The content and modes of occurrence of arsenic and its distribution in Yunnan coal of China as well as its partitioning behavior during the coal separation process were investigated. The following laboratory equipment such as proximate analyzer, ultimate analyzer, sulfur analyzer, inductively coupled plasma mass spectrometer and the methods including sequential chemical extraction process, screening analysis, float-and-sink analysis, heavy liquid separation, and progressive release flotation were frequently used during the research process. The coal sample has a high sulfur content of 8.21%, and its arsenic content is 15.1 μg/g, which is within the range of the mild enrichment level. Content relationship among the various modes of occurrence in order is sulfide-associated form (47.38%) > organically bounded form (18.09%) > silicate-associated form (17.51%) > carbonate-associated form (12.04%) > ion-exchangeable form (3.84%) > water-soluble form (1.14%). The sulfide-associated form is the dominant mode of occurrence of arsenic in the raw coal, which means arsenic has an affinity to sulfur. Arsenic in the sulfide-associated form mainly occurs in the inorganic sulfide minerals (especially in pyrite). Besides, the arsenic content increases with the decrease of coal particle size, and arsenic is concentrated in high-density products. There is a good correlation between the removal rate of arsenic and clean coal ash in either gravity separation or flotation, and arsenic removal rate of 57.96% and 70.77% could be obtained through gravity separation and flotation, respectively. In order to ensure arsenic removal rate and clean coal yield, a combined approach of physics and chemistry should be developed.

As one of the dangerous trace elements in coal, mercury has become an advanced research hotspot in coal purification. The content of ash, sulfur, and mercury of raw coals from four high-Hg coal preparation plants located in the main production areas of China was tested. To distinguish the connection between mercury content and the content of ash or sulfur, person correlation analysis and regression correlation analysis were performed by SPSS. The modes of occurrence of mercury in coal samples of these four plants were studied by sequential chemical extraction procedure (SCEP). The laws of mercury removal during coal preparation were researched by analyzing the mercury contents of feedings and products from the four plants. Results showed that mercury in coal was significantly associated with inorganic constituents or sulfur or both of them. The sulfide associated form is the dominant mode of occurrence of mercury in coals. Removal statistical laws of mercury during the process of preparation can be summarized as follows: (1) mercury in coal tends to concentrate in the fine fraction; (2) dense medium separation has a more obvious effect on mercury removal than flotation; (3) qualified clean coal products with up to standard mercury content are difficult to obtain by a conventional gravity separation or flotation process. This work facilitates understanding of the modes of mercury occurrence in other Chinese coals, and provides a foundation for removal of mercury in clean coal technology.

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