Four things to consider when converting a ball mill from overflow to grate discharge

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.

Mill power draw

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.

If a mill has available power, conversion of the discharge arrangement from overflow to grate can be beneficial in allowing increased throughput or a finer product size.

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.

Mill product

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.

Mill throughput

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.

Conversion costs

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.

Determining the best option

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.