Tonnage
250tph
Material
Sand & Gravel, Claybound Aggregates, Limestone, Crushed Rock, Quarry Waste
Output
2.4mm to 6mm | concrete sand | 63 micron to 2.4mm sand product | 63 micron to 1.2mm washed sand
THE CHALLENGE
CDE commissioned a new wash plant for C.O. Williams to process material from an existing crushing operation at Lear’s Quarry in Barbados. The company operates a 250 tons per hour crushing plant processing coral limestone. This produces over 35,000 tons of aggregates per month and the company also produces more than 18,000 tons of asphalt per year.
The existing crushing plant produces a large quantity of minus 6mm material with very high fines content and as a result this material was stockpiled as a waste product on site. “The waste from the crushing operations was an ongoing concern for us and the high fines content required that we look at a wash plant to see if we could recover any material which could be used either as construction sands or in our own asphalt production” explains Neil Weekes, General Manager at C.O. Williams.
There are waste stockpiles approaching 500,000 tons of the minus 6mm material to be processed using the CDE wash plant and the crushing plant produces another 500 tons per day of this material. An analysis of the waste stockpiles revealed a material with 20% minus 63 micron. A further sieve analysis revealed that the feed to the sand washing plant (minus 2.4mm) would contain 36% minus 63 micron material. This required a customised double pass washing system to ensure this would be reduced to a level that would allow the washed sand products to have a commercial application. The space available on site was also limited and access to the volumes of water required to feed a wash plant to do this job satisfactorily was also an issue.
THE SOLUTION
Taking all of this into consideration CDE designed a project that includes the M2500 mobile washing plant with the required customised dual pass sand washing system. The plant also includes closed circuit water recycling with the introduction of an AquaCycle A400 thickener and a side beam filter press. The final plant measures 45m x 29m and as a result of the inclusion of the side beam filter press has eliminated the requirement for settling ponds.
“Without the introduction of the AquaCycle thickener and filter press this project just would not have been possible” explains Neil Weekes. “The space we had available at Lear’s quarry was very restricting and it is only the compact, modular nature of the CDE design that allows us to generate value from existing waste stockpiles and ensure that in future we are able to deal with the minus 6mm material from our crushing operations in an efficient manner.”
The new wash plant is producing a range of products which C.O. Williams are using in a number of different applications – an 2.4mm to 6mm material is being used in concrete production as is the 63 micron to 2.4mm sand product. The 2.4mm to 6mm material is also being used in asphalt production. The 63 micron to 1.2mm washed sand is used in masonry processes for block work and plastering.
The Process
The M2500 accepts 100 tons per hour of crushed coral limestone which is delivered to the top deck of the integrated double deck rinsing screen. The top deck removes the +6mm material to an oversize stockpile. This only represents 10 tons per hour (10%) of the feed material. The bottom deck of the screen produces a 2.4mm to 6mm material which is also stockpiled via the integrated stockpile conveyor. This product makes up 31 tons per hour of the feed (31%).
The minus 2.4mm material is delivered from the screen sump to the first cyclone on the EvoWash sand washing plant. This is the first pass where the cyclone allows for the efficient removal of the minus 63 micron material. The 63 micron to 2.4mm material is discharged to the split dewatering screen and a 1.2mm to 2.4mm mesh sand product is dewatered and stockpiled via the integrated conveyor. This represents 13 tons per hour of the feed material (13%).
The split screen is set up for the 0 to 1.2mm material to fall through to the EvoWash sump before being pumped to the second cyclone for a further wash. The second side of the split dewatering screen accepts the cyclone underflow and produces a dewatered 63 micron to 1.2mm sand product. This represents 26 tons per hour of the feed material (26%). A proportion of this 63 micron to 1.2mm sand product is blended with the 1.2mm to 2.4mm sand from the first side of the split dewatering screen to produce a 63 micron to 2.4mm product. The dual pass allows for the proportion of minus 63 micron material in the sand product to be reduced from 36% in the feed to 3% in the final product.
Meanwhile the overflow from both cyclones delivers the wastewater containing the minus 63 micron material to the AquaCycle thickener. The waste is discharged at the highest point of the EvoWash sand washing plant which allows for gravity feed to the AquaCycle thickener and eliminates the requirement for an additional sump and pump.
As the wastewater enters the thickener tank it is mixed with a pre-mixed flocculant from the FlocStation poly plant. This forces all the fine particles to join together and sink to the bottom of the tank while the clean water overflows the peripheral weir and is sent to a concrete water storage tank before being recirculated to the wash plant. Introduction of the AquaCycle is the first stage in the water recycling process and recovers 85% of the process water for re-use.
“The AquaCycle achieves significant water recycling on its own and without the filter press the waste sludge would then be delivered to on site settling ponds” explains Conor McCollum, CDE Project Manager on this project. “The space restrictions at the C.O. Williams site necessitated that a further sludge management stage was introduced as there simply wasn’t the space available to accommodate settling ponds.”
The waste sludge from the AquaCycle thickener is first delivered to a concrete buffer tank before being sent to the side beam filter press. The filter press accepts 20 tons per hour of solids and has 110 plates. This is pressed under 15 bar pressure to produce a filter cake with 80% dry solids content.
The water extracted from the sludge at this point is also returned to the water storage tank for recirculation to the wash plant. The combination of the AquaCycle and side beam filter press achieves 90% water recycling from the plant and reduces the volume of top up water required to feed the wash plant to 36m3 per hour. The filter cake is discharged to a bay below the filter press enclosure which can be accessed by a loading shovel for removal of the waste material.
“We are actively looking at completely eliminating waste from the operation by finding an outlet for the filter cakes” explains Neil Weekes of C.O. Williams. “We hope to have some success in the coming months with potential customers in the agricultural sector.”
THE RESULTS
Rapid set-up and a compact footprint
The modular nature of the CDE design was critical with this project as a result of the specific conditions prevailing at Lear’s Quarry. “The space available at the quarry was the stumbling block for us whenever we were exploring a solution to the volume of crushed limestone dust that we were producing” explains Neil Weekes of C.O. Williams. “It was only as a result of seeing the CDE equipment at ConExpo in Las Vegas that we started to see a solution to this problem and the ability to integrate closed circuit water recycling as part of the plant allowed us to move to a situation where we are now maximising our product yield from Lear’s Quarry. Not only does this make our operation more profitable but it ensures the most efficient extraction and processing of the quarry reserves and makes our operation more sustainable in the long term.”
The modular nature of the processing plant also ensures that process efficiency is maximised according to Conor McCollum of CDE. “The efficiency of every transfer point within the system further increases the potential for C.O. Williams and all of our customers to maximise product yield and minimise operational costs which makes for a very compelling return on investment. The civils design also allows for the M2500 to be raised by 500mm in order to maximise the stockpile capacity and reduce transport movements on site.”
The CDE design also includes integrated plant walkways which facilitate a complete plant walkthrough for essential plant inspection and maintenance. These make it easy to walk through the complete processing plant from the feed point to the filter press which reduces the time required for plant maintenance and ensures that production time is maximised for C.O. Williams.
An alternative sand source in Barbados
The dominant mineral in Barbados is limestone of the kind processed by C.O. Williams and according to estimates from the Government of Barbados reserves are in excess of 30 billion tons. However, sand is in shorter supply with current extraction focused in the Walkers area on the north-eastern coast. The Government of Barbados website reports that “there is only a safe period of about 10 years remaining before adverse effects of the coastline environment occur.” While sand reserves are available in other areas of the island these are not as easily accessible as the sand in these areas is more siliceous and consolidated.
Taking all of this into account the development of an alternative sand source from the coral limestone being processed by C.O. Williams represents a way to reduce pressure on existing sand reserves and provide an alternative sand product for use in construction applications. “We recognised not only the business opportunity that existed from the more efficient exploitation of our coral limestone reserve but also for the potential that existed to protect long term sand and aggregate supply by introducing the advanced processing technologies offered by CDE” explains Neil Weekes. “Our introduction of the AquaCycle thickener and filter press technology is further evidence of our commitment to building the future for Barbados in a sustainable manner by employing the latest technologies available to us.”
Sir Charles Williams started the C.O. Williams company in 1960 using a single tractor to offer simple earth moving and agricultural cultivation services. Since this time the company has grown to be one of the leading civil engineering and highway construction companies providing services across the Caribbean from the company headquarters in Barbados as well as regional offices in Antigua and St. Lucia.
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