Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2000-02-07
2001-03-13
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S554000
Reexamination Certificate
active
06200454
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for producing sodium persulfate, which is widely employed in industrial fields at the present time as a polymerization initiator for polyvinyl chloride and polyacrylonitrile and as a treatment agent for a printed wiring board.
As a general process for producing sodium persulfate, there is known a production process by the reaction of ammonium persulfate and sodium hydroxide in an aqueous solution. It is necessary in the above-mentioned process that in the first place ammonium persulfate as a starting raw material be produced by an electrolysis method, and the resultant ammonium persulfate be concentrated and separated by vacuum crystallization, centrifugal filtration or the like and then taken out as a crystal. At this time, the solution containing the crystal (usually referred to as “mother liquor”) is mixed with the liquid produced at a cathode and is used as a starting raw material for an anolyte.
The ammonium persulfate thus obtained is re-dissolved in the next step, and is transferred to the step of reaction with sodium hydroxide. In the aforesaid reaction step, a solution containing sodium persulfate is produced, then is concentrated and separated by vacuum crystallization, centrifugal filtration or the like and is subsequently taken out as a crystal. As mentioned hereinbefore, the process for producing sodium persulfate by the reaction of ammonium persulfate and sodium hydroxide necessitates quite long production steps and a number of steps, and, moreover, lowers the yield of the objective sodium persulfate based on the ammonium persulfate, thereby making itself far from economically advantageous.
Under such circumstances, several attempts have been made to produce sodium persulfate by direct electrolysis without passing through ammonium persulfate. For instance, Japanese Patent Application Laid-Open No. 56395/1975 (Sho-50) describes a process for producing sodium persulfate by the use of sodium hydrogensulfate as a starting raw material, which process, however, is impractical because of an extremely low current efficiency in the electrolysis.
In addition, Japanese Patent Publication No. 31190/1980 (Sho-55) describes a process for producing sodium persulfate by means of electrolysis through the use of a neutral starting raw material for an anolyte in the presence of ammonium ions, which process, however can not be said to be economical because of a low current efficiency being about 70 to 80% in the electrolysis. Further, the above-mentioned process suffers such disadvantages that the ammonium ions being contained in the objective crystal increase the content of nitrogen components in the objective sodium persulfate, and that the process necessitates a minute and attentive cleaning step in order to satisfy the ordinary requirement for the quality of sodium persulfate as the finished product, namely a purity of at least 99% and the content of nitrogen components of at most 0.1%. In spite of a number of efforts and endeavors having heretofore been directed towards the improvement of the production process, it is the real situation that an economical process for producing sodium persulfate has not yet been developed.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the problems involved in the conventional processes for producing sodium persulfate as described hereinbefore, and at the same time to provide a process for producing sodium persulfate in an industrially advantageous manner.
As the result of intensive research and development carried out by the present inventors under such circumstances in order to overcome the aforesaid disadvantages, there has been found a process for producing sodium persulfate which comprises the steps of producing ammonium persulfate by electrolysis at an anode in the presence of sodium ions, directly adding sodium hydroxide to the resultant liquid produced at the anode to produce sodium persulfate (reaction step), and concentrating and separating the resultant sodium persulfate. It has also been found that by the use of a starting raw material for an anolyte which coexists with sodium ions and which is obtained by mixing the liquid produced at a cathode with the crystallization mother liquor formed by concentrating and separating the resultant sodium persulfate, there is obtained a current efficiency in electrolysis for ammonium persulfate which surprisingly exceeds the current efficiency for ammonium persulfate without coexisting with sodium ions. The present invention has been accomplished by the above-mentioned findings and information.
That is to say, the present invention relates to a process for producing sodium persulfate which comprises the step (1) of electrolyzing, at an anode, a solution containing ammonium sulfate and the step (2) of producing sodium persulfate from the resultant liquid produced at the anode and sodium hydroxide.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, a detailed description will be given of the process according to the present invention. In the electrolysis step, that is the step (1) in the process of the present invention, there is used a solution containing ammonium sulfate as a starting raw material for an anolyte, preferably a solution containing ammonium sulfate and further containing sodium ions, in particular, a solution comprising 15 to 40% by weight of ammonium sulfate, 5 to 20% by weight of sodium sulfate and 0.1 to 20% by weight of sodium persulfate, the solution just mentioned above in the form of an aqueous solution having a concentration range in which the concentration of the ammonium sulfate is higher than that of the sodium sulfate. Preferably the starting raw material for the anolyte contains a necessary amount of a polarizer, which is exemplified by thiocyanate, cyanide, cyanate and fluoride. The starting raw material for the anolyte is not necessarily required to be regulated neutrally, and it may contain a free acid, which does not influence the current efficiency at all. There is used as a starting raw material for a catholyte, 10 to 80% by weight of an aqueous solution of sulfuric acid containing ammonium sulfate having a concentration range of 0 to 35% by weight. The use of the aqueous solution having a concentration outside the above-mentioned range is disadvantageous because of a low current efficiency obtained therefrom.
The electrolytic cell to be used for the process according to the present invention may be either a diaphragm cell which is partitioned with an alumina diaphragm and is widely industrially used, or a filter press-type electrolytic cell which is partitioned with ion exchange membranes. The anode is made preferably of platinum, and there is usable a material having chemical resistance such as a carbon electrode. The cathode is made preferably of lead or zirconium, and there is usable a metallic electrode having acid resistance such as stainless steel. The current density of the surface of the anode is at least 40 A/dm
2
. The temperature inside an electrolytic cell is 15 to 40° C. The temperature therein, when being lower than said range, gives rise to a fear of salt deposition from the solution. The solubility of a salt increases with a rise in the temperature of the solution, but an unreasonably high temperature therein is unfavorable because of the liability of the produced persulfate to hydrolysis reaction.
The reactor to be used for the step (2) in the process according to the present invention for reacting the liquid produced at the anode with an aqueous solution of sodium hydroxide, is not specifically limited provided that it is usable under reduced pressure, and may be equipped with an agitator. The amount of sodium hydroxide to be added to the liquid produced at the anode is the amount necessary to turn all the cations contained in said liquid to sodium ions. The reaction temperature is 15 to 60° C., preferably 20 to 50° C. The reaction temperature, when being lower than said range, brings about such adverse influences as the deposition of ammoni
Kajiwara Shoichiro
Kimizuka Ken-ichi
Tsuruga Takamitsu
Antonelli Terry Stout & Kraus LLP
Mitsubishi Gas Chemical Company Inc.
Phasge Arun S,.
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