A Process for removing toxic heavy metals to produce the high purity NH4H2PO4 and KH2PO4 from a crude phosphoric acid

Abstract

The use of chemical fertiliser, in particular a phosphate, in the agricultural sector is an essential in a modern production of crops. However, low grade fertilisers containing toxic heavy metals could be accumulated in soil and introduced into our food chain eventually. For sustainability, it is important to use the high purity fertilisers in the agricultural sector. In this context, we propose a hydrometallurgical process to remove impurities, including toxic heavy metals, to produce the high purity monoammonium phosphate (MAP) and monopotassium phosphate (MKP) from a crude phosphoric acid (approx. 50% P2O2). The proposed process is incorporated with a series of precipitations, ion exchange, activated carbon adsorption, and evaporating to crystallise the final product. All impurities in final products were removed by over 90% and controlled under marketable specifications. As and Cr were removed further below to less than 1 mg/L in the final solution for adjusting the capacity of carbon was identified. Optimised conditions identified were then utilised in 40 L size pilot-scale reactors and produced the high purity MAP and MKP, respectively. The proposed process is expected to contribute for producing the high purity MAP and MKP by relatively simple technique and cost-effective than a conventional solvent extraction process from the crude phosphoric acid. The use of high purity phosphate fertilisers can aid sustainability for the agricultural industry by reducing accumulating the toxic heavy metals in soil in a wide sense.