Ball charge in ball mill

BALL CHARGE LOADING – IMPACT ON SPECIFIC POWER CONSUMPTION AND CAPACITY By: IEEE-IAS Cement Industry Committee David S. Fortsch Manager of Milling Technology - Senior Process Engineer FLSmidth, Inc.

ABSTRACT In determining the proper mill size required to meet a targeted production rate, many factors are evaluated including: length to diameter (L/D) ratio, individual compartment lengths, rotational velocity, liner type, ball gradation and percent filling degree. This paper examines the effects of changing mill filling degree with respect to specific power consumption (kWh/ton) and product throughput as well as the impacts that High Efficiency Separators (HES) have had on the design of cement grinding systems. The research concludes that lowering the ball mill filling percentage negatively affects mill sizing and increases capital and installation costs of milling equipment. INTRODUCTION Many theoretical approaches to comminution theory have been developed over the last century and a half. As early as the 1860’s Rittinger tried to explain the relationship between the inputted power and the change in surface area from feed to product size material. Kick in the 1880’s and then Bond, in the 1950’s furthered this theory and developed a relationship between the power input to material and the new crack tip length. Today, Bond’s theory is still fairly accurate in determining power requirements in open circuit ball mills as well as closed mill circuits with conventional separators. As separator technology has developed, from static separators, to first generation, to today’s third generation high efficiency separators, the Bond theory has had to be modified to account for the improvements in grinding efficiency due to the use of separator technology. By the late 1980s high efficiency separators had been introduced to the cement industry and their effect on specific power consumption was drastic. Consequently the theories and rules developed for open mill circuit or conventional separator circuit required re-examination for modern mill circuit design. Today there are over 350 ball mills installed in the USA yet there is still a deficiency in cement produced. To meet this need there has been a resurgence in increasing the current grinding capacity of many of the existing plants as well as new plants altogether. To this end, this paper will address modern ball mill designs. Past ball mill studies have evaluated the importance of many factors in ball mill grinding efficiency including ball size distribution, mill length to diameter ratio, compartment lengths, mill rotational speed, and mill filling percentage. These areas will be addressed in this paper once again. However, the importance of inter-relationships among the individual pieces of equipment in the complete mill system (mill + separator) and their effect on the resultant grinding efficiency will be stressed more fully. To this point, when mill internal designs are based solely on ball mill operation alone, instead of the complete grinding system, they fail to achieve an optimized grinding system both physically and economically.

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DESIGNING THE RIGHT MILL SIZE FOR THE RIGHT APPLICATION For the remainder of the paper, a closed circuit system involving a ball mill and a high efficiency separator (HES) will be evaluated. In order to evaluate the effects of ball mill loading on specific power consumption and production it is necessary to discuss the actual design basis upon which the equipment sizes in a mill system are selected. This includes the following parameters: -

Material Grindability Mill Power Draw Determination High Efficiency Separator Mill Shell Liner Design Mill Grinding Media Size Gradation Mill % Loading and Specific Power Consumption

Material Grindability Input energy is required in order to reduce material from feed to product sized material. For any material this energy can be expressed in terms of output production. This resultant term is described as a material’s grindability and is expressed in terms of specific power consumption or kilowatt-hours per ton (kWh/ton). In order to compare various materials for grinding, a standard test using a Bond mill has been generally used. The results of this testing are represented by index figures. Different materials can then be compared consistently when evaluating them on the basis of a Bond Work Index (BWI). Based on the BWI, a mill design input power is then determined by multiplying the BWI by the desired throughput. With today’s third generation high efficiency separators, the Bond Work Index has had to be manipulated. Several mill design manufacturers have developed their own standardized testing methods based on similar principals to Bond in order to predict specific power requirements of ball mills in circuit with these separators. Mill Power Draw Determination Using the desired production rate and the expected specific power consumption figure from the grindability testing, the net power requirement delivered by the ball mill is determined. There are several methods of calculating the net mill power draw for an industrial mill. One such method is expressed in the following equation: N=kxFxnxµxDxa Where:

N = Net power (kW) k = Constant for unit conversion F = Amount of charge (mt) n = Mill revolutions per minute (rpm) µ = Torque factor D = Effective inside diameter of mill compartment (m) a = distance from mill center to center of gravity for the charge

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The torque factor (µ) is a dimensionless figure that correlates the power equation above to the amount of rotational lift exerted onto the grinding media. Higher figures indicate more rotational lift and therefore correspond to higher power draw figures. The change in this value is dependent primarily on the liner type but also the ratio of material in the mill to the grinding media. Lifting linings (step) typically have torque factors between 0.7 and 0.73. For corrugated lining this figure is between 0.62 and 0.69. As compared to corrugated lining, classifying liners have a slightly lower torque factor (i.e. higher slip). In addition, due to the thickness difference between corrugated and classifying liners there is a reduction of available mill volume by up to 10%. The amount of charge (F) is typically determined by using a mill filling degree between 28 and 35% of the available mill volume. The limitation on the maximum filling is determined based upon the mill inlet and outlet opening sizes as well as the mill shell mechanical designs. More rigidly designed mills in the past have been capable of higher loadings (>40%) however due to the capitol costs of such mills these are not frequently seen today. High Efficiency Separator One of the biggest changes in the cement grinding industry since the late 1970s is the conversion from conventional first generation separators to High Efficiency Separator (HES) technology. The conversion of milling systems from conventional first generation separators or even static separators to third generation high efficiency separator has shown the tremendous interdependence between ball mill optimization and classifier efficiency. Since the advent of the high efficiency separator, ball mill operational efficiency has improved dramatically and in degrees much greater than a few percent. Design rules and guidelines that were once used for conventional separators have had to be revised. The HES has had the following benefits on finish milling systems: -

Increased capacity for the same mill power input Reduced specific power consumption Improved product characteristics Lower product temperature Improved quality control Greater system flexibility

Beforehand, it was difficult to obtain consistent engineering tools (grinding theories) that applied to all systems because the separator inefficiency decreased the mill capacity, often to a severe extent. The HES has eliminated the separator restriction on mill system optimization and industrial theory advancement. The HES is used in mill systems with wide ranging characteristics thus providing an opportunity to study several variables, without separator manipulation or suppression. Clinker grindability of course is one of the main factors in determining a throughput given a specific mill size. By using standard grindability figures and then correlating this information with empirical data, an improved grindability correlation was obtained as a result of the HES creating a baseline equation in determining the specific power consumption as follows: Y Power Consumption (kWh/mton) = C x (Blaine) / Grindability

Where:

C and Y are a constants Grindability is determined in standardized testing

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Use of this formula allows the estimation of full mill capacity potential based on HES results. This yields an end-point or goal for mill optimization. A large majority of HES retrofits achieved this result without additional changes to the milling system. Tromp curves and particle size improvements have all been similar, showing strength gain with very low coarse particle residues. Depending on the mill system in question, changes to mill internal should be optimized. Mill Shell Liner Design There are several mill liner suppliers with numerous liner designs. Each design assures optimal grinding efficiency depending on the grinding system. Whether it is wet or dry, single or multiple chamber, crushing or attrition grinding, classifying or non classifying a lining system can be designed for each application. In the cement industry, the standard modern designed closed mill circuit consists of a two compartment mill. The first compartment is typically lined with a step or lifting liner which accentuates the crushing action required in the first compartment. In the second compartment, the standard lining is either a corrugated (wave) or classifying lining. The corrugated lining accentuates the tumbling motion of the finer charge of the second compartment increasing the attrition grinding action. Classifying lining is used to assist in classification of the media, whereby larger media is found closer to the center diaphragm and smaller media is located toward the discharge of the mill. Due to the introduction of the HES, classifying liners (without corrugation) have not shown to be beneficial to mill throughput or specific power consumption. This is perhaps best exemplified by mill systems that added classifying liners after a HES was installed. The results of this study showed that the mill performance was not improved. In fact, there have been are several that experienced a decrease in capacity. This phenomenon was first realized several years ago when statistical analysis did not yield any difference for ball mills with and without classifying liners in systems that had a HES. Results from analysis of 26 plants with HES technology are shown in TABLE 1. The table shows that the industrial average power consumption, when it is normalized at equal grindability, is higher for classifying liners, or certainly, at best, equal to corrugated or wave liners. TABLE 1: Comparison of straight and classifying liners versus specific power consumption in various plants Straight Liners Plant No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Average Range

@3600 cm^2/gm KWH/mton Actual 28.7 26.7 31.7 27.8 29.5 35.8 30.7 27.6 35.7 30.9 30.8 34.9 32.5 32.8 24.8 32.8 32.8 31.0 24.8-35.7

Classifying Liners Plant No. 1 2 3 4 5 6 7 8 9

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@3600 cm^2/gm KWH/mton Actual 28.7 34.2 33.1 33.1 27.3 32.8 30.6 41.0 37.3

33.1 27.3-41.0

Classifying liners will not increase a mill system capacity above the potential of a HES. Their contribution, if any, is not additive. In fact, they will probably decrease it for the following reasons: Low Friction - such liners have a higher slip and thus develop a lower torque and cannot draw as much power as other liner designs at the same volume loading. The power draw may be 4% - 10% less with classifying liners due to the lower friction. To draw the same power they require a higher volume loading which requires more grinding media, reduced mill air velocity and can result in reverse classification if the volume loading is too high. This means that the full mill power potential may not be attainable for some or all of the previous reasons. In the case of a new mill, a larger mill would be required. Anomalies - some mills equipped with classifying liners have an operational sensitivity which results in unclassified or reverse-classified ball charges. These systems are sensitive to oversized clinker pieces that have a tendency to accumulate and clog the ball charge causing the cement to flow around or over the ball charge instead of through it. In other cases, the mill grinding condition appears normal but the specific power consumption may be above average, e.g. more than 40 kWh/t 2 at 3600 cm /g product fineness. Variable Porosity - the effect of the sorting or classifying creates zones of different material flowability where the finest ball size zone controls the flow which is different for the larger ball sizes, in contrast to a 'mixed charge' with flowability being independent of position in the mill. The low porosity zone at the discharge of the mill is unequal compared to the other zones in the same chamber. The low 2 porosity zone may have a surface area of 45 m /t while maintaining an overall chamber average of 34 2 2 m /t. A mixed charge, as seen in a corrugated lining, can have a higher average surface of 37 m /t and also have a higher porosity for uniform material flow and a lower tendency for clogging with a higher potential circulation factor. It should be noted that the classifying liner systems were successful in improving the mill performance in the past. The explanation is that the classifying liner allowed more efficiency in grinding, by locating finer ball charge at the discharge end of the ball. This increased media surface area increased the grinding efficiency and reduced the transport velocity through the circuit. The end result was a mill product that was finer in size and lower in quantity (lower circulating loads). The resultant reduction in circulating load allowed grossly inefficient conventional separators to appear to operate “more efficiently”, which resulted in higher production. The proper explanation became obvious after the HES was introduced. Modern HES are designed with much higher efficiencies, greater than 75% as compared to conventional separators that were 40% or lower. By improving the efficiency in the separator performance, the ball mill does not have to re-work material that is product sized and therefore the overall mill loading is reduced. It is possible that classifying liners can increase mill productivity for a conventional separator before a HES is installed because the circulating load is reduced. However, the same improvement can be reached with a finer ball charge and a corrugated lining provided the diaphragm has the proper specifications and position. It is also important to note that an improvement with classifying liners will later limit the potential gain with a HES for the reasons mentioned earlier.

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Grinding Media Size Gradation Another area of improving the grinding efficiency of a ball mill is in improving the ball mill grinding media size gradation. Through two techniques, theoretical modeling (Bond) and empirical data, an optimum ball size gradation which improves the operational efficiency of the ball mill can be determined. Literature has shown that increases in media surface area increases the throughput of a mill system. For the second compartment ball charge using a ball gradation between 30 mm and 15 mm will provide the optimum grinding efficiency. Typically the material size distribution at the partition between the first and second compartments of a ball mill should not be more than 0.5% retained on 2 mm sieve size. The limitation on the smallest ball size is the discharge grate slot width opening. Typically a ball size that is smaller than twice the size of the discharge grate slot width is not selected. Mill % Loading and Specific Power Consumption Ball mills are relatively simple grinding devices from an operational point of view. Typical operational philosophy is to put as much grinding media in a rotating tube as it can take and then introduce a feed material. If the feed material is softer than the grinding media, it will be reduced in size. If, after some time, the grinding media is worn then a simple replacement of media is made. This paper is a proponent of this approach. When it comes to designing milling equipment however there are some considerations that should be evaluated. One of these is the maximum design loading of a mill. Today’s ball mills are typically designed to operate with ball charge fillings up to 35% by volume. There are cases where this figure is higher, for example with mill shells designed for higher loadings, but these cases are becoming more rare, with the additional cost of steel becoming prohibitory of this design. As mill loadings are reduced there is a commensurate reduction in mill power draw per given mill size. In order to maintain the same throughput, the mill sizes; diameter and length, are required to increase. The result is a change in mill design. This can lead to additional cost in capital investment for the end user. Alternative design theories have been presented which ascribe to the point that ball mill grinding efficiency is improved with a lower grinding media level (~