As demand increases for quality recycled paper, equipment manufacturers and researchers are working to improve the repulping process, where paper is broken down into fibers and ink and other contaminants are detached from the fiber’s surface.
Two contaminants have particularly wreaked havoc for recyclers and processors: ink and tacky substances in pressure-sensitive adhesives and coating binders (stickies).
The de-inking phase is key to making a good recycled product, especially for printing and writing papers. They are being used in more advanced applications, and in short-term applications, requiring both quality and affordable recycled paper, says Richard Venditti, a professor in the Paper Science program at North Carolina State University. Venditti researches the repulpability and recyclability of papers.
“For instance, these papers are used on tickets with bar codes, and if there is residual ink it can cause problems in readers, so whiteness and absence of dark contaminants are important to interface with the computer recognition systems,” Venditti says.
With these and other short-term paper applications, combined with trends such as printing of more images and photos, mitigating stray inks is increasingly important.
Pulpers use a process called fiber-fiber rubbing to extract as much fiber as possible while detaching contaminants including ink from fibers’ surface. New pulpers are designed to improve this process, but it’s a balancing act to be able to detach the ink while ensuring the fibers are not cut.
Following fiber-fiber rubbing the detached ink is removed through either flotation or washing-de-inking.
With flotation, ink attaches to air bubbles that float to the top of a tank; the ink and bubbles are skimmed off, leaving clean fiber. Now there is an industry trend to introduce more and smaller air bubbles for more effective de-inking.
“Technology developers are trying to disperse air in many small bubbles with a lot of surface area so ink can attach to the bubbles, to separate them from fibers. Though even with improvements, currently removal efficiency can vary a lot,” Venditti says.
Washing de-inking is more common than flotation. Through this process ink is dispersed in water; small particles of ink are strained through a wire mesh when the pulp is washed, while the good fiber stays on the mesh.
A problem is that the wire mesh is resistant to de-inking. Fibers can form a thick pulp mat on top of the mesh and trap ink, so the challenge in washing de-inking is removing the water without forming a mat. New design improvements involve cylinders that quickly rotate wire mesh, enabling placement of thin layers and mitigating thick pulp mats that trap ink. The speed is key when going to thinner mats in order to maintain the same production rate.
If there is remaining ink in pulp after this process, that ink is probably still attached to fibers, but there is another recourse that serious paper mills use after flotation and washing to detach residual ink. It’s a process called kneading, which is a mechanical action involving more fiber-fiber rubbing. Kneading is followed by another stage of washing and flotation.
“This technology is powerful and evolving, with equipment manufactures looking at better ways to operate kneaders whether machine design, chemicals to improve kneading, or other approaches,” Venditti says.
In addition to the technical challenges tied to quality, there are environmental considerations when dealing with inks, most of which are petroleum based.
They contain plastic used to bind carbon black, which gives ink its pigment. These microplastics release into wastewater during de-inking, limiting reuse of that water and making it challenging to safely discharge it to the environment after its last use. Processors remove ink from water through a clarification process leveraging chemicals [flocculating agents] that bump efficiency of the process to remove ink and other solids; these agents bridge particles to form large blocks of solids that float or sink easily, explains Venditti.
Buckman, who provides specialty chemicals to the paper industry, focuses on de-inking through a process that involves ink collection and flotation, but an even bigger focus for Buckman is stickies in pressure-sensitive adhesives and coating binders.
“Stickies are the number one problem with boxboard, and there’s more boxboard recycled than mixed office waste with ink on it,” says Daniel Glover, Innovation director Pulp & Paper, Buckman.
“Stickies are a big part of why people do not use more recycled fiber; they limit applications,” he says.
Especially challenging for processors is that they stick to the paper machine wires, felts, and dryer cans used during the paper making process and they can cause holes and breaks in paper.
“When paper breaks on the machine you have to stop, so it costs downtime. Depending on how much paper is being run through, it can cost processors $5,000 to $100,000 per hour,” Glover says.
The glue-like chemicals are ester-based which are the problematic materials. Buckman developed enzyme technology to break ester bonds, reducing the materials’ stickiness.
“Recyclers and processors depend on the price of recycled fiber because paper mills will use recycled if the price is right but go with virgin otherwise.
So, recyclers tend to buy what’s dirty and cheap, but then they have to figure out how to get the quality up,” Glover says.
“If you can de-ink and remove stickies and other contaminants in fiber you can use more and reduce cost, increasing profits.”
Key is a good combination of well-maintained, modern cleaning equipment and the best available chemical technology.
“The ROI is the ability to use cheaper, dirtier recycled paper. You spend on the chemicals [and machine improvements], make higher quality products, and can make more of it because you have maximized production efficiency and improved off-quality material. You have a cleaner, marketable product,” Glover says.