The effect of clay on gypsum filterability in the nitrophosphate process

The influence of clay added to Khouribga phosphate rock during the dissolution with nitric acid in the nitrophosphate process, upon the filterability of gypsum (calcium sulphate dihydrate) was studied. The precipitation of the acidulate with an aqueous ammonium sulphate containing solution was investigated in a continuously operated bench scale reactor system. It was found that the filtration time of a gypsum-crystals-containing filter cake with a height of 50 mm could be reduced by up to a factor of 5 by adding approximately 0.5 percent by weight clay in relation to the amount of phosphate ore. Based on our experiments we can calculate that the addition of clay in the range 0.4–3 percent by weight to the phosphate ore can increase the production capacity of a rotary tilting-pan filter system for separation of the gypsum from the mother liquor by more than a factor of two. Moreover because in this range of clay concentrations the capacity becomes almost constant we may conclude that for this system the production capacity is relatively insensitive to fluctuations in the clay dosage system.