a cathode (Reactions 20,21).Mn(IV)-L + e- (from electrode) --> Mn(111)-L Rxn 20Mn(111)-L + e (from electrode) -- > Mn(II)-L Rxn 21Or through the use of a chemical reductant (Reactions 22,23).Mn(IV)-L + e (from reductant) --> Mn(11)-L Rxn 2232Chapter 3. The Proposed &stem and Research ObjectivesMn(111)-L + e (from reductant) > Mn(11)-L Rxn 23(-L represents gluconate ligand)The proposed manganese complex/oxygen redox cycle is as presented in Figure 5.mytil fnu^/TX nl 4 ^02 fr_.0^% 4-w ^OH -->^2‘...74,314‘mu.12^H02H2O^< + e (from cathode) <mnIII2(Gm3) 4(OH)2 4 ^02 + 30H ^mnIV2 (GH3) (OH)2 6 H02-H2O^< + e- (from cathode) < Figure 5. Proposed Redox Cycle for the Continuous Generation ofHydrogen Peroxide From Manganese Gluconate and Oxygen.The benefit of such a system would include the generation of the relatively expensive hydrogenperoxide from oxygen [2]. The redox cycle involving manganese gluconate and oxygen mayalso promote peroxide oxidation reactions via non-free radical pathways [2]. If the proposedredox cycle promotes peroxide oxidation reactions via non-free radical pathways it couldminimise the detrimental effect of free radicals on the cellulosic component of the pulps. Thiswould improve the selectivity of the delignification process.The work described in the following chapters investigates the ability of the proposedmanganese complex/oxygen redox cycle to operate in the bleaching environment and topromote, via the peroxide produced, the delignification of chemical pulp.33- -WE CE SCE N2 or 02PinePotentiostatRDE 3WatanabeWX446X-Y Recorder^<WE - Working electrodeCE - Counter electrodeChapter 4. Experimental Apparatus and Procedure.4.1 Cyclic Voltammetry.Cyclic voltammograms were determined with a Pine Potentiostat RDE 3 in conjunction with aWatanabe WX446 X-Y recorder. The system used three electrodes. A platinised titaniumelectrode served as a counter electrode with a saturated calomel electrode (SCE) as thereference electrode. The working electrode consisted of either a graphite rod, a platinum wire,or a copper-mercury amalgam plate electrode.The set up was as described below (Figure 6).Figure 6. Cyclic Voltammetry Set-upOne liter of sample solution was prepared in air and added to the CV jar. The three electrodeswere then placed in the solution The solution was then purged with nitrogen for at least 1034Chapter 4. Experimental Apparatus and Procedureminutes before the measurement was taken. To examine the influence of oxygen on the systemoxygen purge was available. After being purged with oxygen for the desired time the solutionwould be purged with nitrogen and the measurement taken. The oxygen was removed soelectrode reactions involving oxygen could be eliminated from the results. All CVmeasurements were taken at room temperature (23 °C).The preparation of the manganese gluconate solutions was a step-wise procedure. First thegluconate ion was formed through the addition of glucono-lactone to an aqueous solution of 1M sodium hydroxide. For gluconate concentrations greater than 10 mM the gluconolactonewas added as a solid and for concentrations of 10 mM and less as a solution (0.1 M). Then themanganese ion was added as a 0.1 M solution of manganese perchlorate. The solution could beprepared under nitrogen if the manganese (II) solution was required. All solutions wereprepared at room temperature.35Chapter 4. Experimental Apparatus and Procedure4.2 Spectrophotometry.The spectrophotometric measurements were made with a HACH DR/2000 Direct readingspectrophotometer. The glass vials containing the solution to be measured were cleanedthoroughly with hot water and distilled water between measurements. The sample absorbancewas measured relative to a blank sample solution containing the sodium hydroxide and theligand components only.The solutions prepared all contained 1 mM manganese and 5 mM glucono lactone in 1MNaOH. When a manganese II complex was required, to ensure no oxidation of the complex tothe III or V forms, the solution was prepared under nitrogen purge. The solutions wereprepared using the stepwise procedure described in section 4.1. The manganese III and IVgluconate solutions were prepared through the addition of potassium ferricyanide to amanganese II gluconate solution prepared under nitrogen. The stoichiometry being as shown inTable 5 below [22].Reaction Mol of K3Fe(CN)6/mol of Mn IonII --> III 1.0II --> IV 2.0Table 5. Stoichiometry for Manganese Complex Oxidations withK3Fe(CN)6.The samples were measured directly upon removal from the reactor or flask to avoid excessivereaction with oxygen in the air. The path length was 2.5cm.36Chapter 4. Experimental Apparatus and ProcedureFor the initial determination of absorption vs wavelength for the system the solutions wereprepared as above and their absorption at different wavelengths determined.For the spectrophotometric experiments the electrochemical cell set up was as for the bleachingruns (Section 4.4). The cathode was a platinised-titanium electrode. The solutions wereprepared as above and poured into the catholyte side of the reactor. All experiments werecarried out at room temperature, 23 °C. The 5 experiments utilising spectrophotometricanalysis are described in Table 6 below.Experiment InitialManganeseOxidationStateGas^Purge15 cc/min@ STPCurrentAmpsPulpPresent1 II 02 0.0 No2 III N2 1.0 No3 IV N2 1.0 No4 II/III 02 1.0 No5 II/III 02 1.0 YesTable 6. Description of Spectrophotometric Experiments.The solutions for experiments 4 and 5 were prepared in air and therefore contained a mixtureof II and HI manganese oxidation states.37Chapter 4. Experimental Apparatus and Procedure4.3 Hydrogen Peroxide Analysis.The reactor set-up was as for the bleaching runs (Section 4.4). All experiments were carriedout in the absence of pulp. The solutions were prepared in the stepwise procedure described inSection 4.1.In Experiment 1 a 1 M solution of NaOH was subjected to 02 purge and a current of 2.0 A ona graphite cathode and periodically tested for the presence of hydrogen peroxide. InExperiment 2 a 1 M solution of NaOH containing 1 mM manganese and 5 mM gluconate wassubjected to 02 purge and a current of 2.0 A on a graphite cathode and tested for the presenceof hydrogen peroxide. Finally, in Experiment 3 a 1 M solution of NaOH containing 1 mMmanganese and 5 mM gluconate was subjected to 02 purge and a current of 2.0 A on aplatinised titanium cathode and tested for the presence of hydrogen peroxide.The solution was periodically analysed for hydrogen peroxide following the titration proceduredetailed in Appendix 5.38MembranePowerSupplyCathode Anode Gas170mmMembraneChapter 4. Experimental Apparatus and Procedure4.4 Pulp Delignification Trials.The equipment for the electrochemical bleaching runs was as shown below in Figure 7.Support PlateFigure 7. Diagram of Equipment Used in Electrochemical BleachingTrials.The reactor consisted of a cylindrical divided reactor with a diameter and height of 170 mm.The reactor was placed in a constant temperature bath. The reactor was made of stainless steelcoated with PTFE to prevent metal ion contamination. The two chambers of the reactor wereseparated by a cation membrane (Nafion 214) supported by two perforated plexiglass plates.The anode was a 316 stainless steel plate. The cathode material could be varied (graphite,stainless steel, platinised-titanium etc). The cathode area was 140 cm 2 . A DC power supply(Anatek 6030) was used for the experiments involving currents of up to and including 4.0 A.Higher currents were obtained through the use of a SORENSEN DCR 40-25B power supply.39Chapter 4. Experimental Apparatus and ProcedureA variable speed motor with a three blade plastic stirrer provided the mixing during theexperiments. Gas was purged into the reactor through a glass tube connected to the sourcecylinder and a flow meter with Teflon tubing. The temperature of the system was regulated bysituation of the reactor in a constant temperature bath.Preparation of Chemicals and Pulp.The source of the manganese ion was manganese (II) perchlorate hexahydrate obtained fromthe Aldrich Chemical Company. The gluconate ion was obtained through the reaction ofgluconolactone with sodium hydroxide obtained from the Aldrich Chemical Company andFischer Scientific respectively. The other chemicals used (hydrogen peroxide 30% solution,sodium sulphate, sulphuric acid, magnesium sulphate and sodium silicate) were obtainedthrough BDH Chemicals. All chemicals were reagent grade and were used without furtherpurification.All solutions were prepared using distilled water to 1 1 total volume. For the runs at pH 14 thesolution was made up with 40 g/1 of sodium hydroxide (NaOH). The gluconate ligand wasprepared through the addition of gluconolactone to the sodium hydroxide solution. For ligandconcentrations greater than 10 mM the gluconolactone was added as a solid and forconcentrations of 10 mM and less as a solution (0.1 M). This solution was then allowed toattain the temperature required in the bleaching run. Finally the desired manganeseconcentration was attained through the addition of an appropriate amount of 0.1 M manganeseperchlorate solution (0.1 M) also at the bleaching run temperature. For the runs at pH 11 theinitial solution was 0.5 M in sodium sulphate (Na2SO4). The ligand was then added and the40Chapter 4. Experimental Apparatus and ProcedurepH of the solution adjusted to 11 through the dropwise addition of concentrated NaOH. Themanganese was then added. Finally the pH of the solution adjusted to 11 through the dropwiseaddition of concentrated NaOH.The solution was then placed in the catholyte chamber of the reactor with 10 oven dry gramsof unbleached Kraft softwood pulp. The consistency of the pulp suspension was therefore 1 %.The anolyte chamber contained 1 M NaOH. The pulp slurry was purged with gas (02 or N2)at atmospheric pressure, mixed homogeneously and subjected to the desired current for thereaction duration.For the trials involving chemical reductants the stainless steel reactor was replaced with a glassbeaker and the electrode system was not employed.The pulp used in the experiments was obtained from a previous researcher Mr Yuan-ShingPerng. The original properties were a Kappa No of 28.5 and a viscosity of 32.5 cP. The pulpwas a mixture of 35% hemlock, 30% fir and larch, 15% cedar, 10% spruce and balsam fir and10% lodgepole pine.The pulp samples were prepared by the weighing of 11.00^0.01 g of pulp from theconditioning room (= 10 g of oven dry pulp). This was thoroughly dispersed and rinsed, thenprepared as a handsheet in the handsheet former.41Chapter 4. Experimental Apparatus and ProcedureIf a DTPA wash was required the pulp was added to 1 1 of distilled water containing 0.4 %DTPA. The mixture was stirred and left to stand for at least 15 minutes at room temperature.The pulp was then rinsed in distilled water and a handsheet was prepared.After reaction the pulp is removed from the reactor, rinsed thoroughly and formed into ahandsheet.If the colour generation in the reactor indicated a manganese precipitate a sulphurous acidwash was required. The pulp was added to a 0.25 - 0.3 % solution of the acid (approximately100 ml/g pulp) stirred and left to stand for at least 5 minutes at room temperature. The pulpwas then rinsed and a handsheet prepared.The pulp was then analysed for Kappa number and pulp viscosity using pulp industry standardmethods Tappi T236 cm-85 and T230 om-89 respectively [9].42Chapter 5. Experimental Results and Discussion.5.1 Cyclic Voltammetry.5.1.0 Introduction. Cyclic voltammetry is a useful tool for the investigation of electrochemical systems. Thefollowing experiments utilise cyclic voltammetry to investigate the ability of the electrodematerials employed in the delignification trials to promote the reduction of the higher oxidationstates of the manganese complex. The ability of the electrodes employed to promote thatreduction is necessary in the proposed redox cycle for the continuous in-situ generation ofhydrogen peroxide. Cyclic voltammetry can also indicate the appearance of new species in thesystem and therefore may give an indication of the production of hydrogen peroxide.43Chapter 5. Experimental Results and Discussion5.1.1 Results and Discussion. Results from the use of a platinum wire electrode in a manganese gluconate solution indicatecathodic peaks at -0.19 V vs SHE and -0.51 V vs SHE and anodic peaks at 0.06 V vs SHEand -0.21 V vs SHE (Figure 8).E •a)C.)Potential (V vs SHE)Figure 8. Cyclic Voltammogram for the Manganese Gluconate Complex Formed in thePresence of Oxygen and Deaerated with Nitrogen. (10 mM Mn, 50 mM Gluconate, PlatinumWire Working Electrode, 1 M NaOH, 22 °C, Scan Rate 5 V/min)These peaks were not evident in the sodium hydroxide only voltammogram (Figure 9).44A-t 0 ^5-Potential (V vs SHE)-0.8Chapter 5. Experimental Results and DiscussionFigure 9. Cyclic Voltammogram for 1 M NaOH Deaerated with Nitrogen. (Platinum WireWorking Electrode, 1 M NaOH, 22 °C, Scan Rate 4 V/min)The peaks evident in Figure 8 are believed to correspond to the oxygen/hydrogen peroxidecouple (Reaction 19, -0.19 V vs SHE cathodic / 0.06 V vs SHE anodic) and the manganese III--> IV couple (Reaction 16, -0.51 V vs SHE cathodic / -0.21 V vs SHE anodic).45Chapter 5. Experimental Results and Discussion02 + H2O + 2e- < -- > H02- + OH- E°= -0.08 V vs SHE Rxn 19mnIV_L + e- < -- > Mnill-L E = -0.30 V vs SHE* Rxn 16(L- denotes gluconate ligand)(*- measurement for 1 mM Mn, 0.1 M Gluconate, 0.3 M NaOH, 1 atm, 25 °C [25].)The shape of the voltammogram closely resembles a voltammogram presented by Richens et.al. [22] for a similar system.Although not as clear the cyclic voltammograms on the graphite and copper-mercuryelectrodes also show peaks in the area of the potential at which the peaks were noted for theplatinum wire electrode (Figures 10,11).46Chapter 5. Experimental Results and DiscussionPotential (V vs SHE)Figure 10. Cyclic Voltammogram for the Manganese Gluconate Complex Formed in thePresence of Oxygen and Deaerated with Nitrogen. (1 mM Mn, 5 mM Gluconate, CopperMercury Amalgam Working Electrode, 1 M NaOH, 22 °C, Scan Rate 10 V/min)EPotential (\ s SHE)Figure 11. Cyclic Voltammogram for the Manganese Gluconate Complex Formed in thePresence of Oxygen and Deaerated with Nitrogen. (5 mM Mn, 50 mM Gluconate, GraphiteWorking Electrode, 1 M NaOH, 22 °C, Scan Rate 4 V/min)47Chapter 5. Experimental Results and DiscussionThe potential range of the cyclic voltammograms is determined by the hydrogen evolutionreaction (Rxn 18). The potential at which this reaction occurs is dependent on the overpotentialfor that reaction on the electrode material employed.2H20 + 2e- < > H2 + 20H - E° = -0.83 V vs SHE Rxn 18The hydrogen evolution reaction occurs at approximately -1.8 V vs SHE for both the graphiteand copper-mercury amalgam electrodes (Figs 10,11) and approximately -1.0 V vs SHE forthe platinum wire electrode (Figure 12). Figure 12 differs from Figure 8 with respect to thescan rate and range. The scan rate and range were altered in Figure 8 to more clearly indicatethe peaks indicating electrode reactions.485•-5-Potential (V vs SHE)Chapter 5. Experimental Results and DiscussionFigure 12. Cyclic Voltammogram for the Manganese Gluconate Complex Formed in thePresence of Oxygen and Deaerated with Nitrogen. (10 mM Mn, 50 mM Gluconate, PlatinumWire Working Electrode, 1 M NaOH, 22 °C, Scan Rate 4 V/min)The results on the platinum wire electrode clearly indicate the presence of the manganese111/IV couple. This indicates that the reduction of manganese IV to manganese 111 will occuron the electrode surface. Although not as discernible, peaks evident in the graphite and copper-mercury amalgam results may indicate this couple is also active on their surfaces. There is noindication of the presence of the manganese III/11 couple on any of the electrode materialsindicating that reaction does not occur readily on those electrode surfaces. The hydrogenperoxide/oxygen couple is evident in the platinum wire results. The solutions are prepared inthe presence of oxygen. Upon removal of the oxygen from the solution via nitrogen purge thehydrogen peroxide/oxygen couple is still present. Therefore the hydrogen peroxide is produced49Chapter 5. Experimental Results and Discussionupon preparation of the complex in air. The presence of the couple indicates the production ofhydrogen peroxide in the presence of the manganese complex. Again the peaks in the graphiteand copper-mercury amalgam results are not as discernible.The results indicate the presence of hydrogen peroxide and the manganese III <--> IVreaction. This is evidence of the ability of an electrochemical system to influence the reductionof the manganese IV complex to the manganese III form. There is no evidence of themanganese III - II reaction. This may be due to the more negative potential required on theseelectrode surfaces [22].505.2 Spectrophotometry.5.2.0 Introduction. In the following experiments spectrophotometry will be utilised to further indicate the abilityof an electrode employed to promote the necessary reduction of the higher oxidation states ofmanganese in the complex. Spectrophotometry will also be used to indicate the likelypredominant form of the manganese complex (II, 111 or IV) in the solution under conditionstypical of those employed in the pulp bleaching trials.5.2.1 Results and Discussion. The results of the absorbance vs wavelength measurements are given in Figure 13 and aresimilar to results from literature [24]. It can be seen that no separate absorption peaks areevident for the individual oxidation states of the manganese. Therefore the spectrophotometricanalysis will be utilised in the following manner.51Absorbance2Mn (II) GluconateMn (III) Gluconate^ Mn (IV) Gluconate10.50400^440^480^520^560^600^640^680Wavelength (nm)1.5Chapter 5. Experimental Results and DiscussionFigure 13. Absorption Spectrograph for Manganese Gluconate Complex. (1 mM Mn, 5 mMGluconate, 1 M NaOH, 23 °C, path length 2.5 cm)The wave length chosen for the following experiments was 520 nm. The absorbance of themanganese II form has to be measured very quickly following its removal from the flask dueto its fast reaction with oxygen in the air (absorbance increase over time). The absorbancepresented in Table 7 differs from the absorbance in Figure 14. The absorbance given in Table7 represents the measurement taken as soon as possible after removal from the reactor. Theabsorbances of the 1 mM manganese complex solutions used in following analysis are given inTable 7 below.52Chapter 5. Experimental Results and DiscussionManganese OxidationStateAbsorbance@ 520 nm6(1/mol cm)II 0.041 16.4III 0.374 149.6IV 1.012 404.8Table 7. Absorbance Values for the Manganese Gluconate Complex (1 mM Mn, 5 mMGluconate, 1 M NaOH, 23 °C, path length 2.5 cm)For the following experiments the predominant oxidation state of manganese will bedetermined using the above absorbance measurements. If the absorbance of a solution 1 mM inmanganese is approximately zero (ie the absorbance of the manganese II complex) thepredominant manganese form will be considered to be manganese II. If the absorbance isapproximately 0.4 (ie the absorbance of the manganese III complex) the predominantmanganese form will be considered to be manganese III. Similarly for an absorbance ofapproximately 1.0 the predominant form will be considered to be manganese IV.Experiment 1 is designed to determine the predominant ionic form of manganese in amanganese gluconate solution during its oxidation with oxygen in the absence of current. Theresults shown in Figure 14 indicate the rapid rate of reaction of the manganese II complex withoxygen. After about 1 minute the absorbance of the solution indicates the predominant ionicform of manganese to be manganese IV.53Chapter 5. Experimental Results and Discussionbsorbance • 520 nm1.20.80.60.40.20^10^20^30^40^50^60^70Time (mins)Figure 14. Absorption Profile for Manganese (II) Gluconate Complex in the Presence ofOxygen. (1 mM Mn, 5 mM Gluconate, 0 A, 02 15 cc/min @ STP, 1 M NaOH, 23 °C, NoPulp Present)Experiments 2 and 3 are designed to determine the ability of the platinised titanium cathodeemployed to reduce the manganese III and IV complex forms. In these two experimentsoxidation was suppressed through the use of a nitrogen purge.The results of experiment 2 (Figure 15) in which the manganese III form was initially presentindicate the predominant ionic form to be manganese III throughout. There is no evidence ofthe reduction of the manganese III complex to the manganese II form.54Chapter 5. Experimental Results and DiscussionAbsorbance • 520 nm10^20^30^40^50^60^70Time (mins)Figure 15. Absorption Profile for Manganese (111) Gluconate Complex in the Presence ofCurrent. (1 mM Mn, 5 mM Gluconate, 1.0 A on Platinised Titanium Cathode, N2 15 cc/min@ STP, 1 M NaOH, 23 °C, No Pulp Present)The results of experiment 3 (Figure 16) in which the manganese IV form was initially presentindicate the ability of the system to bring about the reduction of the manganese IV complex tothe manganese III form. This is indicated by the decrease in absorption of the solution fromapproximately 1.0 to 0.48. However the results of experiment 3 show no evidence of thereduction of the manganese III complex to the manganese II form. Even after 17 hours (1020mins) the absorbance measurement (0.486) indicates the predominant ion to be the manganeseIII form.1.210.80.60.4 -0.20055Chapter 5. Experimental Results and DiscussionAbsorbance • 520 nm1.2i10.80.60.40.20^200^400^600^800^1000^1200Time (mins)Figure 16. Absorption Profile for Manganese (IV) Gluconate Complex in the Presence ofCurrent. (1 mM Mn, 5 mM Gluconate, 1.0 A on Platinised Titanium Cathode, N2 15 cc/min@ STP, 1 M NaOH, 23 °C, No Pulp Present)The above results correspond to the results from the cyclic voltammetry tests in that they donot indicate the electrochemical reduction of the manganese III complex to the manganese IIform at the cathode. The results do indicate the ability of the cathode to reduce the manganeseIV form to the manganese III form.Experiments 4 and 5 were designed to indicate the likely predominant ionic form of themanganese in the reactor during the pulp bleaching trials. In these experiments oxygen purge isemployed to promote the oxidation of the manganese complex (as evidenced in experiment 1),also current is applied to promote the reduction of the manganese complex (as evidenced in56Chapter 5. Experimental Results and Discussionexperiment 3). The results presented in Figure 17 indicate the absorbance to be between thevalues for the manganese III and manganese IV forms both in the presence and absence of pulpand also indicates the likely ionic than of the manganese to be a mixture of the above twoforms in the bleaching solution. This is probably due to the combination of the rapid oxidationof the complex by the oxygen and the slower electrochemically promoted reduction of themanganese IV complex to the manganese III form.Absorbance e 520 nm1.210^20^30^40Time (minx)---9-- No Pulp PresentX Pulp Present50^60 70Figure 17. Absorption Profile for Manganese Gluconate Complex in the Presence of Oxygenand Current. (1 mM Mn, 5 mM Gluconate, 1.0 A on Platinised Titanium Cathode, 02 15cc/min @ STP, 1 M NaOH, 23 °C)The results of the spectrophotometric experiments indicate the electrochemical reduction of themanganese IV complex to the manganese III form. There is no indication of the furtherreduction of the manganese III complex to the manganese II form. These results support the57Chapter 5. Experimental Results and Discussionresults of the cyclic voltammetry tests. The results of the spectrophotometric experiments alsoindicate the rapid reaction of the manganese complex with oxygen. Furthermore they indicatethat in the bleaching system the complex will indeed operate in a cycle in which the manganeseIII is continuously oxidised to manganese IV by oxygen then reduced electrochemically to themanganese III form. The participation of the manganese complex in this redox cycle isrequired for the continuous generation of peroxide in the proposed system.585.3 Hydrogen Peroxide Analysis.5.3.0 Introduction. The following experiments involve the analysis of a solution containing the manganesegluconate complex, under conditions typical of those in the bleaching environment, for thepresence of hydrogen peroxide.5.3.1 Results and Discussion. Experiment 1 was designed to indicate if there would be significant hydrogen peroxideproduction at the graphite cathode surface during the pulp delignification trials in the absenceof the manganese gluconate complex via Reaction 19. The results of Experiment 1 are shownin Table 8. They indicate the presence of hydrogen peroxide in the NaOH solution. As can beseen in Table 8 the results indicate a concentration of 0.0005-- > 0.0006 M hydrogen peroxidepresent in the NaOH solution with no large increase in concentration over time.59Chapter 5. Experimental Results and DiscussionTime(mins)H202 Concentration(M)0 0.00055 0.000630 0.0006Table 8. Hydrogen Peroxide Concentration in NaOH Solution. (2.0 A on Graphite Cathode,02 15 cc/mm @ STP, 1 M NaOH, 25 °C, No Pulp Present)Under the conditions described above it appears that very little hydrogen peroxide is producedvia this route.The results of Experiment 2 shown in Table 9 indicate the presence of hydrogen peroxide,during electrolysis with oxygen, in the solution containing the manganese gluconate complex.As in Experiment 1 there is no large increase in peroxide concentration over time.Time(mins)H202 Concentration(M)0 0.0015 0.001330 0.0012Table 9. Hydrogen Peroxide Concentration in a Manganese Gluconate Complex UnderBleaching Conditions. (1 mM Mn, 5 mM Gluconate, 2.0 A on Graphite Cathode, 02 15cc/min STP, 1 M NaOH, 25 °C, No Pulp Present)It can be seen that a higher concentration of hydrogen peroxide is obtained in the presence ofthe manganese complex (Table 9) than in the absence of the manganese complex (Table 8).60Chapter 5. Experimental Results and DiscussionThe results of Experiment 3 which employed a platinised titanium cathode are presented inTable 10.Time(mins)H202 Concentration(M)0 0.00085 0.001230 0.0014Table 10. Hydrogen Peroxide Concentration in a Manganese Gluconate Complex UnderBleaching Conditions. (1 mM Mn, 5 mM Gluconate, 2.0 A on Platinised Titanium Cathode,02 15 cc/min STP, 1 M NaOH, 25 °C, No Pulp Present)The results indicate the presence of hydrogen peroxide in the solution. The concentration ofperoxide detected being similar to the concentration detected in Experiment 2 which employeda graphite cathode. Also as in Experiment 2, there is no large increase in peroxideconcentration over time.The results indicate that in the absence of the manganese complex the production of hydrogenperoxide at the graphite cathode was small under the conditions employed. The results indicatethe presence of hydrogen peroxide, during electrolysis with oxygen, in the solutions containingthe manganese complex under bleaching conditions but in the absence of pulp. There was littledifference in the use of a platinised titanium or graphite cathode. However there is not thesteady peroxide concentration increase expected. This may be due to the decomposition of thehydrogen peroxide produced into free radical species. The ability of the free manganese ion tocatalyse peroxide decomposition is known [20] and may be responsible for the peroxide61Chapter 5. Experimental Results and Discussiondecomposition suspected. Decomposition reactions due to the combination of the presence ofmanganese and the high alkalinity [10] may also be of importance.625.4 Pulp Delignification Trials.5.4.0 Introduction. The results of the cyclic voltammetry and spectrophotometric analysis indicated the ability ofan electrode system to promote the reduction of the manganese IV complex to the manganeseHI form. The literature review indicated that hydrogen peroxide would be continuouslygenerated in such a system via the proposed manganese complex/oxygen redox cycle. Thecyclic voltammetry analysis indicated the presence of hydrogen peroxide as did the hydrogenperoxide analysis. The following experiments are designed to investigate the ability of theproposed manganese complex/oxygen redox cycle to delignify chemical pulp and to determinethe important variables with respect to this process.63Chapter 5. Experimental Results and Discussion5.4.1 Results and Discussion. Metal Ion Effect. Approximately midway through the electrochemical bleaching trials it became necessary toclean the reactor due to suspected metal ion contamination subsequent to the use of a leadcathode. A cleaning procedure, described by Landucci [31], using hot sodium bi-sulphitesolution was employed. The results of the bleaching trials prior to the use of the lead electrodeand after the hot sodium bi-sulphite wash differed in certain respects. The difference can beexplained in terms of metal ion effects; however, it is necessary to separate the bleachingexperiments into two groups. Group 1 (Table 12) were the electrochemical bleachingexperiments carried out prior to the use of lead cathode. Group 2 (Table 13) were thoseelectrochemical bleaching experiments carried out after the clean with hot sodium bi-sulphite.Subsequent to the use of a lead cathode it became impossible to reproduce some earlier resultsas indicated in Table 11.Run Notes KappaNumberR115 Prior to use of lead cathode 16.9R157 After use of lead cathode 19.0Table 11. Results Prior to and After the Use of a Lead cathode. (10 mM Mn, 50 mMGluconate, 4.0 A on Graphite Cathode, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours,Pulp DTPA Washed)64Chapter 5. Experimental Results and DiscussionThe Kappa number of the pulp from a run following the use of the lead cathode wassignificantly higher than the Kappa number of a run, under identical conditions, carried outprior to the use of the lead cathode. It was thought that some contamination due to the use ofthe lead cathode was responsible for this effect. The reactor was therefore cleaned using hotsodium bi-sulphite (20 % solution, for 3 hours at 90 °C, finally thoroughly rinsed withdistilled water) and a new membrane and support screens installed.The results of the Group 2 runs (ie. those completed after the bi-sulphite wash) differed fromthe Group 1 runs (those completed prior to the use of the lead cathode). Tables 12 and 13indicate the results of factorial designs and show the differences between the two groups.Run Catalyst Current KappaNumberViscosity(cP)R115 + + 16.9 10.1R112 + - 19.5 27.9R140 - + 18.6 21.2R107 - - 19.8 21.7Table 12. Results of Factorial Experiment.Catalyst;(+) 10 mM Mn, 50 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Current;(+) 4.0 A on Graphite Cathode, (-) 0 A,(Group 1, 02 15 cc/min @ SIT, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)65Chapter 5. Experimental Results and DiscussionRun Catalyst Current KappaNumberViscosity(cP)2-16 + + 17.5 21.22-21 + - 19.7 32.12-18 - + 19.8 30.62-22 - - 19.6 29.7Table 13. Results of Factorial Experiment.Catalyst;(+) 10 mM Mn, 50 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Current;(+) 10.0 A on Platinised Titanium Cathode, (-) 0 A,(Group 2, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)The results indicate the Kappa number responses of the 2 sets of experiments to be similar. Inboth cases the run with catalyst (+) and current (+) had the lowest Kappa number (Runs R115and 2-16).The difference between the two groups lies in the viscosity responses. For Group 2 theviscosity is approximately 30 cP in the runs in the absence of catalyst (Runs 2-18 and 2-22).The viscosity of the run with catalyst but in the absence of current is also similarly high (Run2-21, 32.1 cP). In the run with both catalyst and current the viscosity falls to approximately 21cP.In the Group 1 runs the viscosity is approximately 21 cP for the runs in the absence ofcatalyst, approximately 28 cP for the run with catalyst but no current and approximately 10 cPfor the run with current and catalyst. The catalyst with current runs exhibit the lowest viscosityin both groups.66Chapter 5. Experimental Results and DiscussionThese effects are explained through the action of hydrogen peroxide thought to be producedvia the proposed manganese complex/oxygen redox cycle. It is noted from Chapter 2 that thecontinuous production of hydrogen peroxide from the manganese complex/oxygen redox cyclerequires the combination of 3 components: 1) oxygen, 2) the manganese gluconate complexand 3) a method to reduce the higher oxidation states of the manganese complex (ie, reductionat the cathode). The runs in the factorial experiments in which all 3 components are present(ie. runs R115 in Table 12 and 2-16 in Table 13) exhibit a significantly larger reduction inKappa number and viscosity than in the runs in which one or more of the components areabsent (ie runs R112, R140, R107 in Table 12 and 2-21, 2-18, 2-22 in Table 13).It is therefore argued that the hydrogen peroxide produced in the manganese complex/oxygenredox cycle is responsible for the larger Kappa number and viscosity reductions observed.For the runs in the absence of catalyst (with or without current) a significantly higher viscosityis observed in the Group 2 design. It is believed that there was a contaminant absent in theGroup 2 design responsible for the viscosity loss evident in the Group 1 design. As noted inChapter 2, metal ions play an important role in cellulose degradation. The results shown inTable 14 indicate the effect of metal ion addition on runs in the absence of current andcatalyst. The reactor system employed in the experiments shown in Table 14 consisted of aglass beaker with 02 purge and mixing. There was no catalyst (manganese gluconate) presentand no current was applied.67Chapter 5. Experimental Results and DiscussionMetal IonAddedConcentrationmMKappaNumberViscosity(cP)None n/a 19.8 30.6Fe2+ 1.0 19.6 20.0Fe2+ 0.1 20.1 20.8Mn2+ 1.0 19.4 31.5Mn2+ 0.1 19.7 30.8Cu2+ 1.0 19.7 21.1Cu2+ 0.1 19.6 21.5Table 14. Effect of Mea Ion Addition on Bleaching Result. (Fe2+ from FeSO4, Mn2+ fromMn(C104)2.6H20, Cu from Cu(C104)2.6H20, 02 15 cc/min Q STP, 1 M NaOH, 50 °C,3 hours, Pulp DTPA Washed)The results indicate that small concentrations of iron or copper ions can produce a similarreduction in viscosity to that noted in the Group 1 runs. The metal ions act via their catalyticreaction with oxygen in the system to produce free radicals species responsible for cellulosedegradation. It is therefore assumed that a contaminant, likely a metal ion such as copper oriron, was present in the Group 1 runs and that the hot bi-sulphite wash removed thatcontaminant. The contaminant was responsible for the approximately 10 cP viscosity drop inthe Group 1 runs in the absence of catalyst. A similar viscosity loss is not evident in thecorresponding Group 2 runs.This contaminant is also responsible for the larger viscosity drop in the catalyst/current runs inGroup 1 when compared to Group 2. In Group 1 the viscosity is affected by both hydrogen68Chapter 5. Experimental Results and Discussionperoxide produced through the combination of catalyst and current and the free radicalsproduced by the metal ion contaminant present. In Group 2 the viscosity is thought to beaffected only by the hydrogen peroxide produced by the catalyst and current.For the Group 1 set of experiments the higher viscosity of the run with catalyst and no currentwas probably due to the prevention of cellulose degradation by the free manganese ion. Thiseffect was noted by Gilbert et. al. [32] during oxygen bleaching. This effect is not evident inthe Group 2 design as the absence of the contaminant means there is little viscosity loss andtherefore no preventative effect is obvious.In summary the viscosity responses in Group 1 designs are affected by the presence of acontaminant, likely a metal ion such as copper or iron, causing increased viscosity loss. Theviscosity responses in Group 2 indicate that this metal ion contaminant is no longer present.With this understanding both groups of experiments can be used to give an indication of theeffectiveness of the proposed manganese complex/oxygen redox cycle in pulp delignification.69Chapter 5. Experimental Results and DiscussionEvidence of Catalytic Delignification. The following section indicates the ability of the proposed manganese complex/oxygen redoxcycle to bring about significant delignification.The results of the Group 1 factorial designs presented in Tables 12 and 15 and the Group 2factorial design presented in Table 13 are at a temperature of 50 °C. The results indicate thatfor any of the factorial experiments with oxygen, the run combining current and catalystcauses a significantly larger Kappa number reduction than the other runs in that experiment(approximately 29-- > 17 compared to 29-- > 20). This Kappa number reduction isaccompanied by an increased viscosity loss in both groups (the anomaly for the catalyst/nocurrent run in Group 1 has already been discussed).Run Catalyst Current KappaNumberViscosity(cP)Rill + + 17.5 10.9R112 + - 19.5 27.9R116 - + 19.6 18.2R107 - - 19.8 21.7Table 15. Results of Factorial Experiment.Catalyst;(+) 10 mM Mn, 50 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Current;(+) 2.0 A on Graphite Cathode, (-) 0 A,(Group 1, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)70Chapter 5. Experimental Results and DiscussionThe current/catalyst interaction effect is significant in the analyses of the factorial designspresented in Tables 11, 12 and 15 (see Appendix 1). This indicates the combination of currentand catalyst is a requirement for the observed Kappa number reduction and viscosity loss.In the proposed redox cycle the application of current is required for the reduction of thehigher oxidation states of the manganese complex to facilitate the continuous generation ofhydrogen peroxide. The decomposition of hydrogen peroxide into free radical species is aprerequisite for its delignification reactions [10]. Hydrogen peroxide decomposition products(free radicals) are also responsible for cellulose degradation reactions.Thus if hydrogen peroxide was being continuously produced by the proposed manganesecomplex/oxygen redox cycle and then decomposing into free radical species this would explainthe significant Kappa number and viscosity reduction with oxygen in the presence of thecatalyst and current in both groups.It therefore appears that the proposed manganese complex/oxygen redox cycle is operating andthe hydrogen peroxide produced is capable of delignifying chemical pulp. However thedelignification is accompanied by a substantial viscosity loss.71Chapter 5. Experimental Results and DiscussionEffect of Cathode Material. The following section indicates the ability of the cathode material to influence the operation ofthe proposed manganese complex/oxygen redox cycle.Table 16 indicates the effect of cathode material in Group 1 experiments. For the cathodematerials tested there is a negligible effect on the Kappa number in varying the cathodematerial. There is however a significant effect in varying the cathode material on the viscosityof the bleached pulp. The use of a graphite cathode resulted in a significantly lower viscosity(10.1 cP) than the use of the other cathode materials. The runs utilising a stainless steel orplatinised titanium cathode had a viscosity of approximately double that of the run with thegraphite cathode (19.6 and 20.8 cP compared to 10.1 cP).Run Cathode Material KappaNumberViscosity(cP)R115 Graphite 16.9 10.1R142 Mercury-Copper amalgam 17.4 14.8R138 Stainless Steel 17.4 19.6R143 Platinised Titanium 17.5 20.8Table 16. Effect Cathode Material on Bleaching Result. (Group 1, 10 mM Mn, 50 mMGluconate, 4.0 A, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)The results of the Group 2 experiments (shown in Table 17) also indicated that the use of agraphite cathode caused a significantly lower viscosity in the bleached pulp, than the use of a72Chapter 5. Experimental Results and Discussionstainless steel or platinised titanium cathode (17.3 cP compared to 22.4 and 21.1 cP). Thedifference was smaller than in the Group 1 experiments.Run Cathode Material KappaNumberViscosity(cP)2-15 Graphite 17.6 17.32-17 Stainless Steel 17.3 22.42-19 Platinised Titanium 17.5 21.1Table 17. Effect Cathode Material on Bleaching Result. (Group 2, 10 mM Mn, 50 mMGluconate, 10.0 A, 02 15 cc/min STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)This effect may be due to metal ion contaminants in the porous graphite cathode causinggreater peroxide decomposition or to the ability of the graphite cathode to catalyse the desiredmanganese complex reduction reaction to a greater extent than the stainless steel or platinisedtitanium cathode.In an alkaline environment the potential a cathode can attain for a given current is limited bythe hydrogen evolution reaction (Reaction 18).2H20 + 2e- <--> H2 + 20H- E° = -0.8277 V vs SHE Rxn 18The potential/current profiles for the three electrode materials tested are given in Appendix 3.For a cathodic reaction the electrode potential must be held at a lower (more negative)potential than the reversible electrode potential. If the reversible potential or overvoltage73Chapter 5. Experimental Results and Discussionrequired for the manganese complex reduction and thus continuous hydrogen peroxidegeneration is very negative then the stainless steel and platinised titanium cathode systemswould not be able to promote that reaction to the same extent as graphite. This is because theirlimiting hydrogen evolution reaction occurs at a higher potential.74Chapter 5. Experimental Results and DiscussionEffect of Oxygen. In the proposed manganese complex/oxygen redox cycle oxygen is required for the continucusgeneration of hydrogen peroxide. The following section indicates the effect of the oxygenpurge.Table 18 indicates the effect of oxygen on a Group 1 run. It indicates the desired Kappanumber reduction (29 > 17.5) requires oxygen. The viscosity drop also requires the oxygento be present.Run Gas Purge (15 cc/min) KappaNumberViscosity(cP)R115 Oxygen 16.9 10.1R136 Nitrogen 22.7 32.6Table 18. Effect of Oxygen on Bleaching Result. (Group 1, 10 mM Mn, 50 mM Gluconate,4.0 A on Graphite Cathode, 02 15 cc/mm @ STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPAWashed)Table 19 indicates the effect of oxygen on a Group 2 run. Again the observed Kappa numberreduction (29 > 17.5) and the viscosity drop requires the presence of the oxygen.75Chapter 5. Experimental Results and DiscussionRun Gas Purge (15 cc/min) KappaNumberViscosity(cP)2-16 Oxygen 17.5 21.22-35 Nitrogen 22.0 34.0Table 19. Effect of Oxygen on Bleaching Result. (Group 2, 10 mM Mn, 50 mM Gluconate,10.0 A on Platinised Titanium Cathode, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours,Pulp DTPA Washed)These results indicate that oxygen is an essential component of the system causing the observedKappa number and viscosity reductions. As the proposed manganese complex/oxygen redoxcycle requires oxygen for the production of hydrogen peroxide this indicates the proposedredox cycle is indeed operating.Also the absence of viscosity loss in the Group 1 run in the presence of nitrogen (Table 19)indicates the contaminant also required oxygen to bring about the observed viscosity loss. Thissupports the view that the contaminant was a metal ion as the metal ion catalysed generation offree radicals from oxygen (Rxn 10) would not occur in the absence of oxygen.76Chapter 5. Experimental Results and DiscussionEffect of DTPA Wash. DTPA (Diethylenetriaminepentaacetic acid (sodium salt)) was utilised in prewashing the pulpdue to its known ability to remove metal ions in the unbleached pulp [10]. The effect of theDTPA wash on the important metal ions is indicated in Table 20.Metal Metal Content^(ppm)Unwashed Pulp Washed PulpCopper 1220 6.05Iron 36.7 20.0Manganese 7.1 1.5Table 20. Effect of DTPA Wash On Metal Content of Pulp.(Analysis by Quanta Trace Laboratories Inc. (Vancouver))For the important metals in pulp bleaching (copper, iron and manganese) [16] the DTPA washbrings about a reduction in concentration. In the case of copper, this reduction is especiallypronounced (1220 --> 6.05 ppm).Table 21 indicates the effect of the DTPA wash in Group 1 experiments. It shows the DTPAwash is required to bring about the reduction in Kappa number and viscosity in the runs withcurrent and catalyst.77Chapter 5. Experimental Results and DiscussionRun DTPA Wash KappaNumberViscosity(cP)R113 Yes 17.8 9.6R117 No 22.0 20.4Table 21. Effect of DTPA Wash on Bleaching Result. (Group 1, 1 mM Mn, 5 mM Gluconate,2.0 A on Graphite Cathode, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours)This indicates that for the conditions described the hydrogen peroxide thought to beresponsible for the Kappa number and viscosity reductions is not produced in the absence ofthe DTPA wash. It appears the DTPA wash is necessary to allow the production of hydrogenperoxide from the manganese complex.Table 22 indicates the effect of the DTPA wash in Group 2 runs. Again it is evident the DTPAwash is required to bring about the Kappa number reduction and viscosity loss.Run DTPA Wash KappaNumberViscosity(cP)2-2 Yes 18.1 20.82-67 No 19.4 32.0Table 22. Effect of DTPA Wash on Bleaching Result. (Group 2, 10 mM Mn, 50 mMGluconate, 4.0 A on Graphite Cathode, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours)Table 23 also indicates the effect of DTPA wash on Group 2 runs but the cathode is made ofplatinised titanium in this case. The results differ from those utilising a graphite cathode in thatthe Kappa number reduction and viscosity loss occur in both the washed and unwashed pulp.78Chapter 5. Experimental Results and DiscussionThe unwashed pulp exhibits a greater viscosity loss, the Kappa numbers however arecomparable. Also Table 23 shows the effect of repeated DTPA washes. It is clear there is nobenefit accrued in washing the pulp more than once.Run DTPA Wash (Number of Washes) KappaNumberViscosity(cP)2-31 No 17.1 15.62-16 Yes (1) 17.5 21.22-60 Yes^(5) 17.8 21.1Table 23. Effect of DTPA Wash on Bleaching Result. (Group 2, 10 mM Mn, 50 mMGluconate, 10.0 A on Platinised Titanium Cathode, 02 15 cc/min STP, 1 M NaOH, 50 °C,3 hours)The results in Table 23 (platinised titanium cathode) can be explained with reference to Table20. As discussed in Chapter 2 the catalytic decomposition of oxygen by metal ions is wellknown. The high concentration of copper in the unwashed pulp and its reaction with oxygenmay be responsible for the extra viscosity loss observed. After the DTPA wash the metal ioncontent of the pulp (especially copper) is reduced and this extra viscosity loss is not observed.Also as regards the repeated DTPA wash the literature evidence suggests that any metal ionsremaining in the pulp after one DTPA wash are likely to be bound to the pulp and are notcapable of taking part in the reactions responsible for viscosity loss [18]. Therefore there is noobserved benefit in washing the pulp any more than once.79Chapter 5. Experimental Results and DiscussionThe results in Tables 21 and 22 can not be so easily explained. Although not fully understoodit may be the surface of the graphite cathode fouls in the presence of unwashed pulp, affectingthe electrode reactions promoting hydrogen peroxide formation (ie. manganese complexreduction). These reactions are suppressed and so the Kappa number reduction and viscosityloss are not observed. In the case of the DTPA washed pulp this fouling does not occur, thereactions proceed and the Kappa number and viscosity are appropriately affected. As the onlynoted difference between the washed and unwashed pulp is the metal ion content (particularlywith regard to copper) it is assumed that the metal ions cause the fouling on the graphiteelectrode. It may be that the reduction of the copper 2 + to copper metal occurs on the surfaceof the graphite cathode. The reduction of manganese (IV) to manganese (III) (Reaction 19)might then not occur on this copper plated surface. This fouling effect (ie. copper reduction)does not appear to occur on the platinised titanium electrode.The DTPA wash is necessary for the removal of the metal ions. These metal ions are thoughtto suppress the manganese reduction reactions at the electrode surface or prevent H02generation on the graphite cathode. On the platinised titanium electrode it is thought the metalions present in unwashed pulp do not prevent H02 - generation by the manganesecomplex/oxygen redox cycle, but promote viscosity loss through the generation of free radicalsin their reaction with hydrogen peroxide.80Chapter 5. Experimental Results and DiscussionEffect of Catalyst Concentration. The following section indicates the effect of the manganese complex concentration on thebleaching system.Figures 18 and 19 show the results of varying the catalyst concentrations at various currents.The Kappa number (Figure 18) and viscosity (Figure 19) responses for the two catalystconcentrations do not differ significantly. The plots indicate that changing the catalystconcentration from 1 mM to 10 mM does not significantly alter the response. This is evidenceof the catalytic ability of the manganese complex in the peroxide generating system.Kappa Number252015100^2— 10mM Mn/50mM Ligand-±- 1mM Mn/5mM LigandX No Catalyst Present4^6^8^10^12Current (A)Figure 18. Effect of Catalyst Concentration on Kappa Number. (Group 1, Ligand Gluconate,Graphite Cathode, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 Hours, Pulp DTPA Washed)81Chapter 5. Experimental Results and DiscussionFigure 19. Effect of Catalyst Concentration on Viscosity. (Group 1, Ligand Gluconate,Graphite Cathode, 02 15 cc/min STP, 1 M NaOH, 50 °C, 3 Hours, Pulp DTPA Washed)82Chapter 5. Experimental Results and DiscussionEffect of Hydrogen Peroxide Stabilisers. The following section indicates the effect of known peroxide stabilisers. The use of chemicalstabilisers and the adjustment of the system conditions are examined with respect to theirability to improve peroxide stability. The chemical stabilisers used are sodium silicate andmagnesium sulphate, both commonly used in mechanical pulp brightening to stabilise thehydrogen peroxide bleaching solution. Also the temperature and pH of the system are varied asthese are factors are known to influence the stability of hydrogen peroxide bleaching solutions.Effect of Chemical Stabilisers. Table 24 indicates the effect of the chemical stabilisers. It can be seen the Kappa number andviscosity are both significantly higher in the runs with chemical stabiliser added.Run Group Stabiliser KappaNumberViscosity(cP)R113 1 None 17.8 9.6R124 1 0.01 M Na2SiO3.5H20 18.9 21.32-14 2 None 18.5 21.12-68 2 0.001 MgSO4 19.9 30.0Table 24. Effect of Chemical Stabilisers. (1 mM Mn, 5 mM Gluconate, 2.0 A (Group 1) and4.0 A (Group 2) on Graphite Cathode, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours,Pulp DTPA Washed)83Chapter 5. Experimental Results and DiscussionBoth the silicate and magnesium stabilisers are thought to act through the removal of thetransition metal ions (copper, manganese and iron) responsible for peroxide decompositionreactions via Reaction 8 [15]. Both the stabilisers act therefore to suppress the decompositionof the hydrogen peroxide produced via the manganese complex/oxygen redox cycle [15], inthis way preventing the peroxide decomposition necessary for delignification and viscosityloss. Also it may be the stabilisers react with the manganese in the solution and thereforeinterfere with the formation or action of the manganese complex. This would prevent thegeneration of hydrogen peroxide via the proposed redox cycle and therefore the associatedKappa number reduction and viscosity loss are not observed.84Chapter 5. Experimental Results and DiscussionEffect of pH. Table 25 indicates the effect of reducing the pH of the solution to pH 11. This is a pH atwhich the hydrogen peroxide is more stable [17]. (The pK a of Reaction 1 is 11.6 at 25 °CRun Catalyst Current KappaNumberViscosity(cP)2-43 + + 24.1 33.92-40 + - 24.1 33.82-42 - + 24.6 32.32-41 - - 26.5 35.4Table 25. Results of Factorial Experiment at pH 11.Catalyst;(+) 1 mM Mn, 5 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Current;(+) 0.1 A on Platinised Titanium Cathode, (-) 0 A,(Group 2, 02 15 cc/min STP, 0.5 M Na2SO4 + NaOH to pH 11, 50 °C, 3 hours, PulpDTPA Washed)The results indicate no beneficial effect of the combination of catalyst and current at this lowerpH. This may be due to low hydrogen peroxide production at this pH as the peroxideproduction rate increases with increasing alkalinity [2].85Chapter 5. Experimental Results and DiscussionEffect of Temperature. Table 26 indicates the results of a factorial experiment at room temperature. The results showno beneficial effect of catalyst or current at this temperature.Run Catalyst Current KappaNumberViscosity(cP)R92 + + 22.9 33.1R96 + - 22.3 32.4R99 - + 22.3 22.6R97 - - 23.1 23.4Table 26. Results of Factorial Experiment at 20 °C.Catalyst;(+) 1 mM Mn, 5 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Current;(+) 1.0 A on Graphite Cathode, (-) 0 A,(Group 1, 02 15 cc/min @ STP, 1 M NaOH, 20 °C, 3 hours, Pulp DTPA Washed)It is likely that at this temperature the decomposition of the peroxide produced via theproposed redox system is low (decomposition is considered a prerequisite for peroxidedelignification reactions). For this reason the effects on Kappa number and viscosity evident atthe higher (50 °C) temperature are not apparent.86Chapter 5. Experimental Results and DiscussionRun Catalyst Current KappaNumberViscosity(cP)2-52 + + 10.0 11.62-53 + - 11.2 13.32-54 - + 9.2 12.92-55 - - 9.6 14.4Table 27. Results of Factorial Experiment at 90 °C.Catalyst;(+) 10 mM Mn, 50 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Current;(+) 1.0 A on Platinised Titanium Cathode, (-) 0 A,(Group 2, 02 15 cc/min @ STP, 1 M NaOH, 90 °C, 3 hours, Pulp DTPA Washed)In the factorial experiments carried out at 50 °C it was found the current/catalyst run in eachfactorial experiment had a significantly lower Kappa number than the other runs in thefactorial experiment (see Tables 11,12,15). For a 3 hour factorial experiment at 90 °C (Table27) the results indicate the current/catalyst run does not have a significantly lower Kappanumber than the other runs in the factorial experiment. The viscosity is however slightlylower. Table 27 indicates the catalyst actually slightly inhibits the delignification reaction. It isthought that the production of hydrogen peroxide at 90 °C is only evident in its effect on theviscosity of the bleached pulp. At this temperature the hydroxide ion/oxygen combination issufficient to reduce the Kappa number to around 10. At 90 °C the complex is not stable for 3hours as evidenced by the observed colour change (red of complex --> green/brown pulpcolouration). As this was likely to affect the results a further factorial design for a shorter timeperiod (1 hour) was carried out. The results of which are presented in Table 28.87Chapter 5. Experimental Results and DiscussionRun Catalyst Current KappaNumberViscosity(cP)2-58 + + 16.0 14.62-59 + - 16.8 18.02-56 - + 15.7 19.72-57 - - 15.6 20.8Table 28. Results of Factorial Experiment at 90 °C.Catalyst;(+) 10 mM Mn, 50 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Current;(+) 1.0 A on Platinised Titanium Cathode, (-) 0 A,(Group 2, 02 15 cc/min @ STP, 1 M NaOH, 90 °C, 1 hour, Pulp DTPA Washed)As with the 3 hour design the combination of catalyst and current has no effect on the Kappanumber although the viscosity is markedly lower. It is believed the hydrogen peroxideproduced by the combination of the manganese complex and the current is only evident in itseffect on the viscosity of the pulp at this temperature. It was apparent that at this temperaturethe hydroxide ion/oxygen combination was sufficient to cause the Kappa number reductionobserved.In summary the results indicate the use of the peroxide stabilisers prevents the generation ofthe hydrogen peroxide in the proposed system or prevents the necessary peroxidedecomposition reactions. Lowering the pH to 11 and decreasing the temperature to roomtemperature had the effect of suppressing the peroxide decomposition rendering the systemincapable of promoting delignification. At the higher temperature of 90 °C the peroxide88Chapter 5. Experimental Results and Discussiongenerated affected only the viscosity, the hydroxide ion/oxygen combination at thistemperature being sufficient to cause the Kappa number reduction observed.89Chapter 5. Experimental Results and DiscussionEffect of Current. This section indicates the effect of current on the proposed system. Figures 20 and 21 give theresults of two sets of experiments (Group 1 and Group 2) showing the reduction in Kappanumber and viscosity versus the applied current.Figure 20. Effect of Current on Kappa Number. (Group 1; 1 mM Mn, 5 mM Gluconate,Graphite Cathode, 02 15 cc/min @ STP , 1 M NaOH, 50 °C, 3 Hours, Pulp DTPA Washed.Group 2; 10 mM Mn, 50 mM Gluconate, Platinised Titanium Cathode, 02 15 cc/min @ STP,1 M NaOH, 50 °C, 3 Hours, Pulp DTPA Washed)90Viscosity (cP)35 ^3025201510 --1— Group 1-9-- Group 2Chapter 5. Experimental Results and Discussion0^2^4^6^8^10^12Current (A)Figure 21. Effect of Current on Viscosity. (Group 1; 1 mM Mn, 5 mM Gluconate, GraphiteCathode, 02 15 cc/min @ STP, 1 M NaOH, 50 'V, 3 Hours, Pulp DTPA Washed. Group 2;10 mM Mn, 50 mM Gluconate, Platinised Titanium Cathode, 02 15 cc/min STP, 1 MNaOH, 50 °C, 3 Hours, Pulp DTPA Washed)The results indicate that in the presence of catalyst, the application of current produces a largerreduction in Kappa number and pulp viscosity than is evident in the absence of current. Thisindicates that current is required to continuously generate the hydrogen peroxide responsiblefor these changes through the reaction of the manganese complex and oxygen. The results alsoappear to indicate that increasing the current above a certain value does not result in anyfurther reduction in Kappa number or viscosity. For the Group 2 results, increasing the currentbeyond 4.0 A yields little further reduction in Kappa number or viscosity.91Chapter 5. Experimental Results and DiscussionThe current above which further Kappa number reduction and viscosity loss are not observedcorresponds to the current required to promote the regeneration of the manganese complex andthus continuously generate the hydrogen peroxide responsible for the Kappa number andviscosity reductions.At very high currents (10.0 A) it should be noted the production of hydroxide ions via thehydrogen evolution reaction (Rxn 18) may be of importance. Table 29 indicates the results ofthe addition of hydroxide ion to simulate the production of that ion at a current of 10.0 A (seeAppendix 4). The total amount of hydroxide added was 1.12 mols, added as 11 equal additionsof 0.108 mols (added as solid NaOH) at time t = 15, 30, 45,..,165 mins. It shows that theaddition of 1.12 mols of hydroxide ion (Run 2-33) brings about a similar Kappa numberreduction to that observed in the 10.0 A run (Run 2-16) without the viscosity loss associatedwith the current/catalyst combination.Run Conditions KappaNumberViscosity(cP)2-18 10 A, No Catalyst 19.8 30.62-16 10 A, Catalyst 17.5 21.22-37 0 A, No Catalyst, OH ^Added 17.8 28.22-33 0 A, Catalyst, OH Added 16.9 30.8Table 29. Effect of Hydroxide Addition on Bleaching Result. (Group 2, Catalyst 10 mM Mn,50 mM Gluconate if used, Total OW Added 1.12 mols if added, Platinised Titanium Cathode,02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)92Chapter 5. Experimental Results and DiscussionIn the presence of catalyst, if no current flows, none of the manganese complex is regeneratedto participate in the continuous generation of hydrogen peroxide. Therefore in the absence ofcurrent (Run 2-33) the viscosity is high compared to the run in the presence of current (Run 2-16) where the production of peroxide affects the viscosity.The amount of hydroxide ion added is calculated from the theoretical production of that ion,via the hydrogen evolution reaction, at a current of 10.0 A. It can be seen that the addition ofthis hydroxide ion can produce a Kappa number reduction similar to the reduction evident inthe catalyst/current run. It would seem that the hydroxide ion produced at 10.0 A isresponsible for the Kappa number reduction. It should be noted however that in the absence ofcatalyst, a current of 10.0 A does not produce the reduction in Kappa number (Run 2-18,Kappa No = 19.8) evident when the theoretical amount of hydroxide produced at that currentis added (Run 2-37 Kappa No = 17.8). This implies that the actual production of hydroxideion via the hydrogen evolution reaction is much smaller than the theoretical production anddoes not influence the Kappa number reduction. The actual production of hydroxide ions maybe affected by electrode reactions involving the pulp extractives (ie. those compoundsextracted from the pulp fibres during the bleaching reaction) [33].To investigate the effect of the hydroxide ions thought to be generated at 10.0 A, Table 30shows the result of the addition of the stoichiometric requirement of acid to neutralise thetheoretical production of hydroxide ions produced at 10 A (see Appendix 4). The acid(112SW was added to a current/catalyst run at 10.0 A.93Chapter 5. Experimental Results and DiscussionRun Conditions KappaNumberViscosity(cP)2-16 10 A, Catalyst 17.5 21.22-61 10 A, Catalyst, H+ Added 20.6 22.4Table 30. Effect of Acid Additign on Bleaching Result. (Group 2, Catalyst 10 mM Mn, 50mM Gluconate if used, Total H' Added 1.12 M, if added, Platinised Titanium Cathode, 0215 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)Table 30 indicates that the run involving the neutralisation of the OW ions (generated by thecurrent) by the acid (Run 2-61) does not exhibit the reduction in Kappa number evident in thecurrent/catalyst run (Run 2-16). There was significant viscosity loss apparent in both runs.As noted above, the actual production of hydroxide ions is thought to be lower than thetheoretical value. Therefore the addition of 1.12 mols of H + ions would affect the alkalinityof the bleaching solution (ie. lower the alkalinity) producing the effects on Kappa number andviscosity noted [5].The factorial experiment shown in Table 31 is designed to determine the importance of oxygenand catalyst with respect to the OW thought to be produced at 10.0 A. It is apparent thatoxygen is required for the added OW to be effective in reducing the Kappa Number. It shouldbe noted that in the absence of the peroxide producing catalyst/current combination noviscosity loss is evident.94Chapter 5. Experimental Results and DiscussionRun Gas Purge AddedHydroxideCatalyst KappaNumberViscosity(cP)2-36 - - - 23.9 37.12-27 + - - 20.1 28.72-35 - + - 22.0 34.02-37 + + - 17.8 28.22-34 - - + 24.3 39.92-28 + - + 19.7 29.92-29 - + + 22.2 38.12-33 + + + 16.9 30.8Table 31. Results of Factorial Experiment.Catalyst;(+) 10 mM Mn, 50 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Added Hydroxide;(+) 1.12 mols, (-) 0 mols,Gas Purge; (+) Oxygen, (-) Nitrogen,(Group 2, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hour, Pulp DTPA Washed)Also it can again be seen that the addition of 1.12 mols of hydroxide ion can produce a Kappanumber reduction similar to the reduction evident in the catalyst/current run at 10.0 A(compare Runs 2-16 Table 29 and 2-33 Table 31). The presence of catalyst has little effect onthe results of the hydroxide ion addition (Runs 2-37 and 2-33).It is thought then, that the production of hydroxide ions at 10.0 A is smaller than thetheoretical production and that the reduction in Kappa number and viscosity, noted in runswhere the manganese complex and current are combined, is due to the peroxide continuouslyproduced in the proposed redox cycle.95Chapter 5. Experimental Results and DiscussionTable 32 indicates the effect of hydrogen peroxide addition on the bleaching result. Theperoxide was added to simulate the effect of the production of peroxide in the bleachingenvironment. The total amount of peroxide added was 0.15 mols, added as 3 equal additionsof 0.05 mols (5 ml of 10 M H202) at time t = 0, 1, 2 hours. It can be seen that in thepresence of catalyst the addition of the peroxide (Run 2-30) brought about a similar reductionin both Kappa number and viscosity to the reductions noted in the catalyst/current run (Run 2-16). This is very strong evidence that the proposed redox couple is operating and the peroxideproduced is capable of causing the results attributed to it.Run Conditions KappaNumberViscosity(cP)2-16 10 A, Catalyst 17.5 21.22-30 0 A, Catalyst, H202 Added 17.2 18.8Table 32. Effect of Hydrogen Peroxide Addition. (Group 2, Catalyst 10 mM Mn, 50 mMGluconate, Total H202 Added 0.15 mols (if added), Platinised Titanium Cathode, 02 15cc/min STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)The results indicate the production of hydrogen peroxide via the current/catalyst interactionand show the ability of this peroxide to affect both the Kappa number and viscosity of thebleached pulp.96Chapter 5. Experimental Results and DiscussionTime Dependence. The following experiments examine the effect of reaction time upon the bleaching results.Figure 22 indicates the change in Kappa number and viscosity over an 8 hour period. Theyshow an initial sharp decrease in both Kappa No and viscosity followed by a levelling off.Because of this effect there is little benefit in continuing the reaction beyond 5 -- > 6 hours. Itshould be noted that the complex does not remain stable beyond approximately 5 -- > 6 hoursat 50 °C. Therefore shown in Figure 22 is a run (Run R137) in which the catalyst is renewedafter 3 hours so the complex is present throughout a 6 hour reaction period. There is noincrease in delignification evident in this action.9784^6Time (hours)-)K R137 (Kappa Number)0 R137 (Viscosity25201510500 2 1035302520151050Chapter 5. Experimental Results and DiscussionKappa Number^ Viscosity (cl.30^  40Figure 22. Time Dependance. (Group 1, 10 mM Mn, 50 mM Gluconate, 4.0 A on GraphiteCathode, 02 15 cc/mm @ SW, 1 M NaOH, 50 °C, 3 Hours, Pulp DTPA Washed)It would appear that under these conditions the catalyst/current combination is not effective inpromoting Kappa number reduction (or viscosity loss) beyond 5-6 hours. It is thought that thisis because the peroxide generated is incapable of reducing the Kappa number (or viscosity)beyond a certain value. That value representing the limit of reaction under the conditionsdescribed.98Chapter 5. Experimental Results and DiscussionEffect of Manganese and Ligand Only. The following experiments investigate the ability of the manganese complex to prevent theviscosity loss evident in Group 1 runs in the absence of current as described earlier.From Table 33 it can be seen that the protection of viscosity under the prescribed conditions(R112) also occurs when the metal alone (R119) or ligand alone (R123) are present. The metalis more effective in this respect.Run Conditions KappaNumberViscosity(cP)R107 No Metal, No Ligand 19.8 21.7R112 Metal, Ligand 19.5 27.9R119 Metal, No Ligand 20.1 31.4R123 Ligand, No Metal 19.9 26.5Table 33. Effect of Metal and Ligand on Bleaching Result.Metal; 10 mM Mn added as a solution of 0.1 M Mn(C104)2.6H20,Ligand; 50 mM Gluconate added as a 0.1 M solution,(Group 1, 02 15 cc/min @ STP, 1 M NaOH, 50 °C, 3 hours, Pulp DTPA Washed)The noted effect of manganese protection is known and described earlier (Chapter 2). Themechanism of ligand only protection is thought to be due to its action as a sacrificial protector.It is a sacrificial protector as the viscosity-degrading free radicals attack both the free ligandand the cellulose structures effectively reducing the damage to the cellulose.99Chapter 5. Experimental Results and DiscussionThe protection effect of the manganese complex is therefore ascribed to a combination of theability of its 2 components to protect against cellulose degradation.100Chapter 5. Experimental Results and DiscussionUse of Chemical Reductants. This section examines briefly the use of chemical reagents to reduce the higher oxidation statesof manganese rather than the use of the electrochemical method. Ascorbic acid and hydrazinesulphate were tested as potential candidates due to their known ability to reduce the manganesecomplex [2].The results of the factorial design experiments are given in Tables 34 and 35. The ascorbicacid trials (Table 34) indicate no benefit in the use of the reductant, the blank run (R85) havingthe lowest Kappa number. It would appear that the ascorbic acid is incapable of participatingin the reduction of the manganese complex to generate hydrogen peroxide under theseconditions.Run Catalyst Reductant KappaNumberViscosity(cP)R88 + + 25.5 29.3R83 + - 24.4 28.8R90 - + 25.7 29.2R85 - - 24.4 28.1Table 34. Results of Factorial Experiment.Catalyst; (+) 2.5 mM Mn, 25 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Reductant;(+) 0.1 M Ascorbic Acid, (-) 0 M Ascorbic Acid,( 02 15 cc/min @ STP, 0.5 M NaOH, 30 °C, 1 hour)101Chapter 5. Experimental Results and DiscussionTable 35 for the hydrazine sulphate indicates that the reductant alone (R84) exhibits the largestKappa number reduction and that the viscosity of R84 is also affected detrimentally. In theabsence of the reductant the manganese complex does not improve the bleaching response .(Runs R83 and R85). Where the catalyst and reductant are both present (Run R82) the effect ofthe catalyst is to retard Kappa number reduction and viscosity loss when compared to thereductant only run (Run R84).Run Catalyst Reductant KappaNumberViscosity(cP)R82 + + 23.5 11.8R83 + - 24.4 28.8R84 - + 20.8 7.2R85 - - 24.4 28.1Table 35. Results of Factorial Experiment.Catalyst;(+) 2.5 mM Mn, 25 mM Gluconate, (-) 0 mM Mn, 0 mM Gluconate,Reductant;(+) 20 mM Hydrazine Sulphate, (-) 0 mM Hydrazine Sulphate,( 02 15 cc/min @ STP, 0.5 M NaOH, 25 'V, 1 hour)Shown in Table 36, Run 2-62 shows the effect of hydrazine in the absence of oxygen andindicates that oxygen is a requirement for the Kappa number and viscosity reductions evidentin Run R84.102Chapter 5. Experimental Results and DiscussionRun Conditions KappaNumberViscosity(cP)R84 No Catalyst, Reductant, 02 20.8 7.22-62 No Catalyst, Reductant, N2 24.7 28.1Table 36. Effect of Nitrogen on Bleaching Result.Reductant;(+) 20 mM Hydrazine Sulphate,(02 or N2 15 cc/min @ STP, 0.5 M NaOH, 25 °C, 1 hour)It appears that the hydrazine sulphate is also incapable of operating in a cycle with themanganese complex to generate hydrogen peroxide. The presence of the manganese complexhas the effect of reducing the viscosity drop (compare Runs 82,83). This is probably due to theability of the manganese complex to protect against cellulose degradation. However, thepresence of the manganese complex also reduces the Kappa number drop (compare Runs82,83). It appears the action of the hydrazine sulphate alone with oxygen is responsible for theKappa number and viscosity reductions evident in the factorial experiment.103Chapter 6. General Discussion.The proposed manganese complex/oxygen redox couple involved the use of the manganesegluconate complex to delignify chemical pulp through the in-situ production of hydrogenperoxide from oxygen in an alkaline environment. It was proposed to bring about the reductionof the higher oxidation states of manganese via the use of an electrode system. The ability ofthis redox couple to operate in the bleaching environment was examined and the followingobservations were made.The experiments employing cyclic voltammetry indicated the presence of the manganese III/IVcouple; this indicated the manganese IV- > III reduction would take place on the cathodesurface. There was no evidence of the manganese II/III couple. The experiments employingcyclic voltammetry indicated the presence of hydrogen peroxide in the sample solution.Analysis of the bleaching solution also indicated the presence of hydrogen peroxide in thesolution. The ability of a cathode to promote the reduction of manganese IV to the manganeseIII form was evidenced in the experiments employing spectrophotometric analysis. Similarly,spectrophotometric experiments showed no evidence of the manganese III to manganese IIreduction. The experiments indicated the predominant forms of the manganese gluconatecomplex in the bleaching environment were likely to be the manganese III and IV forms.It was found that the combination of the manganese gluconate complex, oxygen and sufficientcurrent in an alkaline environment produced a significantly larger Kappa number and viscosity104Chapter 6. General Discussionreduction in the chemical pulp, than in the absence of the manganese gluconate complex.Analysis of the factorial design experiments indicated the combination of catalyst and currenthad a significant effect on Kappa number and viscosity reduction. This effect is attributed tothe action of the peroxide produced in the proposed redox cycle. Free radical species generatedfrom the peroxide are thought to be responsible for the effects on Kappa number and viscositynoted, as peroxide stabilisers added to prevent peroxide decomposition inhibited thedelignification and cellulose degradation reactions. Metal ions were found to be very importantwith respect to the delignification process. The addition of Cu 2+ or Fe2+ ions to runs in theabsence of the manganese complex was found to promote viscosity loss in the pulp, probablydue to free radical generation catalysed by the metal ions. It is believed a metal ioncontaminant such as Cu2+ was responsible for increased viscosity loss in certain experimentalruns. The addition of Mn2+ ions in the absence of the manganese complex was found toinhibit viscosity loss, an effect attributed in the literature to the adsorbtion of transition metalions on the surface of precipitated manganese oxides.In summary, the evidence presented in this study suggests the proposed manganese gluconatecomplex/oxygen redox cycle is indeed operating to delignify the pulp. It appears themanganese III and IV forms of the manganese gluconate complex are involved in the redoxcycle. Oxygen is required to oxidise the manganese III form to the manganese IV form and theperoxide anion while sufficient current is required to bring about the reduction of themanganese IV form to the manganese III form. It is noted that free radicals species, thought tobe generated in the bleaching environment from the peroxide, are probably responsible for thedelignification and cellulose degradation reactions observed.105Chapter 7. Conclusions and Recommendations.Conclusions.It was found that the combination of the manganese gluconate complex, oxygen and sufficientcurrent in an alkaline environment produced a significantly larger Kappa number and viscosityreduction in the chemical pulp, than in the absence of the manganese gluconate complex. Thiseffect is attributed to the action of the peroxide produced in the proposed manganesegluconate/oxygen redox cycle. It had been hoped the manganese complex/oxygen redox cyclewould promote the delignification of chemical pulp via non-free radical pathways, thusreducing cellulose degradation. No evidence of this effect was noted; indeed, it is likely thatthe hydrogen peroxide produced via the manganese gluconate complex was decomposed to freeradical species causing the delignification and cellulose degradation observed.The manganese gluconate complex/oxygen redox cycle proved to be a capable oxidisingsystem which had a significant effect on both the cellulose and lignin components of pulp.However the redox couple was not particularly selective compared to conventional oxygenbleaching. Further research may improve the effectiveness and selectivity of the manganesegluconate complex/oxygen redox cycle.106Chapter 7. Conclusions and RecommendationsRecommendations.The following recommendations are suggested for future work in the delignification ofchemical pulp utilising the manganese gluconate/oxygen redox couple:1) Investigation of the reduction of the manganese HI complex to the manganese II form. Thisreduction would increase the potential peroxide production.2) Further investigation of the reduction of the manganese (IV) complex to the manganese (III)form. Improvement would increase the potential peroxide production.3) Incorporation of a method of measuring the production of, and type of, free radical speciesin the reactor chamber. This may indicate conditions promoting more selective delignification.4) An in-situ method of measuring the peroxide concentrations within the reactor would behelpful in determining the optimum operating conditions.107Bibliography. [1] Allison, R.W., "Peroxide Delignification of Alkaline Pulps from Pinus Radiata",Proceedings Tappi Pulping Conference (Toronto), Tappi Press, 319-326 (1982).[2] Lim, P.K. Cha, J.A and Fagg, B.S., "Use of Manganese(II)- Polyol Complexes ToAccelerate High-pH Peroxide Oxidation Reactions", Industrial Engineering ChemicalFundamentals 23(1), 29-33 (1984).[3] Britt, Kenneth W., Ed., "Handbook of Pulp and Paper Technology", 2nd Edn., VanReinhold Company, 3-36 (1970).[4] Kolimam, F.F.P. and Cote, W.A., "Principles of Wood Science and Technology",Springer, Chap. 1-2 (1968).[5] Casey, James P., Ed., "Pulp and Paper Chemistry and Chemical Technology", 3rd Edn.,John Wiley & Sons, Vol 1 86 (1980).[6] Rydholm, Sven A., "Pulping Processes", Interscience Publishers, 598 (1965).[7] Rapson, W. Howard., Ed., "The Bleaching of Pulp", TAPPI Monograph Series No 27,Tappi Press, 8-16 (1963).[8] Singh, A., "Phenoxyl- and Oxy- Radicals and their Role in Oxygen Delignification", TappiProceedings 1987 International Oxygen Delignification Conference, Tappi Press, 111-118(1987).[9] TAPPI Test Methods, TAPPI, 1991[10] Francis,R. C., "Hydrogen Peroxide Delignification And Brightening of Chemical Pulps",Bleaching: A Tappi Press Anthology, Tappi Press, 413-420 (1986).[11] Gierer, J., Jansbo, K. Yang, E. and Yoon, B.-H., "On the Participation of HydroxylRadicals in Oxygen and Hydrogen Peroxide Bleaching Processes", Appita 6th Int. Symp. onWood and Pulping Chem. Proc. (Melbourne), Vol 1 93-97 (1991).[12] Singh, A., "Delignification Mechanisms Using Oxygen And Hydrogen Peroxide", 1990Oxygen Delignification Symposium, Tappi Press, 53-59 (1990).[13] Gustaysson, R. and Swan, B., "Evaluation of the Degradation of Cellulose andDelignification During Oxygen Bleaching", Proceedings TAPPI Non-sulpur PulpingSymposium (Madison), Tappi Press, 43-51 (1974).[14] Lindeberg, G. and Walding, J., "Reactions of Nitrated Kraft Lignin in an AlkalineOxygen Bleaching Stage", Proceedings TAPPI Pulping Conference (Toronto), Tappi Press,Book 2 251 (1986).[15] Singh, R.P., Ed., "The Bleaching of Pulp", 3rd Edn., Tappi Press, Chap. 7 (1979).108Bibliography[16] Landucci, L.L., "Effects of Transition Metals in Oxidative Delignification", Chemistry ofDelignification with Oxygen, Ozone, and Peroxides, May 1975, Grath et al, Eds.,.189-98(1975).[17] Singh, R.P., Ed., "The Bleaching of Pulp", 3rd Edn., Tappi Press, Chap. 8 (1979).[18] McDonough, T.J., Kirk, R.C., Backlund, B. and Winter, L., "Catalysis in PeroxideDelignification", 1987 International Oxygen Delignification Conference, 165-171 (1987).[19] Gierer, J., "Chemical Aspects Of Delignification" International Symposium on Wood andPulping Chemistry, II(12)-11(17) (1981).[20] Smith, P.K. and McDonough, T.J., "Transition Metal Ion Catalysis of the HydrogenPeroxide Oxidation of a Lignin Model Compound ", International Symposium on WoodChemistry and Pulping Chemistry Technical Papers, 215-220 (1985).[21] Landucci, L.L., "Electrochemical Behaviour of Catalysts for Phenoxy RadicalGeneration", Tappi Journal 62(4), 71-74 (1979).[22] Richens, D.T., Smith, C.G. and Sawyer, D.T., "Sorbitol and Related Polyol Complexesof Manganese(II), -(III), and -(IV): Redox and Oxygenation Equilibria", Inorganic Chemistry18(3), 706-712 (1979).[23] Bodini, M.E. and Sawyer, D.T., "Electrochemical and Spectroscopic Studies ofManganese(II), -(111), and -(IV) Gluconate Complexes. 2. Reactivity and Equilibria withMolecular Oxygen and Hydrogen Peroxide", Journal of the American Chemical SocietyDecember 98(26), 8366-71 (1976).[24] Bodini, M.E., Willis, L.A., Riechel, T.L. and Sawyer, D.T., "Electrochemical andSpectroscopic Studies of Manganese(II), -(III), and -(IV) Gluconate Complexes. 1. Formulasand Oxidation-Reduction Stoichiometry", Inorganic Chemistry 15(7), 1538-43 (1976).[25] Magers, K.D., Smith, C.G. and Sawyer, D.T., "Polarographic and Spectroscopic Studiesof the Manganese(11), -(III), and -(IV) Complexes Formed by Polyhydroxy Ligands",Inorganic Chemistry 17(3), 515-23 (1978).[26] Linde, D.R., Ed., "CRC Handbook of Physics and Chemistry", 64th Edn., CRC Press,C601 (1983).[27] Gilead, E., Kirowa-Eisner, E., Penciner, J., "Interfacial Electrochemistry. AnExperimental Approach", Addison-Wesley Publishing Company Inc, Chap. 9 368-396 1975.[28] Vogel A.I., "Textbook of Quantitative Inorganic Analysis", 4th Edn., Longman, 355(1978).[29] Murphy, T.D., "Design and Analysis of Industrial Experiments", Chem. Eng. 84(12),168-182 (1977).109Bibliography[30] Box, G.E.P., Hunter, W.G., and Hunter, J.S., "Statistics for Experimenters", Wiley,Chap. 10 (1978).[31] Landucci, L.L., Sanyer, N., "Influence of Transition Metals in Oxygen Pulping", TappiJournal 58(2), 60-63, 1975.[32] Gilbert, A.F., Pavlovova, E., and Rapson, W.H., "Mechanism of MagnesiumRetardation of Cellulose Degradation during Oxygen Bleaching", Tappi J. 56(6), 95-99 (1973)[33] Lund, H., Baizer, M., Eds., "Organic Electrochemistry", 3rd Edn., Marcel Dekker Inc.,787-808, 1991.110Appendix 1. Analysis of Factorial Designs. The analyses presented in this appendix tabulate the effects (as calculated from Equations 8,9and 10, Section 2.8) of the variables in the factorial experiments presented in Chapter 5.4.Below the table of effects the significant effects on the measured response (Kappa number andviscosity) are noted. The significant effects are those for which the confidence interval doesnot contain zero.For all the analyses presented, except the analysis in Table 44, the 95% confidence intervalsfor Kappa number and viscosity were ±0.58 and ±3.64 cP respectively. The 95% confidenceintervals for Kappa number and viscosity for Table 44 were ±0.82 and • ±5.15 cPrespectively.Main Effects KappaNumberViscosity(cP)Catalyst (C) -1.0 -2.5Current^(I) -1.9 -9.2Interaction (C*I) -0.7 -8.7Table 37. Factorial Analysis of Data from Table 11.Significant Effects.Kappa Number: catalyst, current and catalyst/current interactionViscosity: current and catalyst/current interaction111AppendicesMain Effects KappaNumberViscosity(cP)Catalyst (C) -1.1 -3.5Current^(I) -1.0 -5.0Interaction (C*I) -1.2 -5.9Table 38. Factorial Analysis of Data from Table 12.Significant Effects.Kappa Number: catalyst, current and catalyst/current interactionViscosity: current and catalyst/current interactionMain Effects KappaNumberViscosity(cP)Catalyst (C) -1.2 -0.6Current^(I) -1.1 -10.3Interaction (C*I) -0.9 -6.8Table 39. Factorial Analysis of Data from Table 15.Significant Effects.Kappa Number: catalyst, current and catalyst/current interactionViscosity: current and catalyst/current interaction112AppendicesMain Effects KappaNumberViscosity(cP)Catalyst (C) -1.5 0.0Current^(I) -1.0 -1.5Interaction (C*I) 1.0 1.6Table 40. Factorial Analysis of Data from Table 25.Significant Effects.Kappa Number: catalyst, current and catalyst/current interactionViscosity: noneMain Effects KappaNumberViscosity(cP)Catalyst (C) -0.1 9.8Current^(I) -0.1 -0.1Interaction (C*I) 0.7 0.8Table 41. Factorial Analysis of Data from Table 26.Significant Effects.Kappa Number: catalyst/current interactionViscosity: catalyst113AppendicesMain Effects KappaNumberViscosity(cP)Catalyst (C) 1.2 -1.2Current^(I) -0.8 -1.6Interaction (C*I) -0.4 -0.1Table 42. Factorial Analysis of Data from Table 27.Significant Effects.Kappa Number: catalyst and currentViscosity: noneMain Effects KappaNumberViscosity(cP)Catalyst (C) 0.8 -4.0Current^(I) -0.4 -2.3Interaction (C*I) -0.5 -1.2Table 43. Factorial Analysis of Data from Table 28.Significant Effects.Kappa Number: catalystViscosity: catalyst114AppendicesMain Effects KappaNumberViscosity(cP)Gas Purge^(G) -4.5 -7.9Added Hydroxide(H) -2.3 -1.1Catalyst^(C) -0.2 -2.7Interaction (G*H) -0.3 +1.3Interaction (G*C) -0.5 -0.8Interaction (H*C) -0.2 -0.7Interaction(G*H*C) -0.1 -0.0Table 44. Factorial Analysis of Data from Table 31.Significant Effects.Kappa Number: gas purge and added hydroxideViscosity: gas purgeMain Effects KappaNumberViscosity(cP)Catalyst^(C) -0.1 0.4Reductant (R) 1.2 0.8Interaction (C*R) -0.1 -0.3Table 45. Factorial Analysis of Data from Table 34.Significant Effects.Kappa Number: reductantViscosity: none115AppendicesMain Effects KappaNumberViscosity(cP)Catalyst^(C) 1.4 2.7Reductant (R) -2.3 -19.0Interaction (C*R) 1.4 2.0Table 46. Factorial Analysis of Data from Table 35.Significant Effects.Kappa Number: catalyst, reductant and catalyst/reductant interactionViscosity: reductant116AppendicesAppendix 2. Calculation of Response Error. Runs KappaNumberViscosity(cP)sk sv rR100, R106 18.26, 18.76 0.354 2R101, R105 19.71, 19.93 0.156 2R110, R104 19.42, 19.59 0.120 2R103, R107 19.99, 19.83 0.1131 2R139, R140 21.22, 24.27 2.157 2R138, R141 19.55, 21.08 1.082 22-11, 2-12 22.43, 22.21 0.156 22-16, 2-19 21.16, 21.07 0.064 2Table 47. Table for Response Error Calculation.The response error for duplicate runs (Y1, Y2) for Kappa number and viscosity (sk and s v) arecalculated from5 ((y 1_y2)2/2) 1/2The pooled estimation of response error for both Kappa Number and viscosity responses iscalculated froms2 = E(ri-l)si2/E(ri-1)117AppendicesWith, Degrees of Freedom (v),For Kappa Number response,For viscosity response,v = E(ri-1)s=0.21, v=4s=1.21, v=4118AppendicesAppendix 3. Electrode Studies. To determine the effect of electrode material on the electrode potential resulting from anincrease in current the following experiment was carried out.The equipment used in these experiments was as shown in Figure 6. A Standard CalomelElectrode (SCE) was held close to the cathode in the reactor. A high impedance voltmeter wasconnected to the SCE and the cathode. In this way measurements of electrode potential weretaken.The solution in the reactor was prepared as described in Section 4.4. The applied current wasset at the desired level (as read from the power supply), 1 minute was allowed to elapse andthen the electrode potential (vs the SCE) was read from the voltmeter. The results are indicatedin Figure 23.1190 1 2^3Applied Cell Current (A)4 5Electrode Potential (V va SHE)0.5—*-- GraphiteH-- Stainless Steel4( Platinised Titanium-2AppendicesFigure 23. Electrode Potential vs Applied Current. (1 mM Mn, 5 mM Gluconate, 1 M NaOH,23 °C, No Pulp Present)Figure 23 indicates that for the cathode materials tested the limiting hydrogen evolutionreaction (Reaction 18) occurred at different electrode potentials. The hydrogen evolutionreaction on the graphite electrode occurred at the most negative electrode potential.120AppendicesAppendix 4. Hydroxide Ion Production at 10.0A. Hydroxide ions are produced at the cathode via the hydrogen evolution reaction (Reaction 18).2H20 + 2e- < > H2 + 2011 -^Rxn 18The theoretical production of hydroxide ions is calculated from,OW produced = It/zF^(17)where t - duration of reaction (s)z = electron stoichiometric coefficientI = current (A)F = Faraday Constant (= 96500 c/mol)At 10.0 A for 3 hours, the hydroxide ion production is,OW produced = 10.0x(3x3600)/1x96500 = 1.119 molsThe production of hydroxide ion is simulated through the addition of a total of 1.119 mols ofNaOH (corresponds to 44.77g NaOH) to the bleaching solution.As the production of hydroxide ions is 1.119 mols, the acid (H +) required to neutralise theOW produced is 1.119 mols (corresponds to 1.119/2 mols of sulphuric acid)121AppendicesAppendix 5. Hydrogen Peroxide Titration Calculation 128]. Hydrogen peroxide reacts with permanganate via Reaction 24.2Mn04- + 5H202 + 6H+ <--> 2Mn2 + + 502 + 8H20^Rxn 24The titration procedure described below is based on the above equation and a method presentedin literature [28].A 5 ml sample of solution was drawn from the reactor.This was added to 200m1 of ice-cold 0.5 M H2SO4 in a 250 ml flask.2 drops of ferroin indicator were then added.The sample was then titrated with 0.01 N KMnO4 where, lml of 0.01N KMnO4 titratedcorresponds to a sample concentration of 0.001M H202 .122Glossary. Cellulose : The main chemical component in the cell walls of plants. It is the fibrous substancethat remains after the non-fibrous portions, such as lignin , have been removed from the pulpduring the cooking and bleaching operations.Chemical Pulp : Wood pulp produced by removing lignin during the cooking of wood chips ina digester. The Kraft or sulpite processes are examples of processes producing chemical pulp.Consistency : A measure of the fibrous material in pulp suspensions. It is expressed as aweight percentage.DTPA : Diethylenetriaminepentaacetic acid.EDTA : Ethylenedinitrilotetraacetic acid.Fibre : An elongated, tapering, thick walled cellular unit which is the structural component ofwoody plants.Handsheet : A single sheet of pulp prepared in the laboratory by draining water from a pulpsuspension on a screen-covered sheet mould. It is used in testing the properties of pulp.Kappa Number : A value obtained using a standard test procedure which indicates the lignincontent of pulp.Kraft Pulp : Wood pulp produced by the sulphate chemical process using cooking liquorcomposing primarily of sodium hydroxide and sodium sulphide.Lignin : A coloured organic substance which acts as an interfiber bond in woody materials. Itis removed along with other organic materials during the pulping and bleaching stages.Mechanical Pulp : Pulp produced by reducing pulpwood logs and chips into their fibrecomponents by the use of mechanical energy. eg  via the use of a grinding stone.Pulp Extractives : The products of lignin and cellulose oxidation, found in the bleachingsolution.PTFE : Polytetrafluoroethane.Redox Cycle : Chemical/electrochemical process involving oxidation and reduction reactions.SCE : Saturated Calomel Electrode, V(vs SCE) = 0.24 + V(vs SHE) at 25 °C.SHE : Standard Hydrogen Electrode.TAPPI : Technical Association of the Pulp and Paper Industry.123GlossaryViscosity : A value obtained using a standard test procedure involving the measurement of theviscosity of the pulp in a solvent. This indicates the length of the cellulose molecules in thepulp sample.124

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