Use of Air Cooling Method for Slurry Cooling for WPA Production Units

Abstract

One of the most important issues in wet phosphoric acid technology is maintenance of the set temperature of slurry. Excess process heat is taken away mainly by evaporation of water from slurry. There are two main ways of cooling – evaporation under vacuum and air cooling method. Both ways have advantages and disadvantages. The main advantages of air cooling method are: it’s rather simple, and requires low capital cost for implementation, use of cooling water for condensation of secondary steam is excluded, cooling tower isn't required for cooling water, etc. Since 1980-s JSC “NIUIF” is engaged in elaboration and improvement of air cooling systems for slurry cooling. As a result of these researches air cooling units of various types for dihydrate and hemi-hydrate processes of WPA were constructed. Use of air cooling method is an essential part of elaborated by JSC “NIUIF” hemi-hydrate WPA process technology from Kola apatite concentrate, as well as high-temperature dihydrate process from Karatau phosphate rock which are carried out at temperatures of 90-94°C. Modern system of air cooling is comprised of: low pressure (LP 800-1000 mm) slurry supply axial pump of high capacity (5000-10000 m3/h), air cooling unit, FDF and/or IDF. Due to optimization of heat&mass exchange conditions and design of air cooling unit (ACA), moisture saturation of cooling air can be achieved up to high extent (up to 400 g/kg of dry air) that allows to minimize its consumption. High consumption of slurry for cooling provides rather low gradient of temperatures (∆t ≤ 3°C) at inlet and outlet from ACA and rather low manual labor efforts for cleaning of the equipment from incrusting sediment. The loop of supply for cooling can be combined with circulation loop inside the reactor that contributes to homogenization of slurry in reactionary volume. Air cooling units for slurry cooling based on researches by JSC “NIUIF” are successfully introduced at modernized and newly constructed WPA systems in Russia, Belarus, Kazakhstan, Lithuania. WPA single line process system capacity with air cooling of reactors reaches 1500 tpd of Р2О5.