Introduction to Quarries
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Session 2.1: Introduction
This session gives an introduction to different types of quarrying and processes involved. The first activity is a video which describes a large granite quarry in the UK and briefly introduces the various processes involved. A more detailed description is then given of the different types of operation, focussing on hard rock aggregates quarries, and with a definition of what actually makes an aggregate material.
Activity 2.1.1 Seeing inside a quarry
Lafarge-Tarmac’s Mountsorrel Quarry – the largest Granite quarry in Europe.
The sound quality of this YouTube video is not excellent, but you should still be able to hear the commentary and understand the various descriptions.
Since this video was produced, Lafarge have merged with Tarmac to form Lafarge-Tarmac.
Activity 2.1.2 The elements of quarrying
What is a quarry?
Quarries are places where materials are extracted from the surface of the earth. They usually refer to operations where the material is to be used in construction, whereas open-pit mining often refers to metal mines and opencast mining usually refers to coal. Sub-surface mines consist of underground tunnels or shafts and most of these relate to metals or coal, although construction materials are occasionally extracted from underground, and may sometimes be referred to as underground quarries (just to confuse the issue).
A large hard rock quarry
Some quarries produce rock which is extracted or cut into large blocks (e.g. Portland limestone) or to be cut into slices to form “cladding” on the front of buildings (e.g. granite or marble). In these quarries there is unlikely to be a blasting, and certainly no crushing and screening, so these operations have not been considered in the EE-Quarry project.
The processes used in quarrying
The processes used to recover the material will depend on the characteristics of the deposit. Very often, it will be covered with soils and other material known as overburden, which is not suitable for use as aggregate. This has to be removed and stored for later use in the restoration process once the extraction has finished.
Hard rock deposits will usually have to be broken out of the ground using explosives, although some weaker rocks can be excavated directly into dump trucks or broken using a “ripper” attachment on an excavator or on the back of a dozer. The objective is to fragment the rock to a size suitable for loading and transporting to the plant for further processing, and to do it in a way which is safe and minimises environmental impacts.
Blast in a limestone quarry
Once the material has been removed from the ground, it has to be transported to the processing plant via a conveyor belt or vehicle. The increased usage of modular, mobile processing equipment has meant that some or all of the required processing and loading of vehicles for delivery may now take place within the extraction area itself. This equipment is able to follow the extraction area as it develops within a quarry, but can also be moved from quarry to quarry fairly easily if required. However, a lot of quarries, particularly the larger ones, will have static fixed plant away from the extraction area, where the raw feed material is processed to produce the products required for the market.
A large excavator loading a dump truck in a granite quarry
It is a combination of the nature of the raw material and the requirements of the market that determines the type of processing that is required. The main elements of aggregate processing are breaking the rock into smaller sizes or fractions (if that is required), and then separating the rock fragments into the different size particles required by the customer. These sizes can range from metres, e.g. large blocks for coastal defences, which would not go through the crushing process, to fractions of a millimetre where the particles may have gone through a multi-stage crushing process to produce the right size (and shape) of particle. Generally, particles above 4 mm are referred to as gravels, and particles between 4 mm and 0.075 mm are referred to as sand. The silts and clays below this size are usually considered to be waste products with little commercial value.
A crusher (HSI) not installed and opened up to see the blow which rotate and hit the rock to break it
Of course the nature of the particles of a similar size can vary in terms of their relative shape and dimensions, and this may affect their suitability for different end uses. Sand-sized particles can either be obtained by separating that fraction from a natural sand and gravel deposit, or it can be manufactured by crushing larger particles down to the appropriate size. The shape of the rock particles will be partly determined by the properties of that particular rock type, but also by the type of crushing used which can be based on compression forces or impact forces.
The separation of the gravel fractions usually takes place using a series of screens (large sieves) on which particles either pass through or are retained. The different fractions can then be sold directly or blended to obtain a specific particle size distribution (see Activity 2.1.3). The screening often takes place dry, but water can be used to clean the aggregate on the screens if required.
Screen used for separating different sized particles
Where water is almost always required is in the production of sand, where a wide variety of processes and equipment use water to separate the sand fraction from the finer silt and clay particles.
One type of sand plant at a sand & gravel quarry
Activity 2.1.3 What are aggregates?
By far the highest volume of building material extracted is known as aggregate, and the definition of rock particles as aggregates is based on their purpose or function, rather than the rock’s classification by type, age or origin. Put simply, they are granular materials which form part or the whole of a building or engineering structure. As we saw in Session 1.2, this can be as an unbound material, or it can be mixed with other materials known as binders, which then forms concrete or asphalt.
A wide variety of rock types are suitable for use as aggregates, including hard rocks of metamorphic, igneous and sedimentary origin which need to be blasted and crushed to the appropriate sizes or fractions. Quarries producing other types of material such as cement, chalk or gypsum (some of which have uses outside construction) may also require fragmentation of the rock, and so these are all relevant to the EE-Quarry project.
Natural sand and gravel deposits, which often occur as unconsolidated or loose material associated with ancient or recent river systems, can also be a good source of aggregate material. Some countries in Europe recover deposits from offshore coastal waters using dredgers, but most deposits are operated on land. They usually don’t require any blasting, and the amount of crushing required is often small, although that will depend on the size of the particles in the deposit, and the size required by the local market. Although deposit assessment was part of the EE-Quarry project, and is extremely important in optimising product in sand and gravel operations, it was not possible to include them in the overall assessment of energy efficiency.
A thin sand and gravel deposit
Not all rocks are suitable for use as aggregates, as they must possess certain physical and chemical properties. For example, the Pennant sandstone in South Wales can make an excellent top surface material for roads as it has a high polished stone value which resists wear. But many sandstones are totally unsuitable for use as aggregate, because they are too soft and readily break down.
Over the last 20 years, recycled (previously used) and secondary aggregates (by-products of other mineral or industrial processes) have become increasingly important as a replacement for primary aggregates which is material extracted for use as aggregate for the first time. In 2012, nearly 10% of European aggregates were provided by recycled or secondary materials, and in the UK this reached 29%.
Different rock types will produce aggregates with different properties, which will be suitable for a range of different applications. But very often a range (or mixture) of sizes are also required, which is why crushing and screening are so important in the aggregates industry.