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Thermal reclaimer developed to reduce energy costs.

A venture between a Canadian firm and three gas utility companies resulted in the creation of a stacked-bed sand recovery system.

The steady escalation in the use of chemical binders by foundries and the parallel need to reduce rising sand purchase and disposal costs are growing concerns for many metalcasters, even those not now pressed for dumping sites.

With the typical Canadian foundry spending $200,000-500,000 a year on sand-related expenses, economical sand reclamation has never been more vital, particularly for chemically bonded sands. As a joint venture between Gudgeon Brothers, Ltd., London, Ontario, and a consortium of three gas utilities over a four-year period, an energy-efficient sand reclamation furnace was developed to burn off the chemical binders to make chemically bonded sand reusable. The compact unit can be fired by natural or propane gas.

The system, developed initially in conjunction with Ontario's Union Gas Technology Transfer and Development department, is a compact, stacked-bed thermal reclamation unit that tackles energy concerns by achieving the lowest Btu/lb possible. By stacking the two beds, heat discharged from the lower cooling bed maintains temperatures in the upper heating bed. Hot gases leaving the top bed pass through a heat exchanger that preheats combustion air for the natural gas burners. Heat is constantly recycled to supplement the furnace burners and lower fuel consumption. A programmable logic controller (PLC) automatically scans and records the reclaimer's operations and a graphics panel displays the state of the machine in real time.

The combustion bed is positioned (stacked) directly above the cooling bed. The bed is fluidized when air is forced upwards through a bulk particulate such as sand. As the air flows around the sand, it begins to bubble and, at this point, the sand can flow like a liquid. A fluidized bed is ideal for transferring heat from combustion gases to sand, prompting good mixing and a uniform temperature. The sand can be held at a preset temperature for any time period.

An auger feeds resin-coated sand into the upper chamber, where it is heated by combustion gases from burners immersed in the bed. The sand quickly rises to the optimum combustion temperature of the resin, about 1250F. The excellent heat transfer of a fluidized bed rapidly breaks down sand lumps as the binder melts and is burned. The "boiling" action of the fluidized sand also causes an intergranular rubbing action (friction) between the sand grains. The combined action of the burning and intergranular friction completely removes the combustible chemical and dust residues.

After treatment in the upper fluidized bed, the clean, hot sand passes through a weir into a lower bed. It cools rapidly to 86F by fluidizing air and water-cooled tubes, and is ready for reuse.

Although some fluidized bed systems use electricity, it was discovered that hot gases transfer heat more efficiently than the solid surfaces of electric heating elements.

Figure 1 shows the relative simplicity of this design. Another larger unit for an Ohio foundry operates 24 hours/day to reclaim 72 tons of sand. Smaller units of similar design are being introduced for use in smaller foundries.

New sand costs average $30-50/ton and about $50-90/ton to dispose of waste sand. Environmental controls mandate that reclamation machines have measurable emissions from the heating/burning process that are below limits set by U.S. and Canadian environmental agencies.

New machines require no special operator skills and operate unattended except for required maintenance. Initial reclamation costs were set at $5/ton including factors for energy and maintenance. Foundries, however, have reported actual operational costs average much lower at about $3.60/ton.
COPYRIGHT 1993 American Foundry Society, Inc.
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Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Bex, Tom
Publication:Modern Casting
Date:May 1, 1993
Words:602
Previous Article:Reclaiming chemically bonded sands: a technology review.
Next Article:Research, planning aid selection of sand reclamation system.
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