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Sands of time and time again

NRRI creates opportunity for foundry sand recovery

ME Electmetal

Sand casting to make metal parts is at once ancient and thoroughly modern. Today in Duluth’s west end, ME Elecmetal casts 145 tons per day, supplying large industrial components for Minnesota’s mining industry, as well as other special order castings.

And to make high quality components, the foundry needs high quality sand. The best sand, and the most expensive, is a heavy mineral known as zircon. It has the highest fusion point (almost 5,000 degrees Fahrenheit) doesn’t expand in the mold and has high thermal conductivity. NRRI is helping the Duluth foundry make the most of their valuable zircon with an innovative recovery process.

Zircon sand costs $1.64 a pound while silica sand is purchased at 2 cents a pound. The cheaper sand is used for the bulk of the casting mold and zircon is used for only the most intricate parts of the mold. But after the cast is set and the metal part is removed, the zircon gets mixed with the silica sand. Tom Waring, quality manager at ME Elecmetal, asked NRRI to help them find a way to separate the zircon from the silica sand so that it can be reused.

The old tried and true "gold panning" process where water separates lighter contaminants from the heavier gold was considered, but the foundry required a non-water based process. This was the challenge given to NRRI Engineer Tim Hagen.

First, Hagen let the sand particle size separate 75 percent of the silica away from the zircon by running it through a 70 mesh screen. Then, knowing that zircon sand has a specific gravity of 4.7 and pure silica is 2.65, Hagen tried to work with the most elemental force of all — gravity.

With a leased vibrating air table — think, air hockey — positioned at a slope, Hagen watched as the lighter silica moved away from the heavier zircon which responds to the vibration by travelling uphill and concentrating against a barrier where it falls off and is collected. After some fine-tuning of the slope, vibration frequency, air flow and feed rate, Hagen showed the process to Waring at the foundry.

"It was like the brown colored zircon particles had marching orders to go uphill on the table while the whiter silica migrated downward," said Hagen. "It was amazing to watch as the particles separated by specific gravity class."

Analysis showed that the air table process delivered 88 percent pure zircon. The project team agreed to scale-up the process and the equipment was moved to the foundry where Hagen trained their staff to separate larger quantities. They successfully recovered almost 300 pounds of zircon with an average purity of 80 — 85 percent, valued at about $500.

The next task was to find out if that was pure enough zircon concentrate to make high quality metal parts. The project team made bolt cores in a casting with the recovered zircon sand. It worked well, meeting stability and quality penetration standards. Waring is ready to move ahead with an engineering study to develop a process flow sheet and consider equipment purchase options to fully integrate the process into the foundry.

"The separation method, the dry air table, works for our application," said Waring. "Now we have to figure out the next steps. What will the system look like on the plant-scale? What size equipment do we need?"

NRRI’s assistance will help ME Elecmetal reduce cost, recycle resources and continue to produce high quality metal components for their customers.


Terms:

Specific gravity (also called relative density) is the ratio of the mass of a solid or liquid to the mass of an equal volume of distilled water at 4 degrees Celsius.

Zircon (zirconium silicate) is a mineral belonging to the group of nesosilicates. Zircon sand is a compound of approximately two-thirds zircon oxide (Zr2O) and one-third silica.

Sand casting is a metal casting process that uses sand as the mold material. ME Elecmetal uses a vacuum molding process for ferrous and non-ferrous metals. Unbound sand is compacted and hardened around the pattern making a mold into which molten metals can be poured. The first evidence of molten casting dates back to Assyrian King Sennacherib (704 — 681 BC) who cast massive bronze sculptures.