AU613758B2 - Method for removing sodium oxalate from caustic aluminate liquors - Google Patents

Description

,;1 COMMONWEALTH OF AU A PATENTS ACT 19529 COMPLETE SPECIFICATION

(ORIGINAL)

Class Application Number: Lodged: Complete Specification Lodged: o Accepted: o,.i Published: Briority Related Art: Form Int. Class Name of Applicant: Address of Applicant: VEREINIGTE ALUMINIUM-WERKE AKTIENGESELLSCHAFT Georg-von-Boeselager-Strasse 25, D-5300 Bonn 1, Germany Actual Inventor: NEIL BROWN Address for Service: XQWT A"-f--AA9 Watermark Patent Trademarlk: Attorneys 50 QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

Complete Specification for the invention entitled: METHOD FOR REMOVING SODIUM OXALATE FROM CAUSTIC ALUMINATE L-QUORS The following statement is a full description of this invention, including the best method of performing it known to i t T y P METHOD FOR REMOVING SODIUM OXALATE FROM CAUSTIC ALUMINATE LIQUORS o 0 o *00 0 0 900 0o c 0 '00 0 00 *0 o a 00 TECHNICAL FIELD The present invention relates to a method for removing sodium oxalate from the caustic aluminate liquors of the Bayer process for producing alumina from bauxite.

BACKGROUND OF THE INVENTION 0 In the Bayer process, bauxite is contacted with 20 recycled caustic aluminate liquor at ele-rated temperatures and 0 pressures to extract the alumina content of the bauxite. The resulting slurry thus contains dissolved alumina and undissolved red mud iron oxides, silicates, titanium oxide, etc.). The red mud is separated leaving a clear caustic 25 aluminate solution known as "pregnant liquor" which is then seeded with aluminum trihydroxide to precipitate approximately half of the dissolved alumina content. The precipitated aluminum trihydroxide is chen separated from the caustic aluminate solution. A portion of the precipitated aluminum trihydroxide is recycled to be used as seed for subsequent precipitation of aluminum trihydroxid6 and the remainder is recovered as product. The remaining caustic aluminate solution (hereinafter referred to as "spent liquor") is either recycled in the process for further alumina recovery as it is, or is in part concentrated by evaporation prior to recycling to the bauxite extraction step.

The bauxite used in the Bayer process contains organic substances which dissolve wholly or in part during bauxite

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r I 2 digestion. The organic substances degrade to lower molecular weight compounds under the influence of the high caustic con- Scentration and elevated temperatures experienced during bauxite digestion. Thus, the Bayer liquor may include various organic carbon compounds ranging from high molecular weight humic-type compounds to final degradation products such as sodium oxalate.

Sodium oxalate presents a special organics problem in that it is the only one of the many degradation products formed which accumulates to a concentration exceeding its solubility in solution. Caustic aluminate solutions are thus supersaturated with respect to sodium oxalate and are, to some extent, stabilized in this condition by the presence of the other organic carbon compounds in solution.

Sodium oxalate is thus a major impurity in the caustic s aluminate liquor of the Bayer process. As long as it stays dolt, dissolved in solution, sodium oxalate is considered to be

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ot relatively harmless. Difficulties arise however when the r dissolved sodium oxalate crystallizes at the temperatures and caustic concentrations employed at the end of the precipitation cycle of the Bayer process, the precipitation of product aluminum trihydroxide. The crystalline sodium oxalate which forms can interfere with particle agglomeration and stimulate the formation of fine new crystals of aluminum trihydroxide.

Thus, the presence of crystalline sodium oxalate has a deleterious effect on the particle size of the product aluminum trihydroxide. Moreover, the presence of crystalline sodium oxalate can cause difficulties in the filtration of aluminum trihydroxide slurries.

t A characteristic of modern alumina plants is that the aluminum trihydroxide precipitation circuit is divided into two parts. In the first part, the finer aluminum trihydroxide particles are subjected to relatively rapid size enlargement by an agglomeration mechanism whilst in the second part the agglomerates are consolidated into strong particles with crystal growth the main operating mechanism. The aluminum trihydroxide particles subjected to the agglomeration process are generally washed free of any crystalline sodium oxalate prior to

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3 precipitation, whereas aluminum trihydroxide entering the growth precipitators agglomerated particles recycled coarse material) is not subjected to any washing procedure.

The water used to wash aluminum trihydroxide seed and product particles can contain significant amounts of dissolved sodium oxalate. Typically, aluminum trihydroxide in the unwashed condition may contain 0.1 1.0% sodium oxalate, with respect to the overall quantity of aluminum trihydroxide. By concentrating wash waters by wash-water evaporation, it is possible to subsequently recrystallize the sodium oxalate which can then be separated and disposed of. Thus, seed washing constitutes a recognized method of removing sodium oxalate from I, the Bayer process (see, for example, Roberts et al., U.S.

Patent No. 3,372,985). It can be appreciated, however, that 15 sodium oxalate removal by the 'Seed Washing' process can only I function when crystalline sodium oxalate is already present in i 5 t 1 the aluminum trihydroxide precipitation circuit. In other |i words, the aluminum trihydroxide precipitation circuit must be, to some degree, in difficulty before 'Seed Washing' can work.

j 20 Other techniques have been developed and are used for removing sodium oxalate from the caustic aluminate liquors of the Bayer process. These generally involve seeding systems of v csome type and exploit the well-known sodium oxalate solubility relationships, particularly the temperature and caustic concentration dependencies (see, for example, Sato et al., U.S.

Ij Patent No. 3,649,185 and Fujiike et al., French Patent No.

i 2,405, 901).

Probably the most effective approach, at least in terms of consistently removing sufficient sodium oxalate from the process to maintain sodium oxalate, in both the dissolved and solid states, at acceptably low levels, is side-stream crystallization of sodium oxalate by seeding partially concentrated spent liquor by liquor evaporation, as opposed to wash-water evaporation (see, for example, Carruthers et al., U.S. Patent No. 4,038,039 and Yamada et al., U.S. Patent No. 4,263,261).

The long-standing problem of the progressive contamination and deactivation of the sodium oxalate seed crystals by the other t' 1 4 organic carbon compounds present in solution has been overcome by the employment of a suitable wash process which regenerates Sthe activity of the seed crystals and maintains a crystal form suitable for filtration and separation of the crystalline sodium oxalate (see, for example, Yamada et al., Light Metals Conf. Proceedings (1973) 745-754). Alternatively, techniques have been developed for removing the harmful organic contaminants prior to seeded crystallization of sodium oxalate (see, for example, Gnyra, U.S. Patent No. 4,275,043 and Lever, S 10 U.S. Patent No. 4,275,042).

Thus, side-stream crystallization of sodium oxalate from caustic aluminate liquor suitably concentrated by partial evaporation provides an attractive alternative to the 'Seed Washing' process for sodium oxalate removal. Not all alumina 15 plants, however, are equipped with liquor evaporators (in Sr*, itself a desirable goal due to the energy intensive nature of .liquor evaporation). This applies particularly to alumina r plants operating the tube digestion system.

Therefore, what is needed in the art is a method for S' ,20 sodium oxalate removal which achieves at least the equivalent to that of seeded crystallization of partially evaporated liquor without the need to preconcentrate using liquor evapora- I tion.

CUMMARY OF THE INVENTION -*ie The present invention has several objects includj, but not limited to, the following: to provide a new method for removing sod oxalate from caustic aluminate liquors of the ayer process in such a way that partial evaporati' of the liquor prior to sodium oxalate crystalli ion is not necessary; to provide a new ethod for removing sodium oxalate from caustic a lnate liquor of the Bayer process in such a y that contamination of the aluminum trih roxide precipitation circuit with crystalline sodium oxalate no longer occurs; to provide a method f-enhancing-te stability -of

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The present invention therefore provides a process for removing solium oxalate from a caustic aluminate liquor produced through bauxite digestion using the Bayer process, the liquor including aluminum trihydroxide and so&ium oxalate therein, the caustic aluminate liquor entering an aluminum trihydroxide precipitation circuit for removing the aluminum trihydroxide, leaving a spent liquor which is recycled to the bauxite digestion step, the process further including separating and removing an amount of sodium oxalate from the spent liquor, the sodium oxalate separation comprising the steps of: Sa. washing the removed aluminum trihydroxide obtained from the precipitation circuit essentially free of crystalline sodium oxalate; 15 b. adding a portion of the washed aluminum trihydroxide as seed in the precipitation circuit, the absence of solid sodium oxalate preventing further crystallization of sodium oxalate within the precipitation circuit, thereby raising the dissolved sodium oxalate concentration of the spent liquor over a period of time to a level not exceeding the metastable solubility of sodium oxalate; c. taking a side-stream from the spent liquor after aluminum trihydroxide precipitation and before the bauxite digestion step; d. adding sodium oxalate crystals as seed t thereto, thereby crystallizing sodium oxalate in an amount C which is at least equal to an amount of fresh sodium oxalate produced in the Bayer process from organic substances during bauxite digestion.

e. separating the crystalline sodium oxalate, leaving an oxalate depleted liquor; and f. combining the depleted liquor with the spent liquor for return to the bauxite digestion step. Preferably 1 4b the process also includes the step of adding an organic polymer at the end of the aluminum trihydroxide f precipitation circuit and before removal of the precipitated aluminum trihydroxide to stabilize against sodium oxalate crystallization.

Preferably the removed aluminum trihydroxide of the precipitation circuit is washed until the measured sodium oxalate content is 0.05%.

Preferably wherein the dissolved sodium oxalate concentration of the spent liquor rises due to the substantial absence of crystalline sodium oxalate in the aluminum trihydroxide precipitation circuit.

It is further preferred that the amount of sodium Soxalate seed crystals present in the side-stream crystallization is at least 10 times the amount of new sodium oxalate crystallized; and that the side-stream crystallization is carried out for about 30 min. to 120 min.

at a temperature of about 50-65 0

C.

Preferably the an organic polymer is added in an 2Q amount sufficient to prevent stimulated nucleation of sodium oxalate by the washed aluminum trihydroxide of the precipitation circuit and in an amount of up to about S; mg/l expressed as organic carbon equivalent.

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elevated concentration of dissolved sodium oxalate by the S addition, as and if required, of small amounts of suitable organic polymer.

Accordingly, a process for removing sodium o alate from the caustic aluminate liquor of the Bayer proce is disclosed which comprises the steps of: washing essentially all of t product aluminum trihydroxide of the precipitato n circuit free of Q_ 10 crystalline sodium oxalate; allowing the diss olved sodium oxalate concentration of the main p cess liquor stream to rise to a level which enabl side-stream crystallization to economically remo the required amount of sodium oxalate and at the s a time maintain the aluminum trihydroxide precipitati n circuit free of crystalline sodium oxalate; and, i required, enhancing the stability of the aluminum rihydroxide precipitation with respect to the elevated concentration of dissolved sodium oxalate by the addition The present invention describes a new method for achieving the aforementioned objectives. This is done by initially washing all of the aluminum trihydroxide free of crystalline sodium oxalate. The dissolved oxalate concentral ct l tion of the main process liquor stream then rises to a level which enables side-stream crystallization to economically remove sufficient sodium oxalate to enable steady operation of •the plant aluminum trihydroxide precipitation circuit free of crystalline sodium oxalate. Moreover, it has been found that the stability of the aluminum trihydroxide precipitation with respect to the elevated dissolved sodium oxalate concentration can be enhanced, as and if necessary, by the addition of small amounts of a suitable organic polymer.

SV on th asicauiat iur fteBae rce3 sdicoe whic comrise thestep of I 6 BRIEF DESCRIPTION OF THE DRAWINGS I Th Figure is a schematic illustration of the inventive Smethod for removing sodium oxalate from caustic aluminate liquors of the Bayer process.

DETAILED DESCRIPTION OF THE INVENTION The present invention describes a method for removing sodium oxalate from a caustic aluminate liquor of the Bayer process by side-stream crystallization while avoiding the need to partially evaporate the liquor prior to sodium oxalate crystallization.

A typical distribution of sodium oxalate in an aluminum trihydroxide precipitation circuit of an alumina plant which uses the seed washing system as the major means of sodium oxalate removal is as follows: j dissolved sodium oxalate 2.6 3.0 g/l i crystalline sodium oxalate 0.5 1.0% 1 At a caustic concentration of abodt 130 g/l (as the presence of the 0.5 1.0% of crystalline sodium oxalate provides a steady outlet for about 0.2 g/l of sodium oxalate i from the main process liquor by seeded crystallization. At steady state, this same 0.2 g/l corresponds approximately to the input of sodium oxalate to the process from the organic carbon substances which enter the process with the starting bauxite.

Thus, it has been found that when essentially all the aluminum trihydroxide is washed free of the crystalline sodium oxalate, the dissolved sodium oxalate concentration can rise to I and maintain a concentration of at least 4.0 g/l without crystallization of sodium oxalate occurring under aluminum trihydroxide precipitation conditions.

Using the well-known solubility relationships for sodium oxalate in the caustic aluminate liquor of the Bayer process, it can be readily calculated that for a solution 1 calculated with respect to aluminum trihydroxide present at 300-350 g/l.

.g TS;Tr;;P~oa8 ,,ta

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concentration of 130 g/l Na20free and a temperature of the supersaturation the crystallization driving force) with respect to sodium oxalate has been increased by a factor of about 100% from about 75% to about 150% supersaturated).

C Cnjr -C UALATE UXALAjT SUPERSATURATION CONCENTRATION IN LIQUOR

UALATAE

EQUILIBRIUM

SOLUBILITY

COXALATE

EQUILIBRIUM

SOLUBILITY

7.62 exp. [0.012T 0.016FS 0.011 C0 3 2 Vt

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Vte where T is liquor temperature in °c FS is liquor free soda concentration (expressed as g/l Na 2

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C0 3 2 is liquor carbonate concentration (expressed as g/1 Na 2 0) (See Brown et al., Light Metals Conference Proceedings (1980) 20 107).

According to the method of the prior art, the spent liquor of the Bayer process would require to be concentrated by partial evaporation from 130 g/l to 156 g/l Na2Ofree, an increase in liquor concentration of 20% in order to obtain the same increase in supersaturation with respect to sodium oxalate.

When caustic aluminate liquor at 4.0 g/l sodium oxalate is treated in side-stream crystallization at 600C with a 50 g/l 30 seed charge of previously crystallized sodium oxalate with a retention time of 30-60 min., the dissolved sodium oxalate concentration can be lowered to 2.0 g/1.

Thus, by continuously treating one tenth of the plant liquor flow (taken at the spent liquor stage following aluminum trihydroxide precipitation), the removal rate essentially matches the 'input' of sodium oxalate to the process.

It is recognized that operating at elevated concentrations of dissolved sodium oxalate in the complete absence of crystalline sodium oxalate is a fundamentally different situation from that where crystalline sodium oxalate is present ini the aluminum -trihydroxide precipitation circuit. There is no longer the easy outlet for sodium oxalate due to the r 8 presence of seed crystals. It is thus important to establish how high the sodium oxalate concentration of the process liquor can be allowed to rise without resulting in a sudden explosion of new sodium oxalate crystals, which must be avoided.

Appropriate experiments carried out at the laboratory level show that the dissolved sodium oxalate concentration can rise to 4.5 g/l approaching 200% supersaturated) before stimulated nucleation of sodium oxalate occurs due to the presence of the aluminum trihydroxide particles.

1 0 Further experimental work at the laboratory level has shown that the addition of small amounts of a sodium polyacrylate (such as Nalco product M8081) hinders the stimulated nucleation of sodium oxalate by solid aluminum trihydroxide.

The method of the present invention is illustrated in 15 the Figure which shows a schematic diagram of the main Bayer process with the side-stream sodium oxalate removal system of the present invention. Bauxite is first fed to an extractor and combined with a spent caustic aluminate liquor. During Sbauxite digestion, alumina is dissolved as well as other 20 organic compounds. The resulting slurry is then filtered, i removing the undissolved impurities known as red mud, leaving a clear caustic aluminate solution rich in aluminum trihydroxide r (pregnant liquor). -The pregnant liquor is cooled and seed Sadded to precipitate the aluminum trihydroxide, forming an agglomerate which is grown and strengthened to product size, f C and removed by filtration, leaving a spent liquor for recycle to the bauxite extractor. The product aluminum trihydroxide is then separated and water washed, preferably in a drum filter, with a portion returned as seed to the pregnant liquor, completing the precipitation circuit.

Introduced into tii process is a sodium oxalate removal step wherein approximately 1/10 of the main recirculating spent caustic aluminate liquor of the Bayer process is taken for side stream crystallization. This spent liquor has an elevated sodium oxalate concentration by virtue of having washed all of the aluminum- trihydroxide of the precipitation circuit free of crystalline sodium oxalate, preventing sodium oxalate crystali. K 9 lization in the aluminum trihydroxide precipitation circuit.

Thus, the spent liquor of raised sodium oxalate concentration "i is brought into contact with sodium oxalate seed crystals supplied in an amount of about 50 g/l, or for an exemplary circuit, an amount at least equal to that required to crystallize at least 2 /1 sodium oxalate at a temperature of about The residence time within the crystallizer depends upon the temperature and quantity of seed crystals used, but is optimally within the range 30-120 min.

In the flow sheet of the Figure, the present invention is shown in the form of a continuous process which is preferred, but the present invention may be operated in either a batch process or a semi-continuous process.

Next, the present invention will be described further 15 in terms of the following examples, which are intended to f: illustrate the basis of the invention without limiting its scope.

EXAMPLE 1 't A Bayer process spent liquor having a composition which is approximately 134 g/l Na20 free, approximately 90 g/l A1 2 0 3 having a slightly elevated dissolved sodium oxalate concentration of about 3.2 g/l, was introduced in aliquots of 800 ml to a series of three 1 liter capacity polyethylene bottles. To each was added 300 g/l of aluminum trihydroxide solids containing different amounts of crystalline sodium oxalate. The bottles were then closed and the slurries rotated end-over-end in a onstant temperature water bath at 60°C for 44 hours. At the end of the experiment, all solids were analyzed for the presence of crystalline sodium oxalate. The following results were obtained: START END Crystalline Sodium Crystalline Sodium Oxalate in Al(OH) 3 Oxalate in Al(OH) 3 0.50 0.68 0.25 0.30 0.01 0.01 The results show that the presence of the crystalline sodium oxalate in the starting aluminum trihydroxide promotes the crystallization of further sodium oxalate during precipitation due to the seeding effect. In the absence of any Smeasurable crystalline sodium oxalate at the start, there is no formation of new crystalline sodium oxalate, when holding the slurry for extended periods at 60 0 C under aluminum trihydroxide precipitation conditions.

EXAMPLE 2 Spent liquor having the same composition as in Example 1 was adjusted to have dissolved sodium oxalate concentrations of 4.0 and 4.6 g/l respectively. The liquors were then held for extended periods at 60°C in the presence of 200 g/l waterwashed aluminum trihydroxide from the Bayer process. The experiment was carried out in the laboratory in the same way as that described in Example 1.

15 The results obtained were as follows: STARTING RESIDENCE DISSOLVED CRYSTALLINE SODIUM OXALATE TIME SODIUM OXALATE SODIUM OXALATE I CONCENTRATION CONCENTRATION IN Al(OH) 3 (hours) 20 4.0 0.01 36 3.9 0.01 46 4.1 0.01 16 4.6 0.01 I 4.6 24 4.4 0.01 S36 4.5 0.01 i In the absence of crystalline sodium oxalate in the seed Al(OH) 3 the results indicate that a spent liquor with a Ssodium oxalate concentration of up to about 4.6 g/l is stable i with respect to sodium oxalate under aluminum trihydroxide precipitation conditions.

EXAMPLE 3 Spent liquor of approximately the same composition as that in Example 1 was adjusted to have a sodium oxalate composition of 4.1 g/l, and then seeded with 50 g/l of crystalline sodium oxalate (separated from the Bayer process) which had first been 'washed' free of other organic contaminants by exposing the sodium oxalate crystals briefly, in accordance with the prior art, to a dilute caustic solution undersaturated 11 with respect to sodium oxalate. Thus, 2-3 weight percent of the seed crystals were removed in the wash process.

Sodium oxalate crystallization was carried out on the seed crystals in cyclic experiments at 60 0 C for 60 min. with periodic washing of the sodium oxalate crystals according to the aforementioned procedure. Dissolved sodium oxalate concentration and filtration time of the slurry [constant temperature filtration at 60°C using Nr. 595 0 125 mm filter paper (Selecta: Schleicher Schull)] were measured at the end S 10 of each cycle.

The following rec-lts were obtained: Sodium Oxalate Cycle No. Concentration Filtration Time (min.) 1 2.1 16 S2 2.4 22 3 2.3 34 Oxalate 20 'washed' 12 S4 2.2 23 2.3 S Oxalate S 'washed' 6 1.9 37 The filtration time of the starting 'washed' sodium oxalate was 13 min.

The results indicate that the 50 g/l sodium oxalate seed charge was barely sufficient to achieve the required decrease in dissolved sodium oxalate concentration of 2 g/l.

Here, an increase in the seed charge or increased residence time would have lowered the dissolved sodium oxalate concentration to within the range 1.5-2.0 g/l. The expected poisoning effect of adsorbed organics did not seriously interfere with oxalate crystallization. Rather, the effect was greater on the measured filtration time after crystallization, the filtration time responding more positively to the applied 'wash' procedure. Branching of the oxalate crystals in response to adsorbed organics is believed to be responsible for poorer filtration, with the branch structures- collapsing under the influence of the 'wash' procedure.

i I i 12 EXAMPLE 4 Spent liquor (composition: Na2Ofree 135 g/l; Al203 92 g/1) was adjusted to a sodium oxalate concentration of 4.8 g/l at 700C. 500 ml of this liquor was introduced into each of three polypropylene vessels. To two of the vessels was added 100 g of washed Al(OH) 3 and to one of these was further added mg/l (expressed as organic carbon) of sodium polyacrylate (Nalco M8081 having a molecular weight of about 4 x 106). To begin the experiment, the three vessels were closed and placed in a constant temperature water bath at 70 0 C. They were then rotated end-over-end at 8 RPM and the temperature lowered to 39 0 C over a period of 41 hours. Samples were removed for sodium oxalate analyses as a function of time. The results were as follows: 015 I Time Temp. Sodium Oxalate Concentration (g/l) I (Hours) Liquor Liquor Liquor Al(OH)3 Al(OH)3 +I Sodium Polyacrylate 0 70 4.8 4.8 4.8 4 65 4.7 4.7 4.8 60 4.8 3.7 26 50 4.8 3.9 4.6 39 41 4.6 2.5 2.6 i l" The presence of A1(OH) 3 stimulates the formation of new sodium oxalate crystals at a temperature of 60-65°C. When sodium polyacrylate is used, however, the onset of new crystal formation is delayed until the temperature has decreased below until the supersaturation with respect to dissolved sodium oxalate is 200%.

EXAMPLE 2 Spent liquor (composition: 131 g/1 Na20free; 86 g/1 A1 2 0 3 was adjusted to a sodium oxalate concentration of g/l at 60°C. 500 ml of liquor was introduced into each of two polypropylene vessels along with 100 g of washed Al(OH) 3 To one of these was further added 30 mg/l of sodium polyacrylate.

The experiment was carried out at a constant temperature of 60°C with samples removed for sodium oxalate analyses as a function of time. The results were as follows: t 1 I 13 Time Temp. Sodium Oxalate Concentration (g/l) (Hours) Liquor Liquor Al(OH) 3 Al(OH) 3 Sodium Polyacrylate 0 60 4.5 4 60 4.5 14 60 4.2 4.6 60 3.9 4.6 The results confirm that the presence of Al(OH) 3 in the liquor can stimulate the formation of new sodium oxalate crystals in liquor containing 4.5 g/l dissolved sodium oxalate.

In the presence of added (30 mg/l) sodium polyacrylate, the spent liquor is stable with respect to sodium oxalate for at least 20 hours at 60 0 C. The results confirm the temperaturetime dependence of the nucleation process.

EXAMPLE 6 t The experiment of Example 5 was repeated under identical conditions except that unwashed Al(OH) 3 containing 0.89% i crystalline sodium oxalate was used. The results were as *follows: Time Temp. Sodium Oxalate Concentration (g/l) (Hours) Liquor Liquor unwashed Al(OH) 3 unwashed Al(OH) 3 Sodium Polyacrylate 0 60 4.5 4 60 4.4 4.2 14 60 4.2 4.1 60 4.1 3.8 The results show that the added (30 mg/l) sodium polyacrylate is ineffective in stabilizing the dissolved sodium oxalate in the liquor. Presumably, this is due to the relatively large presence of Al(OH) 3 solids on which the polymer adsorb thus 'diluting' its effectiveness with respect to crystalline sodium oxalate.

The addition of large amounts of polymer to reach a level where sodium oxalate crystallization is inhibited is impractical due to the resulting viscosity increase of the liquor which .decreases the subsequent filtration rate of the aluminum trihydroxide slurry.

I C i i 14 EXAMPLE 7 The experiments of Examples 5 and 6 were repeated under the same conditions except for the order of solids addition to the vessels prior to the start. In one case, 200 g/l washed Al(OH) 3 and 1 g/1 crystalline sodium oxalate were suspended in spent liquor and then 30 mg/l of sodium polyacrylate added. In the second case, 1 g/1 crystalline sodium oxalate was suspended in spent liquor, 30 mg/l of sodium polyacrylate added to the suspension followed by the 200 g/l washed Al(OH) 3 The results S 10 subsequently obtained at 60*C and retention times of up to hours were as follows: Time Temp. Sodium Oxalate Concentration (g/l) (Hours) Liquor Liquor washed Al(OH) 3 Sodium Oxalate Sodium Oxalate Sodium Polyacrylate Sodium Polyacrylate washed Al(OH) 3 .0 60 4.5 4 60 4.3 4.6 14 60 4.1 4.4 60 3.8 4.4 The results confirm that sodium polyacrylate does inhibit crystallization of sodium oxalate in the mixed solids suspension provided the polymer molecules are given the opportunity to 'see' the crystalline sodium oxalate.

EXAMPLE 8 Bayer liquor (composition: Na2Ofree 136 g/l; A1 2 0 3 118 typical of that entering the growth precipitation section of a modern alumina plant, was adjusted to a sodium oxalate concentration of 3.2 g/l and seeded with 200 g/l of washed Al(OH) 3 the latter having a particle size distribution typical of that of the industrial process. Precipitation tests were carried out in the previously described equipment (with a liquor volume of 500 ml in each vessel) at 60 0 C for 24 hours as a function of added sodium polyacrylate (15 and 30 mg/l, expressed as organic carbon equivalent). The following results were obtained:

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Sodium Liquor Liquor Particle Size Analyses Polyacrylate A1 2 0; Sodium Wt.%>90um Wt.%<45um No. of Addition Conc n Oxalate Particles Conc'n per g (mg/1) (g/l) 0 97.1 3.2 43.4 14.4 3.19x10 6 96.6 3.3 45.0 15.8 3.01x10 6 30 96.6 3.2 44.0 16.7 3.22x10 6 I 10 Seed A1(OH) 3 46.7 15.6 3.91x10 6 The results indicate that the addition, in small 1 amounts, of sodium polyacrylate under 'Growth' precipitation conditions has no significant effect on liquor productivity or particle size of the product aluminum trihydroxide.

Thus, the samples show that, provided the aluminum trihydroxide of the precipitation circuit is maintained free of crystalline sodium oxalate contamination, then the spent liquor or near spent liquor of the precipitation circuit is stable with respect to sodium oxalate at concentrations of the latter which are sufficiently high to allow an economical removal of sodium oxalate by side-stream crystallization.

The inhibiting action of sodium polyacrylate on homogeneous nucleation of sodium oxalate and crystallization of sodium oxalate seed crystals in caustic aluminate liquor is already established (see Lever, Travaux du Comite International pour l'etude des Bauxites, de 1'Alumine et de 1'Amuninium. Vol.

13, Nr. 18 (1983) 335-344). It is also known that sodium polyacrylate is effective at low additions (concentration of maximum effectiveness is 60 mg/l, expressed as equivalent organic carbon), and over a wide range of molecular weights.

However, as the present invention shows, sodium polyacrylate additions up to 30 mg/l are surprisingly ineffective as inhibitor of sodium oxalate crystallization where a relatively small quantity of crystalline sodium oxalate is present in admixture with a relatively large amount of crystalline Al(OH) 3 In comparison with the seed washing process and the side-stream crystallization process using partially evaporated S liquor, the method of the present invention is superior in that -r t 16 no liquor is required; the aluminum trihydroxide precipitation circuit of the Bayer process is completely free of contamination by crystalline sodium oxalate; and the aluminum trihydroxide precipitation circuit can be stabilized in the 'crystalline oxalate-free' condition by the addition of small amounts of an organic polymer such as sodium polyacrylate.

Although the invention has been shown aind described with respect to detailed embodiments, it should be understood K 10 by those skilled in the art that- the present invention is not limited to the detailed description but rather includes any equivalent modifications thereto as will suggest themselves to those skilled in the art. It is therefore intended that the following claims cover such modifications as fall within the Si' 15 spirit and scope of the invention.

Claims (6)

1. 2. The process of Claim 1, further comprising: a. adding an organic polymer at the end of the aluminum trihydroxide precipitation circuit and before removal of the precipitated aluminum trihydroxide to stabilize against sodium oxalate crystallization.
2. 3. A process according to Claim 1 wherein the removed aluminum trihydroxide of the precipitation circuit is washed until the measured sodium oxalate content is 0.05%.
3. 4. A process according to Claim 1 wherein the dissolved sodium oxalate concentration of the spent liquor rises due to the substantial absence of crystalline sodium S oxalate in the aluminum trihydroxide precipitation circuit. S 5. A process according to Claim 1 wherein the amount of sodium oxalate seed crystals present in the side-stream crystallization is at least 10 times the amount of new sodium oxalate crystallized.
4. 6. A process according to Claim 1 wherein side-stream Scrystallization is carried out for about 30 min. to 120 min. at a temperature of about 50-651C.
5. 7. A process according to Claim 2 wherein an organic polymer is added in an amount sufficient to prevent stimulated nucleation of sodium oxalate by the washed j aluminum trihydroxide of the precipitation circuit. IC _JC IU**1~~~HYYll;aUII~U~ ~-^IPIP~ 19
6. 8. A process according to claim 7 wherein the organic polymer is sodium polyacrylate added in an amount of up to about 30 mg/l expressed as organic carbon equivalent. DATED this 8th day of April 1991. VEREINIGTE ALUMINIUM-WERKE AKTIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN, VICTORIA 3122 AUSTRALIA IAS:JJC:KG (7.12) Ct;t Cr C C 4 i C 4 0* 9 r 0 a*64 R N A o Y