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Zinc oxide production through reprocessing of the electric arc furnace flue dusts
Journal of Environmental Chemical Engineering 1 (2013) 600–603
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Zinc oxide production through reprocessing of the electric arc furnace flue dusts Michel Shengo Lutandula a,*, Guylain Ngoy Kashala b a b
Chemistry Department, Faculty of the Sciences, University of Lubumbashi, P.O. Box 1825, Likasi Avenue, City of Lubumbashi, Katanga Province, People’s Republic of Congo ‘‘Ge´camines’’ – Electric Foundry of Lubumbashi, Division of Metallurgy, City of Lubumbashi, Katanga Province, The Democratic Republic of Congo
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 March 2013 Accepted 27 June 2013
The smelting of slag in view producing copper and cobalt alloys achieved at the ‘‘Big Hill Smelter in Lubumbashi’’ (DRC) generates Electric Arc Furnace flue dusts as by-product. These EAF dusts are highly toxic wastes since they consist essentially of low-grade zinc oxide powders wherein one finds lead (9.11%) as the major impurity. That is why, they are looked at as hazardous wastes that must be either stored in specialized landfills or reused as secondary raw materials. This research aims at producing zinc oxide through reprocessing of the EAF flue dusts. The treatment suggested is based on three operations: leaching (290 mg/L H2SO4, 130 min) of the EAF flue dusts in view dissolution of zinc (99.42%), precipitation of zinc (90%) at pH 7.5 with sodium hydroxide and calcination (300 8C, 15 min) to produce zinc oxide (93.39%). The recovery of zinc oxide from these hazardous wastes is a cost-saving method for their sound management and disposal. It is though that this practice could contribute to the conservation of resources and the environment safeguard. ß 2013 Elsevier Ltd All rights reserved.
Keywords: EAF flue dusts Reprocessing Zinc oxide Resources conservation Environment safeguard
Introduction The Electric Foundry of Lubumbashi (EFL) is the first stateowned smelter that was built in 1910 in the Katanga province of the Democratic Republic of Congo (DRC) [1]. It was producing copper Blister (99.5%) through the smelting of ores and the converting of copper mattes (Cu2S - FeS). The EFL belong to the ‘‘Ge´camines’’ which is the biggest state-owned mining company in the DRC. During the operating period of the EFL (1911–1993), about 13,000,000 tons of slag were discarded as the process wastes, stored nearby the smelter and named the ‘‘Big Hill of Lubumbashi’’ [2,3]. The production of copper and cobalt alloys at the ‘‘Big Hill Smelter in Lubumbashi’’ through reprocessing of slag was launched in 2000. This new smelter was designed to process 4,000,000 tons of slag from the ‘‘Big Hill of Lubumbashi’’ (2.2% Co, 1.3% Cu and 6–8% Zn) with the aim to produce annually more than 4,000 tons of cobalt and 2500 tons of copper. Consequently, it was expected to generate 15,000 tons of EAF dusts as by-product [2,3]. At present, the ‘‘Big Hill Smelter in Lubumbashi’’ generates daily 40 tons of the EAF flue dusts, which consists of low-grade zinc oxide where one finds lead (8%), iron, cadmium, copper, cobalt, and germanium to very low concentrations [4]. Based on the partnership contract signed at the
* Corresponding author. Tel.: +243 0818422753/0995084289; +32 0488860021. E-mail addresses: [email protected], [email protected], [email protected] (M.S. Lutandula). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.06.027
smelter creation, the EAF flue dusts together with the secondary slag (2% Zn) are sent back to the EFL. Unfortunately, they are neither locally reprocessed nor soundly managed in spite of their high content in zinc and their hazardous nature. The EAF flue dusts are reputed to be hazardous since they can release pollutants to the environment when stored in the open air or abandoned in a moist environment [5–7]. As the matter of fact, the EAF flue dusts can produce the airborne particles or release toxic metals through weathering, erosion and leaching by rainfall and winds. In the more industrialized countries, sophisticated techniques have been designed for the sound management of the EAF flue dusts such as recycling, the storage in specialized landfill, retention of the toxic metals through introduction of dusts in a siliceous phase or vitrification, the chemical stabilization of the EAF dusts prior to disposal or their incorporation in building materials, etc. [7–17]. This research aims at producing zinc oxide through reprocessing of the EAF flue dusts generated by the ‘‘Big Hill Smelter in Lubumbashi’’. It is though that EAF flue dusts can be used as secondary raw materials considering their contents in zinc and lead. To achieve this objective, the EAF flue dusts of our focus were sampled, assayed for copper, cobalt, zinc, cadmium, nickel, iron and germanium and leached using sulphuric acid as solvent in view dissolution of zinc and removal of lead through precipitation as sulphate. Subsequently, the dissolved zinc ions were recovered through precipitation as hydroxide. The obtained precipitate has undergone dehydration to produce calcine (ZnO).
M.S. Lutandula, G.N. Kashala / Journal of Environmental Chemical Engineering 1 (2013) 600–603
601
EAF dusts (88.07 % ZnO) 70.4 % Zn - 9.11 % Pb
Materials and methods Chemical analyses of the EAF flue dusts Two grams of the EAF flue dusts from the ‘‘Big Hill Smelter in Lubumbashi’’ was heated at 105 8C in a Memmert steamroom until constant weight and dissolved using aqua regia in view assessing for the contents in moisture and metals (Zn, Pb, Cd, Fe, Co and Cu) using a Perkin Elmer 2280 spectrophotometer. The residue given by the leaching of the EAF flue dusts was washed, dried, and weighted for the analysis of silica. A Spectro Genesis ICP-SAA device has enabled analysing germanium. Furthermore, an aliquot of a dried sample of the EAF flue dusts was heated at 400 8C in a mittens oven (45 min) in view assessing for the loss in weight through ignition. The results given by the above-mentioned analyses are shown in Table 1.
Leaching at 25 °C H2SO4 Conc.: 100 - 500 g/L Time: 30 - 150 min
Residue and PbSO4
Solid-Liquid separation
Impurities removal M(OH)n
Leach liquor purification (25 °C, pH 5.5) Precipitation of zinc (25°C, pH = 6.5 - 8.5)
Zn(OH)2
Leaching solution (60 mL)
NaOH solution (40 g/L)
NaOH solution (40 g/L) Raffinate
Calcination (300 °C, 15 minutes)
Leaching of the EAF flue dusts Two grams of the EAF flue dusts (Fig. 1) were stirred (300 rpm, 25 8C) with 60 mL of a leaching solvent consisting of sulphuric acid in view dissolution of zinc and lead precipitation via the reactions (1) and (2): ZnOðsÞ þ H2 SO4 ! ZnSO4 þ H2 O
(1)
PbOðsÞ þ H2 SO4 ! PbSO4 ðsÞ þ H2 O
(2)
The concentration of sulphuric acid (100–500 g/L) in the aqueous phase and time (30–150 min) were varied during the dissolution of zinc contained in the studied EAF flue dusts. The measure of the concentration of zinc liberated in the aqueous phase versus time and the concentration of sulphuric acid has provided the data that were processed using the Matlab 7.1 software to obtain the results depicted in Figs. 2 and 3.
Zinc oxide (93.39 % ZnO) 75.01 % Zn - 2.77 % Pb Fig. 1. Flow sheet for the reprocessing of the EAF flue dusts.
The precipitate formed was dehydrated through heating at 105 8C in a steamroom and calcination during 15 min in a mittens oven (300 8C) in view producing zinc oxide as shown in the reaction (5): ð300
ZnðOHÞ2 ðsÞ
C
and 15 minÞ
!
ZnOðsÞ þ H2 OðgÞ
(5)
The purity of zinc oxide produced through reprocessing of the EAF flue dusts generated by the ‘‘Big Smelter in Lubumbashi’’ was spectrophotometrically assessed and compared with that of the starting material.
Leaching solution purification and recovery of zinc Results and discussion Prior to the recovery of zinc ions through precipitation in the form of hydroxide, the leaching solution has undergone purification (pH 5.5) using sodium hydroxide (40 g/L) in view removal of the unwanted ions of the metals (Fig. 1) based on the reaction (3) below [18]: Mnþ þ nOH Ð MðOHÞnðsÞ
(3)
In the above reaction, M may represent iron, lead or copper and n the valence. Furthermore, the leach liquor pH (6.5–8.5) was gradually raised through an incremented addition of sodium hydroxide (40 g/L or 12.09 g) to neutralize the leaching solution and precipitate zinc ions as shown in the reaction (4) below: Zn2þ þ 2OH Ð ZnðOHÞ 2ðsÞ
Chemical analysis of EAF dusts The chemical composition of the EAF flue dusts from the ‘‘Big Smelter in Lubumbashi’’ is shown in Table 1. The analyzed EAF flue dusts consist essentially of zinc (70.74%) which is present in the oxidized state with the moisture that averages 14%. When heated at 400 8C, the dried EAF flue dusts
(4)
Table 1 Chemical composition of the EAF flue dusts from the ‘‘Big Smelter in Lubumbashi’’. Compound or element
Proportion (%)
Zn Pb Si Fe Cu Co Ge Moisture Insoluble matters Weight loss at 400 8C
70.74 9.11 1.6 0.84 0.12 0.006 0.001 13.79 8.7 4.73
Fig. 2. Concentration of zinc versus the leach liquor acidity and time.
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M.S. Lutandula, G.N. Kashala / Journal of Environmental Chemical Engineering 1 (2013) 600–603
Fig. 3. Recovery of zinc versus the leach liquor acidity and time.
Leaching of the EAF flue dusts in a sulphuric acid solution Based on the chemical composition of the studied EAF flue dusts (Table 1), lead appears to be the impurity of concern since it can affect the grade of zinc oxide. That is why the leaching of the EAF flue dusts was achieved using a solution of sulphuric acid in view removal of lead through precipitation as sulphate. The concentration of zinc liberated in the aqueous phase by the EAF flue dusts versus time and the concentration of sulphuric acid in the leaching solvent are depicted in Fig. 2. The amount of zinc dissolved by the leaching solvent augments until a maximum as time passes and the concentration of sulphuric acid increases (Fig. 2). Whatever the time of reaction between the EAF flue dusts and the leaching solvent, two highest dissolutions of zinc (19.84 and 23.51 g/L) are achievable whether one uses sulphuric acid to high concentrations (400–500 g/L) or not (200– 300 g/L). The first maximum dissolution of zinc (19.84 g/L) is observed after 40 min (Fig. 2) at the condition that the leaching solvent contains sulphuric acid to a concentration as high as 420 g/ L. When the leaching solvent is less concentrated in sulphuric acid (295 g/L), some more time (130 min) will be required in order to reach the second maximum dissolution of zinc (23.51 g/L). It is obvious that a compromise between time and the concentration of sulphuric acid will be required for rendering cost-effective the recovery of zinc oxide through reprocessing of the EAF flue dusts. Indeed, the use of the strongly acidic leaching solvent appears to be a timesaving option. On an economic standpoint, the dissolution of zinc contained in the EAF flue dusts using the less acidic leaching solvent seems to be an attractive option. From what precedes, it can be stated that the extent to which the EAF flue dusts release zinc to aqueous phase during the leaching depends both on the content in sulphuric acid and time. For the best dissolution of zinc (99.42%), a leaching solvent containing 290 g/L in sulphuric acid and 130 min (Fig. 3) are suggested because their correspond to a low dissolution of lead (0.007742 g/L) contained in the EAF flue dusts (Fig. 4). However, it is worth stressing that the more acidic leaching solvent has caused the lowest dissolution of lead (0.003205 g/L)
which corresponds to the second achievable maximum dissolution of zinc (19.84 g/L). It can be stated that the achievement of the EAF flue dusts leaching under the more acidic conditions has enabled minimizing the dissolution of lead. Whatever the concentration of sulphuric acid, the amount of the dissolved lead gradually decreases in the leaching solvent until a minimum comprised between 90 and 120 min (Fig. 4). When the content in acid sulphuric of the leaching solvent is kept at 200 g/L, the lowest concentration (0.006417 g/L) of lead is observed in the aqueous around 90 min. The increase in the concentration of lead in the leach liquor, which is observed beyond 120 min, might arise from the dissolution of lead sulphate (PbSO4). This dissolution of the precipitate is thought to result from the prolonged stirring of the leaching solution that interferes on the equilibrium (6): PbSO4 ðsÞ Ð Pb
2þ
þ SO2 4
(6)
Precipitation of zinc ions in the form of hydroxide The amount of zinc ions precipitated versus the aqueous phase pH is given in Fig. 5. It is obvious that the concentration of zinc ions released to the aqueous phase decreases via precipitation as pH augments.
100
Recovery of zinc and lead (%)
undergo a loss in weight of roughly 5%. Apart from zinc, the mean chemical species also present in the EAF flue dusts are lead (9.11%), silica (1.6%), iron (0.84%), and cadmium (0.12%). Worth is to stress that copper, cobalt, and germanium are also present in the EAF flue dusts but to concentrations that range from 10 to 60 ppm.
Fig. 4. Concentration of lead versus the leach liquor acidity and time.
Residual zinc (g/L) Zn Pb
80 60 40 20 0 6
7 8 pH of precipitation
Fig. 5. Recovery of zinc and lead versus pH of precipitation.
9
M.S. Lutandula, G.N. Kashala / Journal of Environmental Chemical Engineering 1 (2013) 600–603 Table 2 Chemical composition of the zinc oxide from the reprocessing of the EAF flue dusts. Compound or element
EAF flue dust (%)
Zinc oxide (%)
Change (%)
ZnO (as total Zn) Pb Fe Cd Cu Co Ge Si
88.07 (70.74) 9.11 0.84 0.12 0.006 0.001 0.001 1.6
93.39 (75.01) 2.77 0.086 0.14 0.005 0.0009 0.0008 0.37
+6.04 69.59 94.25 +16.67 83.33 90.00 80.00 76.86
oxide as raw material in the production of zinc metal or paints. As for lead which was removed from the leaching solution through precipitation as lead sulphate and considering its toxicity, it remains a toxic metal of great concern that must be recovered and could eventually used in the manufacture of batteries. In the days to come, the fate of the leaching residue and solution will be also considered and addressed for their safe handling and disposal. Acknowledgements
However, the amount of the coprecipitated lead remains low (<5%) until pH 7. Instead, one finds in the precipitate nearly 43% of zinc liberated in the leaching solvent by dissolution of the EAF flue dusts. Beyond pH 7.5, lead begins to be increasingly removed from the leaching solvent since its proportion in the precipitate has augmented from 11 to 44%. Additionally, one observes a decrease in the recovery of zinc that might be related to the alkalinity buildup in the aqueous phase leading to the increased dissolution of the zinc hydroxide [Zn(OH)2] which is transformed in zincates ions ZnO2 2 via the reaction (7): ZnðOHÞ2 ðsÞ þ 2OH Ð ZnO2 2 þ 2H2 O
603
(7)
Based on what precedes, it can concluded that the best precipitation of zinc ions occurs at pH 7.5 seeing that the smallest concentration of the unrecovered zinc (2.47 g/L) is observed in the aqueous phase. This optimal pH of zinc precipitation is reached through neutralization of the leaching solution using 12.09 g of sodium hydroxide. However, it is worth mentioning that the recovery of zinc based on precipitation using sodium hydroxide reveals to be a reagents-consuming operation since about 302 mL of the neutralizing solution were required to complete the reaction. Nonetheless, it is worth stressing that on average 90% of zinc was precipitated together with the smallest recoverable amount of lead (11%). Calcination of the precipitate to produce zinc oxide The zinc precipitate was dehydrated through heating at 300 8C in view conversion into zinc oxide. The obtained product has the chemical composition given in Table 2. It consists of high-grade zinc oxide (93.39%) obtained through the removal of nearly 70% of lead initially observed in the starting material. Additionally, the majority of metals observed in the EAF flue dusts were on average removed until 85% of their initial concentrations. Conclusion The recovery of zinc oxide through reprocessing of the EAF flue dusts produced by ‘‘the Big Hill Smelter in Lubumbashi’’ appears to be an attractive option for the sound management of hazardous wastes from industrial processes in the Katanga province. The reuse of the EAF flue dusts, which are presently looked at as low-cost secondary raw materials can contribute to the conservation of resources and the environment safeguard. Indeed, it enables recovering zinc oxide (93.39%) as an economic by-product through the leaching (290 g/L H2SO4, 130 min) of the EAF dusts, precipitation of zinc ions at pH 7.5 using NaOH - 40 g/L and it calcination to produce calcine (300 8C, 15 min). In the days to come, we suggest to optimize the neutralizing conditions of the leaching solvent and focus on the use of the recovered zinc
The authors are grateful to the authorities of the Electric Foundry of Lubumbashi (EFL) for their support and collaboration. Mr Kifite Ngoie is acknowledged for his implication in the achievement of the preliminary work, which has enabled the authors initiating the present research. References [1] N.M. Kalenga, J. Frenay, K. Mukendi, P. De Donato, T.A. Kaniki, Inventory of sites of production, storage and disposal of mineral wastes in Katanga and assessment of their environmental impacts, A report related to a scientific project of cooperation (2005.N86312PS508) between the University of Lubumbashi (UNILU), the University of Liege (ULg) and the National Polytechnic Institute of Lorraine (INPL) funded by the Agence Universtaire de la Francophonie (AUF), 2006; pp. 49–51. [2] G.J. Coakley, The Mineral Industry of Congo (Kinshasa), U.S. Geological Survey Minerals Yearbook, 2002, pp. 10.1–10.9. [3] C. Andrews, B. Bocoum, D. Tshimena, Democratic Republic of Congo, Growth with governance in the mining sector, The World Bank Report No. 43402-ZR, May 2008, Oil/Gas, Mining and Chemicals Department, AFCC2, Africa Region, 2008; pp. 106–107. [4] J.P. Milesi, S.F. Toteu, Y. Deschamps, J.L. Feybesse, C. Lerouge, A. Cocherie, J. Penaye, R. Tchameni, G. Moloto-A-Kenguemba, H.A.B. Kampunzu, N. Nicol, E. Duguey, J.M. Leistel, M. Saint-Martin, F. Ralay, C. Heinry, V. Bouchot, J.C. Doumnang Mbaigane, V. Kanda Kula, F. Chene, J. Monthel, P. Boutin, J. Cailteux, An overview of the geology and major ore deposits of Central Africa: Explanatory note for the 1:4,000,000 map ‘‘Geology and major ore deposits of Central Africa’’, Journal of African Earth Sciences 44 (2006) 571–595. [5] A.R.E. Hollagh, E.K. Alamdari, D. Moradkhani, A.A. Salardini, Kinetic analysis of isothermal leaching of zinc from zinc plant residue, International Journal of Nonferrous Metallurgy 2 (2013) 10–20. [6] O. Ruiz, C. Clemente, M. Alonso, F.J. Alguacil, Recycling of an electric arc furnace flue dust to obtain high grade ZnO, Journal of Hazardous Materials 141 (2007) 33–36. [7] M.A.T. Alsheyab, T.S. Khedaywi, Effect of electric arc furnace dust (EAFD) on properties of asphalt cement mixture, Resources, Conservation and Recycling 70 (2013) 38–43. [8] T. Sofilic, V. Novosel-Radovic, S. Cerjan-Stefanovic, A. Rastovcan-Mioc, The mineralogical composition of dust from an electric arc furnace, Materiali in Tehnologije 39 (5) (2005) 149–154. [9] E. Kruglick, Electric arc furnace dust recycling apparatus and method, Patent Application Publication number and date: N8 US 2013/00142722 A1 & Feb. 21, 2013, United States, 2013; pp. 1–7. [10] A.I. Al-Negheimish, R.Z. Al-Zaid, A.M. Alhozaimy, Method for manufacturing concrete containing electric arc furnace dusts, Patent number and date: N8 US 8,152,917 B2 & April.10, 2012, United States, 2012; 1–6. [11] G. Fedorko, A. Pribulova´, P. Futa´sˇ, D. Baricova´, P. Demeter, Compacting of fly dusts from cupola and electric arc furnace, Metalurgija 51 (2012) 1, 63-66. [12] M. Castellote, E. Menendez, C. Andrade, P. Zuloaga, M. Navarro, M. Ordonez, Radioactively contaminated electric arc furnace dust as an addition to the immobilization mortar in low and medium-activity repositories, Environmental Science & Technology 38 (2004) 2946–2952. [13] A.J.B. Dutra, P.R.P. Paiva, L.M. Tavares, Alkaline leaching of zinc from electric arc furnace steel dust, Minerals Engineering 19 (2006) 478–485. [14] M. Pelino, A.M. Karaman, P. Pisciella, S. Crisucci, D. Zonetti, Vitrification of electric arc furnace dusts, Waste Management 22 (2002) 945–949. [15] M. Liebman, The treatment and disposal of electric arc furnace dust in North America, AIM Market Research, Pittsburgh, 2006, pp. 1–12. [16] L.J. Licis, R.C. Bermark, Onsite recycling of electric arc furnace dust: the Jorgensen steel facility, Report on the process developed by Washington State Department of Environmental Quality and evaluated under the Waste Reduction Innovative Technology Evaluation program, Washington, 2005. pp. 5–8. [17] X. Sun, J-Y. Hwang, X. Huang, The microwave processing of electric arc furnace dust, Journal of Metallurgy 60 (10) (2008) 35–39. [18] J. Jandova´, J. Maixner, T. Grygar, Reprocessing of zinc galvanic waste sludge by selective precipitation, Ceramics-Silika´ty 46 (2) (2002) 52–55.
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Zinc oxide production through reprocessing of the electric arc furnace flue dusts Michel Shengo Lutandula a,*, Guylain Ngoy Kashala b a b
Chemistry Department, Faculty of the Sciences, University of Lubumbashi, P.O. Box 1825, Likasi Avenue, City of Lubumbashi, Katanga Province, People’s Republic of Congo ‘‘Ge´camines’’ – Electric Foundry of Lubumbashi, Division of Metallurgy, City of Lubumbashi, Katanga Province, The Democratic Republic of Congo
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 March 2013 Accepted 27 June 2013
The smelting of slag in view producing copper and cobalt alloys achieved at the ‘‘Big Hill Smelter in Lubumbashi’’ (DRC) generates Electric Arc Furnace flue dusts as by-product. These EAF dusts are highly toxic wastes since they consist essentially of low-grade zinc oxide powders wherein one finds lead (9.11%) as the major impurity. That is why, they are looked at as hazardous wastes that must be either stored in specialized landfills or reused as secondary raw materials. This research aims at producing zinc oxide through reprocessing of the EAF flue dusts. The treatment suggested is based on three operations: leaching (290 mg/L H2SO4, 130 min) of the EAF flue dusts in view dissolution of zinc (99.42%), precipitation of zinc (90%) at pH 7.5 with sodium hydroxide and calcination (300 8C, 15 min) to produce zinc oxide (93.39%). The recovery of zinc oxide from these hazardous wastes is a cost-saving method for their sound management and disposal. It is though that this practice could contribute to the conservation of resources and the environment safeguard. ß 2013 Elsevier Ltd All rights reserved.
Keywords: EAF flue dusts Reprocessing Zinc oxide Resources conservation Environment safeguard
Introduction The Electric Foundry of Lubumbashi (EFL) is the first stateowned smelter that was built in 1910 in the Katanga province of the Democratic Republic of Congo (DRC) [1]. It was producing copper Blister (99.5%) through the smelting of ores and the converting of copper mattes (Cu2S - FeS). The EFL belong to the ‘‘Ge´camines’’ which is the biggest state-owned mining company in the DRC. During the operating period of the EFL (1911–1993), about 13,000,000 tons of slag were discarded as the process wastes, stored nearby the smelter and named the ‘‘Big Hill of Lubumbashi’’ [2,3]. The production of copper and cobalt alloys at the ‘‘Big Hill Smelter in Lubumbashi’’ through reprocessing of slag was launched in 2000. This new smelter was designed to process 4,000,000 tons of slag from the ‘‘Big Hill of Lubumbashi’’ (2.2% Co, 1.3% Cu and 6–8% Zn) with the aim to produce annually more than 4,000 tons of cobalt and 2500 tons of copper. Consequently, it was expected to generate 15,000 tons of EAF dusts as by-product [2,3]. At present, the ‘‘Big Hill Smelter in Lubumbashi’’ generates daily 40 tons of the EAF flue dusts, which consists of low-grade zinc oxide where one finds lead (8%), iron, cadmium, copper, cobalt, and germanium to very low concentrations [4]. Based on the partnership contract signed at the
* Corresponding author. Tel.: +243 0818422753/0995084289; +32 0488860021. E-mail addresses: [email protected], [email protected], [email protected] (M.S. Lutandula). 2213-3437/$ – see front matter ß 2013 Elsevier Ltd All rights reserved. http://dx.doi.org/10.1016/j.jece.2013.06.027
smelter creation, the EAF flue dusts together with the secondary slag (2% Zn) are sent back to the EFL. Unfortunately, they are neither locally reprocessed nor soundly managed in spite of their high content in zinc and their hazardous nature. The EAF flue dusts are reputed to be hazardous since they can release pollutants to the environment when stored in the open air or abandoned in a moist environment [5–7]. As the matter of fact, the EAF flue dusts can produce the airborne particles or release toxic metals through weathering, erosion and leaching by rainfall and winds. In the more industrialized countries, sophisticated techniques have been designed for the sound management of the EAF flue dusts such as recycling, the storage in specialized landfill, retention of the toxic metals through introduction of dusts in a siliceous phase or vitrification, the chemical stabilization of the EAF dusts prior to disposal or their incorporation in building materials, etc. [7–17]. This research aims at producing zinc oxide through reprocessing of the EAF flue dusts generated by the ‘‘Big Hill Smelter in Lubumbashi’’. It is though that EAF flue dusts can be used as secondary raw materials considering their contents in zinc and lead. To achieve this objective, the EAF flue dusts of our focus were sampled, assayed for copper, cobalt, zinc, cadmium, nickel, iron and germanium and leached using sulphuric acid as solvent in view dissolution of zinc and removal of lead through precipitation as sulphate. Subsequently, the dissolved zinc ions were recovered through precipitation as hydroxide. The obtained precipitate has undergone dehydration to produce calcine (ZnO).
M.S. Lutandula, G.N. Kashala / Journal of Environmental Chemical Engineering 1 (2013) 600–603
601
EAF dusts (88.07 % ZnO) 70.4 % Zn - 9.11 % Pb
Materials and methods Chemical analyses of the EAF flue dusts Two grams of the EAF flue dusts from the ‘‘Big Hill Smelter in Lubumbashi’’ was heated at 105 8C in a Memmert steamroom until constant weight and dissolved using aqua regia in view assessing for the contents in moisture and metals (Zn, Pb, Cd, Fe, Co and Cu) using a Perkin Elmer 2280 spectrophotometer. The residue given by the leaching of the EAF flue dusts was washed, dried, and weighted for the analysis of silica. A Spectro Genesis ICP-SAA device has enabled analysing germanium. Furthermore, an aliquot of a dried sample of the EAF flue dusts was heated at 400 8C in a mittens oven (45 min) in view assessing for the loss in weight through ignition. The results given by the above-mentioned analyses are shown in Table 1.
Leaching at 25 °C H2SO4 Conc.: 100 - 500 g/L Time: 30 - 150 min
Residue and PbSO4
Solid-Liquid separation
Impurities removal M(OH)n
Leach liquor purification (25 °C, pH 5.5) Precipitation of zinc (25°C, pH = 6.5 - 8.5)
Zn(OH)2
Leaching solution (60 mL)
NaOH solution (40 g/L)
NaOH solution (40 g/L) Raffinate
Calcination (300 °C, 15 minutes)
Leaching of the EAF flue dusts Two grams of the EAF flue dusts (Fig. 1) were stirred (300 rpm, 25 8C) with 60 mL of a leaching solvent consisting of sulphuric acid in view dissolution of zinc and lead precipitation via the reactions (1) and (2): ZnOðsÞ þ H2 SO4 ! ZnSO4 þ H2 O
(1)
PbOðsÞ þ H2 SO4 ! PbSO4 ðsÞ þ H2 O
(2)
The concentration of sulphuric acid (100–500 g/L) in the aqueous phase and time (30–150 min) were varied during the dissolution of zinc contained in the studied EAF flue dusts. The measure of the concentration of zinc liberated in the aqueous phase versus time and the concentration of sulphuric acid has provided the data that were processed using the Matlab 7.1 software to obtain the results depicted in Figs. 2 and 3.
Zinc oxide (93.39 % ZnO) 75.01 % Zn - 2.77 % Pb Fig. 1. Flow sheet for the reprocessing of the EAF flue dusts.
The precipitate formed was dehydrated through heating at 105 8C in a steamroom and calcination during 15 min in a mittens oven (300 8C) in view producing zinc oxide as shown in the reaction (5): ð300
ZnðOHÞ2 ðsÞ
C
and 15 minÞ
!
ZnOðsÞ þ H2 OðgÞ
(5)
The purity of zinc oxide produced through reprocessing of the EAF flue dusts generated by the ‘‘Big Smelter in Lubumbashi’’ was spectrophotometrically assessed and compared with that of the starting material.
Leaching solution purification and recovery of zinc Results and discussion Prior to the recovery of zinc ions through precipitation in the form of hydroxide, the leaching solution has undergone purification (pH 5.5) using sodium hydroxide (40 g/L) in view removal of the unwanted ions of the metals (Fig. 1) based on the reaction (3) below [18]: Mnþ þ nOH Ð MðOHÞnðsÞ
(3)
In the above reaction, M may represent iron, lead or copper and n the valence. Furthermore, the leach liquor pH (6.5–8.5) was gradually raised through an incremented addition of sodium hydroxide (40 g/L or 12.09 g) to neutralize the leaching solution and precipitate zinc ions as shown in the reaction (4) below: Zn2þ þ 2OH Ð ZnðOHÞ 2ðsÞ
Chemical analysis of EAF dusts The chemical composition of the EAF flue dusts from the ‘‘Big Smelter in Lubumbashi’’ is shown in Table 1. The analyzed EAF flue dusts consist essentially of zinc (70.74%) which is present in the oxidized state with the moisture that averages 14%. When heated at 400 8C, the dried EAF flue dusts
(4)
Table 1 Chemical composition of the EAF flue dusts from the ‘‘Big Smelter in Lubumbashi’’. Compound or element
Proportion (%)
Zn Pb Si Fe Cu Co Ge Moisture Insoluble matters Weight loss at 400 8C
70.74 9.11 1.6 0.84 0.12 0.006 0.001 13.79 8.7 4.73
Fig. 2. Concentration of zinc versus the leach liquor acidity and time.
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Fig. 3. Recovery of zinc versus the leach liquor acidity and time.
Leaching of the EAF flue dusts in a sulphuric acid solution Based on the chemical composition of the studied EAF flue dusts (Table 1), lead appears to be the impurity of concern since it can affect the grade of zinc oxide. That is why the leaching of the EAF flue dusts was achieved using a solution of sulphuric acid in view removal of lead through precipitation as sulphate. The concentration of zinc liberated in the aqueous phase by the EAF flue dusts versus time and the concentration of sulphuric acid in the leaching solvent are depicted in Fig. 2. The amount of zinc dissolved by the leaching solvent augments until a maximum as time passes and the concentration of sulphuric acid increases (Fig. 2). Whatever the time of reaction between the EAF flue dusts and the leaching solvent, two highest dissolutions of zinc (19.84 and 23.51 g/L) are achievable whether one uses sulphuric acid to high concentrations (400–500 g/L) or not (200– 300 g/L). The first maximum dissolution of zinc (19.84 g/L) is observed after 40 min (Fig. 2) at the condition that the leaching solvent contains sulphuric acid to a concentration as high as 420 g/ L. When the leaching solvent is less concentrated in sulphuric acid (295 g/L), some more time (130 min) will be required in order to reach the second maximum dissolution of zinc (23.51 g/L). It is obvious that a compromise between time and the concentration of sulphuric acid will be required for rendering cost-effective the recovery of zinc oxide through reprocessing of the EAF flue dusts. Indeed, the use of the strongly acidic leaching solvent appears to be a timesaving option. On an economic standpoint, the dissolution of zinc contained in the EAF flue dusts using the less acidic leaching solvent seems to be an attractive option. From what precedes, it can be stated that the extent to which the EAF flue dusts release zinc to aqueous phase during the leaching depends both on the content in sulphuric acid and time. For the best dissolution of zinc (99.42%), a leaching solvent containing 290 g/L in sulphuric acid and 130 min (Fig. 3) are suggested because their correspond to a low dissolution of lead (0.007742 g/L) contained in the EAF flue dusts (Fig. 4). However, it is worth stressing that the more acidic leaching solvent has caused the lowest dissolution of lead (0.003205 g/L)
which corresponds to the second achievable maximum dissolution of zinc (19.84 g/L). It can be stated that the achievement of the EAF flue dusts leaching under the more acidic conditions has enabled minimizing the dissolution of lead. Whatever the concentration of sulphuric acid, the amount of the dissolved lead gradually decreases in the leaching solvent until a minimum comprised between 90 and 120 min (Fig. 4). When the content in acid sulphuric of the leaching solvent is kept at 200 g/L, the lowest concentration (0.006417 g/L) of lead is observed in the aqueous around 90 min. The increase in the concentration of lead in the leach liquor, which is observed beyond 120 min, might arise from the dissolution of lead sulphate (PbSO4). This dissolution of the precipitate is thought to result from the prolonged stirring of the leaching solution that interferes on the equilibrium (6): PbSO4 ðsÞ Ð Pb
2þ
þ SO2 4
(6)
Precipitation of zinc ions in the form of hydroxide The amount of zinc ions precipitated versus the aqueous phase pH is given in Fig. 5. It is obvious that the concentration of zinc ions released to the aqueous phase decreases via precipitation as pH augments.
100
Recovery of zinc and lead (%)
undergo a loss in weight of roughly 5%. Apart from zinc, the mean chemical species also present in the EAF flue dusts are lead (9.11%), silica (1.6%), iron (0.84%), and cadmium (0.12%). Worth is to stress that copper, cobalt, and germanium are also present in the EAF flue dusts but to concentrations that range from 10 to 60 ppm.
Fig. 4. Concentration of lead versus the leach liquor acidity and time.
Residual zinc (g/L) Zn Pb
80 60 40 20 0 6
7 8 pH of precipitation
Fig. 5. Recovery of zinc and lead versus pH of precipitation.
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M.S. Lutandula, G.N. Kashala / Journal of Environmental Chemical Engineering 1 (2013) 600–603 Table 2 Chemical composition of the zinc oxide from the reprocessing of the EAF flue dusts. Compound or element
EAF flue dust (%)
Zinc oxide (%)
Change (%)
ZnO (as total Zn) Pb Fe Cd Cu Co Ge Si
88.07 (70.74) 9.11 0.84 0.12 0.006 0.001 0.001 1.6
93.39 (75.01) 2.77 0.086 0.14 0.005 0.0009 0.0008 0.37
+6.04 69.59 94.25 +16.67 83.33 90.00 80.00 76.86
oxide as raw material in the production of zinc metal or paints. As for lead which was removed from the leaching solution through precipitation as lead sulphate and considering its toxicity, it remains a toxic metal of great concern that must be recovered and could eventually used in the manufacture of batteries. In the days to come, the fate of the leaching residue and solution will be also considered and addressed for their safe handling and disposal. Acknowledgements
However, the amount of the coprecipitated lead remains low (<5%) until pH 7. Instead, one finds in the precipitate nearly 43% of zinc liberated in the leaching solvent by dissolution of the EAF flue dusts. Beyond pH 7.5, lead begins to be increasingly removed from the leaching solvent since its proportion in the precipitate has augmented from 11 to 44%. Additionally, one observes a decrease in the recovery of zinc that might be related to the alkalinity buildup in the aqueous phase leading to the increased dissolution of the zinc hydroxide [Zn(OH)2] which is transformed in zincates ions ZnO2 2 via the reaction (7): ZnðOHÞ2 ðsÞ þ 2OH Ð ZnO2 2 þ 2H2 O
603
(7)
Based on what precedes, it can concluded that the best precipitation of zinc ions occurs at pH 7.5 seeing that the smallest concentration of the unrecovered zinc (2.47 g/L) is observed in the aqueous phase. This optimal pH of zinc precipitation is reached through neutralization of the leaching solution using 12.09 g of sodium hydroxide. However, it is worth mentioning that the recovery of zinc based on precipitation using sodium hydroxide reveals to be a reagents-consuming operation since about 302 mL of the neutralizing solution were required to complete the reaction. Nonetheless, it is worth stressing that on average 90% of zinc was precipitated together with the smallest recoverable amount of lead (11%). Calcination of the precipitate to produce zinc oxide The zinc precipitate was dehydrated through heating at 300 8C in view conversion into zinc oxide. The obtained product has the chemical composition given in Table 2. It consists of high-grade zinc oxide (93.39%) obtained through the removal of nearly 70% of lead initially observed in the starting material. Additionally, the majority of metals observed in the EAF flue dusts were on average removed until 85% of their initial concentrations. Conclusion The recovery of zinc oxide through reprocessing of the EAF flue dusts produced by ‘‘the Big Hill Smelter in Lubumbashi’’ appears to be an attractive option for the sound management of hazardous wastes from industrial processes in the Katanga province. The reuse of the EAF flue dusts, which are presently looked at as low-cost secondary raw materials can contribute to the conservation of resources and the environment safeguard. Indeed, it enables recovering zinc oxide (93.39%) as an economic by-product through the leaching (290 g/L H2SO4, 130 min) of the EAF dusts, precipitation of zinc ions at pH 7.5 using NaOH - 40 g/L and it calcination to produce calcine (300 8C, 15 min). In the days to come, we suggest to optimize the neutralizing conditions of the leaching solvent and focus on the use of the recovered zinc
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