Portable plant takes on winter’s crushing challenge
Portable plant takes on winter crushing challenge
July 28, 2008 By Andy Bateman
Given the choice, most aggregate producers will avoid winter production with the weather’s unpredictable impact on production schedules and plant productivity. One portable crushing contractor battled the weather to successfully meet customer demand.
iven the choice, most aggregate producers will avoid winter production with the weather’s unpredictable impact on production schedules and plant productivity. One portable crushing contractor battled the weather to successfully meet customer demand.
Wes Carrick is a production supervisor for crushing contractor J.G. Stewart Construction, most recently in charge of a large portable spread in Miller Paving Ltd.’s Carden Quarry near Brechin, Ont. Poor winter weather is particularly hard on portable crews and equipment such as this which have little protection from the elements. Despite near record snowfalls in February and March, Carrick’s team achieved 550 tonnes/h to 770 tonnes/h throughput, not bad considering few plants in that part of the country ran at all in the same period.
The spread seen by Aggregates and Roadbuilding occupied a large footprint with three crushers, three screens, associated process conveyors and up to twenty additional portable conveyors, with the latter carrying product to stockpile locations designated by the customer. Finished products included Granular A base material, 19 mm clear, HL3 clear and screenings.
Using stone baled over from the quarry’s top bench, a Volvo L330E wheel loader maintained a continuous supply of feed material to the crushing spread, with a primary haul of about 75 m and some 12 tonnes of well fragmented shot rock in each bucket. The L330E discharged into the feed hopper of the plant’s Metso Svedala 32×48 primary jaw crusher, equipped with a pedestal boom and Teledyne TB725X breaker to reduce any oversize in the feed when necessary. Material from the jaw was conveyed to an ElJay 6×16 horizontal screen where any 25 mm minus material was separated and conveyed to a granular base stockpile, while material larger than 25 mm was directed to a Hazemag AP-S 1320K horizontal shaft impact crusher. The upper and lower curtains of the impactor were set at 102 mm and 52 mm respectively to reduce the 152 mm x 25 mm crusher feed to 52 mm minus. Product from this crusher was conveyed to an inclined Cedarapids 8×20 screen fitted with impact screens on the top deck, combination 24 mm and 22 mm screens on the middle deck and 6.3 mm screen cloths on the bottom deck. Material retained on the screen’s middle deck was directed to a Universal 150 horizontal shaft impact crusher, operating in closed circuit with the screen and having curtain settings of 64 mm, 38 mm and 19 mm. Meanwhile, the minus 19 mm plus 6.3 mm size fraction passing the screen’s middle deck and retained on its bottom deck was further separated into 19 mm clear, HL3 and minus 6.3mm screenings by a Dillon 6×16 screen located further downstream. From there, the HL3 product was stockpiled by a programmable ThorStack2 150-8 telescopic portable radial stacker to minimise segregation.
As a further process refinement, screenings from the Cedarapids screen discharged into a bin equipped with an adjustable discharge gate, allowing some of the screenings to be blended back into the granular base material and so maximise the yield of finished product.
This large spread was powered by five separate generator sets, with a combined power output of some 2250 kW or over 3000 hp. Carrick estimates that about 16 000 litres of diesel fuel were consumed each week by the Caterpillar 3306, two 3406’s, a 3412, and 3512 engines driving the generators, adding that increased generation capacity was partly due to the additional portable product conveyors. Carrick also adds that, depending on primary haul distances, two loaders were often used for the primary haul to maintain the plant’s high daily output This operation demonstrated the importance of well fragmented and loose feed material to efficient running. The resulting steady and balanced material flow through each stage of the production process boosted productivity and helped to minimise unscheduled shut downs. Winter primary loading can be problematic in a quarry if raw feed mate-rial is drawn from a shot in which ice and snow have accumulated. In such situations, productivity is often reduced as the primary loader consumes additional time and fuel digging frozen material out of the shot. For the processing plant, the potential consequences are erratic material flow with crushers and screens starved of material or, at the other extreme, plugs up or overloads caused by frozen material. Either way, plant productivity is reduced.
In a sand and gravel pit, steady feed rates likewise facilitate high plant output, but there are additional potential safety issues associated with a frozen pit face. Here, high faces may be become vertical or even overhung if the face material is temporarily bound together by ice and a loader continues to remove material from the bottom. When temperatures rise, however briefly, there is then the possibility of large chunks abruptly breaking away from the face.
For both quarry and pit applications, the winter production rates of portable plants are often impacted by longer start up and shut down times although experienced operators keep these delays to a minimum. At the beginning of a shift, it may be necessary to clear snow or ice from exposed equipment and, especially in very cold weather, it will often be necessary to run the plant empty to allow conveyor belts to warm and oil to circulate. After snow, it can take some time to clear haul roads and the face before the plant sees its first bucket of feed and after heavy falls the clearing process may rule out any production at all. Even if the plant gets running, early shut down may be necessary at the end of a shift to remove built up frozen fines at transfer points, rock boxes and under conveyors.
Some of the most challenging winter conditions occur when daily temperatures are cycling around freezing point. In these conditions, wet or sticky material can accumulate on screens and conveyors during the day and freeze in place overnight. Fortunately, recent developments in both screen media and belt cleaning technology mean that operators now have effective tools to tackle both issues. For screening, advanced media is now available to keep blinding to a minimum and provide effective dry separation of fine sizes, even in marginal weather conditions. The good news continues with belt cleaning, where operators can choose from a wide range of scrapers, brushes or air knives, either alone or in combination, to keep belts clean and running. Effective belt cleaning is particularly important on portable plants with conveyor tail pulleys mounted very low to the ground. In this situation, fine material carried back on the underside of the belt can build up around the tail pulley and beneath the belt itself. Left unattended, material can accumulate to the point where both belt and pulley are partially buried, resulting in accelerated belt wear, belt mistracking and even premature pulley bearing failure. Good belt cleaners will reduce this build up, but it can be difficult to eliminate entirely. To deal with clean up these in hard to reach areas, producers utilise skid steers with small buckets or flat plates, or in Stewart’s case, a Caterpillar IT 18B Integrated Tool carrier fitted with a large flat plate.
One of the few advantages of winter operating is improved screening efficiency, provided the weather stays cold and clear. In sub-zero temperatures, there is no free water available to bind particles together, allowing fine sized particles (including piggy back fines on larger material) to more easily separate from larger particles. The result is improvement screen output and finished product cleanliness. Even this advantage has its downside though, as increased quantities of fugitive dust are
usually emitted from crushers, screens, transfer points and products stackers. With water sprays not an option in freezing temperatures, plant design to minimise the free fall of material at transfer points and stockpile conveyors will reduce dust emissions. Some producers enclose crushers and screens, either partially or fully, and install discharge socks on product stackers. Others experiment with alternative dust suppression methods such as fogging, calcium chloride and other chemical technology with varying degrees of success. n
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