John A. Meech

Enhanced dissolution of minerals: stored energy, amorphism and mechanical activation

A theoretical evaluation of the origin and role of stored energy in the phenomenon of mechanically-activated dissolution of finely milled minerals has been undertaken. The changing mechanical behaviour of minerals with decreasing particle size is examined, leading to the treatment of stored energy in terms of the generation and retention of crystalline imperfections (dislocations) during extended milling. Development of amorphism, and polymorphism, is treated as a direct consequence of decreased periodicity in the crystal lattice that is associated with an increased accumulation of dislocations. Quantitative estimates of stored energy are presented and used to quantify changes in dissolution kinetics in terms of a decreased activation energy of dissolution and hence, an increase in the relative rate of dissolution.

Fracture toughness and surface energies of minerals: theoretical estimates for oxides, sulphides, silicates and halides

Theoretical estimates of the ideal fracture toughness and surface energies of 48 minerals have been modelled by treating them as ionic solids, using the Born model of bonding. Development of the toughness model required calculation of the crystal binding enthalpy from thermodynamic data and the use of published elastic constants for single crystals. The principal minerals studied were oxides, sulphides and silicates, plus a few halides and sulphates. The study showed grain boundary fracture is most likely in singlephase polycrystalline minerals. However, the fracture toughness for grain boundary cracking in pure minerals is not significantly lower than that for intragranular cracking. The computed critical stress intensity values for intragranular cracking, KIC, ranged from 0.131 to 2.774 MPa m 1=2 . The critical energy release rates for intragranular cracking, GIC, ranged from 0.676 to 20.75 J m � 2 . The results are discussed with relevance to mineral comminution, including energy considerations, particle impact efficiency, and lower limiting particle size. 2002 Elsevier Science Ltd. All rights reserved.

Effect of thioligands on plant-Hg accumulation and volatilisation from mercury-contaminated mine tailings

This study investigated the effect of thioligands on mercury (Hg) volatilisation and plant accumulation for Brassica junceaplants grown in mine tailings collected from artisanal gold mines in Brazil (the Serra Pelada mine) and China (the Gold Mountain mine). Plants were treated with either (NH4)2S2O3or NH4SCN and enclosed in gas-tight volatilisation chambers. Elemental Hg released from substrates was captured in a two-trap system containing 5 KMnO4dissolved in 2N H2SO4. Mercury accumulation was enhanced in the presence of (NH4)2S2O3 for plants grown in GM tailings. There was no significant increase in the plant-Hg accumulation after application of NH4SCN to the SP tailings. Volatilisation from planted substrates was not affected by the application of thioligands to either GM or SP mine tailings. Mercury volatilisation from planted substrates was significantly higher than from control substrates. Abiotic (photoreduction) and biotic (microbial interactions) factors might be linked to the enhanced plant effect on Hg volatilisation. There was no significant correlation for the Hg mass released from substrates and the amount of Hg uptake by roots and translocated to shoots. Our results indicate that volatilisation and plant-Hg accumulation are two independent processes. Thiosulphate-induced plant-Hg accumulation may be a potential tool for the phytoextraction of Hg contaminated soils but there are risks of groundwater contamination by Hg-containing leachates.

Enhanced dissolution of minerals: Microtopography and mechanical activation

Abstract A theoretical evaluation of the role of microtopography in the phenomenon of mechanically activated dissolution of finely milled minerals has been undertaken. Attention was directed to the role of surface steps produced by fracture and abrasion of milled particles. It was estimated that the activation energy for dissolution at the edge of steps is at least 20% less than that for dissolution from terraces (flat regions) between the edges, leading to enhanced dissolution of steps. The relative rates of dissolution at steps and terraces depend upon the particular mineral/solution process chemistry through the effects of process temperature (T) and the magnitude of the activation energies for dissolution. Enhanced dissolution of particles containing stepped surfaces was modelled by including the fraction, α, of dissolution sites on step edges relative to the total number of surface dissolution sites. The fraction, α, is expected to be mineral-sensitive through the effects of fracture and cleavage behaviour on step formation. The mean diameter, D M , of particles subject to enhanced dissolution by microtopography factors was shown to lie in the micron (μm) to sub-micron range. This range is consistent with particle sizes where enhanced dissolution by mechanical activation has been reported.

Phytoextraction as a tool for green chemistry

Abstract The unique chemical and physical properties of metals mean that they are extensively utilized by industry in a huge variety of applications, including electronics, materials, industrial catalysts and chemicals. The increased consumer demand from a growing population worldwide with rising aspirations for a better life has resulted in concerns over the security of supply and accessibility of these valuable elements. As such, there is a growing need to develop alternative methods to recover them from waste repositories, current or historic, both for hazard avoidance and potentially, as a new source of metals for industry. Phytoextraction (the use of plants for the recovery of metals from waste repositories) is a green and novel technique for metal recovery, which, if done with the goal of resource supply rather than hazard mitigation, is termed “phytomining”. The ability for plants to form metallic nanoparticles as a consequence of phytoextraction could make the recovered metal ideally suited for utilization in green chemical technologies, such as catalysis. This review focuses on a multidisciplinary approach to elemental sustainability and highlights important aspects of metal lifecycle analysis, metal waste sources (including mine tailings), phytoextraction and potential green chemical applications that may result from the integration of these approaches.

MMCX—An expert system for metal matrix composite selection and design

Abstract Metal Matrix Composites (MMCs) are a relatively new class of engineering materials that offer the designer an opportunity to tailor properties to meet specific requirements. The paper summarizes the development of an expert system which supports engineers in the selection and design of metal matrix composites. The system consists of a dynamic hypertext interface integrated into an expert system developed within the COMDALE/X environment. Mechanical and thermophysical property data for matrix alloys, reinforcement materials, and MMCs are stored in databases accessed by the expert system. Mathematical models which utilize constituent material properties to determine effective composite properties are managed by the system to design metal matrix composites and fill property gaps. Effective elastic modulus, thermal conductivity, coefficient of thermal expansion. Poissons ratio, shear and bulk moduli mathematical models for particulate, whisker, short fiber and fiber composites are contained in a spread sheet managed by the system. Although the physical and mechanical properties may often limit the constituent selection, it is the chemical reactivity of the ceramic reinforcement with the matrix alloy either during fabrication or service which will generally control the final matrix/reinforcement combination. As a result, constituent material compatibility has been determined and incorporated in the system. A database of appropriate reinforcement coatings for applicable matrix/reinforcement systems is also helpful. The system also includes information on suitable manufacturing techniques. A case study demonstrating the use of the system for selecting an MMC for cryogenic applications is presented.

Enhanced dissolution of minerals: Conjoint effects of particle size and microtopography

Abstract The study extends previous fundamental modelling work on the influence of surface microtopography on enhanced dissolution (mechanical activation) of milled particles. Specifically, it analyses quantitatively the conjoint effects of particle size and the fraction ( α ) of dissolution sites associated with steps on the overall mass dissolution behaviour of fine particles. The study shows clearly that microtopography-enhanced dissolution is likely to be most evident for leaching processes exhibiting high activation energies of dissolution (∼70 kJ mol −1 or higher) and unlikely to be evident in particle sizes larger than a few μm. Experimental determination of representative α -values obtained from scanning electron micrographs of three crushed minerals (sphalerite, ilmenite and chalcopyrite) suggest that the limiting milled particle size for significant microtopography-enhanced dissolution lies within the range from ∼ 0.2 to ∼0.4 μm .

Reduction of Mercury Emissions from Gold Mining Activities and Remedial Procedures for Polluted Sites

The modern gold rush in developing countries began in the 1980s. It has involved millions of people who have become artisanal miners in order to escape complete social marginalization. In Latin America today, over one million people are involved directly with artisanal gold mining, producing between 115 and 190 tons of gold (Au) and, as a consequence, emitting over 200 tons of mercury (Hg) per year to the environment (Veiga 1997). Including Africa and Asia, the number of artisanal gold miners may be as high as 6 million. Most developing countries face enormous social and environmental problems derived from poor mining practices when considered together with the lack of economic alternatives.

HgEx: a heuristic system on mercury pollution in the Amazon

HgEx is an Expert System that addresses the complex problems surrounding the pollution of the Amazon with Hg by informal mining operations. The system integrates information on biology, chemistry, geochemistry, medical, social and political issues in order to evaluate contamination for a single site or region. The program attempts to diagnose the possibility of hazardous Hg transformations such as oxidation and methylation. An extensive tutorial section containing over 1500 electronic pages provides guidelines for Hg monitoring fieldwork, sampling and analytical steps, amalgamation practices improvements and amelioration procedures for a polluted site. The system is structured for use by either highly-skilled personnel or those without technical-training.

An interactive simulation model of human drivers to study autonomous haulage trucks

Abstract Driverless haulage trucks have recently been developed for open pit mines. To predict the benefits of an Autonomous Haulage System (AHS), a deterministic/stochastic model has been created to compare AHS to a manual system by estimating benchmarked Key Performance Indicators (KPIs) such as productivity, safety, breakdown frequencies, maintenance and labor costs, fuel consumption, tire wear, and cycle times. The goal of this paper is to describe the driver/autonomous sub-models that function within a virtual 24/7 open pit mine operating with 9 trucks and 2 shovels to move ore to a crusher and waste rock to a dump.