The most commonly used oxidizing agents are oxygen, nitric acid, ferric ion, concentrated H2SO4, chlorine water and sodium hypochlorite (Habashi, 1999a).
Also cupric ions (Cu2+) are used as oxidizing agents (Lundström, 2009). Copper, nickel and cobalt can form ammine complexes when oxygen is used as an oxidant in a solution of water, ammonium hydroxide and a dilute acid. This aqueous oxidation is a very slow process under ambient conditions and hence increased pressure is needed to accelerate the process. Pressure oxidation leaching processes are however beyond the scope of this study and focus is laid on leaching at ambient pressure. The leaching mechanisms in different leaching conditions for different chalcopyrite samples are collected in Table III.
TABLE III
Sulfide*Dissolving mediumT/°CpHMechanismExtra notesSource CpH2SO4, Cr2O72- 50-97-Oxidative dissolutionatmospheric pressureAydogan et al. (2006) CpH2SO4, NaNO380-Oxidative dissolutionatmospheric pressureSokić et al. (2009) CpH2SO4, Fe2(SO4)3850.8-1.9Oxidationatmosphericpressure,redoxpotential controlled by oxygen or nitrogen additionKoleini et al. (2011) CpH2SO4, Fe2(SO4)3751-2OxidationLi et al. (2010) CpHClO4851Oxidationatmospheric pressureHarmer et al. (2006) Cp, Ar, Pe, Pr, Py
Ammonium thiosulfate, ammonium sulfate, ammonia water, cupric sulfate, hydrogen peroxide, HCl
25-Complexformation and oxidationatmospheric pressureFeng & Van Deventer (2002) CpH2SO4,FeSO4,CuSO4 Fe2(SO4)340-Reductive/oxidativeatmospheric pressureHiroyoshi et al. (2000)
TABLE III Leaching mechanisms for different sulfides in different leaching solutions. * Sulfides: Ar=arsenopyrite (FeAsS), Cp=chalcopyrite, Pe=pentlandite, Pr=pyrrhotite, Py=pyrite
Based on this table and a research made on chalcopyrite leaching (Lundström, 2009) focus is laid on the leaching of sulfides in the presence of an oxidizing agent. Many of the studies shown in Table III have used ferric ions as oxidizing agents. In the presence of an oxidizing agent dissolution takes place through transferring of electrons. The dissolution of chalcopyrite takes place through protonation, oxidation, a combination of reduction and oxidation or through complex formation.
Nicol et al. (2010) have done an extensive review on the dissolution of chalcopyrite in chloride solutions. The dissolution mechanism of chalcopyrite depends on the leaching media used. The mechanism can be a redox, a non-oxidative or a mechanism in which first reduction occurs and secondly oxidation (from hereon called reductive/oxidative). The non-oxidative dissolution model can be extended to cover the whole reaction and in this case the model is called a non-oxidative/oxidative model. Most of these mechanisms mentioned by Nicol et al. (2010) are also represented in Table III and hence also apply for non chloride leaching agents.
3.2.1 Oxidative dissolution
Oxidative dissolution by ferric or cupric ions can be represented by an electrochemical model composing of an anodic reaction and a cathodic reaction (Gómez et al., 1996; Nicol et al., 2010). Oxidative dissolution can also be called a redox reaction since both an anodic and a cathodic reaction take place simultaneously. These both reactions are represented by equations 7 and 8, respectively. The ions of chalcopyrite dissolve into the solvent in which ferric ions or cupric ions are used as reducing agents.
CuFeS2 → Cu2+ + Fe3+ + 2 So + 5e- (7) 4Fe3+ + 4e = 4Fe2+ or 4Cu2+ + 4e = 4Cu+ (8)
Aydogan et al. (2006) presented that the oxidative dissolution of chalcopyrite in sulfuric acid with dichromate ion addition produces, presumably due to different
stoichiometry of the equations, either elemental sulfur as shown in equation 9 or sulfate according to equation 10. In the oxidative dissolution of chalcopyrite either elemental sulfur or sulfate is formed. The chromate ion acts as a reducing agent.
6CuFeS2 + 5Cr2O72- + 70H+↔ 6Cu2+ + 6Fe3+ + 12S + 10Cr3+ + 35 H2O (9) 6CuFeS2+ 17Cr2O7
+ 142H+↔6Cu2+ + 6Fe3+ + 12SO4
+ 34Cr3+ + 71 H2O (10)
When sulfides are leached using nitrate as leaching agent elemental sulfur is formed. The sulfides undergo oxidative dissolution according to equations 11 or 12 (Sokić et al., 2009). The study by Vračar et al. (2003) proposes that equation 12 would be the more probable reaction to occur. This has been explained by a more negative value for the change in Gibbs energy.
3MeS + 2NO3-
+8H+ = 3Me2+ + 3S + 2NO + 4H2O (11) MeS + 2NO3-
+4H+ = Me2+ + S + 2NO2 + 2H2O (12)
3.2.2 Protonation
The opposite of oxidative dissolution namely non-oxidative dissolution can also be called protonation. In protonation copper is detached from the chalcopyrite through proton attacks as described in equation 13 (Nicol et al., 2010; Kimball et al., 2010).
CuFeS2 + 4H+ = Cu2+ + Fe2+ + 2H2S (13)
However, as described in Chapter 3.1 the formation of elemental sulfur is preferred and hence reaction 13 can be extended in order to obtain the overall oxidative dissolution process. The overall reaction can be obtained by combining equation 13 with equation 14 (Nicol et al., 2010).
H2S + 2Fe3+ = S + 2Fe2+ + 2H+ (14)
3.2.3 Reductive/oxidative dissolution
The reductive/oxidative dissolution model mentioned by Nicol et al. (2010) is a model proposed by Hiroyoshi et al. (2000). This model is for ferrous-promoted chalcopyrite leaching. The dissolution of copper occurs in sulfuric acid in the presence of cupric and ferrous ions through two steps shown in equation 15. The first step is the reduction of chalcopyrite by ferrous ions to form Cu2S as shown in equation 16. Cu2S can then be oxidized by dissolved oxygen or ferric ions to cupric ions as shown in equation 17.
CuFeS2 Cu2S Cu2+ (15)
CuFeS2 + 3Cu2+ + 3Fe2+ = 2Cu2S + 4Fe3+ (16) 2Cu2S + 8Fe3+ = 4Cu2+ + 8Fe2+ + 2S (17)
With reference to the chosen case study, pentlandite and chalcopyrite are studied in more detail. The main sulfide minerals found at the case study are pentlandite, pyrite and chalcopyrite. Much research has been made on the recovery of chalcopyrite, while pentlandite has not been subjected to as many detailed studies.
Since copper and nickel are the valuable metals to be recovered from the tailings samples, following chapters deal with leaching of these sulfidic minerals in more detail. A summary of the leaching studies made for pentlandite and chalcopyrite are shown in Table IV. These studies will be used as references when choosing leaching reagents for the experimental part of this work.
TABLE IV Leaching studies conducted for pentlandite and chalcopyrite.
Metal sulfide Leaching reagent Reference pentlandite 1 mol/dm3 FeCl3 +
citric acid Hansen et al. (2005)