Screening Technology of Limestone for Power Plant Desulfurization

　　In today's increasingly stringent environmental protection requirements, desulfurization technology is an indispensable method. Whether it is in-furnace desulfurization or spray wet desulfurization, limestone (calcium carbonate) is essential. Generally, users purchase finished limestone products directly from the market, while many also build their own limestone production lines to meet desulfurization needs.

　　Limestone screening is one of the international screening challenges. The limestone powder-making process generally uses two-stage crushing, and some use grinding machines. The crushing equipment and screen form a closed-loop cycle. Particles smaller than 1mm under the screen are discharged as qualified size, while larger particles on the screen return to the crusher for further crushing. Users need to be reminded that in a closed-loop cycle, only when a large amount of finished product is screened out can an equal amount of new material be fed in to participate in the crushing and screening cycle; otherwise, the system will perform useless work.

　　Limestone has a relatively large specific gravity and requires small particle size, making screen selection extremely difficult. To achieve higher screening efficiency, internationally, stainless steel woven screens with high open area are generally used. When processing 1mm particle size, the screen wire diameter is about 0.3-0.6mm. If there is iron in the feed, it will instantly damage the screen. Coupled with limestone's high abrasion coefficient, the screen becomes very fragile.

　　Known wear-resistant screens include: steel plate perforated screens, polyurethane strip slot screens, small perforated comb-tooth rainbow hole screens, stainless steel trapezoidal welded screens, cast screens, and polymer polyethylene screens. Although very wear-resistant, they cannot be chosen due to their relatively low open area. Screen open area is the fundamental guarantee of screening efficiency. A decrease in open area means a multiple increase in screening area, and the screen's external dimensions and energy consumption will eventually reach levels unacceptable to users.

　　We collaborated with local military-industrial enterprises to develop a composite stainless steel sintered screen, which involves applying a coarser screen layer with 20mm openings beneath the original screen. The two screens are sintered together at high temperature, increasing the screen's rigidity. The surface layer of the screen in contact with the material does not reduce its open area at all, ensuring screening efficiency.

　　Having a better screen is not enough. If conventional designs are used, for example, when processing 60T of feed, the screening area must be designed to at least 12㎡, with an effective width of 2000mm and an effective length of 6000mm, which is a sizable piece of equipment. If two or three layers are used, maintenance of the lower or middle layers is extremely difficult, requiring removal of the upper screen. At this point, the screen machine's weight will exceed 20T, and the dynamic load caused by the large vibrating mass is also a negative factor.

　　Through decades of exploration in screening technology and industrial simulation experiments, we concluded that screening small particle limestone does not require a long screening surface length; even 500mm is sufficient. Fine materials can be screened very thoroughly.

　　After repeated experimental comparisons, to achieve higher screening efficiency, we have derived several important experiences and verified them in equipment manufacturing.

　　1: Guarantee of screening area and screen open area.

　　Screening area and screen open area are the fundamental guarantees of screening efficiency. To achieve higher screening efficiency, we use a multi-layer design, which can be designed with five layers, each spaced 80mm apart. The total width and length of the screen machine generally do not exceed 1500mm, forming an approximately square structure, with the drive designed below the screen machine. The composite sintered screen we selected not only has high strength but also maintains a high open area.

　　2: Installation of a screen cleaning system.

　　Small particle limestone is prone to clogging. Limestone moisture does not exceed 1%. As long as it does not absorb moisture, limestone is not sticky. Clogging mainly manifests as material jamming or hole jamming. This requires designing a self-cleaning striking device under the screen, which can simulate manual tapping to clear blockages without causing fatigue damage to the screen surface.

　　4: Focus on material distribution.

　　How to ensure full-width feeding and uniform distribution on each screen layer becomes the key technology to reduce wear and improve the utilization rate of each screen surface. Through research and development, we designed a multi-point, multi-layer distributor to be used with the screen machine. The material layer on the screen surface is thinner and more evenly distributed. Combined with the self-cleaning striking mechanism, screening efficiency can reach over 90%, which is an impressive indicator.

　　5: Quick-change screen structure.

　　Although a unique composite sintered screen is used, natural wear is still inevitable. We designed a drawer-type screen replacement solution that allows quick installation and removal and fast locking, making screen replacement easier.

　　Conclusion:

　　Compared with the old heavy-duty screen machine, the newly designed five-layer limestone screen does not reduce the screening area at all, but the material runs for a shorter time on the screen surface, the material layer on the screen is thinner and more uniform, screen wear is greatly reduced, and screen replacement is easier.

　　The new limestone screen machine not only has higher screening efficiency but, more importantly, its processing capacity is more than twice that of the old screen machine, and the equipment is more compact. The distributor and screen machine form a completely closed small system, responding to strict environmental dust control requirements, maintaining a leading position in international limestone screening technology.