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The coarsening of Bayer alumina trihydrate by means of crystallization modifiers
Powder Technology, 11 (1975) 101-105 0 ElsevierSequoia S.A., Lausanne- Printedin
The Nether!ands
The Coarsening of Bayer Alumina Trihydrat.e by Means of Crystallization Modifiers
B. GNYRA Chemical N.
Division,
Arvida
Works,
AIuminum
Company
of Canada Ltd.,
Arvida,
Quebec
(Canada)
BROWN
Alcan
Research
Center,
Aluminum
Company
of Canada Ltd.,
Arvida,
Quebec
(Canada)
(ReceivedJuly 19, 1974) ._
SUMMARY Modem aluminium smelting practice requires that Bayer plants produce coarse, sandy non-dusting alumina. However, the coarseness of Bayer products is normally achieved at the expense of lower liquor productivities. In the course of work carried out on the so-called “poisons” of alumina trihydrate seed, it was discovered that minute additions of fine calcium carbonate to Bayer seed slurries at the time of seeding can considerably decrease the fine fraction in the resulting final products without affecting the yield of alumina. The action of calcite in the precipitation is that of a crystal-growth modifier resulting in an improved particle agglomeration and making the agglomerated particles in the products more resistant to breakage in mixing and pumping operations.
a result of secondary nucleation [l] and crystal breakage [ 21 during crystallization. In addition, crystal breakage occurs outside the crystallizers during the pumping of alumina trihydrate slurries. Fines removal is achieved during crystallization by the mechanisms of intercrystallization [3,4] and agglomeration 16 - 71. The extent of Bayer fines generation and their removal can be controlled to a certain estent by changes in operating temperatures, seed charges and the mode of mixing in the crystallizers. However, a coarse product can only be obtained at the expense of the crystallization yield, so that the overall economics of the Bayer process is strongly dependent on the efficiency of fines removal during crystallization. This paper describes a novel technique whereby the efficiency of fines removal during crystallization can be increased by the addition of mirute amounts of calcium compounds such as CaCOs, CaO and CaClz as crystallization modifiers.
INTRODUCTION In the industrial production of coarse, sandy alumina trihydrate (Al,0s.3H20) by the Bayer process, the generation and removal of fine crystals is of considerable economic importance since an excess of fines in the crystalline product can cause severe dusting and handling difficulties during calcination to alumina and its subsequent electrolytic reduction to aluminium. Very fine alumina triiydrate crystals can be formed in seeded caustic aluminate liquors as
EXPERIMENTAL TECHNIQUE In order to simulate the crystallization step of the Bayer process in the laboratory, experiments were carried out in nickel bottles of about 1 litre capacity rotating end-over-end in a constant temperature water bath to provide just enough agitation to prevent settling of solids during crystallization. The composition of the solutions used was in the usual industrial range of 160 - 220 g/l caus-
i02
tic concentration (expressed as NazCOs equivalent) and alumina to caustic weight ratio in solution of 0.600 - 0.660. The Bayer seed crystals used had a specific surface area of 1400 1500 cm’/g and contained 75 - 90 wt. % of -44 pm fines. Seed charges varied between 35 and 85 g/l. Reagent grade calcium compounds, added in the concentration range 0 - 200 mg/l, were particulate CaCO, (calcite), CaO and CaCl,. The calcium compounds were added with the seed crystals to the supersaturated caustic aluminate solution at the beginning of run. All runs were carried out isothermally at a temperature of 66” C with a residence time of 24-32 hours. Product size distributions were determined using micromesh sieves. To simulate in the laboratory the crystal breakage which occurs during pumping and mixing operation in the industrial practice, a 30 70 alumina trihydrate water slurry of each product was fed to an apparatus which consisted of a miniature constant speed centrifugal pump connected to four feet of one inch diameter gIass tubing assembled in vertical position. After a recirculation period of 5 15 minutes, the slurry was removed and the extent of crystal breakage determined by measuring the net increase in the -44 pm fraction and the net decrease in the +74 ,um fraction. The examination of particle microstructure was carried out using a scanning electrcn microscope.
RESULTS
The effect of CaCOs on crystallization yield is shown in Fig. 1. Note that in the presence
6
I2
RESIDENCE
RESIDENCE
TIME
IHOURS,
Fig. 2. Crystallization yield as a function time and added CaO and CaCl,.
of residence
of CaCOs there is an induction period the length of which depends upon the amount of CaCO, added. For an addition of 100 mg/I of CaCO,, however, the crystallization yield after 24 hours is more or less the same as that obtamed in the absence of CaCOs. This can be attributed to the fact that secondary nucieation takes place during the induction period, creating new surface area which produces a surge in the crystallization rate [2]. This acts to maintain the crystallization yield at the desired level. The effects of CaO and CaClz on crystallization yield proved to be somewhat erratic. The induction period, when apparent, was generally less marked than that produced by the equivalent amount of CaCOa. Moreover, the crystallization yield after 24 hours was usually less than that obtained in the absence of calcium compounds. Some typical results obtained from CaO and CaClz additions are shown in Fig. 2.
I8 ADDED
TIME (HOURS,
Fig. 1. Crystallization yield as a function time and added CaC03.
of residence
coco3
(m/l
Fig. 3. Product size as a function fore and after crystal breakage.
I
of added CaC03
be-
103 EFFECT OF THE DIFFERENT SOLUBILITIES OF THE THREE
SEED
CHARGE
(g/l
I
Fig. 4. Construction showing increase in crystallization yield made available by adding 100 mg/l CaCOs tc Bayer caustic aluminate solution.
Comparative tests with equivalent amounts of CaCOs,
CaO and CaCl,
showed
that CaCO,
produces the coarsest product crystals while exerting the least inhibiting effect on crystallization yield after 24 hours. The effect of CaCO, on product size distribution, before and after crystal breakage, is shown in Fig. 3. The plots of weight percent +74 pm material uersus amount of CaCO, added exhibit well-defined maxima at a CaCO, concentration of about 100 mg/l. In practical terms, the significance of this improved coarsening is that to obtain the same increase in product size in the absence of CaCOa would have necessitated a 15% reduction in seed charge. In addition to improving product size, the CaCO, additions result in stronger product crystaIs, which are more resistant to crystal breakage. When the increased product strength is taken into account, it can be shown that it is possible to actually raise crystallization yield whilst maintaining product size distribution at the desired level. An
CALCIUM
COMPOUNDS
The reason for the different effects of CaO, CaCl, and CaCO, undoubtedly lies in their different solubilities in caustic aluminate solutions. It is known that CaO and CaCl, are much more soluble than CaCOs [3]. However, under the conditions of temperature and solution composition employed in the Bayer crystallization process, CaO and CaCl, are quickly transformed to crystalline hydrated aluminates whose solubilities are significantly less than that of CaCOs [S]. In seeded caustic aluminate solutions, particulate CaCO, disperses readily, and releases calcium ions into solution as it slowly dissolves. However, on dissolution, CaCO, probably never attains its equilibrium solubility value since calcium ions in solution are adsorbed on alumina trihydrate crystal surfaces faster than they are released into solution. Continuation of this dissolution-deposition process until the source of CaCO, is depleted explains why it is possible to dissolve 200 mg/l of CaCOs in seeded caustic aluminate solution without formation of any solid phase hydrated calcium aluminates. In the case of CaO and CaC12, the effect on crystallization is actually that of their much less soluble reaction products - hydrated calcium aluminates - which will supply calcium ions more slowly than CaCOs but for a Ionger period of time. The transformation to hydrated calcium aluminates takes place too rapidly to allow adequate dispersion of calcium ions into solution. The erratic behaviour of CaO and CaC12 relative to CaCOs could be attributed to pockets of high calcium ion concentration formed initially. The probability of any of these calcium ions “finding” an alumina trihydrate seed crystal surface will be strongly dependent upon operating parameters such as crystallite size of calcium compound, quantity of alumina trihydrate seed crystals, level of agitation and degree of solids dispersion.
example of this procedure is shown in Fig. 4. The effects of CaO and CaCl, on product size distribution and strength were erratic; improved coarsening generally occurred only when an induction period was apparent during the early stages of crystallization. It appears therefore that the mechanisms involved are the same for CaO, CaCls and CaCOs, the latter
generally acting in the most potent and reproducible manner.
SCANNING
ELECTRON
MICROSCOPY
The beneficial effects of small amounts of calcium on product size distribution can be explained in terms of its action as a crystallization modifier. Scanning electron microscopy (Fig. 5) shows that calcium appears to adsorb preferentially on the basal planes (001) of the seed
5. Scanning electron micrograph showing growth layers and secondary nuclei formation crystal surfaces, magnification 6000. Fig.
crystal on seed
crystals during the initial stages of crystallization. This is rezsonable, since the basal planes of alumina trihydrate crystals will have a high electrostatic field strength normal to the surface because the outermost layer contains ions of one kind only [9]. Preferential adsorption on (001) would therefore tend to lower surface energy. Figure 5 shows a thickening of the crysta!
growth
layers on (001)
crystal surfaces
as they
Fig. 6. Scanning electron micrographs showing the effect of calcium carbonate on alumina trihydrate particle structure: (a) and (b) no CaC03 added, magnifications 1200 and 2300, (c) and (d) 100 mgil CaCOa added, magnifications 1200 and 2300.
105
are “pinned” by calcium adsorbed at active sites. It also shows a multitude of tiny crystallites on the alternate prismatic crystal surfaces. This roughening of (001) faces and the appearance of secondary nuclei on the other faces could explain the improved size and strength of the resulting hydrate particles. Two seed particles having a large number of potential intercrystallization centers, in the form of rough faces and small crystallites, should agglomerate more readily than particles undergoing a more orderly crystal growth. Selective adsorption of calcium impurity on (001) crystal surfaces and secondary nucleation lead to a more compact, rounded-off particle structure (Fig. 6.)
CONCLUSION
In the alumina industry, it has been generally accepted in the past that calcium compounds present in Bayer solutions are potent inhibitors of crystallization and as such are to be avoided as much as possible [ 53. This paper, however, has described a technique whereby calcium compounds can be used as an aid to improving crystallization yield (by virtue of increased seed charges) whilst maintaining the desired
product size distribution. has been filed.
A patent application
ACKNOWLEDGE>IENTS
The authors wish to thank Mr. S. Ostap for helpful discussions and are indebted to the Aluminum Company of Canada Ltd. for permission to publish this paper.
REFERENCES N. Brown, J. Cryst. Growth, 16 (1972) 163. N. Brown, paper presented at meeting of I.C.S.O.B.X.. Bar&a Bystrica, Czechoslovakia, June 1979. S.I. Kuznetsov and V.A. Derevyankin, Physical Chemistry of the Bayer Process, ~Ioscow, 196-i. S. hlaricic and I. hlarkoreic, 2. Anorg. Allgem. Chem., 276 (1951) 193. T.G. Pearson, The Chemical Background of the Aluminium Industry, Monograph No. 3, Royal Institute of Chemistry, London, 1955. J. Scott, in Estractive hletallurg>- of _Muminum, Vol. 1, Interscience, New York. 1963. C. Misra and E.T. White, J. Cryst. Growth, S (197 1) li2.
S N.T. Chaplin. paper presented at the 100th Annual Meeting of _&ME, New York, Februaq: 1971. 9 K. Wefers and G.BI. Bell, Tech. Paper No. 19. .\lcoa Reasearch Laboratories, 19i9.
The Nether!ands
The Coarsening of Bayer Alumina Trihydrat.e by Means of Crystallization Modifiers
B. GNYRA Chemical N.
Division,
Arvida
Works,
AIuminum
Company
of Canada Ltd.,
Arvida,
Quebec
(Canada)
BROWN
Alcan
Research
Center,
Aluminum
Company
of Canada Ltd.,
Arvida,
Quebec
(Canada)
(ReceivedJuly 19, 1974) ._
SUMMARY Modem aluminium smelting practice requires that Bayer plants produce coarse, sandy non-dusting alumina. However, the coarseness of Bayer products is normally achieved at the expense of lower liquor productivities. In the course of work carried out on the so-called “poisons” of alumina trihydrate seed, it was discovered that minute additions of fine calcium carbonate to Bayer seed slurries at the time of seeding can considerably decrease the fine fraction in the resulting final products without affecting the yield of alumina. The action of calcite in the precipitation is that of a crystal-growth modifier resulting in an improved particle agglomeration and making the agglomerated particles in the products more resistant to breakage in mixing and pumping operations.
a result of secondary nucleation [l] and crystal breakage [ 21 during crystallization. In addition, crystal breakage occurs outside the crystallizers during the pumping of alumina trihydrate slurries. Fines removal is achieved during crystallization by the mechanisms of intercrystallization [3,4] and agglomeration 16 - 71. The extent of Bayer fines generation and their removal can be controlled to a certain estent by changes in operating temperatures, seed charges and the mode of mixing in the crystallizers. However, a coarse product can only be obtained at the expense of the crystallization yield, so that the overall economics of the Bayer process is strongly dependent on the efficiency of fines removal during crystallization. This paper describes a novel technique whereby the efficiency of fines removal during crystallization can be increased by the addition of mirute amounts of calcium compounds such as CaCOs, CaO and CaClz as crystallization modifiers.
INTRODUCTION In the industrial production of coarse, sandy alumina trihydrate (Al,0s.3H20) by the Bayer process, the generation and removal of fine crystals is of considerable economic importance since an excess of fines in the crystalline product can cause severe dusting and handling difficulties during calcination to alumina and its subsequent electrolytic reduction to aluminium. Very fine alumina triiydrate crystals can be formed in seeded caustic aluminate liquors as
EXPERIMENTAL TECHNIQUE In order to simulate the crystallization step of the Bayer process in the laboratory, experiments were carried out in nickel bottles of about 1 litre capacity rotating end-over-end in a constant temperature water bath to provide just enough agitation to prevent settling of solids during crystallization. The composition of the solutions used was in the usual industrial range of 160 - 220 g/l caus-
i02
tic concentration (expressed as NazCOs equivalent) and alumina to caustic weight ratio in solution of 0.600 - 0.660. The Bayer seed crystals used had a specific surface area of 1400 1500 cm’/g and contained 75 - 90 wt. % of -44 pm fines. Seed charges varied between 35 and 85 g/l. Reagent grade calcium compounds, added in the concentration range 0 - 200 mg/l, were particulate CaCO, (calcite), CaO and CaCl,. The calcium compounds were added with the seed crystals to the supersaturated caustic aluminate solution at the beginning of run. All runs were carried out isothermally at a temperature of 66” C with a residence time of 24-32 hours. Product size distributions were determined using micromesh sieves. To simulate in the laboratory the crystal breakage which occurs during pumping and mixing operation in the industrial practice, a 30 70 alumina trihydrate water slurry of each product was fed to an apparatus which consisted of a miniature constant speed centrifugal pump connected to four feet of one inch diameter gIass tubing assembled in vertical position. After a recirculation period of 5 15 minutes, the slurry was removed and the extent of crystal breakage determined by measuring the net increase in the -44 pm fraction and the net decrease in the +74 ,um fraction. The examination of particle microstructure was carried out using a scanning electrcn microscope.
RESULTS
The effect of CaCOs on crystallization yield is shown in Fig. 1. Note that in the presence
6
I2
RESIDENCE
RESIDENCE
TIME
IHOURS,
Fig. 2. Crystallization yield as a function time and added CaO and CaCl,.
of residence
of CaCOs there is an induction period the length of which depends upon the amount of CaCO, added. For an addition of 100 mg/I of CaCO,, however, the crystallization yield after 24 hours is more or less the same as that obtamed in the absence of CaCOs. This can be attributed to the fact that secondary nucieation takes place during the induction period, creating new surface area which produces a surge in the crystallization rate [2]. This acts to maintain the crystallization yield at the desired level. The effects of CaO and CaClz on crystallization yield proved to be somewhat erratic. The induction period, when apparent, was generally less marked than that produced by the equivalent amount of CaCOa. Moreover, the crystallization yield after 24 hours was usually less than that obtained in the absence of calcium compounds. Some typical results obtained from CaO and CaClz additions are shown in Fig. 2.
I8 ADDED
TIME (HOURS,
Fig. 1. Crystallization yield as a function time and added CaC03.
of residence
coco3
(m/l
Fig. 3. Product size as a function fore and after crystal breakage.
I
of added CaC03
be-
103 EFFECT OF THE DIFFERENT SOLUBILITIES OF THE THREE
SEED
CHARGE
(g/l
I
Fig. 4. Construction showing increase in crystallization yield made available by adding 100 mg/l CaCOs tc Bayer caustic aluminate solution.
Comparative tests with equivalent amounts of CaCOs,
CaO and CaCl,
showed
that CaCO,
produces the coarsest product crystals while exerting the least inhibiting effect on crystallization yield after 24 hours. The effect of CaCO, on product size distribution, before and after crystal breakage, is shown in Fig. 3. The plots of weight percent +74 pm material uersus amount of CaCO, added exhibit well-defined maxima at a CaCO, concentration of about 100 mg/l. In practical terms, the significance of this improved coarsening is that to obtain the same increase in product size in the absence of CaCOa would have necessitated a 15% reduction in seed charge. In addition to improving product size, the CaCO, additions result in stronger product crystaIs, which are more resistant to crystal breakage. When the increased product strength is taken into account, it can be shown that it is possible to actually raise crystallization yield whilst maintaining product size distribution at the desired level. An
CALCIUM
COMPOUNDS
The reason for the different effects of CaO, CaCl, and CaCO, undoubtedly lies in their different solubilities in caustic aluminate solutions. It is known that CaO and CaCl, are much more soluble than CaCOs [3]. However, under the conditions of temperature and solution composition employed in the Bayer crystallization process, CaO and CaCl, are quickly transformed to crystalline hydrated aluminates whose solubilities are significantly less than that of CaCOs [S]. In seeded caustic aluminate solutions, particulate CaCO, disperses readily, and releases calcium ions into solution as it slowly dissolves. However, on dissolution, CaCO, probably never attains its equilibrium solubility value since calcium ions in solution are adsorbed on alumina trihydrate crystal surfaces faster than they are released into solution. Continuation of this dissolution-deposition process until the source of CaCO, is depleted explains why it is possible to dissolve 200 mg/l of CaCOs in seeded caustic aluminate solution without formation of any solid phase hydrated calcium aluminates. In the case of CaO and CaC12, the effect on crystallization is actually that of their much less soluble reaction products - hydrated calcium aluminates - which will supply calcium ions more slowly than CaCOs but for a Ionger period of time. The transformation to hydrated calcium aluminates takes place too rapidly to allow adequate dispersion of calcium ions into solution. The erratic behaviour of CaO and CaC12 relative to CaCOs could be attributed to pockets of high calcium ion concentration formed initially. The probability of any of these calcium ions “finding” an alumina trihydrate seed crystal surface will be strongly dependent upon operating parameters such as crystallite size of calcium compound, quantity of alumina trihydrate seed crystals, level of agitation and degree of solids dispersion.
example of this procedure is shown in Fig. 4. The effects of CaO and CaCl, on product size distribution and strength were erratic; improved coarsening generally occurred only when an induction period was apparent during the early stages of crystallization. It appears therefore that the mechanisms involved are the same for CaO, CaCls and CaCOs, the latter
generally acting in the most potent and reproducible manner.
SCANNING
ELECTRON
MICROSCOPY
The beneficial effects of small amounts of calcium on product size distribution can be explained in terms of its action as a crystallization modifier. Scanning electron microscopy (Fig. 5) shows that calcium appears to adsorb preferentially on the basal planes (001) of the seed
5. Scanning electron micrograph showing growth layers and secondary nuclei formation crystal surfaces, magnification 6000. Fig.
crystal on seed
crystals during the initial stages of crystallization. This is rezsonable, since the basal planes of alumina trihydrate crystals will have a high electrostatic field strength normal to the surface because the outermost layer contains ions of one kind only [9]. Preferential adsorption on (001) would therefore tend to lower surface energy. Figure 5 shows a thickening of the crysta!
growth
layers on (001)
crystal surfaces
as they
Fig. 6. Scanning electron micrographs showing the effect of calcium carbonate on alumina trihydrate particle structure: (a) and (b) no CaC03 added, magnifications 1200 and 2300, (c) and (d) 100 mgil CaCOa added, magnifications 1200 and 2300.
105
are “pinned” by calcium adsorbed at active sites. It also shows a multitude of tiny crystallites on the alternate prismatic crystal surfaces. This roughening of (001) faces and the appearance of secondary nuclei on the other faces could explain the improved size and strength of the resulting hydrate particles. Two seed particles having a large number of potential intercrystallization centers, in the form of rough faces and small crystallites, should agglomerate more readily than particles undergoing a more orderly crystal growth. Selective adsorption of calcium impurity on (001) crystal surfaces and secondary nucleation lead to a more compact, rounded-off particle structure (Fig. 6.)
CONCLUSION
In the alumina industry, it has been generally accepted in the past that calcium compounds present in Bayer solutions are potent inhibitors of crystallization and as such are to be avoided as much as possible [ 53. This paper, however, has described a technique whereby calcium compounds can be used as an aid to improving crystallization yield (by virtue of increased seed charges) whilst maintaining the desired
product size distribution. has been filed.
A patent application
ACKNOWLEDGE>IENTS
The authors wish to thank Mr. S. Ostap for helpful discussions and are indebted to the Aluminum Company of Canada Ltd. for permission to publish this paper.
REFERENCES N. Brown, J. Cryst. Growth, 16 (1972) 163. N. Brown, paper presented at meeting of I.C.S.O.B.X.. Bar&a Bystrica, Czechoslovakia, June 1979. S.I. Kuznetsov and V.A. Derevyankin, Physical Chemistry of the Bayer Process, ~Ioscow, 196-i. S. hlaricic and I. hlarkoreic, 2. Anorg. Allgem. Chem., 276 (1951) 193. T.G. Pearson, The Chemical Background of the Aluminium Industry, Monograph No. 3, Royal Institute of Chemistry, London, 1955. J. Scott, in Estractive hletallurg>- of _Muminum, Vol. 1, Interscience, New York. 1963. C. Misra and E.T. White, J. Cryst. Growth, S (197 1) li2.
S N.T. Chaplin. paper presented at the 100th Annual Meeting of _&ME, New York, Februaq: 1971. 9 K. Wefers and G.BI. Bell, Tech. Paper No. 19. .\lcoa Reasearch Laboratories, 19i9.