Recovered Fiber and Printability (Finale)

I. Paper's "Second Life": Recovery as the Paper Industry's Second Raw-Material Base

Modern papermaking is long past the simple model of "cutting trees to make pulp." In Europe, the recovered-paper recycling rate has reached about 70%; for containerboard and corrugated grades it is even higher. Recovered paper is classified by end use into several main categories: OCC (old corrugated containers), ONP/OMG (old newspapers/old magazines), MOW (mixed office waste, with deinking value), plus clean streams such as mill broke and trimmings. To a paper mill, recovered paper is "semi-finished fibre" that has already had its lignin removed and needs no cooking — used well, it saves both wood and energy.

But recovered paper is no free lunch. It carries printing inks, adhesives, plastics, fillers and the "fatigue" left by repeated cycling. So the core task of the recovery line is to separate out these contaminants, "rescue" the fibre, and at the same time delay as much as possible the strength loss caused by fibre ageing. This episode first covers the two main routes — OCC pulping and deinking — then the most stubborn problems, stickies and fibre ageing, and finally printability and the future.

It must be stressed that the recycling rate has an upper limit. With each cycle the fibre shortens a little and stiffens a little; after 5–7 cycles its strength contribution is essentially exhausted and virgin pulp must be added. This is also why "100% recycled" is physically impossible — the system needs the "blood" of fresh fibre.

II. The OCC Pulping Line: Breaking Cartons Back into Fibre

OCC (old corrugated containers) is the largest-volume recovered grade, used mainly for linerboard, fluting medium and board. Its recovery line starts with the hydrapulper: at 10%–15% consistency and 45–60 °C warm water, rotor shear defibers the cartons into a fibre suspension. High-consistency pulping reduces fibre cutting and preserves strength, while warm water softens adhesives for easier subsequent dispersion.

The defibered stock passes through coarse screening (hole screen) to remove large contaminants such as plastic flakes and string, then enters fine screening (slot screen, slot width 0.15–0.25 mm) to reject fine plastics and unslushed "screen rejects." Next come washing/thickening to remove sand and filler fines, then thermal dispersing (90–130 °C) — the key step of the OCC line: at high temperature, tacky contaminants such as hot-melt adhesives are softened and broken into sub-visible micro-particles, losing their agglomeration ability and dispersing evenly so they no longer do harm.

The OCC line normally does not deink (cartons carry little ink to begin with); the focus is contaminant removal and strength preservation. For grades requiring brightness (such as white-top linerboard), flotation or hydrogen peroxide bleaching is added downstream. The line yield, affected by contaminant content, typically falls in the 80%–92% range.

III. Deinking (DIP): Bringing Office Waste Paper Back to "White"

Waste papers containing printing ink, such as ONP/OMG and MOW, follow the deinking (DIP) route, whose goal is to detach ink particles from the fibre and carry them away with froth/wastewater, restoring brightness and cleanliness. The chemistry of deinking centres on an alkaline-oxidative system: NaOH (1%–2%) saponifies the ink binder, H₂O₂ (1%–2%) bleaches and protects the cellulose, Na₂SiO₃ acts as a buffer and silicate-scale inhibitor, and a saponifier/collector is added to stabilize the detached ink into floatable particles.

Physically, deinking is flotation: micro-bubbles are introduced into the stock, ink particles hydrophobically attach to the bubbles and rise to form a froth layer that is scraped off. Cascaded multi-stage flotation cells can raise brightness from the dull yellow of recovered stock all the way to 60%–70% ISO (MOW, with less ink and higher brightness, can go higher). Washing, thickening and sometimes a peroxide reinforcement bleach (P stage) follow to top up brightness.

Deinking effluent, containing filler, fine fibre and ink, is the main source of wastewater in the recovery line; it is typically clarified by dissolved-air flotation (DAF) and then recirculated or treated for discharge. Here lies a circulation paradox: the tighter the water closure, the more dissolved/colloidal substances (DCS) accumulate, which in turn hurts deinking efficiency and paper-machine runnability, so moderate discharge and treatment are required.

IV. Stickies and Fibre Ageing: Two Chronic Wounds of the Recovery Line

Stickies are the most troublesome contaminant for recyclers — pressure-sensitive tacky particles from tape, self-adhesive labels, hot-melt adhesives (PSA) and binding glue. They are soft, sticky, can foul equipment and form "oil spots" on the sheet that cause ink rejection in printing. The control approach is a "four-pronged axe": screening to remove large pieces, washing to remove fine ones, dispersion (thermal dispersing) to fragment them, and fixation (adding talc/bentonite/fixatives to embed and deactivate the tackiness). Testing by TAPPI extraction or image analysis grades them as macro (>100 μm) and micro (<100 μm); the latter is more hidden yet more lethal.

The other chronic wound is fibre ageing. With each cycle, fibres shorten, stiffen and embrittle because of corner wear and exhausted fibrillation, so they no longer readily fibrillate during refining and the paper strength declines cycle by cycle. Empirically, after 5–7 cycles the fibre's strength contribution tends to exhaustion, and virgin pulp must then be added. This is also the core constraint of recycled-paper furnishing: while cutting cost with OCC/DIP pulp, one must hold the strength floor (e.g., ring crush for board, tensile for graphic paper).

V. Printability: Paper Must "Serve" Three Kinds of Printing Presses

Paper is made for the printing press, yet different printing methods make very different demands on it. Offset transfers by water-ink repulsion and is most demanding on surface strength: if the sheet's fibre/filler bond is weak, the high-speed blanket will "pick" it (measured as the picking speed on an IGT tester); it also requires balanced ink acceptance and absorption, a neutral sheet pH (acidic paper tends to cause linting and plate blinding) and moisture balance (sheet dimensional change from humidity causes misregister).

Gravure uses engraved cells that pick up ink directly and demands an extremely smooth surface (otherwise uneven cell inking produces "white spots"), and is sensitive to static/conductivity (static causes misting and mottling). Digital printing (e.g., electrophotographic/inkjet) cares most about relative humidity and surface energy: inkjet requires the surface to wet and spread yet absorb quickly (pigment inks often need a precoat or pore structure), while electrophotography requires the surface resistance in a suitable window to prevent backgrounding or transfer failure.

In one phrase: printability is not a single metric but a combination punch of "surface strength × smoothness × absorption × dimensional stability × charge." The sizing, coating and calendering done by the mill in EP09 are essentially laying the foundation for this combination punch.

VI. The Future of Paper: Biorefinery, Low-Kappa and Plastic-Free Coatings

As we approach the finale, let us look at industry directions. Biorefinery upgrades the pulp mill from "paper producer" to "chemicals + materials producer": lignin can be converted into carbon materials and phenolic resins; hemicellulose can be fermented into furfural and xylitol; extractives become biomass energy. This tightens the mill's carbon and chemical loops.

On the pulping side, low-kappa high-brightness cooking is being pushed (less chlorine, less bleach chemical, lower AOX); on the finishing side, plastic-free barrier coatings — replacing PE/PP extrusion coatings with all-cellulose or bio-based coatings to make recyclable liquid-packaging board, answering the anti-plastics wave. Dry forming explores a forming path that eliminates massive dewatering, hitting head-on the pain point from EP07/EP08: "95% of water leaves at the wire, drying takes 60%–70% of energy."

The carbon footprint is an unavoidable ledger: fossil-based paper products have a carbon intensity of about 0.6–1.2 tCO₂e/t paper, where the energy mix (whether biomass fuel is used, and how clean the purchased electricity is) matters most. Raising the recycled-fibre ratio, using biomass energy and lowering unit energy consumption are the three main axes of mill decarbonization. Paper is moving from "use and discard" toward a material of "circularity + low carbon + functionalization."

Thus the ten lectures are complete: from the hydrogen-bond essence of the fibre network in EP01 to today's recovered-fibre loop and printability, we have seen the full life of "a sheet of paper" from forest to printed product and back to fibre. Next comes a one-picture overview of the whole series.

VIII. Quality Metrics and Furnishing Principles of Recycled Pulp

Recycled pulp is not "the purer the better" but "good enough" — furnished by grade. The core metrics of OCC pulp are freeness (CSF, reflecting subsequent refining potential), ring crush strength (the key strength of board) and contaminant content (especially macro/micro grading of stickies); DIP pulp is judged by brightness, dirt count (points/m²), residual ink concentration and strength retention. Furnishing follows an iron rule — "make up the strength shortfall with virgin": the top layer of linerboard can use high-brightness DIP for appearance while the core uses OCC for cost; graphic paper replaces part of the virgin hardwood with DIP, but must hold the floor on tensile and opacity, otherwise printing will show through.

The daily work of the furnishing engineer is to find a feasible solution among "recycled-fibre ratio, cost and strength." The recovered-paper market (imported OCC price, maturity of the domestic recovery system, exchange rate) directly determines the mill's profit elasticity — which is why large mills prefer to build their own recovery networks to lock in recovered-fibre sources. For the system, the higher the recycled-pulp share, the tighter the white-water closure and the heavier the DCS accumulation, which in turn raises purification cost — the furnishing ratio is never an isolated number but the balance point of the entire recovery line.

Also watch the ash cycle: fillers in recovered paper (calcium carbonate, kaolin) repeatedly experience acid-base environments during recovery, partially dissolve and reprecipitate as calcium carbonate and the like, affecting both white-water hardness and sizing. Ash management of recycled pulp is another hidden constraint in furnishing — many cases of "poor sizing" should actually be blamed on ash fluctuation rather than the starch itself.

IX. Detection and "Fixation" Technology of Stickies

Stickies are graded by particle size into macro (>100 μm, visible to the eye or by image analysis, prone to oil spots on the sheet that cause ink rejection) and micro (<100 μm, hidden yet aggregating to do harm on the machine or in printing). The TAPPI extraction method (e.g., UM-267 solvent extraction and weighing) gives the total amount, while image analysis gives the size distribution and morphology; only together do they tell both "how much" and "what it looks like." Beyond the screening, washing and dispersion mentioned above, the key on-site control is the "fixative": talc, bentonite and cationic polymers adsorb or embed the tacky particles, depriving them of adhesion and letting them exit with white water or disperse harmlessly.

The control target is to push macro stickies down to an acceptable floor and prevent their cyclic enrichment in the system — stickies most fear "accumulating more and more," so the white-water closure level must be dynamically adjusted alongside fixative dosage. A common pitfall is to focus only on the screen-removal rate while forgetting that dispersed micro-particles can re-agglomerate; therefore the thermal-dispersing temperature, fixative addition point and dose, and white-water exchange rate must be tuned as a coupled system rather than by single-point optimization.

X. Biorefinery and Carbon Footprint: The Mill's Future Ledger

Biorefinery upgrades the pulp mill from single-product papermaking to a "materials + energy" platform: lignin via pyrolysis or catalysis can make carbon-fibre precursors and phenolic-resin substitutes; hemicellulose hydrolysis and fermentation make high-value chemicals such as furfural and xylitol; extractives serve as biomass boiler fuel. This raises both the value per tonne of pulp and its carbon-sequestration capacity, turning by-products once "burned or landfilled" into revenue items. On carbon footprint, fossil-based paper products emit about 0.6–1.2 tCO₂e/t, where the energy mix of purchased power and steam matters most — replacing fossil fuels with biomass (black liquor, bark, waste wood) cuts a large chunk of emissions.

Plastic-free barrier coatings (all-cellulose nanofibrils or PLA-type bio-based) make liquid-packaging board recyclable and compostable, directly meeting global anti-plastics policies; dry forming tries to bypass the structural energy consumption of EP07/EP08 — "95% of water removed at the wire, drying takes 60%–70% of total machine energy" — using air-laid or dry forming to avoid massive dewatering. The future of paper is a trinity of "circularity + low carbon + functionalization" materials science, no longer just the traditional manufacturing of "turning wood into a writing surface."

Worth adding: these transformations are not distant visions — several mills in the Nordics and North America already use lignin by-products for district heating and asphalt modification, and demonstration lines for hemicellulose-to-furfural have run for years. For domestic mills, the entry point of biorefinery often starts from the mature closed loop of "black-liquor multi-effect evaporation + recovery boiler," first capturing energy self-sufficiency, then gradually extending to high-value chemicals. The path is clear; only the pace differs. This also means future mill process engineers must understand both pulping/papermaking and urban logistics and water treatment — recovered-fibre recovery twists chemicals, environment and municipal affairs into one line.

XI. Water Circulation and Wastewater Treatment of the Recovery Line

The recovery line is where high water consumption and high pollution load concentrate, especially DIP deinking tailwater containing ink, filler, DCS and residual chemicals. The mainstream treatment is dissolved-air flotation (DAF) clarification: a flocculant is added so suspended solids and ink flocs rise and are scraped off; the supernatant is recirculated to pulping and washing, and the sludge is dewatered and sent to energy recovery or landfill. The tighter the closure, the more water saved, but dissolved/colloidal accumulation worsens deinking and papermaking, so a moderate "discharge — make-up" must be kept to maintain system balance. In recent years "near-zero discharge" concentrates and splits salts from circulating water via multi-effect evaporation, further lowering intake — at the cost of evaporation energy, back to the old saying from EP08: the bottleneck of papermaking is, in the end, water and energy.

From a broader perspective, recovered-fibre recovery binds "papermaking" and "urban minerals" together: the daily recovered-paper volume of a city of one million is enough to support a medium board line, and recovery-system efficiency directly determines a regional mill's raw-material cost and carbon intensity. This is also why the industry treats the recycling rate as a more critical strategic metric than single-machine capacity — it is both a cost moat and a subtraction item on the carbon ledger. For technicians, the hardest part of the recovery line is that "the raw material changes daily": today's incoming OCC is glue-heavy, tomorrow's MOW is ink-heavy, so recipe and operation must adjust in real time with the waste-paper mix. Building a correlation model of "waste-paper composition — chemicals — quality" is the core capability for recovery-line stability and the yardstick that separates an ordinary operator from a senior process engineer.