11 August 2020
Problems encountered when washing and dewatering sand include the discharge of wet products; product-sized sand sent to waste; too much time spent cleaning out settling ponds, and water usage.
We recently caught up with Willem du Plooy, business development manager from the CDE team in South Africa to discuss the benefits of considering cyclone technology compared to traditional bucket wheel systems for South African construction and materials processing companies.
Until recently, construction and materials processing companies in South Africa had to rely on bucket wheel wet processing technology to separate materials into different grades of sand.
Although bucket wheels have been essential to the quarrying industry over the years, increasing demand for sand and aggregates products, as well as increasingly stringent regulatory requirements, mean that operators must produce a wider range of materials in larger quantities and in less time to protect their return on investment and to turn a profit.
Here at CDE, we have developed an alternative solution for construction and materials companies, in the form of cyclone-based systems which, according to du Plooy, “guarantee a strong competitive edge and fast return on investment”.
Bucket Wheel Technology
“Bucket wheel technology, in its prime when resources were abundant and the price of sand was high, is fast becoming a much less attractive proposition when compared to new systems,” says Du Plooy.
He says bucket wheels accept material either directly from the aggregates screening operation, or are fed from classification tanks in applications where there is excess fine material in the -75/-63 micron range, which must be removed.
The operation of the screw pushes the sand fraction up the elevated chamber while the water and finer particles pass through the overflow weir. The overflow weir, which is closer to the feed point, then provides the mechanism for fines removal.
In bucket wheel classifiers, sand slurry is introduced near the wheel, where the coarse sand settles and is lifted by the rotating wheel. The rotary bucket elevator scoops the sand from the tub and drains the water from the sand during transfer. The wastewater flows over weir at the back of the bucket wheel and the solid fraction is separated in the wastewater slurry.
“The limited capacity at the feed point of these technologies means that control over the volumes of water required for accurate material classification is difficult,” says Du Plooy. “The inability to use sufficient volumes of water means that fines are not efficiently removed and 100 – 300 micron fractions are lost to ponds or to the water treatment phase, along with the overflow, making the sand product coarser.”
He says the bucket wheel’s settings must be adjusted regularly to mitigate the risk of inaccurate material classification.
As this method of classification depends on settling time, the cut point must often be chosen above the threshold to obtain a maximum amount of allowable silt in the final product.
Diverting excess material to settling ponds requires considerably more space to accommodate them, and the classification inefficiencies increase as the proportion of fines in the feed material increases.
Clearing out settling ponds to recover lost material requires plant downtime in addition to the operating costs incurred when reprocessing waste material. High maintenance and restricted throughput aside, the sand product typically discharged contains between 23% and 25% moisture, placing stockpiles at risk of contamination if located in proximity to one another.
“As a consequence, stockpiling the final product requires double or even triple handling to move the material to separate stockpile areas. Stockpiles take long to build and to dry to saleable standards, and return on investment becomes a significant issue,” says Du Plooy.
He says cyclone technology is now replacing bucket wheel technology as a “natural evolution” of materials processing brought about by modern innovations. CDE has developed cyclonic materials technology to the point where it guarantees the removal of the undesired fraction (e.g. -74 micron) while retaining quality fines and maximising dewatering of the product to between 10% and 15% moisture.
The company has been a proponent of cyclone technology since 1992 and has delivered over 2,000 materials washing operations projects globally.
“Cyclone technology involves the use of centrifugal force as opposed to gravity to classifying materials,” says Du Plooy. “After testing to determine the proportion of silt material to be removed from the raw feed, the specification of a cyclone set-up is calculated to ensure that the client’s EvoWash™ hydrocyclone-based system provides control over silt cut points.”
The design of the cyclone unit allows for an increased number of parameters which, says Du Plooy, ensures improved accuracy on the material cut point. The silt exits the overflow at the top of the cyclone with the wastewater, to be sent to settling ponds or a water recycling system. The sand fraction exits the cyclone at the bottom, ready for dewatering and stockpiling.
Retaining all available quality fines in the raw feed reduces the space required for settling ponds and so benefits both the customer and the environment.
“Product yield is maximised and operational costs reduced,” says Du Plooy. “The production of sand and aggregates is guaranteed to be within specifications, in line with the requirements of the customer or their downstream processes.”
“For instance, a 70 tph machine typically loses 20% sand to the pond which equates to 14 tph of sand lost, resulting in 140 t/day and 3500 t/month. If sand is worth R100 per tonne, that is a loss of R350 000 per month. With the lost sand recovered from the silt pond, a CDE EvoWash could pay for itself anywhere between 6 to 12 months.
High Moisture Content
The solution to bucket wheels’ limited ability to dewater sand products effectively lies in using a high-frequency dewatering screen to dewater the material in one pass, so removing the need for re-processing and double handling,” says Du Plooy.
“This allows for a sand product with a moisture content typically ranging from 10% to 15%,” says Du Plooy. “As a result, the product is ready for market straight from the belt, and the sand is turned into revenue in the shortest possible time.”
He says the return on investment is fast thanks to a reduced spares requirement, efficient water management, and longer and more reliable running time.
“For instance, if sand is worth R100 per tonne, a 70 tph machine loses R7000 for each hour it is out of action due to maintenance or repairs. Our products are designed specifically to require minimum attention, minimal civils and a minimal number of operators so that they can be left to simply produce clean sand and aggregates at the required rate.”
Process water recycling
He says CDE’s AquaCycle™ water management solution accelerates return on investment by maximising production efficiency, minimising the loss of valuable fines, and reducing water and energy costs.
“The AquaCycle™ thickener minimises costly water consumption by ensuring that up to 90% of process water is recycled for immediate recirculation.”
Once the feed material has been washed and classified, waste is sent to the AquaCycle thickener tank, where a small amount of polyelectrolyte flocculent is added to the water via an automatic dosing station forcing fine particles to settle to the bottom of the thickener tank.
“The clean water at the top overflows the weir and is stored in the AquaStore tank before being re-circulated,” says Du Plooy. “The result is an efficient water recycling system which requires only a 10% top-up water supply.”
He says waste sludge is discharged into a buffer tank where a motorised rake in constant rotation ensures the material does not settle and solidify. If further dewatering is required, a filter press or decanter is added to the wet processing solution to eliminate the need for settling ponds.
Du Plooy says all water thickeners are not “born equal” and their efficiency results from years of research, development, and refinement.
“Every aspect of its design is considered carefully so that the system is responsive to the site-specific needs of its owner.”
He adds all CDE equipment is pre-wired and pre-tested before despatch, to allow for plug-and-play operation, and is designed for rapid assembly and set-up to start processing material within days of arrival on site.
“These design considerations and factory acceptance tests reduce install time significantly and accelerate return on investment, a real proposition when the CDE AquaCycle thickener is introduced to a washing operation. In fact, RoI with a CDE AquaCycle is typically achieved within six to nine months.”
Settling ponds are a known source of revenue loss due to their high maintenance costs, site footprint, and downtime, but also because high-value fines can be lost and trapped at the bottom of ponds.
“The requirement for settling ponds is greatly reduced by recycling up to 90% of the process water. The addition of a filter press tailings management system eliminates that need entirely. When combined with a CDE filter press custom-built fines management system, this figure increases to 95%, removing the need for settling ponds altogether.”
Ground Breakers, South Africa
South African sand and aggregates producer Ground Breakers required an upgrade of the existing bucket wheel system to address the loss of valuable materials to ponds and excess moisture in the final products.
The company’s JC Janse van Vuuren and Johan Meintjes explain: “Our quarry site in Lanseria faced a lot of challenges: we had limited water sources and space to install a proper wash plant. Silt dams are no solution as they take up valuable mining space.”
The market required clean washed concrete sand but the company’s bucket wheel was not delivering a quality product. In addition, fines were being lost to settling ponds, wasting material that could not be sold without dredging the ponds and re-processing it through the bucket wheel.
The sand product typically discharged from the Ground Breakers’ bucket wheel system contained between 23% and 25% moisture, which meant that stockpiling the final product required double or triple handling to move the material to a separate stockpile area.
The CDE Solution
Ground Breakers’ feed material was tested at CDE Global’s laboratory. The resulting analysis results showed that the customer could achieve savings by adopting cyclone technology to eliminate the loss of fines to ponds.
This, says CDE’s Du Plooy, “would help retain every grain of sand available in the system, reduce the size of the settling ponds, and shorten maintenance time.”
He says CDE’s technology could also add a plaster sand product to Ground Breakers’ offering. Plaster sand has higher commercial value than the river sand originally processed.
CDE then presented its Combo all-in-one wet processing and water recycling system as a solution to the customer’s requirements.
According to Du Plooy, the Combo would allow the client to produce two high-quality sands simultaneously from the raw feed, including plaster and river sand, at a much faster return on investment.
“Incorporated water management would ensure that the final products were dewatered to an average 12% moisture content, making them ready for market straight from the belts. As an added benefit, the fully integrated CDE AquaCycle thickener would allow for up to 90% of the process water to be recycled directly into the system for near-independence from fresh water supplies.”
Following a collaborative design process with the customer, CDE then developed and built a Combo modular plant to transform 60 tph raw feed into washed, 0 – 6 mm river sand and 0 –1,5 mm plaster.
The Ground Breakers bucket wheel was not reducing moisture in the sand products effectively and fines were lost to ponds.
To address these limitations, the Combo high-frequency dewatering screen was provided to dewater the material in one pass, removing the need for re-processing and double handling.
Once the sand slurry with the silt material is discharged from the hydrocyclones, it is delivered to the screen which is sized according to the customer’s capacity requirements. This allows for the production of a sand product with a moisture content typically ranging from 10% to 15%.
The result was a product is ready for market straight from the belts, in turn converting sand into revenue in the shortest possible time.
“An added benefit of the Combo system is that, although the plant packs a powerful punch, it requires a very small footprint, making it easy to relocate, even in remote areas,” says Du Plooy.
Janse van Vuuren and Meintjes agree: “Our CDE Combo is compact and highly efficient. It produces two properly washed products and our customers are happy that the moisture content is so low. This means a considerable cost saving to them, as we generally sell per tonne.”
They say the need for silt dams has been reduced because the system basically works as a full-circuit unit, retaining fines and recycling used water for immediate re-use in the system.
“The Combo on the Ground Breakers site,” says Du Plooy in conclusion, “has allowed the company to maximise yield and reduce its operational costs. The production of sand and aggregates is guaranteed to be within specifications, with no risk of cross-contamination when two products are processed simultaneously.”