Rice hulls could nourish Silicon Valley

Rice, the food that feeds the world, may one day also satisfy the semiconductor industry’s hunger for raw materials. The tough hull that encases rice grains contains 20 percent silicon dioxide, or silica, and rice millers generate millions of tons of hulls each year.

To extract the silicon that occurs naturally in this agricultural waste, scientists need to identify the silicon-containing compounds. A team of scientists in Brazil has used nuclear magnetic resonance (NMR) to confirm previous work demonstrating a silica gel—amorphous silica bound to water—in rice hulls.

Moreover, the NMR work shows that some of the silicon attaches to carbohydrates—especially lignin—in the plant. It’s “the first direct evidence about the connection of silicon to organic groups in natural materials,” says study coauthor Jair C.C. Freitas of the Universidade Federal do Espírito Santo in Vitória.

The researchers find similar attachments in another agricultural waste product that contains silicon. Their report on a discarded layer from the native Brazilian babassu coconut and the rice hulls will be published in Chemistry of Materials.

Harvested rice grains start out “rough”—that is, covered by a hard hull, explains Wayne E. Marshall of the Department of Agriculture’s Southern Regional Research Center in New Orleans.

Rice millers strip off the hull and remove a layer of bran to turn brown rice into white. To make quick-cooking rice, processors then parboil the grains, which removes the sticky starch. Since 20 percent of the rice is hull, the discarded hulls provide an abundant source of fuel that is burned for the parboiling, Marshall says. The ash that remains from the hulls is rich in silica, so it’s a potential source of silicon for industry, says Freitas.

The Brazilian team used NMR to track the burning of rice hulls. For temperatures up to 1,000ºC, they found that the carbon seems to form a matrix with amorphous silica scattered throughout and some silica bound to it. Above 1,200ºC, at the temperatures where the rice hulls burn, the researchers saw silica becoming crystalline. At 1,450ºC, silicon carbide begins forming, they report. What looks like silicon carbide might actually be a second crystalline form of silica, says Andrew Proctor of the University of Arkansas in Fayetteville, who has examined silica in rice hulls using infrared spectrometry and X-ray diffraction.

Amorphous silica in unburned hulls dissolves readily in water, says Proctor. Silica bound to organic materials would be more difficult to extract. Only with extremely effective purification techniques, not yet mastered, could this new source of silicon rise from the ashes.