TECHNICAL FIELD

The present invention relates to the ozone bleaching of chemical paper pulp, especially paper pulp which is produced by the kraft pulp process. The inven¬ tion concerns a method for utilizing the remaining ozone content in the residual gas after the bleaching by using the residual gas for the purifying treatment of condensate from a process for producing paper or paper pulp.

STATE OF THE ART AND PROBLEMS

Owing to the increasing interest in the environment and understanding of the ecological cycle in nature, there is a great desire both among consumers and among producers to decrease the discharges of pol¬ lutants arising from human activities. In recent years, very strenuous efforts have been made to decrease the discharges from our pulp and paper mills, and major advances have indeed been made. The demand of the mar¬ ket for paper which has been bleached without using chlorine has led to alternative bleaching chemicals, such as ozone, persulphuric acid, peracetic acid and hydrogen peroxide, coming into use.

A goal for pulp producers is to create "the closed mill", in which discharges are minimized and return of liquid flows to the fibre line, and chemical recovery, are maximized. In order to be able to achieve this, it is advantageous for the bleaching process not to con¬ tain elemental chlorine or chlorine dioxide since, among other reasons, chlorine has a corrosive effect on the equipment and can form toxic compounds if it is combusted in the recovery boiler. In other respects, returning washing liquid in countercurrent to the fibre stream also places great demands on the purity of the liquid. Some washing stages are more sensitive than others to impurities.

For this reason, there is often the need for some form of purification of the different condensates which can be utilized as washing liquid. In this context, a dis¬ tinction is made between different types of condensate since they have different contents of impurities.

When the black liquor is extracted from the digester, the pressure is reduced somewhat and the liquor is flashed, resulting in steam being driven off. The steam is accompanied, for example, by terpenes, methanol and reduced sulphur compounds which, to a large extent, accompany the steam which condenses after cooling has taken place. In this way, a cooking depart¬ ment condensate is formed from which turpentine is nor¬ mally separated off in a decanter. The black liquor is normally concentrated by evaporation in several stages. The units, which are termed effects, are numbered in accordance with the route of steam supply within the plant. Thus, live steam is supplied to the first effect and the steam which has been obtained from liquor evporation in preceding stages is used as a heat source in the subsequent stages. This is possible owing to the gradual decline in pressure. The heat content which remains in the steam from the final effect is condensed in one or more surface condensers. By means of allowing the steam from the liquor evaporation to condense out step-wise on different heat surfaces in the plant, it is possible to segregate, on the one hand, heavily contaminated condensates, to be purified, for example, in a stripper column, and, on the other hand, very pure condensates which can be used directly in the mill without odour treatment or some other form of purification.

Cooking department condensate and heavily con- taminated condensate from the black liquor evaporation is normally conveyed to a steam stripper for purifica¬ tion. A steam stripper usually separates off methanol,

ethanol, terpenes and malodorous sulphur compounds

(hydrogen sulphide, methylmercaptan, dimethyl sulphide and dimethyl disulphide, etc.) very efficiently. The contaminants which have been separated off are obtained in a concentrated stream which has to be dealt with. It is usually conveyed away and combusted. Condensate which has been treated in a steam stripper often has a very low content of COD. Malodorous compounds which remain in the stripped condensate are, apart from traces of hydrogen sulphide, a number of terpenes such as ocimene, alfapinene, delta-3-carene and decanal. These substances do not smell as badly as the sulphur compounds which are otherwise common in malodorous con¬ densates. The evaporation condensates which are not so heavily contaminated are often termed "pure conden¬ sates". The problem is that they, nevertheless, contain some malodorous impurities. Recently, several malo¬ dorous substances, for example dimethyl trisulphide, 2,3-dimethylphenol, and a variety of trithiolanes and trisulphides have been discovered in these condensates in addition to those which were previously known. It is therefore desirable to purify these condensates as well in order to avoid problems, inter alia with the working environment, when closing the kraft mill.

Many studies have been carried out to investi¬ gate the problem of contaminated condensates from the pulp industry, and also the problem of contaminated water in general, and many suggestions for solving these problems have indeed been presented. The dif¬ ferent methods which exist can be classified into the following groups: phase change (e.g. stripping and adsorption with active charcoal), biological treatment, thermal or catalytic oxidation and chemical oxidation. The present invention relates to the chemical oxidation group which, inter alia, encompasses the oxidizing

agents oxygen, chlorine, potassium permanganate, peroxides and ozone.

In Pulp and Paper Manufacture, pp. 407-409,

Noyes Data Corporation 1977, Marshall Sittig describes how oxidizing gases, such as chlorine gas, chlorine dioxide, oxygen and ozone, can be used in the odour treatment of condensates from the kraft process.

A method for purifying condensates from the kraft process has been patented by means of SE-C-462 169. In this method, the condensates are treated concurrently, in a column, with an oxygen- containing gas in the presence of active charcoal as catalyst. Any oils and solid substances which are present have first to be separated off, and the catalyst has to be regenerated at regular intervals.

The patent GB 1 405 317 describes an ozone treatment method for a desulphurization process. The ozone oxidizes sulphur compounds to sulphate and decreases the content of COD in the waste water. Ultrox International has several patents (US

4,780,287, US 4,792,407, US 4,849,114 and US 4,941,957) for the treatment of gas or water which relate to a purification method which combines ozone, hydrogen peroxide and UV light. The method makes use of the fact that the UV light breaks ozone and hydrogen peroxide down into very reactive OH radicals which, in turn, break organic matter in the water down into carbon dioxide, water and inorganic salts.

In general, it can be stated that the pre- viously known methods for carrying out chemical oxi¬ dation suffer from the disadvantage that they require extensive equipment and additional management of oxida¬ tion chemicals which are expensive to- purchase or pro¬ duce and are difficult to handle.

SOLUTION AND ADVANTAGES

By means of the present invention, a method has been produced for making use of the residual gas from ozone bleaching and utilizing its remaining content of ozone for treating contaminated and malodorous conden¬ sate so that this condensate can be reused in the pulp manufacturing process.

During ozone bleaching, the pulp which is to be bleached is thoroughly mixed with ozone in a carrier gas consisting of oxygen. Ozone is a very reactive chemical which reacts rapidly with the lignin in the pulp. As a rule, the economically most advantageous method is to supply a quantity of ozone which is slightly greater than that which is stoichiometrically required for the bleaching. As a result, a small quan¬ tity of unreacted ozone is often left in the gas, which ozone has to be conveyed away and destroyed in a so- called ozone destroyer. It has now been found that this residual ozone can be used very expediently in a method for treating malodorous condensates, especially those condensates which are relatively pure with regard to their contents of methanol and COD. While ozone has a limited effect on methanol and COD content, its effect on odour is relatively large. The method will be described below with reference to Figure 1, which shows a preferred embodiment of the invention, and Figure 2, which shows a device for separating off the residual gas from ozone bleaching. After cooking and washing, the pulp is treated with ozone which has been generated within the mill from air or oxygen. This treatment with ozone takes place at a pH of approximately 3, a pulp concentration of between 8 and 12%, a pressure of 8 bar and a tem- perature of 50°C. The ozone gas is mixed in with the pulp in a first mixer (1) (Figure 1) and the mixing effect is subsequently amplified by the pulp having to

pass through one or more additional mixers (2), preferably without any further admixture of gas. After the last mixer in the ozone stage, the pulp is con¬ veyed, via a pipeline, to a blow tank (3) . Conse- quently, there is no requirement for a special reactor because of the dwell time.

The residual gas, which principally consists of oxygen with an ozone content of approximately 0.5%, is separated off from the pulp in conjunction with the latter being conveyed to the blow tank, for example by means of a cyclone (4) (Figure 2) in accordance with SE-A-9200183. Fibre is preferably removed from the separated residual gas, for example by means of the gas having to pass through liquid in a liquid seal device (5) of the type which is described in SE-A-9200183.

After that, the residual gas (6) is conveyed to a device for condensate treatment, preferably a coun- tercurrent absorption tower (7) containing packing material, intermediate partitions or the like for creating the maximum area of contact between gas and liquid. Condensate (8), preferably one of the purer liquor steam condensates from the black liquor evapora¬ tion, is introduced at the same time into the upper part of the absorption tower. In this context, it is advantageous if the condensate which is arriving is heated, for example by exchanging heat (13) with the departing condensate (9) and, where appropriate, by making use of heat from the ozone destroyer (12) . Vola¬ tile substances are then given off more readily and can be attacked by the ozone or oxygen. It is also advanta¬ geous if, in association with entering the absorption tower, the condensate is subjected to a decline in pressure so that a stripper effect is achieved.

In the countercurrent procedure, the condensate first meets a gas which principally consists of oxygen since the major part of the ozone has been consumed previously in the absorption tower. Like the ozone, the

oxyger. is a positive oxidizing effect on impurities in the condensate, even if the oxidizing effect of the oxygen is not as great as that of the ozone. The oxygen is able not only to decompose some COD and malodorous substances in the condensate but also, at the same time, like an air stripper, to drive off readily vola¬ tile substances which are then broken down either in the absorption tower or in the ozone destroyer. In addition, when the condensate is oxidized with ozone, its pH is lowered, whereupon the equilibrium of several sulphur compounds, for example hydrogen sulphide, is displaced so that the compounds are given off into the gas phase. This results in the malodorous sulphur com¬ pounds being attacked more efficiently by the ozone and the oxygen.

The condensate flows down through the tower (7) and, in doing so, meets gas of gradually increasing ozone content. Impurities and malodorous organic sub¬ stances are oxidized by the ozone, resulting in the departing condensate (9) having a substantially less offensive smell, a lower COD content, a lighter colour and greater clarity.

If a higher concentration of ozone is required in the gas, additional ozone (10) can be supplied to the absorption tower, preferably at the same end of the tower as the condensate outlet. The purified condensate (9) can, for example, be utilized in washing stages which place relatively high demands on purity and free¬ dom from odour, for example lime sludge dilution, lime sludge washing or peroxide-containing bleaching stages. Gas (11) which leaves the tower (7) is conveyed to an _ozone destroyer so that any accompanying volatile sub¬ stances, or any remaining unused ozone, can be destroyed. There is also a line which passes from the point of residual gas separation (4, 5) directly to the ozone destroyer, for use if condensate treatment is not required.

The major advantage of the method according to the invention is that the remainder of a chemical which is utilized internally for bleaching, i.e. ozone, can be used in another process stage. As a result, use of the ozone is optimized at the same time as condensate is purified.

An advantage of treating the condensate, in addition to that of providing the opportunity of reusing condensate in the process, which represents a simplification when increasing the closure of the mill, is that it improves both the working environment and the environment surrounding the mill. Nowadays, the surrounding population can sometimes be very annoyed by malodorous gases emanating from a pulp mill. It is also advantageous, for example in association with the manufacture of food and liquid packaging, to be able to utilize mill condensate in washing stages in the fibre line without the products being given a residual taste or odour.

ALTERNATIVE EMBODIMENTS

In the method according to the invention, the ozone stage is carried out at a pH of less than 7, preferably of approximately 2-4. At the same time, the pressure on the pulp should be at least 8 bar, the tem¬ perature at most 60°C preferably 20-40°C, and the concentration of the pulp should be 6-14%, preferably 8-12%.

While the carrier gas preferably consists of oxygen, air can also be used. Naturally, the greater the amount of- ozone which remains in the residual gas after the ozone bleaching the better it is for the con¬ densate treatment. The residual gas should preferably contain at least 0.3% by weight, preferably at least 0.5% by weight, and even more preferably at least 1% by weight, of ozone.

Apart from being carried out in an absorption tower, the condensate treatment can also be carried out, for example, in a tank having a stirrer, to which the gas is supplied while stirring. The condensate can also be mixed with the gas in a mixer of the static type and subsequently conveyed into a contact tower having some form of mechanism for mixing the condensate and the gas and for maintaining a turbulent flow. Another alternative is for the condensate which is to be treated to constitute liquid in the liquid seal (5) for separating out fibres from the residual gas from the ozone bleaching. This makes it possible to combine gas treatment and condensate treatment in one and the same unit. In this case, the condensate (8) is preferably supplied continuously to the water seal. Yet another variant is for the condensate to be supplied to the pulp suspension before, or at the same time as, the latter is bleached with ozone so that the condensate treatment takes place at the same time as the actual ozone bleaching.

Even if it is preferred to use the present invention to purify condensates which, relatively speaking, already belong to the purer condensates in a hypothetical pulp mill, the possibility also exists of utilizing the principle for the odour treatment of more contaminated condensates before they are conveyed away, for example, either for biological purification and discharge, or to the dissolver tank in association with causticization.

Naturally, the invention can be applied to any other process water whatever and also, for example, to effluent from a bleaching stage before it is conveyed onwards in the process. While the invention has been described here on the basis of a hypothetical kraft pulp mill, the same positive effects will be achieved, for example, in the

case of a sulphite pulp mill or an integrated paper and pulp mill.

The invention is not limited to the embodiments described here and can be varied within the scope of the subsequent patent claims. A plant for condensate treatment in analogy with the invention can also be advantageous in which the ozone is replaced by other oxidizing agents which are used in bleaching, for example peracetic acid, persulphuric acid (Caro's acid) or hydrogen peroxide. Alternatively, such an oxidizing agent can be used to complement an ozone plant according to the invention.