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Other impurities, such as chloride ions, significantly affect the adhesion of desulfurized gypsum. Potassium and sodium ions are even more harmful to the quality of desulfurized gypsum. However, under normal operating conditions, the levels of these impurities in the final product are typically low and may 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 system reduces the size of the reaction tower and minimizes auxiliary equipment, leading to lower costs and operational expenses. However, this simplification comes at the expense of reduced desulfurization efficiency and the quality of the by-product gypsum. This method was first applied at the first thermal power plant in Taiyuan, China.
(2) Particle characteristics of desulfurization gypsum processed by ball press: Natural gypsum appears in block form, while desulfurized gypsum is usually a moist powder. Its particle properties differ significantly from those of natural gypsum after grinding. The main differences include:
1. **Particle Shape**: Natural gypsum, when ground, has irregular shapes, while desulfurized gypsum consists of well-defined crystal structures, often appearing as columns, fibers, flakes, or hexagonal plates.
2. **Particle Size Distribution**: Figure 2-2 illustrates the particle distribution curve for both natural and desulfurized gypsum. After grinding, natural gypsum exhibits a broader range of particle sizes, including both coarse and fine particles. In contrast, desulfurized gypsum has a narrow particle size distribution, with few fine particles. Its specific surface area is only 40%–60% of that of properly ground natural gypsum. Even after calcination, the particle characteristics remain largely unchanged, leading to poor flowability when mixed with water, significant particle separation, and volume expansion. Therefore, desulfurized gypsum used for building materials must be modified and finely ground to increase the proportion of fine particles and enhance its specific surface area.
3. **Grindability Differences**: Due to the difference in grindability between impurities and gypsum, the coarse particles in naturally ground gypsum are mainly impurities, whereas in desulfurized gypsum, the coarse particles are mostly gypsum, and the fine particles are mostly impurities.
(3) Influence of desulfurization gypsum ball press on major 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 does not perform well, these substances can mix easily during dust removal. They appear as porous, round, or elongated black particles, which are easy to distinguish from gypsum using the naked eye or magnifying glass. During calcination, coke and soot remain unchanged. When mixed with water, they float on the surface due to their low density, causing problems in plaster production. They tend to accumulate at the interface between gypsum and paper, reducing adhesion and damaging foam, increasing the bulk density of the board. When used in plaster, they cause black spots, affecting appearance and making it difficult to wet walls, thus impacting construction quality. Adding about 0.1% calcium carbonate can effectively remove them.
2. **Alumina and Silica**: If present as coarse particles, they are harder than gypsum, affecting grinding efficiency and increasing wear on equipment. In ceramic molds, their hardness impacts the service life of the material. When used as a filler in paper, adhesives, or plastics, these hard particles reduce processing efficiency and damage equipment, affecting product gloss.
3. **Iron Compounds**: Iron compounds in desulfurized gypsum come from the absorbent and are divided into coarse magnetic iron and fine non-magnetic iron. Coarse magnetic iron can be removed by a separator, but fine non-magnetic iron remains and affects the color of the gypsum. The iron content should not exceed 0.15%.
4. **Calcium Carbonate and Magnesium Carbonate**: Unlike natural gypsum, some of these compounds convert to oxides during calcination. If used in gypsum boards, the resulting oxides can raise the pH of the slurry above 8.5, affecting adhesion between the paper and gypsum. Therefore, the content of these compounds in desulfurized gypsum used for gypsum boards should be limited to 1.5% or less.
5. **Trace Elements and Radioactive Elements**: Studies show that the levels of trace and radioactive elements in desulfurized gypsum are far below acceptable limits, making it suitable for use as a building material, similar to natural gypsum.
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