Journal of Cleaner Production 199 (2018) 891e899 Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro Green and efﬁcient utilization of waste ferric-oxide desulfurizer to clean waste copper slag by the smelting reduction-sulfurizing process Zhengqi Guo, Jian Pan*, Deqing Zhu**, Feng Zhang School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, PR China a r t i c l e i n f o a b s t r a c t Article history: Received 13 December 2017 Received in revised form 13 July 2018 Accepted 21 July 2018 Available online 23 July 2018 This research proposed an efﬁcient, innovative and environmentally friendly technology named smelting reduction-sulfurizing process, which uses one waste (waste ferric-oxide desulfurizer) to treat another (waste copper slag). In the new process, the waste ferric-oxide desulfurizer was employed not only as a sulfurizing agent to sulfurize and collect the copper lost in copper slag, but also as a reductant to reduce the magnetite to “FeO” and thus improve slag ﬂuidity. It was revealed that 90.81% Cu was recovered and enriched in copper matte under the smelting conditions. The matte contained 15.87% copper, 20.25% S and 49.56% Fe, which can be returned to the copper smelting process as a feeding. Meanwhile, the removal rate of hazardous elements, such as Ni, Pb, Zn, As, Sb, Bi and Hg, from the initial copper slag by the new process was also determined, and the elimination rate of those elements was all over 90%.The leaching toxicity was used to further evaluate the environmental impact of the cleaned slag, indicating that the concentrations of toxic element ions in the leachate are all much lower than the thresholds, which conﬁrmed that the cleaned slag with trace toxic elements is safe and harmless. © 2018 Elsevier Ltd. All rights reserved. Keywords: Waste copper slag Waste ferric-oxide desulfurizer Slag cleaning Copper recovery 1. Introduction Copper is one of important base metals required for various applications in modern industries, and nearly 80% of copper in the word is produced by pyro-metallurgical process (Potysz et al., 2015; Gbor et al., 2000). Approximately 30 million tons of smelting slag is estimated to be produced and dumped around the word annually (Shen and Forssberg,2003; Heo et al., 2013). Typically, the slag contains not only valuable metals such as Cu, Co, Ni, Pb, Zn, but also many undesirable hazardous metals like As, Sb, Bi, Hg (Shibayama et al., 2010). Unfortunately, of these, more than 80% is dumped directly in many countries without treatment, which not only wastes the limited resources extravagantly but also occupies precious land and even poses potentially damages to the ecological environment (Gorai et al., 2003; Chen et al., 2016). In recent years, extensive researches have been carried out to clean the copper slag. Basically, they can be classiﬁed into two main categories, namely hydrometallurgical process and pyro- * Corresponding author. Peace Building, School of Minerals Processing & Bioengineering Central South University, 410083 Changsha, Hunan, PR China. ** Corresponding author. Peace Building, School of Minerals Processing & Bioengineering Central South University, 410083 Changsha, Hunan, PR China. E-mail addresses: firstname.lastname@example.org (Z. Guo), email@example.com (J. Pan). https://doi.org/10.1016/j.jclepro.2018.07.203 0959-6526/© 2018 Elsevier Ltd. All rights reserved. metallurgical process (Krishna et al., 2015; Al-Jabri et al., 2002). Hydrometallurgical processes are effective methods to extract metals from copper slags including ﬂotation, leaching, roasting followed by leaching and bioleaching (Subrata et al., 2015; Li et al., 2008; Sandeep et al., 2015). But it is very difﬁcult to treat the waste water generated from that, and its production efﬁciency is limited. However, pyro-metallurgy, as a widely used technology in today's production of base and precious metals, is mainly used for slag treatment (Huiting and Forssberg, 2003; Hughes, 2000; Jung et al., 2016; Tang et al., 2015; Zhou et al., 2015). Moreover, if the pyrometallurgical process is employed to clean the slag, the slag may be treated at its molten stage once it is discarded from the furnace before it cools down (Dirk et al., 2008; Guo et al., 2016a, b; Guo et al., 2018). Thus, processing in the molten condition to recover the valuables is more energy efﬁcient. Maweja et al. had attempt to clean the copper smelting slag from a water-jacket furnace by direct reduction (Maweja et al., 2009). But more coal were used to obtain the higher metals recovery, resulting in unacceptably high iron content in matte phase. Mikhail and Matusewicz reported that pyrite concentrate, as a collector phase, was employed to improve cleaning of copper slag (Mikhail and Webster, 1992; Matusewicz and Mounsey, 1998) in sulfurizing process. Indeed, it is an effective way to clean slag to produce low grade matte. However, the excessive addition of pyrite concentrate inevitably causes higher Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 cost. Moreover, the iron content in the matte produced in this process is too high, leading to a difﬁculty in further converting process. Li et al. reported the gypsum was used as a sulfurizing agent to sulfurize and recover copper and cobalt from coppercobalt smelting slag (Li et al., 2017; Jeong et al., 2016), however, the removal of toxic elements was neglected. Therefore, this investigation is set to develop an eco-friendly and cost-effective process for selective extraction of valuable metals from copper slag and maximized removal of toxic elements simultaneously. Besides, we have noticed that waste ferric-oxide desulfurizer(WDS), as a kind of solid residue, is generated from coal-derived gas industry with high temperature H2S removal process (Ren et al., 2010). With the rapid development of the coal-derived gas industry, iron oxides desulfurizer has been attracted more attention due to their low cost and high desulfurization performance (Liu et al., 2013). Concomitantly, the more and more waste ferricoxide desulfurizer is produced, which is under considerable pressure of green treatment and effective utilization. Unfortunately, it has been rarely reported to process waste ferric-oxide desulfurizer with high efﬁciency and environment protection so far. Hao et al. reported that WDS can be applied in sulfuric acid production area due to its high sulfur content (Hao and Wang, 1992). Qian reported that WDS could be recycled and prepared to iron oxide red, which is an excellent pigment and has ﬁne qualities such as covering power, coloring power, heat resistance, resistance to solvent, acid fastness, innocuity and anti-rust (Qian, 2010). The WDS Generally, it primarily contains iron sulﬁde (FeS, Fe2S3), sulfur (S2), calcium carbonate(CaCO3), calcium sulfate(CaSO4), carbon (C) and etc (Hao and Wang, 1992). Thus, it bears the potential to be used as a sulfurizing agent, ﬂux, or reductant that may play important roles in cleaning process of smelting slag replacing the traditional sulfurizing agent, such as pyrite and calcium ﬂux. In this study, a novel method named smelting reductionsulfurizing process was proposed to recover copper and eliminate other hazardous metals simultaneously from copper slag. In this new slag cleaning process, the waste ferric-oxide desulfurizer was employed as a sulfurizing agent to sulfurize and capture the metals into matte phase from the slag at molten stage. Then the separation between matte and secondary slag took place in the crucible by density difference at high temperature, with the heavier matte dropping to the bottom of the crucible. The matte phase carrying valuable metals and hazardous elements would be further processed for reﬁnement. And the cleaned slag containing less toxic elements has the potential to be utilized as an eco-friendly, clean functional material, which can avoid land occupancy and minimize environmental risks. 2. Experimental 2.1. Materials 2.1.1. Copper slag Copper slag used in this study was collected from Tongling Nonferrous Metals Group Holding Co., Ltd, Anhui, China. Chemical compositions of copper slag are shown in Table 1. It can be seen from that the copper slag contains 2.71 wt.% Cu, 0.48 wt.% Ni, and some toxic elements, including Pb, Zn and As, are also observed in it. Phase compositions of the copper slag were determined by X-ray diffraction (XRD)analyses and the result are shown in Fig. 1. It indicates that the used slag is mainly composed of magnetite and fayalite. The distribution of copper in associated minerals in smelting slag were carried out based on solubility difference of various phases in solvent and results are shown in Table 2. As can be seen from that 50.55% of copper is present in the form of copper sulﬁde, 24.35% of copper exists in metallic copper and the proportion of copper in oxide form entrapped in slag is as high as 25.09%. 2.1.2. WDS The chemical composition of the WDS from Baosteel Group Corporation is shown in Table 3. WDS mainly comprises 32.33 wt.% S, 17.91 wt.% CaO and 7.89 wt.% Fe, which was used in this work as a replacement of sulfurizing agent and ﬂux. In addition, it also contains 5.82% C, which might be suitable to substitute some of the reductant. The XRD patterns shown in Fig. 2 conﬁrms that the WDS consists of crystalline gypsum, pyrite, sulfur with calcite and some unidentiﬁed minor phases. In smelting reduction-sulfurizing process, sulfur, existing in CaSO4, FeS2 and S2, would transfer into and ﬁx in the metal sulﬁdes through contacting and reacting with metal oxides. Meanwhile, CaCO3, as a kind of ﬂuxes, can decrease the viscosity of the molten slag by breaking its silica bonds, which can accelerate the settling of matte particles and minimize the metal losses with mechanical entrapment of matte (Guo et al., 2016a, b). 2.1.3. Reductant Blind coal, which was crushed and screened to a size of 1.0 mm, served as the reductant. Its ash chemical composition, proximate analysis and ash fusibility analysis were determined by GB/T212e2008 and GB/T219-2008, and results are shown in Tables 4 and 5, respectively. 2.2. Experimental methods 2.2.1. Smelting reduction-sulfurizing process All samples were dried, ground and sieved to yield a particle size below 1.0 mm prior to the smelting process. In the smelting reduction-sulfuring step, for each test, 200 g ground slag and a M M-Fe3O4 Magnetitie F-FeSi2O4 Fayalite F Intensity/Counts 892 F F 10 20 30 F F F F M F M F MF 40 F M F 50 Two-theta/deg M M F 60 70 80 34 35 36 Two-theta/deg Fig. 1. XRD patterns of the copper slag. Table 1 Chemical analysis of copper smelting slag (Wt.%). Element Fetotal Ni Cu Pb Zn As SiO2 Al2O3 CaO MgO Cr S P LOI Copper slag 39.56 0.48 2.71 0.68 1.93 0.043 26.61 3.71 2.50 1.05 0.04 0.83 0.01 0.98 Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 893 Table 2 Distribution of copper in associated minerals (mass fraction, %). Mineral Copper oxide Metallic copper Copper sulﬁde Combined copper oxide Cutotal Content Fraction 0.63 23.24 0.66 24.35 1.37 50.55 0.05 1.85 2.71 100.00 Table 3 Chemical analysis of WDS (Wt.%). Element Fetotal Cr SiO2 Al2O3 CaO MgO C S P LOI WDS 7.89 0.01 0.86 0.31 17.91 0.21 5.82 32.22 0.01 38.02 G-CaSO4Gypsum G Intensity/Counts I-FeS2 G Sulfur S-S2 G 10 C & 6 G I, , G Pyrite C-CaCO3 Calcite 6 20 G, , I , ,G 30 40 50 Two-theta/deg S 60 70 80 27 28 29 30 Two-theta/deg Fig. 2. XRD patterns of the waste ferric-oxide desulfurizer. Table 4 Main chemical compositions of blind coal ash/wt.%. Fe2O3 SiO2 Al2O3 CaO MgO P S 0.757 1.24 0.36 1.12 0.06 0.0006 0.58 certain amount of coal (0e8%) and WDS (0e20%) were mixed evenly and then charged into a 200 ml corundum crucible. Subsequently, the corundum crucible was put in a vertical tube furnace (as shown in Fig. 3) and heated to ﬁxed temperature with 10 C/min heating rate under high purity nitrogen (N2) atmosphere, and then kept at that temperature (1200 C-1400 C) for various durations (0.5 h-4 h) to allow for homogenization of the melt. After the smelting reduction-sulfurizing, the molten slag was cooled to room temperature under the protection of N2, and then it was removed from the furnace. Subsequently, the crucible was broken to separate matte and cleaned slag very carefully. Finally, both cleaned slag and matte were weighed and chemically analyzed by ICP-AES, AAS (Atomic Absorption Spectroscopy) and chemical titration. Test methods of the content of total iron (TFe), ferrous iron(Fe2þ) and metallic iron(MFe) were shown in papers (Meng and Wang,1980; Huang, 1989). The content of Fe3þ was calculated by mass balance of iron. Fig. 3. Schematic diagram of the experimental apparatus. The chemical species analysis method for determining the copper phases is based on solubility difference of different phases in solvent. Addition of appropriate inhibitors can effectively inhibit dissolution of speciﬁc species, and the detail method can be seen in some references (Liu et al., 2015; Xing and Liu, 2014; Kun, 1974). 2.2.2. Toxicity characteristic leaching procedure (TCLP) test The cleaned slag was crushed to <9.5 mm for toxicity characteristic leaching procedure (TCLP, USEPA, 2017) (U.S. EPA, Part 261). The TCLP test of cleaned slag was carried out in a buffered acetic acid under the conditions of pH 4.93 ± 0.05,100 g cleaned slag obtained from smelting reduction-sulfurizing process, liquid/solid ratio 20, stirring speed 30 ± 2 rpm, leaching time 18 ± 2 h and temperature 22.3 ± 3 C. After leaching, the leachates were ﬁltered by glass ﬁber ﬁlter with pore of 0.6e0.8 mm. The leaching liquor were detected for potential toxic content by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin Elmer, Optima 3000). Table 5 Proximate and fusibility analysis results of soft coal. Ash fusibility analysis/ C Proximate analysis/% Mad Aad Vad FCad DT ST HT FT 12.98 4.49 30.41 52.12 1332 1376 1450 1469 (Mad：Moisture；Aad：Ash；Vad：Volatile Matter ；FCad：Fix Carbon；DT：Distortion Temperature；ST：Soften Temperature；HT：Hemispherical Temperature；FT： Flow Temperature). 894 Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 2.2.3. Characterization Crystalline mineral compositions of samples were revealed by X-ray powder diffraction (XRD, RIGAKU, D/Max-2500). Optical microscopy (Leica DMRXP) and scanning electron microscopy (SEM, JEOL, JSM-6490LV) with energy dispersive spectroscopy (EDS, JEOLJSM-6490LV) were employed to identify the phases of samples while the combination of latter two techniques was applied to determine the elemental quantitative of each phase (Chen et al., 2017a, b). likely reduction reactions are as follows: Fe3O4(s)þC (s) ¼ 3FeO(l)þCO(g) (6) DGq ¼ 31.52e0.045T (KJ/mol) (7) 3Fe3O4(s)þFeS(l) ¼ 10FeO(l)þSO2(g) (8) DGq ¼ 85.43e0.053T (KJ/mol) (9) And DGq also can be shown as follows: 3. Results and discussion DGq ¼ -InKq 3.1. Thermodynamic analysis As can be seen from Table 2, some copper dissolved in slag as the form of oxidation state. In order to simplify the reaction mechanism of reduction-sulfurizing process, Cu2O was used to represent copper oxides dissolved in slag. Fig. 4 shows the variation in the standard free energy(DGq) with the temperature for the sulfuring reduction of copper oxides and iron oxides. It can be seen from that the standard free energy variation for the reactions 1e5 is negative over the temperature range of 1100e1400 C, indicating that the sulfurizing reaction is thermodynamically possible. Hence, it is possible to sulfurize the copper oxides in the slag through adding the WDS. Cu2O(l)þCaSO4(s)þ4C(l) ¼ Cu2S(l) þCaO(s)þ4CO(g) (1) FeO(l)þCaSO4(s)þ4C(l) ¼ FeS(l)þCaO(s)þ4CO(g) (2) 2Cu2O(l) þ1.5S2(g) ¼ Cu2S(s)þSO2(g) (3) 4FeO(l)þ3S2(g) ¼ 4FeS(l)þ2SO2(g) (4) Cu2O(l) þFeS(l) ¼ Cu2S(l) þFeO(l) (5) In addition, Fe3O4 in the slag also plays a critical role to affect the cleaning efﬁciency of copper slag. Generally, low content of Fe3O4 is beneﬁcial to decreasing the viscosity and improving the ﬂuidity of molten slag (Sun and Huang, 1992; Cui., 1978; Davenport et al., 2002). Hence, the reduction treatment of copper slag to decrease the content of Fe3O4 is necessary by adding reductants in view of some magnetite existing in the initial slag (as seen Fig. 1). The most (10) Where: DGq is the Gibbs free energy of the reaction (9); Kq is the equilibrium constant of the reaction (9); Kq can be expressed by that: kq ¼ a10 Feo PSO2 (11) a3Fe3o4 aFeS Through the equations (9), (10) and (11): . DGq a3Fe3o4 aFeS ¼ a10 Feo PSO2 e (12) Where: aFe3O4 is the activity of Fe3O4 in molten slag; aFeO is the activity of FeO in molten slag; aFeS is the activity of FeS in molten slag; PSO2 is the partial pressure of SO2 in smelting process; DGq is the Gibbs free energy of the reaction (9); e is the nature constant, 2.71828. When the temperature, aFeO，PSO2 is ﬁxed, the relationship between aFe3O4 and aFeS can be expressed by combining the equations (9)e(12). Therefore, the relationships between a(FeS) and a(Fe3O4) in the molten slag with different value of a(FeO) were calculated by Eq. (12). In case of SO2 partial pressure of 102 atm, the relationships between a(FeS) and a(Fe3O4) in the molten slag with different value of a(FeO) were calculated by Eq. (12), and the results are present in Fig. 5. It can be seen from that with an increase in a(FeS), the a(Fe3O4) is decreased signiﬁcantly, which means magnetite is readily reduced to FeO by WDS. Therefore, the addition of reductants (Coals and WDS) can minimize the amount of Cu lost into slag by decreasing the content of magnetite in smelting slag and 1.0 -20 a(FeO)=0.35 a(FeO)=0.40 a(FeO)=0.45 a(FeO)=0.50 a(FeO)=0.55 -40 0.8 -80 a(Fe3O4) Ƹ*©.-PRO -60 -100 -120 -140 -160 -180 -200 600 Eq.(1) Eq.(2) Eq.(3) Eq.(4) Eq.(5) 800 0.6 0.4 0.2 1000 1200 1400 Temperatureć Fig. 4. Variation of standard Gibbs free energy DG of several reactions with temperature obtained by thermodynamic calculations using HSC 6.0 software. 0.0 0.0 0.2 0.4 a(FeS) 0.6 0.8 Fig. 5. Relationship between a(Fe3O4) and a(FeS) by thermodynamic calculations using HSC 6.0 software. Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 3.2. Smelting reduction-sulfurizing process for Cu recovery As shown in Table 1, copper is the main valuable metal in copper slag, which should be considered ﬁrst for recovery. Hence, the effects of smelting reduction-sulfurizing condition on recovery of copper were explored systematically. The effect of WDS addition on grade and recovery rate of Cu was investigated and results are shown in Fig. 6(a). It is clearly seen that with an increase of dosage of WDS from 0 to 15%, the recovery of Cu is increased signiﬁcantly from 41.64% to 91.67%, Cu grade of matte and cleaned slag is elevated gradually from 12.21% to 15.87%, 1.69%e0.31%, respectively. When the dosage of WDS is over 15%, the recovery and grade of copper keep constant. It implied that the addition of WDS is favorable to recover copper entrapped in initial smelting slag, which plays an important role in the reductionsulfurizing process. The reasons for the great inﬂuence of WDS on smelting reduction-sulfurizing process might lie in two aspects. On one hand, the WDS, as a sulfurizing agent, can ensure sulfuration of all copper existing in the oxide form within the initial slag so that it enters the matte phase thoroughly. On the other hand, the addition of WDS enables to provide enough collector phase(matte) to clean the slag, which allows to separation from slag easily. However, excessive WDS would sulfurize more “FeO” into matte phase, resulting in a decrease of copper grade of matte. Over consideration of grade and recovery, an optimum dosage of WDS addition was 95 20 (a) 90 90 10 Recovery of Cu Cu grade in matte Cu grade in cleaned slag 50 40 20 0 5 10 15 (c) 85 80 Recovery of Cu Cu grade in matte Cu grade in cleaned slag 70 18 95 16 90 14 85 10 1.6 0.8 0.4 55 0.0 Temperature/ 1350 1400 1.6 1.2 60 50 1.2 1300 Recovery of Cu Cu grade in cleaned slag Cu grade in matte 65 0.0 60 1250 70 55 65 1200 12 75 0.4 12 75 15 80 0.8 0.4 0.0 0 20 Dosage of WDS/% Recovery of Cu/% 1.2 0.8 30 90 1.6 Grade of Cu/% Recovery of Cu/% 70 60 18 85 15 2 4 Dosage of coke/% 6 8 (d) 16 14 Grade of Cu/% Recovery of Cu/% Recovery of Cu/% 80 (b) Recovery of Cu Cu grade in matte Cu grade in cleaned slag 80 75 10 1.6 70 1.2 65 0.8 60 55 0.0 12 Grade of Cu/% 100 recommended as 15%. In addition, the reductive atmosphere is very important to smelting reduction-sulfurizing process. In order to improve copper recovery through adjusting the reaction atmosphere, the effect of coal dosage on recovery and grade of copper were conducted and the results are show Fig. 6(b). It indicates that the copper recovery increases obviously with the addition of coal from 0 to 3% and performs the best of 90.81% at 3% coal dosage, then maintains constant when the dosage of coal increases to 4.5%. Accordingly, the grade of copper in cleaned slag and matte present a similar tendency to the copper recovery with the increase of coal dosage. However, when the coal dosage elevates 8%, each index deteriorates sharply. It is implied that an appropriate reductive atmosphere is beneﬁcial for generation of matte, while excessive enhancement of reductive atmosphere would be detrimental to slag cleaning. In smelting reduction-sulfurizing process, with the addition of coal, the “Fe3O4” in the slag is gradually reduced to “FeO” and thus the content of FeO increases, leading to the improvement of ﬂuidity of the molten slag. It is also essential for the liberation and settling of mechanically entrained copper matte and metallic copper inclusions. However, excessive addition of coal would cause that the “FeO” is further reduced to metallic “Fe”, resulting in high viscosity and poor ﬂuidity of the molten slag. Therefore, 3% coal dosage is suitable for cleaning the copper slag. Undoubtedly, smelting temperature has a signiﬁcantly effect on the recovery of valuable metal. Fig. 6(c) shows the effect of smelting Grade of Cu/% improving the ﬂuidity of molten slag. 895 0.4 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 4.5 Duration/h Fig. 6. Effect of process parameters on grade and recovery of Cu. a-Effect of WDS dosage on grade and recovery of Cu (Smelting reduction-sulfurizing at 1300 C for 2 h with 3% coke addition); b- Effect of coke dosage on grade and recovery of Cu (Smelting reduction-sulfurizing at 1300 C for 2 h with 15% WDS); c- Effect of smelting temperature on grade and recovery of Cu (Smelting reduction-sulfurizing for 2 h with 15% WDS and 3% coke); d- Effect of smelting duration on grade and recovery of Cu (Smelting reduction-sulfurizing at 1300 C with 15% WDS and 3% coke). 896 Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 anions in the slag, decrease the viscosity and improve the ﬂuidity of slag, which contributes to collision, aggregation and separation of matte particles. What is more, increasing the temperature also weakens interfacial tension between matte and slag, leading to a decrease in the dissolution and mechanical inclusion of matte in slag. Consequently, from a practical point of view, the smelting temperature is ﬁxed at 1300 C in the subsequent experimental series. The effect of smelting duration on copper recovery and grade is shown in Fig. 6(d), which indicates that copper recovery and grade of matte increases as prolonging duration from 0.5 h to 2.0 h while the copper grade of cleaned slag decreases accordingly, and then keep steady. It is implied that insufﬁcient smelting duration cannot ensure sulfurizing reaction of Cu2O carry out thoroughly, resulting in the poor cleaning efﬁciency of smelting slag. Moreover, the full setting of the matte also requires enough time. Thus, the smelting time should be performed for 2 h. Removal rate/% 100 80 60 40 20 0 Ni As Sb Pb Zn Bi Hg Elements Fig. 7. Removal rate of hazardous metals with various dosages of WDS. (Smelting reduction-sulfurizing at 1300 C for 2 h with 3% coke). Percent of element present in different phases/% The removal rate of various toxic elements in copper smelting slag, especially hazardous elements such as Ni, Pb, Zn, As, Sb, Bi, Hg, was determined and calculated according to Eq. (13), and the results are shown in Fig. 7. Matte phase Slag phase Vopor phase 100 80 3.3. Removal of hazardous elements (Ni, Pb, Zn, As, Sb, Bi, Hg) 100% 60 40 Where Rt is the removal rate, M1 is the mass of cleaned slag, g1 is the content of relative metal in the cleaned slag, M2 is the mass of initial waste copper slag, g2 is the content of relative metal in the initial slag. As can be seen from Fig. 7, the dosage of WDS is believed to have a prominent effect on the removal of Ni, As and Sb. As increasing the dosage of WDS from 0 to 15%, the removal rate of Ni, As and Sb obviously ascends from 43.21% to 98.94%, 45.56%e96.81% and 52.23%e95.51%, respectively. The results indicate that the higher dosage of WDS is beneﬁcial to eliminate those metals, which could be because Ni, As and Sb minerals in copper slag were reduced and sulfurized to metal state or sulﬁde, and then transferred and settle down to matte phase. However, WDS seems to have a light inﬂuence on the eliminate of Pb, Zn, Bi and Hg. The removal rate of those metals is above 80% even though smelting reduction without WDS. During the smelting reduction process, the bismuth, lead, zinc and mercury minerals maybe reduced into vapors of Bi, Pb, Zn and Hg due to their low vaporization temperature. Hence, they can be readily collected by cooling the exhaust gas. In order to further investigate the eliminate behaviors of hazardous metals in reduction-sulfuring process, the element distribution in different phase and chemical compositions of the initial copper slag, matte and cleaned slag are shown in Fig. 8 and Table 6, respectively. 20 6 5 4 3 2 1 0 Ni As Sb Pb Elements Zn Bi Hg Fig. 8. Elements distribution in different phases (slag phase-cleaned slag). (Smelting reduction-sulfurizing at 1300 C for 2 h with 15% WDS and 3% coke). temperature on copper recovery was investigated and results. As can be seen from that, the copper recovery prominently ascends ﬁrstly and then changes slightly as the smelting temperature increasing from 1200 C to 1300 C. Meanwhile, the copper grade of matte maintains around 15% while the copper grade of cleaned slag decreases gradually from 1.25% to 0.31% with the temperature elevating from 1200 C to 1300 C. Afterwards, the copper recovery and grade keep constant with further increasing the temperature. As is well-known, high temperature can break bonded silicate 1 M1 g1 M2 g2 Rt ¼ (13) Table 6 Chemical analysis of initial copper smelting slag, matte and cleaned slag. Elements Cu Ni Pb Zn As Sb Bi* Hg* Initial copper slag Matte Cleaned slag 2.71 15.87 0.31 0.48 3.13 0.006 0.68 0.84 0.002 1.93 0.80 0.009 0.088 0.543 0.003 0.036 0.23 0.002 45 87.22 1.20 31 4.36 4.29 Fe SiO2 Al2O3 CaO MgO P S Cl Cr K Na Ti V 39.56 49.56 37.18 26.61 1.56 29.87 3.71 0.69 3.98 2.5 0.21 5.34 1.05 0.033 1.26 0.01 0.13 0.042 0.83 20.25 1.74 0.04 0.025 0.048 0.04 0.15 0.015 0.58 0.064 0.69 0.24 0.293 0.005 0.049 0.017 0.147 0.049 0.008 0.01 Bi*, Hg*(1 106), Others (1 102). Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 were calculated according to the conservation of mass. Combined with Table 6, it is found from Fig. 8 that, besides Cu, other elements, including Ni, As, Sb and S, were also enriched into the matte phase. In addition, the majority of Pb, Zn, Bi and Hg were removed by reduction and volatilization, which agrees well with the results present in Fig. 7. Consequently, the matte can be returned to the corresponding smelting process for further recovery of these valuable elements, and the cleaned slag containing FeO, SiO2, CaO and Al2O3 and trace impurities may be reused as a raw material for cements. The vapor phase percent (VPP) is calculated by based on the mass balance of chemical composition between origin copper slag, matte and cleaned slag. And the calculated equation is shown as follow: VPP ¼ 1 M1 g1 M3 g3 M2 g2 M2 g2 100% (14) Where VPP is the vapor phase percent, M1 is the mass of cleaned slag, g1 is the content of relative metal in the cleaned slag, M2 is the mass of initial waste copper slag, g2 is the content of relative metal in the initial slag, M3 is the mass of matte, g3 is the content of relative metal in the matte. Thereinto, the percent of relevant elements present in gas phase 3.4. Leaching toxicity of cleaned slag The cleaned slag was achieved under the optimum conditions of smelting temperature 1300 C, smelting duration 2 h, 15% WDS and 3% coal. The toxicity of cleaned slag was also determined by Toxicity Characteristic Leaching Procedure (TCLP) test and the results are present in Table 7. The concentrations of toxic element ions in the leachate are all much lower than the thresholds prescribed in USEPA, which implies the cleaned slag obtained by the new process is harmless and can be used as raw material for safe cement production. Table 7 TCLP test results of cleaned slag(mg/L). Elements Cu Ni Pb Zn As Sb Bi Cleaned slag Toxicity thresholds 70.95 100.00 0.97 1.00 0.28 5.00 1.39 70.00 0.38 0.50 0.15 NS ND NS NS: not stated in regulation, ND: not detected. (a) F F 15% WDS Intensity/Counts M F F F F F F FF M 10% WDS F 5% WDS M M F FMF F 34 20 30 40 50 Two-theta/deg (b) 33 32 2+ 3+ 31 Ratio of Fe /Fe in the cleaned slg 10 2+ 30 29 28 0 5 10 15 20 Dosage of WDS 16 60 70 0.28 (c) 12 10 8 6 4 2 0 5 10 15 20 Dosage of WDS F 80 34 14 0 F F M Withou WDS M F 9 LVFRVLW\RIVODJ3Dg 6 F M Content of Fe in the cleaned slag F M F F F F F F 897 35 M F 36 Two-theta/deg (d) 0.24 37 ć ć ć ć 0.20 0.16 0.12 0.08 0 5 10 15 20 Dosage of WDS/% Fig. 9. XRD patterns and content of Fe2þ and Fe3þ in the cleaned slag. (a-XRD patterns, b-content of Fe2þ, c-ratio of Fe2þ/Fe3þ, d-viscosity of slag, Smelting reduction-sulfurizing at 1300 C for 2 h with 3% coke). 898 Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 3.5. Characterization of resultants In order to further reveal the behavior of WDS in smelting process, the crystalline compositions, and microstructural characterization of cleaned slag and matte were analyzed using XRD, and optical microscopy combined with SEM-EDS, respectively, and the results are shown in Figs. 9e12. It was observed that from Fig. 9 with an addition of WDS in smelting reduction-sulfuring process, the peaks of fayalite are sharply intensiﬁed, while that of magnetite are weaken, even disappeared in the slag, which is caused by the reduction of magnetite to “FeO” by WDS and coal. Meanwhile, the content of Fe3þ and Fe2þ, and the ratio of Fe2þ/Fe3þ were analyzed and calculated, the results shown in Fig. 9 (b)-(d) further conﬁrms that WDS is capable to decrease the content of Fe3O4 while increase the content of “FeO” in the slag, which is beneﬁcial to improving the futility of molten slag. The theoretical viscosity of slags at various temperatures calculated by FactSage 7.0 (as shown in Fig. 9(d)), further conﬁrmed that the addition of WDS can decrease the viscosity of molten slag. The general expression for free-setting velocity of matte particles in smelting process can be shown as follow: Vs ¼ 2gr 2 ðrm rs Þ=9m (15) Where: g is the gravitational acceleration, m/s2; r 2 is the diameter of matte particles, m; rm is the density of matte, Kg/m3; rs is the density of smelting slag, Kg/m3; m is the viscosity of the molten slag, Pa‧s. As can be seen from Eq. (15), with a decrease of the viscosity of the molten slag, the free-setting velocity of matte particles through the molten slag is dramatically increased. This may ultimately Fig. 10. Optical micrographs(a) with corresponding SEM-EDS results(b) of cleaned slag. Intensity/Counts M S S- FeS W-FeO C-CuFe2S3 M-Cu S S C W S W C 10 20 30 40 50 60 Two-theta/deg Fig. 11. XRD patterns of copper matte. 70 80 Fig. 12. Optical micrographs(a) with corresponding SEM-EDS results(b) of matte. result in a lower entrainment of matte particles in the slag and a higher recovery of copper. In addition, the slag after being cleaned primarily contains fayalite phases. These results are also in good accordance with the thermodynamic analysis. In order further conﬁrm the phase and element compositions of the cleaned slag, optical microscope technique were employed. As shown in Fig. 10, there are three mineral phases can be obviously detected in it: entrapped matte, fayalite and magnetite. Comparing with the initial slag, the ratio of magnetite in cleaned slag are markedly decreased, while that of fayaite phase is correspondingly increased, which matches well with the XRD results shown in Fig. 9(a). Moreover, the rare slight bright spots, as the matte phase, are still observed in the slag. Nevertheless, the amount of matte particles entrapped in cleaned slag has seen a dramatic decline, which also offers stronger evidence that copper lost in initial copper slag can be efﬁciently recovered via the new process. On the other hand, the XRD patterns of matte obtained by the new process present in Fig. 11 shows that the main mineral phases are iron sulphide (FeS), wustite and cubanite(CuFe2S3), and the copper, as the minor phases, is also detected in matte. It suggests that most of copper in the initial slag were sulfurized and collected as the form of sulﬁde in the matte phase by WDS. These results were further veriﬁed by optical micrographs with corresponding SEM-EDS of matte. As shown in Fig. 12(a), four mineral phases are identiﬁed: matte, wustite, matte and alloy phase. The phase compositions of the matte obtained from Fig. 12 are basically consistent with those from the XRD patterns shown in Fig. 11. Especially, it is noted that for the point 3, it represents the alloy phase. The EDS analysis results shows that it contains 4.14% As, 1.43% S, 2.65% Pb, 15.65% Sb, 10.23% Fe, 44.41% Ni and 21.19% Cu. It is implied that some toxic elements, such as As, Pb, Sb and Ni, were also reduced and sulfurized together with copper and iron oxide and then transferred into matte phase during cleaning process. As a result, the content of those hazardous elements in the cleaned slag is decreased obviously, which further conﬁrms that the removal of Z. Guo et al. / Journal of Cleaner Production 199 (2018) 891e899 hazardous elements can be realized and the cleaned slag was successfully achieved by the new process (as shown in Figs. 10 and 11). Therefore, this smelting reduction-sulfurizing process is very effective to clean the copper slag. 4. Conclusions Based on the experimental results above, it can be concluded that the smelting reduction-sulfurizing process is an effective technology to clean the copper slag and recover the valuable metals from that. Our main ﬁndings can be summarized as follows: (1) Waste ferric-oxide desulfurizer applied as a sulfurizing agent to treat the copper slag is efﬁcient and environmentally friendly, which enables the copper oxides dissolved in initial slag to transform copper sulﬁde and then enriched in matte phase. Meanwhile, it also can decrease the content of magnetite in slag and improve slag ﬂuidity. Hence, the recovery of copper is increased signiﬁcantly and the copper lost in slag as mechanical entrapment is decreased correspondingly. (2) Under the optimum conditions of smelting temperature 1300 C, smelting duration 2 h, 15% WDS and 3% coal, 90.81% Cu was recovered and enriched in copper matte, which can be returned to the copper smelting process as a feeding. 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