Sharp focus on wood sorting

Photos courtesy of Tomra Group

Tomra Recycling says it has taken significant steps to strengthen its position in the wood recycling segment by building a dedicated team to offer sensor-based sorting solutions “to drive the development of waste wood recycling through technology and process improvement consulting.”

In an interview with Construction & Demolition Recycling (C&DR), Fabrizio Radice, Tomra Recycling’s head of global sales and marketing, was among three Tomra executives helping to describe the company’s progress toward its goal to offer automated scrap wood sorting systems.

Joining Radice for the early 2022 interview were Jose Matas, Tomra’s segment manager for the wood market, and Ty Rhoad, Tomra’s regional sales director for the Americas.

(C&DR): What market or government circumstances are driving demand for this advanced wood sorting technology?

Jose Matas (JM): Over the previous years, we have been observing an increasing demand for recycled wood that is driven by both the panel as well as the building and construction industries, both of which have recognized the enormous potential of recycled wood as a material. For instance, using recycled wood in the production process of particleboard (as one of the main application areas of recycled wood) comes with numerous benefits for the manufacturers.

First, they no longer rely on the availability of fresh wood, which is increasingly hard to get and, in addition, much more expensive than recycled wood. Second, fresh wood is generally more humid and requires more energy in the drying stage of the production process. Using scrap wood, thus, leads to greater energy savings and higher profitability for the particleboard manufacturer.

Furthermore, particleboard manufacturers are looking to manufacture particleboard of superior quality. But to produce high-quality particleboard, the collected waste wood must be freed from contaminants, properly sorted and classified by wood type. Our sensor-based sorting units for wood applications offer a solution to this. Equipped with the latest technologies, they remove inert materials and metals in a first sorting step and further purify the presorted materials (separating wood by type) in a second step. Particleboard manufacturers usually target a clean wood A fraction, since this is nonengineered and provides the necessary qualities and features to produce superior quality particleboard with a high level of salability.

Another driver of the increased demand for recycled wood comes from energy producers. They are using recycled wood that does not fulfill the quality requirements as woody biomass in energy production. Biomass proves to be a sustainable and economically viable alternative to energy production with finite fossil fuels. This development is increasingly supported by policies and subsidies granted by some governments, thus additionally driving the demand for recycled wood.

Ty Rhoad (TR): From a North American perspective, we see that the costs for lumber as a resource are at a historical high and that wood is still the No. 1 material used in the construction industry. But whereas demand has not waivered, supply has. To date, most wood waste is burned and used for energy production. While this approach diverts the materials from landfills, a lot of valuable material we could recycle and use to produce new materials is lost.

C&DR: What markets are viable for the “clean” wood that is separated from the coated or “dirty” wood? What is the fate of the “B” grade product?

JM: Wood can be classified into four categories that are characterized by different purity levels. Their respective purity is the decisive factor that determines for which application and markets the materials can be used. In Europe, wood is classified into classes A, B, C and D.

Class A is nonengineered waste wood, thus the cleanest material class. Its origin ranges from residues of sawmills, leftovers from forest cleaning to wood manufacturing and the packaging industry. Being the cleanest of all wood grades, it is perfectly suitable to produce high-quality wood-based panels, as well as animal bedding. Unlike other wood grades, wood A can be used in all applications.

Wood in the B class, on the other hand, is engineered and considered a “mixed grade,” although not featuring halogenated coatings. It generally comes from business waste as well as from construction and demolition operations. It can be used also in the production of some types of wood-based panels, animal bedding and landscaping.

The lower grades, C and D, are not suitable for manufacturing professional or consumer products. C is highly contaminated and originates from municipal collections or transfer stations. Its main application is energy production in biomass plants.

Wood D, on the other hand, is hazardous waste coming from agricultural fencing, telegraph poles and railway sleepers. Due to its hazardous nature, it must be disposed of at special facilities with a particular license to accept and burn it.

TR: In North America, the clean wood fraction may also be used for particleboard manufacturing and to make synthetic silk (textiles).

Wood B, on the other hand, is able to go into plasticized wood products. It cannot be burned in many states due to emissions from the glues.

The challenge for A wood beyond use as fuel is a matter of creating a more reliable and profitable off-take market for it—and the opportunities for both recyclers and particleboard manufacturers are there.

C&DR: What types of optical technology are being deployed to make the separation?

Fabrizio Radice (FR): The separation of wood by material types (Wood A versus Wood B, or nonprocessed wood versus processed wood) and the recovery of medium-density fiberboard (MDF) from waste wood require advanced technologies. We recommend deploying a combination of X-ray technology for a first sorting step followed by near infrared (NIR) and deep-learning-based sorting systems for further separation into wood classes. Integrating a combination of both systems allows for the recovery of single wood fractions and has proven to generate the highest quality results, enabling the materials to be further processed and recycled.

At Tomra, we offer both solutions. Our customers have been using our high throughput sorting solution X-Tract, which harnesses the power of dual-energy X-ray transmission (XRT) with high-resolution sensors, for ages. After the waste wood has been shredded, chipped and screened, X-Tract removes the inert material (glass, stones, ceramics, etc.) and metals from the waste wood stream based on the material’s atomic density and regardless of the material’s thickness.

Over the past few years, we have observed an increase in quality requirements and been approached by many customers who aim to use recycled wood of a much higher purity level in their production processes and for the manufacturing of high-quality chipboard. Since the increase in quality requirements entails the need to remove further materials, such as engineered wood composites as well as polymers, we had to find a new solution to serve the current as well as the future needs of the market and our customers.

Our in-house team of industry experts, application engineers and software developers collaborated to develop a new deep-learning-based application to sort wood chips by type. Gain is an add-on to our AutoSort units and enables the separation of diverse types of wood-based materials at higher purity levels, which is not possible using conventional NIR or X-ray technology only.

C&DR: What type of technology is being used to conduct the sorting process?

FR: The separation/ejection of the materials in both sorting steps (with X-Tract and the AutoSort used in combination with Gain) is carried out by high power valve blocks. Based on the sorting decision taken by the software, the valves generate a powerful and fast air blast to eject the material.

The ejection precision—and consequently the sorting quality—depends heavily on the correct choice of the valve block.

We offer numerous sensor configurations to best meet our customers’ sorting requirements.

C&DR: To what extent is uniform piece/object size important, and how is this size uniformity best achieved?

FR: The size of the material is an important factor contributing to sorting precision and recovery rates. The most common material size ranges between 15 to 60 millimeters (mm), or one-half inch to 2.5 inches, for main applications. Fines sorting can be done between 5 to 15 mm (one-quarter to one-half inch).

In order to process the ideal sizes, raw waste wood must be shredded into sizes up to 100 mm (3.9 inches) in the first step, and big impurities (metals, stones) can be removed. In a second step, the material undergoes an additional shredding process and is reduced to grain sizes between 5 to 60 mm (one-quarter inch to 2.5 inches)—a size that is used to produce particleboard. Finally, disc screeners separate the materials into different target sizes depending on the application. Sensor-based sorting solutions conduct further purification.

More information on Tomra Recycling is available at www.tomra.com/en/sorting/recycling.