Fine grading - aggregates in Mountsorrel
Michael Forrest talks to Ian Brown, National Production Manager of Lafarge Aggregates, about the ongoing rock supply found at the long-serving Mountsorrel Quarry, in Leicestershire, UK.
Aggregates are one of the lowest cost mined materials in the UK, and one of the most used, supporting the built environment. Production of aggregates in the UK in 2006, according to the British Geological Survey, was over 240Mt, although today this is 35% lower due to the recession. Sixty per cent is from hard rock, the remainder are sands and gravels from alluvial and marine resources. Hard rock has an advantage in consistency, and in known parameters in terms of compressive strength and chemical homogeneity.
Mine gate prices are not excessive at around £10 per tonne, depending on market demand. This leaves little margin, meaning aggregate production has to be efficient to support the capital and investment cost of mining.
A few multinational companies, including the established French company Lafarge, dominate production in the UK. Their Mountsorrel operation, 10 miles south of Loughborough, UK, is based on an Ordovician-aged (450Ma) hornblende granodiorite, which, at the time of intrusion, formed part of an island arc later to be buried by Mesozoic sediments. Today, the Mountsorrel granodiorite outcrops in an area underlain by Triassic desert sediments, and marls and forms a topographic high adjacent to the Soar Valley, Leicestershire, UK.
The rock has been quarried since Roman times, and on an industrial scale since the mid 1800s, where transport links made it ideally located to supply the industrial Midlands. The first large-scale link was via the Grand Union canal where a series of locks allowed barge loading. By 1860, the Great Central Railway and the London North Eastern Railway provided rail links within five kilometres, from which a light rail line was run directly into the quarry.
‘This location has allowed Mountsorrel to prosper through all economic cycles. The price of aggregates is heavily dependent on bulk transport costs and Mountsorrel is and has been well served,’ says Ian Brown of Lafarge Aggregates, Leicestershire, UK. ‘It is also directly on the “stone line”, that geological trend which separates the softer rocks, clays and sands to the south and east from the harder and older rocks to the north and west, and provides the nearest hard rock source to the capital.’
The Mountsorrel Granite Company began quarrying here in 1854. Its main products were granite setts, kerbs and head stones, transported via rail to the main line across the Soar river and floodplain, along a brick viaduct which is now a listed building.
By 1900, 600 workers in four sites within the quarry footprint produced over 250,000t of aggregates. However, the 1970’s saw the largest expansion and set the framework for the present day operation. During that period, the Buddon Wood quarry was developed with a capacity of 2.5Mt per year. Concurrent with the expansion was the installation of what was the world’s largest primary gyratory crusher, which is still in operation today. ‘These developments laid down the plan of operation that is still relevant today,’ states Brown.
Lithologically, the Mountsorrel granodiorite is a homogeneous rock that is near surface over an area of one kilometre squared. Constraints on long-term expansion are man-made – the Swithland Reservoir to the west, expensive housing to the north and south, and the River Soar to the east. Nevertheless, says Brown, ‘some 125Mt has been mined with another 90Mt in measured reserves. The maximum production occurred in 1989 when projects such as the M25 were in full swing. The quarry then produced close to 6.5Mt, well above today’s production of around 4.5Mt per year’.
The production method has changed little since the 1970’s, a reflection on the excellence of the original flow sheet rather than a lack of investment.
Heavy duty operation
Quarrying is achieved through cyclical drill and blast with loading and trucking to the primary crusher. Two rigs operate an Atlas Copco L8 DTH equipped drill and new top hammer L7 Coprod unit. Although DTH is favoured in UK quarrying, the top hammer drill is actually more fuel efficient and trials are being conducted to ascertain its long-term effectiveness. One of the challenges is the highly abrasive nature of the rock.
‘In most blast hole drilling, one could expect several thousand metres before bit replacement. At Mountsorrel we replace or re-sharpen bits every 300m, which with 15m holes gives only 20 holes. Blast holes are charged with bulk emulsion explosives supplied directly by EPC and fired by non-electric detonators.’
‘We are concerned with optimising fragmentation without disturbing our neighbours through excessive vibration. We have four monitoring sites around the quarry to measure blasting activities,’ adds Brown.
Post-blasting two O&K RH120E, 15m3 bucket, 150-litre diesel/hr excavators load eight 100t Caterpillar and Komatsu trucks that haul the rock to the crusher. The loader, through dropping a five-tonne steel ball on the larger boulders whilst the trucks are circulating, breaks oversized blocks. A five-day 11-hour shift work pattern is maintained, with essential maintenance completed on Saturdays. Bucket teeth wear, as well as diesel, is a significant cost.
The primary crusher is a Nordberg 60x102 gyratory, installed in 1974 it has a capacity in excess of the quarry production of 20,000t per day. The machine can take up rocks of up to 1,000mm, reducing them to -300mm. The reduction in production of fines is a constant concern and one shared by the Mineral Industry Sustainable Technology (MIST) programme, managed by the Mineral Industry Research Organisation, Solihull, UK. The scheme has the objective to reduce the 20Mt of quarry fines produced in the UK, annually. Lafarge has taken the objectives on board and has with sophisticated materials handling reduced their production to a minimum.
The Nordberg crusher has a centre cone of 90t that revolves in an eccentric motion trapping rock against the armoured manganese steel lining. Recent developments in foundry technology have enabled Lafarge to double the life of crusher wear items to two years.
The most sophisticated part of the operation is the screening and secondary crushing. The market for aggregates has evolved and customers demand tailor-made product at a mass-market price. To reach that objective Lafarge has built on past infrastructure and now operates computer-controlled secondary crushing and screening plants that can advance or recycle material until the correct size and tonnages are achieved. Belt weighing equipment and flow rates, all monitored by instrumentation and CCTV, allow ‘packets’ of aggregate to be directed to a loading system for trucks or trains.
By constant adjustment of flow through the system, a batch of 30t of 10mm-20mm sized rock can be followed by several hundred tonnes of coarse railway ballast. Surge piles allow the production flow to be stabilised and sized rock can be directed to the ‘toastrack’, a series of compartments at the loading point that can fill a 25t road truck in a matter of minutes. Greater handling rates are available at the railhead. Here a three kilometre conveyor system, using the brick viaduct, moves material continuously or in batches, 20 seconds delayed, to bins to be loaded onto trains. The 38t to 70t units are loaded by shunting under a conveyor loading point using a remote-controlled locomotive. A workforce of two then completes loading at night.
‘As the second largest (Glensanda is the largest in tonnage terms) producer, Mountsorrel is heavily reliant on positive cash flows and investment to maintain production. However, we are most proud of our second to none safety record,’ says Brown.
Granite House, Watermead Business Park, Syston, Leicestershire, LE7 1WA. Tel: +44 (0)844 561 0037. Website: www.lafarge-aggregates.co.uk