Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
2001-03-02
2004-06-22
Nguyen, Dean T. (Department: 3629)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S063000, C162S066000, C162S027000, C162S028000
Reexamination Certificate
active
06752904
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the removal of lignin from lignocellulose containing material comprising treating the lignocellulose containing material prior to a high-yield pulping process with a delignifying gas comprising gaseous compounds comprising at least one oxygen atom.
BACKGROUND OF THE INVENTION
In high-yield processes, or mechanical pulping processes as they also are referred to, the objective is to disrupt the structure of the lignocellulosic containing material in fragments of variable dimensions by the use of mechanical treatment optionally in the presence of chemicals.
High-yield pulps are produced by grinding or refining of the lignocellulose containing material and thereby essentially all of the fibres constituents are retained in the pulp.
High-yield pulping processes encompasses grinding, screening and refining processes. In the grinding process, washed logs are forced against a rotating grinding disk to tear the lignocellulose containing material into small pieces which are then further processed by screening or refining to produce mechanical pulp. In the refining process, screened washed chips pass between rotating disks or a rotating and stationary disk to shred the lignocellulose containing material into pulp. The resultant pulp may then be further refined, screened and cleaned to produce the desired mechanical pulp.
One aspect of high-yield pulping is to rip or shred the lignocellulose containing material into fibres which consumes energy. This energy is measured as the power required to rotate the grindstone or the refiner disk(s).
A significant portion of the total energy used in high-yield pulping process is required to break or soften the lignin bonded to the cellulose and hemicellulose fibres. Weakening or breaking down the lignin structure prior to the grinding or refining process results in significant reductions in the power requirement to produce the finished high-yield (mechanical) pulp.
Energy, usually measured in terms of electrical power, is a major component in the cost of producing high-yields pulps. Thus, the pulp production cost increases directly with the cost of energy.
Therefore, one advantage of the present invention is to provide a pretreatment process that softens or breaks the lignin contained within the lignocellulose containing material resulting in a substantial reduction in the energy required to produce high yield pulp.
U.S. Pat. No. 4,172,006 refers to the pretreatment of wood chips with oxygen prior to adding a cooking liquor.
U.S. Pat. No. 4,750,973 relates to a process for reducing carbohydrate losses in sulphate pulping of wood using sodium hydroxides and sodium sulfide, wherein the wood is pretreated in presence of water with oxygen gas and nitrogen oxides.
GB 567774 discloses a process for the treatment of cellulosic raw material where wood chips are contacted with a aqueous solution of a wetting agent prior to subjecting the chips to a solution containing sodium chlorite thereby using sufficient acid to insure the liberation of chlorine dioxide.
WO 8908165 refers to a method for the pretreatment of wood chips with sulphur dioxide gas prior to alkaline deligification operations.
DE 1049220 discloses a method comprising subjecting wood chips to carbon acid before sulfite cooking.
JP 49020241 refers to a pulping process comprising the steps of reacting chlorine dioxide or a mixture consisting of chlorine dioxide and chlorine with wood chips in the presence of water soluble cellulose derivatives and thereafter removing inter alia the oxidised lignin by extraction.
U.S. Pat. No. 5,474,654 refers to a delignification process where chlorine dioxide gas is used on pulp obtained from pulping processes such as chemical kraft, sulfide or mechanical processes.
Common to all prior art documents cited above is that the pretreatment is carried out before a chemical pulping process.
U.S. Pat. Nos. 3,591,451 and 3,919,041 disclose the use of gaseous chlorine dioxide subsequent a pretreatment step which may be either mechanical, chemical or a combination thereof.
SUMMARY OF THE INVENTION
In accordance with the present invention it has surprisingly been found that a substantial reduction in the energy required to produce high yield pulp can be obtained by providing a process according to the claims. More specifically, the invention relates to a process for the removal of lignin from lignocellulose containing material comprising treating the lignocellulose containing material prior to a high-yield pulping process with a delignifying gas, wherein the gas comprises gaseous compounds comprising at least one oxygen atom.
DETAILED DESCRIPTION OF THE INVENTION
Suitable lignocellulose containing material used in the present invention can be any lignocellulose containing material derived from natural sources such as softwood, hardwood, gum, straw, bagasse and/or bamboo. The physical state of the lignocellulose containing material is not critical, however, a physical state providing a large surface area is preferred that maximises penetration of the delignifying gas and optionally processing chemicals. Suitably, the lignocellulose containing material is in the form of chips with a size which is governed by the process equipment and process parameters.
Further advantages of the present invention are apparent from the specification.
The lignocellulose containing material is suitably treated according to any method known to the skilled artisan which renders the diffusion of the delignifying gas within the fibre source to the lignin more effective such as steaming and/or evacuation.
According to the present invention the lignocellulosic material is treated with a delignifying gas comprising gaseous compounds comprising at least one oxygen atom. Suitably, the delignifying gas comprising chlorine dioxide, nitrogen oxides, carbon dioxide or sulphur dioxide or mixtures thereof. Nitrogen oxides include nitrogen monoxide, nitrogen dioxide, polymeric oxides and double molecules thereof, e.g. N
2
O
4
or N
2
O
3
.
The delignifying gas mixture is suitably a non-liquid containing gas.
According to one preferred embodiment the delignifying gas comprises chlorine dioxide. The chlorine dioxide containing gas may contain other gases such as nitrogen, oxygen, air or steam or mixtures thereof. The chlorine dioxide containing gas may also contain small amounts of chlorine, however, the gas is suitably substantially free from chlorine, preferably having less than 10% by volume, more preferably less than 1% by volume of chlorine.
The use of a delignifying gas such as a gas comprising chlorine dioxide overcomes several unsolved problems. Treatment of the lignocellulosic containing material with solutions containing for example chlorine dioxide is limited by the rate of chlorine dioxide diffusion through the solution to the fibre source followed by the diffusion of the chlorine dioxide within the fibre source to the lignin. The result is a slow delignification process that works primarily on the fibre source surface and an aqueous effluent stream containing chlorine dioxide, dissolved fibre components and chlorides, which is difficult to treat in an environmentally correct manner.
It has been found that a gas comprising chlorine dioxide does not have the diffusion barriers that limit the process when a solution is used. The gas passes rapidly and uniformly into the fibre source resulting in even delignification throughout the material. Furthermore, there are no aqueous effluent streams.
Preferably, the gas comprising chlorine dioxide is applied on lignocellulose containing material free from any surrounding aqueous solution. Preferably, the moisture content of the lignocellulose containing material is from about 30 weight % up to about 60 weight % based on oven dry material, more preferably from about 40 up to about 50 weight %.
The chlorine dioxide containing gas employed in the present invention is suitably produced using a chlorine dioxide generation process as described in the U.S. Pat. Nos. 4,770,868, 5,091,197 and 5,380,517
Bryant Patrick S.
Hutto Dwane S.
Sokol John C.
Akzo Nobel N.V.
Burke Michelle J.
Nguyen Dean T.
Parker Lainie E.
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