WO1999027179A1

WO1999027179A1 - Method for bleaching cellulose in a closed system using caro's acid

Info

Publication number: WO1999027179A1
Application number: PCT/SE1998/002117
Authority: WO
Inventors: Erik Bengtsson; Peter BJÖRKLUND
Original assignee: Kvaerner Pulping AB; Kvaerner Pulping Technologies AB
Current assignee: Metso Fiber Karlstad AB
Priority date: 1997-11-24
Filing date: 1998-11-23
Publication date: 1999-06-03
Anticipated expiration: 2000-05-24
Also published as: SE9704298D0; AU1358199A; SE9704298L

Abstract

The present invention relates to a method for bleaching cellulose in a closed system in one or more stages using Caro's acid. The method is characterized in that to maintain the sulphur and sodium balance, the production of Caro's acid, and of the sulphuric acid necessary for producing the latter, are included in the closed system.

Description

TITLE:

Method for bleaching cellulose in a closed system using Caro's acid.

TECHNICAL FIELD:

The present invention relates to the bleaching of cellulose with Caro's acid in a system which is closed or partially closed with respect to the bleaching, the digestion of the cellulose and the recovery of the chemicals. The method is distinguished by the fact that it comprises one or more bleaching stages using Caro's acid.

PRIOR ART:

The production of cellulose, for example by the sulphate method or sulphite method, and the bleaching of cellulose have always entailed environmental problems, on the one hand because such large quantities are produced and have led to substantial emissions, and, en the other hand, because the chemicals used and the emissions are not environmentally friendly or are directly toxic and can have unpleasant odours. To remedy this situation, attempts have been made, on the one hand, to close the production methods by recirculating the used chemicals and, on the other hand, to replace the previously used bleaching chemicals, chlorine and chlorine compounds, with other compounds which are not toxic to nature. These essentially include oxygen, hydrogen peroxide and ozone.

Despite the fact that these bleaching agents are not harmful to nature and in most cases give satisfactorily bleached pulp, attempts have nevertheless been made, for various reasons, to supplement or replace these agents with other newer bleaching agents. The reason behind this is that it is often difficult to achieve the high brightness units which are desired while at the same time maintaining a high pulp strength, and it has always been attempted to make the processes less expensive and to reduce the emissions. Extensive closure of the systems has also led to a number of problems which have had to be solved in various ways.

The new bleaching agents which have begun to be used are mainly peracetic acid, peroxysulphuric acid

(Caro' s acid) , cyana ide and enzymes which bleach by means of so-called bio-bleaching. The bleaching always takes place in several stages and often with intermediate washing. At some stages, such as peroxide bleaching, a high pH is needed, whilst the ozone stage is carried out on the acid side. Various chemicals therefore have to be added to regulate the pH during the bleaching process.

TECHNICAL PROBLEM:

Since both peracetic acid and Caro' s acid are unstable chemicals, it is advantageous to produce these in situ in part of the plant, so that storage and transport can be avoided. The peracetic acid is an expensive chemical, for which reason Caro's acid is preferred. Both Caro's acid and the acetic acid are easy to produce in situ, but since Caro's acid is produced from sulphuric acid and hydrogen peroxide, large quantities of sulphur have to be added to the bleaching process. Since both the bleaching process and the digestion and recovery process must as far as possible be closed, then, when Caro's acid is used, the sulphur content in the system will increase. To maintain the correct sodium/sulphur ratio in cellulose production, sodium must be added to the system, in most cases in the form of sodium hydroxide. The addition of sulphur with Caro's acid, and the addition of sodium to maintain the sodium/sulphur ratio, marginally exceeds the system's natural losses. The excess of chemicals can be bled out as sodium sulphate, an extremely costly solution with regard to chemical losses. The problem with the excess sulphur must be dealt with, and it has even been proposed to exploit the excess sulphur by means of electrolysis of sulphate. This is described in Svensk papperstidning/Nordisk cellulosa No. 3, 1996, on page

45. However, this is an extra process in the plant and brings with it further problems, and for this reason there has long been a need to solve the problem of the sulphur balance in a closed or partially closed system for production and bleaching of cellulose using Caro's acid.

SOLUTION:

Thus, according to the present invention, a method has been developed for bleaching cellulose in a closed or partially closed system in one or more stages using Caro's acid, which method is characterized in that the production of Caro's acid is included in the closed or partially closed system. According to the invention, the sulphuric acid needed for producing Caro's acid can be produced from uncondensed gases rich in sulphur, as is described in French Patent 9303831.

According to the invention, sulphur-containing and uncondensable gases from the evaporation of waste liquor and from cellulose digestion and/or from heat treatment of the thick liquor in a superconcentrator, for example, represent the source of sulphur needed for producing Caro's acid. According to the invention, it is expedient for the bleaching stage with Caro's acid to have a pH of between 3 and 6 and a temperature of between 70 and 140°C.

According to the invention, the bleaching stage with Caro's acid can also include an addition of peracetic acid which can, for example, represent half the acid mixture.

To raise the pH in the actual bleaching stage and after the bleaching stage with Caro's acid, totally oxidized white liquor is preferably used according to the invention. The totally oxidized white liquor can be produced, for example, by any one of the methods which are described in US Patents 5 382 322, 5 500 085 and 5 439 556. In this way, a balanced level of sulphur and sodium is achieved in the closed system.

According to the invention, an advantageous bleaching sequence comprising Caro's acid is one which is characterized by the stages OP-CAQ-PO, where 0 stands for oxygen, P for hydrogen peroxide, CA for

Caro's acid and Q for complexing agent.

DETAILED DESCRIPTION:

As has been stated above, the invention involves bleaching cellulose in a closed system using Caro's acid in at least one bleaching stage, Caro's acid being produced in situ with preservation of the sulphur balance in the system.

Cellulose is produced according to one of the known methods by means of wood chips being cooked in an alkaline liquid, after which the released cellulose is filtered off from the cooking liquid or waste liquor. The unbleached cellulose is then subjected to a bleaching procedure while the waste liquor is evaporated in a number of so-called effects for concentrating the waste liquor which, in the concentrated state, i.e. with a high dry substance content, is combusted in a soda recovery unit for generating thermal energy and managing the chemicals in melted form. The waste liquor also contains, in addition to dissolved lignin, a large quantity of sulphur and sodium compounds, mainly in the form of sodium sulphide and sodium hydroxide.

In the evaporation of the waste liquor in the evaporation effects, a large amount of water vapour is thus created, which vapour also contains sulphur compounds in the form of hydrogen sulphide, mercaptans and the like. The amount of sulphur which can come from these evaporation stages can be somewhere in the region of up to 3 kg of sulphur per tonne of pulp produced.

According to the invention, this amount of sulphur can be dealt with by means of the sulphur-rich vapour being combusted so that sulphur dioxide is obtained with a concentration of about 1% by volume in the waste gas. By means of further combustion with the aid of a vanadium oxide catalyst, the sulphur dioxide is combusted to sulphur trioxide. This mixture of sulphur trioxide and water is then conducted into a tower with circulating 60% strength sulphuric acid. The S0₃ gas is absorbed and forms sulphuric acid which is incorporated in the circulating acid and removed continuously from the plant. The concentration of the circulating acid is regulated with the aid of the temperature and possible addition of water. The pure, sulphur-free waste gas is allowed into the atmosphere after passing through an aerosol separator. The 60% strength sulphuric acid is then concentrated to a concentration in the region of 93% by weight, which is a suitable concentration for production of Caro's acid.

As has been mentioned above, the amount of sulphur which is available from the evaporation plant amounts to somewhere in the region of 3 kg of sulphur per tonne of pulp. However, this is not sufficient to permit production of a sufficient amount of Caro's acid, for which reason a further source of sulphur must be used. This source consists of sulphur from a further heat treatment of thick liquor, for example in a so-called superconcentrator . The vapour which develops in such a superconcentrator is very much richer in sulphur and represents somewhere in the region of 5 - 10 kilos of sulphur per tonne of pulp. By subjecting this vapour, richer in sulphur, to the same processes as the vapour from the evaporation plant or combining it with the latter, it is possible to achieve a combined amount of sulphur of 8 - 13 kg per tonne of pulp. This is a sufficient amount for the sulphur balance in the system to be maintained upon total closure and optimum use of Caro's acid.

If, however, the bleaching and cooking stages are included in a completely closed system, the metals which are dissolved out from the wood will gradually accumulate. Some of the metals are undesirable in bleaching because they cause disintegration of active bleaching chemicals, for example hydrogen peroxide, and can even degrade the cellulose fibres. According to the invention, it may therefore be expedient to make the system partially closed by tapping off some of the circulating liquid to the outlet, with some of the undesired metals then disappearing. However, in this case it is necessary to replace the sulphur with, for example, bought-in sulphuric acid, and the sodium with, for example, bought-in NaOH.

However, the system can be made completely closed without accumulation of the metals originating from the wood. This is because these can be made to precipitate on the cellulose in an alkaline medium. It is therefore possible to raise the pH at the end of the acidic Caro's acid stage without separating the acidic liquid from the cellulose, which is done at present in order to save alkali. The metals then follow the cellulose out of the system.

The sulphuric acid produced from the gases in the evaporation system is then used for in situ production of Caro's acid which will be used in the bleaching stage. Caro's acid is produced in a known manner by reaction between hydrogen peroxide and sulphuric acid, with Caro's acid and water being obtained. The reaction between these reactants is not complete and the equilibrium is displaced in favour of the final Caro's acid if the reactants have a high concentration. To obtain a high conversion of hydrogen peroxide, which is the most expensive raw material, an excess of sulphuric acid is used.

When using 93% strength sulphuric acid and 70% strength hydrogen peroxide in the molar ratio of 1.6:1, a 70% strength conversion of hydrogen peroxide to Caro's acid can be achieved. While the sulphuric acid is produced in situ from sulphur which is included in the system, the hydrogen peroxide can either be produced by traditional methods also in situ or it can be bought in. The production of Caro's acid is known per se and has been described in both the scientific literature and in patent specifications. The optimum amount of Caro's acid which should be produced and included in the bleaching method is about 15 kg per tonne of pulp.

Caro's acid can be used in one or more stages in the bleaching sequence. Since it disintegrates and loses oxygen at pH values higher than 6, the most suitable initial pH value is 4 - 6, which drops to about 3 at the end of the stage. Liquor should be added to control the final pH value since the natural final pH value in an unbuffered Caro's solution is about 1.5 - 2.0. The optimum pH value in bleaching is about 5 - 6 and the temperature can be kept fairly high, between 70 and 90°C. Since Caro's acid bleaches in approximately the same way as peracetic acid, these acids can be mixed in a bleaching stage and used in preferably similar quantities. After the acid bleaching stage with Caro's acid, a basic hydrogen peroxide stage may be appropriate. The pH must then be raised by further addition of liquor. Sodium hydroxide was previously bought in and used. However, according to the invention, white liquor can be used, which is obtained from the chemical recovery in a soda recovery unit, but since this consists of a mixture of sodium sulphide and sodium hydroxide, the sulphide will be oxidized in the bleaching stage and thus use up unnecessary hydrogen peroxide. Thus, according to the invention, it is advantageous to use a so-called totally oxidized white liquor for this purpose. In this totally oxidized white liquor, the sulphur is present as sulphate. Since this sulphur and sodium come from the closed process, it does not add any new sulphur or sodium and thus does not disturb the sulphur or sodium balance in the process.

The invention can be implemented in various bleaching methods. It is possible to have one or more stages with Caro's acid. An example of complete bleaching comprising this acid stage consists of the sequences OP-CAQ-PO. This means that the pulp is first bleached in a known manner in a stage which is dominated by oxygen with admixed hydrogen peroxide, followed by the stage with Caro's acid, and finally a stage with peroxide and oxygen where the peroxide predominates .

By means of the present invention, it has thus been possible to solve the problem of the chemical balance in cellulose production in a closed system using the extremely useful and inexpensive Caro's acid as bleaching agent. The system according to the present invention gives a low bleaching chemicals cost and it is suitable for all installations for cellulose production. It also allows different bleaching sequences to be used.

The invention is not limited to the embodiment shown, but can be varied in different ways within the scope of the claims.

Claims

PATENT CLAIMS

1. Method for bleaching cellulose in a closed or partially closed system in one or more stages using Caro's acid, c h a r a c t e r i z e d i n that the production of Caro's acid is included in the closed or partially closed system.

2. Method according to Claim 1, c h a r a c t e r i z e d i n that sulphur-containing, uncondensable gases from the cellulose digestion and/or from the evaporation of waste liquor from the cellulose digestion and/or from heat treatment of the thick liquor in, for example, a superconcentrator represent the sulphur source which is needed for production of Caro's acid.

3. Method according to either of Claims 1 and 2, c h a r a c t e r i z e d i n that the pH in the bleaching stage with Caro's acid lies between 3 and 6 and the temperature is between 70°C and 140°C.

4. Method according to any of Claims 1 to 3, c h a r a c t e r i z e d i n that the bleaching stage with Caro's acid also includes an addition of peracetic acid.

5. Method according to any of Claims 1 to 4, c h a r a c t e r i z e d i n that the pH is regulated completely or partially with the aid of totally oxidized white liquor.

6. Method according to any of Claims 1 to 5, c h a r a c t e r i z e d i n that the bleaching sequence is OP-CAQ-PO.