Pulp and Paper Mills

Lime in pulp and paper mills

Lime is used in several different applications within pulp mills.

Here are the two most common applications:

The first is for the sulfite-pulping process. This process uses acid to dissolve the glue (lignin) that holds the fibers (cellulose) together. When acidic water discharged from this process is neutralized, lime is the reagent of choice in order to safely elevate the pH of the effluent stream prior to discharge.

The second is the Kraft-pulping process. This, the most modern means of making pulp, uses caustic soda (NaOH), among other chemicals, to dissolve the lignin. Kraft mills have two internal loops in their system working together to generate caustic soda. The first involves the caustic soda itself. Freshly made caustic soda is pumped to large chambers where it is mixed with wood chips under heat and pressure. The caustic soda attacks the lignin and, in the process, is converted to sodium carbonate (Na2CO3). This sodium carbonate is then pumped to a large vessel that holds slaked lime (Ca(OH)2). The two react to form caustic soda and calcium carbonate (CaCO3). The caustic soda is pumped off to once again mix with the wood chips, and the calcium carbonate is conveyed to a kiln where it is heated to convert it to lime (calcium oxide or CaO). This lime is then mixed with water, forming slaked lime, and the sodium carbonate returning from the pulping process is mixed with it forming caustic soda; and the process is started all over again.

Graymont supplies a significant amount of tonnage of quicklime for both processes. It is critical that the supplied quicklime be as pure as possible in order to provide maximum benefit with minimal impact from trace elements.

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