Kraft mills can turn dissolved wood organics into cash by recovering tall oil and turpentine—using gravity, acid, vacuum, and smart condensers—and the payoff runs from tens of dollars per ton of pulp to multi‑percent revenue bumps with short paybacks.
In the Kraft (sulfate) process, the same cook that makes pulp also liberates a pair of valuable by‑products: tall oil from “soap” in black liquor and “sulfate turpentine” from digester vapors. With crude tall oil (CTO) pricing around ~$2,700–$3,000 per tonne by mid‑2025 (chemanalyst.com) and U.S. turpentine at ~$3,345/ton in Q1 2025 (chemicalindustrypricingreports.wordpress.com), mills that capture these streams routinely add meaningful dollars per ton of pulp.
The gains are not theoretical. Typical tall oil yields on resinous softwoods run on the order of 30–50 kg CTO per OD (oven‑dry) tonne of pulp, translating to roughly ~$80–$150 per ton of pulp at recent prices (resourcewise.com; chemanalyst.com). Turpentine adds more: even modest yields on the order of 10–30 kg per ton of pulp imply ~$60–$300 per ton of pulp at ~$6–$10/kg market values (U.S. ~$3,345/ton; China ~$2,020/ton; Brazil ~$1,740/ton) (U.S.; China; Brazil), though actual turpentine yields are often lower than tall oil.
Policy tailwinds are real: EU and U.S. programs (EU RED II, CORSIA, U.S. IRA) classify tall oil as a qualifying advanced biofuel feedstock, lifting demand for HVO/renewable diesel and SAF and helping support CTO pricing (resourcewise.com; chemanalyst.com).
Kraft by‑products and definitions
In Kraft pulping, softwood chips release organics into “black liquor,” including soap (sodium salts of fatty and resin acids) and volatile terpenes. Separating and acidifying the soap yields crude tall oil (CTO), while condensing the digester’s relief vapors produces “sulfate turpentine” (pulpandpaperonline.com). CTO is typically distilled further into market fractions; turpentine can be sold as is or refined.
Soap separation parameters (black liquor)
After washing, mills concentrate black liquor to roughly 30% solids and remove tall‑oil soap via gravity decanters or skim tanks, because the soap (less dense) floats on the liquor. Tests identify an optimal 28–32% dry solids window for skimming—soap droplets coalesce and rise most efficiently at ~28–32% solids (doczz.net). Industry reviews echo that “the optimal dry solids content for soap skimming is 28–32%” (doczz.net).
Some mills dose flocculants or natural colloids to agglomerate fine soap micelles and improve gravity separation (researchgate.net). Patented methods even deploy ultrafiltration membranes to concentrate residual soap and recycle it to the skimmer, though gravity decanters remain standard (researchgate.net).
Acidulation chemistry and CTO composition
Collected soap is acidulated—typically with sulfuric acid (H₂SO₄)—to convert sodium soaps (RCOO–Na⁺) to free acids, yielding CTO and a spent aqueous phase rich in Na₂SO₄ (gypsum) and water. The core reaction is: RCOONa + H₃O⁺ → RCOOH + Na⁺ + H₂O (researchgate.net). If calcium is present, the step can generate CaSO₄; to reduce sulfuric acid use, some processes opt for sodium sesquisulfate (researchgate.net).
Crude tall oil is a dark, viscous liquid, typically ~40–50% resin acids and ~40–50% fatty acids, with 5–15% unsaponifiable neutrals (researchgate.net; doczz.net). One study states CTO is ~38–53% fatty acids and ~38–53% resin acids (researchgate.net).
Vacuum distillation and fraction uses
CTO is typically vacuum distilled into five cuts: “heads” (light volatiles), a fatty‑acid fraction, distilled tall oil (DTO, a mid‑boiling mixture), a rosin (high‑purity resin acids), and a heavy pitch residue (studylib.net). The heads often contain light hydrocarbons and any residual turpentines; small overhead heads cuts may be redistilled to recover terpenes or turpentines.
The fatty‑acid fraction (TOFA) goes into soaps/detergents, and the rosin fraction feeds adhesives, printing inks, rubber compounding, or paper sizing; pitch can be burned as fuel or further processed (e.g., to sterols) (studylib.net). Quality control (e.g., density, acid number) and multi‑stage trains (columns, condensers, decanters) drive purities. Value‑add options include “de‑warming” or hydrogenating rosin to reduce color, and esterifying or hydrogenating CTO fatty acids—including catalytic routes to biodiesel (researchgate.net). Some mills blend or fractionally crystallize rosin for paper sizing. Where markets soften, CTO is sometimes used directly as boiler fuel or as a low‑cost raw material (e.g., for soap making) (researchgate.net).
Turpentine recovery train (digester relief)
During the digester blow (pressure relief), approximately 80–85% of the wood’s inherent turpentine vaporizes and is routed to a dedicated recovery system (pulpandpaperonline.com). Relief gases first pass through a separator to knock out entrained fibers/black liquor droplets (pulpandpaperonline.com) and then a condenser.
The condenser’s biphasic condensate (water–turpentine) flows to a decanter. Because turpentine has a specific gravity of ~0.82–0.88, it immiscibly floats and is skimmed off, while the aqueous underflow exits as liquid effluent (pulpandpaperonline.com). Decanter designs provide sufficient residence time—typically at ~110–120°F—to ensure phase separation (pulpandpaperonline.com), with level/flow adjustments handled by specialized internal arrangements (pulpandpaperonline.com). Noncondensed volatiles (e.g., CO₂, H₂S) are sent on to the recovery boiler, and overall the system typically recovers ~80–85% of turpentine generated (pulpandpaperonline.com).
Optional turpentine refining steps
Crude sulfate turpentine carries sulfur compounds and heavier pitch oils from the Kraft environment. While often sold “as is” as a solvent or raw material, pilot‑scale treatments—washing with base or oxidants followed by fractional distillation—can lower sulfur and heavy tars and yield streams enriched in α/β‑pinene or terpineol (bioresources.cnr.ncsu.edu). Many buyers use unrefined sulfate turpentine for paints/varnishes, adhesives, fragrances, or as a chemical feedstock (bioresources.cnr.ncsu.edu).
Market context and price signals
Tall oil is a specialty chemical with tight supply—only ~187 Kraft mills are currently extracting CTO globally (resourcewise.com). Tall oil rosin alone was roughly US$1.1 billion in 2022 and is projected to reach ~$1.4 billion by 2030 (CAGR ~2.8%), with U.S. rosin at ~$307 million in 2022 and China forecast to grow fastest (globenewswire.com; globenewswire.com). The adhesives segment is expected to reach ~$574 million by 2030 (globenewswire.com).
Policy incentives for advanced biofuels (EU RED II, CORSIA, U.S. IRA) are stimulating CTO demand for HVO/renewable diesel and SAF (resourcewise.com). This “regulatory pull” coincides with mid‑2025 North American FOB CTO prices of ~US$2,700–$3,000 per tonne (chemanalyst.com), and with the yield range (30–50 kg CTO per OD tonne of pulp—roughly 30–70% of the extractives) it implies ~$80–$150 per ton of pulp revenues (resourcewise.com).
Turpentine markets are smaller but buoyant and volatile: Q1 2025 prices averaged ~$3,345/ton in the U.S., ~$2,020/ton in China, and ~$1,740/ton in Brazil (U.S.; China; Brazil). Even modest yields (on the order of 10–30 kg per ton of pulp) can add ~$60–$300 per ton of pulp, though actual yields are often lower than tall oil.
Mill profitability and recovery boiler impacts
At pulp capacities above ~100 kt/year, building or upgrading tall‑oil facilities is described as almost always profitable; one industry analysis puts new plant payback at ~1–3 years and modernization at ~2–5 years (pulp-paperworld.com; pulp-paperworld.com). Piping soap to an acidulator also frees recovery‑boiler capacity: rather than burning dilute soap, the boiler can fully combust black liquor, often enabling a pulp‑rate increase (pulp-paperworld.com).
Industry commentary is blunt: “selling tall oil brings more income… than selling the soap as such,” and separating soap “improves overall efficiency” (pulp-paperworld.com). For mills that had been burning soap, diverting it to CTO can yield both higher revenues and extra internal steam/electricity generation (pulp-paperworld.com), with elevated CTO prices (~$2.7–3.0k/t) reinforcing the economics (chemanalyst.com).
Regional outlooks and scale effects
In emerging bioeconomies—including Indonesia—these dynamics are similar. Major Indonesian producers (APP, APRIL, etc.) generate millions of ADT (air‑dry tonnes) of pulp annually. Even without local regulations specific to tall oil, global biofuel incentives and strong export markets make by‑product recovery economically sensible. Bulk tall oil and turpentine from Indonesian mills likely follow international practice: mechanical skimming, acidulation, and decantation producing CTO and sulfate turpentine of comparable quality and value (pulp-paperworld.com; chemanalyst.com).
For large mills, effectively recovering and purifying tall oil and turpentine—through gravity skimming at ~28–32% solids, chemical acidulation, vacuum fractionation, blow‑tank condensation, and decantation—can add tens to hundreds of millions in product value, commonly a multi‑percent revenue increase with payback measured in a few years (doczz.net; pulpandpaperonline.com; pulp-paperworld.com; chemanalyst.com).
