US2456935A - Refining of volatilizable metals - Google Patents

Description

Dec.,2l, 1948. H. F. FISHER REFINING OF VOLATILIZABLE META-LS Filed Nov. 5, 1946 2 Sheets-Sheet 1 INVENTOR. HAPMON F. FISHER Dec. 21, 1948.

Filed Nov. 5} 1946 H. F. FISHER REFINING OF VOLATIL IZABLE METALS 2 Sheets-Sheet 2 FISHER AGENT Patented Dec. 21, 1948 1 1 :Harmon FHFiSher, Sunnyvale, Calif.,- assignor. to i The Permanente Metals Qorporation, Oakland,

Califl, a corporation of Delaware.

' -This invention relates to a method for recovering volatilizable metals from materials con 'taining "the sam'e.= More particularly, it relates.

.toii thei vaporization of volatilizable metals from substantialamounts of non-volatilizable impurities by a method wherein the heat require'd'to vapot iae th'e -metal aisi supplied directly to the materialicontaining the metal by means of a carrirgas i A r The invention is applicable generally tothe refining of volatilizablermetals such as--zinc, the 'alkali metals, the alkaline earth metals,"etc., which" may be separated by vaporization; from concomitant non-volatilizable impurities. For purposes of illustrationfhowever. the. invention will be "described with particular reference to the treatmentbf magnesium; It is especially suited to the recovery of magnesium from carbothermic -magnesium=dust. M 1 v In the carbothermic process for the production of meta'llic magnesium a 'mixture of magnesium ox'ideand carbon'is subjected to a' temperature i-n the order of: 2000 0. whereby magnesium and :carbon monoxide are formed according to the following reaction: i i

" MsQ+QA=M ---'-:Si-nce this reactionzis reyersible it is necessary to cool the magnesium and carbon monoxide to :a point'where they are stable; in the presence of feachz other. This cooling or chilling t operation :is rapidly effected withthe aid of large quantities ioiaauchilling medium such as naturalgas, liquid hydrocarbons, hydrogen. or certain of the noble gases: -aHowever, even when good chilling. ef- .ficiency, is achieved there is some reversion of mthesreactionso thatthe product recovered com- -;.prises a mass ofabout fifty per cent metallic ;magnes ium with the remainder magnesium oxide ;and;carbon in the ratio of about 60 to 40. 2 There;-

are also relatively small amounts of carbides present inaddition to lesser amountsofcalcium,

si-liqmri; .sodium, .ir0n.;and aluminum anditheir ,eoxides y.Theqmaterial is finely divided and the individual particles range insize forthe most i part from several microns to a small fraction of one micron. In theindustry this product is usually referred to as. carbothermic magnesium di t; 13

The ecovery of and team magnesia of Application November 5, 1946, Serial No. 707,819

, 9 Claims. (c1. s- -cs) use of special: equipment, and asa consequence it. must be handled with considerable care and precaution In the process of purifying metallic mag.- nesium; of; the type produced by the carbothermic process, thecoperation in the past has beento placethelfinely. divided magnesium together with its accompanying impurities .(i. 'e. magnesium oxide, magnesium carbide, carbon, silica, etc.) into apelleting machine and then to feed the resulting pellets into aretort; The retort was hermetically sealedand then lowered intoapitfurnace and evacuated to decrease the. distillation" temperature. Any volatile binders and impuritieswere'first distilled ofi at. a low temperature and then the temperature was raised until the magnesium metal was volatilized. Because of the. evacuated interior of the. retort and. be-

cause of the, low heat conductivity of the charge stockv and the high. heat capacity of the-retort,

the, process, was extremely slow. The; retorts were subjected to high temperatures because of the necessity of transferring heat through the shell of the retort. and thence by radiation through the'i poorly conducting pieces of charge stock containing the magnesium-.1. The. capital investment win heavyzplant equipment was extremely high-and equipment life: was short because of thejsevere conditionsimposeduponit.

It is an object of the presentinvention to provide a method for the distillationof volatilizable metals'whereby the heat required for the distillation of the metal-from the charge stock is carried directly to the charge stock by a carrier gas. An additional object is to provide "a method for the distillation of metals which is rapid and .etficient,-and :which does not require. the use: of expensive plant equipment or severe operating conditions that are highly destructive to plant equipment.

,In practicing the present inveritionga stream of carrier gas; is firstheated to a temperature above the'boiling temperature of the volatilizable gmetal' sought in pure .form, and then the heated carrier gas is circulated through a bed of-the chargestockcontained in an insulated chamber or retort. Continued circulation of theliot carrier'gascauses a rapid rise in the temperature of the charge stock until the boiling temperature of the volatilizable metal is approached. At these elevated temperatures the metal'exhibits an appreciable vapor pressure and, 'inthe presence of the stream of carrier gas it vaporizesf-at ;a rapid rate. The metal vapors are conveyed'by the carrier gasto a condenser where the metal is 3 condensed to the liquid or solid state .and separated from the carrier gas. The recovered carrier gas is recycled under pressure through a suitable heating zone and fed back through the charge stock.

In the event that any dust is mixed with the charge stock pellets, fragments, or other agglomerates it fnia'ly be blown out o'fj'thchafg'e stock" early in*the"lieating cycle by seiieral-blast of car= rier gas. The dust particles picked up by the suitable dust separating means.

In a modified embodiment oi the preseiitinverr tion, a preheating step maybe-interposedwer (or from other recovery sources), is circulated through the charge stock agglomeratestolieat- 2O obviated, and the metazl walls and heat insulatthem to a temperature somewhat below the melting: point of the. metal .tohbe'tvaporizedz As described abovervolatile binderssuch mineral permits a simple and economic "separation of dust rromtheicarrier gas when such separation' of=-dust is necessary. If appreciable amoun'ts of dust are present with theimetal vapors fin the carrier gas that enters the condensen'the condenser maybe- :come' r fouled T: by accumulated dust' particles; and the rate of heat transfer to the cooledcondensing surfaces will be decreased.v In addition,-the en trained. dust may contaminate the 1 recovered um metal with impurities ancl'thereby decrease-its Ivalue.

Dust's'eparators,',such asfiltrs and cyclones,

employed in" the process-of the inventionmust be amaintained at temperatures abovei'the condenser temperatures. By the use :of a. dust collector in the primary preheat? ga'ssstream, lower temperatures may be used on thedust collector. The pri- 'mary preheat :gas stream, which is better adapted for freeing-the charge "stock of dust, may be sent throughthe'.pelle'tedcharge stock at a higher velocity than the secondary vaporization carrier gas stream, which carries;metalfvapors I that mustibe condensed.

The carrier-gases employed in'th'e present pro'c-= -ess are prefera'b'ly inert. Hydrogen, becauseof its-inert character to' magnesium and. most ofthe volatilizable metals, has been found to be'very satisfactory. Such inert-gases as helium, and argon may also be used.

Pressures above atmospheric are' conveniently used in the process. thereby minimizing: the danger of'air infiltration into the carrier' gas-"system. The process may, however, be operated at pressures near o'r far'belowatmospheric pressure" byrusing suitable precautions to prevent leakage 'ofair-intotl'ie system, i z

v The carrier gas ma befl'ie'ated by 'uny Suitable means such as refractory pellets'ofth'e eontained in Royster 'st0ves,'o'r bylectrical esistance elements, similar-to enhances-ma eof silicon carbide: which may be saf ly-operated "up t'o'ZSUO" F; Metallic resistors may be' used, prov ded the upper temperature lin'iits' -aretvithifi the safe working range of the anoy.--

heating may rio also be accomplished by radiant heater tubes made of high temperature all-y. These tubes are sealed off from the carrier gas in the heating furnace. Combustion of fuel within the heater tubes supplies the necessary heat.

Since all the high temperature heat required in the instant process is supplied directly to the carrier! gas from within' the" encwsed: system rather than frdm heat' supplied' tbthe' outside walls of portions of the equipment, none of the carrier gas blasts are then separated from thecarrier gas by the use of cyclonefilters or otherri-ietal walls of the equipment-such as retorts, re-

,tort shells eteare at high temperatures during the operation ofthe process. It is, therefore, possible to line the :hot gas ducts and the vaporiza- 5"tion retor'tswitlihigh temperature refractory and 'i'irsulatiirgliningssuch as, for example, carbon i r 1-rthe hottest zones, and magnesia in the low temperature zones. Thus heat losses are reduced, triene'ea for expensive heavy plant equipment is ing material: are' not destroyeclabyroxldatieii or reduction reactionsr. The carrierwg'asprocesszof theinventiohfmazytbe operated irr ei-ther:batch or continuoussmanner.

. Examples fofrboth modes at operationl'wlll-ibesdexscribed:

Fig: 1 is :a descriptive :drawing: of a -dis'tillatlon 'or't'vaporizatiom retort .which may be used ;-in. c0n-, nection with a batch type operation ofstheinven- 'tio'n.-

A gas-sealed chargingrhopper' l0; into-which funnel 'l l empties; isequipped with :plugzvalves'tl'z and I3 and-is mountedontopofr'retortt-lQ5 Between-the .top of retort l 4 and plug valvel'l 3 there ,3; is installed a' spe'ctacle'slide valve:l5.

Retort I 4=iscomposed of atop.=-section? I Grantee bottom section 1| 1: Both top; ands=bottom sectlons of the: retort are composed of join-ed.- eylind-i ieal sections and truncated cone sections asz'shown in sFig: 1;- Ifhe-wallsofsection Morton-section l6 areslightly inclinedinward fromthe verticalland form' atruncated l conerather: than a cylinder. Gpening into thetop. andebottom'isections: oflth'e retort are inlet :ducts l9 andlllr respectively. Spectacle slide valves 2|, 22 andi2'3:arafitted to ducts l9 and zlllandto the outlet at the bottom of the retort. Highterfipfatiife'alloy plug cocks 24; 2.5;: and" 2 6 areinstalle fzadjaeentztoitnazsfieota cl'e slidecvalv'es.

5o 7 The: wallsnofthetop: ancl'bottomasectronss [5 and" I6" of the retorti and: of ducts are: composed of i' a. steel 1 'outen'stilli lin'ed withirefr'actory. -'carboni=.brick liniiigri'baclied up: with carbon: black heat-insulation:

wand; 20

Near the tbp of the bottom-'rsectiomtFof theretort tare fastened: two -hinged isemi eirc'ular irate bars 21.: The:grateebarssiare niade offing-1i exiliieratureialloyv'andi' the: forged hinges on the gratestare oil -cooledr The spctaeleeslile valves 6 be used tovadvantage:

I Charging dry pellets-fragments; other alijiiierated forms of carbothermic magnesium-(Hist 0 fareenareedanteruiirr rf n until"'it" 'is"filledf 'rne ewe-rooted;into-charging opiiei l'o valve 12." Irrtliejhopiir tlie' cli'afge 5 is urge-a; free-trait with the earner 56 wnememgrngasedfiifiiete; tia-itelz ins -teen passing a ae-upward through th 7 Preheatz'ng If preheating is practiced, a preheating carrier .gas streamis passed through the bed of charge stock. Valves l3, I5, 23 and 26 are closed and valves 2|, 22, and are opened to pass thehot gas through duct 29 uputhrough the cold charge stock and out through duct l9. By this. process the retort and charge are warmed up to a uniform temperature of about 1204? F., the melting point of magnesium. At this temperature the vapor pressure of magnesium is less than about 2 mm. of mercury and only a small amount of vaporization occurs. i

During this preheating step loose dust and broken pellets are swept out. of the pellet 'bed and into a dust separator. It is preferable, at

.the beginning of the preheating stage, to briefly scavenge the retort charge with a high velocity blast of carrier gas for the purpose of removing dust from the charge bed. If the chargestock contains a volatile material such as a hydrocarbon binder this material is conveniently vapor- ,ized from the charge during the preheating step.

Vaporizing After the preheating cycle has been completed, the preheating gas stream is shut ofi and a stream of vaporizing carrier gas, having a higher temperature, is circulated through the retort in the same manner as described for the preheating carrier gas stream. This circulation is continued untilsufiicient heat is supplied to vaporize the metal in the dry pellets, fragments or other ag-,.

glomerated forms. Practically the entire metal contents of the charge stock is vaporized and carried away in the hot carrier gas to a condenser (not shown) where the magnesium is recovered by condensing it to the liquid or solid state.

The temperature of the carrier gas used on magnesium charge stock should not be appreciably below 2015 F. for rapid vaporization of the metal. When the temperature of the carrier gas is appreciably below 2015 F. a much larger vol- 'ume ofcarrier gas-is necessary'to vaporize a given mass of magnesium then when carrier gas having a temperature above 2015 F. is employed.'

Carrier gas temperatures as high as 2035 to 2050 F. are preferable for rapid rates, of vaporization and have the additional advantage that smaller rate of gas circulation will, in turn, depend upon the depth and temperature of the charge stock as well as the size of charge stock agglomerates, radiation conditions, etc. The optimum rate of carrier gas circulation is determined experimentally for each set of conditions.

Residue cooling After the vaporization cycle is completed, the

carrier gas stream is shut off and a stream of cold carrier gas is circulated through the hot bed of residue in the retort in the same manner as described for the. previous gas circulation cycles. The cold carrier gas cools the hot residue and is itself heated sufficiently to beused as a preheating carrier gas stream in another retort charged with cold ag lomerated charge stock. The cold carrier gas stream is circulated until the residue bed is cooled to 1200 F. or any other desired temperature.

Discharging After the residue and the retort have been cooled to the desired temperature the cooling gas stream is shut ofi, valves 2l, 24, 22 and 25 are closed and valves 23 and 26 are opened to a residue receiver (not shown) which was previously purged with a blanketing gas. grate bars .21 are then lowered as shown by dotted lines in Fig. 1 and the residue drops down out of the retort into the residue receiver. The retort is then ready to be charged again as described above. I

The above cycles of operations are essentially the same for the recovery of other volatilizable metals than magnesium except that difierent temperatures are employed. 7

Fig. 2 is an illustrative drawing of an apparatus which may be employed to continuously evaporate volatilizable metals in accordance with the method of the invention.

In operation with agglomerated carbothermic magnesium dust in the form of pellets, for example, the gas tight, inert gas filled surge chamber II] is continuously charged with stock from .gas locked feed bins (not shown) through pipe H. A continuous, gradual movement of the charge stock is maintained downward in rectangular retort tube l2, which is about six times as wide as it is thick. As the charge stock moves into the zone of preheat carrier gas duct 13, it is preheated by a cross-stream of inert preheat carrier gas (having a temperature of about 1250" F.) passing through duct l3 and into cyclone separator It. In cyclone separator I4, any. dust that is picked up from the downward movingcharge by the preheat carrier gas is separated from the gas, and recycled through a pelleting or agglomerating station to the feed bins. The preheat carrie r gas leaving cyclone separator; l4,

through pipe [5, may be recirculated as cooling gas in the manner described below. 1

In its continuous movement through retort tube [2, the preheated charge stock next enters the zone of vaporization carrier gas duct 16, where it is contacted by a cross-current ofhot inert vaporization carrier gas having a temperature of 2100 to 2150 F. or more. While passing slowly through the vaporization carrier gas cross-current at the mouth of duct 16, the charge stock is heated sufiiciently to cause substantially complete vaporization of the magnesium from the stock into the stream of hot vaporization carrier gas. The magnesium is recovered from the hot vaporization carrier gas stream by passing the stream through a condenser (not shown) wherein the magnesium vapor is condensed to either the liquid or solid state.

From the mouth of duct I6, the hot residue fragments or agglomerates continuously pass to the zone at the mouth of duct 11. Here they give up a substantial portion of their heat content to a cross-current of cooling carrier gas. This cooling carrier gas may be circulated from Hingedmerges-2s .yeionex separator outlet! pipe: i51 After" it. is heatedzibyither'esiduenit'imay be -pa'ssedtinto pre heatroarrierrgas duet; l 3

The cooled". residue? agglo'merates: are: moved downwardn continuously" through? am adjustable discharge mechanism: (not shown) and i into a residue: receiver (not: shown): connected: tow-the bottom of retort -tube I21:

Iiiisimilar batch'ior continuous. methods of 10pera-tion, other tolatilizabl -nietalsisuehias sodium, potassium, rubidium, .cesi-um, barium; strontium, calcium and Zinc,.may. be recovered from carbothermic reduction dusts, and the like, and the revenue-m is' riotiiitende'd' to' be limited 1 to the i eiz it is particularly advantageous mreceverifig thelatterriietal shite-several of th'e diffieulties r'ieour-i-t'ered with magnesium Manet-encountered with an other readil-ytolatiliiable metals".

The processor the invetition' is not intended 'l-imitdto pelleted carbothermic dust of iiable metals, Bilit' is" applicable t the glomerates or fragments composed of volatihzabl'e metals such as Zinc, mercury, barium, strontium, sacrum; sodium, potassium, rubidium, cesium, magnesium,- cadmium, bismuth, arsenic, antimony, radium, se1eniu'm, etc., and containing nonwolatiliz'able impurities such as carbon, mtaloxide's, and other inorganic compounds.

1. In the method of refining a mixtu're of a -vol-atilizab'le metal and less volatile solid impurity, wherein themixture is heated to atemperature suflicierit to vaporize themetalbut insufficient to vaporii'e the impurity, and the metal vapor is separated from the-unvaporized impurity; the impr'oveinent which comprises: supplying the heat er vaporizationof the metal directly-t0 a bed of agglorheratesof 'said mixture by meansof a preheatdiriert gas, by passing a stream-ofthe hot preheated gas through said bed or the mixture at a temperature and at a ratesu'c'h that the metal "exerts a substantial vapor pressure,- thereby va eri'zm the metal from said mixture into- 1 said stream ofhot'ihert'gas at a substantial rat-e 'bii'virtue of'the heat transferred'from' the hot preheated gas to the bed of the mixture; and separating the metal from the hot inert gas stream.

2. The improvementof-claim 1, wherein said ls's' -volati1e impurity is composed largely ofthe oxide of the volatilizablemetal and a reducing agentcapab'le of'reduc'ing the o'xide'to the metal at a high temperature, and the temperature of" themixture of iiiertgas'andfihely divided Solids is' we1l below temperatures at which substantial red uetidm of; the oxide by." thereducing; agent occurs butiis sufficientlyhigh to producesubstam tial :vapori'zation of thefmetal; i

3-: In athe'meth'od of refinmgaimixture omnag nesium metal and 7 less: volatile sol-id" impurity composed largelmofmagnesium oxide: and carbon, wherein the mixture is heated to a temperature sufficient to vaporize the metal but insufiicient to 'vaperize-thedmpurity, anelithe metaivapor: is

separated fro'rnitlie unvapori'ze'd' impurity; the 1m:- provemeht which cor'riprisesf supplyin'g 'the heat e'ri vaporization of the? metal idire'etly to' a bed of a'g'gl'emerates 'of said mixture, bypassing a'stream ofin'ert' -ga's; preheated to a temperature 1 within the ranae of aboutc2015 -"to 2050 F;, through said bed of the :mixture at a rate "such thitithe metal exerts asubstantialwapor pressure; therebyzvaporizingrthametalfrom said mixture: into said stre'an 'i'of hot -inert=;eas at a substantial rate by virtue of the heat transferredfrom the hot-Cpre heate'dgas twthe'ibed of the mixture'; andis'eparating the metalifrom the'hotainertigasstream:

4 The zimprovemerit of f claim 3-; wherein said inerizgas is -hydrug en;

5: The improvement of 1 claiml3'; wherein irrertigas is alnoble' 'gas'.

6." Thea m'etho'dwf refining-1a solid mixturenof volatilizable metal" and. less'volatile impurity, which comprises e'stahlishing'a'cyelic fiow 'of a' gas Said .whichiisinert with respect tot't-he' metal, .heating the gas during the course ofits flow to a temperature at which the metal exerts a substantial vapor pressure but below theztemperatures -of substantial vaporization of said impurity, passing a stream of the' hot' inert gas, after itahas been thus preheated; through a-bed of agglomerate's of said mixture at such a rate to effectsubstantial heat UNITED sTM-F's PATEN' 7 Namev Date Ogorzaly l Jan. .29, :1946

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