Analysis of gypsum calcination characteristics
From : clirik Date : 2019-02-21 10:58
1. Low temperature slow calcination
Refers to the calcination method when the temperature of the material during calcination is less than 150 ° C and the material stays in the furnace for several tens of minutes or more. Such as continuous wok, fluidized bed roaster. The calcination method is that the dihydrate gypsum is gradually dehydrated into a hemihydrate gypsum when heated, and the optimum dehydration temperature is selected according to the purity of the dihydrate gypsum, and the automatic control system adjusts the furnace temperature, the water vapor partial pressure, the material residence time, etc. to the most The stable state makes the quality of the calcined product uniform and stable. Most of the calcined products are semi-aqueous, very small amount of anhydrite gypsum III, dihydrate gypsum and anhydrite gypsum II. The crystal water content is generally 5.5-6.0%, and the individual is >4.0%. The calcined building gypsum powder is stored in the powder store for use, and is widely used in powder, plate and other products. The practice of continuous wok at home and abroad for many years has proved that the physical and chemical indicators of the calcined products can meet the quality standards and are one of the reliable equipment for gypsum calcination. The fluidized bed roaster has no practical data in China, but from the Australian side, it can calcine a good quality building gypsum. Some domestic chamber calcination processes are similar to this process, and they are also unique. They are also worthy of promotion and application in the field of chemical gypsum calcination.
2. Rapid calcination
It refers to the calcination method in which the calcined material temperature is greater than 160 ° C and the material stays in the furnace for several seconds. It has been used abroad for many years and is widely used in the production of high-speed gypsum board production lines and gypsum blocks, with good results. The calcination method is that the dihydrate gypsum is rapidly dehydrated after being heated, and the semi-aqueous or anhydrous III is formed quickly. Because the material temperature is high, the proportion of anhydrous III is large, and the type III anhydrite is unstable phase. It is easy to absorb moisture in the humid air to form a semi-aqueous phase. Therefore, in this calcination method, there are some cooling devices, such as Peter Mill, which has a special cooling device for introducing cold air; Shashiji Grinding has a spiral forced-type aging machine, which is used to reduce the material temperature and reach the material. The purpose of aging. For rapid calcination, the atmosphere in the furnace is very important. When the moisture content of the hot gas in the furnace is very small, there is relatively more anhydrous gypsum III in the calcined finished phase, such as the bitumen of coal burning, the anhydrous gypsum of the product III. Up to 30% or more; Beijing Guohua's Shashiji grinding and calcining products only have 3 to 3.5% of crystal water, and most of the products calcined in the Sde test furnace are anhydrous gypsum III, which indicates that the calcined products are in an "over-fired" state. After cooling and aging, the phase composition changes and the semi-aqueous phase increases. In the phase composition of the product obtained by the use of natural gas as a fuel, the amount of anhydrous III is only 10-15%, which is due to the large amount of water generated in the combustion of natural gas, and the rationality of containing the "exhaust gas" on the other hand. Recycling will increase the partial pressure of water vapor in the furnace, which is beneficial to the transformation and stability of phase components. Therefore, in addition to adjusting the optimal dewatering temperature, the rapid calcination method should also calculate the moisture content of the combustion gas according to the type of fuel, and determine the partial pressure of water vapor in the furnace. If it is small, it should be applied with external humidification. The method is to increase the humidity of the atmosphere in the furnace; or adopt the measures such as forced aging. The most prominent feature of rapid calcination is high production efficiency and low energy consumption. The quality of the product can be guaranteed through good cooling and aging in production.
3. Medium speed calcination
This is the residence time of the material in the kiln for a few minutes, ten minutes, and the material temperature is between 140~165 °C. This way is between the two. The typical calcining equipment is a rotary kiln. As early as the mid-1970s, China's first gypsum board production line adopted this method. The calcined natural gypsum is granular, and the quality of the product is not uniform due to the particle grading factor. And stable. However, after selecting the better grinding equipment in the kiln and adding good aging measures, the product quality can be guaranteed. When calcining the industry to produce gypsum, in addition to considering a certain drying zone in the design of the kiln, the appropriate grinding equipment should be selected according to the particle grading of the material to improve the proportion of the particle grading before and after calcination, so that the physical properties of the product are more superior. At present, in this way, calcined industrially-produced gypsum, such as phosphogypsum and desulfurized gypsum, are in trial production, and technical parameters and performance test data are yet to be accumulated and deepened.
4. Regarding the particle grading problem of calcined gypsum in the calcining industry
Industrially-produced gypsum exists in the state of “fine powder”. For example, the particles of desulfurized gypsum are concentrated in 40~60 microns, the particle size distribution is single, the specific surface area is small, and the rheology is poor. This particle gradation will affect the product quality. The first solution is to increase the “grinding” measures to complete the dehydration and change the particle grading in the calcination process. After the calcined product with the "crushing" process, the particle size distribution curve is greatly changed, and the product performance is improved.
5. Principles for selecting calcination methods and equipment
The first is the type and traits of gypsum.
Whether it is natural gypsum or industrially-produced gypsum; it is granular or powdery; if it is powdery, it should use fast calcining equipment; if it is granular, it should use equipment with long residence time in kiln.
Type of terminal product
If it is to produce gypsum building materials, such as various gypsum boards, blocks, etc., it is advisable to use rapid calcination equipment. As the building gypsum produced by the rapid method is hardened by condensation, it can improve production efficiency and accelerate the turnover of the mold; if it is to produce gypsum cementitious materials, such as plaster, gypsum adhesive, gypsum joint material, etc., slow calcination should be used. Equipment, because the slow calcination of the product is more difficult to coagulate and harden, which is beneficial to reduce the dosage of admixtures and reduce the production cost; if both types of products are produced, they are selected according to the leading products. In the end, we should also weigh the comprehensive technical and economic indicators such as the cost performance of various calcining equipment.
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