Menetelmä sulfaattimenetelmällä tehtävässä titaanipitoisten raaka-aineiden liuottamisessa muodostuvan jätehapon käsittelemiseksi käyttämällä kalsinointijätekaasujen lämpöä

The invention relates to a process for working, in a manner which saves energy and material, the waste acid which accumulates during the production of titanium dioxide and which contains 20-24% by weight H2SO4 and dissolved metal sulphates. This waste acid is brought, in a circuit, into direct contact with hot, unpurified calcination waste gases and preconcentrated to a concentration of 26-29% by weight H2SO4, and the concentration of this acid is maintained constant in the circuit by supplying additional waste acid and unpurified calcination waste gas. A residue which essentially contains titanium dioxide and calcium sulphate and also a relatively small quantity of silicon oxide, precipitates out in connection with this. After, where appropriate, having added anhydride seeds to the preconcentrated acid, the residue is separated, preferably in a membrane filter press, freed from most of the calcium sulphate by washing with water and returned to the process for titanium dioxide production. When using relatively iron-rich waste acids, for example waste acids which are derived from ilmenite, a mixture of water-containing metal sulphate, which is termed "green salt" and which predominantly contains iron (II) sulphate heptahydrate, is separated from the preconcentrated acid, which has been separated from the residue, by means of cooling crystallisation. This step is omitted with iron-poor acids. After that, the acid is concentrated up to a content of 60-70% by weight H2SO4, in association with which metal sulphates ("filter salts") precipitated out, with these metal sulphates likewise being removed. The acid is returned to the raw material digestion, and the filter salt can be cleaved thermally, with the cleavage gas being worked up into a high-percentage sulphuric acid. Alternatively, the filter salt can be neutralised. During the preconcentration and concentration steps, at least 80% by weight of the calcium sulphate which is originally present in the waste acid can be precipitated out and removed.