OCCURRENCE AND ENRICHMENT OF TRACE ELEMENTS IN MARINE OIL SHALE (CHINA) AND THEIR BEHAVIOUR DURING COMBUSTION

The Shengli River-Changshe Mountain oil shale zone represents the potentially largest marine oil shale resource in China. With the aim to have better knowledge of the distribution of trace elements in oil shale, and their behaviour during oil shale combustion and gasification, 28 raw Changliang Mountain oil shale and marl samples, 19 oil shale combustion residue samples and 23 samples of minerals isolated from Changliang Mountain oil shale are studied for trace element content. The oil shale samples from the Changshe Mountain area are characterized by high ash yield (54.69-86.75%) and total organic carbon (TOC) content (2.20- 13.44%). The contents of Se, Mo, Cd, As, B, and Ni in raw oil shale samples are 3.78 to 28.44 times their upper continental crust values. The enrichment of an element in oil shale seams may be a function of that association and the origin of oil shale fractions. Some trace elements in the Changshe Mountain oil shale show susceptibility to release into the atmosphere, including about 93% Hg and 20-30% As, Ba, Be, Co, Cr, Nb, and Sc. The behaviour and migration of trace elements during oil shale combustion strongly depend on their mode of occurrence in the rock. Based on Pearson's coefficients of correlation between elements, as well as cluster and isolated mineral analyses, the main modes of occurrence of trace elements in marine oil shale are distinguished: 1) clay mineral affined - B, Ba, Be, Cr, Cs, Ga, Hf, Li, Nb, Rb, Sc, Sn, Ta, Th, V, W, Zr, REEs; 2) organic affined - As, Mo, Se; 3) Fe-bearing mineral affined - Bi, Co, Cu, Ni, Pb, U; 4) calcite affined - Cd, Sr and 5) fossil affined - Cd, Zn.