Role of galvanic interaction in selective leaching of nickel from copper flotation concentrate.
Bacterial leaching of metal sulphides apparently requires the attachment of bacterial cells to metal sulfides. Different hypotheses were proposed by many authors regarding the mechanism of leaching by Tf (G. Urbano et al, 2007; Sand et al., 2001; Hansford and Vargas, 2001; Donati et al., 1988). Two modes of bacterial attack were primarily distinguished which could be termed as direct and indirect oxidation mechanisms. Fe acts 3+ 2 as an oxidant in the indirect mode whereas [O.sub.2] acts as the one in direct mode.
Other possible mechanism of leaching is metal dissolution by galvanic interaction between different mineral phases. Galvanic interaction leads to accelerated corrosion of one of the members of an electrochemically coupled mineral ores. Such type of mechanism exists in a heterogeneous system with different rest potentials and operates by transportation of electrons from anodic area of the mineral to some constituents of bacterial respiratory chain that serves as cathode(Guozhi et al, 2006; Cruz, 2005).
Chemical oxidation rates mainly depend on electrode potentials (EP) of different mineral phases and redox potential ([E.sub.h]) of the leaching medium. Bacteria would preferentially oxidize the mineral phase with lower EP i.e. sulfide anode, due to formation of galvanic couples. Lower values of EP and higher values of [E.sub.h] indicate faster oxidation rates of Tf. The electrochemical reactions involved are
MS [??] [M.sup.2+] + [M.sup.o] + [2.sup.-] (anode)
[O.sub.2] + 4 [H.sup.+] + 4 [e.sup.-] [??] 2[H.sub.2]O (cathode)
Presence of Tf in the leaching medium, will promote the galvanic dissolution process due to its ability to transform elemental sulfur to sulfate. Tf can modulate the galvanic effect by acting as a catalyst in oxidizing dissolved ferrous iron to ferric in acidic conditions or catalyze the oxidation of various metal sulfides by attaching to their metal lattice.
Several researchers studied the role of the galvanic interaction between chalcopyrite and pyrite during bacterial leaching of low-grade wastes, which led to corrosion of chalcopyrite and passivation of pyrite leaching. Pesic and Kim(1991); Tributsch et al., (1981); Berry et al., (1978). However, reported literature regarding the role of galvanic interaction in leaching of nickel is sparse (Natarajan, 1999).
Presently, the copper concentrate is dumped in vacant places causing hazardous problems to the environment. Hence, an attempt has been made to study the selective leaching of nickel by focusing the attention on galvanic interactions and its effect on leaching of nickel.
Materials and Methods
Copper flotation concentrate
The representative samples of copper flotation concentrate were obtained from UCIL(Uranium Corporation of India Ltd), Jaduguda, India. The concentrate sample after grinding was subjected to sieve analysis. The chemical composition of the concentrate sample and different mineral phases analyzed by X-ray diffractometry are given in Table.1 and Table.2. respectively.
Tf-44 and Tf-231 strains were obtained from Agharkar Research Institute, Pune, India. Tf strains were activated and regularly subcultured on modified 9K medium. The composition of the medium (g)- ammonium sulfate-2.0, magnesium sulfate-0.5, di potassium hydrogen orthophosphate-0.025, ferrous sulfate- 40 in 1liter of distilled water. pH of distilled water was first adjusted to 2.5 (Paknikar and Agate, 1995).
Bacterial leaching technique
Bacterial leaching was carried out in standard Erlenmeyer flasks (100mL, 250mL, 500 mL) filled to 40% of the capacity with contents of leaching medium (M9[K.sup.-] medium (without ferrous sulfate) + pulp density). The leaching medium was sterilized in an autoclave at 121[degree]C and 15psi. The temperature of the biological incubator shaker was o kept at 30-32[degree]C and the pH was set to 2.3. Homogeneity of the pulp was achieved by agitation. The pulp (leaching medium) was inoculated with Tf culture under aseptic conditions and kept for incubation in a shaker for a period of 30 days. The decrease in the medium volume due to evaporation was compensated by adding distilled water. Total contents of each flask were filtered through Whatman No.1 filter paper. The filtrate was used for the analysis of soluble Ni, Cu. Parallely, controls were maintained using 1% thymol/ [Hg.sub.2][Cl.sub.2] as bactericide.
Various metals present in the concentrate sample leach liquor and leach residue were analyzed by Atomic absorption spectroscopy (Perkin-Elmer). Different mineral phases of the concentrate were analyzed by X-Ray Diffraction technique. Bacterial growth was estimated indirectly interms of ferrous consumption. Ferrous recovery during leaching period was analyzed by calorimetric method. Change in pH and Eh were monitored using calomel and platinum electrodes respectively.
Results and Discussion
Leaching of metals by Tf, involves oxidation of elemental and reduced sulfur compounds that leads to acidification of the leaching medium. Under optimal conditions of leaching (Table.2), a maximum of 83.5% of nickel leachability was obtained (Figure. 1).
The copper flotation concentrate contains Cu, Ni, Co, Fe which were associated with chalcopyrite, pentlandite, pyrite and pyrrhotite phases respectively. The presence of different mineral phases led to galvanic interactions, which were responsible for the preferential leaching of nickel. Presence of Tf cells further accelerated the galvanic interaction by facilitating the continuous supply of Fe:Fe cycle, providing an environment with high redox potential (Table.5). Copper dissolution was started only from 15 day of incubation period while maximum nickel leachability was obtained within 15 days of incubation period of leaching.
The results of X-ray diffraction (XRD) analysis reveal the presence of chalcopyrite (CuFe[S.sub.2]), pentlandite [[(Fe Ni).sub.9] [S.sub.8]] as major phases and pyrite (Fe[S.sub.2]), pyrrhotite (FeS), violarite [[(Ni Fe).sub.3] [S.sub.4]] as minor phases in the copper flotation concentrate prior to leaching(Table.3). However, chalcopyrite, still remained as the major phase in the leach residue and nickel bearing pentlandite was visible in trace amounts along with pyrite (Table.4).
Chalcopyrite with Ep 250 mV acts as cathode in the presence of pentlandite (Ep-180mV) and pyrrhotite (Ep-120mV). Nickel present as pentlandite was associated with pyrite and pyrrhotite. In the presence of cathodic substances (chalcopyrite), pentlandite acts as anode. Primarily, the cathodic nature of chalcopyrite was the reason behind the slow process of dissolution of copper. The extraction of nickel from pentlandite was faster because of the association of iron with it (Mason and Rice, 2002; Ekmekci, 1997).
[FIGURE 1 OMITTED]
Other reason for biological acceleration of galvanic interaction was the chelation of cysteine secreted by Tf with pyrite, which altered the redox potential of Fe2+:Fe3 couple, thus accelerating the oxidation of pyrite sulfides as + indicated by Rojas and Tributsch, (2001). Increase in acid consumption coincided with an increase in redox potential and the concurrent leaching of nickel, suggesting partial oxidation of pyrrhotite. Similar results were obtained by Ahonen & Tuovinen, (1995).
The major reaction of pyrite in bacterial oxidation process is shown below.
4Fe[S.sub.2] + 15 [O.sub.2] + 2[H.sub.2]O [left and right arrow] 2[Fe.sub.2][(S[O.sub.4]).sub.3] + 2[H.sub.2]O
This oxidation reaction requires energy of 24.2 MJ/ kg S to release 1.88 kg [O.sub.2]/kg S and generate 0.76 kg [H.sub.2]S[O.sub.4]/kg S [Hayward et al., 1997].
Pentlandite [(Ni, Fe).sub.9] [S.sub.8] in which nickel is replaced by cobalt in proportions of Ni: Co=50:1 was leached by Tf according to the following equation [Rao, 1989; Torma, 1987].
[(Ni,Fe).sub.9][S.sub.8] + 17 5/8[O.sub.2] + 3 1/4 [H.sub.2]S[O.sub.4] [right arrow] + 4 1/2 NiS[O.sub.4] + 2 1/4 [Fe.sub.2](S[O.sub.4]) + 3 1/4 [H.sub.2]O
Different reactions involved during the galvanic dissolution process of copper and nickel bearing flotation concentrates are [Natarajan and Iwasaki, 1983] as follows.
Fe[S.sup.[arrow right]] [Fe.sup.2+] + [S.sup.o] + 2[e.sup.-]
[[[(Fe,[Ni.sub.1]).sub.9] [S.sub.8]].sup.[arrow right]] 9 [Fe.sup.2+] + 9[Ni.sup.2+] + 8[S.sup.o] + 18 [e.sup.-]
[O.sub.2] + 4[H.sup.+] + 4[e.sup.-[arrow right]]2[H.sub.2]O
Eq-1 and eq-2 are anodic and eq-3 is cathodic that takes place on the chalcopyrite or pyrite surface. In pyrite oxidation, acid forms as a result of the following two reactions.
Fe[S.sub.2] +7/2 [O.sub.2] + [H.sub.2]O [sup.[arrow right]][Fe.sup.2+] +2[H.sup.+] + 2S[O.sup.2-][O.sub.4]+2[e.sup.-]
Fe[S.sub.2] + 14 [Fe.sup.3+] + 8[H.sub.2][O.sup.[arrow right]]15[Fe.sup.2+] + 16 [H.sup.+] + 2S[O.sup.2-][O.sub.4]
Chalcopyrite was protected cathodically, facilitating selective leaching of nickel in the order followed by copper. In chalcopyrite, both the oxidizable metal moiety and sulfide moiety are simultaneously attacked (Liu et al. 2007).
4CuFe[S.sub.2] + 17[O.sub.2] + 10[H.sub.2][O.sup.[arrow right]] 4[Cu.sup.2+] +4Fe[(OH).sub.3] +8[H.sup.+] + 8[O.sup.2- ][O.sub.4]
The slow rate of Cu leaching could also be attributed to the fact that primarily, adsorbed Tf cells were involved in attacking sulfide surface by direct mechanism and the chemical leaching is insignificant. However, for all sulfide minerals present in this ore, the data emphasizes that the fastest leaching rates are achieved under oxidized conditions and low pH values, indicating bacterial activity is responsible principally for oxidative leaching of sulfide minerals.
Galvanic interaction plays a major role in preferential leaching of metal from ores and flotation concentrates. Selective leaching of nickel from the copper flotation concentrate was possible due to the development of galvanic couple between copper containing chalcopyrititic phase and nickel containing pentlandite. Copper dissolution started after the maximum leachability of nickel was obtained under optimal conditions of leaching by Tf. This clearly indicates the role of galvanic interaction in selective leaching of nickel from copper flotation concentrate.
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Corresponding Author: P. Ravindra, University College of Technology, Osmania University, Hyderabad, India--500 007 E-mail: firstname.lastname@example.org Mobile: 9866240917 Tele phone: +91-40-27098901 Tele Fax: +91-40-27098472
(1) K. Bharathi, (1) M. Lakshmi Narasu and (2) P.Ravindra
(1) Centre for Biotechnology, Institute of Science and Technology, Jawaharlal Nehru Technological University, Kukatpally, Hyderabad, India--500 072
(2) University College of Technology, Osmania University, Hyderabad, India--500 007
(1) K. Bharathi, (1) M. Lakshmi Narasu and (2) P. Ravindra: Role of Galvanic Interaction in Selective Leaching of Nickel from Copper Flotation Concentrate: Adv. in Nat. Appl. Sci., 2(2): 68-72, 2008
Table 1: Elemental composition of copper floatation concentrate. Elements S Fe Cu Ni Mo Composition(%) * 31.5 29.5 21.4 2.73 1.38 The remaining percentage is contributed by traces of other elements such as M g, Ca, K , Ti, Cr, P, Co, Cs, Ba, H f, Ir, Pt, Au, Pb, Zn, M o, Zr, N b, Ru, Sn, Te, U and siliceous and carbonaceous gangue minerals. Table 2: Optimal parameters of nickel leaching Leaching parameter Optimal value Temperature 30[degrees]C pH 2.3 Pulp density 10% Agitation 140 rpm Residence time 20 days Inoculum size 10% Table 3: Mineral phases present in the concentrate (prior to leaching). Sample Major phases Minor phases Copper concentrate Chalcopyrite Pyrite (FeS), Pentlandite Pyrrhotite [[(Fe,Ni).sub.9] ([Fe.sub.1-x]S), (CuFe[S.sub.2]) Violarite [S.sub.8] [[(Ni Fe).sub.3] [S.sub.4]] Table 4: Mineral phases present in the leach residue of the concentrate. Sample Major phases Minor phases Copper concentrate Chalcopyrite Pentlandite (leach residue) ([CuFeS.sub.2]) [[(Fe Ni).sub.9] [S.sub.4]] Hydronium jarosite [[H.sub.3]o [Fe.sub.2] [([SO.sub.4)].sub.3] O[H.sub.6])] Table 5: Redox potentials of the pulp during bacterial leaching (Tf- 44). Residence Sterile Tf-231 Tf-44 time (days) control 2 345 350 350 4 320 320 320 6 340 380 380 8 348 420 420 10 342 480 480 12 340 492 492 14 339 500 500 16 335 510 510 18 340 538 538 20 342 550 550 22 340 525 525 24 339 515 515
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|Author:||Bharathi, K.; Narasu, M. Lakshmi; Ravindra, P.|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||May 1, 2008|
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