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Green Treatment of Cyanide Tailings Using a “Filter Press Backwash–Chemical Precipitation–Gaseous Membrane Absorption” Method

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Green Treatment of Cyanide Tailings Using a “Filter Press Backwash–Chemical Precipitation–Gaseous Membrane Absorption” Method applied sciences Article Green Treatment of Cyanide Tailings Using a “Filter Press BackWash–Chemical Precipitation–Gaseous Membrane Absorption” Method Jingmin Yan 1,2, Yanhua Wang 1,2, Yubo Tu 3, Peiwei Han 1,4, Xiang Liu 1,2 and Shufeng Ye 1,4,* 1 State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; [email protected] (J.Y.); [email protected] (Y.W.); [email protected] (P.H.); [email protected] (X.L.) 2 School of Chemistry and Chemical Engineering of University of Chinese Academy of Sciences, Beijing 100049, China 3 State Key Laboratory of Solid Waste Reuse for Building Materials, Beijing Building Materials Academy of Sciences Research, 100041 Beijing, China; [email protected] 4 Innovation Academy for Green Manufacture, Chinese Academy of Sciences, Beijing 100190, China * Correspondence: [email protected]; Tel.: +86-13911828468 Abstract: Based on a “filter press backwash–chemical precipitation–gaseous membrane absorption” process, treatment of harmless cyanide tailings was conducted using cyanide tailings from a gold smelting enterprises (Yunnan Province, China) as the research object. The effects of air-drying time, backwash water parameters, initial pH of acidification, NaHS dosage, cyanide-containing water flow rate, and gaseous membrane stages on the process were investigated. Chemical composition, X-ray diffraction, and X-ray photoelectron spectroscopy analyses of the copper products were carried Citation: Yan, J.; Wang, Y.; Tu, Y.; out. Results showed that the copper content in the copper product was 54.56%, and the chemical Han, P.; Liu, X.; Ye, S. Green Treatment of Cyanide Tailings Using composition was mainly CuSCN, CuS, Cu2S, and CaSO4. Five cycles of experiments were carried out a “Filter Press BackWash–Chemical under optimal conditions; the results showed that the process can make the treated cyanide tailings Precipitation–Gaseous Membrane meet the requirements of the technical specification for pollution control of cyanide leaching residue Absorption” Method. Appl. Sci. 2021, in the gold industry (TSPC) standard for storage in a tailings pond and a have certain stability. The 11, 2091. https://doi.org/10.3390/ average recovery rate of copper and total cyanide in elution water was 97.8% and 99.89%, respectively, app11052091 and the average removal rate of thiocyanate was 94.09%. Academic Editor: Bart Van Keywords: cyanide tailings; harmless; gaseous membrane; wastewater der Bruggen Received: 27 January 2021 Accepted: 14 February 2021 1. Introduction Published: 26 February 2021 The cyanide gold extraction method was proposed by the British scientist MacArthur Publisher’s Note: MDPI stays neutral in 1890 [1]. It is a gold extraction process that first dissolves the gold in the ore with a dilute with regard to jurisdictional claims in cyanide solution, replaces it with zinc powder, and then smelts it into gold ingots [2]. The published maps and institutional affil- cyanide extraction process inevitably produces a large amount of cyanide-containing waste, iations. and the annual discharge of cyanide tailings of China’s gold industry exceeds 24.5 million tons. The technical specification for pollution controls of cyanide leaching residue in the gold industry (TSPC) was not promulgated in China until 1 August 2018 [3]. The cyanide tailings produced in the gold industry are mainly disposed of in open-air stockpiling and tailings pond storage, which not only occupies a large amount of land, but also affects Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the environment and sustainable development of the mining area [4,5]. Cyanide tailings This article is an open access article are also a secondary resource, containing a large number of valuable metals such as gold, distributed under the terms and silver, copper, iron, sulfur, and so on [6,7]. However, if valuable resources are directly conditions of the Creative Commons recovered, the residual cyanide in the cyanide tailings will not only harm the health of Attribution (CC BY) license (https:// the workers, but also affect the recovery rate of the valuable metal resources. Therefore, creativecommons.org/licenses/by/ harmless treatment of cyanide tailings is not only beneficial to environmental protection, 4.0/). but also beneficial to the recovery of valuable resources. Appl. Sci. 2021, 11, 2091. https://doi.org/10.3390/app11052091 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 2091 2 of 16 Under the current call of energy conservation, emission reduction, and environmental protection, it is of great practical significance to study the cyanide treatment method in the cyanide tailings of gold enterprises [8]. At present, cyanide treatment technologies mainly include electrochemical oxidation, chemical oxidation, adsorption, biological methods, etc. [7,9–12]. These methods mainly convert cyanide into non-toxic, harmless, and recy- clable substances through chemical or physical reactions, and then the treated harmless tailings and water are recycled to reduce the harm of cyanide to the environment. Chen et al. [10] used electrochemical oxidation to oxidize cyanide in the cyanide tailings pulp to CO2 and N2, and the metal cations were reduced and deposited at the cathode. The removal rates of total cyanide (TCN), free cyanide (CN−), Cu, and Fe were 80.17%, 84.91%, 84.10%, and 90.91%, respectively. Hou et al. [11] found that the cyanide tailings could meet the backfilling requirements of TSPC after being co-treated with sodium metabisulfite and hydrogen peroxide. The best conditions for the decomposition of cyanide from cyanide tailings are treatment in 0.5 g/L Na2S2O5 at pH 10 for 3 h and then 2 mL/L H2O2 is added to the tailings at pH 9 for 4 h. Bahrami et al. [12] utilized gilsonite to adsorb cyanide in cyanide wastewater; a maximum adsorption of 61.64% was obtained in the size range of −1 + 0.5 and −2 + 1 mm of gilsonite. However, these methods have some disadvantages, such as a high consumption of ingredients, harsh operating conditions, difficulty achieving industrial application, no recovery of cyanide in cyanide tailings, and so on. In this study, green treatment of cyanide tailings with the “filter press backwash– chemical precipitation–gaseous membrane absorption” method results in the treated cyanide tailings meeting the requirements of TSPC to enter the tailing pond for stor- age. In this process, a filter press with a reverse washing function is used to realize the harmless treatment of cyanide tailing pulp, the copper element in elution water is recovered by acidification precipitation technology combined with vulcanization precipitation tech- nology, the cyanide in eluent water is recovered by a hollow fiber hydrophobic membrane, and the water after membrane treatment is returned to the pressure filter backwash process as backwash water to realize the recycling of water in the system. The process not only provides an efficient and environmentally friendly method for harmless cyanide tailings but also recovers a certain amount of copper and cyanide during the treatment process, realizing the water recycling in the system without the generation of secondary pollutants. Furthermore, there is also an opportunity for the subsequent recovery of valuable metals from the treated cyanide tailings. 2. Materials and Methods 2.1. Materials and Equipment The object processed in this study is cyanide tailings slurry with a slurry concentration of 38%, obtained from a mining company in Yunnan. The experimental sulfuric acid was industrial grade 98% concentrated sulfuric acid from the plant. The industrial grade (70%) sodium hydrosulfide was provided by Nantong Ruijia Chemical Co., LTD., (Nantong, China). The 99% flake sodium hydroxide was provided by Wujiang Xianglong Chemical Co., LTD., (Suzhou, China). The experimental equipment included a CJWA-5/4/30 countercurrent washing ma- chine; 1 m3 PPH reaction tank with a stirring device; F500 × 2500 lye spray system; 2 m3 diaphragm plate and frame filter press; and 5-um-filter-hole precision filter. The model number of the gaseous membrane assembly is ETN-6X28 (including two hollow fiber gaseous membranes, cyanide-containing water storage tank, lye storage tank, lye pump and cyanide-containing water pump, and two security filters). 2.2. Experimental Set-Up and Operation 2.2.1. Filter Press Backwash Process The whole process of the “filter press backwash–chemical precipitation–gaseous mem- brane absorption” method is illustrated in Figure1. The cyanide tailings were transported to the mixing barrel in front of the filter press in the form of slurry and were driven into Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 16 Appl. Sci. 2021, 11, 2091 The whole process of the “filter press backwash–chemical precipitation–gaseous3 of 16 membrane absorption” method is illustrated in Figure 1. The cyanide tailings were trans- ported to the mixing barrel in front of the filter press in the form of slurry and were driven into the countercurrent washing machine through the slurry pump. The feeding pressure the countercurrent washing machine through the slurry pump. The feeding pressure was was about 0.6 MPa. When there was no obvious filtrate outflow in the filtrate pipeline, the about 0.6 MPa. When there was no obvious filtrate outflow in the filtrate pipeline, the filter filter chamber was judged to be basically full. The feeding time was 5–10 min. After the chamber was judged to be basically full. The feeding time was 5–10 min. After the feeding feeding was complete, an air compressor blew air for 0.5–3 min (primary air-drying), and was complete, an air compressor blew air for 0.5–3 min (primary air-drying), and the the filter press filtrate and the primary air-dried blow-off liquid were returned to the cya- filter press filtrate and the primary air-dried blow-off liquid were returned to the cyanide nide process together as the filter barren solution. Then, the backwash pump was turned process together as the filter barren solution.
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