Abstract:
Difficulties in defluorination and deep purification of wet process phosphoric acid bottleneck its use in the production of toothpaste grade calcium hydrogen phosphate. A two step process for purification of wet process phosphoric acid is proposed which utilizes pre defluorination by precipitation and deep defluorination by neutralization with slaked lime. The effect of defluorination is examined with the quantity of potassium chloride and the concentration of phosphoric acid. Moreover, active silicon oxide is of assistance to defluorination. The ratio of phosphorus pentoxide content to fluorine content in so treated that phosphoric acid can be more than 100. In a further purification step by neutralization with slaked lime, most of the fluorine and other impurities in the phosphoric acid deposit are separated at a definite pH value. The filtrated phosphoric acid, in which the ratio of phosphorus pentoxide content to fluorine content is more than 3 000, can therefore be employed to produce toothpaste grade calcium hydrogen phosphate. In the proposed process, about 30 percent of phosphorus pentoxide in the raw wet process phosphoric acid will come into the production of toothpaste grade calcium hydrogen phosphate.