Patent GB-357580-A (GB)
Title: Method of leaching phosphate rock
IPC: C05B11/06
Filed: 1930-06-23
Abstract: In a process for leaching phosphate rock with nitric acid and potassium sulphate a solution containing phosphoric acid produced by a separate process is added together with the phosphate rock and the nitric acid or is added to the reaction mixture obtained by dissolving the phosphate rock in the nitric acid whereupon the potassium sulphate is added.  The phosphoric acid solution may be obtained by leaching phosphate rock with sulphuric acid, by leaching phosphate rock with sulphuric acid and ammonium sulphate, in which case a mixture of phosphoric acid and ammonium phosphate is obtained, or by electrothermal or thermal processes.  In a preferred embodiment phosphate rock is treated with sulphuric acid and washing liquid from a preceding operation; the reaction may be carried out at a low temperature and the dihydrate of calcium sulphate CaSO4,2H2O which is obtained converted into the semihydrate 2CaSO4,H2O by heating the reaction products or the reaction may be carried out at such a high temperature that the semihydrate is directly obtained.  The reaction products are then mixed with the reaction mixture obtained by dissolving phosphate rock in nitric acid and potassium sulphate which may be suspended in water or in washing liquid added; the amount of potassium sulphate being in deficit in relation to the lime content of the solution and the temperature being kept so high that calcium sulphate formed in the reaction between potassium sulphate and calcium nitrate crystallizes as semihydrate.  The reaction mixture is cooled and diluted with washing liquid and when the semihydrate has recrystallized to dihydrate a further quantity of potassium sulphate is added to precipitate the remainder of the lime.  Alternatively phosphate rock is treated with sulphuric acid under such conditions that the calcium sulphate produced crystallizes as semihydrate.  The reaction product is cooled and diluted with washing liquid and the semihydrate left to crystallize to dihydrate.  The mixture is added to the reaction mixture obtained by dissolving phosphate rock in nitric acid and the potassium sulphate is added, the temperature being kept sufficiently high to prevent the crystallization of the potassium nitrate but sufficiently low to allow the calcium sulphate to crystallize as dihydrate.  The calcium sulphate is separated and washed, the washing liquid being returned to the process.  The remaining solution of phosphoric acid and potassium nitrate may be neutralized with ammonia and evaporated to give a product which may be used as a fertilizer or a portion of this solution may be freed from part of its potassium nitrate by crystallization and added together with the phosphoric acid containing solution to the reaction mixture obtained by dissolving phosphate rock in nitric acid.  To ensure the production of gypsum of a suitable crystalline shape it is stated that an ammonium salt should be present during the formation of the dihydrate and if the solution does not contain an ammonium salt, a minor quantity of ammonium salt may be added e.g. ammonia may be added to the washing liquid added to promote the recrystallization of the semihydrate into dihydrate. Specification 338,079 [Class 111, Sewage &c.], is referred to.  The Specification as open to inspection under Sect. 91 (3) (a) states that a solution containing an ammonium salt, e.g. the phosphate, sulphate or nitrate or a solution containing phosphoric acid and an ammonium salt may be added to the reaction mixture obtained by dissolving the phosphate rock in nitric acid instead of the phosphoric acid containing solution and also that the potassium sulphate may be added in the solid state.  This subject-matter does not appear in the Specification as accepted.ALSO:In a process for leaching phosphate rock with nitric acid and potassium sulphate a solution containing phosphoric acid produced by a separate process is added together with the phosphate rock and the nitric acid or is added to the reaction mixture obtained by dissolving the phosphate rock in the nitric acid whereupon the potassium sulphate is added.  The phosphoric acid solution may be obtained by leaching phosphate rock with sulphuric acid, by leaching phosphate rock with sulphuric acid and ammonium sulphate, in which case a mixture of phosphoric acid and ammonium phosphate is obtained, or by electrothermal or thermal processes.  In a preferred embodiment phosphate rock is treated with sulphuric acid and washing liquid from a preceding operation; the reaction may be carried out at a low temperature and the dihydrate of calcium sulphate CaSO4.2H2O which is obtained converted into the semihydrate 2CaSO4.H2O by heating the reaction products or the reaction may be carried out at such a high temperature that the semihydrate is directly obtained.  The reaction products are then mixed with the reaction mixture obtained by dissolving phosphate rock in nitric acid and potassium sulphate which may be suspended in water or in washing liquid added; the amount of potassium sulphate being in deficit in relation to the lime content of the solution and the temperature being kept so high that calcium sulphate formed in the reaction between potassium sulphate and calcium nitrate crystallizes as semihydrate.  The reaction mixture is cooled and diluted with washing liquid and when the semihydrate has recrystallized to dihydrate a further quantity of potassium sulphate is added to precipitate the remainder of the lime.  Alternatively, phosphate rock is treated with sulphuric acid under such conditions that the calcium sulphate produced crystallizes as semihydrate.  The reaction product is cooled and diluted with washing liquid and the semihydrate left to crystallize to dihydrate.  The mixture is added to the reaction mixture obtained by dissolving phosphate rock in nitric acid and the potassium sulphate is added, the temperature being kept sufficiently high to prevent the crystallization of the potassium nitrate but sufficiently low to allow the calcium sulphate to crystallize as dihydrate.  The calcium sulphate is separated and washed, the washing liquid being returned to the process.  The remaining solution of phosphoric acid and potassium nitrate may be neutralized with ammonia and evaporated to give a product which may be used as a fertilizer or a portion of this solution may be freed from part of its potassium nitrate by crystallization and added together with the phosphoric acid containing solution to the reaction mixture obtained by dissolving phosphate rock in nitric acid.  To ensure the production of gypsum of a suitable crystalline shape it is stated that an ammonium salt should be present during the formation of the dihydrate and if the solution does not contain an ammonium salt, a minor quantity of ammonium salt may be added e.g. ammonia may be added to the washing liquid added to promote the recrystallization of the semihydrate into dihydrate. Specification 338,079, [Class 111, Sewage &c.], is referred to.  The Specification as open to inspection under Sect. 91 (3) (a) states that a solution containing an ammonium salt e.g. the phosphate, sulphate or nitrate or a solution containing phosphoric acid and an ammonium salt may be added to the reaction mixture obtained by dissolving the phosphate rock in nitric acid instead of the phosphoric acid containing solution and also that the potassium sulphate may be added in the solid state.  This subject-matter does not appear in the Specification as accepted.