Investigation of the Carbochlorination Mechanism of Mullite from Fly Ash
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TION
HIGH-ALUMINA fly ash is collected from coalfired power plants, and has an alumina content of 35 to 50 wt pct.[1] The yearly report on integrated resources use in China (2016) stated that annual production of fly ash in 2015 was about 620 million metric tons. Given its high alumina content, i.e., over 45 wt pct and total available resource of 15 billion metric tons,[2] high-alumina fly ash is a promising alumina extraction source.[3] Furthermore, because of high annual production (about 50 million metric tons) and low comprehensive use (about 20 pct), environmental pollution caused by fly ash is becoming a pressing problem, and is receiving increasing attention.[4] Traditionally, the main methods for alumina extraction have been alkali[5,6] and acid methods.[7] In the alkali method, alumina in fly ash is converted to sodium aluminate to enable its extraction from the fly ash.
LONG WANG, TING-AN ZHANG, GUO-ZHI LV, JING-ZHONG ZHANG, ZHI-HE DOU, WEI-GUANG ZHANG, LI-PING NIU, and YAN LIU are with the School of Metallurgy, Northeastern University, Shenyang 110819, China and also with the Key Laboratory of Ecological Metallurgy of Multi-metal Intergrown Ores of Ministry of Education, Shenyang 110819, China. Contact e-mail: [email protected] Manuscript submitted November 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
However, the high sintering temperature needed in the alkali method consumes vast amounts of energy, and the large amounts of Ca-based by-products, generated by causticization of high-alumina fly ash lead to serious environmental problems.[8] In the acid method, alumina is converted to aluminum sulfate or aluminum chloride to facilitate production.[9] However, the acid method causes serious equipment corrosion and acid smog problems, therefore expensive acid-resistant and airtight processing equipment is needed.[10] In addition, iron oxide and calcium oxide are difficult to separate from alumina. These methods fail to achieve comprehensive use of the valuable elements in fly ash. Some studies have shown that a metal oxide can be converted to the corresponding metal chloride in the presence of carbon and chlorine gas by carbochlorination.[11] Carbochlorination metallurgy is based on the high reactivity of chlorine, the high volatilities, and low boiling points of metal chlorides.[12–14] Carbon has active catalytic sites for the formation of highly reactive gas intermediates.[15–17] Manukyan[18] studied the mechanism of chlorination of metal oxides by chlorine, and reported that chlorination of oxides in carbonthermal media occurred via a two-step mechanism, i.e., formation of oxycarbochlorides followed by chloride formation. Because of the disadvantages of traditional methods,[19,20] more efficient, flexible, and environmentally friendly processes are needed for fly ash treatment. A new process for fly ash treatment, based on
carbochlorination metallurgy,[21] with carbon as the reductant and chlorine gas as the chlorination agent, has been proposed.[22] This new technique solves the key problems associated with tra
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