Large horizontal grinding mills, which include semi-autogenous (SAG) mills as well as ball mills, represent vital machines that are used in modern mineral processing plants. Today’s mining industry relies heavily on big mills with large capacities since they allow for high ore processing volumes per day. Yet, the process of operation of such huge machines is associated with great mechanical and electrical stress. The sudden start results in serious mechanical shock, heavy torque and large current consumption that might cause damage to drives and electrical equipment. Therefore, reliability requires the use of an effective protection system. To prevent mechanical failures and production downtime one should use solid-state drive acceleration, load angle monitoring, and high-pressure lubrication.
Direct across-the-line starting causes high voltage spikes and torque impulses in the drive train of the mill. This results in faster gear wear, possible cracking of the girth gears and overloading of the drive chain. The problem can be solved with the help of solid-state motor controllers like Rockwell Automation SMC and STC. They provide for reduced voltage impulse during the start that results in gradual torque generation and mechanical engagement. Wound-rotor induction motors are successfully protected by Liquid Resistance Starters (LRS) that optimize the non-linear dipper speed control and thus reduce critical area current spikes and avoid thermal overload of the motor.
The structural stability of idled grinding mills is adversely affected by the formation of solid loads resulting from ore and grinding media. The situation where the load forms a frozen charge is a potential hazard that arises after a shutdown of about twenty minutes. Major structural damage is caused by running with a loaded cemented load on the shell wall. The situation where the charged falls from a higher angle causes major damages to shell liners, trunnion bearings, and mill structure due to impact damage. The most advanced technology of load condition monitoring includes Siemens Trouble-free Mill Start (TMS) and SDG Mill Safe Start technologies. Both the technologies make use of two current transducers to monitor the motor performance to calculate angles at which the loads rotate. Automatic tripping is achieved before load attains an angle of 75-85 degrees.
Lubrication system is mandatory because of the presence of heavy trunnion bearings. The systems from companies like Bijur Delimon are characterized with the provision of high-pressure jacking pumps. The system lifts the heavy mill trunnions using the pumps before the motors are started in order to create a good oil film that protects against dry friction and metal-to-metal wear. It also ensures proper filtration and cooling of the oils during operation to maintain optimum viscosity and removes abrasive particles. It is equally important to ensure the safety of the electrical system. The use of solid-state overload relay such as Furnas ESP100 provides precise thermal protection and phase loss detection. There is provision of customized trip classes and rotor side monitoring to detect arcing and jammed conditions. Inspection of supply voltage, contactors, and control wiring minimizes unexpected thermal overload trips and equipment failures.
A reliable grinding mill operation requires an integrated approach for physical and electrical protection of the equipment. Smooth motor starts with solid-state controllers minimize mechanical shock and electrical surge damage. Frozen charge protection prevents damages of liner and shell liners due to load drops. High-pressure lubrication systems and solid-state overload relays protect the vital mechanical components and motors from wear and tear.
