The Influence of Integrated Gravity Circuit on the Efficiency of Gold Extraction at a Carbon-in-Leach Plant

In gold ores, the precious metal particles may occur as nuggets (> 0.5 mm) and down to sub-microscopic particles. Coarse particles are generally recovered by gravity concentration before leaching the bulk material with sodium cyanide to enhance leaching efficiency. A mine in West Africa operates a carbon-in-leach (CIL) plant where a gravity circuit has recently been installed. The purpose of this study was to investigate the effect of the gravity-integrated circuit on the overall efficiency of metallurgical operations in the CIL plant. In this investigation, a quantitative research approach was selected to determine the variables in analysing the influence of the integrated gravity circuit on the efficiency of gold extraction. The general plant efficiencies before and after the installation of the Gravity Recoverable Gold (GRG) circuit were investigated, with a focus on the milling circuit, CIL circuit, and reagent consumption in the various circuits. A model was constructed using multiple linear regression analysis, and the relationship between the variables was determined. From the results, the mill's throughput increased from 13.5 million tonnes per year to 13.9 million tonnes per year after the installation of the GRG circuit. In models 1 and 2, all coefficient p-values were less than the 5% significance level chosen for the study. Cost-benefit analysis of reagent use before and after gravity installation showed that the plant consumption of reagents decreased from 43 264 – 36 481 tonnes, 13 144 – 10 141 tonnes, 1 779 – 1 538 tonnes, 3 208 – 1 551 tonnes, 9 274 – 8 045 tonnes for lime, sodium cyanide, activated carbon, hydrochloric acid and caustic soda respectively. Again, the GRG circuit dramatically reduced the gold loadings onto activated carbon with an overall reduction in tailing grade and increased the gold recovery rate and purity by 1% and 2%, respectively. Overall, the mine's annual ore processing capacity increased by 2.34 %. Therefore, installing additional Knelson concentrators (gravity units) can be key to addressing the excess gravity-recoverable gold suspected to exist in the circuit with continuous checks and balances performed

Oxidation Behaviour and Bio-oxidation of Gold-bearing Sulphide Ores: Oxygen Capabilities and Challenges

The paper presents an overview of bio-oxidation of sulphidic refractory gold ores prior to gold cyanidation. This review discusses several factors, more importantly, oxygen requirements and oxygen limitations in biological oxidation of various sulphide minerals associated with gold ores. The availability of sufficient oxygen in bio-oxidation systems can speed up and enhance the oxidation of sulphide minerals, allowing the gold to be liberated for further extraction. However, oxygen supply and its low solubility in water have been the major limiting factors in bio-oxidation processes. More importantly, oxygen limitations are found to affect the rate of sulphide oxidation and the volume of materials that can be treated for gold leaching. First, the paper discusses the influence of dissolved oxygen on the oxidation behaviour of various sulphide minerals found in refractory gold ores or concentrates. Further discussed are the limiting factors in relation to dissolved oxygen during bio-oxidation of refractory gold ores. This review demonstrates that oxygen availability is a major challenge and therefore, oxygen enhancement techniques or strategies are vitally needed. As well, the review serves to inspire new research into efficient strategies to enhance oxygen availability

Fungal-Transformation of Surrogate Sulphides and Carbonaceous Matter in Refractory Gold Ores: Revisited

In the recovery of gold from refractory gold ores, pretreatment is required to decompose sulphides and liberate occluded gold before cyanidation, and to deactivate carbonaceous matter and prevent it from adsorbing dissolved gold. Until the past three decades, most commercial pretreatment processes had been by abiotic means. Biological pretreatment methods on commercial basis is therefore a recent phenomenon, and several researches are underway to assess the ability of different biological agents in the breakdown of sulphur and carbonaceous matter (CM) in gold ores. This paper, which is a revisit of an earlier one, presents an overview of on-going research aimed at assessing the capability of the fungus, Phanerochaete chrysosporium, to degrade sulphides and CM. Surrogate carbonaceous materials (lignite, bituminous and anthracite coals) and pure sulphides (pyrite and arsenopyrite) were used to model the behavior of CM and sulphides in refractory gold ores. To monitor the extent of biotransformation, preg-robbing test was conducted on the as-received and treated CM, and sulphide sulphur in the residual sulphidic materials was also determined. From an initial preg-robbing effect of 125 µg of gold per gram of CM, the ability of CM to preg-rob gold reduced by 70-95% in the order of lignite<bituminous <anthracite within 21 days of treatment, whereas there were 18% and 39% oxidation of sulphide sulphur in pyrite and arsenopyrite respectively. XRD examination of the treated anthracite confirmed reduction in the graphitic structure of carbon following fungal transformation. Similarly, there was a decline in the major sulphide peak after microbial pretreatment. The results indicate that the fungus biotransforms through destruction of the ordered structure, followed by introduction of oxygen groups. The amorphous nature, thus generated, inhibits the uptake of aurocyanide ions by CM, while enhancing the affinity of cyanide for the oxidised sulphide material in subsequent cyanidation treatment. The findings contribute to knowledge on novel and technically viable alternative methods for oxidative pretreatment of refractory gold ores. Keywords: Phanerochaete Chrysosporium, Carbonaceous Matter, Metal Sulphides, Biotransformatio

Geometallurgical Studies on Gold Ore for Enhanced Comminution and Leaching

Many gold processing plants are experiencing challenges as mining pits are becoming deeper, rocks are getting harder and more complex polymetallic and refractory ores are being encountered. The variations in the characteristics of ores lead to deviations from the established parameters, and these affect gold extraction efficiency. This paper presents a study where geological characteristics of the ore types from some mining pits were used to ascertain the influence of ore blends on improving the performances of comminution and leaching circuits. To achieve this, mineralogical, comminution, gravity recoverable gold and leaching investigations were conducted on fresh and weathered ore samples and their blends. Mineralogical study showed that the main rock types associated with the mine pits were dolerite, phyllites, conglomerates and sandstone. The dominant minerals were quartz, plagioclase, with traces of pyrites. The Crushability Work Indices of the rocks were between 30 and 37 KWh/t, which are generally higher than the maximum design value of 31.9 kWh/t, and this situation will pose throughput challenges in that section. The Bond Ball Mill Work Indices of the blends tested were between 16.4 kWh/t and 9.6 kWh/t and a blend ratio of 85% fresh and 15% weathered was found to have a Bond Ball Work Index almost equal to the design value of 14 kWh/t. With gold assays of 2.5 g/t for dolerite, 2.1 g/t for phyllite, 3.7 g/t for sandstone and 3.4 g/t for conglomerate, the gravity recoverable gold was in the order of sandstone 36% > phyllite (31.5%) > dolerite (29.5%) > conglomerate (18%). The overall gold recoveries were in the sequence of conglomerate (95%), sandstone (94%), phyllite (92%) and dolerite (87%). This information could be utilised in developing a proactive plant operations strategies for an operating plant in order to ultimately manage the plant and enhance achievement of set targets. Keywords: Geometallurgy; Ore Blends; Characterisation; Communition Circuit Performance; Gold Recover

Enhancing food security in Northern Ghana through smallholder small ruminant production and marketing

Key words Livestock markets, technical and institutional constraints, innovation systems, veterinary services; smallholder farmers; structural adjustment, scaling out, co-learning, supplementary feeding, herd growth, food security, positive deviants, commercialization, Ghana. Smallholder livestock production systems in sub-Saharan Africa and other developing countries are very important because a large share of the rural poor keep livestock, livestock can contribute to improved soil fertility and household food security, and there is rapid growth in demand and market for livestock products. As elsewhere, almost 80% of smallholder farmers in Northern Ghana keep livestock, especially small ruminants (i.e. sheep and goats). The high demand for livestock products at the national level remains unexploited by the farmers. This thesis sought to examine 1) the salient technical and institutional constraints that hinder innovation with respect to improved production and market participation of smallholder small ruminant farmers in Lawra and Nadowli districts in Northern Ghana, and 2) how previous interventions and farmers themselves sought to address the constraints. The thesis proceeded from a broad diagnosis of the technical and institutional constraints to market participation. Then, detailed empirical studies were conducted on the emergent issues. Three main constraints were prioritized by farmers i.e. water shortage during dry season, high mortality and theft of livestock. The corresponding institutional limitations include weak structure of veterinary services delivery, and weak traditional and formal justice delivery structures. As a result of the constraints, a majority of farmers keep livestock to support crop production and consequently invest minimally in animal husbandry. Few farmers changed from minimal to moderate investment as a result of accumulated experiences and recorded significant herd growth, but then, did not enlarge the volume of their market share. The main conclusion was that household food security is the principal driver of smallholder small ruminant production and that market demand is only a marginal driver. The thesis also showed that self-organization of smallholder producers and the support systems in which they are embedded was quite effective in response to the different institutional constraints that these face, but that self-organisation has not been captured and built upon in interventions. Furthermore, farmers select or generate intervention elements that were useful to resolving the constraints they cared about. However, interventionists were often inflexible in the planning and implementation of projects and paid little attention to feedback and changes in the broader social and institutional surroundings. Among other things, the thesis recommended a more inclusive approach to livestock development and interventions to resolve the broader social and institutional conditions that hem in smallholder farmers to uncompetitive husbandry practices.</p

Teachers’ Concerns Towards the Implementation of ICT Curriculum in Basic Schools in Kumasi Metropolis

The study sought to assess concerns of teachers regarding the implementation of ICT curriculum in basic schools in Kumasi metropolis. It also examined the relationship between gender and teachers’ stages of concerns. A multi-stage sampling procedure was adopted in selecting the sample for the study. The Stages of Concern Questionnaire was modified and used to gather data from 181 male and 165 female ICT teachers. Both descriptive and inferential statistics were used in analysing the data. Results showed that teachers generally had high consequence and informational concerns and low concern at awareness stage. This indicates that teachers were more bothered on the impact of the ICT curriculum on their students and also lacked requisite knowledge and skills in implementing the curriculum. Moreover, the independent sample t-test conducted revealed statistically significant relationship between gender and informational, management, consequence, collaboration and refocusing concerns. It was recommended among others that GES should embark on rigorous educational programmes in order to upgrade teachers’ knowledge and skills on the ICT curriculum. Keywords: Basic school teachers, Gender, ICT curriculum, Teachers’ Concern

A parallel hybrid modular multilevel converter for high voltage DC applications

Reliability and efficiency of power transmission has been at the forefront of research for some time and is currently being given critical consideration due to the increased dependence on electrical energy. With the increased demand for electricity, engineers are considering different methods of supply arrangement to improve the security of electricity supply. High Voltage Direct Current (HVDC) transmission is a technology that avails itself for distance power transmission, interconnection of asynchronous networks and cross sea or offshore power transmission. The main element of an HVDC system is the AC/DC or DC/AC power converter. Recently, a new breed of power converters suitable for HVDC transmission has been the subject of considerable research work. These converters are modular in structure with high efficiency and their operation results in higher power quality, with reduced filtering components when compared to the use of Line Commutated and two-level or three-level Voltage Source Converter (VSC) based transmission systems. One such modular circuit is the Parallel Hybrid Modular Multilevel Voltage Source Converter (PH-M2L-VSC). This research investigates the operation and control of the PH-M2L-VSC for HVDC applications. Control schemes supporting the operation of the converter as would be expected of an HVDC VSC are proposed, including operation with an unbalanced AC network. Simulation results from a medium voltage demonstrator and experimental results from a laboratory scale prototype are presented to validate the methods proposed and enable a performance comparison to be made with other topologies

Detoxification of Cyanide Wastewater by Cyanotrophic Organisms: the case of Phanerochaete chrysosporium

Abstract Cyanide, a carbon-nitrogen radical, is a major building block in many industries including pharmaceuticals, petrochemical and gold processing. In the gold extraction industry, cyanide has been the universal lixiviant for over a century due to better understood process chemistry, among others. Industries that discharge cyanide-laden effluents are mandated to keep concentrations below 0.2 mg/L to prevent death by cyanide-intoxification, which occurs when cyanide binds to key iron-containing enzymes and prevent them from supplying oxygen-containing blood to the tissues. Techniques used to attenuate cyanide in wastewater can broadly be grouped into chemical, physical and biological methods.&nbsp; In recent times, attention has been placed on biotechnological methods, which make use of cyanotrophic microorganisms to clean up cyanide-contaminated environments. This paper reports on studies set out to assess the ability of Phanerochaete chrysosporium to degrade cyanide under different conditions including changes in cyanide concentration, culture mass, time, closed system and open system. At the end of 24-hour contact in an open agitated system with initial pH of 11.5, a control experiment using 100 mg/L cyanide revealed a natural attenuation of 15% with pH decreasing to 9.88, while the best myco-detoxification of 85% was achieved by contacting 100 mg/L cyanide with 0.5 g culture mass, translating into degradation capacity of 17.2 mg/g (milligram of cyanide per gram of culture) with pH reducing to 8.4 in 24 hours. The degradation could be based on a number of mechanisms including hydrolysis to HCN, oxidation to cyanyl radical and cyanate due to natural attenuation through atmospheric contact, and secretion of organic acid, oxidative enzymes, and hydrogen peroxide by the fungus. &nbsp; Keywords: Cyanotrophic Organism, Myco-Detoxification, Cyanide-Laden Effluents, p

Design optimization of a short-term duty electrical machine for extreme environment

This paper presents design optimisation of a short term duty electrical machine for extreme environments of high temperature and high altitudes. For such extreme environmental conditions of above 80⁰C and altitudes of 30km, thermal loading limits are a critical consideration in machines, especially if high power density and high efficiency are to be achieved. The influence of different material on the performance of such machines is investigated. Also the effect of different slot and pole combinations are studied for machines used for such extreme operating conditions but with short duty. In the research, A Non-dominated Sorting Genetic Algorithm (NSGAII) considering an analytical electromagnetic model, structural and thermal model together with Finite Element (FE) methods are used to optimise the design of the machine for such environments achieving high efficiencies and high power density with relatively minimal computational time. The adopted thermal model is then validated through experiments and then implemented within the Genetic Algorithm (GA). It is shown that, generally, the designs are thermally limited where the pole numbers are limited by volt-amps drawn from the converter. The design consisting of a high slot number allows for improving the current loading and thus, significant weight reduction can be achieved

Metal Sorption Capabilities of Two Common Plants in Tropical Wetlands - Bambusa vulgaris and Raffia bambusa

In most developing countries, careless disposal of waste dry cell batteries and Ni-Cd batteries from mobile phones and rechargeable lamps has led to an increase in some metal contaminants including copper, lead, nickel and cadmium in soils and water bodies. These metals have the potential to cause serious, and sometimes, irreversible health effects if they are consumed. Unfortunately, in most cases, either the water bodies are not treated prior to consumption or the water treatment methods do not target the removal of these metals. This study therefore sought to mimic the bio-filtering effect of wetlands by assessing the capabilities of two common plants in tropical wetlands; Raffia bambusa and Bambusa vulgaris to remove Cu2+, Pb2+, Ni2+ and Cd2+ from wastewater. Spectroscopic characterisation indicated that some of the functional groups on the biomass were the OH, C=O and COOH, and R. bambusa appeared to have more pronounced groups than B. vulgaris. The results show maximum sorption of 95% Cu and Pb by biomass derived from roots of these plants. Sorption of Cd and Ni were lower at about 40%. By using a 3-stage sorption system of live plants, sorption of Cd and Ni increased up to 80%. Keywords: Heavy Metals, Sorption, Biomass, Bambusa vulgaris, Raffia bambus