Leaching of low-grade manganese ores by using nitric acid and glucose: optimization of the operating conditions

Abstract

Manganese extraction from low-grade ores was investigated by using glucose as a reducing agent in dilute nitric acid medium. Ore characterisation by X-ray and SEM denoted that manganese is present as a mixed oxide Mn 7O 13 (6MnO 2·MnO) surrounding and joining together aluminosilicate fragments. The effects of temperature, particle size, glucose and nitric acid concentrations were investigated and compared with previous results using H 2SO 4. The operating conditions of leaching were optimised to obtain maximum manganese extraction and purity by performing sequential factorial designs and evaluating the statistical significance of the different factors by analysis of variance (ANOVA). The optimised conditions are 90 °C, stoichiometric nitric acid, −20% below stoichiometric glucose and 295–417 μm size fraction. The stoichiometric reagent concentrations were evaluated according to the following reaction assuming all manganese in the ore as Mn(IV): C 6H 12O 6+12MnO 2+24H +=6CO 2+12Mn 2++18H 2O Optimised leaching conditions with HNO 3 give up to 99% manganese extraction, <0.05% iron dissolution and reduced reagent consumption compared to previous tests using H 2SO 4.