Tadashi Chida

Dissolution rates of amorphous silica in highly alkaline solution

Cement is an essential materials to construct the subsurface radioactive waste disposal system. However, cementitious materials alter the groundwater pH to highly alkaline condition about 13. To comprehend the effect of such a hyperalkaline condition on the repository surroundings, this study focused on the dissolution rates of amorphous silica at [NaOH]=10-1 mol-dm-3. The used samples were three kinds of pure commercial silica and a natural silica scale which was obtained from inside wall of the hot-water pipe of a geothermal power plant. The observed dissolution rates were interpreted with using the model, which assumed that the particle sizes decrease with the progress of dissolution. Moreover, due to the particle size distribution anticipated in the natural silica scale, this analysis assumed it contained particles with various initial diameters. In the results, (1) all pure silica samples and at least 60wt% of the silica scale showed good agreement of the activation energy of the dissolution in the range of 77 through 88kJ-mol-1 in the highly alkaline solution, (2) these rate constants were of the order of 10-8-10-7 mol-m-2.s-1 at around 310 K and were definitely larger than those already reported for quartz, (3) the specific surface area based on BET method was revealed to be an important factor to give the main difference in the dissolution rates between the synthetic silica and the natural silica.

Hydrodynamic dispersion and mass transfer in unsaturated flow

Tracer response experiments were carried out using a packed bed of glass beads, where fluid flowed as either a single phase (water) or two phases (water and gas) under steady unsaturated conditions. The results depended on the water saturation and could be divided into two regions at the critical water saturation, that is, the saturation at which the gas phase began to be immobile. To determine the hydrodynamic dispersion coefficients, the experimental responses were compared with the numerical results of a one-dimensional advection-dispersion equation. These results demonstrated that the hydrodynamic dispersion coefficients increased with a decrease in water saturation. Furthermore, the rates of the increase in the dispersion coefficients above the critical saturation were different from those below it. The following mathematical relations between the Peclet number, Pe, and the water saturation, Sw, were obtained from the results of the two unsaturated flow experiments: Pe=0.8Sw1.2 for single-phase flow and Pe=0.9Sw3.1 for two-phase flow. When Sw=1 in the relation for single-phase flow, the value of Pe is in a range that has been reported previously. The relation for two-phase flow was used to analyze the mass transport of the two-phase tracer (ammonia-water solution) through unsaturated beds.

Inhibitory Effect of High Concentrations of Ferric Ions on the Activity of Acidithiobacillus ferrooxidans

The influence of high concentrations of ferric ions on the biochemical activity of Acidithiobacillus ferrooxidans was studied using intact cells. The specific oxidation rate of ferrous ions decreased with increasing ferric ion concentration. Lineweaver-Burk plots revealed typical competitive inhibition kinetics, because the slopes varied with the ferric ion concentration. A linear relationship between the slope and the square of the ferric ion concentration revealed that the iron-oxidizing enzyme system of A. ferrooxidans was competitively inhibited by about two molecules of ferric ion. The kinetic equation based on this inhibition model agreed with the experimental observation at a high ferric ion concentration where the bacterium is usually exposed in bioleaching and biooxidation plants.

Identification of geothermal reservoir structure analyzing tracer responses using the Two-Fractured-Layer model

Abstract A model to analyze tracer test data is proposed. It consists of a fractured layer and a slightly-fractured layer, where tracers move between the layers in proportion to their concentration gradients. It is shown that the slightly-fractured layer plays an important role in the movement of fluids and transport of tracers in fractured rock masses. The concepts behind the proposed model for identifying geothermal reservoir structure from the analysis of tracer returns and its application to field data are described. The results demonstrate that one main flow path can explain tracer responses showing two peaks or a long tail. The model allows us to calculate the flow fraction and the fluid flow velocity through the slightly-fractured layer from the tracer returns observed in geothermal wells.

Enhancement of the specific growth rate of Thiobacillus ferrooxidans by diatomaceous earth.

Diatomaceous earth is an effective carrier with a large surface area on which Thiobacillus ferrooxidans adsorbs, and enhances the oxidation rate of ferrous ions by the bacterium. The adsorption of T. ferrooxidans on diatomaceous earth was expressed by the Henry isotherm, and reached equilibrium within five minutes. The enhancement was due to an increase in the growth rate of the adsorbed cells, which were so active that their specific growth rate calculated using a Monod-type equation was higher than that of the free cells in the liquid phase.

Polysulfide reduction using sulfate-reducing bacteria in a photocatalytic hydrogen generation system

A hydrogen generation process using photocatalytic reactions has been proposed. In this process, hydrogen sulfide is a source of hydrogen and is turned into polysulfide. In order to establish the cyclic operation of a photocatalytic hydrogen generation system, it is necessary to convert polysulfide back into hydrogen sulfide with a small energy input. This paper proposes the use of sulfate-reducing bacteria (SRB) for the regeneration of hydrogen sulfide. Batch cultivation of natural source SRB samples were carried out using a culture medium containing polysulfide as the only sulfur compound source. SRB produced hydrogen sulfide from several kinds of polysulfide sources, including a photocatalytic hydrogen generation-produces solution. Production lag phase and production rate of hydrogen sulfide were affected by initial polysulfide concentration. SRB activity was inhibited at high initial polysulfide concentrations. SRB enrichment culture T2, exhibited the highest hydrogen sulfide production rate, and was able to utilize several kinds of organic matter as the electron donor. The results suggest the possibility of using large biomass sources, such as sewage sludge and the raw garbage in a hydrogen generation system. We developed speculative estimates that an SRB based hydrogen generation system is feasible.

Estimation of the radionuclide transport by applying the mean, the standard deviation and the skewness of permeability

A new method for estimating the mass transport by using the stochastic values (the arithmetic mean, the standard deviation and the skewness) of permeability is presented. Generally, detail of permeability distribution cannot be obtained except for moments of the distribution. Also, measurement results of permeability for the rock matrix including cracks or fast flowpaths do not always follow the log-normal distribution frequently applied. In such a situation, we must evaluate the characteristic permeabilities for the whole or some regions of the disposal site including the accessible environment. The authors have investigated the characteristic permeability on the basis of some probability density functions of permeability, applying the Monte Carlo method and FEM. It was found that its value does not depend on type of probability density function of permeability, but on the arithmetic mean, the standard deviation and the skewness of permeability [1]. This paper describes the use of the stochastic values of permeability for estimating the rate of radioactivity release to the accessible environment, applying the advection-dispersion model to two-dimensional, heterogeneous media. When a discrete probability density function (referred to as ‘the Bernoulli trials’) and the lognormal distribution have common values for the arithmetic mean, the standard deviation and the skewness of permeability, the calculated transport rates (described as the pseudo impulse responses) show good agreements for Peclet number around 10 and the dimensionless standard deviation around 1. Further, it is found that the transport rates apparently depends not only on the arithmetic mean and the standard deviation, but also on the skewness of permeability. When the value of skewness dose not follow the lognormal distribution which has only two independent parameters (the mean and the standard deviation), we can replicate the three moments estimated from an observed distribution of permeability, by using the Bernoulli trials having three independent parameters.

Numerical Experiments of MEOR with the microbe producing water-soluble polymer

Our group has carried out the various studies and experiments on MEOR process with the microbe producing water-soluble polymer. Based on these efforts, the basic performance and mechanism in this process were well simulated by computation experiment in the previous paper.In this paper, our group carried out the case studies of this process using the computation experiments and suggested the important conditions and its sensitivity of the effect.First, the effect of reservoir conditions was estimated by changing permeability distribution, oil viscosity and oil saturation. The ratio of the enhanced oil recovery by this process was high at the all conditions of reservoir. It should be noted that this process was effective in any types of reservoir without restrictions of permeability distribution, oil viscosity variety. It was shown that the application of this process in the earlier stage of development was more effective because oil recovery was accelerated in the earlier stage by increase of oil saturation.Next, the effect of cultivation conditions was estimated by changing nutrient concentration, shut-in time for incubation and fluid property. The ratio of the enhanced oil recovery became higher by increase of nutrient concentration, but they had not proportionality relation. Therefore, it is necessary to optimize nutrient concentration for target reservoir by estimating the economical efficiency. It was suggested that taking shut-in at least for 2 days was one of necessary procedure for higher enhanced oil recovery.Third, the effect of injection conditions was estimated by changing injection volume, rate of microbe, nutrient solution and injection cycle of the solution and water. It was shown that those conditions must be optimized by considering economical efficiency for target reservoir.In conclusion, it was suggested that the enhanced oil recovery by this MEOR process can be optimized by selecting suitable conditions to reservoir characteristics and by controlling conditions of cultivation and injection.

Bio-Heap Leaching of Primary Copper Sulfide Ore by JOGMEC

JOGMEC has been carrying out the study on primary copper sulfide ores leaching for recovering copper economically and efficiently by heap leaching. In our study, we have been using the primary copper sulfide ore produced in an IOCG deposit. The ore is characterized by high iron content and high acid consumption in leaching. For the optimization of the leaching, the conditions such as ore size, agglomeration, pH and irrigation rate of leaching solution were examined with column leach tests. The best result was over 80 % extraction of copper in 150 days with leaching solution of 1 M sulfuric acid and temperature of 45 °C. In this experiment, the fines of the ore sample were removed before putting it into the column to keep the permeability of the ore bed. In these column leach tests, the Fe2+/total-Fe ratio of PLS decreased gradually by the activity of naturally grown iron-oxidizing bacteria. The bacteria in PLS, which have the ability of iron-oxidizing and sulfur-oxidizing, were identified by next-generation sequencing as Acidithiobacillus caldus and Sulfobacillus thermosulfidooxidans. We also carried out bench-scale tests with about 200 tons of the primary copper sulfide ores. Iron-oxidizing and sulfur-oxidizing bacteria were identified in the leaching solution as same with column leach tests. It is considered that the bacteria worked in the leaching solution and on the surface of the ores.

Effect of Sulfuric Acid Concentration on Chalcopyrite Concentrate Chemical Leaching

This study reports the effect of sulfuric acid concentration on chalcopyrite chemical leaching in very simple H2SO4 solution systems ranging from 23g/L to 30g/L, with 2.5% chalcopyrite concentrate at 30°C. Copper extraction from chalcopyrite increases with an increase in sulfuric acid concentration, e.g. 86%, 90% and 92% after 96 days at 23g/L, 25/L and 27g/L H2SO4 solution respectively. Sulfur element formed on the surface of chalcopyrite was very porous as the result of an electron probe microanalyzer (EPMA). Copper extraction, however, leveled out at 35% after 20 days when the sulfuric acid concentration was higher than 28g/L on 25g/L of chalcopyrite concentrate. Sulfur element was detected by X-ray analysis as only a leaching reaction product. The passivation may be caused by thick elemental surface formed on the surface of chalcopyrite.