Production of sodium chromate from chromite ores

摘要

Sodium chromate is produced from noritic chromium ores i.e ores having a Cr2O3 content of 40 to 55% and a Cr to Fe ratio within the range 1,3 to 2,3, by forming a mixture of powdered ore (preferably of average particle size 20 to 75 microns), hydrated non-dolomitic or dolomitic lime, soda ash and water, in the proportions equivalent to 60 to 80 parts of hydrated non-dolomitic lime of 70% CaO content or 75 to 115 parts of hydrated dolomitic lime of 45% CaO content and 50 to 70 parts of water per 100 parts of ore and approximately the stoichiometric amount of soda ash required to react with the chromium content of the ore to produce sodium chromate. Small pellets produced from the mixture are heated to a temperature of 480 DEG to 600 DEG C., then roasted by passing them through an oxidizing atmosphere rising in temperature from 535 DEG -815 DEG C. to 1090 DEG -1205 DEG C. and then leached to remove the sodium chromate therefrom. Preferably the hydrated dolomitic lime if finely divided i.e, particles pass through an 80 mesh screen (U.S. standard) and preferably the constituents of the mixture to be subjected to pelleting have a particle size of 20 to 125 microns. Although it is preferable to dry the ore before pulverizing it and then mixing the pulverized ore with the other pelleting constituents, the ore may also be ground wet in the presence of quicklime whereby slaked lime is produced in situ. The pellets preferably have an average size not exceeding 1/2 inch in their longest dimension. Preferably the pellets are heated to a temperature of 480 DEG to 600 DEG C. by passing them through the exit gas stream from the kilns at a temperature of 535 DEG to 760 DEG C. In the Figure, a rotating pan pelleting machine 10 is continuously fed with dry mix from line 13 and water from line 14 and the pellets, 16, formed are continuously fed to a foraminous conveyer 18. Hot gases from the pellet-inlet end 25 of rotary kiln 26 flow through dust chamber 21 heating the pellets conveyed therethrough by conveyer 18. The heated pellets are discharged at 27 into the inclined rotating kiln 26 and tumbled therethrough countercurrent to the flow of hot oxygen-containing gases produced by burning a suitable fuel supplied by burner 28. Secondary air is introduced at 29 (by arrow 30). The hot pellets are discharged into leach tank 31, each having a perforated base, and weak sodium chromate solution is added to cover the pellets and percolates through them. Strong sodium chromate solution is withdrawn and the leached residue may be used as a diluent in the pelleting operation.