Pyrite alkaline leaching

In the present study the behavior of some of the principal coal minerals was traced during alkaline leaching. This was accomplished by leaching several types of coal and by leaching individual minerals, which are commonly present in coal, with various alkalis. Different alkali concentrations and temperatures were employed. The solid reaction products were characterized by x-ray diffraction (XRD) and by dissolution in mineral acids. Results achieved with Illinois No. 6 coal are discussed, since they are representative, and the results of Individually leaching quartz, kaolinite, and iron pyrite... 

In a third set of experiments, -38 pm pyrite particles were leached with hot alkaline solutions to study the conversion of iron pyrite to iron oxide and soluble sulfur species. In each experiment, 5 g. of acid-cleaned pyrite was leached with 120 ml. of alkaline solution for 1 hr. In addition to analyzing the solid residue by XRD, the total sulfur content of the leachate was determined in order to estimate pyrite conversion. 

When the pyrite residue from the alkaline leaching step was treated with acid, all of the hematite dissolved in either hot hydrochloric acid or hot sulfuric acid. However, the hematite appeared to dissolve incompletely or very slowly in cold sulfuric acid. 

Under our experimental conditions, nitric acid dissolved iron pyrite but not the hematite which was formed during the alkaline treatment step. More rigorous alkaline treatment conditions were more effective than less rigorous conditions for converting the iron pyrite into hematite which could not be easily removed by the nitric acid. Consequently, when rigorous alkaline treatment conditions were applied to Lower Kittanning coal, the residual hematite more than made up for the other minerals which were extracted so that the overall reduction in ash content for the two-step process was no better than for HNO, leaching alone. 

Previous work has shown that ash-forming mineral matter, including iron pyrites, can be removed from coal by leaching the finely divided material with a hot caustic solution under pressure followed by washing with a dilute mineral acid (1-6). Recently, similar results have been achieved in leaching fine-size coal with hot sodium carbonate solutions (J7). In both cases, quartz appears to dissolve in the hot alkaline solution, while clay minerals and iron pyrite are converted into acid-soluble compounds which are removed in the acid washing step. Acid also removes any carbonate minerals which are present in the coal. 

Table V. Results of Leaching 5 g. Acid-cleaned Pyrite with 120 ml. Alkaline Solution for 1 hr.

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