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Low fines crushing process,Quarry fines and waste during rock crushing process

Quarrying , and the associated processing operations , inevitably lead s to the production of quarry fines. The amount produced depend s upon the geology, the rock type quarried, the efficiency of the extraction and processing operation and the local market for quarried products. Quarry fines , defined by BS EN standards, are the inherent fraction of an aggregate passing 0.06 3 mm (63 microns) . Many quarries also refer to their (sub – economic) fine aggregate (finer than 4 mm) as quarry fines (or quarry dust). The term is used here to denote both fine aggregate and quarry fines (material < 63 microns) . Quarry fines can be considered a mixture of coarse, medium and fine sand material, a nd silt / clay (s ilt and clay are often known collectively as ‘ filler ’ ).

The taxes on waste disposal and on production of primary aggregate materials have encouraged the use of secondary materials as aggregate , but have depressed the market for quarry fines. Many quarries in the UK have large stockpiles of fines they cannot sell. Also, large demand for high – specification fine aggregate, and aggregate with specific shape characteristics, has resulte d in a n increase in fines production.

Production of quarry fines

Quarry fines are generated by processes related to blasting , pr ocessing, handling and transport ation . Particle shape , as well as grading and fines content , are a function not only of the crushing process but also of the mineral composition and texture of the rock. As a rule of thumb, a coarse – grained rock will generate fewer fines than a fine – g rained rock . This is because it takes less energy to separate individual mineral particles than it does to crush them. Also, minerals with low abrasion resistance ( i.e. softer mate rials ) will breakdown more readily than harder materials and produce more fines.

The amount of fines generated during blasting may be as high as 20% ; a dapting the blasting process to produce larger broken rock fragments can reduce the gener ation of blasti ng – related fines. However, this may lead to an increase in secondary breakage, downstream crushing costs and equ ipment maintenance costs. I f fragmentation is too great , an excess o f fines could result . Blasting tends to be optimised according to handling and crusher efficiency criteria rather than fines minimisation.

Most quarry fines are produced during the crushing, milling and screening of quarried rock to produce single – size aggregate (ranging from 20 mm to 6 mm) and other products. Crushing of quarried rock is carried out in s tages, with the primary crushing stage typically carried out using jaw crushers or gyratory crushers an d subsequent (secondary and tertiary) stages by cone or impact crushers . Fines production increases with an increase in the number of crushing stages . Multiple (three or four) stages are often used to keep th e reduction ratio at each stage relatively low. Although, t his minimises fines generation at each stage, the cumulative fines production may be higher than a process using fewer st ages with higher reduction ratios. Table 1 indicates the fines content generated at each stage of the crushing process. T he proportion of fines produced varies with the type of rock and also the type of crusher used. The figures are estimates, largely based on operator experience and gathered during a survey of quarrying operations.

Table 1: Quarry fines produced at each crushing stage

Low fines crushing process

Low fines crushing process

The type of crusher used also directly controls the amounts of fines produced. It is well known that impact crushing produces more fine s than compressive crushing. Impact crushers , such as horizontal and vertical shaft impactors, tend to produce 25 – 30% more fines than compressive crushers, such as jaw crushers and cone crushers . The type of impact applied in a vertical shaft impact crusher also influence s the pro duction of fines. Rock – on – rock interaction helps to improve particle shape and reduce crusher wear but leads to increased fines production. Rock – on – metal interaction produces fewer fines while maintaining a cubical product but leads to greater crusher wear .

In the past, quarries produced a range of single – size aggregate products up to 40 mm in size. However, the recent trend has been for highly specified aggregate , typically finer than 20 mm. I t is not unusual for material coarser than 10 mm to be stockpiled and recrushed on demand. The production of aggregate with a smaller particle – size has had a dramatic effect on the proportion of fines produced ; a 40 mm top size results in 5 to 10% fines, 20 mm top size, 15 to 20% fines and 10 mm top size, 35 to 40% fines.

Quarry fines statistics

The distribution of quarry fines stockpiles is not uniform across the country. The markets for bulk materials are usually local to urban centres and quarries in remote areas may have problems finding markets for fines. This is further complicated if there are local sources of alternative materials; such as slate waste in North Wales and china clay sand in south – west England. Fines are a particular problem in quarries producin g aggregates with a high Polished Stone Value (>55) from sands tones or certain igneous rocks.

Some areas (such as the limeston e quarries in the Mendips or igneous rock quarries in Leicestershire) produce a large volume of quarry fines due to the massive scale of the operations and there may be a substantial excess of fines. In other locations, however, there may be a local shorta ge of fines for a specific market (such as limestone fines for a block plant) and local quarries may work to increase fines production by the use of impact crushers. Alternatively, fines may be imported from other quarries or regions (where commercially vi able) or a quarry fines substitute may be used to meet a local demand.

Quarry fines are a direct conseq uence of aggregate production. H owever , in recent years fines stockpiles have been increasing and this has created a problem for many companies. The r ise in the Aggregates Levy to £1.95 per tonne will likely encourage the greater use of alternatives and these stockpiles may be set to grow ever larger. Many companies are focused on finding uses for quarry fines but as yet , this has not created a market d emand that will consume the many millions of tonnes of quarry fines being produced annually.

Quarry fines minimisation is an alternative approach; this would help to tackle the problem by reducing the amount of quarry fines produced to a more manageable a mount. The key benefits of quarry fines minimisation include a reduction in waste production, an increase in mineral resource use efficiency and an increase in the production of saleable aggregate.

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