T. Subbaiah

Electrolytic manganese dioxide (EMD): a perspective on worldwide production, reserves and its role in electrochemistry

Electrolytic manganese dioxide (EMD) is the critical component of the cathode material in modern alkaline, lithium, and sodium batteries including electrochemical capacitors and hydrogen production. In terms of environmental and cost considerations, EMD is likely to remain the preferred energy material for the future generation, as it has been in recent decades. Diminishing fossil fuels and increasing oil prices have created the need to derive energy from sustainable sources. The energy storage device from alternative and inexpensive sources, such as low grade manganese ores, has a niche in the renewable energy and portable electronics market. Despite vast manganese sources along with the current activity in producing modified EMD materials from secondary sources, to a surprise, India mostly imports EMD to meet its demand. Keeping this in view, a comprehensive review has been prepared on the synthesis, physical and electrochemical characterization of EMD produced from synthetic solutions and secondary sources. This review summarizes the available EMD sources in the world including Indian deposits and the recent investigations of fundamental advances in understanding the electrochemical mechanism involved in aqueous rechargeable batteries and electrochemical capacitors, thus leading to an improved energy storage performance, which is essential for their long term use in storing renewable energy supply.

Recovery of copper from a surface altered chalcopyrite contained ball mill spillage through bio-hydrometallurgical route

Bioleaching studies for chalcopyrite contained ball mill spillages are very scarce in the literature. We developed a process flow sheet for the recovery of copper metal from surface activated (600 °C, 15 min) ball mill spillage through bio-hydrometallurgical processing route. Bioleaching of the activated sample using a mixed meso-acidophilic bacterial consortium predominantly A. ferrooxidans strains was found to be effective at a lixiviant flow rate of 1.5 L/h, enabling a maximum 72.36% copper recovery in 20 days. Mineralogical as well as morphological changes over the sample surface were seen to trigger the bioleaching efficiency of meso-acidophiles, thereby contributing towards an enhanced copper recovery from the ball mill spillage. The bio-leach liquor containing 1.84 g/L Cu was purified through solvent extraction using LIX 84I in kerosene prior to the recovery of copper metal by electrowinning. Purity of the copper produced through this process was 99.99%.

Bio-hydrometallurgical processing of low grade chalcopyrite for the recovery of copper metal

A process flowsheet was developed to recover copper metal from the lean sulfide ore of copper available at Malanjkhand, Hindustan Copper Limited (HCL), India. Copper pregnant leach solution (PLS) obtained from bio-heap leaching of chalcopyrite containing 0.3% copper was purified through solvent extraction (SX) and the copper recovered by electrowinning (EW). The copper-free raffinate obtained from SX stripping unit was returned back to the bioleaching circuit. The purity of the electrolytic copper produced at pilot scale was found to be 99.96%. During electrowinning, the effect of flow rate of electrolyte on current efficiency and energy consumption was also studied.

Physico-chemical properties of copper electrolytes

A systematic study was undertaken to determine the diffusion coefficient(D) for Cu+2 in the CuSO4-H2SO4 system at different Cu and H2SO4 concentrations and temperatures. An empirical equation for predicting theD value was developed and checked for its validity. Conductivities, densities, and viscosities of copper electrolytes were measured in a wide range of Cu and H2SO4 concentrations and temperatures and the results are reported. Such information also was gen-erated for complex solutions containing the impurities Ni, Co, Fe+2, Fe+3, and Mn. These prop-erties were calculated further using empirical equations and compared with the measured values.

Electrodeposition of manganese dioxide: effect of quaternary amines

The effect of quaternary ammonium salts (tetraethyl ammonium bromide, tetrapropyl ammonium bromide, and tetrabutyl ammonium bromide) on the structural, morphological, and electrochemical characteristics of electrolytic manganese dioxide (EMD) obtained from acidic aqueous sulfate solution has been investigated. Physical characterization of the EMD was achieved by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential thermal analysis, and Fourier transform infrared spectroscopy. The charge–discharge profile of the materials was determined to evaluate their potential for alkaline battery applications. The presence of these quaternary ammonium salts as organic additives in the solution increased the current efficiency while decreasing energy consumption during electrochemical deposition of manganese dioxide (MnO2). All the additives influenced the discharge characteristics of the EMD samples significantly, producing a cathode material with increased cumulative discharge capacity relative to EMD prepared in the absence of additives. This is attributed to the ability of the additives to affect the particle size and morphology, and therefore electrochemical activity, of electrodeposited materials; the effects in the case of the additives investigated in this work were positive, producing a material with potential application to battery technology.

Mass transfer rates in an electrochemical cell

Abstract Local mass transfer conditions in an electrochemical cell were obtained at various spatial locations with turbulence created by electrolyte circulation, or by an impeller, or by employing both simultaneously. For the cell with electrolyte circulation both the limiting current density and mass transfer coefficient varied with the cathode heights and three zones for their distribution in the cell were obtained. An empirical equation correlating mass transfer coefficient with residence time is proposed. The mass transfer coefficient has been correlated with specific power. Three hydrodynamic zones were also obtained where the turbulence was created by an impeller. An equation for correlating the mass transfer coefficient with the impeller speed is suggested.

Effect of some common impurities on mass transfer coefficient and deposit quality during copper electrowinning

Abstract The effects of impurities such as Fe2+, Fe3+, Ni2+, Co2+, Mn2+ and LIX64N on copper electrowinning were investigated. These impurities were found to affect the physicochemical properties of the electrolyte adversely, which ultimately caused a decrease in the limiting current density and mass transfer coefficient. Examination of the cathode copper deposited from such electrolytes showed that changes in the physicochemical properties of the copper electrolyte not only affected the mass transfer conditions during copper electrowinning but also effected the surface morphology and crystal orientation of the cathode copper significantly. The presence of any of the metal ions studied promotes the growth of pyramidal and planar structures, while restricting the growth of ridge-type structures.

Improved ionic mass transfer in an electrochemical cell in presence of turbulence promoters

Abstract In industrial operations, reduction in equipment size can increase productivity, lower energy inputs and improve purity of the product. The presence of turbulence promoters in an electrochemical cell can increase the intensity of local turbulence, which enhances the mass transfer coefficient at the transfer surface and the limiting current density. The regularity of their geometrical configuration induces a pseudo-uniformity of the spatial distribution of the local mass transfer coefficients. High mass transfer coefficients can be achieved at much lower flow rates when turbulence promoters are placed in the cell. Limiting current data were obtained using a diffusion-controlled electrode reaction at point copper electrodes fixed on the wall of the electrolytic cell in the presence of rectangular turbulence promoters. The improvement in limiting current density, and hence mass transfer coefficient, due to the presence of turbulence promoters is assessed. The effects of the flow rate of the electrolyte, height of the promoters and spacing between the promoters were studied.

Influence of Synthesis Temperature on the Growth and Surface Morphology of Co3O4 Nanocubes for Supercapacitor Applications

A facile hydrothermal route to control the crystal growth on the synthesis of Co3O4 nanostructures with cube-like morphologies has been reported and tested its suitability for supercapacitor applications. The chemical composition and morphologies of the as-prepared Co3O4 nanoparticles were extensively characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Varying the temperature caused considerable changes in the morphology, the electrochemical performance increased with rising temperature, and the redox reactions become more reversible. The results showed that the Co3O4 synthesized at a higher temperature (180 °C) demonstrated a high specific capacitance of 833 F/g. This is attributed to the optimal temperature and the controlled growth of nanocubes.

Separation of iron and zinc from manganese nodule leach liquor using TBP as extractant

Abstract In this work, the separation of total Fe and Zn leading to the purification of Mn from manganese nodule leach liquor were carried out from hydrochloric acid solution by tributylphosphate with methyl iso-butyl ketone (phase modifier) dissolved in kerosene. The simulated solution comprised of 0·29 mol L−1 Mn, 0·12 mol L−1 Fe, 0·087 mol L−1 Zn and 2·3 mol L−1 HCl was used for the study. Different parameters such as extractant and acid concentrations affecting the optimum extraction condition for the investigated metal ions were examined. The McCabe-Thiele plots for extraction and stripping were constructed and scrubbing studies were carried out. Total Fe was first separated and co-extracted Zn in the loaded organic was removed by scrubbing with ferric chloride solution; and the remaining Zn was then removed by tributylphosphate. Finally, extraction of 100%Fe and 99·7%Zn; and about 99·5% stripping efficiencies in both the cases were achieved.