The invention relates to a procedure for the treatment of filter cakes obtained during the purification of chemical substances in liquid form, especially during the purification of phosphoric acid obtained by wet methods, such as for the preparation of tripoly-phosphates.
In is known per se that phosphoric acid can be obtained by the decomposition of phosphate ores with sulphuric acid, whereby on the one hand phosphoric acid is obtained, and secondly gypsum (CaSO.sub.4.2H.sub.2 O). The phosphoric acid from the decomposed material, which is obtained after removal of the majority of the gypsum, still contains a large number of impurities such as titanium, zinc, cadmium, manganese, chromium, vanadium, lead, uranium, fluorine, copper, arsenic and rare earth metals. However, these impurities do not cause any interference during the processing of the phosphoric acid to give phosphate fertilizers.
For certain purposes, for example for the preparation of tripolyphosphates or other phosphate compounds, it is necessary to use a much more highly purified phosphoric acid, which is achieved by the purification of phosphoric acid obtained by wet methods.
During this purification operation different filter cakes are formed which contain significant quantities of impurities in the form of vanadium, mercury, uranium, fluorine and the like. Because of the presence of heavy metals and fluorine, these filter cakes must not be discharged to open water courses so as to prevent environmental pollution whilst dumping on open ground is also forbidden because of the possibility of leaching during storage in the atmosphere caused by rainfall and the like.
Of the various filter cakes which are obtained during the further purification of phosphoric acid the following can be mentioned:
(a) an initial filter cake which consists of 80-85% gypsum, plus small quantities of various metals such as iron, aluminium, magnesium, chromium, vanadium, copper, also a small quantity of carbon; PA0 (b) a second filter cake which is deposited after removal of the first filter cake which consists mainly of gypsum, the said second filter cake containing sodium phosphate, sodium aluminium silicate, iron hydroxide, magnesium silicate, calcium fluoride, chromium trioxide, zinc oxide, plus for example titanium, cadmium, manganese, vanadium, mercury, lead, rare earth metals, uranium and carbon; PA0 (c) a third filter cake consisting of disodium phosphate, ferric hydroxide, magnesium fluoride with for example significant quantities of vanadium pentoxide plus for example titanium, cadmium, manganese, chromium, mercury, lead, lanthanum, uranium and carbon, all dependent on the raw material from which the phosphoric acid has been prepared.
The latter-mentioned cake, which consists mainly of disodium phosphate, ferric hydroxide, magnesium fluoride and vanadium oxide is obtained as filter cake during the final purification of the phosphoric acid.
The filter cakes which are formed during the purification of phosphoric acid obtained by wet methods are extremely large, so that attempts are being made to find a solution which obviates the problems mentioned above.