[zinc oxide Summit] Super Technology paste: read the Application Technology of Zinc oxide in Ceramic products

SMM11, 10th: zinc oxide is an important raw material for ceramic chemical flux. in the ceramic industry, zinc oxide is widely used in the translucent glaze of brick glaze and coarse pottery and the transparent coarse glaze of process tableware. Especially in the building ceramic wall and floor tile glaze and low temperature ceramic glaze consumption is more. At the 2018 (sixth) China Zinc oxide Industry chain Trading Summit hosted by Shanghai Nonferrous Network, Mr. Cai Ruinien, Technical Director of Guangdong Sanshui Dahong Glaze Co., Ltd., introduced the application of zinc oxide in glaze and the application technology of zinc oxide in ceramic products.

The appearance of zinc oxide is white solid, chemical formula ZnO, molecular weight is 81.38, its flash point is 1436 ℃, melting point is 1975 ℃, boiling point is 2360 ℃, density is 5.606g cm, soluble in acid and strong base, insoluble in water. Zinc oxide is a commonly used chemical additive, which is widely used in the manufacture of plastics, silicate products, synthetic rubber, lubricating oil, paint, ointment, adhesive, food, battery, flame retardant and so on. Zinc oxide has large band gap and exciton binding energy, high transparency and excellent optical rotation at room temperature. It has been used in liquid crystal displays, thin film transistors, light emitting diodes and other products in the semiconductor field. In addition, micro-particles of zinc oxide as a nano-material also began to play a role in related fields.

Production process of zinc oxide:

Direct law: (American law)

Using zinc concentrate as raw material, after removing impurities such as lead, cadmium and sulfur by high temperature oxidative roasting, coal (as reducing agent) is added and pressed into pellets, and then reduced at high temperature in the furnace. The volatile zinc vapor is oxidized with oxygen introduced into the air to form zinc oxide. The product quality of zinc oxide by direct method is not only related to the process used and the quality of zinc concentrate, but also related to the quality of coal, because the volatile zinc vapor is directly affected by the combustion products of coal. The content of zinc oxide produced by direct process is generally between 90% and 95%.

Indirect law: (French law)

The zinc slag or zinc ingot is heated in a high temperature resistant crucible to 600 to 700 ℃ and melted so that it is converted into zinc vapor at a high temperature of 907 ℃ (usually about 1000 ℃) and oxidized by hot air. After the production of zinc oxide is cooled and separated by cyclone, the fine particles are collected by cloth bag, and the zinc oxide is obtained by indirect method. The product quality of zinc oxide by indirect method and the content of heavy metals in zinc slag and zinc ingots directly affect the content of heavy metals in the products. The content of zinc oxide produced by indirect process is 99.5% and 99.7%.

Wet method: divided into acid method and NH3 (chemical formula) method

Zinc oxide is produced by acid method using zinc sulfate or zinc chloride as raw materials, after removing impurities, adding sodium carbonate solution to form basic zinc carbonate precipitation, after rinsing, filtering and drying, the prepared dry powder is calcined to get zinc oxide. The prepared zinc oxide has a large specific surface area, which is called active zinc oxide.

The production of zinc oxide by NH3 (chemical formula) method is to use secondary zinc oxide as raw material, add NH3 (chemical formula) water to soak at a certain temperature for one hour, add a certain proportion of ammonium bicarbonate to react for two hours, add Na2S to remove impurity and add metal zinc powder to replace it. The replaced metal and mixed salt were completely removed by filtration, and finally zinc oxide was obtained by evaporative roasting.

Application of Zinc oxide in Glaze

The function and use of zinc oxide in glaze:

Zinc oxide has a strong melting effect in the glaze, which can reduce the expansion coefficient of the glaze, improve the thermal stability of the product, increase the gloss and whiteness of the glaze, and improve the elasticity of the glaze. It can increase the luster of glaze while expanding the melting range. However, it is not suitable to use in black glaze or green glaze containing chromium.

Zinc oxide is mainly used in glaze in the following aspects:

1. Used as flux: the number of electrons in the outermost layer of Zn2+ is 18, and the polarization deformation of Zn ~ (2 +) increases the composition of covalent bond and weakens the bond force of Si-O. Therefore, the addition of zinc oxide can reduce the viscosity of glaze at high temperature and make the ceramic brick more smooth after firing. When zinc oxide is used in the frit glaze system, the general dosage is between 5% and 10%, but the amount in the raw glaze is rarely more than 5%, and the relative dosage in the glaze throwing system is a little more.

2. Used as opacifying agent: because zinc oxide has a high refractive index of 2.00 and 2.03, and because zinc oxide can form zinc spinel ZnO Al2O3 crystal with Al2O3. Therefore, adding zinc oxide to the glaze with high Al2O3 content can improve the opacification of the glaze. In the formula of zinc-containing opacified glaze, the addition of Al2O3 can improve the whiteness and opacification of the glaze. The addition of SiO2 can improve the luster of the glaze.

3. As a crystallization agent: in the art glaze crystallization glaze, zinc oxide is an indispensable crystallization agent, in the melting glaze rapid cooling, formed into a larger crystal pattern, very beautiful. In lead-zinc crystal glaze, the amount of zinc oxide is as high as 20 to 30%.

4. Used to make ceramic color: because of its strong melting effect, zinc oxide can be used as flux, mineralizer and glaze carrier of ceramic color. Zinc oxide can form spinel structure with R2O3 oxide at high temperature. For example, the brown series is composed of Fe2O3, Cr2O3 and ZnO. After calcination at high temperature, three kinds of spinel, ZnFe2O4, ZnCr2O4 and FeCr2O4, are formed. Through the combination of different ratios, the better colors such as black brown, red brown and yellow brown can be emitted in zinc-containing glaze. In addition, zinc oxide can react with cobalt oxide at high temperature to form ZnO ·CoO, which enhances the color of cobalt blue.

Application of Zinc-containing Glaze in Ceramic products

Through long-term research, some ceramic enterprises in Foshan have found that zinc oxide can also reduce the water absorption of ceramic wall and floor tiles. Industry insiders believe that in the ceramic glaze formula using zinc oxide powder with a fineness of about 1000 mesh, due to the fineness of particle size, the specific surface increases, so that the water absorption rate decreases. Of course, the higher the number of items, the higher the price of zinc oxide.

Application of zinc glaze in building ceramics:

1. Zinc oxide and lead oxide are commonly used raw materials in nugget formula because of their excellent melting effect and wide firing range in the field of third degree burning at low temperature of 700 ℃ to 1000 ℃. Zinc is a better substitute for lead when there is a requirement for lead and cadmium.

2. In the field of wall bricks with a medium temperature of 1000 to 1150 ℃, the melt is still the main body of glaze application, in which the zirconium white melt needs more zinc oxide to help melt and adjust the smoothness because of the high viscosity of zirconium. The addition amount of zinc oxide is generally 7 to 12%. The corresponding transparent melting block because the high temperature viscosity is not as high as zirconium white melting block, so the addition of zinc is relatively normal, generally in 5 to 10%. There are also a lot of low zinc melts in the market, but at the same time of reducing the cost, the firing range of application is relatively narrower, at the same time, low taste 80 zinc, 90 zinc, 95 zinc and so on are also widely used in low cost melts.

3. In the field of floor tile with high temperature of 1150 ℃ and 1250 ℃, the main application theme of glaze is raw glaze, supplemented by molten block, so the amount of zinc oxide is much less than that of wall brick system. However, in recent years, the concept of wall climbing has led to a lot of shrinkage in the wall brick market, and the market share of glazed tiles as the main popular trend has greatly increased, so the total amount of zinc oxide in the field of floor tiles is increasing.

Application of zinc glaze in daily ceramics:

The emergence of daily ceramics can be said to be due to people's demand for daily life, people are the most exposed, but also the most familiar porcelain, such as tableware, tea sets, coffee sets, wine sets, rice utensils and so on. The main index of daily ceramic products related to human health is the dissolution of heavy metal elements such as lead and cadmium.

At present, the high-grade daily porcelain is bone porcelain, which is generally fired by high temperature and low temperature glaze. In addition to the material of its own body, the temperature, moisture and penetration of bone porcelain is mainly represented by the melting pieces of bone porcelain, in which the taste of zinc oxide is very important, because lead and zinc are associated minerals, so whether the lead element can be screened out, It will have a great influence on the dissolution of lead heavy metal elements.

Application of zinc glaze in sanitary ceramics:

Sanitary ceramics refer to ceramic utensils for sanitation and cleaning bathrooms. The production process of sanitary ceramics is generally fired at the temperature of 1250 to 1280 ℃. The mud with prescribed properties was prepared by grinding with kaolin, highly plastic clay, quartz and potassium feldspar as the main raw materials, adding water and a small amount of electrolytes. The basic glaze materials are feldspar, quartz, limestone, dolomite, talc, magnesite, zinc oxide and carbonic acid Ba (chemical formula), and zircon and tin oxide are used as opacifying agents for white glaze.

Zinc oxide is mainly used as a cosolvent in sanitary ceramics. Due to the requirement of whiteness, more zirconium silicate or aluminum brighteners are added to the formula, so the high temperature viscosity is high, and the addition of zinc oxide can effectively reduce the high temperature viscosity. It is of great help to firing and glaze texture.

In addition, in recent years, sanitary ceramics to raw glaze-based glaze use habits, gradually with the addition of high-temperature melts. On the one hand, it can reduce the firing temperature to save energy and reduce consumption, on the other hand, it can make the glaze feel more warm and moist.