A new technology for producing silica compounds from agricultural waste could reduce both cost and emissions compared with existing processes. Producing high-purity silicon compounds—which are used to polish the wafers that go into electronics, to strengthen tires and as an abrasive in toothpaste—can be an expensive and carbon-intensive process that requires heating mined silicon metal and anthracite coal to 3,500 degrees Celsius in an electric arc furnace.

University of Michigan Materials Science and Engineering Professor Richard Laine has developed a technique to extract silica out of the hulls left over from processing rice. The approach could save six tons of carbon emissions per ton of silica compounds produced and 90 percent of the costs compared with the current process.

Hundreds of millions of tons of rice hulls are produced globally every year, much of it burned to produce electricity. The ash that’s left over contains high levels of silica. Some of this ash is used in construction or as insulation, but much of it is dumped in landfills.

Ash from rice hulls burned to produce electricity contains high levels of silica. Image credit: Morguefile.com.Laine's method uses two relatively inexpensive and easily sourced chemical compounds to extract the silica from rice hull ash: ethylene glycol (antifreeze) and ethanol (grain alcohol). Antifreeze combined with a small amount of sodium hydroxide is introduced at the start of the process to weaken the chemical bonds between the silica and the rice hull ash. This dissolves the silica into a liquid solution.

The solution is then heated to 390 degrees Celsius, forming a polymer of silica and antifreeze that is filtered to remove the ash. Although this stage requires energy, Laine says it is more than offset by the energy produced when the rice hulls are burned at the beginning of the process. And because the carbon released when the hulls are burned was previously absorbed by the rice plant, the process is considered carbon-neutral.

Grain alcohol is added at the end of the process. It is chemically similar to antifreeze, so it swaps in to replace the antifreeze, which is recycled. The liquid silica then can be distilled from this second solution and used to make a high-purity precipitated silica product for industrial use.

Laine has formed a company called Mayasil to commercialize the technology. The company is in the process of building a plant to develop a scaled-up manufacturing process. If the scale-up is successful, Laine says that it may fundamentally change the way silica products are made and used. “I think eventually, we’ll be producing high-purity silica and other silicon compounds right next to the rice fields,” Laine says. “It will be possible to process rice and produce high-grade silica in a single location with little or no carbon footprint.”