Desulfurization gypsum standard

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Desulfurization gypsum standard
The initiator and main completion unit of the standard preparation is Beijing General Research Institute of building materials. The preliminary scheme proposed at present is as follows:
1. According to the quality and use of FGD gypsum, the product requirements are classified as grade I and grade II.
2. Free water: less than 10% and less than 15%.
3. Calcium sulfate dihydrate: as the main classification index of desulfurized gypsum, the content of calcium sulfate dihydrate is estimated to be 80% and 90%.
4. Magnesium salt and sodium salt soluble in water: the classification is limited in the standard.
5. Chloride: it shall be determined according to the European Association standard (< 0.01%) after the desulfurization gypsum verification test in different regions.
6. Calcium sulfite hemihydrate: the European Association standard is less than 0.5%.
7. PH value: basically neutral, 5-9.
8. Color: graded according to use requirements.
9. Odor: this indicator is recommended.
10. Average particle size: it will be defined in the standard (the European Association standard is more than 60% of 32 microns).
11. Composition of combustible organic matter: whether it is included shall be determined after test verification.
12. Al2O3 and SiO2: whether they are included or not shall be determined after test verification.
13. Fe 2O 3: whether it is included or not shall be determined after test verification.
14. Radioactive elements: it shall meet the requirements of the national standard GB6566-2001 building materials radionuclide limits.
15. Toxicity: according to bekert, J; Einbrodt.H; Fischer. M: comparison of natural gypsum and flue gas desulfurization gypsum to determine toxic substances [1990] as a test method.
Quality parameters Expressed as : Unit Quality Criteria Free moisture H2O % by weight < 10 Calcium sulphate dihydrate CaSO4 x 2H2O % by weight > 95 (1) Magnesium salts, water soluble MgO % by weight < 0.10 Sodium salts, water soluble Na2O % by weight < 0.06 Chloride Cl % by weight < 0.01 Calcium sulphite hemihydrate CaSO3 x 1/2 H2O % by weight < 0.50 pH 5 - 9
Quality of desulfurized gypsum:
Desulfurized gypsum is an industrial by-product gypsum obtained by desulfurizing and purifying the flue gas generated after the combustion of sulfur-containing fuels (coal, oil, etc.). The formation process is as follows: the sulfur-containing flue gas after dust removal is introduced into the absorber, the fine lime or limestone powder forms a slurry, the flue gas is washed in the absorber by spraying, and reacts with the sulfur dioxide in the flue gas to form calcium sulfite (CaSO3 • 0.5h2o), and then a large amount of air is introduced to forcibly oxidize the calcium sulfite to calcium sulfate dihydrate (CaSO4 • 2H2O), The gypsum suspension from the absorber is dehydrated by concentrator and centrifugation, and the final product is wet and loose fine particles with high gypsum dihydrate content and residual water content of 10 ~ 20%.
Desulfurized gypsum has a high grade and is different from natural gypsum in chemical composition, especially in impurity composition. Due to the fuel used in the combustion process (especially coal) and the lime / limestone used in the washing process, there are often impurities such as carbonate, silica, magnesium oxide, alumina, sodium oxide (potassium) in the desulfurized gypsum. When some impurities exceed a certain content, they will affect the quality of gypsum products. For example, a desulfurized gypsum product had "anti frost" on its surface in a humid environment. After chemical analysis, the "frost" contained MgO, Cao, SO3. The main phase measured by X-ray diffraction was MgSO4 • 4H2O. Its formation is that MgSO4 in the product is a soluble substance, which can be separated from the inside of the product to the surface. When the humidity in the air is high, it absorbs moisture to form magnesium salt. In addition, when the content of Na2O and K2O is high, Na2SO4 will also be formed. Salts of different crystal water, such as Na2SO4 • 10h2o (commonly known as mirabilite) or CaSO4 • K2SO4 • H2O double salt, will precipitate to the surface of the product to form white crystals. This kind of back frost (back alkali) phenomenon not only affects the appearance of the product, but also seriously affects the adhesion of gypsum and the covering layer, such as separating the protective paper and paste core of gypsum board, rough or even falling off the surface of the coating layer of gypsum block products, resulting in product quality problems.
Other impurities, such as smaller particles of Fe and incompletely burned pulverized coal particles, will affect the whiteness and bonding performance of products.
[Abstract]: FGD gypsum is a by-product of wet FGD technology used in thermal power plants. In recent years, with the vigorous implementation and promotion of wet flue gas desulfurization as the mainstream technology in China, the emission of flue gas desulfurization gypsum is increasing day by day. However, due to the adverse effect of soluble impurities in FGD gypsum on its performance, the resource utilization of FGD gypsum in many building materials fields is hindered. In view of this, the physicochemical characteristics of FGD gypsum and the influence of impurities contained in FGD gypsum were first studied in this paper.