Rogers, M.G.

Aluminium and alumina operations, aluminium smelters, fabrication and anodising plants produce corrosive wastewater streams that contain dissolved aluminium salts and suspended solids. Neutralisation prior to disposal creates a gelatinous precipitate of contaminated aluminium hydroxide sludge. Separation, and disposal of this sludge must satisfy environmental protection requirements.

Many of these operations are located close to, or in heavily populated regions where aluminium is used as a building and construction material and in fabrication plants with anodising facilities for corrosion protection. This particularly applies to Europe, USA and Japan where the high cost of disposal of large quantities of hazardous sludge is a major environmental problem.

A process has been developed for converting the hydroxide sludge solids into large dense crystalline particles that can be thickened, filtered and recycled for disposal or re-treatment.

The technique involves chemical treatment and flocculation of the effluent in the presence of previously precipitated solids. Under specific conditions the changes brought about by the chemical reactions cause agglomeration, ageing and consolidation of the solids where the smaller particles disappear while the larger particles grow larger at an apparent higher density. These coarser particles settle faster, thicken to a higher underflow density and produce slurry with a higher filtration rate.

The procedure has application in the treatment of dilute liquor streams from alumina refineries, aluminium smelters, fabrication and anodising plants. The process, which partially converts aluminium hydroxide to gibbsite, can be applied to a wide range of metal hydroxides, including titanium hydroxide (rutile), iron hydroxides (goethite, hematite and magnetite), magnesium hydroxide (periclase) and mixed hydroxides.