Filtration is a very general physical treatment method used to remove insoluble materials from liquids. Since the solid matter in the liquid is often a substance which is fine, amorphous, soft and easily clogging the pores of the filter cloth, if it is separately filtered, it often causes difficulty in filtration, the filtrate is unclear, and the filter residue layer cannot be formed (only on the surface of the filter cloth) Problems such as the formation of some slurries on the surface cannot be applied. Adding a filter aid to the solution, or pre-coating a filter aid on the surface of the filter cloth, can significantly improve the condition, the filtration speed is faster, the filtrate is clear, the filter residue is relatively tight and can be detached from the filter cloth. The most commonly used filter aid in various industries is diatomaceous earth.
In some cases where diatomaceous earth is used for filtration, activated carbon is also added to simultaneously adsorb and remove colored substances in the solution and other substances which adversely affect the quality of the product. In the domestic advanced beverage factory, the sucrose used is first dissolved into 50% to 55% concentration, activated carbon and diatomaceous earth (0.2% to 0.3% of the ratio of sugar to sugar), and stirred for 10 to 15 minutes. Filter to achieve full clarity and then use the clear sugar solution to make soda and other beverages.
Diatomaceous earth is a naturally occurring mineral. It is mainly composed of the siliceous part of the sediments of the remains of ancient diatoms and other single-celled micro-organisms, which are processed into products. The main component is SiO 2 .nH 2 O, and the color is white, gray, yellow, gray, and the like. It has a lot of voids inside, light and soft, hardness of 1 to 1.5, density is generally 1.9 ~ 2.3g / cm 3, the drying is 0.4 ~ 0.7g / cm 3, a porosity of up to about 90%, is easy to grind powder. Diatomaceous earth has a strong adsorption capacity and good filterability and chemical stability.
China's Shandong, Jilin, Zhejiang, Yunnan, Sichuan, Jiangxi and other provinces are rich in diatomite, and there are many factories (some of which are produced according to foreign brands and technologies). Due to the different raw materials and processing methods, diatomaceous earth has many different products and brand names; the same production plant often has several products with different properties.
The basic product of diatomaceous earth is that the diatomaceous earth ore is subjected to beneficiation , ground and dried (usually twice), and separated by a pre-separation and a cyclone separator to obtain a fine powdery product. The quality indicators for different grades of products are as follows:
These products have a moisture content of less than 10%, a pH of 5 to 8, and a specific surface area of ​​15 to 17 m 2 /g.
The refined product of diatomaceous earth is also subjected to calcination or fluxing roasting treatment. The calcined product is obtained by selecting the raw materials and calcining at a high temperature of 700 to 900 ° C; the fluxing roasting product is to add the selected raw materials to an appropriate amount of flux and calcined at a high temperature of 900 to 1200 ° C, both of which are subsequently pulverized and Screening. The calcined product is orange, pink to reddish brown, and the fluxing roasting product is pinkish white. In the refined product, SiO 2 is higher than 87%, Al 2 O 3 is less than 3.5%, Fe 2 O 3 is less than 1.5%, CaO is less than 0.35%, MgO is less than 0.35%, organic matter is less than 2%, and water soluble matter Below 0.3%, hydrochloric acid solubles are less than 1.8%, and lead and arsenic are less than 4 mg/kg. The flux of the fluxed roast has a high pH of about 10.
There are many micropores inside the diatomaceous earth, which are visible in the microscope. The pore volume of the original soil is 0.4 to 0.9 mL/g, and the pore volume of the refined product is 1.0 to 1.4 mL/g, and the specific surface area is 20 to 70 m 2 /g. Therefore, it has good adsorption properties, especially good at adsorbing suspended particles in the trapping solution. Filtration of the solution with celite afforded a clear filtrate.
The true density of diatomaceous earth is 2 to 2.5 g/mL, and the bulk density is 0.3 to 0.5 g/mL. Its pore size varies by product and method of manufacture. The pore size of the calcined product is small, such as 3 to 8 μm, and the pore diameter of the fluxed calcined product is large, such as 11 to 16 μm.
Different diatomaceous earths have different permeability to liquids because of their different pore sizes. Permeability is an important parameter for the performance of diatomaceous earth in Darcy. The calcined product is 0.03 to 0.35, and the fluxing calcined product is 0.9 to 5.5. The former type of diatomaceous earth is mainly used to treat low viscosity liquids, and the latter type is used for high viscosity materials.
Diatomaceous earth is widely used in sugar factories abroad, especially for the filtration of high viscosity materials such as syrup. Add it to the sugar liquid, or let the filter pass through the mixture of diatomaceous earth and water before filtering the sugar liquid, form a "pre-coating" of diatomaceous earth on the filter cloth, and then filter the sugar liquid, and then the solution The suspended matter is retained on the diatomaceous earth layer. Filtration of these sugar liquids is often difficult to form a sludge layer in a filter without adding diatomaceous earth.
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