Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing organic compound
Reexamination Certificate
2001-08-27
2003-02-11
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing organic compound
C205S444000, C205S554000, C205S494000, C205S746000, C205S758000, C205S759000, C205S776000
Reexamination Certificate
active
06517699
ABSTRACT:
1. Technical Field
The present invention relates to a method for producing polysulfides by electrolytic oxidation. Particularly, it relates to a method for producing a polysulfide cooking liquor by electrolytically oxidizing white liquor or green liquor in a pulp production process.
2. Background Art
It is important to increase the yield of chemical pulp for effective utilization of wood resources. A polysulfide cooking process is one of techniques to increase the yield of kraft pulp as the most common type of chemical pulp.
The cooking liquor for the polysulfide cooking process is produced by oxidizing an alkaline aqueous solution containing sodium sulfide, i.e. so-called white liquor, by molecular oxygen such as air in the presence of a catalyst such as activated carbon (e.g. the following reaction formula 1) (JP-A-61-259754 and JP-A-53-92981). By this method, a polysulfide cooking liquor having a polysulfide sulfur concentration of about 5 g/l can be obtained at a selectivity of about 60% and a conversion of 60% based on the sulfide ions. However, by this method, if the conversion is increased, thiosulfate ions not useful for cooking, are likely to form in a large amount by side reactions (e.g. the following reaction formulae 2 and 3), whereby it used to be difficult to produce a cooking liquor containing polysulfide sulfur at a high concentration with a high selectivity.
4Na
2
S+O
2
+2H
2
O→2Na
2
S
2
+4NaOH (1)
2Na
2
S+2O
2
+H
2
O→2Na
2
S
2
O
3
+2NaOH (2)
2Na
2
S
2
+3O
2
→2Na
2
S
2
O
3
(3)
Here, polysulfide sulfur which may be referred to also as PS-S, is meant for sulfur of 0 valency in e.g. sodium polysulfide Na
2
S, i.e. sulfur of (x−1) atoms. Further, in the present specification, sulfur corresponding to sulfur having oxidation number of −2 in the polysulfide ions (sulfur of one atom per S
x
2−
) and sulfide ions (S
2−
) will generically be referred to as Na
2
S-state sulfur. In the present specification, the unit liter for the volume will be represented by l.
On the other hand, PCT International Publication WO95/00701 discloses a method for electrolytically producing a polysulfide cooking liquor. In this method, as an anode, a substrate surface-coated with an oxide of ruthenium, iridium, platinum or palladium, is used. Specifically, a three-dimensional mesh electrode composed of a plurality of expanded-metals is disclosed. Further, PCT International Publication WO97/41295 discloses a method for electrolytically producing a polysulfide cooking liquor by the present applicants. In this method, as the anode, a porous anode at least made of carbon is used, particularly an integrated body of carbon fibers having a diameter of from 1 to 300 &mgr;m is used.
It is an object of the present invention to produce a cooking liquor containing polysulfide ions at a high concentration by an electrolytic method from a solution containing sulfide ions, particularly white liquor or green liquor in a pulp production process at a high selectivity with a low electrolytic power while minimizing by-production of thiosulfate ions. Further, it is an object of the present invention to provide a method for producing a polysulfide cooking liquor under such a condition for the electrolytic operation that the pressure loss is small and clogging is minimum.
DISCLOSURE OF THE INVENTION
The present invention provides a method for producing polysulfides, which comprises introducing a solution containing sulfide ions into an anode compartment of an electrolytic cell comprising the anode compartment provided with a porous anode, a cathode compartment provided with a cathode, and a diaphragm partitioning the anode compartment and the cathode compartment, for electrolytic oxidation to obtain polysulfide ions, characterized in that the porous anode is disposed so that a space is provided at least partly between the porous anode and the diaphragm, and the apparent volume of the porous anode is from 60% to 99% based on the volume of the anode compartment.
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, the porous anode is disposed so that a space is provided at least partly between the porous anode and the diaphragm, and the apparent volume of this porous anode is from 60% to 99% based on the volume of the anode compartment. Here, the volume of the anode compartment is the volume of a space defined by the effective current-carrying surface of the diaphragm and an apparent surface of the portion of the stream of an anode solution most distanced from the diaphragm, in other words, an apparent surface of the portion of the anode solution stream which flows most distantly from the diaphragm. The space to be formed between the anode and the diaphragm, may be formed over the entire effective current-carrying surface or may be formed at a part thereof. In a case where clogging is likely to take place when a solid component having a large particle size enters into the electrolytic cell, this space is preferably continuous as a flow path. If this apparent volume exceeds 99%, the pressure loss tends to be large on the electrolytic operation, or suspended substances are likely to cause clogging, such being undesirable. If the apparent volume is less than 60%, the amount of the anode solution flowing through the porous anode tends to be too small, whereby the current efficiency tends to be poor, such being undesirable. Within this range, the electrolytic operation can be carried out with a small pressure loss without clogging while maintaining a good current efficiency. This value is more preferably set to be from 70 to 99%.
Further, the present inventors have found that a space on the diaphragm side will provide an unexpected effect. It is considered that the electrode reaction of the anode in the present invention takes place substantially over the entire surface of the porous anode, but at a portion of the anode close to the diaphragm, the electric resistance of the solution is small, and the current tends to flow readily, whereby the reaction proceeds preferentially. Accordingly, at such a portion, the reaction tends to be mass transfer rate controlling step, whereby by-products such as thiosulfate ions or oxygen, tend to form, or dissolution of the anode is likely to occur. However, if a space is provided between the porous anode and the diaphragm, the linear velocity of the anode solution through this space tends to be high, the flow rate of the solution at a portion on the diaphragm side of the anode increases as induced by this flow, and the material diffusion at the portion of the anode close to the diaphragm will be advantageous, whereby it is possible to effectively control the side reactions.
Further, by this space, the flow of the anode solution tends to be smooth, and there will be a merit that deposition tends to scarcely accumulate on the anode side surface of the diaphragm.
As the porous anode to be used in the present invention, those having various shapes or made of various materials may be employed. Specifically, carbon fibers, carbon felts, carbon papers, metal foams, meshed metals or meshed carbon, may, for example, be mentioned. A metal electrode having modification with e.g. platinum applied to the surface, is also suitably employed.
In the present invention, the above electrolytic operation is preferably carried out under such a pressure condition that the pressure in the anode compartment is higher than the pressure in the cathode compartment. If the electrolytic operation is carried out under such a condition, the diaphragm will be pressed to the cathode side, and the above-mentioned space can readily be provided between the porous anode and the diaphragm.
The porous anode of the present invention preferably has a physically continuous three-dimensional network structure. The three-dimensional network structure is preferred, since it is thereby possible to increase the anode surface area, and the desired electrolytic reaction takes place over the entire surface of the
Andoh Tatsuya
Nanri Yasunori
Shimohira Tetsuji
Tanaka Junji
Watanabe Keigo
Asahi Glass Company Limited
Wong Edna
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