|Cyclone Sample Mill|
Principle of Operation
The Cyclone Sample Mill uses high velocity air flow, an abrasive surface, and centrifugal forces to grind various materials. See Figures 1 and 2. The impeller is rotated at a high speed creating a high velocity flow of air to propel the material particles against the abrasive surface.
When material is added to the Mill, the rotation of the impeller and centrifugal force throw the particles to the perimeter of the grinding chamber where the air flow pushes them along the abrasive tungsten carbide surface. When the particles become small enough to overcome their angular velocity, they move with the air flow out of the grinding chamber and are deposited in a sample collection bottle by the cyclone action. The air flow prevents material accumulation, and any need to clean out the Mill between samples is eliminated. The air flow also minimizes heating of the material and avoids any thermal degradation.
The Mill creates a relatively uniform particle size. The screen only has an indirect effect on the particle size, because most particles ordinarily do not escape the grinding chamber until they are small enough to flow with the airstream. Occasionally impact with the impeller throws a larger particle toward the grinding chamber exit. The screen provides an upper limit of the size particles that can exit. Finer screens also reduce the air flow and the particle size the air stream can carry.
An optional Sample Feed Controller provides an uniform feed rate of material into the Mill. This significantly increases the uniformity of the particle size of ground samples. It also makes sample addition more convenient and eliminates the possibility of Mill overloading, which is the main cause of belt failure.
The Cyclone Sample Mill uses an induction motor and belt drive to maintain nearly constant rpm speed. It is relatively quiet and has a high grinding c capacity. It rotates
the impeller at a uniform 12,600 rpm, using a 3/4 H.P. totally enclosed induction motor. When operated with 50 hz power, the Mill runs about 5/6 the speed as when operated at 60 Hz.
Additional information about the Mill, materials millable, sample feed into the Mill, initial and final particles sizes, etc. may be found in the color brochure for the Mill.
Place the Mill in the desired location. Verify that a screen and the cyclone air separator are in place, then place the cover on top of the Mill. Secure the cover by tightening two clamps on diagonally opposite sides of the Mill simultaneously. Tighten the remaining two clamps. Plug the Mill into a suitable power outlet. Attach the Sample Feed Controller, if one is used.
Position a sample collection bottle under the cyclone body by depressing the spring-loaded bottle support, then turn the Mill on.
With bin gate in place, slowly add 20 to 30 grams of sample into the sample bin. DO NOT add sample before the Mill is turned on. Otherwise, it will stall, and the belts will be ruined. Adjust the sample bin feed rate for about 2 to 3 grams per second by adjusting the bin gate. Lock the gate in position with the thumb screw. Do not feed samples at a rate fast enough to slow the motor down. Overloading the motor weakens the belts very rapidly and lowers the rate that material can be fed into the Mill without further overloading it. If you need to replace belts more often than after 20 to 30 thousand samples, the feed rate is probably too high. If the Sample Feed Controller is being used, the sample may be added to the Sample Feed Controller Hopper (funnel) all at one time.
When no more sample appears in the cyclone body leading into the sample collection bottle, the Mill can be turned off. If the Mill is left running continuously, press the air inlet plug down momentarily while changing the sample collection bottle. Otherwise, dust will be expelled into the room.
Seal the bottle with the snap-cap, then shake thoroughly to obtain a homogenous mixture.
DO NOT OPERATE THE MILL WITH A SCREEN IN PLACE. The grinding chamber will be damaged by sample abrasion.
IF THE MOTOR SLOWS DOWN, THE FEED RATE IS TOO FAST. This is the main cause of belt failure. NOTE: Severe overloading only once may cause belt slippage and loss of grinding capacity. Proper feed rate is automatic when the Sample Feed Controller is used. See
Figure 3. Uniform feed rates lead to a more uniform particle size and improved accuracy for NIR analysis. Use of the Sample Feed Controller is, therefore, highly recommended when NIR instruments are to be used for testing.
The maximum dimension of sample particles fed into the Mill should not exceed 5 mm (1/4 inch) unless the particles are of low mass such as leaves or forages. Large particles may be reduced by a variety of means including blenders, coffee grinders, and other special mills. Foliage, leaves and stems may be fed directly into the Mill when the optional Forage Cover is used.
Larger quantities of material can be conveniently ground and collected using a rip-stop
nylon bag (35-0311) connected to the lower portion of the cyclone body in place of sample collection bottle. Other bottles may be used with adapters or by modifying the cyclone body or bottle support, or both.
Occasionally, because of high moisture or oil content or other properties of materials, the self-cleaning action of the Mill is hampered (typically over 15% moisture or 20% oil). In these cases, additional air flow through the Mill is recommended. To increase the air flow, a canister-type vacuum cleaner may be connected directly to the air outlet in place of the air filter assembly. Contrary to expectation, this will decrease sample loss and improve performance because of the increased centrifugal forces caused by the higher air flow.
When materials contain so much oil or moisture that increasing the air flow is not sufficient, grinding may still be accomplished with a modified Cyclone Sample Mill. Please contact UDY Corporation about this alternative.