Dust Control Handbook for Industrial Minerals Mining and Processing

Wet Control MethodsPREVENTION AND SUPPRESSION APPLICATIONSWet dust control systems can be very effective and are less costly to install and operate, but theymay not be feasible due to characteristics of the mineral, subsequent processing steps, orcustomer specifications. Also, when operations are in northern climates, freeze protection isnecessary, and ice buildup can create additional safety hazards for workers.As discussed more fully in Chapter 2—Wet Spray Systems, the use of water to control dust maybe classified into prevention applications and suppression applications. Prevention is theapplication of water to prevent dust from becoming airborne. Suppression is the use of water towet dust particles which have already become airborne, increasing their mass and causing themto settle more rapidly.In general, prevention is more effective than suppression [NIOSH 2003; USBM 1978].However, when the wetted material is subject to further size reduction as in crushing operations,effective prevention requires application of additional water to the dry—and larger—surface areaof the material exposed by the size reduction process. This may create a complication becauseadditional application of water to improve prevention may cause the material to become too wet,interfering with efficient handling, subsequent processing operations, or product specifications.As a result, some trade-off between wet dust control and process efficiency is often unavoidable.This trade-off can necessitate the use of other control approaches to achieve acceptableconditions.Suppression of respirable airborne dust using water, usually through sprays directed into the dustcloud, is not always highly efficient. It is difficult with hydraulically atomizing spray nozzles toproduce water droplets small enough to suppress respirable particles effectively. Appropriatelysized water droplets can be produced with air-atomizing nozzles, but this method requires asource of compressed air to atomize the water, and the very small nozzle orifices are subject toclogging, requiring increased maintenance attention. Additionally, air-atomized spraying, due tothe volume of the air/water mixture released, typically requires an enclosed area to avoidspreading dust into surrounding areas. All of these factors increase operating costs.Dry Control MethodsControl of dust generated by transport (see Chapter 5—Conveying and Transport) andprocessing of minerals can be achieved through containment, exhaust, and cleaning of dusty air.This approach, termed dry control, can be more costly to install and operate than wet controlmethods, but can be very effective. Additionally, dry dust control may be necessary when theproduct is adversely affected by the addition of water, such as trona or clayey materials.Dry dust control must create conditions that will prevent the escape of dusty air from thecontrolled space to areas occupied by workers. This control is achieved by using exhaustventilation to create a negative air pressure inside the controlled space relative to outside of thecontrolled space (see Chapter 1—Fundamentals of Dust Collection Systems). The amount ofventilation necessary to achieve control is affected by:112 Crushing, Milling, and Screening