Case Study
Grinding Mill

Grinding Mill Application

When processing minerals in mines, the first step is the excavation of earth to get mineral ore. This ore can be in the form of huge chunks of rocks together with small particles. So obviously, the grinding process is a crucial stage in any mining operation. A mill is a grinder used to break rock-solid or hard materials into smaller pieces and finally into desired powdery fineness by means of shear, impact and compression methods.

Grinding mill machines are thus an essential part of many primary mining processes.

Depending on the type of mineral, usually, two types of grinding mills are deployed for crushing large chunks of ore – Semi-Autogenous Grinding (SAG) mill or Ball Grinding mill. Both grinding mills use more or less the same simple principle: hard steel balls falling on top of each other which can crush large chunks of rocks between them! (fig.1). However, to get the desired thickness of the ore for further processing, this crushing is done in two stages – the SAG mill is used to crush larger pieces into intermediate smaller sizes (fig. 2) and then the Ball mill is used to crush them further to get the desired level of pulverised dust thickness (fig 3).
 

fig. 1

fig. 2

fig. 3

Typically, a SAG mill has about 20% of the space filled with metal balls whereas a Ball mill has up to 40% of space filled with metal balls. The SAG mill relies on repeated collisions between the metal balls and the ore to crush the material into smaller pieces. Ball mill, on the other hand, achieves pulverisation by the constant rotation of the cylinder causing friction, and thus the collision between the balls and the ore. One typical concern in grinding mills is that of ‘Frozen Charge’ wherein material inside a mill solidifies if the mill is stopped for a longer duration. When restarted, this heavy solidified mass does not cascade evenly but suddenly free falls vertically causing heavy damage to the lining of the mill and the mill structure.

Both, the SAG mill and Ball mill, can carry a few hundred tons of load inside them and hence, they are rotated by large heavy duty medium voltage motors.

In most modern mines, it has become quite normal to use variable frequency drives (VFD) to operate the SAG mill motor or Ball mill motor at controllable speeds rather than motors running at one fixed speed. Speed variation gives mill operators better control of the process and reliable consistency of the end product from the mills. VFDs convert the standard 50 Hz supply frequency to an adjustable variable frequency to enable motors to run at corresponding speeds and usually incorporate motor protection features too. Customers generally benefit if motors and VFDs are from the same supplier and are matched for optimum performance.

Enter Toshiba.

Toshiba has decades of experience in supplying the required heavy-duty medium-voltage motors to the mining industry. Toshiba’s MV motor design incorporates precision Japanese engineering practices for excellent results. Further, to enhance the overall system performance, Toshiba uses its popular medium voltage variable frequency drives (MV VFD) to control the speed of these high-quality motors. However, to meet customer-specific requirements, Toshiba can supply VFD + Motor in various configurations, as shown below:

1.

 

2.

Case Study:

A mining customer of Toshiba approached the experts in our MV department of Toshiba with a real problem: Starting and controlling two 6 kV Ball mill MV motors and one 6 kV SAG mill MV motor with variable frequency drives which had to be installed in a very small available space. Further, the customer was also concerned about the ‘Frozen Charge’ and the damage it causes to the mill lining. After understanding the customer’s requirement and the site limitations, Toshiba engineers developed a unique solution that involved the advanced technique of ‘drive synchronisation and transfer’.
 

 

With the first Ball mill motor running directly, the second Ball mill motor was started with the MVG2 drive and the same method of synchronisation and transfer was used to connect it to the utility supply. With MVG2 now free for the SAG motor, it was ramped up to the operator’s set speed and all three motors were in operation.

The reason to select the MVG2 drive was not just for its ability to synchronise and transfer. MVG2 was also equipped with a special software option to detect the presence of a ‘Frozen Charge’ in the mills. This is done by analysing the estimated torque of the motor as it accelerates. If a cascading load is not detected within a predefined angle, the MVG2 drive trips to prevent harmful and/or permanent damage to the mill cylinder lining due to the free falling of the frozen charge.

Drive synchronisation and transfer feature enabled Toshiba to use just one drive instead of three individual drives. This immediately solved the customer’s space problem. Further, built-in ‘Frozen Charge’ protection within the MVG2 drive enabled the customer to avoid other expensive and space-consuming detectors.

The customer was very happy when the entire plant operated exactly as he wanted.

The site is now running for a few years without any problems.

With the use of better quality materials, a strictly controlled manufacturing environment and expertise in testing & installations, Toshiba has gained valuable experience over the years. No wonder, customers are simply delighted with the outcome.