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Other impurities, such as chloride ions, significantly affect the adhesion of desulfurized gypsum. Potassium and sodium ions are even more detrimental to its quality. However, under normal conditions, the concentrations of these impurities in desulfurized gypsum are low and typically do not require special attention.
In addition to the conventional limestone/lime-gypsum wet flue gas desulfurization system, there is also a simplified version known as the "wet fast desulfurization" system. This approach reduces the size of the reaction tower and minimizes auxiliary equipment, leading to lower costs and operational expenses. However, this simplification also results in reduced desulfurization efficiency and lower quality of the by-product gypsum. The first thermal power plant in Taiyuan, China, adopted this simplified method.
(2) Particle Characteristics of Desulfurization Gypsum: Natural gypsum typically appears in block form, while desulfurized gypsum is a moist powder. Its particle properties differ significantly from those of natural gypsum after grinding. Key differences include:
1. **Particle Shape**: Natural gypsum particles are irregularly shaped after grinding, whereas desulfurized gypsum consists of well-defined crystalline structures, often appearing as columns, fibers, flakes, or hexagonal plates.
2. **Particle Size Distribution**: As shown in Figure 2-2, the particle distribution of natural gypsum is more uniform, with both coarse and fine particles present. In contrast, desulfurized gypsum has a less varied particle size, with a concentrated distribution and limited fine powder. Its specific surface area is only 40%–60% of that of properly ground natural gypsum. Even after calcination, the particle characteristics remain largely unchanged, which can lead to poor flowability when water is added, causing particle separation and stratification. This makes it unsuitable for direct use in building gypsum without modification and additional grinding to increase the fine particle content and improve the specific surface area.
3. **Grindability Differences**: Due to the difference in grindability between impurities and gypsum, natural gypsum often contains impurities in its coarser particles after grinding. In desulfurized gypsum, however, the opposite is true — the coarser particles are mostly gypsum, while the finer ones tend to be impurities.
(3) Influence of Desulfurization Gypsum on Main Impurities:
1. **Coke and Soot**: These are combustible organic materials. According to European standards, the content of combustible matter in desulfurized gypsum should not exceed 0.1%. If the electrostatic precipitator is inefficient, they may mix easily with the gypsum. They appear as porous, round, or elongated black particles and are easy to distinguish from gypsum using the naked eye or magnifying glass. Coke and soot do not change during calcination and float on the surface due to their low density. When used in gypsum board production, they accumulate at the interface between the gypsum and paper, affecting adhesion and foaming performance. They can also cause black spots on plaster surfaces, reduce product gloss, and negatively impact construction quality. Adding about 0.1% calcium carbonate can help eliminate them effectively.
2. **Alumina and Silica**: If present as coarse particles, they are harder than gypsum and can reduce grinding efficiency and increase wear on equipment. Their presence affects the durability of gypsum when used in ceramic molds and can damage processing equipment when used as fillers in paper, adhesives, or plastics, reducing product quality and surface finish.
3. **Iron Compounds**: These originate from the absorbent and can be classified into coarse magnetic iron and fine non-magnetic iron. Coarse magnetic iron can be removed using separators, but fine non-magnetic iron remains and can discolor the gypsum. The iron compound content should not exceed 0.15%.
4. **Calcium Carbonate and Magnesium Carbonate**: During calcination, some of these compounds may convert to calcium oxide and magnesium oxide. This increases the pH of the gypsum slurry, potentially affecting adhesion if the pH exceeds 8.5. Therefore, their content in gypsum board production should be limited to 1.5% or less.
5. **Trace Elements and Radioactive Substances**: Studies have shown that the levels of trace elements and radioactive substances in desulfurized gypsum are far below acceptable limits. Like natural gypsum, it is safe and suitable for use as a building material.
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