Turning agricultural waste to useful silica compounds

Producing high-purity silicon compounds is today an expensive and carbon-intensive process but that could soon change, thanks to a new technology from University of Michigan that can produce the same silica compounds from agricultural waste.

Silica and other silicon compounds are essential for many products that we use every day. They are used to polish the silicon wafers that go into electronics, to strengthen tyres, to make paints and paper white, as an abrasive in toothpaste, and yes, in countless tiny packets of silica gel beads.

But current method of silicon compound production requires heating mined silicon metal and anthracite coal to 3,500 degrees Celsius in an electric arc furnace, making it an expensive and high carbon-intensive process.

Developed by Richard Laine, professor of materials science and engineering, the new technique is believed to be the first simple, inexpensive chemical method for producing high-purity silica compounds from agricultural waste.

"I think eventually, we will be producing high-purity silica and other silicon compounds right next to the rice fields," Laine said.

"It will be possible to process rice and produce high-grade silica in a single location with little or no carbon footprint. It is really very exciting," Laine noted.

Much of the world's agricultural waste contains silica. While the new process could be used to produce silica and silicon-containing chemicals from many types of agricultural waste, Laine focused on using the hulls left over from processing rice.

The hull is the outermost layer of the rice grain. It is removed when rice is processed.

But while the world is awash in silica-rich rice hull ash, getting that silica out has proven to be a major challenge. The difficulty stems mostly from the incredibly strong chemical bond between silicon and oxygen, one of the strongest that exists in nature.

Laine found two easy and inexpensive ways to break that bond: ethylene glycol, or antifreeze, and ethanol, or grain alcohol.

The antifreeze combined with a small amount of sodium hydroxide weakens the chemical bonds between the silica and the rice hull ash at the beginning of the process, dissolving the silica into a liquid solution.

The solution is then heated to 390 degrees Celsius, forming a polymer of silica and antifreeze.

Grain alcohol is then added at the end of the process. It is chemically similar to antifreeze, so it easily swaps in to replace the antifreeze, which is then recycled.

The liquid silica can then be distilled out of this second solution and used to make a high-purity precipitated silica product for industrial use.

Laine has formed a Michigan company, Mayasil, to commercialize the technology.

The findings were published in the journal Agewandte Chemie.