1. A plant-floor view: the other path of the power-hungry giant
Chemical pulp mills save chemicals through recovery, but the mechanical-pulp plant is a veritable "power-hungry giant." Walking into a TMP plant, what you hear is not the roar of a recovery boiler but the near-screaming high-frequency noise of refiner after refiner — that is the energy of whole logs being ground straight into pulp. The underlying logic of mechanical pulp is the opposite of chemical pulp: it hardly dissolves any lignin, instead mechanically tearing and fibrillating the wood cells, so yield can reach 85–96% with almost zero fiber loss; the price is that tearing the fibers out of the wood requires enormous mechanical energy, with power consumption routinely over a thousand kWh/t.
The appeal of this route lies in "bulk, opacity and stiffness" — newsprint, SC magazine paper, LWC base stock and board core layers are full of it. But how to reckon the power cost, at what refining degree strength starts to drop, and why TMP is so much stronger than the old stone groundwood (SGW), are what this episode makes clear. We start with the family tree, lining up the several mechanical pulps and their pedigrees.
2. The family tree: from grindstone to chemi-mechanical pulp
Mechanical pulp is a continuously evolving family; the further along, the more chemicals are involved, the yield drops slightly but strength and brightness improve. Arranged by "increasing chemical use, decreasing yield" one gets a clear lineage:
- SGW (Stone Groundwood): the oldest, wood bolts pressed against a rotating grindstone to be ground directly, yield 93–96%, almost zero chemicals. Advantages: high light-scattering coefficient, good opacity; drawbacks: few long fibers, low strength, prone to yellowing.
- RMP (Refiner Mechanical Pulp, atmospheric): replaces the grindstone with a refiner, pulping at atmospheric pressure, still high yield, fiber quality slightly better than SGW, but limited improvement in energy and strength.
- TMP (Thermo-Mechanical Pulp, preheated): chips are first softened in a preheater at 120–130 °C steam, then enter the refiner. Thermal softening makes lignin tougher and fibers easier to peel along the wall layers; long-fiber retention improves markedly, tensile and tear strength are significantly higher than SGW, making it the mainstay of newsprint.
- CTMP (Chemi-Thermo-Mechanical Pulp): on the TMP basis adds Na₂SO₃ impregnation (about 1–3%), yield 85–92%. The sulfite pre-treatment further softens lignin and protects fibers, lifting strength and brightness a notch, commonly used for SC paper and board core layers.
- CMP (Chemi-Mechanical Pulp): a chemi-mechanical pulp leaning more toward chemical pre-treatment, with higher chemical use and lower yield, between TMP and chemical pulp.
The essence of the lineage is a trade-off between "yield" and "chemical/energy cost": the further right, the lower the yield, but fiber quality, brightness and strength rise a step. Which to choose depends on whether the end paper grade wants bulk and opacity (SGW/TMP) or strength and printability (CTMP/CMP).
3. Energy consumption: the lifeline of this line
In the operating cost of a mechanical-pulp plant, power consumption is the absolute majority, so the specific energy consumption (SEC, kWh/t pulp) is the most core KPI of this line. Typical ranges: SGW about 1200–1600 kWh/t; TMP at newsprint grade about 2000–2800 kWh/t; CTMP (including chemical treatment) about 1800–2500 kWh/t. A comparison makes clear why it is called power-hungry — chemical-pulp cooking steam is only 1.5–2.5 GJ/t (about 400–700 kWh/t equivalent), while mechanical pulp's power consumption is several times that.
Behind this is a physical essence: chemical pulp "dissolves" lignin through chemical-bond breaking, with a low activation-energy threshold; mechanical pulp uses mechanical work to peel and fibrillate fibers out of the wood composite structure, requiring large amounts of elastic-plastic deformation and fracture energy to be "hammered" directly into the material, so its energy-efficiency ceiling is much lower. This is also why mechanical-pulp plants are extremely sensitive to electricity price — many TMP lines are built in hydro/cheap-power regions.
Lower energy is not always better — it is in tension with strength. Too light refining, fibers are not sufficiently fibrillated, bonding is poor and strength low; too heavy, fibers are cut and fines surge, and strength falls back. So the floor target is "the lowest SEC at a given strength target," not simply saving power. The curve below makes this clear. And precisely because power accounts for such a large share, mechanical-pulp plants are highly sensitive to electricity price: many TMP lines are located in hydro-rich or low-price regions, even with their own power plants, turning energy cost from the biggest weakness into a relative advantage.
4. Mechanism: thermally softened lignin and fibrillation
Why does TMP's added preheating greatly improve strength? The key is lignin's glass-transition temperature (Tg). At room temperature lignin is a hard glassy state, and hard grinding only cuts fibers; when chips are preheated in 120–130 °C steam, lignin absorbs water and its Tg drops markedly, shifting from glassy to rubbery with greatly increased toughness. Then the refiner's shear force more easily peels fibers along the S1/S2 cell-wall layers rather than cutting them in half — this is "fibrillation": the fiber surface is brushed out into a large number of microfibrils, and both specific surface area and flexibility rise, so hydrogen-bond bonding points explode in the finished sheet.
The more thorough the fibrillation, the better the interfiber bonding, and tensile, burst and stiffness all benefit. But the other side of the coin: refining is a progressive damage process; over-refining over-cuts fibers and produces large amounts of fines (short fractions). Fines can fill gaps and raise light scattering and opacity, yet contribute almost no long-range strength; once the fines share crosses a threshold, sheet strength falls back as freeness drops — precisely the microscopic root of the "strength peak then fall" in Fig. 2.
From a fiber-morphology view, fibrillation occurs mainly at the S1/S2 interface of the secondary wall and within the S2 layer's microfibril bundles; the brushed-out microfibrils can reach nanometer scale, multiplying the single fiber's specific surface area. This "hairy" surface interlocks during sheet formation, raising hydrogen-bond point density — the physical essence of mechanical pulp's strength. Notably, fibrillation and fiber cutting often occur simultaneously in the same refining zone; the only difference is which way the plate geometry and consistency conditions lean — which again confirms that "large attack angle + high consistency" is the process combination that protects long fibers and promotes fibrillation, while low consistency and narrow gap more easily slide toward cutting.
So mechanical-pulp quality control is essentially finding a balance between "fibrillation (good)" and "cutting/fines (bad)." Preheating preserves the long fibers, but the plate gap and energy distribution decide where the balance point lies. This leads to the next section's three refining-control elements.
Dry lignin Tg is about 130–150 °C, but after absorbing water (the refiner's high-consistency environment contains free water) the Tg can drop to the 80–100 °C range — which is exactly why TMP preheating at 120–130 °C works: the lignin is fully softened but not yet largely degraded. If temperature goes higher and time longer, hemicellulose begins to hydrolyze and fiber strength is damaged, so preheating is the delicate window of "soften without degrading."
The reason S1/S2 interlayer peeling matters is that the softwood tracheid S2 layer accounts for about 80% of the wall thickness; peeling along the interlayer preserves the long-fiber body intact, whereas SGW's grindstone low-temperature hard grinding produces large amounts of cutting and dust.
5. The three refining-control elements
The refiner is the heart of mechanical pulp; its output quality is decided by three operable variables, colloquially the "three elements" on the floor:
1. Plate gap
The gap directly decides shear intensity. A large gap means fiber defibration dominates with little cutting, good long-fiber retention but insufficient fibrillation; a small gap means high shear, thorough fibrillation but easy over-cutting. Modern refiners control the gap with a hydraulic or motor-load closed loop, linked with the SEC setpoint.
2. Consistency
Mechanical pulp is divided into low-consistency (3–5%) and high-consistency (20–35%) refining. High-consistency refining is dominated by fiber-fiber friction and rubbing, giving good fibrillation and little cutting, preserving bulk and strength; low-consistency refining has strong fiber-plate direct action and is easier to cut. The main stages of TMP/CTMP are both high-consistency — a structural reason their strength beats SGW.
3. SEC staging
This is the key trick for TMP to preserve long fibers: split the total energy into two stages. The first stage (often ~1000 kWh/t magnitude) does "defibration" at low plate pressure and high consistency, aiming to separate chips into individual fibers without over-refining; the second stage does "fine refining" at even higher consistency, dedicated to fibrillation. Two-stage load distribution preserves long fibers and raises strength better than single-stage hard grinding. The flow diagram below makes it clear.
6. Applications: each grade's destination
Different mechanical pulps have clear divisions of labor among paper grades because of differences in strength, bulk and opacity:
- SGW: high light-scattering coefficient, excellent opacity, suited to strength-undemanding, opacity-heavy newsprint and some printing-paper base stocks, but prone to yellowing and low strength.
- TMP: good long-fiber retention, strength clearly better than SGW, the mainstay of newsprint (often blended with DIP), SC magazine paper and LWC base stock; high bulk and stiffness.
- CTMP: further raises strength and brightness, used for SC paper, board core layers (providing bulk and stiffness), absorbent products, and in tissue furnishes to improve softness and absorbency.
- CMP: a chemi-mechanical pulp leaning toward higher strength/brightness, used for coated base-paper base stock needing better printability.
On the floor, mechanical pulp is often blended with chemical pulp and DIP by performance and cost: chemical pulp gives "skeletal strength," mechanical pulp gives "bulk, opacity and cost advantage." For example, an LWC coated paper uses bleached chemical pulp on the top layer to ensure printing and coating, and TMP/CTMP on the bottom layer to prop up bulk and stiffness — this "layer-wise pulp use" routine is the core means of balancing cultural-paper cost and performance.
In packaging board, CTMP's position is especially prominent. The core layers of linerboard and corrugating medium use large amounts of CTMP or TMP, because it is bulky, stiff, low-cost, and can absorb the compression cushion the core needs; as long as the top layer uses sufficiently strong kraft to guarantee ring crush and burst, using mechanical pulp in the core greatly cuts cost. Tissue furnishes also often add CTMP or BCTMP to improve softness, absorbency and creping feel, but too high a blend damages strength and causes dusting, and must be balanced with market pulp. It can be said that wherever "bulk is wanted but extreme strength is not," mechanical pulp is the cost trump card.
The refiner consists of a pair of plates (one moving, one stationary) engraved with radial bars and grooves. Key geometric parameters include: attack angle (affects cutting vs. rubbing), bar-width/groove-width ratio, plate diameter (decides tip speed and throughput), and refining-zone taper. A large attack angle leans toward rubbing (fibrillation), a small one toward cutting; high-consistency refining often chooses a large attack angle to protect long fibers.
Tip speed often reaches 30–50 m/s, and a single motor can be several MW or even 10 MW class; the motor load and plate gap together form the closed-loop control of SEC. Worn plates need periodic re-grooving, otherwise SEC silently creeps up and quality drops.
7. Evaluating refining quality: what to measure
Mechanical-pulp quality cannot be judged only by SEC; a set of laboratory indicators must quantify "how well it is refined." The core one is freeness (Canadian Standard Freeness, CSF), expressed in mL — the lower the value, the finer the refining and the harder the water drains. TMP newsprint pulp's CSF often lands in the 100–300 mL range; lower means finer, better strength tendency but slower drainage and higher drying energy. The floor tracks it online or offline with a freeness tester, a direct window on refining degree.
Better than freeness at reflecting "fiber preservation" is fiber fractionation. Using a Bauer-McNett or Pulmac fractionator to grade by fiber length, a high share of the long-fiber fraction (e.g., +30 mesh or R14/R28 fraction) indicates the refiner did not cut fibers and fibrillation is sufficient, giving good tensile and tear strength. Fines content (the fraction passing fine screens) inversely indicates the degree of over-refining — too many fines both drag down strength and raise light scattering and opacity, requiring a trade-off.
Finished-sheet performance targets vary by grade: newsprint values bulk and opacity with medium strength demand; SC/LWC require higher tensile and surface strength to withstand high-speed printing. So on the same TMP line, adjusting SEC and the two-stage split switches between "bulk type" and "strength type" — the refiner operator's signature skill. A practical tip: at a given CSF, pursuing a higher long-fiber fraction preserves strength and controls energy better than simply lowering CSF.
Quick reference for quality evaluation
- CSF freeness: 100–300 mL (TMP newsprint grade), lower = finer, slower drainage.
- Long-fiber fraction: higher +30 mesh / R28 share → better tensile and tear.
- Fines: too high indicates over-refining; strength falls, opacity rises.
- Bulk: mechanical pulp typically 2.0–3.0 cm³/g, far above chemical pulp — its biggest selling point.
- Light-scattering coefficient: SGW/TMP high (40–60 m²/kg), good opacity.
Here is a plant-wide energy chain to warn about: the finer the refining (lower CSF), the slower the sheet drains, the heavier the dewatering burden on the wire and press sections, and the more the drying section's steam consumption rises. In other words, the SEC "saved" in the refining stage may be paid back partly as extra steam "burned" in the drying stage — especially for high-mechanical-pulp products like newsprint, refining fineness must be planned together with drying capacity. The floor often draws this trade-off as a "refining degree – drying energy" curve to set the optimal CSF, rather than pursuing low freeness in isolation. This also explains why TMP lines often use high-consistency refining rather than blindly fine refining: high-consistency refining drains better at similar strength, optimizing whole-mill energy.
8. Yellowing and mechanical-pulp bleaching: protecting color, not removing lignin
Mechanical pulp contains large amounts of residual lignin, the root of its high yield and good bulk, but also its weak spot — the phenolic structures and conjugated carbonyls in lignin oxidize under light and oxygen into quinone chromophores, the sheet gradually yellows and brightness falls, called "yellowing" in the industry. Newsprint yellowing over time has its root here. So the bleaching logic of mechanical pulp is fundamentally different from chemical pulp: chemical-pulp bleaching is "selective oxidative removal of residual lignin," while mechanical-pulp bleaching is "color protection" — touching the lignin skeleton as little as possible, only destroying formed chromophores or preventing their formation.
The mainstream means is hydrogen peroxide (H₂O₂) bleaching, with sodium silicate, DTPA/EDTA and other stabilizers complexing transition-metal ions (iron, manganese, copper catalyze the useless decomposition of H₂O₂). After H₂O₂ bleaching mechanical pulp can reach 78–82% ISO brightness and somewhat suppress yellowing. For higher brightness or better stability, sodium dithionite (Na₂S₂O₄, commonly called "hydrosulfite") reductive bleaching can be added, specifically reducing quinone chromophores, but reductive bleach tends to revert under light and oxygen and must be paired with an oxidative sequence.
It must be stressed that mechanical-pulp bleaching neither removes lignin nor raises yield — it is purely an "appearance and stability" investment. For products like newsprint with non-extreme brightness demand, often only light H₂O₂ bleach or even none is done; for high-end products like SC/LWC, the bleaching cost is worth spending. This is also why mechanical-pulp brightness targets (78–82% ISO) are markedly lower than chemical-pulp ECF bleach at 86–90% — not that it cannot reach higher, but lignin remains, and bleaching too high is neither economical nor stable.
In alkaline conditions with transition metals present, H₂O₂ undergoes Fenton-type useless decomposition, wasting chemicals and possibly causing hydroxyl-radical pitting on fibers. So before H₂O₂ bleaching, mechanical pulp must chelate away Fe/Mn/Cu with DTPA or EDTA (0.2–0.5% on oven-dry pulp), then use sodium silicate (water glass) to provide alkaline buffering and adsorb metals, pushing H₂O₂'s useless decomposition to a minimum. This "chelation + stabilization" package is the key to protecting both brightness and yield in mechanical-pulp bleaching, and also a different fiber-protection logic from the chemical-pulp oxygen stage (which uses MgSO₄ to protect viscosity). If the floor skips chelation and adds bleach directly, brightness often fails to rise while chemicals double — the account is clear.
Stringing this episode together: mechanical pulp is a process of trading "electricity" for "yield and bulk"; the lineage from SGW to CTMP keeps shifting between yield and fiber quality; SEC is the lifeline, two-stage refining and high consistency are the tricks to preserve long fibers; and yellowing and bleaching remind us that the more lignin retained, the more extra money must be spent maintaining appearance stability. Next episode enters washing, screening and bleaching, to see how chemical pulp's "delignify to the end" bleach climbs to 90% brightness.
Finally, an economic perspective for grade selection to help the floor judge the route: mechanical pulp has a high unit power cost (2000–2800 kWh/t), but high yield, wood savings, and no large recovery investment; chemical pulp has lower yield and needs recovery, but high strength, large brightness potential, and recovery can also sell steam and power. So when the product wants "bulk, opacity, low cost, medium strength" (newsprint, board core), mechanical pulp wins; when it wants "high strength, high brightness, good printability" (high-end cultural paper, specialty paper), chemical pulp is irreplaceable. The two are not competitors but complementary — modern mills are almost all "chemical-pulp skeleton + mechanical/recycled-pulp filling" blends, pushing each tonne's combined cost and performance to the optimum. Understanding this boundary is what truly reads mechanical pulp's position in the value chain.
Key points of this episode (quick-number memory)
- Family lineage: SGW yield 93–96% → RMP → TMP (preheat 120–130 ℃) → CTMP (Na₂SO₃ 1–3%, yield 85–92%) → CMP.
- SEC: SGW 1200–1600, TMP newsprint grade 2000–2800, CTMP 1800–2500 kWh/t — the lifeline of this line.
- Mechanism: lignin Tg drops → rubbery state → fiber peels along S1/S2 → fibrillation; over-refining → fines ↑ → strength peak then fall.
- Three refining elements: plate gap, consistency (high 20–35%), SEC staging (stage-1 ~1000 kWh/t defibration, stage-2 fine refining).
- Two-stage refining protects long fibers: stage-1 defibration, stage-2 high-consistency fine refining, long-fiber rejects recirculated.
- Application split: SGW/TMP give bulk and opacity, CTMP/CMP give strength and brightness; layered blending with chemical pulp cuts cost.
- Refiner tip speed 30–50 m/s, single-unit power up to several MW~10 MW class; plates need periodic re-grooving when worn.
Whether chemical or mechanical pulp, after cooking/refining both face the same hurdle: wash the spent liquor and impurities out of the pulp, then bleach the residual lignin to the target brightness. Next episode we enter the bleach plant, clarifying how dilute–extract washing and displacement washing recover black liquor, how pressure-screen slots 0.15–0.35 mm remove knots and grit, and how oxygen delignification (O stage) and the mainstream ECF sequence O-D₀-(EOP)-D₁ are configured in pH, temperature and kappa. How AOX emission drops from >5 kg/t to <0.3 kg/t will also be reckoned.