US2950236A - Electrolytic production of magnesium metal - Google Patents

Description

Aug. 23, 1960 L. G. DEAN ET AL 2,950,236

ELECTROLYTIC PRODUCTION OF MAGNESIUM METAL Filed June 24, 1957 2 Sheets-Sheet 1 INVENTORS.

9 2 Lloyd 6. Dean I I Char/e5 14 ME Cafe/zen rates Pater Humid-0..

Patented Aug. 3, 1250 1 2 2,950,236 of electrolysis, comprising MgCl KCl and LiCl where- ELECTROLYTIC PRODUCTION OF MAGNESIUM METAL Lloyd G. Dean and Charles W. McCutchen, Lake Jackson,

Tern, assignors to The Dow Chemical Company, Mid- 5 land, Mich, a corporation of Delaware Filed June 24, 1957, Ser. No. 667,505 2 Claims. (Cl. 204-70) The invention relates to electrolytes for and methods of electrolytic production of magnesium. relates to molten salt mixtures as electrolytes having a density less than that of molten magnesium and to the method of producing magnesium therefrom.

Magnesium is currently produced on a large scale by the electrolysis of a fused salt bath containing magnesium chloride. The magnesium chloride of such a bath is usually derived from the ocean or brines of deepwells. Some magnesium chloride is derived from the naturally occurring mixed salt, carnallite: KCl-MgCl -6H O.

These electrolytic baths currently employed have a density greater than that of molten magnesium. Unless the cathode in the bath consist of a molten metal or an alloy which alloys with molten magnesium produced by the electrolysis, the liberated magnesium rises to the surface of the bath and is removed therefrom in recovering the magnesium so produced. The use of an alloying cathode entails a complex subsequent separatory step to recover the magnesium therefrom and is not economically feasible. In current practice, therefore, the magnesium is permitted to rise to the surface. gaseous products of the electrolysis are also lighter than the bath, and therefore rise to and evolve from the surface, elaborate means for preventing recombination of the magnesium with the evolving gaseous products have been found necessary.

A further difiiculty encountered with baths heavier than molten magnesium is that the magnesium, as it collects at the surface of the bath, is exposed relatively unprotected from air which gives rise to a constant threat of burning of the magnesium, particularly at higher operating temperatures. As a result, a significant amount of the liberated magnesium is reoxidized and unrecoverable. Furthermore, the efiiciency of known electrolytic methods of producing magnesium has not been satisfactory as shown by the number of kilowatt-hours of energy required to produce a pound of magnesium. This undesirably low eficiency is due largely to the low conductivity of the electrolytic bath used.

In view of the difficulties attendant upon conventional methods of electrolytic production of magnesium, it is a desideratum in the art to provide an improved electrolyte for and method of producing magnesium electrolytically.

We have discovered that by electrolyzing, with a submerged cathode, a fused salt bath, having a density less than that of molten magnesium metal at the temperature It especially 10 Since the 30 trolyte.

in the molar ratio of MgCl to KCl is greater than 1:1 but is less than 3:1 and the molar ratio of LiCl to the combined molar weights of MgCl and KCl is greater than 4:2, magnesium metal is produced out of contact with the atmosphere below the surface of the electrolyte in a highly efficient and convenient manner and is recoverable without significant loss.

We prefer to have present in the bath a fluoride of an alkali or alkaline earth metal in an amount up to about 1.5 percent (i.e. about 0.75 to 1.0 percent of the fluoride fraction) by weight of the bath. There may also be present in the bath the chlorides of sodium and calcium up to about 4 percent of each and the chlorides of strontium and barium up to about 2 percent of each by weight.

The magnesium chloride content of the electrolyte is to be at least 5 weight percent because codeposition of lithium with the magnesium at the cathode becomes objectionably excessive at less than 5 percent by weight of magnesium chloride in the electrolyte. On the other hand, the percentage of magnesium chloride is to be no higher than that which will result in the molten magnesium being of greater density than the electrolyte at the temperature of electrolysis. A density differential of at least 0.034 g. per cc. is recommended. If the density of the electrolyte is not at least 0.034 g. per cc. less than the molten magnesium, the magnesium deposition tends to become erratic due to the tendency of some of the magnesium to remain suspended in the elec- The desired density difierential is obtained by maintaining the percentage of components of; the electrolyte according to the invention.

In the accompanying drawing,

Figure l is a plan view of an electrolytic cell with which the invention may be practiced.

Figure 2 is a sectional elevation along line 22 of Figure 2.

Figure 3 shows graphically the limits of the components of the electrolyte of the invention in molar percentages.

Figure 4 shows graphically the limits of the components of the electrolyte of the invention in weight percentages.

The molar ratio of MgCl to KCl of 1:1 is 95.2:74.6 by weight and the molar ratio of 3:1 is 286.7:74.6 by weight. The molar ratio of LiCl to MgCl +KCl of 4:2 is 169.6:169.8 by wei ht or about 50 percent LiCl when the molar ratio of MgCl to KCl is 1:1 and is 169.6: 180.1

by Weight or about 48.6 percent LiCl when the molar v TABLE I Percentage composition of electrolyte [When MgCl, is at the Lower Limit, i.e., 5%.]

Molar Ratio of MgOl; to K01 Is Just Over 1:1 Molar Ratio of MgCl to K01 Is Just Under 3:1

. M. P Molar Molar Wt. Moles Per Molar ii Pe r nt ltiii mi Percent Ratio Percent Gm. Percent 2 .0525 2. 3 Just Under 1. 5 5.0 0. 0525 2. 3 i I. i 0525 2. s 0. 5 1. a o. 0175 o. 7 91.1 2.1500 95. 4 9a. 7 2.2100 97.0

[When LiCl is at Lower Limit, i.e., .Tust Over 4 moles of LiCl to 2 moles of MgOl +KOl.]

EiiBYi IIII i 333 3233i 0.0 10.0 0.1421 8% LiCl just Over 4 so. 0 1. 179 4. 0 4s. 6 1.146 66% 3 It is recommended'that the percentage of MgCl not be above about 36% nor the LiCl be below 50% because the adverse effects on cell efiiciency of the decreasing conductivity and on cell operation by the increasing density of the electrolyte which accompanies higher than 36% MgCI or lower than 50% LiCl are appreciable.

By referring to Figures 3 and 4 of the drawing, the molar percentage and weight percentage limits set out in the above table may be readily understood. The

shaded area of Figure 3 shows the limits of the components of the electrolyte of the invention in molar-pen centages. The shaded area of Figure 4 shows the limits the components of the electrolyte of the invention in weight percentages.

The electrolyte of the invention has a lower viscosity and surface tension than the heavier-than-magnesium electrolytes currently in use. The lower viscosity and surface tensionaidin separating the magnesium metal from the electrolyte and sludge. i [The electrolyte of the invention having any of the percentage compositions set out-hereinabove will show a marked improvement in conductivity over any other electrolytes useful in the production of magnesium known by the inventors to be currently used; However, the electrolyte of the invention which has the highest conductivity and hence greatest efficiency is one having as low a percentage of magnesium chloride as is permitted without objectionable codeposition of; lithium and/or potassium Withthe magnesium.

A portion of the magnesium metal which collects at the cathode may serve as a molten metal cathode which and outlet pipe 8 for egress of combustion gases of the heating burner. in cover 5 is opening 9 for admission of feed and access to the pot. Opening 9 is provided with a removable cover 10. Outletl'l is provided for egress of chlorine and othergases, if any, formed during electroly sis. Extending through an opening in cover 5 and into pot 3 is anode 12;. The degree to which anode 12 extends is controlled by a block and tackle assembly (not shown). Drain assembly 13 including a valve as shown is provided at the lower portion of pot 3. Current leads is and 3.5 are connected to the anode and cathode respectively by suitable terminals 16, and 17 respectively. iacking j gland 18, is provided about the anode at the opening in makes a highly efiicient contact surface for further recovery of magnesium from the electrolyte. The cell for use in practicing the invention may be of simple design and construction in that no especial provision need be made to keep the chlorine separate from the magnesium produced since the-latter is separated from the chlorine byabarrier of electr olyte. Thecell is easily operated, requiring a small degree ofcontrol since the magnesium metal coalesces well, cell voltage and current remain in a substantially" steady state, fire hazard is reduced, and oyerflow of moltenmagnesium from the cell is very unlikely. Voltage requirements are lower than for cells employing known electrolytes of greater density than the magnesium being produced because of the higher conductiyity of the electrolyte and closer 'permissible spacing of anode and cathode as a result of reduced likelihood of recombination; of magnesium and chloride ions in the submerged region of the anode. The formation of sludge which collects at the bottom of the cell and which is associated with the electrolysis ofsalts is substantially less than in cells employing known electrolytes. In the production of magnesium by electrolysis, anodes of carbon-or graphite are slowly consumed. Electrolysis employing the electrolyte of the invention results in a sub stantial reduction in anode consumption over methods employing electrolytes of greater density than molten magnesium at the temperature of electrolysis.

The invention then consists of the improvedelectrolyte and method of producing magnesium therefrom herein fully described and particularly pointed out in the claims, reference being'made to the accompanyingdrawing.

Referring to, Figures 1 and*2 .ofthe drawing in detail,

' there is-shown steel shell 1 'inclosingrefractory furnacesetting 2. Iron pot 3, having 'a flanged rim, is placed in the furnacesettingp The pot holds electrolyte 4, formu lated in accordance with the invention,fand also serves if;

" Wt. of Mg possible V 7 =percent cathode efiiciency'acoording to Faradays 0. J V r I D ecomposition voltage, viz., 2.7 for' MgCl,

asthe cathode of the cell. The top' of the cell is-provided with ceramic-lined steel cover-5. About the intenor of pot 3 is ceramiclining -6- extending from cover-- 5 downwardly into electrolyte 4a sufiic'ient distance to fall-below the operating'level 'of the" electrolyte. This protects the iron pot from interaction with chlorine formed during the electrolysis. Furnace settihgjzcontains-inlet R m? 7 for ntmdupinahea .asfromahumer .(not shown).

cover 5.

In carrying out the invention, the furnace setting and pot are heated preferably to a temperature above the melting point of'magnesium. The heated pot is then charged either with the electrolyte, formulated in accordance with the invention, or, its separate-ingredients. In charging the pot initially as with the separate ingredients itis advantageous to firstiintroduce the lowest melting point ingredient (LiCl) followed by the other ingredients. If desired, the pot maybe charged with the electrolyte ingredients in the unmelted state and thereafter melted by heat admitted through inlet pipe 7. .After the pot is charged, the electrolyte is brought to asuitable operating temperature above the melting point of magnesium, the electrolyte then being completely molten except for incidental refractory and insoluble impurities normally present in the electrolyte ingredients when not completely removed by previous purification. A. generally desirable operating temperature is in the range of about 660 to 1 900 C.; a preferable temperature rangeis about 700 to 800 C.

Electrolysis is effected by applying a suitable potential across-the anode andcathode of the cell, making the pot the cathode as shown, so as to passdirect current through the electrolyte. The strength of the current is not sharply critical. For example, a current density at the anode in the range of about 3 to.30 amperes per square inch of submerged anode surface may be used.- A preferable current densityof the anode is 4 to 15 amperes per square inch. Cathodecurrent densities may vary from those of the anode dependent upon operating conditions, e.g., the relative areas of each contacting the electrolyte V Astheelectrolysis proceeds, the magnesiumliberated at the cathode sinks in the electrolyte and accumulates at the bottom of the cell to form the molten body of magnesium'19.v Chlorine is liberated .at the anode and rises to the surface of the electrolyte where it is withdrawn from the cell, as through outlet 11. The accumulated molten magnesium may be removed by means of a dipper or a siphon inserted through port 9 or by means of the drain lineassembly 13 by openingthe valve therein.

When an-electrolyteof the invention, for example, one having a composition by weight of about 12% MgCl;;, 8%. KCl, 1% CaF and 79% LiCl, is electrolyzed in a cell similar to that shown in the drawing, at 750 C., having an anode tocathode spacing of 1.5" and a current density of 10 amp/sq. in. at the anode and 8 amp/sq.

e in. at'the cathode and a cell voltage of 4.4 volts, there is a obtained a currentefficiency at-the cathode as high as lated according to the formulae:

and a power efi'iciency of, at least when calcu- Wt. OfMg produced 00 cell voltage Xpercent cathode eflfieiency=percent .power effioieney It is understood that the cell of Figures 1 and 2 is illustrative of but one form of cell for use in the practice of the invention and that other forms of cells and modifications of that shown may be used with the novel electrolyte according to the method herein described. The design and construction may vary widely. It may be much more complex and elaborate. The cathode and anode may have any of various configurations. Instead of a single anode and cathode, as shown, there may be a plurality of anodes and cathodes. A cell chamber which is lined with an insulating ceramic material and containing a graphite or high-melting non-alloying electrode extending upwardly from the bottom of the chamber may be used. In such a cell, except for the brief start-up period, molten magnesium collecting on the graphite or metal electrode serves as an efficient molten metal cathode. These and other variations will be apparent to those skilled in the art.

Although the invention encompasses any molar ratio of MgCl to KCl which is greater than 1:1 and less than 3:1 and any molar ratio of LiCl to MgCl +KCl which is greater than 4:2, with from between 0.5 and 1.5 percent by weight of CaF- the weights usually employed are those set out in Table II below.

TABLE I11 Composition of Electrolytes in Densities of Electrolytes Percent by Weight MgCh LiC1+1% KO] 750' 0. 800 0. 900 0.

100 percent Magnesium Metal 1. 567 1. 557 1. 518

Table II shows the densities of various baths from which the difierential between the density of the given electrolyte and molten magnesium can be ascertained by subtraction. In the preferred embodiment of the invention the density differential should be 0.034 g. per cc.

2 As the percent of salts other than the essential com- TABLE II ponents of the bath increases, the operating range necessarily narrows since the other salts, e.g., CaCl or NaCl, which may be present in the bath are heavier than LiCl. Component i g 'ggg From the data shown in Table III it is evident that the g by Weight 3; density difierential increases as the temperature is advanced so that slightly higher concentrations of MgCl MgOh 7 to 25 can be tolerated at higher temperatures.

g 53 In the electrolysis of the electrolyte of our invention, 02th.... .0 the magnesium chloride is dissociated, chlorine collecting 35 at the anode and molten magnesium at the cathode. Electrolytes which are illustrative of the electrolyte of the in- The densities illustrative of molten salt baths for the e i l are 5610111 in Table IV and examples Offlle fill/61k production of magnesium metal according to the inven- 1011 employrng the electrolytes Of Table IV (whereln the tion and the density of molten magnesium are set out electrolyte used agrees in number w1th the number of the in'Table III below. The percentage composition of the 40 example) to produce metallic magnesium in a cell, simibaths shown in Table III are those percentages designated lar to that shown in Figures 1 and 2, are setout in by dots on Figure 4. Table V.

TABLE IV Composition of electrolyte L101 KCl MgCl;

CaF Molar Molar Ratio Electrolyte N0. Wt. Wt, Ratio, of L101:

M019 Per- Mole Wt. Mole Wt. Percent MgClnKCl MgCl -i-KCI Percent cent Percent Percent Percent Percent TABLE V [Current conditions and results at 800 C. and electrode spacing 013 inches] Avg. Current Den., Kwh. per Length Average Average Amp/111. Cathode Lb. or Conduc- Example 01 Run Current Voltage Current Mg. Protivity in inHours inAmp. inVolts Efiiclency duced Mhos Anode Cathode As the magnesium chloride in the electrolyte becomes depleted during the electrolysis, it is replenished either atintervals or continuously to maintain'the desired pro portion of it'in the electrolyte: Also appropriate additions of lithium chloride m ay be made it needed as shown by routine tests of the electrolyte. A fluoride .of an alkali or alkaline earth metal is preferably present in an amount up to 1 percent 015 the fluoridefraction- (F) to facilitate, coalescence of the magnesium. Calcium fluoride, lithium fluoride and magnesium fluoride are the fluorides usually employed. Calcium fluoride is preferred and may conveniently be added as fluorite in the preferred range of 1.0 to 1.5' percent of fluoride by weight of the fused bath.

During the electrolysis a small amount of non-metallic insoluble sludge may accumulate at the bottom of the cell. The sludge may be removed either by a dipper as in the case of the magnesium produced or by draining through the drain assembly. 1 3. The fluoride addition already mentioned also aids in setting the sludge which stratifies below the molten magnesium in the cell. Desludging operations are necessary after protracted operation but less frequently than is now required in current practices.

Melting the magnesium chloride feed and adding the resulting melt to thecell oflers certain advantages ver its'being added as a solid? When added as a solid, especially in large'additions, occasionally fairly largeportions of unmelted magnesium chloride fall to the floor of the cell and thus temporarily raise'the' density of the bath near the cathode. i

It has been observed that the electrolyte of,the invention tolerates a higher percentage of the impurities most frequently found in feed for electrolytic cells for f the productionof magnesium than do conventional electrolytes. Impurities'up to 1 M2 percent normally present in thefeed such as magnesium oxide, and traces of salts and oxides of other metals among which are those of'iron, copper, nickel, silicon, manganese, lead, titanium, boron, aluminum, and chromium may be tolerated. Since'most of these metals deposit at a lower;

potential than does magnesium, the specifications of the magnesium to be produced will predetermine the permissible amounts of such metals.

Conductivity of commonly used electrolytes in the production of magnesium is about 2.25 miles per c'cl, whereas the conductivity of the, electrolyte of this invention is 4.7 mhos per cc. at 800 C. The power efliciency is consequently much better when the electrolyte of higher con ductivity is used. Power efliciencies approaching "60 perpower efiiciency of about 3 2 percent which is theefficiencyof present electrolytic magnesium production processes. The per poundi cost of magnesium is correspondingly reduced by use of our electrolyte. Sludge deposits are less when our electrolyte'is employed. It the feed to the bath is anhydrous to the extent of containing only about 0.5 percentwater, graphite consumption of the anode is in the l'owrange of 0.015 pound and less. of graphite per pound of magnesium produced.

It is apparent that a number of advantages over known electrolytes and methods of using them stem from practicing the, invention, Among these advantages are: recovery of the. magnesium in a' well coalesced mass at V the bottom of the cell away from chlorine and oxygen,

a molten cathode inherent in the process immediately after electrolysis begins, lower viscosity and surface ten sion of the electrolyte facilitating separation of the mag nesium from electrolyte and sludge and removal from the cell, a smooth and easily controlled operation, simplified cell design, reduced sludge formation, reduced anode consumption, and increased conductivity and. power efficiency over electrolytes in current use resulting in a substantialreduction in operating costs.

Having described the invention, what we claim and desire to protect by Letters Patent is:

1. The method of producing magnesium metal electrolytically, employinga cathode submerged in the elec-' trolyte, by electrolyzing magnesium chloride in a molten salt bath at a temperature between 660 and 900 C., said bath consisting essentially by weight of between 7 and 25 percent MgCl between 5 and 20 percent KCl, between about 67 and 89 percentLiCl, and about 1 percent of a fluoride selected 'fromthe classconsisting of cent aremade possible by our invention in contrast to the CaF MgF and UP, thereby causing magnesium metal to collect at the submerged cathode, and recovering the metal thus produced.

2. The method of'claim 1' wherein the temperature of electrolysis is between 700 and 800 C; 7

Claims (1)

1. THE METHOD OF PRODUCING MAGNESIUM METAL ELECTROLYTICALLY, EMPLOYING A CATHODE SUBMERGED IN THE ELECTROLYTE, BY ELECTROLYZING MAGNESIUM CHLORIDE IN A MOLTEN SALT BATH AT A TEMPERATURE BETWEEN 660* AND 900*C., SAID BATH CONSISTING ESSENTIALLY BY WEIGHT OF BETWEEN 7 AND 25 PERCENT MGCL2, BETWEEN 5 AND 20 PERCENT KCL, BETWEEN ABOUT 67 AND 89 PERCENT LICL, AND ABOUT 1 PERCENT OF A FLUORIDE SELECTED FROM THE CLASS CONSISTING OF CAF2, MGF2, AND LIF, THEREBY CAUSING MAGNESIUM METAL TO COLLECT AT THE SUBMERGED CATHODE, AND RECOVERING THE METAL THUS PRODUCED.

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