2. Working principle

The filtering agent slurry is circulated continuously until a layer of filtering agent is settled on the filtering medium, the process is named as "pre-coating", and then small amount of filtering agent is added to the original liquid regularly, it is called " addition". With the conducting of filtering, filtering agent is mixed with suspended solid impurities in the original liquid, and settled on the pre-coating layer, thus, the new pre-coating layer will form continuously, the fine filtering　agent particulate has a great amount of fine channels which can block the impurities suspended, but the clean liquid can pass through without resistance.

3. Use and main points
　　

1) Selection of types

　　The selection of type of filtering agent is am important link, due to the difference in filter, operation environment, turbidity and production capacity (selection is based on yeast and flocculating protein for beer production), different types of products made by our company can be selected flexibly to meet different requirement, the mixing of coarse and fine diatomite should achieve the stipulated turbidity target set for the filtered liquid.

2) Pre-coating of filtering agent

　　 The main function of the pre-coating layer is to avoid blocking by keeping impurities from direct contacting with filtering medium, and let the filtered liquid to reach satisfactory clarity at the beginning stage, and help wash filtering medium after the end of the filtering cycle. In order to develop stable pre-coating layer, there is certain requirement for the size distribution of the filtering agent, so that it can be of bridging. The proportion of particulates with size greater than the porosity of filtering medium should at least account for 1% to 5% of the filtering agent to be used as pre-coating layer, the thickness of the pre-coating layer will depend on the pre-coating method and filtering equipment.

　　 Pre-coating:Pre-coating is to pre-coat the filtering agent on the filtering medium, which is the most key link among the whole filtering process. The pre-coating slurry ( filtering agent and clear liquid) is fully mixed in pre-coating cylinder, in the meantime, the concentration of slurry is subject to the filtering area and the volume of the pre-coating cylinder ( the volume of the mixed slurry should be 1.25 times the sum of the volume of filter cavity and the volume of connecting duck), and the concentration should not be lower than 3%(filtering agent and clean liquid), or it will be difficult to pre-coat. The concentrated pre-coating slurry is pumped into filter, the pre-coating is carried out in the light of the filtering area, either too quick or too slow may cause the unevenness and falling off.

　　Pre-coating amount:The pre-coating amount is in positive proportion to filtering area, the pre-coating layer should take place with 2-4 mm thick. Generally, pre-coating should be carried out in two or three times (based on the amount of yeast and flocculating protein before filtering for beer production, and viscosity for other liquid). Two-time pre-coating: 1) coarse diatomite pre-coating; 2) mixed coarse and fine diatomite pre-coating, or fine diatomite pre-coating. Three-time pre-coating: 1) coarse diatomite pre-coating; 2) mixed coarse and fine pre-coating; and 3) fine diatomite pre-coating. The proportion of coarse and fine diatomite pre-coating for beer application can refers to the amount of yeast and flocculating protein, duration of yeasting and turbidity after filtering. The concentration of pre-coating slurry ranges from 8% to 10% for vacuum drum filter, the thickness of the pre-coating layer 75 to 105mm, and 16 to 24 kg filtering agent is required for every square meter of the filter surface.

　　Notes:（1） After the pre-coating layer is finished, the residual gas in filtering system should be discharged; 2) The inner pressure of the filter should be stable ( especially from pre-coating to normal filtering, during the switch of valves), so as to avoid the filtering layer bubble and falling off, once it occurs, the liquid at inspection window will turn turbid quickly, at this time, the pre-coating cycle should carried out again, and filtering is conducted after the filtrate turns clear.