"I think eventually, we'll be producing high-purity silica and other silicon compounds right next to the rice fields. It will be possible to process rice and produce high-grade silica in a single location with little or no carbon footprint."Richard Laine, University of Michigan
Silicon-based compounds such as precipitated silica are essential for many products that we use every day. Hundreds of thousands of tons of them are produced every year. They're used to polish the silicon wafers that go into electronics, to strengthen tires, to make white paints and paper, as an abrasive in toothpaste, and in the countless tiny packets of silica gel beads used to absorb moisture in packaging for new products.
Producing high-purity silicon compounds is today an expensive and carbon-intensive process that requires heating mined silicon metal and anthracite coal to 3500°C in an electric arc furnace. That could soon change, though, thanks to a new technology from the University of Michigan that can produce the same silica compounds from agricultural waste.
Developed by Richard Laine, a professor of materials science and engineering, the new production process could save approximately six tons of carbon emissions per ton of silica compounds produced. Laine estimates that the process, which is detailed in a paper in Angewandte Chemie, costs less than 90% of the current one, with virtually no carbon footprint.
Much of the world's agricultural waste contains silica, and the search for a practical way to extract it stretches back 80 years. 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 and is removed when rice is processed. Hundreds of millions of tons of the hulls are produced around the world every year. Many are burned to produce electricity, and 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 simply dumped in landfills.
Although 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 has now found two easy and inexpensive ways to break that bond: ethylene glycol, or antifreeze, and ethanol. When combined with a small amount of sodium hydroxide, antifreeze can weaken the chemical bonds between the silica and the rice hull ash at the beginning of the process, dissolving the silica into a liquid solution.
This solution is then heated to 390°C, forming a polymer of silica and antifreeze. While this stage does require energy, it's 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 to be carbon-neutral.
Ethanol is then added at the end of the process. Because it's chemically similar to antifreeze, it easily swaps in to replace the antifreeze, which is then recycled. 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. Headquartered in Ann Arbor, it's currently in the process of building a ‘pre-pilot’ plant that will be used to develop a scaled up manufacturing process. If the scale-up is successful, Laine predicts that it will 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 said. "It will be possible to process rice and produce high-grade silica in a single location with little or no carbon footprint. It's really very exciting."
Laine recently received the 2015 Michigan Green Chemistry Governor's Award from the Michigan Department of Environmental Quality for this work. Mayasil is a spinoff of Mayaterials, a company Laine founded in 2003 in conjunction with University of Michigan Tech Transfer. Mayasil holds a patent on the silica production process.
This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.