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MINERAL COMPOSITION OF ESTONIAN OIL SHALE SEMI-COKE SEDIMENTS
Abstract
The Estonian kukersite is the largest oil shale resource in the world used industrially today. The majority of mined oil shale is utilized for electricity and heat production. About 20\% of all of the oil shale extracted annually is used for retorting shale-oil and shale-gas, after which large amount of environmentally hazardous solid retort residue – semi-coke is left over. Production of 1 tonne of shale-oil is accompanied by almost 3 tonnes of semi-coke waste. During more than 80 years of operation over 110 million tonnes of semi-coke have been deposited near retorting plants but so far very little is known about the mineral composition of this waste. However, the environmental as well as the reusability aspects of that waste depend besides of its organic material content also on the composition of its mineral matter.This study presents new data on mineral composition of semi-coke and its spatial variation in waste heaps. The study is based on mineralogical study carried out on fresh and hydrated semi-coke samples by means of X-ray diffractometry. The composition of semi-coke reflects the changes of mineral matter during the retorting process. The changes with the terrigenous and carbonate matter are almost negligible during the main phase of retorting; only dehydration and partial transformation of clay minerals and decomposition of sulfur compounds occur. At the final step of retorting (900–1000 °C) a slag-like material forms, which consists of amorphous and Ca-silicate phases. The deposition and subsequent hydration causes the change in the composition of semi-coke. The most notable change is the formation of ettringite. The composition of mineral matter in semi-coke waste heap is relatively uniform. The variations are probably due to both physical and chemical separation during deposition of the sediment and to different diagenetic/hydration processes.
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