Article pubs.acs.org/IECR
Synthesis of Different Manganese Oxides Using SO2/O2 Gas Mixtures at Different Temperatures Simón Bello-Teodoro,† Roberto Pérez-Garibay,*,† and Jocelyn Bouchard‡ †
Cinvestav-IPN, Av. Industia Metalúrgica 1062, Ramos Arizpe, Coahuila C.P. 25900, México LOOP (Laboratoire d’Observation et d’Optimization des Procédés), Département de Génie des Mines, de la Métallurgie et des Matériaux, Université Laval, Québec City, Québec Canada
‡
ABSTRACT: The aim of this work is to study the kinetics and required operation conditions for the formation of various manganese oxides using the SO2/O2 gas mixture as the oxidant agent. This promising approach presents some obvious economical advantages over traditional pyrometallurgical routes. Experimental evidence demonstrate that increasing the temperature of the reaction allows modifying the oxidation potential, thus facilitating the oxidation product synthesis. However, at 25 °C by varying the SO2/O2 ratio and the gas mixture flow rate, it was not possible to modify this potential. It was also observed that manganese dioxide (MnO2) is formed between 20 and 50 °C (activation energy (Ea) = 14.33 kJ/mol), oxyhydroxide of manganese (MnO·OH) is obtained between 55 and 65 °C (Ea = 25.14 kJ/mol), and manganese(II, III) oxide (Mn3O4) is produced between 70 and 90 °C (Ea = 12.44 kJ/mol). The magnitude of these activation energy values is characteristic of processes controlled by the diffusion of the gaseous reagents.
1. INTRODUCTION Manganese plays an important role in ferroalloy production because it is used as a deoxidizing agent, and is employed as well as an additive in steel, aluminum, and copper alloys. New outlets appeared also recently for manganese dioxide, for primary and secondary battery production. The increase in the demand of manganese has motivated the recovery from secondary sources such as nickel laterite waste effluents, or from leaching low-grade minerals. Oxidative precipitation with inexpensive oxidants such as an SO2/O2 mixture is highly selective for the recovery of manganese and appears to be the most promising method for future research and development.1 Only a few studies have been reported on manganese oxidation using the SO2/O2 gas mixture. Most of them were focused on the manganese removal of the solution, as it is considered as an impurity in leach liquors of several metals (Zhang et al.,2,3 Mulaudzi and Mahlangu,4 Menard and Demopoulos5), or on the treatment of the waste stream of hydrometallurgical processes to comply with the environmental discharge regulations.6 In almost all these studies, the formation of MnO2 as a reaction product has been reported; however, Bello-Teodoro et al.7 recently observed that other oxidized manganese species precipitate as well when the oxidation temperature is increased above 50 °C. However, the authors specified neither the temperature range of formation, nor the kinetics of each reaction. The aim of this work is to study the kinetics and operating conditions leading to the formation of various manganese oxides, i.e. MnO2, MnO·OH and Mn3O4, using a SO2/O2 gas mixture as the oxidant agent. Synthetizing Mn3O4 typically involves pyrometallurgical processes, but this research proposes a new hydrometallurgical route requiring lower temperatures. Section 2 presents the current status of understanding. In Section 3 is presented the methodology and a description of the © 2014 American Chemical Society
experimental setup. Section 4 presents the results and the analysis of the most relevant aspects found in this study. The conclusions are presented in Section 5.
2. CURRENT STATUS OF UNDERSTANDING Various oxidants for Mn(II) precipitation have been studied and applied. These include ozone, SO2/O2 mixtures, as well as Caro’s, peroxydisulfuric, hypochlorite, and chlorate acids. The main challenges related to the oxidative precipitation method are the cost, the corrosive nature, control and handling of some oxidants, which can seriously handicap the practical feasibility of manganese recovery from diluted solution. With this respect, a SO2/O2 (O2 in air) oxidizing mixture is relatively inexpensive, and is considered to be the most suitable oxidant for this purpose.1 SO2/O2 mixtures have demonstrated to form powerful oxidizing reagents in the presence of various metal ions. The oxidizing power of SO2/O2 is surprising to some extent, because SO2 is normally considered a reductant, and oxygen, a mild oxidant.8 For manganese precipitation in the system described by the Pourbaix diagram presented in Figure 1, Menard and Demopoulos5 presented four theoretical oxidation reactions, depending on Eh and pH, i.e., Mn 2 + + 2H 2O → Mn(OH)2(s) + 2H+
(1)
3Mn 2 + + SO2(g) + O2(g) + 4H 2O → Mn3O4(s) + SO24 − + 8H+ Received: Revised: Accepted: Published: 7965
(2)
January 8, 2014 March 21, 2014 April 25, 2014 April 25, 2014 dx.doi.org/10.1021/ie500095s | Ind. Eng. Chem. Res. 2014, 53, 7965−7970
Industrial & Engineering Chemistry Research
Article
Characterizing the precipitate formed by Mn(II) oxidation with SO2/O2 gas mixtures is also a topic of interest. Menard and Demopoulos5 identified the formation of a poorly crystallized precipitate consisting most likely of a birnessitelike phase (26% Mn) and the coprecipitation of other metals from the solution. Mulaudzi and Mahlangu4 produced Mn precipitates from cobalt leach liquors and reported the formation of solids with small amounts of Mn (
