By Paul Holroyd, head of Primo (Sand and Aggregates), CDE | All images by CDE
CDE’s technology assists Ground Breakers to make significant savings by adopting cyclone technology to eliminate the loss of fines to ponds.
South African construction materials producer Ground Breakers is the brainchild of two businessmen with in-depth experience of plant hire, transportation logistics and quarrying operations, JC Janse van Vuuren and Johan Meintjes. The innovative duo developed a full-circle business model where quarry materials are reprocessed and re-used in situ to eliminate transportation and dumping costs as well as to promote environmentally-friendly practices and structures.
A leading supplier of building material including G5, G6, G7, topsoil, building sand, river sand and plaster sand, Ground Breakers, started to produce materials for the local construction market in November 2016 at its Lindley quarry in Gauteng province. A bucket wheel was put to work to transform the quarry’s raw feed of decomposed granite into construction sand and aggregates, with additional low-value fill or bedding products. Encouraging best practice across the industry, the company also offers site clean-up and plant hire services to other quarry operators keen on using a standalone scalping screen to repurpose spoils or site rubbles on their sites as and when required rather than incur transportation and input costs.
To fully achieve the sustainable and profitable vision of the company’s owners, an upgrade of the existing bucket wheel system was required to tackle the loss of valuable materials to ponds and excess moisture in the final products. Janse van Vuuren and Meintjes explain, “Our quarry site in Lanseria faced a lot of challenges; we had limited water sources and limited space to put up a proper wash plant. Any silt dams we dug was taking away valuable mining area.
“The market required a clean washed concrete sand, and our bucket wheel was not delivering a quality product. We also lost a lot of fines to the settling ponds, wasting material that we could not sell without dredging the ponds and re-processing it through the bucket wheel. We knew that to grow the business we had to find a more sustainable and efficient washing solution.”
The limitations of bucket wheel technology
Although bucket wheel technology was widely used in its prime when sand resources were abundant and the price of construction materials was high, it is now being replaced by more advanced technology to address shortcomings such as the difficulty to control the volumes of water required for accurate material classification due to the limited capacity at the feed point. Indeed, as bucket wheels struggle to process sufficient volumes of water to achieve the desired cut points, fines are not efficiently removed and 100 to 300 micron fractions are lost to ponds or to the water treatment phase along with the overflow, making the sand product coarser. To mitigate the risk of inaccurate material classification, bucket wheels’ settings must be adjusted on a regular basis. Diverting excess material to settling ponds requires considerably more space to accommodate the latter, and classification efficiency decreases as the proportion of fines in the feed material grows. The time then required for clearing out settling ponds to recover lost material requires long periods of plant downtime.
Outside of the issue of high maintenance costs for a restricted throughput, the sand product typically discharged from the Ground Breakers’ bucket wheel system contained between 23% to 25% of moisture. This high moisture content meant that stockpiling the final product required double – sometimes triple – handling (to move the material to a separate stockpile area).
Following a visit by CDE experts, Ground Breakers’ feed material was tested at CDE’s laboratory. Based on the analysis results, CDE engineers established that the customer could make significant savings by adopting cyclone technology to eliminate the loss of fines to ponds. This would help to retain every valuable grain of sand available in the system and reduce the size of the settling ponds as well as maintenance time. In addition, CDE technology could add a plaster sand product to the company’s offer, which has higher commercial value than the river sand currently processed.
After considering the site’s footprint and the customers’ requirements, CDE presented the Combo all-in-one wet processing and water recycling system as the most appropriate solution to the customers’ requirements. The Combo would allow them to produce two high-quality sands simultaneously from the raw feed including plaster sand and river sand, for a much faster return on investment. Incorporated cutting-edge water management would ensure that the final products would be dewatered to an average 12% moisture, 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 developed and built a Combo modular plant to transform 60 tonnes per hour of raw feed into washed 0-6mm river sand and 0-1.5mm plaster.
Cyclone technology – the state-of-the-art separation methodology
Although bucket wheel technology has been undeniably essential to the growth of the construction industry in South Africa, cyclone technology is now replacing it on the market as ultra-colour HD has replaced black and white TV – as a natural evolution of materials processing brought about by modern innovations from forward-thinking research engineers.
A leading innovator in cyclone technology, CDE has been designing and delivering over 2 000 projects globally since 1992 to increase the efficiency of materials washing operations.
Cyclone technology involves the use of centrifugal force to classify materials rather than gravity. After extensive testing to determine the proportion of silt material to be removed from the raw feed, the specification of a cyclone set up is precisely calculated to ensure that each client’s new hydrocyclone-based system, such as the Ground Breakers’ all-in-one Combo wet processing and water recycling plant, provides unrivalled control over silt cut points and optimal water recycling. Due to the design of the cyclone unit, an increased number of parameters are available to ensure complete accuracy on the material cut point. The silt exits the overflow at the top of the cyclone with the wastewater, to be sent to the integrated AquaCycle water recycling system. The sand fraction exits the cyclone at the bottom, ready for dewatering, stockpiling and immediate dispatch to market.
Willem du Plooy, business development manager for CDE Southern Africa says, “CDE is fast growing its footprint in Africa as sand and aggregates producers are upgrading their installations to enjoy the benefits of wet processing technology in terms of more efficient production and higher product quality.
“With the respective successes met by the CDE EvoWash sand washing plant and the AquaCycle thickener in South Africa, more operators are now looking for a user-friendly solution that incorporates both plants. Ahead of the curve, CDE expert engineers have created a patented all-in-one system, the Combo. Pre-wired and pre-tested before dispatch to allow for rapid set up, the Combo has a capacity range of 50 to 500 tonnes and boasts a unique modular design to combine feeding, grading, washing, water recycling and stockpiling onto one compact chassis. The Combo technology is so efficient that the final products are ready to be sold directly on exiting the system.”
The Ground Breakers’ bucket wheel was not effectively reducing moisture in the sand products and fines were lost to ponds. To address these limitations, the Combo’ high-frequency dewatering screen dewaters 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 customers’ specific capacity requirements. This allows for the production of a sand product with a moisture content typically ranging from 10% to 15%. As a result, the 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 only requires a very small footprint, which makes it easy to relocate, even in remote areas and urban settings.
Janse van Vuuren and Meintjes agree, “Our CDE Combo is compact and highly efficient. It produces two properly washed products and our customers are very happy that the moisture content is so low. This means a considerable cost saving to them, as generally we sell per tonne.”
“Because the system works basically as a full-circuit unit, retaining fines and recycling used water for immediate re-use in the system, the need for silt dams has been significantly reduced and we have been able to preserve valuable mining areas.
“Switching from a bucket wheel system to the Combo has been conducive to considerable cost savings, as generally we sell per tonne. The additional -2mm product has many uses, from plastering exterior walls to making paving bricks. It has increased our profitability considerably.
Du Plooy concludes, “The new Combo on the Lanseria site 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, and in line with the requirements of customers or their downstream processes.
“At CDE we put all our passion and expertise in developing equipment that is tailor-made for each client to the highest quality standards. Return on investment is fast thanks to reduced requirements for spares, highly efficient water management as well as longer and more reliable running time.
“CDE’s products are specifically designed to require minimum attention, minimal civils and a minimal number of operators thanks to its one-touch technology, so that it can be left to simply produce clean sand and aggregates at the required rate.
“It was a pleasure to support Ground Breakers in installing the next generation of wet processing equipment to allow the company to achieve the full potential of its vision and its quarry site.”
Future of Sand Washing Conference in Johannesburg in collaboration with ASPASA