OH&S News

Breaking The Bottleneck

If your screening operations are inefficient, much of your profit may be falling through the cracks. Consider that if a screen is only 75 per cent efficient, then 25 per cent of the material within the  desired product size range is being rejected with the oversize material. Vibrating screens  must be properly selected, designed and applied. If not, they may end up being the biggest bottleneck within an operation.

Certainly there are those who mistakenly view the screen as an auxiliary or secondary piece of equipment. As such, they may focus on the initial capital expenditure as the primary factor in determining which screen to implement within a given application – but experts will stress that it is never advised to scrimp on the size or quality of a screen. Efficient screening is the biggest link to high-capacity, multiple-product output.

The current trend is toward larger screens to increase capacity within larger plants. ?Most producers want more tonnes per hour across the screen. However, the key to optimum screening is maximising capacity without losing efficiency,? says Joe Schlabach, vice president of marketing and sales for Indiana-based Deister Machine Company Inc. ?That involves both art and science.?

The science is that of stratification, or how effectively the vibrating screen causes material to stratify, allowing larger materials to remain on a top deck, and smaller  particles to fall through the openings of the screening surface.

The art lies in the tuning, tweaking and synchronising of screen setups to maximise both capacity and efficiency in a number of given applications.

The following examples illustrate how customised screening solutions access that special combination of art and science to break production bottlenecks in a variety of applications.

An Ohio-based operation employs 16 Deister screens. Seven of those screens are located on a custom-made bucket ladder dredge that stays afloat by a 156ft long by 44ft wide pontoon. The dredge alone (rated capacity of 908kg per hour) has a large processing plant that has more screening capacity than many sand and gravel plants.

Designing the screening operation on the dredge demanded a unique approach – one that was more flexible. The extensive system separates oversize, gravel and fines for dewatering while giving the operator better control of gradations, and allowing the ability to send whatever percentage of sand ashore that is desired.

Deister engineers created a system that is flexible, reliable and ensures plenty of capacity. It starts with a 5ft x 8ft vibrating grizzly that removes plus-8-inch rocks and clay, which are dumped back into the lake behind the bucket ladder. The scalping screen must withstand significant impact as it is a very severe application. Unlike the steady feed of a conveyor, material is dumped 3m out of a 425-litre bucket, dropping in a fairly compacted chunk, one bucket after another.

Material then moves to a splitter chute, which directs it to two 7ft x 20ft triple deck screens. Uniquely, the middle deck and the bottom deck on the screens have the same size openings. However, the middle deck contains a splitter section that allows part of the material to pass to the bottom deck. The remaining middle deck has a double deck urethane system with solid lower panels so that the material passing through the upper panels can go into a special set of chutes and into the under pan along with the material that is passing through the bottom deck. This method was used after discovering that other dredging systems were having bottlenecks at the primary screens due to a silt problem.

Gravel from the primary screens travels to a 48-inch conveyor that runs down the centre line of the ship and dumps to a floating conveyor system that moves the material ashore to a raw storage pile where it is tunnel-reclaimed into the main plant.

Fines and water go to four sumps equipped with 8-inch pumps. The slurry runs through eight 20-inch cyclones.  Underflow from the cyclones moves to four 6ft x 12ft fine sand recovery screens.

The cyclones dewater, but not well enough to allow transport to shore. While the sand leaves the cyclones still fairly wet, it is handled efficiently by the screens. The operator explains that the material on the dewatering screens has to travel uphill to get off the screens.

?It actually has a dam at the end of it,? he says, ?so the material depth can be five inches deep as it pushes up the deck. With half-millimetre slotted openings, the water is pulled down to about 14 per cent moisture. Any sand passing through these screens recirculates until the filter bed  becomes heavy enough to recover the sand.?

In addition to requiring less manpower, the dredge is designed to deliver environmental benefits. It is electric-powered by a 400-hp DC motor that can move 27 buckets per minute. This removes the chance for fuel spillage from diesel-powered equipment. Also, the dredge is customised for quieter operation. All screens are rubber-lined or urethane.

Recently, a Tennessee-based stone and gravel operation broke ground in preparation for its latest wash plant. The new plant is designed with two major goals in mind – to be environmentally sound, and to process an average of 1000 tonnes per hour of raw input. Its construction began with the excavation of 365,760m3 of earth to level the 0.12km2 site, and to create carefully designed drainage and pond systems that prevent any unwanted discharge of plant process water. One of the biggest challenges was engineering the plant, stockpiles and ponds to fit a smaller footprint.

Each of the three screens at the new plant are Deister BHM inclined washing/rinsing screens, which are engineered to ensure day-to-day reliability through features such as dual vibrating mechanisms and baked epoxy-coated springs.

As to material flow at the site, pit run material travels through a splitter box to enter either of two three-deck 8ft x 20ft screens for desanding. The plant manager knew from past experience that each 2m x 6m screen would be more than large enough to handle 453 metric tonnes  per hour, for a total of 907 metric tonnes per hour. And they were running about 30,283 litres per minute over the  primary screens.

After desanding, material enters a 12ft x 48 ft classifier, followed by processing within dual 54-inch sand screws. The plus-1/8-inch and plus-3/16-inch material is fed into dual log washers. From there, material is conveyed to a three-deck 7ft x 20ft screen for the rinsing and sizing of products such as #57, minus-1 1/2-inch, and plus-3/8-inch material.

The plant manager had already had a long-term relationship with Deister. ?They know our material is very high in clay content, so we?ve got to have a lot of water. They know what we?re trying to accomplish and what products we need,? he says.

As a result, he says it is advantageous to ?standardise? his equipment and parts, particularly in the integral area of screening.  ?With multiple locations, I think it is better to be standardised. I have one set of parts on the shelf for eight different locations. I don?t have to keep a big inventory. It is a maintenance training issue too. All the guys are familiar with this screening equipment,? he adds.

To increase capacity and throughput, a New Jersey-based granite quarry recently upgraded its screening circuits by replacing older single-drive units with new dual-drive screens customised specifically for the plant and its applications. In 2007, four new Deister finishing screens were added: three triple deck 6ft x 20ft units and one single deck 6ft x 20ft screen. Screening upgrades were required as previous single-drive screens couldn?t handle the tonnage and keep a consistent stroke, so the operation was seeing a lot of carryover. On the smaller opening screens particularly, it was tough to get the material clean and the inconsistency could put the product out of spec, which would cost some tonnage.

With the four new finishing screens, improved gradation has been one of the most favorable results. The facility is able to use smaller openings in its screens and still get the finished material as  clean, if not even cleaner, than expected. That delivers more options as to the cuts that can be made – and adjustments can be made to the screens to really finetune the gradation. But importantly, the product is cleaner and more consistent and they aregetting consistent tonnage across the screen.

Five additional screens are also being added to the primary and secondary circuits, which include four triple deck 8ft x 20ft units and one single deck 7ft x 20ft screen – again, all dual-drive units replacing less efficient single-drive models. Plus, the upgrade to triple deck screens, particularly in the primary position, will allow increased throughput as the middle deck acts as a ?relief deck? between the top and bottom deck, preventing the bottom deck from overload.

Each new screen is designed as a custom fit within the plant, allowing the crew to take the old screens out and drop the new ones in with little-to-no fabrication. Deister designed the screens to meet the exact specifications of the discharge and feed chutes. All the screens were delivered on bases that would sit directly on top of the existing structures, allowing a pick-and-place type of installation.

Each screen is mounted on what Deister calls an H-beam base, which is set at a 20-degree incline. The base allows for a complete setup of the screen at the factory – with all the springs, and the media – ensuring an easy installation with virtually no work to do on the chutes.

Proper screen specification involves making sure that the manufacturer understands your production goals and is supplied with complete application data. Armed with accurate information, the manufacturer can customise the screen setup for maximum performance.

Managing the art and science of screening is all about the details – and it is well worth the effort – as breaking the screening bottleneck delivers the big benefits of capacity, reliability and profitability.

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