Operations & Uses
Sifting, in its simplest form, employs the well-known principle that, in a dry material made up of various particle sizes, agitation of the material will cause the smaller particles to move to the bottom of the mass. When the material is confined in a container having a bottom opening, or openings, all the particles small enough will tend to pass through the openings while the larger particles will be retained.
The speed and efficiency with which this separation by particle size takes place is largely determined by the individual size and weight of the smaller particles, the pressure exerted on them from above by the large particles, the amount and type of agitation employed, and the number of openings available.
Sifter Parts & Service screens are the result of engineering experience and skill in incorporating the above outlined characteristics into a modern sifting machine, capable of continuous, efficient operation under normal service conditions.
In operation, the Gyratory Sifter receives the material to be processed at a rate which does not exceed the capacity of the sifter. The sifter box becomes the container or containers, for the material, and the sieves provide the openings through which certain particles are able to pass. The movement necessary to cause the smaller particles to reach the openings and pass through them is provided by a motor drive which rotates the sifter box at a rate of speed which has been found to provide the most efficient results.
As the particles pass through the sieve openings, they fall onto an inclined collecting tray which directs their flow to a lower sieve or to a sifter outlet. The particles too large to pass through the sieve openings pass over the end of the sieve and onto another sieve or to another sifter outlet.
To prevent any material from lodging in or blocking the sieve openings, cleaners are frequently provided which act upon the underside of the sieve cloth. Various types of sieve cleaners are available, but the most common method is to use small rubber balls. These balls ride on a coarse wire screen and are confirmed in compartments. When the sifter is in motion, the balls bounce constantly from the ball carrying wire to the underside of the sieve to dislodge any particles blocking the sieve openings.
Cloth stocking legs provide dust-tight, flexible connections at the sifter inlets and outlets and prevent material spillage.
Although the specific purposes for which a sifter is used may vary widely with individual installations, sifter applications may be regarded as belonging in one or more of the following groups: Scalping, Sifting, Bolting, Grading, Sizing, Screening, Rebolting and Proof Sifting.
Term used to remove lumps or foreign material such as sticks, twigs, strings and tags, from material that is much finer than the screen on which it is being scalped. Due to the large opening or mesh size, in comparison to product size, this operation is usually performed very easily at a
fresh rate with a high throughput on a small screen area, and the efficiency of a separation would be high.
2. Sifting, Screening, Bolting, Sizing or Grading:
These would be terms used in separating a material by its actual size when a reasonably close job of separating is desired. Depending on the material, of course, and its own characteristics, this is not nearly as fast a job as scalping. Throughput would not be as high; more screen area would be required. The size of screen openings is the actual size of the product separation, or approaches it, and the efficiency will depend upon the amount of sieve area and the time the material is on the screen. This is actually to classify or grade the components of the material being sifted. As in the case of many products where only particles of a specified size are acceptable for use, sale or for further processing.
3. Rebolting or Sifting:
To remove the formations or lumps which may form in a material which has been kept in a storage for some time. This is similar to scalping in that a material usually much finer than the mesh opening is involved in the process. An example of rebolting would be the rebolting of flour – just ahead of packaging or shipping in which strings, tags, insect larvae, eggs and insect fragments, are removed before releasing the product to the customer.
4. Proof Sifting.
This is actually a final check on product quality. It provides a final check on a product after it has been processed; in many cases, it has been sifted earlier. This would be either a scalping or grading job, as above, depending on the installation.
In many applications, it is desirable to accomplish more than one of the above types or degrees of separations. For those installations, each of the Sifter Parts & Service Sifters can be furnished with one or more screens to deliver various separations.
Generally speaking, the Sifter Parts & Service Sifters find application in four locations with respect to any process.
1. Ahead of the Process.
Here are the advantages can be two-fold. First, for sanitation to scalp off foreign material from incoming product; second, to remove the material which is desired size before grinding. This can increase capacity or quality of the grinding.
2. Within the Process.
The main function of a sifter within the process is to classify according to particle size. Additional advantages can be seen as increased productivity on existing grinders or reducing the size of the mill necessary on new installations. This can be accomplished by removing the desired product before regrinding the oversize.
3. After the Process.
This application would be similar to that previously mentioned. The sifter could be used to remove over and undersize material from good product. The sifter could also classify the finished product into several fractions or grades for packaging. An example would be the screening of cracked corn into several sizes – coarse, medium, fine and meal.
4. After Storage.
It is often possible that the finished material, after classification, is moved into storage bins or hoppers, to await packaging or bulk loadout. During this storage, there is always the possibility of contamination with foreign material – or because of pressure and moisture, a tendency to form lumps. Many processors find it advantageous to use a CS-1 Sifter just ahead of final packing or bulk loadout.