This paper gives the review of oxide coated titanium electrodes prepared by thermal decomposition for oxygen evolution in industrial electrolysis such as electrowinning and electroplating. The electrode is prepared by calcination of a precursor solution containing platinum group metals (Ir, Ru) and valve metals (Ta, Ti) on a titanium substrate. The crystallographic structure and surface morphology of the oxide coating depend on the pretreatment of the substrate, the composition and preparation procedure of the precursor solution, thermal decomposition temperature, the coating thickness, and others. The catalytic activity for oxygen evolution is strongly influenced by the structure, particle size, and distribution of platinum group metal oxides such IrO2 and RuO2; a low temperature thermal decomposition produces a mixture of amorphous platinum group metal oxides and valve metal oxides in which platinum group metal oxides are uniformly dispersed in nano size in amorphous valve metal oxide matrix. This unique coating structure results in a high catalytic activity for oxygen evolution in acidic solutions, because of the increase in active surface area. The overpotential for oxygen evolution is reduced compared to traditional Pb alloy electrodes or crystalline oxide coated titanium electrodes, so that the cell voltage of zinc or copper electrowinning is significantly decreased and some unwanted side reactions, i.e., anodic deposition of PbO2 or MnO2 induced by oxidation of Pb (II) or Mn (II) contained in electrowinning solutions, is suppressed. The commercial production and application to electrowinning plants of amorphous oxide coated titanium electrodes for oxygen evolution are in progress, and this new technology is expected to realize a more environmentally friendly electrowinning process.
Limestone is one of the most important mineral resources, and it is only one mineral which can be fully supplied by the domestic product. Generally limestone mines in Japan located at mountainous regions, and the combination between bench-cut method and the carry-out system using vertical shaft is widely used in the domestic mines. Generally certain amount of ores is stored in the vertical shafts temporally after blasting excavation, and they are carried out from the bottom of the vertical shaft. However, the blockage in the vertical shafts sometimes occurred and some examples are recently reported. Once the blockage in the shafts is occurred, it will cause serious problems towards the mine operations, and a lot of works and dangerous procedures are necessary to solve the problems. It is thought that blockage is influenced by the cohesion due to clay minerals thrown into the vertical shafts, however, the quantitative and precise reason is not revealed yet. In this study, the process of blockage occurrence was simulated by Discrete Element Method (DEM). In the DEM analysis, discontinuous circular particles are generated, and the individual movement of each particle is calculated by forward difference method. Since the movement of each ore is similar to the procedure, it is suitable method to simulate the blockage process in the vertical shaft. Here we assumed standard shaft size and ore size distributions, the conditions which causes blockage in the vertical shafts, such as cohesion and friction coefficient, were discussed. Moreover, various kinds of shaft shape including inclined shafts were also simulated, and those influences to the blockage occurrence were discussed. It was found that the cohesion between ores was the main factor to cause blockage, and that minimum cohesion exited towards any values of friction coefficient. It was also found that the wider diameter of the shafts is effective in order to avoid from blockage. The shapes of shafts are also important factor, and it was found that the blockage often occurs where the diameter suddenly becomes smaller.
There are many research results on creep and relaxation aiming at investigating the time-dependent behavior of rock. However, rock mass around gateways in mines or underground structures deforms accompanied with both stress and strain change. Fukui et al. (1992) proposed the generalized relaxation test where both stress and strain change keeping the ratio between them constant. In this study, computer simulation was carried out to explain their results. The non-liner Maxwell model was used in the simulation and the calculated results well coincided with the experimental results except those in high stress level in which strain/stress rate were considerably larger than the calculated results.
In order to develop an energy-saving insoluble anode for Zn electrowinning, Pb-based anodes in which catalyst powders of low oxygen evolution overpotential were spread over the Pb substrate were prepared by powder-rolling method and the effect of the catalyst powder on lowering the anode potential was investigated. It was clearly shown by galvanostatic electrolysis and anode polarization measurement that the RuO2 powder dispersed to the Pb substrate was effective in reduction of oxygen evolution overpotential. The anode potential of Pb-RuO2 anode fell remarkably around RuO2 content of 0.60 mass%, and the anode potential of the Pb-based anodes containing 0.70-1.50 mass%RuO2 was 300-350 mV lower than that of the pure Pb anode. The activation energy of the oxygen evolution reaction of Pb-RuO2 anodes containing 0.50 or 1.00 mass%RuO2 was only 24-26 kJ mol-1, whereas those of pure Pb and Pb-0.70mass%Ag anodes were 62.7 and 35.5 kJ mol-1 respectively. Therefore, it was judged that RuO2 could accelerate the oxygen evolution reaction more efficiently than Ag as the electrode catalyst. XPS analysis also showed that the existence ratio of PbO2 on the Pb-RuO2 anode surface after electrolysis for 18.0 ks was higher than those of the pure Pb and the Pb-0.70mass%Ag anodes. Consequently, the chemical conversion from Pb to PbO2 might be also concerned with lowering the anode potential of the Pb-based anodes.