Travelling light - lowering energy and conveyor belt costs
Long-distance transportation of minerals from the mine site is a key contributor to rising energy costs for mining companies. Gerard van den Hondel from global aramid fibre manufacturer Teijin Aramid looks at new materials developments that could reduce energy use of belt conveyers.
Most minerals travel a long distance from their point of extraction to point of use. Typically their journey involves trucks, conveyor belts, trains and boats. While on short hauling distances truck transport is very flexible, the heavy weight and high fuel consumption of these vehicles has led to a trend to shift to conveyor transport. Furthermore, distances within mines are tending to increase – typically, the longer a mine is in operation, the further away extraction moves from the processing point, for example in the case of a power plant built next to a coal deposit. Longer hauling distances are one of the reasons that global energy use in mines is increasing by 6% a year. Naturally, mining companies are seeking to counter the associated increase in energy cost. An additional challenge is the license to operate from society. Can the use of fossil energy be diminished and carbon emissions lowered – on transport at the mine, can something be done to reduce the energy use of belt conveyors?
To answer this question, it helps to go back a century or so to when belt conveyers first came into use. The first conveyor belts were small and at that time, rubbers were reinforced with cotton. Today, natural cotton fibre is replaced by modern, man-made materials such as polyester and polyamide, which are light, strong, resilient and abundantly available.
However, they suffer one serious drawback – low stiffness modulus. Long belts have extreme elongations – consider a 5km conveyor installation that has a 10km looped belt. If this belt elongates by 3% under running tension, 300 metres of excess belt length is formed that needs to be taken up. For this reason, long belts are reinforced with another material that has been used in belts for many years – steel, in the form of embedded cables. But in addition to its high modulus, steel is characterised by a substantial density of 7.8kg/L and, as is the case with any moving system, more weight means more energy consumption. So, are there any solutions to reduce the mass of conveyor systems?
Are aramids the answer?
The name aramid derives from aromatic polyamide. Like any man-made high-strength fibre, its strength is achieved by carefully aligning strong molecular chains along the direction of the force. For aramid, the result is a fibre with strength close to that of steel (see page 39). A big difference, however, is the weight – aramid has a density of 1.45kg/L, which is five times lighter than steel. Aramids are also three times stronger than polyester, which is currently the second most dominant reinforcement material in conveyor belts.
Since they first came into use in the 1970s, aramids have moved from specialist applications to a wide range of uses where high strength, high modulus and low weight are required. Their use in ballistic protection is widely known, from bulletproof vests that need to be light for comfortable wearing to lightweight ballistic shields in ground vehicles or aircrafts. Less visible but equally widespread is the use of aramids to reinforce car engine hoses (they are able to withstand the heat, chemicals and vibration around a car engine), drive belts (that require high strength and fatigue resistance for long life) and brake pads (where heat-resistant aramid replaces asbestos).
Typical moving reinforced rubber products using aramids are high-performance car tyres, which require low weight, dimensional stability and high strength to withstand the forces while driving at high speeds.
Similarly, the use of aramid in conveyor belts means the whole system weight can be substantially reduced, as was verified by Professor Gabriel Lodewijks of Delft Technical University, the Netherlands. Lodewijks calculated this weight reduction using an existing conveyor belt installation in South Africa, used to transport coal from mine to power plant at the Optimum Colliery operation in Mpumalanga province. This installation, which was run with steel cable reinforced belts, was recalculated for comparison with aramid fabric-reinforced belts. The results were interesting, with the belt weight reducing from 32kg/m to 19kg/m. This in turn reduced the system weight (carrying belt, coal load and empty belt on return side) from 170kg/m to 144kg/m, equivalent to 15% less moving mass.
On a long conveyor belt, more than 60% of driving energy is lost in indentation rolling resistance. The term refers to the hysteresis loss when a rubber belt passes over its support rollers and deforms. A solution to this is sulfron, a recently developed aramid-based rubber ingredient that lowers hysteresis when used in the running layer of a conveyor belt.
In the same study, this hysteresis reducing aramid was included in the belt drive power calculation. The combination of low-weight aramid reinforcement and reduced rubber rolling resistance was shown to lower belt power consumption by 25%.
The questions is, how relevant is this saving? In the case of the South African installation, which covers a distance of 15km using four conveyor belts, under full load the belts use 4,000kW – the equivalent energy consumption of a town with 20,000 inhabitants. Therefore the energy saving on a single large conveyor installation is equivalent to the domestic electricity use of 5,000 people. When this 1,000kW saving is applied 24/7, it adds up to an energy bill hundreds of thousands of dollars lower at the end of the year for a single conveyor operation.
That’s the theory, now let us next consider some practical cases. There are a number of short aramid belts currently in operation around the world, as well as some long-distance ones. One example is a 2.6km belt at Bulgarian power plant Maritsa Istok-2, where an important design consideration was low power consumption. This pipe conveyor (closed belt against spillage) was estimated to be 44kg/m with steel reinforcement, but by reinforcing it with aramid, the belt weight was reduced to 29kg/m. This equates to a moving system weight reduction from 162kg/m to 132kg/m (carrying belt, plus payload, plus returning belt), which is 18% less moving mass. In simplified form, the resistance is linear, so around 18% less power is required to drive the fully loaded belt. Since 2002, the 2.6km aramid reinforced pipe conveyor has run without a problem, and in 2007 Maritsa Istok decided to install a second aramid belt of 5.8km, which became operational in early 2009.
In a second case at the Compagnie des Phosphates de Gafsa (CPG) phosphate mining operation in the Gafsa basin, in the south of Tunisia, steel was again replaced with aramid. The site had two conveyer belt installations with lengths of three and four kilometres that originally ran with one metre wide, steel-reinforced belts. To improve corrosion resistance, these were replaced with a fabric reinforced belt in 2000. The available take-up length of the installation was sufficient for replacement with aramid-reinforced belts with low elongation. After 10 years of successful operation, in June 2010 the belts needed replacing due to rubber wear.
Measuring energy consumption of a conveyor belt is a complex calculation – drive power changes over time with belt load, operating temperature, belt speed and other factors. While belt weight is easy to determine and to equate in terms energy use, for rolling resistance it is far harder to produce a clear energy saving figure. To address this, several installations are currently being planned and built that will be closely monitored for energy use to generate savings data.
For mine operators, keeping installation downtime to a minimum is key, and so an alternative belt reinforcement will only be considered if reliability is proven. This is of greatest concern in long conveyors, which are assembled from individual rubber belt sections of several hundred metres each, the weakest links being the splices where sections are connected.
In a bid to prove reliability, splicing company SMC Industrial and researchers at the University of Hannover, Germany, are currently working together to measure belts on static and dynamic strength. Repetitive stress equivalent to the forces on an operating belt is applied to the test specimens and, after a defined number of load cycles, residual belt strength is measured.
In practice, aramid belts have a long track record for reliability and it is hoped that these tests will add up-to-date, reliable and extensive splice strength data. This should support the wider acceptance of aramids in conveyors, and that their low weight can substantially reduce energy costs and, therefore, make mineral transportation a more sustainable operation.
For more information, email Gerard van den Hondel firstname.lastname@example.org