Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Hydrolysis of aluminum sulphate solutions at high temperatures 1971

You don't seem to have a PDF reader installed, try download the pdf

Item Metadata

Download

Media
UBC_1971_A7 N54.pdf [ 3.83MB ]
Metadata
JSON: 1.0078959.json
JSON-LD: 1.0078959+ld.json
RDF/XML (Pretty): 1.0078959.xml
RDF/JSON: 1.0078959+rdf.json
Turtle: 1.0078959+rdf-turtle.txt
N-Triples: 1.0078959+rdf-ntriples.txt
Citation
1.0078959.ris

Full Text

HYDROLYSIS OF ALUMINUM SULPHATE SOLUTIONS AT HIGH TEMPERATURES BY CVETKO NIKOLIC D i p l . Ing. (Chemical Engineering) University of Belgrade, 1963 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE In the Department of METALLURGY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February 1971 In presenting t h i s thesis in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for s c h o l a r l y purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or p u b l i c a t i o n of t h i s thesis for f i n a n c i a l gain shall not be allowed without my written permission. Department of M e t a l l u r g y The University of B r i t i s h Columbia Vancouver 8, Canada Date A p r i l 28, 1971 - i i - ABSTRACT Normal and acid aluminum sulphate solutions containing about 6.0 gr/1 of aluminum and up to 50 gr/1 SO^ were hydrolyzed until equilibrium was reached in the temperature region 125-250°C. Under the equilibrium conditions the only stable solid phase observed in equilibrium with a liquid phase of various compositions was basic aluminum sulphate with nominal formula 3A1203.4S03.9E^0. A small portion of the ternary diagrams for the system A^O^-SO^-K^Q at 225°C and 250°C was constructed. A mixture of aluminum sulphate and other metal sulphates, K„S0., NaoS0,, Li.SO., FeSO. and CuSO. 2 4 2 4 2 4 4 4 i.e. was hydrolyzed at 225°C in order to find the effect of these salts on hydrolysis. The overall hydrolysis reaction was found to occur according to the chemical equation; ekl^ + 4HS0. + 14H„0 • 3Al o0 o.4S0„.9H o0 + 14H+ 4 2 2 3 3 2 The equilibrium constants at 125, 150, 175, 200, 225 and 250°C were determined. Finally a mechanism for the hydrolysis of aluminum sulphate was proposed. - i i i - ACKNOWLEDGEMENT The author wishes to express sincere thanks to Dr. E. Peters f o r his continuing guidance and i n t e r e s t i n th i s project. His thanks i s extended to fellow graduate students, p a r t i c u l a r l y Mr. D. Jones and the tech n i c a l s t a f f of the Department of Metallurgy for t h e i r h e l p f u l discussions and assistance. F i n a n c i a l support from the National Research Council of Canada i n the form of a Research Assistantship i s g r a t e f u l l y acknowledged. - iv - TABLE OF CONTENTS Page 1. INTRODUCTION 1 2. A REVIEW OF THE LITERATURE .. . 3 2.1 The Fe 20 3-S0 3-H 20 System 3 2.2 The A1 20 3~S0 3-H 20 System 6 2.3 The M 20-A1 20 3-S0 3-H 20 System 10 3. EXPERIMENTAL 12 3.1 Experimental Technique 12 3.2 Materials Used 18 3.3 Preparation of Solutions for Hydrolysis 21 3.4 Chemical Analysis 22 4. RESULTS AND DISCUSSION 24 4.1 The E f f e c t of Temperature on Hydrolysis of Aluminum Sulphate Solutions 25 4.1.1 Hydrolysis of Aluminum Sulphate Solutions at 125°C 25 4.1.2 Hydrolysis of Aluminum Sulphate Solutions at 150°C 28 4.1.3 Hydrolysis of Aluminum Sulphate Solutions at 175°C 30 4.1.4 Hydrolysis of Aluminum Sulphate Solutions at 200°C 32 4.1.5 Hydrolysis of Aluminum Sulphate Solutions at 225°C 34 - V - Page 4.1.6 Hydrolysis of Aluminum Sulphate Solutions at 250°C 36 4.2 The E f f e c t of Sulphuric Acid Concentration on Hydrolysis of Aluminum Sulphate Solutions 38 4.2.1 Hydrolysis of Aluminum Sulphate Solutions with I n i t i a l S0 3:A1 20 3 Ratio > 3.0 at 250°C. 38 4.2.2 Hydrolysis of Aluminum Sulphate Solutions with I n i t i a l S O ^ A l ^ Ratio > 3.0 at 225°C... 40 4.3 The Equilibrium Constants 42 4.4 Ternary Diagrams for the System A1 20 3-S0 3-H 20 . 51 4.5 Mechanism of the Hydrolysis of Aluminum Sulphate ... 55 4.6 The E f f e c t of A l k a l i Metal Sulphates on Hydrolysis of Aluminum Sulphate at 225°C 56 4.6.1 The E f f e c t of Lithium Sulphate on Hydrolysis of Aluminum Sulphate at 225°C 58 4.6.2 The E f f e c t of Sodium Sulphate on Hydrolysis of Aluminum Sulphate at 225°C 60 4.6.3 The E f f e c t of Potassium Sulphate on Hydrolysis of Aluminum Sulphate at 225°C 61 4.7 The E f f e c t of Divalent Metal Sulphates on Hydrolysis of Aluminum Sulphate Solutions at 225°C 65 4.7.1 The E f f e c t of Copper Sulphate on Hydrolysis of Aluminum Sulphate Solutions at 225°C 67 4.7.2 The E f f e c t of Ferrous Sulphate on Hydrolysis of Aluminum Sulphate Solutions at 225°C 69 - v i - Page 4.8 Appl i c a t i o n of the Hydrolysis Process 71 5. CONCLUSIONS . 74 APPENDIX I SOLUBILITY OF ALUMINUM SULPHATE 75 APPENDIX II ANALYSIS OF THE SOLID PHASE 76 APPENDIX III X-RAY DIFFRACTION PATTERNS 77 LITERATURE 81 - v i i - LIST OF TABLES Table Page 1 Hydrolysis of aluminum sulphate solutions at 125°C... 26 2 Hydrolysis of aluminum sulphate solutions at 150°C... 28 3 Hydrolysis of aluminum sulphate solutions at 175°C... 30 4 Hydrolysis of aluminum sulphate solutions at 200°C... 32 5 Hydrolysis of aluminum sulphate solutions at 225°C... 34 6 Hydrolysis of aluminum sulphate solutions at 250°C... 36 7 The effect of excess sulphuric acid on hydrolysis of aluminum sulphate at 250°C 40 8 The effect of excess sulphuric acid on hydrolysis of aluminum sulphate at 225°C 42 9 Equilibrium constant for the reaction HSO^ J H + + S u ^ T • 45 10 Hydrolysis constant of water 46 11 Equilibrium hydrolysis of normal aluminum sulphate in the temperature range 125-250°C 7̂ 12 Equilibrium constants for hydrolysis of aluminum sulphate solutions with excess sulphuric acid at 225°C and 250°C 47 13 Equilibrium constants for the hydrolysis reaction of aluminum sulphate 49 14 Solution.composition of 250°C 51 15 Solution composition at 225°C 52 I | | 16 Equilibrium constants for reaction Al + ^ 0 £ AKOH)^4" •+ H + 56 l - v i i i - Table Page 17 Hydrolysis of aluminum sulphate-lithium sulphate solution at 225°C 58 18 Hydrolysis of aluminum sulphate-sodium sulphate solution at 225°C 6 1 19 Hydrolysis of aluminum sulphate-potassium sulphate solution at 225°C 6 3 20 Ionic radii of the ions involved in the system 65 21 Hydrolysis of aluminum sulphate-copper sulphate solution at 225°C 6 7 22 Hydrolysis of aluminum sulphate-ferrous sulphate solution at 225°C 6 9 I-A Solubility of Al o(S0.) o in water 75 2 4 3 I-B Solubility of aluminum sulphate in aqueous solutions of sulphuric acid at 25°C 75 I-C Solubility of aluminum sulphate in aqueous 10% ̂ SO^. 75 II Analysis of the solid phases 76 III-A Diffraction pattern of the precipitate obtained at 150°C 77 III-B X-Ray diffraction pattern of the precipitate obtained at 175°C 77 III-C X-Ray diffraction pattern of the precipitate obtained at 200°C 78 III-D X-Ray diffraction pattern of the precipitate obtained at 225°C 78 III-E X-Ray diffraction pattern of the precipitate obtained at 250°C 79 - ix - Table Page III-F X-Ray diffraction pattern of the precipitate obtained from FeSO.-Al-CSO.K solution at 225°C 79 4 2 4 3 III-G X-Ray diffraction pattern of potassium alunite obtained at 225°C 80 II1-H X-Ray diffraction pattern of sodium alunite obtained at 225°C 80 III-I X-Ray diffraction pattern of lithium alunite obtained at 225°C 80 - X - LIST OF FIGURES Figure Page 1 Fe 20 3-S0 3-H 20 system at 200°C 4 2 Fe 20 3-S0 3-H 20 system at 140°C ' 5 .3 A1 20 3-S0 3-H 20 system at 220°C 7 4 Experimental Apparatus 13 5 YSI-602 Termistor C a l i b r a t i o n 14 6 Transducer C a l i b r a t i o n 1.5 7 Pressure Dependence on Temperature 16 8 Heating Time to Reach the Working Temperature 17 9 Hydrolysis of Aluminum Sulphate at 125°C 27 10 Hydrolysis of Aluminum Sulphate at 150°C 29 11 Hydrolysis of Aluminum Sulphate at 175°C 31 12 Hydrolysis of Aluminum Sulphate at 200°C 33 13 Hydrolysis of Aluminum Sulphate at 225°C 35 14 Hydrolysis of Aluminum Sulphate at 250°C 37 15 Hydrolysis of Acid Aluminum Sulphate at 250°C 39 16 Hydrolysis of Acid Aluminum Sulphate at 225°C 41 17 Phase Diagram of the A l ^ - S O ^ H 0 System 43 18 Equilibrium Hydrolysis 48 19 The Change of Equilibrium Constant with Temperature .. 50 20 A1 20 3-S0 3~H 20 System at 250°C 53 21 A1 20 3-S0 3-H 20 System at 225°C 54 22 Hydrolysis of Lithium-Aluminum Sulphate Solution at 225°C 59 23 Hydrolysis of Sodium-Aluminum Sulphate Solution at 225°C 62 - x i - Figure Page 24 Hydrolysis of Potassium-Aluminum Sulphate Solution at 225°C 64 25 Hydrolysis Yields Dependence on Ionic Radii 66 26 Hydrolysis of Copper-Aluminum Sulphate Solutions at 225°C 68 27 Hydrolysis of Iron-Aluminum Sulphate Solutions at 225°C 70 28 D.T.A. of 3Al203.4SO3.9H2O 72 1. INTRODUCTION The aluminum industry was the f i r s t to use a pressure hydro- m e t a l l u r g i c a l process (The Bayer process) on a large scale. Caustic soda solutions are used to dissolv e aluminum from bauxites and alumina i s separated. Even though caustic solutions are not very s e l e c t i v e and the Bayer process i s l i m i t e d to low s i l i c a ores, alumina i s produced commerically only by t-his process. This has imposed on the aluminum industry the task of transportation of bauxties over a long distance because the producing countries do not have bauxites of t h e i r own. The production of aluminum i n countries such as Canada and the United States i s today very s e n s i t i v e to the a v a i l a b i l i t y of bauxites from foreign countries. With a desire to recover aluminum from clay minerals, processes have been considered,from time to time, that are applicable to common North American clays for the domestic production of alumina. Such processes might also be useful i n the treatment of c e r t a i n mine waters from copper dump leaching, which contain dissolved aluminum sulphate, among other sulphate s a l t s . Any new process producing alumina must compete i n p r i c e with the Bayer process. This l i m i t s new leaching processes to cheap reagents. Sulphuric acid i s the favourite reagent considered f o r alternate - 2 - * alumina production processes and an acid process developed by C.S.I.R.O. has been found to produce aluminum sulphate solutions which can be hydrolysed to yield basic aluminum sulphate. Most of the work in hydrolysis has been done with basic aluminum sulphate solutions, or solutions containing a l k a l i metal sulphates and under "process conditions". Because of this there i s practically no data on equilibrium hydrolysis available in the literature. Some equilibrium hydrolysis work was carried out on the Fe^O^-SO^-H^O system. With some exceptions A^O^-SO^-H^O and Fe20.j-S03-H20 systems precipitate similar compounds during hydrolysis. The separation of basic aluminum sulphate salts from aluminum sulphate solutions is not well understood either from a kinetic or thermodynamic point of view. This work is concerned with hydrolysis reactions of aluminum sulphate solutions that occur in the temperature range 125-250°C, and attempts were made to study both precipitation rates and the f i n a l equilibria. Commonwealth Scientific and Industrial Research Organization - Melbourne - 3 - 2. A REVIEW OF THE LITERATURE It is generally known that above a temperature which is a characteristic of the metal ion involved, a sulphate salt becomes less soluble as the temperature i s further increased. E.T. Carlson and C.S. Simons"^ have reported the necessity of temperature increase from 200 to 275°C to achieve selective extraction of nickel and cobalt from laterite ore. At 200°C,even though nickel and cobalt sulphate solubility was high enough to give good recoveries, the selectivity was rather poor because of the high solubility of aluminum and iron sulphates. They noticed that the SO^tA^O^ ratio in the residue was about 1:1 and suggested that Al^O^ and SO^ exist in i t as a compound. 2.1 The Fe 20 3-S0 3-H 20 System At elevated temperatures f e r r i c sulphate solutions hydrolyze to 2 precipitate f e r r i c oxide or basic salts. Posnjak and Merwin have studied the Fe^^-SO^-H^O system at temperatures up to 200°C. At 200°C only three solid phases exist in equilibrium with various compositions of the liquid phase. ¥e^0^ is in equilibrium with dilute solutions of ferric sulphate. The stable phase in equilibrium with a somewhat higher ̂ concentration of sulphuric acid in solution is a basic salt of the composition Fe„0„.2S0_.Ho0. Finally, at high concentrations F e 2 ° 3 ~ S 0 3 " H 2 ° SYSTEM AT 2 0 0 C° - 5 - F e 2 0 3 F i q . 2 • F e 2 0 3 - S 0 3 - H 2 0 SYSTEM AT ! 4 0 C ° - 6 - of sulphuric acid i n solution the stable phase i s the normal s a l t , Fe 20 3.3S0 3 (Fig. 1). With decreasing temperature the isotherms become more complicated and at 140°C, there are f i v e s o l i d phases i n equilibrium with different compositions of the l i q u i d phase (Fig. 2). With further decrease i n temperature the equilibrium isotherms become s t i l l more complicated. At 130°C, f e r r i c oxide monohydrate replaces the anhydrous f e r r i c oxide, and at 110°C anhydrous f e r r i c sulphate disappears, and two acid s a l t s are stable ( F e ^ . 4S0 3 > 3H20 and F e ^ . 4S03.9H20) . 2.2 The A1 20 3-S0 3-H 20 System 3 P.T. Davey and T.R. Scott have studied the A^C^-SC^-H^O system under process conditions i n order to determine the optimum conditions for production of alumina. Their experiments approached equilibrium to varying degrees. Solutions with i n i t i a l S0 3:A1 20 3 r a t i o less than 3.0 and high i n aluminum content were hydrolyzed at temperatures of 100-220°C. In this temperature range the only stable phase observed, i n equilibrium with various l i q u i d phases, was basic aluminum sulphate of the nominal formula 3A1 20 3«4S0 3 >9H 20. At these temperatures, the precipitated phase was i n equilibrium with acid solutions of aluminum' sulphate. Only a limited number of experiments were performed for periods of time long enough to reach the equilibrium. The equilibrium saturation curve for this system at 220°C i s given i n Fig. 3. This saturation curve was obtained by hydrolyzing aluminum sulphate solutions with i n i t i a l S0„:A1„0„ r a t i o less than 3.0.  - 8 - 4 Basset and Goodwin have studied the system A l ^ - S C ^ - t ^ O at 25°C at t h i s temperature concentrated solutions of aluminum sulphate can be c r y s t a l l i z e d to y i e l d e i t h e r hydrated aluminum sulphate A ^ (SO^) ^. 1 6 ^ 0 or the basic s a l t A l 2 O 2 . 2 S O 2 . H H 2 O depending on SO^iA^O^ r a t i o of the i n i t i a l s o l u t i o n . These c r y s t a l l i n e materials tend to occlude mother- li q u o r and are r e a d i l y soluble i n water, which makes i t d i f f i c u l t to remove impurities by washing without r e d i s s o l v i n g a s u b s t a n t i a l amount of the s o l i d phase. 5-9 T.R. Scott et a l . have proposed an acid process for recovery of aluminum from ores low i n bauxite content and high i n s i l i c a . The stages of the new process are described as follows: a. Digestion Recycled l i q u o r containing aluminum sulphate i s used to recover extra aluminum from an ore at 180°C. b. Modification A liquor from the digestion stage i s treated with fresh bauxite to give a so l u t i o n of basic aluminum sulphate with SO^A^O^ r a t i o less than 3 .0. c. Reduction Modified l i q u o r i s treated with sulphur dioxide to reduce soluble i r o n to the ferrous state at 100°C. d. Hydrolysis Reduced l i q u o r i s hydrolyzed at 220°C i n the absence of oxygen to produce basic aluminum.sulphate. e. C a l c i n a t i o n Basic aluminum sulphate i s heated, pr e f e r r a b l y i n stages at 1200 °C-1300°C, to y i e l d the required a-alumina and a mixture of sulphurous gases for r e c y c l i n g . J.L. Henry, and G.B. King"^ have studied the system A^O^-SO^-E^O - 9 - at 60°C. At t h i s temperature two basic s a l t s were found to be i n equilibrium with solutions of aluminum sulphate having an SO^A^O^ r a t i o very close to 3.0. The s a l t Al^O^.2S0.J.HH^O i s i n equilibrium with solutions of pH about 2.5 and A^O^. SO^. 61^0 i s i n equilibrium with solutions of pH 2 . 5 - 3 . 2 0 . At very low concentrations of free sulphuric acid i n s o l u t i o n normal aluminum sulphate i s the stable phase at 60°C. R.A. Chaves, V.V. Kavelin and B.P. Sobolev"'"^ have studied the side reactions occurring i n the sulphuric acid process for extracting n i c k e l and cobalt from Cuban l a t e r i t e s and have revealed the presence of the following compounds: hematite a-Fe^O^, boehmite y-A^O^.H^O, aluminochromite FeCCrAl^O^, basic i r o n sulphate Fe^O^. 2 S O . J . H 2 O , basic aluminum sulphate 3 A I 2 O.J. ASO^. 9^0 and hydronioj a r o s i t e 3Fe203.4S0.j. 9H20. The larges t amount of basic aluminum sulphate i s formed i n the autoclave where the preheated pulp at 250°C mixes with concentrated sulphuric a c i d , i n d i c a t i n g very high reaction rate at t h i s temperature. Beside aluminum and f e r r i c sulphate s a l t s , other t r i v a l e n t and 1 2 quadrivalent metal sulphates can be hydrolyzed. At room temperature vi o l e t - c o l o u r e d chromium sulphate solutions are stable, but at high temperatures these are converted i n t o green complexes i n s o l u t i o n . When SO^iC^O^ r a t i o i n the i n i t i a l s o l u t i o n i s less than 3.0 hydrolyzed chromic oxide i s p r e c i p i t a t e d but only at temperatures higher than 180°C. Even at high temperatures solutions of lanthanum sulphate do not hydrolyze r e a d i l y below pH 7. - 10 - Vanadium i s almost completely p r e c i p i t a t e d even i n the presence of f r e e s u l p h u r i c a c i d to y i e l d ^O,.. In the absence of oxygen at 100°C, the s o l u t i o n s are s t a b l e , but p r e c i p i t a t e V^O^ at temperatures above 200°C. Scandium could be separated from rare earths provided that the s o l u b i l i t y product of the scandium compound was exceeded at the temperature and i n the s o l u t i o n s used. Indium can be separated from z i n c e l e c t r o l y s i s l i q u o r s under the same c o n d i t i o n s as f o r scandium. Titanium and zirconium sulphates hydrolyze r e a d i l y without recourse to autoclave c o n d i t i o n s . 2.3 The M ^ A ^ O -SOy-H^O System* Under otherwise the same c o n d i t i o n s , the presence of a l k a l i metal sulphates w i l l i n c r e a s e the h y d r o l y s i s y i e l d of a b a s i c s a l t from aluminum sulphate s o l u t i o n s . V.S. Sazhin, A.K. Z a p o l s k i i and N.N. 13 Zaklarova have stud i e d the i n f l u e n c e of ammonium, sodium, and potassium sulphates on h y d r o l y s i s of aluminum sulphate s o l u t i o n s . The degree of h y d r o l y s i s was increased i n the f o l l o w i n g s e r i e s : (NH^^SO^ < Na.SO. < K oS0.. 2 4 2 4 14 S. Br e t s z n a j d e r , J . Boczar, J . P i s k o r s k i and J . Porowski have studied the h y d r o l y s i s of aluminum sulphate s o l u t i o n s w i t h a d d i t i o n of sodium hydroxide at 179-285°C f o r 0-240 min. The p r e c i p i t a t e d s o l i d phase was found to be Na 20.3A1 20 3.4S0 3 >7H 20. Excess sodium hydroxide * M 20 = a l k a l i oxide; M = L i , Na, K, NH^ - 11 - greatly increased the y i e l d . Higher temperature and more d i l u t e solutions gave higher y i e l d s . There was l i t t l e change i n y i e l d for periods longer than 30 min. 15—18 A.K. Z a p o l s k i i et a l . studied the e f f e c t of potassium sulphate from 0-167.5 gr/£ K^SO^ on hydrolysis of aluminum sulphate s o l u t i o n containing 308 gr/£ A l ^ S O ^ at 170-250°C. The degree of hydrolysis sharply increased at a l l temperatures for K^SO^ concentration from 0-1 mole/1. Further increase i n K^SO^ concentration had l i t t l e influence on hydrolysis. In the absence of K^SO^ basic aluminum sulphates of constant composition (3kl^0^. ^SO^. 7^0) were formed. Potassium a l u n i t e K 2 [ A 1 6 ( S 0 4 ) 4 ( 0 H ) 1 2 ] ' > was p r e f e r e n t i a l l y formed at 230-250°C from solutions containing 1 mole of K^SO^, or more, per mole of A ^ C S O ^ ) ^ - At temperatures 170-190°C a les s basic s a l t S^SO^.3A1 20 3.4S0 39H 20 was observed. The system Na 20-A1 2Q 3-S0 3-H 20 was studied at 200°C under process conditions i n order to determine the conditions for recovery 19 of alumina from ores. The p r e c i p i t a t e d compound was found to have a formula very close to Na 20.3A1 20 3>4S0 3.6H 20. K 2 [ A 1 6 ( S 0 4 ) 4 ( 0 H ) 1 2 ] = K 20.3A1 20 3.4S0 3.6H 20. - 12 - 3. EXPERIMENTAL 3.1 E x p e r i m e n t a l T e c h n i q u e A s h a k i n g a u t o c l a v e ( F i g . 4) made o f z i r c o n i u m was used f o r a l l h y d r o l y s i s e x p e r i m e n t s . The a u t o c l a v e was p l a c e d i n an aluminum b l o c k c o n t a i n i n g f i v e h e a t e r s (100 W. each) t o p r o v i d e good h e a t d i s t r i b u t i o n . The t e m p e r a t u r e was c o n t r o l l e d by a 71 model " T h e r m i s - temp" t e m p e r a t u r e c o n t r o l l e r to within ± 6.1 °C. The t e m p e r a t u r e c o n t r o l l e r was used w i t h a YS1-602 s e n s i n g p r o be w i t h a range o f 80 t o 250°C, and was i n d e p e n d e n t l y c a l i b r a t e d a g a i n s t a p r e c i s i o n m ercury thermometer i n o i l f o r each t e m p e r a t u r e used. The c a l i b r a t i o n c u r v e i s shown i n F i g . 5. I n t h e low p r e s s u r e r e g i o n t h e p r e s s u r e i n t h e s y s t e m was m o n i t o r e d by a t r a n s d u c e r and r e c o r d e d by a "Heath" s e r v o - r e c o r d e r . F o r t h e h i g h p r e s s u r e r e g i o n , a p r e s s u r e gauge was u s e d . A c a l i b r a t i o n c u r v e f o r t h e t r a n s d u c e r i s shown i n F i g . 6. The v a r i a t i o n o f p r e s s u r e w i t h t e m p e r a t u r e i s g i v e n i n F i g . 7. The h e a t i n g t i m e v a r i e d w i t h t e m p e r a t u r e and t h i s v a r i a t i o n i s shown i n F i g . 8. F o r each r u n , t h e a u t o c l a v e was f i l l e d w i t h 75 ml o f s o l u t i o n l e a v i n g about 30 cc of gas volume i n t h e a u t o c l a v e . re corder tronsduc er aotoclov e J ? temperature controler I c ooIe r sampling tube Fig. 4- EXPERIMENTAL APAR AT US   - 16 - •00 150 200 250 C° Fjg. 8. HEATING TIME TO .REACH THE TEMPERATURE - 18 - Samples of hydrolyzed s o l u t i o n were taken from the cooling system and the sampling time x^as between 2 and 3 minutes depending on temperature. To make sure that the samples represented the so l u t i o n from the autoclave, the f i r s t 3 ml of s o l u t i o n from the sampling tube were discarded. Immediately a f t e r the sample was taken i t was f i l t e r e d and i t s pH measured. Most of the hydrolysis product was p r e c i p i t a t e d on the autoclave walls and remained i n s i d e a f t e r the l i q u i d was removed. The autoclave was then cooled with running water to room temperature and opened. The p r e c i p i t a t e was removed from the autoclave walls mechanically by shaking the autoclave with glass b a l l s and a few m i l l i l i t e r s of d i s t i l l e d water. Af t e r being f i l t e r e d and washed, the p r e c i p i t a t e was dried for 24 hrs at 105°C. After the s o l i d phase was removed the autoclave was washed with sodium hydroxide s o l u t i o n and then successively with d i l u t e sulphuric acid and water. To evaporate the re s i d u a l water, i t was then heated and again allowed to cool to room temperature. 3.2 Materials Used Aluminum sulphate, c r y s t a l , reagent, B&A, Al (S0 4) .18H20; M.W. 666.45 Maximum Limit of Impurities Insoluble 0.005% Free acid (H SO.) 0.20% Chloride (Cl) 0.002% Arsenic (As) 0.00005% - 19 - Heavy metals (as Pb) 0.001% Iron (Fe) 0.002% Substances not p r e c i p i t a t e d by NH^OH as sulphates 0.20% Lithium sulphate, granular reagent B & A Li oS0..H.O M.W. 127.96 2 4 2 Maximum Limit of Impurities Insoluble 0.010% Chloride (Cl) 0.002% Ni t r a t e (N0 3) 0.001% Heavy metal (as Pb) 0.0005% Iron (Fe) 0.0002% Potassium (K) 0.05% Sodium (Na) 0.10% Potassium sulphate, c r y s t a l , reagent B & A K.S0.. M.W. 174.27 2 4 Maximum Limit of Impurities Insoluble 0.005% Chloride (Cl) 0.001% Nitrogen Compounds (as N) 0.005% Arsenic (As) 0.00005% Calcium, Magnesium and R 0 3 p r e c i p i t a t e 0.020% Heavy metals (as Pb) 0.'0005% Iron (Fe) 0.0002% Sodium (Na) 0.005% - 20 - Sodium sulphate, anhydrous, granular reagent B & A Na„S0. M.W. 142.05 z 4 Maximum Limit of Impurities Insoluble 0.010% Loss on i g n i t i o n 0.50% Chloride (Cl) 0.002% Nitrogen compounds as (N) 0.0005% Arsenic (As) 0.0001% Calcium, magnesium and R 0 3 p r e c i p i t a t e 0.010% Heavy metals (as Pb) 0.0005% Iron (Fe) 0.0005% Ferrous sulphate, c r y s t a l , reagent B & A FeSO..7Ho0 M.W. 278.03 4 2 Maximum Limit of Impurities Insoluble 0.005% Chloride (Cl) 0.001% Phosphate (PO^) 0.001% Copper (Cu) 0.005% Ferric- Iron (Fe'' ') 0.01% Manganese (Mn) 0.05% Substances not pr e c i p i t a t e d by NH.0H 4 0.050% Zinc (Zn) 0.005% - 21 - Cupric sulphate, granular c r y s t a l s , B.D.H. reagent CuSO..5H„0 M.W. 249.69 4 2 A l k a l i e s (suphated) not more than 0.5% Chloride (Cl) not more than 0.005% Iron (Fe) not more than 0.08% Aluminum "AnalaR" B.D.H. A l M.W. 26.98 Acid ins o l u b l e matter passes test Iron (Fe) 0.004% Copper (Cu) 0.005% S i l i c o n (Si) 0.01% Tota l nitrogen (N) 0.002% 3.3 Preparation of Solutions f o r Hydrolysis The aluminum sulphate s o l u t i o n was prepared by d i s s o l v i n g hydrated aluminum sulphate Al^(SO^)^.I8H2O i n d i s t i l l e d water. A s o l u t i o n of approximately 6.0 gr/£ of aluminum was made having 302^1^02 r a t i o of about 3.0, and a pH of 3.10. The s o l u t i o n was f i l t e r e d before i t was used f o r hy d r o l y s i s . Solutions of the lowest pH (pH = 0.70) used i n t h i s work were prepared by d i s s o l v i n g A^C^SO^) ̂ . I8H2O i n d i l u t e sulphuric acid. Any so l u t i o n between these two l i m i t s (pH = 0.70 and pH = 3.10) was prepared by mixing selected ratios of the two. Solutions containing a l k a l i n e metal sulphates and divalent metal sulphate s a l t s were prepared by mixing the so l u t i o n of the corresponding s a l t with aluminum sulphate s o l u t i o n . Solutions of normal aluminum sulphate were discarded a f t e r 7 days of - 22 - storage. Aluminum sulphate solutions containing excess sulphuric acid were kept for a few weeks. Solutions containing a l k a l i n e metal sulphates were prepared just before using them. I n i t i a l and f i n a l solutions were analyzed f o r aluminum, sulphate, and added metal ions. The pH of the solutions was measured before and a f t e r hydrolysis. The sulphate content was determined by p r e c i p i t a t i o n as BaSO^. 20 * Aluminum was determined by complexing with EDTA and then back t i t r a t i n g the excess EDTA with zinc sulphate using dithizone as an i n d i c a t o r and 1 M CH oC00H-l M CHoC00NH. s o l u t i o n as a buffer. 3 3 4 Standard aluminum sulphate s o l u t i o n was made from "Analar" aluminum by d i s s o l v i n g i t i n sulphuric acid. 3.4 Chemical Analysis * disodium s a l t of ethylene diamine Na-OOCH„C CHoC00H H00CHoC CH^COONa diphenylthiocarbazone .H H I S II H H Divalent i r o n i n the aluminum sulphate s o l u t i o n was analyzed by I | | | i t i t r a t i n g Fe with CeCHSO^)^ to oxidize i t to Fe using 1,10- orthophenanthroline ferrous sulphate (Ferroin) as an i n d i c a t o r . In t h i s case the aluminum content of the s o l u t i o n was determined from the differe n c e i n EDTA used to t i t r a t e both i r o n and aluminum. Copper and aluminum sulphate s o l u t i o n was analyzed by t i t r a t i n g l i b e r a t e d 1^ with ^2820^ to determine the copper content. The aluminum content was determined from the di f f e r e n c e i n EDTA used to t i t r a t e both copper and aluminum. The p r e c i p i t a t e d phase was also analyzed for aluminum and sulphate. The s o l i d phase i s soluble i n strong sulphuric acid and strong sodium hydroxide, and therefore the same methods as for the solutions were used for analysis. Water content was taken as the differe n c e between the t o t a l weight and the aluminum and sulphate content. L i , Na and K content was determined by the "EEL" flame photometer. ,c—c c — C.> // \ // \ c c — C \ / \ ' C = N N=C C [ F e ( C 1 2 H g N 2 ) 3 ] ++ cation; red colour 41 + 2Cu ++ 2CuI + I 2 I 2 21 - 24 - 4. RESULTS AND DISCUSSION The in t e n t i o n of the work presented i n t h i s thesis was to study the change i n equilibrium hydrolysis of aluminum sulphate solutions with temperature, sulphuric acid concentration and other cations. I t i s generally known that the amount of p r e c i p i t a t e d phase i s determined by the concentration of aluminum and sulphate i n the i n i t i a l s o l u t i o n 3 for a given temperature. In the present work the concentration of aluminum i n the i n i t i a l s o l u t i o n was not varied i n order to make i t possible to e s t a b l i s h the e f f e c t of other f a c t o r s . Results obtained are discussed under the following c l a s s i f i c a t i o n s : 4.1 The e f f e c t of temperature on hydrolysis of aluminum sulphate solutions. 4.2 The e f f e c t of sulphuric acid concentration on hydrolysis of aluminum sulphate solutions. 4.3 The equilibrium constants 4.4 Ternary diagrams for the A^O^-SO^-f^O system. 4.5 Mechanism of the hydrolysis of aluminum sulphate. 4.6 The e f f e c t of a l k a l i metal sulphates on hydrolysis of aluminum sulphate. 4.7 The e f f e c t of divalent metal sulphates on hydrolysis of aluminum sulphate solutions. 4.8 Application of the hydrolysis process. - 25 - 4.1 The E f f e c t of Temperature on Hydrolysis of Aluminum Sulphate Solutions At ordinary temperatures, concentrated solutions of aluminum sulphate w i l l only c r y s t a l l i z e a basic s a l t i f the SO^iA^O^ r a t i o i n the i n i t i a l s o l u t i o n i s le s s than 3 .0 . 14 S. Bretsznajder has obtained normal aluminum sulphate from clay (calcined at 770-820°C for 1 hr)by leaching i t with d i l u t e sulphuric acid for 10-12 hrs at 80°C and cooling the f i l t r a t e to c r y s t a l l i z e 3 the normal aluminum sulphate hydrate. I t was reported by T.R. Scott that at temperatures higher than 110°C hydrolysis of aluminum sulphate solutions with i n i t i a l SO^A^O^ r a t i o less than 3.0 p r e c i p i t a t e s c r y s t a l l i n e material "basic aluminum sulphate" (B.A.S.). This compound i s r e l a t e d to the mineral a l u n i t e , having a formula expressed as 3AI2O2.4SO2.9H2O with a t h e o r e t i c a l composition as follows: ^L2°3 ~ 38.8%; S 0 3 = 40.6%; HyO = 20.6%. The temperature e f f e c t on hydrolysis was determined by hydrolyzing d i l u t e solutions of aluminum sulphate with i n i t i a l S 02 :A l202 r a t i o of about 3.0 i n the temperature range 125-250°C. Retention time i n these experiments was as long as 16 hrs. The equilibrium was determined from the hydrolysis curves when there was no further change i n the aluminum concentration with prolonged time. 4.1.1 Hydrolysis of Aluminum Sulphate Solutions at 125°C Hydrolysis experiments at t h i s temperature were done with u n f i l t e r e d solutions of pH = 3 .10. Results obtained were not reproducible. When the same solutions were f i l t e r e d , and then hydrolyzed, r e s u l t s were more uniform. Hydrolysis y i e l d s at equal time i n t e r v a l s were less for - 26 - f i l t e r e d s o lutions, and, i n both cases, not s u f f i c i e n t for chemical analysis. For times which were long enough to e s t a b l i s h equilibrium, hydrolysis y i e l d s were almost the same for both the f i l t e r e d and the u n f i l t e r e d s o l u t i o n s , thus suggesting that a nucleation process plays an important r o l e i n -the k i n e t i c s of the reaction at t h i s temperature. Results of the analysis of hydrolyzed solutions are shown i n Table 1 and F i g . 9. '» Table 1. Hydrolysis of aluminum sulphate solutions at 125 C. Time at 125°C hrs Al i n s o l . gr/Ji SO. i n s o l . 4 gr/i> A l prec. gr/£ SO^ prec. pH S0 3/A1 20 3 i n the pre- 1.00 6.09 - 0.01 . - 2.98 - 3.00 5.89 - 0.21 - 2.53 - 5.00 5.80 31.70 0.30 0.70 2.24 1.31 6.00 5.69 31.33 0.41 1.07 1.80 1.47 7.00 5.68 31.34 0.42 1.06 1.80 1.41 12.50 5.76 31.30 0.34 1.10 1.78 1.82 13.75 5.65 31.38 0.45 1.02 1.79 1.27 16.50 5.68 31.38 0.42 1.02 1.78 1.36 Sta r t i n g s o l u t i o n : 32.40 6.10 + + 0.3 gr/£ 0.05 gr/£ S0. = A pH = 3.10  - 28 - 4.1.2 Hydrolysis of Aluminum Sulphate Solutions at 150°C F i l t e r e d solutions of aluminum sulphate with i n i t i a l SO^rA^O^ r a t i o of about 3.0 were hydrolyzed at 150°C for d i f f e r e n t times. Results obtained are presented i n Table 2 and F i g . 10. From F i g . 10 equilibrium seems to be reached i n 6.0 hrs and the SO^tA^O^ r a t i o of the p r e c i p i t a t e d compound remains unchanged with prolonged times. The amount of p r e c i p i t a t e d compound was not s u f f i c i e n t f o r chemical analysis but the X-ray d i f f r a c t i o n pattern indicated that basic aluminum sulphate existed at t h i s temperature (see Appendix III-A). Table 2. Hydrolysis of aluminum sulphate solutions at 150°C* Time at 150°C A l i n s o l . SO, i n s o l . 4 A l prec. SO. prec. 4 PH s o 3 :A1 20 3 hrs gr/£ gr/£ gr/£ i n the prec. 1.00 5.28 30.44 0.82 1.96 1.67 1.34 2.00 5.07 29.96 1.03 2.44 1.55 1.33 3.00 4.84 - 1.26 - 1.45 - 4.00 4.76 - 1.34 - 1.45 - 6.00 4.64 28.65 1.46 3.75 1.40 1.44 10.00 4.64 28.60 1.46 3.80 1.38 1.46 16.00 4.65 28.64 1.45 3.46 1.44 1.34 Starting s o l u t i o n : 32.40 gr/2, SO 6.10 gr/£ A l pH = 3.10  - 30 - 4.1.3 Hydrolysis of Aluminum Sulphate Solutions at 175°C Filtered solutions of aluminum sulphate with i n i t i a l SO^iA^O^ ratio of about 3.0 were hydrolyzed at 175°C and results obtained are presented in Table 3 and Fig. 11. The precipitated phase has a ratio of SOyA^O^ of about 1.32 (calculated from Fig. 11) corresponding to the ratio of basic aluminum sulphate. Analysis of the solid phase confirmed the presence of the basic aluminum sulphate (see Appendix II, for Chem. Anal, and Appendix III-B for X-ray d i f f . patterns.). Table 3. Hydrolysis of aluminum sulphate solutions at 175 C Time at Al in sol. SO in sol. Al prec. SO prec. pH SO :A190» 175°C 4 * hrs. gr/Jl gr/>! gr/£ gr/£ in the prec. 1.00 4.45 28.53 1.54 3.87 1.30 1.32 2.00 3.96 27.26 2.14 5.14 1.27 1.35 4.00 3.68 26.76 2.42 5.64 1.18 1.31 6.00 3.56 26.39 2.54 6.01 1.15 1:33 10.00 3.55 26.40 2.55 6.00 1.15 1.32 16.30 3.50 26.40 2.60 6.00 1.15 1.30 * Starting solution: 32.40 gr/£ S0 4 6.10 gr/£ Al pH = 3.10 J ' " 1 - J 1 I > I ! * ' ' ' ' I L _ 0 • 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 Fig II. HYDROLYSIS OFALUMINIUM SULPHATE AT 175 C° t imeinhrs . - 32 - 4.1.4 Hydrolysis of Aluminum Sulphate Solutions at 200°C Solutions with i n t i a l SO^ikl^O^ r a t i o of about 3.0 were hydrolyzed at 200°C for as long as 18 hrs. Results obtained are shown i n Table 4 and F i g . 12. The equilibrium hydrolysis y i e l d i s higher than at 175°C. The SO^iAl^O^ r a t i o of the p r e c i p i t a t e d compound calculated from F i g . 12 i s about 1.31. A n a l y s i s of s o l i d phase shows that the p r e c i p i t a t e d compound i s the basic aluminum sulphate (see Appendix II for Chem. anal, and Appendix III-C for X-ray d i f f . pattern). Table 4. Hydrolysis of aluminum sulphate solutions at 200°C * Time at 200°C hrs. A l i n s o l . gr/2, SO, i n s o l . 4 gr/£ A l prec. gr/£ SO, prec 4 gr/£ pH S0 3:A1 20 3 i n the prec. 1.00 3.30 25.78 2.80 6.62 1.07 1.33 2.00 3.01 24.98 3.09 7.42 1.03 1.35 4.00 2.71 24.32 3.39 8.08 0.99 1.34 6.00 2.51 23.91 3.59 8.49 0.98 1.33 6.45 2.51 23.89 3.59 8.51 0.98 1.33 18.00 2.40 23.65 3.70 8.75 0.98 1.33 S t a r t i n g s o l u t i o n : 32.40 gr/SL S0~ 6.10 gr/£ A l pH = 3.10 0 I 2 3 4 5 6 7 8 9 10 II Fig.12. HYDROLYSIS OF ALUMINIUM SULPHATE AT 2 0 0 C 12 13 14 15 ° timsinhrs. 16 - 34 - 4.1.5 Hydrolysis of Aluminum Sulphate Solutions at 225°C F i l t e r e d solutions of aluminum sulphate with i n i t i a l SO^iA^O^ r a t i o of about 3.0 were hydrolyzed at 225°C and r e s u l t s obtained are shown i n Table 5 and F i g . 13. Hydrolysis y i e l d s are very high. The p r e c i p i t a t e d phase has an S O ^ A l , ^ r a t i o 1.33 (calculated from F i g . 13). Analysis of the s o l i d phase showed the composition of basic aluminum sulphate (see Appendix II for Chem. anal, and Appendix III-D for X-ray d i f f . pattern). Table 5. Hydrolysis of aluminum sulphate solutions at 225°C Time at 225°C hrs. A l i n s o l . gr/l SO. i n s o l . 4 gr/H A l prec. gr/n SO, prec. 4 gr/X, pH SO i n 3 : A 1 2 ° 3 the prec. 1.00 1.89 22.45 4.21 9.95 0.89 1.33 1.00 1.88 22.35 4.22 10.05 0.92 1.34 2.00 1.80 22.31 4.30 10.09 0.92 1.32 2.00 1.73 21.55 4.37 10.85 0.89 1.39 3.00 1.65 21.96 4.45 10.44 0.88 1.32 5.00 1.58 21.40 4.52 11.00 0.89 1.37 8.40 1.44 21.36 4.66 11.04 0.85 1.33 16.00 1.48 21.63 4.54 10.77 0.85 1.33 * Starting s o l u t i o n : 32.40 gr/j> S0 4 6.10 gr/Z A l pH = 3.10 Fig. 13- HYDROLYSIS OF ALUMINIUM SULPHATE AT 225 C* timemhrs - 36 - 4.1.6 Hydrolysis of Aluminum Sulphate Solutions at 250°C F i l t e r e d solutions of aluminum sulphate were hydrolyzed at 250°C and i t was found that the amount of p r e c i p i t a t e d basic aluminum sulphate on heating the autoclave to t h i s temperature was comparable to the amount of basic s a l t p r e c i p i t a t e d i n 16 hrs at 225°C. This suggests that the reaction rate above 225°C must be very high. Results obtained at th i s temperature are shown i n Table 6 and F i g . 14. The SO^A^O^ r a t i o of the p r e c i p i t a t e d compound i s 1.33. Analysis of the s o l i d corresponds to the basic aluminum sulphate (see Appendix II for Chem. anal, and Appendix III-E for X-ray d i f f . pattern). Table 6. Hydrolysis of aluminum sulphate at 250°C* Time at 250°C hrs. A l i n s o l . gr/£ SO, i n s o l . 4 gr/£ A l prec. gr/£ SO^ prec. gr/£ PH S0 3:A1 20 3 i n the prec. 0.00 1.52 21.57 4.58 10.83 0.85 1.33 1.00 0.97 20.27 5.13 12.13 0.80 1.33 3.30 0.88 20.27 5.22 12.13 0.80 1.31 6.30 0.91 20.13 5.19 12.27 0.79 1.33 * Starting s o l u t i o n : 32.40 gr/2, SO 6.10 gr/£ A l pH = 3.10 o ca < 6.0 J 0 I v o \ ll 5.0 1 4.0 3.0 H 2.0 H I.OJ \ \ O A l Q S0| V PH PH 3.0 o 1-30 2.0 4 -28 - 26 24 1.0 -I o Co 22 h20 18 16 1 1 ' 1 • 1 1 L 1 1 I I I • • 3 1 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 Fig. 14. HYDROLYSIS OF ALUMINIUM SULPHATE AT 250C° timeinhrs. - 38 - 4.2 The E f f e c t of Sulphuric Acid Concentration on Hydrolysis of Aluminum Sulphate Solutions Most of the work on hydrolysis of aluminum sulphate was done on solutions having i n i t i a l SO^Al^O^ r a t i o l e s s than 3.0. The reason for t h i s was to obtain high y i e l d s i n a short time and at low tempera- tures. If hydrolysis i s to be used as a p u r i f i c a t i o n process for leach solutions higher concentrations of sulphuric acid would be expected. In the work presented i n t h i s thesis a s e r i e s of experiments with solutions containing about 6.0 gr/£ aluminum and d i f f e r e n t concentrations of sulphuric acid was done. Hydrolysis curves i n the previous part were used to determine the retention time for the two temperatures 225°C and 250°C. These two temperatures were chosen because of the high y i e l d s at short times with solutions having an SO^rA^O^ r a t i o of about 3.0. 4.2.1 Hydrolysis of Aluminum Sulphate Solutions with I n i t i a l S0 3:A1 20 3 Ratio > 3.0 at 250°C F i l t e r e d solutions with excess sulphuric acid were hydrolyzed at 250°C for 4 hrs. The highest S0 3:A1 20 3 r a t i o used i n t h i s set of experiments was about 4.63. Results obtained are shown i n Table 7 and Fig . 15. As the concentration of excess acid i n the i n i t i a l s o l u t i o n was increased the amount of p r e c i p i t a t e d b a s ic aluminum sulphate decreased. i ! - 39 - ~ ~ i r ~ 22 24 —T" 28 n 1 1 1 1 r 30 32 34 36 38 26 '0 o o 0 v SÔ jn sol. gr/1. O Al 0 S ° 4 VPH IO 2.0 — L — —r- 1.0 3.0 4.0 AI in sol. gr/1. + Fig.15. HYDROLYSIS OF ACID ALUMINIUM SULPHATE AT2 50C * 2.0 finalpH - 40 Table.7. The e f f e c t of excess sulphuric acid on hydrolysis of aluminum s ulphate at 250°C. pH SOT i n i t . 4 A l i n s o l . SO, i n s o l . 4 A l prec. SO^ prec. pH i n i t i a l gr/£ gr/Jl gr/£ gr/£ gr/A f i n a l 0.70 50.08 3.31 43.43 2.79 6.65 0.56 1.00 44.26 2.17 34.97 3.93 9.29 0.60 1.50 37.05 1.25 25.45 4.85 11.60 0.70 2.00 34.08 1.00 22.05 5.10 12.03 0.76 2.50 33.21 0.98 21.13 5.12 12.08 0.78 3.10 32.40 0.91 20.13 5.19 12.27 0.80 4.2.2 Hydrolysis of Aluminum Sulphate Solutions with I n i t i a l S0 3:A1 0 3 Ratio > 3.0 at 225°C F i l t e r e d solutions of aluminum sulphate were hydrolyzed at 225°C for 6 hrs. I n i t i a l solutions used i n this set 0 f experiments were the same as solutions used at 250°C. Hydrolysis y i e l d s increased with i^preasing pH of the i n i t i a l s o l u t i o n . Results obtained from t h i s set of experiments are presented i n Table 8 and F i g . 16. - 41 - X o. 3.0 H 22 ~i— 26 30 I i p 3 4 — r — 3 8 i i 4 2 S O 4 in soLgr/l. 1.0 - 4 2 0 3.0 • 4.0 A l in sol.gr/1. f- ,-. . ~ IX) 2 0 pH f i n a l Fig. 16. HYDROLYSIS OF ACID ALUMINIUM SULPHATE AT 2 25C° - 42 - Table 8. The e f f e c t of excess sulphuric acid on hydrolysis of aluminum sulphate at 225°C. pH SO. i n i t . 4 A l i n s o l . SO. i n s o l . 4 A l prec. SO^ prec. pH i n i t i a l gr/£ gr/£ gr/a gr/A gr/£ f i n a l 0.70 50.08 4.94 47.04 1.16 3.04 0.65 1.00 44.26 3.62 38.35 2.48 5.91 0.70 1.50 37.05 2.33 28.37 3.77 8.68 0.78 2.00 34.08 1.87 24.10 4.23 9.98 0.83 2.50 33.21 1.63 22.60 4.47 10.61 0.84 3.10 32.40 1.50 21.52 4.60 10.88 0.85 4.3 The Equilibrium Constants Ap p l i c a t i o n of the phase r u l e to the system Al^O^-SO^-H^O requires that at constant temperature, a s o l u t i o n composition e x i s t s which i s invariant when i n equilibrium with two s o l i d phases, e.g. A^O^nR^O and 3A1 20 3.4S0 3.9H 0 (B.A.S. - the most basic s a l t ) . If the s t a r t i n g s o l u t i o n has a composition below t h i s point, the f i r s t phase to p r e c i p i t a t e i s always the oxide, but the s o l u t i o n composition w i l l change i n the d i r e c t i o n of the invar i a n t point u n t i l the basic s a l t also p r e c i p i t a t e s . The l o c a t i o n of the invar i a n t point P (Fig. 17) w i l l change with the temperature. Fig.I 7 . PHASE DIAGRAM OF THE A I 2 O 3 - S O 3 - H 2 O SYSTEM - 44 - If the solution composition i s below the invariant point P at the temperature T and above the invariant point at any temperature higher than T, then the oxide w i l l precipitate i n the early stages of heating and at later stages the basic salt w i l l precipitate while the oxide w i l l tend to redissolve. Eventually the oxide may completely disappear i f the composition of the solution i s such that i t is above the invatiant point. In the work of T.R. Scott, a second phase (A^O^m^CO was not observed because the hudrolysis was carried out at high concentrations of aluminum in solutions and i t can only be expected to precipitate from dilute solutions of basic aluminum sulphate. The X-ray diffraction patterns did not show the presence of A^O^ in the solid phase and therefore i t can be concluded that the solution composition is above the invariant point where only one phase exists in equilibrium with the liquid phase. The X-ray diffraction pattern of the precipitate obtained at 125°C, was very poor so that no conclusion about the presence of aluminum oxide could be made. If there i s only one solid phase in equilibrium with liquid phase at a l l temperatures from 125°C up to 250°C, then the same hydrolysis reaction should occur. For such a reaction, the slope of a plot of log [equilibrium constant] vs. 1/T°K should be linear. 21 R.G. Robins has reported values for the second dissociation constant of sulphuric acid up to about 200°C which are in good agreement with the values calculated from the correspondence principle. Values of these constants, are given i n Table 9. The presence of aluminum sulphate w i l l shift the equilibrium to the l e f t i n the reaction 4SC>4" J H + + SO^". Therefore only HSO^ ions w i l l exist in solution. Assuming that aluminum sulphate is completely dissociated, and that only HSO, ion can exist - 45 - - -»• + Table 9. Equilibrium constants f o r the reaction HSO, H + SO. 4 4 T°C 125 150 175 200 225 250 log K -3.53 -3.89 -4.40 -4.72 -5.95 -6.05 i n s o l u t i o n i n the temperature range .125-250°C the hydrolysis reaction can be written as: 6A1 4 4 4" + 4HS0. + 14KL0 — — 3Al o0„.4S0 o.9H o0 + 14H + (1) 4 2 — 2 3 3 2 The equilibrium constant f o r t h i s reaction can be expressed as: K = ( 2 ) [ A 1 W ] D [ H S 0 4 " ] ^ An expression for the v a r i a t i o n of the equilibrium constant K with temperature i s derived by combining -AG0 = RT in K (3) and 9T ^ T ; J P T2 K ' Therefore ,9 in K. = d&n K = AH°_ ,,-\ 9T ;P dT 2 U J K l assuming thatAH° i s v i r t u a l l y independent of temperature. Equation (5) can be also written as - 46 - din K AH° . . d l / T = R { b ) Thus, i f £n K i s p l o t t e d against 1/T the slope of the curve at any point i s equal to -AH°/R. From the expression for the equilibrium constant (2) i t follows that - l o g K = 14pH + 6 log [ A l + + + ] + 4 log [HS0 4~] (7) The pH of the hydrolyzed solutions was measured at room temperature and as such i t i s d i f f e r e n t from the pH at which the r e a c t i o n occurs. The change i n the d i s s o c i a t i o n constant of water (Table 10) i s such that the pH of the s o l u t i o n should decrease with increasing temperature. Table 10. Hydrolysis constant of water T C° 25 60 100 150 200 250 300 350 -log K 14.0 13.05 12.21 11.65 11.30 11.18 11.19 11.33 True pH value at any temperature can be defined thermodynamically but i t cannot be e a s i l y measured. The pH change which occurs i n heating or cooling an e l e c t r o l y t e s o l u t i o n i s due to the hydrolysis and change i n a c t i v i t y of a l l ions i n s o l u t i o n . Assuming that the pH of hydrolyzed solutions remains unchanged with temperature the equilibrium constant for the hydrolysis reaction can be calculated at each temperature. - 47 - Results of the equilibrium hydrolysis of normal aluminum sulphate solutions i n the temperature range 125-250°C are presented i n Table 11 and F i g . 18. Results i n Table 7 and Table 8 were also used to c a l c u l a t e the equilibrium constants for hydrolysis of aluminum sulphate solutions with excess sulphuric acid. Calculated values of log K are presented i n Table 12. Table 11. Equilibrium hydrolysis of normal aluminum sulphate i n the temperature range 125-250°C. Temp. Aluminum i n s o l . Sulphate i n s o l . pH °C gr/a gr/Ji 125 5.68 31.35 1.78 150 4.65 28.65 1.42 175 3.50 26.40 1.15 200 2.45 23.80 0.98 225 1.46 21.60 0.85 250 0.90 20.20 0.79 Table 12. Values of log K for the hydrolysis of aluminum sulphate solutions with excess sulphuric acid at 225°C and 250°C I n i t . pH 0.70 1.00 1.50 2.00 2.50 3.10 log K 2 2 5 -3.43 -2.96 -2.42 -2.16 -1.93 -1.77 l o g K250 -1.00 0.00 0.513 0.503 0.374 0.35 - 48 - - 49 - Mean equilibrium constants f o r 225°C and 250°C were calculated from Table 12 and were found to be: log K 2 2 5 < > c = -2.44 K = 3.62 x 10 log K 2 5 0 o c = 0.302 K = 2.0 Table 13 presents the equilibrium constants f o r hydrolysis reaction of aluminum sulphate i n the temperature range 125-250°C. Table 13. Equilibrium constants f o r the hydrolysis reaction of aluminum sulphate 1/T x 10 4 log K 1/T°K T°C 25.10 -18.92 1.2 x 10" 1 9 125 23.60 -13.20 6.3 x I O - 1 4 150 22.30 - 8.63 2.3 x 10" 9 175 21.10 - 5.05 8.9 x I O - 6 200 20.05 - 2.44 3.6 x 10~ 3 225 19.10 +• 0.302 2.0 . 250 Since the plot of log K against 1/T gives a st r a i g h t l i n e (Fig. 19) i t i s most probable that there i s only one s o l i d phase i n equilibrium with the l i q u i d phase. Therefore the s o l u t i o n composition i s above the invariant point P (Fig. 17) at a l l temperatures. ,9 ~^~0 3 Zi ?i 24 . ^ 5 Fig. 19.THE CHANGE OF EQUILIBRIUM CONSTANT WITH TEMPERATURE - 51 - 4.4 Ternary Diagrams f o r the System Al^O^-SOy-H^O Some data i s a v a i l a b l e on the hydrolysis of basic aluminum sulphate 3 at temperatures up to 220°C, but no useful data i s a v a i l a b l e on hydrolysis of acid aluminum sulphate solutions. Experimental data from Section 4.2 was used to c a l c u l a t e the equilibrium l i q u i d composi- t i o n i n acid aluminum sulphate solutions at 225 and 250°C. Ternary diagrams were constructed for the water corner of the A1 20 3"S0 3-H 20 system at 225 and 250°C. Tables 14 and 15 show the composition of the i n i t i a l and f i n a l solutions at 250 and 225°C. Table 14. Solution composition at 250°C Sta r t i n g l i q . comp. wt % E q u i l . l i q . comp. wt % A1 20 3 so 3 H 20 A1 20 3 so 3 H 20 . 1 1.12 4.10 94.78 0.51 3.52 95.98 2 1.12 3.59 95.29 0.40 2.84 96.76 3 1.12 3.00 95.88 0.23 2.07 97.70 4 1.12 2.77 96.11 0.184 1.79 98.03 5 1.12 2.69 96.20 0.18 1.72 98.10 6 1.12 2.64 96.24 0.168 1.63 98.20 - 52 - Table 15. Solution composition at 225°C Starting l i q . comp. wt % E q u i l . l i q . comp. wt % A1 20 3 so 3 H 20 A 1 2 0 3 so 3 HgO 1 1.12 4.10 94.78 0.91 3.81 95.28 2 1.12 3.59 95.29 0.66 3.11 96.23 3 1.12 3.00 95.88 0.43 2.31 97.26 4 1.12 2.77 96.11 0.345 1.96 97.69 5 1.12 2.69 96.20 0.30 1.84 97.86 6 1.12 2.64 96.24 0.276 1.75 97.97 The data from Tables 14 and 15 are presented g r a p h i c a l l y i n F i g . 20 and 21. The i n t e r s e c t i o n of the i n i t i a l and f i n a l l i q u i d composition l i n e s suggests the p o s s i b i l i t y of the presence of a second phase i n the system at 250 and 225°C. The second phase i s probably a normal aluminum sulphate but i t could not be detected i n any of the experiments from the X-ray d i f f r a c t i o n patterns of the s o l i d . Although a number of experiments with solutions of the composition corresponding to the points A^ and A 2 i n F i g . 20 and 21 were done, the presence of a second phase could not be unambiguously proven. Acid aluminum sulphate solutions (compositions A^ and A 2 - Figs. 20 and 21) were hydrolysed f o r predetermined times, and s o l u t i o n samples were taken. The autoclave and the remaining material was cooled to room temperature, and the r e s u l t i n g s o l u t i o n was analyzed. From the difference i n s o l u t i o n compositions, the S0 3:A1 20 3 r a t i o of the re-dissolved p r e c i p i t a t e was calculated. It was - 53 - - 54 - - 55 - found to be slightly increased but i t was far from the SÔ .-AĴ Ô ratio of a normal salt. 4.5 Mechanism of the Hydrolysis of Aluminum Sulphate In aqueous solutions aluminum sulphate i s at least partially dissociated into i t s constituents. Al 2(S0 4) 3(aq) ^ 2A1 + 3S04 (8) The Al ion in solution w i l l react with ^ 0 to give: Al"1"1"1" + H20 * AKOH)"1^ + H + (9) The concentration of A1(0H) in solution w i l l depend on the concentra- -H-tion of Al and H in the i n t i a l solution. A1(0H) cation in solution reacts with HS04 ion to give the basic aluminum sulphate: eAKOH)"1"1" + 4HS04" + 8H20 —»- 3A1203.4S03.9H20 + 8H + (10) 22 The equilibrium.constant for reaction (9) was reported by Helgeson -4. 75 and i t i s 10 * for dilute solutions. Because of the change i n the dissociation constant of water with temperature this constant w i l l change, and become more and more positive as the temperature is increased. Helgeson gives the equilibrium constant values for the reaction: i A1(0H) >- Al + OH (11) - 56 - Reaction (9) i s simply a combination of reaction (11) and H 20 - — v H + + 0H~ (12) Knowing the equilibrium constants at temperatures up to 300°C for reactions (11) and (12) the equilibrium constant for reaction (9) can be calculated from the r e l a t i o n : log K 9 = log K 1 2 - log K n Calculated values f o r are given i n Table 16. I | | Table 16. Equilibrium constants f o r reaction A l + R^O «- A1(0H) + + + H + Temp °C 25 50 60 100 150 200 . 250 300 log K -4.75 -3.87 -3.53 -2.26 -0.84 +0.63 +1.97 +3.31 P r e c i p i t a t i o n reaction (10) i s a fast reaction as can be seen from the p r e c i p i t a t i o n curves at high temperatures where i t reaches equilibrium during the heating period. 4.6 The E f f e c t of A l k a l i Metal Sulphates on Hydrolysis of Aluminum Sulphate at 225°C When sulphates of L i , Na or K are present i n aluminum sulphate solutions during h y d r o l y s i s , corresponding a l u n i t e s of the general form - 57 - M2O.3Al2O2.4sO3.6H2O are precipitated instead of basic aluminum sulphate. Since the solubility of these compounds is less than the solubility of basic aluminum sulphate ("Hydrogen Alunite") the hydrolysis yields are higher and very close to 100%. 3 P.T. Davey and T.R. Scott have reported complete precipitation of aluminum in 30 min.at 220°C from solutions with i n i t i a l SO^iA^O^ ratio of. 2.87. The ratio of I^SO^:Al2(SO^)3 calculated on the basis of aluminum present in starting solution was 2.16. The precipitated compound was of a composition corresponding to K2O.3Al2O3.4SO3.6H2O. 13 V.S. Sazhin, A.K. Zapolskii and N.N. Zakharova have studied hydrolysis of aluminum sulphate solutions having an i n i t i a l molar ratio of I^SO^iA^CSO^^ of 0.33. Concentration of aluminum sulphate in solution was 308 gr/& (0.9 M), having S 0 3 : A 1 2 0 3 ratio of 3.0. Such solutions were hydrolyzed for 1 hr in the temperature range 175-250°C. Maximum hydrolysis yield at 250°C was about 82%. Their results show the presence of two solid phases. In the temperature range 175-190°C, basic potassium-aluminum sulphates of the form K^SO^.3AI2O3.4SO3.9H2O are precipitated. At temperatures higher than 230°C, alunites are formed with the formula K̂ O .3AI2O3.4SO3.6H2O. In the temperature range 190-230°C they have reported a mixture of the two. In order to see the effect of a l k a l i metal sulphates on hydrolysis of dilute solutions of aluminum sulphate, solutions with M 2 S 0 4 : A l 2 ( 3 0 ^ ) 3 ratio of about 0.5 containing 6.10 gr/£ aluminum with S 0 3 : A 1 2 0 3 ratio of about 3.0 were hydrolyzed at 225°C until equilibrium was established. ^SO^ corresponds to I^SO^, Na2S0^ and K 2 S 0 4 . - 58 - 4.6.1 The Effect of Lithium Sulphate on Hydrolysis of Aluminum Sulphate at 225°C Aluminum sulphate-lithium sulphate solution containing 6.12 gr/£ aluminum, 38.30 gr/£ SO^ and 0.815 gr/£ lithium was hydrolyzed at 225°C until equilibrium was reached. The results obtained are presented in Table 17 and Fig. 22. Table 17. Hydrolysis of aluminum sulphate-lithium sulphate solution at 225°C* I | | = — Time at Al in sol SO, in sol L i in sol Al prec SO, prec L i prec pH 225°C hrs gr/£ gr/£ gr/£ gr/£ gr/£ ,gr/£ 0 1.74 27.87 0.44 4.38 10.43 0.375 0.82 1 1.29 26.71 0.40 4.83 11.59 0.415 0.78 2 1.16 26.48 0.39 4.96 11.82 0.425 0.75 3 1.07 26.36 0.39 5.05 11.94 0.425 0.71 3 1.11 26.36 0.39 5.01 11.94 0.425 0.73 5 1.05 26.37 0.39 5.07 11.93 0.425 0.72 8 1.05 26.36 0.39 5.07 11.94 0.425 0.72 Starting solution: 38.30 gr/£ SO 6.12 gr/£ Al 0.815 gr/£ L i pH = 3.05 o c < 6.0 -I ?\ D L i -1.0 36 S.O'- 3.0- 1.0 - O Al 0 S O 4 40_| ;i\ V v V — _ 3 0 ^| -0.6 2 8 o o c (0 » . x — c: o ^ a. •_"] o to 34 V PH _ 0 8 3 2 26 2 0 - j V • n n n n - 0 . 4 24 - O O - 22 - -0.2 2 0 • VO 0 1 2 3 4 5 6 7 8 9 10 II 12 13 time in hrs. Fig. 2 2 . HYDROLYSIS OF LITHIUM-ALUMINIUM SULPHATE SOLUTION AT 2 25C° - 60 - The p r e c i p i t a t e d compound was analyzed (see App. II) and i t s composition was found to be L i2O.3A l2O2.4SO2.6H2O. In a four component system such as t h i s , i t i s normal to have at l e a s t two s o l i d phases i n equilibrium with the l i q u i d phase, but since the concentration of L i + ions i n s o l u t i o n i s high enough to p r e c i p i t a t e a l l the aluminum present i n the s t a r t i n g s o l u t i o n as l i t h i u m a l u n i t e , only one s o l i d phase i s p r e c i p i t a t e d . The X-ray d i f f r a c t i o n pattern for both basic aluminum sulphate and basic lithium-aluminum sulphate i s almost the same, so i t i s d i f f i c u l t to d i s t i n g u i s h between the two (see App. III-K and I I I - E ) . 4.6.2 The E f f e c t of Sodium Sulphate on Hydrolysis of Aluminum Sulphate at 225°C. Aluminum sulphate-sodium sulphate s o l u t i o n containing 6.12 gr/£ Al 4 - 1" 4", 2.625 gr/£ Na + and 38.14 gr/£ S 0 4 = was hydrolyzed at 225°C. About 5.0 gr/£ of aluminum was p r e c i p i t a t e d while heating the system to 225°C. When the equilibrium was reached about 98% of the aluminum present i n the i n i t i a l s o l u t i o n was p r e c i p i t a t e d as a basic s a l t . The r e s u l t s obtained i n t h i s set of experiments are presented i n Table 18 and F i g . 23. Analysis of p r e c i p i t a t e d s o l i d phase shows the composition corresponding to Na2O.3Al2O2.4SO2.6H2O (see App. I I ) . The X-ray d i f f r a c t i o n pattern of t h i s compound i s the same as for basic aluminum sulphate and l i t h i u m a l u n i t e (see App. III-H). - 61 - Table 18. Hydrolysis of aluminum sulphate-sodium sulphate solution at 225°C* Time at 225°C hrs A l i n s o l gr/a SO. i n s o l 4 g r / i l Na + i n s o l g r / i l A l prec g r / i l SO^ prec g r / i l Na prec g r / i l pH 0 1.05 26.06 1.44 5.07 12.08 1.185 0.77 1 0.41 24.58 1.00 5.71 13.56 1.625 0.70 2 0.24 24.19 0.925 5.88 13.95 1.70 0.69 2 0.32 24.06 0.925 5.80 14.08 1.70 0.69 4 0.17 24.04 0.925 5.95 14.10 1.70 0.63 8 0.13 23.84 0.925 5.99 14.20 ' 1.70 0.63 * = Starting solution: 38.14 gr/Jl SO^ 6.12 gr/Jl A l 2.625 g r / i l Na pH = 3.09 4.6.3. The Effect of Potassium Sulphate on Hydrolysis of Aluminum Sulphate at 225°C Potassium sulphate-aluminum sulphate solution containing 6.12 g r / i l aluminum, 4.465 gr/£ potassium and 38.14 gr/£ SO^ was hydrolyzed at 225°C. About 97% pr e c i p i t a t i o n occurs i n heating the system to 225°C. 100% p r e c i p i t a t i o n occurs i n only a few minutes at 225°C. Results obtained are presented i n Table 19 and Fig. 24. Thev.precipitated compound was analyzed and i t was found to correspond to the formula K 20.3A1 20 3.4S0 3.6H 20 (see App. I I ) . 0 I 2 3 4 5 6 7 8 9 10 II 12 13 timeinhrs. Fig. 2 3. HYDROLYSIS OF SODIUM-ALUMINIUM SULPHATE SOLUTION AT225C° - 63 - Table 19. Hydrolysis of aluminum sulphate-potassium sulphate s o l u t i o n at 225°C* Time at 225°C hrs A l i n s o l gr/£ SO, i n s o l 4 gr/£ K i n s o l gr/Jt A l prec gr/£ SO, prec 4 gr/£ K prec gr/£ pH 0 0.21 .24.06 1.61 5.91 14.08 2.855 0.69 1 o.oo • 23.54 1.51 6.12 14.60 2.955 0.67 2 0.00 23.56 1.51 6.12 14.58 2.955 0.66 3 0.00 23.54 1.51 6.12 14.60 2.955 0.66 Starting s o l u t i o n : 38.14 gr/£ SO^" 6.12 gr/il A l 4.465 gr/£ K pH = 3.09 When a l k a l i metal sulphate-aluminum sulphate solutions are hydrolyzed at 225°C only one s o l i d phase i s found to be i n equilibrium with the l i q u i d phase. The s o l i d phase corresponds to the formula 13 M 20.3A1 20 3.4S0 3 <6H 20. This i s contrary to the r e s u l t s of V.S. Sazhin. If there i s any temperature l i m i t i n formation of M20. 3A1 20 3« 4S0.J. 6H 20 i t should be below 225°C and not 230°C. The most probable.second phase i n such mixed solutions would be the basic aluminum sulphate under the condition that there i s Insufficient M+ present i n the i n i t i a l s o l u t i o n to p r e c i p i t a t e a l l aluminum i n the form of a l u n i t e . The reaction for • the hydrbthermal p r e c i p i t a t i o n of alunites can be written as ekl^ + 4HS0 " + 2M+ + 12H.0 *• M o0.3A1.0 o.4S0„.6H o0 + 16H + 4 2 -< 2 2 3 3 2 (13) o « - < 5.0 A 4.0 - 3.0 - 2.0 - 1.0 - 0.0- 0 6 k V \ ^ o — o — o \ \ O v ^ • — o — o — o - O Al _: o v •» o c - n"t~: o O ^ « pH (A o c x - 36 D K 3.0 -0.8 34 0 S 04 - 32 - 2.0 -0.6 30 -V PH *• • • — 28 • 1.0 -0.4 26 "• - 24 OJO -0.2 2 2 • 20 ON 4>- 0 I 2 3 4 5 6 7 8 9 10 11 12 time in hrs. Fig. 24. HYDROLYSIS OF POTASSIUM—ALUMINIUM SULPHATE SOLUTION AT 225 C° - 65 - The e f f e c t of a l k a l i , metal sulphate s a l t s i s such that the hydrolysis y i e l d s are increased i n the following s e r i e s K > Na > L i . This series i s the same i f the i o n i c r a d i i of the L i , Na, and K are compared. Table 20 shows the i o n i c r a d i i of the ions involved i n t h i s system. Fi g . 25 shows the s t r a i g h t l i n e r e l a t i o n between the i o n i c r a d i i and p r e c i p i t a t e d amount of aluminum i n heating the system to 225°C. Table 20. Ionic r a d i i of the ions involved i n the system Element Type of Ionic radius A l prec. Radius A° % H 1 + 0.00 48.00 A l 3 + 0.57 Fe 3 + 0.67 L i 1 + 0.68 71.80 Fe 2 + 0.80 60.50 Cu 2 + 0.80 61.40 Na 1 + 0.98 83.00 K 1 + 1.33 96.80 4.7 The E f f e c t of Divalent Metal Sulphates on Hydrolysis of Aluminum Sulphate Solutions at 225°C As a general r u l e , ions of divalent metals do not p r e c i p i t a t e with basic aluminum sulphate. Many t r i v a l e n t or quadrivalent metals p r e c i p i t a t e as oxides, hydroxides or basic s a l t s under the same - 66 - - 67 - conditions as for hydrolysis of aluminum sulphate. The e f f e c t of copper and i r o n was investigated at 225°C mostly because of t h e i r presence i n many leach s o l u t i o n s . 4.7.1 The E f f e c t of Copper Sulphate on Hydrolysis of Aluminum Sulphate Solutions at 225°C A copper- sulphate-aluminum sulphate s o l u t i o n containing 6.10 gr/£ aluminum, 3.58 gr/£ copper and 37.92 gr/£ SO^ was hydrolyzed at 225°C. There was p r a c t i c a l l y no copper p r e c i p i t a t i o n detected i n these experiments. Results obtained are presented i n Table 21 and F i g . 26. Table 21. Hydrolysis of aluminum sulphate-copper sulphate s o l u t i o n at 225°C* Time at 225°C A l i n s o l Cu i n s o l SO. i n s o l 4 A l prec Cu prec SO^ prec pH hrs. gr/il gr/Jl gr/£ gr/jl gr/£ gr/£ 0 2.35 3.56 29.02 3.75 0.02 8.90 0.94 1 1.48 3.58 26.92 4.62 0.00 11.00 0.85 2 1.23 3.57 26.32 4.87 0.01 11.60 0.82 4 1.12 3.56 26.06 4.98 0.02 11.86 0.82 6 1.10 3.58 26.06 5.00 0.00 11.86 0.82 Starting s o l u t i o n : 6.10 gr/£ A l 3.58 gr/£ Cu 37.92 gr/£ S0 4 pH = 3.0 ON 00 I 0 I 2 3 4 5 6 7 8 9 10 I I 12 time in hrs. Fig.26. HYDROLYSIS OF COPPER-ALUMINIUM SULPHATE SOLUTION AT 225 C° - 69 - 4.7.2 The Effect of Ferrous Sulphate on Hydrolysis of Aluminum Sulphate Solutions at 225°C A ferrous sulphate-aluminum sulphate solution containing 6.12 gr/£ aluminum, 3-26 gr/SL iron and about 38.29 gr/£ SO^- was hydrolyzed at 225°C. Some iron was precipitated in the form of ¥e^0^ due to atmospheric oxidation of ferrous iron. Results obtained in this set of experiments are presented in Table 22 and Fig. 27. The increased precipitation of aluminum can be attributed to the common ion effect. The bivalent ions are not compatible with the alunite lattice and do not tend to replace the hydrogen ions in basic 3 aluminum sulphate.. T.R. Scott has reported that no contamination of basic aluminum sulphate occurs in the presence of Mg, Cd, Zn and Ni even at high concentrations. Table 22. Hydrolysis of aluminum sulphate-ferrous sulphate solution at 225°C* Time at Al in sol Fe in sol SO. in sol Al prec Fe prec SO, prec pH hrs gr/£ gr/£ gr/Jl . gr/£ gr/Jl gr/Jl 0 2.42 2.69 29.44 3.70 0.57 8.85 0.97 1 1.57 2.68 27.48 4.55 0.58 10.81 0.82 2 1.40 2.69 27.04 4.72 0.57 11.25 0.80 4 1.15 2.66 26.42 4.97 0.60 11.87 0.82 6 1.12 2.69 26.40 5.00 0.57 11.89 0.80 Starting solution: 6.12 gr/£ Al; 3.26 gr/l Fe; 38.29 gr/£ SO pH = 3.0 I 8 - - i 1 i — J 1 i ' « » ' • ' • ' ' ' 0 I 2 3 4 5 6 7 8 9 10 II 12 time in hrs. Fig. 27 HYDROLYSIS OF IRON-ALUMINIUM SULPHATE SOLUTION AT 22 5C° - 71 - 4.8 A p p l i c a t i o n of the Hydrolysis Process Basic aluminum sulphate of the form 3A1 20 3.4S0 3.9H 20 i s the product of high temperature hydrolysis of aluminum sulphate solutions with a wide range of i n i t i a l S O ^ A l ^ r a t i o s . A basic s a l t of the form M2O.3Al2O2.4sO2.6H2O i s a high temperature h y d r o l y s i s product of aluminum sulphate solutions containing a l k a l i metals ( L i , Na or K). Thermal decomposition of such basic s a l t s y i e l d s alumina as a f i n a l product which can be used f o r aluminum production. Thermal decomposi- t i o n of a few samples of basic aluminum sulphate was followed by D.T.A. A t y p i c a l D.T.A. curve i s shown i n F i g . 28. The f i r s t endothermic peak i n the temperature range 200-400°C corresponds to the loss of i n t e r s t i t i a l l a t t i c e water. 9 The peak at about 450°C corresponds to the dehydroxylation process. This process i s completed at temperatures around 600°C where anhydrous basic aluminum sulphate 9 e x i s t s . At temperatures above 600°C, SO^, SO2 and O2 are evolved y i e l d i n g amorphous alumina which subsequently c r y s t a l l i z e s to y-alumina at higher temperatures. Y -Alumina i s transformed to a-alumina at temperatures of 1000°C and higher. A few samples of K2O.3Al2O2.4SO2- 6H2O were calcined at about 1000°C for 2 hrs. The decomposition of these samples was followed by the weight l o s s . Results of one of the decomposition experiments i s shown below. 1.0811 gr K 20.3A1 20 3.4S0 3.6H 20 > 0.5857 gr K 2S0 4 + A l 0 + , sulphurous t gases 0.5857 gr K 2S0 4 4- A1 20 3 iffg^ 0.365 gr A1 20 3 o 200 4 0 0 6 0 0 Fig. 2 8 . D . T . A . O F 3AI2O34SO39H2O - 73 - In leaching processes at high temperatures where aluminum is present in solution i t precipitates as a basic salt in autoclaves and pipelines"'"''" which necessitates "forced" shutdowns of the process in order to clean autoclaves and/or pipelines. With increasing sulphuric acid concentration and decreasing temperature the solubility of basic aluminum sulphate was found to increase. Therefore running concentrated solutions of sulphuric acid through the system at low temperatures should dissolve the precipitate. In dump leaching solutions aluminum is present as aluminum sulphate. If aluminum is not recovered the viscosity of the solution w i l l increase. To remove the, aluminum from solution, a part of the liquor after cementation can be treated in an autoclave to precipitate basic aluminum sulphate. Liberated acid from the hydrolysis can be recycled to the heap. - 74 - 5. CONCLUSIONS 1. D i l u t e solutions of normal and acid aluminum sulphate hydrolyze when heated above 125°C to y i e l d a basic s a l t of the nominal composition 3A1 20 3,4S0 3.9H 20. 2. Hydrolysis y i e l d i s a function of the temperature and i n i t i a l concentration of sulphuric acid i n s o l u t i o n . 3. Only one s o l i d phase was found to be i n equilibrium with the l i q u i d phase i n the temperature region 125-250°C, but there are some i n d i c a t i o n s that a less basic s a l t may e x i s t i n the system. 4. When L i , Na or K ions are present i n solutions of aluminum sulphate a basic s a l t of the form M 20.3A1 20 3«4S0 3.6H 20 i s p r e c i p i t a t e d . 5. The hydrolysis y i e l d s are increased i n the presence of a l k a l i metal sulphates i n the following s e r i e s : K > Na > L i . I | 6. Aluminum can be s e l e c t i v e l y p r e c i p i t a t e d i n the presence of Cu and Fe"1"1" as 3A1 20 3 > 4S0 3-9H 20. 7. Both basic aluminum sulphate and a l u n i t e s can be calcined at temperatures above 1000°C to y i e l d alumina. - 75 - APPENDIX I SOLUBILITY OF ALUMINUM SULPHATE I-A S o l u b i l i t y of A l (SO .K i n water Temp. °C 20 30 40 50 60 70 80 90 100 gr A 1 2 ( S 0 4 ) 3 26.7 28.8 31.4 34.3 37.2 39.8 42.2 44.7 47.1 100 gr sat. s o l . I-B S o l u b i l i t y of aluminum sulphate i n aqueous solutions of sulp u r i c acid at 25°C. gr A 1 2 ( S 0 4 ) 3 27.82 29. 21 26.2 19.5 11.6 4. 8 1. 5 1.0 2.3 4.0 100 gr sat. s o l . gr H 2S0 4 0.0 5. 73 10.0 20.0 30.0 40. 0 50. 0 60.0 70.0 75.0 100 gr sat. s o l . I-C S o l u b i l i t y of aluminum sulphate i n aqueous 10% H SO Temp. ° C 30 42 50 gr A 1 2 ( S 0 4 ) 3 14.52 16.45 18.77 100 gr sat. s o l . - 76 - APPENDIX II ANALYSIS OF THE SOLID PHASES Time at Temp. % A l 0 % SO % M00 % H O Sta r t i n g hrs. C pH 6.00 175 38.7 37.2 - 24.1 3.10 10.00 175 38.5 39.6 - 21.9 3.10 16.30 175 38.7 39.7 - 21.6 3.10 4.0 200 38.6 39.5 - 21.9 3.10 6.0 200 38.8 39.9 - 21.3 3.10 18.00 200 38.7 40.1 - 21.2 3.10 1.00 225 38.8 40.2 - 21.0 3.10 5.00 225 38.9 40.8 - 20.3 3.10 16.00 225 39.1 41.6 - 19.3 3.10 0.00 250 38.8 40.3 - 20.9 3.10 1.00 250 38.7 40.2 - 21.1 3.10 3.30 250 39.0 40.5 - 20.05 3.10 6.30 250 39.2 40.9 - 19.9 3.10 6.00 225 39.0 . 40.6 - 20.4 2.5 6.00 225 38.7 40.6 - 20.7 2.0 6.00 225 38.9 40.8 - 20.3 1.5 6.00 225 . 38.9 40.2 - 20.9 1.0 6.00 225 39.2 41.7 - 19.1 0.70 4.00 250 38.9 40.6 - 20.5 2.5 4.00 250 38.8 40.7 - 20.5 2.0 4.00 250 38.9 41.2 - 19.9 1.5 4.00 250 39.2 41.6 - 19.20 1.0 4.00 250 38.9 40.8 - 20.3 0.70 L i 2 0 3.00 225 39.90 42.00 4.00 14.10 3.05 Na 20 2.00 225 38.40 40.10 7.70 13.80 3.09 8.00 225 38.50 40.20 7.80 13.50 3.09 K 20 0.00 225 37.05 38.39 11.36 13.20 3.09 0.00 225 36.91 38.91 11.38 12.80 3.09 1.00 225 37.12 38.48 11.30 13.10 3.09 - 77 - APPENDIX III X-RAY DIFFRACTION PATTERNS III-A Diffraction pattern of the precipitate obtained at 150°C Prec. obtained in 10 hrs. Prec. obtained i n 16 hrs d A° I/xo d A° I/To 5.71 7 5.71 7 5.03 2 5.03 2 3.55 5 3.57 5 3.01 1 3.01 1 2.27 4 2.27 4 1.909 3 1.908 3 1.76 5 1.755 5 1.654 7 1.659 7 1.566 7 1.570 7 1.496 5 1.500 5 1.325 7 1.329 7 1.290 6 1.299 6 1.220 7 1.217 7 1.171 7 1.171 7 1.150 7 1.147 7 III-B X-ray diffraction patterns of the precipitate obtained at 175 Prec. obtained in 2 hrs. Prec. obtained in 16.5 hrs d A° I / I 0 d A° 1 / T o 5.71 7 5.75 1 5.034 2 5.12 2 3.56 5 3.56 5 3.01 : 1 3.04 1 2.87 7 2.486 8 2.27 4 2.30 4 2.22 8 1.908 3 1.920 3 1.757 5 1.770 5 1.654 7 1.654 7 1.566 7 1.570 7 1.495 5 1.500 5 1.380 8 1.325 7 1.330 7 1.293 6 .1.290 6 1.219 7 1.217 7 1.174 7 1.172 7 1.150 7 1.150 7 - 78 - III-C X-ray diffraction pattern of the precipitate obtained at 200°C. Prec. obtained in 18 hrs. d A° . I/I 5.72 7 5.03 2 3.56 5 3.04 1 2.87 7 2.27 4 2.22 8 1.910 3 1.756 •5 1.654 7 1.565 7 1.490 5 1.387 8 1.290 6 1.217 7 1.171 7 1.151 7 III-D X-ray diffraction pattern of the precipitate obtained at 225°C Prec. obtained in 3 hrs at 225°G d A° I 7 I 0 d A° I/I 5.61 8 1.736 6 5.02 2 1.666 8 4.80 2 1.641 8 3.43 6 1.550 8 3.03 1 1.546 8 2.932 1 1.479 4 2.806 4 1.4227 8 2.440 8 1.362 8 2.304 4 1.316 8 2.241 3 1.281 4 2.191 8 1.205 8 1.925 8 1.195 8 1.880 3 1.161 8 1.767 8 1.132 8 1.758 6 - 79 - III-E X-ray d i f f r a c t i o n pattern of the precipitate obtained at 250°C Prec. obtained i n 0.0 hrs. Prec. obtained i n 3.5 hrs. d A° ^ 0 d A° I / T o d A° I / x o d A° l / 1 o 5.61 8 1.765 8 5.61 8 4.02 2 1.736 6 5.01 2 1.742 6 4.80 2 1.665 8 4.85 2 1.646 8 4.27 8 1.557 8 - 1.555 3.40 6 1.479 4 3.46 6 1.483 4 3.01 1 1.424 8 3.01 1 - 2.91 1 1.367 8 2.95 1 1.369 8 2.79 4 1.317 8 2.81 4 1.317 8 2.45 8 1.286 4 2.46 8 1.286 4 2.296 4 1.208 8 - 1.208 8 2.24 3 1.200 8 2.24 3 - 2.187 8 1.167 8 1.895 8 1.167 8 1.929 8 . 1.138 8 1.139 8 1.879 3 III-F X-ray d i f f r a c t i o n pattern of the precipitate obtained from from FeSO -Al„(SO 4 2 solution at 225°C Prec. obtained in4.0hrs at 250°C d A° 1 / T o d A° I/I 5.08 2 1.558 4 4.80 2 1.501 5 3.03 1 1.481 2 2.915 1 1.423 5 2.294 5 1.371 3 2.234 1 1.314 4 1.918 4 1.209 3 1.888 2 1.1969 5 1.768 - 5 1.1867 5 1.737 2 1.1364 5 1.637 4 1.1054 5 - 80 - III-G X-ray diffraction pattern of potassium alunite obtained at 225°C Prec. obtained inO.Ohrs. at 225°C d A° I/I d A° I/I r 5.71 5 1.560 8 5.01 2 1.546 8 3.49 3 1.489 3 3.01 1 1.422 8 2.90 8 1.380 8 2.47 8 1.316 8 2.29 3 1.281 5 2.20 5 1.205 5 1.888 2 1.161 6 1.736 2 1.132 6 1.641 8 III-H. X-ray diffraction pattern of sodium alunite obtained at 225°C Prec. obtained in 2.0 hrs. at 225°C d A° X / I 0 d A° 5.70 5 1.746 2 5.01 2 1.644 8 3.496 3 1.554 8 3.01 1 1.538 8 2.91 8 1.505 3 2.45 8 1.281 5 2.212 3 1.205 5 1.896 2 1.161 6 III-K X-ray diffraction pattern of lithium alunite obtained at 225°C Prec. obtained in 3.0 hrs. at 225°C d A° : Wo d A° w „ 5.68 : 5 1.736 2 5.01 2 1.641 8 3.491 3 1.554 8 3.01 1 1.535 8 2.90 8 1.489 3 2.44 8 1.281 5 2.20 3 1.205 5 1.887 2 1.161 6 - 81 - LITERATURE 1. E.T. Carlson, C.S. Simons, Met. Soc. of A.I.M.E., Extractive Metallurgy of Copper, Nickel and Cobalt. International Symposium, New York, February 15-18, 1960. 2. Posnjak, E., Mervin, H.E., J. Am. Chem. Soc. 44, 1965-1994 (1929). 3. P.T. Davey and T.R. Scott, Aust. J. Appl. Sci. 13, 229-241 (1962) 4. Basset, H. and Goodwin, T.H., J. Chem. Soc., 2239-2279 (1949). 5. T.R. Scott, Paper presented at the Annual Conference of the Institute, August 14-24, 19|>3, Melbourne. 6. T.R. Scott, Extractive Metallurgy of Aluminum, Vol. 1, Alumina, International Symposium on the Extractive Metallury of Aluminum, February 18-22 (1962), New York. 7. T.R. Scott, Research Appl. Ind. 14, 50-54 (1961). 8. P.T. Davey and T.R. Scott, Nature 195, 376 (1962). 9. P.T. Davey, G.M. Lukaszewski and T.R. Scott, Aust. J. Appl. Sci. 14, 137-154 (1963). 10. Henry, J.L. and G.B. King, J. Am. Chem. Soc. 72_, 1282-1286 (1950). 11. R. Acosta Chaves, V.V. Karelin, and B.P. Sobolev, Tsvet. Met. 1968, 41 (4), 50.53. 12. T.R. Scott, Internal Report, Division of Mineral Chemistry C.S.I.R.O., Melbourne. 13. V.S. Sazhin, A.K. Zapolskii and N.N. Zakharova, Zh. P r i k l . Khim., 41 (7), 1420-1423 (1968). 14. S. Bretsznajder, J. Boczar, J. Piskorski, and J. Porowski, Prezmysl. Chem. 11, 89-93 (1955). 15. A.K. Zapolskii and G.I. Tsarenko, Ukr. Khim. Zh. 1969, 35 (8), 866-868. - 82 - 16. V.S. Sazhin, A.K. Zapolskii, N.N. Zakharova and A.I. Volkovska, Ukr. Khim. Zh. 1966, 32 (1), 95-100. 17. A.K. Zapolskii, Ukr. Khim. Zh. 33 (8), 805-809 (1967). 18. A.K. Zapolskii, V.S. Sazhin, N.N. Zakharova and A.I. Volkovska, Ukr. Khim. Zh. 32 (11), 1222-7 (1966). 19. H.G. Iverson and H. Leitch. Report of Investigations 7162 (1968). 20. Erkki Wanninen and Anders Ringbom, Analytica Chimica Acta Vol. 12 (1955), 308-318. 21. R.G. Robins, LR 80 (NST) Report of the Mineral Sci. and Technology Div. of the Warren Spring Laboratory. 22. H.C. Helgeson, Am. J. of Sci. Vol. 267, 1969, 729-804.

Cite

Citation Scheme:

    

Usage Statistics

Country Views Downloads
Canada 25 0
United States 18 5
Singapore 13 0
India 12 0
Iraq 10 10
Cambodia 8 0
Unknown 7 1
Russia 5 0
Norway 5 4
Estonia 5 0
France 5 1
New Zealand 4 0
Kyrgyzstan 4 0
City Views Downloads
Unknown 70 19
Calgary 20 0
Mountain View 6 2
Ankara 4 0
Tallinn 4 0
Phumi Preah Haoh 4 0
Bristol 4 0
Delhi 3 0
Delft 3 0
Tokyo 3 0
Beijing 3 0
Mumbai 2 0
Ruislip 2 0

{[{ mDataHeader[type] }]} {[{ month[type] }]} {[{ tData[type] }]}

Share

Share to:

Comment

Related Items