M. Valix

Coalbed methane sorption related to coal composition

Abstract Gas sorption by coal is closely related to its physical and chemical properties, which are, in turn, governed by coal type and rank. The role of coal type (sensu maceral composition) is not fully established but it is clear that coal type may affect both adsorption capacity and desorption rate. Adsorption capacity is closely related to micropore (pores Desorption rate investigations have been performed using selected bright and dull coal samples in a high pressure microbalance. Interpretation of results using unipore spherical and bidisperse pore models indicate the importance of the pore structure. Bright, vitrinite-rich coals usually have the slowest desorption rates which is associated with their highly microporous structure. However, rapid desorption in bright coals may be related to development of extensive, unmineralised fracture systems. Both macro- and micro-pore systems are implicated in the more rapidly desorbing dull coals. Some dull, inertinite-rich coals may rapidly desorb due to a predominance of large, open cell lumina. Mineral matter is essentially nonadsorbent to coal gases and acts as a simple diluent. However, mineral-rich coals may be associated with more rapid desorption. Coal rank and type (maceral composition) per se do not appear to be the critical factors in controlling gas sorption, but rather the influence they exert over pore structure development.

Fungal bio-leaching of low grade laterite ores

Abstract In this study, the mineral leaching ability of heterotrophic fungi, through secretion of organic acids, of various laterite ores was studied. Ores subjected to the leaching process included saprolite, weathered saprolite, limonite and nontronite. Strains of Aspergillus and Penicillium were found to be the most efficient organisms. Nickel extraction of up to 36wt%, cobalt of 54wt% and iron of 0.76wt% were achieved using a direct leaching where metals were leached in the same phase where bio-acids were produced. Extractions were compared with chemical leaching, where leaching up to 79.5wt % Ni, 71 wt% Co and 50wt% Fe were leached. The variation in recovery was found to be strongly influenced by the ore mineralogy and the metal loss through electro-sorption properties of the ores. Effective and selective leaching by certain strains for cobalt and nickel, which was not otherwise observed in the chemical leaching process, suggest that microbiological activity, apart from bio-acid production is participating in the leaching process.

Carbonisation of bagasse in a fixed bed reactor: influence of process variables on char yield and characteristics

Carbonisation experiments on samples of sugar cane bagasse were conducted in a static fixed bed reactor to determine the effect of process variables such as temperature, heating rate, inert sweep gas flow rate and particle size on the yield and composition of solid product char. Experiments were performed to the final temperatures of 250–700°C with heating rates from 5 to 30°C/min with nitrogen sweep gas flow rate of 350 cc/min. Additional tests were aimed at studying the effect of different flow rates of nitrogen sweep gas from 0 to 700 cc/min during carbonization and different particle size fractions of bagasse. The results showed that as the carbonisation temperature was increased, the yield of char decreased. The reduction in yield was rapid up to a final temperature of 500°C and was slower thereafter. The yield of char was relatively insensitive to the changes in heating rate and particle size. Increasing the sweep gas flow rate to 350 cc/min reduced the yield of char. It appears the presence of inert sweep gas reduced secondary reactions which promoted char formation. The proximate analysis of the char suggests that fixed carbon and ash content increased with temperature. The char obtained at temperatures higher than 500°C have high carbon content and is suitable as renewable fuel and for other applications. The carbonization of bagasse has the potential to produce environmental friendly fuels and can assist in reducing deforestation for the production of charcoal.

Heavy metal tolerance of fungi

The tolerance of fungi strains including Penicillium funiculosum, Aspergillus foetidus, Penicillium simplicissimum for different heavy metals, which could be leached, from nickel laterite ores (Ni, Co, Fe, Mg and Mn) was studied. These strains were exposed to heavy metals up to 2000 ppm. The tolerant strains were selected by repeated subculturing in petri dishes with increasing metal concentration in the medium. The degree of tolerance was measured from the growth rate in the presence of the various heavy metals and compared to a control, which contained no heavy metals. It appears that Penicillium funiculosum and Aspergillus foetidus were the most tolerant to the heavy metals and exhibited strong growth often exceeding the control. Penicillium simplicissimum showed the least tolerance particularly for Ni and Co. A growth pattern, which was consistent for each strain under various heavy metals, was observed as a function of time. The growth pattern of the fungi exhibited a lag, retarded, similar and enhanced rate of growth in the presence of heavy metal relative to the control. The similarity in the pattern appears to suggest the tolerance development or adaptation of the fungi for heavy metals.

Study of phase transformation of laterite ores at high temperature

The phase transformation of laterite minerals reduced at temperatures up to 800 °C and the phase reversibility with cooling were investigated in this study. The phases were studied by in situ synchrotron radiation based X-ray powder diffraction with temperature resolved studies. The degrees of reduction were confirmed by a leaching of reduced ore with ammoniacal solution. Key features, which were observed in this study, include the role of dehydroxylation in releasing nickel and cobalt from the main gangue minerals in which these metals are incorporated. The transformation of hydrated gangue, goethite in limonite to magnetite and the serpentine minerals to the olivine phases in the saprolite and control of these phase transformations, in particular the recrystallisation of magnesium silicate to the forsterite phase were also investigated.

Adaptive tolerance behaviour of fungi in heavy metals

Certain metals are essential to biological actions. However all metals, whether essential or inessential, will tend to show toxicity at certain levels. Previous attempts to biologically leach nickel oxide ores are hampered by poor tolerance of the strains to heavy metals, which suggest the need for new strains with resistance to high concentrations of heavy metals. This study investigates the development of training strategies, which is attempting to address this issue. The heavy tolerance development of Aspergillus niger, Penicillium simplicissimum, Aspergillus foetidus and Aspergillus carbonarius strains in the presence of Ni, Co, Fe, Mg and Mn was studied up to concentrations of 2000 ppm. The adaptive behaviour was measured from the rates of initial growth and death rate of the strains with time. The effect of type of metal, the metal concentration and the type of strains on this adaptive behaviour was investigated. The results indicate that these initial adaptive behaviours are reflections of the strains tolerance development with increasing metal concentration. A high growth followed by a weaker death rate appear to lead to tolerance development of strains in increasing metal concentration often exceeding the control. Whereas a strong growth followed by an equivalently high death rate appear to lead to poorer tolerance. The relative toxicity of the various metals to the strains was used to select a strain that would be most suitable in leaching nickel laterite ores.

The effects of mineralogy on the biological leaching of nickel laterite ores

The amenability of limonite ore, the iron rich nickel laterite phase, was observed to be less than saprolite, the silicate rich phase, when subjected to biological leaching with heterotrophic microorganisms. To understand this difference, controlled leaching of the various nickel laterite minerals was conducted using citric acid to mimic the chemical action of the organisms and subsequent characterisation of the leached residues. The raw ore and the leached mineral residues were examined using high-resolution synchrotron based X-ray diffraction and optical microscopy. The possible contribution of electrosorption and solid porosity was also considered. The results suggest that although electrosorption contributes to nickel and cobalt losses, it is the structural changes induced by the acid in the minerals which effects the recovery of the metals. Citric acid was found to dehydroxylate saprolite minerals resulting in an unstable nickel oxide and thus making it susceptible to acid attack. This behaviour was not observed in the limonite ore, which was found to be poorly leached by citric acid.

Dye adsorption onto activated carbons from tyre rubber waste using surface coverage analysis.

Two types of activated carbons from tyre char (with or without sulphuric acid treatment) were produced via carbon dioxide activation with BET surface areas in the range 59-1118 m(2)/g. Other characterisation tests include micropore and mesopore surface areas and volumes, pH, and elemental compositions, particularly heteroatoms such as nitrogen and sulphur. They were correlated to the adsorption capacity which were in the range of 0.45-0.71 mmol/g (untreated) and 0.62-0.84 mmol/g (acid-treated) for Acid Blue 25. In the case of larger-sized molecules like Acid Yellow 117, capacities were in the range of 0.23-0.42 mmol/g (untreated) and 0.29-0.40 mmol/g (acid-treated). Some tyre carbons exhibit a more superior performance than a microporous, commercial activated carbon (Calgon F400). By modelling the dye adsorption equilibrium data, the Redlich-Peterson isotherm is adopted as it has the lowest SSE. Based on the surface coverage analysis, a novel molecular orientation modelling of adsorbed dyes has been proposed and correlated with surface area and surface charge. For the acid dyes used in this study, molecules were likely to be adsorbed by the mesopore areas.

Effect of sulfur on the mineral phases of laterite ores at high temperature reduction

Recovery of nickel and cobalt from nickel laterites processed according to the Caron process is hampered by reduction of iron in limonite ores and formation of fosterite in saprolite ores. The Caron process involves the reduction of the ores at temperatures between 700 and 800 °C followed by ammoniacal leaching. The presence of sulfur has been found to markedly improve the Ni and Co recoveries from laterite ores. In this study, limonite and saprolite laterite ores were reduced in the presence of an activator, in the form of elemental sulfur. The mineral phases resulting from the reduction, both in the presence and absence of sulfur were related to recoveries and selectivity obtained from laboratory reduction and leaching tests. The mineral phases were examined using X-ray diffraction with a synchrotron radiation source. The role of sulfur and the history of reduction in effecting the increase in recovery and selectivity of nickel and cobalt was investigated in this study.

The electro-sorption properties of nickel on laterite gangue leached with an organic chelating acid

In this study the affinity of nickel ions on laterite gangue as a function of pH was investigated as an alternative reason for nickel losses observed during the bioleaching of nickel laterite ores. In microbiological extraction of nickel from laterites, dissolved nickel concentration in the leaching medium at the end was observed to be lower than at the beginning. It has been suggested that this was associated with the uptake of metal by the biomass of the fungi. This study demonstrated that electrosorption of nickel does contribute to metal loss during bioleaching. The surface charge and the type of minerals present in the ore gangue were shown to have an important influence on the adsorption capacity. The analysis of the process suggest that control of pH during the bioleaching process is critical in optimising the leaching process.