Modeling the contribution of specific grinding pressure for the calculation of HPGR product size distribution

Vale, one of the largest mining companies in the world, has prioritized the development of HPGR technology for practical application in its current projects. An existing model for the HPGR, capable of predicting product size distributions, has been evaluated under distinct grinding conditions for one feed material. The effect of grinding pressure and feed size distribution were investigated. The model response showed a clear dependency of product size distribution with specific grinding pressure. As a result, specific grinding pressure was incorporated into the model, allowing for predicting product size distribution at practical values of this important process parameter. Based on this result, a characterization procedure was envisaged so as to produce parameters for the model. The procedure does not require complex experimental procedures, and all of the testing can be carried out in an expedited form in an instrumented bench-scale HPGR, using small samples of about 10 kg. The only analyses required are size distributions. The model was implemented in the Modsim? plant-wide simulator, with facilities to predict product size distribution for any roll diameter, length and speed of an HPGR machine.     

High pressure grinding rolls (HPGR) applications in the cement industry

In this study, the performance evaluation studies in five cement grinding circuits, in which HPGR is used in various configurations, were presented. Sampling surveys were performed around the circuits followed by the determination of the size distribution of the samples down to 1.8 μm using a combination of sieving and laser sizing methods. The results showed that the specific energy consumption of the circuit decreases as the size reduction achieved by the HPGR increases. As given in the case studies when the size reduction ratio (F₈₀/P₈₀) changed from 308.2 to 4.4, the specific energy consumption of the HPGR was 8.02 and 4.05 kWh/ton, respectively. Since various configurations offer rather different ball mill feeds, the best usage of HPGR could be attained by optimization of operating parameters of both ball mills and air classifiers.     

Comparison of open and closed circuit HPGR application on dry grinding circuit performance

A conventional cement grinding circuit is composed of a two compartment tube mill, a mill filter which collects the fine material inside the mill and a dynamic air separator where final product with required fineness is collected. In general the material fed to the circuit has a top size of 50 mm which is very coarse for the ball mill. For this purpose, later in 1980s, high pressure grinding rolls (HPGR) has found applications as a pregrinder which increased throughput of the grinding circuit at the same fineness.In early applications, HPGR was operated in open circuit. But later as the operating principle of the equipment based on the compression, some portion of the HPGR discharge recycled back to improve efficiency of the mill or operated closed circuit with classifiers. Within this study effect of open and closed circuit HPGR applications on dry grinding circuit performance was examined. For this purpose sampling studies around three different cement grinding circuit were completed. In the first study, a circuit including open circuit HPGR, ball mill and air separator was sampled and chosen as the basic condition. As the final product size distribution is important for grinding circuit, model structure of each equipment was developed. The second and third surveys were carried out around closed circuit HPGR operation with V and VSK separator to develop models for the separators. Finally the separator models were used in basic condition to simulate closed circuit HPGR application.It was understood from the studies that closed circuit HPGR operation improved the overall circuit efficiency at the same final product fineness by reducing the specific energy consumption.     

Stirred milling kinetics of siliceous goethitic nickel laterite for selective comminution

The objective of this study is to determine how grinding conditions affect the breakage rate with respect to the sample mass, major elements, and minerals present in siliceous goethitic (SG) nickel laterite. This information is helpful in determining the optimal grinding conditions for selective comminution and nickel upgrade. The kinetics of batch wet grinding of nickel laterites with feed sizes of 2.38–1.68, 1.68–1.18, 1.18–0.85, 0.85–0.6, 0.6–0.42, 0.42–0.3, 0.3–0.21, and 0.21–0.15 mm were determined using a Netzsch LME4 stirred mill under the following conditions: 1000 rpm, 50% charge volume, 150.0 g of solid. The grinding behaviour of the majority of the feed samples was non-first-order due to the fast breakage rate of soft minerals and the low breakage rate of hard minerals in the feed. Therefore, an enrichment of the soft mineral was obtained in the underscreen product by selective grinding. The effect of selective grinding on Ni upgrade was evaluated by looking at grinding time, feed size, and product size. Optimum grinding time with respect to Ni upgrade was 0.25 min for SG nickel laterite samples. Generally, grinding larger particles and/or collecting finer product size yielded better Ni upgrade results. The effect of selective grinding was evaluated by the changes of the major soft and hard minerals for the selected samples. Selective grinding was also examined with respect to the major element weight ratio (e.g. Si/Ni for SG nickel laterite). With respect to Ni upgrade, the best result was achieved from the 1.18–0.85 mm feed on the −400 mesh product after grinding for 0.25 min. The Ni grade increased from 0.73% to 1.30% (upgrade 76.8%), with 14.4% Ni recovery; the Mg grade increased from 1.30% to 3.96% (upgrade 205.6%); the Si grade decreased from 28.7% to 16.2%.     

Adsorptive removal of arsenic from river waters using pisolite

This work presents the experimental results for arsenic removal from aqueous solutions using pisolite as a natural inorganic sorbent, a waste mineral product from Brazilian manganese ore mines. A pisolite sample was submitted to physical and chemical characterization; particle size analysis by screening, X-ray diffractometry, X-ray fluorescence, surface area determination by the Brunauer–Emmett–Teller (BET) method and atomic absorption spectrophotometry (AA) for the determination of the species concentration in the pisolite and in the aqueous solution samples from the experiments.Column and batch tests to contact pisolite and aqueous feed solutions were carried out for evaluation of the pisolite’s performance as a natural sorbent for arsenic removal. Experiments using activated pisolite and aqueous feed solutions prepared with Velhas River water were also performed. In the column system, 1.0 g of pisolite removed 1.41 mg of As (4.05% As extraction) from 630 ml of the aqueous feed solution and 1.0 g of activated pisolite extracted 3.51 mg of As (11.6% As extraction). Results for the batch tests with 100 ml of aqueous feed solution and 1.0 g of pisolite removed 1.29 mg of As (24.7% As extraction) and 1.0 g of activated pisolite extracted 3.17 mg (58.2% As extraction).     

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.     

Liberated 0–10 μm particles from sulphide ores,their production and separation—Recent developments and future needs

The general types of industrial behaviour of liberated valuable sulphide minerals in the 0–10 μm size fraction are described and tests to explore on-site at a concentrator the general nature of any deficient behaviour are discussed. In addition, the general types of behaviour of the gangue minerals in this size fraction are also described in terms of their mechanisms for recovery.These behaviours are also discussed in terms of the type of grinding device and grinding environment in which they were produced and the target grinding product size. Major recent improvements in the industrial capabilities of grinding devices are discussed.Present difficulties in characterization of particles in the 0–10 μm range are described and desirable improvements for the future are outlined. Further, desirable improvements for industrial processing of valuable minerals in this size range are discussed along with some remedial actions.     

Comparison of energy efficiency between ball mills and stirred mills in coarse grinding

Stirred mills are primarily used for fine and ultra-fine grinding. They dominate these grinding applications because greater stress intensity can be delivered in stirred mills and they can achieve better energy efficiency than ball mills in fine and ultra-fine grinding. Investigations were conducted on whether the greater performance of stirred mills over ball mills in fine grinding can be extended to coarse grinding applications. Four different laboratory ball mills and stirred mills have been tested to grind seven ore samples with feed sizes ranging from 3.35 mm to 150 μm. A case study on full scale operations of a 2.6 MW IsaMill replacing the existing 4 MW regrind ball mill at Kumtor Gold Mine in Kyrgyzstan is also included. This paper summarizes the major findings from these investigations.     

Modeling breakage rates in mills with impact energy spectra and ultra fast load cell data

A new era in modeling particle size distribution in grinding mills started at the beginning of 2000s. A direct estimation of breakage parameters became possible via computation of collision energy by discrete element method (DEM) and material breakage data.The material breakage data can be obtained for primary modes of breakage. In this study, impact and abrasion are assumed to be the primary modes of particle breakage, which are readily studied in the laboratory. The impact breakage mode is studied in a drop-weight apparatus and in a specialized device known as the ultra fast load cell. The abrasion mode of breakage is studied in a laboratory scale ball mill. Next, the particle breakage versus energy data is converted into breakage rates via impact energy spectra of the grinding mill computed by a DEM code. The fundamental material breakage information is converted into energy based breakage distribution function.The verification of the modeling concepts is shown for a 90 cm laboratory scale ball mill. In the batch mill, approximately a 10 kg mass of limestone in the 30 mm size is ground with around 100 kg of 50 mm steel ball charge. The breakage rate and the breakage distribution functions constitute the parameters of the energy based batch population balance model. It is shown that accurate particle size distribution predictions are possible with this modeling approach for different grinding regimes.     

Value adding limestone to filler grade through an ultra-fine grinding process in jet mill for use in plastic industries

In this study, ultra-fine grinding of limestone was carried out in jet mill using four levels of classifier rotational speed and grinding pressure. The holdup amount was determined during the grinding process, while the feed rate was kept constant at 8 kg/h. The ground product was characterized for its particle size and shape. In addition, the mechanochemical effect on the ground product was characterized through XRD. The particles size of the ground product ranged from 2.21 μm to 7.29 μm, demonstrating various particle shapes such as cubical, angular, and elongated. The degree of crystallinity of the ground product ranged from 54.5% to 93.7%. Afterwards, the ground product was incorporated as filler in polypropylene (PP), and its performance was characterized for mechanical properties. After conducting the test work, we find that the PP filled with ground limestone exhibited excellent thermal and mechanical properties. The composite flexural modulus, impact strength, tensile strength, and elongation at break were 2.1 GPa, 42 kJ/m², 22.75 MPa, and 21%, respectively, when loaded up to 20%. It likewise exhibited CTE value of 57.2 ppm/°C.     

Dual energy rapid scan radiography for geometallurgy evaluation and isolation of trace mineral particles

Procedures are described for the evaluation and isolation of trace mineral particles from sample populations at the ppm level by dual energy rapid scan radiography using an X-ray microtomography instrument (XMT). After calibration of the XMT, the samples are split into narrow size fractions, each size fraction distributed/assembled on projection plates, and then the plates placed in the sample holder of the XMT for irradiation at two energy levels (dual energy analysis). In this way, for example, more than 200,000 particles of 250 × 150 μm in size can be interrogated in less than 1 h and the composition of particles containing high density mineral phases estimated. These trace particles can be isolated for 3D analysis by high resolution X-ray microtomography and/or selected for surface characterization using XPS, TOF/SIMS, or other analytical instruments. Rapid scan radiography can be used for the examination of drill core samples, tailings samples, or any other particulate sample containing trace mineral particles.     

Disintegration mode of bauxite and selective separation of Al and Si

This paper reports research on solving a problem in China concerning the most suitable particle size for flotation and the optimum particle size of the concentrate required for the subsequent Bayer Process. The results showed that different types of grinding medium led to different grinding effects, the following being the order of the selective grinding effect: (column + ball) > ball > rod. A novel process of “selective grinding-flotation” has been developed and has been successfully put into industrial application in China from 2003.     

The effects of chamber diameter and stirrer design on dry horizontal stirred mill performance

This paper focussed on investigating the effects of chamber diameter and stirrer design on cement grinding performance of a horizontal type dry stirred mill. Within the scope, pilot scale test works were undertaken with two different chamber diameters (20.4 cm and 26.4 cm) having the same length and three different stirrer designs (wing, cross and disc) having the same diameter (16 cm). The chamber diameter tests were performed at the same stirrer design, media size and media filling. The studies concluded that, the use of larger chamber improved the grinding efficiency since 31.8% and 35.8% less energy was consumed than the smaller mill at the RR_d50 of 1.41 and 1.66 respectively. This behaviour of the larger mill can be attributed to the increased gap distance between the chamber wall and stirrer edge. With regards to stirrer design, the statistical evaluations, grinding results and temperature measurements all indicated that the disc design of stirrer ground the particles more effectively at high energy levels (>40 kW h/t). The use of the disc design reduced the energy consumption by 21% (at RR_d50 of 3.5). This was attributed to dissipation of energy as heat since the temperature measured for the wing and cross types were higher than the disc type.     

Exploratory modelling of grinding pressure within a compressed particle bed

With increasing industry interest in high pressure roll grinding (HPGR) technology, there is a strong incentive for improved understanding of the nature of grinding pressure that exists in the interior of a compressed particle bed. This corresponds to the crushing region of the HPGR. The relationship between applied pressure (stress) to the particle bed and induced pressure (stress) within particles and at contact points between particles is of particular interest. A detailed parametric investigation is beyond the scope of this exploratory paper. However, this exploratory investigation does suggest some interesting behaviour.The compressed particle bed within an 80 mm diameter piston has been modelled using Particle Flow Code for three dimensions. PFC3D is a discrete element code. The total number of simulated particles was 1225 and 2450 for two beds of different thickness. Particle diameters were uniformly distributed between 4 and 4.5 mm. The results of the simulations show that stress intensity within the simulated particle beds and within the observed particles increased with increase of the applied stress. The intensity of the average vertical stress in the selected particles tended to be comparable with the intensity of the pressure applied to the surface of particle bed and was only occasionally higher. However, the stress at contact points between particles could be several times higher. In a real crusher, such high stress amplification at contacts will quickly decrease due to local crushing and a resultant increase the size of the contact area. Therefore, its significance is likely to be relatively small in an industrial context.The modelling results also suggest that failure within the particle bed will progress from the crushing surface towards the depth of the bed.     

Effect of grinding parameters on the rheology of pyrite–heptane slurry in a laboratory stirred media mill

The influences of wet ultra-fine grinding parameters on the rheological behavior of pyrite–heptane slurry in a laboratory stirred media mill were investigated with solid concentration, dispersant dosage, grinding time and carbon numbers of organic acid as dispersant. The results reveal that when the solids concentration is increased from 64 wt% to 79 wt%, the rheological behavior of slurry with 1 wt% of stearic acid transforms from Bingham characteristic to the pseudoplastic one with a yield stress. The Casson model fits well for the experimental data. And the apparent viscosity of the pyrite–heptane slurry increases exponentially with increase solid concentration at a given shear rate. The increase of viscosity is propitious to reduce the particle size of pyrite. When the solid concentration is 64 wt%, stearic acid is superior to octadecanol for the reduction of the slurry viscosity, and the slurry with 1 wt% of stearic acid possesses the best flowability. The extrapolated Bingham yield stress with dispersant almost stays constant when the dosage is over 2 wt%. Besides, the rheological behavior and particle size are also related to the grinding time and carbon numbers of organic acids as dispersant.     

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.     

The effect of feed moisture on the comminution efficiency of HPGR circuits

The comminution efficiency of high-pressure-grinding-rolls (HPGRs) is a well described function of a number of feed parameters including grindability, abrasion index, granulometric composition, top size and particle size distribution. Far less studied is the effect of feed moisture. This paper investigates both the overall and the specific comminution efficiency of a circuit consisting of a pilot HPGR unit followed by a batch ball mill as a function of the moisture level in the HPGR feed. Forsterite olivine sand (−7 mm) supplied by Sibelco Nordic was used as feed material. The results showed that the relationship between moisture and crushing efficiency for both the HPGR and the circuit can be described successfully by means of a parabolic function. Dry material, as well as that with the highest moisture content, showed the lowest particle size reduction ratios irrespective of the specific grinding force level. The paper also analyses the phenomenon of flake generation and shows that the feed moisture influences the flake content in the coarser size fractions of the HPGR product.     

Determination of the concentrations of calcium and magnesium released from fluid inclusions of sphalerite and quartz

This study investigates the morphology of fluid inclusions trapped within a natural sphalerite and its closely intergrown quartz, and determines the total concentrations of calcium (Ca) and magnesium (Mg) released from the inclusions after grinding. The results indicated that numerous fluid inclusions with sizes from few microns to dozens of microns exist in the sphalerite and quartz. The inclusions contain abundant Ca and Mg. However, the concentrations of Ca and Mg in the inclusions of quartz are significantly lower than those of sphalerite. The inclusions were broken during the grinding process, releasing Ca and Mg to the solution; these released concentrations increased with the increase of grinding time. The maximum concentrations of Ca and Mg released from the fluid inclusions were 61.19 ppm and 5.23 ppm for sphalerite, and 3.28 ppm and 0.21 ppm for quartz, respectively. This study provides new understanding for the source of Ca and Mg in flotation pulp.     

Multicolumn solid phase extraction with hybrid adsorbent and rapid determination of Au,Pd and Pt in geological samples by GF-AAS

A novel hybrid adsorbent (HA) composed of cellulose fiber, activated carbon, and anion exchange resin Dowex 1 × 8 was prepared for the preconcentration and separation of noble metals, namely, gold (Au), palladium (Pd) and platinum (Pt), in geological samples. The optimal experimental parameters, such as flow rate, sample volume and interfering ions, were investigated. The accuracy of the method was confirmed by added/found method for tap and sea water, and evaluated by analyzing certified reference materials with good agreement. Under the optimal experimental conditions, the detection limits (3σ criteria) of the developed technique were 0.008 ng mL⁻¹ (Au), 0.017 ng mL⁻¹ (Pd) and 0.014 ng mL⁻¹ (Pt) and the sample throughput reach to 30 samples every eight hours. Moreover, the adsorption capacity of HA for Au, Pd and Pt was determined to be 48.2, 35.9 and 29.8 mg g⁻¹, respectively.     

Determination of the milling parameters of a platinum group minerals ore to optimize product size distribution for flotation purposes

Most concentrators desire to operate under optimal design configuration that guarantees high mineral recovery and low operational costs. The optimal design configurations are determined through studying the material to be milled in a laboratory mill under standard conditions. This is achieved through determining the selection and breakage function parameters and applying the mathematical simulation of the grinding process in order to optimize the size reduction process. The desired particle size is determined by the downstream processes, in our case, flotation. To this end, three mono-size classes feeds 850–600 μm, 600–425 μm and 425–300 μm of a platinum ore were ground using three different ball sizes (10, 20 and 30 mm) in a laboratory mill for the grinding times 0.5, 1, 2, 4, 8, 15 and 30 min. The data collected was used to determine breakage and some of the selection function parameters. The remaining parameters were back-calculated within the population balance model framework. The parameters were then used to obtain the product size distribution (PSD) that was later compared with the experimentally measured one. The milling kinetics for the desired size class for flotation was also simulated.There was a good match between the predicted and the experimentally measured PSD. The results of the milling done for further 60, 90, 120 and 240 min to validate the simulated milling kinetics from the determined parameters also showed good match between the simulated and the experimental one. This further confirms the validity of the determined parameters. From this, it becomes possible to determine the grinding conditions for optimal flotation.