System Spruces Explosive Mess

Thu, 07/17/2008 - 11:47am

Minnesota Explosives Co. manufactures bulk explosives that are used to blast taconite rock into small pieces—the first step in recovering iron ore. To produce the explosive compounds, sodium nitrate is added to a high-temperature corrosive solution of ammonium nitrate. Previously, the ammonium nitrate was pumped into a large open box (or slush box) in which the solution was circulated. An operator then used a forklift to position a bulk bag containing sodium nitrate granules above the slush box to discharge.

This operation frequently resulted in plugging the pump and hoses in the vessel’s recycle loop, causing the ammonium nitrate solution to overflow the slush box and spill onto the floor. Furthermore, it required two operators—one to run the forklift, another to watch the slush box and stop the filling process to prevent overflows.

“We wanted a system that eliminated spillage of solution,” says Jon King, Minnesota Explosives’ general manager of operations. So the company replaced the open slush box with an enclosed material-handling system consisting of a bulk bag discharger, a flexible screw conveyor and two 230,000-lb. capacity tanks in which to blend the material with the ammonium nitrate solution.

In the new process, liquid ammonium nitrate is charged into two insulated blending tanks agitated and heated by steam coils. When the temperature drops below the set point, the agitators are activated, and steam is fed to the coils to maintain the desired temperature. A flexible screw conveyor then transports the granular sodium nitrate from the bulk bag unloader to heated tanks, where it dissolves into the ammonium nitrate. The resulting solution is transferred to a blend table and combined with a mixture of four fuels, one of which is an emulsifier. The emulsification is pumped to a holding tank, and later, to another area where solid ammonium nitrate prills are added via an auger. The final product is then discharged into a delivery truck.

Each batch consumes 10,000 lbs. of sodium nitrate supplied in five 2,000-lb. bulk bags, which are emptied using a bulk bag unloader manufactured by Flexicon Corp. The unloader has a removable bag-lifting frame that connects to straps on the bulk bag, and is hoisted by forklift into a cradle at the top of the unloader frame. As the bag empties and elongates, Pop-Top™ bag extension devices mounted on the top of the frame stretch the bag into a cone shape, promoting total discharge of material from the bag into the hopper.

The interface between the bag spout and the 3-ft.³ hopper consists of a Spout-Lock® clamp ring positioned atop a Tele-Tube® telescoping tube. The clamp ring creates a high-integrity, dust-tight connection between the clean side of the bag outlet spout and equipment. The telescoping tube is raised pneumatically, allowing the clamp ring to connect with the bag spout, and then lowered, applying continual downward tension to elongate the bag and keep the spout taut, preventing material in the spout from bulging outward or falling inward.

The granular sodium nitrate is conveyed from the hopper to the blending tanks by three Flexicon flexible screw conveyors. Each 20-ft. conveyor consists of a stainless steel spiral rotated within a flexible 4.5-in. outer tube by a 7.5-HP electric motor with a gearbox located at the discharge end of each unit. The intake end of each conveyor is equipped with a U-shaped trough that serves as a charging adapter. A section of the rotating screw passes through the trough, and the screw propels the material through the tube. The outer tube of the first conveyor is attached to the hopper with a compression coupling, and the intake end of the screw extends into the hopper.

The first conveyor transports material from the hopper at a 45-degree incline and discharges it into the charging adapter of a second identical conveyor connected to it; together they move the material over a horizontal distance of about 28 ft. The second conveyor discharges into a diverter valve, which feeds the product either directly into one blender or into the intake adapter of a third conveyor for transport to a different blender located 16 ft. away.

Capacitive proximity sensors monitor material levels in the hopper and throughout the system, signaling a controller to start and stop the conveyors. Level sensors in the transition regions trigger the shutdown of a conveyor if the subsequent transition is full; when the high-level condition has cleared, the controller restarts the conveyor.

The new process eliminated slush box-related spills, improving plant hygiene while reducing material waste. “It’s much easier to sweep a small amount of granular material than to clean up liquid spills, which had to be washed down into a sump and treated with the wastewater,” King points out. “And we drop any granules we collect into the blender, so no material is wasted.” Costs were also reduced by eliminating the need for an individual to monitor slush box levels. n
More information is available by contacting Flexicon Corp., Bethlehem, PA, at 888.353.9426, sales@flexicon.com or www.flexicon.com.?

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