Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 55.4.11 Decadmation of H 3 PO 4 by reactive extractionDescriptionThe decadmation process was installed in the example plant in 1973 and was applied after a wetdecomposition of ores with sulphuric acid. An earlier version of this process included severalcounter stream extraction steps using isopropanol as the solvent and the initial intention was toremove the organic components from the raw phosphoric acid. At the beginning of the 1990s,the quality standards for metal contents, especially cadmium, were increased. Hence theexample plant developed a process to be applied prior to the extraction steps already installed.The decadmation can be described as follows:• cadmium is removed from the raw phosphoric acid by an extraction step using an organicsolvent consisting of a mixture of inert solvent/amines or ammonium compounds with achlorocomplex as a counterion• the organic phase is separated• cadmium is removed from the organic phase by re-extraction with an aqueous phase(containing hydrochloric acid and a chlorocomplex of, e.g. Zn or Fe. The mechanism can beunderstood as the swapping of the chlorocomplex and cadmium between the two phases• the phases are separated from each other• cadmium is removed from the aqueous phase.The ideal process temperature is between 15 und 25 ºC. The process can be applied incontinuous or batch mode. A specific prerequisite condition for the application of the process isthat the phosphoric acid is produced by the use of H 2 SO 4 from phosphate rock, and not by usingHCl or HNO 3 . Applied to phosphoric acid with P 2 O 5 contents of between 28 to 58 % w/w,cadmium can be reduced by 95 %.Cadmium is removed from the aqueous phase by precipitation as a final process step afterdecadmation. This includes the following:• physical separation of the aqueous phase from the organic phase• distillation of the dilute hydrochloric acid solution• cooling of the concentrated solution resulting in precipitation of cadmium compounds in afine cristalline modification• decantation of the precipitated cadmium compounds.The remaining filtrate after removal of hydrochloric acid and cadmium is fed back to the initialraw phosphoric acid and hydrochloric acid is recycled to the extraction step. Hence, the liquidsare handled in a closed cycle.Achieved environmental benefits• decadmation efficiency of 95 %.Cross-media effects• high energy consumption.244 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers