Process for producing polysulphide by means of oxidizing sulphide in spent liquors from kraft cellulose cooking

The invention relates to a process for producing polysulphide by means of oxidizing sulphide in spent liquors from kraft cellulose cooking. The process is characterized in that green liquor is firstly enriched in sulphide and then oxidized.

BACKGROUND OF THE INVENTION 
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 is 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 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 U.S. Pat. No. 
5,082,526. 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 U.S. Pat. No. 4,024,229. 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.

SUMMARY OF THE INVENTION 
There has, therefore, long been a need to improve the known processes in 
order to obtain a higher yield of sodium polysulphide without using a 
sensitive catalyst, and a process has therefore been developed, in 
accordance with the present invention, for producing polysulphide by means 
of oxidizing sulphide in spent liquors from kraft cellulose cooking, which 
process is characterized in that green liquor is firstly enriched in 
sulphide and is then oxidized. 
It is expedient for the green liquor to be partially causticized prior to 
the enrichment in sulphides. 
According to the invention, calcium particles (lime sludge) may be present 
in the liquor both in association with the enrichment with sulphide and 
during the oxidation. 
When the green liquor is causticized, it can, according to the invention, 
be causticized up to as much as 50% of the normal degree of 
causticization. 
According to the invention, green liquor sludge can be added prior to or 
during the oxidation. 
Black liquor can also be added to the green liquor prior to the oxidation. 
According to the invention, the green liquor to which black liquor has been 
added and which has been oxidized to a high polysulphide content may 
expediently be used for impregnating wood chips which are to be cooked. 
The enrichment with sulphides can be achieved, for example, by sodium 
carbonate being crystallized out and filtered off from the green liquor. 
The green liquor, which can be partially causticized and contain lime 
sludge, is evaporated, in association with which sodium carbonate 
precipitates out and can be filtered off together with the lime sludge. In 
this way, a sulphide-rich liquor is obtained which can be further 
causticized to form white liquor. Such an enrichment of the green liquor 
with sulphide is described in Swedish Patent no. 500748. The green liquor 
is expediently causticized by this procedure up to 50% of the normal 
degree of causticization before the sulphide is oxidized to polysulphide 
in accordance with the present invention. 
When the smelted chemicals from the recovery boiler are dissolved to form 
green liquor, a so-called green liquor sludge is obtained. This consists 
of solid constituents, principally calcium carbonate and magnesium 
hydroxide, but also manganese hydroxide, iron salts, aluminium salts and 
the like, in addition to carbon particles and possibly other ash 
substances as well. According to the invention, this green liquor sludge, 
which contains manganese ions, inter alia, can be added prior to or during 
the oxidation of sulphide to polysulphide. In this way, a higher yield is 
obtained by virtue of the catalytic effect of the green liquor sludge, 
principally due to the manganese content of the sludge. 
The sulphide-enriched green liquor can also be treated with black liquor 
prior to the oxidation and then be oxidized. All the constituents of the 
green liquor sludge are present in the black liquor and these constituents 
will then exert a catalytic effect during the oxidation of the 
sulphide-enriched green liquor. 
This oxidized mixture of green liquor and black liquor is especially 
suitable for impregnating wood chips which are to be cooked in accordance 
with the kraft method. 
The invention is not limited to the abovementioned embodiments and can be 
varied in different ways within the scope of the patent claims.