GB1591695A - Process for the removal of metal ions from aqueous solution - Google Patents

Description

(54) PROCESS FOR THE REMOVAL OF METAL IONS FROM AQUEOUS SOLUTION (71) We INDALEX LIMITED a British company of 19 St. James's Square, London SW1Y 4LD do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the removal of metal ions from aqueous solutions such as the acidic or alkaline effluent liquors produced in metal finishing operations.

There is a considerable problem in removing metal ions from aqueous solution in effluent treatment systems particularly from anodizing and metal plating effluents. Acidic effluents may be neutralised with such materials as lime or sodium hydroxide and alkaline effluents, such as anodizing effluents, may be neutralised with mineral acids to produce a suspension of metal hydroxide. The fine and often gelatinous nature of the thus-produced suspensions makes them difficult to treat by means of the more conventional filtration/settling techniques. We have now discovered that an effective metal ion separation procedure for such effluents can be achieved by means of the procedure according to this invention.

This invention consists in a process for removing metal ions from aqueous effluent solutions, comprising; (a) neutralising an alkaline aqueous solution containing metal ions to a pH value at which the metal ions are substantially completely precipitated as oxides or hydroxides, to produce a suspension of solids in an aqueous liquid.

(b) treating this suspension with small rising gas bubbles so as to float a froth of solid particles with associated gas bubbles to the liquid surface, and (c) centrifuging this froth to separate solid particles from gas and liquid to give a solid product and a subtantially-clear liquid.

Preferably the suspension is subjected to a settling step, after step (a).

Preferably the suspension is produced by the neutralisation of an aqueous alkaline solution containing metal anions by means of a mineral acid, such as sulphuric acid.

Where aluminium salts in an alkaline medium are involved, as in treating alkaline anodizing wastes, the pH to which neutralisation is effected should preferably be within the range of 6.5 to 10.

Preferably the gas bubbles are produced by dissolving gas (preferably air) under pressure in a liquid which is added to the suspension and then releasing pressure to allow gas bubbles to escape, thus producing a froth or float containing solids, some liquid and the air. A polyelectrolyte may be added to assist this flotation process.

Preferably the gassing is carried out in a flotation cell having a skimmer for removing froth from the liquid surface.

Preferably a polyelectrolyte is introduced into the centrifuge to reduce the flocdestructive effect of the shear produced by acceleration of the feed material.

Preferably the separation process according to this invention is associated with a subsequent liquid disposal or re-circulation step. The solid product may be valuable or may be disposed of e.g. by dumping.

The invention will be further described by reference to the accompanying single figure of diagrammatic drawing which represents, purely schematically, a process flow sheet for treating alkaline effluent leaving an anodising plant, to remove metal ions therefrom.

Anodising waste consisting of aluminium salts in an alkaline aqueous medium (pH 10-14) is neutralised with waste sulphuric acid from the anodising process, to pH 6.5 to 7.5 and the heavier solid particles are settled out in sedimentation tank 1. The residual suspension of solids (largely alumi nium hydroxide) in water is passed to the centre of a circular air-flotation cell 2 and mixed with water containing dissolved air, which is then allowed to escape thus producing rising gas bubbles which carry a froth of solid particles with associated gas/liquid to the surface of the liquid in the cell. This froth is removed from the surface by overflow or skimming and is passed into centrifuge drum 3 spinning at about 3000 r.p.m.

The drum 3 is operated in the horizontal mode as shown, so that solids are thrown torwards one end and discharged while relatively clear liquid is discharged from the other end. The produced solids are wet (about 20% by weight dry solids) aluminium hydroxide. Polyelectrolyte is added to the froth entering the centrifuge and this serves to reduce the tendency of the centrifugal force to break the solid flocs within the centrifuge. The substantially clear liquids leaving the flotation cell and centrifuge may be combined and recycled or discharged to drain.

WHAT WE CLAIM IS: 1. A process for removing metal ions from aqueous effluent solutions comprising; (a) neutralising an alkaline aqueous solution containing metal ions to a pH value at which the metal ions are substantially completely precipitated as oxides or hydroxides, to produce a suspension of solids in an aqueous liquid.

(b) treating this suspension with small rising gas bubbles so as to float a froth of solid particles with associated gas bubbles to the liquid surface, and (c) centrifuging this froth to separate solid particles from gas and liquid to give a solid product and a substantially-clear liquid.

2. A process as claimed in claim 1 wherein the suspension is subjected to a settling step after step (a) and before step (b).

3. A process as claimed in claims 1 or 2 wherein the suspension is produced by the neutralisation of an aqueous alkaline solution containing metal anions by means of a mineral acid.

4. A process as claimed in claim 3 wherein the metal anions include aluminates and the neutralisation is effected to a pH value between 6.5 and 10 to produce a suspension of aluminium hydroxide particles.

5. A process as claimed in claim 1, wherein the gas bubbles are produced by dissolving gas under pressure in a liquid which is added to the suspension.

6. A process as claimed in claim 5 wherein the froth is produced in a flotation cell having a skimmer for removing froth from the liquid surface.

7. A process as claimed in claim 1 wherein a polyelectrolyte is introduced into the froth passed to the centrifuge to reduce the floc-destructive effect of the shear produced therein.

8. A process as claimed in claim 1 wherein the clear liquid is re-circulated to an anodizing process.

9. A process for separating finelydivided solids from suspensions substantially as hereinbefore described with reference to the accompanying drawing.

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. nium hydroxide) in water is passed to the centre of a circular air-flotation cell 2 and mixed with water containing dissolved air, which is then allowed to escape thus producing rising gas bubbles which carry a froth of solid particles with associated gas/liquid to the surface of the liquid in the cell. This froth is removed from the surface by overflow or skimming and is passed into centrifuge drum 3 spinning at about 3000 r.p.m. The drum 3 is operated in the horizontal mode as shown, so that solids are thrown torwards one end and discharged while relatively clear liquid is discharged from the other end. The produced solids are wet (about 20% by weight dry solids) aluminium hydroxide. Polyelectrolyte is added to the froth entering the centrifuge and this serves to reduce the tendency of the centrifugal force to break the solid flocs within the centrifuge. The substantially clear liquids leaving the flotation cell and centrifuge may be combined and recycled or discharged to drain. WHAT WE CLAIM IS:

1. A process for removing metal ions from aqueous effluent solutions comprising; (a) neutralising an alkaline aqueous solution containing metal ions to a pH value at which the metal ions are substantially completely precipitated as oxides or hydroxides, to produce a suspension of solids in an aqueous liquid.

(b) treating this suspension with small rising gas bubbles so as to float a froth of solid particles with associated gas bubbles to the liquid surface, and (c) centrifuging this froth to separate solid particles from gas and liquid to give a solid product and a substantially-clear liquid.

2. A process as claimed in claim 1 wherein the suspension is subjected to a settling step after step (a) and before step (b).

3. A process as claimed in claims 1 or 2 wherein the suspension is produced by the neutralisation of an aqueous alkaline solution containing metal anions by means of a mineral acid.

4. A process as claimed in claim 3 wherein the metal anions include aluminates and the neutralisation is effected to a pH value between 6.5 and 10 to produce a suspension of aluminium hydroxide particles.

5. A process as claimed in claim 1, wherein the gas bubbles are produced by dissolving gas under pressure in a liquid which is added to the suspension.

6. A process as claimed in claim 5 wherein the froth is produced in a flotation cell having a skimmer for removing froth from the liquid surface.

7. A process as claimed in claim 1 wherein a polyelectrolyte is introduced into the froth passed to the centrifuge to reduce the floc-destructive effect of the shear produced therein.

8. A process as claimed in claim 1 wherein the clear liquid is re-circulated to an anodizing process.

9. A process for separating finelydivided solids from suspensions substantially as hereinbefore described with reference to the accompanying drawing.