GB865306A - Metal recovery process - Google Patents

Abstract

Metal sulphides which are insoluble in dilute acids, especially germanium sulphides, are recovered from alkaline coal combustion residues in a process comprising washing the residues with water, treating the resulting solution with a water-soluble sulphide, and acidifying, preferably to below pH 5, to precipitate the metal sulphides. The initial wash water may contain some or all of the water soluble sulphide, and the final acidified solution may be heated to below boiling, and preferably above 180 DEG F., to hasten the sulphide precipitation. The water-soluble sulphide is preferably an alkali or alkaline earth metal polysulphide. The preferred acid for adjusting the pH is hydrochloric acid, but sulphuric, phosphoric, nitric, and glacial acetic acids may also be employed. In the case of germanium, the amount of sulphide employed is preferably in an amount sufficient to add sulphur in an amount from 0.02% to 0.2% by weight of the original coal residue. If the coal residue is normally non-alkaline, an artificial alkaline residue suitable for processing may be obtained by adding alkali, for example soda ash, lime, potash, or caustic soda, to the coal prior to burning. Other metals values which may be recovered include antimony, arsenic, copper, lead, iron, aluminium, gallium, titanium, uranium, gold, tin, platinum, and cadmium. The sulphide precipitated may be freed from free sulphur by leaching, for example with carbon disulphide, or by distillation or controlled oxidation. The sulphide precipitate may be roasted to give the oxide.