Liquor in papermaking? Yes, but not the liquor in a glass. The most common process in the pulp and paper industry is kraft pulping, an efficient process that reuses nearly all the chemicals and byproducts from the wood to power the process itself. When cooking wood to break down into pulp, a recovery boiler burns and collects liquid runoff from the wood in order to generate power, burn off organic waste, and help treat more wood for easier pulping. This runoff is called liquor and occurs in different varieties at different stages of the process.

Wood chips are cooked using chemicals and heat to make it easier to extract the fibers necessary to make paper goods. Before cooking begins, however, the wood chips are treated with a chemical byproduct called white liquor that breaks down the wood. This initial cooking stage retains about half of the wood, which is now pulp. The rest dissolves and mixes with used chemicals to become the first of the liquors, a liquid stream called black liquor.

1. Black Liquor

Black liquor comes in two forms: solid black liquor, which can be used for fuel, and weak black liquor, a liquid that can be concentrated to produce more fuel. This fuel is substantial to the process; over 1.3 billion tons of weak black liquor are processed per year, and over 200 million tons of black liquor solids are burned in recovery boilers. This makes black liquor the fifth most important fuel in the world and the most important biofuel. The pulping process depends on it to remain efficient, save costs, and reduce its environmental impact.

2. Green Liquor

When black liquor is burned for energy, chemicals settle in the bottom of the recovery boiler and dissolve in water. This produces green liquor which is used to further break down the wood pulp into a usable product. Then the runoff from this is mixed with lime to produce white liquor.

3. White Liquor

This is perhaps the most crucial substance used in the kraft pulping process. White liquor is the active ingredient that makes the initial cooking work. It’s a strongly alkaline substance that makes it possible to begin breaking down the wood chips, which are saturated with this highly caustic substance and steamed to soften them before cooking begins.

Pulping wood depends on this continued cycle. Chemicals released and washed off the wood are boiled to concentrate them and reused to treat the wood, so that it can break down further and further into a usable product. Maintaining this cycle efficiently by making the most of these substances and the energy they produce is the process that paper mills rely on to stay in operation.