Mill Power Research Articles

Effect of fluid medium density on ball mill torque, energy distribution and grinding kinetics

Dolomite was ground in a batch ball mill filled completely with fluids having densities ranging from nearly zero to almost 3000 kg/m 3. The measured power draft was in good agreement with the torque-arm model for tumbling mills, modified to account for buoyancy effects on the grinding charge. The results show that in the case of three liquid fluids the grinding path remains invariant even though the energy input is throttled significantly with increase in fluid density. Extrapolation to a hypothetical fluid of density equal to that of the grinding charge permits an approximate breakdown of the energy consumption pattern in the mill. There is a threshold power level below which hardly any grinding takes place. It further appears that the overall rate of grinding is a linear function of mill power rather than directly proportional to power.

Large mill power outages caused by potential transformer ferroresonance

Investigative procedures relating to a subtle 30 Hz ferroresonant instability of high-voltage potential transformers, which has caused several costly power outages to a large paper mill are reported. Included are unusual field oscillograms of this instability. Corrective measures were developed which have been implemented. An alternate recommended approach is to be incorporated in a similar high-voltage supply to a new mill to eliminate this instability as a possible jeopardy to selective ground-relay protection. >

Detection and prediction of overload conditions in ball mill grinding

The identification and quantification of incipient overload conditions in mills have recently been found to be very significant in the design of tumbling grinding mills. To investigate these phenomena, a series of experiments was conducted in a Plexiglas mill operated in both dry and wet modes. For on-stream measurement of the mill charge weight, volumetric filling, dynamic angle of repose, and mill power, an electronic system was developed. The mill was filled to various levels with 19.1-mm (3/4-in.) ceramic balls as grinding media. As feed material, grindable limestone or 3.2-mm (1/8-in.) nongrinding ceramic balls were fed at various steady-state rates to determine the incipient overload conditions. The prediction of overload has apparently been reduced to predicting the feed rate that results in 50% mill filling.

On-Line Estimation of Maximum Power Draught in Semiautogenous Mills

In the operation of semiautogenous grinding mills a certain mill loading which produces maximum mill power draught, must not be exceeded because in such case a progressive degradation of the mill performance sets on. With the purpose of avoiding such event, in this paper a method is developed for estimating the maximum power draught of a semiautogenous grinding mill with respect to the volumetric contents of ore and water, in spite that these variables are not measured. The maximum power estimate is found by fitting linear dynamic models to input-output data measurements, first to estimate the derivative of power draught with respect to the feed flows, and then to estimate the maximum power draught. These models are periodically updated so that updated estimates are provided. The mill should then be operated in such a way that a safe margin is kept between mill power and the maximum power estimate. To this end the derivative estimate provides a useful additional reference. An important feature of the method is that it matches well with control strategies which aim at obtaining maximum mill power draught, although it may also be used in combination with stabilizing control systems. Tests of the method, provided by simulation, point to the possibility of good results if an industrial mill is tested.

Relationship between breakage parameters and process variables in ball milling — An industrial case study

The performance of the secondary ball mill at the New Broken Hill Consolidated Ltd. concentrator is analysed using the perfect mixing model and an ore-specific breakage distribution function. This function was determined from single-particle breakage tests using a computer-monitored twin pendulum apparatus. The ratio of the breakage rate to the normalized discharge rate, r/d ∗ , determined for the ball mill using the ore-specific breakage distribution function for a range of grinding conditions is related to the mill power consumption. The mill power consumption is related to the percentage of mill volume occupied by the ball charge and to the percentage of solids in the mill feed.

Reactions to Subordination: Attitudes of Southern Textile Workers

Previous studies of southern textiles have emphasized acquiescence in the face of mill power or integration through paternalism as the basis of worker attitudes. Factor and regression analyses of data on textile workers in one mill town reveal little to support these assumptions. Approval of the company or the union, of managerial capitalism or large-scale change reflects perceived self-interest rather than deference or acquiescence. White, better paid, and older workers support the company rather than the union. Black and less well-paid workers see the social structure as rewarding merit with substantial opportunity. The findings augur poorly for effective union pressure on the company because: union support is concentrated among those easiest to replace and with fewest good employment alternatives; and those whose loyalty to the town would lead them to fight for change rather than seek a job elsewhere have no special tendency to support the union.

Reactions to Subordination: Attitudes of Southern Textile Workers

Previous studies of southern textiles have emphasized acquiescence in the face of mill power or integration through paternalism as the basis of worker attitudes. Factor and regression analyses of data on textile workers in one mill town reveal little to support these assumptions. Approval of the company or the union, of managerial capitalism or large-scale change reflects perceived self-interest rather than deference or acquiescence. White, better paid, and older workers support the company rather than the union. Black and less well-paid workers see the social structure as rewarding merit with substantial opportunity. The findings augur poorly for effective union pressure on the company because: union support is concentrated among those easiest to replace and with fewvest good employment alternatives; and those whose loyalty to the town uvould lead them to fight for change rather than seek a job elsewhere have no special tendency to support the union. This paper examines the attitudes of objectively disadvantaged industrial workers to learn how they make sense of, adjust to, and possibly act on their circumstances. Using data on textile workers, the analysis summarizes the relationships among attitudes toward opportunity, inequality, employer, union, job, town, and politics; and it attempts to uncover the social bases of these attitudes. Observers have long asked why southern textile workers for the most part have failed to respond to the low status, poor wages, unsafe conditions, and monotony of their jobs by any substantial efforts to better themselves individually or collectively (Simpson). While the last two de*An earlier version of this paper was presented at the 1983 Southem Sociological Society meetings. I appreciate the helpful comments of L. Richard Della Fave, Virginia Hiday, Nelson Reid, Linda Reif, Joel Rosch, Michael Schulman, Mary Scheuer Senter, Donald TomaskovicDevey, Rhonda Zingraff, and anonymous referees on earlier drafts. The data were collected in cooperation with Michael Schulman, Rhonda Zingraff, Carrie Knowles, and Joel Rosch with funds from a Faculty Research and Professional Development grant at North Carolina State University. Address correspondence to the author, Department of Sociology and Anthropology, North Carolina State University, Raleigh, NC 27695-8107. ? 1986 The University of North Carolina Press

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.

The effect of slurry density on breakage parameters of quartz, coal and copper ore in a laboratory ball mill

Batch wet grinding of 20 × 30 mesh feeds of quartz and copper ore in a laboratory ball mill gave first-order or non-first breakage of this size depending on conditions, with acceleration of breakage rate occurring for lower slurry density and deceleration at higher. Smaller sizes in the slurry appeared to break in a first-order manner, with normalized primary breakage distributions. Breakage of a coal was first order for all sizes. Rates of breakage went through a maximum at 45 vol.% solids for all three materials, but the maximum was most pronounced for coal and least for quartz. Higher slurry densities changed the primary breakage distributions, giving proportionally more fine material. The decrease in overall breakage rates at higher slurry density corresponded to decrease in net mill power, implying a change in the tumbling action in the mill. No physical model is proposed for the variation of breakage rates of the top size, but investigations are continuing.

Grinding Mill Power Consumption

Equations and associated procedures for estimating power draw are inconsistent. The various methods are discussed critically. Topics discussed are: diameter exponent; loading; speed correction factors; buoyancy effects; liner wear, and effect of load dilation. Balance between effective load weight and torque-arm length determines direction of power change at critical operating points (e.g. incipient overloading). Consistent procedures for calculating the effect of charge weight on power draw in different mill types are given. Error analysis shows that the accuracy of power estimation and data correlation may depend as much on the accuracy of measurement of mill dimensions and operating variables as on the particular equation used. An equation based on the torque-arm model, and providing a compromise between existing equations, is proposed. It is simple and suitable for correlating data and estimating power draw. A data-base of power related parameters documenting mill scale-up during the past few decades ...

An analysis of ball-and-race milling part III. Scale-up to industrial mills

The scale-up of breakage parameters measured in the Hardgrove mill to large mill diameters, using a simulation model for continuous milling, is discussed. Mill capacity for race diameter D in meters is ▪ where Q H is mill capacity determined by the model for the Hardgrove mill. The total mill power at normal grinding conditions for coal is ▪ where η m is motor efficiency. The variations in fineness of grind with mill capacity, Hardgrove Grindability Index, classifier setting, and air—coal ratio are described. Results from an E1.7 mill agree with model predictions.

パルプ工場におけるプロセス開発の原動力，エネルギーの問題

Review of the impact that the increasing cost of energy has had on the pulp mill process is given, and various data are available.Energy balance of logical or hypothetical kraft pulp mills is discussed in comparison of old and modernized types. As to the process development for medium consistency pulping, the ideal ultimate solution would be that the digester pressure moves the pulp at blow consistency to storage through washingh, screening and bleaching. Many other process developments in pulp industry are described.It is startling how different results can emerge from the interaction between process selection and the cost of fuel and power in the hypothetical mills, which can be seen guided by internal pricing of enegy.We do already have the knowledge to build a wood pulp mill which is entirely independent of both fossile fuel and external power, and has a substantial excess of both heat and power. The process can be developed so, that the heat demand is reduced to about 50%, and the power demand to about 65% of the present in regard to the present average mill.

An analysis of ball-and-race milling. Part I. The hardgrove mill

The basic principles of mill action were investigated in the batch Hardgrove ball-race machine, modified by the addition of a torque device. It was found that the torque to drive the mill after correction for bearing friction was almost proportional to load M on the balls and independent of rotational speed ω, so that mill power ??? Mω. The first-order specific rates of breakage were proportional to M and ω. Torque increased as the power in the race increased, then reached almost a constant value, as did the absolute rate of breakage. As fines accumulated in the charge breakage became non-first-order, showing a slowing-down effect for all sizes. An easy method of simulation of this condition was demonstrated.

An analysis of fine dry grinding in ball mills

The kinetics of dry grinding of several cement clinkers and two coals were investigated in a laboratory tumbling ball mill. The kinetic process is first-order at first, but the rates of breakage decrease as fines accumulate in the bed. It was demonstrated that the slowing of the breakage rates applies to all sizes in the mill, indicating that the cushioning action of fines affects the whole breakage process, even though mill power remains constant. Tests on cleaning or non-cleaning the balls showed that the major factor was not the build-up of a coating on the balls. Radio-tracing tests showed that the effect was not due to pelletizing of fines into larger particles. The quantitative magnitude of the cushioning action was different for different materials. It is, therefore, postulated that cushioning is affected not only by air trapped in the bed of fine particles but also by the cohesive attraction of fine particles, which is a function of the material.

Realtime Digital Computer State Estimator for a Hard Rock Milling Circuit

This paper describes the development of non-linear stochastic dynamic models of the unit operations in a large, complex milling circuit which are combined to form the basis for building a two-stage extended Kalman filter. The filter provides optimal parameter and state estimates of the operating plant. A description is given of the techniques used for implementing the estimator on a digital computer such that on-line estimates are generated in real time. Sufficient additional real-time computing ability is available to generate control inputs based on the plant estimates. The filter was shown to predict adequately the pebble load in the mills (this is the most useful variable for pebble feed control), water hold-ups, solids hold-ups and particle size distributions around the circuit from measurements of pebble mill power draft, overflow fraction -75 µm and specific gravity, rod mill feed rate and sump discharge specific gravity. The single most important difficulty encountered was the bias introduced by inaccurate modelling of the units and the filter segmentation or “decentralisation” that is necessary because of real-time considerations.

Automatic Control of Continuous Autogenous Grinding

A very important problem in autogeneous grinding control is the determination of the mill charge x0, which represents maximal energy consumption of fines. This optimal mill charge depends upon the ore quality, and is subject to variations over time. In this paper a control system which maintains stable mill operation at maximal mill power consumption in spite of ore quality variations is described. Results from full scale test in an industrial mineral dressing plant are included.

Operating Problems, Analysis, and Improvements of the Dynamic Performance of a Kraft Mill Power System

A new mill is supplied by a weak power system subject to numerous faults. The faults cause supply-voltage dips which result in production interruptions. The major problems that were experienced, the studies that were conducted, and the alterations made to improve the dynamic performance of the utility and mill power systems are described.

The Measurement of Power in Early Steam-Driven Textile Mills

Between the years 1769 and 1810, all the stages in conversion of cotton wool into woven cloth were mechanized so that they could all be worked by power. At the same time, beginning with James Watt's patent of 1769, the steam engine underwent a radical transformation, so that by 1784 relatively efficient rotative engines were becoming available. In the cotton industry, such engines were soon being used in considerable numbers. The market for cotton cloth was

SUGAR PLANTATIONS AND YEOMAN FARMING IN NEW SOUTH WALES1

Between 1865 and 1911 the sugar industry of the northeastern corner of New South Wales, Australia, was organized according to a variety of production systems: plantations, central mills and independent farms, small mills, and co-operatives. The authoritarianism of the plantations, based on gang labor, conflicted with the dominant desire to establish an equalitarian yeomanry in the region. Co-operation between yeoman and capitalist produced the central mill system, which is now universal, but petit bourgeois settlers of moderate capital attempted to maintain their independence by erecting mills of their own. Scottish Presbyterians were dominant in the latter group; their concentration on the lower Clarence resulted in a high density of mill power in the area, in contrast to the dispersion north of the Richmond where water transport was unavailable. The plantations were located in areas where it was possible to aggregate abandoned holdings and where population was sparse; the stability of the small farm pattern on the lower Clarence made it unnecessary for the capitalist millers actually to own land and employ field labor, but they did exert considerable control over the pattern of land use within their mill areas. Whereas the organization of the plantations created a nucleated settlement pattern, the other systems dispersed habitations and the small mills similarly dispersed processing activity.

Sulfur dioxide removal and recovery from pulp mill power plants.

The use of soda ash liquor to scrub SO2 rich power plant flue gases was studied using an Airetron pilot scrubber with a maximum capacity of 3000 cfm. The relative effects of the major operating variables— temperature, soda ash concentration, and the gas/liquid flow ratio—on the absorption phenomenon were determined. Orthogonal factorial experiments were used to derive a response function relating mass transfer values to operating variables. The economics of a full scale NSSC installation are discussed.