Recycling of Cement Kiln Dust As a Raw Material for Cement

environments

Article Recycling of Cement Kiln Dust as a Raw Material for Cement

Minhye Seo, Soo-Young Lee, Chul Lee and Sung-Su Cho * Plant Engineering Center, Institute for Advanced Engineering, Yongin 17180, Korea; [email protected] (M.S.); [email protected] (S.-Y.L.); [email protected] (C.L.) * Correspondence: [email protected]; Tel.: +82-31-330-7688

Received: 24 September 2019; Accepted: 16 October 2019; Published: 17 October 2019

Abstract: Cement kiln dust (CKD) is a major by-product of cement manufacturing and has the potential to be recycled as a raw material if the high concentrations of chlorine and potassium are removed. This study tested four leaching solutions (distilled water and three organic acids) and determined the optimum reaction conditions. At a liquid/solid (L/S ratio) of 10, the removal efficiency of formic, citric, and oxalic acid was higher than that of distilled water, but at L/S 20, distilled water also achieved a high removal efficiency of Cl ( 90%) and K ( 70%). In addition, to minimize the ≥ ≥ discharge of wastewater after leaching, the efficiency of ion-exchange resins for the recovery of leaching solution was tested. When the cation- and anion-exchange resins were arranged together, more than 95% of both Cl and K contained in the leaching solution could be removed. Leaching solution without Cl and K was found to have a high leaching efficiency even after being recycled three times, resulting in a significant reduction in wastewater emissions.

Keywords: cement dust kiln; recycling; leaching of Cl; K; leaching solution; ion exchange process

1. Introduction Cement kiln dust (CKD) is a by-product of the cement manufacturing process and has traditionally been considered as an industrial waste product. Global cement production capacity in 2017 was ~4.99 billion tons per year [1,2], while the CKD production rate ranged from 54 to 200 kg per ton of produced cement clinker [3]. CKD is composed of fine, powdery solids and highly alkaline particulate material, and is similar in appearance to Portland cement. Its size distribution and chemical composition depends on production factors such as raw material, processing method, fuel, kiln type, cement type, and dust collection method (e.g., cyclones, bag filters, or electrostatic precipitators). Finer particles tend to exhibit a higher sulfate and alkali content. In general, CKD has a lower CaO and SiO2 content than typical Portland cement (Table1). Compared with cement, CKD is characterized by higher alkalis (especially potassium), and chloride. The high alkali content precludes the recycling of CKD into kilns, as this would exceed the maximum allowable value [4]. The increasing use of alternative fuels has driven a rise in chlorine input into kiln processes, increasing the importance of chlorine bypass systems in order to avoid operational problems. In particular, using waste fuel can increase the toxic metal content of the subsequent CKD [5]. A high chlorine content in CKD can lead to various problems in its reuse. In general, the maximum chloride content of cement is 0.10%, while the chloride contained in CKD may contain 0.35–15.4 wt%, depending on its raw materials [6]. Chlorine is highly corrosive, and high levels of it in cement can encourage steel corrosion. Several studies have reported that CKD can effectively improve soil strength and can be used as an alternative to lime for soil stabilization [6,7]. Its alkaline properties and good absorption capacity stabilize waste (instead of cement or lime) by reducing the moisture content and increasing the bearing capacity [8]. In addition to this, it has also been injected into an asphalt concrete mixture and

Environments 2019, 6, 113; doi:10.3390/environments6100113 www.mdpi.com/journal/environments Environments 2019, 6, 113 2 of 7 used as a mineral filler. However, CKD still contains corrosive substances such as chlorine, requiring a suitable technology to render them harmless. Some cement plants simply send CKD to landfills, but the increasing annual stockpile of CKD and the high cost of its disposal requires an innovative solution that will allow CKD to be used in construction. Lanzerstorfer [9] reported that chloride concentration from cement kiln bypass dust (CKBD) depends on the particle size and can be removed by air classification. However, air classification is only a pretreatment and has the disadvantage of simultaneously requiring a leaching process. Thus, this study tested methods for the removal of chlorine and potassium ions from CKD in order to allow its reuse as a cement raw material, and evaluated the removal efficiency of various leaching conditions and the feasibility of recovering the leaching solutions through an ion-exchange system.

Table 1. Typical chemical composition of Portland cement and cement kiln dust (CKD).

Content CaO SiO2 K2O Al2O3 Cl− Ref. 64 22 0.3 5 <0.1 [6] Portland cement (%) 63 20 - 5 - [10] 62 21 0.5 4 - [11] 38–50 11–16 3–13 3–6 0–5 [12] CKD (%) 60.56 6.1 2.56 1.37 2.75 [13]

2. Materials and Methods The CKD used in this study was obtained from Sampyo Cement, South Korea; it consisted mainly of CaO with significant amounts of chloride, potassium, and sulfate anions (Table2). The Cl and K ions were leached using four agents: distilled water (DW) and the organic acids citric acid (99.5%, Daejung), oxalic acid (99.5%, Junsei), and formic acid (85%, Daejung). The reaction times (1–30 min) of each 200 mL distilled water (DW) and 0.5 M organic-acid leaching solution were compared. The leaching test of CKD was performed by changing the ratio of liquid and solid to 10 and 20. The CKD had a low specific gravity and had limitations in mixing and reacting in a liquid phase, making it difficult + to further lower the L/S ratio. The removal efficiency of Cl− or K ions can be estimated using the following Equation (1):

Leached Cl− (or K−)conc. in CKD (g) Removal Eﬃciency (%) = 100 (1) Cl− (or K−)conc. in CKD (g) ×

The Cl and K ions leached from the CKD were configured in order to recycle the leaching solution through the ion-exchange reaction (Figure1). An ion-exchange resin was used in conjunction with an ion-exchange resin (SAR20MBOH, Samyang Co., Ltd.) and cation-exchange resin (SCR-BH, Samyang Co., Ltd.), respectively. Then, the CKD (L/S: 20) was added to the deionized leaching solution through the ion-exchange resin, thereby removing Cl and K ions from the CKD. The leaching and removal efficiency of the Cl and K ions from the CKD was evaluated using X-ray fluorescence (XRF-1800, Shimadzu) after pelletizing with B2O3. The ion content in the leaching solution was then measured by an ion chromatograph (DX-500, Dionex). Scanning electron microscopy (MIRA3, Tescan) and energy dispersive spectroscopy (Bruker, XFlash 6130) were used to analyze the morphology and impurities of the ion-exchange resins surface. Environments 2019, 6, 113 3 of 7 Environments 2019, 6, x FOR PEER REVIEW 3 of 7

Figure 1. FlowFlow diagram diagram for for the the leaching leaching and ion-exchange process of cement kiln dust.

Table 2. Properties of CKD powder from SAMPYO cement in South Korea. Table 2. Properties of CKD powder from SAMPYO cement in South Korea. Element CKD (wt%) Element CKD (wt%) Cl 39.10 Cl K39.10 17.85 Ca 18.38 K Si17.85 12.62 S 3.51 Ca Al18.38 2.95 Fe 2.76 Si LoI *12.62 2.83

S * Loss on Ignition (1053.51◦ C).

3. Results and Discussion Al 2.95 The removal efficiencies ofFe Cl and K were ~87% and2.76 ~80%, under 0.5M formic acid. The removal efficiencies were highest for formic acid, followed by oxalic acid, citric acid, and DW; the reaction time LoI* 2.83 varied little between the solutions (Figure2). As shown in Figure2, the leaching reaction occurred quickly and it can be seen that the leaching* Loss rate on Ignition was not (105 increased °C). any longer even though the reaction time increased. Suzuki et al. [14] reported that the salt produced, indicated by K2O, could be recovered 3.by Results more than and 53% Discussion by applying NaCl solution. In this study, however, potassium was able to remove moreThe than removal 80% in efficiencies 5 min. Similar of Cl results and K have were been ~87% found and ~80%, in other under studies 0.5M on formic organic acid. acid The leaching removal for efficienciesthe purpose were of waste highest recycling for formic [15,16 ].acid, Musariri followed et al. by [15 oxalic] reported acid, thatcitric citric acid, acid and and DW; malic the acid reaction both timeproduced varied a leaching little between rate of aboutthe solu 70%tions within (Figure 10 min 2). in As a cobalt shown leaching in Figure experiment. 2, the leaching In this study, reaction the occurredleachingEnvironments ratequickly did 2019 notand, 6, x increaseFORit can PEER be significantlyREVIEW seen that the over leaching time. rate was not increased any longer even4 ofthough 7 the reaction time increased. Suzuki et al. [14] reported that the salt produced, indicated by K2O, could be recovered by more than 53% by applying NaCl solution. In this study, however, potassium was able to remove more than 80% in 5 min. Similar results have been found in other studies on organic acid leaching for the purpose of waste recycling [15,16]. Musariri et al. [15] reported that citric acid and malic acid both produced a leaching rate of about 70% within 10 min in a cobalt leaching experiment. In this study, the leaching rate did not increase significantly over time.

FigureFigure 2. Ion 2. Ion removal removal eﬃ efficiencyciency of of CKD CKD asas aa functionfunction of of the the leaching leaching solution; solution; (a) chloride (a) chloride and ( andb) (b) potassium.potassium. Reaction Reaction conditions: conditions: L /L/SS (Liquid (Liquid solidsolid ratio) == 10,10, 25 25 °C.◦ C.1 atm. 1 atm.

To optimize the safety of CKD recycling, it is important to use solvents that are easy to handle and with minimal environmental concerns. Of the leaching solutions used in this study, DW is the easiest to handle, so its ion removal efficiency was further evaluated by reducing the CKD injection amount to 5 wt% (Figure 3). The removal efficiencies of Cl and K were ~90% and ~83%, respectively, for 1 min of extraction and were similar for up to 5 min of extraction, after which the remaining Ca content was analyzed. The leaching rate increased to 85% for up to 5 min, but Ca began to dissolve in the leaching solution at longer reaction times. This clearly indicates that the extraction reaction time is an important parameter, with an optimal value of ≤5 min in a fixed-bed column. Golpayegani et al. [17] also reported that the chloride or potassium leaching time was an important parameter in an experimental model considering stirring rate, concentration, and temperature.

Figure 3. Ion removal efficiency of CKD with distilled water (DW) as a function of the reaction time; (a) removal of Cl, K, and concentration of Ca, (b) concentration of Cl and K including leached solution. Reaction conditions: L/S = 20, 25 °C. 1 atm.

The ion-exchange capacity was higher at lower flow rates; at 20, 40, and 80 mL/min, the removal rate of Cl ions was ≥99%, 97%, and 70%, and that of K ions was ≥99%, 97%, and 80%, respectively (Figure 4). Liu et al. [18] reported an 80% removal efficiency using the anion-exchange method for the removal of chlorine ions from zinc-production wastewater, meeting the legal requirements for wastewater recycling. Methods for Cl ion removal include chemical precipitation, flocculation, solvent extraction, membrane separation, and ion exchange. The latter has many advantages, including high reaction time, simplicity, and low construction/operation cost [18]. In this study, the Cl and K eluted from CKD was removed easily and effectively via ion-exchange resin, and the

Environments 2019, 6, x FOR PEER REVIEW 4 of 7

Environments 2019, 6, 113 4 of 7 Figure 2. Ion removal efficiency of CKD as a function of the leaching solution; (a) chloride and (b) potassium. Reaction conditions: L/S (Liquid solid ratio) = 10, 25 °C. 1 atm. To optimize the safety of CKD recycling, it is important to use solvents that are easy to handle and withTo minimal optimize environmental the safety of concerns. CKD recycling, Of the leachingit is important solutions to use used solvents in this study,that are DW easy is theto handle easiest toand handle, with minimal so its ion environmental removal efficiency concerns. was further Of the evaluated leaching bysolutions reducing used the CKDin this injection study, amountDW is the to 5easiest wt% (Figureto handle,3). The so its removal ion removal e fficiencies efficiency of Cl wa ands further K were ~90%evaluated and ~83%,by reducing respectively, the CKD for injection 1 min of extractionamount to and5 wt% were (Figure similar 3). forThe up removal to 5 min efficiencies of extraction, of Cl after and whichK were the ~90% remaining and ~83%, Ca respectively, content was analyzed.for 1 min of The extraction leaching and rate were increased similar to 85%for up for to up 5 tomin 5 min, of extraction, but Ca began after to which dissolve the in remaining the leaching Ca solutioncontent was at longer analyzed. reaction The times. leaching This rate clearly increased indicates to that85% thefor extractionup to 5 min, reaction but Ca time began is an to important dissolve parameter,in the leaching with ansolution optimal atvalue longer of reaction5 min in times. a fixed-bed This clearly column. indicatesGolpayegani that the et al. extraction [17] also reportedreaction ≤ thattime theis an chloride important or potassium parameter, leaching with an timeoptimal was value an important of ≤5 min parameter in a fixed-bed in an column. experimental Golpayegani model consideringet al. [17] also stirring reported rate, that concentration, the chloride and or potassium temperature. leaching time was an important parameter in an experimental model considering stirring rate, concentration, and temperature.

Figure 3. Ion removal efficiency of CKD with distilled water (DW) as a function of the reaction time; Figure 3. Ion removal efficiency of CKD with distilled water (DW) as a function of the reaction time; (a) removal of Cl, K, and concentration of Ca, (b) concentration of Cl and K including leached solution. (a) removal of Cl, K, and concentration of Ca, (b) concentration of Cl and K including leached solution. Reaction conditions: L/S = 20, 25 C. 1 atm. Reaction conditions: L/S = 20, 25 °C.◦ 1 atm. The ion-exchange capacity was higher at lower flow rates; at 20, 40, and 80 mL/min, the removal The ion-exchange capacity was higher at lower flow rates; at 20, 40, and 80 mL/min, the removal rate of Cl ions was 99%, 97%, and 70%, and that of K ions was 99%, 97%, and 80%, respectively rate of Cl ions was ≥≥99%, 97%, and 70%, and that of K ions was ≥≥99%, 97%, and 80%, respectively (Figure4). Liu et al. [ 18] reported an 80% removal efficiency using the anion-exchange method for (Figure 4). Liu et al. [18] reported an 80% removal efficiency using the anion-exchange method for the removal of chlorine ions from zinc-production wastewater, meeting the legal requirements for the removal of chlorine ions from zinc-production wastewater, meeting the legal requirements for wastewater recycling. Methods for Cl ion removal include chemical precipitation, flocculation, solvent wastewater recycling. Methods for Cl ion removal include chemical precipitation, flocculation, extraction, membrane separation, and ion exchange. The latter has many advantages, including high solvent extraction, membrane separation, and ion exchange. The latter has many advantages, reaction time, simplicity, and low construction/operation cost [18]. In this study, the Cl and K eluted including high reaction time, simplicity, and low construction/operation cost [18]. In this study, the from CKD was removed easily and effectively via ion-exchange resin, and the optimal operation speed Cl and K eluted from CKD was removed easily and effectively via ion-exchange resin, and the with a fixed-bed column was 40 ml/min. The potential treatment capacity was 267 g-K/L-resin and 255 g-Cl/L-resin, respectively. Environments 2019, 6, x FOR PEER REVIEW 5 of 7

Environments 2019 6 optimal, operation, 113 speed with a fixed-bed column was 40 ml/min. The potential treatment capacity 5 of 7 was 267 g-K/L-resin and 255 g-Cl/L-resin, respectively.

Environments 2019, 6, x FOR PEER REVIEW 5 of 7

optimal operation speed with a fixed-bed column was 40 ml/min. The potential treatment capacity was 267 g-K/L-resin and 255 g-Cl/L-resin, respectively.

Figure 4. Ion removal efficiency for recycling of spent leaching solution. Figure 4. Ion removal efficiency for recycling of spent leaching solution. SEM analysis of the resin before and after ion exchange showed that cation-resin size ranged from 100 to 700 µm and anion-resin size ranged from 500 to 800 µm (Figure5). After the reaction, the ion-exchange resin had a rougher surface than before, and surface analysis using energy dispersive spectroscopy (EDS) showed that fresh and spent resin had different components. After the ion exchange, the resin surface contained 12.9% K and 13.5% Cl and the ions in the solution were effectively removed (Figure4). Figure 4. Ion removal efficiency for recycling of spent leaching solution.

N O S Al Si K Ca Fe

(a) Fresh 27.2 37.9 34.8 - - - - - Cation resin (%) (b) Spent - 42.6 20.2 3.0 2.5 12.9 13.3 5.5

N O F Cl Ca

(a) Fresh 27.2 37.9 34.8 - - - - - Cation resin (%) (b) Spent - 42.6 20.2 3.0 2.5 12.9 13.3 5.5

N O F Cl Ca

Figure 5. SEM images and EDS analysis of ion-exchange resins; (a) fresh cation-resin, (b) spent cation-resin, (c) fresh anion-resin, (d) spent anion-resin. Environments 2019, 6, x FOR PEER REVIEW 6 of 7

Figure 5. SEM images and EDS analysis of ion-exchange resins; (a) fresh cation-resin, (b) spent cation- resin, (c) fresh anion-resin, (d) spent anion-resin.

SEM analysis of the resin before and after ion exchange showed that cation-resin size ranged from 100 to 700 μm and anion-resin size ranged from 500 to 800 μm (Figure 5). After the reaction, the ion-exchange resin had a rougher surface than before, and surface analysis using energy dispersive spectroscopyEnvironments 2019 (EDS), 6, 113 showed that fresh and spent resin had different components. After the 6ion of 7 exchange, the resin surface contained 12.9% K and 13.5% Cl and the ions in the solution were effectively removed (Figure 4). AfterAfter leaching, thethe K K and and Cl Cl ions ions in thein solutionthe solution were were removed removed by ion by exchange, ion exchange, while the while leaching the leachingsolution solution was recovered was recovered and reused and up reused to three uptimes to three (Figure times6). (Figure The removal 6). The eremovalfficiencies efficiencies of K and of Cl Kextracted and Cl extracted from the CKDfrom werethe CKD found were to befound similar, to be indicating similar, indicating that leaching that solution leaching can solution be effectively can be effectivelyrecycled. Recycling recycled. the Recycling leaching solutionthe leaching can significantly solution can reduce significantly the wastewater reduce generated the wastewater from the generatedCKD treatment, from the which CKD can treatment, have a positive which can effect have on thea positive process effect operation. on the process operation.

Figure 6. Leaching test of CKD with recycled leaching solution. Reaction conditions: Reaction Figure 6. Leaching test of CKD with recycled leaching solution. Reaction conditions: Reaction conditions: L/S = 20, 25 C. 1 atm. conditions: L/S = 20, 25 °C.◦ 1 atm. 4. Conclusions 4. Conclusions This study assessed the use of different leaching solutions for removing chloride and potassium fromThis CKD study to allow assessed its reuse the inuse cement of different manufacturing. leaching solutions Although for organic removing acids chloride were twice and aspotassium effective fromas distilled CKD to water, allow the its latterreuse wasin cement selected manufacturing. for further tests Although due to its organic higher acids handling were safetytwice as and effective ease of asrecycling. distilled Distilled water, the water latter was was able selected to remove for overfurthe 90%r tests of Cl due and to over its higher 70% of handling K within safety 1 min underand ease the ofcondition recycling. of Distilled L/S 20. In water the leaching was able reaction, to remove the over Ca concentration90% of Cl and inover the 70% CKD of increasedK within 1 up min to under 5 min, thehowever, condition Ca wasof L/S eluted 20. In at the>5 min,leaching reducing reaction, theCa the content Ca concentration in the CKD. in It the was CKD confirmed increased that up control to 5 min,of the however, reaction timeCa was is an eluted important at >5 min, parameter. reducing the Ca content in the CKD. It was confirmed that controlFurthermore, of the reaction Cl time and Kis an could important be removed parameter. from the leachate through ion exchange to allow leachingFurthermore, solution recyclingCl and K and could to minimize be removed wastewater from the generation. leachate through The ion-exchange ion exchange capacity to allow was leaching255 g-Cl /solutionL-resin and recycling 267 g-K and/L-resin to minimize under a wastewater flow rate of generation. 40 mL/min, The and ion-exchange all showed similar capacity removal was 255effi cienciesg-Cl/L-resin of 95% and or 267 more g-K/L-resin when recycled under and a flow eluted rate threeof 40 timesmL/min, after and ion all exchange. showed similar removal efficiencies of 95% or more when recycled and eluted three times after ion exchange. Author Contributions: Conceptualization, M.S. and S.-S.C.; methodology, M.S. and S.-Y.L. and C.L.; formal Authoranalysis, Contributions: M.S., C.L. and S.-Y.L.;Conceptualization, writing—original M.S. draftand S.-S.C.; preparation, methodology, M.S.; writing—review M.S. and S.-Y.L. and and editing, C.L.; M.S. formal and analysis,S.-S.C.; project M.S., C.L. administration, and S.-Y.L.; S.-S.C.writing—original draft preparation, M.S.; writing—review and editing, M.S. and S.-S.C.; project administration, S.-S.C. Funding: This study was supported by the Energy Development Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resources from the Ministry of Trade, Industry and Energy, Korea (20182010202100). Conflicts of Interest: The authors declare no conflict of interest. Environments 2019, 6, 113 7 of 7

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