R.K. Amankwah

Improved grindability and gold liberation by microwave pretreatment of a free-milling gold ore

Abstract The gravity concentration of gold is very efficient if the metal is fully liberated and the particle size of the gold is relatively coarse. Liberation is usually achieved by comminution, but due to the association of the gold with the other minerals in the ore, overgrinding occurs in conventional comminution circuits and slime generation leads to inefficient recovery. The liberation of minerals can be improved by adding grinding aids, which modify the mechanical properties of the ore and allow breakage at lower stress levels. In this research, microwave pretreatment was used to augment the grinding of a free-milling gold ore containing quartz, silicates and iron oxides. Under microwave irradiation, selective heating of the different mineral components resulted in thermal stresses that caused cracking. These intergranular and transgranular fractures were confirmed by scanning electron microscopy. After microwave processing, the grindability of the ore was improved and the crushing strength and the Bond Work Index were reduced by 31˙2% and 18˙5%, respectively. In addition to the enhanced grindability, gold was released from the matrix of the host minerals at a coarser size, resulting in a significant increase in free gold recovery by gravity concentration. For a gold ore with a head grade of 6˙4 gt−1, the gold recovery improved from about 28% to 40% after microwave pretreatment.

Going for gold: transitional livelihoods in Northern Ghana

This article critically reflects on what impact a supported and formalized artisanal and small-scale mining (ASM) sector could have in Northern Ghana, where poverty is deeply-rooted, the outcome of decades of government neglect. Since country independence in 1957, numerous attempts have been made to improve the living standards of the populations in the country’s North but deteriorated human resource bases and shortages of infrastructure have limited their effectiveness. A recent upsurge in ASM activity, however, has catapulted the region on to another – previously unimaginable – growth trajectory entirely. As findings from research carried out in the township of Kui in Bole District of the country’s Northern Region illustrate, ASM has injected considerable wealth into many of Ghana’s Northern localities, in the process, helping to stabilize their economies and in the process, alleviating the hardships of tens of thousands of farm-dependent families. The intensification of support to, and the formalization of, ASM, could prove to be an important step toward eradicating a poverty problem that has plagued this region of sub-Saharan Africa for more than a century.

The application of direct smelting of gold concentrates as an alternative to mercury amalgamation in small-scale gold mining operations in Ghana

Mercury is used in small-scale mining to amalgamate gold particles, facilitating their separation from heavy sands. The negative environmental/health-related effects of mercury in mining communities in Ghana and other countries have generated research interest into development of safer alternatives. This study tested direct smelting as an alternative to amalgamation. In laboratory investigations, direct smelting yielded 99.8% recovery against 97% for amalgamation. A locally-fabricated furnace, sika bukyia, was used in field tests, yielding an average recovery of 98.3% compared to 88% for amalgamation. Direct smelting has the potential to replace amalgamation and retorting because it is effective, easy, quick and transparent.

The identification and testing of a method for mercury-free gold processing for artisanal and small-scale gold miners in Ghana

A study of the ASM sector of Ghana established the practices and attitudes of gold miners and proposed an alternative to mercury amalgamation. The study showed that miners were aware of health hazards associated with mercury but continued to use it because they knew of no credible alternative. It was realised that any process to replace amalgamation must be very efficient to capture gold around 50-100 μm. To be acceptable to the miners the method must be: easy, quick, cheap, transparent and suitable for processing small batches of concentrate. Direct smelting was selected as the technique of choice.

Microwave calcination and sintering of manganese carbonate ore

Abstract Manganese carbonate ores are becoming an increasingly important source of manganese. Usually these ores undergo thermal pretreatment which involves calcination and agglomeration using fossil fuels as the major energy source. In this study, the application of microwave energy for the pretreatment step was investigated. The real and imaginary permittivities of a material determine its microwave absorption characteristics and both the permittivities of the ore were relatively low but increased rapidly with temperatures above about 500 °C. A significant peak was observed during the calcination process and this was attributed to the migration of ions in the lattice. The degree of microwave absorption and thus the sample temperature increased with increasing processing time, microwave power and sample mass. Under the optimum microwave conditions, sintering temperatures of over 1500 °C could be achieved. The calcine was a relatively good microwave absorber and the addition of a small percentage to the feed resulted in improved microwave coupling. The average uniaxial compressive strength of the sintered material was 1.75 MPa and the tumbler index was about 90. Les minerais de carbonate de manganèse sont une source de plus en plus importante de manganèse. Habituellement, ces minerais subissent un prétraitement thermique qui implique la calcination et l’agglomération en utilisant des combustibles fossiles comme majeure source d’énergie. Dans cette étude, on a étudié l’application d’énergie par micro-ondes pour l’étape de prétraitement. Les permittivités réelle et imaginaire d’un matériau déterminent ses caractéristiques d’absorption des micro-ondes. Les deux permittivités du minerai étaient relativement basses, mais augmentaient rapidement avec des températures au-dessus d’environ 500 °C. On a observé un pic important lors du procédé de calcination que l’on a attribué à la migration d’ions dans le réseau. Le degré d’absorption des micro-ondes, et ainsi la température de l’échantillon, augmentait avec une augmentation du temps de traitement, de la puissance des micro-ondes et de la masse de l’échantillon. Sous les conditions optimales des micro-ondes, on pouvait obtenir des températures de frittage de plus de 1500 °C. Le calcinat était un absorbeur de micro-ondes relativement bon et l’addition d’un petit pourcentage à la charge a résulté en un couplage amélioré des micro-ondes. La résistance moyenne à la compression uniaxiale du matériau fritté était de 1.75 MPa et l’index du “gobelet” était d’environ 90.

Contours of Risk: Spatializing Human Behaviors to Understand Disease Dynamics in Changing Landscapes

We echo viewpoints presented in recent publications from EcoHealth and other journals arguing for the need to understand linkages between human health, disease ecology, and landscape change. We underscore the importance of incorporating spatialities of human behaviors and perceptions in such analyses to further understandings of socio–ecological interactions mediating human health. We use Buruli ulcer, an emerging necrotizing skin infection and serious health concern in central Ghana, to illustrate our argument.

Assessing the Parameters for Optimum Biotransformation of Carbonaceous Matter by Phanerochaete chrysosporium

Refractory carbonaceous gold ores pose challenges during cyanidation of gold due to the presence of carbonaceous matter (CM) which preg-robs dissolved gold, and this results in a decreased recovery. In an on-going study, the fungus, Phanerochaete chrysosporium has been used to deactivate CM and decrease its ability to preg-rob aurocyanide. P. chrysosporium has been reported to biotransform CM by surface oxidation, which leads to disruption of the continuous graphitic structure necessary for adsorption, and alteration of the pores suitable for aurocyanide adsorption. The present study focused on using anthracite coal as surrogate for CM, and establishing fungal-treatment parameters that bring about maximum decrease in preg-robbing by the anthracite-grade CM. The results indicate that P. chrysosporium can survive in several growth media, and possesses the ability to deactivate anthracite in a wide range of time, pulp density, temperature, pH, and level of agitation. A processing time of 5-7 days at pH 4 and 37°C gave the best conditions. The best pulp densities for stationary and shake culturing respectively were 60% and 25%.

The effect of comminution equipment on gravity gold recovery in small-scale mining operations

As a strategy to improve gold recovery in the small-scale mining industry of Ghana, comminution equipment utilised in grinding was investigated. Morphological studies show that gold particles liberated by hammer mills were globular, disc mills produced cigar-shaped particles, and stamp mills created flaky units. Up to 5% of the gold particles liberated using a disc mill were coated with iron due to smears from the grinding surfaces. Based on shape-factors, the efficiency of subsequent gravity concentration operations increased in the order of flakes < cigars < spheres. A proposed flowsheet increased the gold extraction by over 15%.

Utilization of Microwave Energy in the Calcination and Agglomeration of Nsuta Manganese Carbonate Ore

The application of microwave energy in the calcination and agglomeration of manganese carbonate ore from Nsuta, Ghana, has been investigated. The real and imaginary permittivities, which may be used to determine a substances response to microwave heating, were measured and the permittivities of both the ore and calcine were relatively low at low temperatures. Beyond 600oC, both the real and imaginary permittivities increased more rapidly with that of the calcine reaching relatively high values. The degree of microwave absorption and thus the temperature attained by the samples during microwave heating increased with sample mass, processing time and incident microwave power. Processing temperatures of over 1500oC could be attained under optimum microwave coupling. Calcination and agglomeration tests were performed and the calcine obtained readily absorbed microwaves. The tumbler index of the agglomerated material was 90 and the average crushing strength was 2550 KN. Ghana Mining Journal Vol. 9 2007: pp. 54-61