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Process for recovery of metal values from alnico scarps by electro-chemical leaching technique
Separation and Purification Technology 169 (2016) 78–82
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Process for recovery of metal values from alnico scarps by electro-chemical leaching technique G. Prabaharan a, S.P. Barik b,⇑ a b
Department of Chemistry, Maharaja Sayajirao University of Baroda, Vadodara, Gujarat, India Resources Recycling, University of Science and Technology, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 April 2016 Received in revised form 27 May 2016 Accepted 28 May 2016 Available online 28 May 2016 Keywords: Electro-chemical leaching Alnico scraps Synergistic effect Cobalt
a b s t r a c t This paper reports the study on the dissolution behavior of the alnico scraps generated during the manufacture of alnico permanganate magnets. Composition of the scrap used in the study was: 26.09% Co, 16.36% Ni, 47.02% Fe and 8.04% Al. Electroless leaching of the alnico scraps with sulphuric acid had no significant influence on the extraction of metals and which was due to the low surface area and less activity of the scraps. In order to improve the leaching efficiency, electric current was introduced to the system. Various experimental factors such as time, sulphuric acid concentration and current density were studied to determine the best conditions for the dissolution of the metal values. Effective leaching of scrap was due to the synergistic effect of ferric sulphate and temperature both generated during leaching with the current and sulphuric acid concentration in the system. Based on the leaching efficiency, they could be order as Ni > Fe > Co > Al. Under optimum leaching conditions (sulphuric acid 10% (v/v), current density 400 A/m2 and equilibrium time 4 h), more than 99% of the metal content was dissolved. Based on the process steps involved, a flow sheet was proposed for the commercial application. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Extensive exploitation of high grade ore of cobalt and nickel, its depletion and complication in processing of low grade ore leads to the processing of secondary sources to recover these valuable metals. Generally secondary sources of cobalt and nickel include spent catalyst, spent batteries, glass dusts, and alnico scraps [1–4]. Each secondary resource is having its own drawbacks in processing. Major drawback with spent catalyst and glass dust are huge amount of siliceous matrix and low cobalt/nickel content, which leads to generation of vast solid waste and low productivity [5]. In the case of spent batteries, there are issues like collection and storage of batteries and burning of flammable electrolytes [6]. The drawbacks with alnico scraps are: (i) poor leaching kinetics and (ii) evolution of hydrogen gas during leaching [7]. However, it is still attractive in industrial perspective due to its high content of cobalt and nickel. Hence recovering of these metals from such magnetic alloy scraps is essential as it not only influences the economic efficiency due to high content of the desired metals but also minimizes the resources management. In order to have a better understanding, the revenue contribution from alnico scraps is
⇑ Corresponding author. E-mail address: [email protected] (S.P. Barik). http://dx.doi.org/10.1016/j.seppur.2016.05.044 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.
compared with the primary source of cobalt and presented in Table 1. The revenue generation from the alnico scrap is almost close to the high grade ore of cobalt [8]. However, the availability of high grade ore is very less and the average value of cobalt from heterogenite is around 6–10% [9], which is almost 3 times less than alnico. Alnico scrap generated during the manufacturing of permanent magnets is quite rich both in nickel and cobalt along with a large amount of iron. Even though it is rich in both nickel and cobalt, a very few studies have been reported on the processing of such scraps [10], which might be due to the poor leaching kinetics of the metallic scraps. The poor leaching kinetics of magnetic alloy scraps is due to its low surface area and less activity. In order to improve the leaching kinetics, it is required either to increase the surface area by reducing the size or increasing the activity by introducing an oxidizing environment. But size reduction by pulverization is practically not feasible due to huge noise pollution, high energy consumption and damage of hammering materials. It is possible by atomization (water/air), but requires high energy operation. A number of processing approaches for recovering the valuable metals from varieties of scraps in hydrochloric acid leaching medium using different oxidants such as cupric chloride, ferric chloride and ozone have been proposed [10–12]. Alex et al. reported on the leaching of alnico scraps (fine powder) at 70 °C at a pulp density of 17% (w/v) for 5 h using 0.4 M cupric chloride
79
G. Prabaharan, S.P. Barik / Separation and Purification Technology 169 (2016) 78–82 Table 1 Comparison of alnico scrap vs cobalt ore. Alnico scrap
% Abs, g/kg Unit price, Rs Value/ kg of alnico scrap
High grade cobalt ore
Low grade cobalt ore
Co
Ni
Fe
Al
Co
Fe
Cu
Co
Fe
Cu
26.09 260.9 1500 391.4
16.4 164 593 97.0
47.0 470 NA NA
8.04 80.4 NA NA
34.2 342 1500 513
5.13 51.3
14.1 141 350 49.35
3.96 39.6 1500 59.4
10.6 106
6.45 64.5 350 22.6
Total value/kg
488.4
0
562.4
solution under continuously flowing oxygen. Though these processes performed a better leaching yield of metals, but due to the corrosive nature and difficulty in storage of chlorine gas, its use has been restricted so far [13]. Moreover, the process is applicable only for alnico scraps in the form of fine powders not for metallic alloys. Sulphuric acid is less hazardous in nature as compared to other inorganic acids. Due to its commercial viability and less hazardous in nature, sulphuric acid has been widely used in many specific areas of base metal extraction and processing from different sources. However, to date no work has been done on the processing of alnico scraps (metallic) using sulphuric acid. Electro dissolution is a simple technique and easy to handle and control [14]. The application of such technique has been employed in processing of metallic materials containing Fe, Cu, Ni, Co, W, Al, etc. [15–19]. The above reported studies were performed by taking either metal alone or its alloy using different acid/alkali solution. Moreover, no studies on the electro dissolution of alnico scrap (metallic) are reported yet. Thus, the current study focuses on the application of electro dissolution of alnico scraps with sulphuric acid for the techno-commercial feasibility. The effects of major parameters such as time, sulphuric acid concentration and current density on dissolution of various metals from alnico scraps were studied and discussed in detail. In addition, leaching of the alnico scrap was compared and discussed both by electroless and electrolytically.
0
82.0
Graphite (-)
Element
Power supply
Electrolyte
Lead (+)
Alnico scraps
Fig. 1. Schematic diagram of electro dissolution of alnico scraps.
Weighed amount of the alnico scarps were taken inside an anode pot and required amount of concentration sulphuric acid was added and the experiment was carried out for a predetermined time. In the case of electroless leaching experiments, same cylindrical cell was used under agitation with the help of an over head stirrer. Periodically 5 ml of sample was withdrawn, filtered, diluted and analyzed to determine the metal ion concentration in the solution by Atomic Absorption Spectrophotometer (AAnalyst-400, Perkin-Elmer).
2. Materials and methods 3. Results and discussion 2.1. Materials 3.1. Electroless leaching of alnico scraps The sample (alnico scrap) was procured from a Korean recycling company. Shape of the sample was cylindrical with variable particle diameter. Weighed amount of sample (as received) was digested using a mixed acid composed of HClO4, H2SO4 and HNO3 followed by dilution and analysis. Table 2 presents the chemical composition of the sample used in this study. Analytical grade of H2SO4 and distilled water were used in the preparation of the feed for the leaching study. 2.2. Leaching study Electro-chemical leaching experiments were carried out in a cylindrical cell (capacity 2 L) made of fiber reinforced plastic (FRP) containing lead anode inside at the bottom and cathode (graphite, 20 150 mm2) from the top at the center. Both electrodes are connected to a rectifier through a copper wire. The schematic diagram of electro dissolution of alnico scraps is shown in Fig. 1.
In order to investigate the dissolution kinetics of alnico scraps, experiments were performed without current by varying reaction time from 4 to 40 h at room temperature using 20% (v/v) sulphuric acid under agitation. The results are shown in Fig. 2. It can be seen that the leaching kinetics of the metals was very slow and at the end of 40 h of leaching time, a maximum leaching efficiency of 21.1% Co, 6.1% Ni, 3.5% Al and 34.6% Fe was observed. At about 2 103 kg/h of alnico scraps was dissolved out, which was not practically feasible for industrial practices. 20% H2SO4 is nearly 4 times of the stoichiometric requirement. Moreover, the leaching efficiency was low even after 40 h. Further increase of sulphuric acid concentration will lead to unreacted acids in the system, and will consume additional base to neutralize in the next step of the process during iron removal by jarosite precipitation, which will not be economical. So the acid concentration was limited to 20% as maximum. Since electroless leaching had no significant
Table 2 Chemical composition of alnico scrap. Element
Co
Ni
Fe
Al
Mn
Mg
Zn
Cr
Mo
Acid insoluble
%
26.09
16.36
47.02
8.04
0.09
0.01
0.11
0.01
0.04
0.96
80
G. Prabaharan, S.P. Barik / Separation and Purification Technology 169 (2016) 78–82
40
14
Leaching efficiency, %
Leaching efficiency, %
Ni
30
Al Fe
20
10
Ni 10
10
20
30
40
50
Al Fe
8 6 4 2 0
0 0
Co
12
Co
0
5
10
15
20
[H2 SO4 ], %(v/v)
Time, h
Fig. 4. Effect of sulphuric acid on leaching of metals.
Fig. 2. Electroless dissolution behavior of alnico scraps.
influence on the extraction of metals from alnico scraps and hence, studies were carried out with the aid of electric current to improve the leaching efficiency.
3.2.1. Effect of time The leaching kinetics of alnico scraps was performed out by varying the leaching time (2–12 h) using 10% (v/v) H2SO4 and current density 200 A/m2. As shown in Fig. 3, the leaching kinetics of all the metals except Al was very fast up to 8 h of leaching time and remains unchanged over the next 12 h. At the end of 8 h of leaching time, the leaching efficiency of the metals such as Co, Ni, Fe and Al were 9.2%, 12.85, 11.3% and 6.15, respectively. Hence 8 h of leaching time was chosen for the rest of the experiments.
25
Leaching efficiency, %
3.2. Electro-chemical leaching of alnico scraps
30
Co Ni
20
Al 15
Fe
10 5 0
0
100
200
300
400
500
Current density, A/m2 Fig. 5. Effect of current density on leaching of metals.
3.2.2. Effect of concentration of sulphuric acid In order to improve the leaching kinetics, concentration of sulphuric acid was varied from 5 to 20% (v/v) while keeping the rest of the conditions as: time 8 h and current density 200 A/m2. With the increase in acid concentration extractions of Co, Ni, Al and Fe increased as shown in Fig. 4. Above 10% (v/v) sulphuric acid, there was no significant effect on the leaching efficiency of metals. This might be due to the saturation of leach liquor with metal ions which is evident from the formation of crystals of ferric sulphate in the liquor. In the case of 10% (v/v) sulphuric acid, formation of ferrous sulphate was observed during dissolution of iron (Eq. (1)). However, as per the Eq. (2), formation of ferric sulphate was observed with increasing concentration of sulphuric acid [22]. Moreover, this ferric sulphate has poor solubility in water leading to the formation of crystals. 15
Leaching efficiency, %
Co 12
Ni Fe
9
Al 6 3 0
0
2
4
6
8
10
12
Time, h Fig. 3. Effect of time on leaching of metals.
14
! Fe þ H2 SO4 FeSO4 þ H2 ! 2Fe þ 3H2 SO4 þ 6Hþ 2 Fe3þ þ 3SO2 þ 6H2 O
ð1Þ ð2Þ
So, 10% (v/v) sulphuric acid was chosen for the rest of the experiments. The volume of leach liquor was replaced periodically (4 h) with fresh 10% (v/v) sulphuric acid when the leach liquor reached to pH 2. 3.2.3. Effect of current density The effect of current density on leaching of metals was studied in the range of 50–400 A/m2. The rest of the parameters were kept as: time 8 h and sulphuric acid concentration of 10% (v/v). The results are presented in Fig. 5. As can be seen, the leaching efficiency of all the metals followed an increasing trend in the varied range. At the end of 8 h of leaching time and current density 400 A/ m2, the extraction of the Co, Ni, Al and Fe were 18.4%, 27.2%, 12.1% and 21.6%, respectively. Periodically (2 h), the volume of the liquor was replaced with 10% (v/v) sulphuric acid as the liquor pH reached to 2. During optimization of the above three parameters (time, sulphuric acid concentration and current density), a common trend (Ni > Fe > Co > Al) was observed by comparing the leaching efficiency of individual metal ions. The lowest leaching efficiency of Al and Co might be due to the formation of acid resistant oxide layers at the anode which is absent in the case of Ni and Fe. 4. Comparison of electroless and electrochemical leaching of alnico scraps The leaching efficiency of metals obtained at the optimum leaching conditions (time 8 h and 10% (v/v) sulphuric acid) for both
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G. Prabaharan, S.P. Barik / Separation and Purification Technology 169 (2016) 78–82
electroless and electrochemical methods have compared. The compared results are presented in Table 3. In the case of electrochemical leaching, the leaching efficiencies of Co, Ni, Al and Fe are more than 5.8, 20.9, 20.1, and 5.3 times of electroless leaching, respectively. In order to understand the leaching behavior of alnico scraps in the process of electro leaching, the leaching efficiency of metals in gram equivalent by both theoretical and actual was compared. From the results shown in Table 4, the power loss decreased with respect to time up to 9 h. Above 9 h, the leaching efficiency of metals is more than the theoretical value. This is quite abnormal. It is expected that the gram equivalent leached out in electrochemically should match with the combination of gram equivalent leached out by chemically and electrolytically together. On the contrary, the gram equivalent leached out by electro chemically is higher than that of the combination of chemically and electrolytically. This might be due to the combined effect of ferric sulphate and temperature (50 °C) generated during electro leaching [16,20,21].
Table 3 Comparison of electroless and electrochemical leaching of alnico scraps in sulphuric acid. Elements
Cobalt Nickel Aluminum Iron
Leaching efficiency, % Electroless
Electrochemical
3.2 1.3 0.6 4.1
18.4 27.2 12.1 21.6
Similar observation like the autocatalytic effect of Fe3+ ion was also reported during electro dissolution at high rate by Srivastava et al. [16]. This synergistic effect (ferric sulphate, temperature, current and sulphuric acid) was absent at the beginning of the reaction as the leaching medium consists of only sulphuric acid 10% (v/v) and current, which evident from the power loss. However, during leaching the increase of the concentration of ferric sulphate and temperature with time leads to the synergistic effect which is inferred from decrease of the power loss. Above 9 h, there was no power loss and the leaching efficiency was more than the theoretical value till 18 h. Above 18 h, the synergistic effect reached equilibrium. Thus the combined effect of ferric sulphate, temperature, current and sulphuric acid lead to the electro-chemical leaching process as a commercially feasible option for the alnico scraps. Based on the above studies, a flow sheet (Fig. 6) was also proposed for the commercial application of the processing of alnico scrap. In the above flow sheet, solvent extraction and electro winning generally operated as a continuous process. Since the proposed electro dissolution method can also be a continuous process which can comfortably fit in the flow sheet. To confirm the commercial feasibility of the continuous leaching process, 10% (v/v) sulphuric acid was fed continuously using a peristaltic pump to the cell at a flow rate of 16 mL/min to maintain the leach liquor generated at a pH of 2. The result of the continuous leaching operation was presented in Fig. 7. Concentration of the metal ions particularly Co and Fe, the major metal ions in the leach liquor was almost constant throughout the process, which indicates that the electro leaching can be a continuous process.
25
Table 4 Synergistic effect of electro leaching.
2 4 6 9 11 13 15 18 21 23
Total gram equivalent leached out (electrochemically)
Total gram equivalent leached out (chemically)
Expected gram equivalent (theoretically)
Difference, % (synergistic effect)
1.25 3.26 3.77 8.76 13.31 17.25 21.32 25.41 29.54 33.12
0.14 1.32 1.99 2.81 3.56 4.33 4.97 5.92 6.95 7.75
1.49 2.98 4.48 6.34 8.02 10.01 11.49 13.36 15.59 17.46
23 24 14 4 14 20 29 31 31 31
20
[Metal], g/L
Time, h
15 10
Co 5 0
Fe 0
2
4
Fig. 7. Continuous leaching of alnico scrap.
Alnico scraps
H2SO4 (10% v/v)
Spent electrolyte
6
Time, h
Electroleaching
Jarosite precipitation
Fe & Al Cake
Solvent Extraction of Co by Cyanex 272
Nickel Precipitation
Coblat electrowinning
Nickel carbonate
Cobalt metal Fig. 6. Proposed flow sheet for the processing of alnico scrap.
Soda ash
8
82
G. Prabaharan, S.P. Barik / Separation and Purification Technology 169 (2016) 78–82
5. Conclusion The leaching of metals by both chemical and electro-chemical method was studied. From the study, electro-chemical method showed a better leaching efficiency of metals (Fe, Co, Ni and Al) than electroless leaching method. The leaching conditions are optimized and under optimum leaching conditions (sulphuric acid 10% (v/v), current density 400 A/m2 and equilibrium time 4 h), more than 99% of the metal content was dissolved by continuous operation method. The effective leaching was due to the synergistic effect of ferric sulphate and temperature both generated during leaching with the current and sulphuric acid concentration in the system. This study creates the scope for recycling secondary metallic resources by electro-chemical method as process parameters such as grinding of the material, agitation are not required. In addition, there will be no generation of solid waste after leaching with complete recovery of metals and cobalt leach liquor can be produced on the continuous scale. Based on the above study, a complete flow sheet has been proposed for the commercial application of the process. Acknowledgements The authors are thankful to Prof. U.W. Lee, President, University of Science and Technology, Daejeon, Korea for his kind permission to publish this article. References [1] S.P. Barik, K.H. Park, P.K. Parhi, J.T. Park, Direct leaching of molybdenum and cobalt from spent hydrodesulphurization catalyst with sulphuric acid, Hydrometallurgy 111–112 (2012) 46–51. [2] J. Jandova, H. Vu, P. Dvorak, Treatment of sulphate leach liquors to recover cobalt from waste dusts generated by the glass industry, Hydrometallurgy 77 (1–2) (2005) 67–73. [3] A. Chagnes, B. Pospiech, A brief review on hydrometallurgical technologies for recycling spent lithium-ion batteries, J. Chem. Technol. Biotechnol. 88 (2013) 1191–1199.
[4] T. Xu, X. Zhang, Z. Lin, B. Lu, M.A. Chunmei, X. Gao, Recovery of rare earth and cobalt from Co-based magnetic scraps, J. Rare Earths 28 (2010) 485–488. [5] M. Marafi, A. Stanislaus, Spent hydroprocessing catalyst management: a review: part II, advances in metal recovery and safe disposal methods, Resour. Conserv. Recycl. 53 (2008) 1–26. [6] D.C.R. Espinosa, A.M. Bernardes, J.A.S. Tenório, An overview on the current processes for the recycling of batteries, J. Power Sources 135 (2004) 311–319. [7] X. Fan, W. Xing, H. Dong, J. Zhao, Y. Wu, B. Li, W. Tong, X. Wu, Factors research on the influence of leaching rate of nickel and cobalt from waste superalloys with sulfuric acid, Int. J. Nonferr. Met. 2 (2013) 63–67. [8] L. Jun, L. Linyan, X. Shengming, C. Ruan, Recovery of copper and cobalt from low grade heterogenite with reductive acid leaching method, Chin. J. Nonferr. Met. 22 (2012) 305–309. [9] http://www.tuttglobal.com/i/cobalt-ore/all.htm. [10] P. Alex, A.K. Suri, C.K. Gupta, Proc: Waste Processing and Recycling in the Mining and Metallurgical Industries of Canadian Inst. of Metallurgist, vol. 2, Vancouver, British Columbia, 1995. pp. 47–56. [11] R.K. Jana, V. Kumar, A.K. Saha, K.V. Rao, B.D. Pandey, Premchand, Processing of tungsten alloy scrap for the recovery of tungsten metal, in: Proceedings: NsEWM, NML, Jamshedpur, 1996, pp. 94–98. [12] G.J. Casado, C.M. Cruells, M.E. Juan, V.A. Roca, O.J. Vinyals, Selective Metal Leaching Process, EP 1281779 A2, 2003. [13] C.K. Gupta, T.K. Mukherjee, Hydrometallurgy in Extraction Processes, vol.1, 1990, p. 185. [14] T. Tsuru, Anodic dissolution mechanisms of metals and alloys, Mater. Sci. Eng., A 146 (1991) 1–14. [15] E.Y. Kim, M.S. Kim, J.C. Lee, K. Yoo, J. Jeong, Leaching behavior of copper using electro-generated chlorine in hydrochloric acid solution, Hydrometallurgy 100 (2010) 95–102. [16] P.K. Srivastava, R. Srivastava, P.K. Barhai, High rate of anodic dissolution of iron work piece during electrochemical machining, Int. J. Mech. Eng. 41 (2013) 1110–1116. [17] V.V. Malyshev, S.A. Butov, Separation of cobalt and tungsten carbide by anodic dissolution of hard alloys in phosphoric acid solutions, Russ. J. Appl. Chem. 76 (2003) 383–386. [18] M.D. Merre, H.L. Anvers, Separation or Nickel Contained in Nickeliferrous Cobalt Alloys, U.S. Patent 2,651,562 A, 1953. [19] A. Banu, M. Marcu, O. Radovici, C. Pirvu, M. Vasilescu, Electro dissolution studies of three aluminum alloys in acid, neutral and alkaline solutions, Rev. Roum. Chim. 51 (3) (2006) 193–198. [20] T. Hirato, H. Majima, Y. Awakura, The leaching of chalcopyrite with ferric sulfate, Metall. Mater. Trans. B 18 (1987) 489–496. [21] P.R. Taylor, J.P. Vanderloop, An investigation into the leaching kinetics of cobaltite ore with ferric sulfate solutions, Metall. Mater. Trans. B 11 (1980) 83– 88. [22] G. Svehla, Vogel’s Textbook of Macro and Semimicro Qualitative Inorganic Analysis, Fifth ed., 1979, p. 241.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Process for recovery of metal values from alnico scarps by electro-chemical leaching technique G. Prabaharan a, S.P. Barik b,⇑ a b
Department of Chemistry, Maharaja Sayajirao University of Baroda, Vadodara, Gujarat, India Resources Recycling, University of Science and Technology, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 April 2016 Received in revised form 27 May 2016 Accepted 28 May 2016 Available online 28 May 2016 Keywords: Electro-chemical leaching Alnico scraps Synergistic effect Cobalt
a b s t r a c t This paper reports the study on the dissolution behavior of the alnico scraps generated during the manufacture of alnico permanganate magnets. Composition of the scrap used in the study was: 26.09% Co, 16.36% Ni, 47.02% Fe and 8.04% Al. Electroless leaching of the alnico scraps with sulphuric acid had no significant influence on the extraction of metals and which was due to the low surface area and less activity of the scraps. In order to improve the leaching efficiency, electric current was introduced to the system. Various experimental factors such as time, sulphuric acid concentration and current density were studied to determine the best conditions for the dissolution of the metal values. Effective leaching of scrap was due to the synergistic effect of ferric sulphate and temperature both generated during leaching with the current and sulphuric acid concentration in the system. Based on the leaching efficiency, they could be order as Ni > Fe > Co > Al. Under optimum leaching conditions (sulphuric acid 10% (v/v), current density 400 A/m2 and equilibrium time 4 h), more than 99% of the metal content was dissolved. Based on the process steps involved, a flow sheet was proposed for the commercial application. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Extensive exploitation of high grade ore of cobalt and nickel, its depletion and complication in processing of low grade ore leads to the processing of secondary sources to recover these valuable metals. Generally secondary sources of cobalt and nickel include spent catalyst, spent batteries, glass dusts, and alnico scraps [1–4]. Each secondary resource is having its own drawbacks in processing. Major drawback with spent catalyst and glass dust are huge amount of siliceous matrix and low cobalt/nickel content, which leads to generation of vast solid waste and low productivity [5]. In the case of spent batteries, there are issues like collection and storage of batteries and burning of flammable electrolytes [6]. The drawbacks with alnico scraps are: (i) poor leaching kinetics and (ii) evolution of hydrogen gas during leaching [7]. However, it is still attractive in industrial perspective due to its high content of cobalt and nickel. Hence recovering of these metals from such magnetic alloy scraps is essential as it not only influences the economic efficiency due to high content of the desired metals but also minimizes the resources management. In order to have a better understanding, the revenue contribution from alnico scraps is
⇑ Corresponding author. E-mail address: [email protected] (S.P. Barik). http://dx.doi.org/10.1016/j.seppur.2016.05.044 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.
compared with the primary source of cobalt and presented in Table 1. The revenue generation from the alnico scrap is almost close to the high grade ore of cobalt [8]. However, the availability of high grade ore is very less and the average value of cobalt from heterogenite is around 6–10% [9], which is almost 3 times less than alnico. Alnico scrap generated during the manufacturing of permanent magnets is quite rich both in nickel and cobalt along with a large amount of iron. Even though it is rich in both nickel and cobalt, a very few studies have been reported on the processing of such scraps [10], which might be due to the poor leaching kinetics of the metallic scraps. The poor leaching kinetics of magnetic alloy scraps is due to its low surface area and less activity. In order to improve the leaching kinetics, it is required either to increase the surface area by reducing the size or increasing the activity by introducing an oxidizing environment. But size reduction by pulverization is practically not feasible due to huge noise pollution, high energy consumption and damage of hammering materials. It is possible by atomization (water/air), but requires high energy operation. A number of processing approaches for recovering the valuable metals from varieties of scraps in hydrochloric acid leaching medium using different oxidants such as cupric chloride, ferric chloride and ozone have been proposed [10–12]. Alex et al. reported on the leaching of alnico scraps (fine powder) at 70 °C at a pulp density of 17% (w/v) for 5 h using 0.4 M cupric chloride
79
G. Prabaharan, S.P. Barik / Separation and Purification Technology 169 (2016) 78–82 Table 1 Comparison of alnico scrap vs cobalt ore. Alnico scrap
% Abs, g/kg Unit price, Rs Value/ kg of alnico scrap
High grade cobalt ore
Low grade cobalt ore
Co
Ni
Fe
Al
Co
Fe
Cu
Co
Fe
Cu
26.09 260.9 1500 391.4
16.4 164 593 97.0
47.0 470 NA NA
8.04 80.4 NA NA
34.2 342 1500 513
5.13 51.3
14.1 141 350 49.35
3.96 39.6 1500 59.4
10.6 106
6.45 64.5 350 22.6
Total value/kg
488.4
0
562.4
solution under continuously flowing oxygen. Though these processes performed a better leaching yield of metals, but due to the corrosive nature and difficulty in storage of chlorine gas, its use has been restricted so far [13]. Moreover, the process is applicable only for alnico scraps in the form of fine powders not for metallic alloys. Sulphuric acid is less hazardous in nature as compared to other inorganic acids. Due to its commercial viability and less hazardous in nature, sulphuric acid has been widely used in many specific areas of base metal extraction and processing from different sources. However, to date no work has been done on the processing of alnico scraps (metallic) using sulphuric acid. Electro dissolution is a simple technique and easy to handle and control [14]. The application of such technique has been employed in processing of metallic materials containing Fe, Cu, Ni, Co, W, Al, etc. [15–19]. The above reported studies were performed by taking either metal alone or its alloy using different acid/alkali solution. Moreover, no studies on the electro dissolution of alnico scrap (metallic) are reported yet. Thus, the current study focuses on the application of electro dissolution of alnico scraps with sulphuric acid for the techno-commercial feasibility. The effects of major parameters such as time, sulphuric acid concentration and current density on dissolution of various metals from alnico scraps were studied and discussed in detail. In addition, leaching of the alnico scrap was compared and discussed both by electroless and electrolytically.
0
82.0
Graphite (-)
Element
Power supply
Electrolyte
Lead (+)
Alnico scraps
Fig. 1. Schematic diagram of electro dissolution of alnico scraps.
Weighed amount of the alnico scarps were taken inside an anode pot and required amount of concentration sulphuric acid was added and the experiment was carried out for a predetermined time. In the case of electroless leaching experiments, same cylindrical cell was used under agitation with the help of an over head stirrer. Periodically 5 ml of sample was withdrawn, filtered, diluted and analyzed to determine the metal ion concentration in the solution by Atomic Absorption Spectrophotometer (AAnalyst-400, Perkin-Elmer).
2. Materials and methods 3. Results and discussion 2.1. Materials 3.1. Electroless leaching of alnico scraps The sample (alnico scrap) was procured from a Korean recycling company. Shape of the sample was cylindrical with variable particle diameter. Weighed amount of sample (as received) was digested using a mixed acid composed of HClO4, H2SO4 and HNO3 followed by dilution and analysis. Table 2 presents the chemical composition of the sample used in this study. Analytical grade of H2SO4 and distilled water were used in the preparation of the feed for the leaching study. 2.2. Leaching study Electro-chemical leaching experiments were carried out in a cylindrical cell (capacity 2 L) made of fiber reinforced plastic (FRP) containing lead anode inside at the bottom and cathode (graphite, 20 150 mm2) from the top at the center. Both electrodes are connected to a rectifier through a copper wire. The schematic diagram of electro dissolution of alnico scraps is shown in Fig. 1.
In order to investigate the dissolution kinetics of alnico scraps, experiments were performed without current by varying reaction time from 4 to 40 h at room temperature using 20% (v/v) sulphuric acid under agitation. The results are shown in Fig. 2. It can be seen that the leaching kinetics of the metals was very slow and at the end of 40 h of leaching time, a maximum leaching efficiency of 21.1% Co, 6.1% Ni, 3.5% Al and 34.6% Fe was observed. At about 2 103 kg/h of alnico scraps was dissolved out, which was not practically feasible for industrial practices. 20% H2SO4 is nearly 4 times of the stoichiometric requirement. Moreover, the leaching efficiency was low even after 40 h. Further increase of sulphuric acid concentration will lead to unreacted acids in the system, and will consume additional base to neutralize in the next step of the process during iron removal by jarosite precipitation, which will not be economical. So the acid concentration was limited to 20% as maximum. Since electroless leaching had no significant
Table 2 Chemical composition of alnico scrap. Element
Co
Ni
Fe
Al
Mn
Mg
Zn
Cr
Mo
Acid insoluble
%
26.09
16.36
47.02
8.04
0.09
0.01
0.11
0.01
0.04
0.96
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40
14
Leaching efficiency, %
Leaching efficiency, %
Ni
30
Al Fe
20
10
Ni 10
10
20
30
40
50
Al Fe
8 6 4 2 0
0 0
Co
12
Co
0
5
10
15
20
[H2 SO4 ], %(v/v)
Time, h
Fig. 4. Effect of sulphuric acid on leaching of metals.
Fig. 2. Electroless dissolution behavior of alnico scraps.
influence on the extraction of metals from alnico scraps and hence, studies were carried out with the aid of electric current to improve the leaching efficiency.
3.2.1. Effect of time The leaching kinetics of alnico scraps was performed out by varying the leaching time (2–12 h) using 10% (v/v) H2SO4 and current density 200 A/m2. As shown in Fig. 3, the leaching kinetics of all the metals except Al was very fast up to 8 h of leaching time and remains unchanged over the next 12 h. At the end of 8 h of leaching time, the leaching efficiency of the metals such as Co, Ni, Fe and Al were 9.2%, 12.85, 11.3% and 6.15, respectively. Hence 8 h of leaching time was chosen for the rest of the experiments.
25
Leaching efficiency, %
3.2. Electro-chemical leaching of alnico scraps
30
Co Ni
20
Al 15
Fe
10 5 0
0
100
200
300
400
500
Current density, A/m2 Fig. 5. Effect of current density on leaching of metals.
3.2.2. Effect of concentration of sulphuric acid In order to improve the leaching kinetics, concentration of sulphuric acid was varied from 5 to 20% (v/v) while keeping the rest of the conditions as: time 8 h and current density 200 A/m2. With the increase in acid concentration extractions of Co, Ni, Al and Fe increased as shown in Fig. 4. Above 10% (v/v) sulphuric acid, there was no significant effect on the leaching efficiency of metals. This might be due to the saturation of leach liquor with metal ions which is evident from the formation of crystals of ferric sulphate in the liquor. In the case of 10% (v/v) sulphuric acid, formation of ferrous sulphate was observed during dissolution of iron (Eq. (1)). However, as per the Eq. (2), formation of ferric sulphate was observed with increasing concentration of sulphuric acid [22]. Moreover, this ferric sulphate has poor solubility in water leading to the formation of crystals. 15
Leaching efficiency, %
Co 12
Ni Fe
9
Al 6 3 0
0
2
4
6
8
10
12
Time, h Fig. 3. Effect of time on leaching of metals.
14
! Fe þ H2 SO4 FeSO4 þ H2 ! 2Fe þ 3H2 SO4 þ 6Hþ 2 Fe3þ þ 3SO2 þ 6H2 O
ð1Þ ð2Þ
So, 10% (v/v) sulphuric acid was chosen for the rest of the experiments. The volume of leach liquor was replaced periodically (4 h) with fresh 10% (v/v) sulphuric acid when the leach liquor reached to pH 2. 3.2.3. Effect of current density The effect of current density on leaching of metals was studied in the range of 50–400 A/m2. The rest of the parameters were kept as: time 8 h and sulphuric acid concentration of 10% (v/v). The results are presented in Fig. 5. As can be seen, the leaching efficiency of all the metals followed an increasing trend in the varied range. At the end of 8 h of leaching time and current density 400 A/ m2, the extraction of the Co, Ni, Al and Fe were 18.4%, 27.2%, 12.1% and 21.6%, respectively. Periodically (2 h), the volume of the liquor was replaced with 10% (v/v) sulphuric acid as the liquor pH reached to 2. During optimization of the above three parameters (time, sulphuric acid concentration and current density), a common trend (Ni > Fe > Co > Al) was observed by comparing the leaching efficiency of individual metal ions. The lowest leaching efficiency of Al and Co might be due to the formation of acid resistant oxide layers at the anode which is absent in the case of Ni and Fe. 4. Comparison of electroless and electrochemical leaching of alnico scraps The leaching efficiency of metals obtained at the optimum leaching conditions (time 8 h and 10% (v/v) sulphuric acid) for both
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electroless and electrochemical methods have compared. The compared results are presented in Table 3. In the case of electrochemical leaching, the leaching efficiencies of Co, Ni, Al and Fe are more than 5.8, 20.9, 20.1, and 5.3 times of electroless leaching, respectively. In order to understand the leaching behavior of alnico scraps in the process of electro leaching, the leaching efficiency of metals in gram equivalent by both theoretical and actual was compared. From the results shown in Table 4, the power loss decreased with respect to time up to 9 h. Above 9 h, the leaching efficiency of metals is more than the theoretical value. This is quite abnormal. It is expected that the gram equivalent leached out in electrochemically should match with the combination of gram equivalent leached out by chemically and electrolytically together. On the contrary, the gram equivalent leached out by electro chemically is higher than that of the combination of chemically and electrolytically. This might be due to the combined effect of ferric sulphate and temperature (50 °C) generated during electro leaching [16,20,21].
Table 3 Comparison of electroless and electrochemical leaching of alnico scraps in sulphuric acid. Elements
Cobalt Nickel Aluminum Iron
Leaching efficiency, % Electroless
Electrochemical
3.2 1.3 0.6 4.1
18.4 27.2 12.1 21.6
Similar observation like the autocatalytic effect of Fe3+ ion was also reported during electro dissolution at high rate by Srivastava et al. [16]. This synergistic effect (ferric sulphate, temperature, current and sulphuric acid) was absent at the beginning of the reaction as the leaching medium consists of only sulphuric acid 10% (v/v) and current, which evident from the power loss. However, during leaching the increase of the concentration of ferric sulphate and temperature with time leads to the synergistic effect which is inferred from decrease of the power loss. Above 9 h, there was no power loss and the leaching efficiency was more than the theoretical value till 18 h. Above 18 h, the synergistic effect reached equilibrium. Thus the combined effect of ferric sulphate, temperature, current and sulphuric acid lead to the electro-chemical leaching process as a commercially feasible option for the alnico scraps. Based on the above studies, a flow sheet (Fig. 6) was also proposed for the commercial application of the processing of alnico scrap. In the above flow sheet, solvent extraction and electro winning generally operated as a continuous process. Since the proposed electro dissolution method can also be a continuous process which can comfortably fit in the flow sheet. To confirm the commercial feasibility of the continuous leaching process, 10% (v/v) sulphuric acid was fed continuously using a peristaltic pump to the cell at a flow rate of 16 mL/min to maintain the leach liquor generated at a pH of 2. The result of the continuous leaching operation was presented in Fig. 7. Concentration of the metal ions particularly Co and Fe, the major metal ions in the leach liquor was almost constant throughout the process, which indicates that the electro leaching can be a continuous process.
25
Table 4 Synergistic effect of electro leaching.
2 4 6 9 11 13 15 18 21 23
Total gram equivalent leached out (electrochemically)
Total gram equivalent leached out (chemically)
Expected gram equivalent (theoretically)
Difference, % (synergistic effect)
1.25 3.26 3.77 8.76 13.31 17.25 21.32 25.41 29.54 33.12
0.14 1.32 1.99 2.81 3.56 4.33 4.97 5.92 6.95 7.75
1.49 2.98 4.48 6.34 8.02 10.01 11.49 13.36 15.59 17.46
23 24 14 4 14 20 29 31 31 31
20
[Metal], g/L
Time, h
15 10
Co 5 0
Fe 0
2
4
Fig. 7. Continuous leaching of alnico scrap.
Alnico scraps
H2SO4 (10% v/v)
Spent electrolyte
6
Time, h
Electroleaching
Jarosite precipitation
Fe & Al Cake
Solvent Extraction of Co by Cyanex 272
Nickel Precipitation
Coblat electrowinning
Nickel carbonate
Cobalt metal Fig. 6. Proposed flow sheet for the processing of alnico scrap.
Soda ash
8
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5. Conclusion The leaching of metals by both chemical and electro-chemical method was studied. From the study, electro-chemical method showed a better leaching efficiency of metals (Fe, Co, Ni and Al) than electroless leaching method. The leaching conditions are optimized and under optimum leaching conditions (sulphuric acid 10% (v/v), current density 400 A/m2 and equilibrium time 4 h), more than 99% of the metal content was dissolved by continuous operation method. The effective leaching was due to the synergistic effect of ferric sulphate and temperature both generated during leaching with the current and sulphuric acid concentration in the system. This study creates the scope for recycling secondary metallic resources by electro-chemical method as process parameters such as grinding of the material, agitation are not required. In addition, there will be no generation of solid waste after leaching with complete recovery of metals and cobalt leach liquor can be produced on the continuous scale. Based on the above study, a complete flow sheet has been proposed for the commercial application of the process. Acknowledgements The authors are thankful to Prof. U.W. Lee, President, University of Science and Technology, Daejeon, Korea for his kind permission to publish this article. References [1] S.P. Barik, K.H. Park, P.K. Parhi, J.T. Park, Direct leaching of molybdenum and cobalt from spent hydrodesulphurization catalyst with sulphuric acid, Hydrometallurgy 111–112 (2012) 46–51. [2] J. Jandova, H. Vu, P. Dvorak, Treatment of sulphate leach liquors to recover cobalt from waste dusts generated by the glass industry, Hydrometallurgy 77 (1–2) (2005) 67–73. [3] A. Chagnes, B. Pospiech, A brief review on hydrometallurgical technologies for recycling spent lithium-ion batteries, J. Chem. Technol. Biotechnol. 88 (2013) 1191–1199.
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