Australasian Mining Review
Issue link: http://ebook.aprs.com.au/i/37922
92 Fundamental considerations for dust control Wet dust control systems use water sprays however that’s where the simplicity ends The material you are dealing with: Materials respond to moisture differently. Too much moisture can mean sludge and mud, causing potentially hazardous conditions and costly maintenance problems. Applying approx. eight litres of moisture per tonne of ore, for example, will provide adequate wetting and won’t cause process or production issues. T he material will determine whether chemicals should be added to the water to improve suppression. Coal repels water and usually requires a chemical additive to increase absorption. At processing, most dust particles created during breakage, are not released into the air. The dust stays attached to the surface of the broken material. Adequate wetting is critical to ensure the dust stays attached. Plus, partially processed minerals and coal maybe more sensitive to moisture than unprocessed material. Stationary or moving? Drop size and spray angle can affect surface cover range when spraying stationary material. Drop size and drop velocity affect coverage when spraying moving material. These factors must be considered when selecting and positioning spray nozzles. Capturing airborne dust: Dust capture is most effective when dust particles collide with water drops of an equivalent size. Drops that are too large won’t collide with the smaller dust particles and drops that are too small, evaporate too quickly and release the captured dust particles. So understanding the particle size of the dust is critical to effective system design. General guidelines for dust particle size are below, but additional research may be necessary, depending on the specifi cs of the material being processed. Diameter in microns: • Ground limestone: 10 to 1000µm • Coal dust: 1 to 100µm • Cement dust: 3 to 100µm • Pulverised coal: 3 to 500µm Capturing airborne dust with water sprays is most effective in areas with little air turbulence. If wind is a factor, nozzles that produce larger drop sizes are better able to resist drift. Rollback dust is a signifi cant problem and usually comes from under the dumping mechanism on front- end loaders, crushers, grinders, cutting heads and entrances to scrubbers. It may require a separate suppression system. The best solution? • Plain water systems are typically the least expensive and easiest to design and implement. • Adding surfactants to water will lower the surface tension and allow better interaction between water and certain types of dust that resist water absorption. • Foam systems use less water but usually require compressed air. • Binders agglomerate particles together after the moisture evaporates. However, binders can cause clogging and build-up on nozzles, conveyors and other equipment. Water-soluble binders can cause environmental problems should run-off occur. The advantages and disadvantages of various solutions. Available water: Water conservation is no longer optional in most areas. So it is important to specify nozzles that minimise overspray to avoid water waste. Controls may be required to ensure the system operates only when needed. Many options are available, ranging from simple solenoid valves for on/off, to sophisticated controllers that monitor a wide range of operating conditions and make automatic adjustments. Poor quality water can be problematic in many dust control applications. Strainers may be required even when using a clean water supply, because contaminants can be introduced to the water from eroding pipes. Poor quality water will also require more frequent nozzle maintenance. Spraying Systems Co has extensive downloadable literature on dust suppression at www.spray.com.au or www.spray.co.nz. by Igor Zlateski www.spray.com.au [Dust Suppression & Spray Nozzles] _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _