The cake obtained by digestion is dissolved in cold water or in dilute acid recycled from the process. A low temperature must be maintained (<85 °C) to avoid premature hydrolysis, especially with the product from ilmenite. Air is blown in to agitate the mixture during dissolution. With the ilmenite product, the TiO2 concentration of the solution is 8-12 wt.%, and with the slag product 13-18 wt.%.
The trivalent iron is hydrolyzed together with the titanium compounds, and adheres to the titanium oxide hydrate. Therefore, all of the Fe3+ has to be reduced to Fe2+ by scrap iron during dissolution of the ilmenite product, or immediately afterwards. Reoxidation of the iron during subsequent processing is prevented with a small amount of Ti3+ being present until the end of the hydrolysis. Alternatively, reduction of Ti4+ to Ti3+ can be carried out in part of the solution under optimized conditions; this concentrated Ti3+ solution is then added in a controlled manner to the reaction solution [2.28]. In solutions obtained from titanium slag, the Ti3+ content of the solution must be decreased by oxidation with atmospheric oxygen so that no loss of yield occurs during hydrolysis.
With both ilmenite and titanium slag, mixed digestion can be carried out in which the Ti3+ content of the slag reduces all the Fe3+ to Fe2+. The dissolved products obtained from the separate digestion of ilmenite and titanium slag can also be mixed [2.29, 2.30].