US2439805A - Method of electrowinning manganese - Google Patents

Description

Patented Apr. 20, 1948 MANGANESE Herbert It. Hanley, Rolla, Mo., and James Jacobs, Boulder City, Nev.,

No Drawing. Application August 4,1942;- Serial No. 453,542 It V 2 Claims. (o1.'204--105) (Granted under the act of March 3, 1883, as

This invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to us of any royalty thereon.

Our invention relates to improvements in the art of winning manganese from aqueous solutions, and more particularly to an improved anode for use in the electrodeposition of manganese, the use of which is a marked contribution to the electrolytic managnese process.

The anode problem in the electrolytic manganese process is of vital importance. In a process where metal is plated from solutions using insoluble anodes, the choice of the material to be used in the anodes is very limited. There are .two main anode reactions which might occurjwhich would be detrimental to the process.

First, the anode must be insoluble, and second, the reactions occurring at the anode must not interfere with the process. Usually, though not necessarily, the anode product during electrolysis are oxidation compounds formed by the strong oxidizing atmosphere of probably nascent, oxygen.

The first consideration, namely, the insolubility of the anodes, is very well met by the use of lead anodes in sulphate solutions. However, lead does not meet the second requirement when used in the electrolytic manganese process. When manganese sulphate-ammonium sulphate solutions are electrolyzed using lead anodes, a considerable amount of the manganese in solution is rendered insoluble, perhaps as the oxides, the main one apparently being MnOz.

The following figures are given to show how serious this precipitation of manganese is to the electrolytic manganese process.

The Bureau of Mines reported in their electrolytic manganese work that for every three pounds of manganese produced as cathode metal there was one pound of manganese precipitated at the anodes. This amount of manganese precipitated constitutes a very serious problem, not only in keeping the electrolytic cells in proper operating condition, but finally in adding considerably to the cost of making cathode c1ectro lytic manganese.

The following figures show how this is true:

Taking 2000 pounds of cathode metal, there would be 667 pounds of manganese precipitated at the anodes as insoluble manganese compound;

The precipitate assays approximately 40% Mn, so there wouldbe 1334 pounds of dry manganese compounds suspended in the cell :anolyte. It would be necessary to filter these compounds from the anolyte, and as the wet filter cake assays t amended April :30, 1928; 370 G. 757) least moisture, there would be roughly 2600 pounds of the wet cake. In order to wash the ammonium s'ulphatebearing anolyte from the wet cake at least one ton of waterwould be necessary. This water wash would be a problem, and it is very possible that evaporation of it would be necessary in order to keep down plant solution' volumes. V

In order to recover the manganese in the wet filter cake a reduction roast would be necessary to convert the high manganesecompounds, such as M17102, to the acid soluble oxide MnO.

The manganese oxides on precipitating in the electrolytic cells also oiier a serious problem in the fouling of the diaphragm coverings. Also, periodically, the cells would-have to be shut down and cleanedof the manganeseoxides which build up in the cell bottoms.

The above explanation makes clear the desirability oiobtaining an anode which will prevent the precipitation or the manganese compounds. This kind ofanode would definitely cut down on the cost of producing electrolytic manganese.

There is one other feature in the electrolytic manganese process, asitnow stands, which is very undesirable and which can be overcome by use of an anode which would not allow precipitation of the oxides of manganese."

As the process is now operated the anodes and cathodes are separated by cloth diaphragms. The reason for this'is that manganese cannot be plated commercially from highly acid solutions. In the electrolytic cell arrangement the cathodes are placed inside the diaphragms and the anodes outsi de the diaphragms. As 27 cathodes are usually used in each electrolytic cell, and as about 12 cells are necessary to produce one ton of cathode manganese per day, this means 324 cathodes. Each'of these cathodes has its own independent feed line, and as the solution inside the diaphragm holds onlyone-half cubic foot of solution it is very evidentthat considerable supervision must be maintained in order to properly feed the cells. I Now, withan anode whichdoes not precipitate the manganese" oxides, the cathodes could be placed outside the diaphragms and the anodes inside the diaphragms. This would cut the feed lines'to the'cells from 2'7 to oneand at the same time insure a more uniform solution around the cathodes. The volume of solution per cathode jumpsfrom one-halfa cubic foot to four cubic feet, or eight times as much; As the cathode metal is the product from cells which is desired, the above change would make for steadier cathode p ting oonditionsg'and at the same time reduce procedure is expensive and the cloth quickly deteriorates and ruptures.

Colin G. Fink (Anodes for Electrowinning Manganese, The Electrochemical. $00., Preping 76-5;

September 14, 1939) developed an alloy of leadthe sameconditions, the 1% silver-lead anodes so tin-cobalt which will prevent manganese from becoming oxidized at the anodes. However, this alloy is very expensive and difficult to prepare, as

several steps are necessary in its preparation.

The principal object of our invention is the provision of a. relatively inexpensive anode which will solve the problems discussed. 7 I V The anode of our invention is easytoprepare, is of comparatively low cost, is insoluble in manganese sulphate-ammonium sulphate solutions, and minimizes precipitation of manganese anodic compound... This anode comprises a lead-silver alloy and research work'overa long periodhas proven its adaptability to the electrolytic manganese process.

A certain amount oithe manganesecompounds are formed initially on the alloy anode. .Preforming is indicated and more iullydiscussed in the experimental data appended.

Lead-silver alloysv form-a eutectic mixture at approximately 2% to 2%% silver, 97 72,. to 98% lead (International Critical Tables, vol. 2, p. 414; Metals Handbook, Am. Soc. for Metals, 1939 ed'., Cleveland, Ohio, p. 1523). We propose usually to use silver ina concentration about the eutectic or below. Experience in other fields ithe use of Pb-Ag alloy anodes is old, e. g., U. S. 1,851,219; 1,759,493 and 1,587,438) seems to indicate that higher proportions of silver may be undesirable and the'cost of silyer would appear-to render it inadvisable toexceed about 3%,- sofar assilver content is concerned, a proportion. which we have found experimentally togive very desirableresuits, We; therefore, donot limit ourselves to the use of very low percentages of silver inthe practice of our invention.

The practice of our invention may be illustrated by reference te-actualtest data given by way of example;

Several anodes were preparedby melting-tgether 99 of lead and 1% of'srlver and. casting the alloy so produced into an open: mold. The anodes s0 produced analyzed approximately 1% silver, balance lead. As cast, they were 1%." thick; and were trimmed to have a surface areasomewhat less,- thanthe surface area of. the oath odes with which they were used. Several anodes so produced werermounted in a cell, and the anodes preformed by" operating the cell for 24 hours; During .the'flrstiourj hours manganese was precipitated atsucliianodes; and at the endof this period the" surface was covered-with a blue-black'film oniwhich a dark; scale had started to form. 'I'hisrscalezincrea'sed during the prefor'rning' period. At? the; endtofifthe period" the scale was removecr'learej being exercised not: to rupture the. blue-blaclcfilmrzand theianodes werethen'weighed- The ce1ls,"using7 the anodes: prepared as above, were" operatedusing .asolution containing 140 grams perliter ofam-moniuin sulphate and 25 grams per liter of manganese asthe sulphate and 0.2 to- 0.5-gra-rn=perli-ten of sulphur dioxide-in the- :Eeed. A- ten gram strip was. taken. and; the an lyte' contained 15 grams per liter of manganese and the acid concentration was about 20 grams per liter. The cells were operated with a current density at the anodes of either 28 amperes or 58 amperesper square foot.

As the cell was operated there was a very small amount of manganese dioxide formed at the anode, but after the first days operations, the anolyte was very clear. A dark brown scale built up On the anode surface as plating continued.

Comparedwith other anodes employed under produced permitted operation at satisfactory 'curren-t efiiciency, that is, consistently above and it appears that more than adequate current efiiciencies may be maintained. The tests ran were not particularly concerned with current efificiencies, however, but rather with Mp0s formation and anode losses. The latter were very low, and unquestionably may be reduced ,further by selection of most optimum conditions;

In another series of tests, we produced a number of anodes containing 2 /2% silver, balance lead, and treated them in the same manner as described in connection with the '1% silver anodes. These alsoshowed desirable results from the standpoint of efficiencies and costs, particularly from the standpoint of low MnOz production and low anode losses.

Other anodes containing other low percentages of silver produce the same desirable advantages, but in our experimental work we utilize'dprimw rily 1%.and 2 /g% silver' and ran many tests to determine the relative advantageous characteristics of thesetwo embodiments.

For the benefit'ofthose skilled in the art, we show below the results of operations over a period of approximately two. weeks on certain anodes produced in accordance with the invention, these anodes havingv been selected at random from the group produced, and being identified in the three tables by number. {In the tables, 0. D. stands for current. density in amperes per square foot.

The other symbols are conventional.

TABLE I C'ell data 4 Amp. Wt. Anode Percent. Anode Wt-AHOdG Hours Cathode No. Ag G. D. Operated MIL g Scale g.

a 1 26 4725 3135. 7 133. 5 4. 1 28 4725 3125.7 1179.0 25 2a; 1 4725 2956.7 218.5 2.5 28 4725 2966.7 196.5 1 5a 3363 1958 49.0 1 53 3363 1958 40.0 2. 5 58 3363 1051. 2 43. 0 2. 5 58 3363 1981. 2 45. 5

TABLE II Analysis: of anode scale Anode-No: fg 3 Pb. M11. Ag Ca TABLE III Calculatzons from cell data Ratio Pep Per. of Mn cent Per- Per- Ag Pb Anode m Anode 1n Loss in cent cent Loss Loss No. A C. D. Scale Anode Loss Loss 'lon Ton g to Mn Wt Ag Pb Mn Mn Dep.

From the results shown in the preceding tables it can be seen that when the cells are operated with the higher current density on the anode-s, the amount of scale formed is markedly decreased, thereby lowering the silver and lead loss and the amount of manganese precipitated at the anode. The average voltage for the 1.0% and 2.5% silver anodes at 28 current density was 4.00 volts, and at 58 current density it was 4.26 volts. The increased power cost due to the higher operating voltage when using the high current density on the anodes would more than be compensated for by the lower anode cost resulting from the small loss of lead and silver.

Very little difierence could be determined between the efiiciency of 1% and 2.5% Ag anodes and, since the initial investment would be smaller if the 1% Ag anodes were used, it is believed that on the basis of these tests, the 1% Ag anode would be the most logical composition to use. Although scale formedduring the preforming in the tests, this would not be necessary in actual practice. The preforming of the anodes was done merely to keep the cells clear of precipitated manganese compounds, but it could be dispensed with without damaging the electrolytic circuit. The calcium present in the anode scale was due to gypsum crystals being precipitated upon the anode. cuts of the scale is obvious.

The results show definite trends and represent the maximum losses that would be encountered with silver-lead anodes. It is believed that, as the time of operation is increased after the darkbrown scale is formed, the silver and lead losses would be greatly decreased. Based solely upon the results shown, however, the anodes of the present invention are definitely superior to other anodes, such as the lead anodes of the prior art, and in some respects superior to the so-called Fink anodes. The type of solution used, current densities employed, and other details of operation are entirely illustrative, as the anodes of the present invention are adapted for use under any conditions found suitable for the efilcient electrowinning of manganese.

It must not be assumed that the anodes of our invention can be produced and used only in the form of cast shapes. The anodes may be extruded, rolled, drawn, extended. in short,

The origin of other constituformed in any usual way, and may even be produced in the form of clad anodes wherein only the surface is lead-silver.

We have described the anodes of our invention as being cast, but they may be fabricated in any suitable way so long as their composition is substantially uniform. While an alloy of lead and silver, the latter in relatively small proportions, is eminently suitable, other alloying constituents not deleterious to the results desired, such as tin, for example, may be added, and, of course, the silver content may be increased if desired. The base of the alloy, preferably lead, must be a current conducting material insoluble in the anolyte, with or without other constituents, but containing at least a relatively small proportion of silver to minimize the formation of insoluble manganese compounds.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. In the method of electrowinning manganese wherein an electrolyte of manganese and ammonium sulphate is electrolyzed in a diaphragm cell wherein the current efiiciency is greater than 50%, the improvement which is characterized by employing an anode of lead alloyed with one to three percent silver and of less surface area than the cathode; initially operating the cell at an anode current density above about 28 to about 58 amperes per square foot, and exceeding the cathode current density due to the difference in their areas while maintaining the current eniciency above 50%; initially depositing a protective film on the anode under said operating conditions for a period of about 24 hours and continuing said electrolysis under said operating conditions for a period of about two Weeks to form a scale composed principally of oxides of manganese and lead at the anode and reduce the manganese deposited as oxide on the anode to less than 1% of the metal deposited on the cathode.

2. The method as set forth in claim 1 wherein the anode current density is maintained at about 58 amperes per square foot.

HERBERT R. HANLEY. JAMES H. JACOBS.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,759,493 Tainton May 20, 1930 2,361,143 Lente et al. Oct. 24, 1944 OTHER REFERENCES Transactions of the Electrochemical Society,