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There are some theoretical studies and discussions on the process and application of producing type II anhydrous gypsum with industrial by-product gypsum in China, but the large-scale application in industry is rare. Anhydrous II gypsum powder has been used in most countries such as Europe, Japan and the United States, and is widely used in self leveling, plastering gypsum, gypsum putty, pasting gypsum powder and other application fields; Without in-depth development in China, the application of anhydrous II gypsum powder cannot be popularized on a large scale. The main factors are market cognition, production technology, supporting construction technology, policy orientation, etc., while the key factor is to provide complete production process and low cost of production to realize large-scale production. The production, process and market application of anhydrous II gypsum have been applied abroad, such as the application of self leveling, which provides a good reference for the R & D and application of anhydrous II gypsum in this field in China.

Anhydrous gypsum obtained by calcining phosphogypsum at a low temperature below 350 ℃ is anhydrous III. due to the hexagonal transformation of type III anhydrous gypsum into orthorhombic crystal system and many crystal defects, it has strong hydration ability and fast hydration speed. However, the over burned gypsum calcined at high temperature (850 ℃) has no crystal defects, increased density and weak hydration ability. After adding composite activator, Improve the hydration ability of type II anhydrous gypsum. The gypsum powder obtained by activation, hydration, reinforcement, waterproof and other processes has a strength of more than 425 cement, a bending resistance of 11mpa and good water resistance.

Practice has proved that phosphogypsum can remove impurities in phosphogypsum by high-temperature forging in the right direction. Other impurity removal methods are incomplete, unstable and costly. Anhydrous type II gypsum powder is formed by calcining phosphogypsum without water washing at 850 ℃. This phosphogypsum production process is simple and reliable. It is one of the effective ways to recycle phosphogypsum non washing building materials.

Calcination of anhydrous II gypsum at high temperature mainly involves calcination temperature, particle size of dihydrate gypsum, calcination temperature and particle size of calcined raw materials, which are closely related to calcination methods. The calcination methods of anhydrous II gypsum include rotary kiln, fluidized bed and flash gas flow calciner. In Europe, flash gas flow calcination is the mainstream calcination method. Flash gas flow calcination is a rapid calcination method, and the calcination time is only 6 ~ 10 seconds. The advantages of flash air flow calcination are complete calcination and energy saving. About 1.1 million kcal (phosphogypsum free water < 18%) is used to calcine 1 ton of gypsum powder. Moreover, there is no dust pollution.

The process flow of anhydrous II gypsum produced by rapid calcination is as follows:

The process is that the pre calcined dihydrate gypsum is sent to the drying system, and the waste heat of calcined tail gas is used to dry the dihydrate gypsum. The dried dihydrate gypsum enters the cyclone preheater for preheating, and then enters the flash calciner. The type II anhydrous gypsum formed by high-temperature calcination is separated by the separator, then enters the bag filter for secondary recovery, and then enters the cooling system, Enter the storage bin for use. The calcination heat source adopts gas, natural gas, etc.

Effect of calcination temperature on the properties of anhydrous gypsum. With the increase of calcination temperature, the water paste ratio of standard consistency of type II anhydrite decreases, the setting time is significantly prolonged, the hardening bulk density and strength increase, and the softening coefficient increases with the increase of calcination. When the temperature is above 850 ℃, eutectic phosphorus in calcined phosphogypsum precipitates. Organic matter evaporation eliminates the harmful effects of eutectic phosphorus and organic matter.

In the process of flash calcination of phosphogypsum, the temperature of phosphogypsum is gradually increasing through drying and several stages of swirling preheating. Phosphogypsum powder enters the main body of the calciner at 400 ~ 500 ℃ and rapidly rises to 850 ℃, and then discharged from the furnace into the recovery system. Type II anhydrous gypsum calcined in flash calciner is uniformly heated without over burning and under burning.

[] the strength of gypsum powder calcined in medium-sized calciner exceeds that of 425 cement, the flexural strength reaches 11mpa, and the water resistance is very good.