Taming the Terrain
Innovative suspended conveyor systems allow high-speed, continuous material transport in areas where tough topography may be too difficult for conventional haulage
By Russell A. Carter, Managing Editor
Conveyor systems, in a way, are relative newcomers to the industry’s typical capital- cost considerations when planning a mine. They’ve been an important part of both surface and underground mining for decades—but they’ve often been regarded as commodity-type parts and pieces that don’t warrant the attention paid to primary production equipment such as trucks, shovels and loaders, for example.
That philosophy is changing as producers look for ways to to cut operating costs, get workers ‘off the equipment and out of the mine’ for safety and financial reasons, and establish or expand operations in areas that aren’t well-suited for conventional truck haulage. A prime example of the industry’s growing interest in nonconventional haulage is iron ore producer Vale’s Carajás Serra Sul S11D mine, located in the Amazon region of northern Brazil. The mine employs shiftable conveyor belts instead of off-highway trucks to transport ore from the mine to the processing plant—the first time a “truckless” solution has been applied on a large scale at an iron ore mine. Vale determined early on that if the S11D mine were to be operated with truck haulage, it would need about 100 off-highway trucks, with corresponding needs for diesel fuel and other consumables and deleterious emission levels in an environmentally sensitive region.
Design Data: Earlier is Better
With conveyor systems becoming a more important consideration in mine capital- expense planning, whether involving an in-pit crushing and conveying approach or high-capacity overland systems to replace haulage roads, at least two trends seem to be emerging that can potentially help mine owners save money and improve overall efficiency in conveyor-centric operations: First, mine operators are becoming more aware of the importance of providing timely, accurate information to conveyor system designers and suppliers in order to ensure proper system sizing; and second, an expanding portfolio of conveyor-component and system innovations enables customers to select conveyor technologies suited to a wide range of applications and site conditions, including material transportation routes that may be extremely challenging or impassable for conventional haulage vehicles.
Addressing the need to supply conveyor vendors with early and accurate information to ensure proper system sizing, Timothy Felts, an engineer with Cat Conveyor Systems, pointed out in a 2015 presentation* that: “Detailed information must be obtained to eliminate conveyor sizing being reduced to a ‘best guess’ scenario. Lack of information leads to equipment being improperly sized. This guessing exposes the manufacturer and customer to risk. This best guess scenario will, at times, leave manufactures literally left guessing at critical conveyor systems components that are vital to customer operations. Guessing exposes the manufacture to warranty concerns, liability, and also exposes the end customer to downtime, production loss, revenue loss and maintenance concerns. Financial loss and/or time lost impacts the end user and the manufacturer.
“This risk must be evaluated, understanding that conveyor systems can cripple an operation if the equipment fails or is interrupted, especially in comparison to other transportation solution failures. Thus, it is critical to ensure the proper conveyor system solution is sized and installed. The system is optimized for the application during early site development or application development.
“With better information upfront, manufacturers can provide the best solution, execution, product support and on time delivery, and ultimately this results in customer satisfaction,” Felts explained.
Innovative conveyor designs also provide mine owners and operators with an expanded scope of choices when it comes to solving problems posed by difficult terrain, recurrent harsh weather and other situations that can render conventional haulage impossible or inefficient. In particular, cable-based suspended systems have found a niche in the marketplace for these types of of applications. Here are a few examples:
Knowing the Ropes
The El Limon-Guajes gold mine (ELG), 180 km southwest of Mexico City, Mexico, is a high-grade, dual open-pit operation extracting ore with an overall resource grade of 2.65 g/t. It’s located in the Guerrero Gold Belt, a highly prospective zone in which the mine’s owner, Canadian intermediate-sized producer Torex Gold, also is advancing the Media Luna advanced exploration project.
ELG started up in December 2015 and announced commercial production in late March 2016. ELG will, according to Torex, be among the largest and lowest cost gold mines in the world with expected life of mine (LOM) average annual production of 370,000 oz of gold at a LOM AISC of $616/oz.
Over the course of the mine’s predicted eight-year life, ore taken from two pits—El Limón and Guajes—will be processed in a central plant. The El Limón pit, however, is situated on a sharp ridge approximately 400 m above the plant. During the mine planning phase, one of the issues that had to be addressed was the best method of transporting the ore from El Limón down to the valley below—costs, operational safety and impact on the environment all had to be taken into account.
Construction started early in 2014 on a Ropecon system that extends 1.3 km along the 400-m vertical drop with just one tower structure, unloading 1,000 mt/h of ore onto a stockpile near the plant. Doppelmayr’s assembly team accomplished the installation in just six months and the RopeCon system began operation on schedule in March.
The RopeCon concept uses a flat belt with corrugated side walls to transport material. The belt, moving at 3.3 m/sec, is driven and guided by a drum at both ends. To account for the steep vertical rise and incline between the El Limón pit and the valley, the belt was fitted with cleats to prevent material slippage. “Axles” are mounted at regular intervals along the belt, with wheels at each end that run on anchored track ropes, which guide the belt.
Ropecon’s track ropes, or cables, are supported by tower structures, resulting in a minimal ground footprint. The system includes three track rope pairs in total, with the bottom pair supporting the empty belt and the pair in the middle carrying the loaded belt. The third track makes it possible for workers in an inspection vehicle to access any point along the line. Almost all moving parts are attached to the belt and therefore pass through the stations at regular intervals, allowing service and maintenance work to be performed at a central point.
The El Limón RopeCon system generates about 1 MW of power from regenerative braking, which contributes to a reduction of overall operating costs. This, and the fact that no fossil fuel is required to operate the system, is, according to Doppelmayr, among the advantages offered by RopeCon over conventional truck haulage—bolstered by elimination of dust and noise emissions as well as enhanced site safety. Its controls can be integrated into the mine’s central control system.
The El Limón RopeCon system’s single tower structure is 44 m tall to achieve the discharge height required for the stockpile. The span between the loading station and the tower structure is 1,200 m with a conveying angle of 32°.
Available on Cable
In another example of a suspended-belt conveyor solution providing enhanced economics for a mining operation, cement producer Holcim Brazil was looking for an alternative to truck transport of limestone and clay material extracted from a quarry at its Barroso operation.
In general, haul trucks account for the major share of overall surface mining equipment costs, with fuel consumption the primary operating cost associated with trucks. Several factors contribute to fuel consumption and associated CO2 and other combustion-related emissions, such as payload, speed, engine power, idle time, fuel quality, road surface, tire performance, operator driving style, maintenance and truck design characteristics.
At the Barroso operations, calculations determined that, to transport 4.5 million mt/y of raw material, assuming a distance of 25 km between the quarry and the cement plant and a one-hour hour cycle time, annual CO2 emission would amount to a substantial 8,500 mt/y.
Holcim had a stiff set of requirements for any solution that didn’t involve truck haulage. The system had to provide: high capacity (1,500 mt/h); avoidance of geological and environmental impacts; a “suspended” solution so as not to impinge upon a preserve area; elimination of intermediate stations to minimize personnel requirements; and no need for fencing and/or surveillance along the line of travel, along with convenient inspection and maintenance access.
The cement company ultimately chose a solution proposed by Agudio S.p.A., an Italian company founded in 1861 and now part of Poma-Leitner Group, specializing in engineering and construction of customized, cable suspension-based material transportation systems. Agudio’s engineers applied a number of disciplines to analyze Holcim’s needs, including cable, belt and mechanical analysis design along with geological and constructability studies.
The solution they proposed was designed to deliver 1,500 mt/h of limestone and clay over a 7.2-km-long Flyingbelt system that extends from the quarry to the production plant. Created and patented by Agudio in 2007, the Flyingbelt is an aerial transport system for bulk materials, which utilizes cables and a conveyor belt set in a deep, semicircular conveying trough.
Agudio’s sales and marketing manager, Stefano Cattaneo, told E&MJ that the Flyingbelt concept, in general, offers a number of advantages for achieving cost and emission reduction as well as energy efficiency in appropriate applications.
The major benefits accrue from:
• Low environmental impact – Flyingbelt requires significantly fewer supporting structures than standard conveyor systems.
• Flexible route design – It can adapt to highly problematic terrain, bypassing obstacles with supports that can be spaced up to 1,000 m apart, and able to negotiate horizontal bends.
• Reduced capex, opex and maintenance costs – Components are standard conveyor and modular parts specifically designed for simple and rapid assembly. Power consumption is reduced compared with other solutions and maintenance can be done “online” from a dedicated vehicle specifically designed for these operations.
• Energy efficiency – Compared with standard transportation by truck, Flyingbelt is much more efficient; its power requirements are low and, in some cases, even positive when downhill travel results in regenerative braking.
• CO2 reduction – The efficiency of the system can be regarded as a CO2 reduction technique when compared with standard truck haulage emissions.
The Barroso installation is the longest Flyingbelt in the world, comprising a single, 14.4-km-belt that runs at a speed of 4 m/sec from the loading station to the unloading station—both designed by Agudio— and suspended by four sets of tracking cables strung over 18 supporting towers. Visual inspection and maintenance can be performed from any of four maintenance vehicles that travel on the suspension system and have been designed in accordance with international guidelines for safe transportation of people.
In terms of environmental impact reduction, Agudio reported that the Barroso Flyingbelt has a power consumption of 1,400 kWh per 3,000 hr/y of operation. Taking into consideration the IEA composite electricity/heat factor for Brazil (0.089 kCO2/kWh), Agudio said that translates into an emission total of about 375 mt/y—providing a net savings of 8,125 mt/y, or more than 95% over truck haulage.
Skip Conventional Haulage Costs
Recently, the Industrial Solutions business area of thyssenkrupp unveiled its new, energy- balanced, steep-angle “Skip Way” conveying system at MINExpo 2016 in Las Vegas. The system makes it possible to transport ore or overburden from the lower level of a mine to higher elevations by the shortest and most direct route possible, reducing the mine’s carbon footprint.
By using a cable system to transport ore, the Skip Way system is intended to reduce operating costs and carbon dioxide emissions by reducing the number of haul trucks in operation. The deadweight of the skips is fully counterbalanced at all times, so energy only needs to be expended to raise the minerals to the top of the mine.
According to thyssenkrupp, the system is highly adaptable to mine terrain and can be used in harsh weather conditions when truck haulage would typically be suspended. The system also complements thyssenkrupp’s Integrated Skip Conveying and Crushing System, which has handling rates of up to 5,000 mt/h for uncrushed ore or overburden. In that concept, skips run in opposite directions on a steel track over a steep slope and can either empty their payloads of up to 250 mt into the feed hopper of a semimobile crusher or reload ore or overburden into mine trucks at a bin with an apron conveyor discharge.