GB749097A - Improvements in or relating to a method for treating alkali metal chromates - Google Patents

Abstract

Alkali metal chromates are reduced with hydrogen in a manner similar to that described in the parent Specification by passing a stream of hydrogen in contact with a dry granular mass of alkali metal chromate particles which are initially maintained at a temperature of 250 DEG C. to below 300 DEG C. and thereafter maintained at a temperature above 300 DEG C. and below the fusion point of the reaction mass up to a maximum of 600 DEG C. and below the continued until the reaction of the chromate and hydrogen is complete as determined by the amount of hexavalent chromium remaining in the reaction mass, fusion of the mass being avoided at all times. The reaction product may be leached with water at about 100 DEG C. thereby dissolving undesirable impurities and the dried solid may be employed in metallurgical exotherm alloying mixes. Alternatively the so obtained dried leached reaction product may be calcined at a temperature of 600 DEG C. to 1000 DEG C. in an oxidizing atmosphere, preferably air, whereby the chromium complex formed by the hydrogen reduction is broken down to a mixture of chromic oxide and alkali metal chromate, the latter being removed by a hot water leaching leaving a solid residue of pure chromic oxide. The starting material may be pure crystalline anhydrous sodium chromate for example, or impure sodium chromate containing sodium aluminate, hydroxide, carbonate, sulphate and silicate all of which with the exception the hydroxide are removed by water leaching of the reduced reaction product. The sodium hydroxide concentration however is preferably reduced to below 15 per cent by weight of the starting material such as by treating the crude material with carbon dioxide or by extracting the sodium hydroxide with a low molecular weight aliphatic alcohol. Alternatively if the sodium hydroxide is not removed diluents such as the leached reaction product or chromic oxide may be added to the reaction mass to prevent fusion. The hydrogen employed is preferably pure but diluents such as nitrogen and carbon dioxide may be included, the latter serving to form sodium carbonate with the sodium hydroxide which may be present. The reaction may be effected in a tubular furnace and preferably with agitation. According to an example crude sodium chromate containing about 17 per cent hydroxide is extracted with absolute methyl alcohol to decrease the hydroxide concentration and the filter cake fed to a tubular reactor and heated to 240 DEG C. in the presence of hydrogen gas. The temperature is raised to about 260 DEG C. when reaction commences and reaction is continued at this temperature until about 10 per cent of the sodium chromate has been reacted; thereafter the temperature is allowed to rise to 310 DEG C. and maintained at this temperature until reaction is completed as indicated by the cessation of water vapour in the effluent gases from the reactor. The solid reaction product is leached with hot water, filtered and dried and thereafter calcined at a 1000 DEG C. for 1 hour and leached to remove the sodium chromate reformed. Another example refers to the counter-current treatment of the chromate with hydrogen in a tubular reactor fitted with an agitator wherein the reactor temperature at the chromate feed end is 250 DEG C. to 270 DEG C. and rises to 375 DEG C. at the hydrogen feed end.