Increasing Material Cohesiveness
The final common method used to minimize dust is to increase the cohesiveness of the material; that is, the material’s “desire” (or ability) to stick together. The properties of the material must be altered in order to increase its ability to stick to itself. A real-life example of improved cohesiveness would be beach sand versus desert sand. Both types of sand have approximately the same size particles in solid form. Desert sand does not stick to itself; the particles can easily fracture off and become airborne as dust. The added moisture content of beach sand increases cohesion; the particles will stick together and not become airborne when the material is dropped.
A simple way to increase cohesiveness is to introduce water or another binding agent to the material. Care must be taken when applying moisture to a bulk material. If water is applied to the top of material lying on a pile or a conveyor belt, it will only wet the outside of the material. When this material is disturbed, by being reclaimed from a stockpile or by traveling through a transfer point, the particles are arranged and dry surfaces are exposed to the air. Dust can then be released from these dry surfaces. The ideal application point for moisture addition is in free fall. This allows the water to penetrate the material and contact more of the material’s surface.
The advantages of the application of water include the residual effect of the dust suppression. A wet material will retain its elevated cohesion level (and hence, the inability to generate dust) for as long as the material remains wet.
A disadvantage of the application of water is the large amount of water required to thoroughly wet most materials. The drawback is that, because of wet material sticks to itself and system components, the moisture can create problems, including the blinding of screens, the plugging of chutes and the carryback of material on the belt. Even the efficiency of a crusher is reduced with wet material. When designing a material-handling system or considering suppression as a solution for dust problems, the effect of added moisture must be considered.
Another concern of applying water is the added performance penalty that comes from wetting down a product that must be heated or burned. Individual operations must decide if the cost of fugitive dust is greater than the thermal penalty of suppression. An additional issue is that some materials, such as cement, cannot be exposed to water. A thorough understanding of an operation’s material and process is required before a suppression system is selected. One method to minimize the amount of water required for dust suppression is to improve the water’s ability to wet the material with the addition of a surfactant to the water supply. The solution of water necessary to do the job but it increases operating costs.
See Also: Methods of Belt Conveyor Dust Management Part 1 | Part 2 | Part 3
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