Trends with selection and sizing of large grinding mills

Ore grinding accounts for 60 to 80% of the total capital cost incurred in processing plants. The size reduction required to liberate the valuable material from the gangue requires a large amount of energy, and with plant throughputs continually increasing this requires larger and more powerful mills. Incorrect sizing and selection of mills causes large increases in capital cost, decreases comminution efficiency and can affect recoveries in downstream processes if not correctly sized.

 

Figure 1 highlights the zones within a mill during grinding. Depending on the speed of the mill the effective grinding will vary. Highly important to note is the critical speed percentage of a mill. Mills will often run between 65-75% of their critical speed to enable maximum milling efficiency.

 

Higher throughput plants are constantly being designed and commissioned with an increasing number utilising multiple grinding trains to achieve the required throughput. Developments in mill design has allowed for larger capacity mills to be constructed, increasing the possible throughput from a single grinding train.

 

Several types of mills are available to the industry, operating largely around the principles of impacting the ore with a media, either steel balls or itself, to achieve breakage.

 

AG (Autogenous Grinding) mills use no steel grinding media, instead using ore breaking against ore to achieve the required break-age. Testwork is used to determine the amenability of an ore to AG grinding, as this unit will not work with all ores. AG mills currently are available in sizes up to 12.2 m (40ft) in diameter and run at 75 to 85% of critical and can be low or high aspect ratio mills.

 

SAG (Semi-Autogenous Grinding) mills operate similarly to AG mills however feature a small (4-12%) ball charge to assist breakage. Combining mixtures of hard and soft ores to be processed through the same mill achieves high throughputs, while keeping costs from replacement of grinding media down. These are usually variable speed mills. The evolutionary development of pebble crushing, pebble ports and crushing more of the feed has improved the capability of SAG mills. There are significant CAPEX advantages over large crushing plants.

 

High Pressure Grinding Mills (HPGR) are replacing SAG mills on hard ores. The biggest advantaged is reduced power consumption.

 

Ball mills use a high ball charge (35-45%) to achieve the desired breakage with a high pulp density (65-80% solids). Ball mills are typically used in a circuit with an AG/SAG mill to reduce risk compared to a single stage SAG mill. Ball mills can accept feed sizes up to 12.7mm but the oversize feed factor impacts negatively for hard ores.

 

Mill sizes have increased up to 8.2m diameter and were previously limited by mechanical issues. Due to the stresses experienced by the trunnions, through supporting the weight of the mill, these required precise casting and construction. With larger trunnion heads it becomes increasingly difficult to meet the required accuracy of casting, imposing a size limit using trunnion heads. This then imposes limits on mill size and feed / discharge opening diameters.

 

In order to have larger diameter mills, the trunnion supported design was changed to shell supported. This places the mill on multiple hydrostatic bearings against the mill shell, sharing the mill’s load and reducing the stress experienced. This has resulted in a smaller mill foot print and thinner shell plates being required. With the trunnion size no longer an issue, larger feed and discharge openings were possible. Combined with flat mill heads, this allows cheaper manufacture and larger mills to be constructed.

 

Larger mills sizes allow for greater throughput of ore provided there is the power to run the mill. Traditional gear driven mills are limited to 10,400 kW. To pass this limit, gearless mills have been developed that allow far higher power to be reached, up to26, 000 kW. This has made the larger mills with higher throughput feasible.

 

The gearless drive also offers other operating advantages besides higher power output. The drive mechanism allows for far greater speed control of the mill, with gradual power up and down possible, “inching” of the mill to allow precise movement for maintenance and greater stability (no rocking) during maintenance. The gearless drive is also reported to have higher uptime through lower maintenance requirements.

 

Gearless mills however are more expensive and electrically complicated than gear driven; it is up to the design team to determine if the advantages outweigh these disadvantages. Hence gearless drive is normally only used on mills that have higher power requirements, where lower power mills will still use geared drives.

 

It is through gearless drive and shell supported bearings that larger and more powerful mills are industrially available. As such it is now slurry transport issues, as opposed to mechanical limits, that limit possible mill sizes.

 

With increasing mill size, focus is given to motor arrangement and control to ensure the most efficient setup is used, as with high power mills even small percentages of inefficiency can have notable effects on operating costs.

 

Variable speed motors use slip energy recovery (SER) or synchronous motors, pulse width modulated (PWM), cycloconverters or synchronous motors with load commutated inverter (LCI). Synchronous motors require shear pins when used with cycloconverters to protect the drive train in the event of a power failure.

 

A key aspect when sizing and specifying large grinding mills is the electrical system, with there being several major issues to consider:

•High power factors improve electrical efficiency and result in lower power cost.

•High voltage motors 6.6-11kV

•All systems must comply with the network provider’s standards.

•Operating above rated mill powers will shorten motor life.

•Power station approval needed prior to starting large mill motors due to initial torque / power load required.

With variable speed control and advanced sensor systems, control circuits now play a vital role in maintaining plant operations. Systems like SCADA (Supervisory Control and Data Acquisition) and Distributed Control Systems (DCS) are becoming more common, providing state of the art systems with high levels of reliability. These systems are used to monitor the process via magnetic flowmeters, nucleonic density meters and level indication. By allowing automated adjustment of mill speeds, greater ability to resolve downstream issues is available. Combining variable speed mills, through gearless drives, with SCADA systems has enabled greater control and information over the grinding circuit.

 

Mechanical issues can be addressed with finite stress analysis, single vs. dual drives, laser alignment, inspections, oil analysis and carrying critical spares.

 

Lubrication systems are continuously monitored and alarmed, use specialised lubricants, interlocking of temperature and flow, use hydrostatic bearings and use oil instead of grease in some applications. From a civil engineering perspective mills need solid foundations and significant changes in layout, mill centre line clearance, floor slopes and access around the mill has improved greatly. Trommels or vibrating screens can be used with a trend back to trommels being seen.

 

Testwork is integral to select and size a mill or milling circuit. Testwork provides multiple comminution parameters that are used in the design of mills, and defines an ore in a way that is comparable to other ores. It is essential to undertake testwork on samples that are representative of the ore to be processed. Incorrect sampling will cause inaccurate data, incorrectly designed and sized mills and could cause a complete failure of a project. Geometallurgy, mineralogy and metallurgical mapping are all important. Parameters tested and used include:

•HPGR testwork.

•Bond Abrasion Index (Ai) – Used to indicate how abrasive an ore is. Useful for determining wear rates of liners etc.

•Bond Ball Mill Work Index (BWI) – Used as a basis for basic comminution energy requirements. Ranges from 5 kWh/t for soft ores, increasing as ore hardness increases to greater than 25 kWh/t.

•Bond Crushing Work Index (CWI) –Defines the ore competency at larger particle sizes.

•Rod Work Index (RWI) – Measure of energy required to grind an ore in a rod mill from 12.5 mm to 1 mm. Ranges from 5 kWh/t for soft ores, increasing as ore hardness increases to greater than 25 kWh/t.

•Unconfined Compressive Strength (UCS) – Analyses the effect of crushing forces on an ore, used in SAG and AG mill performance testing

•SAG/AG Mill Testwork – Uses AMC, DWI, SMC, SPI, UCS, CWI, RWI and BWI to determine if an ore is amenable to AG/SAG mill processing.

 

 It is through use of these testwork parameters that mill sizing is possible. During advanced study stages, such as definitive feasibility studies (DFS), it is highly advisable to undertake pilot testwork on a representative sample to ensure that the milling circuit will perform as required and there is no build-up of critical sized material, material that requires removal and separate grinding to other material.

 

To assist with mill sizing, simulation programs are available. These programs allow the parameters from testwork and knowledge of the ore to be used in developing a simulated milling circuit. This will indicate the performance of the circuit, energy requirements, recirculating loads, estimate stream size splitting and allow indication of differences between different circuits. These findings are then used to select a mill/s and form the basis for the front end design of a plant.

 

Depending on the ore to be processed and downstream requirements milling circuits will have differing arrangements. Common circuits include (Figure 2):

•Single Stage SAG

•Two Stage Crush / Ball Mill

•SAG / Ball Mill

 

A circuit that will work for one process or ore will not work for all and milling circuits will require individual consideration depending on the process and ore to achieve the lowest capital cost and greatest efficiency. As a general rule smaller throughput projects are generally easier to size and design.

 

Advances in design, experience with new technology and growing use of simulations (JKSimMet) for testwork data have enabled larger and higher throughput mills to be designed, developed and utilised in industry.

 

As demand grows for important commodities and throughputs are required to increase there will be a continuing trend to see growing mill sizes commissioned in order to process higher throughputs, and thanks to constant developments with shell support and gearless drives these increases will be feasible.

 

Major mill vendors are Metso, Outotec, Thyssen Krupp, FLSmidth and Citic Heavy Industries.

 

Examples of large mills are Newmont’s Minas Conga project (12.8 x 7.6 m) and Yanacocha (9.75 x 9.75 m). The Sino Iron project has the largest installed AG mills in the world (12.2 x 10.3 m).