Spent shale or spent oil shale (also known as retorted shale) is a solid residue from the shale oil extraction process of producing synthetic shale oil from oil shale. It consists of inorganic compounds (minerals) and remaining organic matter known as char—a carbonaceous residue formed from kerogen. Depending on the extraction process and the amount of remaining organic matter, spent shale may be classified as oil shale coke, semi-coke or coke-ash residue, known also as oil shale ash. According to the European Union waste list all these types of spent shale are classified as hazardous waste.
Oil shale coke was created by chamber ovens which were used for oil shale gas production. Vertical retorts create mainly semi-coke. Most of solid heat carrier processes creates coke-ash residue as the semi-coke created during the process is combusted for the process energy needs.
Spent shale can be used as ingredients in cement or brick manufacture. In Jordan, usage of spent shale for the production of sodium carbonate, ammonium sulfate, and potassium sulfate has been studied.
- Veski, R. (2005). "The Volumes of Spent Oil Shale from Estonian Oil-Shale Processing Units for the Period of 1921–2002" (PDF). Oil Shale. A Scientific-Technical Journal. Estonian Academy Publishers. 22 (3): 345–357. ISSN 0208-189X. Retrieved 2008-10-25.
- Habicht, Jaan; Orupõld, Kaja (2007-10-15). Retorted Oil Shale In Estonia: An Environmental Audit (PDF). 27th Oil Shale Symposium. Golden, Colorado: Colorado School of Mines. Retrieved 2014-11-27.
- Mõtlep, R.; Kirsimäe, K.; Talviste, P.; Puura, E.; Jürgenson, J. (2007). "Mineral composition of Estonian oil shale semi-coke sediments" (PDF). Oil Shale. A Scientific-Technical Journal. Estonian Academy Publishers. 24 (3): 405–422. ISSN 0208-189X. Retrieved 2014-11-27.
- Koel, Mihkel (1999). "Estonian oil shale". Oil Shale. A Scientific-Technical Journal. Estonian Academy Publishers (Extra). ISSN 0208-189X. Retrieved 2007-07-21.
- Qian, Jialin; Wang, Jianqiu; Li, Shuyuan (2007-10-15). One Year's Progress in the Chinese Oil Shale Business (PDF). 27th Oil Shale Symposium. Golden, Colorado: Colorado School of Mines. Retrieved 2011-05-06.
- Resheidat, Musa; Al-Shyoukh, Ali; Khair Al-Nawasrah, Mohammad (2007-10-15). On the Use of Oil Shale beyond the Production of Oil and Gas (PDF). 27th Oil Shale Symposium. Golden, Colorado. Retrieved 2014-11-27.