Dufferin implements cost effective improvements, breaks with tradition

By Andy Bateman, Engineering Editor

Process changes at a Dufferin Aggregates quarry combine a new production approach with equipment from other company operations to deliver cost-effective solutions.

Material arriving at the secondary plant first passes over a Simplicity 8x20 triple-deck scalping screen.

Dufferin Aggregate's Carden quarry, located 6 km east of Brechin, Ont. supplies granular base materials as well as clear sized products to the rapidly growing residential and commercial market of Markham, currently 90 km to the south and getting nearer all the time. To meet this year's supply commitments, the operation ran right through last winter to build product inventory ahead of a project shutdown that began on the first of May and is scheduled to last for seven weeks. The reconfigured spread is expected to deliver a 10 per cent increase in overall plant throughput as well as an average 10 per cent increase in the ratio of clear sized products. Additional benefits include increased flexibility in product mix, new finished product take off points and a reduction in overall plant operating and maintenance costs.
      The principal changes take place in the middle of the production process and include the replacement of two existing crushers as well as the addition of new secondary screening capacity, a new surge bin and associated conveyors. Originally envisaged as the replacement of just one crusher, the project scope was expanded significantly by utilising equipment available from other company operations, including some recent acquisitions. As a result, the project now addresses several other process issues with only a modest increase in overall capital cost.
      In a break with tradition, the changes include the first ever installation of a cone crusher at Carden in place of an existing vertical shaft impact crusher. Like many aggregate producers in the area, this operation has historically utilised only impact crushers for their ability to deliver not only good particle shape and high production rates but also effective separation of the deleterious shale found in these dolomitic limestone deposits. On the downside, impact crushers will typically produce more fines than a compression type crusher, such as a jaw or cone, in the same application.
      Dufferin site supervisor Jason Lording explains that the changes are designed to deal with a number of process flow issues affecting the existing spread. At present, there is no intermediate screening in the process between an initial scalping screen and a pair of finishing screens. As a result, the finishing screens are doing double duty as both process and finishing screens, resulting in a heavy recirculating load in the system. Adding to the problem, the existing secondary impact breaker does not provide sufficient reduction, while an ageing vertical shaft impact crusher is creating excess fines and is in need of major repair.

After the changes, a Cedarapids 5048 single horizontal shaft impact crusher will replace the existing Cedarapids 3645H double horizontal shaft impact crusher (centre right), while a Metso HP400 cone crusher will replace the existing Spokane 120 vertical shaft impact crusher (VSI), red frame, lower left.

      To address these issues, a Cedarapids 5048 single horizontal shaft impact crusher will replace the existing Cedarapids 3645H double horizontal shaft impact crusher in the secondary position, while a Metso HP400 cone crusher will replace the Spokane 120 vertical shaft impact crusher (VSI) currently in the tertiary position. Under the revised layout, the replacement crushers will be set up alongside each other and both receive feed from the existing scalping screen via a new surge bin. The new bin is split by a low dividing wall to provide 55 tonnes of surge capacity on the cone crusher side and 70 tonnes on the impact crusher side. The cone feed side of the bin will also receive recirculating load from both crushers, recognising the need to keep the cone choke fed for good particle shape and productivity. In the event that the feed rate to the cone exceed its crushing capacity, the level of material in the bin will rise and overspill into the side of the bin feeding the 5048 impact crusher.
      Material discharged from both crushers will be conveyed to a Metso 7x20 triple-deck screen, positioned in existing open space to the side of the plant. Here, oversize will be recirculated, larger clear size products will be drawn off, and all material passing the screen's bottom deck will go forward to the existing twin finishing screens for final sizing and stockpiling. Additional project features include bin level sensors, variable speed drive conveyor belts beneath the split bin and two new metal detectors.

Benefits of the changes include reduced recirculating load on these two Dillon 6 by 16 triple deck finishing screens.

      Looking at project cost, Lording estimates that the project will completed at an overall cost of just over $900 000 or only $150 000 more than the $750 000 originally budgeted for the Spokane VSI replacement alone.
      Once the project is completed, the plant's overall throughput rate is expected to increase from 540 to 600 tonnes/h. The switch from impact to cone crusher at the tertiary position is expected to reduce fines generation and increase the ratio of clear sizes produced (on average) from 40 per cent to 50 per cent of total plant throughput. Within the process, more crushing will take place at the secondary stage, while maintaining a similar throughput rate of about 300 tonnes/h; the 5048 impact crusher can be adjusted to a 25 mm opening compared to 64 mm for its predecessor. The additional screen will reduce the load on the existing finishing screens, as well as providing extra product takeÆoff points if required. In addition, the cone crusher will reduce the recirculating load by some 150 tonnes/h compared to the existing Spokane VSI. The new cone crusher will further improve the good balance between Carden's sales and production mix. At present, some 19 mm by 4.75 mm clear product is blended back into the crusher run to compensate for the high volume of fines generated by the VSI, and the new cone crusher will reduce the amount of clear product required to bring the crusher run onto specification. Other project benefits include the avoided major repairs to the Spokane VSI and smaller items such as simplified maintenance for the single 300 hp drive on the 5048 impact crusher compared to the two 150 hp motors of its predecessor.
      The primary load, haul and crush process at Carden will remain unchanged in the short term. The deposit here is about 30 m thick, divided about equally between 15 m thick top and bottom benches, with the top bench being worked for all current production. The bottom bench, suitable for concrete aggregates, has not seen significant extraction to date. At the face, a Caterpillar 990 on load and carry duty feeds 900 mm minus shot rock to the portable primary crusher. The primary is fully portable and moved as required for production blasts. This load and carry/movable crusher configuration is popular with a number of producers as it eliminates the need for haul trucks and the loader's haul distance can be kept low by the addition of field conveyors. However, it also has the inconvenience of the frequent crusher moves and is unsuitable for multiÆface working where haul distances exceed the effective range of the loader.
      Carden's primary is a Cedarapids 5348 horizontal impact crusher, fitted with a Rammer E64 secondary breaker to help deal with slabby feed. After crushing, a series of field conveyors carry the 200 mm minus run of primary material to a 200 tonne surge bin between the primary and secondary systems. The bin is fed through a pant leg chute that, once full, directs material to a 7 500 tonne surge pile. Surge material is then fed by loader back into the circuit as required. Lording adds that there are plans on the drawing board to replace the relatively small surge bin with a larger in-line surge pile to provide a buffer between primary and secondary process stages.
      A variable speed drive belt under the bin regulates flow to the secondary plant, where material first passes over a Simplicity 8x20 triple-deck scalping screen fitted with 89 mm opening punch plate, 57 mm and 25 mm Flex-Mat screens on the top middle and bottom decks respectively. Crusher run, either 51 mm minus or 19 mm minus, leaves the secondary system at this point, while the balance goes forward for further crushing. Adjustable doors on the scalping screen are used to alter the split of material going forward, depending on the product size required. After the process changes, more feed will be directed to the 5048 impact crusher when making 51 mm minus crusher run, while most will be sent to the HP400 cone when making 19 mm minus crusher run. Downstream of all the project changes, two Dillon 6x16 triple-deck finishing screens will produce clear sized products retained on their top middle and bottom decks. Meanwhile, the reduced quantity of screenings passing through their 6.3 mm opening V-Max bottom screen cloths will, as before, be blended back with crusher run from the initial scalping screen, together with the reduced quantity of 19 mm x 4.75 mm to make a specification granular base.
      Before going ahead with the switch to cone crusher from impact crusher, Lording reports that a significant amount of work was completed at the project planning stage to verify that Carden quarry was a suitable application for a cone crusher. A full tractor trailer load stone sample was sent off site to Metso Minerals' Milwaukee, Wisc. facility for test crushing in 2000, where test results confirmed that the HP400 would deliver good particle shape and productivity while dealing with any residual shale. On-site testing included a number of loaded stops during normal production runs so that accurate belt samples could be obtained and confirm the actual sizes been produced by the existing crushers and screens at key points in the system. As further insurance, the cone will be positioned in the last of three crushing stages, such that material will have already passed through at least one impact crusher. Early on in the project, tests were completed on the Spokane VSI to assess how changes in both feed size and feed rate affected the ratio of fines in the VSI's product. These test results confirmed that a change of crushing method was necessary, as the ratio of fines produced remained high at between 25 per cent and 30 per cent under any practical loading condition.
      Aggregates & Roadbuilding will be making a second visit to Carden later this year to update readers on the project. Project contractors to Dufferin include Vandergeest Welding Inc. for conveyor structural work and Aggressor Automation, a division of Harley's Electrical Services Ltd. for plant controls. In addition to Jason Lording, key Dufferin personnel on the project include Ron Rye, site foreman; Sam Jeater, east end site manager and Ed Persico, operations manager.
      Dufferin Aggregates, a wholly-owned subsidiary of St. Lawrence Cement Inc., is based in Concord, Ont.

May/June 2003 issue

Aggregates and Roadbuilding Magazine
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