bonds rod mill for work index

bond grindability index test ball mill

A Bond Ball Mill, 12 x 12 of fabricated construction comes with electric motor and gear reducer drive, digital counter, jogging button to provide positioning of drum for loading and unloading, emergency stop button, 44 lb ball charge, stand and receiving pan.

Laboratory grindability tests and commercial grinding results have shown that with many materials the work index as computed from Eq. (1a) does not remain constant for different product sizes P; as P becomes smaller the Wi values may decrease, remain constant, or increase. This has suggested to some investigators that the exponent of the product and feed sizes may not remain constant at , as required by the Third Theory, but may be made greater or less than in individual cases to produce a constant work index. Several grindability tests on each material are necessary to determine this apparently variable exponent.

Without this potential variation the Third Theory work index would not be an accurate criterion of the work input required for size reduction. In practice the work index has customarily been determined at a product size close to that desired, or it has been determined at two or more product sizes and the Wi value at the desired product size P has been found by interpolation. This has proved quite satisfactory, except that the amount of laboratory testing required is increased. A better understanding of the comminution process should reveal the reasons for the work index variations at different product sizes, and make possible more accurate grinding calculations without increasing the number of grindability tests required.

It has recently been shown that for any material the energy input remains substantially constant for each unit of new crack length produced when grinding to different product sizes, even though the work index values varied widely. This confirms the Third Theory and suggests that the work index variations are caused by variations in fine size distributions of the feed and products. These fine size distributions are not indicated by the 80% passing sizes used in Eq. (1) and designated as F and P. The use of the 80% passing size as the sole criterion of particle size assumes that the crushed or ground feed and product both follow a consistent average size distribution ratio. Any variation from these average values on the part of feed or product will cause the work index to increase or decrease with finer grinding.

Variations in the Third Theory work index when grinding the same material to different product sizes are caused by differences in the relationship of the feed exposure ratio to the product exposure ratio. The exposure ratio is a measure of the amount of fines present, and is found from a screen analysis plot on special semi-log paper.

Equations are given by which the work index values at different product sizes can be calculated from the results of a single laboratory test or commercial grind, and the exposure ratios of its feed and product. These calculated values can be checked by grinding measurements at other product sizes. Equations are included for calculation of the specific crack length and crack energy. The relationship between the work index and the crack length produced is shown. The equation is given for using the calculated work index values of feed and product to obtain more accurate values of the work input required.

The Erf: Erp relationships in the above equations are empirical. It should be possible to derive a theoretical relationship from the equation for the crack length previously published. However, this equation is so complicated that any theoretical relationship would probably be quite involved, and the attempt was not made. Eq, (17) can be used to check the crack length relationship.

Operating plant information or pilot plant tests may show that the Erf/Erp ratio is different in the plant than in the laboratory grindability tests. In this case the plant ratio should be used with the calculated standard work index to check the plant work index from Eq. (10).

Application of the equations shows that the principal cause of work index variations at different product sizes is the difference in the natural size distribution of feed and product, and that the Third Theory work index remains constant when these are taken into account. Much remains to be done in explaining why certain materials break to form widely different amountsof fines, and why the Er values of certain materials may differ widely at various feed and product sizes, as well as the effect of natural grain sizes upon the Er values. The need for more studies along this line is evident.

The work Index Wi as previously determined at any specified 80% passing product size is still the most practical criterion of the actual work input required for reduction to near that size. Measurement of the exposure ratios of feed and product can increase the accuracy of work input calculations at feed and product sizes far removed from those tested.

bond work index - an overview | sciencedirect topics

The Bond work index is not solely a material constant but is influenced by the grinding conditions. For example, the finer the grind size desired, the higher is the kWh/t required to grind to that size. Magdalinovic [38] measured the Bond work index of three ore types using different test screen sizes. He produced a correlation between the mass of test screen undersize per revolution, G, and the square root of the test screen size, D:

The constant K2 is also dependent on ore type and ranged from 1.4 to 1.5. A regression of Magdalinovics data including the feed 80% passing size gives an average value of 1.485 for K2. If we extend this relationship to any sample of screened material then this gives an approximate estimate of the 80% passing size as 67.3% of the top size. This compares with a value of 66.7% of the 99% passing size obtained from data in Table3.3.

Using Magdalinovics method, from the results of a Bond work index test at a single test screen size, the constants K1 and K2 can be calculated and from these values, the work index at any test screen size can be estimated.

An alternative approach to determine the effect of closing screen size on the Bond ball mill work index (BWi), in the absence of extensive test work, is to use computer simulation. The batch grinding process has been modelled using the sizemass balance approach (Austin [37], Chapter11) and if we can do this, then we can effectively simulate the Bond ball mill work index test. Yan and Eaton [39] measured the selection function and breakage distribution parameters for the Austin grinding model and demonstrated the BWi simulation with soft and medium/hard ore samples. The measured BWi was 14.0 and 6.6kWh/t for the medium/hard and soft ore, respectively, at a closing screen size of 106 m compared with the simulated values of 13.2 and 5.6kWh/t.

The ability to simulate the Bond work index test also allows examination of truncated ball mill feed size distributions on the work index. For grinding circuits where the feed to a ballmill is sent directly to the classifier and the cyclone underflow feeds the ball mill (see Figure3.10), a question arises as to whether this practice will alter the ball mill work index (BWi) of the material being ground and hence have an impact on the energy used in the mill for grinding. Some might conclude that a higher percentage of coarse material in the mill feed will increase the amount of material that needs to be ground to produce the end product and hence it will affect the BWi. Others, in the absence of contrary evidence, assume that there is no change in the work index. Figure3.11 shows the typical circuit represented by the standard Bond work index correlation and Figure3.10 represents the scalped or truncated feed case.

The procedure for the work index test bases the BWi value on the calculation of new fines generated in the test. This means that the fraction of fines in the feed should not influence the test result significantly, if at all. For example, for a sample with 20% of 300 m material in the feed, if this is not scalped out of the fresh feed, then the mill charge, at 250% circulating load will contain 0.2/3.5 or 5.7% of 300 m in the mill charge compared with 0% for a scalped fresh feed, at a closing screen of 300 m. This should not have a great influence on the production of new fines unless the test was carried out in a wet environment and the fines contained a high percentage of clays to affect the viscosity of the grind environment. Thus for a Bond test (dry test), the difference between the scalped and unscalped BWi result is expected to be minor. In a plant operation where the environment is wet and clays are present, a different result may be observed.

Tests carried out to confirm this have clouded the water a little. Three rock types were tested with scalped and unscalped feeds with two samples showing higher BWi values for the scalped ore and the other sample showing a lower value [40].

In the work index test simulation, it is easy to change the closing screen size to examine the effect on the BWi. The results of such a simulation are shown in Figure3.12 where the simulated test was performed at different closing screen sizes and different scalping sizes. This shows that for scalping sizes at or below the closing screen size of the test, the BWi values are not affected. The scalping size of zero refers to the un-scalped mill feed. For scalped screen sizes above the closing screen size, the BWi values start to increase. The increase in BWi is more pronounced at the larger closing screen sizes. At a closing screen size of 300 m and a scalped size of 600 m, the increase in BWi is 4%.

Another outcome of the simulation is the effect of the closing screen size on the work index. As the closing size decreases, the ore must be ground finer, using more energy and producing a higher work index. Further simulations at even larger closing screen sizes show the BWi to increase. This dip in BWi with closing screen size has been observed experimentally, as shown in Figure3.13, with the minimum in BWi occurring at different closing screen sizes for different rock types [41,42].

Bond impact crushability work index (CWi) (Bond, 1963) results reported for iron ores vary from hard iron ore (17.7kWh/t) to medium hardness iron ore (11.3kWh/t) and friable iron ore (6.3kWh/t) (Table 2.11; Clout et al., 2007). The CWi for hard iron ores typically overlaps with those reported for BIF (taconite) iron ores while the range in values in Table 2.11 covers that for different types of iron ores and materials reported earlier by Bond (1963), with some relevant data in Table 2.12.

The most widely used parameter to measure ore hardness is the Bond work index Wi. Calculations involving Bonds work index are generally divided into steps with a different Wi determination for each size class. The low energy crushing work index laboratory test is conducted on ore specimens larger than 50mm, determining the crushing work index (WiC, CWi or IWi (impact work index)). The rod mill work index laboratory test is conducted by grinding an ore sample prepared to 80% passing 12.7mm ( inch, the original test being developed in imperial units) to a product size of approximately 1mm (in the original and still the standard, 14 mesh; see Chapter 4 for definition of mesh), thus determining the rod mill work index (WiR or RWi). The ball mill work index laboratory test is conducted by grinding an ore sample prepared to 100% passing 3.36mm (6 mesh) to product size in the range of 45-150m (325-100 mesh), thus determining the ball mill work index (WiB or BWi). The work index calculations across a narrow size range are conducted using the appropriate laboratory work index determination for the material size of interest, or by chaining individual work index calculations using multiple laboratory work index determinations across a wide range of particle size.

To give a sense of the magnitude, Table 5.1 lists Bond work indices for a selection of materials. For preliminary design purposes such reference data are of some guide but measured values are required at the more advanced design stage.

A major use of the Bond model is to select the size of tumbling mill for a given duty. (An example calculation is given in Chapter 7.) A variety of correction factors (EF) have been developed to adapt the Bond formula to situations not included in the original calibration set and to account for relative efficiency differences in certain comminution machines (Rowland, 1988). Most relevant are the EF4 factor for coarse feed and the EF5 factor for fine grinding that attempt to compensate for sizes ranges beyond the bulk of the original calibration data set (Bond, 1985).

The standard Bond tumbling mill tests are time-consuming, requiring locked-cycle testing. Smith and Lee (1968) used batch-type tests to arrive at the work index; however, the grindability of highly heterogeneous ores cannot be well reproduced by batch testing.

Berry and Bruce (1966) developed a comparative method of determining the hardness of an ore. The method requires the use of a reference ore of known work index. The reference ore is ground for a certain time (T) in a laboratory tumbling mill and an identical weight of the test ore is then ground for the same time. Since the power input to the mill is constant (P), the energy input (E=PT) is the same for both reference and test ore. If r is the reference ore and t the ore under test, then we can write from Bonds Eq. (5.4):

Work indices have been obtained from grindability tests on different sizes of several types of equipment, using identical feed materials (Lowrison, 1974). The values of work indices obtained are indications of the efficiencies of the machines. Thus, the equipment having the highest indices, and hence the largest energy consumers, are found to be jaw and gyratory crushers and tumbling mills; intermediate consumers are impact crushers and vibration mills, and roll crushers are the smallest consumers. The smallest consumers of energy are those machines that apply a steady, continuous, compressive stress on the material.

A class of comminution equipment that does not conform to the assumption that the particle size distributions of a feed and product stream are self-similar includes autogenous mills (AG), semi-autogenous (SAG) mills and high pressure grinding rolls (HPGR). Modeling these machines with energy-based methods requires either recalibrating equations (in the case of the Bond series) or developing entirely new tests that are not confused by the non-standard particle size distributions.

Variability samples must be tested for the relevant metallurgical parameters. Ball mill design requires a Bond work index, BWi, for ball mills at the correct passing size; SAG mill design requires an appropriate SAG test, for example, SPI (Chapter 5). Flotation design needs a valid measure of kinetics for each sample, including the maximum attainable recovery and rate constants for each mineral (Chapter 12). Take care to avoid unnecessary testing for inappropriate parameters, saving the available funds for more variability samples rather than more tests on few samples. Remember that it must be possible to use the measured values for the samples to estimate the metallurgical parameters for the mine blocks in order to describe the ore body, and these estimates will be used in process models to forecast results for the plant. Always include some basic mineralogical examination of each sample.

The expression for computing the power consumption (P) derived theoretically by Rose and English [9] involved the knowledge of Bonds work index (Wi). To evaluate the work index they considered the maximum size in the feed and also the maximum size of particles in the discharge from the crusher. To determine the size through which 80% of the feed passed, they considered a large database relating the maximum particle size and the undersize. From the relation it was concluded that F80 was approximately equal to 0.7 times the largest size of particle. Taking the largest size of the particle that should be charged to a jaw crusher as 0.9 times the gape, F80 was written as

Also, to establish the P80 from the largest product size, Rose and English considered that the largest particle size discharged from the bottom of the crusher would occur at the maximum open set position and hence

For operating a jaw crusher it is necessary to know the maximum power required consistently with the reduction ratio and the gape and closed side settings. The maximum power drawn in a system will occur at the critical speed. Thus for maximum power, Q in Equation (4.51) is replaced with QM from Equation (4.19) to give

The largest size of ore pieces mined measured 560mm (average) and the smallest sizes averaged 160mm. The density of the ore was 2.8t/m3. The ore had to be crushed in a C-63 type jaw crusher 630 440. At a reduction ratio of 4, 18% of the ore was below the maximum size required. Determine:1.the maximum operating capacity of the crusher,2.the optimum speed at which it should be operated.

Finally, a look should be taken at coal elasticity, hardness and strength. However, a particular matter of importance which arises from those consideration is the ease of coal grinding, an important step in whatever coal preparation efforts for further processing. The more fundamental material properties are covered reasonably by Berkowitz (1994), so the discussion here will be limited to coal grindability. For that purpose, use is made of two different indices, both determined experimentally with the material to be ground. One is the Hardgrove grindability index and the other the Bond work index.

The Hardgrove index is determined using the ASTM method D 40971. It involves grinding 50g of the material, e.g. coal, of specified size (1630 mesh cut) in a specified ball-and-race mill for 60 revolutions. The amount of 200 mesh material is measured (w grams) and the index is defined as I = 13+ 6.93w. Thus, the higher the index, the easier is the grinding task. This method loosely assumes that the specific energy consumed is proportional to the new surface generated, following the concept of Rittingers law of comminution.

Berkowitz (1994 p.96) gives a generalized variation of the Hardgrove index with coal rank. According to the variation, anthracites are hard to grind, bituminous coals the easiest, and the subbituminous more difficult, with lignites down to the same low index level as anthracites. It is suggested that the decrease in the index below daf coal of 85% is caused by plastic deformation and aggregation of the softer coal particles, hence reducing the 200 mesh fraction generated by the grinding test.

The Bond work index (Bond, 1960) is based on Bonds law, which states that the energy consumed is proportional to the 1.5 power of particle size rather than the square of Rittingers law. Accordingly, the energy consumed in reducing the particle size from xF to xp (both measured as 80% undersize) is given by

We should note that the higher the value of the work index, the more difficult it is to grind the material. A compilation of data is available, for example, in Perrys Chemical Engineers Handbook (Perry et al., 1984). For coal, one average value is given, with Ei = 11.37 for = 1.63. Bonds law is useful because of the extensive comparative database.

Interestingly, Hukki (1961) offers a Solomonic settlement between the different grinding theories (rather than laws). A great deal of additional material related to grinding, or size reduction, comminution, is available in handbooks, e.g. by Prasher (1987) and research publications in journals such as Powder Technology. A very brief overview of grinding equipment is given in Section 1.5.3.

Rock fragmentation is a consequence of unstable extension of multiple cracks. Theoretically, rock fragmentation is also a facture mechanics problem. Two major differences between rock fracture and rock fragmentation are that (1) rock fragmentation deals with many cracks, but rock fracture deals with only one or a few, and (2) rock fragmentation concerns the size distribution of the fragments produced, but rock fracture does not. There are two important factors in rock fragmentation: (1) total energy consumed and (2) size distribution of fragments. In a study on crushing and grinding, fracture toughness has been taken as a key index similar to the Bond Work Index. Due to many cracks dealt with, rock fragmentation is a very complicated and difficult fracture problem. To achieve a good fragmentation, we need to know how the energy is distributed, which factors influence energy distribution, what is the size distribution, and so on. In practice such as mining and quarrying, it is of importance to predict and examine size distribution so as to make fragmentation optimized by modifying the blast plan or changing the fragmentation system. About size distribution, there are a number of distribution functions such as Weibulls distribution function [11], Cunninghams Kuz-Ram model [12], and the Swebrec function [13]. In engineering practice, how to develop a feasible and simple method to judge rock fragmentation in the field is still a challenging but significant job and will be in the future.

Although the fracture toughness of a rock is very important in rock fracture, the strengths of the rock are also useful in rock engineering. In the following we will see that the strengths and fracture toughness of a rock have a certain relation with each other, partly because of a similar mechanism in the micro-scale failure.

Bong's Work Index is used in Bong's law of comminution energy. It states that the total work useful in breakage is inversely proportional to the length of the formed crack tips and directly proportional to the square root of the formed surface:

where W is the specific energy expenditure in kilowatt-hours per ton and dp and df are the particle size in microns at which 80% of the corresponding product and feed passes through the sieve; CB is a constant depending on the characteristics of materials; and Sp and Sf are the specific surface areas of product and initial feed, respectively. Wi is called Bond's Work Index in kilowatt-hours per ton. It is given by the empirical equation:

where P1 is the sieve opening in microns for the grindability test, Gb.p. (g/rev) is the ball mill grindability, dp is the product particle size in microns (80% of product finer than size P1 passes) and df is the initial feed size in microns (80% of feed passes). A standard ball mill is 305mm in internal diameter and 305mm in internal length charged with 285 balls, as tabulated in Table 2.1. The lowest limit of the total mass of balls is 19.5Kg. The mill is rotated at 70 rev/min. The process is continued until the net mass of undersize produced by revolution becomes a constant Gb.p in the above equation.

To investigate the influence of the coal type on the stampability factor K, stamping tests with eight different coals (C1C8 in Table11.1) were carried out, using the Hardgrove grindability index (HGI) as a measure for the material dependency. The grindability is broadly defined as the response of a material to grinding effort. It can be interpreted as the resistance of the material against particularization. It is not an absolutely measurable physical property of the material. Generally, grindability can be determined either based on product constant fineness method (Bond work index Wi) or on constant useful grinding work method (HGI). The correlation between HGI and Wi can be described by the formula (11.5):

HGI is influenced by the petrographic composition of coal. HGI was developed to find a relationship between petrographic properties and strength of coal particles thus aiming to interpret the coking behavior of coals (Hardgrove 1932). HGI correlates to VM content, and the relationship is empirically specified for most of the hard coals and given with VM from 10% to 38% (db) by Eqs. (11.6) and (11.7):

For the execution of each test, further coal property parameters, particle size distribution and moisture content, as well as the height of fall of the stamp and the number of stamping steps were kept constant, so that the only parameter varied was the coal rank characterized by HGI.

The obtained data of each test was analyzed as described above to calculate the stampability factor K. A higher value for the HGI is equivalent to a lower resistance to stamping, i.e., a better stampability. The determined values of the stampability factor K are plotted against HGI in Fig.11.12.

bond work index tests - grinding solutions ltd

The Bond Low-Energy Impact test can be used to determine the Crusher Work index (CWi), also known as the Impact Work Index. The test determines the impact energy at which a specimen fails and allows approximation of net power requirements for sizing crushers. The open and closed sized settings for given product sizes can also be determined.

The Bond Ball Mill Work Index (BBWi) test is carried out in a standardised ball mill with a pre-defined media and ore charge. The Work Index calculated from the testing can be used in the design and analysis of ball mill circuits

size reduction and energy requirement - sciencedirect

The energy required to liberate a mineral of economic interest from its gangue constituents in the host rock is described in this chapter. The design of equipment use for the purpose is indicated in some details. Standard laboratory tests for determining this energy are described for ball and rod mills systems of grinding. The methods of calculating this energy is illustrated with worked examples.

quantifying grinding efficiency - grinding & classification circuits - metallurgist & mineral processing engineer

What is the best way of measuring andquantifying the grinding efficiency of mineral slurries?I am referring to determining energy savings by improving efficiency. Would the Bond Work Index be the best way?

A metric often encountered in the hard rock business is a measure of the power consumed per unit of desired product e.g. kWh/t of -75 micron material in cyclone overflow (assumes you want to grind to 75 microns). You have to compare the specific electrical energy consumption required for the targeted milling final product particles size.

For many mineral processors, drying and smelting can be just as bad or worse, but grinding is always the most galling, as in 'theory', it should not be so bad!I fully agree that, kWh per tonne of -75 micron (or whatever size) is a good place to start.

Could you please provide a description of your crushing and grinding circuit, indicates particle size in each stage, tell me what type of hardness testing you have performed for your ore, what type of metals are you dealing with .have you performed plant sampling campaigns while measuring energy consumption, do you have the grinding and comminution efficiency curves (based on size by size assay of the feed and discharge and on the energy consumption, do you have the classification efficiency curves for your screen and cyclones (if any).

If the ball mills are in connection with hydrocyclones then the ball mills can't be evaluated by themselves but they need to be assessed as part of the grinding classification system.In any case the evaluation is simple: You need to take a sample of the feed to the grinding-classification system and a sample of the discharge of the system. If cyclones are present then you need to include internal streams such as mill discharge, cyclone feed , cyclone underflow, cyclone overflow. You develop this sampling and measure the consumed energy during the sampling period.You perform this sampling when you are using the reagent and when you are not using it, but you repeat this sampling at least 3 times per condition to discard that random results are making you to take the wrong conclusions.

You perform size analysis of the different streams and then calculate the grinding efficiency and classification efficiency. Make sure that every sampling campaign last at least 4 hours to make sure that the circuit is in steady state. The residence time in the mills is short as well in the cyclones but the steady state for a circuit is not reached in short timein the method that I'm suggesting, you end calculating the Specific Selection Function for grinding in ton/KwH as a function of the particle size plus the grinding efficiency by each size fraction

Check any lab testwork for screen passing size. The bond work index actually changes for a specific ore depending on the product size you screen for. I.e. if you are comparing a 75micron product in the plant with a 125 micron product in the lab.

One way around this is to use the Morrell fine work index as your basis for comparison. This is an attempt to make the milling work index independent of product size. You can use the lab data from a standard bond ball mill test to calculate this alternative index.

Grinding efficiency plays an important part in calculating the work index of any Ore or Minerals. The Lab data should be precise before scaling to pilot scale and this should be repeated to come to final conclusion.Screening at 74 microns is a tedious affair and there should not be any clogging. Standard bond mill tests are the starting point.

Do bottle roll tests to see at what grind you get the best recovery and float runs open and closed circuit using different grind ranges to see at what grind the float is optimized from a recovery point of view... the float will be more challenging as fine grind will liberate more minerals but risk residue losses, course grind could increase residue losses as well, so do the test work to get the optimum grind, after this you can worry about power draw, demand and sizing.

Grinding efficiency could be defined in many ways, useful consumption of energy usually being the parameter to bemaximized.The Bond Work Index is a measure of theoretical power consumption to grind to a certain size while the Operating Work Index can be measured for an industrial system and reductions inOperating Work Indexshow improvements in efficiency. However, milling is a preparation for the downstream process and recovery losses occur in both coarse and fine size fractions so the target size range needs to be identified.Operating Work Indexis not always able to account for this. I have been able to use Functional Performance Analysis to track increases in rate of production of desired product size range material during anoptimizationof a number of mills.

Some very interesting and informative comments from everyone, and thanks for that. We have just run some Bond Index studies and found a reduction in energy requirements with our additive and now need to consider the next step. It is evident from this discussion that we need to give this very careful consideration.

The best way is to quantify the absorbed energy per hour per tone ( Kwh/t) for a given Ore milling practice at a given predetermine hardness and comparing it to the theoretical Power consumption. Care has to be taken to ensure certain influential parameters are kept more or less constant during this survey. such parameters are mathematically represented as

Efficiency is an ambiguous word that needs to be defined for each specific case. For some people it means power to produce one t/h that's minus a certain size. For others it means actual power compared to predicted power from a Bond work index test to grind a ton to 80% minus a certain size. For others, it means ratio of right size material produced compared to total material ground. It's also affected as much by the sizing or classifying device used as by the mill itself.

Operating work indices include motor, drive and grinding mill efficiencies and inefficiencies, therefore, are not directly comparable to work indices obtained from grindability tests performed on the same mill feed, without the application of correction factors.

Mill power as measured in many plants is motor input power, that is, electrical energy going into the motor. It has to be converted to power at the mill pinionshaft. This is done by applying the motor efficiency factor (electrical and mechanical losses) to obtain motor output power. If the plant does not have the motor efficiency data, it can be obtained from the motor manufacturer. When the motor is coupled direct to the pinionshaft, motor output power is mill pinionshaft power. If a speed reducer or other drive element is used between the motor and the pinion shaft, then the efficiency of the units used must be applied to the motor output power to obtain power at the mill pinion shaft.

The grinding efficiency factors should be applied as required to place the operating work index at the same level as the results from grindability tests. The operating work index so calculated is referred to as Wi0c- This operating work index divided by the work index from the grindability test gives a treasure of grinding efficiency as a multiplier of grindability test results.

EF2 - Open Circuit Grinding - when grinding in open circuit ball mills, the amount of extra power required, compared to closed circuit ball milling, is a function of the degree of control required on the product produced. The inefficiency factors for open circuit grinding are given in Table I.

Table II gives a tabulation of EF3 factors for some of the more common mill diameters in both the imperial and metric measuring systems. This table shows that when the mill diameter inside liners is larger than 3.81 meters (12.5') that the diameter efficiency factor does not change and remains 0.914.

EF4 - Oversized Feed - when the grinding mill is fed a coarser than optimum feed, this factor applies to rod milling and ball milling. The most frequent use is with single stage ball milling. This is the one efficiency factor that is directly related to work index as is shown in the following equation:

When available, use the work index from a grindability test at the desired grind for Wi in equation 5. For equation 7, if available, use either the work index from an impact test or a rod mill grindability test, which ever is higher and for equation 8, use the work index from a rod mill grindability test, since these more represent the coarse faction of the feed which is the portion of the feed coarser than optimum. If not available, then use the grindability test results, available.

Without grindability test results, finding the proper work index figure to use in equation 5 is a trial and error calculation which can be programmed for a computer. Using this approach, the work index used in equation 5 should equal the Wioc obtained, after applying EF4 and all other correction factors to the work index calculated from operating data.

This factor generally applies to low ratios of reduction, but its application to high ratios of reduction does not always apply and should be used only if the Wioc Wi grinding efficiency factor indicates that it should be used.

EF7 - Low Ratio of Reduction Ball Mill - the need to use this factor does not occur very often as it only applies to ball milling when the Ratio of Reduction is less than 6. This shows up particularly in regrinding concentrates and tailings. The equation for this is:

EF8 - Rod Milling - a study of rod mill operations shows that rod mill performance is affected by the attention given to feeding a uniform feed size to the mill and the care given to maintaining the rod charge. This efficiency factor cannot be definitely determined. In selecting rod mills based upon power calculated from grindability tests, the following procedure has been recommended :

While this factor is used in selecting rod mills, the inability to measure and define it accurately reduces its value and significance in calculating Wioc and therefore, should probably not be used in determining the efficiency of rod mill performance, However, knowledge of its existence can be helpful in analyzing rod mill performance.

Supervisory, technical and operating personnel studying the operating data and checking the operation of the plant can determine the cause for any inefficient use of power and the acceptability of the product produced.

The Bond equation utilizing work index as the measure of grindability is an accurate, reliable and readily usable method to obtain a consistent measurement of grinding circuit performance. It takes into account variations in feed size and product size with the work index calculated from the operating data reflecting either changes in the grindability or changes in efficiency. Work indices calculated from operating data, when compared to work indices obtained from Bond Grindability tests for the same mill feed, give a direct measure of grinding efficiency. The Bond Equation and the equations for the associated efficiency factors can be used by plant supervisory and technical personnel and can also be used in computer programs for reporting and/or process control. The Bond Equation and work index are useful tools in evaluating grinding circuit performance to help maximize the use of the power delivered to grinding circuits in minerals processing plants.

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bond index ball mill / rod mill bt 100 xl - retsch

A detailed overview and knowledge of the characteristics of a raw material is of utmost importance, especially when planning the layout of a crushing plant. In order to minimize all possible risks extensive trials are necessary to obtain information on the properties of the raw materials. A clear definition of the required crushing capacities and the desired product quality can be precisely determined by using the Bond Index test methods. Using the Bond Index test procedures it is possible to calculate crushing / abrasion behavior of mineral samples. This knowledge is essential to define the required ball mill layout and production capacity.In order to perform the Bond Index Test successfully it is necessary to use pre-crushed sample material as defined below:Module Ball Mill Minerals pre-crushed to < 3.35 mm and sieved Drillcore pre-crushed to < 3.35 mm and sieved Half Drillcore pre-crushed to < 3.35 mm and sieved The Bond Index conforming ball charge consists of: 43 x 1.45 balls67 x 1.17 balls10 x 1 balls71 x 0.75 balls 94 x 0.61 balls The optimum number of grinding balls is 285. However, the ball diameters vary due to wear. Therefore, the total ball number should be adjusted from time to time to ensure a total mass of 20.125 grams. The grinding jar of the Bond Index Ball Mill measures 12 x 12 and has well-rounded corners. Module Rod Mill Minerals pre-crushed to < 12.50 mm and sieved Drillcore pre-crushed to < 12.50 mm and sieved Half a drillcore pre-crushed to < 12.50 mm and sieved The Bond Index conforming rod charge consists of: 6 rods of 1.25 diameter and 21 length2 rods of 1.75 diameter and 21 length The grinding jar for the Bond Index Rod Mill is 12 x 24 in size and has a wave-shaped design. At least 15 to 20 kg sample material is required to simulate a closed grinding circuit in a ball or rod mill. The Rod Mill Work Index (RWI) is used for particle size determination in a size range from 25 mm down to 2.1 mm whereas Ball Mill Work Index (BWI) is used for the range from 2.1 mm down to 100 m.Adjustment of grinding parameters:The operation display permits convenient selection and storage of parameters such as rotation counter, rotation speed, grinding time, start and stop.

The ball and the rod mill basically have the same concept comprising either a 12x12 jar with grinding balls or a 12x24 jar with grinding rods.The jar is attached to a rotating yoke which is driven by a motor and can be placed in three different positions: Upwards for loading, horizontal for grinding, downwards for discharging.To carry out the Bond Index test the pre-defined number of grinding balls or grinding rods is required. The electronic control integrated in the drive is equipped with an overload protection and permits and controls different speeds.During the grinding process the difference in speeds between the balls / rods and grinding jar produces an interaction between frictional and impact forces, which releases the required comminution energy. The interplay between these forces produces a very effective degree of size reduction.