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Disposal of High-Arsenic Waste Acid by the Stepwise Formation of Gypsum

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Disposal of High-Arsenic Waste Acid by the Stepwise Formation of Gypsum RSC Advances View Article Online PAPER View Journal | View Issue Disposal of high-arsenic waste acid by the stepwise formation of gypsum and scorodite Cite this: RSC Adv., 2020, 10,29 Xianjin Qi, Yongkui Li, Longhua Wei, Fengyan Hao, Xing Zhu,* Yonggang Wei, Kongzhai Li and Hua Wang The typical disposal of high-arsenic waste acid is at the expense of discharging a large quantity of hazardous solid waste, resulting in secondary pollution of arsenic. We propose a modified lime/ferric salt method for high-arsenic waste acid disposal by the stepwise formation of gypsum and scorodite at atmospheric pressure. The sulfuric acid in the high-arsenic waste acid is first removed by calcium carbonate generating gypsum, and then the arsenic in the solution is precipitated in form of scorodite. Gypsum with an arsenic leaching concentration below 5 mg LÀ1 is obtained at a final pH of 0.5 in the calcium carbonate neutralization stage. In the second stage, the optimal conditions including a starting pH of 2.0, an Fe/As ratio of 1.5, a reaction temperature in the range of 80–90 C and a reaction time equal to or longer than 8 hours provide an arsenic removal efficiency of 95.34% by the formation of well-crystallized Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Received 21st August 2019 and environmentally stable scorodite with grain sizes in a range of 1–5 mm. The proposed process offers Accepted 2nd December 2019 a promising and facile solution for the low-cost disposal of high-arsenic waste acid in the nonferrous DOI: 10.1039/c9ra06568g metallurgical industry, which enables an efficient arsenic removal with the good accessibility of chemical rsc.li/rsc-advances reagents and facilities. À L 1 with sulfuric acid concentrations ranged from 10 to 200 g 1. Introduction À L 1, which is the most crucial arsenic waste because of its high – Arsenic is a major contaminant or impurity discharged during arsenic mobility and toxicity.3 5 Even a slight leakage of high- This article is licensed under a the extraction of heavy non-ferrous metals from minerals that arsenic waste acid will cause serious arsenic pollution as well are usually associated with arsenic. Arsenic with different as widely safety and environmental concern.6,7 valence states (+3, +5, and À3) can exist in the forms of arsenic Neutralization and sulfurization processes are typical 1,3 Open Access Article. Published on 23 December 2019. Downloaded 9/26/2021 10:08:23 PM. acid, arsenic-compound salt, arsenic-bearing sulde and processes to treat the high-arsenic waste acid. In neutraliza- arsenic oxide with different mobility in nature.1 The arsenic tion process, arsenic in the waste acid is transformed into solid compounds were mainly formed naturally, including arseno- hazardous waste composed of calcium arsenate/arsenite, pyrite, orpiment, realgar pyrite, galena and chalcopyrite.2 The gypsum and other compounds by using Ca-containing 8 huge and increasing emission of arsenic in the exploitation of compounds (CaO, Ca(OH)2 and CaCO3) as precipitators. arsenic-contained minerals, typically heavy non-ferrous metals When sul des (Na2S and H2S) are used to precipitate the arsenic including Cu, Pb, Zn, Ni, Sn etc., is far greater than the recent to produce arsenic sulde, a large quantity of hazardous arsenic market. China is the largest arsenic discharge country because sulde sludge will be discharged. For those two technologies, of the large-scale nonferrous metallurgical industry. 100 000 arsenic is partially xed in the solid hazardous waste. Although tons of arsenic per year is introduced into the smelting process the mobility of arsenic is largely reduced in solid wastes, the from nonferrous concentrates generating an amount of arsenic high risk of arsenic pollution remains for a disposal site of solid solid waste. Among these, about 50% of arsenic is stabilized in hazardous waste and secondary pollution from these solid slag. 8–23% of arsenic is removed through the dust removal arsenic waste is unavoidable during the transportation and system and anode mud in the electrolysis process, which might storage.9 In addition, the further harmless disposal for those be recycled or disposed. However, the most hazardous and arsenic-bearing hazardous waste is extremely expensive. New worthless arsenic in form of high-arsenic waste acid still technology for arsenic xing is urgently desiderated. Arsenic accounts for 16% of the total arsenic emission. The arsenic xation technology includes physical encapsulation and concentrations for the waste acid usually range from 0.5 to 30 g chemical xation. The former is not suitable for treatment of the liquid arsenic waste.10 The transformation of As(III) and As(V) into arsenic minerals with low solubility and high stability is State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, a feasible technology for high-arsenic waste acid treatment.1 Faculty of Metallurgical and Energy Engineering, Kunming University of Science and The Ca- and Fe-associated compounds are the main minerals Technology, Kunming 650093, China. E-mail: [email protected] This journal is © The Royal Society of Chemistry 2020 RSC Adv.,2020,10,29–42 | 29 View Article Online RSC Advances Paper for arsenic immobilization.11 The formation of calcium arsenate atmospheric synthesis method is used to remove arsenic by the – – in a Ca(II) As(V) H2O system could obviously decrease the formation of scorodite using arsenic-bearing solution obtained 12–15 arsenic mobility. But it remains unstable because of from the Stage-I a er oxidation by H2O2. Most arsenic in waste extremely high solubility in acid condition or exposed to CO2. acid can be removed in the form of scorodite in Stage-II. A Crystalline and amorphous arsenate ferric compound with high conventional lime/ferric salt method could be used to treat arsenic can be formed in the Fe–As(V)–H2O system at pHs ltrate-II from Stage-II to produce clean water in Stage-III. 1,16–20 ranged from 1 to 3. Among them, scorodite (FeAsO4$2H2O) Theoretically, solid sludge produced from Stage-III can be shows a high stability,21,22 a high theoretical arsenic content of identied as a kind of general industrial solid waste due to the up to 32%,23 a less volume of slag and it is easy to separate from low concentrations of contaminants. Besides, gypsum produced the solid–liquid mixture. Scorodite is stable in the mildly acidic in Stage-I and scorodite precipitated in Stage-II are classied as environment, while in a neutral to weakly alkaline environment, a general industrial solid wastes due to their low leaching it can easily decompose and release As into solutions. Its toxicities. Moreover, facilities (reactors and liquid–solid sepa- stability can be intensied when scorodite was encapsulated by rations) and raw material essential for this process are the same polyferric sulfate24,25 and aluminum silicate gel,26 etc. The with the a conventional neutralization method,3,51 which will synthesis can be modied by using a catalyst or additives.27,28 greatly enhance its economy and practicability. The treatment Owing to the above properties, scorodite is recognized as the parameters in rst two stages are optimized to obtain high best arsenic-stabilization mineral and attracts widely arsenic removal efficiency and environmentally friendly attentions.16,17,29–42 precipitates (scorodite and gypsum). The relationship between The synthesis of scorodite is of great importance for arsenic structure of precipitates and reaction conditions are explored. removal rate from a arsenic-bearing solution.6,35 The hydro- The performance of this process is also evaluated in a contin- thermal method was proposed using Fe(III) and As(V) starting uous experiment on the basis of optimal parameters. solution at temperatures higher than 125 C in the pHs range of – 43 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 0.2 1.8 to synthesize scorodite. A erwards, atmospheric synthesis method was carried out to produce crystalline scor- 2. Materials and methods odite from Fe(III) and As(V) solution in the temperature range 2.1. High-arsenic waste acid and reference materials from 80 to 95 C by means of controlling super- High-arsenic waste acid, a hazardous acid wastewater, is mainly 29,30,43–47 saturating. The hydrothermal reaction chemistry and produced by heavy nonferrous (Cu, Pb, Zn, Ni, and Sn, etc.) – characterization of ferric arsenate precipitated from Fe2(SO4)3 smelting plant. The high-arsenic waste acid used in this study – – As2O5 H2SO4 solutions in the temperature range of 150 225 C was obtained from a copper smelting plant equipped with Isa 18 – were studied. A synthesis map of arsenate phases in Fe(II) furnace in southwest China. It was produced from the water- 3À– 2À AsO4 SO4 system was proposed and releasing behavior was washing purication of ue gas from the smelting of copper This article is licensed under a also reported. To improve the feasibility in the application, an concentrate aer ltrated by dust removal system. The high- atmospheric scorodite synthesis process was studied under arsenic waste acid contains a massive arsenic and a small 95 C ventilation with oxygen or air oxidation of Fe(II) in the quantity of Zn, Cu, Pb, Sb, and Cd, as shown in Table 1. The presence of As(V) by Fujita.35,45,47 The well-crystallized and À1 Open Access Article. Published on 23 December 2019. Downloaded 9/26/2021 10:08:23 PM. concentrations of H2SO4 and As are 76.56 and 24.50 g L , environmentally friendly scorodite were obtained through respectively. Analytical grade reagents were used for all reac- a low-degree supersaturation precipitation procedure.
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