Operational Stage

 

The planning permission for an operation is implemented when work commences at the site.
Each extraction operation involves:

Site preparation
Extraction
Handling of materials within the site
Primary and secondary processing of the mineral
Dealing with by-products
Transporting products from the site

Site preparation

Preparation of the site includes improvements to road access and securing alternative public footpaths if required; construction of the processing plant, offices and maintenance workshops; erecting secure fencing and posting warning signs at site margins; and the planting of tree screens or construction of marginal bunds (mounds) to help reduce the visual impact and noise emissions from subsequent operations.

Extraction

Aggregates are normally worked in England by surface quarrying. The quarry is commenced by carefully and separately removing soil and subsoil and storing it for later use in bunds at the margins of the working to reduce environmental impacts but also to keep it away from land to be quarried. Any unusable geological material ("overburden") lying above the economic mineral is also removed and placed in bunds or tips.

The method of extraction depends on the nature of the mineral deposit thus hard rock requires drilling and blasting; loose sand and gravel is often extracted using bucket and shovel but in a few cases may be dredged from a water-filled pit; and some deposits of intermediate strength may require selective, rather than regular, blasting. Many sand and gravel deposits are thin so the excavations are relatively shallow. Hard rock can usually be worked to much greater depths thus quarries are worked in a series of levels or "benches" going progressively deeper. The direction of working is influenced by the need to maintain stability of faces and by haulage distance from the plant.

Handling of materials within the site

Excavated material is commonly lifted by front-end loaders at the face and then placed into trucks to be carried to the processing plant but at some workings may carried directly to the processing plant. Designated haulage roads are included in the design of the quarry and may be relocated from time to time as working of the site progresses. At some operations field conveyors may be used to take mineral from the working face to the processing plant, reducing the use of lorries.

Primary and secondary processing

Aggregates used for various purposes have to meet specifications that define the required mix of grain sizes and the absence of any deleterious materials. Therefore the key steps in primary processing are to:

• crush the material, if necessary in one or two stages;
• sort the material using a set of large industrial "sieves" (a step known as "grading") to separate out the constituent grain sizes so that these can be selectively re-mixed in the correct proportions for each intended use of the material; and
• remove material that is too fine grained and any deleterious materials.

However some material is sold without significant processing if it is to be used as low quality construction fill.

Much of the aggregate that is produced is used in products such as ready-mixed concrete, coated in asphalt for use in roads and other surfaces and concrete blocks. Special plant is need for this secondary processing which may be located at the quarry or elsewhere.

Dealing with by-products

In addition to overburden stored before extraction, by-products arise during extraction and processing. Some of these
can be sold but others must be stored on the site. These consist of:

• unsuitable materials found within, or between layers of, the usable mineral deposit which are placed in tips; and
• fine grained material produced during processing which is placed in ponds often called "lagoons".

The construction of tips and lagoons must be undertaken carefully and these structures have to be checked regularly, and maintained if necessary, to ensure that they are effective and safe.

Transporting products from the site

Products are carried to other locations for direct use or for manufacturing of value-added products. This is mainly done within 40, or so, kms of the quarry using heavy lorries, although some value added products such as coated stone are carried further by road. However there are environmental and economic advantages in using bulk transport for heavy materials over longer distances. Some quarries are rail linked and a few have access to marine or river wharfs. However the amount carried by rail or water is limited by the overall capacities of railways and wharves for handling freight.

Lorries may be driven by employees of the mineral operator but, often, are run by independent contract drivers, however the mineral operator remains responsible for ensuring that planning conditions and agreement concerning transport (e.g. agreed lorry routes keeping traffic off narrow roads) are observed.

 

Continued with Post-operationl Stage

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