1. Why wash first, screen next, bleach last
The crude pulp from cooking and blowing is by no means a clean fiber suspension. It exists carrying several tonnes of black liquor per tonne of air-dry pulp; the black liquor holds dissolved large amounts of lignin fragments, organic acids, sugars and cooking chemicals. If the black liquor is not removed first, the subsequent oxygen delignification and bleaching proceed against a background of "high-concentration pollutants," wasting expensive chlorine dioxide and driving the wastewater load straight up. In other words, washing decides recovery economy, screening decides sheet cleanliness, and bleaching decides final brightness — the three steps are interlocked, and any shortcut in one is repaid with interest downstream.
From the engineering layout, washing usually follows right after cooking and blowing as the first gate of the recovery loop; screening can be placed before washing (hot screening) or after (cold screening), depending on pulp grade and plant design; bleaching is an independent section with the highest demands on water, steam and chemical cleanliness. This episode follows the "wash → screen → bleach" order to make clear the equipment, parameters and criteria of each gate.
A frequently overlooked fact: the "yield loss" and "chemical recovery rate" of washing and screening are written directly into the mill's mass balance. A washer that does not clean well, the black liquor carries away not just pollution but also alkali and heat that could have gone to the recovery boiler; a screen that rejects improperly either throws good fiber into the rejects or lets knot debris leak into the accept. So the on-site tuning of these two steps is never "close enough."
2. Washing: dilute–extract solid–liquid separation
The essence of washing is "dilute–extract": first dilute the pulp to a lower consistency with dilute wash liquor or white water so the black liquor in the fiber network is displaced out, then remove the liquid by vacuum, pressure or gravity, leaving a higher-consistency clean pulp. The core indicator is the dilution factor — the wash-liquor volume added per tonne of air-dry pulp washed, usually controlled at 2–3 m³/t in engineering. Too small a dilution factor and it does not clean; too large dilutes the black liquor and burdens the evaporation section. On-site washing tuning is essentially balancing "black-liquor extraction rate" against "evaporation load."
Displacement washing is mainstream
Modern pulp mills widely adopt displacement washing: multiple washers are strung into a counter-current, the dilute wash liquor from the latter stage supplements the former, and the black-liquor concentration rises stage by stage before going to the evaporation station. Common equipment includes the drum vacuum washer (pulp carried on the drum surface, vacuum drawn from inside) and the press (filter-press) washer; the latter squeezes the pulp "dry" under closed pressure with a wire belt or filter plates, with high extraction rate and good sealing, suited to modern low-emission designs. Either way, the goal is to hand back as much black-liquor solids as possible to the evaporation section so the solids content steadily enters the 65–80% evaporation range.
To judge on-site whether washing is clean, look at three numbers: first, whether the residual alkali of the washed pulp (as Na₂O) has dropped to the target range; second, the dissolved-solids loss carried with the pulp, usually required to be controlled at single-digit kg per tonne of pulp; third, the black-liquor extraction rate — kraft lines generally pursue 96–99% alkali recovery, which is the work of washing behind it. The dilution factor, wash-liquor temperature (hot wash helps lower viscosity), and vacuum/pressure are three adjustable knobs; the usual tuning order is first stabilize vacuum, then adjust dilution, finally move the counter-current ratio.
It is worth mentioning that washing's impact on subsequent bleaching is extremely direct: black liquor remaining in the pulp brings a large amount of organic matter that consumes oxidant, making the bleach-chemical dose falsely high. So "the cleaner the wash, the more the bleach saves" is not a slogan but an experience validated by countless mass balances. Pushing down the washing section's water consumption and alkali loss is often the first lever for cost reduction across the whole pulp line.
3. Screening: slot screens as the gate, knot and grit removal
After cooking the pulp contains knots, uncooked particles, sand, metal debris and coarse fiber bundles; if these impurities are not removed they foul the screen plate, damage felts and form dirt spots and holes in the sheet. The core screening equipment is the pressure screen — relying on a screen basket (slotted or perforated) and a high-frequency pulsing rotor to let the accept pass through the slot and retain the coarse reject. For kraft chemical pulp, the typical slot width is 0.15–0.35 mm; mechanical and chemi-mechanical pulps, to preserve long fibers, often use narrower slots or perforated screens.
Three-tier defense: knot removal, coarse screening, fine screening
A complete screening line is usually three-tier defended: the first step knot removal, using a large-hole basket to reject the bulky uncooked knots and coarse debris first; the second step coarse screening, using a medium slot width to retain fiber bundles; the third step fine screening, using a fine slot (often 0.15–0.25 mm) as the final gate, deciding the sheet dirt count. Grit removal is handed to the cleaner (conical centrifuge), which uses vortex flow to fling dense sand and metal debris to the outer wall and settle them out. The pressure-screen feed consistency is generally 1–3%; too high clogs the slot, too low wastes equipment utilization.
The rejects discharged from screening are not simply thrown away. Coarse rejects contain large amounts of good fiber and usually go to a reject screen for secondary recovery, or are sent back to cooking/re-refining; fine-screen rejects go where the pulp grade dictates. The key on-site contradiction is: the harder you reject, the cleaner the sheet, but the faster the yield drops and the more fiber is lost. Engineering controls this with the dual indicators "reject rate" and "reject fiber content," tuning the reject recirculation ratio to a level that guarantees cleanliness without wasting fiber. Be especially careful with hardwood pulp, whose short fibers make over-screening clearly damaging to sheet strength.
The screening section's energy consumption is not low — the pressure-screen rotor is a power hog, but the electricity bill is worth it compared with the quality it preserves. A rule of thumb: stable pulp consistency, temperature and flow before fine screening keep the basket life and screening efficiency stable; any upstream fluctuation makes the slot "jam" or "leak," so the screening section demands high uniformity of incoming pulp.
4. Oxygen delignification (O stage): the low-cost first delignification step
Before entering the bleach tower, putting in a pass of oxygen delignification has been standard on kraft lines for nearly three decades. Its logic is simple: use cheap oxygen under alkaline conditions to oxidize, fragment and dissolve some residual lignin, lowering the kappa number entering the bleach tower by 35–50% first, so the subsequent expensive chlorine-dioxide dose drops greatly. Typical O-stage conditions: temperature 90–100 °C, oxygen partial pressure 0.2–0.6 MPa, alkali (NaOH) 2–3%, and a small amount of magnesium sulfate (MgSO₄) added to protect cellulose viscosity from excessive degradation.
Oxygen delignification can be single-stage (O) or two-stage (O/O) in series; two-stage can remove a bit more lignin more gently while pressing viscosity loss even lower. To judge whether it does well, look at the ratio "how much kappa dropped vs. how much viscosity was kept" — we want much lignin gone and little cellulose gone. MgSO₄'s role is precisely to suppress the alkaline oxidative degradation of carbohydrates; too little and viscosity collapses, too much may introduce ash, so on-site it is dosed at a few per mille of oven-dry pulp.
The kappa number is the indicator of residual lignin in the pulp (higher value = more lignin); viscosity reflects cellulose chain length, i.e., strength potential. The deeper the oxygen delignification, the lower the kappa and the more the bleach saves, but too deep damages viscosity. Engineering does not pursue the lowest kappa but sets a "bleach-entry kappa target," letting the O stage deliver it right to the door. Hardwood pulp, with low initial kappa (14–20), has a high O-stage removal ratio but small absolute value; softwood pulp (kappa 25–32) benefits most from the O stage.
From an environmental perspective, the O stage is the hero of emission reduction: it cheaply handles with oxygen, ahead of time, a large amount of lignin that would otherwise go into bleaching, consuming chlorinated oxidants and producing toxic by-products. In the era without the O stage, bleaching chemicals and water use were much higher.
5. ECF bleaching: the selective oxidation of chlorine dioxide
ECF (Elemental Chlorine-Free bleaching) is today's absolute mainstream for kraft pulp. Its classic sequence is O-D₀-(EOP)-D₁: after oxygen delignification, first a chlorine-dioxide bleaching stage (D₀), with an oxygen-and-pressure-added alkaline extraction (EOP, Enhanced Oxidative Extraction) inserted in the middle, and finally another chlorine dioxide stage (D₁) to finish. Chlorine dioxide (ClO₂)'s excellence lies in "hitting lignin specifically" — it selectively oxidizes lignin chromophores without much harming cellulose, which is exactly why it replaced chlorine gas.
The division of labor between D₀ and D₁
The D₀ stage is the main delignification stage, using a higher kappa factor (about 0.2, i.e., ClO₂ dose tied to the incoming kappa) to oxidize most of the residual lignin, under acidic conditions: pH 3–4, temperature about 50 °C. The EOP stage, under alkaline, oxygen-added and pressurized conditions, further dissolves the lignin fragments generated in D₀ while protecting downstream brightness. The D₁ stage is the fine bleach, pushing brightness to the target, with slightly higher pH (3.5–4.5) and temperature raised to about 70 °C. Sometimes a (D/C) stage (ClO₂ and chlorine mix) is added up front or ozone is used in the middle, but a "full D" sequence — chlorine dioxide throughout — is already the norm.
Kappa factor = actual ClO₂ dose / incoming-stage kappa, the core setting of the D₀ stage. Too low a factor and lignin is not fully removed, downstream brightness cannot rise; too high wastes bleach and may also oxidize cellulose, causing viscosity to drop. For the full softwood O-D₀-EOP-D₁ sequence, total ClO₂ use is roughly 4–6% (on oven-dry pulp). On-site, using an online kappa meter and residual-chlorine/ORP feedback to fine-tune the factor is the standard move for stable brightness and cost control.
ECF's brightness capability is strong: hardwood pulp can be bleached to 88–90% ISO, softwood 86–90% ISO, sufficient for the vast majority of printing and writing papers. Another big selling point is that it presses AOX (Adsorbable Organic Halides) to a very low level.
6. TCF bleaching: the thoroughly chlorine-free green route
TCF (Totally Chlorine-Free bleaching) goes further — no chlorinated oxidant anywhere in the process, sequences such as O-Q-(PO)/(Z-P). The Q stage uses a chelating agent (EDTA 0.2–0.4%) to complex the transition-metal ions in the pulp (iron, manganese, copper), because these metals catalyze the useless decomposition of hydrogen peroxide and ozone; then hydrogen peroxide (H₂O₂) enhanced alkaline extraction (PO stage) is used to decolorize and brighten, or ozone (Z stage) for strong delignification, finally finished with a peroxide (P stage).
TCF has the best environmental image — zero AOX, wastewater easier to treat — but its brightness ceiling and bleaching cost have weaknesses: under equivalent conditions, TCF brightness is usually a few points lower than ECF, and it demands harsher pulp cleanliness and metal-ion control, with no low bleach-chemical cost (especially ozone generation power consumption). So TCF is mostly used in markets with high environmental-certification requirements or for specific pulp grades, while ECF remains the king of cost-performance. Mechanical-pulp brightness improvement, meanwhile, relies almost entirely on H₂O₂, targeting 78–82% ISO.
AOX is the most criticized by-product indicator of chlorinated bleaching. In the 1990s, old chlorinated bleaching's AOX emission could exceed 5 kg/t pulp, while modern ECF/TCF processes have pressed it below 0.3 kg/t pulp, and many advanced pulp mills are even below 0.1. Behind this is the combined result of O-stage adoption, ClO₂ replacing chlorine gas, and closed screening with filtrate recycling. For the pulp mill's environmental assessment and the downstream paper mill's green procurement, the choice of bleaching route is long since not a purely technical issue but a market-access issue.
7. Quick reference for brightness targets and sequence selection
Different pulp grades and end uses vary greatly in brightness demand. The table below puts typical targets and routes together for on-site reference. It must be stressed that all brightness figures refer to ISO-standard blue-light diffuse reflectance brightness, not the various older bases, so cross-mill comparison must read %ISO.
| Pulp grade / route | Typical brightness target (%ISO) | Mainstream sequence | Note |
|---|---|---|---|
| Mechanical pulp (H₂O₂) | 78~82 | H₂O₂ single/two-stage | No delignification, keeps yield and bulk |
| Hardwood kraft ECF | 88~90 | O-D₀-EOP-D₁ | Main furnish for cultural paper |
| Softwood kraft ECF | 86~90 | O-D₀-EOP-D₁ | Strength backbone, ClO₂ ≈ 4–6% |
| TCF pulp | 82~88 | O-Q-PO/P | Zero AOX, higher cost |
Sequence selection has no absolute answer, only fit: pursue low-cost high brightness → ECF, pursue zero-halogen certification → TCF, mechanical pulp essentially locks to hydrogen peroxide. What the floor must watch to the death is "brightness stability" and "yellowing" — if bleached pulp's brightness falls back in storage (yellowing), chances are residual lignin or metal ions were not controlled, and the rework cost far exceeds prevention.
8. Equipment details and on-site tuning points
Having finished the flow, return to the equipment itself. The drum vacuum washer relies on pulp carried on the drum surface and internal vacuum to draw the black liquor away; it fears "drum fouling" and "vacuum leakage" — uneven feed consistency and aged vacuum-box seals both drop the extraction rate; the press (filter-press) washer squeezes the pulp dry under closed pressure, with good sealing and high extraction, suited to new low-emission pulp lines, but clogging and replacement of the filter cloth/plates are daily maintenance focus. Neither type is absolutely better; the drum wins on continuous stability, the press on sealing and high extraction; selection depends on the mill's water-closure degree and black-liquor handling strategy.
Internal construction of the pressure screen
The core of the pressure screen is the rotating rotor and the fixed screen basket. The rotor blades sweep past the basket at high speed, on one hand creating a local negative pressure that "sucks" the accept through the slot, on the other using pulsed flow to prevent fiber bridging and slot clogging. The slot precision of the basket decides the screening lower limit — the narrower the slot, the better it catches fine debris, but the easier it clogs and the lower the capacity. Three keys to on-site maintenance: the basket intact with no damage (one cut slot leaks coarse reject), stable rotor-basket gap (a larger gap collapses screening efficiency), and steady feed consistency and flow. Many screening faults trace back to upstream washing being unclean — high black-liquor viscosity fouls the screen — again proving washing and screening are inseparable.
Engineering logic of closure and filtrate reuse
Modern bleach plants almost all move toward closure: EOP filtrate reused upstream, D₁ filtrate sent to D₀, wash/screen white water recycled, aiming to press fresh-water use and white-water discharge to the minimum. Closure brings benefits but also trouble — dissolved matter and salts accumulate in the system, possibly corroding equipment and breeding deposits. So the closure degree must match purification capacity (e.g., ultrafiltration, evaporation, anaerobic treatment), not the more closed the better. One lesson: first ensure the O stage and washing recover as much organic matter and alkali as possible, then talk about bleach-section closure; get the order wrong and "closure causes accidents."
In the oxygen-delignification reactor, how oxygen disperses uniformly in high-consistency pulp and avoids local over-peroxidation that hurts cellulose is a design difficulty. The common approach mixes oxygen strongly with alkali liquor and stock in a mixer, then enters the tower to react; the tower holds a certain residence time (usually tens of minutes) and 0.2–0.6 MPa oxygen partial pressure to ensure reaction driving force. If mixing is uneven, "some places oxygen excess hurts pulp, some places oxygen shortage fails to delignify" non-uniform delignification appears, showing as viscosity fluctuation and unstable brightness. Two-stage O/O, by adding oxygen and reacting in stages, flattens this non-uniformity and is the preferred choice for high-quality pulp lines.
Finally, a systemic perspective: the total energy and chemical cost of the three steps washing, screening and bleaching all finally convert into the cost per tonne of pulp. The most cost-effective optimization order on-site is usually "first stabilize washing, then optimize screening, then tune bleaching" — because the alkali and heat saved in washing are free, while every drop in ClO₂ dose in bleaching goes straight into the profit sheet. Managing these three gates as one linked line rather than three isolated steps is the essential gap between a mature pulp mill and a novice one.
9. Troubleshooting the three steps
What the floor fears most is not equipment breakdown but parameters silently drifting unnoticed. The washing section must watch "residual alkali" and "black-liquor extraction rate": if the washed-pulp residual alkali rises, first check whether vacuum/pressure has dropped or the dilution factor was reduced, then check the upstream cooking-blow consistency; if the black-liquor extraction rate drops, it is mostly a clogged cloth/basket or a leaking seal. The most typical screening-section signal is "accept dirt count rising" coexisting with "reject fiber content too high" — the former means the basket is damaged or the gap enlarged and leaks coarse reject, the latter means reject-screen recovery is insufficient or over-screening is happening; both point to basket condition and reject ratio.
The bleaching section's faults concentrate on unstable brightness and yellowing. If brightness swings, first see whether the D₀ kappa factor failed to track the incoming kappa fluctuation in time, then check whether the ClO₂ generator capacity and concentration are sufficient; yellowing (brightness falling back in storage) is usually residual lignin or metal ions not controlled, requiring stronger Q-stage chelation or deeper O stage. There is also a cross-section signal: unclean washing carries black liquor all the way into bleaching, making ClO₂ dose falsely high and wastewater COD spike — so when bleach-chemical cost rises abnormally, the first reaction should be to look back at washing, not blindly add more bleach.
This troubleshooting order has a mass-balance basis: every 1% more black-liquor organic matter entering bleaching means the subsequent oxidant must be consumed additionally to "deal with" pollutants that should not have been there, and all are AOX precursors. In other words, a washing-section defect is amplified in the bleaching section into a triple loss of chemicals, wastewater and cost. Taking the washing extraction rate and residual alkali as upstream input indicators of the bleaching section and putting them on the daily dashboard is standard practice in many advanced pulp mills — extending the boundary from "this step qualified" to "downstream benefits" avoids local optimum and global loss.
Key points of this episode · quick-number memory
- Washing dilution factor: 2–3 m³/t; black-liquor extraction target >97%, directly decides recovery economy.
- Pressure-screen slot width: chemical pulp 0.15–0.35 mm; knot removal → coarse → fine three-tier defense, reject recirc. preserves yield.
- Oxygen delignification O stage: 90–100 °C, O₂ partial pressure 0.2–0.6 MPa, NaOH 2–3%, kappa drop 35–50%.
- Main ECF sequence O-D₀-EOP-D₁; D₀ kappa factor ≈0.2, pH 3–4, 50 °C; D₁ pH 3.5–4.5, 70 °C.
- Softwood ECF brightness 86–90% ISO, hardwood 88–90%, mechanical-pulp H₂O₂ bleach 78–82%.
- TCF (O-Q-PO/P) zero AOX, cost about 10–30% above ECF, brightness slightly lower 82–88%.
- AOX emission: from >5 kg/t in the 1990s down to <0.3 kg/t modern, via O stage + ClO₂ + closed filtrate.