Supervisors: TÜ ÖMI professor Kalle Kirsimäe, PhD ja TÜ ÖMI vanemteadur Argo Jõeleht, PhD
Opponent: dr. Alexander Thomas Jakubick (Wismut GmbH, Chemnitz, Saksamaa)
Otherwise plain relief of Ida-Virumaa is contrasted by high artificial heaps near Kohtla-Järve and Kiviõli. These heaps accommodate huge amounts of shale-oil retorting wastes that are mainly composed of mineral matter remaining after processing of oil shale, but also contain leftover organic material that give the waste its dark color. Two types of waste are formed, which depend on the retorting technology - coarse-grained semi-coke and fine black ash. Both types are environmentally hazardous, thus they should be landfilled in a way that guarantees mechanical stability and environmental safety of landfills. These properties depend on strength and permeability that are controlled by mineral composition and alteration of the waste after disposal.
The current thesis focuses on the relationship between mechanical properties, cementation and mineral alteration of oil shale retorting wastes. During retorting some minerals in found in oil shale decompose into unstable compounds. These compounds start to react and form new cementing phases that bind granular material together, thus contributing to its strength. The study shows that the strength properties of semi-coke highly depend on the formation of cementive mineral ettringite. Ettringite, however, is not stable in the long term and the cementation has occasionally disappeared in old semi-coke dumps. This suggests that semi-coke should be considered as granular material and the additional strength cannot be relied on. The alternative solution could be mixing and co-disposal of semi-coke and oil shale ash. If the ash content in the mixture exceeds the capacity of semi-coke pores, intensive formation of cementing minerals causes cracking. These cracks increase permeability and leached water can easily infiltrate into the surroundings. If the ash content stays low, the strength and stability of semi-coke is improved. Black ash, the new type of retorting waste, is not landfilled separately so far; however, the increasing volumes of this type of waste have raised the need for well-planned disposal method. Different methods were imitated in a laboratory experiment. It was found that black ash has good self-cementing properties and its strength exceeds that of semi-coke manyfold. Also the cementing phases in black ash are chemically more stable compared to semi-coke.