Bulk Bag System Increases Refractory
Manufacturer's Batching Accuracy

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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