Phase changes in mineral matter of North Dakota lignites caused by heating to 1200 °C

Abstract

Changes in structures of minerals taking place in lignitic coals during combustion were investigated by first concentrating the mineral matter in the coal by low-temperature ashing and then heating the mineral matter at 100 °C intervals from 200 °C–1200 °C and analysing the major mineral phases by X-ray powder diffraction. Samples of high and low sodium contents were analysed to determine differences in mineral phases with varying sodium contents. Quartz and bassanite were identified as major phases in the low-temperature mineral matter of all ten lignite samples, and pyrite and calcite were identified in eight of the ten samples. Kaolinite was the only clay mineral identified and appeared in nine of the ten samples. Those samples with a sodium oxide content of 8.56 wt % or greater, showed sodium nitrate as a major mineral phase in the low-temperature mineral matter. When the mineral matter was heated to higher temperatures, quartz was a major phase at 1200 °C in five of the samples, and was stable to 1000 °C in all of the samples. Anhydrite was a major mineral phase in all samples from 600 °C–800 °C, appearing in some of the samples as low as 200 °C, and persisting to 1100 °C in some samples. Hematite was found to be a major phase in seven of the ten samples, having an overall temperature range from 300 °C–1000 °C. Magnetite was detected in the range from 800 °C–1200 °C with hercynite forming as a major mineral phase, after magnetite, in two of the samples at 1200 °C. The solid solution series gehlenite-akermanite was found in all ten samples from 1100 °C–1200 °C although they appeared in some samples at 900 °C. Samples of high sodium content formed sodium sulphates at intermediate temperatures and sodium silicates at higher temperatures. Low Sodium samples formed bredigite, a calcium silicate, at higher temperatures.