Contribution to the understanding of the three-product cyclone on the classification of a dual density platinum ore

Includes bibliographical references (p. [234]-246).The detrimental effects exhibited by the conventional hydrocyclone in classifying ores that contain different density components motivated the AMIRA P9 project to look into modifications to the conventional hydrocyclone that can improve separation by component density. In this project a cyclone termed the three-product cyclone has been developed and tested. The three-product cyclone is a modification of the conventional hydrocyclone with an additional vortex finder termed the inner vortex finder inserted concentric to the existing one, termed the outer vortex finder, resulting in three products from the same hydrocyclone. The three-product cyclone produces a finer overflow stream, an intermediate overflow stream comprising fine high density particles and medium sized light particles, and a coarse underflow stream. The work presented in this thesis involved designing an overflow arrangement which allowed interchangeable inner vortex finders to be tested, and a special rig for industrial scale experiments using a 600mm diameter hydrocyclone

Different perspectives of dynamics in comminution processes

The performance of a comminution and classification process depends on the design and configuration of each process unit, the configuration of the plant, the design of the control and physical properties of the incoming feed. Comminution processes should be designed to have a stable and efficient production over a wide range of conditions. However, demands from the management, operational cost, investment, maintenance or any other related field can result in process alterations that are not beneficial for the stability of the circuit and, therefore, utilization and efficiency of the production. Furthermore, advanced process control and optimization rely heavily on understanding the dynamic behaviour of the process in achieving a more stable and consequently efficient process. This review aims to explore different dynamic aspects from particle, bulk, unit and process perspectives, their origin, and what consequences they may have on the operation. The aim is to illustrate a holistic view of process dynamics that should be considered when evaluating circuit performances and identifying risk zones that affect the process, considering the state-state performance and dynamic behaviour. Based on that, several dynamic related issues were formulated and ranked by experts within the field to get a subjective perspective. Issues such as process control design and configuration, ore variability, segregation and upstream disturbances ranked high in possible gains for comminution processes

Investigation and modelling of the progression of zinc leaching from large sphalerite ore particles

X-ray Computed Tomography (CT) was used to follow the progression of Zn leaching in a number of individual sphalerite ore particles, which were subjected to a long-term simulated heap bioleaching environment. The ore was prepared by two different modes of comminution – HPGR at 90 bar and cone crusher – and individual particles were selected from three different size fractions. Investigation of the reacted fraction of Zn vs distance from the centre of each particle indicated that leaching from large particles leads to near complete conversion near the surface, but only partial conversion in the zones that are closer to the centre of particles. The cores of the cone-crushed particles show hardly any conversion at all, especially in the larger particle sizes. Mathematical analysis shows that leaching from the large particle does not follow the shrinking core model. It is shown that the progression can rather be described by a combined reaction-diffusion process progressing through the network of cracks and pores closer to the particle surface. Extent and depth of this network are a function of particle size and comminution method. A simplified rate model is proposed that describes the extent of leaching as a function of time in terms of a set of parameters that can all be related to just particle size and crushing mode

Investigation of particles with high crack density produced by HPGR and its effect on the redistribution of the particle size fraction in heaps

The application of comminution technology such as the High-pressure grinding rolls (HPGR), which is able to generate a high density of cracks in the ore particles, is favourable for leaching processes. Extraction of metallic values by the heap leach process, can take place on the particles with partial exposure of mineral grains, if it can provide sufficient surface front for chemical attack by leaching solution. The aim of this study was to assess the benefits of high crack density in the ore particles produced using the HPGR and how it could diminish due to inadequate percolation of the leaching agent. A zinc ore was comminuted using HPGR at three different pressure settings and with a cone crusher for the control experiment. Subsamples from the (+23/-25, +14/-16, +5.25/-6.75 mm) size fractions were characterized and packed into leach reactors. The reactors were stopped from time to time to investigate the progress of crack and micro-crack growth and its effect on metal extraction using the X-ray computed tomography (CT). The results are validated with those obtained using traditional techniques such as SEM and QEMSCAN. Investigation of the leach reactors residue indicated significant changes in the particle size distribution (PSD) of initial feed toward the fine size fraction. The residues from the reactors leaching the material prepared using the HPGR product contained more fine particles than the reactors, which were fed by cone crusher product. These differences were up to 10.3%

Extended grinding curves are essential to the comparison of milling performance

The technique of developing curves of how the operation and product of a mill responds to a change in one of the prime operating variables is presented. This has been investigated to the great benefit of research into the comparison of mill performance when an operating parameter of interest is changed. These ‘grindcurves’ can be used to select the correct conditions for meaningful comparative tests. It is shown that the conclusions drawn from pilot work can be totally changed by shifting the mill to operate under optimal conditions for the comparison. The use of the technique in quantifying SAG mill performance is demonstrated

Circulation rate modelling of mill charge using position emission particle tracking

A model linking the circulation rate of charge particles with physical mill parameters (load fraction, shoulder angle and friction) has been developed and tested using experimental data derived from positron emission particle tracking (PEPT). The model parameters are obtained directly from the in situ flow field of the PEPT tracer particles. The model formulation, methodology for model parameter correlations and comparison of circulation rate model with direct measurement from PEPT forms the focus of this paper

Redesigning SAG Mill Recycle Crusher Operation

Recycle crushing is used extensively to boost SAG mill throughput, by rapidly reducing the competent pebbles that would otherwise build up in a SAG mill. Although applied with considerable success, it is the Authors observation that the crushers are generally poorly utilised: operating well below installed power; switching on and off; and set at large closed side settings. Application of good design and control practice has the potential to increase crusher efficiency in terms of reduction ratio and thus in turn increase SAG mill throughput and unload overloaded Ball mills to open up circuit capacity. Examples of operating practice and potential are provided to illustrate this

Mineralogical Factors Affecting the Dense Medium Separation of Nickel Sulfide Ores

Dense medium separation (DMS) is often used to reject a large portion of gangue material upfront to create cost and energy savings during processing. As lower-grade ores with complex mineralogy are being increasingly exploited, the properties of the gangue minerals begin to play a more important role in the upgrading of the ore. It is therefore important to understand these mineralogical factors to be able to select suitable processing routes for specific ore types. Two nickel sulfide deposits in southern Africa were chosen as case studies to understand differences in DMS efficiency when applied to different ores: Ore A and Ore B. Both ores showed nickel upgrades using DMS and the products were then characterized using QEMSCAN, with the aid of X-ray diffraction and electron probe microanalysis. Overall, particle density remains the main control on the separation, followed by sulfide texture, with massive and net-textured sulfides having larger grain sizes and therefore better liberation than disseminated sulfides. In addition to the concentration of sulfides, primary and secondary silicate minerals are separated by their density differences, which can affect the recovery of finely disseminated sulfides associated with them. Particle size is also important in DMS, with material near the cut-point density separating on size rather than density. The understanding of the mineralogical properties affecting DMS can aid in the prediction of the suitability of DMS for different ore types