US3018233A

US3018233A - Producing manganese by fused salt electrolysis, and apparatus therefor

Info

Publication number: US3018233A
Application number: US7586A
Authority: US
Inventors: Jay Y Welsh; Marlyn W Milberg; Harold R Peterson
Original assignee: Manganese Chemicals Corp
Current assignee: Manganese Chemicals Corp; Chemetals Inc
Priority date: 1960-02-09
Filing date: 1960-02-09
Publication date: 1962-01-23
Anticipated expiration: 1979-01-23

Description

3,018,233 SALT 1962 J. Y. WELSH ETAL PRODUCING MANGANESE BY FUSED ELECTROLYSIS, AND APPARATUS THEREFOR 3 Sheets-Sheet 1 Filed Feb. 9, 1960 1952 J. Y. WELSH ETAL PRODUCING MANGANESE BY FUSED SALT ELECTROLYSIS, AND APPARATUS THEREFOR 5 Sheets-Sheet 2 Filed Feb. 9, 1960 Jan. 23, 1962 J. Y. WELSH ETAL PRODUCING MANGANESE BY FUSED SALT ELECTROLYSIS, AND APPARATUS THEREFOR,

3 Sheets-Sheet 3 Filed Feb. 9, 1960 United States Patent 3,018,233 PRODUCING MANGANESE BY FUSED SALT ESECTROLYSIS, AND APPARATUS THERE- R Jay Y. Welsh, Marlyn W. Milberg, and Harold R. Peterson, Brainerd, Minn, assignors to Manganese Chemicals Corporation, Minneapolis, Minn., a corporation of Minnesota Filed Feb. 9, 1960, Ser. No. 7,586 12 Claims. (Cl. 204-64) This invention relates to the production of metallic manganese by electrolysis from fused salt systems, and is concerned with an improved process of, and apparatus for producing manganese, and alloys of manganese, very low in carbon by fused salt electrolysis.

It heretofore has been proposed-U.S. Patent No. 704,393, Simonto produce manganese metal by electrolysis of a fused bath comprising calcium fluoride and a lower oxide of manganese. The metal product inherently was rich in carbon (78%)-due to the use of a carbon cathode and to the use of a feed containing free carbon-and hence had nothing to commend it over the conventional pyrornetallurgical carbon reduction process.

More recently, it was proposed-An U.S. Patent No. 2,398,589, Mitchellto lower the carbon content of the product metal by using a non-carbonaceous cathode and by lining with non-carbonaceous material that part of the cell which in the carrying out of the process normally would contact the product metal. However, the process entailed maintaining the fused bath (which consisted mainly of cryolite and sodium fluoride containing dissolved magnesia or alumina) at a temperature below the melting point of manganese whereby the product formed as an ingot of solidified-bath stock containing reduced manganese, from which ingot the reduced metal subsequently had to be separated by melting in a separate vessel. The ingot, as produced in the electrolytic cell, contained from 25% to as much as 50% by weight of solidified electrolyte. When concentrated and purified, the final metallic product contained about 0.8% of carbon along with substantial concentrations of aluminum, silicon and other alloying impurities, and hence did not meet the need for low-carbon manganese of high purity.

According to the present invention, low-carbon manganese is produced, from a lower oxide of manganese, by a process which comprises establishing a fused electrolytic bath whose compositioncon"isting essentially of a mixture of oxides with at least 50% by weight of calcium fluoride-is stable against decomposition and volatilization at any temperature up to at least 1500 C. and which is molten and without solid phases at the desired operating temperature; confining this electrolytic bath within a solid skull of its own composition and above and contiguous to a pool of molten manganese; adding lower oxide of manganese (i.e., MnO, reduced MnO ore, or MnO slags, consisting largely of MnO) to the bath while an electric current is passed through the latter from a carbon anode partially immersed in the electrolytic bath-to the surface of said pool of molten manganese as cathode; maintaining the electrolytic bath at a temperature above the melting point of manganese and at such a temperature as to maintain molten at least the upper surface of the underlying pool of manganese; and intermittently tapping molten manganese from said pool.

3,018,233 Patented Jan. 23,, 1962 The expression to maintain molten at least the upper surface of the underlying pool of manganese is intended to mean that the pool of Mn should largely be molten, but that a frozen metal skull at the periphery of the metal pool is tolerable. As the process is carried out commercially, practically the whole pool-including of course the upper surface -is maintained molten and sufficiently superheated that the product metal is freely fluid and readily may be tapped.

In the carrying out of this process, the pool of molten manganese is supported on and contacted by a solid layer composed essentially of one or more refractory oxides which is or are non-reactive with molten manganese; Cathodic electrical contact with the molten metal pool is established and maintained by the provision of a conductive metal bar embedded in the refractory oxide layer and extending through the Wall of the cell. As will readily be appreciated, the composition of this cathode contact bar should be compatible with the molten metal which it contacts, in order that any melting of the upper part of the bar will not result in undesirable alloying with the molten manganese. Since iron is not (usually) an objectionable alloying element in manganese metal, the cathode contact bar may be formed of iron or mild steel.

It is an important feature of the process (and apparatus) of the present invention that the heat balance of the cell must be such that the molten electrolyte largely is containedsave for the underlying molten metal pool on which it is supported-within a skull of its own composition. This will be obvious from a consideration of the fact (1) that carbon cannot be used as a container because of contamination and the further fact (2) that no known ceramic material is inert in, or not attacked by, contact with fused fluorides and oxides. In practice,

the proper heat balance is attained by (a) making the cell in the form of a wide dish, (b) by establishing and maintaining an electrolyte composition suitable (1) as to MnO solubility and (2) as to melting point (or, more properly,

melting range) with respect to the melting point of the product metal and (3) as to its electrical conductivity. Thus, the fused electrolyte must contain sufiicient oxides to prevent high voltage anode effects, and must be capable of dissolving suflicient MnO that manganese is preferentially electrolized at the cathode.

Again, and apart from temperature influence, a suitable solubility of MnO (in the bath) depends upon the proper combination of oxides in the bath composition, the presence of oxides having acid characteristics apparently being essentialor, at least, very desirablefor practical cell operation.

As regards the melting point (or range) of the fused electrolyte, practical operation dictates that the melting point cannot be appreciably lower than that of the product metal, since otherwise a suitable flux skull cannot be trolyte contain a substantial amountat least 50% byweight-of a fluoride, and economic as well as melting point evaluation singles out CaF as being the commercially' most important, ifnot the only practical fluoride. It is noted, in this connection, thatthe electrical resistance of fused oxides-including MnO-is some 20 times that of CaF with dissolved MnO in it. While it is possible to operate the cell with an electrolyte below 50% in CaF and high in A1 if the cost of electricity were unimportant, it appears that a Cal-* percentage below 55 60% is generally impractical because of the relatively low conductivity of the fused electrolyte.

As regards electrical operation, the cell may be oper ated at anode current densities as high as 10,000 amps/sq. ft. or higher; or, as low as may be necessary to maintain a temperature level suitable for maintaining the electrolyte (and pool of product metal) freely fluid; in commercial operation, the normal anode current density is within the range LOGO-3,000 amps/sq. ft. The cathode current density is not critical. The operating voltage of the cell normally falls within the range 5.0-6.5 volts-lower voltages being consistent with larger cell units.

The carbonaceous anode may be formed from coal carbon, graphite, or-preferably-petroleum coke carbon, e.g. Soderburg anodes made from petroleum coke carbon. Use of this latter material is indicated in the event a very high purity product is desired.

The intermittent step of tapping molten manganese from the maintained cathodic pool thereof may be effected by conventional oxygen lancing as practiced in blast furnace operation: preferably, however, it is effected by electrically melting a metal rod insert provided, for this purpose, in an aperture or tapping port in the aforesaid layer of inert refractory oxide on which the pool of molten product metal is supported, allowing a desired portion (less than all) of the molten product metal to flow out of the pool through the thus-opened aperture, and then re-sealing the tapping aperture by inserting a cold (fresh) metal rod insert into said aperture. melting of the metal (e.g. iron, or mild steel) rod insert is effected by passing a high current (either AC. or DC.) through the rod. This tapping method is superior to lancing because it is precise, easily carried out, and results in almost no deterioration of the tapping port.

The product metal, as tapped from the cell, is characterized by a very low concentration of carbon, values as low as 0.060.05% carbon having been attained in actual practice, whilst the contents of other impurities-not purposely added as alloying componentsdepend upon the purity of the feed (i.e., upon the purity of the MnO- containing material added to the fused oxide-fluoride bath). Manganese metal assaying over 99.0% Mn readily can be produced by the carrying out of this process, the customary impurities-over and above an exceedingly small amount of carbonbeing iron and silicon.

ELECTROCHEMISTRY OF PROCESS The electrochemistry of the process of the present invention has a superficial similarity to that of the Hall process of reducing aluminum, the chemical equations representing the over-all cell reaction being expressible simply as follows:

Moreover, both in the Hall process and in the present process the basic purpose of the fused electrolyte is to supply an electrically conductive solvent for the oxide of the product metal. But here the similarities stop, because the chemistry within tthe fused electrolyte is markedly different.

The solubility of MnO in the fused electrolyte of this process is largely the result of compound formation between the MnO and the acid oxides present in the fused electrolyte. It is intended that none of the Mn is in di- The rect combination with fluorine. The compound formation of MnO with the acid oxides is an essential feature of the process, since it sufficiently inactivates the MnO to prevent it from directly reacting with carbon. If this were not the case a reaction would occur between the M and stray non-anodic carbon to form objectionable quantities of high carbon metal in the product. The acid oxides are equally important in facilitating the rapid and positive solution of MnO.

Amongst operable oxides with acid characteristics are B 0 (e.g. in the form of borax glass Na OB O or borax), A1 0 TiO SiO or a combination of these or similar oxides.

It is believed that the chemistry involved at the cathode can be resolved into an evaluation of the stabilities of the various oxides. Any alkali or alkaline earth metals present in the fused electrolyte, if cathodically reduced, will displace all of the metals under consideration by virtue of their electromotive force. If it be assumed, then, that it is unimportant whether or not sodium, calcium or magnesium may be an intermediate in the cathode reaction, the selectivity of the cathode discharge reaction can be evaluated on the basis of the free energy of formation of the various oxides that may be present, as follows:

Thus, the generalized reaction of manganese metal with the respective oxide may be set up as a basic of comparison:

Then the table below can be developed:

AF of reaction This indicates that Ni, W, Mo, Cu, Co, Fe and Cr will be displaced in preference to Mn in that order, while V, Ti, Si, B and AI will be displaced after Mn with increasing difficulty as listed. The value of CO is indicated to show that the over-all cell reaction for manganese should proceed at a very low theoretical voltage.

However, inasmuch as one is concerned with compound formation in the fused electrolyte, the values in the above table may be shifted somewhat depending upon the stability of the compound formed. Thus, the stability of MnO is significantly increased by the formation of Mno'Bzog.

It is, important to note that B 0 is sufiiciently more stable than MnO (+24 kcal.) that boron is not electrolized until the manganese level is very low or in the event of extremely high cathode current densities. Silicon with a free energy value of +15 kcal., on the other hand, represents more of a problem. In order to avoid excessive silicon in the product, as SiO builds up in the fused electrolyte from the ore feed, it is essential that the current density be kept reasonably low or if a particularly high silica ore is involved it may be necessary to include in the feed a compound which will increase the SiO stability, CaO, A1 0 or MgO for example.

It has been found that thevalue of the electrolyte composition is depreciated by the presence therein of substantial concentrations of alkalies, particularly, alkali carbonates. The alkalies tend to lower the melting range of the electrolyte composition to a critical range, and moreover, the alkali carbonates tend to react with metallic manganese according to the equation which reaction results in the undesirable addition of carbon.

It has been found, further, that the reduction of Slo to elemental silicon should be prevented by tying up the silica in the form of a silicate. In this connection, it has been discovered that excess CaO in the electrolyte composition tends to form, with the SiO content, an eutectic having an undesirably low melting point, but that tying the Si0 with A1 0 is much more advantageous.

CELL FEED The cell feed varies widely depending upon the metal or alloy being produced and also in terms of compensating adjustment in the fused electrolyte to correct for impurities inherent in ores of lower grade. Several cases are discussed below which demonstrate the flexibility of the system.

(A) Two special cases exist in which, for all practical purposes, the fused electrolyte maintains a constant composition over long periods of operation.

(1) Where the feed is a high purity metal oxide, MnO, for example.

(2) Where the feed is in the form of a metallic anode bar.

A combination of these two special cases represents a practical system for the production of special alloys. For example high purity MnO can be electrolyized using a carbon anode while an anode bar of Cu of Ni is simultaneously fed into the cell at a controlled rate to produce a Cu--Mn or Ni-Mn alloy of any desired composition.

(B) When the ore feed is impure, contaminating impurities build up in the fused electrolyte. Depending upon the nature of the impurities, the effect may be to raise the melting point and viscosity as well as the electrical resistance (this being true of a high A1 0 contarnination); or, the effect may be to lower the melting point below a practical limit, which latter might result from a very high alkali content or certain combinations of CaO and SiO and/ or A1 0 and SiO In either event it is necessary to feed CaF (sometimes borate) along with the ore, and-to tap offthe excess electrolyte thereby produced, so that the fused electrolyte is maintained within a practical operating composition. Below 50% CaF is normally considered impractical.

(C) Certain alloys involve metals which cannot be practically fed as anodic rods, boron for example, in which case the feed is made to include the alloying oxides blended in the proper proportion to give the desired alloy.

In a consideration of the cell feed, it is to be noted that the inclusion therein of significant amounts of higher valence manganese-cg. a poorly reduced MnO ore characterized by a high percentage of Mn O obviously leads to inefficient current utilization, i.e., the necessity of more electrons passed per unit of manganese produced, and, therefore, is to be avoided.

The invention will now'be described in further detail in the following examples, and the apparatus aspect thereof described with reference to the accompanying drawings, in which latter FIGURE 1 is a diagrammatic view, in vertical crosssection with-certain parts removed, of a pilot cell operable in the carrying out of the present process;

FIGURE 2 is a diagrammatic view, in vertical crosssection, of a modified form of the cell represented in FIG- URE l;

FIGURE 3 is a generalized detailed view, in longitudinal vertical cross-section, with certain parts removed, of a commercial-sized cell;

FIGURE 4 is a generalized detailed view, in transverse vertical cross-section of the cell shown in FIGURE 3, and

FIGURE 5 is a ternary diagram of the system CaF :Al O :SiO showing in cross-hatched area the range of composition of electrolyte in which the process of the present invention can be carried out.

Referring, first, to FIG. 5, it is to be noted that the line AB represents the composition Al O -SiO below which composition, with respect to alumina, silicon will be electrolyzed. Line B-C represents a temperature boundary set at about 1200 C. Area 1) represents a limited range, below the composition Al O -SiO in which the excess SiO can be chemically combined with Ca() and still maintain adequate melting point.

In FIGURE 1, a relatively shallow vessel 1 of steel constitutes the outer supporting shell of the cell, the vessel being lined--both as regards its Walls and its bottomwith heat-insulating ceramic material 2.

Overlying the bottom insulation is a generally imperfo-rate layer 3 of a refractory ceramic material-such for example as alumina or magnesia brick or their respective ramming mixtures which is inert to molten manganese. There is embedded in layer 3 an electrically conductive metal cathode contact bar 4 at least the upper surface of that part of the bar which is within the shell of the cell being at a height at least as great as is the upper surface of layer 3 so as to be exposed thereabove. Contact bar 4, which extends outwardly from the cell may be formed of any suitable electrically conductive metal, unless a very small amount of iron alloying is intolerable during the initial stages of cell operation. It is to be understood hat after an extended period of operation the cathode contact bars will no longer alloy into the product metal. It is preferred that contact bar 4' be formed of iron or steel. At any suitable locus adjacent the central portion of layer 3 there is fabricated a tapping port 5 formed of high-alumina sintered shapes defining a generally cylindrical tapping aperture 5' normally closed (or sealed) by an iron rod 6.

Above layer 3 and extending about the inner periphery of lining 2 is a back-up wall 7 of dish configuration. This back-up wall is, during use of the apparatus, in contact with a pool of molten metalindic=ated at 8su'p'- ported on layer 3, and'also with a mass of electrolyte conposition 9, 9' (about to be described) the most nearly central part of which latter may, on occasion, be molten: consequently, the back-up wall 7 is formed of a material,

.e.g. high-alumina brick, which is suitably inert both to molten product metal and to the fused electrolyte composition.

The parts described abovetogether with the anode member and confining hood later to be described-constitute the invariant portions of the operating cell. The shallow dish formed by' layer 3 and the sloping sides of back-up wall '7 constitute the receptacle for (a) a pool 8 of molten manganese (product metal) lying upon layer 3 andoverlying said pool-a mass of electrolyte composition 9, 9'. The central and lower portions of the mass of electrolyte composition, indicated at 9, are normally molten, whereas the more outlying portions of said mass, indicated at 9, are normally solidified (i.e., frozen) and constitute the solid, bath-confining skull referred to hereinabove.

A generally cylindrical and vertically disposed hood member It) is supported (by any suitable means, not shown) centrally "above the above-described shallow dish and is so disposed that the lower edge 10' of the hood member is embedded and sealed in the solid skull mass 9. A suitable feed orifice 11 is provided in an upper part of the hood member (either at the top thereof, as shown, or in the upper part of the side wall of the hood member above the upper extremity of the skull mass 9'), which feed orifice normally is closed by a door 12. Centrally disposed in the top of hood member 10 is a relatively large anode orifice 13 through which latter a massive anode'bar 15 extends so that its lower end is immersed in molten electrolyte 9. The annnular space between anode member and anode orifice may, if desired, be substantially sealed by the interpositionof an annular gas sealing means (not shown). The purpose of hood member 10 is to maintain a small positive pressure of reducing gas-consisting largely of carbon monoxideabout the un-immersed but hotter part of anode whereby to protect the anode from oxidation.

The means for supporting anode bar 15, and for adjusting the distance between the lower end of anode bar 15 and the'cathodic pool 8 of product metal, being conventional, are not'shown. At its upper end, anode bar 15 is electrically connected to a conventional anode bus 16.

In a similar manner, contact bar 4 is electrically connected to a conventional cathode bus 17.

Not shown in the drawing, there is provided, above the entire cell, a suction hood of conventional design for positive upward displacement of by-product gases by an enveloping current of fresh air.

In use, a particulate mixture of electrolyte composition components is placed in the aforesaid shallow dish and melted therein by the passage of current from the anode to the cathode contact bar. As soon as a molten bath begins to form, the feed of manganese oxide begins whereby to establish a cathodic pool of molten manganese underlying the flux bath and overlying said cathode contact bar. Thereafter, the feed of MnO to the molten electrolyte continues, with or Without the simultaneous or intermittent feeding in of additional particulate electrolyte components as needed. Continuance of the operation results in the building up of a skull of electrolyte about (i.e., around and above) a central body of molten electrolyte. The manganese oxide is dissolved by the molten electrolyte, and manganese in molten form is deposited at the cathode, thereby progressively'increasing the volume of the cathodic pool. When the volume of molten manganese reaches a predetermined value, the sealing means is removed from the tapping orifice, and a suitable portion of the molten product metal is allowed to flow out of the cell into an ingot mold. Thereupon, the tapping orifice is sealed shut by introducing a cold rod into it, whilst the operation of the cell continues.

In the event the volume of electrolyte in the cell becomes excessive, it is diminished to the desired level, e.g., by opening a flux orifice in the skull and allowing an appropriate amount of molten electrolyte to flow out.

According to FIGURE 2, the contour of the aforesaid shallow dish may be modified to provide therein a centrally disposed well 21, lined with high-alumina brick, adapted to contain all or substantially all of the cathodic pool of molten product metal: Also, the side walls of the metallic outer shell 1 of the cell are extended upwardly, well above the upper level of the electrolyte skull, and are bridged by a cover member 1. In this embodiment the carbon anode 15' is suspended within the cell from a conductive metal shaft 22 which latter is electrically connected to an anode bus and extends into the cell through a central opening 13' in cover 1'.

FIGURES 3 and 4 illustrate essential elements of a cell for use in a fullscale commercial operation of the process. In these figures the cathodic pool of molten product metal and the electrolyte have not been shown, it being understandable that during use of the apparatus a frozen" skull of electrolyte composition would form and persist about a centrally disposed pool of molten electrolyte composition and would extend upwardly from and inwardly over, said pool to seal the lower portion of the hood 10, and simultaneously a pool of molten product metal would form and persist underneath the electrolyte and in contact with the cathode contact bar 4.

In the embodiment illustrated in FIGURES 3 and 4, that part, 31, of the aforesaid shallow dish which is intended, in use, to contact electrolyte is formed of highalurnina brick, whilst the part, 32, intended to be contacted only by molten product metal is formed of comadal ramming mix. Both the alumina brick and the corundal ramming mix layer overlie and are backed-up by a layer, 33, of carbon brick. A mass, 34, of heat-insulating masonry underlies and supports the above-described layers. A feed screw member 36 is disposed in feed orifice 11 for feeding manganese oxide onto the electrolyte.

At 37 is schematically shown a contact member, projecting into anode bar 15, for conducting electrical current from anode bus 16 to the anode bar.

SPECIFIC EXAMPLES Example 1 The apparatus used was that illustrated in FIGURE 2: the upper'portion of the cell containing the fused electrolyte was one foot square, and the molten manganese pool was 3 inches square in horizontal cross-section.

The cell was brought up to operating temperature and to temperature equilibrium by passing an alternating current through a fused CaF electrolyte, a carbon rod acting as one electrode and a manganese melt acting as the other electrode. An initial charge of 500 grams of manganese metal had been added to establish the metal electrode (cathode).

After temperature equilibrium had been established, sutficient Na B O and MnO were added to the fused CaF to give a fused electrolyte containing 10 parts CaF to 1 part Na B O to 2 parts MnO. The AC. current was then disconnected and D.C. current passed as indicated in FIGURE 2. During D.C. operation grams of MnO were fed every l5 minutes. It was necessary, however, because of the high heat loss from such a small cell occasionally to change from D.C. to A.C. current in order to raise the temperature to the desired level.

The pertinent data are summarized below:

Total D.C. amp. hours 2525. Metal produced by electrolysis- 2060 gms. Current efliciency 75%. Average D.C. current 400 amps. Average anode current density- 10,000l4,000 amps/fif Metal analysis:

Mn94.0%. Fe-4.2%. C0.12%.

Balance-Al, Si and traces of other metals.

Example 2 The cell used for this experiment was identical in general design to that shown in FIGURE 1. The anode was 20 inches in diameter and made from carbon. The overall dimensions of the cell itself were 6' x 10 in horizontal cross-section.

The log of the entire run is shown below:

'At this point, some alloying of the iron of the cathode HIGH PURI'IY MnO FEED Percent Percent Analysis of metal product- Avg. Avg. Lbs. of Mn in P10 in Lbs; of Date volts Kva Mn fed fused fused metal Comments as Mn' electroelectro Mn Fe O Si tapped lyte lyte,

9-28- 8. 7 45. 7 270 6.1 2. 2 88. 4 10. 0 117 Melting out tapping port electrically. 929 8. 6 44. 3 248 6. 9 2.0 91. 7 1 10.0 230 Do. 9-30--- 8.7 45.9 250 7.3 23 94. 4.2 135 Do. 1 8. 5 46.1 255 5. 8 1. 6 94. 9 3L 8 150 D0. 10- 8.5 46. 1 289 6.0 2. 7 3. 6 255 Do. 10- 8. 6 46. 5 293 5. 7 2. 7 150 7 D0. 10-4- 8. 7 48. 8 302 5. 8 10-5..... 8. 7 49. 4 315 6. 2 1.9 l1 280 Laneed 1. 5 14 425 Do. 10-6 8. 8 49. 8 313 5. 5 1. 8 12 14 235 Do.

(REDUCED) NATURAL ORE FEED 9.0 48. 8 255 7. 5 97. 4 2. 1 e 15 285 Laneed.

8. 7 47.0 196 7. 3 95. 1 4. 4 17 a .75 250 Melted out tapping port electrically.

8. 7 45. 4 225 6. 5 91. 6 6. 4 e 15 2.0 Do.

8. 8 44. 9 48 9. 2 89. 3 8.0 2. 8 Used up anode.

l The high iron percentage in the initial taps was due to the fact that the mild steel cathode contact bars were alloying into the metal product.

Additional pertinent data calculated from the above log were as follows:

Average anode current density amps/ft? 2500 Average current efficiency (correcting for higher valence manganese and also the contained iron and silicon) percent 93.5 Carbon (anode) efficiency based on C0 evolui n do 96.5

The analysis of the natural ore before reduction fed during the latter part of the run was as follows:

Percent Mn 52.1- Fe 5.7 SiO- 7 .8 A1 0 .3

It might be noted that the silicon electrolyzed during the ore feed portion of the run was high because of a combination of two factors: (1) high current density, and (2) A1 0 and/ or CaO were not present in sufiicient quantity to inactivate the SiO by chemical combination. When electrodes of diameter are employed the current density drops to less than 1500 amps/ft? and the electrolysis of silicon is substantially reduced.

Example 3 contact bar with the Mn of the cathode pool took place.

The size of the CaF pool increasedto a diameter some what in excess of the diameter of the carbon anode, this pool being contained within a solid skull which formed about and above the liquid-roughly, a vessel with a necked-in mouth at the top.

This initial procedure was continued until the cell had reached temperature equilibrium and a pool of sufiicient size had been formed. Then, MnO (reduced ore) was added to the CaF- pool along with another oxide, such as A1 0 or SiO or an oxide of boron, to increase the solubility of MnO in the electrolyte and to increase the 0 content of the fused electrolyte in order to prevent anode passivity or anode effects.

This start-up procedure applies not only to this particular example but also to Example 2 and to the start-up of a multi-anode commercial cell.

At this stage, the passage of AC. current was interrupted and passage of D.C. current between, the carbon anode and the cathode pool was initiated. Thereafter, reduced ore. was fed into the pool of molten electrolyte in proportion to the D.C. current. The reduced ore contained about 2.75% A1 0 and 1.4% SiO and about 2.1% iron as Fe O about 1.9% potassium oxide and balance essen: tially allMnO.

The voltage imposed between the electrodes was main: tained at a uniform level between 7 and 7.5 volts. Also the D.C. currentwas held, asnearly uniform as possible. The approximate. resistance of the fused electrolyte at any particular. stage of operation was therefore an inverse function of the anode-cathode separation. The experimental run was divided into 3 parts, indicated as A, B and C in order to demonstrate various properties of the elec tro-chemical system and each part will, be analyzed separately.

LOG OF EXPERIMENTAL CELL RUN NO.

Percent Percent Percent Days Anode- Pounds Mn in B20 CaF- of cathode D.C Mn fed fused infused in fused Mn in Fe in C in Si in Pounds Flux operaseparaamps as MnO electroelectroeleetroprodprodprod prodmetal temp. Remarks tion tion in ore lyte lyte lyte not not not not tapped of- 4 4, 290 143 10, 0 0 9O 4 4, 080 153 7. 9 0 81, 3 15. 4 .46 1.98 35 2, 290 4 4, 560 234 7.9 0 73 81. 7 15.3 40 2.02 2, 360 3 4. 520' 244 9. 4 0 65 84. 7 12. 7 33 1. 66 220 2, 340 2 4, 6 0 222 7. 8 0 55 87. 5 10.2 27 1. 46 2,350 Solid phase forming. 1 4, 690 240 6. 2 0 58 89. 5 8.0 20 1. 34 330 2, 340 Solid phase present. 1 4, 660 231 5. 1 O 50 91.4 7. 0 17 l. ()4 305 2, 315 Do. 1 4, 550 272 5. 1 0 56 92. 6 6. 0 16 .84 295 D0.

Percent Percent Percent Days Anode Pounds h In in 2 3 CaFz f cathode D.C. Mn fed fused in fused in fused Mn in Fe in O in Si in Pounds Flux operaseparaamps as MnO electroelectroelectroprodprodprodprodmetal temp Remarks tion tion in ore lyte lyte lyte not not not not tapped of 1 4, 690 273 5. 6 0 60 92. 1 6. 7 .16 180 2, 310 Solid phase present.

3 4, 450 262 6. 1 2.0 65 93. 4 6.0 009 160 2, 290 S01511 cphase d.1sappear1ngB0rate 3 5, 530 139 6. 3 6O 30 2, 200 Metal freezing adding lime. 2 4, 920 1. 3 0.2 60 2, 140 Adding lime. 2 5, 200 259 2. 9 O. 1 6O 2, 240 Metal frozen adding lime. 2 5, 530 270 0. 2 60 2, 220 o. 2 5, 100 2. 4 13 Metal frozen.

4 5, 670 120 0. 5 0. 9 4 4, 430 166 3. 0 l. 6 4 4, 380 263 5. 9 3. 5 3. 5 4, 910 279 5. 7 3.0 3 5, 300 288 6. 1 2. 5 3 5, 100 306 6. 9 2. 4 1 5, 170 299 8. 2 2. 6 Metal frozen. 1 5, 430 291 9. 2 7 Do. 1 5, 530 132 7. 3 2.0 Do.

Part A.This portion of the experimental run demonstrates (1) the control of silicon in the product by causing the SiO in the fused electrolyte to form a stable compound with alumina and thus preventing its electrolysis, and (2) the elimination of objectionable solid phases in the fused electrolyte by the addition of sodium borate.

The control of silicon is demonstrated by the fact that the fused electrolyte initially contained an excess of 50 pounds of SiO while the ore feed contained alumina and silica in the amounts of 2.75% A1 0 and 1.4% SiO The log shows that the silicon in the product metal started at about 2%, dropped slowly to about 1% and then suddenly fell to 0.1%. Calculations of the Al O to SiO ratio at this point show it to be one to one molar, indicating that the electrolysis of silicon essentially stopped as soon as the excess SiO over the compound Al O -SiO was removed from the fused electrolyte.

The log also shows under remarks that solid phases developed in the fused electrolyte, presumably from high melting silicates. The presence of this solid non-conductive material in the fused electrolyte raised its electrical resistance as is evidenced in the low anode-cathode separation. The log shows that the solid phases as well as the high resistance were eliminated by the addition of some 2% borate. With borate' added as needed the cell continued to operate satisfactorily in all respects.

It should be noted also for comparison in part C that satisfactory operation was observed when the CaF level in the fused electrolyte was held at 60%.

Part B.This portion of the run demonstrates the adverse effect of adding excessive CaO to the fused electrolyte. Two difficulties resulted as demonstrated by the log: 1) the basic character of the CaO markedly decreased the solubility of the MnO, and (2) the melting point of the fused electrolyte was dropped sufliciently to make it impossible to maintain the product pool in a molten condition. The depression of the melting point was due to the formation of an eutectic'mixture of CaF and CaO-SiO Part C.-This portion of the run demonstrates the difiiculty in operation as a result of a low CaF concentration. V

The log shows satisfactory cell operation as the CaF percentage is'dropped from 90% down to 60%.' At 40% CaF however, two adverse electrical effects are evident: (1) the resistance of the fused electrolyte is high in spite of the fact that no solid phases were present, and (2) the melting point of the fused electrolyte was too low to maintain a molten product pool. (See FIG. 5, showing the low melting eutectic area of the three phase system Al O SiO CaF In general, a small content of iron in the low-carbon product metal is tolerable. In cases, however, where it is essential that the product metal be substantially devoid of iron, other heavy metals, and phosphorus, the process of the present invention advantageously is supplemented as follows:

As the feed to the cell, we provide a synthetic composition which isa slag product of a furnace operation wherein terromanganese is produced, said slag consisting essentially of MnO, 6080% by weight, and the balance consisting essentially of CaF plus A1 0 plus SiO the combined weight of A1 0 and SiO to the weight of CaF being in the ratio of about 2:3.

In preparing this synthetic feed an electric arc furnace isfed with natural manganese ore (e.g., pyrolusite) and sufiiclent carbon to reduce all of the MnO of the ore to MnO plus suflicient additional carbon to reduce a portion (e.g., 20%) of' the total manganese in the ore to metallic Mn, together with CaF in an amount sufficient to meet the above ratio balancing the A1 0 and SiO contents of the ore. In this operation, substantially all of the heavy metals other than manganese, including iron, in the ore are metallized and all sources of carbon as carbonates and of phosphorus are removed from the slag. The form-manganese product of this electric furnace operation is, necessarily, adulterated with carbon, phosphorus and such heavy metals as may have been present in the ore, whereas the slag has been substantially purified with regard to the elements just mentioned. The term-manganese and 'the slag are separately tapped ofi, and the slag is cooled and then crushed to a fineness suitable for feeding to the electrolytic cell.

By resorting to the use of this synthetic feed, the metal product of the electrolytic process is extremely pure not only as regards carbon and silicon contents but also essentially devoid of phosphorus, iron and other heavy metals.

Example 4.-MnCu alloy (A) Cu-Mn alloy consisting of 70% Mn and 30% Cu was prepared in the same physical cell as described in Example 3. The starting procedure described in Example 3 was followed with the exception that the manganese oxide was solubilized with borate instead of SiO After the cell was stabilized on DC. current an additional anodeconnected to the common anode busconsisting of a copper bar was lowered adjacent to the carbon anode and anodically solubilized in the electrolyte. The rate of addition of Cu was controlled by the rate of which the copper bar was lowered into the pool, and the ratio of Cu added per unit of time to the Mn added during the equivalent period was maintained at 3:7. The

DC. current was maintained at an average value of 5,000 ampf., and the cumulative feed of Mn and Cu was controlled by the percentage of Mn in the fused electrolyte: average, 6%. The MnO feed was in the form of high-purity MnO (from MnCO The bath temperature during the run was maintained at about 2300 F.

The product was tapped in th conventional manner, and' upon analysis was found to consist-save for incidental impurities-of Mn and Cu, in the proportion of 70:30.

Example 5 In a repetition of Example 4, the copper bar is replaced by a bar of nickel and the current is increased to 5,500 amps, the other conditions of operation remaining the same. The nickel bar is lowered at a rate to establish and maintain a 1:1 ratio with the Mn added by Way of the feed. The control of the cumulative feed of Mn and Ni is controlled by the percentage of Mn in the fused electrolyte being maintained at an average of about 6% The product metal consists, except for incidental impurities, of Mn and nickel in the ratio of 1:1.

Example 6 In this experiment a Mn-B alloy, containing about 5% B, was produced by the above-described process.

The cell was started by the initial feeding of CaF and A1 (in ratio of 85: 15) whereupon sodium borate and high-purity MnO were fed in amounts to provide a Mn-to-B ratio of 95-5. After an initial period of operation, the sodium borate was substituted by B 0 The current was 5,000 amp., and the temperature of the bath was approximately 2300" F.

The cumulative feed was balanced against the DC. current, with the assumption that the valence charge of manganese in the electrolyte process was 2 and that of the boron was 3.

' The metal product, after tapping from the cell, was found to consist essentially of Mn and B, the content of B being 45% Ina manner similar to that of Examples 4, 5 and 6 above, various 2-component alloys of manganese can be produced at operating temperatures up to about 1300 C., by simply changing the identity of the feed and the feed ratio. A number of such alloys so produced are indicated in the following table:

MnSb All proportions. MnAs All proportions. MnCr Cr range 020%. Mn-Co- Co range 0-75%. MnCu All proportions. MnFe Fe range 060%. Mir-Ni Ni range 080%. MnSi Si range 0- 50%. MnSn All proportions. MnZn Allproportions.

1 Consistant with boiling point of alloy.

We claim:

1. A fused salt electrolytic cell for the production oi Mn, comprising a vessel open at the top and composed of side walls of fused A1 0 or MgO and a bottom of a non-carbonaceous refractory material non-reactive with Mn, said bottom being configured and adapted to support a pool. of molten Mn thereon, a cathode bus, a. metallic conductor bar embedded in said bottom and extending inwardly to the bottom of said pool space and outwardly connected to said cathode bus, a metallic rod embedded in said bottom and extending inwardly to said pool space, a hood member supported above said vessel and having atop opening, a carbon anode extending substantially vertically into said vessel and terminating above and adjacent said pool space, a feed orifice in said hood, a pcripheral body of solidified electrolyte covering said side walls and extending inwardly and upwardly into proximity 14 with said anode and into sealing relation with the bottom of said hood, said peripheral body constituting the side walls of an electrolyte space whose bottom wall is the top of said metal pool, an anode bus, and an electrical conductor connecting said anode bus with said anode.

2. A fused. salt electrolytic cell for the production of Mn, comprising a vessel open at: the top and composedof side walls. of fused A1 0 or MgO and a bottom of a non-carbonaceous refractory material non-reactive with Mn, said bottom being configured and adapted to support a pool of molten Mn thereon, a cathode bus, a metallic conductor bar embedded in said bottom and extending inwardly to the bottom of said pool space and outwardly connected to said cathode bus, a metallic rod embedded in said bottom and extending inwardly to said pool space, a hood member supported above said vessel and having a top. opening, a carbon anode member extending substantially vertically through said hood opening and into said vessel and terminating above and adjacent said pool space, a gas sealing means between said anode member and said hood opening for maintaining within said hood a reducing gas atmosphere, a feed orifice in said hood member, a pcripheral body of. solidified electrolyte covering said side walls and extending inwardly and upwardly into proximity with said anode member and into sealing relation with the bottom of said hoodmember, said peripheral body constituting the sidewalls of an electrolyte space whose bottom wall is the top of said metal pool, an anode bus, and an electrical conductor connecting said anode bus with said anode.

3. A process for the production of low-carbon, lowsilicon manganese from a lower oxide of manganese, which comprises establishing a fused electrolytic bath consisting essentially of calcium fluoride, a lower oxide of manganese and a mixture of inorganic oxides including an acid-acting oxide whose free energy change of reaction with manganese at 1327 C. to form MnO is greater than zero, and a basic-acting oxide whose free energy change of reaction with manganese at 1327 C. to form MnO is greater than zero, said mixture of inorganic oxides having an overall acidic character sufficiently acidic to facilitate the dissolution of a lower oxide of manganese but having a sutficient alkaline content to prevent the substantial electrolysis of silicon, the fused bath containing from about 50% to about by weight of calcium fluoride, and from about 0.5% to about 10% by weight of manganese as a lower oxide of manganese, and having a melting range of from about 1150 C. to about 1300 C., the bath composition being stable against decomposition and volatilization within said temperature range, confining said bath within a solid skull of its own composition above and contiguous to a pool of molten manganese, adding a material containing a manganese oxide to said bath, passing an electric current through said bath from an anode to the surface of said underlying pool of molten manganese as cathode, maintaining the bath at a temperature above the melting point of manganese and at such temperature, within said range, as to maintain molten at least the upper surface of said underlying pool: of manganese, and intermittently tapping molten manganese from said pool, said process being further characterized in; that said pool of molten manganese is supported ona carbon-free layer composed essentially of relatively electrically non-conductive refractory oxide which is non-reactive to manganese.

4. A process for the production of low-carbon, lowsilicon manganese from a lower oxide of manganese, which comprises establishing a fused electrolytic bath consisting; essentially of calcium fluoride, a lower oxide of manganese and a mixture of inorganic oxides including an acid-acting oxide of the group consisting of silicon dioxide and an oxide of boron and a basic acting oxide of the group consisting of alumina, calcium oxide, barium oxide, and magnesium oxide, said mixture of inorganic oxides having an overall acidic character sufficiently acidic 15 to facilitate the dissolution of a lower oxide of manganese but having a sufficient alkaline content to prevent the electrolysis of silicon, the fused bath containing at least 50% but not more than about 90% by weight of calcium fluoride and from 0.5% to by weight of manganese as a lower oxide of manganese, and having a melting range of from 1150 C. to about 1300" C., the bath composition being stable against decomposition and volatilization within said temperature range, confining said bath Within a solid skull of its own composition above and contiguous to a pool of molten manganese, adding a material containing a lower oxide of manganese to said bath, passing an electric current through said bath from an anode to the surface of said underlying pool of molten manganese as cathode, maintaining the bath at a temperature above the melting point of manganese and at such temperature, within said range, as to maintain molten at least the upper surface of said underlying pool of manganese, and intermittently tapping molten manganese from said pool, said process being further characterized in that said pool of molten manganese is supported on a carbonfree layer composed essentially of relatively electrically non-conductive ref actory oxide which is non-reactive to manganese.

5. Process as defined in claim 3, said process being further characterized in that the layer which supports the molten manganese pool includes a solid conductive metal insert extending through said layer, and in that tapping molten metal from said pool is efiected by electrically melting said metal insert.

6. Process as defined in claim 3, in which the content of Calin the bath is within the range of from about 60% to about 65% by wt. and the content of lower oxide of manganese in the bath is within the range of from about 4 to about 7% by wt.

7. A process for the production of low-carbon, lowsilicon manganese from a lower oxide of manganese, which comprises establishing a fused electrolytic bath consisting essentially of calcium fluoride, a lower oxide of manganese and an oxide of boron, the content of manganese as lower oxide of manganese being from about 5.5 to about by wt., the content of oxide of boron being from about 1.6 to about 8% by wt., the content of CaF being the balance save for incidental impurities, said fused bath having a melting range of from about 1150 C. to about 1300 C., confining said bath within a solid skull of its own composition above and contiguous to a pool of molten manganese, adding a material containing a manganese oxide to said bath, passing an electric current through said bath from an anode to the surface of said underlying pool of molten manganese as cathode, maintaining the bath at a temperature above the melting point of manganese and at such temperature, within said range, as to maintain molten at least the upper surface of said underlying pool of manganese, and intermittently tapping molten manganese from said pool, said process being further characterized in that said pool of molten manganese is supported on a carbon-free layer composed essentially of relatively electrically non-conductive refractory oxide which is non-reactive to manganese.

8. Process as defined in claim 3, in which CaO is added to the bath as needed for minimizing electrolysis of silicon.

9. A fused salt electrolytic cell as defined in claim 1, in which the bottom of the cell is formed from a refractory oxide selected from the group consisting of MgO and A1 0 10. In the process defined in claim 3, the improvement which consists in counteracting the formation 'of insoluble solid phases in the bath and promoting solution of MnO in the bath by the addition to the latter of an oxidic compound of boron in an effective amount of about 2% by weight based upon the total weight of the bath.

11. A process for the production of a low-carbon, lowsilicon manganese alloy, which comprises establishing a fused electrolytic bath consisting essentially of calcium fluoride, a lower oxide of manganese and a mixture of inorganic oxides including an acid-acting oxide whose free energy change of reaction with manganese at 1327" C. to form MnO is greater than zero, and a basic-acting oxide whose free energy change of reaction with manganese at 1327" C. to form MnO is greater than zero, said mixture of inorganic oxides having an overall acidic character sufiiciently acidic to facilitate the dissolution of a lower oxide of manganese but having a suflicient alkaline content to prevent the substantial electrolysis of silicon, the fused bath containing from about 50% to about by weight of calcium fluoride, and from about 0.5% to about 10% by weight of manganese as a lower oxide of manganese, and having a melting range of from about 1150 C. to about 1300 C., the bath composition being stable against decomposition and volatilization within said temperature range, confining said bath within a solid skull of its own composition above and contiguous toa pool of molten manganese alloy, adding to said bath a material containing a manganese oxide and a source of alloying metal of the group consisting of heavy metals and oxides of heavy metals, passing an electric current through said bath from an anode to the surface of said underlying pool of molten manganese alloy as cathode, maintaining the bath at a temperature above the melting point of the manganese alloy and at such temperature, within said range, as to maintain molten at least the upper surface of said underlying pool of manganese alloy, and intermittently tapping molten manganese alloy from said pool, said process being further characterized in that said pool of molten manganese alloy is supported on a carbon-free layer composed essentially of relatively electrically non-conductive refractory oxide which is non-reactive to manganese alloy.

12. A process for the production of low-carbon, lowsilicon manganese-boron alloy, which comprises establishing a fused electrolytic bath consisting essentially of calcium fluoride, a lower oxide of manganese and an oxide of boron, said fused bath having a melting range of from about 1150 C. to about 1300 C., confining said bath within a solid skull of its own composition above and contiguous to a pool of molten manganeseboron alloy, adding to said bath a material containing a manganese oxide and an oxide of boron, passing an electric current through said bath from an anode to the surface of said underlying pool of molten manganese-boron alloy as cathode to electrolyze said metals, maintaining the bath at a temperature above the melting point of the manganese-boron alloy and at such temperature within the aforesaid range as to maintain molten at least the upper surface of said underlying pool of manganeseboron alloy, and intermittently tapping molten manganese-boron alloy from said pool, said process being further characterized in that said pool of molten manganese-boron alloy is supported on a carbon-free layer composed essentially of relatively electrically non-conductive refractory oxide which is non-reactive to said manganese-boron alloy.

References Cited in the file of this patent UNITED STATES PATENTS 1,062,430 Becket May 20, 1913 1,534,315 Hoopes Apr. 21, 1925 2,202,012 Long May 28, 1940 2,398,589 Mitchell Apr. 16, 1946 2,865,833 Renner et a1. Dec. 23, 1958 2,915,442 Lewis Dec. 1, 1959 FOREIGN PATENTS 17,190 Great Britain Ian. 19, 1901 of 1900

Claims

3. A PROCESS FOR THE PRODUCTION OF LOW-CARBON, LOWSILICON MANGANESE FROM A LOWER OXIDE OF MANGANESE, WHICH COMPRISES ESTABLISHING A FUSED ELECTROLYTIC BATH CONSISTING ESSENTIALLY OF CALCIUM FLUORIDE, A LOWER OXIDE OF MANGANESE AND A MIXTURE OF INORGANIC OXIDES INCLUDING AN ACID-ACTING OXIDE WHOSE FREE ENERGY CHANGE OF REACTION WITH MANGANESE AT 1327*C. TO FORM MNO IS GREATER THAN ZERO, AND A BASIC-ACTING OXIDE WHOSE FREE ENERGY CHANGE OF REACTION WITH MANGANESE AT 1327*C. TO FORM MNO IS GREATER THAN ZERO, SAID MIXTURE OF INORGANIC OXIDES HAVING AN OVERALL ACIDIC CHARACTER SUFFICIENTLY ACIDIC TO FACILITATE THE DISSOLUTION OF A LOWER OXIDE OF MANGANESE BUT HAVING A SUFFICIENT ALKALINE CONTENT TO PREVENT THE SUBSTANTIAL ELECTROLYSIS OF SILICON, THE FUSED BATH CONTAINING FROM ABOUT 50% TO ABOUT 90% BY WEIGHT OF CALCIUM FLUORIDE, AND FROM ABOUT 0.5% TO ABOUT 10% BY WEIGHT OF MANGANESE AS A LOWER OXIDE OF MANGANESE, AND HAVING A MELTING RANGE OF FROM ABOUT 1150*C. TO ABOUT 1300*C., THE BATH COMPOSITION BEING STABLE AGAINST DECOMPOSITION AND VOLATILIZATION WITHIN SAID TEMPERATURE RANGE, CONFINING SAID BATH WITHIN A SOLID SKULL OF ITS OWN COMPOSITION ABOVE AND CONTIGUOUS TO A POOL OF MOLTEN MANGANESE, ADDING A MATERIAL CONTAINING A MANGANESE OXIDE TO SAID BATH, PASSING AN ELECTRIC CURRENT THROUGH SAID BATH FROM AN ANODE TO THE SURFACE OF SAID UNDERLYING POOL OF MOLTEN MANGANESE AS CATHODE, MAINTAINING THE BATH AT A TEMPERATURE ABOVE THE MELTING POINT OF MANGANESE AND AT SUCH TEMPERATURE, WITHIN SAID RANGE, AS TO MAINTAIN MOLTEN AT LEAST THE UPPER SURFACE OF SAID UNDERLYING POOL OF MANGANESE, AND INTERMITTENTLY TAPPING MOLTEN MANGANESE FROM SAID POOL, SAID PROCESS BEING FURTHER CHARACTERIZED IN THAT SAID POOL OF MOLTEN MANGANESE IS SUPPORTED ON A CARBON-FREE LAYER COMPOSED ESSENTIALLY OF RELATIVELY ELECTRICALLY NON-CONDUCTIVE REFRACTORY OXIDE WHICH IS NON-REACTIVE TO MANGANESE.