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 Zedmark's
Slippery Rock, PA, refractory plant receives truckloads of bulk bags containing
materials such as aluminum oxide and zirconium oxide. The oxides come
in different grades that vary according to particle size and material
form (such as tabular or calcined). Forklift trucks move the palletized
bulk bags to a warehouse and then to processing. During processing, raw
materials are formulated into batches and mixed in a horizontal plow mixer.
The mixed batches are placed in portable bins and moved to casting production
lines. Finally, the batches are cast into specialized refractory shapes
that are eventually used to manufacture glass, steel, cement, and aluminum
products.
Manually handling
paper bags results in dust, bag disposal costs
 Before
1992, Zedmark's batching station consisted of two workers who weighed
and dumped a lot of paper bags into a mixer for each batch. Zedmark received
50 and 100 pound bags of materials on pallets, which forklift trucks moved
to a raised platform in the blending area. Workers lifted the bags and
loaded them into a bag-breaking station mounted on load cells. The workers
weighed each ingredient as it entered the station and discharged each
ingredient individually into the mixer.
Zedmark's concerns about the batching
station included dust in the workplace, the time and manual labor required
to formulate batches, the cost of material lost from spillage and incomplete
bag emptying, empty bag disposal costs, and batch consistency. "Workers
manually handled the bags, and we had some dust in the workplace as a
result," said Zedmark Engineer Bob McCutcheon. Bags sometimes tore, increasing
dust and material loss. "Bag disposal also got very expensive," McCutcheon
said.
Manufacturer seeks
new batching system
 In
1992, Zedmark wanted to develop a system to replace the manual batching
station. Because Zedmark uses 16 basic raw materials in about 30 formulas
with up to seven raw materials each, the company sought a batching system
that would accommodate its variety of production needs. "We were looking
for a flexible system that would rapidly switch between formulas," McCutcheon
said. "We also wanted to eliminate manual bag lifting and dust," he added.
Zedmark also had limited floor space
and found that some batching systems required a separate weighing station
for each ingredient, which would take up too much room. Then Zedmark considered
a more compact system that used bins mounted on load cells. The system
also permitted quick changeover between batching formulas and computerized
batching, which Zedmark preferred. As a result, Zedmark approached the
batching system's manufacturer to discuss design options.
Computerized batching
system uses PLC, PC
After initial meetings with the batching
system manufacturer, McCutcheon started work on a custom-designed system.
The resulting system has eight on-line bulk bag stations and a paper-bag
or drum-dump station for very minor ingredients. All nine stations mount
on load cells for loss-in-weight measurements that enable an IBM PC with
an Allen Bradley PLC to control the materials as they discharge onto a
transfer belt conveyor to the mixer.
 Each
station consists of a bulk bag holding rack that suspends the bag by four
loops on a frame above a bulk bag dumper, which is also supported on
its own frame. Each frame is independently movable by forklift truck for
quick material changes. Each rack includes a dust collection ring that
goes around the bag throat to control dust during initial bag opening.
Each station has a dust collection valve, and a worker opens the valve
to open a bulk bag. Once the bag is opened, the air space between the
bag and the station is sealed, and the valve is closed. The bulk bag
dumper discharges through a flexible sleeve and a slide gate.
Because of Zedmark's variety of batch
formulas, the batching system includes eight additional bulk bag dumpers
to increase system versatility. The system uses a bin ID system to prevent
the interchangeable dispensers from being placed in the wrong station.
Now the computer can tell which material is in which discharge station
and can weigh out the required amount of the right material. "That's one
of the features everyone likes," McCutcheon said. "It reduces the potential
for making a mistake."
The bin ID system identifies each
bin with a binary code in the form of a mechanical bar code; the code
consists of a notched plate that trips limit switches to identify the
bin. "The code identifies that such-and-such bin has such-and-such material
in it," McCutcheon said. The bin IDs are changed every nine months on
average.
To formulate a batch, the operator
selects the formula and the number of batches to be repeated. A three-digit
pneumonic identifies each formula, and the operator can enter and store
new formulas and pneumonics on disk. The operator inputs the desired batch
size, and the computer receives information from the load cells to determine
if enough of the required materials are present to formulate the batch.
If so, the computer controls dispensing of each material in the required
ratio to the transfer belt conveyor. The computer also controls the transfer
conveyor and the mixers to finish the batching cycle.
Batching system
cuts labor and cleanup costs
Since installing the batching system,
labor needs have gone from two workers 10 hours per day to one worker
four hours per day, and the plant is cleaner. The batching system also
eliminates material loss due to incomplete bag emptying and empty bags.
Handling
50 and 100 pound bags of oxides subjected workers to dust. A bulk bag
batching system uses a dust collection ring during bag opening and then
seals in material after the bag is opened.
Batching consistency has doubled,
and Zedmark's quality control department says batches are now always in
specification. All programming and training for the system were supplied
by the batching system manufacturer and done on-site. A support person
was also provided to help with the start-up. The system has been operating
for almost two years, and it has proven so efficient that Zedmark will
move it to its Dover, Ohio, operation and do all batching for both facilities
there.
"We've been getting visitors just
to see the batching system, and the folks in Slippery Rock are upset that
we're going to move it to Dover," McCutcheon said. "They were beginning
to feel like they were in charge of a sci-fi vehicle or something."
Published in October, 1994 Powder & Bulk Engineering magazine.
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