Question 3 of 6: Mechanical Pulping — Furnish Species, Grinding vs. Refining, Bleaching, TMP Screening & Grinder Energy/Water Balance

Question 3: Mechanical Pulping — Furnish Species, Grinding vs. Refining, Bleaching, TMP 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) Jackpine vs. a boreal newsprint species

Jackpine is a resinous softwood — its resin canals carry a high content of pitch (fatty/resin-acid extractives) that, in a mechanical pulping process (no chemical/thermal removal of extractives), remains largely in the pulp and causes severe pitch deposition problems on the grinder stones, refiner plates, wires and dryer cans, plus brightness-reversion and printability defects in the finished newsprint sheet. Jackpine is, however, well suited to the Kraft process precisely because the hot alkaline (NaOH) cooking liquor saponifies these same resin/fatty acids into soluble sodium soaps, which are removed with the black liquor (and recovered as tall oil) rather than remaining in the fibre. A boreal species well suited for mechanical newsprint pulp is black spruce (or balsam fir): it has long, slender tracheids that give high-strength, low-pitch mechanical pulp with good optical (light-scattering) properties for newsprint, without jackpine's resin-canal pitch burden.

(b) Two mechanical pulping methods for newsprint

The two established mechanical-pulping routes are stone groundwood (SGW), where debarked logs are pressed against a rotating abrasive grindstone under a water shower, and thermomechanical pulping (TMP), where wood chips are first steamed/pre-softened under pressure and then refined between rotating discs (refiner plates) in one or two stages. TMP is generally the more favourable modern process: pre-steaming softens the lignin in the middle lamella, so fibres separate with less mechanical damage, giving TMP pulp substantially higher strength (tensile, tear) than groundwood for a comparable freeness — the main disadvantage being TMP's much higher specific energy consumption (typically 1800–2200 kWh/ADMT vs. 1200–1500 kWh/ADMT for groundwood) and higher capital cost (pressurized refiners and steaming vessels vs. simple grinders). Equipment: SGW uses pocket or magazine grinders (logs pressed against a wet grindstone); TMP uses chip pre-steaming vessels/impregnators feeding pressurized disc refiners (commonly two refining stages in series, as in part (e) below).

(c) Bleaching groundwood pulp

The two established methods are hydrosulphite (sodium dithionite, Na₂S₂O₄) bleaching and peroxide (H₂O₂) bleaching. Hydrosulphite bleaching is a mild reductive process run at low consistency (3–4%), near-neutral pH, ambient-to-warm temperature (40–50°C), in a simple retention/reaction tower, and typically gives a modest brightness gain of 4–6 points ISO. Peroxide bleaching is an oxidative process run at higher consistency (10–20%), strongly alkaline pH (≥10, with sodium silicate and DTPA/magnesium stabilizers to control peroxide decomposition by transition-metal ions), elevated temperature (50–70°C), typically in an enclosed retention tower, and gives a substantially larger brightness gain (10–20 points ISO). Peroxide bleaching is the most common method in modern mills because its much larger, more stable brightness gain is needed to meet current newsprint/SC-paper brightness targets, and unlike hydrosulphite its brightness gain does not revert as quickly on storage.

(d) Groundwood, TMP, CTMP and Kraft — process comparison

Groundwood (SGW): highest yield (≈95–98%), lowest cost/energy, but lowest strength and poorest permanence (high residual lignin causes rapid yellowing) — used for short-life printed products (newsprint, directories). TMP: similar high yield (≈90–95%), better strength than groundwood (see (b)), still low permanence (full lignin retained) — used for newsprint and higher-quality mechanical printing papers blended with some chemical pulp. CTMP (chemi-thermomechanical pulp): a mild chemical (usually sodium sulphite) pre-treatment before refining softens/sulphonates the middle lamella further, giving still-higher strength and brighter, easier-refining pulp at a small yield penalty (≈85–92%) — used for tissue, some paperboard and specialty printing grades. Kraft: lowest yield (≈45–55%) and highest cost, but essentially full lignin removal gives the highest strength and best permanence (little yellowing/brightness reversion) — used wherever high strength or permanence is required (packaging, printing/writing, and as reinforcement fibre blended into mechanical-pulp furnishes). Groundwood, TMP and CTMP are NOT typically used for high-quality, high-strength or archival papers, because their high residual lignin content causes ongoing brightness reversion (yellowing) and their fibres are inherently weaker/more damaged than chemically-cooked Kraft fibre; conversely, groundwood/TMP/CTMP have the advantage over Kraft in yield (far less wood per tonne of pulp), energy/chemical cost, and, importantly, opacity and light-scattering (printing) properties — the same fibre/fines structure that limits strength gives excellent sheet opacity, valuable for lightweight printing grades where Kraft pulp alone would be too transparent.

(e) TMP mill unit operations

A typical TMP mill's main unit operations, in process order, are: chip washing/cleaning → chip pre-steaming (softening) → primary (first-stage) disc refining → primary screening (accept/reject split) → secondary (second-stage) refining of the primary reject → secondary screening → latency removal (a hot, low-consistency retention stage that relaxes curled fibres straightened by the refiner) → bleaching (peroxide, as in part (c)) → stock storage/blending for the paper machine.

(f) Grinder energy/water balance — shower water requirement

Given.

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

Approach. All the electrical power drawn by the grinder ultimately appears as friction heat in the vat. An energy balance on the vat — wet wood entering at 25°C, shower water entering at 40°C, grinder power dissipated as heat, everything leaving at the 70°C vat setpoint — is solved for the one unknown, the shower-water mass flow.

1. Wet-wood mass flow. O.D. pulp $=40\times0.90=36.0\ \text{ODt/day}$ (90% AD convention); dividing by the 99% yield gives O.D. wood, then dividing by (1−moisture) gives wet wood: $$\text{O.D. wood}=36.0/0.99=36.36\ \text{ODt/day}\qquad \dot m_{wood}=\frac{36.36/(1-0.42)\times1000}{86{,}400}=\boxed{0.7256\ \text{kg/s}}$$
2. Grinder power. $$P=\frac{1500\ \text{kWh/ADMT}\times40\ \text{ADMT/day}}{24\ \text{h/day}}=\boxed{2500\ \text{kW}}$$
3. Vat energy balance. All streams leave at 70°C; power is dissipated fully as heat ($P/Cp$ acts as an equivalent mass-flow×temperature-rise term): $$\frac{P}{Cp}+\dot m_{wood}(T_{wood}-T_{vat})=\dot m_{water}(T_{vat}-T_{water})$$ $$\frac{2500}{4.18}+0.7256(25-70)=598.1-32.7=565.4=\dot m_{water}(70-40)$$ $$\dot m_{water}=\frac{565.4}{30}=\boxed{18.85\ \text{kg/s}}$$