In dressing plant, coal preparation plant and other industry departments, the crushed materials are mixed together in different size of grains. These materials need to be classified into several grades with similar granularity before further processing. The screening process can be divided into following types:

1. Pre-screening: It is the process that prepared for further processing. In dressing plant, if we take gravity concentration and electromagnetism separation, the ore should be in required size. Therefore, before separating, the ore must be classified in order to helpful for further process. 

2. Independent screening: The product obtained after screening is the finished product, this screening process is called independent screening. For example, in coal preparation plant, the raw coal classified into several types is directly provided for the customer use, this process is applied independent screening.

3. Auxiliary screening: This screening works together with crushing machine. The grain size should meet the requirement of crushing machine after screening, this type of screening is called auxiliary screening. 

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