Patent GB-1094539-A (GB)
Title: A new or improved method of manufacturing phosphoric acid
IPC: C01B25/22
Filed: 1965-07-06
Abstract: In the process of preparing phosphoric acid by treating rock phosphate with sulphuric acid the mineral is treated in a first stage with an amount of acid insufficient to react completely with the whole phosphate content thereof at a temperature such that the calcium sulphate is formed as its semi-hydrate, then in a second stage sufficient sulphuric acid is added to the reaction mixture to complete the reaction and the temperature is maintained in this stage at a value such that the CaSO4 is produced in the continuing reaction in the form of its dihydrate and the semi-hydrate formed in the first stage is converted to the dihydrate, and in a final stage, the CaSO42H2O is separated from the phosphoric acid.  Preferably in the first stage the reaction mixture is maintained at a temperature between the semi-hydrate/dihydrate transition point and the b.p. of the mixture e.g. greater than 90 DEG  C., for a time sufficient to complete the reaction of that stage, e.g. 1 hour, and in the intermediate stage the temperature is maintained at a value below the transition point for a time sufficient to allow the reaction to be completed and the semi-hydrate converted to the dihydrate. In a preferred embodiment, the process is carried out as a continuous process, the residence times in the first and intermediate stages being determined by the relationship between the total through and the size of the vessel in which the reaction is being performed, and the second stage is performed in two steps, the temperature in the second of such steps being maintained at a lower value than that of the first step, and part of the reaction mixture being recycled from the second to the first of such steps, and part from the second step being sent forward to the separation stage of the process.  The temperature of the reaction mixture in the two steps of the second stage of the process is preferably maintained, e.g. at 80 DEG  and 70 DEG  C. respectively by removing a portion of the reaction mixture from the first step, cooling the mixture in a cooler to the temperature of the second step and returning part of the cooled mixture to the first step and part to the second step.  The preferred residence time of the first step of the second stage is between 2 and 4 hours and that of the second step is between 3 and 6 hours. The phosphate ion content of the first and second stages of the process of the invention may be controlled by returned part of the product from the separation stage to these stages.  Preferably an adjustable portion of this phosphoric acid is added directly to the second stage, the remainder being mixed with the H2SO4 added to the first and second stages of the process.  The separation stage preferably incorporates a series of washing stages, to remove H3PO4 from the CaSO4.2H2O p.pt. and the washing sequence is preferably arranged so that the washing liquid of the last washing stage is water and in the other washing stages the p.pt. is washed with the effluent of the next washing stage.  The final effluent from the washing stages is that phosphoric acid which may be returned to the first and second stages of the process.  The temperature of the first stage of the process may be controlled in part by returning to it a portion of the reaction mixture from the second stage and/or by a portion of the washing effluent from the separation stage.