Inefficient screening and poorly maintained screens can affect the success of any operation and its costs, no matter how much material is produced per hour,? commented David Hewitson, the mining and quarrying applications engineer at Australia?s Nepean Rubber & Plastics.
?Lower production rates, poor product quality, complicated and expensive process add-ons, increased wear on other equipment and higher labour costs are all outcomes of poorly performing screens.
{{image2-a:r-w:250}}Frequent shutdowns to replace worn or broken screen media or to clean screens due to blinding, pegging or clogging or having in-spec product trapped in an endless loop?recirculating in closed-circuit systems?all greatly affect production and cost producers millions of dollars in lost revenue each year.?
HIDDEN COSTS
It might not be immediately obvious but if a screen is not operating at its intended capacity, it can severely affect a producer?s bottom line in a number of ways.
A blinded screen can reduce screening efficiency by 70 per cent or more. So, an 8? x 20? (2.4m x 6.1m) screen deck suffering a 70 per cent efficiency reduction suddenly is producing at the level of a 4? x 12? (1.2m x 3.7m) screen deck.
Further, a rule of thumb is that a fully loaded screen deck that is even partially blinded?say just five per cent?will result in a total production loss equal to the percentage of the screen that is affected. This means a single screen deck blinded just five per cent on a 500 tonne per hour plant that produces 1.2 million tonnes per year will lose about 60,000 tonnes of that total production in spec product every year.
At a $10 per tonne selling price, this operation could easily lose $600,000 per year in potential revenue from what seems like a small screen deck problem. This example illustrates the effect realised from only one screen deck on one screenbox. Multiply it by multiple decks and screenboxes for the true result of poor screening efficiency.
{{image3-a:l-w:250}}Poor screening often leads to poor product quality or out-of-spec product, both of which can have far reaching repercussions. On the plant side, these include the additional cost and time to re-screen the material, additional wear and tear on the equipment, the cost of recirculating material through the crusher, lower prices and even lost customers.
For aggregate customers, an out-of-spec product could cause them to face stiff penalties and fines for using inferior stone, as well as delayed project costs while the stone is being replaced, often eliminating any hope for an early completion bonus.
In addition, inconsistent gradations can cause asphalt and concrete mix design complications, typically requiring completed jobs or portions of jobs to be replaced at the contractor?s cost.
The cost of inefficient screening can also severely affect a portable contract-crushing operation. For example, a 350 tonne per hour portable crushing and screening plant working to fulfill a 500,000 tonne contract would generally require 3150 tonnes of production per day within a nine-hour day?and approximately 159 working days to complete the contract, working five days per week for a total of almost 32 weeks.
{{image4-a:r-w:250}}If the plant had even a five per cent reduction in screening efficiency due to blinding or pegging, it could require the contractor to spend eight more days on the job to complete the project. Eight days might not sound like much, until electric power, diesel fuel consumption, wear on the equipment and employee salaries are taken into account.
That is an additional eight days in which a new contract cannot be started. Because income for a portable crushing operation is often based on the number of contracts that can be completed each year, every day lost directly affects income potential.
Viewed in this light, an inefficient screen certainly takes on ominous possibilities.
NOT ROCKET SCIENCE
Although many producers believe the opposite, maximising screening efficiency is not rocket science. Simple, consistent screenbox maintenance?and taking advantage of the more recent screen media solutions available today?can help ensure a higher level of in-spec material production.
?Installing self-cleaning screen media, like Major Wire?s Flex-Mat 3 in tensioned or modular versions, is one of the easiest ways to eliminate blinding,? said Hewitson. ?Polyurethane strips bond the individual wires and allow them to vibrate independently at different frequencies.
This creates more screening action so properly sized material falls through instead of sticking to the wire. Flex-Mat-type screens produce up to 50 per cent more screen capacity, providing even more potential for material to fall through.?
Although there are many different types of screen media, from woven wire and polyurethane to punch plate and rubber, the vast majority of screen media used today is still traditional woven wire. When tensioned screens using woven wire ?fail?, the problem often results from broken woven wire screen media?typically occurring for one or more of the following reasons:
? Poorly tensioned, loose screens that beat themselves against the crown bars at a high frequency.
? Inconsistent crown bar heights that prevent a proper arc for the screen media to tighten and rest against.
? Worn out, damaged or missing clamp bars and rubber channel that allow material to wedge between the screen media and screenbox rails.
? Wash plant spray bars that force a steady stream of water directly onto screen
media wires from a close distance or that run when there is no aggregate on the screen deck, leading to ?drilling? holes
into the wire.
? Excessive impact on the feed end of the screen from material falling too great a distance onto the screen or very large material falling onto small wire diameters, causing premature wear and breakage.
? Off-calibration of a screen?s eccentric throw, damaged springs or rubber mounts or decks affecting a screen?s vibration that prevent an even flow and spread of material across the screen media for maximum throughput.
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All of these problems are relatively easy to spot with frequent visual inspections of the screenboxes and screen media, while they are in operation and when shut down. A formal inspection schedule, similar to those typically practised with the crushing operation, will allow producers to make pre-emptive repairs and adjustments before problems begin to affect production.
Frequent inspections also allow a producer to make repairs or replace parts when it is convenient, not during the height of the production day.
Producers should also check the performance of their woven wire screen media. One of the easiest ways to achieve this is to look closely at the pile of discarded screen media for a history of recurring problems, such as broken wires, wires pulled from the hooks, blinding and/or pegging, or unusual wear patterns.
SCREEN MAINTENANCE
Daily maintenance routine
1. Check the oil level and add oil if necessary.
2. Check the breather caps for build-up.
3. Set the belt-tensioner spring tension at the recommended setting.
4. Free the screenbox of material build-up beneath the vibrating basket.
5. Make sure the guards are in place and in good condition.
6. Check the screen media draw bolts for proper tension.
7. Make sure the snubber rubber is in contact with the snubber plates.
8. Make sure the feed box is free of excessive material build-up.
Weekly maintenance routine
1. Inspect screen media and feed liner for excessive wear.
2. Inspect the springs for excessive wear.
3. Inspect the drive shaft seal for leakage and add fresh grease.
4. Inspect the drive belt and sheaves for excessive wear.
5. Check the tag tension band for damage.
For more advice on how to keep your operation profitable, Major Wire and its Australian dealer Nepean Rubber & Plastics can provide a free screening plant audit. During an audit, a Nepean representative will review the screening plant and compile a detailed report of findings with photographs and clear recommendations of adjustments that will help the operator maximise the screening operation.
Gary Pederson is the vice president of sales for Major Wire Industries Limited, based in Candiac, Quebec, Canada.