Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2000-01-28
2001-07-24
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S554000, C205S746000, C423S562000
Reexamination Certificate
active
06264819
ABSTRACT:
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.
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. 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, thiosulfate ions not useful for cooking, are likely to form by side reactions (e.g. 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+20
2
+H
2
O→Na
2
S
2
O
3
+2NaOH (2)
2Na
2
S
2
+30
2
→2Na
2
S
2
O
3
(3)
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
2
Sx, 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 Sx
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 a 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, WO97/41295 discloses a method for electrolytically producing a polysulfide cooking liquor by the present inventors. 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 obtain polysulfides at a high concentration by an electrolytic method from sulfide ions in a solution, particularly to produce a cooking liquor containing polysulfide sulfur at a high concentration from white 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 electrolysis operation that the pressure loss is small.
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, wherein at least the surface of said anode is made of nickel or a nickel alloy containing nickel in an amount of at least 50 wt %, the porous anode has a physically continuous three-dimensional network structure, and the surface area of the anode per unit volume of the anode compartment is from 500 to 20000 m
2
/m
3
.
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, the anode compartment is provided with a porous anode, of which at least the surface is made of nickel or a nickel alloy containing nickel in an amount of at least 50 wt %, which has a physically continuous three-dimensional network structure, and which has a surface area of from 500 to 20000 m
2
/m
3
per unit volume of the anode compartment. Since at least surface portion of the anode is made of nickel or a nickel alloy, a practically adequate durability can be obtained in the production of polysulfides. Nickel can be available at a relatively low cost, and the elution potential and the formation potentials of its oxide are is higher than the formation potentials of polysulfide sulfur and thiosulfate ions, whereby it is suitable as an electrode material to obtain polysulfide ions by electrolytic oxidation. The surface of the anode in the present invention is preferably made of nickel, and a nickel alloy containing nickel in an amount of at least 50 wt % may be used. More preferably the nickel alloy has a nickel content of at least 80 wt %.
Further, the anode is porous and has a three-dimensional network structure, whereby it has a large surface area, and accordingly when it is used as the anode, the desired electrolytic reaction will take place over the entire surface of the electrode thereby to suppress formation of by-products. Further, said anode is not an integrated body of fibers, but has a physically continuous network structure, whereby it exhibits adequate electrical conductivity as the anode, and IR drop at the anode can be reduced, and accordingly the cell voltage can further be decreased. Further, since the anode has good electrical conductivity, the porosity of the anode can be made high, whereby pressure loss can be decreased.
In the present invention, the surface area of the anode per unit volume of the anode compartment is required to be from 500 to 20000 m
2
/m
3
. Here, the volume of the anode compartment is represented by the volume of a portion partitioned by the effective current-carrying surface of the diaphragm and a current-collecting plate of the anode. If the surface area of the anode is smaller than 500 m
2
/m
3
, the current density at the anode surface tends to be high, whereby not only by-products such as thiosulfate ions are likely to form, but also nickel is likely to undergo anode dissolution, such being undesirable. If the surface area of the anode is larger than 20000 m
2
/m
3
, there are possible problems in the electrolysis operation such that the pressure loss of the liquid may increase, such being undesirable. The surface area of the anode per unit volume of the anode compartment is more preferably within a range of from 1000 to 10000 m
2
/m
3
.
Further, the surface area of the anode is preferably from 2 to 100 m
2
/m
2
per unit area of the diaphragm partitioning the anode compartment and the cathode compartment. The surface area of the anode is more preferably from 5 to 50 m
2
/m
2
per unit area of said diaphragm. The anode is required to be made of nickel or a nickel alloy at least at its surface, and the entire anode may be made of nickel or a nickel alloy. Further, the anode has a physically continuous three-dimensional network structure, and forms a porous structure. The network structure is a physically continuous structure, and may be continuously bonded by e.g. welding. Concretely, porous nickel obtained in such a manner that nickel is plated on the skeleton of a foam high molecular material, and the high molecular material in the inside is removed by calcinations, may be mentioned.
Pores in the anode has an average pore size of preferably from 0.1 to 5 mm. If the average pore size of the pores is larger than 5 mm, the surface area of the anode can not be made large, whereby the current density at the anode surface tends to be high, and accordingly, not only by-products such as thiosulfate ions are likely to form, but also nickel is likely to undergo anode dissolution, such being u
Andoh Tatsuya
Shimohira Tetsuji
Tanaka Junji
Asahi Glass Company Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Phasge Arun S,.
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