Electrocoagulation: a Promising Method to Treat and Reuse Mineral Processing Wastewater with High COD

water

Article Electrocoagulation: A Promising Method to Treat and Reuse Mineral Processing Wastewater with High COD

Gaogui Jing 1,2, Shuai Ren 1,2, Yuesheng Gao 3 , Wei Sun 1,2 and Zhiyong Gao 1,2,* 1 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China; [email protected] (G.J.); [email protected] (S.R.); [email protected] (W.S.) 2 Key Laboratory of Hunan Province for Clean and Eﬃcient Utilization of Strategic Calcium-Containing Mineral Resources, Central South University, Changsha 410083, China 3 Department of Chemical Engineering, Michigan Technological University, Houghton, MI 49931, USA; [email protected] * Correspondence: [email protected]

Received: 23 January 2020; Accepted: 18 February 2020; Published: 21 February 2020

Abstract: Mineral processing wastewater contains large amounts of reagents which can lead to severe environmental problems, such as high chemical oxygen demand (COD). Inspired by the wastewater treatment in such industries as those of textiles, food, and petrochemistry, in the present work, electrocoagulation (EC) is applied for the first time to explore its feasibility in the treatment of wastewater with an initial COD of 424.29 mg/L from a Pb/Zn sulfide mineral flotation plant and its effect on water reuse. Typical parameters, such as anode materials, current density, initial pH, and additives, were characterized to evaluate the performance of the EC method. The results showed that, under optimal conditions, i.e., iron anode, pH 7.1, electrolysis time 70 min, 19.23 mA/cm2 current density, and 4.1 g/L activated carbon, the initial COD can be reduced to 72.9 mg/L, corresponding to a removal rate of 82.8%. In addition, compared with the untreated wastewater, EC-treated wastewater was found to benefit the recovery of galena and sphalerite, with galena recovery increasing from 25.01% to 36.06% and sphalerite recovery increasing from 59.99% to 65.33%. This study confirmed that EC is a promising method for the treatment and reuse of high-COD-containing wastewater in the mining industry, and it possesses great potential for wide industrial applications.

Keywords: mineral processing; wastewater treatment; ﬂotation; electrocoagulation (EC); chemical oxygen demand (COD)

1. Introduction Wastewater from mineral processing remains a headache for the mining industry not only because of its large volume, but more importantly, its hazardous components. Among the complicated residual reagents, the organic reagents receive the most attention due to their high chemical oxygen demand (COD) levels. Direct discharge of wastewater causes serious environmental pollution, while its reuse as feed water without proper treatments causes great harm to the recovery and grade of valuable minerals [1]. To solve this problem, techniques for wastewater treatment have been intensively developed in the mining industry, such as natural settling, flocculating setting, chemical oxidation, adsorption, and biodegradation to remove COD [2]. However, these techniques possess such disadvantages as inefficiency in COD removal, extreme operating conditions, high operation cost, huge instrument investment, etc. It is worth noting that coagulation–flocculation, the most common technique in beneficiation plants because of its economy, usually has a relative low COD removal rate of only 35% [3].

Water 2020, 12, 595; doi:10.3390/w12020595 www.mdpi.com/journal/water Water 2020, 12, 595 2 of 12

Electrocoagulation (EC) has become widely used as a wastewater treatment technology in the past decade and has exhibited a satisfactory removal of COD in diverse fields, such as in textiles [4], the food industry [5], dairying [6], brackish water [7], and potable water [8]. Its advantages include easy operation, small area occupation, and a high degree of automation. In the process of EC, iron or aluminum is usually adopted as the anode material, and each one can lead to a series of similar redox reactions. Taking the iron electrode as an example, simplified reactions occurring in acidic and alkaline solutions can be described as follows [9]: Under acidic conditions: 2+ Fe Fe + 2e− (1) (s) → (aq) + 2H O O + 4H + 4e− (2) 2 → 2 4Fe2+ + 10H O + O 4Fe(OH) + 8H+ (3) (aq) 2 (l) 2(g) → 3(s) (aq) Under alkaline conditions: 2+ Fe Fe + 2e− (4) (s) → (aq) 2+ Fe + 2OH− Fe(OH) (5) → 2 As shown in above equations, the iron anode is used to generate ions in the aqueous medium. The generated iron ions are then immediately hydrolyzed to form Fe(OH)3, monomeric 3+ 2+ 4+ ions, and polymeric hydroxy complexes such as Fe(H2O)6 , Fe(H2O)4(OH) , Fe2(H2O)6(OH)4 , 4+ 2+ Fe2(H2O)8(OH)2 , and Fe(H2O)5(OH) , depending on the solution pH [10]. These hydroxides and polyhydroxides are excellent adsorbents for counter ions and organic pollutants. In some cases, they can even form complexes with some of the organics [11]. Pollutants in the wastewater can be treated by physical and chemical attachment to hydroxides or chemical reactions [12]. After the EC treatment, separation of precipitation from the liquid is traditionally achieved via a filter process. Then, the filtered and separated sludge is usually treated using the incineration and landfill treatments [13]. In this paper, EC treatment of sulfide mineral flotation wastewater was applied for the first time, and the effects of different parameters like anode materials, current density, initial pH, electrolysis time, and additives on COD removal rate were investigated. The treated water was then recycled as feed water in the sulfide ore flotation. For comparison, untreated wastewater and fresh water were used to assess the effects of EC-treated water on the grade and recovery of galena and sphalerite.

2. Materials and Methods

2.1. Samples, Reagents, and Analytical Methods Samples of mineral processing wastewater and actual ores were all from a Pb-Zn beneficiation plant located in Southern China. The plant had three different types of wastewater, namely Pb-Zn flotation wastewater, pyrite concentrate wastewater, and pyrite tailing wastewater, accounting for 70%, 14%, and 16% of the total amount, respectively. In this study, three different types of wastewater samples were mixed at the above ratio to obtain the mixed wastewater samples. The pH and COD values of each wastewater are shown in Table1.

Table 1. Chemical oxygen demand (COD) and pH values of mineral processing wastewater samples in a Pb-Zn beneﬁciation plant.

Sample Name pH COD (mg/L) Pb-Zn ﬂotation wastewater 11.90 434.7 Pyrite concentrate wastewater 9.11 245.3 Pyrite tailing wastewater 8.29 603.2 Mixed wastewater 11.83 424.9 Water 2020, 12, 595 3 of 12

Water 2020, 12, 595 3 of 12 The H2O2 (30% pure), H2SO4, HCl, and NaOH used in the EC experiments were all of analytical grades. The granular activated carbon (GAC) was made from a coconut shell. The H2O2 (30% pure), H2SO4, HCl, and NaOH used in the EC experiments were all of analytical The pH of the solution was measured by a pH meter (Leici, Shanghai, China). The COD values of grades. The granular activated carbon (GAC) was made from a coconut shell. the wastewaterThe pH of were the solution analyzed was by measured fast digestion–spectrophotometric by a pH meter (Leici, Shanghai, method China). (HJ/T The 399-2007). COD values of Thethe wastewater chemical compositions were analyzed of by the fast actual dige orestion–spectrophotometric samples were relatively method simple. (HJ/T SiO 399-2007).2 and CaO were the mainThe non-metallic chemical compositions ingredients of [ 14the]. actual The lead ore samples and zinc were grades relatively in the simple. samples SiO took2 and up CaO 4.17% were and 7.18%the ofmain the non-metallic total, respectively. ingredients Themajor [14]. The metallic lead mineralsand zinc grades were galena, in the sphalerite,samples took and up pyrite. 4.17% Actualand ore7.18% samples of the were total, crushed respectively. to below The 3 mm major using metallic a stainless-steel minerals were hammer galena, and sphalerite, then ground and in apyrite. conical ballActual mill. Theore samples ground were products crushed were to usedbelow as 3 feedmm samplesusing a stainless-steel for ﬂotation tests.hammer and then ground in a conicalDiethyldithiocarbamate ball mill. The ground (95% products purity) were and used sodium as feed n-butylxanthate samples for flotation (92% purity) tests. were purchased from AladdinDiethyldithiocarbamate Industrial, Shanghai, (95% China.purity) Theand CaOsodium (analytically n-butylxanthate pure) (92% and CuSOpurity)4 (analyticallywere purchased pure) usedfrom in Aladdin this study Industrial, were provided Shanghai, by China. Kermel The Company, CaO (analytically Tianjin, China. pure) and Terpenic CuSO4oil (analytically (chemically pure) pure) wasused obtained in this fromstudy Haoshen were provided Chemical by Kermel Reagent, Company, Shanghai, Tianjin, China. China. Terpenic oil (chemically pure) was obtained from Haoshen Chemical Reagent, Shanghai, China. 2.2. EC Experiments 2.2. EC Experiments The EC set-up used in this study is shown in Figure1. The total volume of the electrolytic cell was 250The mL, EC and set-up the experimentsused in this study were is carried shown out in Figure by treating 1. The 200 total mL volume wastewater of the sampleselectrolytic for cell each runwas at room250 mL, temperature. and the experiments Two pairs were of platescarried (length out by =treating6.5 cm; 200 width mL wastew= 5.6 cm;ater thickness samples for= 0.2each cm) wererun placed at room on temperature. the electrolytic Two cell pairs in parallel.of plates The(length anode = 6.5 and cm; cathode width =plates 5.6 cm; were thickness connected = 0.2 cm) to the positivewere placed and negative on the portselectrolytic of the cell QW-MS3010D in parallel. The DC anode power and supply cathode (0–5 plates A, 0–80 were V), withconnected constant to the load positive and negative ports of the QW-MS3010D DC power supply (0–5 A, 0–80 V), with constant current. NaOH and H2SO4 solutions were used to adjust the pH. At the end of the experiment, water load current. NaOH and H2SO4 solutions were used to adjust the pH. At the end of the experiment, was transferred to beakers for two hours of settling, and then the COD value of the supernatant was water was transferred to beakers for two hours of settling, and then the COD value of the supernatant measured. Finally, the sludge produced by the EC was ﬁltered, dried, and weighed. was measured. Finally, the sludge produced by the EC was filtered, dried, and weighed.

FigureFigure 1. 1.Electrocoagulation Electrocoagulation (EC) experimental set-up: set-up: (1) (1) Magnetic Magnetic stirrer; stirrer; (2) DC (2) power DC power supply; supply; (3) (3)electrolytic electrolytic cell; cell; (4) (4) anode anode plate; plate; (5) (5) cathode cathode plate; plate; (6) (6)magnetic magnetic bar-stirrer. bar-stirrer.

The COD removal rate was employed to evaluate the electrocoagulation efficiency, which is calculated according to the following Equation (6).

Water 2020, 12, 595 4 of 12

Water The2020, COD12, 595 removal rate was employed to evaluate the electrocoagulation efficiency, which4 of is12 calculated according to the following Equation (6). R(%) = × 100, COD COD (6) R(%) = 0 − 1 100, (6) COD0 × where R represents the removal rate of COD (%), COD0 represents the initial COD value of the wherewastewater R represents before electroc the removaloagulation rate (mg/L), of COD and (%), COD COD1 represents0 represents the theCOD initial value COD of the value wastewater of the wastewaterafter electrocoagulation before electrocoagulation (mg/L). (mg/L), and COD1 represents the COD value of the wastewater after electrocoagulation (mg/L). 2.3. Flotation Experiments 2.3. Flotation Experiments Fresh water, wastewater, and treated water were then used for flotation at room temperature. The wholeFresh water,process wastewater, can be divided and into treated two water stages: were Galena then flotation used for followed flotation by at sphalerite room temperature. flotation. TheFor wholeeach flotation process experiment, can be divided an into800 twog ore stages: sample Galena was ground flotation by followed a conical by ball sphalerite mill to flotation.obtain a Forproduct each flotationof 85 wt.% experiment, passing 74 an μm. 800 After g ore grinding, sample was the groundore pulp by was a conical transferred ball mill to the to obtain galena a flotation product ofstage, 85 wt.% including passing single-stage 74 µm. After roug grinding,hing, three-stage the ore pulp cleani wasng, transferredand single-stage to the scavenging. galena flotation The tailing stage, includingof galena single-stageflotation was roughing, used late three-stager for sphalerite cleaning, flotation, and where single-stage single-stage scavenging. roughing, The two-stage tailing of galenacleaning, flotation and single-stage was used later scavenging for sphalerite were flotation, adopted. where In both single-stage flotation roughing, stages, the two-stage concentrates cleaning, of androughing single-stage were reground scavenging to were80 wt.% adopted. passing In both44 μ flotationm in a Φ stages,150 × 100 the conical concentrates mill. ofAccording roughing to were the regroundexisting process to 80 wt.% of the passing beneficiation 44 µm in plant, a Φ150 sequential100 conical addition mill. of According regulator, to collector, the existing and processfrother ofto × thethe beneficiationflotation cell plant,were sequentialcarried out, addition as shown of regulator, in Figure collector, 2. After andflotation, frother the to theproducts flotation were cell dried, were carriedweighed, out, and as shownassayed, in Figureand then2. After the recovery flotation, thewas products calculated were on dried,the basis weighed, of the and mass assayed, and metal and thenbalance, the recoveryas shown was in Equation calculated (7). on the basis of the mass and metal balance, as shown in Equation (7).

Unit:g/t raw minerals/800g SH:sodium humate H:50%KBX+50%NA

Grinding:5min30s CaO:6000 H:100 2min terpenic oil:25

2min H:10 Grinding:6min30s CaO:2000 2min H:20 3min CaO:1000 CuSO4:400 midding 4 3min KBX:400 3min CaO:1000 midding 3 1min terpenic oil:15 2min H:2.5 2min KBX:25 Grinding:5min CaO:1000 3min CaO:500 2min SH:100 2min CuSO 4:400 2min KBX:30 midding 2 midding 7 tailing 3min CaO:500

Pb concentrate midding 1 midding 6

Zn concentrate midding 5

FigureFigure 2.2. Flowsheet and corresponding experimental conditionsconditions of batch ﬂotationflotation tests using didifferentﬀerent typestypes ofof water.water.

× ɛ(%) = × 100, (7) ∑(×)

where ɛ is the recovery of product, γi is the yield, and βi is the grade.

Water 2020, 12, 595 5 of 12

γi βi ε(%) = P × 100, (7) n(γ β ) × 1 i × i Waterwhere 2020ε,is 12 the, 595 recovery of product, γi is the yield, and βi is the grade. 5 of 12

3.3. Results Results and and Discussion Discussion

3.1. Effect of EC Parameters on COD Removal Rate of Mixed Wastewater 3.1. Effect of EC Parameters on COD Removal Rate of Mixed Wastewater Unlike anode materials, cathode materials do not have a significant effect on EC efficiency except Unlike anode materials, cathode materials do not have a significant effect on EC efficiency except in electro-Fenton processes [15]. We have learned that stainless steel is the most widely-used cathode in electro-Fenton processes [15]. We have learned that stainless steel is the most widely-used cathode material in industry, considering its economy and durability [16]. In addition, the distance between material in industry, considering its economy and durability [16]. In addition, the distance between the anode and cathode is suggested to be 1~2 cm, according to previous publications and industrial the anode and cathode is suggested to be 1~2 cm, according to previous publications and industrial experience [17]. After verification, 2 cm was the optimal distance to be adopted in our following experience [17]. After verification, 2 cm was the optimal distance to be adopted in our following experiments. As the anode material is a prime consideration for EC technique [18], iron and aluminum experiments. As the anode material is a prime consideration for EC technique [18], iron and plates were used as anode materials in this study. The results of COD removal efficiency with the two aluminum plates were used as anode materials in this study. The results of COD removal efficiency anode materials are shown in Figure3. When using an iron electrode, the COD value decreases from with the two anode materials are shown in Figure 3. When using an iron electrode, the COD value 424.9 to 158.6 mg/L, which is equivalent to a removal rate of 62.7%. However, when an aluminum decreases from 424.9 to 158.6 mg/L, which is equivalent to a removal rate of 62.7%. However, when anode is used, the COD value is reduced to 265.1 mg L, corresponding to a removal rate of 37.7%. an aluminum anode is used, the COD value is reduced/ to 265.1 mg/L, corresponding to a removal rateThe of results 37.7%. indicate The results that indicate in this study, that in Fe this performs study, Fe better performs as the better anode as material the anode of thematerial EC process. of the ECTherefore, process. an Therefore, Fe anode an was Fe adoptedanode was in theadopted subsequent in the experiments.subsequent experiments.

500

424.9 400

300 265.1

200

COD/mgL-1 158.6

100

0 wastewater treated water treated water with Al anode with Fe anode FigureFigure 3. EEffectﬀect ofof anode anode materials materials for for EC treatmentEC treatment on mixed on mixed wastewater wastewater COD value COD (cathode value (cathode material: material:Stainless Stainless steel, current steel, density: current 13.74density: mA 13.74/cm2 ,mA/cm pH: 7.1,2, electrolysispH: 7.1, electrolysis time: 50 min). time: 50 min).

CurrentCurrent density density is is another another impo importantrtant parameter parameter that that determines determines the the amount amount of of coagulant coagulant producedproduced inin thethe reactor reactor [19 ].[19]. It has It ahas significant a significan effectt on effect the EC on e ffitheciency EC andefficiency electricity and consumption. electricity consumption.The experimental The resultsexperimental are summarized results are in summarized Figure4. Figure in 4Figure shows 4. that Figure COD 4 removalshows that rate COD first removalincreases rate and first then increases declines and with then the declines increase with of current the increase density. of current COD removal density. rateCOD increases removal fromrate increases49.41% to from 77.46% 49.41% as the to current77.46% densityas the current rises to density 24.73 mArises/cm to2 .24.73 Then, mA/cm it decreases2. Then, to it 60.42% decreases as theto 60.42%current as density the current continues density to increase continues to 30.22to increase mA/cm to2 30.22. The mA/cm current2 density. The current is related density to the is amountrelated to of thedissolved amount iron of dissolved ions and theiron formation ions and the of Fe(OH) formation3, which of Fe(OH) determines3, which the determines COD removal the COD efficiency removal [20]. efficiencyIt can also a[20].ffect It the can cell also voltage affect through the cell various voltage overpotentials, through various which mayoverpotentials, be conducive which to the may removal be conduciveefficiency ofto thethe COD.removal If theefficiency current of density the COD. is relatively If the current low,the density lack ofis coagulantrelatively low, may the restrict lack theof coagulantremoval e ffimayciency. restrict Therefore, the removal the curve efficiency. presents Therefore, an upward the trend curve at an pr earlyesents stage. an upward However, trend when at thean earlycurrent stage. density However, continues when to increase the current to 32.22 dens mA/cmity2 ,continues the temperature to increase of treated to wastewater32.22 mA/cm increases2, the temperaturesignificantly. ofHigh treated temperature wastewater causes increases the significantly. instability of High coagulation, temperature which causes can leadthe instability to a low ECof coagulation, which can lead to a low EC efficiency. Furthermore, energy consumption grows gradually with the increase of current density [21]. Taking both the COD removal rate and electricity consumption into account, 19.23 mA/cm2 was used as the current density for subsequent experiments.

Water 2020, 12, 595 6 of 12 eﬃciency. Furthermore, energy consumption grows gradually with the increase of current density [21]. TakingWater both 2020 the, 12, COD595 removal rate and electricity consumption into account, 19.23 mA/cm2 was6 usedof 12 Water 2020, 12, 595 6 of 12 as the current density for subsequent experiments.

80 80

70 70

60 60

Removal rate of COD/%

Removal rate of COD/% 50 50

5 1015202530 5 1015202530Current density /mA·cm-2 Current density /mA·cm-2 FigureFigure 4. Eff ect4. Effect of current of current density density on mixed on wastewatermixed wastewat CODer removal COD removal rate (anode rate material:(anode material: Fe, cathode Fe, material:Figurecathode 4. Stainless Effect material: of steel, current Stainless pH: density 7.1, steel, electrolysis pH:on mixed7.1, electrolysis time: wastewat 50 min). ertime: COD 50 min).removal rate (anode material: Fe, cathode material: Stainless steel, pH: 7.1, electrolysis time: 50 min). To evaluateTo evaluate the e fftheect effect of the of initial the initial pH, the pH, wastewater the wastewater samples samples were adjustedwere adjusted to a preset to a preset pH value pH usingvalue sodiumTo evaluate using hydroxide sodium the effect hydroxide or of sulfuric the initialor acid.sulfuric pH, The theacid. e ffwa ectsThestewater effects of initial samples of initial pH value were pH value onadjusted COD on COD to removal a presetremoval rate pH rate are shownvalueare in usingshown Figure sodium in5 Figure. The hydroxide results5. The results show or sulfuric show that the acid.that COD the The COD removaleffects removal of rateinitial rate in pH thein thevalue EC EC process on process COD remains remainsremoval around aroundrate are shown in Figure 5. The results show that the COD removal rate in the EC process remains around 75.0%75.0% in a widein a wide pH valuepH value range range from from 3 to 3 9.to However,9. Howeve whenr, when the the initial initial pH pH increases increases to to 11,11, thethe removal 75.0% in a wide pH value range from 3 to 9. However, when the initial pH increases to 11, the removal efficiencyefficiency presents presents a downward a downward trend trend with with a corresponding a corresponding removal removal rate rate of of 71.08%; 71.08%; thisthis isis probablyprobably efficiency presents a downward trend with a corresp−4 onding removal rate of 71.08%; this is probably becausebecause of the of formationthe formation of soluble of soluble Fe(OH) Fe(OH)4, which, which is is not not conducive conducive to to the the floc floc formation. formation. OnOn the because of the formation of soluble Fe(OH)−4, which is not conducive to the floc formation. On the contrary,contrary, at a neutral at a neutral or acidic or acidic pH, higher pH, higher removal remova ratesl occur rates asoccur most as of most the iron of the complexes iron complexes are formed. are contrary,formed. at Hence, a neutral subsequent or acidic experiments pH, higher removawere carrl ratesied out occur under as mostneutral of pHthe conditions.iron complexes pH 7.1 are was Hence, subsequent experiments were carried out under neutral pH conditions. pH 7.1 was employed formed.employed Hence, in thesubsequent following experiments experiments. were carried out under neutral pH conditions. pH 7.1 was inemployed the following in the experiments. following experiments. 90 90

80 80

70 70

60 Removal Rate of COD/% 60 Removal Rate of COD/%

50 50 24681012 24681012pH pH Figure 5. Effect of initial pH value on mixed wastewater COD removal rate (anode material: Fe, FigureFigurecathode 5. 5.E ffEffectect material: of initialof initial Stainless pH pH value steel,value on current mixedon mixed wastewater density: wastewater 19.23 COD mA/cm COD removal 2removal, electrolysis rate (anoderate (anodetime: material: 50 material: min). Fe, cathode Fe, material:cathode Stainlessmaterial: steel,Stainless current steel, density: current density: 19.23 mA 19.23/cm 2mA/cm, electrolysis2, electrolysis time: 50 time: min). 50 min). Electrolysis time is another important parameter affecting COD removal efficiency [22]. The ElectrolysisresultsElectrolysis of the time effecttime is anotherisof anotherelectrolys important importantis time parameter on parameter the COD aff ectingremovalaffecting COD rate COD removal are removal provided effi ciencyefficiency in Figure [22]. [22]. The6. Figure The results 6 ofresults theshows eff ectof thatthe ofelectrolysis effect the COD of electrolys removal time onis rate the time CODincreases on the removal CODrapidly rateremoval in are theprovided rateinitial are phase, provided in Figure which in6. isFigure Figure due to 66. showsthe Figure increase that 6 theshows CODof coagulant; that removal the COD yet, rate the removal increases growth rate rapidlytrend increases of inthe the CODrapidly initial removal in phase, the rate initial which gradually phase, is due whichslows to the downis increasedue with to the the of increase coagulant; extension of coagulant;of electrolysis yet, thetime. growth When trend electrolysis of the COD time re incrmovaleases rate to gradually 70 min, slowsthe COD down removal with the rate extension reaches a of electrolysis time. When electrolysis time increases to 70 min, the COD removal rate reaches a

Water 2020, 12, 595 7 of 12 yet, the growth trend of the COD removal rate gradually slows down with the extension of electrolysis Water 2020, 12, 595 7 of 12 time. When electrolysis time increases to 70 min, the COD removal rate reaches a plateau at 77.62% Water 2020, 12, 595 7 of 12 becauseplateau the at adsorption77.62% because of organic the adsorption pollutants of reachesorganic anpollutants equilibrium reaches state. an equilibrium The electrolysis state. time The of subsequentelectrolysisplateau at experiments 77.62%time of becausesubsequent was the set ex toadsorptionperiments be 70 min. ofwas organic set to bepollutants 70 min. reaches an equilibrium state. The electrolysis time of subsequent experiments was set to be 70 min. 90 90 80 80 70 70 60 60 50 50 40 40 Removal rate of COD/% 30 Removal rate of COD/% 30 20 20 0 20 40 60 80 100 120 0 20 40Time/Min 60 80 100 120 Time/Min

FigureFigure 6. E6.ff Effectect of electrolysisof electrolysis time time on mixedon mixed wastewater wastew CODater COD removal removal rate (anoderate (anode material: material: Fe, cathode Fe, material:cathodeFigure Stainless6. material: Effect of steel,Stainless electrolysis current steel, density:time current on 19.23density:mixed mA wastew 19.23/cm2 mA/cm,ater pH: COD 7.1).2, pH: removal 7.1). rate (anode material: Fe, cathode material: Stainless steel, current density: 19.23 mA/cm2, pH: 7.1). 3.2.3.2. Eff Effectect of of Additives Additives on on COD COD Removal Removal RateRate ofof MixedMixed Wastewater 3.2. Effect of Additives on COD Removal Rate of Mixed Wastewater UnderUnder the the same same current current density, density, an an increase increase of of supporting supporting electrolyteelectrolyte concentration can can lead lead to to a decreasea decreaseUnder of the of the the interelectrode same interelectrode current resistancedensity, resistance an and increase and thus thus of reduce reduce supporting energyenergy electrolyte consumption. concentration Ghernaout Ghernaout can reported lead reported to thatthata decrease the the addition addition of the of interelectrodeof aa supportingsupporting resistance electrolyte electrolyte and can canth usalso also reduce affect affect energythe the removal removalconsumption. rate rateof COD Ghernaout of COD [23]. Hence, [23 reported]. Hence, the theeffectthat effect the of of additionsupporting supporting of aelectrolyte electrolytesupporting concentrationconcentration electrolyte can on on ECalso EC efficiency affect efficiency the was removal was further further rate studied of studied COD in [23].this in this study,Hence, study, and the and effect of supporting electrolyte concentration on EC efficiency was further studied in this study, and NaNa2SO2SO4 was4 was chosen chosen as as the the supporting supporting electrolyteelectrolyte because of its its low low cost. cost. The The Na Na2SO2SO4 concentration4 concentration of of 1.0,1.0,Na 1.1,2 1.1,SO 1.2,4 1.2,was and and chosen 1.3 1.3 g / g/LLas correspondedthe corresponded supporting to toelectrolyte thethe currentcurrent because densities of its of of low 8.24, 8.24, cost. 13.74, 13.74, The 19.23, Na 19.23,2SO and4 and concentration 24.73 24.73 mA/cm mA/ cmof2, 2, 1.0, 1.1, 1.2, and 1.3 g/L corresponded to the current densities of 8.24, 13.74, 19.23, and 24.73 mA/cm2, respectively.respectively. The The results results of of the the eff effectect of of Na Na2SO2SO44addition addition onon thethe COD removal rate are are shown shown in in Figure Figure 7. 7.respectively. The removal The rates results with of the effectaddition of Na of2 SONa42 additionSO4 are much on the lower COD removalthan those rate without are shown Na 2inSO Figure4. No The removal rates with the addition of Na2SO4 are much lower than those without Na2SO4. No positive positive7. The removal effect on rates COD with removal the addition efficiency of is Na observed2SO4 are by much the addition lower than of Na those2SO 4without, which isNa consistent2SO4. No effect on COD removal efficiency is observed by the addition of Na2SO4, which is consistent with positive effect on COD removal efficiency is observed by the addition of Na2SO4, which is consistent previouswith previous reports [reports24]. The [24]. low The removal low removal efficiency efficiency can be attributed can be attributed to the increase to the ofincrease the passivation of the passivationwith previous layer reports caused [24]. by the The addition low removal of Na2SO efficiency4 [25]. can be attributed to the increase of the layer caused by the addition of Na2SO4 [25]. passivation layer caused by the addition of Na2SO4 [25]. 100 without Na2SO4 100 without with NaNa2SOSO4 90 2 4 with Na2SO4 90 80 80 70 70 60 60 50

Removal rate of COD/% 50

Removal rate of COD/% 40 40 30 8 1216202428 30 8Current 1216202428 density/mA·cm-2 Current density/mA·cm-2 Figure 7. Effect of addition of Na2SO4 on mixed wastewater COD removal rate (anode material: Fe, Figure 7. Effect of addition of Na2SO4 on mixed wastewater COD removal rate (anode material: Fe, Figurecathode 7. material:Eﬀect of Stainless addition steel, of Na pH:2SO 7.1,4 onelectrolysis mixed wastewater time: 70 min). COD removal rate (anode material: Fe,cathode cathode material: material: Stainless Stainless steel, steel, pH: pH: 7.1, 7.1, electrolysis electrolysis time: time: 70 70min). min). Activated carbon is widely used to adsorb organic compounds in wastewater [26]. In order to studyActivated the effect carbonof activated is widely carbon used on theto adsorbCOD removal organic efficiency compounds with in EC, wastewat wastewaterer [26]. samples In order were to study the effect of activated carbon on the COD removal efficiency with EC, wastewater samples were

Water 2020, 12, 595 8 of 12

WaterActivated 2020, 12 carbon, 595 is widely used to adsorb organic compounds in wastewater [26]. In order to8 studyof 12 the effect of activated carbon on the COD removal efficiency with EC, wastewater samples were treated treated by GAC adsorption, EC/GAC adsorption (EC plus GAC in one reactor), and EC + GAC by GAC adsorption, EC/GAC adsorption (EC plus GAC in one reactor), and EC + GAC adsorption (EC adsorption (EC followed by GAC in different reactors) process, and their results are summarized in followed by GAC in different reactors) process, and their results are summarized in Figure8. Figure 8.

FigureFigure 8. E ﬀ8.ect Effect of granular of granular activated activated carbon carbon (GAC) (GAC) treatment treatment on on mixed mixed wastewater wastewater CODCOD removalremoval rate (anoderate (anode material: material: Fe, cathodeFe, cathode material: material: Stainless Stainless steel, steel, current current density: density: 19.2719.27 mAmA/cm/cm2,, pH: 7.1, 7.1, electrolysiselectrolysis time: time: 70 min). 70 min).

TheThe Figure Figure8 shows 8 shows that that the the COD COD removal removal rate rate using using GAC GAC alone alone is is low low (only (only 25.7%) 25.7%) whenwhen thethe GACGAC dosage dosage reaches reaches 6.47 6.47 g/L, g/L, indicating indicating that that GAC GAC adsorption adsorption has has little little e effectffect on on thethe residualresidual reagents in mineralin mineral processing processing wastewater. wastewater. When When wastewater wastewater is is treated treated by by the the EC EC processprocess alone,alone, the COD removalremoval rate rate is 77.62%. is 77.62%. Compared Compared with with EC EC or or GAC GAC alone, alone, both both the the EC EC/GAC/GAC and and EC EC+ + GAC processes can improvecan improve COD COD removal. removal. However, However, it isit worthis worth noting noting that that the the COD CODremoval removal raterate ofof ECEC/GAC/GAC is is higherhigher than than that ofthat EC of+ ECGAC. + GAC. At a At dosage a dosage of 2.94 of 2.94 g/L GAC,g/L GAC, the CODthe COD decreases decreases from from 424.9 424.9 to 74.9 to 74.9 and 83.3 mgand/ L,83.3 corresponding mg/L, corresponding to removal to rates removal of 82.37% rates forof 82.37% EC/GAC for and EC/GAC 80.4% forand EC 80.4%/GAC, for respectively. EC/GAC, respectively. Therefore, the EC/GAC process was adopted in the subsequent experiments. Therefore, the EC/GAC process was adopted in the subsequent experiments. It can be concluded that the adsorption of GAC alone cannot be used as an effective treatment It can be concluded that the adsorption of GAC alone cannot be used as an effective treatment method for sulfide mineral flotation wastewater. In the EC/GAC system, GAC acts as a moving method for sulfide mineral flotation wastewater. In the EC/GAC system, GAC acts as a moving particle electrode to accelerate electron transfer rate and hence improve the processing efficiency particle electrode to accelerate electron transfer rate and hence improve the processing efficiency [27,28]. [27,28]. The main reason is that it is essentially a three-dimensional electrochemical process which is The mainnot stable reason [29]. is that it is essentially a three-dimensional electrochemical process which is not stable [29The]. sludge produced is around 5.8 kg/m3 of wastewater under the optimal processing 3 conditions.The sludge It produced can be considered is around as 5.8 a valuable kg/m of resource wastewater because under of theits high optimal Fe content processing of about conditions. 45.5%. It canFurther be considered research on as the a valuable sludge recycling resource ut becauseilization ofwill its soon high be Fe reported content separately. of about 45.5%. Further research onFurthermore, the sludge the recycling effect of utilization H2O2 concentration will soon beon reportedEC efficiency separately. was studied, and the results can beFurthermore, seen in the Supplementary the effect of H2 MaterialsO2 concentration (Figure S1 on and EC Figure efficiency S2). was studied, and the results can be seen in the Supplementary Materials (Figure S1 and Figure S2). 3.3. Effect of EC Treatment on COD Removal Rate of Different Types of Wastewater 3.3. Effect of EC Treatment on COD Removal Rate of Different Types of Wastewater Through the above experiments, the optimum treatment conditions of mixed wastewater were obtainedThrough (iron the above anode, experiments, pH 7.1, 70 min the electrolysis optimum treatment time, 19.23 conditions mA/cm2 current of mixed density, wastewater and 4.1 were g/L obtainedactivated (iron carbon). anode, Under pH 7.1, these 70 minoptimum electrolysis conditions time,, Pb-Zn 19.23 flotation mA/cm 2wastewater,current density, pyrite andconcentrate 4.1 g/L activatedwastewater, carbon). pyrite Under tailing these wastewat optimumer, and conditions, mixed wastewater Pb-Zn flotation were treated, wastewater, respectively. pyrite concentrate The results wastewater,are shown pyrite in Figure tailing 9. wastewater, and mixed wastewater were treated, respectively. The results are shown in Figure9.

Water 2020, 12, 595 9 of 12 Water 2020,, 1212,, 595595 9 of 12

800 100 CODCOD removalremoval raterate CODCOD beforebefore treatmenttreatment 700 CODCOD afterafter treatmenttreatment 80 603.2 600

500 60 434.7 424.9 400

40 COD/mgL-1 COD/mgL-1 300 245.3 200 removal rateof COD/% 20 removal rateof COD/%

85.9 90.2 100 85.9 72.9 50.7

0 0 Pb-Zn flotation Pyrite concentrate Pyrite tailings mixedmixed wastewaterwastewater wastewaterwastewater wastewaterwastewater wastewater FigureFigure 9.9. EffectEﬀect of of EC EC treatment treatment on on the the COD COD values values and and COD COD removal removal rate ratess ofof four four different diﬀerent types types of ofwastewaters. wastewaters.

TheThe COD removal removal rates rates for for the the above above four four types types of wastewaters of wastewaters are are80.24%, 80.24%, 79.33%, 79.33%, 85.05%, 85.05%, and and82.84%, 82.84%, respectively. respectively. The results The results indicate indicate that the that higher the higher the COD the CODvalue valueof wastewater of wastewater is, the is,better the betterefficiency eﬃciency the corresponding the corresponding EC experiment EC experiment obtains. obtains. The pyrite The tailing pyrite wastewater tailing wastewater with the with highest the highestCOD value COD has value the has highest the highest COD COD removal removal rate, rate, followed followed by bymixed mixed wastewater, wastewater, Pb-Zn Pb-Zn ﬂotationflotation wastewater,wastewater, andand pyritepyrite concentrateconcentrate wastewater.wastewater.

3.4.3.4. Effect Effect ofof WaterWater TypeType onon thethethe GradeGradeGrade and andand Recovery RecoveryRecovery of ofof Pb PbPb/Zn/Zn Sulfide Sulfide Mineral Mineral Flotation Flotation TheThe aboveabove experimentalexperimental resultsresults confirmedconfirmed thatthat ECEC isis anan eeffectiveeffectiveffective techniquetechniquetechnique forfor CODCOD removalremoval ofof thethe sulfidesulfide mineral mineral flotation flotation wastewater. wastewater. To To further further investigate investigate the possibility the possibility of reusing of reusing the EC-treated the EC- watertreated as water the feed as the water, feed flotation water, flotation experiments experiments were carried were out, carried and theirout, and results their are results shown are in shown Tables S1–S3,in Tables respectively. S1–S3, respecti Thevely. comparison The comparison of the Pb /ofZn the grades Pb/Zn and grades recovery and recovery rates using rates diff usingerent different types of watertypes of during water flotation during flotation is presented is presented in Figure in10 Figure. 10.

(%) (a) PbPb GradeGrade 80 PbPb RecoveryRecovery 71.73

60 57.62 51.91

40 36.06 30.54 25.01

0 freshfresh waterwater wastwater wastwater water water afterafter teatmentteatment Figure 10. Cont.

Water 2020, 12, 595 10 of 12 Water 2020, 12, 595 10 of 12

(%) Zn Grade (b) 80 Zn Recovery 68.61 65.33 59.99 60 56.96 56.96 56.33

0 fresh water wastwater water after teatment Figure 10. The grade and recovery of Pb ( a)) and Zn ( b) sulfide sulfide flotation flotation concentrates using using different different types of water.

The Pb recovery recovery using different different types of water water ob obeyseys the the following following order: order: Treated Treated water water (36.06%) (36.06%) > freshfresh water water (30.54%) (30.54%) >> wastewaterwastewater (25.01%), (25.01%), wherea whereass the the order order of of Pb Pb grades grades is: is: Fresh Fresh water water (71.73%) (71.73%) > wastewaterwastewater (57.62%) (57.62%) > treatedtreated water water (51.91%). (51.91%). The The experimental experimental results results indicate indicate that that flotation flotation using treated water can significantly significantly improve the PbPb recovery,recovery, whilewhile thethe PbPb gradegrade isis reduced.reduced. In terms terms of of Zn Zn concentrate, concentrate, a a similar similar Zn Zn grade grade is is obtained obtained with with different different types of waters (around 56%). The The Zn Zn recovery obeys the following order: Fresh water (68.61%) > treated water (65.33%) > wastewater (59.99%). Compared Compared with the the untreated untreated wastewater, wastewater, EC-treated water is more conducive for the increase of Zn recovery without aaffectingffecting thethe ZnZn grade.grade.

4. Conclusions Conclusions This study provides the latest evidence that EC is a a promising promising method for treating treating and recycling mineral processing wastewater. Different Diﬀerent factors we werere investigated to obtain optimal experimental 2 conditions, i.e.,i.e., ironiron anode,anode, pH pH 7.1, 7.1, 19.23 19.23 mA mA/cm/cm current2 current density, density, 70 70 min min electrolysis electrolysis time, time, and and 4.1 g4.1/L g/Lactivated activated carbon. carbon. Under Under the the optimal optimal conditions, conditions, the the COD COD of of the the mixed mixed wastewaterwastewater decreases from 424.9 to 72.9 mg/L. mg/L. The addition of a supporting electrolyte likelike NaNa22SO4 cancan be be harmful harmful to to the the removal removal of COD. The COD removal rate of wastewater withwith the highest COD value is highest. Moreover, Moreover, flotationﬂotation results show that the recoveries of both Pb and Zn using EC-treated water are higher than those of directly directly using using wastewater wastewater or fresh water. The results imply that the EC EC technique technique can can provide provide a new new direction direction for for recycling recycling mineral mineral processing processing wast wastewater.ewater. This This study study provides provides novel novel insights insights into dealinginto dealing with withwastewater wastewater in the in themining mining industry industry,, and and is ismeaningful meaningful in in both both environmental environmental and economical ways.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4441/12/2/595/s1: Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Figure S1: Variation of Figure S1: Variation of COD removal rate with dosage of H2O2 (Anode material: Fe, Cathode material: Stainless COD removal rate with dosage of H2 2O2 (Anode material: Fe, Cathode material: Stainless steel, Current density: steel, Current density: 19.23 mA/cm , pH: 3.0, Electrolysis time: 70 min), Figure S2: Eﬀect of H2O2 on the boundary 19.23between mA/cm2, mud and pH:water, 3.0, Electrolysis Table S1: Flotationtime: 70 min), tests resultsFigure withS2: Effect fresh of water, H2O2 Tableon the S2: boundary Flotation between tests results mud withand water,mixed Table wastewater, S1: Flotation Table S3:tests Flotation results with tests fresh results water, with treatedTable S2: water. Flotation tests results with mixed wastewater, TableAuthor S3: Contributions: Flotation tests Conceptualization,results with treated Z.G. water. and W.S.; methodology, W.S.; formal analysis, Z.G.; investigation, AuthorG.J.; data Contributions: curation, G.J. andConceptualization, S.R.; writing—original Z.G. draftand preparation,W.S.; methodology, G.J.; writing—review W.S.; formal and analysis, editing, Z.G.; Y.G.; supervision, Z.G.; project administration, W.S.; funding acquisition, Z.G. All authors have read and agreed to the investigation,published version G.J.; ofdata the curation, manuscript. G.J. and S.R.; writing—original draft preparation, G.J.; writing—review and editing, Y.G.; supervision, Z.G.; project administration, W.S.; funding acquisition, Z.G. All authors have read andFunding: agreedThe to the authors published acknowledge version of ﬁnancial the manuscript. support from the National Natural Science Foundation of China (51634009, 51774328, and 51404300), the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), Funding: The authors acknowledge financial support from the National Natural Science Foundation of China (51634009, 51774328, and 51404300), the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001),

Water 2020, 12, 595 11 of 12 the Young Elite Scientists Sponsorship Program by Hunan province of China (2018RS3011), the Natural Science Foundation of Hunan Province of China (2018JJ2520), and the National 111 Project (B14034). Acknowledgments: The authors are grateful to the financial support from the National Natural Science Foundation of China (51634009, 51774328, and 51404300), the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the Young Elite Scientists Sponsorship Program by Hunan province of China (2018RS3011), the Natural Science Foundation of Hunan Province of China (2018JJ2520), and the National 111 Project (B14034). All authors appreciate the School of Minerals Processing and Bioengineering, Central South University, Changsha, China for providing the facilities. Conflicts of Interest: The authors declare no conflict of interest.

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