PROCEDURE FOR THE ELABORATION OF A MATERIAL AND ENERGETIC RESERVE OF SOFT ACIDS ATTACKED BY THE ENERGETICS OF TITANIUM DIHYDROXIDE AFTER THE SULPHATE METHOD.

摘要

The invention relates to a process for energy- and material-saving treatment of the diluted acid which is obtained in the manufacture of titanium dioxide and contains 20 to 24 % by weight of H2SO4 and dissolved metal sulphates. This acid is directly contacted in circulation with hot, unpurified calcination off-gases and preconcentrated to a concentration of 26 to 29 % by weight of H2SO4 and the concentration of this acid is kept constant by adding further diluted acid and unpurified calcination off-gases into the circulation. A residue then precipitates which essentially contains titanium dioxide as well as calcium sulphate and a little silica. After previous addition of anhydrite seeds, as necessary, to the preconcentrated acid, the residue is separated off, preferably in a membrane filter press, freed of the major part of the calcium sulphate by washing with water and recycled into the titanium dioxide manufacturing process. In the case of relatively iron-rich diluted acids, for example those originating from ilmenite, a mixture of water-containing metal sulphates, which is called "green salt" and contains predominantly iron(II) sulphate heptahydrate, is precipitated by cooling crystallisation from the preconcentrated acid separated from the residue, and separated off. This step is unnecessary in the case of acids containing less iron. The acid is then concentrated up to 60 to 71 % by weight of H2SO4, metal sulphates ("filter salt") precipitating which are likewise separated off. The acid is recycled to the raw material digestion, and the filter salt can be thermally cracked, the cracking gases being processed to give high-percentage sulphuric acid. Alternatively, the filter salt can be neutralised. During the preconcentration and further concentration, at least 80 % by weight of the calcium sulphate originally present in the diluted acid can be precipitated and separated off.