Steve Davis discusses why we clean belts, the cost of having dirty belts, and some practical guidance on selecting cleaners and keeping belts clean.
Conveyor carryback costs the bulk handling industry billions of dollars every year in clean up, lost product and downtime.
Carryback is the fine material that falls on the floor or structure along the length of the conveyor, which can build up on and below idler rolls. It can damage the conveyor and become a fugitive dust source. There are also safety implications as carryback can lead to injury, serious damage and encrustation.
The costs of carryback can quickly build up. Consider the following: a 1.5-metre-wide belt moving at four metres per second, with 1.2 metres of the belt dirty. If the amount of carryback is one millimetre thick, the volume carried is 17.3 cubic metres per hour or 121,000 cubic metres per year.
In this scenario, there would be potentially more than 100,000 tonnes of coal, or nearly 200,000 tonnes of iron ore on the floor from each conveyor. Reduce the thickness to 0.1 millimetre, and it is still 10,000 tonnes of coal and 20,000 tonnes of iron ore.
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- BULKtalk: Best practice in chute design – Part one
Assuming an iron ore stream includes some 20 similar conveyors from pit to port, and as I write, the value has just passed $100 per tonne. This could be $40 million on the floor, on top of the cost to clean it, plus any resulting damage it causes, adding up to a potential cost of $75 million annually for a single operation. By reducing carryback to 0.01 millimetre, there is the potential to save nearly $70 million a year.
Although many suppliers have developed excellent belt cleaning equipment, keeping belts clean is still a major problem. It’s not possible to eliminate carryback entirely, however belt cleaning equipment can be implemented to the best possible standards.
Ploughs stop dirt (and larger things) from entrapment between belt and pulley, extend lagging life and protect the belt from punctures and the pulley from failure. A missing plough could easily cost several million dollars for pulley and belt replacement and downtime. Pulleys can tear from their supports if tramp enters between belt and pulley. This is a safety hazard on top of the cost of damage.
Getting the best from cleaners
There is no Australian Standard for belt cleaning equipment. In order to get the best out of cleaners, start by selecting one that is fit for purpose from a specialist supplier.
There are very few new ore conveying situations, so select a cleaner type with proven capability in blade type and pressure setting system for the ore on the belt. Understand the requirements, and the differences between wet and dry, coarse and fine, abrasion and the like. Is the belt reversible? Will it have clip or vulcanised splice? Is the pulley crowned? Would an air knife really remove fine wet residue? Is tungsten carbide better than polyurethane for the blades? Is a brush cleaner or belt wash appropriate?
Choose cleaners that make maintenance easier. Self-adjusting blade tensioning gives better performance as precise manual adjustments are time consuming. Robust components that are maintainable while wearing gloves are required.
Install the cleaners according to supplier guidelines. One cleaner is seldom sufficient for best belt cleaning. Primary and secondary cleaners, and possibly more, are usually necessary. Design chutes around the cleaners. Consider the cleaner discharge, and preferably how it can return product to the belt.
Do not allow procurement to buy something different to the original design just to save a few dollars. The additional cost to change the chute to match the new supply, or the ongoing failure in operation when installed incorrectly will be far higher than the saving.
Cleaners start to wear out immediately after commissioning. Cleaner wear rate is high, so for best performance provide good inspection and maintenance access in and around the cleaner. Safe, simple and fast maintenance will get best results.
Consider retracting the components from the chute, which could need wider and lower walkways. Provide access panels for easy condition monitoring. Who wants to lie on grating to peer through a slot to see cleaner condition? Remember that the chute is a guard, and all penetrations must comply with AS/NZS 4024.
I have seen a cleaner installation so well tucked away under the chute that the maintenance team was not aware of it, and others that are completely inaccessible without entering a chute or dismantling structure.
The worst I have seen so far is a site with a zero-maintenance expectation for cleaners and no spare parts. Carryback was extreme after two years operation, and the facility had to pay to dispose to landfill, turning $150 per tonne profit into $275 per tonne loss.
Ploughs
Install ploughs before any belt to pulley nip point, using vee or diagonal ploughs for unidirectional belts, and diagonal ploughs only for reversible belts. Ploughs only work properly if the belt is positively flat where it contacts, and the plough is as wide as the belt. Including safety chains is highly recommended.
The plough must be deep enough to divert the largest rock that could be on the belt. If in doubt, a shrouded plough should keep all particles from the nip point.
Rounded or bull nose vee ploughs may not be best for wet fines, as the particles can build up in front and cause the plough to lift and wear and can damage the belt.
Ploughs discharge to the side of the belt, and many installations have no provision to collect this discharge. It is common to see ploughs bury themselves and the belt at the tail end of a conveyor, while elevated ploughs can throw a large rock several metres.
Ploughs also need maintenance, so access is required. Many ploughs are self-adjusting under gravity. If allowed to wear down to the steel considerable belt damage can result. There are some ploughs with quick-change blades for fast maintenance.
Cleaner types
Cleaner selection is subjective, and previous experience is a good start for selection. I believe there are some differences between the available cleaners. I also believe that any cleaner chosen based on a logical selection process, that is correctly installed, and is simple to maintain will function well.
Maintenance spares must be readily available too. One of the best sites I have seen had cleaners supplied and maintained as a single contract in chutes designed to suit. There are contractors in Australia who can supply such a package.
I am often asked to make a recommendation for a replacement cleaner when the fault is with installation and maintenance. Discuss problems with the supplier first as they have experience to help. Retrofitting different cleaners is fraught with possible operational problems when making assumptions regarding the installation. Replacement cleaners are often installed incorrectly to avoid rework.
Cleaners are not expensive. It is worth spending a few extra dollars to get a good result. There used to be many cleaner and plough designs from non-specialist suppliers. Most of those I have seen were relatively ineffective, but the cost of the engineering was not reflected in the cheaper supply cost.
The majority of cleaners for belts are blade style with some form of blade pressure control. Use supplier guidance for pressure settings, as grinding the blade into the belt under high pressure will wear the blade faster and probably will not improve cleaning. Beyond this, the materials of construction and blade configuration are extremely varied. I have not seen any table that recommends a particular type and application. Selection is subjective. Some suppliers recommend wetting the blades to improve cleaning and reduce wear.
Diagonal pulley cleaners are relatively new. Two steel cords with reversible blades in between are tensioned around the pulley face. Supplier recommended installation and setup for these cleaners is specific and retrofitting to an existing chute may require more chute changes than other types to get optimum performance.
Rotary brushes need space, a drive, bearings and supports, so are more complex and maintenance intensive. Capture of the discharge is different. The brush may clog quickly with fine, wet, sticky materials. Brush cleaners may generate static electricity, which can be a hazard. Rotary ‘finger’ cleaners are also available.
Rappers are useful for sidewall and cleated belts. Air knives can work well with fine carryback such as alumina. Spiral and disc return rolls can remove carryback, but distribute it along the conveyor.
Wash boxes are the ultimate cleaners for difficult carryback. Primary and secondary cleaners are still required with the wash box being the last stage. Wash boxes need space on the return belt for operation, and good access for maintenance. From experience, they can be difficult to retrofit so for some conveyed ores it might be prudent to allow space in the design even if not initially installed. A clean water supply is best, and some method of disposing of the slurry formed. Wet process plants usually can manage a small slurry stream, dry process plants are another matter.
Water jet cleaners are non-contact systems that may be applicable. The slurry resulting has to be captured and treated. Generic spray bars are generally not effective for belt cleaning as nozzle size, spray shape and orientation should be specific.
All cleaners and ploughs remove material from the belt. For best results, address where and how the material will be collected and disposed. Primary and secondary cleaners generally return ‘dribbles’ to the main chute, but this can be difficult in the chute geometry. Consider polytetrafluoroethylene slides, water and vibration to encourage flow on shallow slopes in dribble chutes.
Belt damage, splice damage, belt clips, build-up of materials on pulleys and lagging damage all interfere with the operation of belt cleaners, and will accelerate wear of the cleaner. Lifting the cleaner is not a solution, nor is increasing pressure on the blade. Some cleaners accommodate these issues better than others. Multiple finger cleaners may be more effective than a continuous blade for an irregular belt surface.
Large belt cleaners have large and heavy components for manual handling so consider the weight, location and removal method. A primary cleaner bar with blades for a 2200-millimetre-wide belt will weigh approximately 150 kilograms.
Motors, drives and other mechanisms add complexity. Many cleaners have stored energy in the adjuster from springs, air cylinders and hydraulics; release this energy safely prior to work.
Summary
Install quality cleaners to supplier recommendations and maintain appropriately for best cleaning. No cleaning system is perfect. There will always be some carryback, so consider how this will be collected.
Belt cleaners and ploughs need frequent inspection and regular maintenance to work well. I have not seen a cleaner on-line condition monitoring system that would give advanced warning of wear or other problems, but this would cut out the regular inspection.
Safe, accessible, easy, quick inspection and maintenance will help keep belts clean. Simpler maintenance systems are easier, cheaper and faster to maintain.