The main component of desulfurization gypsum is CaSO4 2H2O, and the impurity components include SiO2, A12O3, Fe2O3, MgO, CaO, etc., in which the CaSO4 2H2O content is generally above 85%, and the free water content is 10%~15%. Desulfurization gypsum is generally neutral and is a powdery material. The density of wet powder desulfurization gypsum is 1.06~1.20g/cm3, and its particle diameter is concentrated in 30-60μm. Its quality depends on the source of the limestone and the production control of the flue gas desulfurization process. Since desulfurized gypsum is wet powder and easy to agglomerate, it does not need to be crushed like natural gypsum before calcination, but it is necessary to dry, disperse and disperse the agglomerated gypsum powder, so that the gypsum particles can be fully mixed with the heating medium Uniform contact ensures a balanced and continuous heat transfer process. Secondly, the particle size distribution of desulfurized gypsum is narrow, and grinding measures need to be added after calcination to ensure a good particle size distribution. Finally, desulfurized gypsum contains a small amount of high-hardness impurities such as SiO2, which has strong abrasiveness on calcination equipment, and direct contact and moving parts need to be treated with wear resistance.
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1. Calcination process of desulfurized gypsum
(1) Low temperature and slow calcination
Low-temperature slow-speed calcination means that the material stays in the calcination equipment for a long time, and the feeding and discharging time is at least tens of minutes. Rotary kiln, indirect fluidized fluidized furnace, etc. are all low-temperature slow-speed calcination equipment. These calcination equipment indirectly heat the desulfurized gypsum through built-in heating tubes, and the heat source generally uses steam, heat transfer oil or hot flue gas. Because it is indirect heating, the heat transfer rate between the material and the heat source is slow, and after the gypsum is heated slowly, it will be slowly dehydrated to form hemihydrate gypsum.
The most prominent advantage of low-temperature slow calcination is that the product quality is uniform and stable. Due to low-temperature calcination, most of its calcined products are hemihydrate gypsum (about 88%), and a very small amount of AⅢ type anhydrous gypsum (about 2%) and dihydrogen gypsum Water gypsum (less than 1%) has crystal water content of 5.5%~6.0%, so the building gypsum powder produced by low temperature and slow calcination has stable quality, stable phase composition, slow coagulation speed, and small water requirement for standard consistency.
(2) High temperature rapid calcination
During high-temperature rapid calcination, the temperature of the heat source is usually higher than 600°C, and the material stays in the calcination device for only tens of seconds. The surface temperature of the gypsum particles is relatively high, and the internal temperature of the particles varies according to the size of the agglomerated particles, such as airflow calcination, Direct rotary kiln calcination and so on adopt high temperature rapid calcination process. The high-temperature rapid calcination equipment is a direct contact between desulfurization gypsum and high-temperature flue gas for heat exchange, rapid dehydration and calcination. Because the fine gypsum powder heats up quickly, it is easy to produce anhydrite AⅢ and anhydrite AⅡ; the medium-fine gypsum powder mainly produces hemihydrate gypsum HH; The calcination temperature still maintains the dihydrate phase. Its phase composition is generally about 70% of hemihydrate gypsum HH, about 10% of anhydrous gypsum AⅢ, and about 3% of dihydrate gypsum, showing a large proportion of anhydrite AⅢ, and even a certain amount of anhydrite AⅡ , reducing the active ingredients of gypsum. Therefore, it is difficult to control the rapid calcination process, the quality of building gypsum fluctuates greatly, the phase composition of the product is unstable, the coagulation speed is too fast, and the standard water demand is high, but the production efficiency is high. Since Type AⅢ anhydrous gypsum is an unstable phase, it is easy to absorb moisture in the air to form a semi-aqueous phase, so an aging cooling device is required in the system. The most prominent advantages of high-temperature rapid calcination are low equipment investment and high production efficiency.
(3) Composite calcination
Composite calcination is a new process developed in the desulfurization gypsum calcination process in recent years, especially in the gypsum board industry. Composite calcination is mainly to simulate slow calcination. The process and time of gypsum calcination are appropriately lengthened to avoid concentrated heating, that is, the method of "one-step calcination" is changed to "two-step calcination". Desulfurization gypsum is processed in two different calcination equipment Heating in sequence in the process, first remove all free water and part of crystal water, and then remove the remaining part of crystal water to produce qualified building gypsum, which can reduce the material temperature and realize the cascade utilization of energy, so as to achieve the same effect as low-temperature calcination. At present, the usual practice is to combine rapid airflow drying and calcining with indirect heating fluidized furnace or compound rapid airflow drying and wok calcination.
2. How to grind desulfurized gypsum
Grinding desulfurized gypsum is usually used desulfurized gypsum mill model HC1300. The desulfurized gypsum grinding bulk material is crushed by the crusher to the feed fineness (15mm-50mm) that can enter the mill. The crushed gypsum small pieces are sent to the storage hopper by the elevator, and then evenly and quantitatively sent to the grinding chamber of the mill by the feeder for grinding. The ground gypsum powder is classified by the classification system, and the unqualified powder is classified by the classifier and returned to the main machine for re-grinding. The fineness powder enters the dust collector with the airflow through the pipeline for separation and collection, and the collected finished powder is sent to the finished product silo by the conveying device through the discharge port, and then uniformly packed by a powder tanker or an automatic packing machine.
Different mill equipment, different production needs, and configuration schemes of the desulfurization gypsum mill production line are different, so the equipment prices are also different. As a manufacturer, Hongcheng Mining is committed to providing tailor-made services for each customer, equipped with reasonable mill prices and selection plans, welcome to leave a message to learn more about new environmentally friendly Raymond mills, vertical mills and other equipment.
