Future Technology and Practices
Future production trends of the UK quarrying industry will be guided by economic and legislative developments with increasing emphasis on energy and
water consumption,
recycling and waste generation and disposal issues.
Climate change is a key driver; it is likely to have a significant direct and indirect impact on the
aggregate industry. Current scenarios indicate that average global temperatures will continue to increase. In Britain results could include more frequent and severe heat waves, drier, warmer summers, milder, wetter winters, and more extreme weather events including extreme rainfall. Direct impacts are therefore likely to include water shortages and/or water excesses. However, in some respects it is the indirect impacts that are likely to be more significant. The strategic response to greenhouse gas emissions and climate change will drive the agenda for energy supply and consumption and also the management of water supplies for the foreseeable future. As the quarrying industry is a major user of energy and water, voluntary and regulated responses to climate change will affect its consumption of energy and water.
Carbon neutrality and offsets
Growing awareness of the risks of climate change has propelled national and local governments, companies and NGOs to take action to manage greenhouse gas (GHG) emissions, including the introduction of carbon offsets and tradable quotas or ‘caps’. At an international level, the main instruments driving the market in GHG emissions are the Kyoto Protocol of the United Nations Framework Convention on Climate Change, which has been ratified to date by 163 countries, and the Emissions Trading Scheme of the European Union.
Carbon brokers such as CO2e (www.co2e.com), EcoSecurities (www.ecosecurities.com) and Natsource (www.natsource.com) are also becoming increasingly prominent as trading via GHG-related exchanges such as the Chicago Climate Exchange (www.chicagoclimatex.com) and Greenhouse Gas Exchange (www.ghgx.org) continues to increase.
Aggregates and carbon trading
The energy used to produce a tonne of aggregate is equivalent to approximately 10kg of CO2; the current cost of offsetting this CO2 is 7 Euro cents or 4 pence (based on the current carbon exchange trading price for a tonne of CO2 which is around €7 or £4.70). Offsetting typically involves tree planting, occasionally in old quarries; one tonne of aggregate would require the planting of one tree. Based on the annual production of aggregate in the UK this would require the planting of 281 million new trees. The CO2 figures are purely based on production and do not consider transportation or ultimate enduse.
Currently, assessment tools used to determine CO2 emissions assume that the same amount of energy is used to produce both primary and recycled aggregate. However, it is likely that more detailed information arising from life cycle inventories and assessments may change this in favour of recycled and secondary aggregate. This will probably mean that there will be increased pressure to reduce the environmental impact of primary aggregate production; one result of this may simply be an increase in the substitution of primary aggregate for recycled and secondary aggregate. What can the UK quarrying industry do to address this situation? How long before the first carbon neutral quarry?
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Energy consumption
Reducing energy consumption has always been a key target for those involved in production; however the main motivation has been controlling costs rather than addressing green issues. There are a number of energy strategies that could be applied in the future:
- Energy efficiency : One means of assessing the amount of energy used to produce aggregate is to determine the embodied energy (or embodied CO2); this refers to the quantity of energy (or CO2 ) required to produce and transport aggregate. Eco-friendly construction projects select the most energy efficient construction materials by auditing embodied energy. The production of aggregate requires 6 to 139 kWh of energy per tonne;
ready mixed concrete, 278 kWh/tonne (largely due to the high energy costs of cement production) and recycled aggregate, 28 to 111 kWh/tonne. The figure for recycled aggregate is surprisingly high but probably reflects the proportion of cement present. It is possible that the embodied energy will become one of the important criteria for future aggregate production; especially as concern over
climate change is one of the key drivers behind the sustainable development ethos of the mineral planning system.
- Energy reduction: Heat-assisted
comminution may be a future means of reducing the energy costs associated with aggregate production. Shock heating and rapid cooling of as quarried rock would promote rock fracturing prior to
crushing. Less energy would be needed to crush the rock; this may also help to reduce the amount of
fines produced. The main drawback is the amount of energy required to heat the rock; cheaper and more efficient methods of heating are needed to make this process viable. Microwave or ultrasound treatment may be a possible way forward.
- Energy avoidance : The use of subsurface grouting to minimise groundwater flow through the rock mass that is, or will be, extracted. This reduces the need to abstract groundwater to keep the workings dry, which may be a substantial energy consuming activity and cost at some sites.
Water consumption
A recent review by the Parliamentary Office of Science and Innovation noted that in most areas of England and Wales, the balance between water users and the environment is currently sustainable (www.parliament.uk/documents/upload/postpn259.pdf). However, in some locations, this is not the case, and excessive use of water is putting water-based ecosystems at risk. While this is not solely an issue for the
aggregate sector, quarrying activities can use and consume significant amounts of water across the operational lifecycle, from initial exploratory investigations through to closure and beyond. For further details on washing plants
click here. Consequently, finding new ways to minimise
water use in general and water consumption in particular will become increasingly important in the short- and medium-term.
As noted elsewhere, there is an ongoing drive to reduce overall water consumption through improved recycling and
reuse and implementation of
good practice. The uptake of low water use / high efficiency washing systems and screens is also increasing across the sector. However, there are a number of additional options for the future, ranging from changes in practice to the use of different technologies. Examples include:
- The capture and storage of clean water entering the site as rainfall can be improved (e.g. capturing water directly from ‘clean’ surfaces such as plant roofs before it can become contaminated with particulates on the ground).
- ‘Pinch analysis’ can be applied to minimise water use and consumption. This is a systematic process analysis tool, originally designed for energy applications, but now extended to the optimisation of water network design and water treatment, recycling and reuse. Pinch analysis allows a user to benchmark their actual water consumption against a theoretical minimum. It can then be used to identify opportunities to save water and move the operation closer to its minimum achievable consumption. Pinch analysis for minimisation of water consumption is being used increasingly in the petrochemical, paper, textile and food industries, but as yet there appears to be little uptake in the aggregates sector (for an example of a case study in the chemicals sector, see
www.envirowise.gov.uk/page.aspx?o=119526).
- Due to the pressure on water resources and the advent of time-limited
abstraction licences it is possible that dry alternatives for fines removal from
sand and
gravel will be adopted. The current perception is that this will be expensive due to the high costs of drying; however, development of high efficiency drying technology may make dry processing a reality in the future.
- Inland or marine brackish/saline waters can be treated and used as a replacement for freshwater supplies.
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Recycling and quarry waste
Aggregates, including
sand,
gravel and
crushed rock, account for 80% of a typical concrete mix; the concrete industry is actively pursuing a policy of recycling concrete in order to reduce the use of these natural resources. Some construction companies are achieving a recycling rate of 70–90% of concrete from their waste streams returning as aggregate (www.sitelines.co.uk/pdfs/22103.pdf).
In many manufacturing industries, products are designed for recycling. The production of
aggregate, and construction products, could possibly be modified to enable easier recycling and improve the properties of recycled aggregate. Aggregate composed of recycled concrete generally has a lower bulk density and higher absorption than natural aggregate; concrete made with recycled aggregate has at least two-thirds the
compressive strength of natural aggregate concrete. This is because recycled aggregate consists of composite particles of natural aggregate and cement. Separation of the cement from the natural aggregate would enable production of a recycled aggregate with properties much closer to those of natural aggregate; the cement could be separated and used as a sand substitute. In the future, it is possible that the aggregate crushing process could be modified to produce natural aggregate that is easier to remove from concrete, has more uniform properties and can be used to produce concrete with properties similar to that made with natural aggregate. Surface modification of aggregate, using biological or polymer coatings, may be a possible way forward. By extension, modification of
quarry fines and other
quarry wastes using physical, chemical or biological methods to produce value-added materials for sale in specialist and bulk commodity markets might be possible.
Future trends in crusher development
Modern
crushers have benefited from a better understanding of the
feed characteristics, machine geometry, crushing chamber design, the relationship between power draw and crushing force, speed of operation and lubrication/ hydraulic system conditions357. Future developments of crushing technology will be driven by the industry focus on:
- higher productivity at reduced costs per tonne (increased profitability)
- higher size
reduction ratios
- reduced stock inventory and ‘just in time’ supply
- improved reliability and availability of plant.
Current tends that will continue into the future include:
- Crusher automation: This can lead to an increase in throughput (up to 30%) compared to manual control. The use of hydraulically activated setting mechanisms allows crushers to be easily and simply integrated into partially or fully automated systems. Automation ensures that the crusher always operates within ideal parameters, promoting the constant
choke-feeding condition that improves liner utilisation and inter-particle crushing required for good particle shape (i.e. cubical shape).
- In-pit crushing: This is already well established in the UK quarrying industry. The use of highly manoeuvrable self-propelled track-mounted crushing and screening plants has reduced, and in some cases eliminated, the need for haulage. This trend will continue and new
mobile plant will be developed.
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Cone crushers: These will become smaller, quieter and more energy efficient.
- ‘Smart’ crushers and screens: This equipment will become more common and performance and condition monitoring will be conducted automatically with data fed back to the operator or even to the equipment manufacturer for routine maintenance or problem solving at a distance.
- Control and instrumentation: Particle-size analysers will determine the size distribution and mass of a material stream, this will be used to control the crusher settings in real time
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Future trends in drying technology
Heating and
drying accounts for between 10 and 25% of industrial energy consumption in developed economies; therefore high-efficiency technologies would make a significant contribution to reducing energy consumption and cutting CO2 emissions. Conventional drying mainly uses rotary convective technologies, which have a relatively low thermal efficiency. Potential high-efficiency drying technology includes the following:
- Microwave heating: This has the advantage that it enables uniform drying, requires less heating time (as low as 1% of that required by conventional heating) and microwave energy is selectively absorbed by areas with greater moisture content.
- Pulse combustion drying: This involves intermittent combustion of the drier fuel; this process is up to 40% more efficient than conventional dryers. Currently, this technology appears to be restricted to spray dryers, for example those produced by Pulse Combustion Systems (www.pulsedry.com); however there may be potential for this to be used for a wider range of materials in the future.
- Insulated dryers: These have a high thermal efficiency but a low capacity throughput; development of this technology would enable an efficient and relatively cheap form of drying.
Blue sky concepts
- ‘Centre for Sustainable Aggregates’: This could be a central research facility co-sponsored by aggregate industry, technology and service providers and other relevant bodies to specifically develop new and innovative technology and management practices (e.g. taking on work that one company or provider would be unable to fund or undertake in isolation). The facility could also have a ‘cross-pollination’ function, assessing and developing relevant technology and concepts used in other sectors. The International Centre for Aggregate Research (ICAR) based at the University of Texas in the USA is one model that could be followed (www.icar.utexas.edu); alongside university courses, it coordinates research projects, facilitates technology transfer and provides access to information on aggregates technology.
- Centralised processing: Environmental and social issues, in particular the growing pressure on water availability, are potential drivers of a radical change in how extraction and processing of aggregates are interlinked. The present standard approach is to process extracted aggregates on-site. However, local and regional water shortages, environmental and social issues and economics may promote a move towards centralised processing in some cases (with the most likely scenario being centralised processing of material from two or more extraction sites operated by the same company). There are still, of course, substantial environmental issues in following this approach, not least of which would be transportation of material to and from the central processing hub. However, the concept may be worthy of an initial scoping study and preliminary economic and environmental cost-benefit analysis for a range of simulated scenarios.
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