Grinding mills, including SAG (semi-autogenous grinding) mills and ball mills, consume approximately 60-70% of the total energy cost of the entire mining operation. The mills are installed with large motors, up to 20+MW, which impart the required energy to successfully process many tonnes per hour and grind the rock to a powder size suitable for recovering the valuable mineral from the gangue. Machines which do not effectively draw the available motor power represent lost opportunity to a mining operation, since less power often means less tonnes. Therefore, the need to make full use of these machines and their installed power is critical, so that this energy is effectively utilised.
Converting a ball mill from overflow to grate discharge involves installing a grate inside the mill at the discharge end. The grate holds the balls inside the mill but allows the slurry to pass through, and it is then pumped out of the mill by the pulp lifters. This is different to an overflow mill which holds the slurry and balls inside until the slurry builds up to a level at which it is possible to overflow out of the discharge trunnion.
The installed motor power for a grinding mill is a key indicator, along with the physical dimensions of the mill itself, of the capacity of the mill to process tonnes of ore. When an overflow mill is not drawing all available motor power during normal operation, this can indicate that there is potential to increase the mill throughput or decrease the mill product size by operating it in a way which allows the additional capacity to be used.
Converting a mill from an overflow to a grate discharge arrangement leads to an increased power draw. This happens because the installation of the grate discharge arrangement enables the mill to run with a lower level of slurry due to it being no longer necessary for the slurry to build up inside the mill until it overflows from the mill discharge. This build-up of slurry in an overflow mill leads to pooling of the slurry in the bottom of the mill as it rotates, a phenomenon known as slurry pooling. This results in a reduced power draw due to the slurry pool introducing a counterweight to the rising charge which lowers the torque required by the motor to rotate the mill. The grate discharge arrangement removes this slurry pooling effect almost completely.
Estimates of the increases in power draw possible with grate discharge ball mills range from 10% to 20%, according to mill size and other operating parameters. The figures from pilot tests and some operating sites are even higher, up to 25% more power draw.
Conversely, if the mill does not have available power then it is still possible to install a grate discharge and to draw the same power by using a lower ball charge. This has the added benefit of reducing media consumption costs.
For an operation challenged by recovery losses due to an overly coarse product size from the grinding circuit, converting the ball mill from overflow to grate discharge can be highly beneficial. Conversion of the mill results in improved breakage inside the mill. The slurry pool in the bottom of an overflow mill as it rotates creates a cushion onto which the balls fall, which reduces the energy of impact of the balls. This reduced impact energy results in a reduced rate of breakage across all size fractions inside an overflow mill compared with a grate discharge mill. If the slurry pool is removed, the balls fall without cushioning and the breakage rates increase.
Additionally, the slurry pool in the bottom of an overflow mill creates a dead zone in which some of the slurry is held up. This dead zone leads not only to a higher residence time of the average particle in the mill, but also a larger variation in the residence time. Pilot tests indicate that the residence time distribution of an overflow mill has a range up to 3-4 times that of a grate discharge mill, depending on mill operating conditions. The disadvantage of a wide residence time variation is that the retention of material for long periods can lead to overgrinding of the fine fraction of the mill contents through attrition and abrasion breakage, generating excessive fines (or slimes) which are difficult to recover downstream. This is despite the overall product size distribution being relatively coarser than that of a grate discharge ball mill. Excessive fines in the mill product leads to a lower recovery which means lower revenue.
The combination of improved breakage and more consistent residence times in a grate discharge mill means that the product from a grate discharge mill is finer and the size distribution narrower, or sharper, than that from an overflow mill. This is beneficial in operations where a finer product is required but the production of slimes from overgrinding is likely to be a costly problem.
As already mentioned, the average residence time in a grate discharge mill is lower than that in an overflow mill. In addition, the discharge rate of product from the grate discharge mill is higher and not dependent on mill feedrate. This combination of lower residence times and higher discharge rates in a grate discharge mill, coupled with the higher power draw and improved breakage rates, means that the throughput is higher in these mills for the same product size.
However, the actual increase in throughput is dependent on quite a few factors, including available motor power, ore properties, feed and product sizes, recirculating load and other operating parameters and all should be taken into account when assessing the potential throughput increase. Additionally, the throughput is dependent on an optimal grate size, open area, relative radial positioning and pulp lifter design. These grate design features influence the discharge capacity of the mill and should be carefully considered when converting an overflow mill.
The potential benefits of converting an overflow mill to a grate discharge will frequently far outweigh the downsides. In operations needing improved grinding circuit performance, the ability to fully utilise the available installed ball mill power will enable higher throughput, a finer grind, or a combination of the two, which will lead in turn to increased production and a corresponding increase in revenue. However, the practical considerations and costs of converting an overflow mill are important factors to consider and a cost-benefit analysis should always be conducted before proceeding.
Costs associated with converting a mill from overflow to grate include the design and retrofitting of the grate discharge arrangement, including pulp lifters. Additionally, the ongoing operating costs associated with replacement of worn grates and pulp lifters, and the corresponding downtime, are part of the analysis.
In some operations, grate discharge mills can lead to unscheduled downtime due to grate failure, blockage of grates due to sticky ores or poor design, and increased ball consumption due to high wear rates of grate apertures causing loss of media from the mill. In addition, not all mills are suitable for conversion.
Finally, a correctly designed pulp lifter with sufficient discharge capacity is very important, to ensure no increased costs due to high wear as a result of flowback, or lost benefits due to slurry holdup and pooling inside the mill.
The potential to make full use of the available power in a ball mill through conversion from overflow to grate discharge often results in the mill being able to process more tonnes per hour or achieve a more suitable product size to increase the overall production, and hence revenue, from an operation. The benefits need to be carefully weighed against the costs associated with implementation, wear parts and potential additional downtime, as well as considering the practical viability of implementing the change in a particular mill.
A reliable mill lining supplier will always consider these factors when offering the best solution for conversion of a ball mill from an overflow to a grate discharge. For suitable mills, Metso Outotec is able to provide modelling of the mill before and after conversion to quantify the process benefits of the conversion, and then design the best discharge arrangement to meet the requirements and ensure no operational issues.
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Allow metocompare:Ball mills can be of the overflow or of the grate discharge type. Overflow discharge mills are used when a product with high specific surface is wanted, without any respect to the particle size distribution curve. Overflow discharge mills give a final product in an open circuit.
Grate discharge mills are used when the grinding energy shall be concentrated to the coarse particles without production of slimes. In order to get a steep particle size distribution curve, the mill is used in closed circuit with some kind of classifier and the coarse particles-known as classifier underflow-are recycled.
In the past, I had worked with +10% as an expected increase created by the conversion. How much capacity you gain by using grate discharge over overflow discharge on a mill =The +20% data comes from an old paper.
BallMills have a very large discharge opening or area and smaller area for incoming feed. The gradient between the incoming feed opening and the discharge near the periphery of the shell provides a faster migration of the fines than the oversize particles. In deep pulp level mills commonly known as overflow mills this migration can not occur since material enters and leaves at the same level by displacement only. Independent tests have shown that regardless of mill shape or design, the discharge product of an overflow mill will be the same no matter at which end the feed enters.
Grate Discharge Ball mills withlow pulp levels benefit from the full impact of the grinding media acting on the ore particles, as it falls into the shallow pulp. With a deep pulp level the grinding media is cushioned in the pulp, thus losing its energy and reducing its grinding ability. Grate Discharge Ball Mills have shown 25% to 45% more tonnage ground and a substantial reduction in power consumed per ton of material compared size for size with overflow mills.
A general statement can be made that the closer the discharge is to the periphery of the shell, the quicker the material will pass through and less overgrinding will take place. This is important in both rod mill and ball mill grinding. First, regardless of how fine a grind is required, overgrinding is costly and undesirable. The ideal condition is to remove the particles as soon as they have reached the optimum size. Secondly, in grinding applications where a minimum amount of fine material is preferred, again a rapid flow through the grinding mill is required. These can be accomplished with the grate for ball mill operations, or the various Grate Discharge discharge arrangements for the rod mill.
The discharge end of the conventional Open End Rod Mill is virtually open as the name implies. As a means of controlling splash and to prevent unruly rods from moving out of the mill a discharge plug or plug door arrangement is furnished. The use of this construction permits pulp to discharge freely around the annular opening between the plug and the discharge trunnion liner. By simple removal of the plug the full large area of the discharge end may be used for re-rodding, inspection of the mill when in operation, and an easy access to the mill interior for relining. This large opening does away with the necessity of manholes for mill entrance as commonly employed in the overflow type mill. The plug door arrangement is a great time saver during re-rodding and re-lining operations.
On smaller diameter Grate Discharge Rod Mills a discharge plug is furnished mounted on the trunnion liner and extending through to line up with the discharge head liners. The larger diameter Grate Discharge Mills are furnished with a discharge housing arrangement independent of the mill. A hinged door is mounted in this housing and easily swings in or out of the discharge trunnion liner. These housings are also used to control the direction of discharge pulp flow leaving the mill. Such flow may be directed to the left, right, or directly below the mill centerline.
The discharge housing is of very heavy construction for strength and rigidly. Maintenance of this housing is kept at a minimum, the only wearing part being the replaceable Manganese Steel plug door liner.
The discharge end of a Grate Discharge Ball Mill is fitted with grate sections approximately 3 thick, made of special heat treated alloy steel developed for this particular application. The grate sections have tapered openings between and 7/8 dependent upon the specific grinding application. These are selected to provide the greatest efficiency for any particular job. The grate sections are held in place by tapered Manganese Steel side clamp bars, a center discharge liner, and end clamp bars. The discharge grates are very simple to install and require no attention during operation. The overall life of the discharge end parts generally is greater than that of the feed head liners or shell liners. The discharge end of the Grate Discharge Ball Mill has at least ten times the discharge opening area, through the grate slots, compared to the common trunnion overflow type mill. The discharge grates are designed to run clean and free of any blinding or choking. The pulp level in the mill may be varied by merely changing the pulp dilution. There is no complicated mechanical arrangement to compensate for pulp level changes. The side clamp bars and center discharge liner besides holding the grate sections in place, act as a means of stirring up the ball charge and reduces the amount of wear on the grate sections. The pulp discharges through the grate slots into a lifter compartment in the discharge head, lined with replaceable wearing parts. This lifting compartment elevates the discharge pulp up to the level of the discharge trunnion liner opening and spills this against a deflecting cone which directs it out through the trunnion liner.
The above is a Grate Discharge Mill head with discharge grates, side clamp bars, end damp bars, and center discharge liners in place. The grates and side clamp bars are accurately ground to fit the machined surfaces of the discharge head lifters.
We have already discussed grinding in a general way and have referred numerous times to the grate dischargeprinciple of grinding. To illustrate roughly this principle, take a certain weight of crushed ore and grind it with a mortar and pestle until all of the ore particles will pass through a 65-mesh screen. Then take a similar sample but this time grind for a few minutes and screen at 65-mesh removing the finished material, then return the oversize particles and grind for another short period of time and repeat the screening operation. You would find that the actual net grinding time required for the second sample is about half the time required under the first condition. This same process takes place in the Grate Discharge Ball Mill. It must be borne in mind that it is the classifier which determines the size of the finished product, not the grinding mill itself. The Grate Discharge Mill permits a quick discharge of the finished material into the classifier which makes the desired mesh size separation and returns the oversize particles to the mill for another pass.Contrary to the usual belief, material does not discharge through the grates at the bottom. In fact it is carried up in the ball load so that the greater portion passes out from the ball load on the upturning side of the mill, in the grate area from about half way below the centerline of the mill, on up to the point where the balls start to leave the shell on their downward paths. This indicates then that the thick pulp carried in the mill is well within the ball mass where the actual grinding is taking place. The discharge grates are not to control the size of particle discharged, but merely to retain the grinding balls within the mill, provide the full discharge area required, and form the steep gradient between the feed entrance and product discharge.
To illustrate the comparison of the grate discharge Ball Mill to an overflow type of mill we are showing on page 31 several actual case histories of installations where the performance of grate discharge mills versus overflow mills have been proven. In each such test, run for long periods of time, the ore characteristics and size of feed were maintained identical so that the tests could be compared under like conditions. It will be noted that in each case the grate discharge Mill provided a high increase in tonnage with a lesser increase in power consumption so that the actual KWH per ton consumed was reduced. From these field examples you can verify the previous statement that an overflow type of mill has somewhere near 70% the capacity of the grate mill. These tests were conducted independently by the actual operating companies involved.
The above tables list some of the most common Grate Discharge Ball Mill sizes. Capacities are based on medium hard ore with mill operating in closed circuit under wet grinding conditions at speeds indicated. For dry grinding, speeds are reduced and capacities drop between 30% to 50% .
The above dimensions are approximate and for preliminary use only. Right hand mills are shown. For left hand mills put drive on opposite side. Drive may also be located at feed end. but clearance of scoop must be considered.
The above dimensions are approximate and for preliminary use only. Right hand mills are shown. For left hand mills put drive on opposite side. Drive may also be located at feed end, but clearance of scoop must be considered.
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