Health & Safety including Dust Control
LEFT: Aggregate silo with filtered inlet, for vehicle loading bays
Much of the nuisance dust from mineral processing machines may be described as “fugitive”; that is, from uncontrolled sources. The dust extraction hoods apparently work well, but areas around machines nevertheless remain randomly dusty. Spillage from screens, unevenly loaded conveyors, loading and bagging points can all give rise
to fugitive emissions, but careful tuning of machines and attention to flow rates can minimise this effect. For example, screens for progressively grading particle sizes may be fitted with enclosure typehoods, with further dust hoods at the discharge spouts. The amplitude and frequency of vibration should be set to the minimum to cope with specified flow rates. Higher vibration amplitudes may increase screening efficiency, but at the expense of creating more airborne dust and particularly adding significantly to noise levels. If the plant is housed in a building, it could be beneficial to add forced ventilation to the enclosed area, in this case maintaining a slight negative pressure within the building, the airflow for this provided by the centralised dust collector to prevent blowing dusty air outside.
Many secondary processes are situated at quarry sites. However, hot mix asphalt production, for example, requires dedicated filtered extraction principally to ventilate directly fired aggregate driers with a smaller extraction requirement from mixers. The moist, dust laden extracted air/gas would typically be passed through a skimmer type pre-separator to remove larger particulate, which would be returned to the mixer. The dust laden air/gas at about 120ºC would then be passed through a bag filter fitted with filter material selected to be resistant to hydrolysis. As the driers are normally directly fired, the air/gas passing into the filter would contain a high concentration of flue gases, with unburnt hydrocarbons, so the filter unit would be thermally insulated to prevent damaging internal condensation. The discharged dust would also be reprocessed for use as filler material.
Mineral extraction may be associated with on-site production facilities such as clay preparation for brick making, cement manufacture or asphalt production. In such cases, with the exception of asphalt mixing, a centralised dust extraction plant could be economic, to serve material crushing, grading, screening and conveying adjacent to the production plant if a compact layout were possible.
Where brick making or cement manufacture is associated with quarry workings, specialist dust extraction solutions would be applied, including for example higher temperature extraction to ventilate brick kilns and to handle cement clinker or ventilate ball mills.
Sound advice
Noise in dust extraction systems is principally generated by the extraction fans. However, potentially annoying lower frequency noise may emanate from the filter body and hopper arising from
40 Solids & Bulk Handling • May 2010
www.solidsandbulk.co.uk
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