AU693247B2 - Process for the removal of organic poisons - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION 9* J 0* C. S* C

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STANDARD PATENT Name of Applicant: Actual Inventor(s): Address for service is: ALCOA OF AUSTRALIA LIMITED IAN ROSS HARRISON and STEPHEN CHARLES

GROCOTT

WRAY ASSOCIATES 239 Adelaide Terrace Perth, WA 6000 Attorney code: WR Invention Title: 'PROCESS FOR THE REMOVAL OF ORGANIC

POISIONS"

Details of Associated Provisional Application No(s): PM8544 The following statement is a full description of this invention, including the best method of performing it known to me:-

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-2- THIS INVENTION relates to a process for the removal of organic poisons from the Bayer process. In particular, the invention relates to a process for the removal of the organic species which are responsible for poisoning the crystallisation step in sodium oxalate removal processes used in Bayer process alumina refineries.

The Bayer process is used for the production of aluminium hydroxide and alumina from bauxite. Sodium oxalate is generally introduced to a Bayer circuit with the bauxite, either as oxalate salts, or oxalic acid or pre-cursor organic materials which are present as plant material which may be in various stages of decomposition. Indeed, the pre-cursor organics often oxidatively degrade in the Bayer circuit to form sodium oxalate.

In the Bayer process, the precipitation of aluminium hydroxide is carried out under carefully controlled conditions of concentration, temperature and seed crystal type and concentration, these conditions determining the morphology, yield and quality 15 of the final product. To induce the aluminium hydroxide precipitation, a liquor rich in dissolved aluminium hydroxide (the green liquor) is seeded with fine aluminium hydroxide crystals to produce larger, well crystallised agglomerates. These S. aluminium hydroxide agglomerates are filtered, washed and subsequently processed to produce alumina.

S S S 20 Unless a sodium oxalate removal process is practised, the concentration of sodium oxalate will increase to levels far greater than the equilibrium concentration of sodium oxalate in aluminium hydroxide precipitation circuit liquors, the increase being to a level where crystallisation of sodium oxalate may occur during aluminium hydroxide precipitation.

If fine sodium oxalate crystals or nuclei are present in the aluminium hydroxide precipitation circuit, the aluminium hydroxide that is produced can be deleteriously affected. Typically, the aluminium hydroxide that is produced in the presence of contaminating sodium oxalate usually will contain high levels of sodium impurities, will be too fine to be an acceptable commercial product or will be sufficiently fine ~-ql-I11 -3as to produce difficulties in separating and processing the solid material in subsequent parts of the Bayer process.

Thus, Bayer process plants have practised sodium oxalate removal. Some Bayer plants practise sodium oxalate removal via a process where a Bayer liquor stream is concentrated by evaporation and Is then seeded with solid sodium oxalate to cause crystallisation of sodium oxalate from the Bayer liquor. A number of other sodium oxalate technologies are also practised or have been developed, including such processes where sodium oxalate is caused to (or allowed to) crystallise at certain stages of the aluminium hydroxide precipitation process to then be washed from the product.

In many of these other sodium oxalate removal processes, the crystallisation of sodium oxalate is inefficient and the sodium oxalate concentration in solutions exiting from the removal process is higher than the equilibrium solubility of sodium oxalate in the Bayer liquor. This severely hinders the production of aluminium 15 hydroxide by requiring very large quantities of Bayer liquor to be treated to S:remove the required amount of sodium oxalate from the circuit. Furthermore, removal of large quantities of sodium oxalate can result in the removal of sodium oxalate poisons from parts of the process which rely on the presence of sodium oxalat9 poisons to prevent the spontaneous crystallisation of sodium oxalate 20 where it is detrimental to the process. This is of particular importance in the S. aluminium hydroxide precipitation circuit, where sodium oxalate crystallisation may be prevented by the presence of sodium oxalate crystallisation poisons.

Indeed, the efficiency of these traditional sodium oxalate removal processes is often reduced considerably due largely to the action of various organic species present in the Bayer liquor, those organic species often being referred to as sodium oxalate crystallisation poisons or simply as organic poisons. These organic poisons reduce the crystallisation rate of sodium oxalate during the later stages of crystallisation and make further gains in sodium oxalate crystallisation yield uneconomic because of the long times required to make minor gains in yield. In fact, a highly poisoned liquor stream can result in exit oxalate 11111~ '1 r -4concentrations two or more times higher than the true equilibrium solubility in Bayer liquor. Thus, any process which increases the sodium oxalate yield achievable in the sodium oxalate removal process is likely to be beneficial to the overall Bayer process.

Therefore, an aim of the present invention is to provide a process for the removal of a substantial portion of the organic species responsible for poisoning, and thus adversely affecting, the sodium oxalate crystallisation processes often used to remove sodium oxalate from Bayer process liquors.

In developing this invention, it was firstly necessary to properly determine the S. 10 precise nature of the problem to be solved. In this respect, the crystallisation of sodium oxalate does not reach the equilibrium solubility in the liquors exiting a sodium oxalate removal process because of the presence of the organic poisons, the poisons severely slowing the crystallisation at levels well above the equilibrium solubility. Furthermore, in seeded sodium oxalate removal processes, 15 as practised in many Bayer refineries, the seeds crystallised, which are recycled in the process, can be affected by the organic poisons and thus may require activation by washing, heating or by other means, before they can be effectively used as seed crystals in the circuit.

It has been found that the organic poisons actually form only a small portion of S: 20 the overall organic concentration in a Bayer liquor. It has also been found that the organic poisons may be chemicals of several classes, including specific sodium salts of carboxylic acids and a range of higher molecular weight species which are present as a result of the partial degradation of humic and fulvic acids derived from plant matter. Also, the organic poisons may be introduced as process additives in various parts of the Bayer circuit.

Furthermore, while various processes have been investigated for the removal of organics from the Bayer process, none provide a suitable method for the removal of organic materials which specifically affect the crystallisation of sodium oxalate, mainly because they rely on the treatment of large quantities of Bayer liquor. In I I particular, because of the diverse nature of the organics which poison the crystallisation of sodium oxalate, and their low concentration in the Bayer liquor, it is not economically feasible to use any of the known organics removal processes to specifically remove from the Bayer liquor the organics which poison sodium oxalate crystallisation. Further still, the high sodium hydroxide concentrations present in virtually all Bayer process liquor streams precludes the use of many processes for removing organic materials from aqueous solutions.

Indeed, we have determined that the removal of organic poisons can considerably increase the sodium oxalate yield during crystallisation, thereby significantly reducing the amount of Bayer liquor which has to be treated for sodium oxalate removal.

Thus, the present invention provides a process for specifically removing organic poisons from the Bayer process, with at least one embodinment thereof providing additionally for the organic poisons to be re-deployed in other parts of the Bayer 15 process to prevent sodium oxalate crystallisation where it is detrimental to the process (for example during aluminium hydroxide precipitation).

Accordingly, the present invention provides a process for removing organic Spoisons from a Bayer process, the removal process comprising the steps of: preparing a soluble sodium oxalate solution by dissolving sodium 20 oxalate solids in a low alkalinity solution; and subjecting the soluble sodium oxalate solution to an organics removal step such that organic poisons are removed therefrom.

The low alkalinity solution may be any aqueous solution capable of dissolving sodium oxalate, but is preferably water or re-claimed lake water from the Bayer process. Furthermore, the dissolution of the oxalate is preferably conducted at temperatures between 250C and 200'C, although temperatures in the range of 500C to 1100C are more preferred. The whole of the solids need not be dissolved.

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-6- The low alkalinity solution preferably has a total alkali level (TA) of less than 120 grams per litre, more preferably less than about 20 grams per litre, but also preferably less than about 5 grams per litre. However, while there may be some undesirable effects present at higher TA levels, it must be appreciated that it may be possible for some poisons removal processes to be operated with "low" alkalinity solutions having TA levels higher than 120 gpl due to the differences in operating parameters and liquor properties. Nonetheless, it remains preferable to operate with low levels of TA to reduce the levels of sodium ions and to thus increase the solubility of sodium oxalate.

The sodium oxalate solids dissolved to prepare the soluble oxalate solution are most likely provided from whatever sodium oxalate removal process is being operated in conjunction with the Bayer process. Thus, if the poisons removal :process were to operate in a plant which practised oxalate removal by the generation and subsequent removal of spherulitic crystals of sodium oxalate, it would preferably be those spherulitic crystals (or any agglomerates or partially formed crystals) that would be utilised. However, in plants practising sodium oxalate removal processes which produce acicular (needle-like), or indeed any other sodium oxalate crystal morphology, such crystals could also be used to produce the soluble oxalate solution.

20 Further, in one embodiment of the invention it is preferred that the sodium oxalate solids used to prepare the soluble oxalate solution are first washed with water to remove as much residual Bayer liquor from the solids as possible. In this form, the solubility of the oxalate is increased and therefore a higher concentration of dissolved oxalate in solution is achievable.

Indeed, in one application of the invention it is envisaged that the poisons removal process will be particularly useful in conjunction with the sodium oxalate removal process of our co-pending Australian patent application PM8542 filed on September 1994 and titled "Process for the removal of sodium oxalatd'. In that situation, the organics removal step of the present invention may follow the I C II -7preparation of the soluble sodium oxalate solution practised in the invention of application PM8542.

The organics removal step of the present invention may be any suitable organics removal process, due primarily to the relatively low sodium hydroxide concentration of the sodium oxalate solution, and the small volume of the resulting liquor stream. For example, an ultra-filtration or filtration process using low molecular weight cut-off membranes may effectively remove large portions of the poisons and allow them to be added to other stages of the Bayer process.

This is particularly beneficial to the aluminium hydroxide precipitation circuit of a Bayer plant where the poisons can prevent the crystallisation of sodium oxalate with aluminium hydroxide. Furthermore, organic poisons removal from the solution containing dissolved sodium oxalate may be practised by the addition of activated carbon to the solution.

However, it will be appreciated that removal of the organic poisons from the 15 solution containing dissolved sodium oxalate may also be achieved by other methods, including a range of other adsorbents, such as magnesium oxide, calcium oxide or any other adsorbent capable of adsorbing organic materials from •the solution containing dissolved sodium oxalate.

Sodium oxalate crystallisation poisons may also be chemically degraded in the 20 dissolved sodium oxalate solution by wet oxidation or treatment with chemical agents such as ozone, sodium hypochlorite or other agents.

The organics removal process thus preferably allows sodium oxalate to remain in solution, but without the organic poisons, or at least with a reduced concentration of organic poisons. The cleansed sodium oxalate solution may then be used as a concentrated sodium oxalate solution for the precipitation of further sodium oxalate from a Bayer liquor stream, such as in the process for removal of sodium oxalate as described in our co-pending Australian patent application PM8542 referred to earlier, or in a sodium oxalate removal process using sodium oxalate seed crystals. Of course, the sodium oxalate precipitated in the presence of the

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-8- Bayer liquor stream will again contain organic poisons which may then be subjected to the poisons removal process of the present invention.

It can thus be seen that by operation of this invention the total concentration of organic poisons in a Bayer circuit can be significantly reduced. This increases the efficiency of a sodium oxalate removal process, and in one embodiment of the invention, can allow the organic poisons to be re-deployed to other parts of the Bayer circuit in order to reduce oxalate crystallisation in, for example, the aluminium hydroxide precipitation circuit.

The following examples illustrate some embodiments of the invention. However, t 10 it must be understood that the following description is not to limit the generality of above description.

EXAMPLE 1 A solution containing approximately 50 gpl of sodium oxalate was made by dissolving Bayer process sodium oxalate cake in water at 1450C. Excess solids were removed by filtration.

The solution was cooled to various temperatures below 800C and then subjected to ultra filtration using a membrane filter. The ultra filtration process was continued until approximately 80 per cent of the solution had passed through the membrane leaving a retentate solution consisting of highly concentrated organic poisons (and other organic compounds which were contained in the oxalate filter cake) and a permeate solution containing a much cleaner solution of sodium oxalate.

The sodium oxalate permeate was then used to crystallise spherical sodium oxalate balls from a concentrated Bayer liquor stream. Crystallisations were carried out by evaporating the concentrated spent liquor feed stream to a total alkali concentration of about 320 grams per litre, cooling in a heat exchanger to about 250 and then adding low TA, high soluble oxalate concentration liquor 1 I I L_ I l -9- (concurrently) at three different ratios to give total sodium oxalate concentrations of about 6, 8 and 9 grams per litre. The results clearly show that the removal of oxalate seed poisons by ultrafiltration can give a significant reduction in the exit sodium oxalate concentration after crystallisation. This experiment is a batch experiment which simulates a single pass in a continuous sodium oxalate process operating in a Bayer plant. As more organic poisons are removed, even larger benefits should be achieved.

Total oxalate Exit oxalate after Total oxalate Exit oxalate after concentration crystallisation (gpl) concentration crystallisation (gpl) (gpl) (gpl) Ultrafiltration Ultrafiltration Solution from Solution from Permeate Permeate Plant oxalate Plant oxalate 5.90 2.74 5.96 2.87 7.99 2.20 8.12 2.42 9.58 2.09 9.07 2.83

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EXAMPLE 2 As further confirmation of the benefits achievable by ultrafiltration of a solution containing dissolved sodium oxalate and organic poisons, an experiment was carried out using a solution made by dissolving sodium oxalate precipitated from a Bayer refinery in water to give a solution containing about 30 gpl of sodium oxalate. The solution was ultrafiltered through a membrane filter and the solutions passing through the filter (permeate) and rejected by the filter (retentate) were compared for their ability to poison a seeded sodium oxalate crystallisation.

The permeate and retentate solutions were compared with the ultrafiltration feed liquor and with crystallisations carried out without the addition of solutions a L *0 *0 0*00

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containing organic poisons. The results again show that oxalate poisons are rejected by the membrane, allowing them to be removed from the Bayer process.

Furthermore, the clear poisoning attributable to the retentate liquor indicates that such liquors may be of benefit in preventing oxalate crystallisation in those parts of the Bayer process where such crystallisation is undesirable. This is particularly the case in the aluminium hydroxide precipitation circuit.

The cry, tallisations were carried out by evaporating a spent Bayer liquor to about 320 grams per litre total alkali concentration (TA) and adding various mixtures of water, pure sodium oxalate solution and the three test solutions to make solutions of equivalent TA and total dissolved sodium oxalate concentration.

Approximately 30 grams per litre of analytical reagent grade sodium oxalate was added as seed to each liquor which was then agitated for about 3 hours at TA Exit oxalate Exit oxalate Exit oxalate Exit oxalate concentration concentration concentration concentration (gpl) (gpl) (gpl) (gpl) Retentate Permeate Feed Liquor No added poisons 220 2.75 1.98 2.11 1.93 253 2.42 1.59 1.68 1.52 2.80 2.68 1.79 1.84 1.68 EXAMPLE 3 The effect of organic poisons removal from solutions containing dissolved sodium oxalate by adsorption on activated carbon was investigated by carrying out sodium oxalate prec'pitations after treating soluble oxalate solutions with a range of activated carbon concentrations.

c, -11 Soluble oxalate solutions were prepared by dissolving Bayer plant sodium oxalate seed in plant condensate (water) and treating 100 mis of the resulting solution with activated carbon for 30 minutes at 70C. The solutions were filtered to remove the carbon and then added to 900 mis of spent liquor. The following results were obtained: Act. Carbon 0 0.25 0,5 0.75 Used (g) Spent Start Oxalate Liquor solution TA (gpl) 334 23 301 300 298 298 295 Exit 4.3 4.01 (start) 2.7 2.0 2.0 2.0 oxalate (start) (gpl) Net 1.4 2.0 2.1 2.1 1.9 Oxalate Yield (gpl) a C. C a a a **5 5*55 a a U. C

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These examples show the effect of removing sodium oxalate crystallisation poisons from a batch sodium oxalate crystallisation. In a Bayer plant installation the process will be used to continuously bleed oxalate crystallisation poisons fron the process, resulting in optimal removal (and possible re-deployment) of the poisons. These results also illustrate the yield benefits achievable through organic poisons removal from solutions containing sodium oxalate solutions.

Illustrated in Figures 1 and 2 are two possible flow sheets showing processes which embody the present invention. In Figure 1, sodium oxalate solids separated from a sodium oxalate removal process are mixed with a low TA aqueous solution 12 (preferably water or plant condensate). The amount of water is preferably sufficient to dissolve all or most of the sodium oxalate at the temperature set by the recycle 14 through the heat exchanger 16. Alternatively, r -12the solution could be heated by direct steam injec or by any other suitable method. In the preferred embodiment of this flow sheet, the sodium oxalate solids should be washed with water prior to dissolution (as represented by stream 13), thereby reducing the amount of residual Bayer liquor present in the cake and maximising the sodium oxalate concentration achievable at the desired maximum temperature in vessel After all, or most, of the sodium oxalate solids have been dissolved in vessel the solution may then be transferred to a thickener 18 (optional) to reduce the pre-filtration requirement for the ultrafiltration stage 20. The thickener underflow solids 22 can be recycled directly back to the dissolution vessel 15 and the thickener overflow solution 24 is then transferred to the ultrafiltration stage The filtration stage 20 should preferably contain a pre-filtration step (not shown) to eliminate any remaining solids capable of fouling the ultrafiltration stage.

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Following this preferred pre-filtration, the solution is subjected to ultrafiltration 15 ultrafiltration stage 20. The ultrafiltration membranes should be made from materials resistant to sodium hydroxide.

SThe retentate solution 26 from the ultrafiltration stage 20 containing the rejected

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organic poisons can be. eliminated from the system, or if desired, can be utilised elsewhere in the Bayer process to reduce the crystallisation of sodium oxalate in 20 such parts of the process where this is undesirable (such as in the aluminium hydroxide precipitation circuit). The permeate solution 28, now containing sodium oxalate with a reduced level or organic poisons, can be used in the oxalate removal circuit. In the preferred embodiment of this invention, the solution can be used in the oxalate removal process of our co-pending Australian patent application PM8542. However, it can be seen that this solution will be beneficial to the performance of other sodium oxalate removal processes practised in Bayer plants.

In the regime shown in Figure 2, the solutions are the same as described in Figure 1. However, in Figure 2 the poisons removal process is based on the -13adsorption of the organic poisons on an introduced substrate in adsorption stage The adsorption substrate may be any adsorbent suitable for the adsorption of organic seed poisons, including activated carbon, magnesium oxide, calcium oxide or other materials.

In some variations of this invention, the thickener circuit will not be required as the adsorbent may be added directly to the seed slurry without removing undissolved sodium oxalate. Depending on the cost of the adsorbing material and the ease by which the material can be regenerated by such processes as washing or by chemical removal of the sodium oxalate precipitate, the adsorbing material may remain in the process and be disposed of during removal of the sodium oxalate S' precipitate. If desired, the adsorbing material may be regenerated and re-used in the process, such regeneration being shown in the solids removal and regeneration stage 32.

o• Finally, it will be appreciated that other modifications and alterations may be 15 made to the processes described above that are also within the scope of the present invention.

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Claims (11)

1. A process for removing organic poisons from a Bayer process, the removal process comprising the steps of: preparing a soluble sodium oxalate solution by dissolving sodium oxalate solids in a low alkalinity solution; and subjecting the soluble sodium oxalate solution to an organics removal step such that organic poisons are removed therefrom.

2. A process according to claim 1, wherein the low alkalinity solution is water or re-claimed lake water from the Bayer process.

3. A process according to claim 1 or claim 2, wherein the dissolution in step is conducted at a temperature in the range of 250C to 200 0 C.

4. A process according to claim 1 or claim 2, wherein the dissolution in step is conducted at a temperature in the range of 500C to 1100C. *oe* A process according to any one of claims 1 to 4, wherein the low alkalinity solution has a total alkali level (TA) of less than 120 grams/litre.

6. A process according to any one of claims 1 to 4, wherein the low alkalinity solution has a total alkali level (TA) of less than 20 grams/litre.

7. A process according to any one of claims 1 to 4 wherein the low alkalinity solution has a total alkali level (TA) of less than 5 grams/litre.

8. A process according to any one of claims 1 to 7, wherein the sodium oxalate solids are provided from a sodium oxalate removal process being operated as a part of the Bayer process.

9. A process according to claim 8, wherein the sodium oxalate solids are washed with water to remove residual Bayer liquor therefrom prior to the dissolution in step A process according to any one of claims 1 to 9, wherein the organics removal step is an organics removal process such as a filtration process using low molecular weight cut-off membranes, an adsorbing process, or a chemical degradation process.

11. A process according to any one of claims 1 to 10, wherein the removed organic poisons are re-deployed to other parts of the Bayer process in order to reduce the oxalate crystallisation therein.

12. A process according to claim 11, wherein the removed organic poisons are re-deployed to the aluminium hydroxide precipitation circuit in the Bayer process 10 to reduce the oxalate concentration therein. S' 13. A process according to claim 1 substantially as herein described in relation to any one of the examples.

14. A process according to claim 1 substantially as herein described in relation to the accompanying Figures. DATED this TWENTY-NINTH day of September 1995. ALCOA OF AUSTRALIA LIMITED Applicant Wray Associates Perth, Western Australia Patent Attorney for the Applicant 111 1 -16- ABSTRACT A process for removing organic poisons from a Bayer process, the removal process comprising the steps of: preparing a soluble sodium oxalate solution by dissolving sodium oxalate solids in a low alkalinity solution; and subjecting the soluble sodium oxalate solution to an organics removal step such that organic poisons are removed therefrom. 0 00 o 9