Thermodynamic Analysis for the Controllability of Elements in the

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Thermodynamic Analysis for the Controllability of Elements in the Recycling Process of Metals Kenichi Nakajima*,† †

Research Center for Material Cycles and Waste Management, National Institute for Environmental Studies, Ibaraki 305-8506, Japan

Osamu Takeda,‡ Takahiro Miki,‡ Kazuyo Matsubae,‡ and Tetsuya Nagasaka‡ ‡

Graduate School of Engineering, Tohoku University, Miyagi 980-8579, Japan ABSTRACT: This study presents the results of chemical thermodynamic analysis on the distribution of elements in the smelting process of metallic materials to examine the controllability of impurities in the pyrometallurgical technique. The results of the present work can give an answer against the frequently given question; “Which impurity element can be removable in metallurgical process?” or “How far can the impurity level be controlled?”. The proposed method was applied to estimate the distribution of 29 elements for a copper converter and 26 elements for a steel-making process and shows the distribution tendency of elements among the gas, slag, and metal phases as well as clarifying which metals can be recovered or removed from secondary resources in metallurgical processes. The effects of temperature, oxygen partial pressure, and slag composition on the distribution ratio of elements were also evaluated, and the removal limit or controllability of impurity in these two processes was presented. This study results in thermodynamic features of various elements in the pyrometallurgical process and also shows, even by varying process parameters such as temperature and oxygen partial pressure, no drastic improvement of removal efficiency should be expected, except for lead and tin in copper.

1. INTRODUCTION Material flow analysis (MFA) is an excellent tool to quantify material balance in specific areas globally,1 nationally,2 regionally,3 and by industry.4 It is also useful to systematically evaluate the flows of substances/materials for resource and waste and the structure of resource consumption. Representative studies for metals, such as aluminum,3 iron,5 copper,6 and chromium,7 including recycled resources and metal scrap, have provided important information for a better understanding of the structure of metal flows. Discussions of the complex web of metals and their linkages (relationships of elemental coexistence in natural and secondary resources), however, have been insufficient in most traditional MFAs, except for a few studies.812 The studies conducted by Murakami et al.8 and Nakamura et al.9,10 incorporated a smelting model to account for metal linkages, including the recovery of silver from the lead metallurgical process. In their extensive works, Verhoef et al.11 and Reuter and Verhoef12 suggested the importance of understanding metal linkages in natural resource processing by introducing the concept of the “metal wheel”, and Schaik13 pointed out the limitation of recycling based on thermodynamic consideration. Even these representative studies, however, do not provide us the quantitative limitations of impurity removal and the recoverability of elements in the recycling of end-of-life (EoL) metal products. In this study, our aim was to develop quantitative criteria for evaluating the possibility of removing impurities during the metal recycling process by considering all the relevant thermodynamic parameters. r 2011 American Chemical Society

One of the best tools to predict and understand the behavior of impurities in the recycling of EoL metal products is a knowledge of metallurgical physical chemistry, which is a well-established academic area studying the production of primary metals or inorganic materials from natural minerals through pretreatment, smelting, and refining with hydro- and pyrometallurgical technologies. Using these types of technologies, it should also be possible to predict the possibility of the removal of impurity or recovery of elements from remelted metal scrap. In our previous articles, extensive discussion was made on the thermodynamic behavior of impurity elements in the metal, slag, and gas phases of the steel-making process (both for basic oxygen furnaces, BOF, and electric arc furnaces, EAF), copper converters, lead blast furnaces, the imperial smelting process (ISP) of zinc and lead,14 the remelting process of aluminum,15 and the remelting process of magnesium.16 In the metallurgical processes, impurities in main metal product are mostly controlled by oxidizationreduction reactions and evaporation except for some special cases. Taking such technological status into account, the distribution ratio of impurity elements between the slag/metal or gas/metal phases was quantitatively evaluated in Received: December 16, 2010 Accepted: April 25, 2011 Revised: April 20, 2011 Published: May 11, 2011 4929

dx.doi.org/10.1021/es104231n | Environ. Sci. Technol. 2011, 45, 4929–4936

Environmental Science & Technology

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Figure 1. Distribution chart of elements among gas, slag, and metal phases for the metal recovery under the simulated atmosphere of the converter of copper smelting.

this study. The most conventional operating conditions for each process of metal have been chosen as process parameters to simplify discussions. To graphically represent the results of our evaluation, a chart named as the “element radar chart”15 plotting the distribution tendencies of the metal elements processed by the methods previously described was proposed. In our previous paper, the distribution ratio of impurity elements among the metal, slag, and gas phases that would occur in a smelting furnace under typical operating conditions was calculated as a first approximation. However, as described later in this article, the removability of impurities from remelted metal by evaporation or oxidation is simultaneously affected by several thermodynamic parameters, such as the Gibbs energy change of the oxidation reaction, total pressure, activity coefficients of the impurity in the metal and its oxide in slag, oxygen partial pressure, and temperature. Activity coefficients are also dependent on the composition of the metal and slag. When the Gibbs energy change of the reaction exhibits a large negative value or when a sufficiently low activity coefficient of oxidation product is achieved by selecting an adequate slag composition, the impurity can be removed even with a small activity coefficient in the metal, which is met in the practical process of refining of primary steel and copper (e.g., dephosphorization of molten steel). Without knowing the effect of several process parameters on the distribution behavior, the removal limit of impurities from the remelted metal cannot be precisely known. In this article, the effects of process parameters, such as temperature, oxygen partial pressure, and slag composition, on the distribution ratio of elements in the steel-making process and a copper converter are theoretically evaluated, and thermodynamic criteria on the removal limit or the controllability of impurities in these two processes are presented. Discussion is extended to the potential effectiveness and limits of extractive metallurgical processing as a recycling method.

2. METHODOLOGY The thermodynamic methodology used in this study is essentially the same as that described in our previous articles.14,15 The most

conventional pyrometallurgical processes, volatilization and oxidization, were selected. In each case, the distribution ratio of a given element M is thermodynamically obtained with following equations MðlÞ ¼ MðgÞ Lg=m ¼

ð1Þ

pM =po aM poM po γ xM ¼ ¼ M M o pSolv =p pSolv pSolv

n MðlÞ þ O2 ðgÞ ¼ MOn ðslagÞ 2 K3 ¼

a MOn aM ðpO2 =p°Þ

Ls=m ¼

n=2

¼

x MOn γMOn γM xM ðpO2 =p°Þn=2

xMOn K3 γM ðpO2 =p°Þn=2 ¼ xM γMOn

ð2Þ ð3Þ ð4Þ

ð5Þ

where poM, pM, pSolv, and po are the partial pressures of the pure element M (Pa), of M dissolved in the main metal product (solvent metal) (Pa), of the solvent metal vapor (Pa), and conversion factor (101325 Pa/atm); aM and γM are the activity and the activity coefficient of M in the pure liquid standard state; xM is the mole fraction of M in the solvent metal; aMOn and γMOn are the activity and the activity coefficient of the oxidation product MOn in slag in the pure solid standard state; xMOn is the mole fraction of MOn in slag; and pO2 is the oxygen partial pressure (Pa). Symbols of l, g, and slag in parentheses in eqs 1 and 3 denote the state of species as in liquid solvent metal, in gas phase, and in slag phase, respectively. K3 is the equilibrium constant of reaction 3 and is given from the Gibbs free energy change of the reaction ΔGo3 by eq 6 ΔGo3 K3 ¼ exp ð6Þ RT where R and T denote the gas constant and temperature (K), respectively. In the oxidation reaction 3, a monocationic oxide species is assumed as an oxidation product throughout this paper. For example, in the case of oxidation of aluminum in the solvent 4930

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Figure 2. Distribution chart of elements among gas, slag, and metal phases for the metal recovery under the simulated atmosphere of converter of steel making.

metal, the product is AlO1.5 rather than its general stoichiometric form of Al2O3. Dissociation reaction of general stoichiometric oxide to monocationic oxide is represented in eq 7 Mx Oy ðslagÞ ¼ xMOn ðn ¼ y=xÞ

ð7Þ

The free energy change of this reaction is reasonably assumed zero.1719 Therefore, ΔGo3 and K3 can be evaluated from general thermodynamic references. The distribution ratio Lg/m and Ls/m quantitatively show how the vapor pressure of M is bigger than that of solvent and how the concentration of oxidation product MOn in slag is higher than that of M in the solvent metal at given condition, respectively. A greater distribution ratio results in easier removal of M into the gas or slag phase. For the calculation of Lg/m and Ls/m in eqs 2 and 5, the parameters were referred from published thermodynamic data. 2027

3. RESULTS 3.1. Element Distribution Tendencies. Figures 1 and 2 show the calculated Ls/m and Lg/m for remelted copper and iron, respectively. In Figure 1, the distribution of 28 elements is shown under a copper converter’s typical operating conditions, that is, at 1500 K and pO2 = 1 101 Pa.30 In Figure 2, the distribution of 26 elements is shown under conventional operating conditions of a BOF or an EAF for steel making, that is, at 1873 K and pO2= 1.9 105 Pa,25,31 where pO2 is calculated from the equilibrium between the metal and the slag. In both figures, the initial impurity content in the copper or iron was set at 1 mol % (xM = 0.01) where γM is constant due to the dilute concentration range. The activity coefficient of MOn was assumed to be unity as a base case. The boundary between metal and gas was assumed to be the condition that satisfies both log(pM/pSolv) = 1 and log(pM) = 3. 32 For the calculation of Ls/m and Lg/m, the activity coefficients γM were sourced from published literature.21,2529 When the temperature and the concentration ranges for the published values of γM differed from the calculated conditions, they were estimated by assuming

the regular solution behavior using eq 6a _ Mix _ XS ΔHM ¼ G M ¼ RT ln γM ¼ Rð1 xM Þ2

ð6aÞ

_ _ Mix where G XS M and ΔH M are the partial molar excess Gibbs energy of mixing and the partial molar enthalpy of solution of M, respectively. The parameter R is the so-called alpha-function, which is a temperature and composition independent constant in the regular solution. Because the values of γM for mercury, rhenium, and rhodium in molten copper are unknown, they were assumed to be unity as a first approximation. The solubilities of calcium, lead, and silver in molten iron are less than 1 mol % at 1873 K, so that the activities and vapor pressures were assumed to be those for the pure state for these elements. Tungsten, boron, aluminum, chromium, gallium, iron, germanium, manganese, indium, magnesium, zinc, and strontium can be transferred to slag from molten copper by oxidation, and mercury can be removed by evaporation (Figure 1). Nickel, rhenium, tin, cadmium, and lead are partially distributed to the slag phase. On the other hand, precious metals (silver, gold, platinum, palladium, and rhodium), bismuth, selenium, tellurium, and antimony remain in the molten copper phase. Most of the elements distributed to the metal phase can be removed or recovered in further processing, such as electrorefining (electrolysis) or hydrometallurgical processes,33 and some of the elements distributed to the gas phase can also be recovered in further processing.34 The recoverable elements are shown as white circles in Figure 1, and the gray circles denote the typical alloying elements for industrial copper products. Most of the alloying elements can be removed or their concentrations can be managed by controlling the processing conditions. More elements tend to be distributed into the slag or gas phase in steel, although tungsten, molybdenum, cobalt, nickel, tin, and copper remain in molten iron. There is essentially no industrial process to recover elements from slag and dust in steel making, with the exception of zinc recovery from EAF dust of carbon steel production.35 Typical alloying elements for steel products are denoted as gray circles in Figure 2. Deoxidizing agents for steel refining, which have a large negative distribution ratio, are shown as black triangles. They are distributed into the slag phase. 4931



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Figure 3. Distribution ratio of elements between metal and slag in a copper converter.

Figure 4. Distribution ratio of elements between metal and slag in a steel-making process.

3.2. Effect of Temperature and Oxygen Partial Pressure on the Distribution Ratio. Figures 1 and 2 visually demonstrate the

elements that tend to enter the metal, slag, and gas phases under conventional metallurgical processing for copper and steel. However, operating conditions vary depending on facilities, scrap grade, and other factors. Therefore, the effects of process parameters on the distribution tendencies were further investigated. Figure 3 shows the effect of temperature and oxygen partial pressure on the distribution ratio of some elements between molten

copper and slag. For evaluating the possibility of change of the distributed phase, we selected tin, lead, nickel, and indium that have a distribution ratio ranging from 103 to 102. The distribution ratios Ls/m for lead decreased with increasing temperature and crossed over zero, indicating lead tends to remain in the metal phase at higher temperatures. However, the effect of temperature on the distribution ratio was not remarkably large. The distribution ratio increased with increasing oxygen partial pressure. For example, most tin remains in 4932



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Figure 5. Variation of the activity coefficients of FeO and MnO in CaO-Al2O3-SiO2 slag at 1873 K calculated by a regular solution model.

Figure 6. Variation of final manganese content in remelted steel scrap with mass of CaO-Al2O3-SiO2 slag at 1873 K. The compositions of slag A and slag B are shown in Figure 6. Initial manganese content in molten steel was 1 mol %.

molten copper at pO2 = 102 Pa, but it tends to be removed into slag at pO2 = 10 Pa. This distribution tendency indicates that a higher pO2 is favorable for the oxidative removal of impurities in a solvent metal. The oxygen partial pressure, however, should be less than the maximum pressure determined by the equilibrium of molten copper as the solvent metal with copper oxide in the slag, as shown in eq 7a 2CuðlÞ þ 1 =2 O2 ðgÞ ¼ Cu2 Oðin slagÞ

ð7aÞ

Figure 7. An example of the removal limit of an element (Pb) in liquid copper by evaporation.

Gibbs energy change of eq 7 gives the maximum oxygen partial pressure at 1500 K as 8.3 Pa, above which significant oxidation loss of molten copper occurs. The effect of temperature and oxygen partial pressure on the distribution ratio between molten iron and slag is presented in Figure 4 for niobium, manganese, vanadium, chromium, tungsten, molybdenum, nickel, tin, and copper, all of which have distribution ratios ranging from 104 to 104. In all cases, the distribution ratio Ls/m decreased with increasing temperature, indicating that the 4933



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Figure 8. Flow of elements associated with the copper production process. Copper, sulfur, and oxygen are the primary materials; minor elements and gangue minerals are not included. PGM denotes platinum group metals, including Ir, Rh, Ru, Os, Pd and Pt.

alloying elements tend to remain in the molten iron phase at higher temperatures. Once again, however, the effect of temperature was not remarkably large. Increasing oxygen partial pressure increased the distribution ratios. The oxygen partial pressure should again be limited to below the Fe/FeO equilibrium (molten iron as the solvent metal with iron oxide in the slag) given by eq 8 FeðlÞ þ 1 =2 O2 ðgÞ ¼ FeOðin slagÞ

ð8Þ

The critical oxygen partial pressure determined by the equilibrium between pure molten Fe and pure solid FeO was calculated to be 4 104 Pa at 1873 K, above which significant oxidation loss of molten iron occurs. Even at a high pO2 (just below the critical value), copper and tin are very hard to remove by oxidation (figure 4b). Copper and tin are both recognized as typical harmful impurities (so-called tramp elements36) for steel. 3.3. Effect of Slag Composition on the Distribution Ratio Ls/m. In the previous sections, the activity coefficient of the oxidation product MOn in slag was assumed to be unity as a base case. However, in practical operations, the slag composition is selected or designed to control the distribution behavior of elements between metal and slag because the activity or activity coefficient changes depending on the slag composition. For this reason, quite a lot of research has been conducted to study the activity coefficients of oxides in various kinds of slags.37 However, most of the studies have focused on conventional impurity elements whose origins are primary raw materials, such as iron ore, copper concentrate, coke, and lime. Thermodynamic information on elements whose origins are secondary resources, such as scrap, remains quite limited. Figure 5 shows the iso-activity coefficient contours of FeO and MnO in CaO-SiO2-Al2O3 ternary slag37 at 1873 K. The contours were obtained by “quadratic formalism” based on the regular solution model.31,38 For the calculation, it was assumed that this

model is applicable for the entire composition range of molten CaO-SiO2-Al2O3 slag containing 5 mol % MnO and FeO. The activity coefficients of MnO and FeO were also assumed to be constant from 0 to 5 mol % MnO and FeO. When molten iron containing manganese reacts with CaOSiO2-Al2O3-FeO slag, the exchange reaction of eq 9 takes place, and manganese transfers as MnO into the slag while a corresponding amount of FeO decreases as the reaction proceeds Mnðin FeðlÞÞ þ FeOðin slagÞ ¼ MnOðin slagÞ þ FeðlÞ ð9Þ The prediction of final manganese content by the refining with CaO-SiO2-Al2O3-FeO slag has been made where two typical slag compositions shown in Figure 6 (Slag A (γFeO = 1.61, γMnO = 0.43) and Slag B (γFeO = 3.22, γMnO = 1.27)) have been selected. The results are plotted in Figure 7 as a function of the amount of slag generated. Because the activity coefficients of MnO and FeO exhibit similar composition dependencies, the final manganese content of the two slags was similar. The final manganese content of the steel decreases almost linearly as the amount of slag generated increases in the range below 120 kg-slag/t-metal. Generally the amount of slag generated for refining of steel is less than 100 kg-slag/t-steel, except for very special cases, so that slag A or B cannot lower manganese in remelted steel below xMn = 0.005 which is the upper limit of manganese in carbon tool steel. Final manganese content will decrease as the amount of slag increases, but this would lead to increased costs and energy consumption. Therefore, in real-world operations, an optimum slag amount is chosen both from the specification of products and the cost performance. The behavior of other elements can also be predicted if the activity coefficients of the reactant and oxidant are known for a given slag system. The variable range of the slag/metal 4934


Environmental Science & Technology distribution ratios was calculated by changing the activity coefficient of MOn in slag from 0.1 to 10 for typical alloying elements (Cr, Co, Mn, Mo, Ni, V, W) in industrial steel products. The calculated ranges are presented as bars on the distribution ratios shown in Figure 2. As the figure indicates, these elements are still not controllable, even by changing the activity coefficients of oxidation products, because the distribution ratios in initial conditions are apart from zero. 3.4. Limitations of Evaporation Refining. As indicated in Figures 1 and 2, some elements can be removed or controlled by evaporation from molten copper or steel, but there are limits to removal by evaporation. The vapor pressure of the objective element decrease while that of solvent metal slightly increases as the evaporation proceeds. As a result, the vapor pressure of the objective element and solvent metal become closer, and preferential removal of the impurity eventually becomes impossible. As a typical example, we calculated the removal limit for lead from molten copper by a vacuum treatment (Figure 7).39 The assumed initial mole fraction of lead in molten copper was 0.01 (3.2 mass%), and the vapor pressure of lead was 249 Pa and that of copper was 0.82 Pa. When this molten copper alloy is treated under vacuum, essentially only the lead evaporates from the molten CuPb alloy and the concentration of lead decreases. As the process proceeds, the lead pressure decreases while that of copper slightly increases. The lowest pressure of about 0.1 Torr (13.3 Pa) can be attained in a metallurgical vacuum facility. Hence, lead in copper will be removed only until its partial pressure reaches the critical value of xPb = 5.3 104 (0.18 mass%) for molten copper at 1500 K. The major use of copper is electrical applications, and it needs a 4N (99.99%) purity to be highly conductive. Therefore, lead should be removed from remelted copper scrap by a different technique if the refined copper is to be used in electrical applications.

4. DISCUSSION The results of the present work can give an answer against the frequently given question: “Which impurity element can be removable in metallurgical process?” or “How far can the impurity level be controlled?”. The element radar chart, shown in our previous article,15 indicates that removing impurities is much more difficult from aluminum and magnesium than from the other metals listed. Even so, some elements are also difficult to remove from molten copper and iron, as shown in Figures 1 and 2. Even by varying process parameters such as temperature and oxygen partial pressure, no drastic improvement of removal efficiency should be expected, except for lead and tin in copper (Figures 3, 4, and 6). The disadvantage of steel making processing, remelting of aluminum, and remelting of magnesium is that the processes cannot effectively remove impurities. Typical alloying elements for steel are national strategic metals in Japan. As shown in Figure 2, not only nickel but also tungsten, molybdenum, and cobalt are very stable in molten iron. This thermodynamic feature strongly indicates that loss of or contamination by these uncontrollable elements could occur in conventional recycling systems of steel, aluminum, and magnesium. The removal or control of impurities is essential in this type of system. In order to effectively use these elements as important alloying elements rather than contaminations, it is important to develop a presorting system for scrap and accurate scrap specifications that take material composition into account. Needless to say, new metallurgical solutions such as

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calcium treatment 40 or nonmetallurgical solutions13 should be developed for sustainable supply of high metal products. This is not the case for copper and lead smelting. The primary metallurgical processing of copper and lead are generally followed by an electro-refining process (electrolysis) and extraction process as shown in Figure 8, which enables the effective removal and recovery of elements from crude metals. Figure 8 shows the flow of elements associated with the copper production process. Copper, sulfur, and oxygen as a primary material, minor elements, and gangue minerals are not included in this figure. Figure 8 shows that some valuable elements, bismuth, gold, silver, and PGMs (platinum group metals), including copper can be extracted into an anode slime in EoL products such as e-waste. In fact, metals nobler than copper, including PGMs, remain in copper and lead after processing in converter (Figure 1). These noble metals can be recovered from anode slime after electrolysis by an extraction process such as a hydrometallurgical process, for example, a solvent extraction process.33 Thus, the contamination does not become an essential problem for copper and lead when scrap containing these noble metals is supplied to the ordinary smelter. Even if PGMs are mixed into the copper scrap from e-waste, they can be finally recovered via anode slime (Figures 1 and 8).

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ81-29-854-2744. E-mail: nakajima.kenichi@nies. go.jp.

’ ACKNOWLEDGMENT This research was partially supported by JSPS (KAKENHI 22360387, KAKENHI 22360218). Financial supports given by the ISIJ Innovative Program for Advanced Technology, The Iron and Steel Institute of Japan, in 2008-2010 and ISIJ Research group, Automobile Recycling from Material Industry's perspective are also gratefully acknowledged. ’ REFERENCES (1) Rauch, J.; Pacyna, J. Earth’s global Ag, Al, Cr, Cu, Fe, Ni, Pb, and Zn cycles. Global Biochemical Cycles 2009, 23, GB2001. (2) Adriaanse, A.; Bringezu, S.; Hammond, A.; Moriguchi, Y.; Rodenburg, E.; Rogich, D.; Schutz, H. Resource flows: the material basis of industrial economies; World Resource Institute: Washington, DC, 1997. (3) Recalde, K.; Wang, J.; Graedel, T. E. Aluminum in-use stocks in the state of Connecticut. Resour. Conserv. Recycl. 2008, 52, 1271–1282. (4) Nakajima, K.; Yokoyama, K.; Nagasaka, T. Substance flow analysis of manganese associated with iron and steel flow in Japanese economy. ISIJ Int. 2008, 48, 549–553. (5) Wang, T.; Muller, D. B.; Graedel, T. E. Forging the anthropogenic iron. Environ. Sci. Technol. 2007, 41, 5120–5129. (6) Graedel, T. E.; Bertram, M.; Kapur, A.; Reck, B.; Spatari, S. Exploratory data analysis of the multilevel anthropogenic copper cycle. Environ. Sci. Technol. 2004, 38, 1253–1261. (7) Jonson, J.; Schewel, L.; Graedel, T. E. The contemporary anthropogenic chromium cycle. Environ. Sci. Technol. 2006, 40, 7060– 7069. (8) Murakami, S.; Yamanoi, M.; Adachi, Y.; Mogi, G.; Yamatomi, J. Material flow accounting for metals in Japan. Mater. Trans. 2004, 45, 3184–3193. 4935


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Thermodynamic Analysis for the Controllability of Elements in the

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