GB748610A - Improvements in or relating to a method for treating alkali metal chromates - Google Patents

Abstract

Alkali metal chromates are partially reduced by passing a stream of hydrogen in contact with a dry granular mass of the chromate particles initially maintained at a temperature within the range of 300 to 350 DEG C. and thereafter maintaining the temperature above 300 DEG C. and below the fusion point of the reaction mass up to a maximum of 600 DEG C. and continuing the contact of hydrogen with the chromate until the reaction is substantially complete as determined by the amount of hexavalent chromium remaining in the reaction mass. The fusion point of the mass rises as the reaction proceeds and the temperatures are such that fusion is at all times avoided. The reduced product after washing to remove impurities may be employed after drying as a chromium containing material for use in metallurgical exothermic alloying mixes, or alternatively the reduced materials after washing may be calcined at above 600 DEG C. in an oxidizing atmosphere for example air, whereby the chromium complex resulting from the reduction treatment breaks down to a mixture of chromic oxide and alkali metal chromate. The latter may be removed by leaching with hot water leaving a solid residue of high purity chromic oxide. The raw material employed, preferably sodium chromate, may be either pure crystalline anhydrous salt or crude sodium chromate for example, crystallized from leach liquors obtained from the alkaline oxidation roasting of chromite ores, which crude product contains as impurities sodium aluminate, hydroxide, carbonate, sulphate and silicate. The aluminate, carbonate, sulphate and silicate impurities are removed by water leaching of the reduced reaction product but the sodium hydroxide content of the starting material is preferably reduced to below 15 per cent by weight either by treating the crude material with carbon dioxide or by extraction with a low molecular weight aliphatic alcohol. Alternatively if the sodium hydroxide is not removed diluents such as the leached reaction product or purified chromic oxide may be added to prevent fusion of the reaction mass. The hydrogen employed is preferably pure but carbon dioxide may be tolerated as a diluent since it reacts with any free sodium hydroxide present to form leachable sodium carbonate. The reduction reaction which is preferably effected at 300 to 400 DEG C., may be carried out in a tubular reactor preferably with agitation, the solid and gas preferably passing in counter current. Examples compare reduction with hydrogen at 315 DEG C. according to the invention, whereby 99.5 per cent of the initial chromate is reacted, with reduction with hydrogen at 650 DEG C. when fusion occurs and only 50 per cent of the initial chromate reacts. Further examples illustrate the effect of sodium hydroxide concentration in the initial material and compare the reduction with hydrogen according to the invention with reduction employing sawdust, sawdust and a hydrogen stream, and reduction with charcoal. Chromates of Li, Na, K, Rb and Cs may be so treated.