Reusing recycled abrasives in waterjet cutting?

Troy Cain plays down his educational background, saying that any advanced schooling he received occurred only in the U.S. Marine Corps, but if you know anything about that branch of the military, you know Marines are famous for adapting and overcoming. Well, it also applies to waterjet cutting.

While Cain is not personally involved in fabricating metal, he has firsthand knowledge of the waterjet cutting process. He has been in the parking lot maintenance business in the Seattle area for 17 years, and as an offshoot of that business, he has been cleaning the cutting beds of waterjet machines in nearby fabricating facilities for several years. Also as part of that business, he was charged with hauling away the sludge (see Figure 1)—comprised of water, abrasives, and metal fragments—and delivering it straight to the landfill.

Now because the Puget Sound area is a hotbed of aerospace manufacturing, these fabricating shops cut a lot of parts on waterjet tables. (Waterjets are the preferred device for cutting these types of parts because they offer high-tolerance cuts and do not create a heat-affected zone during cutting, which could change the original makeup of the metal.) That also means that the shops create a lot of waste material.

Living in the environmentally conscious Pacific Northwest, Cain recognized that there had to be a way to recycle the waterjet cutting byproduct so that it didn’t end up in a landfill. Nearby customers already were interested in recycling their spent abrasive—The Environmental Coalition of South Seattle has honored two of Cain’s waterjet cutting customers for saving more than 2 million pounds of spent garnet from the landfills—but they weren’t using any once-used garnet for waterjet cutting. The problem is that no one has been successful on a large scale in turning used garnet abrasive into a reusable product for waterjet cutting.

Today some recycling of the quicksand-like material takes place, but it results in a product that can be used as a filler only in asphalt and concrete products. Cain said that large-scale recycling efforts that he is familiar with don’t separate the byproduct to leave a garnet material that is good enough to be used as a born-again abrasive for waterjet cutting.

“I began to do a little research,” Cain said. “The only reason that they have to take it to the dump is that it is used in-process. There could be metals in it.

“So I thought if we could get the metals out and dry it, we should be able to recycle this stuff for reuse,” he said.

For the past year Cain said he has taken the waterjet waste material and worked with it in his company’s indoor facility. It wasn’t until recently that he found the right combination of drying time and screening that resulted in separation of the water from the abrasive, the good abrasive from the rest of the materials, and the metal and water from the remaining abrasive that is destined for asphalt and concrete applications (see Figure 2).

In fact, Cain calls the entire process “completely green.” The metals are recycled, and the water is perfectly clear after treatment.

The recycling effort also produces reusable garnet material of about 80 to 90 mesh, or grit, the measurement used to describe the size of the abrasive particle. Cain said that 80 mesh is the preferred size, but some metal fabricators don’t mind the 80- to 90-mesh mix if it saves some money.

Garnet is usually used as an abrasive in a waterjet stream because the fairly hard gemstone is widely available. When individual grains fracture, they expose sharp edges, which are ideal for helping to remove metal in a highly pressurized water stream. Suppliers of garnet typically crush the material as part of processing, exposing the sharp edges desired for waterjet cutting. Cain said the abrasive generated from his recycling process is the suitable size and shape for reuse in waterjet cutting applications.

“Here they are just getting used to the fact that recycled garnet can be used and it works just about as good,” he said, “unless they are cutting with a 50-grit, which is a very large size grit.”

He added that the recycled abrasive also could be used for another untapped market, sandblasting. With the cost of the recycled material being less expensive than unused garnet, Cain said the future looks promising.

“People want to start these recycling facilities all over the place if it works out,” he said.

Leave it to a Marine to overcome a simple recycling challenge.