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Sand reuse is a prime foundry concern.

There was unanimous agreement among the nearly 200 foundrymen attending the recent international Sand Reclamation Conference (March 14-15, Novi, Michigan) that the reuse of foundry sand is one of the highest priority problems confronting the industry.

In the last decade, the pit cost of sand has risen 50-60%, freight costs have gone up 125% and waste sand disposal costs have doubled or tripled. With disposal options severely constricted and costs soaring, the economic advantages of reusing sand have become an operational imperative.

In his keynote address to the conference, Matthew J. Granlund, president of Foundry Systems Control, said used core and molding sand constitutes over 60% of total foundry wastes. "Used" Sand

'Used' sand is a misnomer, he said, in that only the binder on the grain is "used" and not the sand grain itself. Reclaiming the sand involves removing the dead binder coating from each grain, making the sand ready for ultimate rebinding and reuse. With silica sand purchases by foundries running well over 7 million tons a year, the stake in sand reclamation is enormous.

Many mold and core tests using reclaimed sand have yielded castings that are equal to or better than those where new sand was used, Granlund said. This can be attributed in part, he explained, to the elimination (during recycling) of sand grains with poor chemical structure and weak cleavage planes caused by the thermal shock of the casting process and the mechanical shock of shakeout, lump-breaking and scrubbing.

George Good, Casting Div./Ford Motor Co., in recounting his company's long experience with sand reclamation, quoted Roberto Gonzalez (Ford of Mexico) as saying that properly reclaimed sand works like new bank or lake sands, but with the advantages of a better grain size distribution that eliminates undesirable large or excessively fine grains.

Therefore, the bonding and additive materials used to prepare reclaimed sand for cores and molds work on grains essentially free from clays, alkalies, organic materials and sharp edges.This produces a more homogenous, stronger grain surface suitable for use with new bonding and additive materials. "Spent" Sand

"Spent" sand should be considered a raw material whose properties can surpass those of new sand, according to Professor Dietmar Boenisch of the Technische Hochschule's Giesserei Institut, Aachen, Germany, and conference dinner speaker.

To be effective, used core sands must be rid of bentonite, which in this case is considered a major contaminant. When heated above 500C, bentonite loses the properties that make it an excellent sand bonding agent. Reclamation then becomes more difficult.

The bentonite gets into the crevices of sand grains, and, though in minute quantities, represents a potent contaminant of core resins. The most important core process is phenolic urethane coldbox followed by hotbox and shell cores. Ridding these core sands of bentonite dust without losing the sand fines is a problem not yet solved satisfactorily, Boenisch said.

Boenisch advanced the concept of small foundries linking sand reclamation efforts into a single plant that would handle all sand reconditioning at one location. Or, he suggested, simple, mobile sand-cleaning units could be developed to serve the sand reclamation needs of a group of companies.

Gerhard Bauch described a sand reclamation system at Germany's Halberg foundry. The foundry produces 320,000 tons of gray and ductile iron a year. The system represents 20 years: experience in processing spent sand for use in Halberg's core room. The sand reclamation system has been amortized and operates as a profit center.

The reclamation plant processes 12 tons of sand per hour, 120 hours a week. it uses a combination of mechanical, thermal and reclamation processing.

The mechanical phase involves crushing lumps, separating contaminants and storing the refuse sand in holding silos. Next, the sand is calcined to about 1450F to drive off moisture and burn organic materials. The sand is then cooled and pneumatically scrubbed to remove residual clay dust.

Although Halberg's sand reuse operation is large, scaled-down versions using the same reclamation process can be equally effective in reducing a small foundry's sand purchases through an efficient sand reclamation operation, Bauch said.

William Petch, foundry operations consultant, explained that even a very small foundry (1 000 tons of new sand annually) can reclaim sand for reuse. He described a small aluminum foundry that acquired a compactreclamation unit with three times the required capacity to process polyol urethane/sand cores and molds. The reclaimed sand is very similar to new sand, but more workable and requiring 20% less binder, he said.

The equipment was purchased and installed for $200,000, but annual sand operations costs have been reduced from 131,000 to $50,500. Even the recovered fines are reused by the foundry as filler in plaster molding mix.
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Title Annotation:AFS Sand Reclamation Conference
Author:Bex, Tom
Publication:Modern Casting
Date:May 1, 1991
Previous Article:Chill testing of iron; part 1 of 3.
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