MINERALOGY AND TRACE ELEMENT GEOCHEMISTRY OF ENTRAINED-FLOW COAL GASIFICATION RESIDUES FROM NORTHWEST CHINA

With the increasing production of coal gasification residues in China, the investigation of mineralogy and chemistry of coal gasification residues will provide an understanding of the modes of occurrence of potentially hazardous trace elements and rare-earth elements and yttrium (REY), which are valuable for the control of the hazards and the recovery of REY. High volatile C bituminous coals used in the General Electric (GE) and Gaskombinat Schwarze Pumpe (GSP) coal gasification plants located in Ningxia, China, are prepared in a coal–water slurry and then pulverized. Coarse and fine residues were collected from the bottom of the gasifiers and the filters in the syngas purification system, respectively. The concentrations of the trace elements in the samples, including the potentially hazardous trace elements and REY, were analyzed. The chemistry of the inorganic particles in the fine fractions (<75 μm) of the four residues was measured using computer-controlled scanning electron microscopy (CCSEM). The inorganic particles in the coal gasification residues were predominantly silicates. Na-Fe silicates and Mg-Fe silicates were the main silicate phases in the GE gasification residues; however, Mg-Fe silicates and Fe aluminosilicates in the GSP gasification residues had the dominant calculated weight percent (wt%) of the total particles based on particle area. Iron-containing phases such as Fe silicates, aluminosilicates, and iron oxides were generally the main carriers of V, Cr, Mn, Co, Ni, Cu, Zn, As, Sr, Mo, Cd, Sb, Tl, and Pb detected by energy-dispersive spectrometry (EDS). La, Ce, Nd, and Gd were detected by EDS. The coal gasification residues can be regarded as promising REY raw materials for economic development with the sum REY contents (ash basis) in the GE Coarse Residue, GE Fine Residue, GSP Coarse Residue, and GSP Fine Residue measured at 304.9 ppm, 328.5 ppm, 319.4 ppm, and 327.3 ppm, respectively.