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Canada's “Rock to Road”
Magazine
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Fines recovery system delivers new saleable product, reduces cost. A fines recovery system is increasing the output of saleable product and simultaneously reducing operating costs at Dufferin Aggregate’s Aberfoyle, Ont. sand and gravel operation 75 km west of Toronto. (Above) Dewatered product is being produced at an average rate of 25 tonnes/h. (Below) With a solids content of between 75 and 80 per cent by weight, the material can be conveyed and stockpiled like a regular product.
Ron Van Ooteghem, Dufferin’s manager for west sand and gravel operations, explains that the new system is delivering some 45 000 tonnes/y of a new saleable product that has a number of potential applications. Equally important, the same tonnage has been diverted away from the operation’s settling ponds, allowing the interval between pond clean-outs to be stretched from one to three years. The new system receives grey wash water generated by the existing sand washing and classification process. Previously, this wash water was pumped directly to the settling ponds, carrying with it some 75 000 tonnes of fine material, representing about 10 per cent of total annual plant throughput. However, limited pond storage capacity meant that pond excavation was a regular winter event in order to create sufficient storage space for the next production season. On the sales side, Dufferin’s sales and marketing group is exploring a wide range of potential markets applications for the new soft and readily compactable material, sized between the 200 and 400 mesh (75 to 38 micron) sieves. Possibilities include engineered flowable fill, soil remineralization, asphalt filler, product blending, horse track coating, infield clay mix/golf bunker sand, stall linings for cattle, cement blending and electrical cable bedding. Since its commissioning on May 21, 2002, the Derrick Hi-G Dryer fines recovery system has performed as expected, producing the new product at an average rate of 25 tonnes/h and reducing waste fines generation by the same amount. In terms of cost reduction, Van Ooteghem reports that over $100,000 in annual costs for pond cleaning, including equipment, labour and fuel, have been replaced by expected operating costs of less than $ 5000, or $0.065 / tonne for the 75 000 tonnes of fines produced. The fines in the feed to the system are recovered by 102 mm urethane and ceramic lined hydrocyclones, set up in two radial clusters of 20 hydrocyclones each. Feed is pumped the hydrocyclones at 9092 litres/min (2000 Igpm) and at a constant, equal pressure of 35 psi to each. According to the manufacturers, the hydrocyclones make a nominal 400 mesh (38 micron) separation of the solids within the feed, with particles coarser than 400 mesh discharged out of the bottom of the hydrocyclones as underflow. Meanwhile, the clean overflow from the top of the hydrocyclone contains ultra fine materials, that is, finer than 400 and 500 mesh (20 micron) sizes. Typical hydrocyclone capacity is reported to be between 189 and 322 litres/min, and the underflow usually contains between 55 and 65 per cent solids (by weight), sized at between 85 and 95 per cent retained on the 400 mesh sieve. All the hydrocyclone underflow reports to a 4x10 Derrick Hi-G force linear motion screening unit for dewatering. This dewatering screen is driven by dual 2.5 hp motors, mounted above the screen to provide linear action with adjustable angles from zero to 5 degrees uphill. It operates at 1550 rpm with a stroke of 4.86 mm. The resulting acceleration of between 7.5 and 8.0 G further reduces the moisture content of the pre-thickened hydrocyclone underflow, assisted by the use of a urethane screen surface with openings of 25 to 35 mesh (0.5 to 0.7 mm) that provide about 40 per cent open area. The hydrocyclones and the dewatering screen operate in closed circuit, so that any material passing through the screen surface reports back to the initial feed sump, to be recirculated back to the hydrocyclones. In a typical setup, as much as 10 per cent of the hydrocyclone’s underflow is recirculated back in this manner, providing repeated opportunities to screen recoverable solids while also maintaining a consistent solid /liquid ratio in the feed sump. The incremental power requirement for the new system is a modest 105 hp, including the new 100 hp 8x6 pump to deliver feed to the hydrocyclones and the screen drive motors. The system’s integration into the existing sand classification system was relatively simple, requiring only the adaptation of the existing sump discharge to provide feed to the hydrocyclones. Although unlikely, this means that the washing process could revert to the old system should the new system have to be taken off line.
Left: The dewatering screen is driven by dual 2.5 hp motors mounted to provide linear action. Right: The 4x10 dewatering screen generates between 7.5 and 8.0 Gês of force to separate water from the hydrocyclone underflow, increasing the solids content of the product from 55-65 per cent to 75-80 per cent by weight. The ceramic lined discharge tips on the hydrocyclones are said to provide long wear life, with three seasons of operation expected in this application before tip replacement becomes necessary. The manufactures state that the hydrocyclones are both light-weight and economical for ease of change outs, while the radial cluster arrangement is said to deliver the same amount of water, solids and pressure to each hydrocyclone. In addition, each hydrocyclone has its own shut off lever and quick release coupling, so that individual hydrocyclones can be isolated without interrupting the feed to the entire system. The high open area urethane screen reportedly provides long wear life, while its high G forces are said to deliver maximum dewatering efficiency. The dewatered product from the screen typically contains between 75 and 80 per cent solids by weight, a figure that can be adjusted by changing the uphill grade of the screen and thereby adjusting the material’s retention time on the screening media. There is a trade off between production rate and moisture content, given the objective of maximizing the output of product with a low moisture content. A screen operating flat, with relatively low retention time, will produce material at a faster rate and with a higher moisture content than the same screen operating at an uphill grade of 3 per cent. Here the retention time will be longer, resulting in lower numbers for both production rate and product moisture content. Overall, the manufacturers claim production rates of between 4.5 and 32 tonne/h of material having a solids content between 75 and 80 per cent, depending on the application. This material will have a size grading of between 75 and 95 per cent retained on a 400 mesh (38 micron) sieve. Depending on percentage of solids and size gradations within the slurry discharging from the production plant, up to 80 per cent of the solids within the slurry can be effectively extracted. It is also noted that no chemicals are required. As a footnote, Van Ooteghem points out that the new system may provide additional benefits to the operation in the future. “Long term, the system may give us increased flexibility in terms of real estate management - the acreage required for settling ponds and fines storage may be reduced, so freeing up valuable real estate for other possible uses such as plant expansion or rehabilitation. By Andy Bateman Aggregates
and Roadbuilding Contractor Back to Library |
