Washing plants are commonly used by sand and gravel operations to produce clean aggregate;udthey are effective in removing fines (silt and clay) but use a large volume of water. Newudregulations controlling the abstraction of water require quarry operators to demonstrate ‘waterudefficient’ practice as part of the abstraction license renewal process. The use of waterlessudtechnologies for removing fines would go a long way to satisfying this requirement; airudclassification is one such technology. This report details the findings of air classification trialsudcarried out to determine the technical feasibility of using a waterless method for producing cleanudaggregate. This work forms part of the ‘Waterless Fines Removal’ project funded by theudAggregate Levy Sustainability Fund (ALSF).udFour samples of sand were collected from sand and gravel operations (these will remainudanonymous) for laboratory and pilot-scale air classification trials. The laboratory-scale trialsudwere carried out using a Hosokawa Alpine zig-zag air classifier and the pilot-scale trials using audBradley Pulverizer Windsifter.udThe proportion of fines removed by air classification from two of the sites was poor, rangingudfrom 12% to 29% (average of 18%); the main factor causing the lower fines removal figures wasudinsufficient disaggregation of fines during the separation. This highlights one of the advantagesudof using wet processes; the fines are fully dispersed before separation and they are easier toudremove. The other two sites had fines removals ranging from 33% to 84% (average of 58%); thisudis comparable with the fines removal achieved by air classifiers used in other parts of the world.udThere are apparently no recorded cases of quarries in the UK using air classifiers as part of theudfine aggregate production process. There are a few quarries across the world, mainly in dry partsudof the USA, where air classifiers are used to produce manufactured sand; in these cases finesudremoval is typically 50 to 65%.udIn the future, it is possible that crushed rock quarries may adopt air classification to processudquarry fines (material finer than 4mm); this would be especially attractive to limestoneudoperations for production of a saleable fine aggregate and a fine-grained by-product (whichudcould find use as agricultural lime or as mineral filler). It seems unlikely that air classificationudwill be adopted by UK sand and gravel operations in the near future; however given the pressureudon water resources and technological improvements it may be adopted in the longer term.
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