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Material World
Process

Kraft pulping

Dissolving lignin out of wood with sodium hydroxide and sodium sulfide — the dominant route to paper, and a chemical plant that runs largely on its own waste.

Wood is cellulose fibres glued together with lignin, and papermaking is the business of separating them. Kraft pulping does it chemically: wood chips are cooked in sodium hydroxide and sodium sulfide, which break the lignin into fragments soluble in the alkaline liquor while leaving the cellulose largely intact.

The name is the German for strength, and it is earned — kraft fibres are longer and less damaged than those from mechanical pulping, which is why kraft paper takes the load in sacks and corrugated board. The trade-off is yield: roughly half the wood leaves as dissolved lignin rather than as paper.

Uses

Most of the world's chemical pulp, and therefore most packaging board, printing paper and tissue of any quality.

What makes the process economically viable is what happens to the waste. The spent liquor, carrying the dissolved lignin, is concentrated and burned in a recovery boiler: the organic fraction supplies most of the mill's energy, and the inorganic chemicals are recovered from the smelt and returned to the digester. A modern kraft mill is close to energy self-sufficient and recovers upwards of ninety-five per cent of its cooking chemicals, which is why a process that dissolves half its feedstock is nonetheless the dominant one.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

  • Wood — material · as chips, cooked until the lignin dissolves and the cellulose fibres separate
  • Sodium hydroxide — compound · with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor
  • Water — compound · and the pulp and paper industry is among the largest industrial water users there is

produces

  • Paper — material · via pulp — most chemical pulp and therefore most quality paper
  • Resin — material · as tall oil rosin from the black liquor — a by-product of making paper, and now a larger source than tapping

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Kraft pulping back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Kraft pulping → takes as input (with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor) → Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is produced by (in the solar route — the same process, read from the other end) → Brine evaporation → takes as input (the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from) → Seawater
  • Kraft pulping → takes as input (and the pulp and paper industry is among the largest industrial water users there is) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
  • Kraft pulping → takes as input (as chips, cooked until the lignin dissolves and the cellulose fibres separate) → Wood
  • Kraft pulping → takes as input (with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor) → Sodium hydroxide → is sourced from (electrolysis of brine, which produces chlorine in fixed proportion) → Halite
  • Kraft pulping → takes as input (with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor) → Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (as the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
  • Kraft pulping → takes as input (with sodium sulfide, breaks the lignin into fragments soluble in the alkaline liquor) → Sodium hydroxide → is produced by (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite

These are the most distinct paths back. Kraft pulping can be traced through others besides.

Downstream — what it becomes

  • Kraft pulping → produces (as tall oil rosin from the black liquor — a by-product of making paper, and now a larger source than tapping) → Resin → is used in (as the flux in rosin-cored solder wire: it strips the oxide off copper at soldering temperature so the solder can wet the metal, and the smell of soldering is the smell of pine) → Electronics manufacture → is associated with (and the supply chain it produced is the deepest and most concentrated in the world economy) → The semiconductor era complete chain
  • Kraft pulping → produces (via pulp — most chemical pulp and therefore most quality paper) → Paper → is used as (cartons and cases, and the material most plastic packaging is now being asked to justify itself against) → Packaging
  • Kraft pulping → produces (as tall oil rosin from the black liquor — a by-product of making paper, and now a larger source than tapping) → Resin → is a source for (the same material, forty million years later: the volatiles gone and the remainder cross-linked into a network that no longer dissolves) → Amber → is used as (worked since the Upper Palaeolithic, and traded the length of Europe before anyone smelted a metal) → Jewellery and adornment
  • Kraft pulping → produces (as tall oil rosin from the black liquor — a by-product of making paper, and now a larger source than tapping) → Resin → is used as (as a tackifier, and as the birch-bark tar that hafted stone tools 200,000 years ago — the oldest manufactured adhesive there is) → Adhesion and bonding