Study on Kinetics of Vacuum Heat Treatment Process of the Spent Cathode Carbon Blocks from Aluminum Smelters
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RESEARCH ARTICLE
Study on Kinetics of Vacuum Heat Treatment Process of the Spent Cathode Carbon Blocks from Aluminum Smelters Ming‑zhuang Xie1,2 · Hong‑liang Zhao1,2,3 · Ze‑gang Wu1,2 · Wei Liu1,2 · Rong‑bin Li1,2 · Feng‑qin Liu1,2 Received: 19 August 2020 / Accepted: 26 October 2020 © The Minerals, Metals & Materials Society 2020
Abstract The spent cathode carbon block (SCCB) produced from the overhaul aluminum reduction cell is regarded as hazardous solid waste because it contains a large amount of soluble fluoride and small amount of cyanide. Coupling vacuum-heating method is considered an effective way to address the issues of such hazardous waste. Herein, SCCB is processed under the conditions of 3000 Pa and 1673–1973 K. The correlation between soluble fluoride content and treatment time was analyzed. The apparent activation energy of the detoxification reaction was calculated based on the Arrhenius equation. The rate-determining step of the detoxification process has also been analyzed in detail. The results show that the detoxification process is a firstorder reaction. The apparent activation energy was k = 15.08 kJ mol−1, and gas diffusion was the rate-determining step of the detoxification reaction. Graphical Abstract
Keywords Spent cathode carbon block · Vacuum · Dynamics · Rate-determining step The contributing editor for this article was U. Pal. Extended author information available on the last page of the article
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Introduction Almost all primary aluminum worldwide is produced by the Hall-Héroult process. The alumina and fluoride salt is added into the cryolite melt as raw material and additive, respectively [1]. The average life of a reduction cell is generally 5–7 years. The spent lining materials (SPL) generated during the overhaul of the reduction cell is considered as hazardous solid waste because it contains a large amount of soluble fluoride and small amount of cyanide [2–4]. Soluble fluoride and cyanide easily leach into the soil and groundwater when these hazardous wastes are stacked in the open air. Excessive fluoride can cause damage to human bones, while grain yields are significantly reduced when irrigated with fluoride-containing wastewater [5–8]. In recent years, some research work has been performed on the harmlessness and resource utilization of hazardous waste from the aluminum industry [9–17]. Those treatment processes can be divided into hydrometallurgical and pyrometallurgical methods. Most of the hydrometallurgical methods take place in a liquid environment. As much as possible, soluble fluoride is leached to separate and recover the fluoride from the carbonaceous material. For instance, Alcan uses LCL&L process to deal with SPL [18]. A low-concentration NaOH solution was used to extract the fluoride and cyanide, and then, the fluoride ions in the filtrate were precipitated by slaked lime. The cyanide in the filtrate was destroyed by hydrolysis at 180 °C. The problem of SPL resource utilization can partially be solved by this method, but there are al
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