Hydrated titanium dioxide, purified water is an amorphous titanium oxide hydrate, a large number of surface adsorption of water and sulfuric acid intoxication (SO 3), if the direct calcination require higher temperatures, not only the particles are easily sintered, hard, Even the grayish yellow phase, if the temperature is low and the sulfur is not exhausted, the pH, the achromatic power and the oil absorption are not good. Therefore, in the production of pigment grade titanium dioxide, it is necessary to add a salt treatment agent to the hydrated titanium dioxide before calcination, which is also called ore. Chemical agent, this process is called salt treatment or pretreatment.
By salt treatment, the calcination process of hydrated titanium dioxide can be controlled at a lower temperature, so that the particle size and the granules are moderately soft, so that the product has good whiteness, high decoloring power, low oil absorption, strong hiding power, and easy Dispersed high quality titanium dioxide pigment. Meanwhile salt treatment agent has the effect of further promoting agent crystalline form, in the production of rutile titanium dioxide rutile can maintain a high conversion rate; in the production of anatase titanium dioxide to rutile conversion can be suppressed its type, to avoid Produce mixed crystal products. Some salt treatment agents can also change the weather resistance and chalking resistance of titanium dioxide, and even produce various kinds of bottom phase titanium dioxide. Salt treatment, like hydrolysis and surface treatment, is one of the three main methods for designing and producing different types and specifications of titanium dioxide pigments.
1. Salt treatment agent for rutile titanium dioxide
Rutile titanium dioxide can be obtained by thermal conversion of anatase titanium dioxide at high temperature. This phase transition is irreversible and the temperature is very high (above 1050 ° C). At such high temperatures, the titanium dioxide particles are easily sintered and hardened. Deoxidation of the crystal lattice results in a dark, yellowish appearance which is not required for green pigment grade titanium dioxide. Alkali metals and alkaline earth metals are excellent rutile-type accelerators (or positive catalysts), and the smaller the radius of the cation, the stronger the rutile-type conversion promoting effect. At present, the mechanism of action is still unclear. It is generally believed that when the titanium dioxide after salt treatment is calcined, one substance (salt treatment agent) is dissolved in another substance (titanium dioxide) to form a solid solution, and the dissolution process is related to phase transposition. And the crystal geometry of the two substances is similar, that is, there are atomic planes facing the distance in the two crystal lattices, which are close to the surface of the rutile, and play an important role in the anatase-type rutile. The orientation of the titanium atoms accelerates the rate of conversion to rutile, which was confirmed in the study of NiO-promoted rutile conversion.
Promoter can promote conversion of anatase to rutile many, oxides, hydroxides or salts of zinc, titanium, antimony, tin, aluminum, magnesium, barium, bismuth, lithium, nickel, boron and other elements, The rutile type crystal conversion agents in industrial production mainly include the following:
(1) zinc salt
Zinc salt is a strong rutile accelerator and the most commonly used rutile salt treatment agent, mainly in the form of zinc oxide, zinc sulfate and zinc chloride.
The biggest advantage of zinc salt is that it has strong weathering resistance to powdering, which can reduce the temperature of anatase to rutile conversion. For example, adding ZnO at 0.5 °~1.0% (mass) of zinc oxide at 850 °C The anatase type can be completely converted into a rutile type, and the temperature is too high to cause the particles to sinter, lose light, and discolor.
For example, R-930, R-820, and R-830 ​​of Japan's Ishihara Industrial Co., Ltd., which has been imported and used in large quantities, are all general-purpose titanium dioxide powders. One of the common features of their manufacturing process is salt treatment before calcination. At the time, a zinc salt is added as a crystal form accelerator and a crystal form stabilizer. Such brands of titanium dioxide belong to the R2 category in the ISO-591 international standard classification; belong to the category IV in the classification of ASTM-D-476; the classification in Japan JISK-5116 belongs to the rutile category 3.
However, while the zinc salt promotes the conversion of the anatase crystal form to the rutile crystal form, it also greatly promotes the growth of the titanium dioxide particles and affects the improvement of the color reduction power, and the underlying hue is lightly used when the treated product is used in the paint. The red phase, or the viscosity of the coating increases due to the alkaline action of ZnO, and the stability of the storage tank is reduced. Therefore, when using ZnO as a salt treatment agent, other auxiliary treatment agents should be added. For some special requirements, after calcination Also pickled, at least 60% of the zinc can be removed before use.
(2) Titanium dioxide sol
The titanium dioxide sol is usually a calcined seed crystal and an additional seed crystal (secondary seed crystal). Titanium dioxide sol is the rutile accelerator after zinc oxide. Its accelerator ability is not as good as that of zinc oxide, but it has no above characteristics of zinc oxide. More importantly, it can improve the color resistance of the product and improve the titanium dioxide during calcination. the particle shape of the particles regular smooth, soft and easy to sintering, is a very valuable advantages from the perspective of titanium dioxide pigment in terms of performance, thus almost every brand titanium dioxide rutile titanium dioxide was added to both, in an amount of TiO 2 The disadvantage is that the preparation process is complicated.
(3) Magnesium salt
Magnesium salt mainly uses magnesium oxide, which not only can promote the conversion of rutile type, but also accelerates the time when the calcined product reaches neutral pH during calcination, thereby relatively shortening the time required for rutile conversion. The disadvantage is that too much addition is not only ineffective, but also makes the product hue red. Table 1 shows the effect of magnesium oxide on the pH value during calcination. [next]
It can be seen from Table 1 that the amount of MgO added is too large, and the effect is too small. Generally, the amount of addition is 0.2% (mass) of TiO 2 .
Table 1 Effect of magnesium chloride on the pH value of calcination
Salt treatment conditions (addition amount, % of TiO 2 ) | Temperature / °C | Neutral time / min |
MgO, 0.2; TiO 2 sol, 2; K 2 CO 3 , 1; ZnO, 0.2 | 870 | 40 |
MgO, 2.1; TiO 2 sol, 2; K 2 CO 3 , 1; ZnO, 0.2 | 870 | 70 |
MgO, 0.2; TiO 2 sol, 2; K 2 CO 3 , 1; ZnO, 0.2 | 870 | 30 |
MgO, 0.1; TiO 2 sol, 2; K 2 CO 3 , 1; ZnO, 0.2 | 870 | 70 |
TiO 2 sol, 2; K 2 CO 3 , 1; ZnO, 0.2 | 870 | Not reach |
TiO 2 sol, 2; K 2 CO 3 , 1; ZnO, 1.0 | 870 | 30 |
(4) lithium salt
Lithium salts, especially lithium chloride, promote the rutile type very strongly, but because it is rare and expensive, it is rarely used in industrial production.
(5) tin salt
Tin oxide and tin chloride are also positive catalysts for the rutile type, which are also beneficial for improving the whiteness of the product, but are rarely used in industrial pigment grade titanium dioxide, and are mainly used for producing rutile-type mica pearlescent titanium dioxide pigment.
(6) strontium salt
Like ZnO, cerium oxide is the first accelerator used in rutile type, but its rutile type promotes conversion not as strong as zinc oxide. It cannot be converted to rutile at low temperature by cerium oxide, but a small amount of cerium oxide is added. (0.1% or less) can make the product have a blue phase and improve the gloss of the paint film.
(7) Boron salt
Along side Rupert UK (Laport) Company has been tested, 1% phosphoric acid was added boron metatitanic than without metatitanic acid boron phosphate, calcined at the same temperature, its rutile conversion rate can be increased 50 More than %, but not much used in industrial production.
(8) sodium salt
Sodium chloride and sodium sulfate are also positive catalysts for the rutile type. They are good for adjusting the shape of the particles, but they are much less effective in promoting the rutile type than zinc salts, and the product has low color reduction, so it is rarely used now.
2. Crystalline stabilizer of rutile titanium dioxide
Crystalline stabilizers, also known as grain conditioners, or rutile-type auxiliary additives, are mainly used to make the rutile type conversion rate not too fast, making the product particles soft, smooth, and imparted with other characteristics (weatherability, increase). The white and blue bases are equal. The commonly used crystal stabilizers are as follows.
(1) Aluminum salt
Aluminum salt is the most commonly used rutile crystal stabilizer. Aluminum salt is actually a rutile type negative catalyst. It inhibits the conversion of anatase to rutile. The conversion temperature is higher than that of zinc salt when using aluminum salt. It is about 150 °C high. The biggest advantage of aluminum salt is that it can be calcined at a higher temperature (1000~1100 °C), the product whiteness is still good, the product particles are dense and weather resistant, and the aluminum salt is often used in products that cannot use zinc salt. To replace. Another advantage of aluminum is that, at higher titanium content iron ore raw material produced niobium (of Nb2O5) hydrous titanium dioxide, may be added to the trivalent aluminum fifth electronic compensation niobium, niobium to suppress the surface segregation of titanium oxide crystals To prevent the light absorption of bismuth and affect the whiteness, the aluminum salt is generally added in the form of aluminum sulfate.
(2) Potassium salt
Potassium salt is an indispensable crystal stabilizer for rutile titanium dioxide. It is one of the rutile negative catalysts. The use of potassium salt can inhibit the conversion rate of rutile type, reduce the desulfurization temperature, avoid the growth of particles, improve the performance of pigments, The color reduction is enhanced because it also makes the calcined product particles soft, and sometimes such alkali metal additives are referred to as "softeners". The potassium salt is generally added in the form of potassium carbonate, potassium sulfate or potassium hydroxide. The potassium carbonate can neutralize the free acid in the metatitanic acid during the salt treatment, so that the product reaches neutrality during calcination, and the reaction forms CO 2 and H 2 . O volatilization can make the product loose, which is also good for improving whiteness.
(3) Phosphoric acid or phosphate
Phosphoric acid and phosphate are usually added in the form of phosphoric acid and ammonium phosphate (mono or binary). They are also negative catalysts of rutile type. A small amount of P 2 O 5 can increase the whiteness of the product during calcination, making the product soft and pulverized, but Too much addition will cause the decolorization power to drop and the paint performance will be poor.
(4) Other additives
V 2 O 5 , WO 3 , MoO 3 , a-Fe 2 O 3 , Fe 3 O 4 and even some organic acids have the effect of promoting rutile conversion, and the melting point of these oxides is lower than that of titanium dioxide, promoting The larger, but the above oxidants will cause discoloration of the product, so it is generally not adopted in industrial production. [next]
3. Salt treatment agent for anatase titanium dioxide
(1) Potassium salt
Potassium salt is mainly added in the form of potassium carbonate and potassium sulfate. It can block the conversion of anatase to rutile during calcination. It can lower the calcination temperature, improve the whiteness and color reduction of the product, and increase with potassium ions. The calcination temperature is gradually reduced. This is mainly because the alkaline potassium ion accelerates the desulfurization rate. Experiments have shown that 0.0175gk + per 100g of TiO 2 can obtain the best achromatic power, but excessive addition of potassium salt will make the pigment oleophilic. The performance is degraded and the water-soluble salt of the product is increased, which affects the paint performance of the product.
In addition to potassium carbonate and potassium sulfate, potassium chlorate (KClO 3 ) can be used, which functions to decompose oxygen during calcination, keep the calcination process in an oxidizing atmosphere, and it can also retain three residues in metatitanic acid. The oxidation of titanium to tetravalent titanium prevents the low-cost titanium from causing the lattice defects of titanium dioxide and affecting the optical properties of the product.
In addition, the presence of a potassium salt sometimes causes an alkaline reaction of titanium dioxide (pH = 7.5 to 8), which is caused by hydrolysis of potassium titanate formed by the reaction of a potassium salt with metatitanic acid.
(2) phosphate
Phosphate is mainly added in the form of ammonium phosphate or diammonium phosphate, which is an anatase-type main crystal stabilizer similar to the potassium salt. Because phosphorus does not enter the crystal lattice of TiO 2 during calcination, but sucks on its surface. As the TiO 2 particles grow and aggregate, the phosphate on the surface prevents the particles from growing further and maintains it at a certain level. Within the particle size range, this is one of the reasons why the anatase type particle size is smaller than the rutile type. Phosphate also prevents rutile from being mixed into the product during calcination and has a certain weather resistance. In addition, phosphoric acid can react with iron in metatitanic acid to form white iron phosphate and light yellow high-phosphoric acid iron to prevent the formation of brown-red iron oxide, which has the effect of improving the whiteness of the product. Its chemical reaction formula is as follows:
3FeSO 4 +2H 2 PO 4 →Fe 3 (PO 4 ) 2 +3H 2 SO 4
Fe(OH) 3 +H 3 PO 4 →FePO 4 +3H 2 O
Excessive amount of phosphoric acid will affect the colorlessness of the product, and the acidity of the metatitanic acid will increase and the difficulty of desulfurization will increase.
(3) strontium salt
In the anatase titanium dioxide, strontium salt (Sb 2 O 3 ) is added, which can form iron bismuth sulphate with iron in the material, which has the function of shielding iron, can improve product gloss, improve weather resistance, and more importantly, can Prevent the phenomenon of light color mutual transformation, but the dosage can not be large, otherwise it will affect the dispersion, generally only add 0.05%~0.15%.
(4) ammonium salt
The addition of ammonium salt (NH 4 HCO 3 ) can make the product soft, whiteness, water dispersibility, and easy to desulfurize, but adding more will increase the oil absorption.
4. Salt treatment agents for other special varieties
When the enamel of magnesia is used in titanium white powder, the rutile type in the product can be kept at a certain ratio (80% to 83%), and the use effect thereof can be improved.
Cobalt acetate can improve the gloss of enamel and ceramic titanium dioxide, making the product bright and bright.
Magnesium carbonate can increase the electrical insulation performance of the capacitor titanium dioxide. The presence of trivalent titanium in the capacitor production will greatly reduce the electrical properties, and the divalent magnesium ion can avoid the reduction of tetravalent titanium to trivalent titanium.
5. Salt treatment operations and formulations
The addition of salt treatment agent is not a simple mixing, because there is a chemical reaction, and there is also a strong absorption effect of metatitanic acid. Although there is no ideal detection and control means at present, the most important thing in production is uniform mixing and can be formulated. The salt of the solution [H 3 PO 4 , K 2 CO 3 , Al 2 (SO 4 ) 3 ] must be used in a certain concentration, and the salt treatment agent (ZnO, MgO, etc.) which cannot be formulated into a solution should be adjusted with water. Add into a slurry, then stir at room temperature for 1~2h.
The concentration of the titanate slurry during salt treatment also has a large effect on the salt treatment effect. The low concentration of the slurry is relatively uniform, but the effect of some soluble salts is also greatly affected. The low concentration of the slurry is relatively uniform, but some soluble treatment agents (H 3 PO 4 , K 2 CO 3 , etc.) will affect the actual treatment effect when the filtrate is removed with the filtrate; the concentration of the slurry is too high, although it loses less during filtration. However, if the viscosity of the material is large, it is not easy to disperse evenly and also affect the treatment effect. Generally, the slurry concentration (TiO 2 content) is controlled at 270~300g/L. If the problem of stirring and dispersion of thick slurry can be solved, the slurry concentration should be higher. Some factories in foreign countries use spiral conveying and stirring, and the slurry concentration can be increased by 300. ~400g / L, this can also reduce the energy consumption of dehydration during rotary kiln calcination. The domestic Panzhihua Titanium Dioxide Factory used a double-screw agitator for salt treatment, and the slurry concentration was significantly improved.
The salt-treated formula is a scientific and practical work. It is the same as the hydrolysis and seed crystals. When considering the formulation, it is important to match the order between the positive and negative catalysts and the order of addition. It is necessary to consider reducing the calcination temperature and promoting the transformation of the crystal form, and pay attention to not being able to convert too fast, so as to avoid excessive or sintering of the particles, and also pay attention to The effect on the pigment properties (achromatic power, whiteness, weather resistance, softness, etc.) is usually determined by experiment. Excessive addition will not only reduce the purity of the product, but also increase the solubility of the soluble salt and reduce the dispersion. Sex. The general addition amount (mass ratio to TiO 2 ) and the order of addition are as follows:
Rutile type: (a) calcined seed crystal (titanium dioxide sol) added 2% ~ 5%; (b) ZnO addition amount 0.2% ~ 1.5%; (c) MgO addition amount 0.2% ~ 0.5%; (d) The addition amount of K 2 CO 3 is 0.5% to 1%; (e) the addition amount of H 3 PO 4 is 0.01% to 0.02%.
Anatase type: (a) K 2 CO 3 addition 0.5% ~ 1%; (b) H 3 PO 4 addition 0.2% ~ 0.35%.
Titanium dioxide for enamel: (a) MgCl 2 is added in an amount of 0.1% to 0.15%; and (b) CoCA is added in an amount of 0.005% to 0.01%.
Titanium dioxide for capacitors: MgCO 3 is added in an amount of 0.3% to 0.4%.
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