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Abstract
A statistical ‘randomised block design’ full plant trial of magnetic conditioning of flotation feed was undertaken at Rio Tinto’s Northparkes Mine in New South Wales. Magnetic conditioning of flotation feed has been shown to improve the flotation of <38 μm paramagnetic minerals consistent with selectively aggregating these paramagnetic minerals. The process was evaluated using shift composite samples that were analysed for copper and gold. Because the process targets <38 μm minerals, all samples were sized and the size-by-size recoveries were calculated for each sample. The size-by-size analysis targeted the effect on fine mineral, reduced the plant noise and shortened the test period required to get statistically significant results. An increase in gold and copper recovery was found in the <20 μm fraction to a high level of statistical significance. For the >20 and <38 μm size range, the copper recovery increased to a high level of significance, whereas the increase in gold recovery in this fraction was to a lower statistical significance. There was no statistical improvement in the >38 μm fraction for either metal. Approximately half the copper and gold losses at Northparkes are in the <20 μm size fraction. While the sulphide copper minerals chalcopyrite and bornite are known to be paramagnetic; gold is not paramagnetic. Gold’s response to magnetic conditioning, however, is consistent with a number of literature references where, depending on the gold’s mineralogical disposition, gold has been shown to respond to magnetic treatment.
... An unusual technology was tested by Wood et al. (2012) at the Northparkes in Central West NSW seeking to improve gold recovery. Magnetic conditioning was used to induce selective aggregation of paramagnetic minerals. ...
... While flotation has been used as the main pre-concentration process for gold ores for many years, new technologies have been developed to improve its efficiency. Magnetic aggregation has been shown to improve the efficiency of separation by improving gold recovery in the fine fraction of a low grade ore (Rivett et al., 2007). On the other hand, specially designed flash flotation circuits have been shown to increase gold recovery in a number of plants by 2.5e5% (Chadwick, 2011). ...
The environmental profile of gold production with regards to embodied energy, greenhouse gas emissions, embodied water and solid waste burden has been assessed using life cycle assessment methodology. Both refractory and non-refractory ores were considered, with cyanidation extraction followed by carbon in pulp (CIP) recovery assumed for non-refractory ore processing. Flotation and pressure oxidation were included prior to cyanidation for processing refractory ores. For a base case ore grade of 3.5 g Au/t ore, the life cycle-based environmental footprint of gold production was estimated to be approximately 200,000 GJ/t Au, 18,000 t CO2e/t Au, 260,000 t water/t Au and 1,270,000 t waste solids/t Au for non-refractory ore. The embodied energy and greenhouse gas footprints were approximately 50% higher with refractory ore due to the additional material and energy inputs and gold and silver losses associated with the additional processing steps required with this ore. The solid waste burden was based on an assumed strip ratio of 3 t waste rock/t ore, but this ratio varies considerably between mines, significantly influencing the estimated value of this impact. The environmental footprint of gold production (per tonne of gold produced) was shown to be several orders of magnitude greater than that for a number of other metals, largely due to the low grades of ore used for the production of gold compared to other metals.
This paper aims at investigating the gold leaching behaviour of refractory, bio-oxidized flotation gold concentrates, before and after high intensity milling, for enhancing gold extraction. Specifically, the role of factors such as milling time and ceramic balls-to-pulp mass ratio, coupled with corresponding physico-chemical changes on gold extraction yield and leaching kinetics was studied. From the results, high intensity milling of the bio-oxidised flotation concentrate reduced the average particle size by increasing the fraction of fines mainly through abrasion/attrition, and increased surface area by both fine particle and crevice production. Furthermore, notable increase in sodium cyanide consumption was observed with an increase in the mechanical stress deployed, consistent generally with degree of mineral relative amorphisation and reactive gangue mineral exposure. About 1.6–1.8 times variable increase in gold yield was observed after 24 h cyanide leaching of activated samples, irrespective of the impacted mechanical stress and degree of mineral amorphisation.
In flotation practices as the particle size becomes fine, e.g., p<20μm, the process efficiency declines markedly. When flotation of fine particles is necessary, one method to improve the recovery is selective agglomeration of fines. Different agglomerating methods have been introduced and tested; however, one method which has recently received great attention is to aggregate fine particles by magnetic force. A selective aggregation of the fine paramagnetic particles, e.g., sulfide minerals, can be achieved by applying a high intensity magnetic field during flotation conditioning. The method is simple, selective and effective for fines, and has a low operation cost. However, a minimum magnetic susceptibility of fine grains is required for this process. This method has been demonstrated in different sulfide minerals flotation plants. The implementation of the magnetic conditioning device, ProFlote, in both the Cu-Pb and the Zn circuit of Boliden's Garpenberg plant in Sweden, where a complex massive sulfide ore is treated, is discussed herein. The preliminary results have shown an increase in the recovery of value minerals, in particular Zn and Ag and considerable reduction in Zn deportment to the final tail. Increases in Cu grade and decreases in Pb grade in Cu-con have been achieved as well.
This paper discusses the drivers and imperatives for innovative changes in flotation plant practice over the last 30 years. Improvements in characterisation technology have led to improved understanding of contributions to inefficient mineral separation in flotation plant cells and circuits. In some cases this has also led to new process technology and approaches to targeted plant designs which may be applied globally. Technology improvements may drive further developments in characterisation tools and flotation technology.
After bench-scale and pilot-plant tests in which it was shown that wet high-intensity magnetic separation (WHIMS) can achieve good recoveries of gold and uranium from Witwatersrand residues, a production-size machine was installed at a gold mine. WHIMS can be combined with flotation for optimization of overall gold and uranium recoveries. This concept is relevant not only to operations for retreatment of tailings, but for treatment of coarser material. In the latter, there is a saving in energy consumption compared with fine grinding, and a material suitable for underground backfill can be produced. More cost-effective machines under development are also described.
Wet high-intensity magnetic separation (WHIMS) for the concentration of gold and uranium was tested on many cyanidation residues, and on some ores and flotation tailings. The results varied, but many indicated recoveries of over 60 per cent of the gold and uranium. The main source of loss is the inefficiency of WHIMS for material of smaller particle size than 20 mu m. The recoveries in the continuous tests were lower than those in the batch tests. The continuous tests indicated an operational difficulty that could be experienced in practice, namely the tendency for wood chips and ferromagnetic particles to block the matrix of the separator. It was decided that a solution to the problem lies in the modification of the separator to allow continuous removal of the matrix for cleaning. A system has been developed for this purpose and is being demonstrated on a pilot-plant scale. A simple model describing the operation of the WHIMS machine in terms of the operating parameters is described. This should reduce the amount of empirical testwork required for the optimization of operating conditions and should provide a basis for scale-up calculations.
Mineralogy of gold and significance of mineralogy in relation to the commonly used gold recovery processes are discussed. Methods used in the mineralogical assessment of gold ores are described with particular emphasis on problems and techniques specific to these ores. Graphs, tables, and color plates show data.
A full-scale statistically-based on/off plant trial of a new flotation conditioning technology designed to improve fine mineral recovery was undertaken to determine whether the recovery of selected fine sulfide minerals can be increased. The technology is selective for certain sulfide minerals only, including chalcopyrite and sphalerite. The technology has been shown to selectively aggregate the fine chalcopyrite and sphalerite minerals giving increased flotation recovery. The plant trials have shown an increase in the mineral recovery, without decreasing selectivity. Size by size recovery shows a significant increase in the fine (<20 µm) mineral recovery levels, such that the losses in the finest of these fractions were reduced significantly.
The influence of iron hydroxide/oxide coatings on the recovery of gold was studied using 13 stream sediment samples collected from an area of known gold mineralisation. Magnetic products collected from a Frantz isodynamic separator showed higher gold values than did non-magnetic fractions. An acid-treated magnetic product when run through the isodynamic separator yielded a significant amount of non-magnetic component which, when viewed by binocular microscope, showed the presence of gold grains in both bulk sample and in bromoform-separated heavy mineral concentrate. It is suggested that concentrates are treated with 10% HCl prior to magnetic separation where iron oxide coatings are conspicuous.
Results of the experimental investigation of the effect of pH on the recovery of uranium and gold by high-gradient magnetic separation are presented. It has been demonstrated that, by alteration of the surface potential of the minerals present in the uranium-gold tailings, the quality of the magnetic concentrate can be enhanced and the recovery of uranium increased. Gold is observed to follow uranium closely in the magnetic concentrate as a function of pH. The experimental results imply that the highest metallurgical performance of a magnetic separator can be expected at a pH value at which the surface potentials of the valuable and gangue minerals have the same sign, and the pH value is not too far removed from the point of zero charge of the valuable mineral.
From the examination of data from detailed plant surveys and associated laboratory batch testing, the principal effects of particle size in flotation have been identified. The current state of knowledge concerning the role of this variable is discussed in terms of the evidence presented. It is concluded that the minimum degree of hydrophobicity necessary for the flotation of a particle depends upon its size and as a result, recovery-size curves are a valuable diagnostic aid to the assessment of flotation performance. Entrainment is shown to be an important contributory mechanism to the recovery of fine particles which, when coupled with a low rate of genuine flotation, can account for much of the observed behaviour of such fines. The significance of particle size and its consequences in flotation research, in plant operations and in control schemes has been under-rated. The separate conditioning or flotation or both of separate size fractions seems inevitable as ores become increasingly difficult to concentrate.
The review is in three main sections. First, the observation that overall flotation performance deteriorates for the finest particles is confirmed, but the precise effects of particle size on recovery, grade, and flotation rate are shown to be more complex. In the second section an assessment is made of theories which have been proposed to explain the slower flotation rate of ultrafines, the effects of slime coating, and the flotation of slimes with cationic collectors. The last section of the paper gives a critical evaluation of methods which are claimed to improve the flotation of ultrafine particles.
High Gradient Magnetic Separation (HGMS) tests have been carried out on the leached residue of Witwatersrand (S.A.) gold ore to determine the performance of HGMS in the beneficiation of the gold and uranium content (typically of the order of 1 ppm and 100 ppm respectively) of this residue. A number of important findings have emerged. Five-fold concentration of the gold and uranium has been achieved in the magnetic fraction, constituting 15% by weight of the feed sample. The separator consists of a superconduct ing solenoid (max. field 8 Tesla) which has a 73mm diameter room-temperature bore. The slurry is pumped through a perspex canister placed in the bore and containing the matrix material. Woven wire mesh was found to be the most suitable matrix material for this application and was therefore used throughout.
Mineral processing engineers often conduct trials to improve the performance of their plants. A common problem in such trials is detecting real but relatively small improvements or changes in process performance against a background of very noisy data. This large data variance is frequently caused by (among other things) long-term time trends in performance, which can be a result of systematic changes in feed conditions.This paper describes two well-known statistical procedures for dealing with such situations, the paired t-test and the randomised block experiment. These methods are illustrated through their application to three real case studies in base metal flotation plants, all involving “yes-no” decisions, and all using metal recovery as the main performance criterion: 1.1. The evaluation of a new flotation collector in a production plant.2.2. The assessment of two alternative flotation circuit configurations in a pilot plant.3.3. Determination of the value of introducing a regrind stage ahead of a flotation circuit in a production plant.The paper considers the practical problems encountered in these experiments, discusses the compromises sometimes required in analysing imperfect experiments, and shows how the statistical procedures can be used to make good decisions in the face of uncertainty. The formulae and computational procedures are given in full in full to encourage their application to similar situations in the practice of mineral processing.
In this review, it is first attempted to put together the available basic knowledge as relevant to the problem of very fine particles, and secondly to analyse the various surface-based methods. The greater part of this vast literature is of a basic, fundamental nature. An acute need to do more research on the problem of very fine particles from an applied angle is explicit. Considerations of the significance of what has been practically observed with a qualitative theoretical approach may be defined as an applied approach. Such an approach will enable an understanding of why there are lower size limits for various existing processes and a base for further developments.Aggregation of very fine particles by hydrophobic-association, the use of oil as a collector extender in the above, column flotation, and the physical and chemical characterization of the surface composition and heterogeneity are the areas in surface-based methods identified for future research.
Energetic and kinetic criteria for the selective magnetic separation of a useful mineral from a system containing several mineral species are derived, based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. It is shown that both criteria give similar predictions of the onset of coagulation and of the selectivity of magnetic separation. The best quality of magnetic concentration is achieved at a pH value that lies outside the interval between points of zero charge (PZC) of the minerals present, and the highest recovery of the useful mineral is achieved when the working pH value is as close as possible to the PZC of the valuable mineral. The magnetic field must be chosen in such a way that heterocoagulation between the useful mineral and the gangue is suppressed and homocoagulation of the valuable mineral particles is promoted. The predictions of the model are shown to be in very good agreement with the experimental results.
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