=137. Reactions with Fluorids.=—Since calcium fluorid is present in nearly all mineral phosphates, the reactions of this compound must be taken into consideration in a chemical study of the manufacture of acid phosphates. When treated with sulfuric acid the first reaction which takes place consists in the formation of hydrofluoric acid: CaF₂ + H₂SO₄ = 2HF + CaSO₄. Since, however, there is generally some silica in reach of the nascent acid, all, or a portion of it, combines at once with this silica, forming silicon tetrafluorid: 4HF + SiO₂ = 2H₂O + SiF₄. This compound, however, is decomposed at once in the presence of water, forming hydrofluosilicic acid: 3SiF₄ + 2H₂O = SiO₂ + 2H₂SiF₆. The presence of calcium fluorid in natural phosphates is extremely objectionable from a technical point of view, both on account of the increased consumption of oil of vitriol which it causes, but also by reason of the injurious nature of gaseous fluorin compounds produced. Each 100 pounds of calcium fluorid entails the consumption of 125.6 pounds of sulfuric acid. =138. Reaction with Carbonates.=—Most mineral phosphates contain calcium carbonate in varying quantities. This compound is decomposed on treatment with sulfuric acid according to the reaction: CaCO₃ + H₂SO₄ = CaSO₄ + H₂O + CO₂. When present in moderate amounts, calcium carbonate is not an objectionable impurity in natural phosphates intended for acid phosphate manufacture. The reaction with sulfuric acid which takes place produces a proper rise in temperature throughout the mass, while the escaping carbon dioxid permeates and lightens the whole mass, assisting thus in completing the chemical reaction by leaving the residual mass porous, and capable of being easily dried and pulverized. Where large quantities of carbonate in proportion to the phosphate are present the sulfuric acid used should be dilute enough to furnish the necessary water of crystallization to the gypsum formed. For each 100 parts, by weight, of calcium carbonate, eighty parts of sulfuric anhydrid are necessary, or 125 parts of acid of 1.710 specific gravity = 60° Beaumé. In some guanos a part of the calcium is found as pyrophosphate, and this is acted upon by the sulfuric acid in the following way: Ca₂P₂O₇ + H₂SO₄ = CaH₂P₂O₇ + CaSO₄. =139. Solution of the Iron and Alumina Compounds.=—Iron may occur in natural phosphates in many forms. It probably is most frequently met with as ferric or ferrous phosphate, seldom as ferric oxid, and often as pyrite, FeS₂. The iron also may sometimes exist as a silicate. The alumina is found chiefly in combination with phosphoric acid, and as silicate. Where a little less sulfuric acid is employed, as is generally the case, than is necessary for complete solution, the iron phosphate is attacked as represented below: 3FePO₄ + 3H₂SO₄ = FePO₄·2H₂PO₄ + Fe₂(SO₄)₃. When an excess of sulfuric acid is employed, the formula is reduced to the simple one: 2FePO₄ + 3H₂SO₄ = 2H₃PO₄ + Fe₂(SO₄)₃.