Reduction Kinetics of Zinc Powder from Brass Converter Slag by Pyrometallurgical Method Using Hydrogen Gas

Abstract

A pyrometallurgical reduction process for the recovery of copper and zinc from brass secondary slag (BSS) was studied. Specifically, the effect of reduction temperature and time on the conversion to metallic phases was investigated. The brass secondary slag was characterized by X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, automatic elemental analysis, thermogravimetric analysis, and field emission scanning electron microscopy. A two-step reduction of BSS was identified. The step 1 comprised the reduction of ZnO, while the step 2 featured the reduction of ZnO·Al₂O₃. Furthermore, the application of a first-order reaction model with Arrhenius analysis, indicated a conversion of ZnO to Zn(g) that had a rate constant increasing from 1.4 ± 0.13 × 10⁻³ s⁻¹ at 900 °C to 2.18 ± 0.15 × 10⁻² s⁻¹ at 1050 °C. This reaction had an activation energy of 233.2 ± 26.1 kJ/mol. Secondly, the conversion of ZnO·Al₂O₃ to Zn(g) and Al₂O₃ increased from 1.54 ± 0.21 × 10⁻⁵ s⁻¹ at 900 °C to 1.09 ± 0.19 × 10⁻³ s⁻¹ at 1050 °C, and the activation energy was 376.7 ± 22.4 kJ/mol. This reaction mechanism and its associated kinetic data can be applied to optimize the operation conditions of recycling processes for Cu-containing wastes.

1. Introduction

Copper and zinc are important base metals required for various applications in metallurgical and chemical industries. The annual global output of copper and alloy products represents around 16.1 million tons, of which about 15.3 million tons (75–80 %) originate from refined copper, while about 4.2 million tons (20–25 %) are derived from direct melt scrap (Moskalyk P.R. and Alfantazi A.M., 2003).

However, the increasing worldwide demand for zinc and copper has necessitated intensive studies into the recovery and extraction of these metals from different sources (Rudnik E. et al., 2009). Some of these metals are recovered from different secondary slags, e.g., brass dross, brass ash, and flue dusts in electric furnaces. These brass secondary slags (BSS) are by-products obtained during brass scrap smelting and crushing of brass slag. It has been estimated that for every tons of copper production about 2.2 tons of slag are generated, so that 24.6 million tons of slag result from world copper production every year (Gorai B. et al., 2003).

BSS usually contains Cu, Zn, and other minor elements such as Pb, Fe, Ni, Si, and Sb (Jha M.K. et al., 2001). However, these slags contain different level of impurities depending on source. Recovery and extraction of these metals is necessary and important from both economic and environmental viewpoints.

In industries, various methods are used to extract metals from BSS. One of these, the hydrometallurgical process, is used to recover and recycle metals from BSS using different leaching agents, including sulfuric acid (Anand S. et al., 1983), hydrochloric acid (Sharma K.D. and Row B.R.L., 1985), cyanide (Anand S. et al., 1980) acetic acid (Altundogan H.S. and Tumen F., 1997), and ammonia (Sukla L.B. et al., 1986). However, the use of leaching agents is harmful to the human body and can cause environmental pollution.

These drawbacks can be overcome using pyro-metallurgical smelting to recover Cu and Zn from BSS. The top submerged lance (TSL) process is a typical pyro-metallurgy method that was commercialized and researched to improve recovery efficiency. The TSL process provides highly efficient mixing and a heat source for reaction, due to combustion of the fuel and oxygen-mixed gas with the BSS. Moreover, TSL process can easily remove the impurities that are generated through the formation of slag in the upper furnace (Bakker M.L. et al., 2011).

However, it is difficult to directly feed certain raw materials that are fine dusts to a TSL furnace because of dust-scattering during the injection step. To solve this problem, the density of the BSS can be increased during the reduction process using a fluidized bed reactor. This is done in a stage prior to the TSL furnace, and can be effective for increasing process throughput. The reduction rate and reaction mechanism of BSS are very complex to determine because such slags can contain mixtures of copper, zinc, and various elements. Therefore, further research into the thermodynamics and reaction kinetics of BSS recovery processes is necessary to optimize process conditions in fluidized bed reactors.

The aim of this work is to determine the activation energy and reaction mechanism of BSS reduction by applying a pyro-metallurgical method using different reduction gas. Firstly, different parameters affecting the reduction ratio, such as reduction temperature and reduction time, were investigated. Then, based on the results of the reduction, the activation energy and reaction mechanism of BSS were determined using the Arrhenius equation (Igor I. K., 2008).

2. Experimental

The BSS that was used in this experimental was taken from the brass smelter of SEOWON CO. LTD in South Korea. The BSS sample was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, ARL 3410, Switzerland) and X-ray diffraction (XRD, Siemens D-5000, Germany). The carbon concentration of the BSS sample was determined by an automatic elemental analyzer (AEA, Flash EA 1112 series, Thermo Fisher Scientific, U.S.A.). The extent of reduction of the BSS sample was estimated by a simultaneous thermogravimetric analyzer (TGA, TA SDT model 2060, TA Instruments, U.S.A.), under a flow of 5 % H₂ + 95 % N₂, at a flow rate 100 cm³/min. The sample was heated to 850 °C at a heating rate of 5 °C/min.

The reduction of the BSS sample was carried out under a hydrogen atmosphere in a horizontal alumina tube furnace. The hydrogen flow rate was fixed at 100 cm³/min in all experiments. The respective effects of reduction temperature and time on the reaction rates of the brass secondary slags were investigated. Specifically, the reduction temperature was varied from 900 to 1050 °C, and the reduction time was varied from 10 to 480 min. Each sample of BSS, weighing 15 g, was placed in an alumina crucible and then put in the tube furnace. Then, the samples were heated to the desired reduction temperature at a heating rate of 5 °C/min. Finally, the samples were cooled in the furnace.

After reduction, phase analysis of the reduction products was performed by X-ray diffraction, using Cu Kα radiation (40 kV, 30 mA) at step size increments of 0.1°. The microstructures of the reduction samples were observed using a field emission scanning electron microscopy (FE-SEM, JSM 6330F, Jeol, Japan). The respective chemical compositions of copper and zinc were analyzed using energy-dispersive X-ray spectroscopy (EDS, INCA, Oxford, U.K.).

3. Results and discussion

3.1 Characteristic of BSS

The chemical composition of the BSS sample used in this study is presented in Table 1, as determined by ICP-AES and AEA. ZnO and CuO were identified by X-ray diffraction analysis as major components in the BSS, whereas Zn-spinel (ZnO·Al₂O₃) and Al₂SiO₄ were detected as a minor component, as shown in Fig. 1. Fig. 2a shows all the morphological features of the initial BSS sample. The latter consisted mainly of well-distinguishable, submicron-sized ZnO particles (Fig. 2b). The BSS samples were also leached with HNO₃ + HCl (3M: 1M) to measure the other impurities. Bright particles indicate aluminum and silicon compounds, whereas other particles indicate silicon dioxide (Figs. 2c, d).

Table 1 Chemical composition of the initial brass secondary slags (mass%)

Cu	Zn	Si	Al	Pb	Fe	Mn	C	Other
14.74	70.54	3.92	5.7	1.01	0.74	0.32	2.2	0.83

Fig. 1

X-ray diffraction pattern of initial BSS sample.

Fig. 2

SEM morphologies of BSS samples: a) whole morphological features of the initial samples, b) the submicron-size ZnO, c) after ZnO leaching with HCl + HNO₃, and d) EDS analysis results.

The thermogravimetric curve of the reduction process, which was recorded during a linear program at a constant heating rate, is shown in Fig. 3. As shown in Fig. 3, the TGA curve of the initial sample reveals two major weight losses: one has a monotonous weight decrease of 1.3 % in the range of 50–760 °C, and the other is very rapid and occurs at 760 °C with approximately 1.4 % weight loss. The first weight loss is possibly related to the combustion of impurities, evaporation of water, and a chemical reaction between carbon in the BSS sample (2.2 mass% in Table 1) and hydrogen gas. The other weight loss (1.4 %) is due to the reduction of zinc oxide in the BSS sample to zinc gas phase, by hydrogen gas (Gioia F. et al., 1977).

Fig. 3

TGA analysis of the BSS sample in mixed gas (5 % H₂ + 95 % N₂).

3.2 Reduction mechanism of BSS

Fig. 4 shows the XRD patterns of BSS samples prior to and after reduction at various temperatures (900–1050 °C) for varying reaction times (10–480 min). Generally, XRD patterns for reductions at the four temperatures are similar, with the exception of time-scale, indicating that the same reduction mechanism prevails in the range of 900–1050 °C. These results suggest a stepwise reduction mechanism of BSS with hydrogen gas. Thus, based on these XRD results, a two-step reaction was identified. The main reactions at each step are as follows: Step 1 is the ZnO reduction (Eqn. 1), and step 2 is the ZnO·Al₂O₃ reduction (Eqn. 2).

Fig. 4

XRD patterns of BSS, prior to, and after, hydrogen reduction at 1000 °C for various times.

  

$$\text{ZnO}(\text{s})+{\text{H}}_2(\text{g})\to \text{Zn}(\text{g})+{\text{H}}_2\text{O}(\text{g})$$(1)

  

$$\text{ZnO}\cdot {\text{Al}}_2{\text{O}}_3(\text{s})+{\text{H}}_2(\text{g})\to \text{Zn}(\text{g})+{\text{Al}}_2{\text{O}}_3(\text{s})+{\text{H}}_2\text{O}(\text{g})$$(2)

The morphology changes in the BSS samples versus the reduction times at 1050 °C are shown in Fig. 5. It is seen that the initial fine ZnO powder is aggregated at the initial stage of the reaction, after which the [ZnO·Al₂O₃/ Cu] mixture forms a more dense structure as shown in Figs. 5a, b, respectively. The amount of ZnO powder decreased with increasing reduction time because Zn evaporated after reduction by hydrogen gas according to Eqn. 1. Also, it should be noted that Cu powder, having spherical particles of ∼1 μm, appeared after removal of the ZnO at 120 min of the reaction time. Furthermore, there was no significant change in the shape or particle size of the Cu particles, even after 480 min.

Fig. 5

The morphology change in BSS versus hydrogen reduction times at 1050 °C: a) 10, b) 60, c) 120, and d) 480 min.

The distilled Zn was recovered as metallic Zn in the cold zone of the alumina tube, as shown in the inset of Fig. 6a. In the cold zone, it was condensed and formed Zn powder immediately. The latter had spherical particles with a size range of approximately 1–6 μm. The XRD pattern confirms that the recovered powder is composed of pure Zn, without any other components. This is due to much difference in evaporation temperature between Zn (907 °C) and Cu (2562 °C), SiO₂ (2950 °C), Al₂O₃ (2977 °C) and CuO (2000 °C), respectively.

Fig. 6

a) Metal zinc powders recovered from ZnO in BSS at the cold zone of furnace, b) morphology of metal Zn powder, and c) XRD pattern of metal Zn powder.

From the XRD and SEM data, the process of Cu reduction from BSS comprises several stages. The first step is the conversion of brass to Cu with distilled Zn. According to previous research, the brass is generated by the reaction of CuO and ZnO with hydrogen gas at above 600 °C (Kappenstein C. et al., 1996). The concentration of Zn in brass decreased with increasing reduction temperature and times. This was due to loss of Zn by distillation from the brass during the first step. Fig. 7 shows the respective concentrations of Cu and Zn in brass as a function of reduction temperature and reaction time. Here, Cu concentration increases with reduction time; moreover, the rate of increase of Cu concentration is reduced with decreasing reduction temperature as shown in Fig. 7a.

Fig. 7

Concentration change of Cu and Zn in brass by Zn vaporization according to reduction times and temperature: a) Cu, and b) Zn.

The Cu concentration increase is attributed to not only reduction of CuO but also evaporation of reduced Zn. The contribution of Zn is presented in Fig. 7b, where Zn concentration linearly decreases in the early of the reaction time at most of the reduction temperature. In the case of reactions performed at 1000 °C and 1050 °C, all Zn is eliminated by 480 min and 360 min, respectively. Lastly, a significant amount of Zn remained in the Cu phase at 900 °C for all reduction times.

3.3 The kinetics analysis of BSS

In order to understand the reduction mechanism for BSS, a series of kinetic analyses were performed. For this, the extent of BSS reduction was integrated with respect to time and plotted cumulatively as a reaction ratio (α). The reaction ratio was defined as 0 before the reaction and 1 when it was completed. The rate of each reaction was expressed as a function of Arrhenius parameters with reaction model in Eqn. 3:   

$$\text{d}\alpha /\text{d}t=k(T)f(\alpha )$$(3)

where dα/dt is the reaction rate, α the reaction ratio, k(T) is the temperature-dependent rate constant, t is the time, T is temperature, and f(α) is the differential form of the reaction model (Santosh K.U., 2006). We used the first-order reaction model f(α) = 1 − α for this reaction. This model is applicable to a wide range of solid-state reactions: Reactant(s) → Product(s) + Gas (g) (James E.H., 2007). For the purpose of this kinetic analysis, the residual mass at various reduction times was expressed as the reaction ratio (α), by means of Eqn. 4:   

$$\alpha =(M_0-M_{\text{t}})/M_0$$(4)

where M_t represents the mass of the sample at a reduction time t and M₀ is the initial mass of the sample. Therefore, a more general form of the first-order reaction equation can be written as follows:   

$$-\text{ln}(1-\alpha )=k(T)t$$(5)

Thus, by plotting −ln(1 − α) versus t, the rate constant (k) can be determined from the slope and intercept of the fitted straight line. Values for the rate constant (k) calculated for α, in the range 0.2–1, are given in Table 2. The transformation step 2 is not as rapid as step 1, although the amount of ZnO·Al₂O₃ is relative small. Specifically, the rate constant of step 1 increases from 1.4 ± 0.13 × 10⁻³ s⁻¹ at 900 °C to 2.18 ± 0.15 × 10⁻² s⁻¹ at 1050 °C. Meanwhile, the rate constant of step 2 increases from 1.54 ± 0.21 × 10⁻⁵ s⁻¹ at 900 °C to 1.09 ± 0.19 × 10⁻³ s⁻¹ at 1050 °C. The temperature-dependence of the rate constant is traditionally described by the Arrhenius law:   

$$k(T)=A\hspace{0.17em}\hspace{0.17em}\text{exp}-(E_{\text{a}}/\mathit{RT})$$(6)

Table 2 Summary of rate constants k of BSS sample reduction at different temperature and step

	STEP 1 (s−1)	STEP 2 (s−1)
900 °C	1.4 ± 0.13 × 10−3	1.54 ± 0.21 × 10−5
950 °C	5.52 ± 0.72 × 10−3	5.61 ± 0.68 × 10−5
1000 °C	1.16 ± 0.24 × 10−2	3.45 ± 0.52 × 10−4
1050 °C	2.18 ± 0.15 × 10−2	1.09 ± 0.19 × 10−3

In Eqn. 6, A is the pre-exponential factor, E is the activation energy, and R is the universal gas constant. Rewriting Eqn. 6 (Jelic D. et al. 2011),   

$$\text{ln}\hspace{0.17em}k=\text{ln}\hspace{0.17em}k-(E_{\text{a}}/\mathit{RT})$$(7)

The activation energies (E_a) calculated for steps 1 and 2 are 233.2 ± 26.1 and 376.7 ± 22.4 kJ/mol, respectively, as shown in Fig. 8. The significant coefficients of determination (R²) of the activation energy indicated 0.963 and 0.989 as confidence level, respectively. The reaction mechanism, deduced from the kinetic data, is depicted graphically as shown in Fig. 9. In step 1, the reductions of CuO and ZnO begin at 500 °C and 600 °C, respectively. ZnO starts to evaporate as soon as it is reduced to the metallic phase, due to its high vapor pressure, and this evaporation is the main cause of the weight loss in this reaction system. In the temperature up to 600 °C, Cu forms a brass phase with Zn, whereas the brass phase decomposes into solid Cu and Zn vapor at 900 °C, which is step 2. It should be noted that ZnO in the ZnO·Al₂O₃ spinel phase is also reduced in this step; hence, Cu, SiO₂, and Al₂O₃ are the final phase formed in this BSS reduction system. It is supposed that the ceramic impurities are removed by the succeeding smelting process.

Fig. 8

Arrhenius plot for the dominant reaction in the range of 900–1050 °C with hydrogen gas: a) step 1 (ZnO reduction); and b) step 2 (ZnO·Al₂O₃ reduction).

Fig. 9

Schematic diagram of BSS samples reduction mechanism between room temperature and 1050 °C with hydrogen gas.

4. Conclusion

The reduction kinetics of BSS in a hydrogen atmosphere has been studied at 900 °C to 1050 °C, and for the reduction times between 10 min and 480 min. The characterization of the samples was done by means of SEM and XRD analysis. The results indicated that the reduction of BSS samples was found to occur in a two-step process, with recovery of Zn powder (spherical particles, size: approximately 1∼6 μm). The first step involved a rapid transformation of ZnO to Zn (g), whereas the second step involved the slow conversion from ZnO·Al₂O₃ to Zn (g).

A first-order function and Arrhenius analysis were utilized to calculate the rate constants and activation energies for the BSS reduction. In particular, the rate constant for the transformation from ZnO to Zn (g) increased from 1.4 ± 0.13 × 10⁻³ s⁻¹ at 900 °C to 2.18 ± 0.15 × 10⁻² s⁻¹ at 1050 °C with an activation energy of 233.2 ± 26.1 kJ/mol in step 1. In step 2, the corresponding rate constant for the transformation from ZnO·Al₂O₃ to Zn (g) increased from 1.54 ± 0.21 × 10⁻⁵ s⁻¹ at 900 °C to 1.09 ± 0.19 × 10⁻³ s⁻¹ at 1050 °C with an activation energy of 376.7 ± 22.4 kJ/mol. It is believed that further understanding of the reaction mechanism observed in this study can contribute to the further optimization of process conditions in fluidized bed reactors.

Acknowledgements

This research was supported by a grant from the Fundamental R & D Program for Energy (No. 2012T100100092, (“Development of pyrometallurgical technology for low quality urban mining by-products”) funded by the Ministry of Knowledge Economy, Republic of Korea.

Author’s short biography

Tae-Hyuk Lee

Tae-Hyuk Lee is a doctor candidate for Materials Engineering, Chungnam National University, South Korea. He received his M.D. from Chungnam National University, South Korea in 2012. He is now pursuing his Ph.D. in Chungnam National University focusing on stress corrosion cracking of Inconel 690 for steam generator tubes. His research interests include stress corrosion cracking, recycling of metals, and metal matrix composites.

Sin-Hyeong Joo

Sin-Hyeong Lee is a master’s course for Materials Engineering, Chungnam National University, South Korea. He received his B.S. from Chungnam National University, South Korea. His research interests include powder metallurgy materials and processing, in particular non-ferrous materials, such as boron, boron composite, silver and silicon.

Hayk H Nersisyan

Hayk Nersisyan is research professor of Materials Science & Engineering at Chungnam National University in South Korea and Rapidly Solidified Materials Research Center. He received his B.S., M.S. and Ph D degrees from Yerevan State University in Armenia. His major research field is powder metallurgy materials and processing, in particular non-ferrous materials and their composites such as magnesium, titanium, aluminium and copper.

Man-Sik Kong

Man-Sik Kong is senior researcher of Plant Engineering Center, Institute for Advanced Engineering, South Korea. He received his B.S., and M.S degree from Gyeongsang National University and Ph D. degree from Ajou University in South Korea. Dr. Kong’s research interests include powder metallurgy materials, processing and recycling of rare metals.

Jae-Woo Lee

Jae-Woo Lee is director of SEWON CO., LTD, South Korea. He received his B.S. degree from Hanyang University in South Korea. His research interests include recycling process of copper and copper alloy, recovery of metals from urban mine.

Ki-Won Park

Ki-Won Park is head of research center of SEWON CO., LTD, South Korea. He received his B.S. degree from Inha University in South Korea. His research interests include recycling process of copper and copper alloy, recovery of metals from urban mine.

Jong-Hyeon Lee

Jong-Hyeon Lee is Professor in Department of Materials Science & Engineering at Chungnam National University in South Korea and head of Rapidly Solidified Materials Research Center. He received his B.S., M.S, and Ph D degrees from Chungnam National University. His major research field is electro refining, winning, and coating of rare metals. He also studies on powder synthesis by self-propagating high temperature synthesis. He published more than 100 articles and 30 patents.

References

•  Altundogan  H.S.,  Tumen  F., Metal recovery from copper converter slag by roasting with ferric sulphate, Hydrometallurgy, 44 (1997) 261–267.
•  Anand  S.,  Kanta  R.P.,  Jena  P.K., Recovery of metal values from copper converter and smelter slags by ferric chloride leaching, Hydrometallurgy, 5 (1980) 355–365.
•  Anand  S.,  Sarveswara  R.K.,  Jena  P.K., Pressure leaching of copper converter slags using dilute sulphuric acid for the extraction of cobalt, nickel and copper values, Hydrometallurgy, 67 (1983) 1–7.
•  Bakker  M.L.,  Nikolic  S.,  Mackey  P.J., ISASMELT^TM-Application for nickel, Minerals Engineering, 24 (2011) 610–619.
•  Gioia  F.,  Mura  G.,  Viola  A., Experimental study of the direct reduction of sinterized zinc oxide by hydrogen, Chemical Engineering Science, 33 (1977) 1401–1409.
•  Gorai  B.,  Jana  R.K. Premchand, Characteristics and utilisation of copper slag—a review, Resources conservation and recycling, 39 (2003) 299–313.
•  Igor  I. Katkov, Challenge from the simple: Some caveats in linearization of the Boyle-van’t Hoff and Arrhenius plots, Cryobiology, 57 (2008) 142–149.
•  James  E.H., Principles of chemical kinetics 2nd eds., Academic Press, New York, 2007.
•  Jelic  D.,  Tomic-Tucakovic  B.,  Mentus  S., A kinetic study of copper oxide powder reduction with hydrogen, based on thermogravimetry, Thermochimica Acta, 521 (2011) 211–217.
•  Jha  M.K.,  Kumar  V.,  Singh  R.J., Review of hydrometallurgical recovery of zinc from industrial wastes, Resources conservation and recycling, 33 (2001) 1–22.
•  Kappenstein  C.,  Cernak  H.,  Brahmi  R.,  Duprez  D.,  Chomic  J., Copper-zinc oxide catalysts. Part IV. Thermal treatment in air, argon and hydrogen and XRD study of new bimetallic precursors-direct formation of alloys, Thermochimica Acta, 279 (1996) 65–76.
•  Moskalyk  P.R.,  Alfantazi  A.M., Review of copper pyrometallurgical practice: today and tomorrow, Minerals Engineering, 16 (2003) 893–919.
•  Rudnik  E.,  Burzynska  L.,  Gumoska  W., Hydrometallurgical recovery of copper and cobalt from reduction-roasted copper converter slag, Minerals Engineering, 22 (2009) 88–95.
•  Santosh  K.U., Chemical kinetics and reaction dynamics, Springer, New Delhi, 2006.
•  Sharma  K.D. and  Row  B.R.K., An electrolytic process for recovery of zinc dust from melting furnace slag, Hydrometallurgy, 13 (1985) 377–383.
•  Sukla  L.B.,  Panda  S.C.,  Jena  P.K., Recovery of cobalt, nickel and copper from converter slag through roasting with ammonium sulphate and sulphuric acid, Hydrometallurgy, 16 (1986) 153–165.