Scielo RSS <![CDATA[Journal of the Southern African Institute of Mining and Metallurgy]]> vol. 110 num. 5 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<b>Leaner and meaner or keener and cleaner</b>]]> <![CDATA[<b>The preparation of plans and diagrams at South African mines. Where is the boundary between the roles of mine and land surveyors?</b>]]> SYNOPSIS The newly implemented mineral law framework exacerbated the need to clarify the roles and responsibilities of mine and land surveyors in the preparation and registration of plans and diagrams. The main concern is that survey tasks may be performed by persons who are not considered competent under the different laws. The methodology used was to analyse historic and present legislation on the issue in order to formulate a narrow interpretation of the current requirements. The major findings are tabulated to present a summary of who is responsible for which areas of survey. <![CDATA[<b>Prediction of hydrocyclone performance using artificial neural networks</b>]]> Artificial neural networks (ANNs) have found their applications in the modelling of unit operations of mineral processing plants. In this research, laboratory-scale tests were conducted, using a three-inch diameter Mozley hydrocyclone. Main parameters included pressure drop at inlet, solid per cent, vortex and apex diameter were adjusted. The corrected cut size (d50c) and the flow rates of underflow and overflow were determined. Multi layers perceptron (MLP) feed forward network architectures were designed to predict the responses. The results showed a good correlation between experimental and network output, for corrected cut size and flow rates. <![CDATA[<b>CFD simulation and experimental measurement of nickel solids concentration distribution in a stirred tank</b>]]> SYNOPSIS Solids suspension influences the quality of mixing and energy requirement in a solid-liquid system, both of which determine the efficiency of industrial processes such as nickel precipitation. Nickel solids concentration distribution in a stirred tank was investigated using computational fluid dynamics (CFD) and experimental methods. The concentration distribution of the nickel solids was compared with that of sand and glass. The laser Doppler velocimetry (LDV) method was used to measure the velocity field for the liquid-only system and an optical technique was employed to determine the axial solids concentration distribution. Regions of inhomogeneity in the tank were identified. It was found that, for a given solids loading, the solids concentration distribution depended on both particle size and particle size distribution. High solids loadings were investigated and a difference in the concentration distribution pattern was obtained with nickel, flint glass and sand particles. The CFD simulation results highlighted problems that could be associated with some conventional experimental methods of determining solids concentration distribution in a stirred tank. <![CDATA[<b>The effect of feed-coal particle size on the separating characteristics of a gas-solid fluidized bed</b>]]> SYNOPSIS The separating performance of a pilot dry beneficiation system using a gas-solid fluidized bed was investigated with coal from South Africa. The coal used for the study has a low inherent moisture content, a moderate ash content, a moderate volatile content, a low sulphur content and a high calorific value. Its washability is moderate. Experimental results show that the separating quality of the fluidized bed drops gradually as the feedcoal particle size decreases. The probable error, E, values for 50-25 mm, 25-13 mm and 13-6 mm coals were 0.04 g/cm³, 0.06 g/cm³ and 0.09 g/cm³, respectively. The cause of the differences in separating characteristics was analysed by particle dynamics and numerical modelling. Furthermore, the pilot fluidized bed was employed to separate 50-6mm coal. In this experiment the coal ash content was reduced from 23.74% to 11.79%, with a probable error, E, value of 0.07 g/cm³ and a recovery efficiency of 98.26%. This indicates that the fluidized bed is applicable to the separation of coal from South Africa and has good separating performance and wide applicability. <![CDATA[<b>Fluidized bed gasification of selected South African coals</b>]]> An investigation was undertaken to ascertain the suitability of four selected low grade, South African coals for gasification in a bubbling fluidized bed for production of synthesis gas and for the development of integrated gasification combined cycles (IGCC). This study consisted of the characterization of the coals, laboratory evaluation of intrinsic reactivity, and experimentation with a pilotplant fluidized bed gasifier. Results of the characterization experiments show that the selected coals are high in ash, rich in inertinites, very dense (low porosity), with low caking indices and high ash fusion temperatures. Reactivity measurements with a thermogravimetric analyser (TGA), under reaction rate controlling conditions with carbon dioxide, showed that the reactivity of the coal chars decreases with increase in rank of the coal, as expressed by the vitrinite random reflectance and carbon content of the parent coals. The fixed carbon conversion achieved in the fluidized bed gasifier also correlates well with the rank parameter of the coal, with higher conversions being obtained with the lower rank coals. Thermal shattering and attrition of the coal particles produce significant amounts of fines, which correlate with the grindability indices, and no agglomeration (non-caking) was observed. It is concluded that fluidized bed gasifiers are able to utilize typical low grade, high ash South African coals for synthesis gas production and for inclusion in integrated gasification combined cycles for power generation. <![CDATA[<b>The manufacturing of nanoparticles with a plasma process</b>]]> SYNOPSIS Non-transfer-arc linear plasma systems are highly suited to manufacturing nanoparticles. In this paper, the manufacturing of such particles with specially designed plasma systems is discussed. In general, it entails the volatilization of a metal halide that is injected in the high-temperature region of a plasma flame, where it is reacted with a suitable reagent, for example O2, to produce nanooxide powders. Metal powders can also be introduced directly in the plasma. Due to the very short residence time of only milliseconds in the reaction zone, as well as the fact that the reaction takes place in the gas phase, combined with very rapid quenching in the order of 10 000°C.s-1, nanoparticles are produced. At Necsa the manufacturing of such particles has been demonstrated on various scales. A pilot plant for the production of nano-Al2O3, and two pilot plants, one using SiCl4 and the other one using SiF4 as feed material, were built to produce fumed, pyrogenic, nano-SiO2. The alumina plant had a production rate of 8-10 kg.h-1 and the silica plants had production rates of 2 and 10 kg.h-1 respectively. Perfectly spherical alumina particles with a particle size distribution of between 10 and 200 nm were produced on a continuous system. Particle size analysis for the silica by SEM measurements showed a d50 of 60 nm and a particle size distribution of 30-100 nm. The specific surface area was in the range of 170 m2.g-1. Other samples of plasma produced nanoparticles such ZnO and B4C will also be presented. <![CDATA[<b>The plasma manufacturing of titania pigment and nano-titania in a pilot plant</b>]]> SYNOPSIS The establishment of a pilot plant for the continuous production of titania pigment and nano-titania is discussed. The plant was designed, constructed and operated on the Necsa site. The TiO2 production capacity is 7-10 kg.h-1 by the reaction of TiCl4 and O2 at >1000°C. Liquid TiCl4 is sprayed under pressure into a plasma reactor where it evaporates and reacts with O2 to form TiO2. Process optimization has allowed for continuous production without any clogging or blockages. The primary advantage of the plasmaassisted process is the possibility to recover and recycle the chlorine values in high yield as Cl2 for use in the manufacture of TiCl4. The HCl formed in most conventional fossil-fuelled processes makes Cl2 recovery and recycling much more difficult. Particle size distribution analysis of the fine, white product revealed a d50 = 125 nm. The raw product consisted of 60% rutile that could be increased to 80-100% rutile by post-annealing. The specific surface area (SSA) of the pigment was 8-20 m²·g-1(BET). According to the CIE colour index the 'whiteness' (L*>95) of the plasma-produced pigment was better than that of several commercially available pigments. The particle size could be manipulated by controlling the TiCl4 feed rate, gas flow rates, and the particle quench rate and residence time. Nano-sized TiO2 powders with a particles size (d50) as small as 50 nm were also successfully produced in this way. <![CDATA[<b>Plan compliance process for underground coal mines</b>]]> SYNOPSIS This paper presents a unique method of monitoring the effectiveness of the planning process as well as the execution of mine plans. The method primarily determines the degree of deviation from the original mine plans by comparing actual mined areas to initially planned mining areas. The performance of mining operations is also measured by comparing actual and planned tonnages for a particular period. Once the degree of deviation from mine plans has been established the cause/s of the deviation can be determined through a simple fault tree analysis. Accurate identification of the reasons of deviation can aid the mine in selecting possible methods to correct the deviation. It is, however, beyond the scope of this paper to discuss the methods employed in correcting such deviations, as each mining operation will have unique causes of plan deviation and methods of remedying them. Deviations from the plan greatly affect the yields obtainable, which in turn affects the life of mine, thus having negative economic and financial implications. Accordingly, this paper presents a project management method which aims to reduce risk by improving planning systems to ensure optimum overall extraction of coal reserves.