While, a major proportion of the Kraft pulp that is made is used for white or colored papers such as writing and printing papers, and then the pulp needs to be bleached, the process is called paper pulp bleaching process.
After several years of production practice, Anyang Machinery invented the disc filter , which is the latest generation of white liquor recovery equipment. Using the disc filter for pulp washing can save energy, reduce water consumption and fiber loss. Meantime protect the environment. For the superior appearance, disc filters are widely used in white liquor recovery system in paper mills.
We can also produce gravity disc thickener and a series of paper pulp bleaching equipment. All of the equipment are manufactured according to international standards. Meantime, we can provide professional one-stop service, from bleaching equipment design to installation and maintenance. Bleaching involves removing virtually all of the lignin that still remains after cooking, as the lignin contains the chromophoric groups which make the pulp dark. Strictly speaking, bleaching and cooking are both delignification processes, and modern developments have tended to blur the difference between the two processes.
The green color is due to iron impurities which, along with silica and other suspended solids, are removed by treatment with coagulating agents and settlement in a clarifier. The clarification step is followed by a causticizing step in which the green liquor is treated with lime calcium oxide to convert the sodium carbonate to sodium hydroxide, the sodium sulfide remaining unchanged. The sludge from the lime treatment is removed, and the resulting solution is referred to as white liquor which can be reused as cooking liquor in the digester.
The sludge is washed in a lime mud washer and the filtrate used to dissolve additional smelt from the recovery unit. The lime mud is treated in a kiln to yield reburned lime. During the burning operation some calcium oxide and sulfur compounds are collected in the kiln stack and removed by a scrubber in said stack.
Unbleached Kraft, or sulfate, pulps are generally bleached to an acceptable brightness by a multistage process employing chlorine or chlorine-containing compounds such as calcium or sodium hypochlorite and chlorine dioxide. Although chlorine and chlorine-containing compounds have proven to be effective bleaching agents, these compounds are difficult to handle, introduce the problem of corrosion of the paper-making equipment, and render the effluents from the bleach plant incapable of being recovered and recycled.
In addition waste liquors and wash water from the bleach plant incorporating such compounds can cause a serious pollution problem. However, this method of disposing of such effluents has several major disadvantages which have discouraged the use of such a recovery and disposal system for chlorine-containing bleach plant effluents.
The chloride content of the effluent introduces the problems of corrosion, solid build up, volatilization of toxic compounds, etc. Ozone is a well known bleaching agent and has been suggested for use in bleaching wood pulp.
Because of the chloride content or the effluents these processes would not eliminate the pollution problems discussed above. Other systems which have employed ozone in a bleaching process, either alone or after the fibers were pretreated by exposure to U. In accordance with the present invention Kraft pulp is bleached to a high brightness by a bleaching sequence comprising from one to about three treatments with ozone, and a final treatment with an alkaline peroxide solution, the bleach treatments being separated by an alkaline extraction.
The effluent from each of the bleach treatments is collected and either, recycled and utilized in subsequent bleaching operations thereby reducing the amount of fresh water required, or introduced into the chemical recovery system of a Kraft pulping process thereby eliminating pollution which would otherwise be caused by dumping these effluents.
Following each of the ozonation steps the pulp may be washed if desired. The pulp is finally treated with an alkaline solution of hydrogen peroxide such as is commonly used in the bleaching of wood pulp and washed. While it is preferred to subject the pulp to from 1 to about 3 treatments with ozone, the following bleaching sequence is especially preferred in carrying out this invention: ozonation, alkaline extraction, ozonation, alkaline extraction, peroxide treatment, and wash.
It has now been found that the amount of fresh water required for the bleaching operation can be substantially reduced if effluent from the various bleaching treatments is recycled and used in subsequent bleaching operations. It is especially preferred to introduce fresh water into the system at the final wash following the alkaline peroxide treatment and to utilize the effluent from one stage at an earlier stage in a subsequent bleaching operation.
In the especially preferred bleaching sequence, effluent from the peroxide treatment and wash is used in the second alkaline extraction in a subsequent bleaching operation, and effluent from the second alkaline extraction is used in the first alkaline extraction in a subsequent operation. It is preferred not to reuse effluent from the first alkaline extraction, but to treat it as below in the chemical recovery system.
To eliminate stream and air pollution, effluent from the first alkaline extraction, and, if desired, from other bleaching treatments is introduced into the chemical recovery system associated with the Kraft pulping operation.
The effluent can be introduced into this system at any point where fresh water is normally employed. It is preferred to introduce the effluent at the brown stock washer, the smelt dissolving operation, the lime mud washer or the kiln stack scrubber.
If the volume of effluent is large, it may be separated and introduced at more than one of these points. It is especially preferred to introduce the effluent at the brown stock washer.
In order to describe the present invention so that it may be more clearly understood, the following examples are set forth. These examples are set forth primarily for the purpose of illustration, and any specific enumeration of detail contained therein should not be interpreted as a limitation on the concept of this invention.
A mixture of hardwood beech, birch and maple chips was pulped via the Kraft process. Sheets produced in this manner had a brightness, as measured on a General Electric Photometer and expressed in terms of G.
The pulp was washed and again treated with ozone so that the amount of ozone consumed was 0. A sample of the pulp was washed with water and handsheets were prepared as with the unbleached pulp. The brightness of this sheet was equal to The bleach solution was applied so that 0.
Sheets prepared as above had a brightness of from 90 to 91 G. The effluent from the several bleaching operations is collected and introduced into the chemical recovery system of the Kraft pulping process. Midwestern poplar chips were pulped via the Kraft process. Handsheets prepared as in Example I had a brightness of 30 G.
A sample of the pulp was fluffed and subjected to a first ozone treatment as in Example I. The amount of ozone consumed was equal to 1. After washing the pulp was treated with an aqueous solution of 1.
Following a second ozonation in which the ozone consumed was equal to 0. All of these treatments were carried out as in Example I except for the amounts of reagents employed. Handsheets prepared from the pulp had a brightness of A second sample of the unbleached pulp prepared in Example II was bleached by a method identical with that disclosed in Example II except that the fresh water utilized in the second alkali extraction in that example was replaced by the effluent from the peroxide treatment in Example II.
Handsheets prepared from pulp bleached in this manner had a brightness of A third sample of the unbleached pulp prepared in Example II was bleached by a method identical with that disclosed in Example II except for two modifications.
The fresh water in the second alkali extraction was replaced by the wash water from the peroxide treatment in Example III. Also the fresh water in the first alkali extraction was replaced by the effluent from the second alkali extraction in Example III. The effluent from the first alkali extraction is introduced into the chemical recovery system of the Kraft pulping process at the brown stock washer.
A mixture of hardwood chips beech, birch and maple were pulped via the Kraft process.
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