EXTRACTION OF H<I>₃</I>PO<I>₄</I> from Wet Phosphoric Acid By nC4 - nC7 Alcohols

Abstract

Most phosphate rocks are used to manufacture phosphoric acid by digestion with concentrated sulfuric acid by the wet method. Many impurities dissolve also in the acid and then go to the fertilizer which makes this acid unsuitable for food or pharmaceutical uses. Pure phosphoric acid for food purposes is obtained by reduction of the phosphate rocks in an electric furnace to obtain P₂O5 by the dry method. This method consumes a lot of energy and so it is rather expensive. There has been an increased interest in the past few years in purifying commercial wet phosphoric acid to obtain a pure acid. Several methods were developed on the pilot plant or commercial scales but most of the works are still in the developmental stage. This paper studies extraction of H₃PO₄ by nC4 - nC7 alcohols by liquid-liquid extraction. The alcohol mainly used is n-heptanol. The effect of temperature and mixing time on distribution ratio is studied. The results show that nC7 alcohol is a suitable solvent for H₃PO₄ extraction. The temperature has a slight negative influence on extraction. The extraction is enhanced by increasing P₂O₅ and alcohol concentrations. The extraction decreases by increasing the number of carbon atoms or the molecular weight of the n-alcohol, but phase separation is slower. Extraction of uranium and heavy metals is negligible while extraction of fluorine and iron is relatively small.