Bioleaching of nepheline

Abstract

Organic acid leaching of nepheline has been studied under batch and semi-continuous laboratory conditions. Rates of leaching by citric and oxalic acids (as purified compounds and as the mixed product of Aspergillus niger fermentation) have been compared with those of hydrochloric acid for batches of mineral (−37 +53 μm) and acid stirred at 30 min −1 at 26±2°C. For pH values 1.8–3.7 relevant to fermentation, a model of the type 1− 2R 3 − (1−R) 2 3 = kt applied, where R is the fraction of mineral reacted at time t and k is a first order rate constant. Decrease in pH from 3.7 to 2 increased k from approximately 10 −4 min −1 to 10 −3 min −1 for all these acids, citric acid giving marginally the greatest rate. Photomicrographs of partially reacted grains in cross-section showed a combination of compact silica gel and unreacted core of approximately the original particle size, together with an outer “floccose” layer of gel. Kinetic factors in semi-continuous fermentation-leaching have also been measured and related to F/V = 3k (1 − R) 1 3 /[2R(1−(1−R) 1 3 )] , where F is the flow rate, V the volume and k is a working rate constant. Factors in applying fermentation-leaching to organic aluminium salt synthesis are discussed.