Beitrag zur feinstmahlung von industriemineralen

Industrial minerals of a high degree of fineness such as kaolinite, calcite and barite are used to an increasing extent in the chemical industries as filling material in the manu-facturing of paper, paints and plastics or as ingredients of ceramic products. Grains smaller than 5 μm can be produced by dry grinding in a jet mill or by wet grinding in drum mills. To avoid impurities due to wear, these mills are lined with rubber or plastic and ceramic balls are used as grinding bodies.Our investigations in a jet mill show that the fineness of the ground product depends on the feed rate and on the air volume. The grain sizes received in the drum mill are influenced by feed rate, pulp density, speed of rotation and by the grinding bodies (density, size).REM-photographs of mill discharge in the grain-size ranges 10-5 μm and below 5 μm indicate that in both the grains mainly show crystal planes as boundaries. After dry grinding the grains show smooth and clean faces; thus is traced back to the impact effect of the air streams. In wet grinding the fine grains are very prone to form agglomerates.     

A theoretical analysis and control study of open-circuit grinding

Regrinding is an essential step in many mineral processing flowsheets as a final preparatory step for mineral separation. It is normally carried out in open-circuit ball mills or rod mills. Wide fluctuations occurring in the hardness and size distribution of feed materials to regrinding mills result in a nonuniform product fineness and processing inefficiency. Using a previously developed regrinding model, this paper presents the dynamic analysis and design considerations of a control system for open-circuit grinding, using the traditional P-, PI-, and PID-control algorithms. This study shows the utility of simple analysis techniques in designing a mineral process control system with special attention given to the effect of sampling and analysis time on control system performance.     

Slurry flow in mills with TCPL — An efficient pulp lifter for ag/sag mills

The difficulties associated with slurry transportation in autogenous (ag) and semi-autogenous (sag) grinding mills have become more apparent in recent years with the increasing trend to build larger diameter mills for grinding high tonnages. This is particularly noticeable when ag/sag mills are run in closed circuit with classifiers such as fine screens/cyclones.     

Production of carbonate and silica nano-particles in stirred bead milling

This paper presents the experimental results of the mechanical production of silica and carbonate colloidal particles below 100 nm using two types of stirred bead mills (i.e., DCP Superflow 12 and PML H/V). It is shown that the stirred bead mill with very small beads can be used as an efficient equipment for the production of the colloidal particles in nanoscale from the feed materials of several microns in sizes at high energy consumptions. The DCP Superflow mill with high power densities is superior for the effective size reduction and production rate, compared to the conventional PML H/V mill with lower power densities. The smaller particles could be produced by the DCP Superflow mill at the same level of high energy inputs as from the PML H/V mill. The “grinding limit” for the processes in the mills has been discussed.     

Analysis of the gold recovery profile through a cyanidation plant

Leaching in the grinding circuit is currently practiced in plants that process gold ores with low content of cyanide consuming minerals. The high gold recovery observed in the grinding–classification section of the plant is commonly explained by the inherent high initial dissolution rate of gold observed in gold ore cyanidation, or by the intense agitation prevailing in grinding mills. For understanding this leaching behavior of gold ores, the grinding, classification and leaching sections of a gold processing plant are analyzed through reconciliation of operating data. It is found that gold circulation in the grinding circuit is quite different from the flow behavior of other species due to the strong gold separation effect in the hydrocyclones, which produces quite large residence time for gold-rich particles. The results presented in this study demonstrate that the residence time of these particles in the grinding circuit can be as long as in the leaching tanks and might be the dominant factor in explaining the high dissolution of gold in this section.     

Comparative study of microstructural characteristics and stored energy of mechanically activated hematite in different grinding environments

Hematite concentrate was mechanically treated using different milling machines and experimental conditions in air atmosphere. The changes in phase constitution, particles size, specific surface area, lattice parameters and X-ray amorphous phase fraction of activated hematite were determined. It was found that the agglomeration of the particles take place during extended milling with accessible pores for Nitrogen gas. The higher media surface brought about the largest specific surface area whatever milling devices used. After 9 h of grinding with higher media surface, the maximum and minimum specific surface area resulted from the grinding in the tumbling and vibratory mills, accounting for 6.83 m²/g and 18.42 m²/g, respectively. For the same grinding condition, tumbling mill produced the lowest X-ray amorphous phase. The maximum X-ray amorphous material estimated around 85% from the grinding in the planetary mill with higher media surface for 9 h of milling.     

Role of feed moisture in high-pressure roll mill comminution

The application of high-pressure roll mills (HPRM) for industrial comminution has been growing, particularly because of its energy efficiency. Many aspects of HPRM comminution have not been fully characterized because the performance of the machine is affected by numerous parameters that still need to be investigated and optimized. The present paper is concerned with the effect of feed moisture on HPRM performance. The comminution of minus 8-mesh dolomite feed at several different moisture levels was conducted with a laboratory HPRM to delineate the effect of feed moisture on product characteristics, specific energy consumption, and such mill operating parameters as applied load, roll gap and roll speed. The results showed definite effects of moisture on these parameters and the product size distribution, most likely because of lubrication of the particles in the compacting bed as they pass through the roll gap.     

Environmental effects on grinding

The effect of surface-active agents in improving the grinding efficiency for haematite, expressed as a measure of reduction ratio, has been studied. Drying of the feed before grinding has been found to improve the grinding efficiency. The effect of surfactant additions on hardness, microhardness, crushing strength and wet grinding of haematite has shown that dispersants are better than flocculants, and each dispersant has a critical concentration at which the grinding efficiency is maximum.     

A laboratory study of high chromium alloy wear in phosphate grinding mill

Grinding mills are commonly used in the Florida phosphate industry to reduce particle size. The corrosion of metallic grinding media and mill liner is a very serious problem, particularly under acidic conditions as encountered in the Florida phosphate fertilizer industry. A statistical Box–Behnken Design (BBD) of experiments was performed to evaluate the effects of individual operating variables and their interactions on the wear rate of high chromium alloy during phosphate grinding. The variables examined in this study included grinding time, solution pH, rotation speed, mill crop load, and solids percentage. The wear tests were conducted using a specially designed grinding mill whose electrochemical potential can be controlled. The most significant variables and optimum conditions were identified from statistical analysis of the experimental results using response surface methodology (RSM). It has been shown that solution pH had the most significant effect on the wear rate. The optimum process parameters for minimum wear rate were solution pH at 8.7, rotation speed at 61 rpm, solid percentage at 65% and crop load at 58%.     

An Evaluation of the Effects of Primary and Cross‐Contamination during the Preparation of Rock Powders for Chemical Determinations

Quartz crystals from the Batatal site, Minas Gerais, southern Brazil, were used to determine quantitatively primary contamination when submitting the samples to milling processes. Crushing devices used were a primary steel jaw crusher (br1), a secondary tungsten carbide (WC) crusher (br2) and a hydraulic press with stainless steel plates (pr). Fragments with suitable sizes were then ground in different Fritsch planetary mills, equipped with agate, WC and chromium‐steel rings for 7, 4 and 3 min, respectively. Solutions of the powders were analysed initially with the TotalQuant^? method in a quadrupole ICP‐MS instrument, providing semi‐quantitative results for seventy‐five elements. Contamination from crushers and mills was visible in major and minor elements such as Fe, Mn and Ti, and noticeable in trace elements such as Cr, Co, Ni, Cu and also V, Zr, Sc; significant contamination was observed from W, Mo, Co, Ta and Nb (mainly by use of the WC devices). Little or no contamination was observed for Pb, REE, Sr and Rb, elements that are important in routine isotopic determinations. Cross‐contamination was tested by grinding either a granite or a basalt sample, followed by conventional cleaning with mica‐free quartz sand, before the working quartz powder was prepared.     

Influence of shape characteristics of talc mineral on the column flotation behavior

Column flotation, which is a very effective process in mineral processing especially for easily floatable minerals, is one of the most important new developments to emerge in mineral processing technology in the last years. In this study, the flotation behavior of talc products having different particle shapes produced by different grinding mills (ball and rod mill) was determined by using column flotation process. Shape characteristics of the particles were investigated by the two dimensional measuring technique based on the particle projections obtained from the SEM microphotographs using a COREL Draw 10.0 program. The results showed that particles possessing higher elongation and flatness properties were recovered better during column flotation, whilst roundness and relative width had a negative effect on the flotation behavior of the talc mineral studied. Consequently, as the shape of the particles produced by the mill deviated from the ideal sphere, their floatability was increased.     

Performance of Sample Preparation for Determination of Gold in Samples of Geological Origin

We investigate the performance of sample preparation of gold ores using vibratory (bowl, ring and puck type) mills in common use in mineral analytical laboratories. The main criteria for effective grinding are using reduced grinding charge masses ≤ ca. 50% of nominal bowl capacity and using a grinding aid to prevent caking. We show that gold particles of millimetre scale can be comminuted to ≤ 100 µm by grinding in silica flour, bauxite, synthetic carborundum, or mixtures of silica and these materials using times of up to 5 min and that 95% < 50 µm is achievable with extended grinding. This suggests that modified grinding techniques can be used to make sample masses ≤ 5 g viable for routine determination of gold in geological samples. We also demonstrate homogenisation of a gold‐bearing copper sulfide mineral flotation concentrate alone and in mixtures with silica by extended grinding at reduced charge masses. To support this work, we develop a convenient new benchmark of gold ore sample preparation performance ‘G’, an apparent maximum gold particle size interpolated from replicate analytical variance in order to overcome the limitations of laborious sieve fraction analysis of gold particle size. We show useful agreement between G and sieve fraction analysis of gold particle size in samples and test the viability of G experimentally and by analysis of literature data.     

Grinding of mineral mixtures in high-pressure grinding rolls

Attempts at improving comminution machines generally have been directed towards increasing the performance efficiency, particularly increasing throughput rate and decreasing energy consumption. The latest and most successful new comminution technology has been the high-pressure grinding rolls (HPGR), which have proved to be highly efficient in energy consumption and to have a relatively high throughput rate at low steel consumption. Already used extensively in cement plants worldwide, the first HPGRs in the mineral industry were installed in plants processing diamond ores. They are now finding their way into the large-scale base-metal mining industry. Since feed constituents in natural ores vary in their physical properties such as hardness, plasticity and brittleness, the present paper is concerned with an investigation of the behavior of a heterogeneous feed as it passes through the HPGR. The effect of feed composition on operational parameters, the energy efficiency of comminution, and energy distribution among the feed components is addressed. Mineral particles with high hardness act as energy transfer agents in the roll gap and enhance the grinding of softer mineral particles in a mixed feed.     

The back-calculation of specific rates of breakage and non-normalized breakage distribution parameters from batch grinding data

This paper describes a mathematical approach for calculating parameters relating specific rates of breakage and breakage products distribution from batch grinding a known feed for several grinding times. Knowledge of such parameters leads to improved equipment design criteria and consistent operating correlations. The approach involves the use of non-linear optimization with appropriate statistical tests and gives parameter estimates close to those gained from direct experimental measurements. Some results and experiences using the technique are summarized.     

Distribution function of comminution kinetics — Modelling and experimental study

A mathematical model for distribution function is developed considering the distribution of the number of fractures that can occur in a particle. It is shown as a consequence of the model that selection and distribution functions are intimately related through a single parameter — expected number of fractures per unit original size. This parameter can be comparatively easily obtained from batch experiments and can later be used to simulate industrial-scale grinding mills. Predictions of the model are amply confirmed by preliminary experimental results and other published observations. A correlation is developed to predict the parameter of the model from known equipment and operating variables for the particles used in the present study.     

Modelling mineral size reduction in the closed-circuit ball mill at the Pine Point Mines concentrator

Using plug flow material transport and a cumulative-basis rate-of-breakage parameter, overall size reduction through the closed-circuit ball mills at the Pine Point and Gibraltar concentrators was simulated over a wide range of operating conditions. The rate-of-brakage parameter was related to particle size by a power law, the exponent (n) being: Pine Point, n = 1.043 ± 0.026, and Gibraltar, n = 0.747 ± 0.020. The success of this approach probably stems from the high (> 1.5) circulating load ratios encountered.By analogy individual mineral size reduction at Pine Point was examined. A similar rate-of-breakage parameter versus size relationship was found. Pyrite was the hardest mineral, but fine galena was equally resistant. However, the approximation that mineral and overall rates of breakage were the same gave an adequate fit to the mineral size reduction. This was emphasized by combining with a cyclone model to simulate cyclone overflow mineral size distribution. A more accurate cyclone model is shown to be more important in simulating mineral deportment at Pine Point.Complementary laboratory batch grinding tests were conducted on rod mill discharge and ball mill feed samples. Sufficient agreement with the first-order hypothesis was observed to analyse the rate-of-breakage parameter. The kinetics was similar for both samples and in turn similar to the plant-derived kinetics in terms of relative mineral rates-of-breakage and the relationship of the rate-of-breakage parameter with particle size.     

CFD simulation and experimental analysis of flow dynamics and grinding performance of opposed fluidized bed air jet mill

This paper presents a three dimensional Computational Fluid Dynamics (CFD) model to investigate the flow dynamics of solid–gas phases during fine grinding in an air jet mill. Alpine 100AFG fluidized bed air jet mill is considered for the study and the jet milling model is simulated using FLUENT 6.3.2 using a standard k-ε model. The model is developed in GAMBIT 2.3.16 and meshed by tet/hybrid (T-Grid) and Triangular (Pave) meshes. The effects of operating parameters such as solid feed rate, grinding air pressure and internal classifier speed on the performance of the jet mill are analyzed. The CFD simulation results are presented in the forms of dual phase vector plot, volume fraction of phases and particle trajectories during fine grinding process. The mass of ground feed entering and leaving the cyclone (underflow) is also computed by simulation. The proposed model gives realistic predictions of the flow dynamics within the jet mill. Experiments are conducted on the Alpine 100AFG jet mill to study the particle size, morphology and mass of the ground product. The numerical results are found in good agreement with the experimental results.     

Scale up of lifters in ball mills

Liner design and lifter geometry in tumbling mills are very important in comminution kinetics and energy consumption. In terms of the parameters of the batch grinding model, the breakage rate function decreases as lifter size increases while the breakage distribution function is independent of the lifter size. However, the specific energy consumption is not a function of lifter size. Reducing the lifter height from 2.54 cm to 0.32 cm increases the production from this mill by 37%. When the mill is operated without any lifters, the charge apparently slips inside the shell resulting in lower breakage kinetics and higher energy consumption.     

Effect of chemical additives on the dynamics of grinding media in wet ball mill grinding

Recent papers on the role of grinding aids in wet ball mills indicate that certain polymeric aids favorably act by altering pulp rheology, leading to improved production rates at high pulp densities. Analysis of recent data indicates that there can be an improvement in terms of the production of fine particles per unit time, but on the other hand, the production per unit of expended energy remains the same. Our experiments indicate that there is a critical pulp viscosity above which the grinding media tend to centrifuge because the balls cannot be detached from the mill wall during the time of rotation. Under these conditions the mill power draft decreases and a smaller amount of fines is generated. The polymeric additives are able to reduce the pulp viscosity to a value less than the critical value so that the balls no longer centrifuge and the mill draws full power. From an analysis of the force field acting on a ball inside the mill, this critical viscosity can be estimated and the results are in agreement with experiment.     

Kinetics of fine wet grinding of zeolite in a steel ball mill in comparison to dry grinding

Batch wet grinding of zeolite was studied with emphasis on a kinetic study in a laboratory size steel ball mill of 200 mm diameter. The breakage parameters were determined by using the single sized feed fractions of − 850 + 600 µm, − 600 + 425 µm and − 425 + 300 µm for the zeolite samples. The S_i (specific rate of breakage) and B_i,j (primary breakage distribution) values were obtained for those feed size fractions in order to predict the product size distributions by simulation for comparison to the experimental data. The specific rates of breakage values for wet grinding in the first-order breakage region were higher than the dry values reported previously by a factor 1.7 at the same experimental conditions, but the primary breakage distribution (B_i,j) values were approximately the same. The simulations of the product size distributions of zeolite were in good agreement with the experimental data using a standard ball mill simulation program. The wet grinding of zeolite was subjected to slowing-down effect in the mill at 2 min of grinding, corresponding to an 80% passing size of about 400 µm. On the other hand, the slowing down effect in the dry grinding of zeolite was also seen at 4 min of grinding. In addition, effects of some operational parameters on dry and wet grinding of zeolite were determined by simulation using the breakage parameters obtained experimentally.