GB2211832A

GB2211832A - Bayer process liquor purification

Info

Publication number: GB2211832A
Application number: GB8825422A
Authority: GB
Inventors: Mark Edward Keeney; Stephen Charles Grocott
Original assignee: Commonwealth Scientific and Industrial Research Organization CSIRO; Alcoa of Australia Ltd
Current assignee: Commonwealth Scientific and Industrial Research Organization CSIRO; Alcoa of Australia Ltd
Priority date: 1987-10-30
Filing date: 1988-10-31
Publication date: 1989-07-12
Also published as: GB8825422D0; BR8805768A

Abstract

A method of reducing the sodium oxalate content of Bayer process liquors comprises crystallisation in the presence of seed crystals of spherulitic morphology. Processes for the production of seed crystals of spherulite morphology are also mentioned.

Description

"CRYST4LLIS,QTION OF SODI UN OXALATES" The Bayer process is used for the production of aluminium hydroxide and alumina from bauxite. The bauxite is crushed, mixed with a sodium hydroxide containing solution and heated to dissolve aluminium containing species to form sodium aluminate in the solution.

The insoluble residues remaining after this digestion step are commonly known as "red mud" and sand. They consist mainly of sodium aluminosilicates, iron oxides and silica.

They are usually removed and washed by continuous countercurrent decantation.

After removal of the insoluble residues, the solution is cooled and is made to undergo a precipitation reaction which removes a significant proportion of the dissolved sodium aluminate as aluminium hydroxide.

This aluminium hydroxide precipitation is carried out under carefully controlled conditions of concentration, temperature and seed crystal type and concentration.

These conditions determine the morphology, yield and quality of the final product. To induce the aluminium hydroxide precipitation, the liquor is seeded with fine aluminium hydroxide to produce larger, well crystallised agglomerates. These aluminium hydroxide agglomerates are then filtered and washed.

If fine sodium oxalate 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 is too fine and can contain too much sodium impurity to be an acceptable commercial product.

After filtering and washing, the precipitated aluminium hydroxide is calcined (if alumina is the desired final product).

A portion of the spent sodium aluminate solution from the aluminium hydroxide precipitation is then fed to the sodium oxalate removal circuit, usually after undergoing some evaporation to remove water from the solution and increase the solution concentration.

Sodium oxalate removal occurs by feeding the spent liquor to a crystalliser containing seed crystals of sodium oxalate. These seed crystals induce precipitation of further sodium oxalate. This sodium oxalate is usually removed by thickening and/or filtering the sodium oxalate slurry. This removed sodium oxalate can then be disposed of by a number of means including reaction with lime or combustion. Sodium oxalate not disposed by these or other means is usually returned to the oxalate crystalliser to act as seed crystals for fresh incoming spent sodium aluminate liquor.

The morphology of sodium oxalate crystals formed in Bayer process sodium oxalate removal circuits is usually that of needles as is shown in the microphotograph which is identified as Fig. I of the accompanying drawings. These needles are difficult to filter because they tend to interlock and form a restrictive flow path for liquor when being filtered. Also, small needles or parts of needles tend to "blind" filter cloths. Furthermore "needle-like" sodium oxalate, because of its high area to mass ratio is difficult to settle, thicken or centrifuge.

Filtration and settling problems caused by the needle-like morphology of sodium oxalate is a major process problem since the entire Bayer refinery production can be dependent upon throughout of process liquor through the sodium oxalate precipitation circuit.

SUMMARY OF THE INVENTION The invention provides a process whereby sodium oxalate seed crystals are added to a sodium oxalate solution to facilitate filtration and/or precipitation of sodium oxalate crystals having spherulitic morphology. The process may be carried out by combining a sodium oxalate rich stream with a solution that precipitates sodium oxalate or by evaporation or cooling of the sodium oxalate rich stream. One of these streams may contain a substance that causes the sodium oxalate to be crystallised in a spherulitic morphology. In another aspect the invention provides a novel "pipeline nucleator" in which the nucleation and initial growth of the spflerulitic sodium oxalate is caused to happen in a "plug flow" system.This spherulitic sodium oxalate is then used as a seed material for crystallisation o'f further sodium oxalate in a sodium oxalate removal process. The sodium oxalate crystalliser operation of the present invention enables the crystallisers to be operated at high concentration of spherulitic sodium oxalate.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to the removal of sodium oxalate from Bayer liquors by creation of spherulitic sodium oxalate and crystallisation of further sodium oxalate upon these spherulitic seed crystals. For the purposes of this invention, the terms "Bayer process liquors" or "liquor" relate to any sodium hydroxide solution which is generated in the Bayer process. The concentration of sodium hydroxide plus sodium carbonate is expressed as total alkali in units of grams per litre of sodium carbonate equivalent.

Sodium oxalate is extracted and/or formed from the digestion of bauxite in Bayer liquor. Insufficient removal of sodium oxalate from the Bayer liquor causes substantial problems in alumina refineries and particularly in the aluminium hydroxide precipitation circuit.

The present invention is directed to improved removal of sodium oxalate from Bayer process liquors. This goal is achieved by the use of spherulitic sodium oxlate as opposed to the more typical "needle-like" sodium oxalate as seed crystals in the sodium oxalate crystallisers.

Spherulitic sodium oxalate is generated by mixing a solution rich in sodium oxalate with a solution that causes the sodium oxalate to precipitate or by evaporating or cooling the sodium oxalate rich solution. One of these solutions should contain a substance that causes the sodium oxalate to crystallise in a spherulitic morphology. A typical example of spherulitic sodium oxalate is shown in the microphotograph which is identified as Fig. 2 of the drawings.

The solution rich in sodium oxalate can be formed by dissolving a solid that contains a large proportion of sodium oxalate, in water or in a solution that has a low total alkali concentration (TA typically less than 30g/L). The solution that induces the sodium oxalate precipitation upon mixing typically contains a high concentration of a sodium salt, usually sodium hydroxide and carbonate such that the TA is greater than lOOg/L. The substances that force the sodium oxalate morphology to the spherulitic rather than needle-like are suface active substances.

The substances may be synthetic or naturally occurring in the Bayer liquor. Polyacrylate and polyacrylamide polymers and their copolymers serve this purpose well.

Another embodiment of this invention is that the naturally occurring organic substances in the Bayer liquor may also be used for this purpose. High molecular weight Bayer organics such as sodium humates and fulvates are naturally occurring substances whose concentrations may be adjusted at an appropriate stage in the Bayer process in the absence of synthetic crystal modifiers. The morphology of the spherulitic sodium oxalate can be controlled by the ratio of the sodium oxalate rich stream to the high TA stream or by the type and molecular weight of the polymer additive. The morphology can also be controlled by temperature, concentration of sodium oxalate and the addition of nucleation aids such as magnesium salts. The nucleation of spherulitic sodium oxalate can be undertaken in a continuous or batch manner. A novel embodiment of the invention is to carry out the mixing of the two streams in a "pipeline nucleator". This ensures that the nuclei which are formed upon mixing are transported down the nucleator with their parent liquor. This can be contrasted with the situation where the nuclei are held within a large vessel and may grow to excessive size because incoming sodium oxalate rich solution causes growth upon existing nuclei rather than formation of new nuclei.

Examples I - III demonstrate some of the ways in which spherulitic oxalate can be generated. These examples and the processes given above do not exclude variations upon these means. Any process which causes supersaturation of sodium oxalate and the subsequent formation of spherulitic sodium oxalate is implied within these descriptions.

The spherulitic sodium oxalate which has been generated is then able to be used as seed material in an oxalate crystalliser. Examples of the use of this material as seed crystals are given in Examples IV - VI. An example of how the spherulitic sodium oxalate may be employed in a plant sodium oxalate removal circuit is given in Fig. 3 of Example VI.

The spherulitic sodium oxalate is fed to a vessel to act as a seed material for further sodium oxalate crystallisation. Bayer liquor is also fed to this crystalliser vessel and the spherulitic sodium oxalate seed crystals cause sodium oxalate to be depleted from this crystalliser feed liquor by growth upon the seed crystals. The oxalate depleted liquor exits from the crystalliser and is fed to a filter or thickener to remove suspended sodium oxalate. This sodium oxalate is then returned to the crystalliser to act as seed material or is disposed by combustion, reaction with lime or other means.

The use of spherulitic sodium oxalate crystals offers a number of possibilities that do not exist when using "normal" needle-like sodium oxalate. With spherulitic sodium oxalate, solids retention can be readily practised in the crystalliser. This allows the crystalliser to be operated at high seed concentrations and thereby afford increased yield of sodium oxalate. Furthermore, the overflow from the crystalliser may be sufficiently free of solid sodium oxalate that it does not need filtration or thickening. Certainly, the higher settling rate of the spherulitic sodium oxalate compared with needle-like sodium oxalate enables greater use of thickening to replace or reduce the need for filtering sodium oxalate.

Where filtration is still required, less filters or filters with lower surface area can be used because of the superior filtration rate of spherulitic sodium oxalate (Example VII).

An important further aspect of the invention is the allowance for the removal of needle-like sodium oxalate from the sodium oxalate crystallisation circuit.

Needle-like sodium oxalate can form by breakdown of the spherulitic seed crystals. This type of sodium oxalate should preferably be removed entirely from the sodium oxalate circuit otherwise the entire system will eventually convert to needle-like morphology. Sodium oxalate needle removal is embodied in the use of thickeners to concentrate the spherulitic sodium oxalate.

The inferior settling rate of the needle-like sodium oxalate causes it to be lost from the sodium oxalate circuit with the thickener overflow liquor.

In the following examples, illustrations are provided for practising the invention.

EXAMPLE I Deionised water was saturated with pure sodium oxalate at 800 C. A polyacrylate or polyacrylamide polymer or copolymer was added as an aqueous solution to give a final polymer concentration of 1 - 20 ppm. The hot sodium oxalate solution was allowed to cool to ambient temperature with slight stirring. Crystallisation occurred and the solid sodium oxalate was filtered, dried and examined by electron microscopy.

High molecular weight (18 x 106) polyacrylate/acrylamide copolymers (70:30 ratio) at 5 - 15 ppm were found to be the most effective at causing spherulitic sodium oxalate to be formed. In the absence of any polymer, blocks of sodium oxalate were formed.

A similar experiment was carried out using, instead of pure sodium oxalate, sodium oxalate filter cake derived from a Bayer plant. It was found that cooling alone was insufficient to induce homogenous nucleation. However, evaporation induced nucleation and formation of spherulitic sodium oxalate.

EXAMPLE II 45kL of a 200g/L sodium oxalate slurry with TA 80g/L was added to a 700kL vessel. 300kL of a TA 15g/L solution was added to this vessel. The mixture was heated to 950 C to dissolve all of the solid sodium oxalate, thereby producing a solution with a sodium oxalate concentration of approximately 35g/L. This solution was pumped via a pipe to another 700kL vessel. Into this pipe was also pumped, at an equal flow rate, 300kL of spent Bayer liquor of TA 255g/L.

Sodium oxalate spherulites were formed without the requirement for synthetic polymer additives because of the naturally occurring Bayer organics which serve the same function. The yield of the sodium oxalate spherulites can be maximised by the subsequent addition of concentrated sodium oxalate solution.

The spherulitic sodium oxalate produced by this process had a mean diameter of approximately 200um. It provided a settling rate of approximately 30 m/h in spent Bayer liquor at 550 C.

EXAMPLE III The example provides a further example for applying the present invention in an industrial plant employing the Bayer process.

Bayer sodium oxalate is filtered and dissolved at 950C in condensate. The condensate approximates deionised water.

The resultant solution has a TA of less than 5g/L and contains approximately 55g/L of sodium oxalate. It is essential, as in all cases where the sodium oxalate rich solution is being prepared, that no needle-like or other solid sodium oxalate is present in the solution.

This sodium oxalate rich solution is then mixed in at a 1:3 ratio with spent Bayer liquor with a temperature of 550C and a TA of 265g/L. The mixing occurs at the start of a "pipeline nucleator". The pipe provides a short residence time for the 1:3 mixture. Other ratios such as 2:1, 1:1, 1:2 etc can be employed to vary morphology and yeild.

At the end of the pipeline nucleator, a slurry of sodium oxalate spherulites emerges into a growth vessel or vessels which provide further growing time for the sodium oxalate spherulites.

EXAMPLE IV This example depicts the results of laboratory tests in which spherulitic sodium oxalate crystals acted as seed for the crystallisation of sodium oxalate from a spent Bayer liquor.

20g/L of spherulitic sodium oxalate (produced in Example II) was added to spent Bayer liquor at 550 C. The mixture was stirred gently for two hours and the solids were filtered from the solution. 20g/L of these solids were then used in a subsequent crystallisation test using a fresh sample of the. spent Bayer liquor. This process was repeated seven times and at the end of each cycle, the concentration of dissolved sodium oxalate in the spent Bayer liquor was measured. A single similar crystallisation test was also carried out using a sample of typical needle-like Bayer plan sodium oxalate. All of these results are summarised in Table 1.

Table 1. Cyclic sodium oxalate crystallisation tests

LIQUOR SAMPLE SODIUM OXALATE CONCENTRATION IN LIQUOR (g/L NaIC2O4) Before needle cycle 3.80 After " " 1.91 Before 1st sphernlite cycle 3.80 After 1st ' ' 199 Before 7th spherulite cycle 3.80 After 7th " " 1.98 The results in Table 1 indicate that the spherulitic sodium oxalate acts as an efficient seed material.These results were also unchanged in a similar test in which 15ppm of a polyacrylate/acrylamide copolymer (as used in Example I) was added.

In another test, the rate of sodium oxalate removal of spherulite and needle-like sodium oxalate seed was examined. The test conditions were identical to those above. The results are summarised in Table 2.

Table 2. Sodium oxalate crystallisation rates at 20g/L sodium oxalate seed concentration.

TIME SODIUM OXALATE CONCENTRATION (minutes) (g/L Na2C2O4) Needle Test Spherulite Test 0 3.80 3.80 5 2.76 2.70 15 2.11 -2.01 30 1.90 1.98 120 1.91 1.99 It is apparent that the spherulitic sodium oxalate affords a similar crystallisation rate of sodium oxalate as does the needle-like sodium oxalate seed crystals.

EXAMPLE V This experiment was undertaken to examine the possibility of operating a sodium oxalate crystalliser with solids retention so as to maintain a high level of seed density.

A 3L conical bottom, water jacketted crystalliser was filled with spent Bayer liquor and maintained at 550 C.

Spherulitic sodium oxalate was added to give 100g/L of seed crystals. Spent Bayer liquor was pumped to the bottom of the crystalliser at 1L/h to give an average liquor residence time of 3 hours. The overflow liquor from the crystalliser did not contain any spherulitic or needle-like oxalate.

The feed liquor contained 3.5g/L of sodium oxalate. The concentration of oxalate in the overflow liquor was initially 2g/L and slowly rose to 2.5g/L over 4 days.

These results indicate that reduction in surface area of the spherulitic sodium oxalate or poisoning of the surface only occurs slowly. The timeframe for this poisoning is compatible with the operation of a crystalliser in a Bayer plant which also employs a similar degree of solids retention.

In another test, it was found that any needles present are simply elutriated from the spherulitic sodium oxalate by this process. It was practised in a 700kL vessel containing 200g/L of spherulitic and 50g/L of needle-like sodium oxalate The concentration of needle-like sodium oxalate was reduced from 50g/L to less than 5g/L by 1200kL of elutriation with spent Bayer liquor at a flow rate of 200kL/h.

EXAMPLE VI A schematic flowsheet is given in Fig. 3 which demonstrates a possible circuit for implementing the use of spherulitic sodium oxalate as seed material in a sodium oxalate removal circuit. This flowsheet can be considerably simplified, depending upon the precise nature of the sodium oxalate removal requirements and crystalliser vessel design.

EXAMPLE VII Comparative filtration rate tests were carried out on mixtures of spherulitic and needle-like sodium oxalate.

The tests conditions reflected Bayer plant operating conditions for sodium oxalate filtration.

Drying time 12 seconds Initial vacuum 60 kPa Slurry concentrations 20 g/L of sodium oxalate

Claims

CLAIMS 1. A process of reducing the sodium oxalate content of a Bayer process liquor which comprises inducing crystallisation of the sodium oxalate in the presence of seed crystals of spherulitic morphology and separating the oxalate crystals from the liquor.
2. A process as claimed in claim 1 wherein the seed crystals are produced by mixing a solution rich in sodium oxalate with a solution which induces crystallisation of spherulitic sodium oxalate.
3. A process for producing spherulitic sodium oxalate which comprises cooling a solution rich in sodium oxalate under conditions which induce the formation of predominantly spherulitic crystals.
4. A process as claimed in claim 1 wherein the seed crystals are produced by cooling a solution rich in sodium oxalate under conditions which induce the formation of predominantly spherulitic crystals
5. A process for producing spherulitic sodium oxalate which comprises evaporation of a solution rich in sodium oxalate under conditions which induce the formation of predominantly spherulitic crystals.
6. A process as claimed in claim 1 wherein the seed crystals are produced by evaporation of a solution rich in sodium oxalate under conditions which induce the formation of predominantly spherulitic crystals.
7. A process for producing spherulitic sodium oxalate which comprises mixing of a solution rich in sodium oxalate with a solution which induces crystallisation of spherulitic sodium oxalate.
8. A process according to claim 2 or 7 wherein the solution which induces spherulitic sodium oxalate to be formed contains a polymer containing polyacrylate, polyacrylamide or polyacrylate/acrylamide copolymer or any other synthetic sodium'oxalate crystallisation.
9. A process according to claim 2 or 7 wherein the solution which induces spherulitic sodium oxalate to be formed contains naturally occurring Bayer process organic impurities such as sodium humates and/or sodium fulvates but not excluding other Bayer process impurities.
10. A process according to claim 2 or 7 wherein the solution inducing sodium oxalate crystallisation is a concentrated sodium hydroxide solution.
11. A process according to claim 2 or 7 wherein the solution inducing sodium oxalate crystallisation is a Bayer process liquor.
12. A process according to claim 2 or 7 wherein the mixing of the two solutions occurs in a pipe or vessel that ensures plug flow or near plug flow of the mixture so that optimum oxalate morphology is acheived.
13. A process according to claim 2 or 7 wherein the morphology and size of the spherulitic sodium oxalate is controlled by the ratio of the two streams and/or the amount and type of the certain substances or by temperature, concentration of sodium oxalate and the addition of nucleation-aids such as magnesium salts.
14. A process as claimed in any one of claims 9 to 13 wherein the spherulitic sodium oxalate particles have a diameter in the range 10-lOOOum.
15. A process of reducing the sodium oxalate content of a Bayer process liquor substantially as hereinbefore described by way of Example.