Optimizing Fine Circuit Design to Maximize Pyrite Rejection and Enhance Coal Marketability

Abstract

Coal sulfur content is a critical property in both coking and thermal coal marketability. Sulfur level reduction in both markets greatly enhances coal value. In the Northern Appalachian region of the United States, sulfur levels in some potential coking coals are often marginal (1.1%–1.5%). On many occasions, much of the sulfur is concentrated in the form of pyrite in the finer sized fractions (1 mm × 0.045 mm). Traditional fine coal processing circuitry (spirals, flotation) tends to produce high levels of product sulfur. Pyrite reaching the flotation circuit typically floats well, producing high-sulfur froth concentrate. Being able to run at lower densities in the 1 mm × 0.25 mm size fraction reduces sulfur levels considerably. Removal of high-density pyrite by a combination of classification and spiraling allows low-sulfur froth concentrates to be produced. Circuit design and plant results for a recently commissioned plant in Northern Appalachia using this technology are discussed. Similarly in the Illinois basin of the United States, the pyrite concentration in the fines has precluded the use of froth flotation in many plants giving rise to significant yield losses. By incorporating a similar approach of classifying out the pyrite and then treating the desulfured fines in flotation, acceptable concentrates can be generated for the lower sulfur market. Once again the flotation yield is supplemented by taking the concentrated pyrite, which flows to the plant's clean coal effluent streams, and removing this by fine spiraling and floating the reduced-sulfur spiral concentrate.