Conveyor Modifications Lead to Improved Performance

In Mexico’s northeastern state of Coahuila, the country’s largest coal mining complex fuels nearby generating plants that produce about 10% of Mexico’s electrical energy needs. Here, the country’s newest underground mine has found that it can wring yet another 5% out of conveyor downtime to achieve 85% belt availability using a combination of mechanical belt fasteners and belt cleaners designed to work together better. Increased fastener/cleaner compatibility has not only extended the life of both, but also greatly improved cleaning efficiency and eliminated hazardous water pooling around transfers.

Known as MICARE (Minera Carbonifera Ro Escondido SA), this complex is part of the Coal Division of Grupo Acerero del Norte (GAN), which has invested more than US$250 million in new technology for increasing production, quality, safety and environmental control. Among the results are an 80% overall uptime, productivity reaching a monthly 152 tonnes per person, and four consecutive National Safety Awards from the Mexican Chamber of Mining.

Working the Ro Escondido coal seam, covering about 120 km2, MICARE produces sub-bituminous, long-flame thermal coal with a 1% sulphur content. Current operations produce 6.4 to 7.3 million tonnes per year–about 70% of all coal mined in Mexico–with proven reserves of 25 years. Nearly half of MICARE’s output comes from two underground mines, No.4 and No.5, with the new No.6 mine in development. The other half, from two open pits, is extracted primarily by a 4,080-tonne walking dragline, the largest in Latin America.

Newest coal mine in Mexico

MICARE’s No.5 mine, opened in late 1997, is the newest in Mexico. Here, one longwall and four electric continuous miners pursue a 2 m-thick seam between 78 and 127 m below the surface. Production moves out on three 1,016 mm-wide panel belts rated for 1,088 tonnes per hour (t/h) at speeds of about 2.5 m/s. These transfer broadside onto a straight mainline conveyor, on which a 1,219 mm wide belt can carry 1,600 t/h at about 3 m/s. A final transfer turns the coal onto the exit belt, which is 1,372 mm-wide, rated for 2,000 t/h at 3.5 m/s, spans 550 m between end pulleys, and rises to the surface at a 16 incline.

Panel belt lengths vary up to 1,300 m between end-pulley centres. The mainline conveyor ultimately will extend to 1,600 m between end-pulley centres as longwall panels are developed farther away from the exit belt transfer.

Throughout the mine, belts are 12 mm thick, solid-woven PVC construction with 140 kN/m mechanical fastener rating, generally running troughed at 35.

Riveted belt fasteners favoured

By the time MICARE engineers developed the No.5 mine, they had learned some valuable lessons from the four previous developments, says conveyor belt maintenance chief Faustino Rodriguez. “From that earlier experience, we learned the advantages of mechanical rather than vulcanized belt splicing. In mine No.4, we relied on a rivet-attached, hinged-plate belt fastener because it allowed us to shorten panel belts easily by removing sections as the longwall retreated. Equally important, installation with hammer-driven, self-setting rivets made replacement quick and easy.”

Known as the Flexco SR rivet-hinged fastener, it consisted of 38 mm-wide top and bottom plates joined at one end by two broad hinge loops, and secured to the belt end with a staggered pattern of five rivets. These fastener segments are fitted side by side across both belt ends. When both belt ends were brought together, the SR hinge loops meshed and a hinge pin is inserted to complete the joint.

The rivets were a special Flexco design, preassembled with patented, breakaway pilot nails that let the rivets penetrate the belt easily, without damaging carcass fibers. Simple portable tooling captures the nails beneath the splice, allowing the hammer blows to automatically peen the rivets on the underside of each plate.

Splice life nearly doubles

“The SR fasteners in mine No.4 lasted about five months under our conditions,” Rodriquez recalls. “Then our Flexco distributor alerted us to a new version called RAR6LP, which lasted nine months, so when mine No.5 opened, we used RAR6LP there from the start.”

The RAR6LP is dimensionally identical to the earlier SR fasteners, but with plates 24% thinner. This gives the RAR6LP a lower profile that is friendlier to belt cleaner blades, pulleys and idlers. Its reduction in thickness is compensated by an increase in toughness, using a special low-chrome stainless steel called RustAlloy developed for high resistance to acidic mine water and abrasion.

“The RAR6LP also uses a hinge pin of stainless steel cable armoured with a flat stainless wrap,” Rodriguez adds. “This lasts about 60% longer than the bare steel pins we used earlier.”

“While longer splice and hinge-pin life was enough of an advantage at first,” he continues, “we were beginning to want better belt cleaning, and saw that lower splice profile would allow tighter fitting cleaners. Until then, our mines used homemade cleaners with blades made of rubber strips, but we had to adjust them a lot and replace them often, and they never worked very well. So when Flexco came out with their Eliminator belt cleaners, we agreed to try them based our good experience with Flexco splices.”

Cleaning improves 80%

Three Eliminator H-Type head-pulley pre-cleaners were installed at key transfers in the No. 4 mine late in 1999. The H-Type design spans the belt with a row of 200 mm-wide blade segments, separately mounted on pre-tensioned rubber cushions. This lets blade segments self-adjust independently to accommodate irregular belt-surface wear patterns.

“These H-Type cleaners reduced our carryback about 80%,” Rodriguez says, “and their blades are lasting up to seven months, so they’ve cut down our blade replacement work a lot.”

Scallops stretch cleaner life

“Several months after we installed our H-Type cleaners,” he continues, “Flexco brought us yet another fastener improvement called the Scalloped Edge design.” Also made of RustAlloy, the new Scalloped Edge fasteners have leading/trailing edges formed into a concave, scalloped shape with more gradual beveling. These features allow the fastener plate ends to embed deeper into the belt cover, further reducing splice height in installations that do not use skiving (countersinking). Lower profile and less initial leading edge contact area invites belt scraper blades to ramp up and over the plates more smoothly.

“This is very helpful on PVC belting,” Rodriquez explains, “because its surface is harder than rubber cover and doesn’t let other fasteners embed well. We couldn’t use countersinking because solid-woven belt construction doesn’t provide enough top-cover thickness to cut away. When we saw how well scalloped fasteners work on PVC, we began converting both mines No.4 and No.5.”

“Soon we noticed that mine No.4’s belt cleaner blades were lasting about 5% longer, and noticed less noise from splices banging on rollers,” he adds.

When the No.5 mine received its first H-Type cleaner, he found yet another benefit. “The coal here is much wetter than in our other mines, and puts a lot of water on the belts. Too much of it ended up in pools around the transfers, creating an unnecessary hazard. The H-Type cleaner did a great job of ejecting the water onto the receiving belt along with the coal. We added a second H-Type cleaner where the mainline transfers onto the exit belt, so both belts now carry most of the water out of the mine and the pools around the transfers dried up.”

Eleazar G. Castro is the Director General of Flexco de Mexico.

---