Patent ID: 7067100

Claim:
In a process for making aqueous ferric sulfate solution that involves: (a) combining in a closed reaction vessel a mixture of finely-divided ferric oxide, concentrated sulfuric acid and water; (b) heating said mixture to a reaction temperature of from about 130° C. to about 150° C. and, at a pressure of from about 30 PSI to about 40 PSI while agitating said mixture and while controlling the rate of temperature increase at about 5° C. per fifteen minutes; (c) maintaining said mixture at a digestion temperature ranging from about 130° C. to about 150° C. and a pressure ranging from about 30 PSI to about 40 PSI for a period ranging up to from about three to eight hours while continuing said agitation; (d) lowering the temperature and pressure of said mixture; and (e) collecting said aqueous ferric sulfate solution as a product of the process, the improvement comprising water scrubbing waste gas from the reaction vessel and recycling the scrubbing water to the digestion process of step (c).