AU1664292A

AU1664292A - Method for controlling sodium oxalate levels in sodium aluminate solutions

Info

Publication number: AU1664292A
Application number: AU16642/92A
Authority: AU
Inventors: Robert Clegg; Bryan A Hiscox; Jacques Laurier; Jean Victor Sang
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 1991-05-03
Filing date: 1992-05-01
Publication date: 1992-12-21
Anticipated expiration: 2012-05-01
Also published as: AU650747B2; BR9205965A; CA2108785C; WO1992019537A1; IE68425B1; CA2108785A1; IE921423A1

Description

Method for the Control of Sodium Oxalate Levels in Sodium Aluminate Solutions

Technical Field

This invention relates to a method for the removal of

5 organic substances from a sodium aluminate solution in the production of alumina from bauxite using the Bayer process. More particularly, the invention relates to a method for the efficient removal of sodium oxalate in the sodium aluminate solution of the Bayer process.

10 Background Art

As is well known, the production of alumina by the Bayer process comprises the steps of treating aluminous ores such as bauxite (hereinafter referred to as bauxite) with a hot caustic solution, such as a caustic soda

15 solution at a temperature generally above 130°c, to extract the alumina portions contained in bauxite (the digestion step) ; separating undissolved residues such as iron oxide, silicates, titanium oxide and the like as red mud from the slurry obtained in the digestion step (the

20 red mud sedimentation step) ; adding particles of aluminum hydroxide as seed to a clarified sodium aluminate solution after the separation of the undissolved residues (hereinafter referred to as the Bayer solution) to precipitate aluminum hydroxide at a temperature generally

25 between 85° and 50°C. (the precipitation step) ; separating the precipitated aluminum hydroxide from the sodium aluminate solution (the separation step) ; recycling a portion of separated aluminum hydroxide precipitate as seed to the precipitation step; and withdrawing the

30 remaining portion of separated aluminum hydroxide precipitate as product, while recycling the sodium aluminate solution after the separation of aluminum r hydroxide precipitate (hereinafter referred to as the * spent liquor) , as it is or after evaporation, to the

35 digestion step for bauxite.

The process solutions used in the Bayer process contain among other constituents large concentrations of sodium hydroxide, some sodium carbonate, and dissolved alumina in the form of sodium aluminate. Solutions of sodium aluminate that are supersaturated at the precipitation temperatures are referred to as pregnant liquors or solutions; those of lower alumina concentration are the spent liquors or solutions. The concentrations of dissolved alumina and total caustic are usually expressed in terms of the alumina/caustic ratio, in which the alumina is expressed in g/L of A1₂0₃, while the caustic is expressed as g/L of equivalent Na₂C0₃. Usually, pregnant liquors are characterized by an alumina/caustic ratio between 0.55 and 0.75, and spent liquors by an alumina/caustic ratio between 0.25 and 0.50.

The starting bauxite contains various organic substances and these tend to dissolve in the sodium aluminate solution in the digestion step for bauxite. Also, flocculating agents such as starch or a high molecular weight coagulant, such as sodium polyacrylate, may be added to the slurry solution obtained in the digestion step. As a result, organic substances gradually accumulate in the circulated sodium aluminate solution of the Bayer process or in the spent liquor of the Bayer process.

The organic substances in the Bayer solution are present in various forms and include sodium oxalate. Among such organic substances, those having a low solubility in the sodium aluminate solution of the Bayer process, e.g. sodium oxalate, precipitate as crystals and are attached to the surface of the aluminum hydroxide in the precipitation step. When such aluminum hydroxide with sodium oxalate attached is recycled as seed to the precipitation step, the growth of aluminum hydroxide crystals is hindered, so that aluminum hydroxide precipitate having a large grain size and resistant to particle breakdown (attrition) is very difficult to produce. Also, aluminum hydroxide withdrawn as product is contaminated with sodium oxalate, resulting in a lowering of the purity. Moreover, the accumulated organic substances hinder the sedimentation of red mud and the separation of precipitated aluminum hydroxide from sodium aluminate solution, thereby significantly lowering the. efficiency of alumina production. In order to remove organic substances, such as sodium oxalate, from aluminum hydroxide withdrawn as product or aluminum hydroxide used by recycling as seed for the precipitation step of the Bayer process, various methods have been proposed. U.S. Patent 3,372,985 describes a procedure in which all of the alumina hydrate seed is filtered and washed in two stages. In the second stage of washing, sodium oxalate is dissolved in the washing liquor and removed, and then caustic soda is recovered by treating the washing liquor. But, as this method filters and washes great quantities of solids, it requires equipment of large scale and a vast volume of washing liquor.

U.S. Patent 3,899,571 teaches the addition of seed crystals of sodium oxalate to promote the precipitation of sodium oxalate from a spent Bayer liquor solution in which the sodium oxalate solution is supersaturated. Prior to use, these seed crystals are physically separated from the slurry in which they are produced, and subjected to a washing step to restore or improve their effectiveness as seed. This is an uneconomical procedure with the washing step requiring large scale equipment.

U.S. Patent 4,201,749 teaches a process to remove the oxalate containing mother liquor surrounding the desired aluminum hydroxide product. It was discovered that this could be done by washing the product with first a solution concentrated in sodium oxalate to remove the sodium aluminate and second with hot water to dissolve the oxalate retained by the aluminate hydroxide.

U.S. Patent 4,263,261 teaches the recycling of the precipitated products as seed crystals to increase the removal of dissolved sodium oxalate. This procedure is distinguished by the step of dissolving 10 to 50% of the newly precipitated product in an unsaturated aqueous solution of sodium oxalate to remove the impurities which have been coprecipitated with the sodium oxalate, before using the remaining washed crystals as recycled seed in the next cycle of the process.

Australian Patent Publication No. Au-A-35943/89, published July 12, 1989, (Brown) describes a process for removing sodium oxalate by adding sodium oxalate seed to a spent liquor and causing the sodium oxalate to precipitate. However, in order to do this, an additional amount of suitable organic polymer must be added to the saturated sodium aluminate solution to ensure that the sodium oxalate does not precipitate during the precipitation of aluminum hydroxide. British Patent Publication No. 2,211,832, published July 12, 1989, (Keeney et al) describes the removal of sodium oxalate by crystallization in the presence of seed crystals of spherulite morphology. This is achieved by precipitating the seed crystals from a solution containing a substance that causes the sodium oxalate to crystallize in spherical form. The seed crystals of sodium oxalate are added to the solution as a solid.

When sodium oxalate precipitated from spent liquor is recycled as seed to precipitate sodium oxalate, there is the disadvantage that the sodium oxalate seeds lose their activity as seed during recycling due to the adsorption of organics present in the Bayer solution. Washing with water to reactivate the seed is not satisfactory because this cannot be done without dissolving part of the oxalate seed itself. As a result, it is difficult to maintain low oxalate levels in the treated solution and oxalate precipitation can then occur in other parts of the precipitation circuit with resulting detrimental effects. It is the object of the present invention to find a new and simplified procedure for solving the above problems. Disclosure of the Invention The present invention solves the problem of reactivating oxalate seed by avoiding the need to recycle the oxalate seed. Instead, the present invention utilizes a highly concentrated solution of dissolved oxalate to precipitate new and very reactive seed. This new and reactive seed is added to a process liquor or solution circulating through the Bayer process and thereby precipitates out any remaining dissolved oxalate, which is then separated and the clarified liquor is returned to the process. The result is low oxalate concentrations in the liquor and, therefore, precipitation of oxalate in other parts of the precipitation circuit can be avoided. In a particular application of this invention, the coarse aluminum hydroxide seed can be kept free from oxalate contamination which would promote excessive nucleation of alumina hydroxide fines.

Precipitation of sodium oxalate will occur when its concentration is greater than that required for autonucleation. The autonucleation concentration is a function of temperature, concentration of stabilizers, such as dissolved humates, organic compounds and polymers, and the concentration of caustic. However, the autonucleation concentration for most Bayer process liquors is between 3 and 7 g/L sodium oxalate depending upon the above variables, and generally around 4-5 g/L of sodium oxalate.

Thus, the present invention relates to a process for removing sodium oxalate from a sodium aluminate liquor produced through bauxite digestion using the Bayer process, the liquor including aluminum hydroxide and sodium oxalate therein. The sodium aluminate liquor enters an aluminum hydroxide precipitation circuit for removing the aluminum hydroxide, leaving a spent liquor, which is typically recycled to the bauxite digestion step. The sodium oxalate removal procedure of the present invention comprises the steps of (a) continuously generating new and reactive seed particles of sodium oxalate by mixing together an aqueous solution of sodium oxalate and a portion of a Bayer process sodium aluminate liquor to achieve a concentration of sodium oxalate within a range to generate spheroidal particles of sodium oxalate, (b) adding a suspension of the above spheroidal particles of sodium oxalate as seed to a second portion of the Bayer process sodium aluminate liquor, to thereby precipitate the remainder of dissolved sodium oxalate and (c) separating the sodium oxalate precipitate, leaving a liquor free from sodium oxalate contamination.

The Bayer process sodium aluminate liquor to which the present invention relates may be any sodium aluminate solution or liquor of the Bayer process, but is usually a spent liquor from the final stages of classification and typically has an alumina/caustic ratio (Al₂0₃/Na₂C0₃) of about 0.25 to 0.50. The spent liquor may be entirely free of aluminum hydroxide particles, or may include suspended aluminum hydroxide particles which were not removed during classification and filtration.

As stated hereinabove, autonucleation concentration for most Bayer process liquors is between about 3 and 7 g/L sodium oxalate. However, it has been found according to the present invention that there is concentration range of sodium oxalate above this at which spheroidal particles are generated. Typically, the concentration should be at least about 5% above the autonucleation concentration. There is also an upper limit beyond which spheroidal particles are no longer produced and fine needles are formed which are difficult to filter. Thus, the maximum concentration is in the order of about 75% above the autonucleation concentration. However, it is generally preferred to operate in the range of about 10 to 25% above the autonucleation concentration. The best results are obtained with the process of the present invention when the freshly prepared seed particles of sodium oxalate are used directly as seed without any intermediate storage or any intermediate treatment, such as washing.

The spheroidal particles of sodium oxalate may be conveniently separated from their production solution by 5 filtration, sedimentation, hydrocloning, etc. to form a concentrated suspension. This concentrated suspension, without any kind of intermediate treatment, is fed as seed directly into the Bayer process liquor. Alternatively, the entire seed production suspension, including 10 spheroidal particles of sodium oxalate and their mother liquor may be added directly to the main stream of Bayer process liquor.

While the benefits of the present invention can be achieved by adding the sodium oxalate seed crystals to 15 spent liquor circulating through any of the stages of the Bayer process, it is particularly effective at the tertiary classification stage, and at the final precipitation stages.

It is also preferable to utilize a two stage washing 20 to separate the sodium oxalate for further treatment. Brief Description of the Drawings The invention is described with respect to the attached drawings in which:

Figure 1 is a schematic flow diagram showing the 25 relationship of the oxalate removal system of the invention with respect to the precipitation and classification system of the Bayer process, and

Figure 2 is a schematic flow diagram showing an alternative procedure. 30 Best Modes For Carrying Out The Invention

As shown in Figure 1, the numeral 10 represents a tertiary classifier of a traditional Bayer process with a Bayer liquor feed 11 of fine aluminum hydroxide seed and spent liquor from a previous stage. An underflow stream 35 13 is withdrawn from the bottom of classifier 10 and is recycled to feed line 11. Spent liquor is drawn off at 14. A part of recycle stream 13 is drawn off via line 15 and is fed to a primary seed filter 19. This filter is designed for efficient washing of a filter cake. The filter 19 is washed by a wash stream 31 which comprises a filtrate from secondary seed filter 26. This wash is rich in oxalate and low in soda and removes the sodium aluminate liquor retained by the filter cake without removing oxalate crystals. The primary filtrate 20 is fed as a component to oxalate seed generating vessel 17. The primary filter cake 21 from primary filter 19 is transferred to reslurry tank 22 and agitated for a period of 1/2 to 1 1/2 hours where it is reslurried in a stream of secondary filter wash filtrate 25 plus, if necessary, hot condensate make up 23, the flow of which controls the concentration of sodium oxalate in the reslurry liquor. The reslurry tank level is maintained by a draw off pump. The wash filtrate 25 is heated to 65-100^βC by steam 24 whereby solid sodium oxalate present in the vessel 22 is dissolved- The concentration of aluminum hydroxide solids in the reslurry tank 22 is normally uncontrolled, but may be adjusted separately from the oxalate concentration or by the use of the strong filtrate recycle 34 from secondary filter 26. Normally this is necessary only during start up of the process. After a sufficient residence time, a flow of slurry 27 is withdrawn from the bottom of reslurry tank 22 and is filtered by the secondary seed filter 26. This filter is also adapted for washing a filter cake.

In filter 26, the oxalate rich liquor in the filter cake is displaced by a hot condensate wash 28. It is advantageous to take two filtrates from filter 26. The strong filtrate 30 is used in streams 31, 32, 33 and 34. The more dilute later wash filtrate 25 is in part recycled to reslurry tank 22 via line 25 and the washed filter cake 29 is used as fine seed in the main aluminum hydroxide precipitation circuit.

The main filtrate stream may be split in varying proportions between streams 25 and 30 for fine tuning of the process and stream 33 is fed to oxalate seed generating vessel 17, and ultimately returns to the tertiary classifier 10 via line 18. In the reactor 17 which may have a short residence time, the oxalate rich secondary filtrate stream 33 is reacted with Bayer liquor feed 16 and primary filtrate 20 to produce fresh finely divided oxalate crystals which move via line 18 to become part of the feed stream 11 to tertiary classifier 10. These fresh finely divided oxalate crystals effectively strip the dissolved oxalate from the spent liquor in the tertiary classifier.

A stream of oxalate rich secondary filtrate 32 may be directed to the oxalate disposal system the flow rate of which is varied according to how much oxalate should be directed to 17 and this stream may be treated by any technique appropriate to the location, e.g. lime precipitation of calcium oxalate, biological decompo¬ sition, ^'etc. In the case of a plant operating on a relatively high silica bauxite, which necessarily has a large requirement for make up caustic, an appropriate technique is to salt out the oxalate in a relatively pure form using concentrated (about 50% w/w) caustic. The sodium oxalate crystals may then be filtered off, and high caustic filtrate along with some spent liquor is used to prepare the oxalate seed. The oxalate crystals may be sold as a by-product, calcined in the liquor purification unit, etc.

The stream 34, which recycles strong oxalate liquor to the reslurry tank 22, may be used during start up, or as an additional control on the aluminum hydroxide solids concentration in reslurry tank 22. The stream 31 is used as a displacement wash on primary filter 19 to displace spent liquor from the filter cake 21 whilst leaving its oxalate concentration unchanged.

In the flow sheet shown in Figure 2, the numeral 40 represents a Bayer liquor stream from anywhere in a Bayer circuit, preferably a spent liquor after precipitation of the aluminum hydroxide. A side stream 41 of this liquor is cooled in cooler 42 to a temperature between 40 and 70^βC, preferably 50^βc. The cooled outlet stream 43 is divided into two streams 44 and 45. The stream 44 feeds into a reaction vessel 47 which optionally connects to a second reaction vessel 48. The stream 45 feeds into a reaction vessel 46 for the production of sodium oxalate seed. Also feeding into the reaction vessel 46 is a stream 60 of oxalate-rich solution which has been produced in vessel 58. The oxalate-rich stream 60 and the process stream 45 are mixed in vessel 46 in the proper proportions to produce the required spheroidal particles of sodium aluminate to be used as seed. A stream 49 carries a suspension of these particles to reactor vessel 47 where this comes in contact with stream 44 of process liquor. The discharge 50 from reactor vessel 48 flows to a filter 51 with the filtrate from this filter being recycled to combine with process stream 40 and the filter cake 53 being deposited on a conveyor belt system 54 which can move in either direction. This conveyor belt 54 may deliver the solids 53 (1) into vessel 47, or (2) into vessel 58 via discharge 57 or (3) via discharge 55 into disposal area 56.

When the solids product 53 from filter 51 is being used as solids feed 57 to reactor vessel 58, side stream 61 is operated such that solid product 53 is subjected to a displacement wash whereby residual spent liquor is displaced by an oxalate-rich stream from discharge 60. The procedure shown in Figure 2 is of particular interest for use with Bayer liquor which is free of particles of aluminum trihydrate. The oxalate-rich stream 60 is used to raise the oxalate concentration in reaction vessel 46 to 0.25-3.0 gpl, preferably 0.5-1.00 gpl over the critical oxalate concentration in reactor 46. The autonucleation concentration is, of course, that concentration above which sodium oxalate spontaneously crystallizes from unseeded Bayer liquor at the temperature, caustic and impurity concentration existing. This value is normally about twice the absolute solubility of sodium oxalate under the specified conditions.

Additional preferred embodiments of this invention are described in the following non-limiting examples. In these examples the spent Bayer liquor contains:

Caustic, (expressed as Na₂C0₃) 265 g/L Alumina 111 g/L

Sodium Oxalate (Na₂C₂0₄) 3.2 g/L

Temperature 50°C

The oxalate washing solution contains: Caustic (expressed as Na₂C0₃) 10 g/L Sodium Oxalate (Na₂C₂0₄) 30 g/L

Temperature 50°C

Example 1

100 mL of oxalate washing solution was added at the rate of 100 mL/min. to 1000 mL of spent Bayer liquor. The mixture contained 5.64 g/L dissolved sodium oxalate. It was held at 50"C, and stirred continuously. The concentration of dissolved sodium oxalate was measured at intervals, and found to decrease as follows.

Time Elapsed (h) Conc'n Dissolved Oxalate g/L 0 3.2

1/4 1.7

1/2 1.4

1 1.4

2 1.4 There was good removal of dissolved sodium oxalate from the mixture.

The precipitated sodium oxalate was found to be in the form of spheroidal particles. Such spheroidal particles, unlike needle-like particles, are easy to filter or separate by other means from the liquor. It is also noteworthy that the total concentration of dissolved sodium oxalate in the combined solution was 5.64 g/L, which is above the concentration at which autonucleation and spontaneous precipitation occurs.

Example 2

The same procedure described in Example 2 was repeated, but 200 mL, i.e. double the amount of Example 1, of oxalate washing solution was added. This mixture contained 7.6 g/L of dissolved sodium oxalate. The concentration of dissolved sodium oxalate with time varied as follows:

Again there was good removal of dissolved sodium oxalate from the solution. However the precipitated sodium oxalate was in the form of fine needles, which are difficult to filter, and are unsatisfactory for easy separation from the liquid.

This Example shows that an increase of the dissolved sodium oxalate concentration to 7.6 g/L, 90% above the concentration for autonucleation creates an excessive amount of nuclei, which results in the precipitation of many fine crystals. This experiment showed that the morphology of the precipitated crystals is affected by the concentration of dissolved sodium oxalate at the beginning of the crystallization. Example 3 According to the procedure of Example l, 25 g/L of oxalate washing solution was added to 1000 mL of spent Bayer liquor. The resulting mixture contained 3.85 g/L of dissolved sodium oxalate. The concentration of dissolved sodium oxalate with time varied as follows:

There was no removal of sodium oxalate from the solution. This is because the concentration of dissolved sodium oxalate was below the concentration at which autonucleation occurs. Example 4

The same solutions used in Example 3 were again reacted, but in two steps.

In the first step, 25 mL of oxalate washing solution was added to 250 mL of spent Bayer liquor. The concentration of dissolved sodium oxalate was 5.64 g/L. The combined solution was held at 50"C for 15 minutes with stirring.

In the second step, the remaining 750 mL of spent Bayer liquor was added to the solutions prepared in step 1. This gave a solution in which the total sodium oxalate concentration was 3.85 g/L, equivalent to that of Example 3. The variation of dissolved sodium oxalate concentration with time was as follows:

Under these conditions, there is good removal of dissolved sodium oxalate. The precipitated crystals were spheroidal in shape, and filtered easily.

The difference between Examples 3 and 4 is that in the first step, the concentration of dissolved sodium oxalate at 5.64 g/L was sufficient for autonucleation to occur so that there were nuclei on which crystals could grow during precipitation.

In the second step, the addition of more spent liquor reduced the sodium oxalate concentration below the oxalate concentration required for autonucleation. Nevertheless, crystallization continued, because nuclei were already present in the combined solutions.

This examples demonstrates that nuclei must be present in order to precipitate easily filterable crystals. Example 5

The oxalate precipitate obtained in Example 4 was filtered from the liquor. The calculated weight of the sodium oxalate solids was 2.4 g.

This wet unwashed precipitate was added to 1000 mL of spent Bayer liquor. The mixture was held at 50'C with stirring for two hours. The concentration of dissolved sodium oxalate at the end of the time was 2.9 g/L, only 0.3 g/L lower than the initial concentration.

This example showed that aged sodium oxalate crystals present at about 2.4 g/L and which had been precipitated on a preceding experiment and separated by filtration, were not as effective in precipitating the dissolved sodium oxalate as the freshly prepared sodium oxalate crystals. Example 6

Another batch of sodium oxalate crystals were prepared according to the procedure of Example 4. These crystals were washed thoroughly with 300 mL of cold water, in order to remove any occluded liquor and impurities. The calculated weight of the washed sodium oxalate was 2.40 g.

This washed seed was added to 1000 mL of spent Bayer liquor, according to the procedure described in Example 5. The mixture was held at 50^βC with stirring for two hours. At the end of this time, the concentration of dissolved sodium oxalate was 2.7 g/L. This concentration is higher than that obtained in Example 4, and indicates less removal of oxalate occurred.

This example shows that water washed aged sodium oxalate is not as effective a seed for precipitating dissolved sodium oxalate as freshly prepared sodium oxalate seed.

Claims

Claims :

1. A process for removing sodium oxalate from a sodium aluminate liquor produced through bauxite digestion using the Bayer process, which comprises the steps of (a) continuously generating new and reactive seed particles of sodium oxalate by mixing together an aqueous solution of sodium oxalate and a portion of said Bayer process sodium aluminate liquor to achieve a concentration of sodium oxalate of 10 to 75% above the autonucleation concentration within a range to generate spheroidal particles of sodium oxalate (b) adding a suspension of said spheroidal particles of sodium oxalate as seed to a second portion of said Bayer process sodium aluminate liquor, to thereby precipitate the remainder of dissolved sodium oxalate and (c) separating the sodium oxalate precipitate, leaving a liquor of reduced sodium oxalate concentration.

2. A process according to claim 1 wherein the sodium aluminate liquor is a spent liquor from the Bayer process.

3. A process according to claim 2 wherein the spent liquor has an alumina/caustic ratio between 0.25 and 0.50.

4. A process according to claim 1 wherein the concentration is in the range of 10 to 25% above the autonucleation concentration.

5. A process according to claim 1 wherein the autonucleation concentration is about 3 to 7 g/L sodium oxalate.

6. A process according to claim 1 wherein the suspension of spheroidal particles of sodium oxalate is fed to the sodium aluminate liquor directly as formed without any treatment.

7. A process according to claim 1 wherein the spheroidal particles of sodium oxalate are in the form of a suspension which is thickened by sedimentation, filtration of hydrocloning prior to being fed as seed to the sodium aluminate liquor.

8. A process according to claim 1 wherein the sodium oxalate precipitate is separated from the liquor by filtration.

9. A process according to claim 1 wherein two filtration stages are used, each filtration step including a filter cake washing stage.

10. A process according to claim 9 wherein the first filtration stage filter cake is washed with filtrate rich in oxalate from the second filtration stage, said washing serving to remove sodium aluminate liquor retained by the filter cake without removing sodium oxalate precipitate.

11. A process according to claim 1 wherein the sodium aluminate liquor contains both sodium oxalate and aluminum hydroxide and these are removed together by filtration.