Strength and self-cementing properties of oil shale retorting wastes
Date
2013-05-24
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Abstract
Ida-Virumaa tasast pinnamoodi ilmestavad Kohtla-Järve ja Kiviõli külje all kõrguvad tehismäed, kuhu ladestatakse põlevkiviõlitööstuse jäätmeid. Need koosnevad õli tootmisel põlevkivist järele jäävast mineraalainest, kuid sisaldavad ka pisut orgaanikat, mis annab neile iseloomuliku musta värvuse. Sõltuvalt tootmistehnoloogiast tekib kahte tüüpi jäätmeid – jämedateralist poolkoksi ning peeneteralisemat musta tuhka. Mõlemad on keskkonnale ohtlikud, seetõttu on eriti olu¬line, et nad oleksid ladestatud viisil, mis tagab prügilate stabiilsuse ja keskkonnaohutuse. Seda kontrollivad peamiselt tugevus ja veejuhtivus, mis sõltuvad muuhulgas jäätmete mineraalsest koostisest ning selle muutumisest pärast ladestamist.
Käesolevas doktoritöös uuriti õlitööstuse jäätmete mehaaniliste omaduste seoseid tsementeerumisega ja seda kontrollivate mineraalsete muutustega. Õli toomisel kuumutatakse põlevkivi kõrge temperatuurini, mille tõttu tekivad mitmed uusmineraalid. Nende hilisemal reageerimisel toimub tsementeerumine, mis seob algselt pudeda materjali tugevamaks massiiviks. Uuringute tulemused näitavad, et poolkoksiprügilaid moodustava materjali tugevus sõltub olulisel määral tsementeeriva mineraali ettringiidi moodustumisest. Ettringiit ei ole aga pikaajaliselt püsiv ning vanades poolkoksilasundites on kohati lisatugevust andev tsementatsioon hävinud. Seetõttu ei saa poolkoksi puhul pikaajalise lisatugevusega arvestada. Alternatiiviks võib olla poolkoksi ja põlevkivituha segamine ja koosladestamine. Katsete käigus selgus, et kui tuha hulk segus on suurem kui poolkoksi poorid mahutavad, praguneb segu intensiivse tsementeerumise tulemusena oluliselt. Pragude kaudu liiguks aga läbi ohtlike ja leonduvate jäätmete vesi, mis kannaks reostuse pinnasesse ja põhjavette. Seevastu kui tuhka on vähe, suurendab see poolkoksi tugevust ja lasundi stabiilsust. Uuemat õlitööstuse jääki musta tuhka pole seni eraldi ladestatud, kuid kasvavad mahud on tekitanud vajaduse sobiva ladestusmetoodika järele. Selle leidmiseks imiteeriti laborikatses mitmeid ladestamistehnoloogiaid. Selgus, et must tuhk tsementeerub veega kokku puutudes mitmeid kordi tugevamaks kui poolkoks ja seda põhjustavad mineraalid on võrreldes poolkoksiga keemiliselt stabiilsemad.
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.
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.
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Keywords
põlevkivitööstus, tootmisjäätmed, poolkoks, tuhamäed, mehaanilised omadused, tsementeerumine, oil-shale industry, industrial effluents, semi-coke, ash dumps, mechanical properties, cementation