New modular sand & gravel washing plant for Raymond Brown at Roke Manor quarry
Final commissioning has recently been completed on a new CDE modular sand & gravel washing plant at Roke Manor quarry in Hampshire.
The site is operated by Raymond Brown who currently have eleven locations across Hampshire, Dorset, Wiltshire, Oxfordshire and Berkshire. Roke Manor quarry is the latest addition to the group portfolio having been officially opened by Group Chairman, Ron Isaac in November 2014.
“The new site was seven years in development and planning permission was granted in 2011” explains Richard Hoare, Project Manager at Raymond Brown. “Our licence will see us extract 750,000 tonnes of sand and gravel at a rate of 125,000 tonnes per year.”
The permission is accompanied by a 30 year woodland management plan which involves additional tree planting in line with an approved landscaping plan that formed part of the planning permission. The site is located 11 miles from Southampton on the outskirts of the New Forest and it was the sensitivity and environmental responsibility displayed by Raymond Brown which resulted in the successful planning application.
Speaking about the Roke Manor development Steve Clasby, Chief Operating Officer said “This is an exciting new opportunity for Raymond Brown. We pride ourselves on our sustainable and pro-active approach in dealing with our environmental, social and economic impacts.”
The CDE plant
The new washing plant at Roke Manor will process sand and gravel at a rate of 80 tonnes per hour and includes several items of equipment from the CDE portfolio including the AggMax modular logwasher EvoWash sand washing plant, AquaCycle thickener and decanter centrifuge as well as a feed hopper and transfer and stockpile conveyors.
Material is delivered to the plant via the M14 feed hopper with 150mm grid spacings. CDE conducted extensive material testing prior to specifying the plant and the vast majority of material being processed is in the minus 63mm range with 80% typically passing 20mm.
“Raymond Brown were looking for performance guarantees that gave them confidence in the capability of the plant to tackle high fines and clay content in the feed material and produce a range of washed sand and aggregate products that would meet their customers’ requirements” explains Chris McKeown, Technical Manager at CDE. “Our performance statement covers the production of a range of products from the new plant to agreed specifications: 0-4mm concrete sand, 0-2mm building sand, 4-10mm, 10-20mm and 20-40mm aggregates and a +40mm oversize.”
This agreement was reached after extensive material testing at Roke Manor to establish parameters around the likely variations in feed material within the deposit. “It is only through the extensive material testing that we conduct on every project that we are able to deliver washing plants which deliver on our customers’ requirements through the production of consistently graded washed sand and aggregates” explains Matt Bunting, Director of Business Development in the UK for CDE. “This allows us to work with customers on the kind of performance statements included on this project for Raymond Brown.”
The feed material is transferred to the first stage of processing via a 23 metre conveyor with an 800mm belt. At the head of this conveyor a washbox fluidises the material before delivery to the AggMax modular logwasher.
Before material enters the AggMax a double deck horizontal screen is employed as a pre-screen to remove the +40mm oversize material and the -4mm sand from the feed material. The +40mm oversize is stockpiled in a bay underneath the AggMax via an integrated 4m horizontal conveyor with a 650mm belt. The feed material contains an average of 9 tonnes per hour of this oversize material.
While the top deck of the screen removes the oversize material the -4mm material falls through the bottom deck of the screen and is sent to the sand washing phase. This is an important step in protecting the plant explains Matt Bunting. “Logwashers and sand are not a very good combination as if the sand is allowed to get into the logwasher it will result in excess wear on the machine. By removing the sand fraction at this stage we are able to protect the machine, ensuring maximum plant availability and allowing Raymond Brown to maximise their production volumes and the return on their investment.”
Material from the bottom deck of the pre-screen is delivered to the AggMax modular logwasher at a rate of 47 tonnes per hour on average where the material is subjected to high levels of attrition before being discharged to a triple deck sizing screen which is integrated onto the AggMax chassis.
The AggMax is one of the most popular products that CDE offer and is specified on a range of projects from sand & gravel and crushed rock processing to C&D waste recycling and iron ore processing. “The design of the AggMax has been refined over the last 10 years to ensure it continues to offer our customers the highest level of scrubbing performance while also including a range of design features which ensure it is operational for as close to 100% of the time as possible” explains Product Development Manager at CDE, Glenn Sloan.
The most obvious design feature of the AggMax is the arrangement of the paddles in a fan configuration which not only maximises the material on material impact within the machine but reduces shock loads on the bearings to deliver extended equipment life and a reduced power requirement.
The AggMax also features enhanced bearing protection with the main bearing housings located on the outside of the machine. “The AggMax bearing arrangement features 13 levels of bearing protection to eliminate the potential for material and water ingress – protecting the machine and ensuring our customers are able to concentrate on meeting their production targets” explains Glenn Sloan. There is also a bearing temperature sensor which has the capability to automatically shut down the plant if the temperature goes beyond a defined level, maximising equipment life and minimising the time required for essential plant maintenance.
The scrubbed aggregates are discharged onto an integrated triple deck sizing screen to produce the 4-10mm, 10-20mm and 20-40mm aggregate products. These are then stockpiled via three 15m stockpile conveyors with 650mm belts.
Any fines liberated from the aggregates during the attrition phase are combined with the 0-4mm material removed at the pre-screening stage and delivered to the EvoWash sand washing plant. The EvoWash produces two sands for Raymond Brown – 0-4 mm concrete sand and a 0-2mm building sand.
“Due to the relatively high level of fines in the feed material it is necessary to produce two sands in order to ensure production of a concrete sand spec” explains Chris McKeown. “There is a lot of material in the 0-2mm range and the first stage of classification involves the removal of some of this material to allow production of the concrete sand.”
The material is first delivered to a 500mm cyclone and the cyclone underflow is discharged onto the first side of the split EvoWash dewatering screen. The dewatering screen is set up to allow some of the 0-2mm material to fall through to the sump and this is then pumped to a bank of three 250mm cyclones. This not only achieves the required removal of the minus 63 micron fraction but delivers a slurry containing the 0-2mm material to the second side of the split dewatering screen before this is stockpiled.
The waste water containing the minus 63 micron fraction is then delivered to the first stage of water treatment and recycling where the AquaCycle A200 thickener is deployed. The AquaCycle accepts the feed from the cyclone overflow by gravity feed and at the entry point to the thickener it is dosed with flocculent that has been pre-mixed in the integrated FlocStation.
The flocculent causes the very fine particles to bind together and sink to the bottom of the AquaCycle tank while the clean water overflows the peripheral weir and is stored in a steel water tank for recirculation to the washing plant. This recycles 90% of process water and reduces the fresh water requirement to top up only.
“Fresh water for use on the new CDE plant is limited” explains Richard Hoare of Raymond Brown. “Without the integrated water recycling and sludge management system it simply would not have been possible for us to proceed with this investment in the new washing plant and CDE were able to demonstrate significant experience and capability in this area.”
The sludge at the bottom of the thickener tank is conditioned by a set of rakes which rotate around the bottom of the tank. The helps to ensure that the sludge can be easily discharged from the tank but also provides the mechanism by which the automatic sludge discharge function is activated. Once the rakes encounter a specific level of resistance this activates the sludge pump and this is then discharged into a circular steel buffer tank with a capacity of 106m3.
As with the AquaCycle thickener, the sludge buffer tank is also fitted with a set of rakes which, as Chris McKeown explains helps to optimise the performance of the decanter centrifuge. “In order to ensure best performance from the decanter centrifuge it is essential that the density of the sludge is consistent. The rakes in the buffer tank help to maintain this consistency which ensures maximum dewatering of the sludge for maximum water recovery and minimum waste volumes.”
After the sludge has been dewatered a screw conveyor discharges the filter cake to a bay below the centrifuge enclosure. On this plant the centrifuge is sized to accept delivery of 8 tonnes per hour of solids.
The new CDE plant at Roke Manor quarry is operational for 10 hours per day but the decanter centrifuge will run for an additional 2-3 hours per day after material processing has stopped. This helps to clear the sludge buffer tank in advance of the next day’s production. “The advantage of the centrifuge decanter in this instance is the capability for continuous, unmanned operation” explains Matt Bunting of CDE. “This allows Raymond Brown to not only reduce the level of operator intervention but also gives flexibility in the sizing of the centrifuge.”
In addition to the water recovery and recycling benefits offered by the combination of the AquaCycle thickener and decanter centrifuge they also have the effect of significantly reducing the space required on site. This is as a result of eliminating the requirement for settling ponds which are now replaced by the waste bay underneath the centrifuge enclosure where the cake is discharged.
“The complete plant footprint for Raymond Brown including product stockpiles is 57m x 48m” explains Matt Bunting. “If no water recycling and sludge management system was in place the washing plant would require a considerably larger area than this and introduce further cost to the project as a result of the requirement to build secure & safe settling ponds.”
“Another benefit of eliminating settling ponds that can’t be overstated is the health & safety benefits” explains Richard Hoare. “By removing this risk from our site at Roke Manor we are demonstrating our commitment to best practice. The best way to protect your people is to eliminate the risk and that is what we have chosen to do at Roke Manor by introducing this closed circuit water recycling and sludge management system.”
Given the location of Roke Manor quarry the new washing plant puts Raymond Brown in a good position to take advantage of the aggregate supply opportunities that exist in the Southampton and wider Hampshire area.
“We’ve got good access to Southampton at only 11 miles from the city centre” explains Richard Hoare. “We identified the opportunity from an investment at Roke Manor quarry some time ago and are delighted to now have the latest materials washing and classification technology in operation. The will allow us to deliver the highest quality sand and aggregates to the local construction market – something that customers of Raymond Brown have come to expect from us in the last 60 years.