Manufacture of iron blue pigments

Claim 1

IN A PROCESS FOR PRODUCING AN IRON BLUE PIGMENT OF IMPROVED TEXTURE AND TINCTORIAL QUALITIES WHEREIN A SUBSTANTIALLY WHITE IRON BLUE PIGMENT INTERMEDIATE IS PRECIPITATED BY REACTING AN ACIDIFIED AQUEOUS SOLUTION OF FERROUS SULFATE AND AMMONIUM SULFATE WITH AN AQUEOUS SOLUTION OF SODIUM FERROCYANIDE AND THE RESULTING PRECIPITATE IS DIGESTED AND ACIDIFIED, THE IMPROVEMENT WHICH COMPRISES EFFECTING SAID PRECIPITATION, DIGESTON, AND ACIDIFICATION IN A CLOSED REACTION VESSEL WHILE THE REACTION MASS IS SUBJECTED TO VIGOROUS MECHANICAL AGITATION, REQUIRING AT LEAST ABOUT 1.0 H.P. PER 1000 GALLONS OF SUSPENSION, AND TO SUCH OXIDIZING CONDITIONS THAT NOT MORE THAN ABOUT 6% OF SAID PRECIPITATE BECOMES CONVERTED TO THE BLUE, OXIDIZED STATE, AND THEREAFTER CONVERTING THE RESULTING, PARTIALLY OXIDIZED INTERMEDIATE WHILE IN AQUEOUS SUSPENSION TO THE FINAL BLUE PIGMENT.