Optimization of operating conditions in purification of wet process phosphoric acid in a liquid-liquid extraction column

Abstract

Solvent extraction is an effective method for purufucation of wet process phosphoroc acid. The aim of this study was purification of wet-process phosphoric acid using acid grade phosphorus pentoxide and minimization of fluoride and arsenic. The first step included the pre-purification which was done by discoloration of acid and clarification during which the impurities were reduced. Fluoride reduction rate of about 68% was significant at this step. In the following step, liquid-liquid extraction was used to purify the phosphoric acid from previous stage. The parameters of phase ratio, two-phase contact time and the fluid flow regime in a packed column of the fluidized bed were studied using 2-ethylhexanol as a basic solvent. The results of this part have been used as an evaluation of acid extraction yield and separation factor of impurities using an experiment designed by the Taguchi method. Arsenic, fluoride and phosphorus pentoxide concentration were measured using Inductivity Coupled Plasma (ICP) and Ion Chromatography (IC) analysis. The optimal level of parameters and their effect was obtained. By comparing the optimal and worst level in extraction step, it was found that the amount of acid extracted can vary from 23% to 73%. Furthermore, the removal rate of 49% for fluoride and 67% for arsenic in the worst conditions, can increase to the optimal conditions as much as 72% and 93%, respectively. The overal removal rate of 92% for fluoride and 96% for arsenic in two stages were achieved. Increasing of phase ratio, the time as well as increasing the flow regime created optimal conditions in the extraction section.