Jochen Petersen

Ageing of chromium(III)-bearing slag and its relation to the atmospheric oxidation of solid chromium(III)-oxide in the presence of calcium oxide

Slag arising in ferrochromium and stainless steel production is known to contain residual levels of trivalent chromium. As the chromium is normally bound in the slag matrix in various silicate or spinel phases, and hence not easily mobilised, utilisation or controlled disposal of such slag is generally considered unproblematic. Experimental test work with a number of slag materials indicates, however, that very gradual oxidation of trivalent to hexavalent chromium does occur when the slag is exposed to atmospheric oxygen, rendering a quantifiable but small portion of chromium in this much more mobile and toxic form. Mechanisms and rates of the oxidation reaction were investigated in a number of long-term studies using both original slag materials and artificial mixes of chromium and calcium oxides. Powders of these materials, some of them rolled into balls, were left to age under different conditions for periods of up to 12 months. In the slag samples, which contained between 1 and 3 wt.% chromium, 1000-10000 microg Cr(VI) were found per gram of chromium within 6-9 months of exposure to an ambient atmosphere. The rate of the oxidation reaction decreased exponentially, and the reaction could generally be said to have ceased within 12 months. In mixtures of calcium and chromium oxides the oxidation reaction is presumed to occur at the boundaries between chromium oxide and calcium oxide phases through diffusion of oxygen along the grain boundaries and of Cr(3+) across the boundaries, resulting in the formation of calcium chromate. In the slags, where calcium and chromium oxide can form a solid solution, the oxidation is likely to occur at the exposed surface of grains containing this solution.

Principles, Mechanisms And Dynamics Of Chalcocite Heap Bioleaching

Solution is irrigated (continuously or intermittently) over the top surface of the heap and left to seep through the ore bed where it can react with the target minerals. Dissolved metals are then transported with the flowing solution to the bottom of the heap from where they are drained via the drainage system into a collection pond as the pregnant leach solution (PLS). The target metal is removed from the PLS through a suitable technology (by solvent extraction, cementation or adsorption), and the barren solution (usually referred to as the raffinate) returned to the top.

Heap Leaching Technology—Current State, Innovations, and Future Directions: A Review

ABSTRACT Heap leaching is a well-established extractive metallurgical technology enabling the economical processing of various kinds of low-grade ores, which could not otherwise be exploited. However, despite much progress since it was first applied in recent times, the process remains limited by low recoveries and long extraction times. It is becoming increasingly clear that the choice of heap leaching as a suitable technology to process a particular mineral resource, which is both environmentally sound and economically viable, very much depends on having a comprehensive understanding of the underlying fundamental mechanisms of the processes and how they interact with the particular mineralogy of the ore body under consideration. This paper provides an introduction to the theoretical background of various heap leach processes, offers a scientific and patent literature overview on technology developments in commercial heap leaching operations around the world, identifies factors that drive the selection of heap leaching as a processing technology, describes challenges to exploiting these innovations, and concludes with a discussion on the future of heap leaching.

An annual and seasonal characterisation of winery effluent in South Africa.

Winery effluent is known to have a high chemical oxygen demand (COD) and a low pH. In this study, we extensively analysed effluent from two cellars and studied the temporal changes over the duration of a harvest and the duration of a year. We found that ethanol contributes approximately 85% to 90% of the COD of raw winery effluent, with acetic acid being the next significant contributor. The pH showed some dependence on the concentration of acetic acid. The concentration of sodium in the effluent is strongly dependent on the cleaning regime in place at the cellar, and the concentration of potassium has been shown to be linked to the spillage of juice, wine or lees. The data and correlations presented here could allow for an artificial effluent to be prepared easily for research purposes.

The Effect of Total Iron Concentration and Iron Speciation on the Rate of Ferrous Iron Oxidation Kinetics of Leptospirillum ferriphilum in Continuous Tank Systems

In this study the results from a systematic study of the oxidation kinetics of Leptospirillum ferriphilum in continuous culture at total iron concentrations ranging from 2 to12 g/L are reported. In all experiments the steady-state concentrations of ferrous iron were small and comparable, and at least 97% of was as ferric. Surprisingly, the specific ferrous iron utilisation rate decreased with increasing total iron concentration, while yield coefficients increased. It was noted that the biomass concentration in the reactor (as measured by both CO2 uptake rate and cell counts) dramatically increased with increasing total iron concentrations, whereas it stayed more or less the same over a wide range of dilution rates at a given total iron concentration. The experimental data was re-analysed in terms of ferrous iron kinetics using Monod kinetics with a ferric inhibition term. The results confirm that the maximum specific iron utilisation rate is itself a function of ferric iron concentration, declining with increasing concentration. It thus appears that high concentrations of ferric iron stimulate microbial growth while at the same time inhibiting the rate of ferrous iron oxidation. It is postulated that these phenomena are related, i.e. that more growth occurs to reduce the load on the individual cell, possibly by sharing some metabolic functions.

The Effect of Aluminium and Magnesium Sulphate on the Rate of Ferrous Iron Oxidation by Leptospirillum ferriphilum in Continuous Culture

In heap bioleaching the dissolution of gangue minerals from igneous ore materials can lead to the build-up of considerable concentrations of Mg and Al sulphates in the recycled leach solution. This may interfere with microbial ferrous iron oxidation, which drives the oxidation of the target minerals. The kinetics of the oxidation process have been well studied for Leptospirillum and Acidithiobacillus species in tank systems. Although not directly comparable, kinetic parameters derived for tank systems do apply also for heap bioleach conditions. In the present study the effect of solution concentrations of Mg and Al as sulphate at individual concentrations of 0 to 10 g/L and combined concentrations 0 to 16 g/L each has been investigated in continuous culture using Leptospirillum ferriphilum. Increasing the concentrations of the salts increasingly depresses the rate of ferrous iron oxidation and also shifts the viable range more and more into the low potential region. Al significantly reduces the amount of carbon maintained in the reactor (assumed to be commensurate with biomass), whereas Mg actually enhances it at low concentrations. In both cases, however, the rate is always depressed. The results indicate that heap cultures are likely to perform sub-optimally in those operations where build-up of dissolved gangue minerals is not controlled.

An electrochemical study of the dissolution of chalcopyrite in ammonia–ammonium sulphate solutions

Ammoniacal solutions are an effective lixiviant for the oxidative dissolution of some mineral sulphides. A study of the anodic dissolution of chalcopyrite in ammonium sulphate-ammonium hydroxide solutions has been carried out using cyclic voltammetry, chrono-amperometry and rest-potential measurements. The role of the copper(II)/copper(I) redox couple in the oxidation process has been evaluated. Rest potentials have been found not to be affected by oxygen but to increase with an increase in initial copper(II) concentration. This trend remains more or less unchanged with increasing total ammonia (NH3+NH4+) concentration. Cyclic voltammetry analysis shows an oxidation peak/shoulder when anodically polarising the chalcopyrite after attaining rest potential in the presence of copper(II) ions. All reverse sweep curves are more or less identical, but the Tafel slope decreases from around 150 mV/decade at 1M total ammonia concentration to around 120 mV/decade at higher ammonia concentrations. Pseudo-steady-state anodic current densities measured in the absence of copper(II) ions at the rest potentials obtained in the presence of copper(II) ions increase with increasing concentration of copper(II) ions, confirming the positive effect of copper(II) ions on the rate of dissolution of the mineral. Cyclic voltametric and chrono-amperometric data show that in the absence of initial copper(II) ions, the rate of dissolution in the presence of dissolved oxygen is significantly lower. These results confirm that the effective oxidant for the mineral under the conditions of the study is copper(II) and not dissolved oxygen. Coulometric measurements have been used to establish the stoichiometry of the anodic reaction at different potentials as involving approximately seven electrons per mole of chalcopyrite, suggesting the formation of thiosulphate, although thiosulphate ions have not been tested for or identified in solution. SEM and energy dispersive X-ray (EDX) analysis of the mineral surface left to equilibrate over time shows a copper-depleted surface, rich in iron but relatively low in sulphur. The iron in the surface layer can easily be removed by short contact with concentrated acid, which is consistent with the formation of a secondary iron-based precipitation layer rather than an altered mineral phase.

Heap and tank leaching of copper and nickel from a Platreef flotation concentrate using ammoniacal solutions

The technical feasibility of ammonia leaching of platinum group mineral (PGM)-bearing Platreef flotation concentrates was investigated as an alternative method for the recovery of base metals before precious metal recovery. The composition of the concentrate sample included 2·02% Cu and 3·17% Ni, mainly as chalcopyrite and pentlandite. A solution containing 4M NH4OH, 4M (NH4)2CO3, buffered in the pH range 9–10, was used as base case lixiviant in column and batch stirred tank reactor tests, around which the effects of total ammonia concentration and operating temperature were explored further. At optimal conditions nearly 100% Cu and Ni extraction were obtained in the batch stirred tank reactor, and 95% Cu and 60% Ni extraction was obtained in columns. The activation energy for chalcopyrite and pentlandite was 36 and 16 kJ mol− 1, respectively, in both reactors, indicating different reaction pathways for the two minerals. Gas-liquid mass transfer appears to be the limiting reaction step in columns much more than in tanks, and the degree of ammonia loss to air is significant. This initial test work indicates the feasibility of using ammonia leaching for this and similar materials as an alternative route to conventional smelting and refining before PGM recovery, but a number of technical challenges still need to be overcome.

Investigating the Bioleaching of an Arsenic Mine Tailing Using a Mixed Mesophilic Culture

The aim of this study was to investigate the microbial colonization and arsenic leaching kinetics of South Korean mine tailings containing arsenopyrite at fixed temperatures (20°C, 30°C and 45°C) and at ramped up temperatures (25 to 45°C, with a 2°C daily increase). The experiments were conducted in a packed bed of inert granite pebbles coated with the tailings material and leached with a mesophilic culture dominated by Acidithiobacillus caldus (56%), a lesser percentage of Leptospirillum ferriphilum (29%) and Archaea (15%), using 1 g/L ferrous-enriched 0K medium. The ramped-up temperature experiment was conducted in triplicate and columns were sacrificed after different leach periods to study the evolution of microbial species dominating the colonization. The leaching performance was evaluated using the arsenic released into solution, the iron oxidation rates, the pH and the redox potential. The microbial speciation of the culture attached to the solids during the leach experiment was determined upon completion of each experiment. A steady arsenic solubilisation of between 94 and 97% was observed among the various column experiment after 88 days post inoculation. Microbial speciation performed following the leaching of the mineral indicated a shift of microbial communities in the columns when compared to the initial inoculum.

Effect of Alteration on the Mineralogy and Flotation Performance of PPM Platinum Ore

The 2.05-Ga Bushveld Complex in South Africa, host to many lucrative ore deposits (e.g. platinum group elements (PGE)), is surprisingly pristine and unaltered, given its geological age. In some areas, however, there is evidence of low-temperature weathering and alteration, most commonly observed when the ore is near the surface. The Pilanesburg Platinum Mines (PPM) operation in South Africa treats ore from an open pit and routinely suffers from low and erratic PGE flotation recoveries. This article investigates the effect of low-temperature alteration on the mineralogy and flotation performance of the PPM “silicate reef” ore. Bulk density measurements of the various lithologies show a clear decrease in density with decreasing depth, and similarly, a decrease in PGE flotation recovery. Various factors from the mineralogical investigation show supporting evidence that loss of platinum group minerals (PGM) recovery is due to low-temperature alteration. Several recovery improvement strategies are also investigated.