For years sand quarries have used wet wash systems to remove minus 200 mesh (75 micron) fines in sand quarries. Wet systems can include dewatering screens, belt presses, hydrocyclones and wash screws. One thing that they have in common is the need for water, a commodity that is becoming increasingly expensive or impossible to obtain.
An alternative to wet washing quarried stone is the Sturtevant whirlwind air classifier. As the name states, it uses air instead of water as the method of removing the fine powder from the granular sand, so that it can be successfully sold for asphalt or concrete.
Sturtevant Incorporated developed the whirlwind air classifier in the early 1900s, but it was never intended for use in quarries, where the dedusted crushed stone is the main product and the fines are undesirable waste. The whirlwind air classifier was actually designed for closed-circuit milling in the cement industry, where the fine discharge is the main product and the coarse discharge is recycled back to the mill for additional grinding.
Later, the aggregate industry and Sturtevant realised that if the whirlwind air classifier was used in an open circuit, the coarse discharge could be recovered as the dust-free main product. Hence, a new use for the whirlwind air classifier was born, giving many sand producers a choice between wet washing and dry washing.
Quarried stone is usually mined from open pits of naturally occurring rock, such as limestone, basalt, granite, trap rock, sandstone, diabase rock and gabbro stone. After blasting and transport to the processing area, where it is sometimes stockpiled in preparation for crushing, a primary crusher, like a jaw crusher, is used to make the stone manageable for the next sizing steps. After the final crushing is done using a crusher such as a cone crusher, gyratory crusher, or vertical shaft impact crusher, dry screens are used to separate the desired gradations of various stone products. The smallest particles that pass through all the sizing screens are referred to as ?screenings?, having a top size of 0.45cm to 8 mesh but containing significant quantities of minus 200 mesh fines ranging between 10 and 20 per cent.
The crushing process generates an excess amount of minus 200 mesh (75 micron) fines that are undesirable when used in asphalt or concrete products. While some fines are acceptable, the majority must be removed in order for the ?screenings? to be sold as a viable product. A sand producer must then choose between a wet or dry process to remove the fines.
If the stone contains naturally occurring clays or surface moisture that is greater than two to three per cent, the wet system will most likely be necessary. However, if the stone is fairly dry, the whirlwind air classifier provides a great alternative that does not require any water or the complications that come with a wet wash system. To prevent adding moisture to the crushed stone prior to the air classifier, it is very important to limit the amount of water that is used in the quarry for dust control. This can be accomplished by covering conveyors and by using low water- consuming fog mist systems.
The whirlwind air classifier is a compact machine that requires simple support legs, feed and discharge conveyors. It does not require the settling ponds that accompany wet wash systems. It has less land requirement and no pond management issues.
The feed is conveyed into the air separator by gravity. The fines and coarse also exit the air classifier by gravity. Major changes in product fineness are achieved by changing the quantity of internal selector blades. Since the fines removed from the crushed stone are dry, they can also be sold for aglime or filler for various markets.
The whirlwind air classifier incorporates a speed self-contained fan and rejecter blade classification system. Its internal fan design uses low wattage and does not require cyclones, airlocks or baghouse to collect the fines. It is available in 14 sizes, from a feed rate of one tonne per hour at 3.6kW to 800 tonnes per hour at 588kW.
While all mechanical machines processing crushed stone will have wearing parts, whirlwind air classifiers operate at very low speeds (130-300 RPM) for internal pneumatic conveying of the fines, so their maintenance is not as severe as high speed crushers or mills. Whirlwind air classifiers are manufactured with a robust design and the replaceable internal wear protection liners are selected according to the abrasiveness of the stone that will be processed.
Water expense and shortages is becoming a worldwide issue. Sturtevant has installed hundreds of whirlwind air classifiers in US quarries, but there is growing interest in dry air classifier systems from many countries, such as Australia, Canada, Mexico, India, Guam, Cyprus, Europe and the Middle East.
Source: Sturtevant Inc