US2384479A - Process fob the production of - Google Patents

Description

p 1 R. LEP'SOE ET AL 2,384,479

PROCESS FOR THE PRODUCTION OF ANHYDROUS MAGNESIUM CHLORIDE Filed Nov. 24, 1939 ROBL'RT L [P5015 G. a. 0R 77VR a H. SAL 75R.

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Patented Sept. 11, 1945 Robert Lepsoe, Gerald StanIey-Ort'ner, and John Henry Salter, Trail, British Columbia, Canada, assignors, by mesne assignments, to" The i Mathieson Alkali Works,.Inc'.,.a'corporationzof Virginia Application November 24, 1939, Serial No. 305,978

7 Claims.

invention relates to a process for the production of magnesium chloride and is particularly directed to the production of substantially pure, anhydrous magnesium chloride.

@Processes for the electrolysis of fused magnesi- ,um chloride to obtain metallic magnesium therefrom have been in relatively widespread use ,for many years past. It is also well known that magnesite (magnesium carbonate) which occurs in nature alone .orin compounds, such as dolomite iMgCoaCaCoz), can be converted into .chlorideiorm and the resulting magnesium chloride electrolyzed to recover the metallic magnesium in substantially pure form. r

-- Fo'r the successful electrolytic separation of magnesium irom molten magnesium chloride, the

.magnesium chloride-must be substantially free from solid impurities and preferably should contain no water and not more than 1% solids. Magnesium chloride of this quality is suitable for direct additiori to the electrolytic cell, and recovcries of the order of 90% of the magnesium with high current efilciency can be obtained. To illustrate the importance of using a chloride substantially free from solid impurities and water, which latter would be decomposed to form magnesium oxide, it is found that a recovery of only about 80% of themagnesium can be obtained from. magnesium chloride containing 2% solids, while a recovery of only 60% can beobtained [from magnesium chloride containing 4% solids.

.These lower recoveries are due totheretention of electrolyte and of metallic magnesium by magnesium oxide and other insoluble impurities which it is necessary to remove from the cell duri s Operation.

It is manifest fromthe published literature relating to processes for the chlorination of magnesium oxide, such as exemplified on page 175, vol. III of Handbuchder TechnischenElektrochemie-, by Engelhardt, 1 934, that considerable difliculty' has been encountered in the developm'entof processes for the production of anhydrous magnesium chloride from magnesite,

.. Among the-principal. difficulties encountered in "such processes is that of effecting the substantially complete combination of chlorine with the magnesium'of the magnesium oxide.

[IA further difliculty arises from the tendency of the magnesium oxide material to disintegrate during chlorination, whether the material is in the form of lumps oi magnesite or in the form ofbriquettes of finely. ground magnesia, bonded by knownmethods involving theuse of magnesium; chance, carbonaceous 'materials and the gested to obviatefthetlisadvantages arising out of like? These iilagnsium oxide materials tend to disintegrate into particles which are carried away with th e'moltenmagnesium chloride, with the resultthatthe whole charge is converted into an extremely viscous mass which retards, if it does not completely prevent, the further chlorination of such of the un'chlojrinated pieces of the charge asare covered with this viscous material. The magnesium oxide ,thusi entrained impairs the purity of any product obtained anclnecessitates "its further treatment to make it suitable for electrolysis. Similar difficulties are caused by contamination of theg nolten magnesium chloride with finely ground coke or other forms of carbon whether bonded or not.

' *A still 'iurther difliculty arises from the development of high temperature zones in the electrically heated chlorinating furnaces of the prior --art, which tend to volatilize the magnesium chloride as it is formed. The volatilized magnesium chloridemay be carried off in theexit gases from the furnace or condensedto an appreciable extent in co1der. sections of the chargeor furnace to ,form ,viscous zones, thus decreasing .the' efiiciency of; the operation. Under such circumstances, very little, if any, magnesium chloride 'is recoveredin molten form at the tap-hole.

Reference is madein the prior art to the exothermic eactions:

.ltappears thatthe stov reactions have been the-subject or detailed investigation fora number of years past but due'to the above mentioned difficultie 'in. operation, processes based on these reaction have not appeared in practice until recently. f v

In these processesyexpedients have been sug the entrainment oiparticles of magnesium oxide by-themolten magnesium chloride in its downward" cours'eflthroug h thechar'ge. Filtering or irrigation'b'ed's have 'beensuggested toserve the purpose of promoting the chlorination-of the entrained magnesium oxide particles'suspended in .the molten 'magnesium chloride and of separating: out the impurities-and any unchlorinated magnesium oxide sparticles; Alternatively, it has beenQsuggested, in order to. avoid the above disadvantages, that chlorination should be carried out ata temperature belowthe melting point of the magnesium chloride-conditions being ,arranged so that the chloride will fuse and run isium :chloride formed in .a charge 101 thisimaterial quickly oil the unreacted material of the charge at the zone of contact with the irrigation bed, which is maintained at a temperature above the melting point of the chloride. The quick separation of the molten chloride, immediately prior to entering the "irrigation bed, is recommended, presumably to prevent the magnesium chloride from picking up particles of magnesium oxide,

However, operating under these restricted conditions involvesconsiderable difliodlfiies and th-dis ZIO advantage that residue, resulting fwin the use-if i excess magnesium oxide, excess'poke ,impure electrically heated irrigation bed, making continued operation impracticable'lmd,sfurthermore,.- chlorinating at such low temperatures entails the disadvantage that a slowerrate. of chlorination is inevitable.

We have found that it is possible to iefiect the chlorinationof magnesium oxide in such ,a manjner' that 'itheinagnesium chloride 'is recovered in substantially purelan'd ihighly fluidtorm, while obtaining .a high yield and avoiding the 'difliculities hitherto associatedwith the chlorination of magnesium oxide. v

' one of the principal features of, our invention resides in the preliminary treatment to which the magnesiumoxide is subjected before it is charged, into ,the ichlorinatin'g furnace. The magnesium oxide is '.give n}a preliminary treatmentlto prepare .it in agmrm Lin whichit is porous .andxin which it hasisufiicient,:physicalstrength ,to enable it -to ,withstanddisintegration during chlorination. A piece of -magnesium oxide .pre- .pared :in ;the manner disclosed hereinafter retmainsgthroughout .the ,chlorinating operation, as -a single piece which graduallydecreases in size -as its chlorination progresses. Mo1ten :magne- :is pure. :and, :iiurthemnore, rides not 'become 26.011- taminated .during Sits; passage lthrough itheccharge.

$By :employing a"charge ofithis mature, the exh ,A further feature of theinvention resides in I s the. manner in. whichlthe .chlorinating ,process is "operatedto obtain laih'igh reaction velocity and e'fliciency with the result that .at the same :time

economies are efiected'in the-operation of-the process which were hitherto considered impossibleto-realize. V F p p A ,complete understanding oi :the objects=of the present invention and the-manner inywvhich they are=attained may be ,had .from the following :de-

scription' and :acmmpanying drawing in which:

. Figure 1 a cross+sectional side eleva'tion'of an electrical resistanceffurnace suitable for carlrying out the process herein-described; and

Figure 2-is-atop-plan view of theiurna'ce il1us trated in Figure ,1.

Like characters of reference refer to like parts throughout the specification and drawing,

In general, the present process consists of two steps:

,(a) v The preparation .of .the charge containing sstrong ico'herentmorous' pieces of :siritered magnesium oxide and pieces of coke; and

. (b) The chlorination of the charge during which the magnesium oxide is converted into substantially ipurenmd highly fluid molten magnesiumchlorider 1 We have found that magnesium bearing ores 'suhias do1omite',brucite and magnesite, in the {form finely ,ground concentrates or of ore in Jump for finely;ground form, all of which forms may contain impurities such as the oxides of silicon, iron, aluminum and calcium, may be used satisfactorily "as "raw materials for our process.

.-In the-case of a finely ground concentrate, containing magnesium in the form of carbonate, we preferffirslt to calcine the concentrate at about '7O'D C. to produce alightly calcined or caustic magnesium oxide. Water or -a *dilute -acid solution, such asjfor example,wouldcontain 3% sulphur ic acid, is added to form a plastic mixture suitable for moulding in'tobrick 'shapes. The use of sulphuric acid solution increases the strength of the bricks after drying and in the first stages of firing, the firing preferably-being conducted in the' mannerdescribed hereinafter. The useof sodium carbonate, -'Goulac -or similar binding agents gives similar characteristics. The con- "centrate may, if desired, be first moulded, then calcined and fired. Alsogmagnesium bearing ores maybe calcined, sized,moulded and fired, or they may bes'ized, moulded, calcined and =fired at the requisite temperatures. In the latter case, the calcination and firing may take place in oneop- -eration. Alternatively, if the magnesium bear- =ing ores are in lump form, they may becalcined and fired with-theomission-of the mould- 'irr'g step;

The temperature at which the finely ground material-isgiven its calcination has "an important 'bearingon the"subsequentbehaviour of the calcined "material. The lower the temperature of "calcinationfithe lighter, more chemically reactive and wa'tenabs orbent is 'the resultingcalcined' ma- ;terial and weptherefore, 'prefer to "calcine 'the *m'a'gnesium bearing material at a comparatively "-low temperature, about 10'0" C., which, in the next-stage er the ffi'ring, results 'in" a very strong and rlatively porousbrick-which is readilyamenable *to chlorination.

Thebricks, after they have been *inoulded, are dried slowly andthen fired at the requisite tem- --perature which, as described hereinafter, causes "suiiicierrt 'sintering of the material to produce a -strong, p'orous,coherentproduct.

temperature at iwhichthe bricks are fired. -Pure magnesium *oxide "has a melting :point 01' about 28w '=-but, if impure, it softensfand partially "or completely fuses at considerably, lower temfperaturesth actualtemperatures of suchiphysim1 change depending 'upon' "the composition "of "the material being used, Magnesium :sili'cates format temperaturespf 1400*0. and above.

tween 4" and 1 /2".

- ture of-firing is employedf'a product is obtained which lacks strength due to an insufficient proportion of fused materials being present in the Y "mass. This producttends to crumble during chlorination to form'fines' which contaminate the magnesium chloride and cause the operating diificultie's which it is to avoid.

We have found that the most suitable tem- "perature range of firing will vary with different types of magnesium oxide'bearing materials. The product from such firing should preferably be not glassy or vitreous in appearance, though a porous, coherent mass and'only suflicient temperature to produce this'result should be used for reasons mentioned hereinabove. We have found that thepreferred firing temperature for certain magnesium bearing concentrates, which hadbeen caustic calcined and bonded as described hereinabove and for certain magnesium bearing ores, lies Within the range of from 1250 C. to 1400 C.

*The size of the pieces of magnesium oxide "charged into the chlorinating furnace also has an important bearingon the successful chlorination o'f-lthe charge, inasmuch as the speed of the reaction between the chlorine and the magnesium "oxide-is largely determined by the surface area of the pieces exposed to the action of the chlorine. "While this factor governs the maximum size, it

isparticularly desirable to limit the minimum sizesince too large a proportion of fine material in the charge tends to choke the furnace and, H "moreover, such fine material has a tendency to be'washed down with the magnesium chloride,

forming therewith the thick pasty mass, which makes the continued operation of the process extremely diflicult and contaminates the product.

We have found that very satisfactory operating results are obtained when the size of the magnesia brick fed to the chlorinating furnace is be- It will be understood, of course,- that the scope of the invention is not to be limited to these sizes.

nated, has a diameter of approximately 3 millimeters and no difliculties are encountered with pieces of such size.

The size of the'coke, which preferably comprises about 20 ofthe weight of the charge,

' is of the same order as that of the pieces of magand or residue producing impurities.

" The coke, if containing appreciable amounts the object of this invention It is pointed out that even a A" piece, after it has been 90% chloriof hydrocarbons and .water, preferably; should firstl be calcined in. order. :to avoid the possible formation of soot andhydrochloric acid.

We have found that very satisfactory operating result are obtained when an excess of coke is mixed With'the magnesium oxide over the amount actually required to effect the reaction.

In operating with a charge containing pieces of magnesium oxide and coke in the proportion of about 80 to 20 by weight, the chlorination proceeds at a-very rapid rate.

The sized pieces of magnesium oxide and coke are charged into a suitable furnace, such as that described hereinafter, and the temperature of "the lower or chlorinating zone of the furnace is raised by means of internal or external heating to the desired temperature above the melting point of magnesium chloride, but below the point at which appreciable volatilization of the mag nesium chloride will occur. Havingregard to the above limitations, we have found that the higher the temperature above the melting point inthis zone, the more rapid is the rate of chlorination.

We have further found that the most satisfactory results are obtained when the chlorinating zone is maintained at about 1100 C., as the use of this temperature serves to increase the rate of chlorination many times over the rates prevailing in processes operated at temperatures below or near the melting pointiof magnesium chloride. The temperature of 1;l00 C. is well above the meltingrpoint of magnesium chloride, 712 (3., yet not sufficiently high to cause any appreciable volatilization of the magnesium chloride, attack upon the furnace lining or the formation of silicatesor other fusible materials which, if present, would increase operating difficulties, decrease efiiciencies and impair the purity of the product. The top zone-of the furnace, which is much shallower than the chlorinating zone beneath it, need only be maintained at atemperature sufiiciently high to prevent, therein, any appreciable condensation of volatile impurities such as the chlorides of iron and aluminum. When the furnace has been heated to within. the predetermined temperature ranges, chlorine gas is admitted to the chlorinating zone and passed'up through the charge at the desired rate.

As the fluid, molten magnesium chloride, un

' contaminated by entrained impurities, is formed,

magnesium oxide,

it passes downwardly through the charge and is collected at the base of the furnace in a reservoir of coarse lumps of suitable material such as coke, graphite, silica, magnesium silicate, fused charge mixture, or spent charge. The fluid molten magnesium chloride is removed periodically from the base of the furnace in a manner similar to that used in the operation of blast furnaces for the production of various metals.

.-Fresh charge is added to the top zone of the furnace, as the charge below is consumed and sinks downwardly as" the reaction between the magnesium oxide and chlorine proceeds. This procedure is repeated until the furnace becomes full of residue, which isthen removed and the operation is repeated; Alternatively, the operation may be conducted continuously, the residue being removed periodically through a furnace clean-out door provided for that purpose.

Recoveries of magnesium as magnesium chloride of the order of 90% or more can be obtained in our process with substantially no chlorine loss e mm which it is ":by "conducting :the reaction :in a .-shaft:furnace or a fheight ssufilcient :such that tall the is consumed and :suchthat the residue irem'oved trom 'the'sturnace acontains .very ilittle unreac'ted material. :In the event that .theioperation is carried flout in samannensuchtthat theaexit gases icontainzchlorine, this chlorine :may :be srecovered-and passed into a second furnace .or returned to the first furnace.

Referrin'g' to the accompanying drawing, the numeral Z'l indicates 'an electrically heated shait 'furna'ce :lined with :heat :and (chlorine -resi's'tant material 2' such as silica bricks, into which :is ch'arged the sized pieces of magnesium oxide and -coke '"3. M'olten magnesium chloride collects in t-he reservoir :formed in the base of the furnace periodically -w ithdrawn through 'itap hole 4. I

l he' furnace is heated by-means-of-two vertical carbon or graphite electrodes 6 used "as resistors, which-are solidly connected "to theh'orizontal caricon or graphlte connection of relatively large cross-sectional area. :It'w'ill be apparent that the furnaice may be fitted with' any number of ver- 7 "'and provided with openings 1-0 and i l which serve respectively for barringthe charge i'f necessary, andforevacuation ofith'e exit gases. The resistors are-shielded and water-jacketed to prevent their corrosionwhere they enter the furnace.

In "the event that .it is desired to conduct the chlorinating process in,.an externally heated furnace, aretort type furnace, similartoithat used in cokeoven practice, isirequired.

In the operation of the furnace, it is necessary to ensure that .the charge .beheated and maintained within the .highest possible temperature range .having .regard .to the .limitations set out hereinbefore in .orderto conduct the operation of the furnace in the most -eflicient -manner. -We

..have found that temperature TOOHIIOI within this range is necessary for the reasons also set :out

.hereinabove. In addition localzhigh temperature zones, such as would be developed :by arcing, *by points of high electrical resistanceor 'by other causes, must be avoided tozpreventzhigh electrode consumption and the formation of undesirable -materialssuch as secondarily .formed, finely divided -.magnesium-oxide and soot.

. "The composition of the residue ris idependent upon theanalysiso'f the charge fed into 'theifur- :na'ce'and the degree of "chlorination of the charge. A' typical -residue where the process has been operated in a manner described here'in'be'fore, consists of unused coke, saturated with magnesium chloride, and mixed with considerable amounts of impurities such as silica and incompletely chlorinated magnesium oxide. 1" j'jllfiithj operation of the process we have found that we are primarily interested :in the following :consecutive :reactions:

With respect to rate-constant k of reaction A, .Gmelins .Handbuch der :Anorganischen Chemie,

.From these values we .have determined by calculation that :Kl000.C==.60.3.3,nd that-at equilibrium at 10.00 -C. the theoretical gas analysis would, therefore,-show.2.6.% .Oz and 97.4% C12. The-rate of reactionof a reversiblereaction being proportional to the extent to which the system is removed f-rom the state .of equilibrium, it follows that the rateofchlorinationin the case of reaction A must depend upon the difference in oxygen concentrations .at equilibrium .and at any given state-of the system. Where reactions -A and B -.only are concerned, the rate of chlorination, being ,pr0moted:by the vremovalof oxygen :from the system and by the intimate distribution of coke in the charge, is thus governed bythe diffusion rate of oxygen. Where reactions Cand D are, however also involved, the diifusionrate is of much less importance. Of these secondary reactions to depress the oxygen concentration and, providedthere is sufficient coke available, it .is .not essential that the coke be intimately .mixed .in the icharge.

This theory is confirmed by chlorinating pieces ofmagnesium oxide brick feed alone in (an externally heated silica-lined furnace at :a temperature-of about .-1000 C., no reducing agent being present. while magnesium chloride, free frommagnesium oxide, was collected in the :well of the apparatus.

,In a second experiment, .a column of alternate pieces of magnesium oxide brick feed and coke, the pieces .beingabout an inch and .a half insize and touching only .at their upper andlower extremities, wereexposed to.reaction with chlorine and .the chlorinating rate was .much faster due to the secondary reactions thanin the first experi- .ment, where nosecondaryreactions were possible.

.In .a third experiment, an intimate mixture of .pieces .of .coke and .of similarly sized pieces of magnesium oxide brick feed was chlorinated, with .the=result that thesecondary reactions were favouredand the chlorinating rate was increased considerably. In this experiment, in which the coke was distributed throughout the charge, .conditions were similar to .those prevailing in our normal furnace operation.

It will be apparent from the above discussion that, for any given temperature, .the rate of chlorination would be favoured to some extent by the intimacy .of the magnesium oxide and carbon mixture .in the charge. 33riquettedmixtures of finely ground magnesium oxide andcarbon might thus be more reactive than a mixture of coarser pieces of magnesium oxide .and .carbon, if the latter mixture had not sufficient porosity. However, temperaturesbelowithe melting point ofm'a'gnesium chloride have beenrecommendedior the chlorination *of these briquetted materials, since these disintegrate above the melting point "of magnesium chloride, and this Oxygen was .found in the exit gas,

disintegration seriously retards the rate of chrination, impairs V the purity of any product formed, and causes difiiculties which finally make continued operation impossible. The lower temperatures greatly decrease the rate of chlorination. In operating with the type of charge described hereinbefore and at a relatively high temperature, the chlorination is carried out considerably above the melting point of magnesium chloride, such as for example at about 1100 C., thereby gaining the advantage of the greatly increased rate of chlorination without any of the above disadvantages associated with the use of briquettes at such high temperatures.

Carbon monoxide, as is shown in the prior art, may be used as a reducing agent in place of coke, but we have found that the chlorinating rate is, however, only about three-quarters that when coke is used, due to dilution of the chlorine with carbon monoxide, both the driving force and the time of contact being reduced. The carbon monoxide mentioned here is not to be confused with the carbon monoxide of reaction C above, which is only an intermediate reaction product. In this case also, when using a magnesium oxide charge, as prepared above, we have found that a pure magnesium chloride product is obtained directly from the charge.

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

l. The method of producing anhydrous magnesium chloride which comprises reacting magnesium oxide, in the form or strong, porous, coherent pieces formed by heating magnesium concentrates to incipient fusion of the impurities therein, with chlorine gas in the presence of coke while maintaining a reaction temperature above the melting temperature of magnesium chloride.

2. The method of producing anhydrous magnesium chloride which comprises reacting magnesium oxide, in the form of strong, porous, coherent pieces formed by heating to incipient fusion magnesium oxide bearing substances which fuse substantially above the melting temperature of magnesium chloride, with chlorine gas in the presence of coke while maintaining a reaction temperature above the melting temperature of magnesium chloride.

3. A process for the production of anyhydrous magnesium chloride which comprises charging pieces of impure magnesium oxide into a chlorinating furnace, said pieces of magnesium oxide having been formed by heating to a temperature substantially above the melting temperature of magnesium chloride whereby incipient fusion of the impurities occurs to form pieces which are strong, porous and coherent at temperatures above the melting temperature of magnesium chloride, reacting the magnesium oxide with chlorine gas in the presence of coke at a temperature above the melting temperature of magnesium chloride and separately withdrawing substantially pure magnesium chloride from the chlorinating furnace.

4. A process for the production of anhydrous magnesium chloride which comprises charging into a chloriiiating furnace pieces of sintered magnesium oxide and pieces of coke, said pieces of sintered magnesium oxide having been prepared from magnesium oxide sintered at a temperature within the range of from 1250 C. to 1400 C.; maintaining in the chlorinating zone of the furnace a uniform temperature above the melting point of magnesium chloride and in the upper zone a temperature sufficiently high to prevent condensation of volatile metallic chloride impurities from the exit gases; passing chlorine gas through the charge and separately withdrawing substantially pure magnesium chloride.

5. A process for the production of anhydrous magnesium chloride which comprises charging pieces of impure magnesium oxide, sized within a particle size within the range of from inch to 1 inches, into a chlorinating furnace, said pieces of magnesium oxide having been formed by heating to a temperature substantially above the melting temperature of magnesium chloride whereby incipient fusion of the impurities occurs to form pieces which are strong, porous and coherent at temperatures above the melting temperature of magnesium chloride, reacting the magnesium oxide with chlorine gas in the presence of coke at a temperature above the melting temperature of magnesium chloride, and separately withdrawing substantially pure magnesium chloride from the cnlorinating furnace.

6. A process for the production of anhydrous magnesium chloride which comprises heating to incipient fusion magnesium oxide bearing substances whlcn I'use at a temperature substantially above the melting temperature of magnesium chloride, thereby forming pieces oi impure magnesium oxide which are strong, porous and coherent at temperatures above the melting temperature of magnesium chloride, charging the pieces of impure magnesium oxide into a chicrinatlng furnace and reacting the magnesium oxide with chlorine gas in the presence of coke whilemaintaining a reaction temperature above the melting point of magnesium chloride and separately withdrawing substantially pure magnesium chloride from the chlorinating furnace.

7. A process for the production or anhydrous magnesium chloride which comprises sintering magnesium oxide at a temperature within the range of from'1250 C. to 1 i00 C.; breaking the sintered magnesium oxide into pieces of from to 1 /2" size; charging into a chloriiiating furnace the sized pieces of sintered magnesium oxide and similarly sized pieces of coke, maintaining in the chlorinating zone of the furnace a uniform temperature of about 1100 C., and in the upper zone a temperature sufficiently high to prevent condensation of volatile metallic chloride impurities from the exit gases; passing chlorine gas through the charge and separately withdrawing substantially pure magnesium chloride.

ROBERT LEPSOE. GERALD STANLEY ORTNER. JOHN HENRY SALTER.

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