Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Recycling electrolytic product produced during synthesis...
Reexamination Certificate
2001-02-22
2002-12-10
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Recycling electrolytic product produced during synthesis...
C205S496000, C205S554000, C205S471000
Reexamination Certificate
active
06491807
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing sodium persulfate. Sodium persulfate has been widely used in industrial process, for example, as a polymerization initiator for the production of polyvinyl chloride and polyacrylonitrile and as a treating agent for printed wiring boards.
2. Description of the Prior Art
As a general production method of sodium persulfate, the reaction between ammonium persulfate and sodium hydroxide in an aqueous solution has been known (U.S. Pat. No. 3,954,952). However, this process is uneconomical because the yield of sodium persulfate based on ammonium persulfate is low due to a large number of steps required. In addition, the concentration of sulfuric acid in the catholyte feed solution should be lowered to maintain a high solubility of ammonium sulfate to the catholyte feed solution, this increasing the electrolytic voltage, i.e., the unit power cost.
U.S. Pat. No. 4,144,144 discloses a direct electrolytic production of sodium persulfate using a neutral anolyte feed solution in the presence of ammonium ion. In this process, the mother liquor after removing crystallized sodium persulfate is mixed with a cathode product and recycled to an electrolytic step as the anolyte feed solution. Therefore, the electrolysis is conducted in the presence of sodium persulfate which participates nothing in the electrolysis, this increasing the electrolysis voltage and decreasing the current efficiency. In addition, since the resultant sodium persulfate crystals contain nitrogen in higher concentrations, a careful and thorough washing is necessary to purify sodium persulfate to an acceptable level for practical use.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above problems in the prior art and to provide a process for producing sodium persulfate in a low unit power cost and a reduced number of production steps.
After extensive study for solving the above problems, the inventors have found that sodium persulfate is more economically produced by electrolyzing an anolyte feed solution containing sodium sulfate, ammonium sulfate and sodium persulfate, reacting the resulting anode product with sodium hydroxide and crystallizing sodium persulfate by concentration, while recovering ammonia gas liberated from the crystallization step into a cathode product, followed by neutralizing the resulting cathode product with sodium hydroxide and/or ammonia and recycling a mixture of the neutralized solution with sodium sulfate recovered from a crystallization mother liquor as a part of the starting material for the anolyte feed solution.
Thus, the present invention provides a process for producing sodium persulfate, comprising (1) a step of electrolyzing a catholyte feed solution containing sulfuric acid and an anolyte feed solution containing sodium sulfate, ammonium sulfate and sodium persulfate, thereby obtaining a cathode product and an anode product; (2) a step of reacting the anode product with sodium hydroxide in a reaction-type crystallizer, thereby obtaining a reaction mixture; (3) a step of crystallizing sodium persulfate from the reaction mixture by concentration, thereby obtaining a sodium persulfate slurry; (4) a step of separating the sodium persulfate slurry to sodium persulfate crystals and a mother liquor, thereby recovering the sodium persulfate crystals; (5) a step of crystallizing sodium sulfate from the mother liquor, thereby obtaining a sodium sulfate slurry; (6) a step of separating sodium sulfate crystals from the sodium sulfate slurry; (7) a step of recovering ammonia gas liberated in the step (2) into the cathode product obtained in the step (1); (8) a step of neutralizing the resulting cathode product with sodium hydroxide and/or ammonia to obtain a neutralized cathode product; and (9) a step of recycling the neutralized cathode product and the sodium sulfate separated in the step (6) to the step (1) as a part of a starting material for the anolyte feed solution.
DETAILED DESCRIPTION OF THE INVENTION
In the electrolysis step (1) of the process of the present invention, an aqueous solution containing, by weight, 5 to 18% sodium sulfate, 21 to 38% ammonium sulfate and 0.1 to 2% sodium persulfate is used as an anolyte feed solution. The sulfate ratio, sodium sulfate/ammonium sulfate, is preferably 0.1 to 0.9 by weight. When the sulfate ratio is less than 0.1, the available amount of sodium sulfate obtained in the separation step (6) is reduced to increase the unit material cost. A sulfate ratio higher than 0.9 increases the electrolytic voltage to increase the unit power cost. The anolyte feed solution may further contain 0.01 to 0.1% by weight of a known polarizer such as thiocyanate, cyanide, cyanate and fluoride. The catholyte feed solution is a 20 to 80% by weight aqueous solution of sulfuric acid.
The electrolytic cell usable in the present process is not specifically limited so long as it is structured to separate the anode from the cathode by a diaphragm, and a box electrolytic cell or a filter press electrolytic cell is preferably used. The diaphragm for the box electrolytic cell is made of an oxidation resistant material such as alumina. Ion-exchange membranes are preferably used as the diaphragm of the filter press electrolytic cell.
The anode is preferably made of platinum, although anodes made of a chemically resistant material such as carbon are usable. The cathode is preferably made of zirconium or lead, although cathodes made of a chemically resistant material such as stainless steel are usable. The anode current density is 40 to 120 A/dm
2
, preferably 60 to 80 A/dm
2
. A current density lower than 40 A/dm
2
produces a poor current efficiency. A current density higher than 120 A/dm
2
could be used, but uneconomical because a specific power supply equipment is needed due to a considerable heat generation at a bus bar.
The electrolytic cell is operated at 10 to 40° C., preferably 25 to 35° C. Temperatures lower than 10° C. are detrimentally low because sodium sulfate, etc. begin to crystallize to make the process inoperable and an unnecessarily high electrolytic voltage is required. Temperatures exceeding 40° C. are undesirably high because excessive decomposition of the resulting persulfate ion occurs to result in a low yield of sodium persulfate.
Then, the anode product from the electrolysis step (1) is introduced into a reaction-type crystallizer and reacted with an aqueous solution of sodium hydroxide in the step (2), followed by the step (3) where sodium persulfate is caused to crystallize from the reaction mixture by concentration. The reaction-type crystallizer is not specifically limited so long as it is operable under reduced pressure, and a reaction-type crystallizer equipped with an agitator, preferably a double propeller reaction-type crystallizer having a clarification zone is used. The reaction-type crystallizer so constructed facilitates the sampling of at least a part of the liquid therein in the step (3) for crystallizing sodium persulfate.
The crystallization of sodium persulfate in the reaction-type crystallizer is carried out at 15 to 60° C., preferably 20 to 50° C. When the temperature is lower than 15° C., the reaction rate between the anolyte product and sodium hydroxide is low and the coexisting sodium sulfate is likely to crystallize to lower the purity of sodium persulfate crystals. At temperatures higher than 60° C., excessive decomposition of the resulting sodium persulfate occurs to result in a low yield of sodium persulfate. The residence time in the reaction-type crystallizer depends on the desired particle size of sodium persulfate, and generally selected from the range of 1 to 10 hours. The residence time can be shorter than one hour if sodium persulfate with smaller particle size is desired.
Sodium hydroxide is added to the anode product solution introduced into the reaction-type crystallizer in an amount enough to displace at least proton and ammonium ion attributable to by-produced sulfuric acid, ammonium
Kajiwara Shoichiro
Kimizuka Ken-ichi
Kogure Naoki
Tsuruga Takamitsu
Antonelli Terry Stout & Kraus LLP
Mitsubishi Gas Chemical Company Inc.
Valentine Donald R.
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