On the fundamental aspects of apatite and quartz flotation using a Gram positive strain as a bioreagent
Graphical abstract
Highlights
► Environmental bioreagent for mineral flotation. ► Rhodococcus opacus bacteria affect the mineral electrophoretic behavior. ► The flotability of apatite and quartz depends on the pH value. ► R. opacus bacteria act as a biocollector and a biofrother. ► R. opacus presents a relevant potential as a bioreagent for mineral processing.
Introduction
Phosphate rocks are vital nonrenewable resources and are essential components in agricultural fertilizers and phosphorous-based chemicals. In Brazil, about 85% of the phosphate produced is consumed in the fertilizer industry. Phosphate deposits can be divided into three groups: sedimentary, igneous and biogenetic deposits (Alburquerque, 2010).
In recent years, mining industry has been facing several problems that influence the mineral processing, such as the depletion of high-grade ore (Dwyer et al., 2012) and environmental regulations (Díaz-Lópes et al., 2012). The former aspect compels the mining industry to process low grade ores, fine mineral particles and flotation tailings to produce material suitable for a global market (Dwyer et al., 2012). Thus, it has become very important to develop appropriate and environmentally friendly technologies able to complement the conventional techniques used at the mineral concentration. In this context, biobeneficiation is increasing its role in mineral processing. The main purpose of this procedure is to selectivity undertake the removal of undesirable mineral constituents from an ore, through interaction with microorganisms and/or their metabolic products, thus enriching it, with respect to the desired valuable minerals (Natarajan, 2006, Díaz-Lópes et al., 2012, Rao and Subramanian, 2007, Rao et al., 2010). Bioflotation exploits the differences in surface characteristics of solids suspended in an aqueous medium, adjusting and controlling their surface energies and interfacial tensions (Pecina et al., 2009) through the use of microorganisms with hydrophobic properties (Natarajan, 2006, Pecina et al., 2009). Bioflotation is becoming very attractive for presenting a great technological potential, environmental acceptability, flexibility in the choice of microorganisms and especially due to its mineral selectivity (Rao and Subramanian, 2007, Dwyer et al., 2012) and also for processing fine and ultra-fine mineral particles (Kuyumcu et al., 2009).
One of the most important steps in mineral bioflotation is the adhesion of the microorganism onto the mineral surface (Jia et al., 2011, Dwyer et al., 2012). The bacterial adhesion occurs as a net result of attractive and repulsive forces of the cell and mineral surfaces. The interactions that result in such adhesion include electrostatic interactions, acid–base interactions, van der Waals forces and hydrophobic interactions, all of which are determined by the cell-wall and mineral surface properties (Dwyer et al., 2012, Díaz-Lópes et al., 2012, Rao and Subramanian, 2007). A selective bacterial attachment is desired, onto a specific mineral, in order to modify its surface properties, consequently obtaining the separation of the desirable mineral (Chun-yun et al., 2008). This surface modification can be direct or indirect; the direct mechanism involves the adhesion of the bacterial cells to mineral particles, while the indirect mechanism refers to metabolic products which act like surface activator reagents. Both interactions may allow the mineral surface to acquire hydrophobic or hydrophilic properties (Natarajan, 2006, Rao and Subramanian, 2007, Somasundaran et al., 2000).
The literature review shows that the use of bioreagents in mineral processing is still in the early stages. In order to have a better understanding of the fundamental aspects of mineral bioflotation, it is required to significantly increase the research related to the electrophoretic behavior of minerals/bacteria systems, thermodynamic adhesion of different bioreagents onto mineral surface and bioflotation kinetics to name a few.
The Brazilian igneous phosphate deposits are associated with several gangue minerals, particularly quartz, magnetite, carbonates and silicates (Oliveira, 2005). The application of Rhodococcus opacus bacteria in the igneous phosphate processing will depend on the flotation behavior of each mineral present in presence of this bioreagent. The aim of this work was the study of the fundamental aspects of the bioflotation of apatite and quartz using a R. opacus strain as a bioreagent, due to its frothing and collecting behavior. This study considered the influence of initial pH of the aqueous suspension and bacteria concentration on the electrophoretic behavior, contact angle, surface tension and flotability of the minerals before and after interaction with R. opacus bacteria.
Section snippets
Sample minerals preparation
Pure apatite and quartz mineral samples were used in this study. Pure apatite sample was provided by the Centre for Mineral Technology (CETEM) and pure quartz sample was provided by a local supplier (Estrada Mining, Belo Horizonte, Minas Gerais State). The samples were jaw crushed and dry screened to −3 mm. These samples were then dry-ground in a porcelain mortar and wet screened for obtaining the desired size fractions (Table 1). The ground quartz was then washed several times in KOH (0.1 M)
Zeta potential studies
This study deals with the changes in electrophoretic patterns of mineral samples after interactions with R. opacus bacterial at different pH values. Any changes in the surface charge of the mineral were related to adhesion of bacteria cells, these changes could also help to elucidate the interaction mechanisms between the cells and the active site of the mineral surface.
Fig. 1 shows the zeta potential curves of R. opacus cells and mineral particles at different pH values. The surface of the R.
Conclusions
The zeta potential evaluation of the mineral particles before and after the R. opacus interaction showed that the biomass modified the zeta potential profiles of apatite and quartz. This change was more significant in the case of apatite mineral. The surface tension of the bacterial suspension was found to be dependent on the pH solution and the bacterial concentration. The lower surface tension value was observed in the presence of 0.15 g L−1 of biomass and at pH range between 3 and 7. The
Acknowledgements
The authors acknowledge CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), VALE, CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro) for the financial support.
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