Waste

 

Quarry Waste - Introduction

At almost every quarry, the material that must be excavated includes, in addition to the mineral or mineral products that are sold or otherwise exported from the quarry, materials for which there is no market and that remain on site either temporarily or permanently. These materials are collectively defined in European Law as "extractive waste" (Directive 2006/12/EC ). 466

Directive (2006/21/EC) on the management of waste from extractive industries (normally referred to as the Mine Waste Directive ("MWD")) 467 is intended to "lay down minimum requirements in order to prevent or reduce as far as possible any adverse effects on the environment or on human health which are brought about as a result of the management of waste from the extractive industries…".

The requirements of the MWD only apply to extractive waste that is managed in a 'waste facility' and generally excludes materials that are returned to the excavation void for the purpose of restoration.

The MWD has been transposed into law in England and Wales via the Waste Permitting (England and Wales) Regulations 2010 L0296; the competent authority is the Environment Agency. The Environment Agency uses the term "extractive materials" to refer to all non-mineral residues of quarrying and open pit mining. These extractive materials are only defined as "waste" falling to be regulated under the 2010 regulations (i.e. requiring a permit) if they are managed in a 'waste facility'.

The Agency has issued guidance jointly with the CBI Minerals Group that sets out how it is to be decided as to whether extractive material requires a permit.

The residues from aggregate quarrying have traditionally been referred to collectively in England and Wales as 'quarry waste' but, since the transposition of the MWD in 2010, they are referred to as 'extractive materials'.

The terms 'quarry by-products' (www.sustainableaggregates.com) or 'non-waste by-products' (MWD) are used to distinguish these non-soil, non-mineral materials (i.e. extractive residues) that remain on site permanently, when they are (or will be) put to some beneficial use (including the complete or partial backfilling of quarry voids and the improvement of soils or creation of new soils).

Extractive materials arising from aggregates quarrying may be categorised as follows:

• Organic soils
  • It is normally a requirement of quarry planning permissions that organic soils (topsoil and subsoil) are preserved and replaced in restoration, and some other (inert) extractive materials can be used as soil forming or improving materials.
• Extractive materials that are separately excavated
  • Overburden (non-mineral materials occurring above the mineral, typically comprising superficial materials such as glacial deposits, alluvium, etc, but may also include unsuitable rock occurring above the target mineral deposit);
  • Interburden (non-mineral materials occurring within the mineral deposit, such as clay beds in a limestone deposit or silt lenses within a gravel deposit);
  • Underburden (occasionally, for geotechnical reasons or to ensure safe access to part of a site, excavation takes place below the mineral deposit).
• Extractive materials that arise from rejection of mineral at the face, from crushing and/or from dry screening
  • Material may be rejected at the excavation face if it can be visually identified (or otherwise marked out on site). The reasons for such rejection may include contamination by clay (e.g. in limestone deposits affected by karstic weathering, or in fault zones) or material may be unsuitable in terms of its physical or chemical properties (e.g. a sandy or silty zone in a gravel deposit which is uneconomic to work, or a zone of altered or weathered rock that cannot yield aggregate of target quality).
  • Material may be rejected at any point in a crushing and screening process if there is no market for the particular size fraction produced. Typically this applies to 'quarry dust' (material 5mm and finer), for which the market is often limited and/or of no value. Less often, shape may be a factor leading to rejection of processed material.
• Extractive materials that arise from wet processing or management of surface runoff
  • Wet screening or washing of aggregates (whether crushed rock aggregates or sands and gravels) gives rise to fine material (fine sand, silt and clay) in suspension. These materials may settle under gravity from the wash water in lagoons constructed for the purpose (with or without the use of chemical additives that promote 'clumping' of the fine particles to create coarser ones) or, less commonly, may be removed mechanically using filter presses. Where settlement is used to recover suspended solids, the resultant residue typically has a high water content, whilst filter cake can be quite dry. The water from which suspended solids is removed is generally re-circulated in the processing plant.
  • Quarry runoff during rainfall events often picks up suspended solids as it flows over the areas of bare rock or earth that characterise quarry sites. Before discharge into watercourses, such runoff is passed through attenuation and settlement facilities to attenuate storm discharges and ensure that the quality of the water (with respect to suspended solids) is within acceptable limits for the receiving water. Silt accumulates in settlement structures as a further source of extractive material with a high moisture content.

All of the extractive materials that will remain on site (including soils) must be temporarily stored or placed permanently in structures (tips, lagoons or backfill) designed to accommodate the volumes that will arise and to be stable. Progressive restoration is achieved by placing soils, overburden and other extractive materials arising from the excavation and processing of material in one phase directly into excavations completed later. Even where this is possible (e.g. in many sand and gravel operations), soils and overburden stripped from the initial phases usually has to be stored temporarily outside the excavation until there is space to accommodate it within the excavation.

Such temporary tips are often used for screening the operation whilst it is active and removed at the end and placed in the final void. In some deep hard rock quarries where progressive restoration is not an option, all of the soils, overburden and other extractive material arising has to be stored outside the excavation in permanent tips which would normally be landscaped and planted and perform a permanent modification to the landscape around the perimeter of the quarry excavation.

With the exception of gravity silt lagoons, where the interpretation of whether these are extractive waste facilities or not remains in dispute at the time of writing, the implementation of the 2010 regulations effectively exclude all of the temporary and permanent facilities for the storage of the extractive materials listed above for aggregates quarries (see CBI Minerals Group guidance note July 2010).

 

Continued with Pre-Operation.

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