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Protect grinding mills through proper maintenance

Without question, one of the most critical components in a plant’s throughput is the grinding mill. It therefore makes sound economic sense to protect the mill from unnecessary, preventable and costly damage. This is even more important in current times as mills are under even greater pressure, with lower grade orebodies and higher resulting throughputs.

The saying “the bigger they are – the harder they fall” has never been truer than for grinding mills. Nowadays, larger mills require more sophisticated bearings and lubrication systems as they are subjected to higher loads and stresses. Larger mills also have higher throughputs, meaning that the opportunity cost in lost production with an unscheduled shutdown can run into hundreds of thousands of dollars. The question then is; how do you best protect a mill from preventable damage?

Modern control systems

A modern mill should be supplied not only with an up-to-date control system, but also a control philosophy which protects the mill. As a part of this control system, critical mill areas should have transducers to sense, in real time, the condition of each given part.

This is then fed back to the control system which, as part of the control philosophy, will digest the information and make decisions based upon pre-set interlocks. In general, there are three types of interlocks;

1. Permissive interlocks:
Conditions that must be met before a mill is allowed to start (e.g. bearing lubrication pumps must be running to allow the mill to start)

2. Alarm interlocks:
Conditions that will cause the mill to raise an alarm, but not prevent the mill from operating (e.g. bearing temperature lower or higher than normal will trigger an alarm)

3. Trip interlocks:
Conditions that will cause the mill to cease operation (e.g. dangerous bearing temperature will stop the mill from operating)

It is important to understand that the set points for these interlocks are selected to prevent the mill from operating under conditions that have the potential to cause significant damage. It can be tempting for an untrained or inexperienced operator to change these set points or bypass the interlocks to keep a mill operational, however this mindset fails to consider the potential damage this may cause.

Instead, a short investigation to find out why an interlock is tripping and fixing it may save an extended shutdown in the future and thousands of dollars in repair in the process.

For example, an interlock could trip if the oil cooling heat exchanger or its upstream strainer becomes filled with dust. This would, in hot climates, reduce the efficiency of the heat exchange, causing the hydrostatic bearing lubrication oil to increase in temperature.

This would first trip an alarm interlock, warning that the temperature is higher than usual and close to being dangerously high. If this alarm was ignored, and the temperature kept rising, a trip interlock would activate and stop the mill to protect the bearings from damage.

Damage could occur due to the fact that, as oil increases, the viscosity lowers, reducing the oil film thickness at the bearing. Should the trip interlock be bypassed in this condition, the mill would be running with an oil temperature that may allow the bearing’s oil film thickness to reduce to the point where there is journal-to-bearing contact, thereby damaging the bearings and mill journal.

Repairing such damage would mean a shutdown of up to one week (or more depending on spares availability). Had the mill been inspected at the first alarm, the simple process of cleaning dust from the heat exchanger or its upstream cooling water strainer would have solved the problem.

Stand alone mill control systems

In stand-alone mill control panels, interlocks cannot be bypassed so easily. They usually have password protected PLC programs, and require specific knowledge or highly skilled personnel to bypass or change the interlocks. Where the mill control and interlocks are part of the centralised mine control system, bypassing an interlock may be as easy as a few clicks on a mouse.

So, in these situations it may be worth upgrading to a stand-alone control system, or putting the necessary protection in place to prevent unwanted interlock bypass. When purchasing a new mill or refurbishing an older mill, it is important to take into consideration the additional protection a stand-alone mill control system will provide.

It is also worth considering that electrical technology moves at lightning pace when compared to mechanical technology (especially mills), and reputable mill manufacturers are constantly reviewing and updating the best practices of mill operation and control.

Control system upgrades

With this in mind, mill control philosophies and systems may be subject to upgrades over only a short elapsed period of time, so it is always worth speaking regularly to the manufacturers and discussing if your control system can be upgraded to meet current best-practices.

Firmware upgrades are very important, as they are generally released by the PLC manufactures to update the functionality of a CPU, or to prevent a re-occurrence of a recently discovered fault.

The need for a firmware upgrade should be reviewed every six months and will require the services of skilled PLC programmer. If the status of the firmware onsite is known, this review may be able to be performed offsite.

Tips on maintenance of control systems

Site maintenance personnel also need to be aware that the control systems protecting the mill require regular maintenance, which includes the following checks:
1. Ensure that the control system is installed in a protected area, away from the presence of extreme heat, moisture and dust
2. Ensure the panel exhaust fans are operating correctly, and filters cleaned regularly
3. Ensure the panel doors are sealing sufficiently to prevent the ingress of dust and water
4. Ensure that any PLC forces and bridges are recorded and action put in place to remove as soon as possible. Note that bridges and forces should only ever be used in circumstances with no other alternatives
5. Review the system set points regularly to ensure they are correct
6. Ensure the PLC firmware is regularly reviewed and updates downloaded to the PLC


A broken down mill will generally mean an idle plant, or at the very least a highly compromised plant. An up-to-date control system, operated correctly, will assist in preventing break downs. This is only effective, however, when combined with a sense of ownership of the mill (across all levels of mine management and operations).

In such a scenario, there is a commitment to monitor, investigate and repair faults as (or before) they occur, rather than blindly bypassing the control system. With such an approach, when we protect our mills by mindset rather than by accident, responsible care of the mill is probable, rather than accidental, and increased overall mill availability will be the result.

[By Daniel Braithwaite (, Outotec.]

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