In CERENA's 20 years seminar series, this month, Professor António Fiúza talks about "An integrated alkaline carbonate leaching process for producing APT (Ammonium Paratungstate) from Scheeliter".

In mining and metallurgical industries, scheelite high-grade ores and concentrates are preferentially leached by sodium carbonate at high temperature and pressure. The production of ammonium paratungstate (APT) from these liquors is complex and involves several operations: elimination of co-extractable impurities, solvent extraction by ion association using a tertiary amine, solvent regeneration and crystallisation. The main issues in this process are the prior removal of co-extractable impurities and the inhibition of tungstic acid formation during solvent extraction. Integrated studies proposing a unified, flexible flowsheet that addresses these problems are scarce.

To overcome these setbacks, two different purification methods for the leaching solutions were tested. The procedure that proved more efficient, avoiding the formation of crud induced by the residual colloidal silica, is based on additions of magnesium and aluminium, whose amounts were optimised. However, solving this problem led to another unexpected issue: occasionally, a gel-like precipitate formed during extraction. It was verified that this product was tungstic acid. Research showed that its formation could be avoided by regulating the intensity of the amine sulphatization. It was verified that molar ratios of sulfuric acid to amine greater than 2 hinder the formation of tungstic acid, and that higher extraction efficiencies are obtained for ratios greater than 8.

APT crystals with grades in W higher than 60% were produced by slow cooling crystallisation with simultaneous evaporation. Chemical characterisation of the crystals was performed by Raman spectroscopy.