Wet grinding, dispersion and grading of titanium dioxide

Titanium dioxide pigment wet milling fractionation prior to the surface treatment is a very important part, since the titanium dioxide coarse particles by pulverization after calcination although Raymond, but still there is a coarse particles or aggregates of particles of a certain number of 10μm ~ No It is crushed, especially when crushed with Raymond mill. Many crushed fine particles are pressed together to form agglomerated particles due to rolling, which requires a large amount of energy to separate. Usually, the fineness of the sieve is measured, usually using a 325 mesh standard sieve, and the pore diameter is about 45 μm. The titanium dioxide pulverized by the Raymond mill generally contains large particles of 0.05% to 0.15% and more than 45 μm, so the coarse particles of 1 to 10 μm are used. by sieve method is not measured out, especially rutile type titanium dioxide particles hard, coarse particles are more easily mixed. DPFields et al. of DuPont Chemicals Department of the United States believe that large particles of 30μm or more mixed in titanium dioxide are sufficient to cause flaws and streaks on the surface of coating film and plastic profiles; particles above 5μm have a significant effect on dispersion; 0.5μm or more Any large particles have a great influence on the gloss of the coating film; the particles of 0.1~0.5μm mainly affect the optical properties such as hiding power, tinting strength and background color (see Figure 1).

The calculation of the opacity backscatter by WDRoss recently shows that the hiding power is almost 4% lost per adhesion of a titanium dioxide particle. According to his calculation, the opacity of a 0.2 μm diameter titanium dioxide particle is 100. The granules are only 96, the three granules are 92, and the four granules are only 88. Figure 2 is a TiO ₂ electron micrograph of different degrees of flocculation.

[next]

The effect of grinding is to further grind the (2), (3) type of agglomerated particles and agglomerated particles in the particle model of Figure 3, which are sintered as a solid polymer and bonded together by face or corner. Aggregates (agglomerated particles), while flocs are composed of smaller particle groups, which are flocculated by adsorption of air or moisture, and the binding force between the particles is very weak, and it is easy to break up, if no dispersion is added. Stabilizers, which are also easily flocculated together, so flocs are also known as "reversible agglomerated particles."

Secondly, in the surface treatment of the envelope, the most ideal state is to envelop the coating agent on each of the primary particles. After the end of such a coating, the flocculation machine is used to decontaminate to ensure each particle. There is a coating layer to achieve the ultimate goal of surface treatment of the envelope. If the coating is not ground and dispersed, it may be a coating of aggregated particles or flocculated particles. Once the coating is crushed by steam, the coating layer is broken, presenting an incomplete envelope state. The intended purpose of the surface treatment envelope is not achieved, so the dispersion must be sufficiently ground before surface treatment.

(1) Wet grinding

Wet grinding is similar to the ordinary static water selection method in obtaining fine particles, but the wet milling production capacity is increased by 145% to 200% under the same conditions.

Wet grinding equipment mainly includes ball mill , vibration mill, sand mill (bead mill), etc. The grinding medium (ball, cylinder) of ball mill and vibrating mill is large, only suitable for grinding larger particles, wet grinding The most popular is the sander, which is a versatile sub-micron wet grinding device that is second only to colloid mills in terms of grinding fineness.

The average primary particle size of the industrial titanium dioxide pigment is about 0.15~0.35μm. It is difficult to pulverize the aggregate to the original particle size by a general mechanical pulverizer, and the steam pulverizer can achieve the requirement of pulverizing to the primary particle, but Since the titanium dioxide particles are very fine, the surface area is large, and the surface free energy is high, the bonding force between the collected particles is very strong, and the crushing energy consumption by the steam jet mill is high and uneconomical. Some studies have done a comparison: If the limit of the fineness of the ordinary mechanical crusher is 10 μm, then the steam mill can be used to crush to 1 μm, and the wet sand mill can be crushed to less than 0.5 μm. Since the surface treatment of titanium dioxide generally adopts a wet coating, a certain dispersion and concentration of TiO ₂ aqueous dispersion is required before coating, so wet grinding is the most effective ultrafine pulverization method for titanium dioxide so far, and the sand mill is wet pulverized. The main equipment for the preparation of high dispersions.

The sander has vertical and horizontal type, and has open and pressurized operation mode. The inner lining is generally made of polyurethane wear-resistant rubber. This material has good elasticity and wear resistance 9 times higher than natural rubber. It is better than styrene-butadiene rubber. 1 to 3 times higher. Abrasive sand mill may be used glass beads, ceramic beads, alumina beads and zirconia, zirconia beads produced by the fineness of a uniform method fused, smooth surface, high hardness, titanium dioxide is ideal for wet grinding with abrasive, mainly The content of ZrO _{2 is} 68.5%, SiO _{2 is} 31.5%, Mohs hardness is 7.2 (rutile TiO ₂ Mohs hardness 6 to 6.5), relative density is 3.96, compressive strength is 710 N, and particle diameter is 0.6 to 1.2 mm.

Wet grinding has special requirements for slurry concentration, viscosity, Ph dispersion state and ball loading (beads). The slurry concentration when wet-milling titanium dioxide is generally controlled at 600~1200g/L, pH value 8~11, bead volume (net volume percentage); sealed vertical 70%~80%, open vertical 60%~70%; Horizontal 80%~85%. It is generally recommended that the diameter of the smallest bead must be more than 2.5 times the size of the exit filter slit, and the spacing between the outer edge of the dispersion disc and the inner liner of the sander must be four times the diameter of the largest bead. Due to the high density of the zirconium beads, from the kinetics, the beads with high density can obtain a large impact energy, so that the rotation speed is not too high, and the linear velocity of the dispersion disk is generally about 14 m/s.

For materials that are difficult to pulverize or have coarser particles, two sand mills can be operated in series because titanium dioxide is not destroyed by excessive grinding. On the contrary, organic pigments have ionic bonds between inorganic pigment crystals due to their intermolecular bonding. Covalent bond bonding is weak, and excessive grinding can affect the quality of organic pigments. For products with higher water-soluble salts, it is best to choose hot wash 1 time before wet grinding, while chlorinated titanium dioxide can be directly ground by sand mill due to the fine particle size of the product after gas phase oxidation. After the classification, the surface treatment is carried out, and the wet pulverization is also advantageous for removing the residual chlorine in the product, but it is preferably steam dechlorination and then wet grinding.

In the past, the method of static water selection was used to obtain fine particles. The gap operation, long time, low water selection rate, and coarse particles selected by water could not be returned to the system. It was separately pulverized, and now wet pulverized and classified. The system is combined with a closed loop to avoid the above shortcomings. [next]

(2) Dispersion

In the wet grinding, the titanium dioxide slurry is kept in a dispersed state, and the effect of wet grinding is good. At the same time, the titanium dioxide must be subjected to a good dispersion state in the surface treatment of the coating. The pH of the uncoated titanium dioxide in the best dispersion state in water is 8-11, so it can be adjusted to the process specified value by alkali (NaOH) or sodium silicate (water glass) before being wet-ground. In addition to adjusting the pH, the dispersing agents may also be added such as: triethanolamine, diisopropanolamine, sorbitol, phosphate, sodium hexametaphosphate, sodium pyrophosphate, sodium carbonate and the like. Sodium hexametaphosphate has a strong dispersion of solid particles in the fine dispersion because it is a linear polyphosphate glass body that forms a soluble complex with metal ions such as calcium and magnesium in the dispersion medium. , to shield the multivalent cations, prevent these positively charged ions from electrically neutralizing with the negatively charged titanium dioxide, and its substructure is as follows:

In order to reduce dispersibility due to the introduction of impurities during dispersion, the water used for wet pulverization requires the use of ion-exchanged water, and its electrical resistivity should be not less than 20000 Ω·cm.

(3) Classification

After the wet-milled titanium dioxide, in order to further obtain a titanium dioxide dispersion having a certain particle size distribution, it is necessary to classify it because general pigment-grade titanium dioxide is not desired to have coarse particles of 5 μm or more, and titanium dioxide for chemical fiber matting is generally not desired. Coarse particles of 2 μm or more. There are two types of grading: dry method and wet method. Although the wet method is slower than the dry method and has higher energy consumption than the dry method, it can keep the particles dispersed in the grading state, and it does not stick to the wall, so it is in titanium. Wet grading is used in the surface treatment of the white powder industry.

Wet grading can be divided into gravity type sedimentation classifiers such as: water tank, Dow type settling tank, thickener, etc. and centrifugal classifiers such as: hydrocyclone, horizontal screw discharge sedimentation centrifuge, etc. And wet mechanical classifiers such as vibrating screens.

Gravity-type settling classifier In China, we used water tanks in the past, calculated the sedimentation height and settling time of the particles according to the Stokes formula, and then separated the two kinds of suspended matter with different particle size distributions at the specified height. Taking rutile titanium dioxide as an example, when it is not a dispersion medium, the viscosity of water at room temperature is η=0.01 poise, the relative density ρ=1, and the relative density of rutile-type titanium dioxide ρ=4.2, calculated according to the Stokes formula, the diameter is It takes 4 min for the 5 μm rutile-type titanium dioxide particles to settle to a height of 1 cm. This method is simple and easy to operate. The disadvantages are gap operation, long settling time, low water selectivity and limited suspension stability. In the past, the gravity settlement of foreign countries generally adopts the Dorr Hydroseparator. Its principle is similar to that of the general Doer type settlement bucket. It is mainly graded by controlling the speed of overflow. One settlement area is 16m ² and the settlement height is 2.7. The continuous gravity sedimentation classifier of m can classify 20t titanium dioxide every day. The overflow liquid can contain no large particles of more than 6μm. This equipment can be operated continuously, but the equipment is heavy, the floor space is large, and the slurry concentration is dilute. Less adopted.

Horizontal spiral discharge decanter centrifuges and hydrocyclones in centrifugal classifiers are the two most widely used centrifugal classifiers in the titanium dioxide industry.

The hydrocyclone is similar to the general cyclone separator (see Figure 4). The material is usually made of hard corundum or 95 porcelain (Mohs hardness up to 9). In industry, dozens of groups are operated in parallel, after swirling and grading. The underflow coarse particles return to the sanding mill. According to the general centrifugal principle, the larger the centrifugal force, the smaller the fractional particle size. The centrifugal force can be calculated according to the following formula:

Where C is the centrifugal force of the particle of mass m; r is the radius of the location of the particle; V _t is the tangential velocity of the particle; m is the mass of the particle. [next]

It can be seen from the above formula that increasing the centrifugal force only by increasing Vt (increasing the feed pressure) to increase the flow rate and reducing the diameter of the cyclone is solved. Therefore, the diameter of the cyclone is generally small, usually 10 to 40 mm. The largest is only 100mm, the feed pressure is 0.15~0.7MPa, and the concentration of pulp washing clothes is 150~300g/L. The hydrocyclone designed by China Institute of Chemical Industry Coatings was used in titanium dioxide factories in Nanjing and Xiamen. 95 porcelain, its specifications are: cyclone diameter D = 15mm; cylinder height H = 34mm; feed port size = 2 × 8mm; overflow nozzle diameter Ñ„ = 3.5mm; bottom nozzle diameter Ñ„ = 1.4mm; The cone angle of the flow cone is 10°. Using this type of hydrocyclone to classify large particles of rutile-type titanium dioxide of 5 μm, the slurry concentration is controlled to be TiO ₂ :H ₂ O=1:8, and the dispersant (polyphosphate) is added in an amount of 0.3% to 0.6%. 2h, in the case of a feed pressure of 0.15MPa, the primary classification rate can reach 85%, and the single production capacity is 100t/a.

The flow ratio R _{f of the} hydrocyclone (ratio of the underflow of the cyclone to the feed amount) is an important parameter when the cyclone is operated. It directly affects the particle size at the time of classification, and its value can be calculated as follows.

Where D _u is the outlet diameter of the bottom of the baffle, and D _e is the diameter of the upper overflow port. This formula is suitable for applications where the feed pressure is greater than 0.05 MPa and D _u /D _e = 0.3 to 0.8.

The hydraulic steamer has simple structure, low cost, no moving parts, small volume, large unit capacity, wide separation particle size and continuous operation. The disadvantages are serious wear and large power consumption, especially the bottom is easy to block, the best. Used in combination with the vibrating screen, the coarse particles are removed through the vibrating screen and then passed through the cyclone, which can reduce the bottom blockage.

The horizontal screw discharge sedimentation centrifuge is a liquid-solid separation equipment with superior performance. This centrifuge has been used as a grading equipment for titanium dioxide in the 1960s. It is a centrifuge that runs at full speed, continuously feeds, and can continuously separate and discharge. The machine has the following forms: one drum is a truncated cone, which is especially suitable for separating solid filter residue; the other drum is cylindrical, which is good for liquid phase clarification and is suitable for particle classification because of its separation. The factor is higher, so the graded particle size can be finer; in addition to this, a column taper, which takes into account the above two characteristics.

The working principle of the horizontal screw discharge sedimentation centrifuge (see Fig. 5) is that the suspension enters the hollow shaft of the screw conveyor at full speed and rotates along the feed pipe, and flows into the drum through the discharge port due to the centrifugal force. The coarse particles in the suspension settle on the inner wall of the drum and gradually fill the gap between the drum and the screw conveyor. Since the rotation speed of the screw is slightly lower than the rotation speed of the drum, the coarse particles are pushed by the screw to the drum. The discharge port of the end is discharged, and then enters the sand mill for wet grinding to form a closed loop system. The filtrate (fine particle portion) is located at the central portion of the drum, and the spiral passage formed along the auger flows to the large end of the drum and is discharged through the overflow baffle.

The graded particle size of the machine can be adjusted by the feed rate and the speed. The production capacity can be adjusted by changing the diameter of the overflow baffle, the feed position, the feed concentration, the feed rate and the difference between the screw and the drum. Because titanium dioxide needs to be graded very fine (2~5μm), it requires high separation factor. For example, the separation factor of foreign Super-D-CantersP-3000 horizontal screw discharge sedimentation centrifuge is as high as 3200. The maximum speed is 4000r/min. This type of machine is also produced in China, most of which is used for liquid-solid (slag) separation. The separation factor is low, and some improvement is needed for the classification of titanium dioxide.

As for the mechanical classifiers such as vibrating screens, they are mainly used for products that do not require too much fineness and particle size distribution due to the limitation of the mesh size of the sieve, or as a material for discarding coarse particles, and can also be used for hydrocyclones. The former coarse separation.