Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Treatment with particular chemical
Reexamination Certificate
2001-05-01
2004-03-09
Alvo, Marc S. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Treatment with particular chemical
C162S071000, C162S072000, C162S076000
Reexamination Certificate
active
06702921
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to pulp and paper making, and more particularly to the bleaching and delignification of pulp.
BACKGROUND OF THE INVENTION
There are three major types of pulping methods known in the Pulp and Paper Industry. The first is Chemical and the second is Mechanical and the third is a Combination of Chemical and Mechanical. Methods to enhance the effectiveness of all three types of pulping methods are always desirable.
In Chemical pulps, sufficient lignin is dissolved to allow the fibers to separate with little, if any mechanical action. However, a portion of the lignin remains with the fiber and an attempt to remove this during digestion would result in excess degradation of the pulp. The degradation is a depolymerization of the cellulose and is measured by determining the viscosity of the cellulose dissolved in special solvents. For this reason from about 3 wt. % lignin to about 4 wt. % lignin is normally left in hardwood chemical pulps and from about 4 wt. % to about 10 wt. % lignin is normally left in softwood chemical pulps after the cook or digestion. The lignin is subsequently removed by bleaching in separate pulp mill operations if completely delignified and whitened pulps are to be produced.
The dominant Chemical wood pulping process is the kraft (“kraft” means strength in German) or sulfate process. In the kraft process, the alkaline pulping liquor or digesting solution contains about a 3 to 1 ratio of sodium hydroxide and sodium sulfide. A stronger pulp is obtained when sodium sulfide is used in combination with sodium hydroxide. This is to be compared with pulp obtained when sodium hydroxide is used alone, as it was in the original soda process. In the kraft process the wood is delignified (pulped) with a solution of sodium hydroxide and the addition of sodium sulfide is beneficial for pulping as well. Key advantages of the kraft process is its great adaptability of pulping many different species of wood and yielding pulps that may be used for a variety of applications.
Another type of Chemical pulping is the “sulfite process”. The sulfite process has several advantages over the kraft process. These advantages include improved yield (45-55%), lower cost cooking chemicals, higher brightness pulps and more easily bleached pulps. However, the sulfite method also has two distinct disadvantages: only a limited number of species can be pulped and the pulps produced are distinctly weaker than those made using the kraft or sulfate process.
In Mechanical pulping, pulp is made predominantly using mechanical methods. The fundamental criteria used in assessing the quality of mechanical pulp is the amount of energy expended per unit of production. Because this energy is difficult to quantify, pulp freeness is most commonly used as a process control parameter. Generally, the more the energy expenditure the lower the freeness of the pulp.
The first step in the Mechanical pulping process is the grinding or refining of wood.
The Stone Groundwood (SGW) process involves making pulp by pressing logs and chips against an abrasive rotating surface. Many years ago the grinding surface used was an actual stone. In current practice specifically designed “artificial pulp stones” are available for the grinding.
A Pressurized GroundWood (PGW) process is where the grinding operation is completely pressurized.
Another type of Mechanical pulping is Refiner Mechanical Pulp (RMP) featuring atmospheric refining with no pretreatment of the wood chips. This process is one of the main mechanical pulping operations.
Thermo Mechanical Pulping (TMP) is a Mechanical pulping process that evolved from RMP and a high temperature process known as the Apslund process. Thermo Refiner Mechanical Pulping (TRMP) is a variation in Thermo Mechanical Pulping. In this case, the chips are preheated under pressure and refining is carried out at atmospheric pressure. TMP and TRMP pulps are stronger than either SCW or RMP pulps.
The third type of pulping process is a Combination of Chemical and Mechanical pulping processes. Two types of Combination processes are ChemiMechanical Pulping and SemiMechanical Pulping. There is little difference between ChemiMechanical Pulping (CMP) and SemiChemical Mechanical Pulping (SCMP). Both processes involve pretreatment of chips with chemicals, followed by mechanical refining. Four different chemical treatments are associated with these processes. These chemical treatments are: sodium hydroxide, sodium bisulfite, sodium sulfite, acid sulfite treatment. These processes are generally used on hardwoods. Chemical treatment weakens the fiber structure allowing fibers to rupture similarly to softwood that is mechanically pulped.
ChemiThermoMechanical Pulping (CTMP) appears to be a full evolution of all Mechanical pulping methods. It includes chemical treatment elevated temperature steaming followed by mechanical refining. This process can produce fibrous raw materials that vary considerably in properties depending upon process conditions such as sodium sulfite concentration, pH, temperature, etc.
With all pulps, “pulp brightness” is a measurement of the ability of a sample to reflect monochromatic (457 nm) light as compared to a known standard, using magnesium oxide (MgO). Since cellulose and hemicellulose are white, they do not contribute to pulp color. It is generally agreed that the lignin left in the pulp after pulping is responsible for the color the pulp. This unbleached pulp has an appearance similar to brown grocery bags. The chromophores are believed to be quinone-like materials formed from the lignin's phenolic groups through an oxidative mechanism. Additionally, heavy metal ions, especially iron and copper, can form colored complexes with the phenolic groups.
There are generally two approaches to removing color. The first uses a selective chemical to destroy the chromophores but not the lignin. The other approach is to use a bleaching system to remove the residual lignin. The bleaching of pulp is the standard method of removing color from pulp. It is current state of the art technology for all Chemical and Mechanical pulps to be bleached.
The bleaching of pulp and the subsequent delignification of pulp is usually performed in several stages, with each stage being referred to by a letter designation. Note, that although all pulps are bleached, only Chemical pulps are delignified using oxygen treatment.
The following table briefly describes the most common stages in a “typical” bleaching process. Note that the stages captured in this table are not necessarily in the order that they are practiced. For example, oxygen delignification is typically never the last step in the process as oxygen delignification leaves the pulp yellowish in color. That is why oxygen delignification is followed by some level of bleaching.
Stage
Description
C-chlorination
Reaction with Cl
2
in an acidic medium
E-Extraction
Dissolution of chlorination reaction products
or
with sodium hydroxide
E
O
adding oxygen with the sodium hydroxide to
or
improve delignifiication and lower the use of
chlorine and chlorine dioxide
E
OP
adding oxygen and peroxide with the sodium
hydroxide to improve delignification and lower
the use of chlorine and chlorine dioxide
H-Hypochlorite
Reaction with sodium hypochlorite in alkaline
medium
D-Chlorine Dioxide
Reaction with ClO
2
in an acidic medium
P-Peroxide
Reaction with peroxides in an alkaline medium
O-Oxygen
Reaction with O
2
at high pressure in an alkaline
medium. Usually used prior to chlorine as a
delignification step.
D
C
or C
D
Mixture of chlorine and chlorine dioxide
Usually the chlorination and extraction stages are carried out in sequence, first chlorinating the lignin compounds and then solubilizing them in the alkaline extraction stage. This is similar to the oxygen stage in that the objective is exclusively to delignify the pulp.
Five or six stages are needed to produce a “full bleach” brightness level of 89 to 91% MgO. Most commonly these stages, in order are CEDED, CEHDED and OCEDED. A brightness of 65% MgO can
Duggirala Prasad Y.
Salmen Kristine S.
Shevchenko Sergey M.
Alvo Marc S.
Breininger Thomas M.
Brumm Margaret M.
Ondeo Nalco Company
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