GB1032205A - Improvements in or relating to the concentration of phosphoric acid - Google Patents

Abstract

An aqueous solution of phosphoric acid is concentrated by removal of water by employing a dessicant comprising an aqueous solution of sulphuric acid to withdraw water vapour from an atmosphere in immediate contact with the phosphoric acid solution, the heat generated in the desiccant being transferred to the phosphoric acid solution. Preferably, there are simultaneous mass transfers of water from a film formed from the phosphoric acid solution to the gaseous medium, and from the gaseous medium to a film formed from the sulphuric acid solution desiccant, the heat generated in the desiccant film by absorption of water being transferred to the phosphoric acid solution. Preferably, the resulting concentrated film of phosphoric acid is returned to the phosphoric acid solution from which fresh films are repeatedly formed, concentrated and returned thereto until the desired degree of concentration is obtained, and the resulting hydrated film of desiccant is returned to the sulphuric acid solution from which fresh films are repeatedly formed, hydrated and returned thereto. The process may be carried out on a batch principle by exposing surfaces of the phosphoric acid solution and desiccant to a common atmosphere within a vessel and providing means of transferring the heat evolved by absorption of water vapour in the desiccant to the phosphoric acid solution; alternatively, the phosphoric acid solution and desiccant may be caused to flow through apparatus in which freshly formed films of the two acids are simultaneously and continuously exposed in a common atmosphere to permit mass transfer of water from the phosphoric acid film to the sulphuric acid film, the acids flowing in such manner that there is good thermal contact between the acids, and the phosphoric acid solution being introduced into the apparatus preferably at above ambient temperature. The sulphuric acid solution contains preferably at least 70%, advantageously more than 90%, by weight acid. The phosphoric acid solution may be that prepared in the "wet process," and more particularly the solution which has been derived by leaching phosphate rock with the diluted sulphuric acid resulting from the above concentration process. The concentration of phosphoric acid solutions is exemplified using two forms of apparatus which are described and illustrated.