06-08 grinding wheel

[Image above] Grinding wheels are widely used in manufacturing to smooth and shape parts. Recovering abrasive grains from rejected and spent grinding wheels can be both economically and environmentally beneficial to manufacturers. Credit: THATLAZYMACHINIST, YouTube

Article by Lisa McDonald and Sabarinathan Palaniyappan

In capitalistic societies, much focus is given to immaterial forms of waste—wasted time, wasted potential, wasted opportunity. Yet when it comes to materials waste, people often do not give a second thought to where their plastic wrappers and used electronics go after disposal.

Fortunately, awareness of the global materials waste problem is growing, along with efforts to combat the problem. Many efforts are taking place within the framework of a concept called circular economy, which aims to move society away from the current linear “take-make-waste” economic model to an economic system that eliminates waste and promotes continual use of resources.

Last year, the International Journal of Applied Ceramic Technology published two special issues on ceramics for the circular economy, and ACerS journals managing editor Jonathon Foreman highlighted several of these articles in a CTT post last month. One topic not covered, however, was the issue of grinding wheel waste.

Grinding is a widely used machining process to ensure the geometry and smoothness of a final part. It involves use of a disc-shaped grinding wheel that is covered in abrasive particles to grind away material from a workpiece’s surface. The abrasive particles are typically ceramic, such as aluminum oxide, silicon carbide, cubic boron nitride, or diamond.

As with any product, waste is generated during the manufacturing process when faulty grinding wheels are discarded. However, waste also occurs during operation because of the way grinding wheels are mounted on the grinding machine. Generally one-third of the grinding wheel is covered up by the flanges, a round flat adapter hub that attaches the wheel to the axle. Because the wheel cannot be used beyond the flange, the portion used for clamping is designated as “waste,” even though it has fresh usable grains in it.

To date, few researchers have investigated ways to recover abrasive grains from the rejected or spent grinding wheels. Sabarinathan Palaniyappan, postdoctoral researcher at the Sri Sivasubramaniya Nadar (SSN) College of Engineering in Chennai, India, is an exception.

Over the past few years, Sabarinathan’s research has focused on recovering abrasive grains from grinding wheel waste using both mechanical and chemical methods. He has specifically focused on recovery from vitrified grinding wheels, which use a glass or glass-ceramic bonding material to fuse abrasive grains to the wheel.

The papers summarized below illustrate some of the work Sabarinathan has conducted on this topic. If you are interested in learning more, or know of a postdoctoral position in the area of ceramic waste recycling seeking applicants, contact Sabarinathan at sabarinathan14010@mech.ssn.edu.in.

Brown alumina abrasive grains: Recovery and application

In an early 2018 paper on recovery of abrasive grains, Sabarinathan and colleagues from SSN College of Engineering and Carborundum Universal Ltd. explored recovering brown alumina abrasive grains using a chemical separation method due to its “optimal” ability to remove the bonding agent.

They chose hydrofluoric acid as the chemical leaching reagent because of its high electronegativity, which allows it to selectively attack the aluminosilicate bond while having a very mild effect on the alumina abrasives. They crushed brown alumina grinding wheels and sieved the particles into three different lump sizes (4.75, 10, and 13.5 mm), which they then reacted with the hydrofluoric acid.

Tests revealed that wheel grade, which indicates the hardness with which the bond holds the abrasive grains in place, had no major effect on grain recovery. However, the lump size of the crushed grinding wheel did, which is “obvious because larger lumps have more grains,” they write.

They also found that too little acid led to insufficient removal of the bond material from the grains, while too much acid led to grain degradation. Regrettably, because the acid reacted with the bond material to form fluorides, “This may reduce the reusability of the acid for further reclamation of grains,” they write.

Ultimately, they determined that the optimal combination of factors for brown alumina abrasive grain recovery is an acid quantity of 100 mL, an immersion time of 1 hour, and a large lump size.

As for what to do with the abrasive grains following recovery, they noted that because abrasives like brown alumina get toughened during vitrification, the grains cannot be reused in the same product. However, they can be used as value-added grains in low-temperature applications, like coated abrasives and resinoid grinding wheels.

In 2019, Sabarinathan and his colleagues published a paper following up on the 2018 study. It explored how well the recovered brown alumina abrasives worked as value-added grains in resinoid grinding wheels.

Instead of using the chemical separation method for recovery described in the 2018 paper, which generates additional chemical waste in the form of fluoride-contaminated hydrofluoric acid, they instead used a modified ball milling process to recover the abrasive grains.

Results from cylindrical plunge grinding of high-speed steel suggested that the accidental toughening of abrasive grains during vitrification is beneficial when reused in resinoid applications. Specifically, the test wheel demonstrated better material removal rate, lower wheel wear, and hence higher grinding ratio than the standard wheel that did not contain recovered abrasives.

The success of the resinoid grinding wheel experiment led Sabarinathan and his supervisor V.E. Annamalai at SSN College of Engineering to conduct another study in 2020, exploring use of recovered brown alumina abrasives in the abrasive water jet machining process (AWJM).

Garnet abrasive grains are the standard abrasives for AWJM. However, studies on using recycled garnet abrasive in the process found it could not compare to fresh grain. “This demands more effective alternative abrasive grain for the AWJM process,” Sabarinathan and Annamalai write.

Experiments on cutting marble and aluminum workpieces using garnet and recycled brown alumina abrasive grains found the recycled alumina improved cutting efficiency by 43% for the marble and by 63% for the aluminum. Additionally, because the recycled alumina has more cutting edges than garnet, it can be used for more than one cutting cycle in the AWJM process.

Typically, the cost of alumina is higher than garnet, so using fresh alumina for the AWJM process would not make sense. But because these alumina abrasive grains are recycled, “the cost of the grain would be the actual recovering cost,” they write.

Recovery of sol gel alumina and cubic boron nitride abrasive grains

In 2020, Sabarinathan and Annamalai explored recovering sol gel alumina abrasive grains using the hydrofluoric acid treatment from the 2018 paper on recovering brown alumina abrasive grains.

Even though the chemical separation method generates additional chemical waste, they preferred this method rather than mechanical crushing for sol gel alumina because it produced purer grains. “The problem faced during mechanical crushing process is there are agglomerates present in the crushed abrasive grains,” they explain in the paper. “So, the chemical separation method is preferred to recover bond free sol gel alumina abrasive grains from the spent vitrified materials.”

Compared to brown alumina, the researchers determined a smaller lump size, higher acid quantity, and slightly longer immersion time (90 minutes) provided the best abrasive grain recovery for sol gel alumina.

This May, Sabarinathan and Annamalai published their most recent paper on recovering abrasive grains. It explores recovering cubic boron nitride abrasive grains using the hydrofluoric acid treatment as well, though in this case the process was carried out two ways—with and without preheating the grains before applying acid—to see if it made a difference.

They found that preheating the cubic boron nitride abrasive grains before chemical treatment resulted in a higher recovery yield, which they suggest is possibly due to elimination of phenolic bonds. “However, to separate the [cubic boron nitride] grains from the grain mixture, more work needs to be done,” they conclude.