AU611715B2

AU611715B2 - Process for decreasing contamination in caustic alkalis in the bayer process

Info

Publication number: AU611715B2
Application number: AU22966/88A
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: 1991-06-20
Anticipated expiration: 2008-09-29
Also published as: AU2296688A; EP0310554A1

Description

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SI

AUSTRALIA

Patents Act E117,13M COMPLEM SPECIFICATIWK

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority ::.Related Art: 0:00 000:0 00 6 660 00 APPLICANT'S REFEREN2CE: Case 1789 66 6 6 6 6 66 Name(s) of Applicant(s): Swiss Aluminium Ltd .,.,9jddress(es) of Applicant(s): Cu -8212, Neuhausen am Rheinfall,

SWITZERLAND.

Address for Service is: 6 PHILLIPS CFJ4CNDE FITZ.PATRICK :Patent and Trade Mark Attorneys 66 367 Collins Street Melbourne 3000 AUSTRALIA ComplEte Specification for the invention entitled: PRO)CESS FOR DECREASING C tTAMIN1ATICH IN CAUSTIC ALKALIS IN TH3E BAYER

PROCESS

Thr e foln sttmn isaflldsrpto7fthsivetoinldn POF Code: 1526/1526 the best method of performing it known to applicant(s): 6003q/1 1 or other witness I I' fy reired E.Wiedmer K.Meyer managers PHILLIPS ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia I P i 1A 1A 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, 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 O0 variety of ways. The substances are acidic compounds, 15 such as, for example, chlorides, phosphates, sulphates, vanadates, fluorides and/or organic salts or acids, in 0* particular oxalates and humic acids.

Since the caustic alkali in the Bayer process is circulated in various steps at various concentrations, the contaminants accumulate at various points up to critical values if they are not removed 0* from the process. Technical and economic problems are associated with the contamination. After accumulation, 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 consequence that the productivity of the alkali, and 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 P18/7/81 PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia -2inaccessible 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 the form of long-chain humic acids or derivatives or 0 reaction products thereof, impart a greyish colour on 15 the aluminium hydroxide. Of particular importance is 0* 0 adversely affect the economic efficiency of the process the oxalate formed during the decomposition phase of the bauxite by renderngdissolution candryst deallicozamposition of um aumum hydrate more difficult and additionally having an adverse and o ther organic acids. This may accumul the crystal adversely affect the economic efficiency of the process rendering crystallization of the are kluminium hydrate more difying critical t hreshold v alues of the contaminantsd alkalis circulated. According to US Patent 4,335,082,ates.

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 since the caustic alkali is destabilized when the humic processes can also be employed for removal of oxalate 00 0 0000 00 @0 0) 0000 0000 0* 00 0 00 0 00~ 00 .00 0 0 00 0 @0 0 0 0 0 00 @000 00 0 0000 0 00 0 00 0 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, thi-u lrecipitating 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 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 20 aluminium hydroxide and keeping the concentration of this contamination within low limits such that it has virtually no effect on the Bayer process.

invention by a process in which the caustic alkal is 25 subjected to electrodialysis using at least one unit comprising an anion-permselective membrane afd a cation-permselective membrane and with fo4rmation of a caustic alkali component, which is ad -ed to the caustic alkali circulating in the Bayer pr ess, and a contaminant component, which is emoved from the process and discarded. The ontaminant component essentially contains the nionic acid radicals of the contaminants, in particular sodium salts of the type mentioned in the in roduction, from the contaminated caustic alkali.

Advan ageous embodiments of the process according o the invention are characterized by the Sfeatur of Claims 2 to The -nven i on--i-bas-ed-a-on------=--arnd Lt: Accordingly, the present invention provides a process for decreasing dissolved inorganic and organic contamination in aqueous contamination containing caustic alkali used in the Bayer process for the preparation of aluminium hydroxide, wherein the contamination containing caustic alkali is subjected to electrodialysis using at least one uni' comprising an anion-permselective membrane and a cation-permselective membrane with formation of a caustic alkali component for addition to caustic alkali circulated 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, in particular sodium salts of the type mentioned in the introduction, from the contaminated caustic alkali.

Advantageous embodiments of the process according to ooo. the invention are characterized by the features of claims S2 to 14.

20 The invention is based on the use of anion- and

U

*u o 0 6 -3a- 4cation-permselective membranes in electrodialysis of the caustic alkali from the Bayer process.

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 according to the overall equation Na 2

CO

3 H20 2NaOH CO 2 Further features and details of the invention are revealed by the drawing, in which, schematically, Figure 1 shows the division of a contaminated caustic •alkali stream into a purified caustic alkali stream and a stream containing the 15 contaminants, Figure 2 shows the course of the reaction in a cell unit comprising an anion-permselective membrane and a cation-permselective membrane, Figure 3 shows a plurality of cell units arranged in S* 20 series, and Figure 4 shows the use of the electrodialysis cell(s) in the Bayer process circuit, In Figure 1, the caustic alkali stream Pv originating from the Bayer process and charged with 25 contaminants is subjected to electrodialysis using at least one anion-permselective membrane and one cationpermselective membrane. After the electrodialysis cell E, a caustic alkali stream Pr which is low in contaminants, and a separate stream V, which is low in alkali and contains the contaminants, are obtained.

By passing the stream Pv several times through the electrolysis cell E or if a relatively large number of units of series-connected electrolysis cells act on the caustic alkali stream Pv it is possible to obtain a purified caustic alkali Pr which essential has the alkali content of the caustic alkali Pv.

TR Figures 2 and 3 show examples of possible r arrangements of electrodialysis units according to the tJ invention, where A indicates an anion-permselective

I

5 membrane and C a cation-permselective membrane. In addition, arrows indicate illustratively and schematically the cell parts into which the contaminated caustic alkali P, can be introduced and 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. The cell units can also be used in a parallel arrangement or in a combination of a series and a parallel arrangement.

Figure 4 shows a greatly simplified view of the caustic alkali circuit in the Bayer process and the use of electrodialysis El 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 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 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.

9 S I t f

Claims

1. A process for decreasing dissolved inorganic and organic contamination in aqueous contamination containing caustic alkali used in the Bayer process for the preparation of aluminium hydroxide, wherein the contamination containing caustic alkali is subjected to electrodialysis using at least one unit comprising an anion-permselective membrane and a cation-permselective membrane with formation of a caustic alkali component for addition to caustic alkali circulated in the Bayer process, and a contaminant component, which is removed from the process and discarded.

2. The process according to claim 1, wherein the electrodialysis is carried out using units comprising cation- and anion-permselective membranes and arranged in series and/or in parallel.

3. The process according to claim 1 or 2, wherein the membranes used are monopolar membranes which are substantially selective for monovalent ions. 20

4. The process according to claim 3, wherein the membranes are selective between 65 and 95% for monovalent ions.

The process according to claim 3, wherein the membranes are selective between 70 and 90% for monovalent ions.

6. The process according to any one of claims 1 to 5, wherein at least part of the contamination-containing caustic alkali is the caustic alkali circulated in the Bayer process and is produced after washing a seed substance for the Bayer process.

7. The process according to any one of claims 1 to 6, wherein at least part of the contamination-containing caustic alkali is the caustic alkali circulated in the Bayer process and is produced after washing red mud produced by the Bayer process.

8. The process according to any one of claims 1 to 7, wherein solid components present in the contaminant component are removed.

9. The process according to claim 8, wherein said solid SO S S* *S 0 5055 50 00 a 05 S S 0* 0* S 5* S. components

10. The aluminium

11. The wherein t recycled

12. The wherein 1 contaminal 10 carbon di out durin process.

13. The dioxide p for treati

14. The herein d drawings. DATED: 2 PHILLIPS Attorneys SWISS ALU 9* a 0 #060 a t a -7 0654Z i I components are aluminium containing components. The process according to claim 9, wherein said aluminium containing components comprise aluminium hydrate. 11. The process according to claim 9 or claim wherein the aluminium-containing components removed are recycled into the Bayer process. 12. The process according to any one of claims 1 to 11, wherein before being subjected to electrodialysis, the contamination-containing caustic alkali is treated with carbon dioxide, and aluminium hydrate which precipitates out during this operation is removed and fed to the Bayer process. 13. The process according to claim 12, wherein carbon dioxide produced when carrying out electrodialysis is used for treatment of the caustic alkali. 14. The process according to claim 1, substantially as herein described with reference to the accompanying drawings. DATED: 2 APRIL, 1991 0S *r 0 0*0 0 *D 0 0*00 0* 00 0 0 0 00 0 *0 0 00 *0 S 0000 0 PHILLIPS ORMONDE FITZPATRICK Attorneys For: SWISS ALUMINIUM LTD. f,)aAAP. A,' 06E T 0/ 4Z =mom