Polymer rail sleepers being tested for the mining industry

1st April 2005 By: peter cromberge

A polymer rail sleeper for use in mines has recently been intro- duced by a local plastics-recy- cling company.

The sleeper, a composite of polyethylene materials, is designed to offer a rugged and durable support for underground railway lines.

Tufflex Plastics, the Germiston-based company that manufactures the sleepers, identified a niche in the market for sleepers with a longer life than the concrete and timber sleepers commonly used in mines. While the US has been using polymer sleepers for years on surface railway lines, the local sleepers have been designed for use in mining environments where pH levels fluctuate greatly and water and humidity levels are high, says Tufflex Plastics marketing director Alan Samuel. Designed for a lifespan of over 30 years, the material is water-resistant and does not warp, degrade or swell upon prolonged exposure to water. The sleepers have a slot under the rail seat area cut at a 1:20 cant in order to accommodate the conical wheels found on rolling stock. This is to maintain full rail wheel contact, which, apart from saving on wear and tear, is also a safety feature, says Samuel. “At present, the sleepers are being tested in AngloGold Ashanti’s and Gold Fields’ mines, and have so far deliver- ed positive results, especially in a derailment situation,” he adds.

While timber sleepers have been used for centuries and have been the traditional mainstay of the mining industry, Samuel explains that price wars forced on manufacturers by pressure from the mining houses, have affected the quality of some sleepers.

“Price jockeying among certain companies has led to the supply of varying qualities of timber sleepers, some with dubious origins and unlikely to last for an acceptable period of time,” says Samuel. He adds that concrete sleepers, the other popular alternative to timber sleepers, are known for breaking when installed incorrectly or when not maintained properly.

Samuel explains that the polymer sleepers can be redrilled and fastenings replaced as the need arises. This is not practically possible with concrete sleepers, while the drilled holes in untreated timber sleepers can sometimes be a source of rot.

“The polymer sleepers come supplied with the fasteners attached – all that is required is for the fasteners to be aligned with the rails and the bolts secured,” explains Samuel The fasteners are hardened and tempered spring-steel secured with galvanised coach screws. Samuel notes that, because they are similar to timber, the polymer sleepers are more labour-friendly and take less time to install. Worker satisfaction on installation has been noted on various underground visits.

Tufflex Plastics is working in partnership with Owen Plastics, a large plastics- recycling company, which is lending its premises and expertise to the manufacturing operation. Produced in all the common gauges and two sizes to cater for axle loads varying from 5 t to 10 t, the black-coloured sleepers are manufactured from recycled plastic and extruded from a low-pressure extrusion machine imported from the Netherlands. The company can also add fibreglass and steelglass to the polymer feed for the sleepers to cater for even higher loadings.

“It is far more cost-effective and environment-friendly to use recycled plastic as opposed to virgin plastic,” says Owen Plastics MD Harry Rombouts.

At a cost of R5-million, the extrusion machine operates around the clock and can produce about 5 000 sleepers a month.

However, Samuel notes that the company expects to install another extrusion machine that will create the capacity to quadruple current pro- duction volumes. Although plastic sleepers are not new to South Africa, the correct formulation is critical for the successful operation of the sleeper “Local manufacturing capacity is essential for producing the volumes necessary to satisfy domestic demand,” says Rombouts. “We aim to be in a position in a few years’ time to manufacture sleepers suitable for mainline use on surface track as is now available in the US.” The polymer sleepers have been introduced as an alternative mainly to concrete sleepers, which are produced by only one company, and Samuel explains that the polymer sleepers are priced higher than their concrete counterparts because of international polymer prices.

However, he notes that the greater cost is offset by a recycle exchange system that is a feature of the sale of the sleepers.

Once the mine has concluded its operations, polymer sleepers can be removed and sold back to the com-pany or other recyclers for use as feedstock.

“Concrete sleepers are difficult to remove, cannot be recycled and generally can only be used as landfill after removal,” says Samuel. He adds that the functionality of wood, upon removal, is also limited to use as a construction material or for firewood. The company is confident that the buy-back scheme will make the material an attractive one for use in mines with a long future.

“The value of the polymer to the mines lies in ease of installation and the durability of the material,” says Samuel.

“In a recent underground inspection of an installation, a derailment at the end of a switch was noted and the only damage to the sleepers were some cut marks with the sleepers still sound and fully functional,” notes Samuel. “From a safety perspective, tests have shown that, once combusted, the material is not harmful on inhalation and registers 1,975 on the toxicity index which is well below the maximum level of five.”