These light-based sensors make glass recycling possible

If you could hitch a ride on a glass bottle or jar that’s sitting in a disposal bin, you’d be in for quite a journey.

Where you and the glass bottle end up depends a lot on where you started. That’s especially true if you live in the US. Some US states recycle over 70% of their glass containers. Other states recycle less than 10%. If you start in one of those low-recycling states, you and your bottle will probably get dumped in a landfill. And that can happen even if you and your glass travel companion originally became acquainted inside a recycling bin.

But if you started in a high-recycling state, like Oregon or Maine, you would likely see your bottle quickly collected, cleaned, sorted, and sent off as crushed glass to a glassmaker to manufacture new bottles and jars. How quickly? A glass container can go from a recycling bin to a store shelf in as little as 30 days, according to the Glass Packaging Institute (GPI), a trade association representing the North American glass container industry.

One of the most important steps in that 30-day trip happens at massive sorting centers. There, you and your bottle might ride along an intricate system of conveyor belts, where automated sorting equipment driven by sophisticated sensors quickly separates your glass—according to color, composition, and other properties—from other types of glass and from everything else zipping along the belt with it.

These high-tech sorters are key parts of the engine that drives glass recycling. Using visible light, X-rays, and other portions of the electromagnetic spectrum, the sensors instantly scan incoming material as it flies by, enabling the sorting machines to process multi-metric-ton quantities of glass per hour. Manufacturers of this gear and the facilities that use it are looking for ways to improve and expand glass-sorting capabilities and reduce the cost. These companies aim to increase the volume of clean, sorted glass that manufacturers can use for new bottles, jars, and other glass products. But new sorting technology alone won’t boost recycling rates. Glass recycling is a multifaceted process driven by economics, local policies, and logistics. All of those factors must favor recycling for rates to increase.

Glass and cullet

Three of the main ingredients in glassmaking have changed very little since artisans began making the material thousands of years ago. Silica sand—mainly SiO₂—accounts for roughly 70% of the weight of today’s common glass formulations. The other components include sodium carbonate (Na₂CO₃), which is known as soda ash, and limestone (CaCO₃). Both of these are common minerals. Glassmakers combine these materials and feed them to a furnace that heats the mixture above 1,500 °C, forming a molten material that cools to form a type of glass known as soda-lime glass. That’s the most common and least expensive type, accounting for roughly 90% of all manufactured glass.

“A fourth ingredient—recycled glass—nearly always goes into the mix,” GPI president Scott DeFife says. Manufacturers of bottles, jars, and other glass products blend clean, crushed glass, or cullet, typically supplied by the recycling industry, with limestone, sand, and other ingredients they deliver to the furnace. The practice benefits glass manufacturers and consumers. It’s also good for the environment, DeFife says.

First, adding cullet means reducing the required amount of raw materials, and it’s better than a one-for-one swap. The high-temperature reaction between silica and the two carbonates, soda ash and limestone, drives carbon dioxide from the minerals as it produces molten glass. So recycled glass weighs less than the minerals added to the furnace to make it. As a result, manufacturers use 1 kg of cullet to replace 1.2 kg of raw minerals, according to Robert B. Hippert, sustainability strategy leader for manufacturing at O-I Glass, a major international manufacturer of glass bottles and containers.

In addition to reducing the consumption of natural resources, using cullet cuts greenhouse gas emissions. With cullet in the mix, manufacturers rely less on the CO₂-emitting reaction between silica and carbonates to make soda-lime glass. Blending the mineral mixture with 10% cullet reduces CO₂ emissions by roughly 5%, Hippert notes. For every 6 metric tons (t) of cullet used in manufacturing, CO₂ emissions fall by 1 t. Adding cullet to the mix also lowers furnace emissions of nitrogen oxides, sulfur oxides, and particulate matter.

Including cullet also benefits manufacturers’ bottom lines. As Hippert points out, for every 10% of cullet blended into the glassmaking feed mixture, the energy needed to run the furnace hot enough to generate molten glass falls by nearly 3%. Operating furnaces at lower temperatures extends their service lives, which reduces operating costs and the price of the glass products.

Because cullet has so many benefits, getting high-quality glass for recycling is important. But not every recycling route yields the same amount of usable cullet.

Most recycling in the US is managed locally and is based on single-stream, curbside collection. In these programs, residents of a given municipality deposit all their recyclable material in a single bin. They commingle glass bottles and jars with paper, cardboard, plastic, aluminum, and other metals. Recycling contractors empty single-stream bins into compacting collection trucks and deliver the loads to materials recovery facilities. Operators at these locations—there are about 400 of them in the US—sort the materials using a multistep process to isolate glass to be sent to cullet processors.

Far less common in the US is multistream recycling. In these programs, consumers deposit glass in glass-only collection bins, separating bottles and jars from other recyclables. This type of collection is more expensive than single-stream collection and requires a high level of consumer education. But it leads to a much cleaner glass stream that requires less processing and generally bypasses materials recovery facilities, instead going directly to cullet processors and then on to glass manufacturers. The same is true of glass recycled in areas where consumers transport it themselves to redemption centers or to drop-off bins.

Despite the cleanup efforts during single-stream recycling, glass exiting that stream tends to be contaminated with other materials at a higher rate than glass from multistream systems. The large difference in quality between glass collected in single-stream and multistream programs leads to a large difference in the rate at which recycled glass is made into new products, DeFife says. About 90% of glass from multistream sources is recycled into new products. For glass from single-stream sources, the number falls to 40%.

Sensors for sorting cullet

Glass manufacturers, some of which receive rail shipments of cullet at the 100 t level, set standards for purity and will not manufacture new glass from material that falls outside those limits. And the types of products that manufacturers make dictate the contaminant limits and color purity of cullet they order.

The companies that produce cullet to meet these needs—the cullet processors—are referred to in the industry as beneficiators. These companies’ facilities rely on a suite of sensor-driven sorters and other automated equipment to purify cullet as tons of crushed glass chunks, often just several millimeters in length, zip by every hour on a series of conveyor belts. Sorting equipment varies from one cullet supplier to the next depending on the intended application—for example, making bottles and jars, which requires color sorting, or fiberglass, which generally does not.

Beneficiators typically start the sorting and cleanup process by removing paper residue—often from container labels—with a device known as a star screen. And they strip bottle caps and other bits of metal from the glass feed using magnets to remove ferrous metals and instruments known as eddy current separators to remove aluminum, copper, and other nonferrous metals.

If too much paper makes it into a glassmaking furnace, Hippert says, it can alter the redox properties and color of new glass products, leaving them off specification. He adds that flecks of metal that slip into cullet will melt, separate from the molten glass, and pool at the bottom of the furnace, corroding the expensive equipment.

After residual metal and paper are removed, optical sorters with intense light-emitting diode light sources and color-sensitive cameras typically take over the sorting process. As the tiny pieces of glass ride on the conveyor, the cameras scan them to assess their color and transparency and instantly trigger valves that fire narrow jets of air to eject unwanted opaque materials, which are often bits of porcelain and ceramics or pebbles.

If a tiny piece of ceramic coffee mug gets mixed into the cullet, it won’t melt in the furnace, says Hugh Summerville, technical manager at KRS Recycling Systems. KRS is a US-based subsidiary of Sesotec, a Germany-based manufacturer of cullet-sorting equipment. He explains that the unmelted ceramic shard may form an inclusion, or defect, in a bottle, leaving it prone to breaking or possibly exploding if it’s pressurized with a carbonated beverage.

The same technology separates cullet by color, producing color-pure streams of clear, brown, and green glass. Manufacturers require color-sorted cullet to produce aesthetically pleasing, uniform containers. “People toss bottles of every color in the bin and it comes back to us sorted by color. It’s like unscrambling an egg,” Hippert says.

Older instruments often mistook dark brown and dark green glass for opaque porcelain and ejected those colors. That error wasted valuable cullet needed for olive oil bottles that block ultraviolet light, says Patrick Potzinger, head of sales for Austria-based Redwave, a major manufacturer of cullet-sorting equipment. He says that newer optical sorters have higher-resolution cameras and improvements in light sources and imaging software that allow the machines to accurately distinguish a wider range of colors and transparencies.

Some cullet processors also use sensors operating with near-infrared light to recognize and remove small bits of plastic, such as pieces of bottle caps. X-ray fluorescence units identify chunks of glass containing lead and other heavy metals as well as heat-resistant borosilicate glass, which is used for cookware. All these materials wreak havoc in glass furnaces used for making bottles and jars, for example, because the impurities’ melting points and densities adversely affect the viscosity and other properties of the molten glass, says Richard Wielgus, general manager at KRS. Some equipment makers have recently phased out X-ray-based systems because of their high cost, replacing them with ultraviolet fluorescence scanners to detect leaded glass and heat-resistant glass.

Other types of sorting methods could further advance glass recycling by better spotting unwanted materials and increasing cullet purity. For example, methods based on Raman and terahertz spectroscopy could better detect hard-to-spot bits of heat-resistant glass and dark plastics, respectively. Meanwhile, laser-induced breakdown spectroscopy, which is an atomic emission method, would be well suited to detecting metal particles. Some manufacturers of glass-sorting equipment say that they are evaluating those technologies but that they are currently too expensive.

Recycling dreams

Even if those types of sensors quickly came to market, US glass recycling rates would not climb overnight.

On average, Americans recycle only about one-third of the 10 million t of glass they dispose of each year, according to GPI. The rest of it ends up buried in landfills. In contrast, Switzerland, Sweden, Finland, and other European countries recycle more than 90% of their waste glass, according to FEVE, the European container glass federation.

The vast differences in recycling rates are not due to some intrinsic materials or chemical property that distinguishes glass containers made in the US from those made in Europe.

Why such big differences? Europe’s progressive legislation and recycling policies, compared with those in the US, play a big role, says Laura Hennemann, vice president for marketing and communications at Strategic Materials, the largest cullet supplier in the US. Laws specifying a minimum recycled content in new products as well as programs involving deposits and refunds on containers “give people the incentive to do the right thing. And that leads to higher glass recovery rates,” she says. Some parts of the US have implemented such policies and seen glass recycling increase. Of the top 10 recycling states by weight of glass recycled, 8 have deposit programs, also known as bottle bills.

Another factor, according to Hennemann, is the large size of the US relative to many European countries. The long distances inside the US negatively affects the economics of glass recycling. Glass is heavy, and collecting it from numerous rural communities and hauling it to a sorting center and then shipping the resulting cullet to a manufacturer are costly. For that reason, in the US, some recycling centers do not recover any glass from the materials they sort because there are no nearby buyers to make it worthwhile, Hennemann explains. In addition, vast open spaces in the US translate to cost-effective landfill rates.

GPI’s DeFife says his organization is working with industry, municipalities, and policy makers to raise the national recycling rate to 50% by 2030. “It’s a stretch goal,” he admits, “but it’s achievable.” Getting there will likely require many changes in today’s recycling practices, such as providing cleaner collection streams and increasing the processing capacity of materials recovery facilities.

Hippert says that if he were granted a wish that would help boost glass recycling, it would be to institute bottle deposit programs in many more locations across the US. “If an empty bottle has cash value, people’s behavior will change,” he says.

Hennemann is optimistic. US consumers’ attitudes about the sustainability of products they buy are changing, she says. “People want to feel good about the package they’re taking off store shelves. They want to know they are doing the right thing.”