With changes in external conditions such as temperature, pressure and moisture, gypsum generates semi-hydrous gypsum with crystalline water (CaSO. 1/2HO) and dihydrous gypsum (CaSO. 2HO), whereas dihydrous gypsum is converted to hemihydrous gypsum by dehydration reaction. Dehydration continues to convert to anhydrous gypsum (CaSO). Due to the different conditions of hydration and dehydration reaction, such as temperature, pressure, the proportion of water is different, the introduction of different impurities gypsum will form more. A variation. These variants have different crystalline water content and crystal phase states. Experiments have shown that the mechanisms of dehydration and hydration are complex and multiple reactions take place at the same time, so both natural gypsum and factory processed gypsum products are mixtures of many variations of gypsum.
It is generally accepted that gypsum has 5 phases and 7 variations:
Dihydrous gypsum (D)CaSO42HO: Dihydrous gypsum (H)CaSO. 2H0(divided into type a and β): Type I anhydrous gypsum (AII)-CaSO4a Type B: Type II anhydrous gypsum (A ID-CaSO4: Type I anhydrous gypsum (AI).
According to the characteristics of various variations, gypsum enterprises have developed a variety of gypsum products with practical value, forming a large scale industry.
Fluoropgypsum is a by-product of hydrofluoric acid produced by fluorogypsum plant using fluorite and concentrated sulfuric acid. When fluoropgypsum is discharged from the reactor, the material temperature is 180 ℃~230 ℃, and the gas temperature is 800'C~1000 ℃. The discharged gypsum is anhydrous calcium sulfate, which can be basically converted into calcium sulfate dihydrate for about 3 months under the condition of sufficient water. There are often unreacted CaF and HSO in the discharged fluorogypsum. Sometimes HSO. The high content makes the discharged fluoropgypsum acidic and can not be discarded directly. To this, our country generally has two treatment methods :1) the lime neutralization method, which is to add water to gypsum slurry, put into the lime neutralization to pH=7 or so discharge. Lime is added to neutralize sulfuric acid and further produce calcium sulfate. When lime is added, only a small amount of Ca(OH) is brought in. Therefore, the purity of fluorogypsum by this treatment method is higher, up to 80% ~90%, which is called lime-fluorogypsum. 2) Bauxite neutralization method, that is, adding bauxite to neutralize the remaining sulfuric acid can recover useful product aluminum sulfate, make it slightly acidic, and then add lime neutralization to pH=7. Then discharge the stack. Bauxite contains about 40% SiO and other impurities, so that the grade of the final discharged fluoropgypsum drops to 70% ~80%, this gypsum is called bauxite fluorogypsum.
As a kind of industrial waste slag, fluorogypsum can not be directly used as building materials because of slow condensation, low strength and serious expansion. If the fluoropgypsum waste residue can be modified and transformed into gypsum building materials, it will certainly play a positive role in the development of building materials industry. In recent years, through the unremitting efforts of Chinese science and technology workers, the research and development of fluorine gypsum has been making new progress, which opens up broad prospects for the utilization of fluorine gypsum resources. Therefore, it is of great significance to make fluorine gypsum meet the requirements of building gypsum by adding various admixtures.
