White cement manufacturing process
White cement is a type of special cement, belongs to decoration cement. In general, Portland cement is greyish-green, as the name suggested, white cement is white.
In the cement manufacturing process, in order to avoid the mixing of colored impurities, calcination of white cement plant mostly use natural gas or heavy oil as fuel. Reduction slag, gypsum and white granulated slag produced by electric steelmaking can also be used to produce white cement without clinker.
White cement property
Whiteness: the whiteness is a typical standard for color, on the market, the whiteness of white cement is required at 80%-84%, on the international market, and whiteness is about 84%-88%.
Fineness: the finer the cement, the more the surface area increases, the faster the cement hydrates and the stronger the cement is. The fineness of cement ensures cement quality.
Strength: strength is another typical standard, there some factors to affect the cement strength, such as constituent minerals of clinker, sintering degree, crystalline state, cement fineness, admixture activity, cement particle gradation, etc.
Setting time: according to the national standard of China, white Portland cement must have initial setting time more tan 45mins, the final setting time is less than 10h.
How to make white cement?
The white cement manufacturing process is similar to the Portland cement manufacturing process, so we introduce the factors that affect cement quality. Just like general cement manufacturing plant, the white cement manufacturing process can be generally divided into six steps:
- Crushing：carbonate composition mostly uses primary crushing. Since a large number of dye oxides are concentrated in the fine particles, it is best to screen the fine particles before feeding them into the cement crusher. The clay ingredients are usually broken individually
- Cement raw materials grinding: most of cement raw materials should be ground in the cement mill, and the grinding body can’t contain iron. As for the fineness, the white cement manufacturing process requires a finer size than that of Portland cement.
- Homogenization: the homogenized raw materials only are promised to have heterogeneity at microscopic scales. The homogenization is very important to the white cement manufacturing process, its multi-component mixtures are generally more prone to segregation than raw materials used in the gray cement production lines.
- Clinker production: There are various combinations of preheating and calcining systems, and which process is better depends on whether the existing plant needs to be rebuilt and how much the plant operators invest in the construction. The cement rotary kiln and cyclone preheater are the recommended group here.
- Cement grinding: cement is made from cement clinker and thickening time control agents like gypsum or anhydrite. The gypsum generally uses jaw crusher, 90% of particles are less than 6mm, the biggest size is 80mm.
- Packing and shipping: the finished cement is generally stored in the cement silo, and for different uses, white cement can be packed or in bulk to transport to the desired place.
Kiln firing system - key to white cement manufacturing process
In the white cement manufacturing process, the kiln firing system is the key to the entire cement production line, which affects the product quality and energy consumption of the entire white cement plant. Therefore, it is important to rationally design the kiln firing system. AGICO CEMENT introduces the design of the rotary kiln firing system on the white cement plant.
Design of preheater system
Because a small amount of fluorite is added as a mineralizer in the white cement raw material to reduce the firing temperature. Fluctuation or improper control of the blending amount in the cement production can cause the liquid phase to appear in advance in the preheater at all levels of the kiln, especially in the high temperature section and the decomposition furnace, which will cause the material to stick. It is especially serious in the place where a little cold wind leaks in and may occur at any time. This is also a feature that must be paid attention to in white cement manufacturing process.
Therefore, in the design of the preheater at each level (fifth stage) of the kiln tail, more attention must be paid to solving the problems of sealing the kiln end preheater system and preventing the accumulation of materials. The solutions are as follows: all the flanged parts of the preheater air pipe flanges are welded to completely prevent air leakage in this part; All levels of unloading air-locking flap valves are made of high-quality heat-resistant steel to reduce the deformation of the valve body at high temperatures to ensure the effect of air-locking; the connection between the cyclone inlet and the air duct at all levels eliminates horizontal or circular air ducts completely, the air pipe is connected with a large inclination angle (with a horizontal angle of 55 °) to avoid the accumulation of material at this part as much as possible.
Design of precalciner volume
The volume should meet the average residence time of the gas in the furnace is equal or more than 5.5s, because the clinker from the kiln is cooled by the water spray, which will cause the temperature of the tertiary air decrease, resulting in a corresponding increase in the fuel combustion time. It is important to ensure that the material entering the kiln has a high carbonate decomposition rate, and reduce the calcination pressure in the cement kiln.
Design of clinker bleaching process
The clinker adopts a combination of water spray quenching and grate cooler. The principle is to promote the reduction of ferric iron in the clinker to divalent iron to increase the whiteness of the clinker.
In actual cement production, the consumption of the water spray is determined by the specific whiteness and intensity control requirements that can meet the quality of the clinker. The design should ensure reasonable configuration and automatic control and operation. This kind of bleaching method can produce all high-quality white cement with a Heinz whiteness of 90 or more and a P · W42.5 or more.
Design of tertiary air duct
Due to spray bleaching, the temperature of the tertiary air will be reduced, in order to compensate and reduce its adverse effects as much as possible. It is necessary to add a layer of 40mm-60mm calcium silicate board in the lining of the tertiary air duct to enhance its thermal insulation effect.
Design of coal mill
According to China’s economic and reasonable situation of using high-quality coal to produce white cement, vertical cement mills or cement ball mills can be considered to help reduce “iron” pollution.
Through the above reasonable design, it is beneficial to reduce the energy consumption on the production line, ensure the production quality of white cement, and produce cement clinker with higher whiteness and strength. AGICO CEMENT has rich experience in cement production line design and supplying cement equipment.
3 Measures To Improve Whiteness Of White Cement
Improve the whiteness of clinker. A weak reduction atmosphere should be used for calcination. The fuel is natural gas, gas or liquid fuel without ash powder.
Improve the whiteness of gypsum. Add some gypsum with higher whiteness in the cement grinding plant, at the same time, increase the fineness of the cement.
Prevent the mixing of sundries. In the calcination, grinding, transportation, packaging, reduce the sundries like the lining material of the grinding mill.
Applications of white cement
White Portland cement is main kind of white cement. low content of ferric oxide and other non-ferrous metal oxide of limestone, clay, silica are used as the main raw material, after high temperature calcination, quenched into cement clinker, adding suitable amount of gypsum (can also add a small amount of white limestone instead of part of the clinker) into cement grinding mill equipped with stone or wear-resistant metal plate and grinding medium.
Uses: white Portland cement is similar to ordinary Portland cement in physical properties, mainly used as decorative materials for buildings, but also for sculpture crafts. The use of white cement also includes putty powder, coating, caulking agent, binder, waterproof coating, terrazzo, artificial stone, floor tile, permeable brick, washing stone, sculpture, wear-resistant floor, color cement and so on
White Cement VS Portland Cement
- Two are silicate cement, their cement raw materials include calcareous materials and salic materials, all materials are calcined by cement rotary kiln into cement clinker, add gypsum and admixture into clinker, which is grounded into white cement or Portland cement.
- Hydration products are basically same, including hydrated silicate gel CSH, Ca(OH)2, ettringite Aft, etc.
- Physical and mechanical performance: strength, setting, rheological property, durability and others. Apply for decoration cement and structural cement.
- High requirement for white cement manufacturing process: it is difficult to make high-quality white cement, which requires high-quality raw materials, strict grinding process and calcination. The white cement calcination temperature up to 100-150℃.
- Cement clinker mineral composition: white cement clinker contains 10-15% tricalcium silicate, 15-20% silicate minerals, basically no content of iron phase minerals, free calcium oxide generally used for Portland cement.
- Admixture: white cement usually use limestone and dolomite, white the admixture of Portland cement include fly ash, slag, and volcanic ash.
- Fineness: the white cement is finer than Portland cement.
- Part of property: the variation trend of the heat of hydration, fluidity, setting time, shrinkage, brittleness is different.