Reusing waterjet cutting abrasive
Recycling systems recover used garnet, but are they worth worth the upfront cost?
For shops that cut using a waterjet machine, the abrasive in the pressurized stream of water, which allows the system to cut away metal, is a significant capital investment. Because of recent technology developments, a fabricator now can recycle the abrasive material and use it several times. But before a shop invests in this type of equipment, it should first consider the types of abrasive material that are available, as well as how the recycling technology works.
Tough economic conditions are making it hard for many metal fabricators to stay competitive. Ideas that will help save money and resources are welcome in any shop. For those shops that cut using a waterjet machine, the abrasive in the pressurized stream of water, which cuts away the metal, is a significant capital investment. Recent technology developments, however, now let a fabricator recycle the abrasive material and use it several times. But before a shop invests in this type of equipment, it should first consider the types of abrasive material that are available, as well as how the recycling technology works.
What Types of Abrasive Material Cut Metal?
Two types of garnet are used as the abrasive material in waterjet cutting of metal: crushed rock and alluvial. The crushed rock has jagged edges that aid in cutting, and the alluvial is rounded like sand in a river bed. Crushed rock comes from several locations, including Idaho and New York, according to Rich Ward, president, WARDJet, Tallmadge, Ohio. Alluvial garnet comes from deposits around the world, but more specifically in Australia, India, and China.
The crushed rock typically costs more. Alluvial goes for 18 to 24 cents per pound, whereas the crushed rock sells for 26 to 46 cents per pound, Ward said.
"When the crushed hard rock impacts the material, these sharp pieces will be more aggressive in cutting through the material," Ward said. "They have most of the same properties for hardness and chemical composition [as alluvial], but the big difference is that alluvial doesn't have the sharp edges."
Both alluvial and crushed rock can have fracture planes inside, but this is more prevalent in garnet that comes from India, according to Ward. For the most part, the version sold in the U.S. doesn't have the fracture planes. As the abrasive runs through the waterjet stream and impacts the metal, the garnet can shatter along the fracture planes into several pieces with sharp edges. As a result, recycled alluvial abrasive, originally rounded, often cuts faster than new alluvial abrasive. The reusable crushed rock, which already was sharp and jagged, still has sharp edges after being recycled. Regardless of the kind of abrasive used, a certain portion will be pulverized when it comes into contact with the material being cut and become too small for reuse.
For most shops, disposing of garnet isn't a problem because it isn't hazardous. However, if a shop cuts lead or any other hazardous material, and the material mixes with the ground-up abrasive, the waste can become potentially hazardous.
"Most shops are cutting metals that aren't considered hazardous," said Bradley Schwartz, regional product manager, Jet Edge, St. Michael, Minn. "You have specialty installations that are cutting nuclear, radiated materials or things like that that have to be disposed of as hazardous."
How Does a Recycling System Work?
One of the main reasons it makes sense to recycle used abrasive material is that some of it doesn't touch the metal during cutting, Ward said. The abrasive goes straight through the jet, which means that some of it is exactly the same after it's been through the machine. The abrasive that impacts the material usually is pulverized, while some of it cracks and creates sharp, new cutting edges.
In recycling technology incorporated in Jet Edge's machines, 50 to 70 percent of the abrasive is not being used, it's actually the core of the jet, according to Schwartz.
"Only 30 to 50 percent of that jet stream is actually being used to cut—the rest is just going into your tank," Schwartz said.
Some of the abrasive, even though it does not come into contact with the material, does become smaller in the process and cannot be used for cutting a second time, according to Ward. "It has been found when doing sieve analysis of used abrasive that as much as 35 percent of some abrasives do not change their size the first time they pass through the cutting head and material," Ward said.
The whole process, which takes the abrasive through the machine and back to a hopper, takes three minutes, Ward explained of his company's patented recycling technology. The machine has a screen that washes the abrasive, with the size of the screen decided by the owner. Ineffective material falls into the holes in the screen and into a hopper as waste material. The abrasive that remains above the screen can be reused. It flows over the top of the screen and down into a dryer, comes out and goes over a secondary splitter, gets pumped to silos, and is ready to be used again.
"It is important that the amount of abrasive being washed and cleaned is balanced with the drying capacity of the recycling system," Ward said.
The splitter divides the fine and coarse grains, or +100 mesh and –100 mesh. The +100 can be mixed with anything that is +100, but –100 shouldn't be combined with +100, according to Ward. This splitter is optional on many recycling systems.
Schwartz suggested that a rule of thumb is to mix recycled garnet 50/50 with new garnet to give it consistent cut quality.
"[The recycling equipment is] drawing in from the abrasive removal system and running through either a single or dual screen," Schwartz said. "The screen is sifting for a certain mesh size. Some users screen for two different mesh sizes, a fine and a larger size. The material is actually being washed as it's running across the screens, and whatever goes through the screens is unusable. Typically, you'll store it and mix it with new garnet when you're ready to use it."
Using the right kind of abrasive, a recycling system should be able to provide in excess of 50 percent recovery, and some fabricators claim to recover as much as 90 percent of their used abrasive, Ward said. But even if a shop will recover at least 50 percent of its abrasive with an abrasive recycling system, is it worth it to absorb the upfront costs of purchasing the machine? Is the system more suitable for some shops than for others?
"If I'm using 5,000 pounds of abrasive a month and it costs me $250 a month to clean out my tank, I'm going to save $720 at 50 percent recovery," Ward said. "The machine costs about $60,000, and if I spend $1,000 a month on payments on the machine, it doesn't make sense. It doesn't pay to try to generate money out of savings you can't make if there isn't enough volume there to save.
"If we go to the other extreme and I'm using 100,000 pounds, the savings are going to be $11,000 a month. Recycling is worthwhile if you're using a large volume. If you're using only 1,000 pounds a month, it's not that effective. If you're using 20,000 pounds or more, you're talking about saving several thousand dollars per month."
Many companies that use minimal abrasive can still consider a recycling system as a future investment, Ward added. These companies should store the used abrasive and stack it up with the ultimate goal of purchasing a recycling system once they have enough to justify the purchase.
Abrasive recycling systems allow fabricators to reuse the material several times. If a shop starts with 100 lbs., for example, it can recycle it and be left with 70 lbs. Then it might recycle it again and have 50 lbs., then 35 lbs., and so on. So the 100 lbs. of abrasive the shop started with could equal 250 lbs. or more, Ward said.
Abrasive recycling systems are set up differently. With Jet Edge recycling technology, the abrasive removal system is put on the waterjet machine to take the garnet out of the tank. Two systems usually are installed—the abrasive removal system and the recycler, according to Schwartz.
"There's two ways [to install it]," Schwartz said. "One is it's possible to just put the recycler on and move a nozzle around your tank to draw out of it without the abrasive material. That's typically not as effective as putting an abrasive removal system on first. That puts the manifold inside the tank, keeps the tank stirred up, and draws the garnet out of the tank."
Some companies that don't recycle their abrasive do use abrasive removal systems. Once the used abrasive is removed, it is generally deposited into a temporary storage hopper before disposal. Some fabricators actually have stored the used abrasive until they have enough volume to justify a recycling system, Ward said. The recycling system is designed to take the abrasive out of the temporary storage hopper, so that instead of being disposed of, the abrasive is reused.
Are more fabricators going to be looking at recycling abrasive? Ward said the technology currently is not used widely, but recycling probably will grow as shops look for ways to save money.
He added that shops should ask four key questions if they are looking at abrasive recycling:
- What recovery percentage will the system deliver?
- Will the system affect the waterjet cutting speed?
- What is the cost per pound of the abrasive when taking into account recycling?
- Can the cost of a recycling system be justified?
"In certain industries, the technology will pay for itself quickly," Schwartz said.
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.