How are plastic, metal and glass recycled and reused and how efficient is the process?
Asked by Shilpa Sarkar, ’11 Houston, Texas
Glass: According to the EPA roughly a quarter of the glass discarded in the United States is recycled. What does find its way to the recycling plant is washed, sorted by color and ground into a substance called cullet. About 90 percent of this cullet is sold to glass manufacturers who melt it at 2,600 to 2,800 degrees Fahrenheit and form it into new products (mainly containers for food and beverages). The other 10 percent ends up in anything from industrial abrasives and construction materials to works of art.
As discussed in the Essential Answer, making products from recycled glass is more energy efficient than starting from raw materials. Because cullet melts at a lower temperature than the components in glass—sand, soda ash and limestone—using one part cullet to nine parts raw materials increases energy efficiency by about 2.5 percent. This means that a product made entirely of recycled glass consumes a quarter less energy than one made entirely of virgin materials.
Metal: About 83 percent of steel in the United States is recycled. For cans specifically, the rate is around 65 percent. More good news: Steel scrap is used to supplement raw ingredients in all new steel made in the U.S. (Worldwide, about 35 percent of steel is made from scrap.) Not only does this save nearly three quarters of the energy it would require to make steel fresh from iron ore, coal and limestone, it also conserves the materials themselves. Each ton of steel recycled saves 2,500 pounds of ore, 1,400 pounds of coal, and 120 pounds of limestone, and preserves habitats from destruction.
Aluminum is the other routinely recycled metal. More than half the aluminum consumed in the United States takes the form of containers and packaging, and about half of these make it into the recycling bin. Virtually all recycled aluminum containers are melted down, formed into blocks called ingots and then rolled thin to make new containers. Because this process renders mining, refining and smelting unnecessary, it takes 95 percent less energy to make an aluminum can from recycled metal than from bauxite ore.
Plastic: Different grades of plastic come stamped with a numerical code that indicates the type of resin—e.g. PET (#1), polystyrene (#6) or ABS (#9). The overall recycling rate for plastics is abysmally low, just 8 percent, but some types are recycled more frequently than others; just less than a third of all high-density polyethylene (HDPE) and PET bottles and jars were recycled in 2011.
After collection, plastic is shredded, cleaned and separated by type. This can usually be accomplished by dumping pieces in liquid and allowing the various grades to segregate themselves by density. ABS plastic, which is used in durable items such as telephones, musical instruments and electronics casings, is tricky because it has the same density as another type of plastic called high-impact polystyrene (HIPS). Fortunately, ABS and HIPS differ in their ability to stick to air bubbles suspended in liquid. When air is whipped into the mixture, HIPS floats to the surface stuck to bubbles, while ABS sinks. This process, called froth flotation, can separate out ABS plastic that is more than 99 percent pure. The plastic is then melted down into pellets called nurdles and sold to manufacturers.
Plastics are often recycled into products that don’t look much like the items they started as. A plastic bottle may be melted down and spun into polyester fiber to make a fleece jacket, or it could end up as a trash bag or a clamshell container for strawberries. Using recycled plastics to make these products translates to a 90 percent energy savings compared to making them from virgin polymers.
However, products made from recycled plastic are often not recyclable. Even with advanced techniques, it is nearly impossible to completely separate the different grades during the recycling process. And even slight contamination by other polymers means that a product can’t endure the process again without losing strength, flexibility and durability—the very traits that make plastic so useful. Only the easiest-to-separate types of plastic such as the clear PET plastic used to make water bottles can be recycled indefinitely. Everything else eventually ends up in landfills—if we’re lucky—or contaminating the oceans.
The takeaway message? By all means keep recycling your plastic, metal and glass and buying products made from recycled materials. Recycling is a huge contributor to resource and energy conservation all around the world. But to enhance your sustainability practices even more, reduce, reuse and recycle wisely.
Anna Hallingstad, '12, works at the National Oceanic & Atmospheric Administration.
