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 (NaOH), among other chemicals,
to dissolve the lignin. Kraft mills have two internal loops
in their system working together to generate caustic. The
first involves the caustic itself. Freshly made caustic is
pumped to large chambers where it is mixed with wood chips
under heat and pressure. The caustic 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
and calcium carbonate (CaCO3). The caustic is pumped off to
once again mix with the wood chips and the calcium carbonate
is conveyed to a kiln where it is heated sufficiently 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; 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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