Recovery of manganese from iron containing sulfate solutions by precipitation
Graphical abstract
The paper covers the recovery of manganese from sulfate leach solution after base metal removal. The process steps studied in this work are shown in the diagram with solid lines.
Highlights
► Mn recovery by precipitation from sulfate solutions. ► Effective Fe separation from Mn with combined air oxidation and carbonate precipitation. ► Precipitation with O2 (air)/SO2 is also effective and economical. ► Method suffers from sensitive feed control and toxic SO2 gas. ► A rather long residence time needed for hydroxide and sulfide precipitations.
Introduction
Iron control plays a significant role in the processing of hydrometallurgical solutions of metal refineries. Iron is one of the most abundant elements in the Earth’s crust and it is present in several sulfide and oxide ore minerals. Iron is also involved in several natural redox processes. In hydrometallurgy, control of iron is difficult due to the rather low oxidation potential of Fe2+ to Fe3+ (−770 mV, compared to 1.5415 mV for Mn2+ to Mn3+) Haynes and Lide, 2011. In acidic solutions, where oxygen (O2) is present, iron is more or less at oxidation state 3+. The presence of Fe3+ in metal recovery with solvent extraction (SX) with commonly used liquid cation exchangers demands special attention in the pH and redox control since Fe3+ has a strong affinity to the formation of coordination compounds with the organic extractants (Ritcey, 2006). An extra SX step for iron separation is possibly needed. Iron treatment from metal solutions with solvent extraction has been studied widely (Biswas and Begum, 1998, Demopoulos and Gefvert, 1984, Hoh et al., 1983, Preston, 1985, Ritcey, 2006, Saji et al., 1998, Sato and Nakamura, 1971, Sato et al., 1985, Vazarlis and Neou-Syngoyna, 1984, Zhou et al., 1989), and different reactants have been utilized in SX plants (Cole et al., 2006, Dutrizac and Monhemius, 1986). The main problem in iron solvent extraction is stripping, which requires strong acids and reduction (Demopoulos and Gefvert, 1984, Lupi and Pilone, 2000, Vazarlis and Neou-Syngoyna, 1984). The use of strong acids entails a risk of reagent decomposition. Moreover, if iron concentration is high, the economic viability of the SX may decrease through large solvent inventory.
In sulfide precipitation, Fe3+ ions reduce to Fe2+, since the sulfide ion (S−) tends to act as a reductant (Wei and Osseo-Asare, 1996, Habashi, 1999). Reductive conditions give the possibility of selective recovery of some metals using precipitation without iron pre-treatment. The iron selectivity is, however, significant only for Ag, Bi, Cd, Cu, Pb, and Zn and at low pH (<3) and cannot be used for Fe–Mn separation (Monhemius, 1977). Moreover, due to environmental concerns and the closed circuits in hydrometallurgy, iron separation is necessary.
From an economic point of view, the ratio of iron to manganese in tailings solutions is critical. Compared to, for example, cobalt, copper or, nickel plants, the Mn process is more cost-sensitive; a result of the less valuable end-product. On the other hand, increased demand for stainless steel and greater use of batteries in electrical devices and vehicles have lead to increased consumption of manganese. (Zhang and Cheng, 2007a). Moreover, metals are increasingly being recovered from low-grade, complex and small-body ores, since most rich ore bodies are already utilized. In this context, it is reasonable to make efforts to recover metals from low concentration solutions. According to Zhang and Cheng (2007b), in mining operations worldwide, about 122 kt Mn is annually leached from laterite ores as side metal and handled as waste. The main difference between the classical Mn processing and this study is the manganese concentrations, which are typically tenfold higher than here in impurity removal. In order to make the recovery of Mn economically possible, cheap reagents and highly optimized energy efficient techniques are needed.
Precipitation, as jarosite, goethite, or hematite, is the most common method currently used to remove iron from leaching solutions. This process is, however, problematical because of the high temperature demands. Furthermore, the gelatinous and metastable solids produced can be difficult to process (Loan et al., 2006). Jarosite and hematite processes are preferred above 100 °C and goethite between 80 and 90 °C (Ismael and Carvalho, 2003). Claassen et al. (2002) and Loan et al. (2006) have reported the possibility of iron removal at temperatures closer to normal hydrometallurgical temperatures (about 60–80°C) using a so-called para-goethite process. The narrow operational range may, however, create practical problems. Jarosite, goethite and para-goethite do not have any further applications, but pure hematite can be utilized as a pigment or in steel making.
The aim of this work is to study manganese recovery from multi-metal sulfate solutions containing Mn, Fe, Ca, Mg, and Na at low concentrations (<5 g/L). The emphasis is on finding the technically and economically most feasible method to refine Mn and remove iron from MnSO4 solutions. Selectivity and consumption of reagents in the precipitation and leaching are significant factors studied. The presence of Ca, Mg and Na in the solution is not of major importance, since these metals can be separated with SX (Pakarinen and Paatero, 2011). The research concludes with the study of iron separation in a continuous pilot process using limestone and air at parameters found in batch experiments.
Section snippets
Precipitation equilibria
Precipitation occurs when the product of the metal and its counter-ion concentrations (activities) rise over a critical value (solubility product, KS) (Eq. (1)). The selectivity of the precipitation can be estimated by comparing the values of the solubility products of different metal salts.Here, M represents a metal cation at oxidation state n+ and A an anion at oxidation state m−. Precipitation diagrams for metals in different systems have been published by Monhemius (1977),
Measurements and modeling
An authentic metal solution after removal of base metals (a tailings solution) was used in the experiments. One aim of the experimental work was to study metal precipitation and solution chemistry. Precipitation of metals was studied by measuring solution concentrations and redox potentials as a function of pH. The measured redox potentials at the experimental conditions are used with the calculated Eh-pH diagrams to estimate the form of the precipitates. Calculations were carried out using HSC
Results and discussion
Precipitation of iron and manganese with different reagents was studied in order to find optimal conditions for the production of a MnSO4 solution free from iron. Both batch and continuous experiments were performed.
Conclusions
Several chemicals can be used to effectively precipitate manganese and iron from sulfate solutions. Soda ash is the most cost-effective reagent for the simultaneous recovery of Mn and Fe, due to its relatively low price, and good filtration and leaching properties. Oxidative precipitation with O2 (air)/SO2 could also be economical, but the disadvantages of this approach are the need for very precise feed control and the use of toxic and corrosive SO2 gas. Hydroxide and sulfide precipitations
Acknowledgements
The financial support and cooperation of Talvivaara Mining Company is gratefully acknowledged. The authors thank Ms. Krista Pussinen and Ms. Anne Hyrkkänen for assistance with the experimental and analytical work.
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