Black liquor is a byproduct of the Kraft process, (also known as Kraft pulping or sulfate process) during the production of paper pulp. Wood is decomposed into cellulose fibers (from which paper is made), hemicellulose and lignin fragments. Black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process.
Early kraft pulp mills discharged black liquor to watercourses. Black liquor is quite toxic to aquatic life, and causes a very dark "Coca-Cola" color in the water. The invention of the recovery boiler by G.H. Tomlinson in the early 1930s, was a milestone in the advancement of the kraft process. Most kraft pulp mills adopted this technology to recover and burn much of the black liquor, generating steam and recovering the "cooking chemicals" (sodium hydroxide and sodium sulfide used to separate lignin from the cellulose fibres needed for papermaking). In fact this enhancement not only managed to deal with pollution load reduction and chemical recovery increase but also made it possible for kraft pulp mills to become self sufficient energetically. In efficient recovery boiler mills the amount of energy is more than necessary to run the process and the surplus can be sold. Approximately 7 tonnes of black liquor (15% solids by weight of which 10% is inorganic and 5% is organic) is produced in the manufacture of one tonne of pulp. Pulp mills producing 1,000 tonnes of pulp per day and more are common. If only 10% of the black liquor generated in pulping escapes the recovery process, then kraft mills can be highly polluting. By 2000, the better kraft mills recovered 99.5% or more of the black liquor, and purified the remainder in biological treatment plants, reducing the environmental impact of the waste waters below the level of scientific significance, except perhaps in very small streams. Even in the 21st century, some tiny kraft mills remained (producing at most a few tons of pulp per day) that discharged all black liquor. However, these are rapidly disappearing. Some kraft mills, particularly in North America, still recovered under 98% of the black liquor in 2007, which can cause some environmental issues, even when biologically treated. The general trend is for such obsolete mills to modernise or shut down.
T.G. Northcote and G.F. Hartman [editors] for Blackwell Science, Oxford. Chapter 12