Autogenous Mills Research Articles

Effect of particle shape and roughness of talc mineral ground by different mills on the wettability and floatability

In this study, shape characteristics of talc particles produced by ball, rod and autogenous mills were investigated using Scanning Electron Microscope (SEM) and expressed by the shape descriptors such as elongation, flatness, roundness and relative width by measuring on the particle projections in two dimensions (2D). Approximately, 139 particles were measured for image analysis from each mill product. Surface roughness values of talc mineral have been expressed by the parameters of R a value on the pelleted surfaces of the particles by employing Surtronic 3 + instrument. The wettability characteristics ( γ c) of talc mineral, produced by three different mills, were also determined by microflotation and the contact angle measurement techniques using the EMDEE MicroFLOT cell and Rame-Hart goniometer, respectively. Some correlations were established between the shape properties, surface roughness values and the wettability characteristics. The results have shown that elongation and smoothness helped to increase the hydrophobicity, while roundness and roughness caused a decrease in hydrophobicity or floatability for the talc mineral studied.

An alternative energy–size relationship to that proposed by Bond for the design and optimisation of grinding circuits

Despite reservations of a number of researchers concerning the applicability of Bond's equation for relating specific energy to the size reduction of rocks, his approach has become an industry standard. Data from a large number of pilot and full-scale autogenous (AG), semi-autogenous (SAG) and ball mill circuits are used to show that Bond's equation does not hold over the particle size range −100 mm+0.1 mm. However, if suitable correction factors are applied its results can be made to approximate observed values. It is shown that a more appropriate relationship relies on a particle size exponent that is a function of size. This new relationship, which does not rely on the need for correction factors, is used to predict the specific energy requirements of a number of autogenous, semi-autogenous and ball mill grinding circuits. The results are compared to those obtained using Bond's equation and are shown to provide a more precise prediction of specific power requirements.

Role of shape properties of calcite and barite particles on apparent hydrophobicity

Shape properties of calcite and barite particles ground by ball, rod and autogenous mills were examined by using scanning electron microscope (SEM). Shape properties have been stated in terms of shape descriptors such as elongation ratio, flatness, roundness and relative width by measuring about 200 particles. The results were correlated with the critical surface tension ( γ c) of wetting determined by contact angle and micro-flotation methods. The correlations set between the shape properties and the wettability for the minerals studied are in the form of “ γ c= a (shape property) + b” type of equations (where a and b are constants). The results revealed that particles with higher flatness and elongation ratio had higher apparent hydrophobicity without being affected by the mill type.

A new autogenous and semi-autogenous mill model for scale-up, design and optimisation

The successful application of modelling and simulation in design and optimisation rests heavily on having a model which is lifelike. In the case of autogenous (AG) and semi-autogenous (SAG) mills this has provided a significant challenge to modellers, due in part to the interaction between feed characteristics and mill performance and also due to the difficulty in obtaining high quality relevant data from full-scale circuits. As a result the literature contains numerous papers on AG/SAG mill modelling, the majority of which contain little or no data to indicate how well the models predict full-scale mill performance. In an attempt to address this problem a new AG/SAG mill model has been developed which incorporates the capability to respond to changes in most design and operational variables. The model can be used as a stand-alone design tool, to scale-up from pilot data and to optimise existing circuits. It predicts throughput, power draw and product size distribution as well as load size distributions and slurry hold-up. The overall structure of the model and the sub-processes that it contains is described. The model is successfully validated using data from over 21 different full-scale circuits.

Predicting the specific energy of autogenous and semi-autogenous mills from small diameter drill core samples

A rock breakage test has been developed that can help predict autogenous (AG) and semi-autogenous (SAG) mill performance in situations where only limited quantities of rock samples are available. The test was originally developed to make use of quartered (slivered) core but whole core, half core and lump material can also be used. Original core diameters in the range 27–85 mm are suitable. The samples, once broken, can be used subsequently for Bond ball mill work index or batch grinding tests, therefore limiting overall sample requirements. The test generates an index (DW i ) that can be used to estimate the throughput of AG and SAG circuits through a combination of power-based and model-based approaches. The model-based approach makes use of the direct relationship between the DW i and the JK rock breakage parameters A and b. The power-based route uses correlations which have been developed between the DW i and the specific energies of a very wide range of operating AG and SAG circuits. Its usefulness also extends to rock mass characterisation in mining applications, as it is also correlated with the point load index/UCS. It is therefore ideally suited for mine-to-mill studies where it can be simultaneously used to predict comminution circuit performance and to augment input to blast fragmentation models. This makes it a useful tool for orebody profiling in greenfield, brownfield and established operations as, in conjunction with a mine’s block model, a detailed picture can built of the blast fragmentation and comminution circuit response.

Variation of critical surface tension for wetting of minerals with roughness determined by Surtronic 3+ instrument

This paper presents a correlation found between the surface roughness and critical surface tension of wetting of calcite and barite minerals. The minerals were ground in ball, rod and autogenous mills to collect samples for further tests (i.e. roughness and wettability). The surface roughness values of those minerals were determined on the pelleted surfaces of the ground powders by employing Surtronic 3+ roughness-meter instrument. The contact angle measurements were also made by a goniometer on these pellets in order to determine the critical surface tension of wetting of calcite and barite minerals. From the experimental work, it is stated that the wettability degree of calcite and barite minerals increase with their increasing surface roughness. The relationships could be stated as: γc=aRa+b type of equations; where γc is the critical surface tension of wetting, Ra is the surface roughness measured, a is the slope and b is the intercept of the regression line.

Variation of critical surface tension for wetting of minerals with roughness determined by Surtronic 3 + instrument

This paper presents a correlation found between the surface roughness and critical surface tension of wetting of calcite and barite minerals. The minerals were ground in ball, rod and autogenous mills to collect samples for further tests (i.e. roughness and wettability). The surface roughness values of those minerals were determined on the pelleted surfaces of the ground powders by employing Surtronic 3 + roughness-meter instrument. The contact angle measurements were also made by a goniometer on these pellets in order to determine the critical surface tension of wetting of calcite and barite minerals. From the experimental work, it is stated that the wettability degree of calcite and barite minerals increase with their increasing surface roughness. The relationships could be stated as: γ c= aR a+ b type of equations; where γ c is the critical surface tension of wetting, R a is the surface roughness measured, a is the slope and b is the intercept of the regression line.

Determination of the shape, morphological and wettability properties of quartz and their correlations

The distributions of particle sizes and shapes are of great importance to industries employing various materials in powdered form. These include many of the raw materials in civil and chemical engineering, pharmaceutical, and mining industries; pigments, metals, ceramics, pills, foods; industries with interests in population of atmospheric dusts, smoke, and grit. The flow of fluids through beds of particles, the flowability of powders, bulk flow, and packing necessitate a considerable knowledge of the size and shape of constituent particles. In this paper, experimental studies to determine the shape properties, surface roughness and wettability of quartz mineral (a very common component many raw materials) were performed for the products of ball, rod and autogenous mills. Shape properties have been stated in terms of shape descriptors such as elongation, flatness, roundness, and relative width by measuring on the projections of particles using scanning electron microscope (SEM). Surface roughness values have been expressed by the parameters of R a value on the pelleted surfaces of the ground powders by employing Surtronic 3 + instrument. The wettability characteristics ( γ c) of quartz mineral determined by microflotation technique using the EMDEE MicroFLOT test tube. Finally, some correlations were found between the shape properties, surface roughness values and wettability. The results have shown that, the γ c values increase with increasing roughness and roundness, i.e., elongated and smooth particles having lower γ c values indicated more hydrophobicity.

Slurry flow in mills: grate-only discharge mechanism (Part-1)

Discharge grates play an important role in determining the performance of autogenous, semi-autogenous and grate discharge ball mills. The flow capacity (grinding capacity) of these mills is strongly influenced by the discharge grate design––open area and position of apertures, as well as the performance of the pulp lifters. As mill sizes have progressively increased and closed-circuiting has become more popular the importance of grate and pulp lifter design has grown. Unfortunately very few studies have concentrated on this aspect of mill performance. To remedy this a series of laboratory and pilot-scale tests were undertaken to study both the performance of grates on their own and in conjunction with pulp lifters. In this first paper of a two-part series the results from the grate-only experiments are presented and discussed, whilst the performance of the grate-pulp-lifter system is covered in the second paper. The results from the grate-only experiments have shown that the build-up of slurry (hold-up) inside the mill starts from the shoulder of the charge, while the toe position of the slurry progressively moves towards the toe of the charge with increasing flowrate. Besides grate design (open area and position of apertures), charge volume and mill speed were also found to have a strong influence on mill hold-up and interact with grate design variables.

Slurry flow in mills: grate-pulp lifter discharge systems (Part 2)

Pulp lifters, also known, as pan lifters are an integral part of the majority of autogenous (AG), semi-autogenous (SAG) and grate discharge ball mills. The performance of the pulp lifters in conjunction with grate design determines the ultimate flow capacity of these mills. Although the function of the pulp lifters is simply to transport the slurry passed through the discharge grate into the discharge trunnion, their performance depends on their design as well as that of the grate and operating conditions such as mill speed and charge level. However, little or no work has been reported on the performance of grate-pulp lifter assemblies and in particular the influence of pulp lifter design on slurry transport. Ideally, the discharge rate through a grate-pulp lifter assembly should be equal to the discharge rate through at a given mill hold-up. However, the results obtained have shown that conventional pulp lifter designs cause considerable restrictions to flow resulting in reduced flow capacity. In this second of a two-part series of papers the performance of conventional pulp lifters (radial and spiral designs) is described and is based on extensive test work carried out in a 1 m diameter pilot SAG mill.

Axial mixing of slurry in an autogenous grinding mill

Direct measurement of load behaviour has provided increased insight into milling operations. A facet of load behaviour of particular importance to simulation and design of mills is the axial mixing of the load. Three different experiments have been conducted on an industrial autogenous grinding mill. The axial mixing model has been fitted to the data obtained from these experiments. It was found that the slurry movement in the mill tends towards rather than a more mixed situation with a vessel dispersion number, N d, varying between 0.03 and 0.45.

A Load-Interactive Model for Predicting the Performance of Autogenous and Semi-Autogenous Mills

The Julius Kruttschnitt Mineral Research Centre (JKMRC) has been involved in the study and modelling of industrial autogenous (AG) and semi-autogenous (SAG) mills for over 25 years. Recent research at the JKMRC has developed a new AG/SAG mill model which is based on charge dynamics. The model relates charge motion and composition to power draw and size reduction. Size reduction is described by considering impact and attrition/abrasion as separate processes. These are linked to energy available in the mill, the charge size distribution and the relative motion of the grinding media. Ore specific energy-breakage are described using laboratory data which are obtained from breakage tests over a wide range of energies and particle sizes. Slurry transport is described using the JKMRC’s latest model which incorporates the effect of grate design. This paper describes the overall structure of the model together with its main sub-processes.

Model-based procedure for scale-up of wet, overflow ball mills part I: outline of the methodology

Bond's method for ball mill scale-up only gives the mill power draw for a given duty. This method is incompatible with computer modelling and simulation techniques. It might not be applicable for the design of fine grinding ball mills and ball mills preceded by autogenous and semi-autogenous grinding mills. Model-based ball mill scale-up methods have not been validated using a wide range of full-scale circuit data. Their accuracy is therefore questionable. Some of these methods also need expensive pilot testing. A new ball mill scale-up procedure is developed which does not have these limitations. This procedure uses data from two laboratory tests to determine the parameters of a ball mill model. A set of scale-up criteria then scales-up these parameters. The procedure uses the scaled-up parameters to simulate the steady state performance of full-scale mill circuits. At the end of the simulation, the scale-up procedure gives the size distribution, the volumetric flowrate and the mass flowrate of all the streams in the circuit, and the mill power draw.

Model-based procedure for scale-up of wet, overflow ball mills part III: validation and discussion

A new ball mill scale-up procedure is developed. This procedure has been validated using seven sets of full-scale ball mil data. The largest ball mills in these data have diameters (inside liners) of 6.58m. The procedure can predict the 80% passing size of the circuit product to within ±6% of the measured value, with a precision of ±11% (one standard deviation); the re-circulating load to within ±33% of the mass-balanced value (this error margin is within the uncertainty associated with the determination of the re-circulating load); and the mill power to within ±5% of the measured value. This procedure is applicable for the design of ball mills which are preceded by autogenous (AG) mills, semi-autogenous (SAG) mills, crushers and flotation circuits.The new procedure is more precise and more accurate than Bond's method for ball mill scale-up. This procedure contains no efficiency correction which relates to the mill diameter. This suggests that, within the range of mill diameter studied, milling efficiency does not vary with mill diameter. This is in contrast with Bond's equation—Bond claimed that milling efficiency increases with mill diameter.

Exploration of mill power modelled as function of load behaviour

A grinding mill power model that is a function of load behaviour parameters was previously calibrated against data collected on an industrial autogenous mill. This model is used to explore the impact of various load behaviour parameters on the power drawn by the mill. This can reveal the optimal operating conditions for a mill. It is shown that premature centrifuging and angle of repose of the load have the most impact on the power drawn by the mill. The power model is also used to investigate the impact of a control strategy on the power drawn by the mill. It is shown that the constant load mass control strategy can lead to sub- optimal mill usage.

The use of the discrete element method and fracture mechanics to improve grinding rate prediction

The Discrete Element Method (DEM) is a numerical technique that can simulate the interaction of discrete particles in dynamic environments such as fluidised beds, jigs, flow in bins, screens and mills. This technique has great application potential in comminution modelling at micro-process level. The DEM is being used to determine the grinding rate of ore particles in an Autogenous mill. The frequency of the contact events and the associated energy dissipations derived from the simulation are used to determine the particle failure rate. The probability of particle failure also depends on the inherent fracture properties of a material; hence fracture tests were conducted on an ore sample using a drop-weight impact test machine to obtain a probability fracture model. A second model that uses energy dissipation spectra from the DEM and probability fracture model calculations has been proposed. Though the model tends to over-predict breakage generally, there are indications that the model is responsive to changes in the load behaviour.

Modification of ball mill simulator models to allow for unequal residence times

Equations are derived to allow for different mean velocities of axial flow for different sizes of particles in fully mixed ball mills. The equations are modified to allow for the exit classification that occurs in grate-discharge autogenous and semiautogenous tumbling mills. An approximate treatment is given that applies for other forms of residence time distribution, provided that the mill contents are close to fully mixed.

An approach to rock size measurement based on a model of the human visual system

This paper describes research into the development of an instrument for the purpose of performing online measurement of rock size distributions using machine vision. Such an instrument would have application in the gold mining industry where it could be used to measure the fragmentation of gold ore on a conveyor belt feed to an autogenous mill, for the purpose of controlling the mill. A computation structure has been developed to identify and delineate rocks in an image for the purpose of measuring their areas. It is based on the human visual system in that it consists of a low-level preattentive vision stage and a higher-level stage of attention focusing. Multiscalar image processing techniques have also been integrated in order to improve the detection of rocks across a wide range of sizes. A performance advantage can be obtained in this way because all the algorithms can be better matched to the size of objects being detected.

Development of an empirical dynamic simulator for the analysis of control strategies in an autogenous grinding circuit : J. Valenzuela, R. Del Villar, M. Bourassa & S. St.Arnaud, CIM Bulletin, 88(995), 1995, pp 60–68

Autogenous grinding is used quite frequently for mineral comminution as a result of its lower capital and operating costs. However, it is more difficult to control than conventional grinding, particularly when the autogenous or semiautogenous mill is the only comminution stage (i.e. no secondary grinding is available). Analysis of the control strategy used then becomes extremely important, a task that is greatly simplified by the use of an empirical dynamic simulator. This paper presents the methodology used in the development and the calibration of such a simulator for the autogenous grinding circuit of an iron ore concentrator. The various steps are described : industrial data acquisition, preliminary treatment of the gathered information, and empirical modelling of the process. Finally, both the actual and the proposed control strategies are compared using simulation results. This exercise has shown that the variability of some of the controlled variables could be reduced through a better handling of the water addition and the use of filters on some of the signals used for control purposes. The reduction in variability of critical variables, such as power draw, would allow their average value to be increased without the risk of exceeding their safety values. Some advanced control techniques were also tested, with interesting results.

Slurry discharge capacity of autogenous and semi-autogenous mills and the effect of grate design

A laboratory based study of tumbling mills has been undertaken in which the mill speed, grate open area and the position of the apertures were systematically varied and their effect on the relationship between water flowrate and hold-up measured. It was found that flow occurred in two distinct zones. One was associated with flow through the grinding media and the other through the pool which developed at higher flowrates in the toe region of the charge. Equations were developed which described the relationship between hold-up and flowrate in each of these zones. These equations were then calibrated using a range of data from pilot and full scale mills where the equilibrium slurry contents and flowrate had been accurately measured. Further validation was then conducted by using sets of independent data. The resultant equations were found to predict the full scale plant data well, covering a range of mill diameters up to 9.6 m and including open as well as closed circuit operations. It is expected that the equations will be of use to plant/equipment designers and researchers interested in modelling the behaviour of autogenous and semi-autogenous mills.