Pulping Processes

Processes at a glance

Yields are oven-dry pulp on oven-dry wood after screening, wood use is cubic metres solid under bark per air-dry tonne at 90 % dry solids, and the basis matters: quoted over bark or per oven-dry tonne the same mill gives a materially different figure. Ranges rather than point values throughout, and the point values used in the fibre-balance model are in the methodology.

Within kraft, softwood sits at the wood-hungry end and eucalyptus at the other; unbleached brown grades reach 65 to 70 % yield. Sulphite spans acid sulphite, magnefite and dissolving grades, and is conventionally quoted in tonnes of wood rather than cubic metres. The mechanical figure is a bracket covering TMP, groundwood and CTMP, since no CTMP-specific factor is published. Mechanical yields refer to Norway spruce, the CTMP energy figure to aspen, and recycled yields to recovered paper input rather than to wood: European mills use 1.10 to 1.30 tonnes of recovered paper per tonne of output.

Chemical pulping

Chemical processes dissolve the lignin that binds the fibres together, along with much of the hemicellulose, and wash it away. The fibres come out intact and largely pure, which is what makes chemical pulp strong, and roughly half the wood leaves as dissolved organics burned for energy rather than as fibre.

• Kraft (sulphate): chips are cooked in sodium hydroxide and sodium sulphide, then washed, screened and usually bleached. The spent liquor is burned in a recovery boiler that regenerates the cooking chemicals and raises steam, which is what makes the process both dominant and energy self-sufficient. See all operating kraft mills on the map.
• Sulphite: cooking in sulphurous acid and bisulphite instead. It gives brighter, easier-bleaching pulp but weaker fibre, and its chemical recovery is more awkward, which is why it has largely given way to kraft outside specialty and dissolving grades.
• Soda: sodium hydroxide alone, without sulphur. Used mainly for non-wood fibres such as straw and bagasse.

Mechanical pulping

Mechanical processes tear the fibres out of the wood with force rather than dissolving what holds them, so almost the whole log becomes pulp. The lignin stays in the fibre, which is why mechanical pulp is bulky and opaque, weaker than chemical pulp, and yellows with light. The energy that chemical pulping gets from burning lignin, mechanical pulping has to buy as electricity.

• Thermomechanical (TMP): chips are steamed to soften the lignin, then refined between rotating discs. The workhorse for newsprint and lightweight publication paper.
• Groundwood (SGW/PGW): whole logs are pressed against a revolving grindstone, at atmospheric or elevated pressure. The oldest pulping method still in industrial use.
• Refiner mechanical (RMP): chips refined without pre-steaming, largely superseded by TMP because refining cold wood damages more fibre.

Chemi-mechanical and semi-chemical pulping

In between, chips get a chemical pre-treatment that softens the lignin before mechanical refining. The fibres separate with less force and less damage, so the pulp bonds better than mechanical pulp while keeping most of the wood.

• CTMP / BCTMP: a mild sulphite impregnation before refining, bleached or unbleached. The result is a bulky, stiff fibre used where thickness matters more than strength.
• NSSC (semi-chemical): neutral sulphite cooking for about 30 minutes at 170 to 180 °C in weakly alkaline liquor, leaving 15 to 20 % residual lignin, followed by refining. The stiffness that residual lignin gives is exactly what corrugated fluting needs.

Recycled fibre processing

Recovered paper is repulped in water, screened and cleaned; where brightness matters, deinking floats or washes out inks and adhesives. Fibres shorten and stiffen each time they go round, so recycled furnish is topped up with virgin fibre in grades that carry load.

Mills running only on recovered paper consume no roundwood and therefore sit outside the wood fibre balance, which is why WoodComp tracks them separately from wood-consuming mills. Many mills blend the two. See methodology.

The database currently tracks 187 operating kraft mills and 26 mills with mechanical or chemi-mechanical lines.