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 storage. 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 technology, 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 machine; 1 m3 PPH reaction tank with a stirring device; Φ500 × 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 membrane 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 transported 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. Then, the backwash pump was turned on, the on,backwash the backwash water parameterswater parameters were adjusted, were adjusted, and a backwash and a backwash pressure pressure of 0.8 MPa of was0.8 MPa used wasfor backwashingused for backwashing the cyanide the tailings. cyanide After tailings. washing, After secondarywashing, secondary air-drying air-drying was conducted was conductedfor 3–25 min. for 3–25 After min. air-drying, After air-drying, the filter the cake filter was cake removed was removed and a leaching and a leaching toxicity tox- test icitycarried; test carried; the washing the washing solution solution and the and secondary the secondary air-dried air-dried blowout blowout solution solution were were used usedas eluent as eluent to enter to enter the the chemical chemical treatment treatmen process.t process. The The influences influences of of primary primary air-drying air-dry- ingtime, time, secondary secondary air-drying air-drying time, time, and and backwash backwash water water parameters parameters (pH (pH value, value, total total cyanide cy- anidecontent, content, multiple multiple (ratio of(ratio the of amount the amount of backwash of backwash water to water the mass to the of mass primary of primary air-dried air-driedcyanide tailings))cyanide tailings)) on the harmless on the harmless effect of cyanideeffect of tailings cyanide were tailings studied were in studied the filter in press the filterbackwash press backwash process. process.

FigureFigure 1. 1. TheThe process process of of the the “filter “ﬁlter press press backwash–chemical backwash–chemical precipitation–gaseous precipitation–gaseous membrane membrane absorption” absorption” method. method.

2.2.2.2.2.2. Chemical Chemical Treatment Treatment Process Process TheThe elution elution water water obtained obtained in in the the filter filter pr pressess backwash backwash process process was was pumped pumped into into the the chemicalchemical reaction reaction tank tank in in which which sulfuric sulfuric ac acidid was was added added to to preliminary preliminary sink sink the the copper, copper, andand after after acidifying acidifying the the sink, sink, sodium sodium hydrosulfide hydrosulfide was was added added to to deeply deeply sink sink the the copper. copper. AfterAfter the the copper copper was was deposited, deposited, the the solution solution was was pressed pressed by by the the diaphragm diaphragm plate plate and and frameframe filter filter press, press, and and the the filter filter cake cake was was a a copper copper product. product. The The filtrate filtrate was was filtered filtered by by a a precisionprecision filter filter and and pumped pumped into into the the cyanidecyanide-containingcontaining water water storage storage tank tank of of the the gaseous gaseous membrane module. The chemical reaction tank was a closed device connected to the alkali membrane module. The chemical reaction tank was a closed device connected to the alkali spray system through negative pressure, and the spilled HCN in the chemical treatment spray system through negative pressure, and the spilled HCN in the chemical treatment process was absorbed by the alkali liquor. In the chemical treatment process, the influence process was absorbed by the alkali liquor. In the chemical treatment process, the influence of the initial pH value of the acidification reaction and the amount of sodium hydrosulfide of the initial pH value of the acidification reaction and the amount of sodium hydrosulfide on the copper precipitation effect was studied. on the copper precipitation effect was studied. 2.2.3. Membrane Treatment Process 2.2.3. Membrane Treatment Process The cyanide-containing water was pumped into the outside of the hollow fiber gaseous membrane,The cyanide-containing and the NaOH solution water was was pumped intointo thethe insideoutside of of the the hollow hollow fiber fiber gaseous gas- eousmembrane. membrane, The twoand flowedthe NaOH in opposite solution directions. was pumped There into were the twoinside security of the filtershollow in fiber front gaseousof the membrane membrane. to The prevent two flowed debris in from opposite blocking directions. the membrane There were hole. two Because security hollowfilters infiber front has of “breathable the membrane and impermeable”to prevent debris characteristic, from blocking the liquidsthe membrane on both sideshole. cannotBecause be hollowmutually fiber soluble has “breathable [13]. HCN inand cyanide-containing impermeable” characteristic, water passed the through liquids theon microporesboth sides cannotof the hollowbe mutually fiber membranesoluble [13]. in HCN gaseous in cy formanide-containing and was absorbed water by passed NaOH through to generate the microporesNaCN, thereby of the reducing hollow fiber the cyanide membrane content in gaseous in cyanide-containing form and was absorbed water [14 by]. NaOH to generate NaCN, thereby reducing the cyanide content in cyanide-containing water [14]. 2.3. Analysis and Characterization The total metal ion contents were measured by inductively coupled plasma-atomic emission spectrometry (ICP-OES, Optima 8000). XRD (MPDDY2094, PANalytical, Nether- Appl. Sci. 2021, 11, 2091 4 of 16

lands) with Cu Ka radiation (l = 0.15418 nm) was conducted with a scanning rate of 5◦ min−1 from 5◦ to 90◦. Copper products were characterized by X-ray photoelectron spectroscopy (XPS) for analysis of chemical and electronic properties. The pH value was detected by a pH meter (pHs-3E, Leici, China). A leaching toxic solution of tailings was pre- pared according to HJ/T 299-2007 “Solid Waste Leaching Toxic Leaching Method—Sulfuric acid nitric acid Method” [15]. The total cyanide in the water sample and leaching toxic solution was analyzed by a colorimetric method.

3. Results 3.1. Raw Material Analysis The current cyanide tailing slurry treatment method is to use a common plate and frame filter press to filter the cyanide tailing slurry. The cyanide tailings after the filter press are directly stored in the tailing pond, and the filtrate is returned to the cyanide gold extraction system. The actual research goal aims to reduce the toxicity of the cyanide tailings before washing to meet the standards for entering the tailings pond in the TSPC. Therefore, the chemical composition and leaching toxicity identification of the cyanide tailings after air-drying was carried out, and the element analysis of the filter barren solution was carried out. The cyanide tailings after primary air-drying were dried at 105 ◦C, and their chemical composition was determined; the results are presented in Table1 . According to the mass ratio of samples before and after drying, the moisture content of cyanide tailings was 20.32%. The standard for cyanide tailings in TSPC to enter the tailings pond for storage [3,16], the elemental analysis of the leached toxic solution of cyanide tailings (after primary air-drying), and the filter barren solution are shown in Table2.

Table 1. Chemical composition analysis of cyaniding tailings after primary air-drying.

Element Au * Ag * Cu Pb Zn Fe As Cd Cr Hg Moisture Content (%) 0.23 38.55 0.372 0.984 0.234 30.16 0.15 0.001 0.006 <0.001 20.32 *: The unit is g/t.

Table 2. Storage standard of cyanide tailings and element content table of cyanide tailing slurry.

Element TCN CN− Cu Fe Pb Zn As Hg Cd Cr Cr(VI) Moisture Standard (mg/L) 5 4 120 120 1.2 120 1.2 0.12 0.6 15 6 22% Leached toxic solution of 53.3 3.9 34.7 0.3 0.01 0.03 <0.01 <0.01 <0.01 0.016 <0.01 20.32% cyanide tailing (mg/L) Filter barren solution (mg/L) 2168 468 1614 0.01 0.03 0.02 0.03 0.01 0.01 0.02 0.01 -

It can be seen from Table2 that the total cyanide content in the toxic liquid extracted from the cyanide tailings after primary air-drying was 53.3 mg/L, the free cyanide content was 3.9 mg/L, the Cu content was 34.7 mg/L, and the Fe content was 0.3 mg/L. The results show that the main reason for the substandard leaching toxicity of the cyanide tailings was the copper–cyanide complex in the cyanide tailings. It can be seen from Table1 that the moisture content of the cyanide tailings after primary air-drying was 20.32%, the Fe content and Cu content in the dry base of cyanidation tailings were 30.16% and 0.372%, respectively, while the Fe content in the toxic leaching solution was only 0.3 mg/L. Combining the content of total cyanide, free cyanide, and Cu and Fe ions in the ﬁlter barren solution in Table2, it can be seen that the main reason for the excessive toxicity of the cyanide tailings was the copper–cyanide complex in the cyanide tailings. And the copper–cyanide complex was mainly distributed in the moisture of cyanide tailings. Therefore, the idea of this research is to replace the remaining high-concentration cyanide-containing water with low-concentration cyanide-containing water in the cyanide tailings after primary air-drying to reduce the toxicity of the cyanidation tailings and make them reach the standard for entering the tailings pond. The elution water undergoes chemical treatment and membrane Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 16

the moisture content of the cyanide tailings after primary air-drying was 20.32%, the Fe content and Cu content in the dry base of cyanidation tailings were 30.16% and 0.372%, respectively, while the Fe content in the toxic leaching solution was only 0.3 mg/L. Com- bining the content of total cyanide, free cyanide, and Cu and Fe ions in the filter barren solution in Table 2, it can be seen that the main reason for the excessive toxicity of the cyanide tailings was the copper–cyanide complex in the cyanide tailings. And the copper– cyanide complex was mainly distributed in the moisture of cyanide tailings. Therefore, the idea of this research is to replace the remaining high-concentration cyanide-containing Appl. Sci. 2021, 11, 2091 water with low-concentration cyanide-containing water in the cyanide tailings after5 ofpri- 16 mary air-drying to reduce the toxicity of the cyanidation tailings and make them reach the standard for entering the tailings pond. The elution water undergoes chemical treatment and membrane treatment to recover copper and cyanide and then returns to the filter presstreatment backwash to recover process copper to realize and the cyanide recycling and thenof the returns backwash to the water. filter press backwash process to realize the recycling of the backwash water. 3.2. Filter Press Backwash Process 3.2. Filter Press Backwash Process 3.2.1.3.2.1. The The Effect Effect of of the the Primary Primary Air-Drying Air-Drying Time Time InIn order order to to explore explore the the influence influence of of the the pr primaryimary air-drying air-drying time time on on the the filter filter press backwashbackwash effect, effect, the the primary primary air-drying air-drying time was tested under the conditions of 0.8 times thethe back back washing washing water, water, pH pH 9 9 of of the the back back washing washing water, water, 10 10 min min of of the the secondary secondary air- air- dryingdrying time, time, and and 1.17 1.17 mg/L mg/L of of the the total total cyanide cyanide concentration concentration of of the the back back washing washing water. water. FigureFigure 22 showsshows thethe leachingleaching toxicity toxicity results results of of backwashed backwashed cyanide cyanide tailings tailings under under different differ- entprimary primary air-drying air-drying times. times. It It can can be be seen seen from from Figure Figure2 2that that when when the the backwash backwash waterwater multiplemultiple was was 0.8, 0.8, the the leaching leaching toxicity toxicity of of the the backwashed backwashed cyanide cyanide tailings tailings was was greater greater than than 55 mg/L mg/L when when the the primary primary air-drying timetime waswas lessless thanthan 0.73 0.73 min min or or more more than than 2.55 2.55 min. min. If Ifthe the primary primary air-drying air-drying time time was was too too short short under under thethe samesame conditions,conditions, then the moisture contentcontent of of the the filter filter cake before backwash backwashinging was higher, and more high-concentration cyanide-containingcyanide-containing water water needed needed to to be be replac replaced,ed, resulting resulting in in a a high cyanide content of backwashedbackwashed cyanide cyanide tailings. tailings. The The filter filter cake cake easily easily formed formed cracks cracks when when the the primary primary air- air- dryingdrying time time exceeded exceeded 1.5 1.5 min, min, which which will will cause cause the the backwash wate waterr to pass through the gapgap preferentially preferentially during during backwashing, thus thus losing the backwash effect. Therefore, the bestbest time time for for primary primary air-drying air-drying was 1.5 min.

FigureFigure 2. 2. EffectEffect of of primary primary air-drying air-drying time time on on the the leac leachinghing toxicity of backwashed cyanide tailings. 3.2.2. The Effect of Multiple Backwashing Conditions

Figure3 illustrates the leaching toxicity results of the backwashed cyanide tailings under different backwash conditions. The experiments were carried out under the following conditions: pH 9 of the backwash water, 1.17 mg/L of the total cyanide concentration of the backwash water, 1.5 min of the primary air-drying time, and 10 min of the secondary air-drying time. As observed in Figure3, the leaching toxicity of backwashed cyanide tailings decreased with the increase in backwashing multiples. When the backwash water multiple increased from 0.5 to 1, the concentration of TCN in the leached toxic solution from backwashed cyanide tailings decreased from 6.8 mg/L to 0.374 mg/L, and the concentration of Cu also decreased from 6.09 to 0.15 mg/L. When the backwash water multiples were 0.5 and 0.6, the total cyanide concentration in the toxic liquid leached from the backwashed cyanide tailings was 6.8 mg/L and 5.26 mg/L, respectively, both of which are higher than 5 mg/L. In order to ensure that the leaching toxicity of the backwashed cyanide tailings Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 16

3.2.2. The Effect of Multiple Backwashing Conditions Figure 3 illustrates the leaching toxicity results of the backwashed cyanide tailings under different backwash conditions. The experiments were carried out under the following conditions: pH 9 of the backwash water, 1.17 mg/L of the total cyanide concentration of the backwash water, 1.5 min of the primary air-drying time, and 10 min of the secondary air-drying time. As observed in Figure 3, the leaching toxicity of backwashed cyanide tailings decreased with the increase in backwashing multiples. When the backwash water multiple increased from 0.5 to 1, the concentration of TCN in the leached toxic solution from backwashed cyanide tailings decreased from 6.8 mg/L to 0.374 mg/L, and the concentration of Cu also decreased from 6.09 to 0.15 mg/L. When the backwash water multi- Appl. Sci. 2021, 11, 2091 ples were 0.5 and 0.6, the total cyanide concentration in the toxic liquid leached from6 ofthe 16 backwashed cyanide tailings was 6.8 mg/L and 5.26 mg/L, respectively, both of which are higher than 5 mg/L. In order to ensure that the leaching toxicity of the backwashed cyanide tailings meets the technical requirements for the disposal of cyanide tailings in the TSPCmeets and the technicalto reduce requirements the water consumption, for the disposal the ofoptimal cyanide multiple tailings of in backwash the TSPC andwater to shouldreduce be the 0.7. water consumption, the optimal multiple of backwash water should be 0.7.

FigureFigure 3. 3. EffectEffect of of backwash water multiples onon thethe leachingleaching toxicity toxicity of of backwashed backwashed cyanide cyanide tailings. tailings. 3.2.3. The Effect of the pH Value of Backwash Water 3.2.3. TheThe leachingEffect of the toxicity pH Value results of Backwash of backwashed Water cyanide tailings under different pH conditionsThe leaching of the backwashtoxicity results water of are backwashed shown in Figurecyanide4a, tailings and the under corresponding different pH elution con- ditionswater components of the backwash are given water in are Figure shown4b. Ain set Figure of runs 4a, wasand conductedthe corresponding under the elution following wa- terconditions: components 0.7 timesare given the in backwash Figure 4b. water, A set 1.17 of runs mg/L was of conducted total cyanide under in thethe backwashfollowing conditions:water, 1.5 min 0.7 times of primary the backwash air-drying, water, and 1.17 10 minmg/L of of secondary total cyanide air-drying. in the backwash Referring wa- to ter,Figure 1.5 4min, when of primary the pH air-drying, value of backwash and 10 min water of secondary was 1 and air-drying. 1.5, the leaching Referring toxicity to Figure of 4,backwashed when the pH cyanide value tailings of backwash exceeded water the standard, was 1 and while 1.5, when the leaching the pH value toxicity of backwash of back- washedwater was cyanide greater tailings than 2,exceeded the leaching the standard, toxicity of while backwashed when the tailings pH value met theof backwash expected waterrequirements. was greater It is than seen 2, that the when leaching the pHtoxici wasty 1of and backwashed 1.5 (see Figure tailings4b), met the totalthe expected cyanide requirements.and copper concentrations It is seen that of when the corresponding the pH was 1 elution and 1.5 water (see Figure were lower, 4b), the and total as the cyanide pH of andthe backwashcopper concentrations water continued of the to corresponding increase, total cyanideelution water and copper were lower, concentrations and as the in pH the ofeluent the backwash remained water stable continued after increasing to increase to a certain, total cyanide extent. When and copper the pH concentrations of the backwash in thewater eluent was remained less than 2, stable the acidity after ofincreasing the backwash to a certain water wasextent. too high,When and theit pH reacted of the directly back- washwith thewater copper–cyanide was less than 2, complex the acidity in the of the cyanide backwash tailings water during was backwashing,too high, and it resulting reacted directlyin a low with copper the contentcopper–cyanide in the elution complex water in andthe cyanide obvious tailings turbidity during of the backwashing, efﬂuent [17]. resultingThere was in an a low off-white copper precipitate, content in the which elution was water a mixture and ofobvious CaSO 4turbidity, CuCN, etc.of the The effluent lower content of copper ions in the elution water indicates that part of copper sediment was left in the backwashed cyanide tailings; therefore, the leaching toxicity of backwashed cyanide tailings could not meet the technical requirements for the disposal of cyanide tailings in the TSPC. The copper content in the elution water increased with the increase in the pH value of the backwash water, and the leaching toxicity of the backwashed cyanide tailings decreased. Therefore, the pH value of the backwash water should not be lower than 2.

3.2.4. Effect of the Total Cyanide Content in Backwash Water Under today’s environmental protection requirements, the production water in a factory is prohibited from being discharged. If clean water is utilized in the press ﬁlter washing process of cyanide tailings, it will inevitably cause the expansion and waste of water resources in the plant area. Therefore, the cyanide tailings backwashing process needs to use treated cyanide-containing water as backwash water. It is necessary to study the effect of the TCN content in the backwash water on the ﬁlter press backwash effect. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 16

[17]. There was an off-white precipitate, which was a mixture of CaSO4, CuCN, etc. The lower content of copper ions in the elution water indicates that part of copper sediment was left in the backwashed cyanide tailings; therefore, the leaching toxicity of backwashed cyanide tailings could not meet the technical requirements for the disposal of cyanide tailings in the TSPC. The copper content in the elution water increased with the increase in the pH value of the backwash water, and the leaching toxicity of the backwashed cyanide Appl. Sci. 2021, 11, 2091 7 of 16 tailings decreased. Therefore, the pH value of the backwash water should not be lower than 2.

FigureFigure 4. 4. EffectEffect of ofthe the pH pH value value of the of thebackwash backwash wate waterr on the on filter the ﬁlterpress press backwash backwash process: process: (a) leaching (a) leaching toxicity toxicity of back- of washedbackwashed cyanide cyanide tailings; tailings; (b) element (b) element content content of elution of elution water. water.

3.2.4. EffectThe conditions of the Total of Cyanide the TCN Content content in in Backwash the backwash Water water were tested under the followingUnder conditions:today’s environmental 0.7 times the protection amount requ of backwashirements, water, the production the pH of water the backwash in a fac- torywater is prohibited at 9, primary from air-drying being discharged. time for 1.5 If min,clean and water secondary is utilized air-drying in the press time filter for 10wash- min. ingThe process leaching of cyanide toxicity tailings, of backwashed it will inev cyanideitably tailings cause the under expansion different and conditions waste of water of the resourcesTCN content in the of plant backwash area. Therefore, water is shown the cy inanide Figure tailings5. Figure backwashing5 shows that process the leachingneeds to usetoxicity treated of cyanide-containing backwashed cyanide water tailings as backwash increased water. with It the is necessary increase in to the study TCN the content effect ofin the the TCN backwash content water. in the backwash Under the wate samer on experimental the filter press conditions, backwash the effect. higher the TCN contentThe inconditions the backwash of the water,TCN content the higher in the the backwash cyanide content water were in the tested water under content the of fol- the backwashed cyanide tailings. In order to make the TCN content in the leached toxic liquid Appl. Sci. 2021, 11, x FOR PEER REVIEWlowing conditions: 0.7 times the amount of backwash water, the pH of the backwash water8 of 16 atof 9, the primary backwashed air-drying cyanide time tailingsfor 1.5 min, less thanand secondary 5 mg/L, the air-drying TCN content time infor the 10 backwashmin. The leachingwater should toxicity be of less backwashed than 21.75 mg/L.cyanide tailings under different conditions of the TCN content of backwash water is shown in Figure 5. Figure 5 shows that the leaching toxicity of backwashed cyanide tailings increased with the increase in the TCN content in the backwash water. Under the same experimental conditions, the higher the TCN content in the backwash water, the higher the cyanide content in the water content of the backwashed cyanide tailings. In order to make the TCN content in the leached toxic liquid of the backwashed cyanide tailings less than 5 mg/L, the TCN content in the backwash water should be less than 21.75 mg/L.

FigureFigure 5. 5. EffectEffect of of the the TCN TCN content content in in the the backwash backwash wa waterter on on the the leaching leaching toxicity toxicity of of backwashed backwashed cyanidecyanide tailings. tailings.

3.2.5.3.2.5. The The Effect Effect of of the the Secondary Secondary Air-Drying Air-Drying Time ToTo better better understand understand the the influence inﬂuence of the of the seco secondaryndary air-drying air-drying time timeon the on filter the press ﬁlter backwashpress backwash effect, effect,the secondary the secondary air-drying air-drying time timeof cyanide of cyanide tailings tailings was wastested tested under under the followingthe following conditions: conditions: the backwash the backwash water dosage water dosage was 0.7 wastimes, 0.7 the times, pH value the pH of the value back- of wash water was 9, the total cyanide concentration of the backwash water was 1.17 mg/L, and the primary air-drying time was 1.5 min. The leaching toxicity results of backwashed cyanide tailings under different secondary air-drying times are shown in Figure 6. As discussed for Figure 6, the leaching toxicity of backwashed cyanide tailings decreased first and then stabilized with the increase in the secondary air-drying time. When the secondary air-drying time was 5 min and 10 min, the total cyanide content in the leached toxic solution of the backwashed cyanide tailings was 5.42 mg/L and 2.29 mg/L, respectively. After the secondary air-drying time exceeded 10 min, leaching toxicity was not signifi- cantly reduced, indicating that the moisture content of cyanide tailings decreased slightly after more than 10 min. Therefore, the secondary air-drying time is preferably 10 min.

Figure 6. Effect of secondary air-drying time on the leaching toxicity of backwashed cyanide tailings. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 16

Figure 5. Effect of the TCN content in the backwash water on the leaching toxicity of backwashed cyanide tailings.

Appl. Sci. 2021, 11, 2091 3.2.5. The Effect of the Secondary Air-Drying Time 8 of 16 To better understand the influence of the secondary air-drying time on the filter press backwash effect, the secondary air-drying time of cyanide tailings was tested under the followingthe backwash conditions: water wasthe backwash 9, the total water cyanide dosage concentration was 0.7 times, of thethe pH backwash value of water the back- was wash1.17 mg/L, water was and 9, the the primary total cyanide air-drying concentration time was of 1.5 the min. backwash The leaching water was toxicity 1.17 resultsmg/L, andof backwashed the primary cyanide air-drying tailings time underwas 1.5 different min. The secondary leaching toxicity air-drying results times of arebackwashed shown in cyanideFigure6 .tailings As discussed under different for Figure secondary6, the leaching air-drying toxicity times of backwashedare shown in cyanide Figure 6. tailings As dis- cusseddecreased for Figure ﬁrst and 6, the then leaching stabilized toxicity with of the backwashed increase in cyanide the secondary tailings air-drying decreased time.first andWhen then the stabilized secondary with air-drying the increase time was in the 5 min secondary and 10 min,air-drying the total time. cyanide When content the second- in the aryleached air-drying toxic solution time was of 5 the min backwashed and 10 min, cyanide the total tailings cyanide was content 5.42 mg/L in the and leached 2.29 mg/L, toxic solutionrespectively. of the After backwashed the secondary cyanide air-drying tailings timewas 5.42 exceeded mg/L 10and min, 2.29 leaching mg/L, respectively. toxicity was Afternot signiﬁcantly the secondary reduced, air-drying indicating time that exceeded the moisture 10 min, content leaching of cyanide toxicity tailings was not decreased signifi- cantlyslightly reduced, after more indicating than 10 that min. the Therefore, moisture content the secondary of cyanide air-drying tailings decreased time is preferably slightly after10 min. more than 10 min. Therefore, the secondary air-drying time is preferably 10 min.

FigureFigure 6. EffectEffect of secondary air-drying timetime onon thethe leachingleaching toxicity toxicity of of backwashed backwashed cyanide cyanide tailings. tailings. 3.3. Chemical Treatment Process 3.3.1. The Effect of the Initial pH of Acidification Conditions In order to study the influence of the initial pH value of acidification conditions on the effect of copper precipitation, the initial pH value of elution was adjusted by sulfuric acid, and the addition of sodium hydrosulfide was controlled to be 0 mg/L. The elemental content analysis of the solution after full reaction with different initial pH values is shown in Figure7. Figure7 shows that the acidification treatment caused the volatilization of HCN to a certain extent, resulting in a decrease in the total cyanide concentration of the solution [18]. The acidification process in elution water at room temperature could be described as [19]: 2NaCN + H2SO4 → Na2SO4 + 2HCN (1)

4NaCu(CN)2 + 2H2SO4 → 2Cu2(CN)2 + 2Na2SO4 + 4HCN↑ (2)

Cu2(CN)2 + 2NaSCN → 2CuSCN↓ + 2NaCN (3) 2+ + Ca + H2SO4 → CaSO4↓ + 2H (4) The effect of acidizing copper precipitation increased with the decrease in the initial pH value. When the initial pH of acidification reaction decreased from 8.64 to 1.5, the concentration of copper ions in the filtrate after the reaction decreased from 704.58 mg/L to 139.86 mg/L. When the pH value was reduced to 1.8, the copper ion concentration did not decrease as the pH value decreased, so the initial pH value of the optimal acidification reaction was 1.8. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 16

3.3. Chemical Treatment Process 3.3.1. The Effect of the Initial pH of Acidification Conditions In order to study the influence of the initial pH value of acidification conditions on the effect of copper precipitation, the initial pH value of elution was adjusted by sulfuric acid, and the addition of sodium hydrosulfide was controlled to be 0 mg/L. The elemental content analysis of the solution after full reaction with different initial pH values is shown in Figure 7. Figure 7 shows that the acidification treatment caused the volatilization of HCN to a certain extent, resulting in a decrease in the total cyanide concentration of the solution [18]. The acidification process in elution water at room temperature could be described as [19]:

2NaCN + H2SO4 → Na2SO4 + 2HCN (1)

4NaCu(CN)2 + 2H2SO4 → 2Cu2(CN)2 + 2Na2SO4 + 4HCN↑ (2)

Cu2(CN)2 + 2NaSCN → 2CuSCN↓ + 2NaCN (3)

Ca2+ + H2SO4 → CaSO4↓ + 2H+ (4) The effect of acidizing copper precipitation increased with the decrease in the initial pH value. When the initial pH of acidification reaction decreased from 8.64 to 1.5, the concentration of copper ions in the filtrate after the reaction decreased from 704.58 mg/L Appl. Sci. 2021, 11, 2091 to 139.86 mg/L. When the pH value was reduced to 1.8, the copper ion concentration9 ofdid 16 not decrease as the pH value decreased, so the initial pH value of the optimal acidification reaction was 1.8.

FigureFigure 7. 7. EffectEffect of of initial initial pH pH value value of of the the acidification acidiﬁcation reaction on the copper precipitation effect.

3.3.2.3.3.2. The The Effect Effect of of Sodium Sodium Hydrosulfide Hydrosulfide Dosage AfterAfter the acidification acidification reaction, 140 mg/L copper copper ions ions were were in the solution, which maymay adversely adversely affect affect the effect of subsequent membrane treatment and the service life of thethe membrane. membrane. Therefore, Therefore, in in this this study, study, sodium hydrosulfide hydrosulfide was added for deep copper depositiondeposition to to improve improve the the copper copper recovery recovery rate rate [20]. [20 In]. Inorder order to study to study the theinfluence influence of the of amountthe amount of sodium of sodium hydrosulfide hydrosulfide on the on effect the effectof copper of copper deposition, deposition, the conditional the conditional exper- imentexperiment of the ofamount the amount of sodium of sodium hydrosulfide hydrosulfide was carried was carriedout under out the under condition the condition that the initialthat the pH initial value pH of valuethe acidification of the acidification reaction reactionwas 1.8. wasThe 1.8.vulcanization The vulcanization process processin elution in waterelution at water room attemperature room temperature could be could described be described as [21,22]: as [21,22]: S2− + Cu2+ → CuS↓ (5)

2− 2− 2− 2Cu(CN)3 + 3H2SO4 + S → Cu2S↓ + 6HCN +3SO4 (6) The elemental content analysis of the solution after treatment with different sodium hydrosulfide dosages is shown in Figure8. It can be seen from Figure8 that when the dosage of sodium hydrosulfide was 50 mg/L, the concentration of copper ions in the solution was reduced from 140.3 mg/L to 70 mg/L, and the copper sinking effect was obvious. With the increase in the amount of sodium hydrosulfide, the concentration of copper ions after treatment was maintained at about 10 mg/L, so the appropriate amount of sodium hydrosulfide was 100 mg/L.

3.3.3. Characterization of Copper Products The chemical composition of the copper products is shown in Table3. The chemical analysis showed that copper products were mainly composed of copper (54.56%) and sulfur (16.58%) and a small number of elements such as calcium and iron. Analysis of copper products by X-ray diffraction (see Figure9) revealed that the main components of the copper products were cuprous thiocyanate (CuSCN), copper sulﬁde (CuS), cuprous sulﬁde (Cu2S), and calcium sulfate (CaSO4)[23]. This is consistent with the element analysis results in Table3. Next, surface chemical states of copper products were characterized by X-ray photoelectron spectroscopy (XPS). The XPS survey spectra shown in Figure 10 indicated the presence of the expected elements: copper, sulfur, carbon, nitrogen, and oxygen. The high-resolution Cu 2p spectra in Figure 10b indicated that the asymmetric peaks of + Cu 2p3/2 were overlapped by two peaks, which were assigned to Cu (932.5 eV) and Cu2+ (933.2 eV) [24–26]. Considering Figure 10c, the spectrum in the sulfur 2p region was Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 16

S2− + Cu2+ → CuS↓ (5)

Appl. Sci. 2021, 11, 2091 2− 2− 2− 10 of 16 2Cu(CN)3 + 3H2SO4 + S → Cu2S↓ + 6HCN +3SO4 (6) The elemental content analysis of the solution after treatment with different sodium hydrosulfide dosages is shown in Figure 8. It can be seen from Figure 8 that when the − dosagedominated of sodium by the peakshydrosulfide of SCN was, including 50 mg/L, S 2p the1/2 concentration(164.4 eV) and of S 2pcopper3/2 (163.2 ions eV) in the [25, 26so-], 2− lutionand the was second reduced peak from at 163.2 140.3 eV mg/L suggested to 70 mg/L, some and contribution the copper from sinking S effect[27].The was minor obvi- 2− ous.peak With at 169 the eV increase is likely in the the amount trace of of the sodium oxidized hydrosulfide, sulfur (SO the4 concentration)[24]. The N of 1s copper peak ions(see Figureafter treatment 10d) had was a binding maintained energy at atabout 398.2 10 eV, mg/L, which so probablythe appropriate corresponds amount to of C≡ so-N diumbonds hydrosulfide [23,28]. was 100 mg/L.

FigureFigure 8. 8. EffectEffect of of sodium sodium hydrosulfide hydrosulﬁde dosage on the copper precipitation effect.

3.3.3.Table Characterization 3. Chemical composition of Copper of copper Products products. The chemical composition of the copper products is shown in Table 3. The chemical Element Cu S Ca Fe Pb Zn Ni As Cd Cr Hg Appl. Sci. 2021, 11, x FOR PEER REVIEWanalysis showed that copper products were mainly composed of copper (54.56%) and11 ofsul- 16 Contents (%) 54.56 16.58 1.305 0.56 0.042 0.027 0.014 0.002 0.001 0.001 0.001 fur (16.58%) and a small number of elements such as calcium and iron. Analysis of copper products by X-ray diffraction (see Figure 9) revealed that the main components of the copper products were cuprous thiocyanate (CuSCN), copper sulfide (CuS), cuprous sulfide (Cu2S), and calcium sulfate (CaSO4) [23]. This is consistent with the element analysis results in Table 3. Next, surface chemical states of copper products were characterized by X-ray photoelectron spectroscopy (XPS). The XPS survey spectra shown in Figure 10 indicated the presence of the expected elements: copper, sulfur, carbon, nitrogen, and oxygen. The high-resolution Cu 2p spectra in Figure 10b indicated that the asymmetric peaks of Cu 2p3/2 were overlapped by two peaks, which were assigned to Cu+ (932.5 eV) and Cu2+ (933.2 eV) [24–26]. Considering Figure 10c, the spectrum in the sulfur 2p region was dom- inated by the peaks of SCN−, including S 2p1/2 (164.4 eV) and S 2p3/2 (163.2 eV) [25,26], and the second peak at 163.2 eV suggested some contribution from S2- [27]. The minor peak at 169 eV is likely the trace of the oxidized sulfur (SO42−) [24]. The N 1s peak (see Figure 10d) had a binding energy at 398.2 eV, which probably corresponds to C≡N bonds [23,28].

Table 3. Chemical composition of copper products.

Element Cu S Ca Fe Pb Zn Ni As Cd Cr Hg Contents (%) 54.56 16.58 1.305 0.56 0.042 0.027 0.014 0.002 0.001 0.001 0.001 Figure 9. XRDXRD patterns of copper products.

Figure 10. XPS spectra of copper products: (a) survey; (b) copper; (c) sulfur; (d) nitrogen.

3.4. Membrane Treatment Process 3.4.1. The Influence of Gaseous Membrane Stages The influence of the number of membrane stages on the effect of membrane treatment was studied under the conditions of 0.3 m3/h of cyanide-containing water and 0.6 m3/h of lye side flow. Figure 11 indicates that when the flow rate of cyanine-containing water was 0.3 m3/h, the total cyanide concentration in the solution could be reduced from 421.21 mg/L to 9.26 mg/L through the primary membrane treatment. After a two-stage membrane treatment, the total cyanide concentration was 1.24 mg/L, and with a continued increase in the stage of membrane, the total cyanide concentration changed little. In order to ensure the effect of the filter press backwash, the total cyanide content in the cyanide- containing water should be reduced as much as possible, so the number of membrane stages should be 2. Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 16

Appl. Sci. 2021, 11, 2091 11 of 16 Figure 9. XRD patterns of copper products.

FigureFigure 10. XPS spectraspectra of of copper copper products: products: (a) survey;(a) survey; (b) copper; (b) copper; (c) sulfur; (c) sulfur; (d) nitrogen. (d) nitrogen.

3.4. Membrane Treatment Process 3.4. Membrane Treatment Process 3.4.1. The Influence of Gaseous Membrane Stages 3.4.1. TheThe influence Influence of of the Gaseous number ofMembrane membrane Stages stages on the effect of membrane treatment wasThe studied influence under theof the conditions number of of 0.3 membrane m3/h of cyanide-containing stages on the effect water of membrane and 0.6 m3 /htreatment wasof lye studied side flow. under Figure the 11 conditions indicates thatof 0.3 when m3/h the of flow cyanide-containing rate of cyanine-containing water and water 0.6 m3/h of 3 lyewas side 0.3 flow. m /h, Figure the total 11 indicates cyanide concentration that when the in flow the solution rate of cyanine-containing could be reduced from water was 0.3421.21 m3/h, mg/L the tototal 9.26 cyanide mg/L through concentration the primary in membranethe solution treatment. could be After reduced a two-stage from 421.21 membrane treatment, the total cyanide concentration was 1.24 mg/L, and with a continued mg/L to 9.26 mg/L through the primary membrane treatment. After a two-stage mem- increase in the stage of membrane, the total cyanide concentration changed little. In order braneto ensure treatment, the effect the of total the filter cyanide press concentrat backwash,ion the totalwas cyanide1.24 mg/L, content and inwith the a cyanide- continued in- Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 16 creasecontaining in the water stage should of membrane, be reduced the as total much cyanide as possible, concentration so the number changed of membrane little. In order tostages ensure should the beeffect 2. of the filter press backwash, the total cyanide content in the cyanide- containing water should be reduced as much as possible, so the number of membrane stages should be 2.

FigureFigure 11. 11. EffectEffect of of number number of of membrane membrane stages stages on on the the membrane membrane treatment treatment process: process: ( (aa)) general general view; view; ( (bb)) the the range range of of −55 toto 35 35 mg/L mg/L in in (a ().a ).

3.4.2. The Influence of the Flow Rate of Cyanide-Containing Water The experimental results of the flow rate conditions of cyanide-containing water under the condition of membrane stage 2 are shown in Figure 12. As can be seen from Figure 12, when the flow rate of cyanine-containing water increased from 0.3 m3/h to 0.9 m3/h, the total cyanide content in cyanine-containing water increased from 10 mg/L to 118.66 mg/L after the treatment with the primary membrane and from 2.66 mg/L to 31.15 mg/L after the treatment with the secondary membrane. When the flow rate of the cyanide- containing water was low, the HCN in the cyanide-containing water had sufficient time to pass through the membrane pores in the form of gas to the other side of the membrane and was neutralized and absorbed by the counter-current NaOH solution inside the hollow fiber membrane. In order to ensure the effect of backwashing, the flow rate of cyanide- containing water during gaseous membrane treatment should be 0.3 m3/h.

Figure 12. Effect of the flow rate of cyanide-containing water on the membrane treatment process.

3.5. Results of the Circulation Experiment In order to verify the feasibility of the whole process, the three processes were connected to carry out the circulation experiment. The process flow is shown in Figure 1. The cyanide-containing water was pumped into the chemical reaction tank, sulfuric acid was added to adjust the initial pH of the acidification reaction to 1.8, and after the full reaction, 0.1 g/L of sodium hydrosulfide was added for deep copper precipitation. After copper Appl. Sci. 2021, 11, x FOR PEER REVIEW 12 of 16

Appl. Sci. 2021, 11, 2091 12 of 16 Figure 11. Effect of number of membrane stages on the membrane treatment process: (a) general view; (b) the range of −5 to 35 mg/L in (a).

3.4.2.3.4.2. The The Influence Influence of of the the Flow Flow Rate Rate of of Cyanide-Containing Cyanide-Containing Water TheThe experimental experimental results results of of the the flow flow rate rate conditions conditions of cyanide-containing of cyanide-containing water water un- derunder the thecondition condition of membrane of membrane stage stage 2 are 2shown are shown in Figure in Figure 12. As 12 can. As be canseen be from seen Figure from 3 12,Figure when 12 the, when flow the rate flow of cyanine-co rate of cyanine-containingntaining water increased water increased from 0.3 fromm3/h 0.3to 0.9 m /hm3/h, to 3 the0.9 mtotal/h, cyanide the total content cyanide in contentcyanine-containing in cyanine-containing water increased water increasedfrom 10 mg/L from to 10 118.66 mg/L mg/Lto 118.66 after mg/L the treatment after the with treatment the primary with the membrane primary membraneand from 2.66 and mg/L from to 2.66 31.15 mg/L mg/L to after31.15 the mg/L treatment after the with treatment the secondary with the memb secondaryrane. When membrane. the flow When rate theof the flow cyanide- rate of containingthe cyanide-containing water was low, water the HCN was low, in the the cyanide-containing HCN in the cyanide-containing water had sufficient water time had tosufficient pass through time to the pass membrane through pores the membrane in the form pores of gas in theto the form other of gas side to of the the other membrane side of andthe membranewas neutralized and was and neutralized absorbed by and the absorbed counter-current by the counter-current NaOH solution NaOH inside solutionthe hol- lowinside fiber the membrane. hollow fiber In membrane.order to ensure In order the effect to ensure of backwashing, the effect of the backwashing, flow rate of cyanide- the flow 3 containingrate of cyanide-containing water during gaseous water duringmembrane gaseous treatment membrane should treatment be 0.3 m should3/h. be 0.3 m /h.

FigureFigure 12. 12. EffectEffect of of the the flow ﬂow rate rate of of cyanide-containi cyanide-containingng water on the membrane treatment process.

3.5.3.5. Results Results of of the the Circulation Circulation Experiment Experiment InIn order order to to verify verify the the feasibility feasibility of of the the whole whole process, process, the the three three processes processes were were con- con- nectednected to to carry carry out out the the circulation circulation experiment. experiment. The The process process flow flow is shown is shown in Figure in Figure 1. The1. cyanide-containingThe cyanide-containing water water was pumped was pumped into the into chemical the chemical reaction reaction tank, tank, sulfuric sulfuric acid acidwas addedwas added to adjust to adjust the initial the initialpH of the pH acidification of the acidification reaction reaction to 1.8, and to 1.8,after and the afterfull reaction, the full 0.1reaction, g/L of 0.1 sodium g/L of hydrosulfide sodium hydrosulfide was added was for added deep forcopper deep precipitation. copper precipitation. After copper After copper sinking, the solution was pumped into the plate and frame filter press through a pneumatic diaphragm pump, and the filter residue became copper products. The filtrate was further filtered through a precision filter, and the precision filter filtrate was pumped into the gaseous membrane module through an acid-resistant pump. During membrane treatment, the flow rate of cyanide-containing water was 0.3 m3/h, the flow rate of lye was 0.6 m3/h, and the number of membrane stages was two. After membrane treatment, cyanide-containing water was pumped as backwash water into the backwash tank of the countercurrent washing machine for backwashing the cyanide tailings. In the filter press backwash, the primary air-drying time was 1.5 min, the backwash water multiple was 0.7 times, and the secondary air-drying time was 10 min. The elution water after the filter press and backwashing was returned to the chemical treatment process for the circulation experiment, and the total cycle number was five. During the experiment, the leaching toxicity of the backwashed cyanide tailings was tested, and the elemental content analysis of the leached toxic solution (LTS), elution water, solution after acidification (SA), solution after vulcanization (SV), solution after primary membrane treatment (M1), solution after secondary membrane treatment (M2), and the lye after gaseous membrane treatment (ML) was carried out. The results of the circulation experiment are shown in Table4. Appl. Sci. 2021, 11, 2091 13 of 16

Table 4. Results of the circulation experiment.

TCN CN− SCN− Cu Fe Ca Mg Steps Process pH (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Initial cyanide water 1011 189.95 99.76 824 0.23 395 3.84 9.66 SA 770 - 4.89 66.5 1.21 294 12.89 1.8 SV 765 5.2 3.5 10.1 1.37 231 13.31 1.8 First cycle M1 48.3 5.2 2.85 10.6 4.16 268 15.23 1.8 M2 1.12 5.2 2.85 10.7 4.21 309 15.29 1.87 ML 947 858.71 201.4 3.43 0.082 0.19 0.029 13.42 LTS 1.18 - - 0.09 0.16 51.1 0.35 10.57 Elution water 751 13.01 51.65 610 0.6 739 8.34 6.6 SA 480 13.01 19.12 80.9 5.75 729 15.3 2.19 SV 476 18.21 8.95 9.43 6.76 717 15.5 2.18 Second cycle M1 10.5 - 2.18 11.3 7.32 746 18.5 1.92 M2 0.96 - 2.85 14.7 8.64 767 20.5 1.94 ML 1525 1256.77 135.68 0.07 0.12 0.51 0.007 13.3 LTS 2.05 - - 0.42 0.19 61.4 0.28 10.95 Elution water 674 78.06 56.5 472 0.3 870 6.69 7.31 SA 307 - 6.24 68.5 6.74 700 20.6 1.95 SV 274 - 0.82 9.75 7.19 720 20.4 1.97 Third cycle M1 7.59 - 0.14 10.2 8.87 737 22.7 1.94 M2 0.617 - 1.5 10.4 8.9 742 23.4 1.94 ML 1806.9 1405.8 143.81 0.31 0.11 0.17 0.011 13.35 LTS 1.4 - - 0.41 0.13 51 0.48 10.68 Elution water 683 96.27 38.09 458 0.64 693 6.59 7.85 SA 381 - 4.89 78.3 3.6 574 20.3 2.05 SV 376 - 5.56 12.9 4.01 529 19.5 2.04 Fourth cycle M1 9.3 - 3.53 11.9 6.8 594 22.1 2.01 M2 0.9 - 3.53 12.5 6.97 600 23 2.01 ML 2169.7 1747.5 115.35 0.56 0.19 1.37 0.037 13.34 LTS 1.53 - - 1.07 0.16 76 0.31 10.95 Elution water 825 91.07 45.55 581 0.24 802 6.73 8.43 SA 465 - 4.89 70.86 3.94 679 19.8 2.03 SV 464 - 2.18 10.74 4.52 657 19.8 2.04 M1 23.6 - 3.53 12.37 8.21 694 24.3 2.02 Fifth cycle M2 0.9 - 4.21 13.33 8.44 696 24.7 2.02 ML 2498.32 2077.94 192.6 3.83 0.11 0.13 0.003 13.36 LTS 2.41 - - 1.74 0.14 59.7 0.28 10.7 Elution water 784.96 189.95 38.77 519.82 1.03 821.77 5.78 9.1

As we can see in Table4, after chemical treatment and membrane treatment of cyanide water in the five cycle tests, the total content of cyanide in the permeating liquid was about 2 mg/L, which can meet the requirements of reverse washing of oxidized mineral cyanide residue. The concentration of thiocyanate and copper in the solution could be reduced to less than 5 mg/L and 15 mg/L, respectively, through the chemical reaction process. During the whole cycle, calcium ions increased from 300 mg/L to about 800 mg/L and then stabilized at about 800 mg/L. This may have occurred because in the backwashing process, acidic backwash water reacts with lime or calcium hydroxide in oxidized mineral cyanidation tailings to form calcium sulfate [29,30]. In addition, part of the calcium sulfate precipitation is discharged with the filter cake, and at the same time, the concentration of calcium and sulfate ions in the solution is close to the saturated solubility of calcium sulfate. In addition, the backwashed cyanide tailings in the five cycle experiments met the requirements for storage in a tailing pond in TSPC. Compared with the existing AVR and SART processes, this process adopts the first acidification and then vulcanization for copper precipitation, which had the advantages of good copper precipitation effect, low consumption of sodium hydride sulfide and no secondary wastewater generation [20,31–33]. And the treated acidic water in this process was used as backwash water to wash the alkaline cyanide tailings, so there was no need to add alkali for neutralization, and there is no acid wastewater discharge. What’s more, the process adopts gaseous membrane to absorb HCN, which had high absorption efficiency and low energy consumption compared with the traditional inflatable volatilization reabsorption [17]. In summary, the “filter press Appl. Sci. 2021, 11, 2091 14 of 16

backwash–chemical precipitation–gaseous membrane absorption” method can realize the green disposal of cyanide tailings.

4. Conclusions It is feasible to use the “filter press backwash–chemical precipitation–gaseous membrane absorption” method for green treatment of cyanide tailings. The main conclusions are summarized as follows: 1. The main reason for the excessive toxicity of the cyanide tailings was the high concentration of copper-cyanide complexes in the cyanide tailings. 2. The effect of the filter press backwash was mainly affected by air-drying time and backwash water parameters. To decrease the total cyanide concentration in the leached toxic solution of backwashed cyanide tailings to less than 5 mg/L, the primary air- drying time should be 1.5 min; in addition, the secondary air- drying time should be 10 min, the pH value of the backwash water must be greater than 2, the backwash water multiple must be greater than or equal to 0.7, and the total cyanide concentration in the backwash water must be greater than 21.75 mg/L. 3. In the process of chemical treatment, the copper element in the elution water was recovered by acidification and vulcanization. The optimal reaction conditions were as follows: initial pH value of the acidification reaction was 1.8 and NaHS dosage of 100 mg/L. The copper products were composed of CuSCN, CuS, Cu2S, and CaSO4. 4. The effect of gaseous membrane treatment was mainly affected by the flow rate of cyanide-containing water and the number of membrane stages. The best conditions were that the flow rate of cyanide-containing water was 0.3 m3/h and the number of membrane stages was two. 5. The process circulation experiment was carried out under optimal conditions to verify the feasibility and stability of the process. In the circulation experiment, the leaching toxicity of the backwashed cyanide tailings reached the TSPC standard for storage in a tailings pond. The average recovery rate of copper and total cyanide in elution water was 97.8% and 99.89%, respectively, and the average removal rate of thiocyanate was 94.09%.

Author Contributions: Conceptualization, J.Y. and S.Y.; methodology, J.Y. and Y.W.; validation, Y.W., P.H. and J.Y.; investigation, J.Y. and X.L.; resources, S.Y. and Y.T.; data curation, J.Y.; writing— original draft preparation, J.Y.; writing—review and editing, J.Y. and S.Y.; supervision, P.H.; project administration, Y.W. and X.L.; funding acquisition, S.Y. All authors have read and agreed to the published version of the manuscript. Funding: We are grateful for the Chinese Academy of Science Project (No. KFJ-STS-QYZD-044) and the National Key R&D Program during the 13th Five-year Plan Period (2019YFC1908405) for support of this work. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to project requirements. Acknowledgments: Thanks are extended to the Yunnan Gold Group for the support of experimental raw materials. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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