Scielo RSS <![CDATA[Journal of the Southern African Institute of Mining and Metallurgy]]> vol. 115 num. 12 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<b>The School of Chemical and Metallurgical Engineering at the University of the Witwatersrand</b>]]> <![CDATA[<b>A right to knowledge</b>]]> <![CDATA[<b>Removal of heavy metals using cassava peel waste biomass in a multi-stage countercurrent batch operation</b>]]> This paper presents a study of the removal of cobalt (Co2+), chromium (Cr³+), and vanadium (V³+) from synthetic effluent solution using cassava waste biomass. Test work was carried out in a multi-stage countercurrent batch system. Single and ternary metal ion systems were studied. A feed inlet concentration of 100 mg/L for each metal ion system was contacted with 0.5 g of cassava waste biomass. The target concentration in the final outlet stream was set against the South African Department of Water and Forestry (DWAF) standards. The results showed that the adsorption capacity was slightly lower for ternary metal ion systems than for single metal ion systems. This was attributed to the greater competition among the metal ions for the occupancy of the binding surfaces on the cassava waste in the ternary systems. Eight adsorption stages were required to meet the targeted limit of 0.5 mg/L for Co2+set by the DWAF. The Cr3+system needed six stages to obtain the targeted limit of 0.1 mg/L, while the V³+ system required four stages to attain the target limit of 0.2 mg/L. In general, cassava waste biomass adsorbed the metal ions in the following order: V3+> Cr3+> Co2+. <![CDATA[<b>Optimization of complex integrated water and membrane network systems</b>]]> Water and energy are key resources in the process and mining industries. Increasing environmental and social pressures have made it necessary to develop processes that minimize the consumption of both these resources. This work considers the synthesis and optimization of water networks through partial treatment of water (regeneration) before recycle/re-use. Two types of membrane regenerators are considered, namely electro-dialysis and reverse osmosis. For each of the membrane regenerators, a detailed design model is developed and incorporated into the water network model in order to minimize water and energy consumption, and operating and capital costs. This represents a rigorous design and accurate cost representation as compared to the 'black-box' approach. The presence of continuous and integer variables, as well as nonlinear constraints, renders the problem a mixed integer nonlinear programming (MINLP) problem. Four cases are presented. The first case looks at the incorporation of multiple electrodialysis regenerators with single contaminant streams within a water network (WN), while the second considers the multiple contaminant scenario. Case 3 examines the incorporation of a reverse osmosis network superstructure within a WN, and case 4 looks at both electrodialysis and reverse osmosis membranes. The developed models are applied to a pulp and paper and a petroleum case study to demonstrate their applicability, assuming both a single and multiple contaminant scenario. The model was solved in GAMS using BARON and DICOPT. The results indicate a wastewater reduction of up to 80% and savings of up to 44% in fresh water intake, 82% in energy, and 45% in the total annualized cost. <![CDATA[<b>The impact of coal quality on the efficiency of a spreader stoker boiler</b>]]> This research establishes the combustion characteristics and efficiencies of South African coals of different qualities and their impact on the performance of a grate spreader stoker boiler. Four different coal samples were tested in the particle size range 6.25 x 25 mm. A detailed investigation involving the boiler operating conditions associated with the physicochemical characteristics of the coals, petrographic properties, and temperature profiles from a thermal camera was conducted. The thermal analysis indicates that there is a strong correlation between thermographic data (combustion behaviour and maximum flame temperatures) and petrographic composition of the coals. This association is not reflected in calorific values and proximate analyses of the coals. In terms of combustion efficiencies, all coals yielded relatively high amounts of unburnt carbon in the fly ash (about 36.90%). The highest steam output obtained was 41.76 t/h at the highest combustion efficiency of 79.13%. The thermographic results obtained from this study led to the conclusion that South African low-grade Gondwana coals undergo delayed ignition and burn at unusually high temperatures (1500-1800°C), which is in contrast to the original belief that the combustion temperature is around 1400°C. <![CDATA[<b>Coal quality and uranium distribution in Springbok Flats Coalfield samples</b>]]> The presence of coal in the Springbok Flats Coalfield (SFC) has been known since the beginning of the 1900s. However, the SFC has not been mined to any degree of economic profit, mostly due to the presence of uranium in the coal. Five boreholes were drilled in the SFC (BH1 to BH5); BH5 intersected two coal zones, the other boreholes intersected one coal zone. Coal samples were collected, selected, and characterized using proximate, ultimate, and calorific value analyses. X-ray fluorescence, instrumental neutron activation analysis, and inductively coupled plasma mass spectrometry were used to determine uranium content. The BH1 intersection and the upper coal zone in BH5 had ash contents higher than 50% and were considered to be primarily carbonaceous shale. BH2 was observed to have better coal quality, resembling typical South African bituminous coal used in local electricity generation. The highest uranium content was found in BH3 (up to 199 mg kg-1, followed by BH2 and BH1. BH4, the upper coal zone in BH5, and the lower coal zone in BH5 all had uranium contents averaging less than 10 mg kg-1. Uranium in the SFC samples was found both in the coal and carbonaceous shale. For all boreholes except BH5, uranium is concentrated within the uppermost 1 m of the coal zone. X-ray fluorescence was the preferred analytical technique since the analysis gave consistent results that compared well with instrumental neutron activation analysis results. <![CDATA[<b>Synthesis of sodium silicate from South African coal fly ash and its use as an extender in oil well cement applications</b>]]> In this work, the use of sodium silicate derived from South African coal fly ash (CFA) in oil well cement (OWC) applications is reported. Silica (SiO2) was extracted from the CFA and used to synthesize CFA-derived sodium silicate (CFA-Na2SiO3), a typical OWC slurry extender. The physico-chemical properties of the CFA-Na2SiO3were compared to those of a commercial sodium silicate (com-Na2SiO3) using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. OWC slurries with varying proportions of cement, distilled water, and 2% CaCl2 by weight of water (BWOW) were prepared and extended using the CFA-Na2SiO3and the com-Na2SiO3at compositions ranging from 0.25-2.5% by weight of cement (BWOC). Rheological properties of the slurries were evaluated using American Petroleum Institute procedures and compared. The physico-chemical properties of the CFA-Na2SiO3are consistent with those of com-Na2SiO3, indicating the purity of the CFA-Na2SiO3. A comparative study of the OWC slurries indicated that the slurries extended with CFA-Na2SiO3have slightly lower densities, lower viscosities, and higher compressive strength than those extended with com-Na2SiO3. This indicates that CFA-Na2SiO3slurries would be easier to pump and preferable where early strength development is critical. This report could be instrumental in providing a way for the beneficiation of South African CFA in the petroleum, oil, and gas industry. <![CDATA[<b>Mechanical behaviour of pack carburized AISI 316L austenitic stainless steel</b>]]> The effect of surface hardening by pack carburizing on the mechanical properties of AISI 316L steel was studied. Pack carburizing with 60 wt% BaCO3, 30 wt% activated carbon, and 10 wt% sodium chloride was carried out at 450, 550, 650, 700, and 750°C for 24 hours and the specimens were furnace-cooled. Tensile, impact, hardness, and fatigue tests were conducted, and SEM and was used to characterize the specimens. The ultimate tensile strength of the as-received steel was similar to specimens carburized at 450 and 550°C (approx. 650 MPa), but decreased from 638 to 603 MPa with further increase in carburizing temperature from 650 to 750°C. Hardness profiles of the specimens treated at 450°C and 550°C were similar to the as-received steel, at approximately 250 HV0.5. Typical case hardening profiles were obtained for specimens carburized at 650, 700, and 750°C. The number of cycles to failure (N) of the as-received specimens was close to 60 000, which was similar to those carburized from 450 to 650°C. Further increase in carburizing temperature decreased the cycles to failure to approximately 25 000 (700°C) and 7000 cycles (750°C). Crack initiation was mainly characterized by cleavage (mode I) for all tested carburized and as-received specimens. Specimens carburized at 450, 550, and 650°C also showed secondary cracking. The final rupture zone contained ductile fracture with dimples, and the specimens showed extensive plastic deformation. <![CDATA[<b>Making sense of our mining wastes: Removal of heavy metals from AMD using sulphidation media derived from waste gypsum</b>]]> This study investigates the recovery of water and the selective removal of valuable metals from acid mine drainage (AMD) using sulphidation media (CaS) derived from waste gypsum. AMD systems containing Fe(II), Ni, Co, Zn, and Pb were investigated using CaS produced from the carbothermal reduction of Anglo Coal waste gypsum at 1025°C to precipitate metals as insoluble metal sulphides. The results show a sulphidation dependence on the pH, sulphide dosage, and metal concentration. The selective sulphi-dation of metals also showed significant dependence on the respective metal sulphide solubility order as a function of pH. According to the Department of Environmental Affairs' South African Waste Information Centre, over 42 million cubic metres of general waste is generated every year in South Africa and mining waste is by far the biggest contributor to the solid waste (about 72%)). Although alarming, these vast quantities of waste also present an opportunity for integrated economic development, particularly in the recycling sector. The major argument has always been that the mining sector generates a large number of waste streams which show strong differences in time, in their treatment methodologies, or even in their spatial distribution. This paper presents a case of a simple strategy for integrated recycling of two mining waste streams and highlights the need for the mining industry to break away from the traditional 'linear' cul-de-sac disposal of wastes and think of new sustainable ways of waste management. <![CDATA[<b>Enhancing study practices: Are first-year students ‘resistant to change'?</b>]]> One of the strategies for trying to reduce attrition among first-year students and for improving their academic performance generally is to include some kind of study skills module in the first-year programme. One of the reasons often given for the relative lack of success of such programmes is the claim that students are 'resistant to change'. This paper presents a study that investigated this claim by interviewing chemical and metallurgical engineering students in a South African university at the beginning and end of their first year. The basis for evaluating the extent to which students' practices appeared to change was a set of six categories of practice identified in a related phenomenographic study on the learning practices of the same students. It was evident from the interview data that even where some change in practice had occurred, the extent of change was somewhat disappointing. For those who reported changing their practice, the primary change driver appeared to be underperformance in the mid-year exam. Underperformance prior to that seemed to exert less force and students did not appear to give very serious attention to class or textual input/activities on study practices. 'Resistance to change' appeared to be implicit in nature and to be more a consequence of overconfidence and the 'momentum' resulting from habit rather than an explicit attitudinal resistance. <![CDATA[<b>Recycling of cemented tungsten carbide mining tool scrap</b>]]> The zinc recycling process (PRZ) and acetic acid leaching (AC) were successfully employed to recycle cemented tungsten carbide mining tool scrap for re-use as production powders. The main success of the PRZ process was that it produced powders which were suitable for manufacturing sintered alloys having properties within the commercial benchmark ranges for WC-6 wt% Co mining-grade tools. Although the powders produced from the AC process were deemed unsuitable for manufacturing the same grade of mining tools, the process cannot be viewed as a failure, since the recycled powders are suitable for different commercial applications. The two recycling processes can therefore be used as complementary processes on an industrial scale. <![CDATA[<b>Titanium carbide-silicon nitride reactions at high temperature</b>]]> The kinetics and mechanism of the chemical interaction between silicon nitride and titanium carbide were investigated using thermogravimetric analysis (TGA). The samples were reacted isothermally at temperatures between 1600°C and 1700°C under nitrogen and argon atmospheres. The extent and rate of reaction increased with increasing temperature under both atmospheres; however, both the extent and rate were higher under argon. Silicon nitride, in the presence of titanium carbide, was thermally stable under nitrogen and the reactions were confined to Si3N4/TiC and TiC/N2 interfaces. Under argon atmosphere silicon nitride dissociated completely to liquid silicon and nitrogen gas within about four hours of reaction time, depending on temperature, and a different reaction mechanism prevailed. The kinetics of interaction between silicon nitride and titanium carbide under nitrogen atmosphere was found to be controlled by the rate of diffusion of nitrogen into the titanium carbide/carbonitride phase. Under argon atmosphere the rate was found to be controlled by the rate of dissociation of silicon nitride. For both cases the mechanisms of reactions were determined in detail and then modelled. <![CDATA[<b>Polyethersulphone-sodalite (PES-SOD) mixed-matrix membranes: Prospects for acid mine drainage (AMD) treatment</b>]]> This article presents the outcome of a preliminary investigation into the application of polyethersulphone (PES)-sodalite (SOD) mixed-matrix membranes for acid mine drainage (AMD) treatment. PES-SOD membranes loaded with different amounts of SOD particles were fabricated using the phase inversion method, and evaluated for AMD treatment. The morphology, phase purity, and surface properties of the SOD particles and the membrane were checked using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, respectively. In addition, the mechanical strength of the membranes was evaluated using a texture analyser. Separation performance (metal ion rejection) of the membranes and the effect of SOD loading on the membrane performance during AMD treatment were also studied. The cations in the AMD (feed stream) and the permeate stream were determined quantitatively using atomic absorption spectrophotometry (AAS). The results of the investigation reveal that mechanical strength (Young's modulus and tensile strength) of the membrane was enhanced at increasing SOD loading. In addition, the membrane flux increased at increasing SOD loadings and the selectivity of the membrane towards Mn2+, Pb2+, Cu2+, Al3+, and Mg2+also increased. The highest membrane rejection of 57.44% was recorded for Pb2+, and the membrane displayed a rejection of 6% towards Mn²+. All the PES-SOD membranes displayed better performance compared to an equivalent unloaded PES membrane. As far as we know, this is the first report on the application of PES-SOD mixed-matrix membranes to AMD treatment. However, optimization of the synthesis protocol and operational conditions is needed to improve the performance of the membrane. <![CDATA[<b>The accuracy of calcium-carbonate-based saturation indices in predicting the corrosivity of hot brackish water towards mild steel</b>]]> Industry has always relied on water's inherent ability to inhibit mild steel corrosion by virtue of its levels of calcium hardness and total alkalinity. This research seeks to verify the application of this principle to brackish water used in industrial systems at moderately elevated temperatures. A brief review is first given of the conventional calcium-carbonate-based scale or corrosion predictive indices. Laboratory corrosion tests were performed at various levels of calcium hardness and total alkalinity, resulting in the generation of an empirically derived nonlinear regression model. The newly developed model and the existing indices were then compared statistically in predicting the corrosivity of brackish water in contact with mild steel at 45°C. The accuracy, broader application, and relevance of the indices are also discussed. <![CDATA[<b>The effect of sodium carbonate on the dispersion behaviour and froth flotation of a nickel ore</b>]]> The effect of sodium carbonate on the flotation performance of a nickel ore was studied and the mechanism investigated. The flotation results show that lizardite minerals in the ore interfere with the flotation of pentlandite, and the addition of sodium carbonate improves pentlandite flotation recovery. The pulp pH did not change with increasing sodium carbonate dosage. Drawing on the literature in this area, combined with the sedimentation test results, sieving test results, and infrared spectra study, it is proposed that carbonate ions, derived from sodium carbonate, interact with lizardite slime and change the interparticle force from attraction to repulsion, resulting in the removal of adhering slimes from pentlandite surfaces. <![CDATA[<b>The influence of water quality on the flotation performance of complex sulphide ores: Case study at Hajar Mine, Morocco</b>]]> As part of the process optimization project of CMG (Mining Company of Guemassa-Marrakech, Morocco), a preliminary study on the effect of water quality on the flotation of galena, sphalerite, chalcopyrite, and pyrrhotite was carried out using asymmetrical fractional factorial design. The multivariable analysis showed that of ten studied factors, six (Cu2+, Zn2+, Mg2+, Ca2+, SO4(2-), and PAX) have a significant influence on the flotation of these sulphide minerals. Graphical analysis showed that high concentrations of Cu2+ (7-14 mg/L) in synthetic process water increased the recovery of galena, chalcopyrite, sphalerite, and pyrrhotite. At low Cu2+ concentrations (0-7 mg/L), sphalerite was depressed. Zn2+ at low concentrations (0-20 mg/L) decreased the recovery of all studied minerals. However, at high concentrations (20-40 mg/L), an increase in chalcopyrite, sphalerite, and pyrrhotite recoveries was observed. Mg2+ (100-200 mg/L) decreased the recovery of galena, chalcopyrite, and sphalerite. Ca2+(1200-2000 mg /L) depressed sphalerite flotation. Sulphate ions (SO4(2-)) enhanced recovery of all the studied minerals. Potassium amyl xanthate (PAX) promoted sphalerite recovery at high concentrations (10-20 mg/L). <![CDATA[<b>Development and optimization of mixed sulphide/oxide copper ore treatment at Kansanshi</b>]]> The Kansanshi Mine (First Quantum Minerals Ltd) treats a mixed sulphide/oxide copper-gold vein deposit. Until June 2009, the sulphide and oxide minerals were respectively recovered by the well established xanthate flotation of sulphides and acid leaching of soluble oxide copper. The mixed (transitional) sulphide/oxide ores were stockpiled in the absence of an economic processing route due to their poor flotation response and high gangue acid consumption. Since June 2009, these mixed ores have been treated by flotation using controlled potential sulphidization. This paper describes the development of the process and its optimization. The effect of the complex mineralogy on the flotation performance is also depicted.