A pulp mill is a manufacturing facility that breaks down wood or other plant material into a mass of fibers called pulp. That pulp becomes the raw ingredient for paper, cardboard, packaging, and a growing number of bio-based products. Some pulp mills operate as standalone facilities that ship bales of dried pulp to paper mills elsewhere. Others are integrated mills, where pulping and papermaking happen on the same site, with logs going in one end and finished paper coming out the other.
How Wood Becomes Pulp
The process starts in the wood yard, where logs are debarked, fed through chippers, and screened to produce wood chips of a uniform size. These chips are the starting material for every pulping method. What happens next depends on whether the mill uses a chemical or mechanical approach, but the goal is always the same: separate the cellulose fibers in wood from the natural glue that holds them together, a rigid polymer called lignin.
The dominant method worldwide is the Kraft process, also called the sulfate process. Wood chips are loaded into a massive pressure vessel called a digester and cooked at around 170°C for roughly two hours in a solution of sodium hydroxide and sodium sulfide. This hot alkaline bath dissolves the lignin and frees the cellulose fibers. What drains out of the digester is a dark liquid (called black liquor) carrying the dissolved lignin and spent chemicals, and a soft mass of fibers that will become pulp.
After cooking, the pulp goes through washing and screening stages that remove residual chemicals and any fiber bundles that didn’t fully separate. At this point the pulp is brown, roughly the color of a grocery bag or shipping box. If the end product needs to be white, the pulp moves to bleaching.
Chemical vs. Mechanical Pulping
Chemical pulping, like the Kraft process, dissolves most of the lignin and produces strong, flexible fibers. The tradeoff is yield: because the lignin is removed, you get less pulp per ton of wood. Mechanical pulping takes the opposite approach. Logs or chips are physically ground against rotating stones or pushed between steel discs that shred the wood into fibers. This keeps nearly all the original material, including the lignin, so the yield is much higher. But the process is energy-intensive, and the resulting fibers are shorter, stiffer, and weaker.
That difference in fiber quality determines what each type of pulp is used for. Chemical pulp goes into products that need strength and durability: office paper, packaging board, tissue. Mechanical pulp works well for products with a short lifespan, like newsprint and advertising flyers, where brightness and bulk matter more than long-term strength. There are also hybrid approaches (called semi-chemical or chemi-mechanical pulping) that use a mild chemical treatment before mechanical refining to get a compromise between yield and fiber quality.
Why Wood Species Matter
Pulp mills choose their wood carefully. Softwoods like pine, spruce, and fir have long fibers, typically 3 to 4 millimeters, which create more contact points between fibers and generally produce stronger paper. Hardwoods like eucalyptus, birch, and aspen have shorter fibers, usually around 1 to 2 millimeters, which yield smoother, more opaque sheets. Most mills blend the two types to balance strength with print quality. Some mills in tropical regions use fast-growing plantation eucalyptus almost exclusively, while Scandinavian and North American mills rely heavily on spruce and pine.
Wood isn’t the only option. Pulp can also be made from bamboo, sugarcane bagasse, straw, hemp, and other plant fibers. These non-wood sources are more common in regions where forestry resources are limited.
Bleaching: From Brown to White
Unbleached pulp still contains small amounts of lignin, which absorbs light and gives the fibers their brown color. Bleaching removes or decolorizes this residual lignin through a series of chemical stages.
Nearly all modern bleached pulp uses a method called Elemental Chlorine Free (ECF) bleaching, which relies on chlorine dioxide and hydrogen peroxide rather than elemental chlorine gas. This shift virtually eliminated the formation of dioxins and furans, toxic compounds that were a serious concern with older bleaching technology. A smaller share of mills use Totally Chlorine Free (TCF) bleaching, which substitutes oxygen, ozone, and hydrogen peroxide. TCF was expected to become the industry standard, but ECF caught up in environmental performance while producing brighter, stronger fibers at lower energy cost. Today, ECF is the dominant bleaching technology worldwide.
The Recovery Cycle
One of the most distinctive features of a Kraft pulp mill is its chemical recovery system. The black liquor that drains from the digester is far from waste. It contains both the dissolved lignin (which is energy-rich) and the expensive cooking chemicals the mill needs to reuse.
The black liquor is concentrated from about 15% solids to 75-80% solids through a series of evaporators, then burned in a recovery boiler. This accomplishes two things at once. The organic material (mostly lignin) combusts and generates steam, which the mill uses for electricity and process heat. Meanwhile, the inorganic chemicals collect at the bottom of the boiler as a molten smelt that gets dissolved, treated, and converted back into fresh cooking liquor. This closed loop means a modern Kraft mill recovers and reuses the vast majority of its chemicals, and the recovery boiler often generates enough energy to make the mill largely self-sufficient in power.
Before recovery boilers were invented in the early 20th century, black liquor was simply dumped into rivers. The recovery cycle transformed pulp mills from major polluters into surprisingly efficient operations, at least in terms of chemical and energy reuse.
Environmental Footprint
Pulp mills are heavy industrial facilities, and their environmental impact is significant even with modern controls. Water use is the most visible issue. The majority of pulp mills consume around 20 cubic meters of water per ton of pulp produced, though the range across the global industry runs from 10 to 300 cubic meters per ton depending on the age and efficiency of the operation. European regulators have set best-available-technology benchmarks aimed at keeping wastewater production near that 20 cubic meter mark.
Air emissions are the other major concern, particularly from Kraft mills. The cooking and recovery processes release sulfur compounds, including hydrogen sulfide and other reduced sulfur gases, that produce the distinctive rotten-egg smell communities near pulp mills know well. These sulfur spikes tend to be worst in early morning and late evening hours, when atmospheric conditions trap emissions close to the ground. Mills also emit particulate matter and volatile organic compounds. Modern mills capture and incinerate most of these gases, but eliminating the odor entirely remains difficult.
Beyond Paper: The Biorefinery Concept
Pulp mills have always produced more than just fiber. Kraft mills generate tall oil (a resinous byproduct skimmed from black liquor) and turpentine as side streams. What’s changing is that an increasing number of mills are positioning themselves as biorefineries, extracting higher-value products from the same wood that once only made pulp.
Lignin is at the center of this shift. Rather than burning all of it for energy, mills can precipitate lignin from black liquor and sell it as a raw material. Its applications are expanding rapidly: adhesive resins, activated carbon, carbon fibers, hydrogels, and even battery components. Lignin can also be broken down into platform chemicals like furfural, organic acids, and polyols that serve as precursors to bioplastics and hydrocarbon fuels. One of the most advanced examples is the Borregaard biorefinery in Norway, which produces ethanol, vanillin, lignin products, and biogas alongside conventional cellulose pulp. Tall oil is being upgraded into biodiesel at other facilities.
This biorefinery model doesn’t replace pulp production. It layers additional revenue streams on top of it, using material the mill was already processing. For an industry built around a single product for over a century, it represents a meaningful expansion of what a pulp mill actually is.