July 18, 1944. H, R NEUMARK ­ y 2,353,782 I ELECTROLYTIC PREPARATION OF SODIUM FERRICYANIDE Filed May 2, 1942 / 3 l? ATTORNEY Patented July 18, 1944 '2,353,782 UNITED STATES PATENT­ OFFICE 2,353,782 ELECTROLYTIC PREPARATION OF SODIUM FERRICYANIDE Hans R.. Neumark, Forest Hills, N. Y., assignor to General Chemical Company, New York, N. Y., a corporation of New York Application May 2, 1942, Serial No. 441,500 9 Claims. (Cl. 204-91) This invention relates to manufacture of so I have »found that the objects of the invention, dium ferricyanide. With respect to removal of sodium hydroxide im It has been proposed to make sodium ferri purity contained in a sodium lferricyanide solu cyanide by electrolytic oxidation of sodium fer tion, maybe accomplished by treating the anolyte rocyanide. Such an operation may be carried out liquor containing sodium hydroxide with certain in a cell equipped with an anode and a cathode agents which function to convert the sodium of surrounded by a porous ceramic diaphragm form the sodium hydroxide to sodium iron-cyanide ing the cathode chamber. On passage of current compounds which are soluble and remainl in so thru the cell, production of sodium ferricyanide lution in the treated liquor, and to convert the is understood to take place as follows: hydroxide component of the sodium .hydroxide to an iron hydroxide precipitate which may be removed from the treated solution by simple fil tration. Thus, I am enabled to substantially According to this equation, it will be noted that completely neutralize or lower the sodium hy as oxidation proceeds, for every mol of sodium. droxide content of anolyte liquorsto any desired ferricyanide formed one mol of NaOH is also extent, to -suit purity requirements of the sodium produced. Approximately half of this NaOH is ferricyanide product, and to transform the so retained in the cathode chamber while the bal diumconstituent of the sodium hydroxideto4 a ance of such NaOH Works thru the porous Walls soluble sodium iron-cyanide and remove from of the diaphragm into the anolyte solution Which,­ the system the hydroxide radical of the sodium on completion of electrolysis, is withdrawn from hydroxide so neutralized, all Without introducing the cell. into the operation any material Which adversely In an operation of this nature, it is not feasible affects purity of the­ ultimate product, and With to carry electrolysis to a point Where substantial out causing loss of sodium or cyanide values by ly all of the sodium ferrocyanide is oxidized to liquor bleed-off as is often necessary in chemical ferricyanide. Hence, a so-called ñnished sodium processes. ferricyanide anolyte solution contains as an im With regard to removal of lsodium ferrocyanide purity, in addition to a substantial amount of so impurity still present in the -sodium ferricyanide dium hydroxide, a generally equal quantity of liquor after the desired neutralization ofsodium unoxidized sodium Íerrocyanide. To my knowl hydroxide, I iind that there are certainherein edge substantially pure sodium ferricyanide, . after described conditions according to> which either in liquid or solid form, has not been com the sodium ferricyanide solutionmay be concen mercially obtainable. Basis for this diñiculty is trated in such a Waythat it becomes> possible to the very high solubility of sodiumlferricyanide precipitate-out of the solution, in a readily sep coupled with the relatively high solubility of the arable form, substantially all or any desired lesser sodium hydroxide and sodium ferrocyanide im >amount of the sodium ferrocyanide impurity, and purities. to accomplish this separation VWithout any sub The principal object of this invention is pro stantial formation of» solid ferricyanide, i. e‘., sub vision of a process for making sodium ferricyanide stantially all of the latter being held in solution. of controllable degree of purity. The invention ~ Briefly, `the process» of the invention ’comprises aims to provide for electrolytic manufacture of electrolytically oxidizing a solution offs'odíum fer sodium ferricyanide by procedure according to ­ rocyanide to form a sodium ferricyanide liquor which it is possible to remove, substantially com -f containing sodium hydroxide ‘and sodium ferro pletely or to any desired lesser extent, the sodium cyanide as impurities, treating'th'e liquor With hydroxide impurity Without introducing extra- > certain `neutralizing agents to convert'the hy neous impurities into the sodium ferricyanide droxide component of the sodium hydroxide to liquor and without requiring losses of sodium or cyanide values. A further object lies in the pro iron hydroxide, removing the latter from'the vision of a process by practice of which it is pos system, concentrating resultingl sodium 4ferri sible to similarly remove, substantially complete 50 cyanideliquor still containing soluble ferrocy ly> or to` any desired smaller degree, the sodium anide impurity according tocerta'inïprocedural ferrocyanide impurity inherently present in a so steps to effect precipitation of sodium f_errocy dium ferricyanide anolyte formed by commercial _, anide impurityy as crystals',,separatingthe latter ly feasible electrolytic oxidation of sodium ferro from the sodium ferricyanide liquor, and'then re cyanide. covering from `the residual. _sodium ferricya-nide 2 2,353,782 solution, substantially pure sodium ferricyanide of the group consisting of ferro and ferri cyanic optionally in liquid or solid form. acids, such salts being reactable with the so In carrying out the invention, in apparatus dium hydroxide contained in the liquors treated such as illustrated diagrammatically on the ac to form soluble sodium iron-cyanide and to pre companying ñow sheet, when starting operation cipitate iron hydroxide. Examples of suitable a. sodium ferrocyanide solution may be formed neutralizing or treating agents are Prussian in make-up tank IU, by dissolving in water blue, f understood to be ferrie ferrocyanide, Na4Fe(CN) 6.101-120 crystals in quantity to form Fea(Fe(CN) s) s; Turnbull’s blue, understood a substantially saturated solution. A sodium fer to be ferrous ferricyanide, Fes(Fe(CN)ß)z; and rocyanide solution as fed into the anolyte cham-Y a more or less white iron salt, understood to be ber II of an electrolytic cell I2 should contain a ferrous ierrocyanide, FezFe<CN> s. certain amount of sodium hydroxide, the function The most satisfactory treating or neutraliz of which is primarily to prevent corrosion of the ing agent is Prussian blue which when added to anode and secondarily to promote conductivity a sodium ferricyanide solution containing so of the anolyte solution. Hence, a desired amount dium hydroxide reacts with sodium hydroxide in of sodium hydroxide from an external source accordance with the following: may be added to the sodium ferrocyanide solu (2) 12NaOH-I-Fe4 (Fe (CN) s) 3*) tion in tank IU, and a typical starting solution 4Fe (OH) 3+3Na4Fe(CN) s may comprise from 200 g. p. l. to 300g. p. l. of Na4Fe(CN)s and from one to 5 g. p. 1. of NaOI-I. Thus, the sodium of the neutralized sodium hy The starting solution is ru-n into the anolyte ' droxide is converted back to soluble sodium fer chamber of the cell provided preferably with a rocyanide which is the essential constituent of a nickel anode and a steel cathode surrounded by starting solution fed into an electrolytic cell and a porous ceramic diaphragm I3 forming a cath ' which ferrocyanide may be eventually recycled ode chamber I 4. Circulation of solution in the thru the process by being returned to tank I0. anolyte chamber, and solution temperature of The hydroxide content of the neutralized sodium the order of 40-50° C. may be_ maintained by hydroxide is converted to ferrie hydroxide which any convenient means. Ordinarily, during the may be ñltered out of the more or less neutral progress of oxidation, temperature should not ized liquor as by ñlter I8. exceed~50° C. in order to prevent decomposition The amount of treating agent to be used de of ferricyanide to iron hydroxide, and the tem pends upon the purity requirements of the ñnal perature maybe kept as high as say 40° C. to sodium ferricyanide product. In some in avoid crystallization Vof sodium ferricyanide. stances, product containing a relatively small ' The .cathode chamber is filled with a weak solu amount of sodium hydroxide may be unobjec tion of NaOH which functions primarily as a tionable, and in other instances it may be de conductor between the cathode and the walls of sired to form a product of best feasibly obtain the surrounding diaphragm. When the current able purity. The NaOH neutralization step of is .turned on', reaction proceeds in accordance the’present process is flexible and facilitates with Equation (1), and sodium ferricyanide is 'either substantially complete elimination of formed.` As previously explained, approximate 40 NaOH or removal of NaOH to any lesser extent ,y ly half of the NaOH formed during the course ’ desired. The amount of NaOH present in an of reaction lWorks its way thru the porous wall anolyte liquor may be, found by analyses and _ of _the diaphragm and into the anolyte solution. ,the theoretical quantity of treating agent needed The electrolytic oxidation operation may be ' to effect the desired degree of puriiication may carried out so as to fomn an anolyte solution 45 be readily calculated. In usual practice of the relatively concentrated with respect to sodium ' process, I find that an ultimate ferricyanide ferricyanide. YThis may be accomplished by ad ' product of acceptable purity with respect to dition to the anolyte liquor, from time to time NaOH content may be obtained when the quan as_oxidation proceeds, of further sodium ferro tity of treating agent employed is such as to cyanìde crystals in quantity to keep the anolyte lower pH of the treated anolyte solution to a solution substantially saturated with respect to value not higher than, 10.