Waterless fines removal : technical feasibility of using air classification to separate fines from sand and gravel

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