GB2032958A - Regenerating Aluminium Anodising Bath Liquor - Google Patents

Abstract

A process for continuously regenerating the bath liquor of an aluminium anodising bath consists in (a) continuously withdrawing bath liquor from the bath, (b) contacting the bath liquor in a continuous liquid- liquid contactor with a solvent stream containing a solvent selective for aluminium, (c) withdrawing from the contactor aluminum-depleted bath liquor and recycling it to the anodising bath, (d) withdrawing separately from the contactor the solvent stream with captured aluminium ions, (e) regenerating the withdrawn solvent stream in a second contactor, and (f) recycling the regenerated solvent stream to the first contactor.

Description

SPECIFICATION Improvements Relating to the Anodising of Aluminium This invention relates to the anodising of aluminium and is concerned with a process for increasing the efficiency of anodising baths.

The anodising of aluminium is a widely used process by which a hard, non-corroding oxide film is desposited on aluminium by making the aluminium the anode of an electrolytic cell containing an acid which is commonly suphuric acid. A small amount of aluminium dissolves in the sulphuric acid and the concentration of aluminium ions in the bath gradually increases. As the aluminium concentration increases the current density required to effect anodization increases and the bath efficiency falls to an unacceptable level at which the bath liquor has to be discarded. Disposal of the spent bath liquor can be a serious environmental problem.

The present invention provides a means by which the bath liquor can be continuously regenerated and recycled for further use.

According to this invention, a process for regenerating bath liquor withdrawn from an aluminium anodising bath and containing acid and aluminium ions comprises passing the bath liquor through a liquid-liquid contactor; passing through the contactor a solvent stream which is substantially immiscible with the bath liquor and which includes a solvent selective for the aluminium ions, the liquor and the solvent stream passing through the contactor in separate phases but being repeatedly brought into contact with one another during their passage; and separately withdrawing from the contactor the solvent stream with dissolved aluminium ions and aluminium-depleted bath liquor. The two liquid phases are caused to flow counter-currently or co-currently through the contactor, which is advantageously of the general type described in U.K. patent specification No. 972035 and others.

Preferably the solvent stream withdrawn from the contactor is continuously regenerated by passing it through a further liquid-liquid contactor in which it is contacted with a stream ofthe same acid as is contained in the bath liquor, and withdrawing separately the regenerated solvent stream substantially freed of aluminium ions and a liquid stream of an aqueous solution of an aluminium salt. The regenerated solvent is recycled to the first contactor.

The invention also includes a process for anodising aluminium in an electrolytic bath wherein bath liquor is continuously withdrawn from the bath, regenerated as described above, and recycled to the bath.

The liquid used to regenerate the spent bath liquor may suitably be a solution of one or more alkyl phosphates in kerosene.

The aluminium sulphate recovered in the second stage of the regeneration process is a commercially useful product. It may be produced as a pure aqueous solution suitable for water treatment, the sizing of paper and other uses, or as a crystallised solid.

The invention will be more readily understood by way of example from the following description of the recovery of aluminium values from anodising bath liquor, reference being made to the accompanying drawing which is a flow diagram of the process.

In the drawing, an anodising bath is indicated schematically at 11. Anodising bath liquor, consisting essentially of sulphuric acid with dissolved aluminium, is continuously withdrawn from the bath on line 12 and fed into a contactor 13, which is as described in British patent specification No. 972035 but modified for reagent injection as described in British patent application 791 7803. Prior to entry into the contactor 1 3, the pH of the liquor is adjusted in vessel 14 to lie within the optimum range for aluminium extraction, i.e. between pH2 and pH6.

A stream of extractant, which is preferably a mixture of mono- and di-ethyl hexyl phosphoric acid (MDEHPA) in kerosene, is supplied to the contactor through line 1 5 and passes through the contactor. The aqueous process liquor and the organic extractant form separate phases in the contactor 13. The aluminium-depleted aqueous phase is discharged from the contactor through line 1 6 while the extractant phase with the captured aluminium ions leave the contactor through line 1 7.

The progressive capture of aluminium ions as the aqueous phase passes along the contactor 1 3 results in the progressive increase in acidity due to the exchange of three hydrogen ions for each aluminium ion extracted. Both the kinetics of extraction and the selectivity of extraction from other metals are detrimentally affected unless the pH of the aqueous phase is maintained within a relatively narrow range (pH2-pH6r. To that end, alkali is supplied through line 1 8 to selected points along the axial length of the contactor 13 in the manner described in British patent specification No. 7917803, the rates of injection being chosen to keep the pH within the desired range.

The extractant phase with captured aluminium values is fed on line 1 7 to a stripping stage, in which the aluminium ions are stripped from the extractant. The stripping stage consists of a second contactor 20 which is similar in construction to the contactor 13 and to which the extractant is fed. A counter-current stream of sulphuric acid is supplied on line 21 as a lower stripping phase and reacts with the extractant phase, to regenerate the extractant and to produce aluminium sulphate which is discharged through line 22 and which is a valuable commodity. The regenerated extractant phase is recycled to line 1 5 with make-up extractant on line 23 as required.The major part of the sulphuric acid on line 21 is fed to one end of the contactor 20, but minor quantities are fed on line 23 and are injected at spaced points along the contactor in order to control the pH within the contactor and to improve the operation of stripping the aluminium from the organic phase.

The aluminium-depleted aqueous phase is discharged from contactor 13 through the line 1 6 and is continuously recycled to the anodising vessel 11.

While the stripping stage is shown and described as being effected in a rotary contactor, it may alternatively take place in a mixer-settler or other equipment having a like function.

1. A process for regenerating bath liquor withdrawn from an aluminium anodising bath and containing acid and aluminium ions, the process comprising passing the bath liquor through a liquid-liquid contactor; passing through the contactor a solvent stream which is substantially immiscible with the bath liquor and which includes a solvent selective for the aluminium ions, the liquor and the solvent stream passing through the contactor in separate phases but being repeatedly brought into contact with one another during their passage; and separately withdrawing from the contactor the solvent stream with dissolved aluminium ions and aluminium-depleted bath liquor.

2. A process for regenerating spent anodising bath liquor according to claim 1, in which the solvent of the solvent stream is a solution of one or more alkyl phosphates.

3. A process for regenerating spent anodising bath liquor according to claim 2, in which the solvent stream is a solution of the solvent in kerosene.

4. A process for regenerating spent anodising bath liquor according to any one of the preceding claims, in which the acid of the bath is sulphuric acid.

5. A process for regenerating spent anodising bath liquor according to any one of the preceding claims, in which the bath liquor and the solvent stream pass through the contactor in a countercurrent or co-current manner.

6. A process for regenerating spent anodising bath liquor according to any one of the preceding claims, in which the solvent stream withdrawn from the contactor with aluminium ions is continuously regenerated by passing it through a further liquid-liquid contactor in which it is contacted with a stream of the same acid as is contained in the bath liquor, and withdrawing separately the regenerated solvent stream substantially freed of aluminium ions and a liquid stream of an aqueous solution of an aluminium salt.

7. A process for regenerating spent anodising bath liquor according to claim 6, in which the regenerated solvent stream is recycled to the first contacted.

8. A process for the regeneration of anodising bath liquor, substantially as described with reference to the accompanying drawing.

9. A process for anodising aluminium in an electrolytic bath wherein bath liquor is continuously withdrawn from the bath, regenerated according to the process according to any one of the preceding claims, and recycled to the bath.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. contactor and to improve the operation of stripping the aluminium from the organic phase. The aluminium-depleted aqueous phase is discharged from contactor 13 through the line 1 6 and is continuously recycled to the anodising vessel 11. While the stripping stage is shown and described as being effected in a rotary contactor, it may alternatively take place in a mixer-settler or other equipment having a like function.

1. A process for regenerating bath liquor withdrawn from an aluminium anodising bath and containing acid and aluminium ions, the process comprising passing the bath liquor through a liquid-liquid contactor; passing through the contactor a solvent stream which is substantially immiscible with the bath liquor and which includes a solvent selective for the aluminium ions, the liquor and the solvent stream passing through the contactor in separate phases but being repeatedly brought into contact with one another during their passage; and separately withdrawing from the contactor the solvent stream with dissolved aluminium ions and aluminium-depleted bath liquor.

2. A process for regenerating spent anodising bath liquor according to claim 1, in which the solvent of the solvent stream is a solution of one or more alkyl phosphates.

3. A process for regenerating spent anodising bath liquor according to claim 2, in which the solvent stream is a solution of the solvent in kerosene.

4. A process for regenerating spent anodising bath liquor according to any one of the preceding claims, in which the acid of the bath is sulphuric acid.

5. A process for regenerating spent anodising bath liquor according to any one of the preceding claims, in which the bath liquor and the solvent stream pass through the contactor in a countercurrent or co-current manner.

6. A process for regenerating spent anodising bath liquor according to any one of the preceding claims, in which the solvent stream withdrawn from the contactor with aluminium ions is continuously regenerated by passing it through a further liquid-liquid contactor in which it is contacted with a stream of the same acid as is contained in the bath liquor, and withdrawing separately the regenerated solvent stream substantially freed of aluminium ions and a liquid stream of an aqueous solution of an aluminium salt.

7. A process for regenerating spent anodising bath liquor according to claim 6, in which the regenerated solvent stream is recycled to the first contacted.

8. A process for the regeneration of anodising bath liquor, substantially as described with reference to the accompanying drawing.

9. A process for anodising aluminium in an electrolytic bath wherein bath liquor is continuously withdrawn from the bath, regenerated according to the process according to any one of the preceding claims, and recycled to the bath.