In this paper, TiO2 particle “beating dispersion-sand grinding” combined process dispersion technology was studied. The dispersion of TiO2 particles was characterized by viscosity and particle size, and the change of size and morphology of TiO2 particles before and after dispersion was observed by transmission electron microscopy. The effects of dispersant type, dispersant dosage, particle size of sanding medium and sanding time on the dispersion effect of TiO2 particles were discussed. The results showed that the optimal dispersion process of TiO2 particles was as follows: sodium silicate as dispersant, adding 0.2% of TiO2 mass fraction, using zirconia bead sand with particle size of 0.4 μm for 25min, average particle size of 265nm, uniform particle size, stable dispersion effect.
After calcination and crushing by rotary kiln, the particle size of titanium dioxide powder is 320 ~ 350nm, and the distribution is wide. Due to the effect of surface interface, volume effect and quantum size effect, a large number of soft and hard agglomerations appear in the powder. This will lead to the subsequent inorganic coating process, titanium dioxide can not be monodisperse state, poor coating effect, and ultimately affect the quality of the product, such as reduced pigment properties and weather resistance of the product. Therefore, the dispersion of titanium dioxide is an important link in the production of rutile titanium dioxide.
There are many literatures on the dispersion mechanism of powders. The dispersion mechanism is studied for different dispersion methods, which are divided into physical dispersion and chemical dispersion. Among them, the physical method is mainly through the design of appropriate mechanical dispersion to improve the effective volume and energy utilization rate, physical dispersion methods include mechanical agitation, ultrasonic dispersion and grinding. Chemical dispersion is by adding ionizable dispersant, dispersant in water dissolved ionized a large number of anions and cations, in the particle surface to form a double electric layer with opposite charges, electrostatic repulsion, prevent particle agglomeration. The dispersion of particles is divided into three kinds according to different dispersion systems: solid particles in liquid phase, solid particles in gas phase, liquid particles in another liquid. The dispersion of titanium dioxide belongs to the dispersion of particles in liquid, and the dispersion process includes the following three stages:
- Particles are wetted by liquid;
- Under the action of mechanical force, the aggregate is opened into primary particles or smaller aggregates;
- By physical or chemical means, the opened primary particles or small aggregates can be dispersed and stabilized to prevent the occurrence of reagglomeration. Because of the particularity of titanium dioxide production process, the ideal effect cannot be achieved by using a single dispersion method. The author tried to combine physical dispersion with chemical dispersion, studied the effects of the type of dispersant, the amount of dispersant, the choice of grinding medium and the grinding time on the dispersion stability of titanium dioxide, formed the combined process of beating dispersion-sand grinding, and determined the better process parameters to improve the quality of titanium dioxide products.
1. Experimental Materials And Instruments
(1) Experimental Materials:
- At the beginning of rutile titanium dioxide (pangang group titanium industry co., LTD.), the single isopropanolamine (basf co., LTD.), sodium silicate (chengdu kelon chemical reagent factory), sodium hexametaphosphate (tianjin branch close the chemical reagent co., LTD.), poly (carboxylic acid dispersant (commercial), polyacrylic acid dispersant (commercial), sodium hydroxide (chengdu kelon chemical reagent factory), All are industrial pure. Different particle size zirconia beads (a domestic zirconium bead factory).
(2) Experimental Instruments:
- Zetasizernano-zs90 Laser Particle size meter (British Malvin Company), Brookfield Rotary viscosity meter (American Brookfield Company), PHS-3 pH meter (Shanghai Thunder instrument Factory), Jem-21000f Field emission transmission electron microscope INCA spectrometer (Japan Electronics Co., LTD.), SF intelligent dispersion sanding machine (Jiangyin Fine Chemical Machinery Co., LTD.).
2. Experimental Process
(1) Titanium Dioxide Dispersion Sanding
Add 300g titanium dioxide powder and stir continuously, add sodium hydroxide solution, adjust pH value to 9.5 ~ 10.0, add 300g zirconia beads, sand and disperse for a certain time in the dispersion sand mill. The dispersed titanium dioxide slurry was obtained by filtration.
(2) Analysis of Tests
- Particle Size Test: 0.5 mL of dispersed titanium dioxide slurry was taken and added into 50mL of sodium hexametaphosphate aqueous solution (mass fraction of sodium hexametaphosphate was 0.05%) for ultrasonic dispersion for 10min. Add 50mL sodium hexametaphosphate aqueous solution to 5mL of the well dispersed solution, and then disperse it by ultrasound for 5min. The final dispersed aqueous solution was taken for particle size detection.
- Viscosity Test: directly measure the strain value of titanium dioxide slurry to shear stress (generated by rotation of a specific type of rotor), in Pa·s. 500mL dispersed titanium dioxide slurry and beaker were measured, and no. 62 rotor was selected at a speed of 30r/min to read the viscosity value. Each sample was tested three times and the average value was taken.
Ⅲ. Results And Discussion
1. Influence of Dispersant Type on Dispersion Effect
The dispersant is added to the solid-liquid dispersion system, so that it is adsorbed on the surface of particles, change the surface properties of particles, so as to change the interaction between particles and liquid medium, particles and particles, resulting in a stable dispersion of the system. The commonly used dispersants are inorganic electrolytes, surfactants and organic macromolecules. The author chooses sodium silicate, sodium hexametaphosphate, monoisopropanolamine, polycarboxylate and sodium polyacrylate as five dispersants for the experiment. The test conditions were as follows :100mL distilled water, dispersant dosage was 0.02g, 10gTiO2 was added after homogenization, stirring for 30min, and Zeta potential of TiO2 slurry was detected. The results were shown in table 1.
As can be seen from Table 1, when sodium silicate, sodium hexametaphosphate and monoisopropanolamine were used as dispersants, the absolute value of Zeta potential of TiO2 slurry was larger. The addition of sodium silicate, sodium hmetaphosphate, monoisopropanolamine and other inorganic electrolytes, on the one hand, increased the double electric layer on the surface of TiO2 particles, and improved the absolute value of Zeta potential on the surface of TiO2 particles; On the other hand, inorganic electrolyte enhances the wetting degree of TiO2 particle surface to water and prevents agglomeration between particles. Polycarboxylate and sodium polyacrylate are polymeric dispersants. The carboxylic groups in their molecular structure can have strong hydrogen bonding with the hydroxyl group on the surface of TiO2 particles, which can make the organic polymer adsorbed on the surface of TiO2 particles, thus forming a layer of molecular protective film, which can induce strong spatial repulsion effect. In addition, when particles with the same charge are close to each other, electrostatic repulsion is generated, which effectively prevents agglomeration between particles. Therefore, the polymer dispersant can reduce the viscosity of TiO2 slurry, but the absolute value of Zeta potential is low.
2. The Influence of Dispersant Dosage on Dispersion Effect
Sodium silicate, sodium hexametaphosphate and monoisopropanolamine were used as dispersants to study the effects of different dispersants on the dispersion effect of TiO2. The test conditions were as follows :500mL distilled water, adding dispersant in a certain proportion, stirring evenly, adding 300gTiO2, stirring for 30min, and testing the viscosity of TiO2 slurry. The test results are shown in figure 1.
As can be seen from Figure 1, the viscosity of TiO2 slurry decreases with the increase of dispersant dosage. However, when the addition of dispersant was increased to 0.25%, the viscosity of TiO2 slurry increased. This is because, with the increase of dispersant amount, anions adsorbed on the surface of TiO2 particles after ionization increase, Zeta potential increases, repulsive force generated by the double electric layer on the surface of particles increases, particle dispersion is better, and viscosity of slurry decreases. With the excess dispersant, too much dispersant is free in the solution, which will lead to the increase of anionic concentration, compression of the double electric layer, and reduction of electrostatic repulsion between particles. At the same time, excessive electrolyte will occur bridging effect, which will aggravate the agglomeration of particles and increase the viscosity of slurry system. Therefore, the optimal amount of dispersant is 0.2%.
3. Influence of Drinding Media on Dispersion Effect
When dispersant is added to TiO2 slurry, it can only open the soft aggregates aggregated by electrostatic force and van der Waals force between particles, but cannot open the hard aggregates formed by the combination of liquid or solid phase Bridges between particles. At this point, it is necessary to open the hard aggregate by means of mechanical grinding, and grind the larger particles fine, and repair the irregular shape of the particles. The author adopts wet sand grinding process, sand grinding operation should control a lot of process parameters, such as the selection of medium, medium size, sand grinding time and so on. TiO2 particles are hard, so zirconia beads are generally selected as the grinding medium. Here, only the ball diameter and grinding time of the grinding medium are studied. The influence of particle size (0.2, 0.4, 0.8 μm) on TiO2 dispersion was studied by using sodium silicate as dispersant. The test conditions were as follows :500mL distilled water, adding dispersant, adding 0.2% (mass fraction), stirring evenly, adding 300gTiO2, stirring for 30min, adding 300g zirconia beads, taking TiO2 slurry in the sand grinding process, and testing its particle size. The experimental results are shown in Figure 2.
Figure 2 shows that the particle size of sand-grinding medium has a great influence on the particle size of products. When the particle size of the medium was 0.4 μm, the particle size of the product was the lowest, and the average particle size decreased to 265nm after 30min. When the particle size of sand-grinding medium is larger or smaller than this particle size, the sand-grinding effect is not ideal, the product particle size is greater than 300nm. When the particle size of the medium is large, the force between the medium and the barrel wall, the medium and the medium is strong, but the force between the medium and the material is relatively weak, which can not achieve the best sand grinding effect. However, when the particle size of sand-grinding medium is small, the effect of buoyancy is relatively large, which offsets part of the energy and reduces the sand-grinding efficiency. Therefore, for grinding materials with different particle sizes, sand grinding media with different particle sizes should be selected, such as TiO2 particle grinding. The optimal particle size of sand grinding media is 0.4 μm.
4. Influence of Sanding Time on Dispersion Effect
Sodium silicate, sodium hexametaphosphate and monoisopropanolamine were used as dispersants to study the effect of different sanding time on the dispersion effect of TiO2. The test conditions were as follows :500mL distilled water, adding dispersant, adding 0.2% (mass fraction), stirring evenly, adding 300gTiO2, stirring for 30min, adding 300g zirconia beads (particle size: 0.4 μm), taking TiO2 slurry in the sand grinding process, and testing its particle size, the test results are shown in Figure 3.
From figure 3 shows that by using single isopropanolamine and sodium hexametaphosphate as dispersant, early grinding with sanding time extended, the average particle size of TiO2 particles decreases, however, as the sand grinding time extended further, instead of the average particle size of TiO2 particles increases, the “inverse grinding” phenomenon, the main reason is that early grinding, Mechanical collision and grinding between TiO2 particles, TiO2 particles and sand grinding medium make the particle size of TiO2 particles smaller, specific surface area increased; With the further progress of sanding, the particles are refined and the specific surface area is further increased, but the specific surface area is not fully dispersed. Therefore, in order to overcome the larger surface energy, TiO2 particles will be reunited under the action of electrostatic force, van der Waals force and mechanical force, thus reducing the specific surface area and reducing the surface tension. When sodium silicate was used as dispersant, there was no “reverse grinding” phenomenon, because sodium silicate could fully disperse TiO2 particles without reagglomeration. The optimal grinding time of sodium silicate is 25min and the limit particle size is 265nm.
5. Morphology Analysis of TiO2 Particles
In order to intuitively observe and analyze the morphology changes of TiO2 particles before and after “dispersion-grinding”, tem analysis was conducted on TiO2 particles before and after “dispersion-grinding”, and the results are shown in Figure 4.
As can be seen from Figure 4, before dispersion, TiO2 particles were in a state of agglomeration, and after “dispersion-grinding”, TiO2 particles were fully dispersed. TiO2 particle size was refined from 300nm to about 260nm; The particle size becomes more uniform and the particle size distribution becomes narrower. The irregular shape is corrected and the shape is more spherical. The particles are monodisperse. TiO2 paste dispersed by the above methods has small particle size, uniform size, single dispersion state and low viscosity, which is conducive to uniform deposition of film coating agent on the surface of TiO2 particles, improve the coating effect, and thus improve the pigment properties and application properties of the product.
- By comparing the influence of different dispersants on the dispersion of TiO2 slurry, sodium silicate and sodium hexametaphosphate were used as dispersants, and the absolute value of Zeta potential of slurry was larger, the viscosity of slurry was lower, and the dispersion effect was the most ideal.
- The particle size of the grinding medium has a great influence on the grinding effect. When the particle size of the grinding medium is 0.4 μm, the particle size of the product is the smallest. After 30 minutes of grinding, the average particle size decreases to 265nm.
- Dispersant plays a very important role in the grinding process, when sodium hexametaphosphate and monoisopropanolamine are used as dispersant, “reverse grinding” phenomenon will occur; However, when sodium silicate is used as dispersant, TiO2 particles can be fully dispersed without agglomeration and “reverse grinding” phenomenon.
- The research results have reference value for the dispersion technology of similar powder particles such as SiO2, kaolin and CaCO3, which are similar to titanium dioxide particles.