Black liquor is a dark, viscous liquid generated as a byproduct of the chemical pulping process used to manufacture paper. It is the spent cooking fluid remaining after wood chips are treated to separate cellulose fibers. Though initially a waste stream, black liquor is now recognized as a valuable resource fundamental to the sustainability of the modern pulp and paper industry. This byproduct contains dissolved organic wood material and inorganic cooking chemicals, which are recovered and reused in a closed-loop system. Utilizing black liquor allows many pulp mills to be nearly self-sufficient in their energy and chemical needs.
How Black Liquor is Generated During Pulping
The production of black liquor is linked to the Kraft pulping process, the dominant method for converting wood into pulp. This process chemically separates desirable cellulose fibers from unwanted wood components, primarily lignin. The initial step involves “cooking” wood chips in a highly alkaline solution known as white liquor.
White liquor is an aqueous solution of the active cooking chemicals, sodium hydroxide (\(\text{NaOH}\)) and sodium sulfide (\(\text{Na}_2\text{S}\)). This mixture is heated under high temperature and pressure within a large vessel called a digester. Under these conditions, lignin, hemicellulose, and other wood extractives are chemically broken down and dissolved into the cooking liquid.
The chemical action dissolves the lignin, which binds the wood fibers together. Once cooking is complete, the mixture is separated into two streams: the purified cellulose fibers (pulp) and the spent cooking liquid. This spent liquid, laden with dissolved organic material and residual chemicals, is the weak black liquor.
The Key Chemical Components of Black Liquor
Black liquor is a complex chemical mixture, typically starting as a dilute solution containing 15 to 20 percent total solids. The solids content is divided into two major categories: organic compounds and inorganic chemicals. The organic fraction, derived directly from the wood, generally accounts for about two-thirds of the total solids.
The largest organic component is lignin, modified and fragmented by the alkaline cooking process. Hemicellulose degradation products and other wood extractives also contribute to the organic matter. This substantial organic content gives black liquor its high fuel value, making it a source of renewable energy.
The inorganic fraction comprises the spent cooking chemicals, primarily sodium compounds. These include residual sodium sulfide and sodium hydroxide, along with sodium carbonate and sodium sulfate formed during the cooking reactions. The presence of these chemicals and dissolved wood components gives the liquor its characteristic dark color and high alkalinity.
Energy Generation and Chemical Recovery
The processing of black liquor serves the dual purpose of energy generation and chemical recovery. Since the weak black liquor stream is mostly water, it must first be concentrated to make it combustible. This is achieved using a series of multiple-effect evaporators, which remove large amounts of water to increase the solids content to between 65 and 80 percent.
This concentrated black liquor is then sprayed into a specialized piece of equipment called the recovery boiler. The organic components, primarily dissolved lignin, combust within the boiler, generating massive amounts of heat. This heat is captured to produce high-pressure steam, which powers turbines to generate electricity and supplies process heat for the mill operations. By burning its own byproduct, the mill becomes highly energy self-sufficient, often covering 60 to 80 percent of its total energy requirements.
The inorganic chemicals do not combust; instead, they melt and collect at the bottom of the recovery boiler as a molten salt mixture called smelt. This smelt, consisting mainly of sodium carbonate (\(\text{Na}_2\text{CO}_3\)) and sodium sulfide (\(\text{Na}_2\text{S}\)), is dissolved in water to form a solution known as green liquor. The green liquor is then clarified and sent to the causticizing plant to begin the chemical regeneration cycle.
In the causticizing process, calcium oxide (lime) is added to the green liquor. This converts the sodium carbonate back into the active cooking chemical, sodium hydroxide (\(\text{NaOH}\)). This regenerated solution, now called white liquor, is filtered and sent back to the digester for the next batch of wood chips. This intricate, closed-loop system drives the economic viability and environmental efficiency of Kraft pulping.