Removal of mineral matter and sulfur from lignites by alkali treatment

Abstract

Studies on coal cleaning showed that it was possible to remove most of the pyritic sulfur and some of the organic sulfur by caustic wash. In the present study, the caustic wash was applied to two different lignites from Turkey: Soma (low ash content coal) and Beypazari (high ash content coal) lignites. The coal samples were treated with concentrated NaOH in Teflon-lined autoclaves. The experiments were carried out at temperatures of 400, 430 and 460 K. The NaOH concentration was varied from 0.5 to 10 M. The treatment time was also selected as a parameter and set at values of 30, 60 and 150 min. From the experimental results, it can be seen that it is possible to prepare clean coal by caustic wash when the process conditions are properly selected. At high temperatures (460 K), about 90% of the mineral matter was removed by washing with caustic solution followed by acid washing for both low ash coal (LAC) and high ash coal (HAC). The coal yields vary from 36 to 94.5% depending on the experimental conditions and the type of coal. The particle size of the coal also affects the efficiency of the chemical washing process. For this reason, some experiments were carried out with different particle sizes of LAC. As the particle size is decreased from 0.9 to 0.16 mm, ash removal is increased from 70.6 to 80.1%. As the particle size decreases the caustic treatment becomes more effective. For the experiments with LAC, a complete analysis of the ash was also carried out. These experiments showed that during caustic wash, most of the silicon dioxide and calcium oxide in the ash were removed. However, most of the iron oxide and sodium oxide remained in the ash.