Conveying Over the Hills and Through the Woods
Reprinted from Power and Bulk Engineering, July 1998
An enclosed belt conveyorwith tight cornering ability transfers lime and carbon lumps across challenging terrain.
Nucor Steel, Charlotte, N.C., produces steel and steel products at its plants and subsidiaries across the US. The company principally operates scrap-metal-based mini steel mills, making products that include hot-rolled and cold-finished steel bars, structural steel products such as joists and deck used for floor and roof systems, and steel fasteners such as bolts and screws.
At their plant in Norfolk, Neb., Nucor produces three grades of steel: special bar quality, cold-headed quality, and merchant quality. Vice president and general manager John Ferriola said steels made at the plant are used in many industries.
"For example, we make the steel for Monroe shock absorbers," he said. "We make all of the steel used in Craftsman sockets. We make the steel for some steering wheel columns for General Motors and the steel used for shafts in the small motor industry."
Scrap metal, steel's primary raw material, is shipped to the plant by railcar. A large magnet lifts the scrap into charging buckets.
Other ingredients - lime and carbon (coal) lumps - also arrive by railcar. Most of the lime lumps range from to 2 inches in diameter and have a bulk density of 67 lb/ft3; some of the lime lumps contain dolomite and have a bulk density of 75 lb/ft3. The carbon lumps are 1 5/8 by 9/16 inches and have a bulk density of 57 lb/ft3.
The lumps are unloaded into silos. A belt conveyor moves the lumps to one of two feeders that dispense them into the charging buckets with the scrap metal.
The buckets are put into a furnace where electricity passes through an electrode to melt the scrap metal. As the metal is melting, the lime and carbon lumps serve as flux materials and react with the metal to form a slag, which is taken out of the molten metal.
Vibrating feeders below the lime and carbon silos load the carbon and lime lumps onto the con veyor belt.
"After we melt the scrap metal, we pour it into billets, which are just big blocks of steel," Ferriola said. "We reheat [each block] and then roll it in a rolling mill to whatever shape we want."
Depending on the grade, the steel is rolled into rounds from 7/32 up to 1 1/2 inches in diameter. For building applications, merchant quality steel is made into angles, squares, octagons, and other forms.
Conventional belt conveyors wouldn't make the grade - or a tight turn
Nucor began the melting-and-casting operation at the Nebraska plant in 1997. As the production line was designed, engineers determined that a unique conveying layout was needed to transport the lime and carbon lumps from the storage silos near the railcars to the charging buckets.
Ferriola said the conveyor needed to travel several hundred feet, often at steep grades, on a route punctuated with such obstacles as railroad tracks and silos. The area couldn't accommodate the width of conventional belt conveyors. In addition, Ferriola said the conveyor had to turn corners with a short radius.
The conveyor's lower roller carries the belt to the load area as the upper roller opens the belt to accept the lumps.
"We needed the belt to run along a building wall, and we had very little clearance," Ferriola said. "So a typical belt conveyor or transfer house wouldn't work - they would be too large for our application."
Ferriola said moving the lumps from the railcars to the charging buckets with a conventional belt conveyor system would require three trough conveyors and one pocket-belt conveyor.
Under similar conditions, pneumatic conveying systems are often the only alternative. Yet Ferriola said he was reluctant to install one.
Besides the current need for c~mveying the lumps, the plant's future pIai.~ call for using iron carbide, a fine metal powder, as a scrap metal substitute. Most installations handling iron carbide use a pneumatic conveying system to move it directly to the furnace, Ferriola said, but iron carbide is very abrasive and can quickly wear through conveying pipe. At the same time, dust and fines result in lost material when iron carbide is moved by belt conveyor. Ferriola wanted an adequate conveying system in place in case the Nucor plant begins to use iron carbide.
A new way to convey
To design their conveying system with current and future needs in mind, Nucor examined proposals from three engineering companies. Each company presented Nucor with conventional belt conveyors, but one company offered a new approach.
"We were looking at three companies, looking for alternative approaches," Ferriola said, "and [one engineering firm] put this one on the table along with a more conventional belt system."
The alternative approach that caught Nucor's attention included an enclosed belt conveyor made in Sweden and recently introduced to the American market. (The conveyor has been sold for 10 years in Europe, Australia, and Africa.) The conveyor offers performance advantages of both a mechanical and a pneumatic conveying system: It provides one continuous conveying line, it can move material up steep inclines and around corners, and it's a closed system that eliminates dust and fines.
Nucor engineers viewed videos of the system in operation before deciding to accept a proposal by Fairfield Engineering, the North American distributor of the SICON belt conveyor.
"[Cost] was not the driving factor in this decision," Ferriola said. "[It was] the design factors, the turning radiuses, the ability to go up on much more of an incline."
"At Nucor, we like to try new things," Ferriola said. "And to be honest, the distributor gave us a deal that would be hard to say no to: If the conveyor did not work, they would remove their equipment and put in conventional equipment for us at the same cost. They guaranteed it to work, basically."
The best of both conveying worlds
In August 1997, the equipment distributor installed the belt conveyor at the plant's melting-and-casting facility. Ferriola characterized the installation as "extremely smooth."
"When we were starting up the melt shop we had issues everywhere, with the exception of the belt," he said. "These guys did right by us."
The conveyor uses a continuous belt loop. As the belt travels, it folds to produce a closed, pear-shaped enclosure for the material. Steel cord reinforcements are vulcanized into the belt's profiles. The profiles serve as tracks for the belt's angled support rollers and vertical guide rollers and absorb all of the conveyor drive system's stress.
When the belt is folded together, one steel cord is above the other. This allows the belt to negotiate a 180-degree turn in a radius less than 1 meter - eliminating the need for transfer points. Depending on the nature of the transported materials, the belt can handle inclines of 35 degrees. In restricted areas, the belt can be given a "serpentine" configuration to cope with steep inclines.
This bottom-up view shows the be It opening to discharge material into the holding hopper Feeders dispense the material (Tom die holding hoppers into the charge buckets.
Since the belt only opens as material is loaded and unloaded, dust and spillage are reduced, solving many environmental problems. The closed belt also protects sensitive loads. Multiple load and discharge points can be designed on one continuous loop. As the belt unloads material, centrifugal force and the belt's elasticity combine to make the belt self-cleaning.
Multiple drive units can be fitted at any curve in the conveyor where the belt cornering is a minimum of 90 degrees. The conveyor can also carry material on the return run, making it a two-way conveyor.
The belt's pear-shaped enclosure absorbs the impact of the material as it's loaded. This allows the belt to accelerate rapidly to high speeds. At Nucor the belt travels 1.8 mIs, transferring material between 50 and 60 lb/ft3 at peak levels of 80 m3/h.
Belt conveyor meets expectations
Installing the enclosed belt conveyor allows Nucor to transport the lime and carbon lumps from railcars across a confined area to the charging buckets in the plant.
"We go around corners. We go right through the melt shop," Ferriola said. "The conveyor kind of snakes its way through. It takes up so little room, it's not in the way at all."
Ferriola said the belt has operated with noticeably less dust than other installations he has witnessed, which leads to a more positive impact on the surrounding environment. This will come in handy if the plant begins to substitute some iron carbide for scrap metal.
"When you transport iron carbide on a conventional belt, you lose a lot of fines and dust. This belt closes on itself," he said. "We think it will be a very good method of conveying iron carbide if we begin using it at this facility."
Since installation, the belt conveyor has required little maintenance. Ferriola said he has nothing but high praise for the conveyor.
"I would highly recommend it," Ferriola said. "It's good equipment. It's been a godsend for us."
Lime/Carbon System with Sicon Conveyor