Recovery of Zinc and Silver from Zinc Acid-Leaching Residues with Reduction of Their Environmental Impact Using a Novel Water Leaching-Flotation Process

Home , Leaching (metallurgy), Zinc smelting

Article Recovery of Zinc and Silver from Zinc Acid-Leaching Residues with Reduction of Their Environmental Impact Using a Novel Water Leaching-Flotation Process

Yunpeng Du , Xiong Tong *, Xian Xie, Wenjie Zhang, Hanxu Yang and Qiang Song

Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] (Y.D.); [email protected] (X.X.); [email protected] (W.Z.); [email protected] (H.Y.); [email protected] (Q.S.) * Correspondence: [email protected]

Abstract: Zinc-leaching residue (ZLR) is a strongly acidic hazardous waste; it has poor stability, high heavy metal levels, and releases toxic elements into the environment. ZLR has potential as a valuable resource, because it contains elevated levels of zinc and silver. In this paper, the recovery of zinc (Zn) and silver (Ag) from ZLR wastes from zinc hydrometallurgy workshops using water leaching followed by flotation was studied. During water leaching experiments, the zinc and copper recovery rates were 38% and 61%, respectively. Thereafter, various flotation testing parameters were optimized and included grinding time, reagent dosages, pulp density, flotation time, and type of adjuster. Experimental results demonstrated this flotation method successfully recycled Ag and Zn. A froth product containing more than 9256.41 g/t Ag and 12.26% Zn was produced from the ZLR with approximately 80.32% Ag and 42.88% Zn recoveries. The toxicity characteristic leaching Citation: Du, Y.; Tong, X.; Xie, X.; Zhang, W.; Yang, H.; Song, Q. procedure (TCLP) results indicated the water-leaching flotation process not only recycled valuable Recovery of Zinc and Silver from metals such as zinc and silver in zinc-containing hazardous wastes but lowered the hazardous waste Zinc Acid-Leaching Residues with levels to those of general wastes and recycled wastes in an efficient, economical, and environmentally Reduction of Their Environmental friendly way. Impact Using a Novel Water Leaching-Flotation Process. Minerals Keywords: zinc-leaching residue; flotation; water leaching; silver; heavy metals 2021, 11, 586. https://doi.org/ 10.3390/min11060586

Academic Editors: Qicheng Feng, 1. Introduction Dianwen Liu and Andrea Gerson Zinc hydrometallurgy, regardless of whether using traditional wet zinc smelting, high temperature, and high acid leaching or direct oxygen leaching, inevitably produces Received: 15 April 2021 a huge amount of zinc-leaching residue (ZLR) [1,2]. ZLR contains a signiﬁcant amount Accepted: 29 May 2021 Published: 31 May 2021 of valuable metals, such as zinc, silver, indium, lead, and copper [3], which means the zinc-leaching slag has high economic and industrial value and places an emphasis on

Publisher’s Note: MDPI stays neutral developing a method for their recovery [4,5]. Most metals in the zinc-leaching residue with regard to jurisdictional claims in occur as sulfates, and except for lead sulfate, almost all of them leach into rainwater [6,7]. published maps and institutional affil- Heavy metal ions and sulfate ions can enter the surface via rainwater and pollute the iations. environment [8], which must be processed by high-temperature smelting. Heavy metals are difficult to decompose and readily accumulate in living organisms, potentially endangering the health of all organisms in the food chain [9]. Zinc is an important component of heavy metal pollutants, as it pollutes both the soil and water [10]. The massive discharge of zinc-containing liquids and solid wastes not only causes serious environmental pollution Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. but negatively impacts the human body and is classified as a hazardous waste [11,12]. This article is an open access article Therefore, the comprehensive recovery of valuable elements from zinc-leaching residues is distributed under the terms and important both from the environmental and resource conservation standpoints. conditions of the Creative Commons Due to extremely high levels of heavy metals and their harm to the environment, Attribution (CC BY) license (https:// these residues usually undergo treatment using adsorption, stabilization, and vitrification creativecommons.org/licenses/by/ to reduce their toxicity [10,13–16]. Furthermore, other pyrometallurgical and hydrometal- 4.0/). lurgical methods, such as hot acid leaching [17], alkaline leaching [18], caustic leaching [19],

Minerals 2021, 11, 586. https://doi.org/10.3390/min11060586 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 586 2 of 11

and transformation roasting [20,21], have been developed to dispose of ZLR. However, these processes suffer from high costs, elevated pollution levels, and immature technology, as well as low metal recovery rates. These studies rarely focus on the recovery of Ag present in the ZLR. The mineral processing industry currently uses flotation to recover valuable minerals from the associated gangue minerals based on surface physiochemical property differences [22]. This technology is inexpensive, easy to operate, and has a high separation efficiency for fine particles [23]. The recovery of valuable metals from ZLR ores by flotation allows for complete resource utilization and reduced environmental harm. The purpose of this study was to explore a clean and effective method to recover valuable metals from ZLR and avoid potential environmental pollution risks. In this study, the recoveries of Zn and Ag from ZLR samples were conducted by water leaching followed by flotation. Metal sulfates readily separate from the ZLR by water leaching, leaving a detoxification residue. Water leaching optimizes the pulp environment during flotation. The influences of grinding time, (C4H9O)2PSSNH4 concentration, (NaPO3)6 concentration, pulp density, and flotation time on Ag and Zn recoveries were also analyzed. Furthermore, the toxicity characteristic leaching procedure (TCLP) was used to investigate the environmental activity of ZLR and flotation tailings for a short contact time. The findings of this study provided a basis for an environmentally friendly and comprehensive utilization of hazardous ZLR.

2. Materials and Methods 2.1. Minerals and Analysis The highly acidic samples of zinc-leaching residue were obtained from a zinc hydrometallurgy smelter in Yunnan Province, China. The samples were dried, mixed, and reduced for subsequent research. Table1 shows the multielement chemical analysis of the samples and the levels of Zn, Ag, and Fe in the sample. Additionally, the residue also contained toxic heavy metals such as Pb, Cu, and Sn. The mineralogical characterizations of ZLR are described in the Results and Discussion. All aqueous solutions were prepared with distilled water and analytical-grade reagents used for ﬂotation.

Table 1. Main chemical compositions of ZLR (mass fraction, %).

Elements Cu Pb Zn Fe Ag* S In* Sn SiO2 Content (wt.%) 0.27 1.99 3.06 5.21 720.9 12.11 82.6 0.40 29.00 *—g/t.

The chemical compositions in the ZLR samples were analyzed by atomic absorption spectrometry (AAS) using the ﬂame technique (SUPER, TAS-990AFG). The levels of heavy metals in the leaching solution were analyzed by inductively coupled plasma (ICP-OES, Opima 5300DVtype, PerkinElmer Waltham, MA, USA). Scanning electron microscope (MLA) and mineral liberation analyzer (MLA) were used to determine the mineralogical characterization and elemental composition of the ore samples, respectively.

2.2. Water Leaching Water leaching dissolved the metal sulfates formed and detoxified the residue. A 100- g sample of ZLR was weighed and stirred at room temperature using a predetermined leaching time (5-20min) and liquid-to-solid ratio (1:1 to 6:1). Upon reaction completion, the filtered residue was dried and subjected to chemical analyses, and the filtrate was analyzed by ICP. Finally, the filtered residue was returned to the hydrometallurgical zinc process and used for flotation. The metal-leaching rates helped evaluate the water leaching effects and was determined as follows: C γ = 1 × 100% C2 Minerals 2021, 11, 586 3 of 11

where γ is the precious metal (or iron) recovery rate; C1 is the zinc, copper, or iron amount in the leaching liquid; and C2 is the total zinc, copper, or iron amount in the roasted residue.

2.3. Flotation Experiments The grinding process used the XMQ-ϕ240×90 conical ball mill produced by the Wuhan Prospecting Machinery Factory (Wuhan, China). Flotation tests were conducted in a hanging tank flotation machine with 1.5-L, 0.75-L, and 0.5-L cell volumes. The 1.5-L flotation cell was used for rougher flotation and cleaning, and the 0.75- and 0.5-L cells were used for flotation cleanings at an impeller speed of 2000 r·min−1. In each rougher flotation test, a certain amount of deionized water was added to the flotation cell. The regulator, collector, and frother of predetermined dosages were added and the slurry stirred for three minutes. Air was introduced at a rate of 0.3 L/h. After flotation, the concentrate, middling, and tailings were filtered, dried, and weighed, and grades of silver and zinc in the product were obtained through chemical analysis. According to the yield and grade of the product, the final calculation yielded the recovery rates of silver and zinc in the product. All of tests were repeated three times under the same conditions; the test results reported in this paper were the averages.

2.4. Toxicity Characteristic Leaching Procedure The toxicity characteristic leaching procedure (TCLP) of heavy metals in the ZLR and tailings were conducted according to the Solid Waste-Extraction procedure for leaching toxicity—sulfuric acid and nitric acid method (HJ/T299-2007) implemented in 2007, China. Additional experiments determined the changes in pH. The ZLR and tailings samples were crushed and sieved through a 9.5-mm sieve to remove oversized particles. A 50-g sample was weighed and heated at 105 ◦C for at least 12 h to stay dry. A sulfuric acid and nitric acid (mass ratio 2:1) solution was adjusted to a pH = 3.20 ± 0.05 and used as the leaching solution (liquid/solid ratio of 10:1). The flask was sealed and put into an oscillator; the flip oscillation process was kept for 18 h, and the filtrate was collected. The heavy metal-leaching toxicity was calculated after determining their concentrations once the separation concluded. All TCLP experiments were carried out in duplicate.

3. Results and Discussion 3.1. Technological Mineralogy The process mineralogy of ZLR yielded the elemental composition, mineral composition, mineral symbiosis, mineral dissociation degree characteristics, and the occurrence of target elements; it also provided a reference to help formulate a mineral processing technology. According to mineralogical studies, the valuable minerals in the ore mainly included sphalerite, zinc sulfate, lead alum, hematite, chalcopyrite, and cassiterite; the gangue minerals mainly comprised diopside, quartz, anhydrite, gypsum, and biotite. Moreover, the independent silver minerals were native silver and argentite, and the independent zinc minerals were sphalerite and zinc sulfate. Tables2 and3 show the forms of silver in ZLR and the relative amounts of various minerals. Native silver and argentite contained 37.09% and 12.52% of Ag, respectively, with 44.10% embedded in other minerals. Table4 summarizes the release rates of the primary ZLR minerals, as well as MLA and optical microscope analyses. The release degree of elemental silver, silverite and sphalerite was very low, which indicated the dissemination size of those minerals was extremely ﬁne and a complex association and dissemination relationship between these minerals. Microscopic images of the target minerals in the ZLR sample are shown in Figure1. These images show that sphalerite crystals were the main Ag carrier mineral and closely associated with gangue such as quartz, talc, and diopside (i.e., a secondary mineral). Native silver and argentite were irregular granular, foggy, and ﬂocculent and distributed in sphalerite cavities or attached to the sphalerite edge. Minerals 2021, 11, 586 4 of 11

Table 2. Relative amounts (%) of the major ZLR minerals.

Chemical Mineral Content Mineral Chemical Formula Content Formula

Argentite Ag2S 0.015 Magnetite Fe3O4 0.31 Elemental silver Ag 0.037 Pyrite FeS2 0.1 Sphalerite (Zn, Fe)S 2.27 Pyrrhotite Fe1−xS 0.04 Zinc sulfate Zn[SO4]·6H2O 6.54 Arsenopyrite FeAsS 0.08 Chalcopyrite CuFeS2 0.27 Diopside CaMg[Si2O6] 18.28 Copper blue CuS 0.13 Quartz SiO2 19.89 Copper sulfate Cu[SO4]·5H2O 0.38 Anhydrite Ca[SO4] 18.05 Cassiterite SnO2 0.49 Gypsum Ca(SO4) 2H2O 11.13 Galena PbS 0.002 Biotite K{(Mg, Fe)3[AlSi3O10](OH)2} 2.84 Lead alum (Pb, Sr)SO4 3.8 Potassium feldspar (K, Na)[AlSi3O8] 2.08 Lead oxide PbO 0.01 Pearl mica CaAl4Si2O10(OH)2 1.63 Elemental sulfur S 3.36 Talc Mg3Si4O10(OH)2 1.54 Hematite Fe2O3 6.32 Pyrolusite MnO2 0.2 Calcite Ca[CO3] 0.04 Dolomite CaMg[CO3]2 0.1 Rutile TiO2 0.01 Barite Ba[SO4] 0.06

Table 3. The physical phase composition of silver in the acid-leaching residue.

Phase Composition Ag Ag2O Ag2SO4 Ag2S Wrapped Ag Ag content(g/t) 278.20 30.95 93.90 13.60 330.80 Phase occupation ratio (%) 37.09 4.13 1.81 12.52 44.10

Table 4. Liberation degrees (%) of the primary ZLR minerals.

Mineral Liberation Degree Mineral Liberation Degree Elemental silver 30.68 Copper blue 82.83 Argentite 9.72 Cassiterite 85.9 Sphalerite 52.93 Lead alum 70.31 Chalcopyrite 81.88 Hematite 72.59

3.2. Water Leaching 3.2.1. Effect of Leaching Conditions on Precious Metal Recoveries Water leaching of the ZLR improved the precious metal recovery rates in the ﬂotation process and optimized the pulp environment. The experiments examined the inﬂuence of leaching time and liquid–solid ratio on the leaching rates of zinc, copper, iron, and silver enrichment (shown in Figure2). The leaching times and liquid-to-solid ratios ( Figure2a,b ) had a greater impact on the leaching rates of copper and zinc but less so for iron. It showed that, under the leaching time (15 min) and liquid–solid ratio (4:1), the recovery rate of zinc, copper, and iron was optimal. Under optimal conditions, the ionic concentrations in the leaching solution are shown in Table5. Most of the leaching solution containing zinc sulfate was recycled into the zinc hydrometallurgy process.

Table 5. Ion concentrations in the leaching solution.

Elements Pb (mg/L) Zn (g/L) Ag (mg/L) Fe (g/L) Cu (g/L) In (mg/L) Content 5.12 3.30 0.15 1.34 0.24 4.15

Table 6. Chemical composition of the water-leached residue (mass fraction, %).

Elements Cu Pb Zn Fe Ag * Content Minerals 2021, 11, x FOR PEER REVIEW 0.103 2.90 1.90 4.864 of 822.4 12 (wt.%) *—g/t.

ZLR sample are shown in Figure 1. These images show that sphalerite crystals were the The ﬁlter residue phases were analyzed and showed the residue no longer con- main Ag carrier mineral and closely associated with gangue such as quartz, talc, and di- tainedopside the (i.e., characteristic a secondary diffractionmineral). Native peaks silver of soluble and argentite sulfates were (Figure irregular3). After granular, water leaching,foggy, the and primary flocculent components and distributed remaining in sphalerite in the ﬁltercavities residue or attached were to CaSO the 4sphalerite, CaSO4· 2H2O, andedge. PbSO 4.

Minerals 2021, 11, x FOR PEER REVIEW 5 of 12

Table 3. The physical phase composition of silver in the acid-leaching residue.

Phase Composition Ag Ag2O Ag2SO4 Ag2S Wrapped Ag Ag content(g/t) 278.20 30.95 93.90 13.60 330.80 Phase occupation ratio (%) 37.09 4.13 1.81 12.52 44.10

Table 4. Liberation degrees (%) of the primary ZLR minerals.

Mineral Liberation Degree Mineral Liberation Degree Elemental silver 30.68 Copper blue 82.83 Argentite 9.72 Cassiterite 85.9 Sphalerite 52.93 Lead alum 70.31 Chalcopyrite 81.88 Hematite 72.59

3.2. Water Leaching 3.2.1. Effect of Leaching Conditions on Precious Metal Recoveries Water leaching of the ZLR improved the precious metal recovery rates in the flotation process and optimized the pulp environment. The experiments examined the influence of leaching time and liquid–solid ratio on the leaching rates of zinc, copper, iron, and silver enrichment (shown in Figure 2). The leaching times and liquid-to-solid ratios (Figure 2a,b) had a greater impact on the leaching rates of copper and zinc but less so for iron. It showed that, under the leaching time (15 min) and liquid–solid ratio (4:1), the recovery rate of zinc, copper, and iron was optimal. Under optimal conditions, the ionic concentrations in the leaching solution are shown in Table 5. Most of the leaching solution containing zinc sul- Figure 1. SEM images of ZLR and the dissemination characteristic of the main minerals. Figure 1. SEMfate images was recycled of ZLR andinto the the dissemination zinc hydrometallurgy characteristic process. of the main minerals. Table 2. Relative amounts (%) of the major ZLR minerals. 70 70 （a）（b） Mineral Chemical Formula Content Mineral Chemical Formula Content 60 Zn 60 Argentite Ag2S 0.015 Magnetite Fe3O4 0.31 Cu Zn Elemental silver Ag 0.037 Fe Pyrite FeS2 0.1Cu 50 50 Sphalerite (Zn, Fe)S 2.27 Pyrrhotite Fe1−xS 0.04 Fe Zinc sulfate Zn[SO4]·6H2O 6.54 Arsenopyrite FeAsS 0.08 40 40 Chalcopyrite CuFeS2 0.27 Diopside CaMg[Si2O6] 18.28 Copper blue CuS 0.13 Quartz SiO2 19.89 30 30 Copper sulfate Cu[SO4]·5H2O 0.38 Anhydrite Ca[SO4] 18.05 Cassiterite SnO2 0.49 Gypsum Ca(SO4) 2H2O 11.13 Metal extraction/% Metal extraction/% 20 20 Galena PbS 0.002 Biotite K{(Mg,Fe)3[AlSi3O10](OH)2} 2.84 Potassium Lead alum (Pb,Sr)SO4 3.8 10 (K,Na)[AlSi3O8] 2.08 10 feldspar Lead oxide PbO 0.01 Pearl mica CaAl4Si2O10(OH)2 1.63 0 0 Elemental5101520 sulfur S 3.36 Talc 1:1 2:1 Mg3Si 3:14O10(OH) 4:12 5:11.54 6:1 Hematite Time/minFe2O3 6.32 Pyrolusite Liquid-solid MnO2 ratio 0.2 Calcite Ca[CO3] 0.04 Dolomite CaMg[CO3]2 0.1 FigureFigure Rutile2.2. Effects ofof leachingleachingTiO parameters 2parameters on0.01 on the the valuable valuableBarite metal metal recovery: recovery: (a Ba[SO) ( timea) time4] and and (b) ( liquid–solidb) liquid–0.06 ratio. solid ratio.

Table 5. Ion concentrations in the leaching solution.

Elements Pb (mg/L) Zn (g/L) Ag (mg/L) Fe (g/L) Cu (g/L) In (mg/L) Content 5.12 3.30 0.15 1.34 0.24 4.15

3.2.2. Chemical Composition and Phase Analysis of Water-Leached Residue Table 6 shows the chemical composition of the sample after water leaching under the optimal conditions. Compared with the raw ZLR, the levels of Zn and other toxic heavy metals such as Cu and Fe decreased; however, the levels of Pb and Ag increased, and their amounts accounted for 2.9% and 822.4 g/t, respectively. The filter residue phases were analyzed and showed the residue no longer contained the characteristic diffraction peaks of soluble sulfates (Figure 3). After water leaching, the primary components remaining in the filter residue were CaSO4, CaSO4·2H2O, and PbSO4.

Minerals 2021, 11, x FOR PEER REVIEW 6 of 12

Table 6. Chemical composition of the water-leached residue (mass fraction, %).

Minerals 2021, 11, 586 Elements Cu Pb Zn Fe Ag *6 of 11 Content (wt.%) 0.103 2.90 1.90 4.86 822.4 *—g/t.

■ CaSO ·2H O 6000 □ 4 2

□ CaSO4 5000 ★ PbSO4

4000 ) -1

3000 ■

Intensity(s 2000 □

□ □ 1000 ■ ■ □ ★ ★ ★ ■ □ □ ★ □★ □ □ 0

0 1020304050607080 2θ（°）

FigureFigure 3.3.XRD XRD patternpattern ofof thethe residueresidue afterafter waterwater leaching.leaching.

3.3.3.3. FlotationFlotation ExperimentsExperiments 3.3.1.3.3.1. Single-FactorSingle-Factor ExperimentsExperiments AA mineralogicalmineralogical analysis analysis of of thethe ZLRZLR showedshowed aa remarkablyremarkably high high liberationliberation of of wrappedwrapped silversilver andand suggestedsuggested that additional additional fine fine grinding grinding was was necessary. necessary. Figure Figure 4a4 ashows shows the the ef- effectsfects of of grinding grinding fineness fineness on on the the flotation flotation results. The flotationflotation recoveryrecovery ratesrates graduallygradually increasedincreased andand thenthen decreaseddecreased withwith increasingincreasing thethe grindinggrinding time.time. TheThe particleparticle sizesize ofof the the ZLRZLR maymay have have been been too too fine; fine; this this created created significant significant mud mud that affectedthat affected the flotation the flotation of silver of andsilver zinc and [24 zinc]. The [24]. results The ofresults the pH of testingthe pH (Figuretesting4 (Figureb) showed 4b) optimized showed optimized recovery ratesrecovery for zincrates and for silverzinc and that silver occurred that atoccurred pH 3–5, at which pH 3–5, agreed which with agreed the other with reports the other [25 ].reports The effect [25]. ofThe the effect collector of the concentration collector concentration on the flotation on th behaviore flotation of behavior the ZLR of is the shown ZLR in is Figure shown4 c.in InFigure the presence 4c. In the of presence ammonium of ammonium dibutyl dithiophosphate, dibutyl dithiophosphate, the recovery the recovery increased increased sharply assharply the collector as the collector concentrations concentrations increased increase to 700d g/t, to 700 exceeding g/t, exceeding 80% at that80% concentration.at that concen- Figuretration.4d Figure shows 4d the shows flotation the resultsflotation at results different at pulpdifferent concentrations, pulp concentrations, with the optimalwith the resultsoptimal at results a 20% at pulp a 20% concentration. pulp concentration. This may This be duemay to be the due fine to naturethe fine of nature the mineral of the particlesmineral particles and that theand slag that needs the slag dispersion. needs dispersion. TheThe effects effects of of different different collectors collectors and and regulators regulators on theon flotationthe flotation were were tested; tested;Figure Figure5a,b presents5a,b presents the residue the residue grades grades and recoveries, and recoveries respectively., respectively.Figure Figure6a,b shows 6a,b shows that using that using only butylamineonly butylamine black black resulted resulted in optimized in optimized recovery recovery rates andrates grades and grades for both for silverboth silver and zinc. and Addingzinc. Adding the regulators the regulators sodium sodium hexametaphosphate hexametaphosphate and sodium and sodium silicate silicate both improved both im- theproved flotation, the flotation, which was which observed was observed as foaming as duringfoaming testing. during Figure testing.5 shows Figure the 5 shows recovery the ratesrecovery and rates grades and for grades silver for at differentsilver at differ timesent and times ultimately and ultimately yielded yielded three minutes three minutes as the optimizedas the optimized rough selectionrough selection time. After time. a After comprehensive a comprehensive study, thestudy, optimized the optimized experimental exper- conditionsimental conditions were: grinding were: grinding time = 5 min, time pH = 5 3–5,min, (C pH4H 3–5,9O)2 (CPSSNH4H9O)42PSSNHconcentration4 concentration = 700 g/t, = (NaPO700 g/t,3) (NaPO6 concentration3)6 concentration = 500 g/t, = 500 pulp g/t, density pulp density = 20%, and= 20%, flotation and flotation time = 3time min. = 3 min. 3.3.2. Closed Circuit Flotation Experiments Based on the above experimental results, using the optimized test conditions, a closed- circuit flotation test of single-stage roughing, two-stage cleaning, and two-stage scavenging was performed. Figure7 shows the detailed flow chart, and Table7 gives the results. Each closed flotation experiment underwent five cycles. The weight of the concentrate and tailings and the grade and recovery of Zn and Ag in the concentrate and tailings remained constant during the last two cycles, which indicated an arrival of a balance between the metal quality and amount. The experimental results indicated the amount of silver in the concentrate was 9256.41 g/t, which corresponded to an 80.32% recovery, while the zinc grade was 12.26%, with a 42.88% recovery. In addition, Table8 shows the concentrate contained enriched copper, and the copper grade was 1.81%. The selective flotation of sphalerite, argentite, and elemental silver from the ZLR was achieved using C4H9OCSSNa as the collector in the absence of an activator. Minerals 2021, 11, 586 7 of 11 Minerals 2021, 11, x FOR PEER REVIEW 7 of 12 Minerals 2021, 11, x FOR PEER REVIEW 7 of 12

100 10 4000 100 10 4000 100 10 4000 100 10 4000 Ag Recovery Zn Recovery Zn Grade Ag Grade Ag Recovery Zn Recovery Zn Grade Ag Grade (a)(a) Ag Recovery Zn Recovery Zn Grade Ag Grade (b)(b) Ag Recovery Zn Recovery Zn Grade Ag Grade

35003500 8080 88 8080 8 8 32003200 30003000 6060 66 6060 6 6

25002500 24002400 4040 44 4040 4 4 Zn Grade(%) Zn Zn Grade(%) Zn Recovery(%) Recovery(%) Recovery(%) Ag Grade(g/t) Recovery(%) Zn Grade(%) Ag Grade(g/t) Ag Grade(g/t) Zn Grade(%) Ag Grade(g/t) 20002000

2020 22 16001600 2020 2 2 15001500

0 0 00 10001000 00 0 0 10001000 012345678012345678 1234567891012345678910 GrindingGrinding time(min)time(min) pHpH

100 10 8000 100100 10 10 40004000 100 10 8000 Ag Recovery Zn Recovery Zn Grade Ag Grade (c)(c) Ag Ag Recovery Recovery Zn Zn Recovery Recovery ZnZn Grade Ag GradeGrade (d)(d) Ag Recovery Zn Recovery Zn Grade Ag Grade

80 8 8080 88 80 8 60006000 3000 3000 60 6 60 6 60 6 60 6 4000 4000 40 4 40 4 40 4 Recovery(%) 40 4 Zn Grade(%) Zn Recovery(%) Ag Grade(g/t) Zn Grade(%) Zn Ag Grade(g/t) Ag

Recovery(%) 2000 Zn Grade(%) Zn Recovery(%) Ag Grade(g/t) Zn Grade(%) Zn 2000 Grade(g/t) Ag 2000 20 2 2000 20 2 20 2 20 2

0 0 0 0 0 1000 0 200 400 600 800 10000 0 0 15 20 25 30 35 0 1000 200 400 600 800 1000 Concentration of (C4H9O)2PSSNH4 (g/t) 15Pulp 20 density(%) 25 30 35 Concentration of (C H O) PSSNH (g/t) 4 9 2 4 Pulp density(%) Figure 4. Effects of the flotation parameters on recovery: (a) grinding time, (b) pH, (c) proportion of (C4H9O)2PSSNH4, and Figure 4. Effects of the ﬂotation parameters on recovery: (a) grinding time, (b) pH, (c) proportion of (C4H9O)2PSSNH4, Figure(d) pulp 4. Effectsdensity. of the flotation parameters on recovery: (a) grinding time, (b) pH, (c) proportion of (C4H9O)2PSSNH4, and and(d) (pulpd) pulp density. density. 100 100 Ag recovery Ag cumulative recovery Ag recovery Ag cumulative recovery Ag grade Ag cumulative grade Ag grade Ag cumulative grade 4000 4000 80 80 3200 3200 ））

60 g/t （ 60 2400 g/t

2400 （ 40 Recovery(%) 40 1600 Ag Grade Recovery(%) 1600 Ag Grade 20 800 20 800

0 0 30 60 90 120 150 180 210 240 0 0 30 60 90 120 150 180 210 240 Cumulative time (seconds) Figure 5. Recoveries andCumulative grades of Ag timein rough (seconds) concentrates as a function of the proportion of the cumulative time. FigureFigure 5. 5.Recoveries Recoveries andand gradesgrades ofof AgAg inin roughrough concentratesconcentrates asas aa function of the proportion of the cumulative time. cumulative time.

Minerals 2021, 11, 586 8 of 11 Minerals 2021, 11, x FOR PEER REVIEW 8 of 12

100 4000 50 10 （a） Ag recovery Ag Grade （b） Zn recovery Zn grade

80 40 8 3000

60 30 6

2000

40 20 4 Recovery(%) Recovery(%) Ag Grade(g/t) Zn Grade(%)

1000 20 10 2

0 0 0 0 （C4H9O）2PSSNH4 （C H O） PSSNH （C4H9O）2PSSNH4 C H NCSSNa·3H2O A31 （C H O） PSSNH （C H O） PSSNH （C H O） PSSNH (C H ) NCSSNa·3H2O A31 4 9 2 4 ( 2 5)2 4 9 2 4 4 9 2 4 4 9 2 4 2 5 2 +Z200 +C4H9OCSSNa +C4H9OCSSNa +Z200

100 4000 50 10 （c） Ag recovery Ag Grade （d） Zn recovery Zn grade

80 40 8 3000

60 30 6

2000

40 20 4 Recovery(%) Recovery(%) Ag Grade(g/t) Zn Grade(%)

1000 20 10 2

0 0 0 0 Na O·nSiO Na O·nSiO Na S Na O·nSiO Na O·nSiO Na S None (NaPO3)6 2 2 2 2 2 None (NaPO3)6 2 2 2 2 2 +(NaPO ) +(NaPO ) 3 6 3 6 FigureFigure 6. 6.Effects Effects of of the the ﬂotation flotation reagent reagent selection selection on on the the recovery: recovery: (a ()a collector) collector on on Ag, Ag, (b ()b collector) collector on on Zn, Zn, (c )(c adjustor) adjustor on on Ag, andAg, (d and) adjustor (d) adjustor on Zn. on Zn.

Table3.3.2. 7. Iron Closed and zinc Circuit distribution Flotation in magneticExperiments and nonmagnetic products. Based on the above experimental results, using the optimized test conditions, a Ag Distribution Zinc Distribution Products Productionclosed-circuit Ratio Ag flotation Grade (g/t) test of single-stage Zinc Grade roughing, two-stage cleaning, and two-stage (%) (%) scavenging was performed. Figure 7 shows the detailed flow chart, and Table 7 gives the Concentrate 6.28results. Each closed 9256.41 flotation experiment 12.26 underwent five cycles. 80.32 The weight of the 42.88 concen- Tailing 93.72trate and tailings 145.2and the grade and recovery 1.45 of Zn and Ag 19.68 in the concentrate and 57.12 tailings Leaching residue 100 822.4 1.90 100 100 remained constant during the last two cycles, which indicated an arrival of a balance between the metal quality and amount. The experimental results indicated the amount of silver inTable the concentrate 8. Chemical compositionwas 9256.41 ofg/t, the whic concentrate.h corresponded to an 80.32% recovery, while the zinc grade was 12.26%, with a 42.88% recovery. In addition, Table 8 shows the con- Elements Ag (g/t)centrate contained Zn enriched copper, and Cu the copper grade was Fe 1.81%. The selective Pb flota- Content (wt.%) 9256.41tion of sphalerite, 12.26 argentite, and elementa 1.81l silver from 6.31the ZLR was achieved 1.98 using C4H9OCSSNa as the collector in the absence of an activator. TheThe phasesphases of of the the silver silver concentrate concentrate are are shown shown in Figurein Figure8 and 8 containand contain diffraction diffraction peaks peaks characteristic of many valuable substances, such as Ag2S, Ag, ZnS, and S, which characteristic of many valuable substances, such as Ag2S, Ag, ZnS, and S, which indicated theindicated enrichment the enrichment of these substances of these substances during ﬂotation. during flotation.

Table 7. Iron and zinc distribution in magnetic and nonmagnetic products.

Minerals 2021, 11, x FOR PEER REVIEW 9 of 12 Minerals 2021, 11, x FOR PEER REVIEW 9 of 12

Production Ag Grade Ag Distribution Zinc Distribution Products Production Ag Grade Zinc GradeAg Distribution Zinc Distribution Products Ratio (g/t) Zinc Grade (%) (%) Ratio (g/t) (%) (%) Concentrate 6.28 9256.41 12.26 80.32 42.88 Concentrate 6.28 9256.41 12.26 80.32 42.88 Tailing 93.72 145.2 1.45 19.68 57.12 Tailing 93.72 145.2 1.45 19.68 57.12 Leaching residue 100 822.4 1.90 100 100 Leaching residue 100 822.4 1.90 100 100

Table 8. Chemical composition of the concentrate. Table 8. Chemical composition of the concentrate. Elements Ag (g/t) Zn Cu Fe Pb Minerals 2021, 11, 586 Elements Ag (g/t) Zn Cu Fe Pb 9 of 11 Content (wt.%) 9256.41 12.26 1.81 6.31 1.98 Content (wt.%) 9256.41 12.26 1.81 6.31 1.98

Zinc acid-leaching residue Zinc acid-leaching residue

Water leaching Water leaching

Zinc enriched solution Leaching residue Zinc enriched solution Leaching residue

Zinc recovery Wet-milling Zinc recovery Wet-milling

Rougher Rougher

Cleaner1 Scavenger1 Cleaner1 Scavenger1

Cleaner2 Scavenger2 Cleaner2 Scavenger2

Concentrate Tailing Concentrate Tailing

FigureFigure 7.7.Flow Flow chartchart forfor zinczinc andand silversilver recoveriesrecoveries from from the the zinc zinc acid-leaching acid-leaching residue. residue. Figure 7. Flow chart for zinc and silver recoveries from the zinc acid-leaching residue.

8000 ● ZnS 8000 ● ZnS ● ● ■ CaSO4·2H2O ■ CaSO4·2H2O ◆ S 6000 ◆ S 6000 □ CaSO

) 4 □ CaSO -1 ) 4 ● -1

● ◆ Fe2O3 ◆ Fe O 4000 ◆ 2 3

4000 ◆ ▲ Ag2S ▲ Ag2S Intensity(s

Intensity(s ▼ Ag ● ▼ Ag □ 2000 ● ★ (Pb,Cr)SO □ 4 2000 ▲★ ★ (Pb,Cr)SO4 ■ ★ ★

▲ ◆ ■ ★ ◆ ▼

◆ ▲ ▼ ▼

◆ ▲ 0 ▼ 0 0 1020304050607080 0 1020304050607080 2θ（°） 2θ（°） Figure 8. XRD pattern of the silver concentrate after flotation. FigureFigure 8. XRD 8. XRD pattern pattern of the of thesilver silver concentrate concentrate after after flotation. ﬂotation.

3.4. Leaching Toxicity The TCLP method simulated the process in which hazardous components enter the 3.4. Leaching Toxicity 3.4.environment Leaching Toxicity via wastewater due to acid rainfall after waste burial or long-term storage. Finally, the leaching toxicity of ZLR and flotation tailings were evaluated (Tables9 and 10 ) and showed that the ZLR concentrations for Cu, Zn, Cd, and Be were 127 mg/L, 1305 mg/L, 21.3 mg/L, and 0.039 mg/L, respectively, all of which far exceeded the established thresholds of 100, 100, 1, and 0.02 ppm, respectively [26], and clearly indicated that ZLR poses a significant biohazard. However, after water-leaching flotation, the Cu, Zn, Cd, and Be concentrations decreased sharply to 2.6 mg/L, 45 mg/L, 0.94 mg/L, and 0.0078 mg/L, respectively, all well below the leaching toxicity limits. Moreover, the leaching concentrations of other impurities such as Pb, Cr, As, etc. were lower than the established thresholds. This was mainly due to toxic elements entering the leachate and flotation tail waters during precious metal recoveries by this water-leaching flotation process [27,28]. Therefore, the final flotation residue poses little environmental harm and can be safely stockpiled for future treatment or recycling. Minerals 2021, 11, 586 10 of 11

Table 9. Results of solid waste corrosion identiﬁcation.

Sample pH ZLR 1.45 Tailing 6.03

Table 10. TCLP test results of the ZLR and water-leaching residues (mg/L).

Regulatory Regulatory Threshold Element ZLR Tailings Threshold (China) (USEPA) Cu 127 2.6 100 — Zn 1305 45 100 — Cd 21.3 0.94 1 1 Pb 3.6 0.3 5 5 Cr 0.11 0.05L 5 5 Hg 0.00015 0.00037 0.1 0.2 Be 0.039 0.0078 0.02 — Ba 0.1L 3.2 100 — Ni 0.17 0.04L 5 — Se 0.0004 0.0008 1 1 As 4.76 0.785 5 5 Fluoride 0.66 0.42 100 — cyanide 0.004L 0.005 5 —

4. Conclusions This paper advances an alternate and environmentally benign method for the treatment of hazardous zinc-leaching residue waste. A combination of water-leaching and flotation recovers zinc and silver from ZLR and ultimately detoxifies the residual ZLR. The following conclusions were drawn: (1) The hazardous ZLR primarily contained Zn, Pb, Cu, Fe, and Ag; sphalerite, zinc sulfate, lead alum, hematite, quartz, anhydrite, and gypsum were the main phases. (2) Water leaching recovered a portion of the precious metals (Zn, Cu, and Fe) from the ZLR and increased the recovery rate of the subsequent flotation tests. The leaching solution was returned to the zinc-smelting process without pollution. (3) Different parameters were optimized during the flotation tests, such as grinding time, pulp density, cumulative time, concentration of (C4H9OCSSNa), and flotation adjusters. These were obtained with one round of roughing, two rounds of scavenging, and two rounds of cleaning. These optimized parameters achieved a silver concentrate grade of 9256.41 g/t and a silver recovery grade of 80.32%. (4) Moreover, this treatment lowered the heavy metal-leaching concentrations below the leaching toxicity limits and rendered the final tailings environmentally benign. This sus- tainable method provided a highly efficient, cost-effective, and environmentally friendly treatment of industrial ZLR waste.

Author Contributions: Y.D. and X.T. conceived and designed the experiments; X.X. and W.Z. performed the experiments and analyzed the date; Q.S. contributed reagents and materials; H.Y. and Y.D. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: The first author gratefully acknowledges financial support provided by the National Natural Science Foundation of China (51764024 and 51764025). Data Availability Statement: Data available on request due to restrictions privacyl. The data provided in this study can be obtained at the request of the corresponding author. As the data needs further research, the data is currently not publicly available. Conflicts of Interest: The authors declare no conflict of interest. Minerals 2021, 11, 586 11 of 11

References 1. Min, X.-B.; Xie, X.-D.; Chai, L.-Y.; Liang, Y.-J.; Li, M.; Ke, Y. Environmental availability and ecological risk assessment of heavy metals in zinc leaching residue. Trans. Nonferrous Met. Soc. China 2013, 23, 208–218. [CrossRef] 2. Ru¸sen,A.; Sunkar, A.; Topkaya, Y. Zinc and lead extraction from Çinkur leach residues by using hydrometallurgical method. Hydrometall. 2008, 93, 45–50. [CrossRef] 3. Altundogan,ˇ H.S.; Erdem, M.; Orhan, R.; Özer, A.; Tümen, F. Heavy Metal Pollution Potential of Zinc Leach Residues Dis-carded in inkur Plant. Turk. J. Eng. Environ. Sci. 2014, 22, 167–177. 4. Turan, M.D.; Altundo˘gan, H.S.; Tümen, F. Recovery of zinc and lead from zinc plant residue. Hydrometall. 2004, 75, 169–176. [CrossRef] 5. Ju, S.; Zhang, Y.; Zhang, Y.; Xue, P.; Wang, Y. Clean hydrometallurgical route to recover zinc, silver, lead, copper, cadmium and iron from hazardous jarosite residues produced during zinc hydrometallurgy. J. Hazard. Mater. 2011, 192, 554–558. [CrossRef] 6. Erdem, M.; Özverdi, A. Environmental risk assessment and stabilization/solidification of zinc extraction residue: II. Stabiliza- tion/solidification. Hydrometallurgy 2010, 105, 270–276. [CrossRef] 7. Li, C.; Zhong, H.; Wang, S.; Xue, J. Leaching Behavior and Risk Assessment of Heavy Metals in a Landfill of Electrolytic Man-ganese Residue in Western Hunan, China. Hum. Ecol. Risk Assess. Int. J. 2014, 20, 1249–1263. [CrossRef] 8. Zeng, F.; Wei, W.; Li, M.; Huang, R.; Yang, F.; Duan, Y. Heavy Metal Contamination in Rice-Producing Soils of Hunan Province, China and Potential Health Risks. Int. J. Environ. Res. Public Health 2015, 12, 15584–15593. [CrossRef] 9. Cao, D.-Q.; Wang, X.; Wang, Q.-H.; Fang, X.-M.; Jin, J.-Y.; Hao, X.-D.; Iritani, E.; Katagiri, N. Removal of heavy metal ions by ultrafiltration with recovery of extracellular polymer substances from excess sludge. J. Membr. Sci. 2020, 606, 118103. [CrossRef] 10. Çoruh, S.; Ergun, O.N. Use of fly ash, phosphogypsum and red mud as a liner material for the disposal of hazardous zinc leach residue waste. J. Hazard. Mater. 2010, 173, 468–473. [CrossRef] 11. Stylianou, M.A.; Hadjiconstantinou, M.P.; Inglezakis, V.J.; Moustakas, K.G.; Loizidou, M.D. Use of natural clinoptilolite for the removal of lead, copper and zinc in fixed bed column. J. Hazard. Mater. 2007, 143, 575–581. [CrossRef] 12. Çoruh, S. The removal of zinc ions by natural and conditioned clinoptilolites. Desalination 2007, 225, 41–57. [CrossRef] 13. Demır, G.; Coruh, S.; Ergun, O.N. Leaching behavior and immobilization of heavy metals in zinc leach residue before and after thermal treatment. Environ. Prog. 2008, 27, 479–486. [CrossRef] 14. Kul, M.; Topkaya, Y. Recovery of germanium and other valuable metals from zinc plant residues. Hydrometallugry 2008, 92, 87–94. [CrossRef] 15. Vahidi, E.; Rashchi, F.; Moradkhani, D. Recovery of zinc from an industrial zinc leach residue by solvent extraction using D2EHPA. Miner. Eng. 2009, 22, 204–206. [CrossRef] 16. Li, Y.-C.; Min, X.-B.; Chai, L.-Y.; Shi, M.-Q.; Tang, C.-J.; Wang, Q.-W.; Liang, Y.-J.; Lei, J.; Liyang, W.-J. Co-treatment of gypsum sludge and Pb/Zn smelting slag for the solidification of sludge containing arsenic and heavy metals. J. Environ. Manag. 2016, 181, 756–761. [CrossRef] 17. Elgersma, F.; Witkamp, G.J.; Rosmalen, G.M.V. Simultaneous dissolution of zinc ferrite and precipitation of ammonium jar-osite. Hydrometallurgy 1993, 34, 23–47. [CrossRef] 18. Dutra, A.; Paiva, P.; Tavares, L. Alkaline leaching of zinc from electric arc furnace steel dust. Miner. Eng. 2006, 19, 478–485. [CrossRef] 19. Xia, D.; Picklesi, C. Microwave caustic leaching of electric arc furnace dust. Miner. Eng. 2000, 13, 79–94. [CrossRef] 20. Holloway, P.C.; Etsell, T.H.; Murland, A.L. Roasting of La Oroya Zinc Ferrite with Na2CO3. Met. Mater. Trans. A 2007, 38, 781–791. [CrossRef] 21. Holloway, P.C.; Etsell, T.H. Recovery of zinc, gallium and indium from La Oroya zinc ferrite using Na2CO3 roasting. Miner. Process. Extr. Met. 2008, 117, 137–146. [CrossRef] 22. Zhang, L.; Khoso, S.A.; Tian, M.; Sun, W. Indium pre-enrichment from a Canadian sulphide ore via flotation technique. Miner. Eng. 2020, 156, 106481. [CrossRef] 23. Gulden, S.; Riedele, C.; Kopf, M.-H.; Nirschl, H. Potential of flotation as alternative separation process in biotechnology with focus on cost and energy efficiency. Chem. Eng. Sci. 2020, 218, 115117. [CrossRef] 24. Zhu, X.N.; Ni, Y.; Wang, D.Z.; Zhang, T.; Qu, S.J.; Qiao, F.M.; Ren, Y.G.; Nie, C.C.; Lyu, X.J.; Qiu, J.; et al. Effect of dissociation size on flotation behavior of waste printed circuit boards. J. Clean. Prod. 2020, 265, 21840. [CrossRef] 25. Foroutan, A.; Abadi, M.A.Z.H.; Kianinia, Y.; Ghadiri, M. Critical importance of pH and collector type on the flotation of sphalerite and galena from a low-grade lead–zinc ore. Sci. Rep. 2021, 11, 3103. [CrossRef][PubMed] 26. GB5085.3-2007. Hazardous Wastes Distinction Standard-Leaching Toxicity Distinction; China Environmental Science Press: Beijing, China, 2007. (In Chinese) 27. Zou, D.; Li, H.; Deng, Y.; Chen, J.; Bai, Y. Recovery of lanthanum and cerium from rare earth polishing powder wastes utilizing acid baking-water leaching-precipitation process. Sep. Purif. Technol. 2020, 261, 118244. [CrossRef] 28. Jiang, G.M.; Bing, P.E.N.G.; Liang, Y.J.; Chai, L.Y.; Wang, Q.W.; Li, Q.Z.; Ming, H.U. Recovery of valuable metals from zinc leaching residue by sulfate roasting and water leaching. Trans. Nonferrous Met. Soc. China 2017, 27, 1180–1187. [CrossRef]