GB801403A - Improvements in or relating to the production of iron oxide and sulfur - Google Patents

Abstract

Iron oxides and elemental sulphur are produced by treating an aqueous acidic feed liquor containing an iron salt with a gas containing free oxygen at elevated temperature and super-atmospheric pressure in the presence of copper sulphide, separating the resultant solids and separately recovering the iron oxides and sulphur therefrom, treating the remaining liquor with a sulphide to produce copper sulphide and recycling at least a portion of said sulphide to the oxidizing step. The iron oxide product is mainly present as ferric oxide and hydroxide and may contain some basic ferric sulphate. The feed liquor normally contains sulphuric acid and may be a pickle liquor or may have been produced by leaching, normally at substantially atmospheric pressure and 25-95 DEG C., a material comprising iron sulphide with an aqueous liquor containing sulphuric acid such as a pickle liquor; the iron sulphide material is obtained by roasting such materials as a mineral containing also copper, lead, zinc, cobalt, nickel, cadmium, aluminium, magnesium, silver, gold or the platinium metals, pyrites, chalcopyrites, pyrrhotite or a concentrate, matte, speiss or industrial by-product or intermediate. In the examples, the above specified ores are roasted at temperatures between 500 DEG and 800 DEG C. in a non-oxidizing atmosphere (to avoid loss of sulphur as sulphur dioxide) preferably not containing excessive amounts of hydrogen, whereby sulphur in excess of that forming FeS is obtained. This sulphur need not be separated from any hydrogen sulphide, if present, but used therewith as indicated below and recovered with the iron oxide. The leaching of the calcine may precede or be conducted simultaneously with the oxidation in the presence of copper sulphide, and when such leaching is employed the feed liquor may consist of or contain an acidic solution containing copper sulphate recycled from subsequent steps. The removal of hydrogen sulphide from the leaching stage may be assisted by steam stripping. When using minerals containing copper, the copper sulphide which remains after leaching is usually removed, but it may be used as part of the added copper sulphide. During the oxidation stage, temperatures should not exceed 130 DEG C. to avoid the solid mixture comprising sulphur becoming sticky, but it is desirable to raise the temperature above the melting point of sulphur for a short period thereafter. After oxidation, the liquor is separated and sufficient copper sulphide for use in a second oxidation step precipitated by means of hydrogen sulphide (e.g. from the leaching and/or roasting stage and possibly containing sulphur as indicated above) or of iron sulphide preferably with hydrogen, the preferred temperature and pressure being 80-100 DEG C. and 10-30 p.s.i.g. The copper sulphide is separated from the remaining liquor which may then be partly or wholly recycled to the leaching step or withdrawn for concentration and recovery of the acid; during concentration ferrous sulphate if present may crystallize and be removed. If some copper is still present in the liquor from which the desired amount of copper sulphide was precipitated, this also may be recovered, e.g. by gas reduction or electrolysis. Gold and other previous metals and lead if present in the starting material remains in the insolubles after leaching. From this mixture sulphur may be removed, e.g. by froth flotation; gold may be recovered by cyanidation. Less acid than will completely dissolve the iron is preferably used in the leaching step; such metals as zinc, cadmium, nickel and cobalt than remain in the residue. When non-ferrous metals are present, the residue from said leaching step may be treated with recycled sulphuric acid in a secondary leaching step under similar conditions, i.e. about 25-100 DEG C. and atmospheric pressure. The hydrogen sulphide liberated is subsequently used to precipitate non-ferrous metal sulphides. The solution comprises zinc and iron sulphates, the former of which may be precipitated as sulphide with hydrogen-roasted barren pyrite calcine and removed whilst the remaining liquor containing iron is returned to the oxidation stage. The residual solids are treated with air, oxygen or oxygen enriched air at 100-350 DEG C. and 400-1000 p.s.i.g. and the residue and liquor again separated. The liquor is treated with hydrogen sulphide to recover copper therefrom, preferably at 25-100 DEG C. and the sulphuric acid so formed is returned to the secondary leaching stage. Lead and precious metals and possibly also sulphur and gypsum are conventionally recovered from the residue. If cobalt or nickel is present, it will remain in the liquor from which copper has been precipitated; as the liquor is recycled the concentration of cobalt or nickel rises and the liquor may be bled off and treated conventionally or as described for zinc. If aluminium or magnesium salts build up in a recycling stream, they may also be recovered after bleeding off a portion and replacing with fresh acid.