 |
Heavy Duty Lump Handling
Bulk materials handling is often referred to as more of an art than a science, and this is certainly true when equipment needs to be designed for materials known to be very difficult to handle, such as hard abrasive materials in lump form.
A variety of industries have a requirement to handle materials in the size range 50-100mm, be they stone in the building products or quarry industry, clinker in the cement industry, slag or metallic lumps in smelters, bottom ash in coal fired boiler plant or a variety of materials in the recycling sector such as glass or hogged wood chips. These materials often present some of the greatest difficulties for a materials handling engineer in that they can be abrasive, unbreakable (in normal conveying) and prone to bridging.
Quite often the products need to be transported in totally enclosed and dust tight systems for environmental reasons and incorporate multiple inlet or outlet points along the length of the route. Many conventional designs of conveyor and elevator are not suitable for such materials or are reliable enough for long term usage.
Transmin has a range of feeding, conveying and elevating equipment specifically designed for these traditionally challenging applications. The designs are based upon proven techniques such as vibrating feeders, screw conveyors, chain conveyors and chain bucket elevators, but also incorporate specialised features to enable the machinery to cope with hard large-sized products.
Engineering Manager, Paul Ingleson explains that "when dealing with materials with a high percentages of lumps, it can be a costly mistake to attempt to install 'standard' conveying and feeding equipment. In many applications, equipment which may operate satisfactorily with 10% lumps, will be completely inoperable at 90% lump, often leading to catastrophic failure of conveying components and drive systems."
Transmin recently incorporated a number of innovative features into its design of twin screw extractors for a 100% lump gypsum and limestone application to ensure maximum reliability and availability. Firstly, to reduce power draw, the screws have been designed counter-rotating to lift in the centre. Research has shown that this absorbs less power than any other alternative. Furthermore, clearances have been increased sufficiently to suit the lump size, and as an added feature, the screw casing tapers gradually away from the screw to reduce the occurrence of material jamming between casing and flight. Even drawdown within the hopper was achieved with a combination of flight pitch variation and flight diameter graduation. The selection of mechanical components and design of shafts and flights has also been carefully reviewed to ensure long life and reliability.
A particularly useful tool in the design of these feeders has been the utilisation of a newly purchased 3D design and modelling package to enhance Transmin's existing design and drafting capacity. It is anticipated that this technology will be further expanded to provide Transmin's mechanical engineers with state of the art facilities for finite element and vibration analysis of new and existing plant and equipment.
Further recent examples of lump machines supplied by Transmin include special twin shaft screw feeders on lump limestone (<75mm) and gypsum at a cement works.
Another recent example of a lump machine supplied by Transmin is a 25m tall 100tph chain and bucket elevator on nickel reverts (slag and nickel agglomerate) and flux at Kalgoorlie Nickel Smelter. The development of this machine was particularly challenging as the material is nearly all lump, the reverts being particularly aggressive due to their irregularly shaped, unbreakable, highly abrasive pieces up to 150mm in size. They are also very dense at approximately 4.0t/m3.
Transmin's new machine replaced a conventional belt and bucket elevator that had been struggling to achieve a reliable performance. The Transmin solution was to design a slow speed gravity discharge elevator, which employed two strands of heavy duty round link chain with buckets mounted between the chains to avoid contact with the material. A low filling efficiency within the buckets was chosen deliberately to minimise spillage and dredging. Naturally, the whole construction was heavy duty with appropriate use of wear resistant materials where required. Other more specific features were included to ensure long-term reliable operation.
Drag chain or 'en-masse' type chain conveyors have also been used traditionally for such applications due to the fact that they are totally enclosed, robust and can incorporate multiple inlets and outlets; however they need to be applied very carefully to ensure adequate performance is achieved. Critical factors include whether or not to select an 'en-masse' style or a wider 'drag chain' type having a lower bed depth of product. A heavy chain with special flight bar attachments is often required to eliminate potential problems such as chain rise, whereby the conveying chain climbs over the material rather than conveying it forward.
Whilst the chain conveyor lends itself to multiple inlets and outlets, these have to be very carefully designed for lump materials to avoid jamming. Transmin uses a special design of outlet gate which has the gate plate form the bottom plate of the conveyor when closed. This design eliminates the pocket in which material can build and promote other problems such as chain rise. In addition, the flush fitting nature of the gate reduces jamming during closure.
Whilst an 'en-masse' chain conveyor in a straight horizontal or straight inclined configuration can be effective, 'en-masse' elevators – i.e. chain conveyors which incorporate a bend and a subsequent inclined or vertical section – are ineffective for this type of application. This style of machine is only suitable for granular materials less than 10mm in size and only when mixed with a high proportion of finer particles.
Specifiers and purchasers must understand that the correct equipment for lump applications can be substantially more expensive to purchase than standard designs, however the initial capital cost can pale into insignificance in the long term and is more than offset by reliable operation. Transmin Marketing Manager Phil Gilbert stated, "We see many examples of attempts to apply standard machines to these difficult applications, which end up failing or proving incapable of achieving design specification. Our approach at the bid stage is always to discuss fully with the client why a particular machine may appear to be more expensive, what features are included and the benefits they deliver, but sadly this isn't always possible when the purchasing decision is purely price driven.
These sorts of applications are not that common and perhaps this contributes to a certain amount of ignorance regarding appropriate equipment because it is highly specialised, which means much knowledge and experience rests with a few companies or individuals.
Good design, like art, often looks simple but the secret lies in the detail. For the handling of lump materials, the details are more important than ever.
|
BACK TO TRANSMIN NEWS ARCHIVE PAGE
