Question 4 of 4: Mechanical Pulping — Furnish Species, TMP vs. Groundwood, Bleaching, Screening & Grinder Energy/Water Balance

Question 4: Mechanical Pulping — Furnish Species, TMP vs. Groundwood, Bleaching, Screening & Grinder Energy/Water Balance (a)–(e) 10 marks each, (f) 20 marks

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Species suitability for mechanical newsprint pulp

Jackpine is generally unsuitable for mechanical (newsprint-grade) pulp because it is a resinous softwood with high pitch (resin/extractives) content and characteristically dense, dark heartwood with numerous knots — the resin causes pitch deposition problems throughout the mill and on the paper machine, and the dark, knotty wood degrades brightness and cleanliness of the resulting sheet. Aspen (a hardwood) is unsuitable because mechanical pulping relies on long, strong softwood-type tracheid fibres for newsprint strength (tear/tensile); aspen's short hardwood fibres, combined with its very low density (poor grinding/refining energy transfer, low pulp yield of usable long fibre), give a weak sheet unable to meet newsprint runnability/strength requirements even though aspen is easy to grind. A boreal species well suited to newsprint mechanical pulping is black spruce (or white spruce/balsam fir) — it has long, strong, light-coloured, low-resin softwood fibres, moderate density (grinds/refines efficiently without excessive energy), and good inherent brightness, making it the classic North American newsprint furnish species.

(b) Mechanical pulping processes for newsprint

The two principal methods are stone groundwood (SGW) and thermomechanical pulp (TMP). In SGW, debarked logs are pressed against a rotating abrasive grindstone in the presence of shower water, which mechanically tears fibres directly from the log surface. In TMP, wood CHIPS (not logs) are first pre-steamed/softened under pressure, then passed through disc refiners (rotating grooved discs) that mechanically defibrate the softened chips into fibres, typically in two refining stages (primary and secondary/rejects refining). TMP is generally the more favourable modern process: the pre-steaming softens the lignin-rich middle lamella, giving longer, less-damaged (higher-strength) fibres and a pulp with distinctly better tensile/tear strength than SGW at comparable freeness, and TMP can use chips (including sawmill residual chips) rather than requiring round logs, improving fibre-supply flexibility. TMP's disadvantage is much higher specific energy consumption (refining energy, on the order of 1.5–2× SGW's grinding energy per tonne) since fibres must be mechanically separated from chips rather than simply abraded off a solid log face; SGW's disadvantage is its lower-strength fibre and its dependence on a steady log (not chip) supply. Equipment: SGW uses grindstones (pocket or continuous grinders); TMP uses steaming vessels/pressurized refiners (disc refiners) in a primary/secondary refining train.

(c) Bleaching groundwood pulp

Two bleaching methods are used on mechanical (groundwood/TMP) pulp: (1) sodium/zinc hydrosulphite (dithionite) bleaching — a mild reducing bleach, typically applied at medium consistency (≈3–5%), near-neutral to mildly acidic pH, ambient-to-moderate temperature (≈40–50°C), short retention (≈30–60 min), using simple mixing/retention towers; it gives a modest brightness gain of roughly 8–12 ISO points and is NOT permanent (some brightness reversion occurs on aging/exposure). (2) Hydrogen peroxide bleaching — an oxidative bleach, typically at higher consistency (10–25% MC/HC), alkaline pH (silicate/NaOH buffered, pH≈9–11), moderate temperature (≈50–70°C), longer retention (1–3 h) in a retention/bleaching tower, requiring peroxide stabilizers (sodium silicate, DTPA/chelant to control transition-metal decomposition of the peroxide); it gives a larger and more durable brightness gain, typically 15–20+ ISO points. Peroxide bleaching is the most common method in modern newsprint/mechanical-pulp mills because it delivers a substantially larger, more stable brightness gain, is compatible with high-brightness grades, and (unlike hydrosulphite) does not require the pulp's metal-ion content to be tightly controlled to avoid poor results (with proper chelation) — hydrosulphite remains used mainly as a lower-cost, lower-brightness-target or touch-up bleach.

(d) Groundwood, TMP, CTMP and Kraft compared

Groundwood and, to a lesser extent, TMP are not used for high-quality, high-brightness, high-strength papers because they retain essentially all of the original lignin, which is inherently low-strength (compared to cellulose/hemicellulose) and photo-oxidizes on exposure to light/UV, causing the characteristic yellowing of newsprint; groundwood in particular also retains a shorter, more fibrillated/damaged fibre population with lower intrinsic strength. Where groundwood/TMP/CTMP genuinely have the advantage over kraft is (i) yield — 85–98% vs. kraft's ≈45–54%, meaning far less wood is needed per tonne of pulp, a major cost and fibre-supply advantage; and (ii) optical/bulk properties — retaining the lignin and the shorter fines-rich fibre fraction gives mechanical pulps much higher opacity, bulk and light-scattering (printing surface) at a given basis weight than kraft pulp, which is exactly why newsprint and many printing/publication grades deliberately use a mechanical (or mechanical + kraft blend) furnish rather than pure kraft.

(e) TMP screening

Reject material from primary screening (long shives, undefibrated chip fragments) is not discarded but is sent to a dedicated reject refiner for further mechanical treatment, then RE-SCREENED (as secondary screening) rather than sent straight back to primary screening, both recovering that fibre as usable pulp and avoiding recirculating coarse material through the primary screens indefinitely.

(f) Grinder shower-water/energy balance

Given.

Find. Shower water flow (kg/s) required to hold the vat at 70°C.

Approach. Essentially all of the grinder's mechanical (electrical) power is dissipated as heat in the pulp/water mass at the grindstone. A steady-state energy balance on the vat — grinding power plus the sensible heat carried in by the wet wood (at 25°C) and the shower water (at 40°C) must equal the sensible heat carried out at the vat temperature (70°C) — is solved for the one unknown, shower water flow, after first sizing the wet-wood feed rate from the production rate, digester-analogous yield, and wood moisture.

1. Wet wood feed rate. O.D. pulp $=40\times0.90=36.0\ \text{t/day}$; at 99% yield, O.D. wood $=36.0/0.99=36.36\ \text{t/day}$; at 42% moisture, wet wood $$\dot m_{wood}=\frac{36.36}{1-0.42}=62.70\ \text{t/day}=0.7256\ \text{kg/s}$$
2. Grinding power. $$P=\frac{1500\ \text{kWh/ADMT}\times40\ \text{ADMT/day}}{24\ \text{h/day}}=\boxed{2500\ \text{kW}}\ (=2500\ \text{kJ/s})$$
3. Vat energy balance. With all streams referenced to Cp=4.18 kJ/kg·°C, the grinding power plus the sensible heat of the incoming wood and shower water must equal the sensible heat of the combined stream leaving the vat at 70°C: $$\frac{P}{Cp}+\dot m_{wood}T_{wood}+\dot m_{water}T_{water}=(\dot m_{wood}+\dot m_{water})\,T_{vat}$$ $$\frac{2500}{4.18}+0.7256(25)+40\,\dot m_{water}=(0.7256+\dot m_{water})(70)$$ $$598.1+18.14-50.79=\dot m_{water}(70-40)$$ $$\dot m_{water}=\frac{565.4}{30}=\boxed{18.85\ \text{kg/s}}$$