Discuss different characteristics of various drying methods

At present, in addition to direct drying and drying, there are constant boiling drying, solvent replacement drying, supercritical drying and freeze drying, as well as spray drying and spin flash drying. Ultrafine silica, also known as silica, is an important fine inorganic chemical product. Because of its excellent properties of reinforcement, thickening, matting, etc., it is widely used in rubber, plastics, coatings, and paints. In many fields such as ink, papermaking, medicine, pesticides, daily chemicals, etc. The preparation methods mainly include gas phase method and liquid phase method. The gas phase method uses silicon halide as a raw material, and the product is directly obtained from the gas phase, there is no problem of drying, so the product performance is very good, the powder particles are fine and do not contain salts. However, it consumes large amounts of energy, complex technology, and expensive products. The liquid phase method, despite its low cost, has poor product quality. The Hypergravity Engineering Research Center of the Ministry of Education of Beijing University of Chemical Technology has used hypergravity technology to develop a new process for the preparation of ultrafine white carbon by wet process. The resulting product has been greatly improved in quality compared to the original wet process product and has obtained relevant patents. The superfine white carbon black products prepared by the high-gravity reaction precipitation method were subjected to various drying methods to study the effects of different process conditions on their agglomeration control. Through various performance index tests, the different characteristics of various drying methods were discussed. The obtained dry powder is used as reinforcing agent and applied in silicone rubber. Through testing various physical and mechanical properties of the rubber compound, the advantages and disadvantages of the drying method are further explained. The selection of industrial equipment and the selection of process conditions are selected in the future. It has a certain guiding significance.

Preparation of white carbon black, acidified, aged, filtered, washed and obtained wet cake, respectively, the following drying experiments: (1) Direct drying and drying. The wet cake was placed in a drying oven and dried at 105e for about 2 h. The filter cake lost most of the water. The larger lumps were crushed and the baking was continued at this temperature for 4 h. The resulting dried powder was ground after 010385 mm. Sieve, sieve product is Sample A. (2) Constant boiling dry. The wet cake was dried by constant boiling, filtered, dried, and sieved through 010385mm. The sample was replaced by solvent of sample B. (3). Take a certain amount of wet filter cake, fully washed and filtered with absolute ethanol, and dry, grind, and sieve as shown in Sample A to obtain sample C. (4) Supercritical drying. The supercritical drying apparatus (self-assembly) was used to supercritically dry the filter cake after washing the alcohol obtained in C. The sample was passed through 010385 mm sieve. (5) Freeze-drying. The resulting wet cake was dried using a CHRIS Alpha 1-2LD freeze dryer with a vacuum of 6 Pa. At this time, the temperature of the freezer was -60e, and the dried product was obtained. The sample was then ground through 010385 mm. E. (6) Spray drying . Take a certain amount of wet cake and mix it with water to prepare a slurry with a solid mass fraction of 15%. Spray dry with a QP3 spray dryer. The inlet air temperature is 160e and the exhaust air temperature is 60e. After drying, the cyclone is separated and collected in a set. In the bottle, the resulting product was ground and passed through a 010385 mm sieve. The product was used as sample F.

The sample characterization was performed on a Hitachi H-800 transmission electron microscope. Transmission electron microscope (TEM) analysis was performed on the wet cake and various dried products. The specific surface area of ​​each sample was measured with an AS-AP2010 type specific surface meter, according to the national standard GB10528- 89 Oil-absorbing values ​​of the obtained samples were tested according to the national standard GB10521-89, and the resulting samples were subjected to a heating reduction test. Image analysis of the resulting dry powders was performed using an IBAS/ò image analyzer. The particle size distribution was examined and the dry powder was observed with an OLYMPUSù51 optical microscope. Observe its shape directly.

Application and testing in silicone rubber Silicone rubber refers to a type of elastomer in which the molecular backbone is SiO inorganic structure and the side groups are organic groups (mainly methyl), belonging to semi-inorganic saturated, hetero-chain, non-polar elasticity. body. Its high temperature and low temperature performance, surface tension is low, has good hydrophobicity, air permeability, insulation properties and excellent aging resistance. Since the tensile strength of silicone rubber is only about 013 MPa, it must be reinforced with a reinforcing agent. The most commonly used silicone rubber reinforcing agent is white carbon black. Now with silicone rubber, white carbon black, Shuangwu five mass ratio of 100B40B215 formula, the silicone rubber reinforcement experiments, and the mechanical properties of the mixed rubber test.

TEM electron micrographs of fine white carbon black gels were analyzed by transmission electron microscopy (TEM). It can be seen from Fig. 1 that the non-dried silica is in the form of a secondary structure of grape clusters, and its primary particle size is less than 20 nm, which is consistent with the conclusions obtained by previous researchers. The preparation of ultrafine carbon black by the method of high gravity carbonization has the characteristics of high stability and good reproducibility, which can provide strong evidence for the industrial application of this research in the future.

In the direct drying and drying process, the moisture gradually loses, the surface of the particles is exposed, and the remaining moisture between the particles forms a capillary phenomenon in the channel, and there is a capillary force. In view of the characteristics of water, the liquid surface bends downwards, and the additional pressure generated by the capillary points to the air, causing the walls of the capillary pores to contract inwards, causing the adjacent particles to pull closer and press tightly together. At the same time, white silica surface exists SiOH structure. During the drying process, a dehydration-condensation chemical reaction can occur between adjacent particles (formula 1), and a hard agglomeration formed by chemical bonds is formed, so that the apparent diameter of the particles becomes larger. The darker part of figure 2 is the more serious agglomeration.

In the process of solvent exchange and drying, when the water is replaced by ethanol with a small surface tension, the capillary force is reduced accordingly, and the formation of chemical bonds is also reduced, and the degree of agglomeration is significantly improved (Fig. 3). For azeotropic drying, n-butanol is used as a constant boiling agent. The azeotropic distillation technique is used to remove the moisture from the filter cake in the form of n-butanol-water azeotrope. The final filter cake is silica and n-butane. The alcohol mixture, n-butanol, has a much lower surface tension than water, reducing particle agglomeration due to surface tension. Moreover, the effervescence during the azeotropic process, the particles are in a state of continuous movement, and the chance of agglomeration and growth is greatly reduced. In addition, under the constant boiling operation conditions, n-butanol can directly replace the hydroxyl groups on the surface of the particles and bind directly to the particles. This combination can weaken the surface tension of the subsequent dealcoholization process. The larger n-butanol also has a steric hindrance, preventing adjacent particles from coming close to each other. At the same time, the n-butanol alcoholic hydroxyl group and n-butyl group also have a certain degree of surface tension. The modification effect. Compared with the constant boiling before and after drying, the TEM photos shown in Figures 4 and 5 show that: 1) The agglomeration state between the particles did not change before and after the azeotropic treatment, and the secondary structure did not change; 2) The products after azeotropic drying were hard The reunion is lighter and basically maintains its original appearance.

The supercritical drying technique uses supercritical fluids to make the drying process in a supercritical state, there is no phase interface between the gas and liquid, and the role of surface tension is eliminated, so that the combination between the particles, the network structure formed by the pores between the particles shrinks The phenomenon of collapse has been greatly reduced, so it must be able to control the occurrence of agglomeration. In the freeze-drying process, water is first frozen at a low temperature to become solid ice. Since the density of ice is less than the density of water, the volume will inevitably expand during the conversion of water into ice, so that the particles of the secondary structure that have been crosslinked together will mutually interact with each other. keep away. Since the primary particles are nano-sized, such expansion makes the effect of increasing the distance between particles very obvious. Finally, the vacuum is used to remove the water directly from the solid state to the gaseous state. Without passing through the liquid phase, the degree of agglomeration is greatly reduced, and the effect is good.

Spray drying is to disperse the suspension into tiny droplets and instantaneously dry it. The water inside the silica secondary structure evaporates quickly, reducing the time and degree of capillary force, and reducing the degree of agglomeration. The ideal condition is that the droplets sprayed by the nozzle quickly vaporize, forming particles only in the droplet, and no cross-linking or chemically bonded agglomeration occurs between the two droplets. It can be inferred that the particle size of the process product depends on the degree of atomization, the higher the degree of atomization, the smaller the particle size, and the narrower the particle size distribution (see Figure 8).

212 The effect of different drying methods on the performance of silica powder removal, the relative position between the particles does not change, leaving a large number of pores after the removal of water, so that not only the outer surface of the particles, there are micropores The inner surface of the structure makes the specific surface area larger. In the dehydration process of other kinds of drying methods, the particle structure breaks down, the particle size decreases, and the surface area slightly decreases. Heating reduction basically meets the requirement of national standard of less than 7%, but because the moisture remaining in the tunnel during the freeze-drying process is reduced due to the capillary force, the vapor pressure is reduced, and the negative pressure provided by the vacuum pump is not enough to completely remove it. Therefore, heating reduction is somewhat high. The oil absorption value is a measure of the degree of development of the powder branch. It refers to the Journal of the Beijing University of Chemical Technology. The silica used as a reinforcing agent requires a certain oil absorption value. The directly dried dry powder undergoes severe agglomeration and the branch chain is inevitable. Reduced, so that the oil absorption value is reduced, other various drying methods, can well control the occurrence of hard agglomeration, better preservation of the original branch chain structure, so the resulting product has a greater increase in oil absorption value, Among them, azeotropically drying products are more than doubled in direct drying. The light microscopy observed that the particle size distribution of the particles accords well with the normal distribution, and is more concentrated, with a standard deviation 316 times smaller than that of the direct drying. The results shown in Fig. 9 indicate that the particles distributed in the 810Lm account for the total The amount of 78% indicates that the product obtained by the carbonation precipitation method not only has a smaller primary particle size, but also has a narrower particle size distribution after drying, and a narrow particle size distribution is also a manifestation of its performance. Especially for azeotropically dried products, the apparent diameter diameter of the product is 2119 Lm, which is only one-eighth that of the direct drying product. This indicates that the use of a better drying method can significantly reduce the apparent diameter of the product particles. (1) When the proportion of raw materials used is relatively small, the specific surface area is higher; (2) The product has a slightly decreased specific surface area after being treated with constant drying. When the concentration of the reaction liquid is high, the degree of supersaturation is large, the primary particles tend to grow, and the particle size of the product increases, resulting in a decrease in the specific surface area. After azeotropic treatment of the product, Liu Haidi's research results show that during drying and dehydration, the resulting additional pressure causes the pore network of the particle network structure to be crushed, resulting in more fine pores, and the pores become mesoporous. The specific surface area of ​​the product has been reduced. The particle size distribution also shows a decrease in the particle size.

Silicone Rubber Application Data Analysis White carbon black is used in silicone rubber for reinforcement. The reinforcing performance is mainly reflected in tensile strength and tear strength. From the physical mechanical performance test data of the rubber compound in Table 2, all can meet the HG1-125-64 standard. The directly dried white carbon has the worst reinforcing performance. All other drying methods are more effective than the direct drying and drying products in these two indicators. The microporous structure of the silica powder after freeze-drying can be absorbed during the compounding process. The high shear force of the mixing causes damage, forming smaller particles and improving the reinforcing performance. At the same time, some of the moisture in the micropores is not completely removed, which is beneficial to the combination of the particles and the rubber, and also improves the reinforcement. The performance is favorable; the supercritical dried powder well preserves the morphology and structure of the original particles, has a small particle size, and has superior reinforcing properties; direct drying of dry Chen Zhitao, etc.: drying of high-gravity carbon dioxide silica and The application of dryness not only resulted in severe hard agglomeration and resulted in the growth of particles. At the same time, due to the lack of surface active groups and no surface hydroxyl groups, the reinforcing properties were the worst. Azeotropic drying greatly weakens the hard agglomeration of the particles, resulting in a very loose powder, small particle size, well-developed microstructure, easy dispersion in the rubber, and surface adsorption of azeotropically treated silica. There is n-butanol, a small amount of active sites of n-butanol and white carbon on the surface, which enhances its lipophilicity, which is favorable for the mixing of inorganic particles and organic compounds, and also improves reinforcing properties. Its tensile strength is 41% higher than that of direct drying, and its tear strength is increased by 46%, which has reached or exceeded the reinforcing performance of commercial gas phase products. The reinforcing properties of acetonitrile treated with azeotropic drying, supercritical drying and freeze drying were all better than those of drying and drying, and some of the data had reached the level of fumed silica. The use of spray-dried silica has better reinforcing properties than commercially available precipitated silica, which is of practical significance for industrialization.

Conclusions (1) The direct drying and drying of white carbon powders occurs in severe hard agglomeration, which adversely affects the physical properties of silica and the reinforcing properties of silicone rubber. Therefore, it is necessary to improve the drying process. (2) The products obtained during the azeotropic drying process are not only greatly reduced from the products whose apparent diameter is more directly dried, but also used as reinforcing agents to add tensile strength and tear to the reinforcing properties of silicone rubber. The strength has been greatly improved, and its performance has been comparable to that of fumed silica produced by the gas phase method. Therefore, from the viewpoint of pursuing excellent performance, the drying method is optimal. (3) Considering the economic accounting and industrialization requirements, the investment in spray drying equipment is small, the operation is simple and flexible, and the product performance can meet the general industrial requirements. It is an economically viable drying method.