Understanding the Kraft Process in Paper Production
The Kraft process, also referred to as the sulfate process or as
Kraft pulping, is a process used to produce paper pulp. The purpose
of the process is to remove the lignin from the cellulose in the
wood fibers that are placed in side larger pressure vessels that
are called digesters. In order to accomplish this, the process utilizes
sodium sulfide and caustic sodium hydroxide.
The Kraft process was developed in 1884 by Carl Dahl, with
the name of the process coming from the German word kraft,
meaning strong. The first kraft mill in the United States was not
built until 1911 and was located in Pensacola,
Florida. Today, the process is used in approximately 80% of
paper production. Those companies using the Kraft process are easy
to recognize by their strong, offensive smell. This is because one
of the byproducts of the process is hydrogen sulfide gas and other
sulfur gas compounds.
A Cyclical Process
The Kraft Process is a cyclical, self-sustaining process. As a
result of the process, a byproduct called black liquor is formed.
It is a combination of the removed lignin, water, and chemicals
used in the extraction process. The black liquor is concentrated
through evaporation after it is produced and then burned in order
to generate high-pressure steam for the mill processes used to make
paper. The inorganic portion of the black liquor then regenerates
the sodium sulphide and sodium hydroxide that is used for pulping.
When softwood, such as conifers, is used in pulping, a soaplike
substance is collected after the process. This soap is then acidified
and used to produce tall oil. Tall oil is a source of fatty acids,
resin acids, and other chemicals. When broken down, certain components
of tall oil called rosin are used in some rubbers, adhesives, inks,
and emulsfiers. Other components, called the pitch, are used as
an adhesive, as a binder in cement, and as an emulsifier for asphalt.
Comparing the Kraft Process to the Sulfite Process
process is another process sometimes used to remove the lignin
from plant fibers. The two processes are different, however, because
the Kraft process uses an alkaline solution. This is more desirable
to paper manufacturers because it is less corrosive and damaging
to the equipment. In addition, the Kraft process is more efficient
at removing the lignin and results in a stronger fiber. On the other
hand, the fiber created with the Kraft process is rougher and contains
more impurities than that created with the sulfite process. This
makes paper made with the Kraft process more difficult to bleach.
Moving the United States Ahead
process played a large roll at moving the United States to the
forefront in the pulp and paper industry. First, the process was
capable of pulping pine trees, which were and still are found in
abundance in the United States. In addition, its cyclical nature
made it possible to recover great amounts of energy. Without the
invention of the Kraft process, the United States would most likely
have never developed into the paper industry giant that it is today.