Reducing waste stockpiles

Caribbean producer C.O. Williams installed a new wash plant to process material from an existing crushing operation at its Lears Quarry in Barbados. The company operates a 250-tph crushing plant that processes coral limestone. This produces more than 35,000 tons of aggregates per month, in addition to the more than 18,000 tons of asphalt the company makes each year.
The existing crushing plant produces a large quantity of minus 1/4-in. material with very high fines content, which 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 that could be used either as construction sands or in our own asphalt production” says Neil Weekes, general manager for C.O. Williams.

There are waste stockpiles approaching 500,000 tons of the minus 1/4-in. material, and the crushing plant produces another 500 tons of it per day.

An analysis of the waste stockpiles revealed a material with 20 percent minus 230 mesh. A further sieve analysis revealed that the feed to the sand washing plant (minus 8 mesh) contained 36 percent minus 230 mesh material. This required a customized 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. Access to the water volumes required to feed a wash plant to do the job satisfactorily was also an issue.

Taking all of this into account, CDE Global designed a solution that includes the company’s M2500 mobile washing plant with the required customized 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 150 ft. x 97 ft. 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,” Weekes says. “The space we had available at Lears Quarry was very restricting and it is only the compact, modular nature of the [plant design] that allows us to generate value from existing waste stockpiles and ensure that in the future we are able to deal with the minus 1/4-in. material from our crushing operations in an efficient manner.”

The new wash plant is producing a range of products that C.O. Williams is using in a number of different applications – an 8 mesh to 1/4-in. material is being used in concrete production, as is the 230 mesh to 8 mesh sand product. The 8 mesh to 1/4-in. material is also being used in asphalt production. The 230 mesh to 16 mesh washed sand is used in masonry processes for block work and plastering.

The process

The M2500 mobile washing plant accepts 110 tph of crushed coral limestone, which is delivered to the top deck of an integrated double-deck rinsing screen. The top deck removes the plus 1/4-in. material to an oversize stockpile. This only represents 11 tph, which is 10 percent of the feed material. The bottom deck of the screen produces a plus 8 mesh to 1/4-in. material, which is also stockpiled via the integrated stockpile conveyor. The product makes up to 34 tph of the feed.

The minus 8 mesh 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 230 mesh material. The 230 mesh to 8 mesh material is discharged to the split dewatering screen and a 16 mesh to 8 mesh sand product is dewatered and stockpiled via the integrated conveyor. This represents 14 tph, or 13 percent, of the feed material.

The split screen is set up for the 0 to 16 mesh 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 230 mesh to 16 mesh sand product. This represents 29 tph, or 26 percent, of the feed material.

A proportion of this 230 mesh to 16 mesh sand product is blended with the 16 mesh to 8 mesh sand from the first side of the split dewatering screen to produce a 230 mesh to 8 mesh product. The dual pass allows for the proportion of minus 230 mesh material in the sand product to be reduced from 36 percent in the feed to 3 percent in the final product.

Meanwhile, the overflow from both cyclones delivers the wastewater containing the minus 230 mesh 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 percent 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 onsite settling ponds,” says John Gallery, CDE Global project manager. “The space restrictions at the C.O. Williams site necessitated that a further sludge management stage be 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 tph of solids and has 110 plates. This is pressed under 15-bar pressure to produce a filter cake with 80 percent 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 percent water recycling from the plant and reduces the volume of top-up water required to feed the wash plant to 132 gallons per minute. 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” Weekes says. “We hope to have some success in the coming months with potential customers in the agricultural sector.”

Rapid setup, compact footprint

The modular nature of the CDE Global design was critical with this project owing to the specific conditions prevailing at Lears 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,” Weekes says. “It was only as a result of seeing the CDE equipment at ConExpo-Con/Agg 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 maximizing our product yield from Lears 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 maximized, according to Gallery.

“The efficiency of every transfer point within the system further increases the potential for C.O. Williams and all of our [other] customers to maximize product yield and minimize operational costs, which makes for a very compelling return on investment,” Gallery says. “The design also allows for the M2500 to be raised by 20 in. in order to maximize the stockpile capacity and reduce transport movements on site.”

The CDE Global 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 maximized.

Alternative sand source

The kind of limestone C.O. Williams processes is the dominant mineral in Barbados. According to estimates from the government, limestone reserves are in excess of 30 billion tons.

However, sand is in shorter supply  with current extraction focused in the Walkers area of Barbados on the northeast coast.

The government of Barbados 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. 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 C.O. Williams processes represents a way to reduce pressure on existing sand reserves and provides an alternative sand product for use in construction applications.

“We recognized 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,” Weekes says.

Sir Charles Williams started C.O. Williams in 1960 using a single tractor to offer simple earthmoving and agricultural cultivation services. Since that 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.

Take note

The combination of the AquaCycle and side beam filter press achieves 90 percent water recycling from the plant and reduces the volume of top up water required to feed the wash plant to 132 gallons per minute.

Information for this story courtesy of CDE Global. More information about C.O. Williams is available at www.cow.bb.