New approach to ‘mining’

Extracting glassmaking raw materials from food waste. Published on September 27th, 2013 | Edited by: Eileen De Guire
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Soda-lime-silica glass made from minerals extracted from food wastes. (Credit: Cornejo and Reimanis; Colorado School of Mines.)

The mission of mining schools is to research new and better ways of extracting useful raw materials to manufacture products. Usually, miners look into the earth for sources of valuable minerals. Researchers at the Colorado School of Mines, expanding the definition of mining, have discovered that the earth will deliver minerals to them in the form of agricultural byproducts.

The food processing industry has a significant problem with waste. The Bible parable about “separating the wheat from the chaff” makes clear what will happen with the “chaff” of the spiritual realm. In the physical realm, chaff disposal is a big problem. Researchers have been working on finding a feasible use for rice hulls and other food wastes at least since the 1990s.

CSM researchers recently filed a provisional patent on a new process for extracting mineral content from a wide variety of food wastes. In a phone interview, Ivan Cornejo said the idea to investigate the mineral content of plant matter came to him when he heard a story on National Public Radio about a calcium carbonate deposit that had been discovered in Britain. According to the report, it was shallow (close to the surface) and, therefore, easy to mine. Unfortunately, there was a natural forest growing on top of it. Cornejo says, “The local government weighed the pros and cons and, in the end, decided to destroy the forest.” That got him thinking. “I knew calcium could be found in waste, so I thought maybe other minerals could be found, too,” he says.

Cornejo and Ivar Reimanis, along with graduate student Subramanian Ramalingam, looked at a variety of food and agricultural wastes and realized they contain useful, extractable amounts of oxides, especially silica, the primary constituent in glass. It turns out the inorganic ash of food waste can provide most of the requisite constituents for ordinary soda-lime-silica glass and perhaps some specialty glass compositions, too. For example, silica is found in rice husks, peanut shells, and corn husks and stems. Peanut shells also contain soda, potassium oxide, magnesium oxide, calcium oxide, and phosphorous oxide. Corn stalk waste yields soda and potassium oxide. Eggshells are rich in calcium oxide. Finding a source of alumina was the most challenging, but eventually the scientists found a plentiful one—coffee. Useful minerals also are found in banana peels, coconut shells, tomato waste, wheat husks, etc.

The process involves pyrolyzing the waste to separate the inorganic compounds, although chemical extraction processes would work, too. The abstract of the provisional patent application says, in part, “By extracting the most common and more needed minerals from these waste streams, a 100% renewable glass, glass-ceramic, and/or ceramic can be made without the need for mining more minerals from the Earth’s crust.”

What is the potential impact of this new way of formulating glass? In an email, Cornejo, who came to CSM from Corning Inc., where he was involved in developing some of its most innovative products, such as Gorilla Glass, Lotus Glass, and Jade Glass, provided some context: “The worldwide production of flat glass in 2009 was 52 million metric tons. If we assume that flat glass is about 70 wt% silica, then you need 36.4 million metric tons of silica.”

Besides flat glass, container glass also represents a huge potential market. A recent analysis by The Drinks Report says the retail market for glass alcoholic beverage containers alone was nearly 200 billion units in 2012 (75% for beer bottles). Add in the global market for soft drink bottles, peanut butter and pickle jars, etc., and the value of a sustainable stream of raw materials becomes obvious. (According to Cornejo, only 20–40% of glass is recyclable, depending on industry.)

Is there enough volume in the food waste stream to meet raw materials demands for flat glass and container glass production? Cornejo thinks so. “The worldwide production of the main three cereals consumed by humans in 2011 was 2,173 million tons worldwide. The wastes coming from these cereals are husks, stems, and cobs, and they are about [20%] by weight of the total production—about 435 million tons of waste. This waste is about 15% (on average) silica, [so] in 2011 we could have obtained about 65 million tons of silica. Much more than is needed for the entire production of flat glass globally!”

Rice generates one of the largest food waste streams and on a global scale. According to data from theUSDA Economic Research Service website (see downloadable spreadsheet “Rice supply and use, selected countries and global trends”), global rice production for the 2012–13 crop year is projected to be about 464 million metric tons. A Singapore-based company, Pte Ltd., says about 20 percent by volume of harvested rice is husks. They claim that risk husks contain 75% or more amorphous silica (which they sell as “white ash”) that is extractable by controlled burning of the husks, which they also say “requires very specific heat and timing to produce the correct product.”

It appears, then, that there is enough agriculture waste to support a sustainable source of raw materials for glassmaking at production levels that also solves a significant global disposal problem. Food wastes are not free, though. As points out, it may be possible to acquire the waste for little or no cost, but there is cost associated with transporting and pyrolyzing it—the so-called embodied energy. Also, significant energy goes into glassmaking itself, and research into refining biomass into fuel may eventually lead to all-food resources for glassmaking.

Cornejo and Reimanis have made several glass compositions demonstrating that plant-sourced minerals can indeed be made into a glass. So far they have focused on simple soda-lime-silica compositions. They have made several formulations, and at least one was made entirely from plant sources. Challenges they have faced along the way include collecting and storing waste, as well as separating oxides.

Many questions still remain. For example, they say a systematic study of the mineral content of plants is needed. Also, they plan to characterize the quality, microstructure, and purity of the extracted raw material and see whether it differs from raw materials obtained by mining. Regarding melting, is this glass fundamentally different from glass made from mined minerals? What are the mineral separation issues?

In closing, Cornejo says the project has been “a great catalyst for the students in the department. They are intrigued and enthusiastic about this weird, weird thing we are doing.”

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