October 14, 2016
Though blessed with most every possible amenity — climate, ocean/gulf beaches, a thriving agricultural economy, etc. — Florida is sorely lacking in a good, available source of aggregates. As a result, suppliers are increasingly looking both out of state and internationally for material to meet their needs and those of their customers. For American Aggregates, LLC, that has meant turning southward to Honduras for an ample, readily available supply of limestone. Once the mined material is loaded onto a freighter and brought to the company’s Port of Tampa location, however, it must be offloaded and transferred by truck to an area where it can be stockpiled for sale. Formerly contracted out to a local firm, that hauling, unloading, and stockpiling effort has been brought in-house with the addition of a pair of major components from Masaba, Inc. The result has been much better control of the overall process and a substantial per-shipload savings.
Hard rock Honduras
Part of the Hunt Ventures group of companies, a venture capital organization established by trucking leader J.B. Hunt, American Aggregates serves as the U.S. arm of a Honduras-based mining operation. According to George “Chip” Doll, Jr., the company’s terminal quality control manager, Hunt’s rationale for purchasing a mine in Central America was both sound and prescient.
Despite a 2010 ruling reaffirming mining rights in Florida’s productive Lake Belt region, reserves from other parts of the state are being heavily tapped, all at a time when demand for limestone is on a steady rise. In addition, a recent study shows the state’s overall demand for aggregate rising from 76 million tons in 2010 to more than 126 million tons in 2030, with better than 95 percent of that material eventually being imported.
“Unlike the material generally found in this area, the limestone we bring in from our Honduras mine is extremely hard, which is what our customers need,” Doll says. “Florida doesn’t have a lot of hard rock left in its reserves, and with development once again on the rise, the demand for such a product is huge. So we, as well as some of our neighboring companies here in the Port, regularly bring in rock from Central America, Mexico, Canada, etc., and sell it for use in road-building applications, as well as in the production of asphalt and concrete.”
Take a load off
At the Honduras location, the newly mined limestone is processed and graded to a number of sizes, including screenings/manufactured sand; #89, which is a 3/8-inch rock; and #57 or 1-inch material. It is then loaded into segregated holds of a transport ship for a three-day trip to the Port of Tampa. Upon arrival, the self-unloading vessel conveys its cargo to a portside hopper which meters and regulates the volumes of material loaded into a steady stream of dump trucks passing below. Once loaded, each truck heads to a nearby 25-acre stockpile area. Getting the ship unloaded in a timely manner is critical, according to Doll.
“There’s an issue called ‘demurrage,’ which is essentially a penalty for exceeding the time it should take to offload the ship,” he says. “And the clock starts running as soon as the ship is docked, so if any kind of breakdown occurs and the allotted time is exceeded, those costs can add up quickly. I’d say that’s as good an incentive as any for a company to have a system in place that they can rely upon.”
Up until about two years ago, American Aggregates contracted with a firm to handle the transfer and stockpiling of the limestone cargo. While effective, the process was hardly economical.
“Suffice it to say that we were paying a lot of money per ton for that service,” Doll says. “And considering that the shipments we get in vary from 55,000 to 60,000 tons each, it’s easy to see why management started to look into purchasing a transfer/stacking system of our own.”
Stability is the difference
The equipment in place at American Aggregates’ Tampa location includes a truck unloader with 72-inch conveyor feeding a 48-inch x 150-foot Magnum Telescoping Conveyor, both from Masaba. According to Doll, while the truck unloader might be the centerpiece of the operation, it was actually the conveyor that steered them toward the manufacturer.
“When we are stacking material, we want the distance of the drop from the conveyor to the pile to be minimized in order to keep the rock size intact,” he says. “That often means fully extending the ‘stinger’ giving us a total length of 150 feet. However, in demos of one other stacker we were considering, when we had that full extension, the back end of the unit seemed to want to lift off the ground — the stability just wasn’t there.”
By contrast, he added, they had no such issue with the unit purchased. He feels that, because the Magnum stacker has so much 1¼-inch plate steel in its design, the overall construction is more substantial. “It has all the stability we need — and then some,” Doll says.
Poetry in motion
American Aggregates’ truck unloading operation is a carefully choreographed exercise in efficiency. Because the truck unloader offers dual-side access ramps — making earthen ramps unnecessary — vehicles are continuously staging, backing into one side or the other, and emptying their payloads.
“Until the ship is fully offloaded, we are round-the-clock operation,” Doll says. “The stream of trucks feeding that unloading/stacking system is non-stop for 60 to 80 hours or better, depending upon the size of the load being delivered by ship — we have had as much as 70,000 tons at one time. So, again, reliability and dependability come into play.”
To help ensure peak performance, every one of the three conveyors on American’s units — one on the truck unloader and two on the stacker — features a pair of motors and two gear boxes. According to Doll, that’s twice what is generally found on other similar equipment. “To some, that might seem like overkill,” he says. “But it just tells us that, even with the heavy screening material, we will not bog these belts down.”
While efficient, the transfer process is fairly straightforward: after loading, the fleet of more than a dozen trucks heads to the stockpiling area where they are directed to queue up to either side of the unloading unit. When it’s their turn, they are guided into the ramp area, raise their dump box, discharge their load of rock, and drive back to the dock to load up again, making way for the next truck.
Discharged material first hits the truck unloader’s built-in grizzly screen, designed to grab any oversized rock, then continues onto the 72-inch wide by 56-foot long conveyor, which deposits it into a collection hopper on the stacking conveyor. From there, it is dropped onto the 48-inch wide belt to begin the trip up to the top of the stockpile, which can reach heights of nearly 53 feet.
“We have a number of options available to us as far as the movement of that telescoping conveyor,” Doll says. “We can program it to follow a specific radial pattern, we can rely on programs provided by the PLC, and we can move it manually. Similarly, the truck unloader has many nice features, including the ability to quickly raise the ramps, allowing them to be easily cleaned of any rock that has been spilled during the unloading process. That alone is a nice time-saver.”
Doll says that, before coming to American Aggregates, he had no prior ship unloading and transfer experience, but studied the operation of the contract firm they used prior to purchasing their own equipment.
“I watched them unload about 20 ships for us, so I knew what to expect and how to approach it,” he says. “But the performance of the Masaba equipment really helped make the transition from subcontracting this out to doing it ourselves a smooth one. We’ve done nearly two dozen ships now — better than 1.4 million tons of limestone — and haven’t had so much as a hiccup out of the unloader and stacking conveyor. They’ve been just outstanding for us. And so is the fact that we are no longer paying for offloading charges. As to durability, I know this: if a hurricane ever blows through while I’m out here, I’m strapping myself to that stacker — I’m certain it’s not going anywhere.”
This article is courtesy of Masaba, Inc.