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.     

Characterising porosity of multi-component mixtures in rotary mills

The measurement of porosity presents a significant challenge in modelling of slurry transport in rotary mills. This is due to the aggressive environment within rotary mills. In this paper, a method of measuring the porosity of mill charge, using the positron emission particle tracking (PEPT) technique, is presented. In this work, multiple particles are tracked, in turn. The packing density of each size component is proportional to the residence time distribution of its representative tracer particle, based on the ergodicity of the system. The charge porosity is a linear combination of the packing densities of individual components. The porosity is modelled as a function of mill geometric and operating parameters—mill speed and filling fraction. The results show correlations between porosity distribution and operating parameters.     

Power draw estimations in experimental tumbling mills using PEPT

Positron Emission Particle Tracking (PEPT) was employed to reconstruct the motion of mono-sized glass beads in an experimental tumbling mill run in batch mode. In each case, the derived trajectory field of a representative tracer particle was used to determine the charge power draw at steady state operation. Two approaches for calculating power draw were considered: the torque of the centre of mass about the mill centre, and the time averaged torque contribution per discrete grid cell summed over the volume of the mill. Results were compared across different operating conditions and particle sizes to measured power.     

Combining Positron Emission Particle Tracking and image analysis to interpret particle motion in froths

Previous research into particle motion in the froth zone has focussed on constructing detailed CFD models that describe the behaviour of particle classes with different properties; density, size and hydrophobicity. These models have been reasonably successful in predicting trends in the separation behaviour and how it can be manipulated. Models of separation sub-processes cannot readily be verified experimentally due to the opacity and fragility of froth systems.Positron Emission Particle Tracking (PEPT) can be applied to particles in froth flotation systems to observe the behaviour of individual particles in a mixed particle–liquid–gas system. However, measuring the particle position alone is not adequate as its behaviour is also affected by instantaneous froth events such as bubble coalescence. To link the observed particle behaviour to the froth behaviour requires multi-modal measurements. Video footage of a rising foam column was recorded simultaneously with PEPT data, so that the PEPT tracer trajectory could be explained in terms of foam structure and events. A time weighting function of cubic splines with kernel width 200 ms was used to remove the effects of signal noise. An ascending 70 μm hydrophilic tracer accelerated within vertical Plateau borders and decelerated in Plateau borders angled away from vertical. The tracer trajectory showed velocity peaks and troughs when it was contained in nodes in a rising foam. When the tracer descended within a foam showing convective roll, coalescence events and subsequent foam deformation directly influenced the tracer trajectory.     

PEPT studies of heavy particle flow within a spiral concentrator

Future improvements of gravity concentrators require an increased knowledge of the mechanics behind the separation, including the motion of the particles. This work details the investigation of particle motion through a spiral concentrator. The results of tracking the motion of individual particles using the positron emission particle tracking (PEPT) technique are described. Tracer particles of different sizes and density were tracked along the trough of a laboratory scale spiral. Multiple passes of one tracer through the spiral are combined to represent the bulk of flow of this particle type and size, with the position and time recorded to allow for the particle trajectory and speed to be determined. Finally, the use of PEPT will be shown to be a powerful method to visualise the behaviour of particles during the concentration process, providing data that will be used for the validation of new models of spiral concentrator performance.     

CFD–DEM modelling of particle flow in IsaMills – Comparison between simulations and PEPT measurements

The IsaMill? is a high speed stirred mill with a horizontal configuration that offers advantages such as energy efficiency and an inert grinding environment. A combined computational fluid dynamics (CFD) and discrete element method (DEM) approach was developed to investigate the particle and fluid flows inside a simplified IsaMill?. The configuration of the mill was simpler than that of an actual IsaMill? and no feed flow or rotor was considered. The CFD–DEM model is a progression from earlier DEM only models of “dry” systems which did not account for the fluid phase. The properties of flows at a macroscopically steady state, such as velocity field, distributions of particle velocity and acceleration in the radial direction and power draw, were analysed. Detailed comparisons were carried out between the simulation results and Positron Emission Particle Tracking (PEPT) measurements under similar conditions. The comparisons showed reasonable agreements, confirming that both techniques can capture the key features of the flow. The discrepancies between simulated and measured results were discussed. The findings indicated that the proposed model can be used to generate microdynamic information that is useful in leading to a better understanding of the underpinning physics of flow inside mills.     

Tracking the motion of media particles inside an IsaMill™ using PEPT

Stirred milling is continually gaining acceptance in the mineral processing industry. The IsaMill? is a high intensity stirred mill with a horizontal configuration and internal classification. The present work describes the use of Positron Emission Particle Tracking (PEPT) to trace the motion of a media bead in a simplified IsaMill? rig. The rig has the same inner dimensions and disc sizes as the real M20 IsaMill? but is a closed unit without any flow through, no product separator and only three discs. The PEPT system has the advantage of being able to obtain detailed charge motion measurements in opaque and aggressive environments such as those encountered in grinding processes. Glass beads (3 mm) and ceramic media (3.5 mm) were tracked over a range of volumetric fillings and rotational speeds. An analysis of the resultant trajectory fields in terms of media location (occupancy), velocity, and acceleration is presented.     

Measurement of shear rates in a laboratory tumbling mill

A methodology for characterising shear rate profiles in tumbling mills based on in situ trajectory fields from Positron Emission Particle Tracking (PEPT) experiments is presented. Experiments were carried out in a laboratory scale mill fitted with lifter bars, pulp lifters and a discharge grate, and run with radioactively labelled particles within re-circulating slurry environments. Using time-averaged velocity distributions of small PEPT tracers, a model for tangential velocity profiles along radial scan-lines is proposed from which the shear rates are derived.     

Speed analysis of quartz and hematite particles in a spiral concentrator by PEPT

Activated quartz and hematite tracers between 1180 and 1700 μm in diameter were tracked in two sections of the first two turns of a spiral concentrator to ascertain information on valuable and gangue particle motion. The tracking took place while the tracer was flowing in a 20% solids by mass iron ore slurry. The direct activation of mineral particles, combined with the use of an adjustable height circular assembly of modular positron emission particle tracking detectors, has made this tracking possible. The tracer particle trajectories and speeds are presented in this paper. An early separation into two streams (concentrate or loss to tailings) can be seen for certain runs of the hematite tracers. Tracers speeds and radial position from the centre of the spiral give more details about the presence of the slurry film’s secondary flow in the middle zone of the spiral trough. Particle inward and outward migration speed in this secondary circulation is presented. The speed of this migration is approximately 0.12 m/s inward and 0.15 m/s outward for the set-up used.     

Tracer studies and breakage testing in pilot-scale stirred mills

A tracer technique is used to provide parameters that describe mixing and breakage in stirred mills. The results are also used to test the accuracy of mixing and breakage models.Tracer studies have been undertaken using a 39-litre vertical Sala agitated mill and a 4-litre horizontal Netzsch mill. The experimental residence time distribution (RTD) of the mills is analysed both in terms of a single mean residence time and a non-linear least squares fit to an optimal number of perf ect mixers of unequal size in series. Results show a strong dependence of RTD on flow rate, minimal dependence on stirrer speed, and support the concept that the RTD's of liquid tracers and solid tracers subject to breakage are similar. A very accurate match to the experimental RTD curves car be achieved with the multiple uneven mixers in series model.Size distribution results from solid tracer tests are used to determine the breakage characteristics of the pilot-scale Sala mill. The population balance model for a single perfect mixer with steady state hold-up is used as the basis for solution of a constrained non-linear optimisation inverse problem for mill breakage rate and breakage function. Experimental results indicate that the breakage rate is first order as hypothesised. The population balance model using optimal breakage parameters provides a reasonable fit to experimental data for cumulative passing percentage as a function of particle size for discrete times. Both the breakage rates and cumulative breakage functions are roughly power law dependent on particle size. Analysis of the size distribution of breakage products indicates that the mode of particle breakage, as indicated by the tracer breakage parameters, is a function of time. This demonstrates that the assumption of time independent breakage parameters in the population balance model may not be valid. Accurate determination of breakage parameters is strongly influenced by the transport characteristics of the slurry through the mill.The work shows that particle breakage in pilot-scale stirred mills is a complex function of both particle and specific mill characteristics. Therefore, in order to gain insight into the appropriate physical processes at work in an industrial scale mill, it is important to perform experiments with and analyse a system that matches the real mill as closely as possible.     

Preparation of iron oxide nanoparticles by mechanical milling

Hematite powder (Fe₂O₃) was ground in a planetary mill and the milling time and mill rotational speed were varied at three levels. The ground products were then characterized, to investigate size reduction and the mechanochemical effect, by X-ray diffraction (XRD) line broadening, specific surface area (SSA) and particle size and morphology analysis. The line broadening technique was used to determine the degree of crystallinity, crystallite size and lattice strain. The milled hematite particles revealed particle sizes in the nanometer range with a specific surface area of 14.96 m²/g. All milled samples exhibited the mechanochemical effect, where the degree of crystallinity ranged from 9.37 to 49.8%. The minimum crystallite size obtained was 17.1 nm with a degree of crystallinity of 9.37% when hematite was ground at 600 rpm for 10 h.     

The effect of ball size on breakage rate parameter in a pilot scale ball mill

The objective of this study is to investigate the effect of ball size on grinding kinetics in a pilot scale ball mill. Six different ball media gradings were tested. Comparative tests were conducted in batch ball mill having 1.2 m diameter and 0.6 m length at constant operating condition of mill such as media mass, mill speed and input specific energy. Feed samples were ground batchwise and representative sample was taken from inside the mill for each determined grinding period. Grinding process in ball mill was modeled and the specific rate of breakage was calculated for the each test. The results indicated that the relationship between different breakage rate and particle size has a maximum for each ball size distribution. Consequently, a new equation to correlate maximum ball size and particle size at maximum breakage occurs is proposed.     

The effect of particle size on acid mine drainage generation: Kinetic column tests

The rate of acid mine drainage (AMD) generation is directly proportional to the surface area and so to the particle size distribution of acid-forming minerals exposed to oxidation. Materials in various particle sizes are subject to weathering processes at field condition; however, the particle size dependent oxidation rate has not been investigated for understanding entire geochemical behavior at a mining site. Therefore, a comprehensive research program was aimed to investigate the effect of particle size on pH variation and acid mine drainage generation using kinetic column tests, and then to find convenient methodologies for upscaling laboratory-based results to the field condition. For this purpose, ore samples collected from Murgul Damar open-pit mining were grinded in three different particle size distributions that are coarse (minus 22.5 mm), medium (minus 3.35 mm) and fine (minus 0.625 mm) sizes, 34 columns were designed in different dimensions for kinetic column tests. It was found that the cumulative concentration of the many constituents measured from medium particles (minus 3.35 mm) are higher than coarser samples due to decreasing specific surface area with increasing particle size. Similarly, because of decreasing of hydraulic conductivity with increasing the fine content, the cumulative concentration of constituents measured from medium particles (minus 3.35 mm) are also higher than finer particles (minus 0.625 mm). Based on statistical and analytical analyses of the results of kinetic column tests, the time required to initiate acid formation at field condition varied between 489 and 1002 days depending on particle size distribution. In addition, considering the effect of particle size and the results of related statistical analysis, main oxidation (SO₄²⁻) and neutralization (Ca²⁺, Mg²⁺, Mn²⁺ etc.) products were also successfully upscaled to the field condition.     

Study of residence time distribution and mill hold-up for a continuous centrifugal mill with various G/D ratios in a dry-grinding environment

It is important to determine the residence time distribution (RTD) of a mill in order to predict the performance of the mill. In this study, the residence time distribution of a continuous centrifugal mill was measured experimentally by using aluminum powder as a tracer in a dry-grinding environment. Aluminum powder was placed into the continuous centrifugal mill chamber as a pulse under a steady-state condition, and then, the discharge time and proportional concentration of the tracer was measured using a metal detector at various G/D ratios and feed rates. During the measurement procedure, the mill product was collected and analyzed with regard to the size distribution to investigate the relationship between the mill product and the characteristics of RTD. In addition, the mill hold-up data was also analyzed after measurement. These test results can be used to predict the mill product size distribution in combination with a kinetic grinding model.     

An empirical power model derived from 3D particle tracking experiments

The kinematic data resulting from 3D particle tracking experiments of a typical bulk charge particle is used to derive an empirical power model that forms the basis for comparison between the DEM and experiment. The model is derived from the center of circulation coordinates and given as a function of mill speed. The wide range of milling configurations coupled with experimental trajectory data accurate to within 0.15 mm in spatial resolution ensure that the comparisons are robust. The power prediction from both the DEM and experiment is done for a 142 mm diameter mill of the same length and charged with 4596 particles with a mean diameter of 6.1 mm.     

The generation of toxic reactive oxygen species (ROS) from mechanically activated sulphide concentrates and its effect on thermophilic bioleaching

Two types of laboratory mills, planetary and vibratory, were used to activate sulphide mineral concentrates mechanically before thermophilic (bio)leaching. These samples were analysed in terms of particle size, surface area, density, SEM, XRD line profile analysis and reactivity. The product particle size distributions indicated different particle breakage mechanisms of the two mills. The surface area for pyrite milled with the planetary mill was three fold that milled in the vibratory mill for the same length of time. Planetary milled samples showed lower densities, up to 4% less for pyrite samples, compared to vibratory milled samples. Particle surface oxidation, observed by SEM, occurred post milling. Surface oxidation products were more prevalent with planetary milled sulphide samples. XRD line profile analysis showed more line broadening effects with the planetary mill. This indicated that more bulk particle-related structural defects were present in the planetary milled samples. The reactivity in acidic solution was measured in terms of the generation of toxic reactive oxygen species (ROS): hydrogen peroxide and hydroxyl radicals. The ROS generation from milled sulphides, normalised to constant surface area loading, increased with increased mechanical activation. The planetary milled samples generated greater ROS per sample surface area than vibratory milled samples, more than 4-fold for pyrite after 60 min of milling. Increased ROS generation was postulated to result from increased surface area defects, solubilisation of iron oxidation products and bulk particle-related defects.The effect of mechanical activation on performance on thermophilic leaching and bioleaching tests was investigated using milled samples at 2% (w/v) pulp density. Short mill times improved leach rates from both mills, up to 7-fold cf. unactivated feed leach rates. Poor bioleaching performance resulted following long periods of mechanical activation (20-60 min). Pyrite and chalcopyrite bioleaching performance decreased dramatically above surface area loadings of 25 and 125 m²/L respectively. Planetary milled samples were less amenable to bioleaching. For pyrite milled for 20 and 60 min and chalcopyrite milled for 40 min, no viable cells were observed following inoculation via fluorescence microscopy, suggesting culture death supported by compromised ferrous iron oxidation. The generation of ROS was postulated to cause poor bioleaching performance under these conditions.     

钢渣粉粒度对复合胶凝材料水化性能的影响

通过控制不同粉磨时间,控制磨细钢渣的比表面积和颗粒度分布,研究了与磨细石英砂凝结时间对比,不同颗粒度分布的钢渣的活性指数,以及配制成复合胶凝材料中的凝结时间和抗压强度,并进行了卧辊磨大磨实验,结果表明:(1)钢渣使复合胶凝材料早期结构发育缓慢,随着钢渣比表面积的增大,其与水分的接触面积增大,导致复合胶凝材料的凝结时间显著的延长,其早期(1～3d)的结构发育缓慢;(2)卧辊磨与实验室结果差距较大,可能原因为卧辊磨配套选粉机钢渣粉粒度分布窄;(3)将钢渣粉的比表控制在300 m2/kg左右,有利于钢渣磨机的台时产量提高,降低钢渣粉的生产成本,增大钢渣粉在水泥中的掺量.     

立式辊磨机对菱镁矿磨矿及浮选效果的影响

立式辊磨机是一种集中碎、粉磨、快速烘干、高效选粉等工序为一体的高效节能环保型设备,具有结构 简单紧凑、工作可靠、流程简单、占地面积小等诸多优点。 针对丹东宽甸某菱镁矿,分别进行立式辊磨机与球磨机磨 矿—浮选试验,通过对比磨矿能耗、磨耗、磨矿产品粒度特性、矿浆中 Fe3+浓度等数据,并比较了药剂制度对闭路试验 精矿指标的差异。 结果表明:立式辊磨机磨矿的能耗仅为球磨机磨矿能耗的 17%左右,立式辊磨机的磨耗约为球磨 机磨耗的 6%左右,同时立式辊磨机磨矿产品中有利于浮选的中间粒级(0. 105~ 0. 045 mm 粒级)含量要比球磨机磨矿 高 2. 39 个百分点、立式辊磨机磨矿矿浆中的 Fe³⁺浓度也仅为球磨机磨矿矿浆的 17. 86%;通过闭路试验精矿指标对比 表明,使用立式辊磨机磨矿,可有效降低油酸钠与六偏磷酸钠用量,并降低精矿中 CaO 品位,提升精矿质量。     

Understanding the influence of HPGR on PGM flotation behavior using mineralogy

Due to the very fine grained nature and complexity of the platinum bearing ores from the Bushveld Complex in South Africa, numerous processing operations have investigated alternate comminution devices that can be used to liberate the platinum group minerals of the Merensky and UG2 ores at a coarser grind, at reduced energy consumption and increased throughput. In this study, the mineralogy and flotation performance of product from the high pressure grinding rolls (HPGR) was evaluated and compared to a conventional ball mill product with the aim of determining whether the HPGR product could be used for flotation without any further grinding. Results show that for both the Merensky and UG2 platinum ores, the HPGR product showed more fines and less coarse content compared to the ball mill product. No conclusive evidence of preferential liberation was observed for samples prepared by particle bed breakage. The best flotation results were obtained from the ball mill product. The results from this study have shown the definite need for an integrated approach for the interpretation of the results that extends beyond just measurements of valuable mineral liberation.     

一段格子型球磨机与平底旋流器分级配置的应用实践

MQG-3660型一段球磨机与改进的φ660平底水力旋流器配置,球磨机处理量达到240 t/h,球磨机利用系数达4.4 t/( m³·h)。该水力旋流器适用于粗粒分级,分级效率高,沉砂嘴不堵塞,具有较广的推广应用价值。