Ferric precipitation and metal sorption in biohydrometallurgical processes

Abstract

Bioleaching is a well-known process for the efficient extraction of base metals from their mineral sulfides, and its long-term viability is reliant on the continual supply of ferric ions (Fe3+). Due to the low solubility of ferric ions at higher pH levels, it tends to precipitate, resulting in the formation of ferric precipitates. The purpose of this work was to assess the impact of initial influent pH values on the surface properties of iron precipitate generated in a bio-oxidation operation and unravel the sorption mechanism by which these precipitates retain metal ions as it relates to a typical bioleach operation. Ferrous ion bio-oxidation experiments mediated by mixed mesophilic culture were carried out in a continuous stirred tank reactor (CSTR) at pH levels of 1.7, 1.9, and 2.2 for 2 weeks at a fixed temperature of 35 °C and 550 rpm stirring speed. The results showed that the mass of iron precipitate generated increased as the initial solution pH increased. The X-ray powder diffraction (XRD) patterns of the recovered precipitates were identified as potassium jarosite (K-jarosite), KFe3(SO4)2(OH)6. The high-resolution X-ray photoelectron spectroscopy (XPS) analysis revealed the binding energies of elemental spectra assigned to Fe(III)–O, OH–, SO42−, and S2− chemical states. The scanning electron microscopy (SEM) morphologies results are composed of aggregates of spherical/round and tabular particles. According to the energy-dispersive X-ray spectroscopy (EDS) results, a trend with a decrease in the total content of Fe, S, and K as the synthesis influent pH increased was observed. The Fourier transform infrared (FTIR) spectra of the precipitates showed the vibrational modes of SO42−; H2O and OH groups; and Fe–O vibrational mode in the jarosite. The thermogravimetric analysis (TGA) studies showed the dehydroxylation of K-jarosite and complete thermal decomposition of yavapaiite. Most importantly, the study revealed an increase in the surface area of the jarosites (from 8.32 – 11.00 m2/g) and decrease in the pore size distribution (from 16.15 – 10.45 nm) as the influent solution pH increases from 1.7 – 2.2, confirming the mesoporous nature of the jarosite particles The feasibility of improving typical biohydrometallurgical operations to minimize copper losses was investigated through the use of biogenic industrial iron precipitate for the uptake of Cu(II) ions from aqueous solutions. The results show that the precipitate is highly heterogeneous. The surface area and average pore diameter were 4.74 m2/g and 11.61 nm respectively. The Cu(II) ion adsorption can be described by both Freundlich and Langmuir adsorption isotherms, with a maximum adsorption capacity of 7.54 mg/g at 30 °C and 150 mg/L. The sorption followed pseudo-second-order kinetics, while the major presence of –OH and –NH2 functional groups initiated a chemisorption mechanism. With estimated activation energy of 23.57 kJ/mol, the obtained thermodynamic parameters of ΔS° (0.034 – 0.050 kJ/mol K), ΔG° (8.37 – 10.64 kJ/mol), and ΔH° (20.07 – 23.81 kJ/mol) indicated the adsorption process was chemically favoured, non-spontaneous, and endothermic respectively. The kinetics and sorption capacity of K-jarosite adsorption for copper ions from aqueous media were studied in relation to surface charge, surface area, and pore size. The point of zero charge of the jarosites was observed to be 2.14 and 2.37 for jarosite samples synthesized at pH 2.2 (Jar-2.2) and pH 1.7 (Jar-1.7) respectively. The results also demonstrated that jarosite could adsorb copper under both positively and negatively charged pH conditions, with more copper being adsorbed when the surface of the jarosite is more negatively charged (when the pHS > pHPZC). Jar-2.2 with a smaller particle size (higher surface area) gave the highest sorption capacity for copper as compared with Jar-1.7, under the same adsorption conditions. The mechanism of copper adsorption onto jarosite powder was by cation exchange and electrostatic attraction. The results from this thesis may provide an insight into the management of biohydrometallurgical processes to minimize metal losses. Thus, the findings stated are applicable in the design and implementation of iron bio-oxidation, precipitation, and treatment of ferric precipitate residues.