Lafarge - Reliable solutions for aggregate conveying

When installing a new plant to produce specialist aggregate blends at its Barnstone works in Nottinghamshire, Lafarge turned to Guttridge Ltd for a range of conveying equipment. Here we take a look at the solutions installed and the design features that have proven especially important in delivering reliable performance.

Operating in 64 countries, Lafarge is the world's leading supplier of building materials with a product range that includes cement, aggregates and concrete. At its Barnstone works in Nottinghamshire, UK, the company bags cement for supply to trade and to the general public and also blends dry sand and cement with aggregates to form specialist mixtures for specific construction applications. Transport to and from the site is by road and the product is typically shipped in 20 kg sacks, although there is facility to deliver products across the 10 - 25kg range.

In 2011, the decision was taken to install a new blending plant at Barnstone to make a range of aggregate based products such as concrete and mortar mixes. A vital element of the project was a series of conveyors for material transport around the plant: six new bucket elevators and a belt conveyor. Following a careful assessment of the options, including an evaluation of the reliability of other equipment already operated on the site, Guttridge Ltd (Spalding, UK) were chosen to provide these machines. A number of sophisticated design features were specified to ensure problem-free, low maintenance operation, for these abrasive streams.

Defining requirements

Figure 1 shows a schematic of the central elements of the blending plant. Bulk feeds - cement, sand and aggregate - are transported from storage up through the building so that they can be sieved under gravity to remove debris and oversized material. From the base of the sieves material is again lifted vertically into the top of the blender, which is a blade/plough mixer that blends the feeds to a uniform product. Material exiting the blender is transported up into large product silos for storage prior to bagging. By controlling the relative proportions of the various feeds, the blending plant can be used to make a wide range of different products following closely defined recipes.

The flow of material through the plant demands vertical lifting at various points, to enable gravity flow down through vessels and unit operations such as the initial debris filter. In addition, certain feed materials are transferred horizontally, some over significant distances. All of the streams routinely handled are highly abrasive, a defining factor in terms of the specification of suitable equipment. In addition, some present a dust hazard, making containment an issue in specific areas.

Bucket elevators for vertical transport

Lafarge identified bucket elevators as the best option for vertical conveying across the new facility, an application that called for a total of six separate elevators. These handle both the feeds to the blending unit and the mixtures produced, at various points across the plant. Reliable and cost-effective with an operating range that comfortably spans the required throughput, bucket elevators are also simple to maintain, an important feature for equipment facing a continuous and demanding duty.

Guttridge had previously supplied a number of bucket elevators to this site and these were already operating successfully and reliably on similar duties. Based on this experience Guttridge engineers were able to suggest new features that would support the attainment of the very highest reliability levels in the new systems - a major goal for the site. The opportunity for shared spares and the benefits of having a local supplier were also influential when it came to choosing a supplier for the conveyors, although cost was, of course, the overarching concern.

Each elevator was uniquely specified to meet the individual application but some general reliability enhancing features are shared by them all. These include:

• Removable Hardox wear plates in the head and boot of the elevator to protect areas of potentially high wear
• Coned, cage bottom pulleys to prevent material getting between the pulley and belt and causing damage
• Rubber lagged top pulleys to reduce the risk of belt slippage
• Stand off bearings with stuffed gland seals at the head and boot to prevent dust ingress and associated bearing damage

Five out of the six elevators were specified with nylon buckets to cope with the highly abrasive materials being handled, the exception being the elevator carrying only cement, the least abrasive stream. The buckets for this machine are steel with wear bands.

The elevators were also designed with sensors to detect and warn of potential operating problems. Diaphragm switches inside each elevator boot, for example, trigger an alarm if the boot fills, an event usually associated with an elevator blockage, while rotational underspeed sensors on the boot shaft provide a relatively sensitive indication of belt slippage. Together these systems help to ensure that any operational problems with the elevators are picked up quickly to minimise the potential impact.

Although these machines are specified to ensure reliable long term operation with minimal maintenance, the issues of access and part replacement were considered during the design stage. As a result certain features have been incorporated to ease any necessary maintenance. Extra access panels make it easier to get to whatever part of the machine needs attention, including individual buckets, while split pedestal bearings simplify bearing replacement.

Belt conveying for long distance transfer

Aside from the elevators for vertical transport there was also a requirement for the horizontal transfer of a cement/sand feed over a relatively long distance. One option was to use several screw conveyors in series but the preference was for a simple solution involving just a single machine. Thoughts therefore turned towards a contained belt conveyor although spillage was a major concern. In addition, in the past conventional belt conveyors had proven problematic with a dusty stream so there was substantial emphasis on achieving complete containment. Bucket elevators provide vertical material transport across the plant.

The system selected was a Kleenbelt conveyor (see figure 3) a unique, well-proven solution for applications where it is particularly important to avoid product spillage or loss of containment. With a throughput range of up to 800 cu.m/hr these conveyors are used across a range of industries to transport materials ranging from rolled oats and rubber crumb to coal fines and fertilizer. Their use in transporting cement is an established application.

Effective sealing of the top section of the conveyor belt is achieved through the use of trough plates and a central supporting roller. Top covers, in combination with belt scrapers and integral dust extraction systems ensure complete product containment. For this application, tungsten carbide belt scrapers were specified to cope with the highly abrasive feed, and, as with the bucket elevators, a performance monitoring device was installed; in this case an underspeed or belt slip detector on the tail end shaft.

Assessing performance

All the supplied systems were commissioned swiftly and easily and have gone on to give the high performance expected from their specification. In particular, the level of product containment delivered by the belt conveyor has been a notable success. Robust and reliable, this unit has a highly effective sealing system and efficiently captures and feeds material onto the belt without spillage. The machines are clearly still in their infancy but expectations of long term beneficial operation are high, based on early experience and the delivery record of other systems. The blending plant is operating smoothly, driven by steady, efficient material transport in all areas.

More generally this is a project that exemplifies how conveyor design and specification is changing to meet the requirements of industries intent on reaching the very highest levels of reliability with minimal manual input. Focusing on reliability, operability and ease of maintenance from the outset leads to the inclusion of a number of features that undoubtedly increase initial capital outlay, but the expectation is that this expenditure will be offset by long term gains. By investing in truly robust equipment, simplifying maintenance, and enabling instant upset detection it is possible to achieve years of trouble-free operation with minimal manual input, maximising the return on investment in high quality conveying equipment.