Fengnian Shi

Effects of slurry rheology on industrial grinding performance

Abstract To determine the effect of slurry rheology on industrial grinding performance, 45 surveys were conducted on 16 full-scale grinding mills in five sites. Four operating variables—mill throughput, slurry density, slurry viscosity and feed fines content—were investigated. The rheology of the mill discharge slurries was measured either on-line or off-line, and the data were processed using a standard procedure to obtain a full range of flow curves. Multi-linear regression was employed as a statistical analysis tool to determine whether or not rheological effects exert an influence on industrial grinding, and to assess the influence of the four mill operating conditions on mill performance in terms of the Grinding Index, a criterion describing the overall breakage of particles across the mill. The results show that slurry influence industrial grinding. The trends of these effects on Grinding Index depend upon the rheological nature of the slurry—whether the slurries are dilatant or pseudoplastic, and whether they exhibit a high or low yield stress. The interpretation of the regression results is discussed, the observed effects are summarised, and the potential for incorporating rheological principles into process control is considered. Guidelines are established to improve industrial grinding operations based on knowledge of the rheological effects. This study confirms some trends in the effect of slurry rheology on grinding reported in the literature, and extends these to a broader understanding of the relationship between slurry properties and rheology, and their effects on industrial milling performance.

Measuring the rheology of slurries using an on-line viscometer

Abstract Slurry rheology is an important property in many mineral processing systems, but its influence is not well understood and it is rarely incorporated into process design, analysis or optimisation. One reason for this is the difficulty of measuring rheology (particulary in plants) and of interpreting the output of such measurements. This paper presents a new procedure for obtaining a full shear rate-shear stress flow curve for unstable slurries using the single bobbin Debex on-line viscometer. It is based on the use of a calibration algorithm which incorporates a correction for turbulent flow in the measurement vessel. It is shown that torque coefficients data from a variety of Newtonian fluids and non-Newtonian slurries fall on a single curve, and it is suggested that this calibration curve (the “TC curve”) is characteristic for a particular instrument configuration. In principle, it can be easily determined using Newtonian fluids alone. It is shown that the Debex on-line viscometer gives flow curves very similar to a conventional laboratory viscometer when this procedure is applied.

A model for slurry rheology

A semi-empirical model has been developed to predict slurry rheology from easily-measured slurry properties. The model demonstrates the complex influence of these properties on rheology, and also permits rheological information to be predicted in cases where it cannot be measured. It is intended for use with slurries commonly encountered in mineral processing. The model first predicts machine output readings of the on-line Debex slurry viscometer at various bobbin rotational speeds, from solids concentration, solids size distribution and temperature. The machine readings and the bobbin rpm are then converted to shear stress and shear rate using the turbulence correction method described in an associated paper. (The simulation of the Debex is a convenience; the method is essentially independent of any particular instrument if the full data reduction procedure is followed.) The model has been applied to 127 sets of Debex viscometer measurements of a variety of slurries totalling more than 1200 data points, with good agreement between the predicted and the measured data. Different slurry types require different model parameter values, but it was found that the rheology of many common slurries could be described by a single set of parameter values over a wide range of conditions. In other cases, adjustments in only one or two parameters were required. The separate effects of solids volume fraction and particle size on slurry rheological nature, simulated using the model and turbulence-corrected by the TC curve procedure, are demonstrated graphically. The influence of various factors on slurry rheology is discussed, and it is shown that a single slurry can exhibit many different rheological natures, depending only on the concentration and size distribution of the solids. This has important implications in mineral processes such as grinding and dense medium separation.

The rheology of flotation froths

A literature review has highlighted the need to measure flotation froth rheology in order to fully characterise the role of the froth in the flotation process. The initial investigation using a coaxial cylinder viscometer for froth rheology measurement led to the development of a new device employing a vane measuring head. The modified rheometer was used in industrial scale flotation tests at Mt. Isa Copper Concentrator. The measured froth rheograms show a non-Newtonian nature for the flotation froths (pseudoplastic flow). The evidence of the non-Newtonian flow has questioned the validity of application of the Laplace equation in froth motion modelling as used by a number of researchers, since the assumption of irrotational flow is violated. Correlations between the froth rheology and the froth retention time, water hold-up in the froth and concentrate grades have been found. These correlations are independent of air flow rate (test data at various air flow rates fall on one similar trend line). This implies that froth rheology may be used as a lumped parameter for other operating variables in flotation modelling and scale up

An energy-based model for swing hammer mills

An energy-based swing hammer mill model has been developed for coke oven feed preparation. it comprises a mechanistic power model to determine the dynamic internal recirculation and a perfect mixing mill model with a dual-classification function to mimic the operations of crusher and screen. The model parameters were calibrated using a pilot-scale swing hammer mill at various operating conditions. The effects of the underscreen configurations and the feed sizes on hammer mill operations were demonstrated through the fitted model parameters. Relationships between the model parameters and the machine configurations were established. The model was validated using the independent experimental data of single lithotype coal tests with the same BJD pilot-scale hammer mill and full operation audit data of an industrial hammer mill. The outcome of the energy-based swing hammer mill model is the capability to simulate the impact of changing blends of coal or mill configurations and operating conditions on product size distribution. Alternatively, the model can be used to select the machine settings required to achieve a desired product

Development of a power-draw model for estimation of the dynamic recirculating load of swing hammer mills with internal classifiers

Abstract A common problem that has been found in modelling of the swing hammer mill with an internal classifier has been that measurement of the dynamic recirculating load is not possible, particularly in full-scale industrial operations. With only the mill feed and product data it has not been possible to find unique parameters for the size-reduction and classification units of the hammermill models. This has made it difficult to establish unambiguous relationships between the fitted model parameters and the operating conditions for simulations and scale-up. Detailed experiments have now been carried out in which a pilot-scale swing hammer mill with various under-screen configurations was used to treat coal. One of the objectives was to understand the relationship between the hammer-mill power draw and the mill operating conditions. Analysis of the data revealed that the net power draw increased significantly with the use of under-screens, implying that the power data are not only a function of the new feed rate but also a function of the internal recirculation. A mechanistic model for swing hammer-mill power draw has been developed using Bernoullis equation. The model assumes that the net power consumed by the hammer mill is used to move the fluidized coal, which comprises new feed and the recirculating load. The model parameters are calibrated to the machine configuration. This approach allows the dynamic recirculating load to be estimated from the measured new feed rate and the mill net power draw. The effects of under-screen configurations and the feed-size distributions on the mill recirculation were thus established.

Some causes of medium loss in dense medium plants

Abstract Medium consumption is a significant component of the cost of operating a dense medium plant. This study aimed to identify and quantify many of the factors contributing to overall consumption, through extensive surveys of four widely differing dense medium mineral separation plants, and associated laboratory experiments. Particular attention was given to the effect of operating density on medium loss. It was concluded that quite modest increases in operating density significantly increased loss from all sources. Density should therefore be minimised, commensurate with process requirements. There is also unequivocal evidence that consumption increases significantly with plant downtime, probably due to housekeeping wastage and overloading of regeneration circuits during the return of dumped medium to circuit. Other conclusions related to the effects of feedrate, ore porosity, medium viscosity, screen washing configuration, screen drainage efficiency and medium size on medium loss. Recommendations for operating practice are given.

The Rheology of Ferrosilicon Dense Medium Suspensions

Abstract Medium rheology is important to the efficiency of dense medium separation processes. An impediment to the selection of appropriate ferrosilicon products for a given DMS process in the past has been the lack of a consistent and comprehensive set of data describing the rheology of the range of manufactured ferrosilicon products. Rheological tests have therefore been conducted using Samancor ferrosilicon dense media over a wide range of media properties and densities, with a modified Debex online viscometer. The data have been successfully fitted to the MV-model that allows apparent viscosity to be predicted as a function of particle size distribution and shape, and medium density and temperature. The results from this procedure provide the expected viscosity ranges for uncontaminated ferrosilicon dense media slurries.

Energy-size reduction of coals in the Hardgrove machine

To simulate the grinding process in an industrial vertical spindle mill, the Hardgrove mill was fitted with a power-measuring meter to investigate the breakage characterizations of different narrow size fractions of coal. Power consumption in the grinding process was precisely recorded at the sampling frequency of 1 second with the help of sampling software for power. The experimental results indicated that the energy input for grinding decreased with increases in time for each sample. Changes in the weight percentage in the top-size interval showed that the first-order law was not satisfied to describe the total grinding process. The new produced fine coal had a negative effect on the breakage behavior of coarse particles. But removal of −0.09 mm size can help to increase the energy input for grinding and the generating rate of −0.09 mm also accelerated in turn. The relationship between t10 and energy input can be described by the classical breakage function. Specific energy consumed by raw coal with a small size was higher when subjected to the same value of t10, due to the relatively higher specific surface area of ground coal at the same breakage ratio of 10.

Investigation of the effects of chrome ball charge on slurry rheology and milling performance

Ten surveys of the ball milling circuit at the Mt Isa Mines (MIM) Copper Concentrator were conducted aiming to identify any changes in slurry theology caused by the use of chrome balls charge, and the associated effect on grinding performance. Slurry theology was measured using an on-line viscometer. The data were mass balanced and analysed with statistical tools. Comparison of the rheogram demonstrated that slurry density and fines content affected slurry rheology significantly, while the effect of the chrome ball charge being negligible. Statistical analysis showed the effects of mill throughput and cyclone efficiency on the Grinding Index (a term describing the overall breakage). There was no difference in the Grinding Index between using the chrome ball charge and the ordinary steel ball charge