Green liquor is a hot, highly alkaline solution used in paper mills as a key step in recycling the chemicals that turn wood into pulp. It forms when molten salts from a recovery furnace are dissolved in water, producing a green-tinted liquid rich in sodium carbonate and sodium sulfide. The name comes simply from its color. Green liquor is not a finished product itself but an intermediate that gets converted into the actual cooking chemical (called white liquor) used to break down wood fibers.
How Green Liquor Forms
To understand green liquor, you need a quick picture of how a kraft pulp mill works. Kraft mills cook wood chips in a strong alkaline solution to dissolve the lignin that holds wood fibers together. That spent cooking solution, now called black liquor, is loaded with dissolved wood compounds and used-up chemicals. Rather than discard it, mills burn the black liquor in a recovery boiler. The organic material provides fuel, and the inorganic chemicals melt into a molten mass called smelt.
That smelt, glowing at extreme temperatures, drops into a dissolving tank filled with water (or a weak wash liquid). The result is green liquor. A typical dissolving tank operates at around 85°C (185°F), and the dissolved solids concentration usually falls in the range of 100 to 150 grams per liter.
What’s in It
The two main chemicals in green liquor are sodium carbonate (Na₂CO₃) and sodium sulfide (Na₂S). In one industrial sample, sodium carbonate measured about 46 g/L and sodium sulfide about 20 g/L (both expressed as sodium oxide equivalents). There is also some residual sodium hydroxide, around 21 g/L in the same sample. The solution is strongly alkaline, with the solid byproduct (called dregs) registering a pH between 10 and 14.
Beyond these main components, green liquor contains small amounts of impurities that mill operators call “non-process elements.” These include potassium, manganese, magnesium, iron, aluminum, zinc, and other metals that enter the system through wood, water, or equipment corrosion. They serve no useful purpose in papermaking and can interfere with pulping and bleaching if they accumulate.
Cleaning It Up: Clarification and Dregs
Before green liquor can move to the next step, those impurities need to come out. The insoluble particles settle or get filtered out as a sludge called green liquor dregs. Older mills use large sedimentation tanks (clarifiers) where gravity pulls the solids to the bottom. Newer mills have largely switched to filtration systems, which separate the dregs more efficiently and take up less space.
The dregs themselves are mostly a carbonate mineral called pirssonite, a combined sodium-calcium carbonate. They also concentrate non-process elements like manganese (over 1,300 mg/kg in one study of a semi-chemical mill) and zinc (around 740 mg/kg). Because of their high alkalinity and metal content, disposing of these dregs responsibly is an ongoing concern for the industry. Most end up in landfills, though researchers have explored using them as soil amendments or in construction materials.
From Green Liquor to White Liquor
The whole point of making green liquor is to regenerate white liquor, the sodium hydroxide and sodium sulfide solution that actually cooks wood chips. The conversion happens through a process called causticizing. Mill operators add quicklime (calcium oxide) to clarified green liquor, triggering a straightforward reaction: the sodium carbonate reacts with the lime to produce sodium hydroxide and a calcium carbonate precipitate, commonly known as lime mud.
The lime mud settles out and gets washed, dried, and burned in a lime kiln to regenerate quicklime, closing yet another loop. The remaining liquid, now rich in sodium hydroxide and still containing its sodium sulfide, is white liquor, ready to cook the next batch of wood chips. This circular system means a kraft mill recovers and reuses the vast majority of its pulping chemicals rather than constantly purchasing fresh ones.
Why It Matters in Papermaking
Green liquor sits at the heart of what makes the kraft process economically viable. Without this chemical recovery loop, mills would need to buy enormous quantities of sodium hydroxide and sodium sulfide for every batch of pulp, and they would lose the energy contained in black liquor’s organic compounds. The recovery boiler that produces the smelt actually generates a significant share of a mill’s total energy.
Getting green liquor chemistry right also directly affects pulp quality. If the dissolving tank runs at the wrong temperature or concentration, or if too many impurities carry over into white liquor, the cooking step becomes less effective. That can mean weaker fibers, higher chemical consumption, or problems in downstream bleaching. Mills monitor green liquor density and total titratable alkali continuously, using inline instruments to keep the process on target.
For anyone outside the paper industry, green liquor is one of those invisible industrial chemicals that plays a surprisingly large role. Kraft pulping accounts for roughly 80% of the world’s chemical pulp production, and every one of those mills runs a green liquor loop day and night to keep the process going.