Studies on the chloridisation of low grade ferruginous manganese ores

Abstract

Hydrochlorination was attempted as a means of chemically beneficiating the low?grade manganese ores which are not amenable to upgradation by normal methods of ore dressing. The samples chosen for the present study were obtained from the Bandur mines in the Bellary District of Mysore State and were found to be a complex mixture of manganese and iron minerals assaying 28.5% manganese and 25.45% iron.

Studies were made on the static bed, and a low?temperature chloridisation process was successfully evolved to effect an almost complete separation of manganese from iron. Dry chloridisation at 360?°C showed that only 50% of the iron could be removed by volatilisation as ferric chloride. Temperatures below the sublimation point of ferric chloride were found to be better suited for obtaining improved chloridisation of both manganese and iron.

The optimum conditions for the chloridisation of manganese were worked out. It was found that chloridisation of an ore sample of –300 mesh at 200?°C for 30 minutes was sufficient to chloridise all the manganese. Separation from iron was subsequently carried out by leaching the chloridised residue with water in the presence of a coagulant, when iron chloride hydrolysed and precipitated out. All the chloridised manganese remained unaffected.

The efficiency of the separation was further improved by resorting to an air roast of the chloridised product. The air roast served the dual purpose of reducing the acidity of the water extract and also converting the iron chloride to oxide, thus minimising the residual iron in the water extract. An almost quantitative separation of iron from manganese was thus achieved. A free?energy diagram of the possible reactions involved in the chloridisation has been drawn to illustrate the salient features of the above process.

Chloridisation studies were subsequently made in a fluidised bed to determine the efficacy of fluidisation on the rate and extent of volatilisation of iron. It was found that only 71% of the iron could be volatilised from a fluidised bed at 360?°C. Fluidisation was possible only between a narrow range of sizes, namely –44 to +100 mesh. Low temperatures, so necessary for an improvement in the overall efficiency of chloridisation, could not be employed as they resulted in caking of the bed leading to non?uniform fluidisation. The results of the fluidisation studies were not up to expectations because both the particle size and the temperature of fluidisation were unfavourable. Maximum removal of 78–80% iron could be obtained by a combination of fluidisation and water leaching of the chloridised residue.

The present work also includes a study of the kinetics of the hydrochlorination of ferric oxide as a point of relevant interest. This was carried out thermogravimetrically. Since the products of chloridisation, namely ferric chloride and water, are both vapours at the temperature under consideration, the loss in weight with time served to indicate the rate of the process. The rates of loss in weight with time obeyed McKewan’s equation, and a calculation of the activation energy showed that between 450 and 600?°C the reaction had an apparent activation energy of 3.5 kcal/mole. The low activation energy obtained suggests a mechanism controlled by diffusive steps.