Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Patent
1997-12-23
1999-10-26
Bell, Bruce F.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
423562, C25B 100, C01B 1718
Patent
active
059721979
DESCRIPTION:
BRIEF SUMMARY
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 in a pulp production process.
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 the 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 active carbon (e.g. 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. Here, polysulfide sulfur which may also be referred to as PS-S, is meant for sulfur of 0 valency in e.g. sodium polysulfide Na.sub.2 S.sub.x, i.e. sulfur of (x-1) atom. Further, the sulfide ion composing polysulfide ion which may also be referred to as Na.sub.2 S as S, is meant for sulfur corresponding to sulfur having oxidation number of -2 in the polysulfide ions, or sulfur of one atom per S.sub.x.sup.2-. Further, in this specification, the unit liter for the volume will be represented by l. However, by this air oxidation method, thiosulfate ions not useful for cooking, are likely to form by side reactions (e.g. reaction formulas 2 and 3), whereby it used to be difficult to produce a polysulfide cooking liquor at a high PS-S concentration with a high selectivity.
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, an electrode substrate surface-coated with one or more oxides of ruthenium, iridium, platinum and palladium, is used. Specifically, a three-dimensional mesh electrode composed of a plurality of expanded-metal layers is disclosed as the anode.
DISCLOSURE OF INVENTION
It is an object of the present invention to obtain highly concentrated polysulfides by an electrolytic method from sulfide ions in a solution, particularly to produce cooking liquor having highly concentrated polysulfide sulfur from white liquor in a pulp production process at high selectivity with low electric power while minimizing by-production of thiosulfate ions.
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, of which at least the surface is made of carbon, a cathode compartment provided with a cathode, and a diaphragm partitioning the anode compartment and the cathode compartment, and carrying out electrolytic oxidation to obtain polysulfide ions.
In the present invention, carbon material is used for at least the surface portion of the anode, whereby the anode has practically adequate durability in the production of polysulfides. Carbon exhibits adequate electrical conductivity as an anode, whereby IR (resistance) drop at the anode can be reduced. Further, the anode used in the present invention has good electrical conductivity and is a porous structure with a large surface area, whereby the desired electrolytic reaction takes place over the entire surface of the electrode thereby to suppress formation of by-products.
BEST MODE FOR CARRYING OUT THE INVENTION
The anode is required to be made of carbon at least at its surface. Namely, the entire anode may be made of a carbon, or carbon may be coated on the surface of a substrate made of other than carbon. It is particularly preferred to use a porous body made of integrated carbon fibers as the anode, whereby a sufficient surface area can be obtained, and an
REFERENCES:
patent: 4108757 (1978-08-01), Fleet et al.
Andoh Tatsuya
Endoh Eiji
Shimohira Tetsuji
Asahi Glass Company Ltd.
Bell Bruce F.
Kawasaki Kasei Chemicals Ltd.
Wong Edna
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