TECHNICAL FIELD:

The present invention relates to a process for producing polysulphide by means of oxidizing sulphide in spent liquors from kraft cellulose cooking. This treatment of spent liquors from the kraft cellulose cooking is designed to raise the quality of the liquor which is once again introduced into the cellulose digester.

STATE OF THE ART: When cellulose is produced by cooking in accordance with the kraft cellulose method, a spent liquor is obtained, after the cellulose has been separated off after the cooking, which is termed black liquor and which constitutes an aqueous solution of dissolved lignin, various other dissolved organic constituents, sodium sulphide, sodium sulphate, sodium thiosulphate, sodium carbonate, sodium hydroxide and, to a lesser degree, calcium carbonate, together with salts or hydroxides of manganese, magnesium, iron, aluminium and similar metals which have been dissolved out of the wood. The black liquor is evaporated, by means of various processes, to such an extent that the concentrated black liquor can be combusted. This combustion usually takes place in so-called recovery boilers and the purpose of the combustion is to extract energy, by means of combustion, from the organic substances and to recover the inorganic substances, which, in smelted form, are removed from the lower part of the recovery boiler. The smelted chemicals from the recovery boiler are then dissolved in water, with a so-called green liquor being obtained. This principally consists of sodium carbonate (from 50 to 60%) and sodium sulphide. However, the sodium sulphide decomposes into sodium hydrogen sulphide and sodium hydroxide.

The green liquor is then causticized by means of adding calcium oxide, whereupon calcium carbonate, termed lime sludge, and sodium carbonate precipitate out. This resulting liquor is termed white liquor and is reused in the cooking of fresh kraft cellulose pulp. A series of different chemical reactions take place in association with the dissolution of the lignin and the defibring of the cellulose during the cooking. These reactions are more or less known. An undesirable chemical reaction is that the cellulose itself can be broken down. In order to suppress such a breakdown to the greatest possible extent, it has been concluded that the liquor which is added in the first part of the cooking should contain polysulphides. These latter prevent the breakdown of the cellulose and consequently increase the yield of the pulp. However, the breakdown can be desirable in the case of a pulp of poor quality which contains a relatively large proportion of hemicellulose, since it is the hemicellulose which is broken down first and the quality of the cellulose which is obtained in the final stage is therefore increased.

Elemental sulphur may also be added in the first part of the cooking instead of the polysulphides. This measure has the intended effect; however, since sulphur : s being added to the process, it must also be removed subsequently, thereby giving rise to problems, especially of an environmental nature. The addition of elemental sulphur is therefore a makeshift solution which is best avoided.

The polysulphide which is added in the first part of the cooking is therefore almost exclusively produced by oxidizing the sodium sulphide present in the spent liquor to give sodium polysulphide to a greater or lesser degree. Various processes have been developed for bringing about this oxidation. One process is described, for example, in American Patent no. 5082526. This process is based on using air or oxygen to oxidize a liquor which consists of white

liquor in which lime sludge particles, that is calcium carbonate, are present. The lime sludge is derived from the causticization of green liquor and particles of the lime sludge are allowed to remain during the oxidation. A partially causticized green liquor can be oxidized in this manner. The reason for allowing at least a portion of the lime sludge to remain is that this sludge constitutes a catalyst for the conversion of sodium sulphide to sodium polysulphide. Another method for producing polysulphide from white liquor by means of oxidation is described in American Patent no. 4024229. In this case, the oxidation of white liquor by air or oxygen is achieved in the presence of a catalyst consisting of activated carbon. Neither in this method nor in other methods does the oxidation of sodium sulphide give rise solely to sodium polysulphide; it also gives rise to a certain quantity of sodium thiosulphate. This latter compound is not desirable and is present in the form of solid particles.

TECHNICAL PROBLEM:

While many processes may be used, per se, for oxidizing sodium sulphide to sodium polysulphide, they result in a low yield, on the one hand, and, on the other, are complicated by the fact that they require the liquor to be of high purity in order to ensure that the catalyst is not poisoned. This is particularly the case with a carbon catalyst, which requires the liquor to be of high purity and the catalyst to be changed frequently.

SOLUTION:

There has long been a need to be able to produce polysulphide by simple means and in high yield in spent liquors, and, in accordance with the present invention, a process has therefore been developed for producing polysulphide by oxidizing sulphide in spent liquors from cellulose cooking, which process is

characterized in that green liquor sludge is used as the catalyst during the oxidation.

According to the invention, green liquor sludge can be added, prior to the oxidation, to ordinary green liquor which has previously been freed from green liquor sludge.

It is also possible for green liquor to be oxidized directly prior to the green liquor sludge being separated off. According to the invention, green liquor sludge can also be used as a catalyst in the oxidation of white liquor.

The constituents of green liquor sludge are already present in black liquor, and therefore, in accordance with the invention, the black liquor can be oxidized directly.

It is expedient, in accordance with the invention, that the oxidized black liquor is used for impregnating wood chips, where appropriate after adding sulphide-enriched green liquor or white liquor.

According to the invention, it is expedient for the green liquor or the white liquor to have been enriched with sulphides prior to the oxidation.

According to the invention, the white liquor, or the green liquor which is partially causticized, can contain particles of calcium carbonate (lime sludge) .

The green liquor sludge which is obtained after the smelted chemicals have been dissolved in water contains solid substances such as calcium carbonate and magnesium hydroxide as the main constituents; however, it also contains manganese hydroxide, iron and aluminium salts as well as other solid substances such as carbon particles. This sludge is allowed to sediment after the dissolution and is always separated off from the green liquor. Since this sludge contains manganese ions and also calcium carbonate (lime sludge) , it has been found, in accordance with the invention, to be suitable for use as a catalyst in the oxidation of sodium sulphide to sodium polysulphide.

According to the invention, some of the green liquor sludge can be added, prior to the oxidation, to ordinary green liquor which has previously been freed from this sludge. However, it is also possible to oxidize the green liquor directly before any of the green liquor sludge has been separated off.

It is also expedient to use green liquor sludge as a catalyst in the oxidation of white liquor. This latter has then previously been freed from green liquor sludge, and the whole quantity, or a portion thereof, can be added to the white liquor prior to oxidation.

The black liquor from a kraft digester contains the constituents which make up the green liquor sludge. It has recently been suggested that it should be possible for wood chips which are to be cooked to be impregnated with black liquor, and it is therefore expedient for this black liquor to have a high content of polysulphide. The black liquor can therefore be oxidized directly without any further addition since all the constituents which make up the green liquor are present in the black liquor. If a particularly high content of polysulphide is desired, sulphide-enriched green liquor or white liquor can be added to the black liquor prior to the oxidation. It is expedient for the green liquor or white liquor to have been enriched in sulphides prior to the oxidation, even if this is not necessary in accordance with the invention.

The white liquor which is to be oxidized with the aid of green liquor sludge can expediently contain lime sludge after the calcination. Green liquor which is partially causticized and which contains particles of lime sludge is also suitable for oxidation after green liquor sludge has been added. The invention is not limited to the abovementioned embodiments, and can be varied in different ways within the scope of the patent claims.