Impact spectra in grinding mills

Monitoring of grinding operations in tumbling mills has been the focus of research for several decades. Charge motion and the breakage field that determine the efficiency of grinding are determined indirectly. An instrumented load cell package that can measure the force of impacts inside the grinding chamber of a mill is developed here. The signal from the instrumented load cell package is interpreted in terms of a histogram termed as an impact spectrum which is a plot of the number of impacts at a specific energy level against the energy. It reflects on the average force regime of the mill. The instrumented load cell package was calibrated against the ultra fast load cell which has been unanimously accepted as a standard to measure single breakage events. The load cell package was successfully used to produce impact spectra in an 8.5 inch lab scale mill. The mill speed and the ball size were varied to study their effect on the impact spectra. A good correlation was found between the process variables and the impact spectra. The load cell package was then used in a 16 inch pilot scale mill. The mill speed, ball size, and mill filling were varied here and a consistent trend between these variables and impact spectra was observed. With a better understanding developed from the initial tests, the design of the load cell package was significantly changed to eliminate noise. With the new design, the impact spectra were re-determined in the 16 inch pilot mill varying the process variables - ball size, mill speed, and mill filling. Finally, it is successfully shown that a change in the operating variables of a mill can be seen in the impact spectra and that this concept can be successfully developed to monitor the grinding operation of industrial mills. The current connections of the load cell package are all hard wired. To adapt it to industrial level it is mandatory to make the load cell package wireless. An initial design of a wireless circuit that is capable of transferring data at the required speed of 1000 kbps was also developed and tested at Cortez Gold Mines (CGM), Nevada.