Biooxidation and precipitation for iron and sulfate removal from heap bioleaching effluent streams

Abstract

Effluents from bioleaching processes cause severe problems if dispersed in the environment since they typically have very low pH values and high sulfate and ferric iron concentrations. Dissolved iron may also interfere with the metal recovery. In the bioleaching circuit, partial removal of dissolved iron and sulfate is needed to alleviate process disturbances. In this study, an integrated, bench-scale process comprising a fluidized-bed reactor (FBR) and a gravity settler was developed for controlled biological oxidation of ferrous iron and precipitative removal of ferric iron and sulfate for use in waste management of heap bioleaching processes. The FBR for iron oxidation by an enrichment culture dominated by Leptospirillum ferriphilum was operated at 37 ± 2 °C. The FBR recycle liquor was partially neutralized with 10 M KOH or 50 g/L CaCO₃ slurry to promote ferric iron and sulfate precipitation. With 6 ± 1.5 g Fe²⁺/L in the feed and KOH-adjusted pH 3.5, the oxidation rate of Fe²⁺ was 3.7 g/L h and 99% precipitation of ferric iron was achieved in the process. Adjustment with CaCO₃ to pH 3.2 slightly decreased the oxidation rate to 3.3 g/L h and 98% of ferric iron precipitated. With 15 g Fe²⁺/L in the feed, the oxidation rate was 7.0 g Fe²⁺/L h coupled with 96% precipitation of ferric iron. A solid solution of jarosite was the main product of ferric iron precipitation with KOH adjustment and with minor amounts of goethite at the higher pH range. Adjustment of the pH with CaCO₃ precipitated ferric iron also as a solid solution of jarosite, and sulfate precipitated also in the form of gypsum (CaSO₄·2H₂O) especially at the higher pH values.

Introduction

In bioleaching processes biological oxidation of iron and sulfur is exploited to solubilize and recover metals from low-grade sulfide ores. Solutions from these processes typically have very low pH values and high sulfate and ferric iron concentrations. Other metals may also be present but the chemical composition of these effluents varies with the source. Dissolved iron and sulfate accumulate in heap bioleaching circuits because of solution recirculation. The accumulation is problematic and excess iron and sulfate need to be removed because they may otherwise interfere with process kinetics due to precipitate formation (Nemati et al., 1998, Watling, 2006) and with the subsequent metal recovery (Dutrizac and Riveros, 2006, Cunha et al., 2008). The final effluents from these processes have to be treated effectively to neutralize the streams and to remove iron and sulfate as stable end products as the effluents cause severe problems if dispersed in the environment (for reviews, see Johnson, 2003, Johnson, 2006). Iron is commonly removed through hydroxide precipitation by adding lime or limestone to increase the pH approximately to 3. This conventional treatment process creates major sludge handling and disposal problems due to the generation of voluminous sludge (Cunha et al., 2008, Dutrizac and Riveros, 2006, White et al., 2006). Jarosite precipitation is also a common chemical iron removal method especially in zinc industry. Jarosite precipitation produces iron precipitates with relatively good settling, filtering and washing characteristics, but various other metal ions may co-precipitate, and elevated temperatures are preferred for the process (Tamargo et al., 1996, Ismael and Carvalho, 2003). Biological and combined biological and chemical iron and sulfate removal systems have been examined, especially for the treatment of acid mine drainage (AMD) as reviewed, for example, by Johnson and Hallberg, 2005, Gaikwas and Gupta, 2008, Kaksonen et al., 2008. These AMD treatment systems often help to decrease the chemical costs and improve the handling characteristics of the sludge produced. Examples of active treatment processes employing iron-oxidizing microorganisms include aerated lagoons and stirred tanks (Umita, 1996), packed-bed bioreactors (Diz and Novak, 1999) and rotating biological contactors (Olem and Unz, 1980, Nakamura et al., 1986). High-rate iron oxidation has been achieved with bioreactors using immobilized iron-oxidizing microorganisms, e.g. packed-bed bioreactors (Diz and Novak, 1999) and rotating biological contactors (Olem and Unz, 1980, Nakamura et al., 1986). In the laboratory scale, a treatment process has been developed consisting of a packed-bed bioreactor for iron oxidation and a fluidized-bed reactor for chemical precipitation of iron onto the surface of seed particles (Diz, 1998, Diz and Novak, 1998). These applications have been developed for treating AMD and, in contrast, a very limited amount of information has been published on the treatment of bioleaching solutions. In our previous study (Nurmi et al., 2009) a bioprocess was developed based on a fluidized-bed reactor (FBR) for Fe²⁺ oxidation by a Leptospirillum ferriphilum dominated biofilm coupled with a gravity settler for precipitative removal of ferric iron. Only few patents have addressed biological iron oxidation for iron removal; e.g., Das and Das (2004) described a process that included simultaneous oxidation and precipitation of iron in hydrometallurgical leach liquors in the presence of Acidithiobacillus ferrooxidans, and Maree and Johnson (1999) designed a bioprocess scheme for the treatment of acidic “raw” (undefined) water containing high concentrations of dissolved ferrous ions.

The solubility of Fe³⁺ is a function of the pH in leach solutions. Generally, Fe³⁺ has an extremely low solubility at pH above 2.5. In sulfate-rich environments Fe³⁺ precipitates mainly as jarosites (Fe(III)-hydroxysulfates, MFe₃(OH)₆(SO₄)₂ where M can be Na⁺, K⁺, NH₄⁺ or H₃O⁺ or a divalent metal ion at ambient temperatures and pressures) at pH values < 4 and as oxyhydroxides and oxides at higher pH values. Goethite, an Fe(III)-oxyhydroxide (α-FeOOH), or schwertmannite, an Fe(III)-hydroxysulfate (ideally Fe₁₆O₁₆(OH)₁₂(SO₄)₂·nH₂O) can also form at similar pH ranges. Jarosite formation is enhanced at high concentrations of monovalent cations, Fe³⁺ and SO₄²⁻ (Nemati et al., 1998, Gramp et al., 2008). The lower threshold pH for jarosite formation is close to pH 1.5, but this depends on the temperature and ionic composition of the solution.

The purpose of this study was to develop a high-rate, high-efficiency process for iron oxidation and precipitation together with sulfate. A combined iron and sulfate removal bioprocess was conceived by employing ferrous sulfate oxidation by iron-oxidizing bacteria followed by precipitation through partial neutralization. The process was developed for the treatment of heap bioleaching effluents. Solid phase samples were characterized in an effort to relate precipitate properties to pH adjustments with two different neutralizing chemicals.