| abstract |
The invention relates to a method for producing gypsum and an iron oxide pigment from the waste acid resulting from the production of titanium dioxide according to the sulfate process. The invention is characterized in that in an initial step the waste acid is partially neutralized using a calcium compound, resulting in the precipitation and possibly direct separation of gypsum. The remaining solution is neutralized further in a second step, with precipitation of a precipitate containing Ti, Al, Cr, V and possibly Fe(III). From the iron sulfate-containing solution obtained after separation of the solid, an iron oxide pigment is produced in a third step by addition of alkaline compounds and possibly ammonia and an oxidizing agent. By adding CaO and/or Ca(OH)2 ammonia is released from the (NH4)2SO4-containing solution obtained after separation of the iron oxide pigment. |