WO2012145797A1

WO2012145797A1 - Recovery of soda from bauxite residue

Info

Publication number: WO2012145797A1
Application number: PCT/AU2012/000443
Authority: WO
Inventors: Peter Smith; Takuo Harato
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2011-04-29
Filing date: 2012-04-27
Publication date: 2012-11-01
Also published as: AU2012248126B2; AU2012248126A1

Abstract

A process for recovery of soda from Bayer process bauxite residue comprises the steps: (a) leaching bauxite residue with sulphuric acid to dissolve soda in DSP in bauxite residue therein; (b) neutralizing the slurry of step (a) with caustic materials to precipitate partially dissolved silica, alumina and calcium, and separating the insoluble portion with precipitated materials by filtration to obtain clear liquor of sodium sulphate (c), processing the resulting liquor by membrane electrochemical process to recover caustic soda and sulphuric acid. The caustic soda is returned to the Bayer process and the sulphuric acid is returned to the leaching process. Consequently, the process can be fully integrated with the Bayer process to reduce caustic soda make-up substantially which would be ordinarily lost. Furthermore the characteristics of the leached and neutralized bauxite residue are changed to reduce its hazardous nature and facilitate improved management of the residue disposal area by the use of clean electric power and non-volatile sulphuric acid.

Description

RECOVERY OF SODA FROM BAUXITE RESIDUE

FIELD OF THE INVENTION

This invention relates to the treatment of waste products of the Bayer process for the extraction of alumina from bauxite.

According this invention, by removing soda from bauxite residue, many of the management problems for residue disposal are solved. Thus, valuable component soda ordinarily discarded in the bauxite residue may be returned to the Bayer process resulting in a significant reduction in the requirement for replacement caustic. The process also reduces the hazardous nature of the remaining residue, thus easing residue management problems and resulting in improvement in overall process efficiency.

BACKGROUND

The Bayer process is the principal method for the production of alumina from bauxite worldwide. In the Bayer process, the bauxite containing aluminium hydroxide or aluminium oxy- hydroxides is contacted with solutions containing caustic soda to dissolve the aluminium values as sodium aluminate while leaving most of the remaining constituents of the bauxite essentially unattacked in solid form.

A part of silica content of the bauxite may also dissolve in the caustic soda solution to form a soluble sodium silicate. This further reacts with the sodium aluminate in solution to form complex hydrated sodium aluminate silicates, known collectively desilication products (DSP). These desilication products are of low solubility in the resulting sodium aluminate-caustic soda solution thereby removing much of the undesirable silica from the solution phase. However, there is the substantial cost associated with the loss of chemically bound soda and alumina in the DSP. After the digestion step, the undissolved part of the bauxite, together with any reaction products, including DSP, (termed "bauxite residue" (BR) or "red mud") are separated from the solution, usually by filtration or sedimentation or a combination of both. After counter-current washing, the bauxite residue is then disposed of, usually to impoundment areas. The clarified caustic-aluminate solution commonly known as "pregnant liquor", is subsequently cooled, diluted, seeded with aluminium trihydroxide crystals (gibbsite) and agitated for a period of time to precipitate a significant fraction of the dissolved alumina as gibbsite. This precipitate is then separated from the resulting spent liquor, which typically still contains in the order of the half the original dissolved alumina. A part of the separated gibbsite may be recirculated as seed material to the gibbsite precipitation, whilst the remainder is washed to recover the soluble valuables from the entrained liquor, and is then suitably calcined to alumina product. The spent liquor may be re-concentrated, impurities removed and new caustic soda added as caustic feed to further digestion.

For disposal, bauxite residue is generally sent to residue ponds or alternatively dry-stacked, either directly or after neutralization of entrained liquor with for example, sea water. The bauxite residue disposal areas are monitored and managed to minimize the effect of the residue on the environment. According to the world wide trend of declining bauxite grades, the processing of higher silica bauxites is likely to increase problems of soda consumption and bauxite residue management. This puts increased pressure on the need for a more sustainable and cleaner alumina industry.

For the purposes of the present application, soda is defined as any sodium salt, but is represented as the oxide (Na₂0). Caustic soda is that portion of soda that exists in the form of hydroxide (NaOH) or aluminate (NaAl(OH)₄). Sodium represents only the sodium metal ion (Na⁺).

A number of methods have been proposed to reduce the cost of soda loss due to the desilication product in bauxite residue. Such procedures (mainly sinter and high temperature hydrothermal processes) are technically complex and require high energy input or reagent consumption.

An alternative method for recovery of the valuable components from bauxite residue involves the treatment of the aqueous bauxite residue, which is in slurry form, with sulphur dioxide (Cresswell:

USP 4,668,485). In this process bauxite residue slurry is treated with sulphur dioxide to dissolve the soda, alumina and silica present in the DSP. After separation of the insoluble portion, the filtrate is heated to selectively precipitate the silica which is then removed to give a solution containing the soluble soda and alumina initially extracted from the bauxite residue. This liquor is causticized with lime, and the resulting caustic aluminate solution may be returned to the Bayer process. Solid calcium sulphite (causticisation residue) is calcined to recover the raw materials used in the process, namely lime and sulphur dioxide.

The recovered caustic aluminate solution after causticisation is much diluted and contains residual sulphite ion. The solution therefore requires evaporation (to concentrate) and treatment by barium compounds (to reduce the dissolved sulphur species) before returning it to the Bayer process. The step of calcining the calcium sulphite is energy intensive. Sulphur dioxide released in this step is potentially harmful to health and therefore must be collected from the calciner and other operations. Such procedures are technically complex, requiring high energy input or reagent consumption, and are not compatible with recent efforts to minimise the environmental impact of production.

In another process (Wightman: WO 94/02417), cation-exchange resin is utilised to achieve neutralisation of the bauxite residue. The process involves contacting a slurry of bauxite residue with a particular cation-exchanger, separating sodium loaded cation-exchanger, regenerating said sodium loaded cation exchanger to liberate sodium values, and recycling the resulting regenerated cation exchanger for contact with further slurry of bauxite residue. When sulphuric acid is applied as eluent on the regeneration step, sodium sulphate is obtained as the regenerant solution. It is mentioned that the liquor can be subjected to further processing, e.g. crystallisation to produce sodium sulphate or

electrohydrolysis to produce sodium hydroxide and sulphuric acid, but details for treatment of the liquor are not presented. The procedure of Wightman is technically complex, and requires expensive cation- exchange resin and a regeneration step prior to further processing of recovery of sodium hydroxide and sulphuric acid.

Recent advances in membrane technology have lead to the use of electrochemical processing e.g. membrane electrolysis or electrodialysis with bipolar membrane for the recovery of soda from wastes containing sodium sulphate from, for example, rayon manufacture and the pulp and paper industry. But so far there is no practical application of acid leaching and membrane electrochemical processing to recover soda from bauxite residue of Bayer alumina production process. A possible reason is that the membrane electrochemical processes for soda recovery require extremely purified feed liquor especially with respect to polyvalent cations such as Ca, Mg, Al, and Fe ions to keep current efficiency high and to prevent clogging of the membrane by precipitation of alkali-earth salts, aluminium or iron hydrates. Bauxite residue contains such kinds of impurities which may cause problems during acid leaching.

It is an object of the present invention to provide a process for the recovery of soda normally contained within the bauxite residue remaining after extraction of alumina from bauxite by the Bayer process which is economical and has good environmental credentials. It is also an object for embodiments of the invention to provide a process which may be fully integrated with the traditional Bayer process. For example, it would be desirable for the soda to be recovered in the form of caustic soda (NaOH) which may be returned directly to the process.

SUMMARY OF THE INVENTION

It has been found that a particular combination of process steps enable the recovery of soda (sodium values) from bauxite residue, which can then be recycled into a Bayer process operation. The process also has the potential advantage of enabling the recovery of another by-product of the process, acid, which may also be recycled. This process has particular application to the Bayer process and the treatment of bauxite residue, but can also find broader application in processes for the recovery of sodium from sodium-containing materials, of which bauxite residue is just one example.

The invention involves the steps of treating of the sodium-containing material, such as bauxite residue, to convert the soda into a soluble form, (for example, through acid leaching of the bauxite residue for soda extraction), neutralisation of the treated product, and membrane electrochemical processing to split the resulting liquor into caustic soda and acid.

Thus, according to one aspect, there is provided a process for recovering soda from bauxite residue, comprising:

(a) treating the bauxite residue to transform at least a portion of the sodium present in the bauxite residue into a soluble sodium salt;

(b) neutralising the product of step (a) with caustic material to yield a neutralised product comprising solids and a liquor;

(c) separating the solids from the liquor to recover a liquor containing the solubilised soda; and

(d) processing the liquor containing the solubilised soda by a membrane electrochemical system to regenerate sodium hydroxide and acid.

The first treatment step may involve:

(a) leaching the bauxite residue with acid in the presence of water to dissolve the sodium present in the bauxite residue.

When this is the form of treatment step used for step (a), then the acid produced in step (d) can be recycled to this step. The sodium hydroxide regenerated in the process can also be recycled to a process requiring sodium hydroxide use, such as the Bayer process.

The acid used in step (a) is suitably sulphuric acid. This soda recovery process may form part of an integrated Bayer process for the production of alumina from bauxite. Thus, according to a second aspect of the invention, there is provided a process for the production of alumina from bauxite, comprising (i) contacting the bauxite with sodium hydroxide to produce a sodium aluminate-containing solution and bauxite residue, (ii) separating the bauxite residue from the sodium aluminate-containing solution, (iii) recovering the aluminium hydroxide from the sodium aluminate-containing solution, and (iv) recovering the sodium from the bauxite residue in accordance with the process described above. In a preferred embodiment, the sodium recovered as sodium hydroxide in the sodium recovery process is recycled to the step in which bauxite is contacted with sodium hydroxide.

In the aspects of the invention described above which involve contacting acid (such as sulphuric acid) with bauxite residue, which may be in slurry form, a portion of the sodium in desilication product is extracted or leached into solution, and then the slurry is re-neutralised by a caustic material. After separation of the insoluble portion, the liquor (or filtrate) is electrochemically processed to recover the raw materials used in the process, namely sodium hydroxide and sulphuric acid. The sodium hydroxide can be returned to the Bayer process and the sulphuric acid is returned to the leaching process.

Consequently, the process can be fully integrated with the Bayer process to reduce caustic soda loss. Furthermore the characteristics of the leached bauxite residue are changed to reduce the hazardous nature of the Bayer residue, easing problems associated with residue management. Bauxite residue management costs may also be reduced through the use of this process.

According to a third aspect, there is provided more generally a process for the recovery of soda from a sodium-containing waste material, the process comprising:

(a) treating the sodium-containing waste material to convert the sodium into a soluble sodium salt;

(c) separating the solids from the liquor to recover a liquor containing the solubilised sodium; and

(d) processing the liquor containing the solubilised sodium by a membrane electrochemical system to regenerate sodium hydroxide and acid.

It will be noted that the soda recovered from the process is recovered as sodium hydroxide.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 illustrates a flow diagram of one embodiment of the present invention using three- compartment electrodialysis with bipolar membrane.

Figure 2 illustrates a flow diagram of a second embodiment of the present invention using two- compartment membrane electrolysis.

Figure 3 illustrates a flow diagram of a third embodiment of the present invention using three- compartment electrodialysis with bipolar membrane to improve soda extraction extent. Figure 4 illustrates a flow diagram of a fourth embodiment of the present invention using three- compartment electrodialysis with bipolar membrane applied to a residue which contains calcium compounds.

Figure 5 illustrates a schematic diagram of a three compartment type electrodialysis which may be utilised in the process according to some embodiments of the invention.

Figure 6 illustrates a schematic diagram of a two-compartment type electrodialysis which may be utilized in the process according to some embodiments of the invention.

Figure 7 illustrates pH changes of bauxite residue after leaching and neutralization.

Figure 8 illustrates one embodiment of continuous leaching and neutralization process.

DETAILED DESCRIPTION OF EMBODIMENTS

According to some embodiments of the invention, there is provided a process for the recovery of soda from bauxite residue. "Bauxite residue" is a term of well known meaning in the art, and is used herein in its broad sense to refer to any solid waste product from bauxite processing, such as a Bayer process operation, containing insoluble soda values. One form of bauxite residue which can be used in the process comprises the combination of the undissolved part of bauxite and precipitated DSP from a Bayer process operation. This combination of undissolved bauxite and DSP may be used in the present soda recovery process either with or without further treatment. One specific form of bauxite residue within this class is desilication product concentrate (DSP concentrate). This is described in further detail below. DSP concentrate may be considered to be a bauxite residue that has been subjected to concentration to concentrate the DSP. The term "bauxite residue" also includes bauxite residue products that have been subjected to pre-treatments, including pre-treatments to reduce the levels of impurities selected from the group consisting of iron, titanium, calcium, magnesium and combinations of one or more thereof, present in the bauxite residue. The reference to "pre-treatment" in this context refers to a treatment performed on the bauxite residue after separation from a Bayer process operation, and prior to step (a) of the present process.

The bauxite residue may be in any suitable form, for example it may be in the form of a slurry, a deliquored cake, filter cake or in lump form.

In step (a), the bauxite residue is treated to transform at least a portion of the sodium present in the bauxite residue into a soluble sodium salt. This may be achieved by a range of different techniques, one of which involves (a) leaching the bauxite residue with acid in the presence of water to dissolve the sodium present in the bauxite residue. Another method for performing step (a) is described in detail below in the context of an alternative embodiment of the invention.

When acid leaching is the form of treatment step used for step (a), then the acid produced in step (d) can be recycled to this step. The acid used in step (a) can be either a mineral or organic acid, but most suitably it will be sulphuric acid.

According to one embodiment, the bauxite residue from the Bayer process is mixed with aqueous acid, such as sulphuric acid. After leaching, a solution now enriched with soda (solubilised sodium) from DSP is neutralized using caustic material. After neutralization the solution, or liquor, is separated from the solids - a residue containing principally iron, titanium-bearing minerals together with some other insoluble minerals and residual un-extracted DSP cage material. This residue may be washed to recover extracted soda and discarded, used as a value added material (e.g. adsorbent, catalyst) or alternatively, may be further treated to recover valuable components such as titanium compounds. It is noteworthy that the pH change of the residue is drastically depressed after the treatment and, therefore, the cost and difficulties of disposal are reduced accordingly.

After solid-liquid separation, the clear solution, now enriched with soda as sodium sulphate is (if necessary, following an optional concentration and/or treatment by chelating resin), sent to electrochemical processing where the solution of sodium sulphate is electrochemically split into caustic soda and sulphuric acid. The recovered caustic soda (sodium hydroxide) is recycled to the Bayer alumina refinery and the recovered acid, such as sulphuric acid according to preferred embodiments, is recycled to the leaching process.

Previous attempts to recover soda from bauxite residue by acid neutralisation resulted in liquors containing many impurities including alumina, silica, iron, and calcium compounds which precluded the adoption of membrane electrochemical processes. These processes also suffered from the requirement of large quantities of acid, making the recovery of soda inefficient.

This invention provides a solution to these problems by a very simple combination of controlled leaching bauxite residue by acid such as sulphuric acid (or the alternative step (a) treatment step described below), neutralization by caustic materials, and after separating residue, processing the resulting solution by membrane electrochemical process to recover caustic soda and acid.

It has been found that purified liquor suitable to supply to membrane electrochemical process can be obtained by the combination of acid leaching (as one example of step (a)) and neutralization in succession, resulting in high yield of soda from bauxite residue, pure enough to supply to membrane electrochemical recovery system directly with minimal loss of leaching acid. The quantity of leaching acid required depends on the composition of DSP, extraction extent of sodium from residue and impurity content in the residue (mainly calcium compounds). The maximum loss of leaching acid does not exceed 20% of the acid for stoichiometric formation of sodium sulphate with extracted sodium except required acid for leaching of calcium compounds in the residue. This is explained in further detail below.

The present process provides a self-established recycling process. According to some embodiments, the process provides for the recovery of soda from a Bayer process, as well as recovery of sulphuric acid. In one embodiment, the process comprises the further steps of:

(e) returning the caustic hydroxide produced in step (d) to a Bayer process operation, and

(f) recycling the acid produced in step (d) to step (a).

In other words, the caustic soda and sulphuric acid produced in step (d) are recycled to a Bayer alumina refinery and bauxite residue leaching step (a) respectively.

The process follows the basic steps of:

(a) leaching of bauxite residue slurry, filter cake or lump in aqueous solution of sulphuric acid,

(b) neutralising the leached slurry by caustic materials, (c) separating the insoluble residue, and

(d) performing membrane electrochemical processing.

Preferably the pH of the product (the solution or slurry) resulting from step (a) is in the range of about 3.5 to about 6. In the first step (a), the minimum pH depends on the characteristics of DSP that varies mainly according to the characteristics of bauxite, digestion process and the processing conditions. The pH is set to fulfil the conditions of maximum extraction of soda from bauxite residue without breakage of DSP cage structure and thus avoid dissolution of silica and alumina in the leached liquor. With the above mentioned conditions, about 40 to about 85% of soda (sodium) will be extracted into liquor from bauxite residue (from the DSP in particular) and almost all materials such as silica, alumina, iron, titanium and calcium compounds remain, for the most part, in the solid. Some material such as silica, alumina and calcium may partially dissolve into liquor, though the concentration of these dissolved species are very low. The concentration of alumina, silica and calcium are controlled through this step to be below 0.1, 0.5 and 1.0 g/L respectively (expressed as Al, Si and Ca). In addition to the increase of impurities, when the leaching pH is lower than about 4, the consumption of acid for leaching and caustic materials for neutralization (next step (b)) increases dramatically.

Sodalite DSP is generally expressed as Na₆[AlSi0₄] ₆.Na₂X (X= C0₃, 2C1, 2A1(0H)₄, S0₄, etc.). It can be supposed that C0₃ is dominant in X for high silica bauxite, and the following explanation is based on X= C0₃. When DSP is leached by sulphuric acid, the reaction can be expressed as follows:

1^st step extraction of sodium

Na₆[AlSi0₄] ₆.Na₂C0₃ + H₂S0₄→ Na₆[AlSi0₄] ₆ + Na₂S0₄ + H₂C0₃ (C0₂† + H₂0) (1) 2^nd step extraction of sodium

Na₆[AlSi0₄] ₆ + x/2H₂S0₄→ Na₆._xH_x [AlSi0₄] ₆ + x/2.Na₂S0₄ (2) Whole reaction, (1) + (2)

Na₆[AlSi0₄] ₆.Na₂C0₃ + (1 +x/2).H₂S0₄→ Na₆._xH_x [AlSi0₄] ₆ + (1+ x 2). Na₂S0₄+ C0₂† + H₂0

(3)

When the residue contains calcium compounds, such as calcium carbonate for example, it will react with sulphuric acid as follows:

CaC0₃ + H₂S0₄→ CaS0₄1 + H₂C0₃ (CO,† + H₂0) (4)

In this case, the consumption of sulphuric acid depends on the extraction extent of sodium from the residue, composition of DSP and calcium content in the residue. The consumption of sulphuric acid for leaching is close to a stoichiometric consumption to form sodium sulphate [equation (3)] and additional acid is consumed to dissolve calcium compounds in the residue [equation (4)]. In the case of batch-wise operation, sulphuric acid is introduced into vessel(s) where it is subjected to suitable agitation to reach the slurry pH within the range of about 4 to about 6. In the case of continuous operation in a series of agitation tanks, the last stage leaching condition is maintained at pH within the range of about 4 to about 6. A diagram showing one possible arrangement for continuous leaching is illustrated in Figure 8. It will be understood that a greater number of leaching vessels may be included in the series. In the second step (b), neutralisation occurs. This refers to a process of raising the pH by addition of a caustic material. The pH is raised to an approximately neutral or slightly alkaline pH. In step (b) neutralization pH is suitably controlled within the range of about 7.0 to about 9.0. In this pH range the partially dissolved silica may precipitate as silica gel, alumina may precipitate as basic aluminium sulphate or aluminium hydrate gel, calcium may precipitate as calcium carbonate and some impurities can be co-precipitated with these gels or adsorbed on the residue, and hence are removed from the liquor. Thus, according to one embodiment, step (b) comprises neutralising the product of step (a) with caustic material to precipitate silica, alumina and calcium out of the liquid phase to yield a neutralised product comprising solids and a liquor. In step (b), the caustic material used for neutralization may, for example, be selected from the group consisting of sodium hydroxide, sodium carbonate, DSP, bauxite residue and combinations of two or more thereof. According to one embodiment, the caustic material used in the neutralization comprises sodium hydroxide obtained in step (d).

In steps (a) and (b), the leaching and neutralization temperature is not limited, but generally leaching and neutralization are preferably performed at a temperature in the range of between about 20°C and 70°C. The temperature may be the same or different in each of these steps.

In step (c) of the process, the liquor, now containing predominantly or only sodium sulphate, is subjected to solid- liquid separation by such a method as filtration, sedimentation, centrifugation or a combination of these unit operations.

In step (d) of the process, liquor from step (c) is subjected to membrane electrochemical processing. Membrane electrochemical processing decomposes sodium sulphate to sodium hydroxide and sulphuric acid. The membrane electrochemical processing may involve electrolysis using ion- exchange membrane(s) or electrodialysis using bipolar membrane(s) and ion-exchange membrane(s). Examples of the process using these alternatives are shown schematically in Figures 1, 2, 3 and 4.

In both forms of electrochemical processing there are two types of processing units, comprising either two- or three-compartment units. With the three-compartment unit, caustic soda is recovered from the base or cathode compartment and sulphuric acid from the acid or anode compartment. In the case of two-compartment unit, caustic soda is recovered from the base or cathode compartment and a mixture of sulphuric acid and diluted sodium sulphate is recovered from the acid or anode compartment.

When electrolysis processing is applied to the liquor, two by-products are produced on electrodes; oxygen on the anode and hydrogen on the cathode. The same by-products are produced in bipolar- dialysis, but the quantity is small compared to electrolysis processing. The choice of electrochemical processing system and the number of compartments are determined from a consideration of economics, maintenance, water balance, and the possibility of further re-processing of gases generated such as application of oxygen to wet oxidation of Bayer liquor, application of hydrogen to fuel cell and to calcination of alumina. Any widely known membranes can be used such as a bipolar membrane, a cation exchange membrane or an anion exchange membrane without any limitation.

We also found that during the operation of both neutralization in step (b) and solid liquid separation in step (c) some leached sodium is absorbed back into solid [this means the reaction (2) is reversible]. The decrease of sodium extraction depends on the leaching pH, neutralisation pH and the retention time at both neutralisation and solid liquid separation. In order to decrease the back-reaction of sodium into solid, the retention time at both neutralization and solid separation operation is desired to be shorter, within about 2 to 5 hours.

In one embodiment of the invention, after leaching, a solution now enriched with soda

(solubilised sodium) from bauxite residue is separated from the solids, and then the solution is neutralised by caustic materials. We found that applying solid liquid separation prior to neutralization is effective to avoid back-reaction of sodium, which may have decreased by about 3-15% of soda extraction extent depending on the leaching pH, neutralisation pH and retention time of the operation. Thus, according to this embodiment, the process comprises separating solids from the product of step (a) and subjecting the resulting liquid to neutralization in step (b). It should be understood that the reference to neutralization of "the product of step (a)" with caustic material is to be read as encompassing the direct product of step (a), or a portion of that product (such as a liquid portion), and in either case the product may be subjected to any other intervening processing steps prior to being subjected to step (b).

We also found that if at least one solid material is added as seed in the neutralisation step, the impurities in the solution can be precipitated out and removed by solid/liquid separation. The impurities may in this case be precipitated out as silica gel, alumina gel and/or basic aluminium sulphate, magnesium hydroxide and calcium carbonate. Thus, according to this embodiment, step (b) comprises neutralizing the product of step (a) with caustic material in the presence of a seed material. After the neutralisation with seed addition, the liquor is separated from the solids. The solids comprise precipitated impurities and seed material. In this embodiment, solid seed material enhances precipitation of the solids, which are mainly made up of silica. Any suitable solid seed material may be used. Suitable examples include one or combination of those selected from the group consisting of DSP, acid leached DSP, bauxite residue, acid leached bauxite residue, separated solids obtained from solid separation step (c) and silica-containing materials such as sodium alumino silicate, aluminosilicate and silica gel. The adding of solid seed material may also be achieved by making the liquor in step (b) intentionally turbid by imperfect solid/liquid separation between leaching and neutralization. The amount of solid seed material that may be added to the product of step (a) is not limited but may generally be in the range of between about 5 and 50g/L.

Lime is used in vast quantities in alumina refineries. It can be used as filter aid (as TCA), as an agent for causticisation (as Ca(OH)₂) of side stream liquor on residue washing train and as an agent for purification of spent or strong liquor to remove phosphorous materials, as some examples. After the treatment the calcium reaction products are discarded with bauxite residue. These calcium reaction products (mainly TCA, calcium carbonate and calcium phosphate) are easily separated from the main stream of bauxite residue to reduce the calcium content in the bauxite residue. The process of the present application is preferably applied to the calcium free or calcium compounds- decreased bauxite residue. The application of the process to such calcium-free or calcium- depleted bauxite residues is advantageous as it decreases the consumption of acid to extract sodium from the residue. In one embodiment of the invention, when the calcium contamination is inevitable and the content in the residue is high and sodium carbonate consumption is high to lower the calcium content in the liquor at the neutralization step (b), the process may comprise a calcium precipitation step following step (c) to precipitate calcium from the liquor. The calcium precipitation step may be a carbonation step to precipitate the calcium as calcium carbonate. In the carbonation step, sodium carbonate can be added to the liquor after separation of solids at step (c) to precipitate calcium ion in the liquor as calcium carbonate. The liquor is then separated from the precipitated calcium carbonate. This can be achieved by sedimentation, filtration, centrifugation or a combination of these unit operations. The carbonation after solid liquid separation decreases the consumption of sodium carbonate at the neutralization step (b) achieving purified liquor of calcium content lower than lOmg/L. The amount of sodium carbonate to be added to the liquor is typically several times that required for stoichiometric formation of calcium ion in the liquor to form calcium carbonate, and generally corresponds to l~5g/L (Na₂C0₃). Carbonation can alternatively be achieved through the contact of the liquor with a carbon dioxide-containing gas, such as flue gas or recovered C0₂ gas. The amount of carbon dioxide gas required in this case can be determined by considering the absorption efficiency of the gas into the liquor, based on the above mentioned stoichiometric requirement.

In another embodiment of the invention, when the calcium content in the residue is high and the acid leached residue is separated after leaching (prior to neutralization) to achieve high extraction extent of soda from bauxite residue, the neutralisation with seed may be carried out by using a mixture of sodium hydroxide with one or more of sodium carbonate, DSP and bauxite residue as the seed material. When sodium carbonate is used, the concentration of sodium carbonate is kept at about 1 to about 5 g/L. As for the pH of the neutralised liquor, it may be raised to over 9.0 after the addition of caustic materials with solid seed material. However we found that impurities of silica, alumina, magnesium and calcium can be removed from the liquor, whilst still achieving very high soda extraction by avoiding the back- reaction of sodium ion into leached residue. It is postulated that the caustic material such as NaOH for neutralisation reagent dissolves aluminium hydroxide or basic aluminium sulphate after neutralisation at pH >9.0 and therefore the upper limit of neutralisation pH is restricted to about 9.0. But when sodium carbonate (or C0₂) is applied for neutralization, a higher pH is allowable because carbonate appears to precipitate aluminium as an insoluble material such as dawsonite (NaAl(OH)₂C0₃).

In another embodiment of the invention, the liquor separated in step (c) may be treated to reduce the concentration of impurity ions selected from the group consisting of calcium ions, magnesium ions, aluminium ions, iron ions and combinations of two or more thereof, prior to step (d). Preferably this treatment step, which may be referred to as a purification treatment, reduces the concentration of, or eliminates, calcium and magnesium ions. The purification treatment may comprise treatment with chelating resin. Treatment with a chelating resin has been found to further decrease the concentration of

(especially) calcium ions and magnesium ions in the liquor.

According to another embodiment of the invention, the liquor obtained from step (c) may be concentrated prior to step (d). This may occur prior to or following the optional purification treatment referred to above. The concentration may be achieved by evaporation. The concentration step increases the efficiency of electrical separation especially in case of membrane electrolysis.

As noted above, the bauxite residue encompasses DSP concentrate. In a preferred embodiment of the process, DSP concentrate, separated from the Bayer refinery, is used as the bauxite residue. According to this embodiment, DSP concentrate, which does not contain other bauxite residue components such as iron, titanium, and calcium materials, is leached in the first step of the process (a). The DSP concentrate can be produced and separated from the main stream at the seeded predesilication process in accordance with the processes described in Kanehara et al. JP S56-160,321-A (1981); Holitt, Crsp et al: WO 98/22390 (1998) or post seeded desilication process in accordance with the processes described in Fulford et al: USP 4,994,244 (1991); Iwase et al: JP S62-230,613-A (1987); Kokoi et al: JP H05-170,434-A (1993); Tanjyo et al: JP H06-172876-A (1994). The disclosures of these references are expressly incorporated herein by reference. Such DSP concentrates typically have very low levels of impurities compared to bauxite residue. This invention is therefore applied most preferably to DSP concentrate.

According to one embodiment, the amount of sodium present in the bauxite residue that is leached from the bauxite residue is a minimum of 25% of that present in the bauxite residue. This amount may be at least 40%, usually at least 50%> and in some cases at least 60% or at least 70%. The amount may be up to 85%) of the amount present in the bauxite residue.

According to one embodiment, the amount of acid required to leach sodium from the bauxite residue is between 100% and 120%> of the stoichiometric equivalent of sodium, according to the level of impurities. This may alternatively be represented as an amount of acid added for leaching per extracted mole of soda being less than 1.20 mol/mol. Although this level is most preferred, in less efficient operations, the level may be up to 1.25 mol/mol or up to 1.30 mol/mol. These figures are calculated after adjusting for calcium content in the bauxite residue (see Example 6).

According to one embodiment, the quantity of caustic material (in one embodiment, sodium hydroxide), required to effect the neutralization step is 30 (mol) % or less compared to the molar amount of acid used in the leaching step. The amount of sodium hydroxide measured in volume may be 50% by volume less, and typically it can be at least 60% less by volume, and in some embodiments at least 70% by volume less than the amount of sulphuric acid used. In another measure, the amount of caustic material required for neutralisation per mole of extracted soda is preferably not more than 0.25 mol/mol, more preferably not more than 0.20 or 0.15 mol caustic material per mol of extracted soda.

According to one embodiment, the liquor produced in step (b) contains at least 30 g/L sodium sulphate, preferably at least 40 g/L, and more preferably at least 70 g/L. The amount of sodium sulphate may be up to 120 g/L.

According to some embodiments, the total amount of impurities Al, Si, Mg, Ca and Fe (in sum) in the liquor produced following step (b) is less than 20 mg/L, typically less than 15 mg/L and more often less than 10 mg/L.

As outlined above, step (a) involves treating the bauxite residue to transform at least a portion of the sodium present in the bauxite residue into a soluble sodium salt. An alternative technique to achieve this, compared to the acid leaching technique described previously, is suited to use in alumina refineries which use bauxite residue as absorbent of sulphur dioxide from stack gas of power station or alumina calciner by wet-absorption. The resulting liquor from these operations containing sodium sulphite and/or sodium bisulphite can be oxidized by air to obtain sodium sulphate, and then the slurry can be neutralized by caustic materials as per step (b) described previously, followed by the above mentioned processing steps (c) and (d). In this aspect of the invention, in step (d), three-compartment unit is preferably applied. The recovered acid is sulphuric acid.

As a consequence of the above, step (a) may comprise:

(ai) using bauxite residue to remove sulphur dioxide from stack gas by wet-absorption, and (aii) oxidising the bauxite residue resulting from step (ai) to convert the sodium sulphite and/or sodium bisulphite present in the bauxite residue resulting from step (ai) to sodium sulphate.

The oxidation may be achieved by oxidation with an oxygen-containing gas, such as air. The bauxite residue is applied to sulphur dioxide removal from stack gas by wet-absorption. The sulphur dioxide is absorbed by the bauxite residue mostly as sodium sulphite and/or sodium bisulphite, amongst other species. The caustic soda of step (e) may be returned to the Bayer process by its addition to the spent liquor stream or to any other stream without evaporation before returning to the Bayer circuit.

EXAMPLES

The process is further described by references to the following examples:

Example 1

Sulphuric acid (47.1 %) was slowly added to a slurry of DSP concentrate (40 g DSP, 120 mL water) with constant stirring at 60 °C to obtain a pH in the range from 3 to 6. The composition of the DSP is shown in Table 1. LOI in Table 1 refers to "loss on ignition" (measured at 1000 °C).

Table 1 Composition of DSP (%)

After 60 minutes the slurry was divided into two portions. The solids and liquor of this portion were analysed after filtration and washing. Another portion was further treated according to step (b). The leached slurry was neutralized by caustic soda (10 % NaOH) with stirring at 60 °C. The neutralization was controlled within the pH 7.4-7.6. After the neutralization the insoluble residue was separated by a membrane filter of pore diameter 0.45 micron meter from liquor and washed with hot water. The composition of the residue after neutralization is shown in Table 2. Table 2 Composition of residue after neutralization (%)

The composition of the leached/neutralized liquor is shown in Table 3.

Table 3 Composition of liquor after leaching/neutralization (g/L; mg/L) U er row after leachin Lower row after neutralization

Thus, the extraction extent of soda from DSP, added amounts of sulphuric acid for the extraction of soda and added amounts of sodium hydroxide for the neutralisation are shown in Table 4.

Table 4 Extracted extent of soda from DSP; added acid for the leaching and sodium hydroxide for neutralisation per extracted soda in final liquor (step (c))

* Leached soda: leached soda at step (a) from bauxite residue (%) ** Extracted soda: final extracted soda into exit liquor after whole processing from bauxite residue (%)

When leaching pH was as low as 3.0-3.5, the leached soda at step (a) was very high (96%) but the extraction extent of soda (after neutralization) decreased to 82-88%. Sulphuric acid was consumed not only to extract soda from DSP but also to dissolve the DSP cage (alumina and silica), Fe₂0₃, and others. Subsequently it also required a larger quantity of soda for neutralization. The extra acid for leaching will form gels or hydroxide precipitates and will be largely wasted. The extra caustic soda for neutralization, though mostly recoverable at electrochemical process (step (d)), increases the required capacity of electrochemical processing.

From this example, it is clear that when DSP was leached by sulphuric acid at the pH range 4-6 and neutralized by caustic soda, soda in DSP was stoichiometrically leached to form sodium sulphate and neutralized by a small quantity of caustic soda. The resulting liquor contained 60-110 g/L Na₂S0₄ and total amounts of impurities such as Al, Si, Ca and Fe were less than 10 mg/L. This liquor is technically and economically acceptable for direct treatment by electrochemical membrane systems, especially electrodialysis with bipolar membrane system. In the case of membrane electrolysis, it would be preferable to concentrate the liquor to as close to saturation of sodium sulphate as possible to increase current efficiency.

Example 2

Sulphuric acid was added to DSP slurry to achieve pH 4.5 with constant stirring at 60 °C. Thereafter the slurry was neutralized by caustic soda as described in Example 1, but in this example, neutralization pH was varied within the range of 5.5-10.0. After filtration of the slurry, the liquor was analysed (Table 5).

Table 5 Composition of liquor after neutralization (pH 5.5~10.0)

It is clear that neutralization after leaching should be controlled to keep the pH within the range of 7.0-9.5. Example 3

Leaching (pH 4.5) and neutralization (pH 7.5) was performed in a similar way to Example 1 except that DSP was applied as the neutralization agent instead of caustic soda. After filtration, the liquor was analysed (Table 6).

Table 6 Composition of liquor after neutralization (neutralized by DSP)

Example 4

Sulphuric acid was added to DSP slurry (40g DSP, 120mL water) to achieve pH 4.0 and retained at 60 °C for 60 minutes with constant stirring. Thereafter the liquor is separated from the residue. After the solid-liquid separation, 20wt% of leached wet residue and sodium hydroxide was added to the liquor to neutralise the slurry and retained it at pH 7.5 for 60 minutes. After the neutralisation the insoluble solids were separated from the liquor by a membrane filter as in Example 1. The composition of liquor that was obtained is shown in Table 7.

Table 7 Composition of liquor after neutralization with seed

(solid-liquid separation was applied after leaching)

Soda extraction extent from DSP was 83%, added amount of sulphuric acid per extracted soda

(mol/mol) was 1.09 and the amount of caustic soda for neutralization was 0.10 (mol/mol).

Example 5

Sulphuric acid (47.1%) was slowly added to a slurry of bauxite residue (100 g residue (A), 300 mL water) with constant stirring at 60°C to obtain a pH 4.5. The composition of the residue is shown in Table 8.

Table 8 Composition of bauxite residue (A) and leached/neutralized residue (%)

After 60 minutes the leached slurry was neutralized by caustic soda (10% NaOH) with stirring at 60°C. The slurry pH was raised to 7.5 by the neutralization. After the neutralization the insoluble residue was separated by a membrane filter same as shown in Example 1 and washed with hot water. The composition of leached residue after neutralization is shown in Table 8. The compositions of both the leached and neutralized liquors are shown in Table 9.

Table 9 Composition of liquor after leaching and neutralization (g L, mg L)

The extraction extent of soda from residue was 59%. The amount of sulphuric acid added for leaching per extracted soda was 1.12 mol/mol. The amount of caustic material (NaOH 10%>) for neutralization per extracted soda was 0.06 mol/mol. All impurities such as alumina, silica, calcium and magnesium were removed by the neutralization step (b). Sodium sulphate concentration of final liquor was about 70 g/L.

Example 6

Sulphuric acid (47.1%) was slowly added to a slurry of bauxite residue (100 g residue (B),

300 mL water) with constant stirring at 60°C to obtain a pH 4.5. The composition of the residue is shown in Table 10.

Table 10 Composition of bauxite residue (B) and leached/neutralized residue (%)

After 60 minutes the leached slurry was neutralized by caustic soda (10% NaOH) with stirring at 60°C. The slurry pH was raised to7.5 by the neutralization. After the neutralization the insoluble residue was separated by a membrane filter same as shown in Example 1 and washed with hot water. Sodium carbonate (10% Na₂C0₃) was added to the filtrate (200 mL) by 8 mL (Na₂C0₃ concentration in the liquor is about 4.0 g/L) and then the precipitated material was filtered. The composition of the residue after neutralization is also shown in Table 10. The compositions of the leached, neutralized and carbonated liquors are shown in Table 11. After the carbonation the liquor pH was 10.5.

Table 11 Composition of liquor after leaching, neutralization and carbonation (g/L, mg/L)

The extraction extent of sodium from residue (B) was 67%>. Required acid per extracted soda for leaching was 1.69mol/mol, and this means that the required acid was 1.69 times bigger than that of stoichiometrically required amount of extracted sodium to form sodium sulphate. When it is assumed that reaction product of both the extracted sodium and the calcium compound in residue with sulphuric acid are sodium sulphate and calcium sulphate respectively, the acid consumption was very reasonable of 1.04 [H₂S0₄/(Ext-Na₂0+CaO in BR), each by molar expression]. Impurities such as alumina, silica and magnesium were removed by the neutralization step and calcium was removed by the carbonation step. Sodium sulphate concentration of final liquor was about 42 g/L.

Example 7

Sulphuric acid (47.1%) was slowly added to a slurry of bauxite residue (100 g residue (B), 300 mL water) with constant stirring at 60°C to obtain a pH 4.0. After 60 minutes the insoluble residue was separated by a membrane filter and washed with hot water. The filtrate (200mL) was neutralized at 60°C with stirring by the addition of caustic materials (sodium hydroxide (10% NaOH, 2mL) and sodium carbonate (10% Na₂C0₃, 4.0mL) and leached bauxite residue (20% of leached residue). The slurry pH was first raised to 10.5 by the addition of the caustic materials and the leached residue, and then decreased to 8.8 during the retention of 60 minutes. After the neutralization the insoluble residue was separated by a membrane filter. The compositions of the leached, neutralized liquor are shown in Table 12. The composition of the liquor without the addition of seed material at neutralisation step is also shown in Table 12.

Table 12 Composition of liquor after leaching and neutralization (g L, mg/L)

The extraction extent of sodium from bauxite residue at leaching step (a) was 84% and that extent after neutralisation was 81%. It is clear that the decrease of soda extraction extent by neutralisation was depressed and clear liquor was obtained by the process.

Example 8

A three-compartment type bipolar membrane electrolyser (ASTOM Co., Japan: ACILYZER EX-3B) was set-up as shown in Figure 5, using a bipolar membrane (Neocepta BP- IE), a cation- exchange membrane (Neocepta CMB) and an anion-exchange membrane (ACM) manufactured by

ASTOM Corporation.

The configuration of membranes was achieved by placing alternating bipolar membranes (BP), anion-exchange membranes (A) and cation- exchange membranes (C) in number of eleven pieces, ten pieces and ten pieces, respectively (bipolar membrane, anion-exchange membrane and cation-exchange membrane possessed effective membrane area of 0.55 dm², respectively, and total membrane area of 6.05 and 5.5 dm², respectively) between a pair of cathode and anode, thereby constituting acid compartments 3, salt compartments 4 and alkali compartments 5.

A liquor produced under the same conditions of Example 4, which included washing liquor of cake and contains 92 g/L of sodium sulphate, was treated by the electrolyser. 500mL of a 0.25 mol/1 sulphuric acid aqueous solution was fed to the acid chambers, 700mL of a solution was fed to the salt compartments, and 500mL of a 0.5 mol/1 sodium hydroxide aqueous solution was fed to the alkali compartments at a linear velocity of 6 cm/sec, respectively, and were circulated. An aqueous solution of 1.0 mol/1 of caustic soda in an amount of 500mL was circulated in series into the anode compartment (6) and the cathode compartment (7). Electrodialysis was carried out at a current density of 10A/dm² for 0.75 hours.

As a result, an aqueous solution of sodium hydroxide (1.9 mol/L) was obtained in an amount of 580 mL from the alkali compartment and an aqueous solution of sulphuric acid (1.0 mol/L) was obtained from the acid compartment in an amount of 550 mL.

In this case, current efficiency for forming caustic soda was 58%. The average cell voltage during this period was 2.4 volts. Therefore, the electric power consumption was 2,400 kwh t-NaOH. The batch-wise electrodialysis was repeated ten times but there was no change in the current efficiency and in cell voltage.

Example 9

A two-compartment type bipolar membrane electrolyser (ASTOM Co., Japan: ACILYZER EX- 3B) was set-up as shown in Figure 6, using a bipolar membrane (Neocepta BP- IE) and a cation-exchange membrane (Neocepta CMB) manufactured by ASTOM Corporation.

The configuration of membranes was achieved by alternatingly placing bipolar membranes (BP) and cation-exchange membranes (C) in number of eleven pieces and ten pieces, respectively (bipolar membrane and cation-exchange membrane possessed effective membrane area of 0.55 dm², respectively, and total membrane area of 6.05 and 5.5 dm², respectively) between a pair of cathode and anode, thereby constituting salt compartments 8 and alkali compartments 5.

The same quantity and quality of sodium sulphate solution (92 g/L, 700 mL) and sodium hydroxide aqueous solution (0.5 mol L, 500 mL) was treated by the electrolyser as described in Example 8.

As a result, an aqueous solution of sodium hydroxide (1.65 mol/L) was obtained in an amount of 550 mL from the alkali compartment.

In this case, current efficiency for forming caustic soda was 40%. The average cell voltage during this period was 1.12 volts. Therefore, the electric power consumption was 1,700 kwh/t-NaOH.

Example 10

Residues produced in Example 1 (leached residue at pH 4.0: 5 g, neutralized residue: 5g), 5 (neutralized residue: 7.5g) and 7 (leached residue 20 g + seed 4 g (after neutralization)) and raw residues (DSP: 5 g, bauxite residue (A): 7.5 g, bauxite residue (B) 20 g) were dispersed with water (100 mL) in plastic bottles with caps (150 mL) and pH were measured periodically. The bottles were replenished with water to keep the liquor level constant. Observed pH changes are shown in Figure 7. The pH change of the residue treated by this invention (after neutralization) is stable at between pH 7~ 10 (depends on the leaching/neutralization conditions) compared to the original residue, which pH increased gradually to pH~13.

Example 11

Four embodiments of the process, incorporated into a Bayer process operation, are outlined in

Figures 1, 2, 3 and 4.

According to the embodiment of Figure 1 , bauxite ( 1) is treated with sodium hydroxide (2) in the Bayer process (3). The DSP (4) or bauxite residue (5) obtained in the process is then leached with sulphuric acid (6) in a leaching step (7) which may be performed in a single stage or multiple stages. The leaching may be conducted in a single vessel or in multiple vessels. The leached product is subjected to neutralisation (9) with caustic material (8) in one or more neutralisation vessels. The neutralised product comprising solids and a liquor is subjected to solid/liquid separation (10), and the residue (12), a solid, is separated from the liquor. The liquor, containing sodium sulphate (11), is subjected to impurity removal in a chelating resin treatment (13). The liquor following this step is subjected to electrodialysis (14), which separates the acid (15), salt (16) and base (17). The acid is recycled to the leaching step, the salt is used partly to wash residue (12), and the rest is sent to waste (18), and the base (17), sodium hydroxide, is recycled to the Bayer process operation as an input sodium hydroxide.

The embodiment of Figure 2 is the same as that for Figure 1, with two modifications. In this embodiment, there is an additional concentration step (19) following separation of the liquor from the solid residue (12). In this embodiment, the method of membrane electrochemical separation involves electrolysis (20) to separate the salt and acid (21) from base (17). The mixture of salt and acid is recycled to the leaching step (7), and the surplus portion of the waste is sent to waste (18).

The embodiment of Figure 3 is the same as that for Figure 1, with two modifications. In this embodiment, there is an additional solid liquid separation step (22) prior to neutralisation (9). In this embodiment, the separated liquor is subjected to neutralisation (9) by the addition of caustic material (8) with seed material (24).

The embodiment of Figure 4 is the same as that for Figure 1, with one modification. In this embodiment, there is an additional carbonation step (26) following separation of the liquor from the solid residue (12). In this modification, calcium ion still remained in the liquor is carbonated by sodium carbonate (25) and separated (27) as residue (28).

Various modifications can be made to the above examples and embodiments within the spirit and scope of the invention.

Claims

THE INVENTION DEFINED BY THE CLAIMS IS AS FOLLOWS:

1. A process for recovering sodium from bauxite residue, comprising:

(a) treating the bauxite residue to transform at least a portion of the sodium present in the bauxite residue into a soluble sodium salt,

(b) neutralizing the product of step (a) with caustic material to yield a neutralized product

comprising solids and a liquor,

(c) separating the solids from the liquor to recover a liquor containing the solubilised sodium, and

2. The process according to claim 1, wherein step (a) comprises leaching the bauxite residue with acid in the presence of water to dissolve the sodium present in the bauxite residue.

3. The process according to claim 2 wherein the acid is sulphuric acid.

4. The process according to claim 2 or claim 3, wherein the acid produced step (d) is recycled to step (a).

5. A process according claim 4 wherein, in the sulphuric acid for leaching bauxite residue in step (a) includes sodium sulphate present in the sulphuric acid produced in step (d) which is recycled to step (a).

6. The process according to claim 1, wherein step (a) comprises:

(ai) using bauxite residue to remove sulphur dioxide from stack gas by wet-absorption, and

(aii) oxidising the bauxite residue resulting from step (ai) to convert the sodium sulphite and/or sodium bisulphite present in the bauxite residue resulting from step (ai) to sodium sulphate.

7. The process according to any one of claims 1 to 6, wherein the pH of the solution resulting from step (a) is in the range of 3.5 to 6.0.

8. The process according to any one of claims 1 to 7, wherein the pH of the solution resulting from step (b) is in the range of 7.0 to 9.0.

9. The process according to any one of claims 1 to 8, wherein, in step (d), the membrane

electrochemical system is electrodialysis system constituted by bipolar membrane and cation- exchange membrane.

10. The process according to any one of claims 1 to 8, wherein, in step (d), the membrane

electrochemical system is electrodialysis system constituted by bipolar membranes, cation- exchange membranes and anion-exchange membranes.

11. The process according to any one of claims 1 to 8, wherein, in step (d), the membrane

electrochemical system is electrolysis system constituted by a cation-exchange membrane.

12. The process according to any one of claims 1 to 8, wherein, in step (d), the membrane

electrochemical system is electrolysis system constituted by a cation-exchange membrane and an anion-exchange membrane.

13. The process according to any one of claims 1 to 12, wherein, in step (b), the caustic material used for neutralization is selected from the group consisting one or more of combination of sodium hydroxide, sodium carbonate, DSP, bauxite residue and combinations of two or more thereof

14. The process according to any one of claim 1 to 13, wherein the leached liquor is separated from solids and the liquor is neutralised with caustic material and seed solid.

15. The process according to any one of claim 14, wherein the seed solid to be added at the

neutralisation step is selected from the group consisting of bauxite residue, leached bauxite residue, DSP, leached DSP, silica minerals and combination of two or more.

16. The process according to any one of claims 1 to 13, wherein, sodium carbonate is added to the liquor separated in step (c) and the reaction product is removed by solid- liquid separation prior to step (d).

17. The process according to any on of claim 1 to 16, wherein, the amount of sodium carbonate to be added to the liquor is several times of stoichiometric formation of calcium carbonate, and generally in the range of l~5g/L (Na₂C0₃).

18. The process according to any one of claims 1 to 17, wherein, liquor separated in step (c) is treated by chelating resin prior to step (d).

19. A process for the production of alumina from bauxite, comprising:

(i) contacting the bauxite with sodium hydroxide to produce a sodium aluminate-containing solution and bauxite residue,

(ii) separating the bauxite residue from the sodium aluminate-containing solution,

(iii) recovering the aluminium hydroxide from the sodium aluminate-containing solution, and

(iv) recovering the sodium from the bauxite residue in accordance with the process according to any one of claims 1 to 18.

20. A process for the recovery of sodium from a sodium-containing waste material, the process

comprising:

(c) separating the solids from the liquor to recover a liquor containing the solubilised sodium; and (d) processing the liquor containing the solubilised soda by a membrane electrochemical system to regenerate sodium hydroxide and acid.