Oil shale-type organic-rich sedimentary rocks can be pyrolysed to produce shale oil. The pyrolysis of oil shale using solid heat carrier (SHC) technology is accompanied by large amount of environmentally hazardous solid residue-black ash-which needs to be properly landfilled. Usage of oil shale is growing worldwide, and the employment of large SHC retorts increases the amount of black ash type of waste, but little is known about its physical and chemical properties. The objectives of this research were to study the composition and self-cementing properties of black ash by simulating different disposal strategies in order to find the most appropriate landfilling method. Three disposal methods were simulated in laboratory experiment: hydraulic disposal with and without grain size separation, and dry dumping of moist residue. Black ash exhibited good self-cementing properties with maximum compressive strength values of >6 MPa after 90 days. About 80% of strength was gained in 30 days. However, the coarse fraction (>125 µm) did not exhibit any cementation, thus the hydraulic disposal with grain size separation should be avoided. The study showed that self-cementing properties of black ash are governed by the hydration of secondary calcium silicates (e.g. belite), calcite and hydrocalumite.
Oil shale ash and semicoke, solid residues from the oil shale industry, are today disposed of separately in landfills which pose a considerable environmental hazard. In the current study, the possibility of codepositing ash and semicoke was investigated in a small-scale field experiment. The purpose of the experiment was to elucidate which mineral changes in the landfilled material occurred and how these changes affected its permeability characteristics. For this purpose five mixtures with different ash-tosemicoke ratios were prepared and placed in the open air. Changes in the dry density, hydraulic conductivity and mineral composition of mixtures were recorded within a period of four months and after one year. During the experiment the mixtures expanded and showed increased permeability due to intensive secondary mineralization. The higher the ash content of a mixture, the more intensive the expansion and, consequently, the higher the permeability, which contributed to an increased infiltration of the leachate and toxic compounds through the landfilled material, thus leading to unfavourable environmental impacts. The above suggests the possibility of co-depositing ash and semicoke, but only if the ash content of their mixture is low enough.
Semi-coke is a solid waste material left after oil shale retorting in oil shale chemical industry. During more than 80 years of operation more than 110 million tonnes of semicoke have been deposited at retorting plants in Estonia but so far very little is known about the mineral composition of this waste. This paper presents new data on mineral composition of semi-coke and its spatial variation in waste heaps. The composition of semi-coke reflects the changes of mineral matter during the retorting process. The changes with the terrigenous and carbonate matter is 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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