May 2013

Lifeline of the mine

Innovations in wire rope construction and monitoring improve performance and availability for deep mines

By Eavan Moore

A construction crew installs a drum hoist, a system well suited for today | Courtesy of ABB

As miners dig deeper, their hoists must keep up with the change in scale. The modern standard of digital control gives systems built-in flexibility to cope with new conditions, but the greatest challenge, and the frontier in research and development, is the rope on which all else hangs.

“It’s a bit cliché to say it, that hoist ropes are the lifeline of the mine,” says Allan Guse, principal engineer, hoisting group, Vale Canada Ltd. “But the fact is, the rope really is the key element of the whole system, around which much of the rest of the system is designed and configured.”

Rope strength and longevity are the factors that limit the efficiency of a hoist system. Typical hoist rope designs surround a steel core or fibre with additional clusters of steel wires. With use, the metal corrodes. On a drum hoist, the coils rub against each other where they wrap around the drum, resulting in wear stresses. Deeper shafts – now extending beyond 3,000 metres – put even more strain on hoist ropes.

It helps that staying on top of rope condition has gotten easier. Canadian R&D firm C-Core developed a camera network that saves the hour or so each day that a person would normally spend visually inspecting the rope on the shutdown hoist. The system, called RopeInspector, is marketed by Bestech. Electromagnetic inspection systems have also advanced. Both CANMET and South African firm Ansys have developed systems that provide ongoing monitoring during normal operation. Ansys’ Continuous Rope Monitoring System is installed at a number of mines, including AngloGold Ashanti’s Moab Khotsong, which boasts the world’s deepest continuous shaft at 3.5 kilometres. The system generates a constant magnetic field to expose broken wires, corrosion and other problems. It shows a basic status display on the operator’s screen, sends out an alert when it sees deviations from its designated safety parameters, and stores more detailed data for reference. “If you can [monitor] that continuously, then you can see little deteriorations starting to happen, and you can take maintenance action ahead of time,” says Guse, who thinks Vale will likely invest in at least one test system in the next couple of years.

Longer rope life

Ideally, of course, the ropes would not break down. In contrast to the standard flattened-strand design, Guse says compacted-strand ropes, used in Xstrata’s Nickel Rim mine and Compass Minerals’ Goderich mine, offer better wear protection properties and, by the manufacturer’s estimate, last twice as long. The strands are drawn through a die tool to smooth the outer surface, reducing the indentation between strands. “The outer surface of the strands being compacted gives a very smooth surface to the strand of the rope, as opposed to conventional ropes, where each individual wire in the outer surface of the strand is exposed,” he explains. “With the smooth outer surface of the compacted strand, there are lower contact stresses.”

But configuration changes cannot change the fact that steel ropes are heavy, which limits their load-bearing capacity. A stronger rope ends up bearing the burden of its own extra weight. It also tends to have a shorter lifespan, since stronger wires seem to invite wire fatigue and age hardening. In response, a design just reaching commercialization by CASAR Drahtseilwerk Saar GmbH uses a lightweight load-bearing core made from synthetic aramid or high-modulus polyethylene fibres. The core is wrapped in the conventional fashion, with steel strands. As a drop-in replacement for existing steel ropes, the hybrid model could carry more tonnes per skip with no change to its external wear properties. The downside is that electromagnetic testing would reveal nothing of the core.

A fully synthetic rope, notes Guse, is further down the road. “Fifty years from now, we’ll probably be wondering how we ever hoisted with steel ropes before,” he says. The design and testing could take another decade – and it could be longer before operators pin their fortunes on plastic – but the all-synthetics would be lightweight and corrosion-free.

A better brake

Paid out into the depths of a two- or three-kilometre shaft, rope engages in risky behaviour. That is when electrical engineers like Klaus Kacy, senior technical consultant at ABB Canada Inc., step in.

A hoist system built after the end of the 20th century will have two key electrical components: a single alternating-current drive, and a programmable logic controller. While the mechanics of hoisting have not changed drastically since the 1930s, the controls are now software-based. Alongside the space and energy efficiency gains, this means manufacturers can easily program sophisticated cycle adjustments, allowances for rope stretching, and brake controls.

Agnico-Eagle’s LaRonde mine, which includes the 2.2-kilometre Penna shaft, was one operation that found itself in need of better braking. At great depth, Kacy explains, a steel rope behaves like a spring: when stopped suddenly, it bounces. For a cage hoist, this has serious implications if the people in the cage are exposed to abrupt deceleration after an emergency stop. After noticing signs that safety catches on its service hoist had been activated, LaRonde called in ABB to do an accelerometer test that gauged excessive deceleration of the cage.

ABB developed and patented a control method that eases the conveyance oscillations. “Controlled rollback” activates when an emergency stop occurs with an upward-moving load. At first, little to no braking torque is delivered; gravity does the braking work. The drum is allowed to roll back, letting the cage fall downwards. At that time, the braking torque gently increases. But the fact that the cage is now moving downwards instead of upwards means that oscillation energy dissipates first, and that the braking system can force a gentler deceleration.

Safety first

Although controlled rollback can benefit friction hoists – a relatively cheap design option effective at depths below 1,600 metres – the technology was initially designed for drum hoists, which are better suited to deep shafts applications.

The factor of safety for rope, calculated by dividing the breaking strength of the rope by the total suspended load, can determine not just the basic hoist design but the legality of new technologies. Ryan Gough, manager of project services at Cementation Canada, remarks that this actually poses a non-technical challenge to deep mining: sometimes industry innovates too fast for regulatory bodies to keep up. It took years for Agnico-Eagle to persuade Quebec regulators that switching from a rope safety factor of five to a factor of four would sacrifice no actual safety, so long as the stringent additional requirements already adopted by the South African Bureau of Standards had been met.

The Blair multi-drum hoist provides another workaround for rope limitations. Designed for depth in the 1950s, the Blair hoist uses two smaller ropes per conveyance instead of one, dividing the weight. Xstrata used a Blair design for a service hoist at its Nickel Rim project in Sudbury and Cementation Canada is preparing to install one in Freeport McMoran’s Grasberg mine. “I think we will see more and more opportunities for the Blair hoist to be used than we have historically,” says Gough, noting that mobile equipment is getting bigger and is demanding more of service hoists.

Faster travel

Ambitious underground mines have high capital costs, which puts pressure on the mine to keep its production running. If it can boost hoisting speed, so much the better. Gough thinks that the brake improvements of the last decade have helped, but he also sees interesting possibilities in the Levelok E-Brake system developed by Horne Group: a hydraulic mechanism that holds the cage steady at shaft stations and automatically brakes in a failed rope situation. “In jurisdictions where safety dogs are required on man-travel conveyances,” he explains, “we normally would operate on wood guides to allow the dogging mechanisms to operate properly, to give us acceptable deceleration rates.” The E-Brake meets Ontario’s deceleration standards for safety dogs on steel guides, potentially allowing cages to operate at higher speeds.

Better brakes, stronger ropes, and continuous safety monitoring are not the flashiest innovations. But they smooth the way for everyday operations, cutting costs and helping define the new ordinary: deeper, faster and bigger.

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