Potassium Phosphates

Basic or neutral phosphates (Rhenania phosphates) may be made directly from rock phosphate by mixing it with potassium chloride, some form of carbon and soda, and heating to over 1000° C. in an electric furnace. One of these products has the composition Ca₂KNaP₂O₈, with some silica. Such products may contain 23 to 31 per cent, of soluble phosphoric acid. Potassium superphosphate has been made by mixing potassium sulphate and calcium carbonate with concentrated phosphoric acid in a lead-lined vessel. The CaSO₄.2H₂O is separated and the filter-pressed solution is evaporated to dryness by steam heat. The residue may be treated with more phosphoric acid and again evaporated. The reaction is expressed by the equation

K₂SO₄ + 2H₃PO₄ + CaCO₃ = 2KH₂PO₄ + CaSO₄ + CO₂ + H₂O