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The Thompson test for determining energy requirement for ball and rod mills utilizes a 12” x 24” rod mill turning at 46 RPM. A 100 lb. rod charge of specified sizes together with 10 lbs of minus 3/8” mineral sample is run for five minus. 8A screen analysis is made on the full, dried head sample before splitting and grinding.) A representative 100 gram portion of the ground mineral is removed and the balance of the sample returned to the rod mill for a subsequent five minute run. Another 100 gram sample is split off and a third five minute run made. This completes the initial 15 minute test. A screen analysis of each of the three 100 gram samples is made. The remainder of the ground mineral is discarded.
…….next “25 minute test” is similar to the have 100 gram samples drawn at the 15.20 and 25 minutes mark. A fresh 10 lbs. of minus 3/8” mineral is used for the test. A “35 minutes, respectively, again starting with a fresh 10 lb mineral sample.
The several screen analysis reports are tabulated and the relative surface area (RSA) calculated for each fraction of the original head sample is totaled. The gain in RSA for each time interval is compared with a “standard” RSA that Mine and Smelter has established. This comparison is then used to establish the grinding energy requirement for the mineral sample.
The three different length grinding tests are run to enable the selection of that time period producing results closest to the actual grind required. Its RSA calculation is then used to determine the energy requirement. The test will often show a variation grindabillity for different time periods. This frequently indicates breaking across natural grain boundaries as the fineness of grind increases. The different length test are also significant when two stage grinding is considered (i.e., rod mill followed by ball mills). The short period time test reflects the coarser rod mill product and helps in the selection of the rod mill. The longer test aids in the ball mill selection.
As with the Bond test limitations in sample splitting, and the relatively small sample size should be kept in mind. Also the standards developed thus far by Mine and Smelter limit the grinding test length to 35 minutes. This restricts the test applicability to those minerals in which the required fineness is achieved within 35 minutes. The engineer would do well to utilize both Bond and Thompson test in analyzing his grinding requirement.
Ball mill speeds are often referred to as a percentage of critical speed. Critical speed is the speed at which an infinitely small particle will just trace the path around the periphery of the mill. Critical speed may also be described as the speed at which a particle’s acceleration due to the centrifugal force of the mill’s rotation is equal to its acceleration due to gravity. The acceleration of a particle in circular motion is developed in basic physics as being equal to the velocity squared divided by the radius. Equating this to the acceleration due to gravity of 32.2 feet per second squared we have:
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