Experimental investigation of iron removal from wet phosphoric acid through chemical precipitation process

In the present work, the combination of experimental investigation and statistical analysis has been used for the first time to study iron precipitation from wet-process phosphoric acid. Firstly, eight types of additives including different carbonate and hydroxide salts were studied in the chemical precipitation reaction in order to decrease the iron content in wet-process phosphoric acid. From different studied reagents, calcium carbonate and calcium hydroxide showed the best to remove ferric ions from crude wet-phosphoric acid. Under the optimum conditions, about 80% and 83% of the iron can be removed at 70 ℃ after the addition of calcium carbonate and calcium hydroxide, respectively. The maximum P2O5 losses from crude phosphoric acid was less than 4% after the addition of calcium hydroxide. Based on the Arrhenius behavior, the activation energy of iron precipitation was calculated about 47.43 and 51.55 kJ/mol for calcium carbonate and calcium hydroxide addition, respectively. The iron state studies showed that the pretreatment process was unnecessary to reduce iron state from ferric to ferrous form before precipitation. Furthermore, the kinetics parameters showed that the iron precipitation process was an endothermic reaction. The XRD analysis of the precipitates produced confirmed the formation of Fe(OH)3, α-Fe2O3, α-FeOOH and FeCO3 through the reaction of iron with carbonate and hydroxide ions. Possible pathways of iron precipitation were proposed. Both calcium carbonate and calcium hydroxide showed good efficiency to remove iron from wet-process phosphoric acid accompanied by moderate P2O5 losses. The Taguchi technique was applied to optimize the parameters using signal-to-noise ratio analysis as well as ANOVA. The analysis showed a greater susceptibility of the response against changes in the value of calcium carbonate, more than calcium hydroxide.