US2398614A - Electrodeposition of manganese - Google Patents

Description

Apr-H 16, 1946.

W. E. BRADT ETAL ELECTED-DEPOSITION OF MANGANESE Filed March 24, 1938 uvsmoasi W/IberEI Bradf,

Patented Apr. 16, 1946 nnnc'rnonarosrrron or MANGANESE Wilber E. Bradt, Orono, Maine, and Harold H.

Oaks, Newport, Wash assignors to Vanadium Corporation of America, New York, N. Y., a corporation of Delaware Application March 24, 1938, Serial No. 197,850

11 Claims. (cl. ens-'45) This invention relates to the electrodeposition of metallic manganese from aqueous solutions containing manganese salts. One object of the invention is to provide deposits of metal closely approaching 100 per cent purity. Another object is to produce deposits that arebright and possess metallic luster. Another object is to establish conditions that will permit deposition of the metal from the same solution for long periods of time, thus establishing a basis for continuous extraction of the metal from its ores and the preparation 01 thick electroplates of the metal. Another object is to make possible the coating of many different metals with manganese metal for protective and decorative purposes and the like. Other objectives will be obvious from the descriptive matter to follow.

During a period of over 75 years, eflorts hav been made to deposit by means of the application of electric current to aqueous solutions of manganese compounds. appreciable amounts of pure metallic manganese. These have been very notably unsuccessful. In some instances, the deposited substance has been 01' low purity, in others of looseor spongy or nonscoherent structure, in others of scaly or powdery nature. Reference has also been made in the literature to the gray or black appearance of these deposits, even in cases where some semblance of compactness has been reported.

Insofar as we have been able to determine, no

electrodeposit of manganese of extremely high purity and considerable thich'less has heretofore been obtained that possesses simultaneously silver-white appearance. noncrystallinity to the unaided eye, compactness, smoothness. coherence. is adherent to a metallic cathode such as copper, is resistant to corrosion and is capable of taking a high polish on buillng. Y

The present invention provides a process whereby manganese may b produced which has all or these desirable properties in marked degree. The color of the manganese deposit is especially important. not only from the standpoint-oi appesrance but also because it is an indication of the purity of the manganese. The dark color ofelectrodeposited "manganese" produced by methods described in the priorart and confirmed by us is due to inclusions of manganese dioxide: This was definitely established by carefully dissolving a dark deposit in very dilute cold hydrochloric acid. Manganese dioxide goes into solution readily only in concentrated hydrochloric acid.

tion of higher valencechlorides of manganese. Pure electrodeposited manganese as produced by our process is silver-white in color. Several oi these silver-white deposits were disolved in hydrochloric acid and titrated with standard potassium permanganate, according to the Volhard method; the results showed percent purity However. color is considered to afford a more delicate indication than chemical analysis of complete absence of oxides, as with increasing oxide content the color of the deposit is no longer silver-white but ranges through gray to brown to black.

' We have carried out a large number of tests employing insoluble (platinum) anodes for. de-

positing manganese from various solutions but have found that the use of insoluble anodes is generally objectionable for several reasons. Any procem using an insoluble anode must necessarily be subject to depletion of the manganese ion during electrolysis. This brings about substantial alteration of the composition of the electrolyte and requires the addition of one or more materials for purpose of adjustment. When using manganese chloride, for example, as the electrolyte with platinum anodes, chlorine and oxygen are normally evolved at the anode. At the same time, manganese dioxide is formed at the anode. These objections are preterably overcome or diminished incur process by using a soluble manganese anode or a combination of soluble manganese anodes and insoluble anodes or by the use input. Further, the use of an insoluble platinum anode, in addition to requiring voltages of '7 to 8, is accompanied by the formation of hydrated manganic oxide. The soluble manganese anode requires approximately 1.5 volts and is accompanied by the iormation o! hydrated manganous oxide. This, moreover, can be prevented or dcto creased by the addition to the manganese chloride solution oi certain ammonium salts-tor example, ammonium chloride. Ammonium chloride is preferably employed in the amount of about 30 grams per liter, but this may be varied so according to conditions from about 10 grams per solving the dark or black deposit in concentrated hydrochloric acid: this color is due to the formaliter to saturation. The following table gives the results of tests employing asoluble manganese anode oi 96% manganese purity, the electrolyte 7 containing manganese chloride and ammonium cu chloride in the proportions given.

aseaeu.

Table Brown hydrated manganic oxides. iormed at anode as soon as electrolysis began and increased in amount as the run proceeded.

Oneiourth oi area metallic, at of Condition of deposit area black and somewhat rough.

Clear (or 6 min. iollowed by formation of manganic oxides.

Initially smooth and bright and well-adhered.

:08 MIJCIIAHIO, 30 NHtCl.

Hydrated manganous oxide formed alter 21 min.

Entire area smooth and silvery in color, though not mirror-like.

- By employing a manganese metal anode in place of a platinum anode, the formation of a precipitate in the electrolyte is greatly retarded.

However, the flaking of black manganese dioxide,

carbon, silicon dioxide and ferric oxide from the,

anode fouled the electrolyte to an appreciable extent. In order to overcome this) a diaphragm separating the electrolyte into anode and cathode chambers was'employed, and it was found that the separation of the anolyte and catholyte caused a plate lighter in color and ire'er from inclusions of manganese dioxide than was obtained previously.

In further studies of chloride electrolytes, par- I ticularly to note the'influence of the concentration of manganese in the electrolyte, tests were made over a range from about 50 grams MnClaAI-IzO up to a concentration represented by saturation at 25 C. In these tests, current densities of both and amperes per square decimeter were used with soluble manganese anodes and copper cathodes. Concentrations in the vicinity of 300 grams of hydrated manganese chloride having shown especially good results, a test was made in a divided cell, using this manganese concentration along with 30 grams per liter of ammonium chloride. At a current density of 20 amperes per square decimeter, a very smooth, bright, silver-white deposit was obtained that was 0.3 millimeter thick and adhered tightly to the cathode; there was some treeing on the edges.

Under the optimum conditions noted below, cur- 1 ranging from '70 to 80%.

The effect of agitating the electrolyte in cells containing diaphragms andin other cells not -form metallic hydroxides or hydrated oxides which may be occluded in the deposit. In these tests also, high current eficiencies in terms of deposited manganese were obtained, as high as 77%. The optimum conditions of electrolysis for the MnC12+NH4Cl electrolyte as established by the foregoing and other tests are as follows:

Anode. Commercial (96 per oontpure) manganese. Cathode OaLeiutlly cleaned and buried copper s so 1 ell Catholytemang anolyte separated y a I porous ap ragm. Electrolyte Solution containing one 400 Cathode current density. 20 ampJdm.

' rent efllciencies were obtained in these tests Treed on edges.

Voltage 1.0 to 4.5 volts. Stirring oi cstholytc Sufllciently vigorous to insure rapid motion of the electrolyte past the su face oi the cathode. Temperature 26 C.

Anode current density"; Below 40 ampJdm.

By carrying out the process under optimum conditions, metallic manganese of 100 per cent purity may be deposited from chloride electrolytes on copper cathodes as good adherent plates of smooth silver-white metal which up to at least 0.3 mm. thickness are devoid of crystals visible to the unaided eye. These plates are capable of a very high polish and have a scratch hardness of 5.5. to 6.0 on the Mohs scale.

Although the optimum current density is about 20 amperes per square decimeter, this may be varied somewhat. On the other hand, too low current densities give basic cathode deposits without metallic luster. Too high'current densities give. black cathode deposits, which are characterized by a lack of adherence. For a high purity product, the current density should preferably be maintained between about 15 and 25 amperes per square decimeter.

Electrolyte temperatures below 10 0. require high voltages and give poor deposits. Temperatures above 40 C. permit the use of low voltages but give foul, poorly adherent plates. The best deposits are obtained in the neighborhood of 26 and the preferred range is from about 20 to Numerous studies have also been made using manganese sulphate electrolytes. Pure, smooth, coherent, silver-white manganese deposits, not visibly crystalline and of thickness comparableto that obtained with chloride baths, may be secured.

a These, plates resist atmospheric oxidation and the corrosive action or laboratory fumes for several years. Cathodic eiiiciencles of 60-75% are normal, using an electrolyte of g./l. MnSO4AHzO plue 75 g./l. (NH4)2SO4 plus 60 g./l. NHdCNS, a cathode current density of 25 ampL/dmfi; a pH of 4.0 to 5.5 and a bath temperature of 25 C.

. In carrying out the work on manganese sulphate electrolytes, a conventional type of cell equipped with a glass stirrer was usually used. Since the soluble manganese anode had been shown by the tests in chloride electrolytes to possess numerous advantages, it was also employed in much of the work with sulphate electrolytes. Its performance was, however, not entirely satisfactory, so that among the tests in sulphate electrolytes were included, some with insoluble anodes. among these being lead, lead sulphate, manganese dioxide, platinum, carbon and graphite.

'lhe lead, lead sulphate and platinum anodes produced basic manganese precipitates, probably due to the high oxygen over-voltage exhibited by these anode materials. 'The manganese dioxide anodes werefound to have very high electrical rev of the bath substantially constant. current was passed through the soluble manga sistance. Commercial carbon rod material, when used as anode, disintegrated rapidly. When graphite anodes were used, conductivity was favorable and disintegration was slow. Howeverr graphite-anodes as well as other insoluble anodes are unsuited to depositing large quantities of metal without adjustment and replenishment of the electrolyte.

Again it was observed that the soluble manwas highly advantageous, it was not completely satisfactory for long time continuous deposition.

We discovered, however, that highly satisfac tory results could be obtained by employing both a soluble manganese ahode and an insoluble anode, such as graphite, in the same bath. By reg-'- ulating the amount of electric current supplied to each type of anode, we could keep the pH value Sumcient nese anode barely to maintain the manganese content of the bath. The remainder of the current was passed through the insoluble graphite anode. By varying the adjustment of these two I current strengths to meet other variables, such as temperature, agitation, addition agents, etc., it was readily possible to control both the hydrogen ion concentration and the manganesecontent of h the bath.

lyte contained in a cell 5. The cell is provided with a stirrer 6. The soluble anode 2 is connected by a line 1 provided with a variable resistance 8, a line 9, a lamp bank In to one branch ll of the positive electrical conductor 12. The insoluble anode 3 is similarly connected by a line IS, a var-'- iable resistance ll, lineklli, lamp bank ,IB to a branch line H, which also is connected to the positive electrical conductor I 2. The cathode 4 A soluble manganese anode 2, an 1 2o amp/din. to 80 ampJdmF.

A study of the effect of the temperature of the electrolyte showed that good deposits were possible between 15 to 30 (2., with best plates at temperatures around 25 C. At higher temperatures deposits showed rather pronounced darkening and a tendency to become rough and to develop trees.

In general, the best results were obtained when stirring produced just enough agitation to prevent the hydrogen evolved from adheringto the cathode or from sliding up its face.

In order to allow free circulation of the electrolyte and at the same time to avoid bath pollution, diaphragms were employed to separate the anolyte and catholyte.

This depositexhibited a silver-white, bright appearance, with extreme smoothness and no spotting. 'As deposition continued, treeing developed at the edge of the cathode, but the main body of the plate was very smooth and silvery-white. Cell voltages had become fairly constant at 4.2 volts. This smooth, white metal did not tarnish in air, when dr and showed little tendency to react with the electrolyte after the current was interrupted.

A large number of addition agents of various types were employed with the manganese sulphate electrolyte. It was found that ammonium .sulphate and ammonium thiocyanate' together exerted a decidedly beneficial eiIect on the nature of the deposits-including a marked reduc- .of 4.0, to 5.5; although good deposition occurred over-a much wider range. Both the pH value and the. metal content of the bath are readily controlled by, varying the ratio of the current passis connected to the negative electrical conductor 18. The conductors I, I3 and I8 are provided with ammeters l9, and a voltmeter 20 is arranged in the line 2i; whichconnects the conductors l8 and I3.

In carrying out the work employing a combination of soluble and insoluble anodes, a series of electrolytes was prepared to contain from to 600 g./l, of MnSOsAHzO. For each of these elecing through the soluble and insoluble anodes. The ratio of current passing through the soluble manganese anode to current passing'through the insoluble graphite anode was in many cases about i 1 to 8, although this ratio may vary considerably. depending upon addition agents, temperature of electrolyte, composition of the electrolyte and other factors.

trolytes another series was prepared in whichthe amount of (NI-I4) 2804 present was varied from zero to 150 g./l. tained indicated that the solution containing 100 g./l. of MIISO4.4H2O with 75 g./l. oi (NI-102804 was most satisfactory. Its hydrogen ion concentration was more readily controlled than in the others; less gassing occurred at the electrodes; and treeing of the deposit was minimized. NH4Cl was equally as effective as (NH4MSO4 in manganese sulphate baths in assisting maintenance of metallic manganese deposits of the desired quality.

The effect of the variation of cathode current The manganese deposits obdensity was next investigated. The values used The optimum conditions for producing a compact, tightly adherent silver-white electro-depos: ited manganese from this bath were found to be as follows:

Cathode... Cafihzlly cleaned and bufl'od copper Soluble anode Commercial (96 per cent pure) manganese rod.

Insoluble anode Acheson graphite od.. Electrical eircuit.. Suitable for independent adjustment or 1 current through each anode. Diaphragm Anodes enclosed in cloth bags. Electrolyte I00 g./l. MnSO|AHrO plus 75 g./l

(NH|)!SO| plus60 g./l. NH (N-S. Cathode current density. 25 ampJdmJ- Initial anode current den 5 ampJdmJ mar. 35 amp./d m1 slty (soluble) Initial anode current den- 30 umpJdm.

sity (insoluble) Initlalcurrent ratio.. Soluble: insoluble-1:5 to 1:8

Agitation Sufilcient to remove hydrogen from cathode surface.

H range 4.0 to 5.6 ath temggratumnnun 25 0. V0 in soluble 4.5 to 8 an a Voltage-4mm insoluble l1 tol2 anode The combination of soluble and insoluble anodes may likewise be-employed'with other elec- 3 Good results were obtained over the range from' removal from the electrolyte.

trolytes. The optimum conditions for a manganese chloride bath are as follows:

Electrolyte A- solution containing 300v to 400 g./l-

MnChAHaO to 60 g./l. N H401. Cathode cariutlly bufled and cleaned copper s so Anode Einsoluble) Acheson graphite rod. Anode soluble) Commercial (96% pure) Mn. Electric circuit Suitable for independent adjustment of current through each anode. Cell Anolyte and catholyte separated by cloth cliaphragms. Guggeat density (cathamp./dm.

o c Initial anode current den- 20 to 30 ampJdm. mar. 40 amp./dm.

sity (soluble). Initial anode current dcn- 15 to 'amp./dm.

sity (insoluble). Voltages (from soluble 4.5 to 5.5 volts.

ano Voltages (from insoluble 7.5 to l2,volts.

' anode). Agitation a.-. Sufilciently vigorous to insure rapid motion of electrolyte past cathode and anode chambers. Temperature 26 0. Initial ratio of current 1:2.

(soluble: insoluble).

With these optimum conditions and thecombination of soluble and insoluble anode current eificiencies as high as those obtained with the soluble anode alone, can be secured, namely 60 to80%. Y

The use of a diaphragm, although preferred, may be dispensed with in some cases, if provision is made for removing the sediment which may accumulate.

Although the preferred cathode metal is copper, manganese can be plated on metals of widely divergent nature, such, for example, as tin, cadmium, nickel, platinum, iron, zinc, aluminum and lead.

Electrodeposits of manganese described in the prior art have been mentioned as being readily oxidizable. In our studies of electrolytes and methods for depositing manganese metal, we have confirmed these reports and noted that the electrodeposits frequently will acquire dark oxidesurfaces in considerably less than one minute after In contrast with these, deposits of; manganese produced in accordance with our method are not readily oxidized. In fact, they are distinctly corrosion resistant and have maintained a good appearance in the atmosphere and also in the atmosphere of a chemical laboratory for five years or more. The degree of corrosion resistance is not equal to that of chromium or of steels rich in chromium orchroto assist in maintaining the desired concentra tion oi manganese.

It is understood that the invention is not limited to the preferred embodiment or manner of practicing the invention, but that it may be otherwise embodied or practiced within the scope of the following claims.

We claim:

1. The process of producing manganese, which comprises electrodepositing manganese from an aqueous solution containing manganese sulphate, said solution also containing an ammonium salt of the group consisting of ammonium sulphate and ammonium'chloride, said solution being substantially free from other depositable metal salts, at a cathode current density of about 20-30 amperes per square decimeter and a. temperature of about 15-30 C., and regulating the pH value of the solution between about 4.0 and 5.5.

2. The process of producing manganese, which comprises electrodepositingmanganese from an aqueous solution containing manganese sulphate and an ammonium salt, said solution being substantially free from other depositable metal salts, at a cathode current density of about 20-30 amperes per square decimeter and a temperature of about 15-30 C., and regulating the pH value of the solution between about 4.0 and 5.5.

3. The process of producing manganese, which comprises electrodepositing manganese from an aqueous solution containing manganese sulphate, an ammonium salt of the group consisting of ammonium. sulphate and ammonium chloride, the solution also containing ammonium .thiocyanate, said solution being substantially free from other depositable metal salts, at a cathode cur- Grams MnSOuiI-IaO 100 (NI-102504 75 NH4CNS said solution being substantially free from other depositable metal salts, at a cathode current density of about 20-30 amperes per square decimeter 4 and a temperature of about 15-30-C., and reguproperties of our electrodeposit as previously dey scribed.

A thorough investigation has been made'of the effect of variation of anode current densities. It has been established that optimum conditions can be maintained over anode current density zones pertinent to each electrolyte under the conditions selected. Specific examples under optimum conditions are as cited above.

Although the optimum pH value of the solutions will vary according to the type of'manganese s'altemployed, the type of addition agents if they are used and upon other factors, we have found that the pH value should in all. cases be lating the pH value of the solution between about 4.0 and'5.5.

5. The process of producing manganese, which comprises employing a soluble manganese anode and electro-depositing manganese from an aqueous solution ontaining manganese, sulphate, said solution also containing an ammonium salt, said solution being substantially free from other depositable metal salts, at a cathode current density of about 20-30 amperes per square decimeter and a temperature of about 15-30" C., and regulating the pH value of the solution between about 4.0 and 5.5.

6. The process of producing manganese, which comprises employing a soluble manganese anode and an insoluble anode and electro-depositing manganese from an aqueous solution containing manganese sulphate, said solution also containing an ammonium salt, said solution being substantially free from other depositable metal salts, at a cathode-current density of about 20-30 amperes per square decimeter and a temperature of about 15-30 C., and regulating the pH value of the solution between about 4.0 and 5.5 by con trolling the proportion ofthe total current which is supplied to each type anode.

7. The process of producing manganese, which comprises employing a soluble manganese anode and an insoluble anode and electrodepositing manganese from an aqueous solution containing manganese sulphate, an ammonium salt of the group consisting of ammonium sulphate and ammonium chloride, the solution also containing ammonium thiocyanate, said solution being substantially free from other depositable metal salts, at a cathode current densityof about 20-30 amperes per square decimeter and a temperature of about 15-30 C., and regulating the pH value of the solution between about 4.0 and 5.5 by controlling the proportion of the total currentwhich is supplied to each type anode.

8. As an article of manufacture, a plate of electrodeposited manganese of approximately 100% purity, which in thick deposits over 0.1mm. in thickness and up to at least 0.3 mm. in thickness is smooth, adherent, silver-white and devoid of crystals visible to the unaided eye.

9. The process of producing manganese, which comprises electrodepositing manganese from an aqueous solution containing manganese salt of the group consisting of manganese sulphate and manganese chloride, said solution also containing an ammonium salt of the group consisting oi! and ammonium chloride, said solution being sub stantially free from other depositable metal salts, at a cathode current density of about 15-25 amperes per square decimeter and at a temperature of about 10 to 40 C., and regulating the pH value of the solution between about 3.0 and 7.3.

11. The process of producing manganese, which comprises electrodepositing manganese from an aqueous solution containing manganese sulphate and an ammonium salt of the group consisting of ammonium sulphate and ammonium chloride, said solution being substantially free from other depositable metal salts, at a cathode current density of about 20-30 amperes per square decimeter and at a temperature of about 15-30 C., and regulating the pH value of the solution between about 3.0 and 7.3.

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