Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Chemical treatment after start or completion of mechanical...
Reexamination Certificate
2001-08-14
2004-08-03
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Chemical treatment after start or completion of mechanical...
C162S030100, C162S065000, C162S068000, C162S072000, C162S076000, C162S077000, C162S079000, C162S080000, C162S090000
Reexamination Certificate
active
06770168
ABSTRACT:
The present invention relates to a substantially sulfur-free process for the production of a chemical pulp from lignocellulosic material and the recovery of chemicals used in said, process. More particularly, the present invention is related to a process for the production of a chemical pulp in which comminuted lignocellulosic material is subjected to oxygen delignification in the presence of an alkaline buffer solution and chemical substances are recovered from the spent liquor and circulated in the process.
BACKGROUND OF THE INVENTION
Current industrial processes for pulping wood and other sources of lignocellulosic material such as annual plants, and processes for bleaching the resultant pulp, have evolved slowly over many decades. To remain competitive, the pulp and paper industry must seek more cost-effective alternatives to the existing capital-intensive technology for manufacturing of pulp. New investment strategies have to be formulated and implemented to increase shareholder value.
Environmental issues have recently come in focus and in spite of significant advances in this area more can be done to improve the environmental performance of pulp mills. Even the best of current technology is unable to completely suppress the odors emitted in kraft mills, or to completely eliminate the emission of gaseous pollutants and COD compounds associated with chemicals recovery and bleaching. The disclosure of new sulfur-free chemicals and more selective delignification methods combined with efficient recovery systems can lead to substantially better returns for the pulping industry along with environmental benefits.
Pulping of wood is achieved by chemical or mechanical means or by a combination of the two. In thermomechanical pulping (TMP), the original constituents of the fibrous material are essentially unchanged, except for the removal of water soluble constituents. The fibers are, however irreversibly degraded and TMP pulps cannot be used for paper products with high strength demand. In chemical pulping processes the objective is to selectively remove the fiber-bonding lignin to a varying degree, while minimizing the degradation and dissolution of the polysaccharides.
Still stronger pulp is obtained in somewhat lower yields by treating wood chips or other cut-up raw material with chemicals before refining. This type of pulp is called chemical thermomechanical pulp (CTMP). When larger amounts of chemicals are used, but yet insufficient to separate the fibers without refining, the pulp is called chemi-mechanical pulp (CMP).
If the ultimate purpose of the pulp is the preparation of white papers, the pulping operations are followed by further delignification and pulp brightening in a bleach plant. The properties of the end products of the pulping/bleaching process, such as papers and paperboards, will be determined largely by the wood raw material and specific operating conditions during pulping and bleaching.
A low lignin pulp produced solely by chemical methods is referred to as a full chemical pulp. In practice, chemical pulping methods are rather successful in removing lignin. However, they also degrade a certain amount of the polysaccharides. The yield of pulp product in chemical pulping processes is low relative to mechanical pulping, usually between 40 and 50% of the original wood substance, with a residual lignin content on the order of 2-4%. The resulting pulp is occasionally further refined in a bleach plant to yield a pulp product with a very low lignin content and high brightness.
In a typical chemical pulping process, wood is physically reduced to chips before it is cooked with the appropriate chemicals in an aqueous solution, generally at elevated temperature and pressure. The energy and other process costs associated with operation at elevated temperatures and pressures constitute a significant disadvantage for the traditional pulping processes.
The two principal chemical pulping processes are the alkaline kraft process and the acidic sulfite process. The kraft process has come to occupy a dominant position because of advantages in wood raw material flexibility, chemical recovery and pulp strength. The sulfite process was more common up to 1940, before the advent of the widespread use of the kraft process, although its use may increase again with the development of new recovery technologies with a capability to split sulfur and sodium chemicals.
Although the purpose of delignification or chemical pulping processes is to significantly reduce the lignin content of the starting lignocellulosic material, the characteristics of the individual processes chosen to achieve the objective can differ widely. The extent to which any chemical pulping process is capable of degrading and solubilizing the lignin component of a lignocellulosic material while minimizing the accompanying degradation or defragmentation of cellulose and hemicellulose is referred to as the “selectivity” of the process.
Delignification selectivity is an important consideration during pulping and bleaching operations where it is desired to maximize removal of the lignin while retaining as much cellulose and hemicellulose as possible. One way of defining delignification selectivity in a quantitative fashion is as the ratio of lignin removal to carbohydrate removal during the delignification process. Although this ratio is seldom measured directly, it is described in a relative manner by yield versus Kappa number plots.
Another way of defining selectivity is as the viscosity of the pulp at a given low lignin content. Viscosity, however, can sometimes be misleading in predicting pulp strength properties, in particular for modem oxygen-based chemical delignification processes.
The classical methods described above for the deligniflcation or pulping of lignocellulosic materials, although each possesses certain practical advantages, can all be characterized as being hampered by significant disadvantages. Thus, there exists a need for delignification or pulping processes which have a lower capital intensity, lower operation costs, either in terms of product yield of the process or in terms of the chemical costs of the process; which are environmentally benign; which produce delignified materials with superior properties; and which are applicable to a wide variety of lignocellulosic feed materials. Such processes should preferably be designed for application in existing pulp mills using existing equipment with a minimum of modifications.
It is known in the prior art that cellulose pulp can be manufactured from wood chips or other fibrous material by the action of oxygen in an alkaline solution. However, the commercial use of oxygen in support of delignificafion today is limited to final delignification of kraft or sulfite pulps.
The oxygen pulping methods considered.in the prior art for the preparation of full chemical pulps can be divided in two classes: two-stage soda oxygen and single stage soda oxygen pulping. Both single stage and two stage processes have been extensively tested in laboratory scale. In the two stage process the wood chips are cooked first in an alkaline buffer solution to a high kappa number after which they are mechanically disintegrated into a fibrous pulp. This fibrous pulp with a high lignin content is further delignified with oxygen in an alkaline solution to give a low kappa pulp in substantially higher yields than obtained in a kraft pulping process.
The single stage process is based on penetration of oxygen through an alkaline buffer solution into the wood chips. The alkaline solution is partly used to swell the chips and to provide a transport medium for the oxygen into the interior of the chip. However, the main purpose of the alkaline buffer solution is to neutralize the various acidic species formed during delignification. The pH should not be permitted to drop substantially below a value of about 6-7. The solubility of the oxygen in the cooking liquor is low and to increase solubility a high partial pressure of oxygen has to be applied.
There are a number of significant potentia
Alvo Steve
Kiram AB
Young & Thompson
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