Chromite Digestion to Alkali Chromates

Only alkaline oxidative digestion is suitable for the industrial conversion of chromite to chromium chemicals. This results in alkali (sodium) chromates  

Sodium carbonate can be in part be replaced by sodium hydroxide. The accompanying constituents are converted into sodium aluminate, sodium silicate, sodium vanadate and consume corresponding quantities of alkali. 

This digestion separates chromium from the associated elements iron, aluminum, magnesium and silicon in a simple process. Post-purification may be necessary to remove the vanadium. 

Prior to digestion, the ore must be finely ground. The chromium ore concentrate enriched by gravitational processes (e.g. spiral classifiers, settling machines etc.) is supplied with a particle size of ca. 0.5 mm. Ball-milling coupled with air classifiers yields a particle size of 0.1 mm. The sodium carbonate is also ground. Since the sodium chromate formed melts at 792°C, the addition of an inert diluent, which can absorb sodium chromate like a sponge, is necessary to enable air to come into contact with the reaction mixture. 

A particularly suitable diluent is the iron oxide residue left after digestion, but calcined pyrites can also be used. A typical digestion mixture consists of  

---