Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Including recovery of organic by-product
Reexamination Certificate
1998-12-03
2001-02-06
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Including recovery of organic by-product
C162S037000, C162S065000, C162S076000, C162S078000
Reexamination Certificate
active
06183597
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for producing a pulp from cellulose-containing material, the pulp itself and its application.
By pulp is meant a cellulose mash that is used in the production of chemical pulp, which, in turn, is used to make paper or cardboard. The cellulose-producing industry today is finding it increasingly difficult to meet the environmental standards and requirements imposed on it. In addition, it is no longer permitted in Germany, by virtue of the environmental regulations of 1990, to use conventional processes for recovering cellulose, such as the sulphite process. In Germany today the sulphite process is the only process wherein sulphur-containing digestion agents are used to dissolve the lignin, which serves as a binder, out of the cellulose-containing material.
One result of efforts to develop more environmentally-friendly processes for recovering cellulose is the so-called Acetosolv process, in which the digestion liquid contains at least 50 percent by weight of acetic acid, to which is added a very small quantity of hydrochloric acid. Following digestion, the recovered pulp is washed with caustic soda and, if required, with organic solvents, in order to more or less completely remove the lignin.
The disadvantages of this process are the relatively high consumption of acetic acid, the use of caustic soda, and the organic solvents that may be needed to wash the pulp.
In accordance with the stringent requirements of the paper industry, the extracted pulp is regularly bleached in a subsequent step, in order to keep the Kappa number at the most below 25. Even the Acetosolv process includes downstream bleaching with peroxide, with respect to which it should be pointed out that high use is not advantageous.
EP 0 325 891 A1 discloses an improvement to this Acetosolv process. The improvement comprises in essence that the pulp be washed following the digestion step, not with caustic soda, but with a C
1-3
-carboxylic acid, or with a mixture of such acids, whereupon the subsequent bleaching step is carried out in the existing acid medium, to which hydrogen peroxide or ozone is added. A carboxylic acid such as, for example, butyl acetate, can be used as the solvent. This patent also discloses that the C
1-3
-carboxylic acid can later be re-used as a digestion liquid.
Disclosed in EP 0 250 422 B1 is a process commonly known as the Milox process, in which bleached pulp is produced from cellulose-containing material. In this process, digestion is accomplished in a peroxyformyl, peroxyaceto, peroxypropional or peroxybutyric acid medium, wherein recovery of peroxic acid is effected in that the acid used is mixed with a relatively large quantity of hydrogen peroxide, whereupon the digestion process continues. Following digestion, the pulp is bleached in an alkali solution to which hydrogen peroxide has been added. The disadvantage of this process, however, is the use of an alkali solution such as caustic soda, as well as the high proportion of hydrogen peroxide required.
Using this prior art as the basis, the present invention aims to develop a process for producing a pulp from cellulose-containing material that is environmentally-friendly, highly economical and efficient.
OBJECT AND SUMMARY OF THE INVENTION
This object is addressed in the process in accordance with Claim
1
. Accordingly, we are surprised to discover that, depending on the type of cellulose-containing material employed, it is necessary only to dissolve the material by using formic acid as the solvent and to cook the liquid at approximately boiling point while employing backflow cooling. Use of suitable cellulose-containing material yields pulp so surprisingly white that subsequent bleaching is unnecessary.
If the cellulose-containing material used is difficult to digest, a preferred embodiment of the proposed process can be employed. This is the case, for example, with deciduous or coniferous wood and also with straw, depending on the use to which the recovered pulp will be put. In this preferred embodiment, the cellulose-containing material is reacted with formic acid and water and then slightly heated by means of an external energy source, whereby backflow cooling is used. Next, a precisely predetermined quantity of hydrogen peroxide is slowly added at a constant rate. This process method, which has never before been disclosed in the prior art, provides for a very economical process, since the exothermic reaction itself supplies the heat required for digestion; i.e. except for start-up activation, no further energy is required. At the same time, control of the reaction is considerably simplified, since only a small, predetermined amount of hydrogen peroxide is added to the digestion solution. By digestion solution in the present invention is meant the totality of solvent, cellulose-containing material and the components thereof that may be dissolved therefrom, such as lignin and sugar. Furthermore, it is advantageous for the execution of the reaction in accordance with the preferred embodiment, that the balance in the reaction of the formation of per acid from formic acid and hydrogen peroxide through the continuous addition of hydrogen peroxide, be constantly shifted toward per acid.
The proposed process comprises that hydrous formic acid be employed in a concentration between approximately 60 and 99 percent by weight. The use of 100% formic acid is not advantageous, since at least a certain percentage of water should be present during pulp production, i.e. during the digestion step. Digestion time should run approximately 30 to 120 minutes, depending on the cellulose-containing material employed.
The predetermined quantity of hydrogen peroxide can be between approximately 1 to 3 percent by weight, preferably from 1 to 2 percent by weight, whereby 1 percent by weight is particularly preferred, relative to the total weight of cellulose-containing material and solvent. It is particularly advantageous in this case that only a very small amount of hydrogen peroxide need be added, which is sufficient on the one hand to ensure that the pulp thus produced meets the brightness requirements of the paper to be manufactured, and on the other hand, that the temperature of the digestion solution be kept in the vicinity of the boiling point of the solvent without further external energy in the form of heat being required. Thus, the characteristics required of the proposed pulp for producing paper can be obtained with a minimal expenditure of energy.
It has proven advantageous if the ratio of liquid to material is in the range of 20:1 to 25:1.
The proposed process can, furthermore, be modified in that, additionally, a gas such as air, oxygen, ozone or a comparable gas or a mixture of two or more of these gases is introduced into the solvent. In this case, the oxidizing power of these gases is utilised in order to facilitate lignin breakdown, which increases brightness and lowers the Kappa number. In addition, the time required for digestion is reduced.
It is proposed that the pulp, following termination of the digestion step, be separated from the solvent, simply by means of screening. In this case, screening is understood in its most general sense, that is, separation by means of a suitable membrane, a filter or a frit, in order to permit the separation procedure to be accomplished in a continuous manner. The pulp, now separated by means of the screening procedure, can be washed with water and/or formic acid. In particular, if the pulp is washed with formic acid, the remaining lignin, which has already been dissolved out of the cellulose-containing material, is floated off. This floating action can be facilitated if the pulp is agitated by means of a stirrer.
If formic acid is used to wash the pulp, such formic acid, together with the solvent from the digestion process, can be recovered by means of simple distillation. The proportion of recovered formic acid is, as a rule, over 95 percent by weight. The remaining formic acid stays in the lignin as a residue.
The pulp, after being
Alvo Steve
Blank Rome Comisky & McCauley LLP
Natural Pulping AG
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