15th International Mineral Processing Symposium

15th International Mineral Processing Symposium, Istanbul-Turkey, October 19-21, 2016

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RECOVERY OF COBALT FROM SULPHATE SOLUTIONS BY PRECIPITATION VIA PERSULPHATE OXIDATION E. Y. Yazici, P. Altınkaya, O. Celep, H. Deveci Hydromet B&PM Group, Div. of Mineral&Coal Processing, Dept. of Mining Eng., Karadeniz Technical University, 61080, Trabzon, Türkiye

Abstract: Cobalt is produced mainly as a by-product from extraction of nickel laterites and copper-cobalt which often treated with sulphuric acid leaching. Selective recovery of cobalt from nickel and/or copper-bearing pregnant leach solutions can be achieved by oxidation of Co2+ to Co3+ since Co3+ precipitates as cobalt hydroxide/oxyhydroxide at pH 3.0. However, it is difficult to oxidise Co2+ to Co3+ due to high oxidation potential of Co3+/Co2+ couple (1.84-1.92 V). In this study, capability of persulphate (S2O82-) with a standard potential of +2.12 V as an oxidant for Co 2+ oxidation in sulphate solutions (0.5 M H2SO4, 1 g/L Co2+) was investigated. Precipitation tests were performed in the presence of potassium persulphate (0.03-0.05 M as K2S2O8) at an initial pH range of 2-5. It was found that increasing the pH allowed higher cobalt recoveries. The increase in concentration of persulphate from 0.03 to 0.05 M led to a significant improvement in recovery of cobalt from 4.3% to 78% even at pH 2. Increase in pH appeared to improve the precipitation of cobalt with its almost complete precipitation (97-99.2%) at pH 5. The results demonstrated that persulphate can be suitably used to oxidise Co2+ to Co3+ for recovery/separation of cobalt from sulphate leach solutions at pH 2-5 over a short period of 15 min. Keywords: Cobalt, Oxidation, Persulphate, Precipitation, Recovery. INTRODUCTION Cobalt is mainly produced as a by-product from nickel and/or copper oxide/sulphide ores (Crundwell et al., 2011; Roberts and Gunn, 2014). Secondary resources (e.g. ewaste, Ni-Co batteries) may also contain significant amount of cobalt and nickel. Hydrometallurgical extraction of Ni/Co from nickel matte/concentrates/ores have, in recent years, gained importance over pyrometallurgical extraction due to high capital/operating costs, environmental limitations (SO2 and dust) as well as lower cobalt recoveries (Fisher, 2011). Sherritt-Gordon process (high pressure ammonia 774


leaching) was developed for treatment of nickel matte and/or nickel sulphide concentrates. Caron process (atmospheric ammonia leaching after reductive roasting) and high pressure acid leaching (HPAL; H2SO4) have been employed for nickel oxides (laterites) (Crundwell et al., 2011; Habashi, 1999; Roberts and Gunn, 2014). In many plants, cobalt and nickel (as well as copper if present) are precipitated as sulphides (using NaHS, H2S) (Eq. 1) or hydroxides (using NaOH, Ca(OH)2, Na2CO3 or preferably MgO) (Eq. 2) to produce an intermediate Co/Ni product i.e. mixed sulphide product (MSP) or mixed hydroxide product (MHP), respectively for further treatment (Crundwell et al., 2011; Gupta and Mukherjee, 1990). M2+ + H2S  MS

(M: Ni, Co, Cu)

(1)

M2+ + OH-  M(OH)2

(M: Ni, Co, Cu)

(2)

Cobalt-nickel separation from leach solutions (or leach liquors of Co/Ni containing MSP or MHP) has been a challenge in hydrometallurgy due to the similar behaviour of cobalt and nickel. Several separation methods including solvent extraction (SX) and ion exchange (IX) have been tested for Co-Ni separation ahead of cobalt electrowinning (EW) (Binnemans et al., 2010; Crundwell et al., 2011; Donaldson and Gaedcke, 1998; Fisher, 2011). Phosphinic acid extractants (commercially available as Cyanex 272, LIX 272 and Ionquest 290) which have high selectivity for cobalt over nickel in sulphate media have been successfully utilised at commercial scale prior to cobalt EW at the expense of low selectivity for copper, zinc, iron(II), cadmium, calcium, magnesium and manganese (Jones et al., 2010). Cobalt extraction from chloride and ammoniacal sulphate solutions has been also widely investigated (Donaldson and Gaedcke, 1998). In addition to its high cost, SX has another shortcoming for sulphate solutions i.e. accumulation of sodium sulphate in tailings dam which results from addition of sodium(e.g. NaOH, Na2CO3) or ammonium-based reagents (e.g. NH4OH) for pH adjustment in extraction stage. This generates an environmental risk that could be prevented by special lining of the dam or recovery of sodium sulphate which add extra important capital/operational cost (Fisher, 2011). New SX reagent Cyanex©301 is capable of synergistic extraction of cobalt and nickel as well as zinc from other impurities (Mn, Mg and Ca) but the presence of copper and ferric should be avoided since these ions are irreversibly extracted and poison the SX reagent. Certain ion exchange resins have been commercially used for selective adsorption of nickel (with Cu and Zn) from cobalt 775


in sulphate solutions (Fisher, 2011). However, due to some technical and economic constraints, ion exchange is more likely suitable for polishing stage to treat low-volume and high-concentration electrolytes to remove Ni, Zn and Cu prior to cobalt EW (Fisher, 2011; Flett, 2004; Gupta and Mukherjee, 1990). Molecular Recognition Technology (MRT), essentially IX with special resins, could remove Fe, Ni, Cu, Cd, Pb and Zn from cobalt-bearing solutions although it is yet to be exploited at industrial scale due to economic reasons (Fisher, 2011). Another method for Co-Ni separation in sulphate solutions is selective oxidation and precipitation of cobalt (Fisher, 2011; Flett, 2004; Jones, 1999). Precipitation pH for cobalt and nickel (Fig. 1) shows that Co-Ni separation is not feasible due to similar precipitation characteristics (i.e. pH 6.7-6.8) of cobaltous (Co2+) and nickel (Ni2+). On the other hand, cobalt can be selectively recovered from nickel at pH 2-5 through oxidation of cobalt from cobaltous (Co2+) to cobaltic (Co3+) state to produce a cobalt oxide/hydroxide compound.

Fig. 1. Precipitation pH of some metals as hydroxide (25ºC) (adopted from Habashi (1999))

Strong oxidants such as chlorine, ammonium persulphate ((NH4)2S2O8), Caro’s acid (H2SO5), bleaching powder (Ca(OCl)Cl), sodium hypochlorite (NaOCl) and ozone are required to oxidise Co2+ to Co3+ (Åkre, 2008; Bhattacharjee et al., 2005; Fisher, 2011; Flett, 2004; Jones, 1999) due to high oxidation potential of Co3+/Co2+ couple (i.e. 1.84776


1.92 V (Lide, 2005; Nicholls, 1973). It is also relevant to note that oxidative precipitation of cobalt can be applied for removal of cobalt from zinc electrolytes prior to zinc EW due to its adverse effect on current efficiency (Lu, 1995). In this study, the utilisation of persulphate (S2O82-) as an oxidant for Co2+ in sulphate solutions (0.5 M H2SO4, 1 g/L Co2+) was investigated. Precipitation tests were performed in the presence of persulphate (0.03-0.05 M S2O82-) at an initial pH range of 2-5. MATERIALS AND METHODS A synthetic acidic sulphate solution (0.5 M H2SO4 and 1 g/L Co2+) was prepared for precipitation tests in deionised-distilled water using sulphuric acid (96%) and cobalt sulphate heptahydrate (CoSO4.7H2O). Solutions (natural pH is 0.88) of 10 mL volume were then added into 15-mL tubes followed by addition of potassium persulphate (K2S2O8) as the oxidant (+2.12 V) in solid form at certain amounts to produce persulphate concentrations of 0.03-0.05 M. pH of the solution was then adjusted to the required level i.e. pH 2-5 using 4 M NaOH. After 15 min. of agitation on an orbital shaker (25ºC), the solution was filtered using cellulose nitrate membrane filters (0.45 µm, Merck) to separate supernatant from precipitate. Initial and residual concentration of cobalt from the solution was analysed using an atomic absorption spectrometer (AAS, Perkin Elmer AAnalyst 400) to calculate cobalt recoveries. RESULTS AND DISCUSSION Fig. 2 illustrates precipitation of cobalt with addition of persulphate (0.03-0.05 M) at different initial pHs of 2-5. Under non-oxidative conditions (i.e. in the absence of an oxidant) Co2+ precipitates as cobalt(II) hydroxide (Co(OH)2, logK=‒14.4 (Jackson, 1986)) (Eq. 3) at pH above ≈7 as shown by Eh-pH diagram of Co-H2O system (Fig. 3) (HSC-Chemistry, 2011). CoSO4 + 2OH-  Co(OH)2 + SO4-2

G0(25°C)= ‒105 kJ

777

(3)


Fig. 2. Precipitation of cobalt in the presence of persulphate at pH 2-5 over 15 min. (0.5 M H2SO4 and 1 g/L Co2+).

Increasing the concentration of persulphate from 0.03 to 0.05 M led to a significant improvement from 4.3% to 78.1% in cobalt recovery at pH 2. While at higher pHs of 35, increasing the persulphate dosage no further contributed to the recovery of cobalt. Increasing pH from 2 to 5 at low concentration of persulphate (0.03 M) induced a substantial contribution to precipitation of cobalt i.e. from 4.3% (pH 2) to 99% (pH 5). The results showed that high precipitation of cobalt can be achieved even at low pHs provided that sufficient oxidant was supplied e.g. in the presence of 0.05 M persulphate 78.1% of Co already precipitated at pH 2. Pourbaix diagram of cobalt presented that under oxidising conditions cobalt starts to precipitate as cobaltic hydroxide (Co(OH) 3, logK=-44.5 (Jackson, 1986)) (Eq. 4) at ≥pH 0 (Fig. 3) (HSC-Chemistry, 2011). 2CoSO4 + S2O8-2 + 6OH-  2Co(OH)3 + 4SO4-2

G0(25°C)= ‒547 kJ

(4)

It is pertinent to note that, in the literature, cobalt(III) precipitate is formulated in different forms including cobalt(III) hydroxide (Co(OH)3), cobalt(III) oxide monohydrate (Co2O3.H2O) or cobalt(III) hydroxide oxide (cobalt oxyhydroxide; CoO(OH)) with the latter claimed to be the most suitable formula (Lieth, 1977; Nicholls, 1973). Cobaltocobaltic oxide (Co3O4; Co2+O.Co3+2O3), which contain both Co2+ and Co3+,can be also formed under slightly acidic solutions at 25°C (Fig. 3). 778


Eh (Volts) 2.5

Co - H2O - System Co(+3a)

Co(OH)3

2.0 1.5

Co3O4

1.0 0.5

Co(+2a)

0.0

Co(OH)2 -0.5

Co H2O Lim its

-1.0

-2

0

2

4

6

8

10

12

14

pH

C:\HSC7\EpH\Co25.iep

Molality ELEMENTS Pressure Fig. 3. Eh-pH diagram Co of Co-H2O system (Co: 0.017 1.000E+00 mol/kg, 1 bar, 25°C) (Dashed 1.700E-02

lines show the stability limits of H2O) (HSC-Chemistry, 2011)

Some researchers (Kim et al., 2013) also investigated oxidation of Co2+ (3 g/L) with sodium persulphate (Na2S2O8) over a period of 120 min. under different conditions of temperature (40-80°C), pH (3-5.5), oxidant addition (stoichiometric ratio:1.1-1.5) (Eq. 5). These researchers found that increasing the oxidation period, pH or temperature positively affected the precipitation of cobalt. Negligible effect of Na2S2O8 addition (at pH 5) was noted presumably due to the high pH tested. This was in consistent with the current findings in that the effect of the oxidant became more apparent at low pHs (Fig. 2). They characterised the precipitate by XRD which indicated that the product was cobalt oxyhydroxide (i.e. CoO(OH)): 2CoSO4 + Na2S2O8 + 4H2O  2CoO.OH + Na2SO4 + 3H2SO4 G0=-93.4 kJ/mol (5) In a recent study (Güler and Seyrankaya, 2016) ammonium persulphate (0.5 M (NH4)2S2O8) as an oxidant was tested on precipitation of cobalt from synthetic CoSO 4 solutions (pH 4) with initial concentrations of 1-125 mg/L Co over 1 h. They found that increasing the concentration of cobalt led to an improvement on the rate and extent of cobalt precipitation with high recoveries (≥99%) at 125 mg/L Co over an initial period of 10 min. They attributed this to the insufficient nucleus formation in diluted solutions of cobalt. Characterisation (XRD) of the precipitates confirmed the formation of cobalt oxyhydroxide (CoO(OH)). The authors also demonstrated that utilisation of ammonium 779


persulphate in leach solutions of a zinc plant residue for precipitation of Fe, Mn and As was efficiently (≥99.9%) removed these impurities under suitable conditions as well as cobalt (70-90%) even from a solution containing an initial Co 2+ concentration of 4.42 mg/L. CONCLUSION Separation of cobalt from nickel has been a challenge in hydrometallurgy due to similar chemical behaviour of these metals in their divalent state i.e. they precipitate at a close pH range of 6.7-6.8. The current study investigated the capability of potassium persulphate (0.03-0.05 M K2S2O8) to oxidise cobaltous (Co2+) to cobaltic (Co3+) state which has a tendency to precipitate as cobalt(III) oxide/hydroxide at a low pH of 3. The tests were carried out using sulphate solutions (0.5 M H2SO4, 1 g/L Co2+) at pH 2-5. High pHs led to an improvement in the precipitation of cobalt by 19-95%, particularly at low concentration of persulphate (i.e. 0.03 M). Increasing the concentration of persulphate from 0.03 to 0.05 M resulted in a substantial improvement in cobalt recovery from 4.3% to 78.1% at pH 2 while at pH 3-5 contribution of persulphate dosage was not apparent. These findings demonstrated that high level of precipitation of cobalt (by up to 99%) from sulphate solutions (pH 2-5) can be readily achieved using persulphate as the oxidant in a short period of 15 min. The results also implied that persulphate could be suitably used in Co-Ni separation from sulphate leach solutions.

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