AU612411B2

AU612411B2 - Process for decreasing contamination in caustic alkalis in the bayer process

Info

Publication number: AU612411B2
Application number: AU22969/88A
Authority: AU
Inventors: Gregory P. Brown; Otto Tschamper
Original assignee: Alusuisse Holdings AG; Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1987-10-01
Filing date: 1988-09-29
Publication date: 1991-07-11
Anticipated expiration: 2008-09-29
Also published as: EP0310553A1; AU2296988A

Description

AUSTRALIA.1 41 Patents Act Cd1W LETE SPECIFICATICfl

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority elated Art.- APPLICANT'S REFERENCE: Case 1788 M me(s) of Applicant(s)t Swiss Ali-mlinium Ltd Address(es) of Applicant(s): F CH-8212, Neuhausen am Rheinfall1, Adress for Service is: PHILLIPS OR1&CDE FITZPATRICK Patent and Trade Mark Attorneys 0 "s 367 Collin Street Meloure 300AUSTRALIA Complete Specification for the invention entitled: PROCESS FOR DECRE.ASNG flNMIATICN IN CAUSTIC ALKALIS IN THE BAYER PROJCESS Our Ref 107332 POF Code: 1526/1526 The following statement is a full description of this invention, including the best method of performing it known to applicanv(s): 6003q/1-1 Process for decreasing contamination in caustic alkalis in the Bayer process The invention relates to a process for decreasing dissolved inorganic and organic contamination in aqueous caustic alkalis in the Bayer process for the preparation of aluminium hydroxide.

The raw materials used in the Bayer process, essentially bauxite and sodium hydroxide solution, .0 contain undesired organic and inorganic substances which are introduced into the Bayer process, in particular during dissolution of the bauxite, and consequently adversely affect the process in a wide 0e.0 variety of ways. The substances are acidic compounds, 00. 15 such as, for example, chlorides, phosphates, sulphates, :0 vanadates, fluorides and/or organic salts or acids, in o particular oxalates and humic acids.

Since the caustic alkali in the Bayer process o is circulated in various steps at various concentrations, the contaminants accumulate at various •00 points up to critical values if they are not removed from the process. Technical and economic problems are associated with the contamination. After accumulation, oO:O they result, since they are produced in the form of sodium salts, in a reduction in the causticity and the content of free caustic alkali, which has the os:. consequence that the productivity of the alkali, and 0 thus the economic efficiency of the process, drops.

Furthermore, execution of the process is hindered by deposits of the undersired substances in tanks, pipes, heat exchangers and other equipment, meaning that cleaning steps must be carried out in order to reestablish the intended production performance. Under certain circumstances, the cleaning steps may be extremely complex, for example in that rinsing with acid must be carried out if the deposits reach coating thicknesses which can only be removed insufficiently by mechanical means or if the deposits are located at ratent ana i raae Mark Attorneys 367 Collins Street Melbourne, Australia 2 inaccessible places.

The economic efficiency of the process is furthermore reduced by the contaminants in that they increase the caustic alkali viscosity, and the pump costs therefore rise. In addition, settling of the hydroxide and the red mud, and removal thereof, are adversely affected.

In addition to the abovementioned disadvantages, which can be blamed on all the contaminants together, there is a large number of further disadvantages attributable to individual contaminants. Thus, organic contaminants, usually in f.s the form of long-chain humic acids or derivatives or reaction products thereof, impart a greyish colour on 15 the aluminium hydroxide. Of particular importance is the oxalate formed during the decomposition phase of the bauxite by dissolution and decomposition of humic and other organic acids. This may accumulate until oversaturation occurs and, when it precipitates, may adversely affect the economic efficiency of the process by rendering crystallization of the aluminium hydrate more difficult and additionally having an adverse affect on the physical properties of the crystal agglomerates.

A large number of processes are known for modifying critical threshold values of the contaminants and for monitoring their concentration in the caustic alkalis circulated. According to US Patent 4,335,082, the caustic alkalis in the process can be treated with sodium hypochlorite or other oxidants in order to decompose the organic substances. In DE-OS 2,415,872, the addition of calcium compounds is proposed for precipitation of the humic acids as insoluble calcium compounds. In a similar way, precipitation reactions using magnesium or barium compounds are proposed in the US Patents 4,046,855 and 4,101,629. The lastmentioned processes can also be employed for removal of oxalate since the caustic alkali is destabilized when the humic I IIII JN_ 3 substances are removed, and the oxalate is thereby precipitated as sodium oxalate. Furthermore, US Patent 3,649,185 proposes removing the oxalate by increasing the caustic alkali concentration by further reduction of caustic soda, thus precipitating the oxalate. The inorganic contaminants can be salted out in an analogous manner. Salting out by evaporative crystallization has also been used in the past, but has only historical significance today.

The processes proposed are relatively complex to carry out, use expensive additives or in some cases require increased consumption of caustic alkali, which ot always increases the production costs and it does not always ensure good product quality.

15 The inventors have therefore set themselves the object of providing a process which is suitable for decreasing, in an economic manner, the inorganic and organic dissolved contamination in aqueous caustic alkalis in the Bayer process for the preparation of aluminium hydroxide and keeping the concentration of *this contamination within low limits such that it has *virtually no effect on the Bayer process.

i- S. Ts-oi eyed _.arn-fto °i invention by a pro ess in which the caustic alkali is subjected to electro ialysis using at least one ~anion- and/or at least\one cation-permselective membrane and at least o e bipolar water-dissociating membrane with formation o a caustic alkali component, which is added to the caus ic alkali circulating in the Bayer process, and a contaminant component, which is removed from the process and discarded. The contaminant component essential y contains the anionic acid radicals of the contaminant from the contaminated caustic alkali. i 1 Advantageous embodiments ofthe process according to the invention are characterized by the features of Claims 2 to 14.

mm n aThd invention eipblaas wadt-n the s3 scf. a biolar

S

SS

*r S 4 *5

S

*OSS

S

55 S S. S Accordingly, the present invention provides a process for decreasing dissolved inorganic and organic contamination in contaminated aqueous caustic alkali in the Bayer process for preparation of aluminium hydroxide, wherein the caustic alkali is subjected to electrodialysis using at least one anion and/or at least one cationpermselective membrane and at least one bipolar, waterdissociating membrane with formation of a caustic alkali component, for addition to the caustic alkali circulating in the Bayer process, and a contaminant component, which is removed from the process and discarded. The contaminant component essentially contains the anionic acid radicals of the contaminants from the contaminated caustic alkali.

A preferred feature of the present invention is that before being subjected to electrodialysis, the caustic alkali is treated with carbon dioxide, and aluminium hydrate which precipitates out during this treatment is removed and fed to the Bayer process.

Preferably, the solid components present in the contaminant component, in particular the aluminiumcontaining components, preferably aluminium hydrate are removed and may be recycled into the Bayer process.

Alternatively, the contaminant component may be subjected to electrodialysis once or several times before being removed from the process and discarded.

An alternative embodiment is that electrodialysis is carried out using at least two series-connected units comprising either a cation- or anion-permselective membrane and a bipolar membrane, the contaminated caustic alkali being fed to a cell part(s) formed from an ion-selective membrane and a part, changed the same as the latter, of a bipolar membrane.

Additionally, the contaminated caustic alkali may be fed to a cell part formed from anode and/or cathode and the membrane adjacent to each.

Furthermore, the electrodialysis may be carried out using at least two series-connected units comprising an anion-permselective membrane and a bipolar membrane, the contaminated caustic alkali being fed to a cell part(s) formed from an anion-selective membrane and an anionic part of a bipolar membrane.

A further embodiment of the invention comprises the i caustic alkali being subjected to electrodialysis using a unit comprising a cation-permselective membrane adjacent to an anode and an anion-permselective membrane and a Sbipolar electrode located between the two, or using a plurality of such units, the contaminated caustic alkali being fed to cell parts formed from the anode and from Scathode and the adjacent ion-permselective membranes and/or to cell part(s) formed from the anion- and cationpermselective membrane.

IAdditionally, a cation-permselective membrane may be inserted between the anion-permselective membrane and the i cathode.

At least part of the contaminated caustic alkali may be the caustic alkali circulated in the Bayer process and produced after washing a seed substance used in the Bayer 20 process. Furthermore, at least part of the contaminated i caustic alkali may be the caustic alkali circulated in the Bayer process and produced after washing red mud produced S by the Bayer process.

The invention is based on the use of a bipolar i 3 1 h -af 7 -3b- 2 membrane in electrodialysi 4 s of the caustic alkali from the Bayer process. Specifically, the reaction, written in general terms, MX H20 HX MOH where M denotes an alkali metal and X denotes the acid radical, occurs. Thus, caustic alkali and the corresponding acid form from the salt as contaminant and water, which is dissociated by the bipolar electrode, in that the cations or anions which have accumulated in the corresponding parts of the cell through the selective membranes react with the dissociation products of the water. For example, the following reactions may occur individually or simultaneously: 15 Na2CO3 H20 2NaOH CO 2 "Na 2

C

2 0 4

H

2 0 NaOH H 2

C

2 0 4 S* NaC1 H20 NaOH HC1 Na 2

SO

4 2H20 2NaOH H 2

SO

4 NaF H 0 NaOH HF The use of the process according to the invention is particularly advantageous when gaseous reaction products which are automatically removed from the further Bayer process are formed from the contaminants. This is particularly the case in the first equation above. Further features and details of the invention are revealed by the drawing, in which, S* S 30 schematically, Figure 1 shows the division of a contaminated caustic alkali stream into a purified caustic alkali stream and a stream containing the contaminants, Figures 2 to 6 show various arrangements of electrodialysis cells, Figure 7 shows the use of the electrodialysis cell(s) in the Bayer process circuit, Figure 8 shows the carbon dioxide treatment with subsequent solids removal, and Figure 9 shows the circulation route of the contaminant

I

.4 I component.

In Figure 1, which shows the simplified basic principle of the invention, the caustic alkali stream P, originating from the Bayer process and charged with contaminants is subjected to electrodialysis using at least one anion- and/or at least one cation-permselective membrane and at least one bipolar membrane which is capable of dissociating water. After the electrodialysis cell E, a caustic alkali stream Pr' 10 which is low in contaminants, and a separate stream V, which contains the contaminants, are obtained. By passing the caustic alkali stream Pr several times through the electrolysis cell E or if a relatively large number of units of series-connected electrolysis 15 cells act on the caustic alkali stream Pv it is possible to obtain a purified caustic alkali Pr whose contaminants no longer affect the subsequent process steps of the Bayer process.

Before subjecting the caustic alkali Pv charged 20 with contaminants to electrodialysis, it can be treated according to the invention with carbon dioxide, as shown by Figure 8, preferably using the carbon dioxide which may be produced when carrying out the electrodialysis, and the solid produced can be removed.

The latter is advantageously carried out by filtration.

The aluminium-containing solid component is fed to the Bayer process. A further process variant according to the invention comprises removing the solid components present in the contaminant component V, in particular the aluminium-containing components, preferably aluminium hydrate. The aluminium-containing components are advantageously recycled into the Bayer process.

It is particularly advantageous economically if the contaminant component is initially circulated once or several times, possibly with removal of solids taking place in each circuit, as shown by Figure 9, where the circulating stream is indicated by V before it is removed from the process as contaminant component

S

S. S S S

I-

6 V and discarded, since it is then possible to feed the alkali and/or aluminium components present in the contaminant component to the Bayer process in optimum manner.

Figures 2 to 6 show examples of possible arrangements of electrodialysis units according to the invention. In addition, arrows indicate illustratively and schematically the cell parts into which the contaminated caustic alkali Pv can be introduced and I the cell parts from which the corresponding reaction products Pr and V can be removed or where the stream/ streams of purified caustic alkali Pr and the stream/ streams of contaminants V are produced.

S 15 Figure 7 shows a greatly simplified view of the caustic alkali circuit in the Bayer process and the use of electrodialysis E l either after washing the red mud and/or the use of electrodialysis E 2 after washing the seed substance. It is of course also possible to 20 employ electrodialysis at other points of the circulation process. However, it has been found that the abovementioned points are particularly preferred since the necessary equipment can be installed without significant operational changes at these points and, on 25 the other hand, the caustic alkali to be purified is present in dilute form, which has a positive effect on the service life of the membranes.

0. 9* U U U a

Claims

1. A process for decreasing dissolved inorganic and organic contamination in contaminated aqueous caustic alkali in the Bayer process for preparation of aluminium hydroxide, wherein the caustic alkali is subjected to electrodialysis using at least one anion and/or at least one cation- permselective membrane and at least one bipolar, water- dissociating membrane with formation of a caustic alkali component, for addition to the caustic alkali circulating in the Bayer process, and a contaminant component, which is removed from the process and discarded. 9 9 9

9.* .9* 9 20 i 9 i i 1 I- F :9 30 2. The process according to claim 1, wherein before being subjected to electrodialysis, the caustic alkali is treated with carbon dioxide, and aluminium hydrate which precipitates out during this treatment is removed and fed to the Bayer process. 3. The process according to claim 2, wherein carbon dioxide produced when carrying out electrodialysis is used for treatment of the caustic alkali. 4. The process according to any one of claims 1 to 3, wherein solid components present in the contaminant component are removed. 5. The process according to claim 4, wherein said solid components are aluminium-containing components. 6. The process according to claim 5, wherein said aluminium-containing components comprise aluminium hydrate. 7. The process according to claim 5 or claim 6, wherein the aluminium-containing components removed are recycled into the Bayer process. 8. The process according to any one of claims 1 to 4, wherein the contaminant component is subjected to electrodialysis once or several times before being removed -7- from the process and discarded. 9. The process according to any one of claims 1 to 8, wherein the electrodialysis is carried out using at least two series-connected units comprising either a cation- or anion-permselective membrane and a bipolar membrane, the contaminated caustic alkali being fed to a cell part(s) formed from an ion-selective membrane and a part, changed the same as the latter, of a bipolar membrane. The process according to claim 9, wherein additionally, the contaminated caustic alkali is fed to a cell part formed from anode and/or cathode and the membrane adjacent to each. 0*

11.i The process according to claim 9 or 10, wherein the electrodialysis may be carried out using at least two series-connected units comprising an anion-permselective membrane and a bipolar membrane, the contaminated caustic S 20 alkali being fed to a cell part(s) formed from an anion-selective membrane and an anionic part of a bipolar membrane.

12. The process according to any one of claims 1 to 8, S wherein the caustic alkali being subjected to electrodialysis using a unit comprising a cation- permselective membrane adjacent to an anode and an anion- permselective membrane and a bipolar electrode located I between the two, or using a plurality of such units, the contaminated caustic alkali being fed to cell parts formed from the anode and from cathode and the adjacent ion-permselective membranes and/or to cell part(s) formed from the anion- and cation- permselective membrane.

13. The process according to claim 12, wherein a cation-permselective membrane may be inserted between the anion-permselective membrane and the cathode. ID 14. The process according to any one of claims 1 to 121, )4 -8- C c -I I- wherein at least part of the contaminated caustic alkali may be the caustic alkali circulated in the Bayer process and produced after washing a seed substance used in the Bayer process. The process according to any one of claims 1 to 12, wherein at least part of the contaminated caustic alkali may be the caustic circulated in the Bayer process and produced after washing red mud produced by the Bayer process. S 9 *5 S S S 59r S* S S

16. The process according to claim 1, substantially as herein described with reference to the accompanying drawings. DATED: 5 APRIL, 1991 PHILLIPS ORMONDE FITZPATRICK Attorneys For: SWISS ALUMINIUM LTD. 'C 99 S. r 04 W 4,