The chemical composition and physical structure of silica gel determine that it has many characteristics that other similar materials are difficult to replace: high adsorption performance, good thermal stability, stable chemical properties, and high mechanical strength. Silica gel is divided into: large-pore silica gel, coarse-pore silica gel, B-type silica gel, fine-pore silica gel according to its pore size. Silica gel is a highly active adsorbent material, which is an amorphous substance, and its chemical formula is mSiO2·nH2O. It is insoluble in water and any solvent, non-toxic and odorless, chemically stable, and does not react with any substance except strong alkali and hydrofluoric acid. Various types of silica gel have different microporous structures due to different manufacturing methods.
1. Safety performance
The main component of silica gel is silica, which is chemically stable and does not burn. Silica gel is an amorphous silicon dioxide. The dust content in the workshop should be controlled at no more than 10 mg/m3. Exhaust air should be strengthened and masks should be worn during operation.
Silica gel has a strong adsorption capacity and can have a drying effect on human skin. Therefore, work clothes should be worn during operation. If the silicone enters the eyes, it needs to be rinsed with plenty of water, and seek medical treatment as soon as possible.
Blue silica gel is poisonous because it contains a small amount of cobalt chloride. Avoid contact with food and inhalation. If a poisoning event occurs, you should seek medical treatment immediately.
Silica gel absorbs water vapor or other organic substances in the medium during use, the adsorption capacity decreases, and it can be reused after regeneration.
Second, the regeneration of silica gel after adsorption of water vapor
After the silica gel absorbs the moisture, the moisture can be removed by thermal desorption. There are many heating methods, such as electric furnace, waste heat heating of flue and hot air drying.
The temperature for removing additional heat should be controlled at 120-180 ℃, for blue glue indicator, color-changing silica gel, DL blue silica gel should be controlled at 100-120 ℃. The maximum temperature during the regeneration of various industrial silica gels should not exceed the following limits:
Coarse pore silica gel should not be higher than 600℃;
The fine-pore silica gel should not be higher than 200℃;
The blue glue index agent (or discoloration silica gel) shall not be higher than 120℃;
Silicone aluminum glue shall not be higher than 350℃.
After the regenerated silica gel, its water content is generally controlled below 2% and can be put into use again.
3. Regeneration after silica gel adsorbs organic impurities
For coarse-pore silica gel, it can be placed in a roaster to gradually increase the temperature to 500-600 ℃, about 6-8 hours until the colloidal particles are white or yellowish brown. For fine-pore silica gel, the firing temperature cannot exceed 200℃.
After the silica gel is absorbed in saturated water vapor to reach saturation, it can be soaked and rinsed in hot water, and can be combined with a detergent to remove waste oil or other organic impurities, and then washed and dried in dehydrated water.
⒊Solvent washing method
According to the type of organic matter adsorbed on the silica gel, an appropriate solvent is selected to dissolve the organic matter adsorbed in the silica gel, and then the silica gel is heated to remove the solvent.
4. Problems that should be paid attention to when regenerating silicone
⒈When drying and regenerating, we should pay attention to gradually increase the temperature to avoid bursting of the rubber particles caused by severe drying and reduce the recovery rate.
⒉ When the silica gel is roasted and regenerated, too high temperature will cause the pore structure of the silica gel to change and significantly reduce its adsorption effect, affecting the use value. For blue glue indicator or discoloration silica gel, the temperature of desorption regeneration should not exceed 120 ℃, otherwise it will lose the color development effect due to the gradual oxidation of the developer.
⒊ The regenerated silica gel should be sieved to remove fine particles to make the particles uniform.
5. Storage and packaging
Silica gel has strong hygroscopicity, so it should be stored in a dry place with a shelf between the packaging and the ground. The packaging includes steel drums, paper drums, cartons, plastic bottles, polyethylene plastic composite bags, flexible container bags, etc. Avoid rain, moisture and exposure during transportation.
Sixth, the use of inorganic silica gel
Macroporous silica gel is generally used as a catalyst carrier, matting agent, abrasive, etc. Therefore, different varieties should be selected according to different uses.
Silica gel can be used as a desiccant, and can be reused. Silica gel is a porous material with different particle sizes formed by dehydration of silicic acid gel mSiO2·nH2O. It has an open porous structure, a large specific surface (surface area per unit mass), and can adsorb many substances. It is a good desiccant, adsorbent and catalyst carrier. The adsorption of silica gel is mainly physical adsorption, which can be regenerated and used repeatedly. Add acid to the alkali metal silicate (such as sodium silicate) solution to acidify it, and then add a certain amount of electrolyte to stir, that is, generate a silicic acid gel; or add acid to a more concentrated sodium silicate solution or Ammonium salts can also generate silicic acid gel. Allow the silicic acid gel to stand for several hours to age, then wash away the soluble salts with hot water, dry at 60-70°C and activate at about 300°C to obtain silica gel. After soaking the silicic acid gel with the cobalt chloride solution and then drying and activating, the discolored silica gel can be obtained. When it is used as a desiccant, it is blue before absorbing water and becomes red after absorbing water. From the color change, we can see the degree of water absorption and whether regeneration treatment is required. Silica gel is also widely used in steam recovery, petroleum refining and catalyst preparation, etc.
Inorganic silica gel is used as a desiccant, which can be specific to the following aspects
1. Mainly used for moisture absorption and rust prevention of instruments, meters and equipment under closed conditions.
2. Used in conjunction with ordinary silica gel desiccant, indicating the degree of moisture absorption of the desiccant and judging the relative humidity of the environment. As a silica gel desiccant for packaging, it is widely used in precision instruments, leather, clothing, food, medicine and household appliances.