Ind. Eng. Chem. Res. 2003, 42, 5813-5818
5813
Mechanochemical Sulfidization of Nonferrous Metal Oxides by Grinding with Sulfur and Iron Jun Wang, Jinfeng Lu, Qiwu Zhang,* and Fumio Saito Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan
Co-grinding nonferrous metal (Cu, Pb, and Zn) oxides with sulfur and iron stimulate solid-state reactions to form nonferrous metal sulfides and iron oxide. The sulfidizing reaction proceeds with an increase in grinding time and is almost complete after 180 min of grinding. The necessary conditions to sulfidize the oxides can be explained on the basis of thermodynamic considerations. A larger negative change in Gibbs free energy accompanies the defined sulfidizing reaction than the formation of iron sulfides. By using the novel sulfidizing process, it is expected that current mineral processing technologies such as flotational and magnetic separations might be used to recover metals from various kinds of wastes or to purify contaminated wastes when the heavy metals contained, such as lead, cadmium, etc., are sulfidized and removed. 1. Introduction Metals are the key materials supporting modern society. They have been widely used, and vast amounts of waste arise upon use of metal-containing materials. In the cases of nonferrous metal such as Cu, Pb, and Zn in Japan, for example, amounts as high as 10-30% of their annual output have been discarded in various types of waste. Recycling of these metals from wastes is highly required from the point of environmental resource preservation and sustainable development. These metals are mainly obtained by processing sulfide minerals such as sphalerite (ZnS), galena (PbS), and charcopyrite (CuFeS2). An efficient technological system already exists to process these sulfide minerals. However, the metals in the wastes are rarely in the sulfide form but rather are oxides and oxidized compounds in addition to the metal itself. If a simple and reliable method to sulfidize the waste were developed, the existing mineral processing methods and metallurgical processes could be applied to treat these synthetic sulfides, making the metal recycling system feasible technologically and economically, compared to the difficulties in the specific development of a new recycling processes for each specific kind of waste.1-5 Methods have been reported for the sulfidization of metals, such as high-temperature reaction and precipitation from solution using H2S and other sulfides.6,7 When the sulfides are used as sulfidizers, it is difficult to avoid the secondary emission of wastes and the release of toxic gases by these methods. In addition, some methods entail acid leaching of the wastes to extract the metals into solution before the precipitation operation. On the other hand, little attention has been paid to the use of elemental sulfur.8 This material has been emitted in byproducts from oil refineries and the mining industry, in addition to occurring naturally, so that an inexpensive supply exists. The use of sulfur as a starting material to treat copper ores has been reported by Yusupov and others.9,10 Because wet milling with water was conducted, H2S was formed during the sulfidizing reaction, so that the process exhibits similar * To whom correspondence should be addressed. Tel./Fax: 81-22-217-5137. E-mail:
[email protected].
disadvantages to the precipitation process using H2S and sulfides. Mechanochemical processing has been applied widely from the treatment of wastes to the synthesis of functional materials including inorganic compounds and metal alloys.11-13 Recently, applications of mechanical treatment to the decomposition of hazardous organic compounds and to the solvent-free synthesis of organic materials have been reported.14-16 In this paper, we report a novel process to sulfidize nonferrous metal oxides: grinding them with sulfur and iron powders. The mechanically induced solid-state reaction results in the formation of nonferrous metal sulfides without observable emission of other hazardous gases. As a reference, other oxides such as TiO2 and SnO2 are also examined. In addition, the necessary conditions to sulfidize the oxides by this process are discussed. 2. Experimental Section All of the samples used, including oxides (ZnO, PbO, CdO, CuO, TiO2, SnO2), sulfur, and iron powders, were chemical reagents supplied by Wako Pure Chemical Industries, Ltd., Osaka, Japan. The reagents were used as received. Oxide was mixed with sulfur and iron at molar ratios according to chemical eqs 1 and 2, respectively
4MO + 4S + 3Fe ) 4MS + Fe3O4 (M ) Zn, Pb, Cd) (1) 4CuO + 8S + 7Fe ) 4CuFeS2 + Fe3O4 (M ) Cu) (2) 2MO2 + 4S + 3Fe ) 2MS2 + Fe3O4 (M ) Ti, Sn) (3) A planetary ball mill (Pulverizette-7, Fritsch GmbH, Welden, Germany) was used for grinding of the mixture. Two grams of the starting compounds was placed into a stainless pot (45-cm3 inner volume) with seven stainless steel balls of 15-mm diameter and subjected to grinding in air at 700 rpm for different periods of time. After grinding, during which no formation of gaseous products was observed, the resulting products were solid
10.1021/ie030046b CCC: $25.00 © 2003 American Chemical Society Published on Web 10/11/2003
5814 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
Figure 2. TG curves of a ZnO-S-Fe mixture ground for different times.
Figure 1. XRD patterns of a ZnO-S-Fe mixture ground for different times.
powders. The ground samples were recovered from the pot in a batch way. The ground samples were characterized by X-ray diffraction (XRD, Rigaku Giegerflex, Cu KR, Tokyo, Japan). Thermogravimetric (TG, Rigaku TAS-200, Tokyo, Japan) analysis for the samples was carried out at 265 °C in air. Morphological change of the sample was observed by a scanning electron microscope (SEM, Hitachi, S-4100, Tokyo, Japan). The reaction was also evaluated quantitatively by measuring the remaining sulfur in the ground samples by washing them with CS2 solvent. One gram of each ground sample was agitated in 100 cm3 of CS2 for 30 min. After filtering, 10 cm3 of CS2 was used to rinse the sample twice. After evaporation of CS2 from the filtrate at room temperature in a draft, the remaining solid sulfur was weighed, and the remaining percentage was calculated. 3. Results and Discussion 3.1. Mechanochemical Sulfidization of Zinc and Lead Oxides. The main reason this study is focused on oxides is that the metals themselves can be sulfidized easily by the mechanochemical process. Our preliminary experiments (results not shown here) confirmed that the reactions
M + S ) MS
(M ) Zn, Pb, Cu)
(4)
occur during grinding, resulting in the formation of sulfides of Zn, Pb, and Cu, when each of the metals Zn, Pb, and Cu was mixed with sulfur at an equimolar ratio and ground for 60 min. In fact, the mechanochemical reactions offer an easy method to synthesize sulfide materials.17-19 This suggests that it is simple to transform the metals in wastes into sulfides. However, when the oxides of these metals were ground with sulfur, sulfidizing reactions were not observed. The method foring transforming these oxides into sulfides is vital in the sulfidizing treatment of waste. Our further research has shown that it requires the use of additives, functioning as a reductant to the
Figure 3. Change in the remaining percentage of sulfur with grinding time.
oxides. Additives such as iron powder are found to be effective in inducing the transformation. Figure 1 shows the XRD patterns of a ZnO-Fe-S mixture ground for different lengths of time. Only the starting materials without observable appearance of new phases are observed in the sample ground for 15 min. When the grinding time is increased to 60 min, new peaks corresponding to ZnS and Fe3O4 are found to appear in the pattern, although the peaks due to ZnO are still observable. All of the phases in the sample ground for 120 min are confirmed to be the products of ZnS and Fe3O4, indicating that the following reaction has been carried out mechanochemically
4ZnO + 4S + 3Fe ) 4ZnS + Fe3O4
(5)
Figure 2 shows the TG curves of the ZnO-Fe-S mixture ground for different times. Because sulfur sublimates when heated, weight loss occurs when sulfur is present in the ground sample. About 17% of the
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5815
Figure 4. SEM photographs of the ZnO-S-Fe mixture ground for different times, as indicated (in minutes).
weight loss is observed from the curve of the sample ground for 15 min. The weight loss is less than the percentage of the originally added sulfur (20.6%), suggesting that a reaction involving sulfur has occurred (at the interface at least) after 15 min of grinding. With an increase in the grinding time, the weight loss due to sulfur sublimation decreases rapidly and becomes difficult to detect when the grinding time exceeds 120 min. A quantitative measurement of the amount of sulfur remaining in the ground sample was performed, and the results are shown in Figure 3. The remaining percentage of sulfur decreases rapidly to 10% after 60 min and reaches 1.8% after 180 min of grinding, indicating that the mechanochemical sulfidization has nearly been completed by that time. Figure 4 shows SEM photographs of the ground samples. Among the starting materials, compared with fine powders of ZnO and S, small grains (under 100 mesh ) 150 µm) of iron powders were used. The general morphology of the sample ground for 15 min is typical of grains with a size of about 40 µm, suggesting that the iron powders have not been pulverized effectively. Instead, the fine powders were found to have adhered firmly to the surface of the iron powders. The SEM photograph of the sample ground for 15 min reveals the main state of the physical mixture of the starting samples. With an increase in grinding time to 60 min and more, morphological changes were observed. The grains were pulverized, and the formation of agglomerates occurred. Moreover, the agglomerates consisted of primary particles less than 1 µm in size according to a magnified photograph not shown here. In other words, an extremely mixed state of the starting materials by means of the mechanochemical treatment is needed to stimulate the solid-state reaction. After confirmation of the sulfidization of ZnO into ZnS, the experiments were extended to include cad-
Figure 5. XRD patterns of the samples of ZnO-S-Fe, PbO-SFe, CdO-S-Fe, and CuO-S-Fe ground for 120 min.
mium oxide (CdO), lead oxide (PbO), and copper oxide (CuO). Figure 5 shows the XRD patterns of samples of ZnO-S-Fe, PbO-S-Fe, CdO-S-Fe, and CuO-S-Fe mixtures, each ground for 120 min. This figure reveals that, in the cases of PbO and CdO, formation of the corresponding sulfides PbS and CdS, respectively, together with Fe3O4, occurred similarly to the case of ZnO, indicating that it is possible to sulfidize these oxides. It is interesting to note that, in the case of CuO, a different phenomenon occurs. It is difficult to obtain a pure phase
5816 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003
Figure 6. XRD patterns of mixtures with ZnO and PbO in different molar ratios ground for 120 min: ZnO/PbO ) (1) 1:0, (2) 3:1, (3) 1:1, (4) 1:3, (5) 0:1.
of CuS. CuFeS2 of the charcopyrite phase always appears with CuS, although the formation of both CuS and CuFeS2 represents the same sulfidization. A single phase of CuFeS2 can be obtained by regulating the molar ratio according to eq 2. Therefore, the formation pattern of CuFeS2 is shown. Figure 6 shows the XRD patterns of ground samples of ZnO and PbO in different molar ratios as the starting materials. In all cases, no existence of the original oxides can be observed after the sulfidizing reaction. Both zinc and lead sulfides are obtained as the reaction products, together with Fe3O4. These results show that mixtures of nonferrous metal oxides can be sulfidized simultaneously. 3.2. Thermodynamic Discussion of the Sulfidizing Reaction. In general, oxides are more stable than sulfides. It is known that natural sulfide minerals gradually transform into oxides when exposed to air. The reverse is not true. This suggests that thermodynamics can be applied to interpret the mechanochemical sulfidizing reactions. First, the changes in the standard Gibbs free energy (∆G298)20 of eq 4 were calculated, giving the results -200.4 (Zn), -97.0 (Pb), -143.7 (Cd), and -53.5 kJ/mol (Cu). Compared to the negative changes of ∆G298 for the reactions given by eq 4, the reactions given by eq 6 exhibit positive changes in each case, with ∆G298 equal to 120.1 (Zn), 91.7 (Pb), 85.6 (Cd), and 74.8 kJ/mol (Cu), respectively.
MO + S ) MS + 1/2O2
(6)
It seems that the positive or negative change in ∆G298 is correlated with the occurrence of the mechanochemically sulfidizing reaction. However, it was found, from the extended investigation, that a negative standard Gibbs free energy change does not ensure that sulfidizing reaction will occur, so an analysis of the thermodynamics was performed in detail. Figure 7 shows the XRD patterns of SnO2/TiO2, S, and Fe mixtures ground for 120 min. In both cases, the
Figure 7. XRD patterns of SnO2/TiO2, S, and Fe mixtures ground for 120 min. Table 1. Comparison of ∆G298 Changes between the Unsuccessful SulfIdization Reactions and the Formation of Iron SulfIdes starting material SnO2 TiO2
reactions
∆G298 (kJ/mol)
2SnO2 + 3Fe + 4S ) 2SnS2 + Fe3O4 SnO2 + 3Fe + 4S ) SnO2 + 2FeS + FeS2 2TiO2 + 3Fe + 4S ) 2TiS2 + Fe3O4 TiO2 + 3Fe + 4S ) TiO2 + 2FeS + FeS2
-266 -364 -53.1 -364
formation of tin/titanium sulfide was not obtained, although tin sulfide can be synthesized by grinding samples containing the two element (tin and sulfur).18,19 The oxides remained in the case of SnO2 or changed into an amorphous sample in the case of TiO2. It is interesting to note that iron sulfides were formed instead. FeS was observed with SnO2 grinding, and both FeS and FeS2 were obtained with TiO2 grinding. On the other hand, the formation of such iron sulfides was not observed when the sulfidizing reaction of the oxide occurred. Therefore, it seems to be necessary to clarify the conditions under which a sulfidizing reaction of the oxide or a formation reaction of iron sulfides occurs. Both reactions of iron sulfide formation and oxide sulfidization were defined, and the changes in ∆G298 were compared and summarized in tables. Table 1 shows a comparison of ∆G298 changes between the unsuccessful sulfidization reactions (in the cases of SnO2 and TiO2) and the iron sulfide formation reactions. Although the defined sulfidizing reactions exhibit negative changes in ∆G298, the absolute values are smaller than those of the iron sulfide formation reactions. On the other hand, the comparison of ∆G298 changes between the successful sulfidization reaction and the formation of iron sulfides shown in Table 2 indicates that all of the successful sulfidization reactions exhibit negative standard Gibbs free energy changes with larger absolute values than the corresponding the iron sulfide formation reactions. This can be summarized as a necessary condition to sulfidize a specific oxide: the
Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 5817 Table 2. Comparison of ∆G298 Changes between the Successful SulfIdization Reactions and the Formation of Iron SulfIdes starting material CdO PbO ZnO CuO
reactions
∆G298 (kJ/mol)
4CdO + 4S + 3Fe ) Fe3O4 + 4CdS 4CdO + 4S + 3Fe ) 4CdO + FeS + FeS2 4PbO + 4S + 3Fe ) Fe3O4 + 4PbS 4PbO + 4S + 3Fe ) 4PbO + FeS + FeS2 4ZnO + 4S + 3Fe ) Fe3O4 + 4ZnS 4ZnO + 4S + 3Fe ) 4ZnO + FeS + FeS2 4CuO + 8S + 7Fe ) 4CuFeS2 + Fe3O4 4CuO + 8S + 7Fe ) 4CuO + 6FeS + FeS2
-673 -364 -649 -364 -535 -364 -1264 -772
defined sulfidizing reaction must exhibit a larger negative change in ∆G298 than the corresponding iron sulfide formation reaction. This thermodynamic condition, however, was found to be a necessary but not a sufficient one upon further investigation. Whether or not an oxide can be sulfidized by this method depends also on the crystal structure. Several other reports have been published on the dependence of mechanochemical reactions on the crystal structures of the starting materials.21,22 3.3. Possible Application to Practical Wastes. If a pure sample of iron is used for this process, cost becomes a concern, making use this approach for metal recovery prohibitive. However, wastes containing iron metal at high concentration are released from various processes,23,24 and it is advisable that these wastes be used as additives for a practical application of this process. For example, tens of millions of used vehicles destroyed per year offer a vast amount of scrap iron. In addition, many recycling processes are also providing iron metal byproducts from various industrial and domestic wastes. The purity of such scrap iron or other byproducts is generally not very high, and the presence of other metals such as copper prevents their direct use as raw materials for steel making. These byproducts offer an ideal additive for our sulfidizing process, where the heavy metals in the scrap iron can be sulfidized as well. As with the use of iron, the use of aluminum powders also brings about the transformation of the oxides into the corresponding sulfides, accompanied by the formation of alumina, as shown in eq 7
3ZnO + 3S + 2Al ) 3ZnS + Al2O3
(7)
Moreover, ∆G298 of reaction 7 is -1223.6 kJ/mol (-407.9 kJ/per mole of ZnO). Although it is economically impossible to use pure aluminum as an additive to treat waste, this method offers a new approach to treat aluminum dross waste, which remains an environmental problem for the aluminum industry and requires proper treatment.25,26 The mechanochemical sulfidization was applied to a real sample of melting fly ash from an incinerator with iron scrap as the additive. Successful sulfidization of the metals contained in the fly ash, such as zinc, was obtained. The experimental details, together with those from a flotation recovery experiment, which is under investigation, will be published later. In general, a novel metal recycling process has been developed: two kinds of waste containing nonferrous metals (in oxide or metal form) and iron/aluminum metals are ground with a sulfur sample to induce solidstate reactions to form nonferrous metal sulfides and iron/aluminum oxides. The existing mineral processing
methods such as magnetic and flotation separation can then be applied to the ground sample to recover the metals. The process also exhibits an environmentfriendly merit because most heavy metals can be sulfidized and recovered to leave the waste as a nonhazardous residue that can be stored without problem or can be used in proper fields. 4. Conclusions The following conclusions can be made on the basis of our experimental results: (1) Sulfidization of nonferrous metal oxides can be achieved by grinding them with sulfur and iron powders at room temperature with iron oxide as the byproduct. Without the addition of iron, such sulfidizing reactions were not observed. (2) There exists the tendency for the formation of iron sulfide during the grinding. The negative change in ∆G298 is not sufficient to cause the reaction to occur. Successful sulfidizing reactions exhibit a larger standard Gibbs free energy change in absolute value than the corresponding formation reaction of iron sulfide. Acknowledgment The authors are thankful to NEDO for the financial support provided to this research project (01A42021d). Literature Cited (1) Nedwed, T.; Clifford, D. A. Feasibility of extracting lead from lead battery recycling site soil using high-concentration chloride solutions. Environ. Prog. 2000, 19, 197. (2) Yoshizaki, S.; Tomida, T. Principle and process of heavy metal removal from sewage sludge. Environ. Sci. Technol. 2000, 34, 1572. (3) Jandova, J.; Stefanova, T.; Niemczykova, R. Recovery of Cu concentrates from waste galvanic copper sludges. Hydrometallurgy 2000, 57, 77. (4) Coetzee, J. W.; Rejaldien, M. Z. The recovery of base metals by ion exchange resin. Miner. Eng. 2001, 14, 433. (5) Parga, J. R.; Valenzuela, J. L.; Muzquiz, G. G.; Ojebuoboh, F. K. Recycling lead to recover refractory precious metals. JOM 2001, 53, 19. (6) Katsuura, H.; Inoue, T.; Hiraoka, M.; Sakai, S. Full-scale plant study on fly ash treatment by the acid extraction process. Waste Manage. 1996, 16, 491. (7) Kim, J. H.; Yang, J. G.; Kunugita, E. Synthesis of processes for recovering metals from incineration fly ash of municipal solid wastes. Kagaku Kogaku Ronbunshu 1997, 23, 47. (8) Alkemade, M. M. C.; Koene, J. I. A. The useful application of sulphur-bound waste materials. Waste Manage. 1996, 16, 185. (9) Omarov, B. N.; Bekturganov, N. S.; Yusupov, T. S.; Antonov, V. A. Role of iron in sulphidization of oxidized copper minerals under grinding conditions. J. Min. Sci. 1994, 30, 409. (10) Omarov, B. N.; Yusupov, T. S.; Bekturganov, N. S.; Sim, S. P. Study of sulphidization of oxidized copper ores during grinding. J. Min. Sci. 1993, 29, 280. (11) Suryanarayana, C. Mechanical alloying and milling. Prog. Mater. Sci. 2001, 46, 1. (12) Takai, S.; Esaka, T. Preparation of functional oxide materials by means of mechanical alloying in view of ionic conductive oxides. Defect Diffus. Forum 2002, 206, 3. (13) McCormick, P. G.; Froes, F. H. The fundamentals of mechanochemical processing. JOM 1998, 50, 61. (14) Braga, D.; Maini, L.; Polito, M.; Mirolo, L.; Grepioni, F. Mechanochemical assembly of hydrogen bonded organic-organometallic solid compounds. Chem. Commun. 2002, 24, 2960. (15) Wang, G. W.; Zhang, T. H.; Hao, E. H.; Jiao, L. J.; Murata, Y.; Komatsu, K. Solvent-free reactions of fullerenes and Nalkylglycines with and without aldehydes under high-speed vibration milling. Tetrahedron 2003, 59, 55. (16) Zhang, Q.; Saito, F.; Ikoma, T.; Tero-Kubota, S.; Hatakeda, K. Effects of quartz addition on the mechanochemical dechlori-
5818 Ind. Eng. Chem. Res., Vol. 42, No. 23, 2003 nation of chlorobiphenyl by using CaO. Environ. Sci. Technol. 2001, 35, 4933. (17) Kosmac, T.; Courtney, T. H. Milling and mechanical alloying of inorganic nonmetallics. J. Mater. Res. 1992, 7, 1519. (18) Balaz, P.; Ohtani, T.; Bastl, Z.; Boldizarova, E. Properties and reactivity of mechanochemically synthesized tin sulfides. J. Solid State Chem. 1999, 144, 1. (19) Balaz, P.; Takacs, L.; Ohtani, T.; Mack, D. E.; Boldizarova, E.; Soika, V.; Achimovicova, M. Properties of a new nanosized tin sulphide phase obtained by mechanochemical route. J. Alloys Compd. 2002, 337, 76. (20) Barin, I. Thermochemical Data of Pure Substances; VCH: Weinheim, Germany, 1989. (21) Zhang, Q.; Lu, J.; Saito, F. Mechanochemical synthesis of LaCrO3 by grinding constituent oxides. Powder Technol. 2002, 122, 145. (22) Zhang, Q.; Saito, F. Mechanochemical synthesis of lanthanum aluminate by grinding lanthanum oxide with transition alumina. J. Am. Ceram. Soc. 2000, 83, 439.
(23) Aboussouan, L.; Russo, P.; Pons, M. N.; Thomas, D.; Birat, J. P.; Leclerc, D. Steel scrap fragmentation by shredders. Powder Technol. 1999, 105, 288. (24) Boom, R.; Steffen, R. Recycling of scrap for high quality steel products. Steel Res. 2001, 72, 91. (25) Drouet, M. G.; Handfield, M.; Meunier, J.; Laflamme, C. B. Dross treatment in a rotary arc furnace with graphiteelectrodes. JOM 1994, 46, 26. (26) Narayanan, R.; Sahai, Y. Chemical interactions of dross with water and water vapor in aluminum scrap remelting. Mater. Trans. JIM 1997, 38, 85.
Received for review January 17, 2003 Revised manuscript received June 26, 2003 Accepted September 5, 2003 IE030046B
