The Refractory or " Fireless Cooker" Method of Producing Magriesium

THE development of huge production facilities and of new or improved processes for manufacturing magnesium from its raw sources has been an outstanding achieve- ment of this war. Furthermore, a t least one new method has reached a stage of pilot production that offers promise. I ts poten- tialities seem to indicatc that a reduction in cost and simplification or elimination of problems incident to the use of at least one of the present processes are quite within the realm of possibility.

Although magnesium of very high purity (99.9 per cent and higher) is obtained, and the rate of recovery reasonable (60 to 70 per cent). the ferrosilicon reduction proccss is open to a scrious objection, which a t the time its use was proposed was thought to be sufficient to raise some doubts as to its suc- cess, a t least on a high production basis. The temperature range a t which the reac- tion is carried out-namely, 2100" to 2150~F.-is very close to the failure tem- perature of the material of which the retort is made. I t is so close that for a time i t was assumed i t might make commercial use of the process an impossibility.

However, a t the time under discussion, the demand for magnesium was so great

This paper is based on research work done by Surface Combustion, under the supervision of the War Metallurgy Committee, for the War Production Board as "restricted' Project NRC-520. I t s publication has been authorized by the Office of Production Research and De- velopment of the War Production Board. Manuscript received a t the office of the Insti- tute Nov. j, 1944; revised July 2 . 1945 Issued as T P 1941 in METALS TECHNOLOGY, Decem- ber 1 9 4 5 .

Listed for the New York Meeting, which was cancrled. ' Director of Research. Surface Combustion

Corporation, Toledo, Ohio.

that it was fclt by those who had to go ahead and produce this essential metal that the emergency warranted their taking a chance on this process. Subsequent develop- ments have fully justified thc soundness of their decision.

Fortunately for the war effort, when the plants were put into operation it was dis- covered thaf although the retorts did col- lapse in rather short ~e r i ods of time, as predicted, they could be blown back into operable shape by application of high- pressure air. This practical method of oper- ationso relievcd thesituation that the prob- lem disappeared. Nevertheless, if thc proccss had not been successful, or the production of magnesium by the elec- trolytic process had not been able to keep pace with demands, our war program would have been seriously impaired.

FIRELESS COOKER'' PROCESS

For this reason a program was set up and assigned for the development of a new method that had been suggested to the War Production Board in 1941. I t is be- lieved justifiable in terming the method "new" for scveral reasons: ( I ) while utilizing the same (ferrosilicon) process as the I'idgeon, hence involving similar rcac- tions, the type of retort or furnace is essen- tially different, (2) the method of operation, reaction temperaturcs, etc., differ materi- ally, and, most important of all, (3) the problems to be overcome in making the process successful were not and could not havc been anticipated by anything pre- viously known about the Pidgcon process. As will be shown later, the briquetting process, the condenser, the furnace lining

94 THE REFRACTORY OR "FIRELESS COOKER" METHOD OF PRODUCING XAGNESIUM

and other parts of the equipment, all had to variety of minor design points-all were be studied and revised before success was virtually unknown. The first period of the possible, even on the limited basis estab- work, therefore, lasted nearly two years- lished by the pilot installation. I t has, until early in 1942. therefore, been deemed advisable to dis- close the method a t this time, especially PRODUCTION OF BRIQUETTES since some of the problems overcome may be of value in other fields of development and may also indicate the path to be fol- lowed in any further development of the process.

The term "fireless cooker" was applied to the process because it most aptly indi- cates the general method involved in ob- taining the desired result. I t consists fundamentally in charging briquettes com- pcsed of dolomite and ferrosilicon (as in the Pidgeon process) into a refractory- lined, vacuum-tinht steel shell, which has - previously been heated to a maximum temperature of z700°F. 'The steel shell is then evacuated and the heat stored in the heated refractory furnishes the necessary heat for reaction, while simultaneously cooling to a temperature of zzoo°F.

The development of the process can most conveniently be divided into three parts: ( I ) laboratory and experimental plant development, (2) preliminary pilot-plant tests, including construction, revisions, etc., and (3) production runs, results, con elusions, future possibilities, and so forth.

LABORATORY AND EXPERIMENTAL PLAKT DEVELOPMENTS

This stage of development was on virgin ground in many ways. Very little really conclusive information was available about the metal itself, especially as to its behavior under the proposed conditions. Reaction rates and temperatures under varying conditions of vacua, heat capacities of the reacting materials and the refractories, under the high temperatures proposed, the effects of magnesium vapor on the furnace materials, means of producing high vacua rapidly, degassing of refractories, mcthods of sealing the apparatus, and an infinite

In preparing the briquettes used in the production of magnesium the procedure is, in gencral, as follows. Dolomite, calcium- magnesium carbonate CaMg(COJ2, is first calcined, leaving a residue of lump lime composed of equal molecular parts of CaO and MgO. The lime is then mixed - with 75 per cent ferrosilicon in such pro- portions that the amount of silicon is chemically equivalent to the amount of magnesium, thus providing the following reaction:

zMgO.Ca0 + Si + (Fe) = a M g (vapor) + CazSi02 + (Fe)

However, it is possible to produce two dis- tinctly different types of briquettes on this same basis; the so-called wet or dry briquettes.

The wet briquettes are made by adding water to the lime before briquetting. 'This produces an extremely hard and tough briquette, which is also very porous after the water is driven off by preheating. C'sing pure dolomite and a perfect mixture, a production of 9.5 lb. of magnesium, at 75 per cent silicon efficiency, is possible per cubic foot of briquettes. Dry briquettes are produced from dry lime, giving a com- paratively fragile but compact briquette, from which a yield of 12.3 lb. of magnesium, a t 75 per cent silicon efficiency, is possihle per cubic foot. The advantages of wet briquettes lie in their ability to resist handling without excessive breakage, in greater reaction rates, in higher ultimate efficicncies, and in reduced briquetting pressures. On the other hand, dry briquettes give a greater yield per cubic foot of retort space, and in the Pidgeon retort process do not require preheating for drying.

96 THE REFRACTORY OR "F~KELESS COOKER" SIETHOD O F PRODUCING MAGNESIUM

REACTION RATES tom) portion could be held a t any tempcr-

Prior to the laboratory for the ature between 1 5 0 0 ~ and ZIOOOF., while fireless cooker method, nothing was known the condensation (upper) part could be held about reaction rates above 2100°F. These a t about 13joOF. Magnesium was distilled

Time, minutes FIG. 2.-CONDEFSATION TEST DATA.

hIagnesium charged, 21 Ib.; magnesium recovered, 20.9 Ib

tests showed the rate to increase rapidly above this temperature, becoming a virtually instantaneous reaction a t 2360°F., the rate being proportional to the number of silicon-vapor molrculrs per unit volume. The influence of silicon and of CaO in the reaction was also fount1 to be important in affecting the reaction rate and availability of the magnesium. The production of mag- nesium from pure MgO and pure ferro- silicon in which the yields mere poor confirmed the effect of CaO in the dolomite.

Much higher production rates thus were to be expected in the refractory method than are obtained by the retort process, because of the much higher temperatures used. This immediately imposed the problem of properly designing a condenser to handle the production rate. T o arrive a t a condenser design, rates and temperatures of magnesium vaporization and conden- sation were determined in a two-zone retort or furnace (Fig. I). The distillation (bot-

and condensed in this retort a t various rates in vacuum under carefully controlled conditions.

Magnesium distillation began a t a tem- perature of IIOOOF. Condensation was in- stantly indicated by an abrupt rise in temperature of the condensing unit and could be followed visibly through a sight glass in the side of the retort. Normal temperature recurred when condensation had been completerl. The amount of magne- sium was then weighed and compared wit11 that originally introduced. The rate of condensation, the amount of heat ab- stracted per pound, and the rate of heat transfer per unit area through the wall of the condenser were thus all determined before the full-scale plant was designed and built. The curves on Fig. 2 give the results of these tests.

Fig. 3 shows the experimental condenser (of 4-in. diameter) with a solid bsll of magnesium condensed upon it. The sodium impurities are condensed on the upper ant1 colder regions. At the right a ball that was

L ' i L 1 11.. .I. -1: u.1,- C S J \ U ~ X : I i ) 115 lrrr,n ( : { ~ I > I 1 I W \ I; ~ T L S .

From left to riplht: I . 0.01-lh. hall produced i11 J. rz-lb. hall prnrluc~rl in b. t 7-11>. ball produrcd in

I! $-in, dia. rctort 10-in. rlia. rt.tr>rt rfmc- TO-in. dia. retort (cut 2. a.,j-111, l~all produced in turcd thrnugh ;ixi:il @ane t l ~ r n r ~ a h center and pol-

4-in. din. rctnrt s l ~ n ~ v i n ~ thc mnjinrslum isI18.d 3. 8-lb. hall p r n d u ~ d in TO-in, structusc) 7. 20-lb. ball produced in

dia. retort j. Ssmc a5 4, unfracturcd to-in. din. retort

98 THE REFRACTORY OR "FIRELESS COOKER" METHOD OF PRODUCING MAGNESIUM

condensed a t too low a temperature is shown. It exhibits a coarse crystal-like structure. Below the large ball is one of intermediate size condensed on smaller equipment. Fig. 4 shows a variety of balls from 0.1 to 2 0 Ib., condensed under various conditions by high condensation rates. Only pure magnesium is condensed if the temperature is above 5o0°F. but below the

condensation temperature of approximately IIOOOF.

Because of its high heat content and great conductivity, Carborundum appeared to be the ideal refractory for use in the "fireless cooker." After a number of tests i t was established that this refractory could

V A C U ~ ~ M PYUP COYWLCIIOII

ALLOV RLTOPI

CLLCTP~C U L L I I H ~ LLLMIHT

O?tl*~l*'. C"LI*O tar Plcursrr buu

tscnrRet burro

E. G . DE CORIOLIS 99

supply heat to the charge a t a sufficiently high rate, and that no deleterious reactions would occur between the refractory and the briquettes or the magnesium vapor a t high temperatures. I t was also established that the magnesium produced would be of a purity comparable with that of the mag- nesium obtained in the Pidgeon retort process.

During this period many other pertinent features of design, such as the vacuum system, the pump, and the closures, were worked out.

Work on the pilot plant, an addition to the pilot-plant building of the blagnesium Reduction Company's plant a t Luckey,

REDUCRON UNIT.

100 THE REFRACTORY OR "FIRELESS COOKER" METHOD OF PRODUCING MAGXESIUM

Ohio (division of National Lead Co.). was temperature is approximately 8oo°F., ob- started toward the end of 1942. By January tained in hairpin U-shaped tubes sus- 1943 the work was completed. The unit pended in an auxiliary furnace. contemplated the production of roo Ib. of Electric heating elements maintain a magnesium per charge, with an over-all temperature of from 1 2 0 0 ~ to r500°F. in cycle of approximately 2 hr., allowing 30 the upper chamber. During the firing min. for production and condensation of period the bottom discharge closure is the magnesium vapor. The expected rate cleaned of all previous contamination and of production was so much higher than also made vacuum-tight. After a temper- contemplated in the laboratory that ature of 2joo°F. has been reached in the segregation of the function of magnesium reaction chamber, firing is stopped, the condensation from that of reaction or briquettes are quickly charged, the burner production was considered desirable. As and flue closures made vacuum-tight and later events showed, coiltrol of the forrner the vacuum pump started. After the pres- was to be a real problem. By March of sure in the reaction chamber has been re- 1943 the plant was completed and ready ducrd to I in. Hg or less, the briquettes in for operation. the charge-holding chamber are dumped

by an cxtcrnally, hydraulically operated valve. This part of the unit was completely

Fig, j shows schematically the o p e r revised subsequently, as described later. ations involved in the process. The vacuum- During the follorving magnesium re- tight shell is fitted with four vacuum-tight duction period the magnesium vapor hydraulically operated closures. The charge passes into the upper chamber and con- and discharge closures are a t the top and denses on the condenser. When the tem- bottom of the unit. Burner and flue closurcs perature in the reaction chamber has fallen are a t the sides. The upper section of the unit contains an electrically heated charge- holding chamber in which thc preheat temperature of the charge of briquettes is maintained prior to dumping into the high- temperature reaction chambcr belorv. The briquettes are externally preheated to approximately I 250°F. belore being placed

to rzoo°F. the reaction is completed, air is allorved to re-enter the chamber, the top closure holding the condensed magnesium ball is raised above the chamber, the flue and burner closures are opened and firing is resumed. During the firing period the bottom closure is lowered and the spent briquettes allorved to fall into the spent

in the upper or holding chamber. The re- briquette buggy. The cycle is then repeated. action chamber is composed of a cross- shaped Carborundum muffle. Surrounding Addilionnl Fentz~res of Design aud Operation this is an annular chamber of Carhorundum used for heat storage. The nlag~lesiurn con- Because of frequent lack of factual denser is suspended axially from thc top data, i t was necessary to project laboratory closure into the charge-holding chamber. results to the full-scale unit. This entailed

• the imposition of safety factors, which in O#eration some cases appreciably exceeded actual

The mode of operation is as follorvs: The requirements. unit is first hcatcd to a temperature of Approximately ro,ooo Ib. of Carborun- z700°F. by burning natural gas with pre- durn refractory is used to provide the heat heated air in the combustion space between storage capacity between the temperatures the Carborundum muffle and the heat- of zjooO and zzoo°F. The bottom closure storage Carborundum. The preheated air provides a double seal made by a tongue

and groove joint thrust home l ~ y the temperature of rgooDF. is low, so that the hydraulic cylinder. Water coolinr: i s pro- life of this retort would !x indefinitely long. vide<! to protrct all rlrhlltar ~ : ~ s k r t s , n tem- perature of 150' bcinr: used to prcvrnt thr CofzJe~rser

condensation of water vnljor frr>rt~ thc Iluv Thc large condensers shown in Fig. 6 are gases. The shell itself was dcs i~ncd to opcr- the result of preliminary exper imen~ . One

FI(>. 6.-1<1 I ! m I, b \ . I I \ < , \ : \

ate Iwtwccn tempcraturcs of 200" znci 3m°F. (Exr~pt for the contlenser anrt lop clmurc shown, thc rlran-infi is accurate.)

To provide heat-exchange capacity, thc Carlmrundum is arranpfrrl in an annular ring, so that i t is l lt ls~il~lc to tran~fcr Ihc heat rapitlly r;trli:~tlnn. Since thcrr is no grcnt t hickncss of thc Carl)orurulurrl, thc material is quickly hrntcrl try thr cntn- bustion gases, which nrr rnaintninrcl at n fiamc tcmpcraturc of 4;oo01:.

In the chargc-hol(1i11g unilt a t ~ l i u top, a heat-misting alloy rrlorE of light jinrlge

is utilized to prr\.etIt tliffu.;ion u i the mag-

nesium vapor into the part of the unit that contains insulating firebrick. The holcling

1 - 1 1 1: 1 ,\.111\-1 - ' \ I"l1l:!~I~11s1).

is 14 in. and thc olhcr 16 in. in dianlctcr, similar in tle.;igr~ to the lahoratnry unit.

Rriqrrcllc Pre~rcalittg

Prcheatitjg oE the briqurttcs prior to charging into the heaf-holding chamhcr was accompliqhed in two small convection furnaces. The briquettes wcrc placed in a cylin(1rical basket with a screen bottom I~c forc thcy \ e r e put into the furnace. Re- circulalrtl flue gases blown at high velocity through ~ h c furnace r e r e found to k capalhu of hctiting th- t~riquettes to a uni- form t~ml~rra turc regardl-s of the diam- ctcr of the basket. The temperature was cnwfullp cuntrctl'ted to prevcnt heatina above thc P C ~ ttmpcrature of the furnace.

€ 0 2 THE REFRACTORY OR "F~RELESS COOKER" MUETIIOD OF PRODVCIXG MAGNESIUBI

Thc laboratory k s t s had shown that the 1048, thc plant was fired, and aftcr an briquettes couIc1 be hcatrd to the proper initial suaking at 2goouF., to tlr!. thc brick- temperature without oxidizing the ferrci- work, and so forth, it wa s heatetl slowly to silicon. and that either hydrated or com- 2300°F. Tests showed satiqfactory vacua

pletely anhydrous dolomitc coulrE Ile u<i=rl. Hanrllin~ ot the briquettcs (rvliich wcrc furnished from the Magnesium Rctiuction Company's plant near by) n=a< fncilitatrd by placing an automatic dump c1rutc in the bottom 01 rach preheat in^ haskct.

Vacuum tcsts coiitluctcrl prior to initial f~ricg shorrcd that rcry satisfactory rrsults upere poszible. Thc. unit mas retlucerl from atmmpheric pn.sGurc to 0.01 in. in S min. The ultimatc prcsrurc of 0 . 0 0 2 5 in. was obtained in 2 2 min. These tcsts wcrc con- ducted prior to the bricking of thc shcll.

After completion of the tests, in March

ant1 control opcrtztiorl. Radiation lmses also were accorrIing to c:~lculations.

J l n ~ ~ r c s i u m Prod t ~ e d

Early i t1 .4lltil r g q the first rnagnniuin 11y this process was produccd-somc 40 to 50 11,. in from 30 to 4; min. ITowevcr, thc conrltbnscr tcmperafurc had t~rcn hcld too Iniv aalld the metal was of thc untlesirahle rrg~tal structure. During the owration of Ixcaking thc vacuum, thc mctal fell into the furnacc, bccarnc i ~ n i t c d and n= lost.

:is might be cry~ccturl, "playing safe" rcsult~d in ove~hoot ing the condenscr tcm- perature thc wcnn(l time the furnace was charged, a few (lays later, and some m a p

nesium condensed in the upper part of the of the lower portion of the furnacc at chambrr. I-lo~~ever, its iorm and purity lower tempera turs obviously rerluccd were satiqfactory. production of msgncsiurn. It also made

FIG. . % : I ~d 9,. ' \ I i 1' 7 8 I '.I' \ I L , ,\Lily. , 7!:LlL2 IJL 1 ( , F JOE4TS.

Sln~1i) tp SroMrrn control of condenser temperature much

nurinr: thvw prrlimInar!* runs, a prob- Icm a r t w that was r\,holly unanticipated and was latrr to pmve serious. In the first run, nnt* of tht- rcmcnts used in the con- struction slaggrtl and drippet! out through thc Imttr)m hung, gluing the Carborundurn facr platr of thc t ~ u n g in i t s clmcd position. IT was also rlfscovcrcd that when anr rnollcn slag rxistcd in the furnace, i t was prnctically impossible to obtain a high vacuum.

Prnduction of magnesium during four sulisequent runs brought out a nurnkr of pnintrj. Thc briquettes could not be com- plc tcly degassed at the charge-hold in^ temperature of I 250' to r4w°F. The rapid rate of heat transfer obtained from the storage refractory to the briqurttcs began the production of magnesium a t prrwurcs far in exccss of 0.2 in. HE. It was Iountl impwsible to eliminate the slaggirlg a t temperatures abox-e z400a1"., and operation

more difificult. I n an attempt to eliminntc thc slag,

thc cntire unit was raised to a tcmperaturc of ?Rm°F. and held at that tcmprraturp for an entire ivcch. Still thcre was no evidence that slagging had been e2imi naled T ~ t s were discon~intzurl thvrclorc until the cauFe of the slagging cnulrl be fountl.

.4dislag~irr s Tcsls rlrrd I I i~~dopmcrr~ Since lahoralnry t-tc hail shown that

purr silicon carhitlc (Carborundurn) docs not slag a t tcrnprratttrr< Pvrn al>n*e the range of operation of thc turrlacc, it was evident that eithrr t h r crmrnt or the rcfractory co~itaincrl s u l ~ tancfi that did slag untlrr the cnntlitic~ns of oprration. .Iko, sincc sonic s l a f i ~ i n ~ had ocrurrcd 'txfnre the trriqurtlw \vl.rrt. inrrotluccd. nci thcr thc dolnmitc nor thc m a g n ~ i u m vapor cr~ulrl Ilc ~vhr>lly r~pnnsi1)lc.

1:xnminntinn crf tmt data from the six pruvious charges intlicatcr! that it rvas

not likely that the production of mag- nesium nor the vacuum was responsible for the slagging. Analyses revealed the various slags to be complex, containing silica, alumina in large proportions, iron oxide, calcium oxide and magnesium oxide. Examination of the spallac brick under- neath the muffle revealed that the bulk of the slagging was coming from the combustion chamber. At no time during the operations had i t been possible to detect any slag running over the inside surfaces of the muffle.

When the Carborundum muffle was removed and access to the heat-storage Carborundum was provided, i t was quite apparent that the silicon carbide cement used to bond the heat-storage brick had melted out of most joints and slagged over most of the inside of the furnace (Figs. 7 and 8). Analysis of the cement used showed :

PER CENT Silicon dioxide. . . . . . . . . . . I I .8 Silicon carbide. . . . . . . . . . . 80.33 Iron oxide. . . . . . . . . . . . . 2 . 8 0 Aluminum oxide.. . . . . . . . 3.27 Calcium oxide.. . . . . . . . . . 0.51 Magnesium oxide. . . . . . . . o. 5 j Other impurities.. . . . . . . . 0.80

The slag that first glued the bottom of the bung was composed of:

PER CENT Silicon dioxide.. . . . . . . . . . jo. o j Aluminum oxide. . . . . . . . . 23.2 2

Calcium oxide. . . . . . . . . . I 7.60 Magnesium oxide.. . . . . . 9.01 Other impurities. . . . . . . . . o. I 2

Subsequent green glassy slags were found to contain:

PER CENT Silica.. . . . . . . . . . . . . . . . . . 61.56 Iron oxide. . . . . . . . . . . . . 3.5 7 Aluminum oxide.. . . . . . . . 24.80 Calcium oxide. . . . . . . . . . . 7.48

. . . . . . Magnesium oxide.. I . 25

Apparently the cement did not slag as a single compound but did so in a selective manner. I t appeared that the lower- melting-point substances melted out of the mixture first, followed by the others

a t successively higher melting tempcra- tures. The slagging material also seemed to become more refractory as the unit was operated. Subsequent tests indicated that the cement melted a t approximately 2570°F. In all cases aluminum oxide was present in various amounts in the slags and in the peeled Carborundum. Since neither the dolomite nor the ferrosilicon contained appreciable amounts of this compound, i t must have been introduced by the cement.

The investigations, which were con- tinued for several months, showed con- clusively that the " impurities," alumina and iron oxide, in the cement as a clay, were responsible for the low fusion point of the cement. I t was also shown that fusion would have occurred even if the furnace had never been exposed to vacuum a t the high temperatures of operation or to the attack of magnesium vapor. Cements free from these materials, also a number of zirconium silicate cements and refractories, were next tested.

Unfortunately, the zircon cements, al- though quite satisfactory otherwise, show- ed evidence of attack by magnesium vapor. Hence their use was not possible. H o w ever, purified. silicon carbide cements were developed, which contained none of the clay ordinarily used as a binder- i.e., less than 3 per cent impurities- whereas the former cement had con- tained up to about 7 per cent. The interior of the unit was also rebuilt with entirely new Carborundum brick. Eventually a purified cement containing less than 2 . j

per cent total aluminum and iron oxides was obtained and proved stable a t tem- peratures as high as 2850°F.

Tests on the effect of the briquettes on the silicon carbide cements, glasses and brick brought out a new point.. Although dolomite powder showed a tendency to fuse, i t did not attack the unglazed brick, but i t did attack the glazed brick. Also, i t was noted that while the briquettes

E. G. DE CORIOLIS 1°5

did not attack the unglazed brick a t first, they did so when fusion of the fer- rosilicon occurred. Attack on the purified cement was noted as well. Previous experience had demonstrated that the briquettes could not be fused in vacuum a t 2700°F., but that if the ferrosilicon were oxidized the briquettes would fuse at that temperature. I t was, therefore, to be expected that if the ferrosilicon in the briquettes is kept from oxidizing in the unit, by discharging directly after the vacuum is broken, no rapid attack will occur.

Revisions to Condenser

Concurrent with the solution of the cement-slagging problem, work was con- ducted toward revising the condensing system. The new condenser was 8 in. in diameter instead of 14 and 16 as pre- viously. Precautions also were taken to make sure that the vapor would be directed against the condensing surface. The top of the charge-holding chamber was necked down so that there was but 3-in. clearance between the condenser and wall of the chamber. The electric heating elements were relocated, to afford better heat control. The bottom bung was reconstructed, to avoid possible sticking.

Measurement of Magnesium Condensalion Rate

The previous method of observing the rate of magnesium condensation by means of a sight glass was far from adequate. In order to follow the condensation process more accurately, a heat exchanger was provided to measure the heat increment in the cooling water that was recirculated through the condenser, the heat in turn coming from the condensation of the magnesium vapor on the condenser sur- face. The system provided a reasonably accurate method of following reaction

progress and afforded a tool for comparing the production rate of the unit under a variety of operating conditions.

Revised Charging Method

I t had been found that the time required to evacuate the unit with the briquette? in the upper chamber was unduly long, requiring about 30 min. Furthermore, after reaching a comparatively low pres- sure before the charge was dumped, the rate of evolution of the gases from the charge into the reaction zone was so great that the pressure quickly rose to one inch after the charge was dumped. This indicated that the degassing accom- plished in the upper chamber was not sufficient and actually of little value in shortening the time to attain high vacuum.

Accordingly, the method of holding the charge in the upper "holding cham- ber," as originally developed, was elimi- nated and the charge dumped directly into the reaction zone a t atmospheric pres- sure. The revised method made i t possible to dump the charge quickly and to place the unit under vacuum within less than one minute after dumping.

"Production" Runs

Revisions were completed and the unit was refired in October 1943. By the middle of the following month some seven charges had been made in the unit. In each the structure of the magnesium was good. Yields up to 60 per cent on the ferrosilicon basis were obtained. The magnesium production rate was carefully determined as a function of time. The condenser proved to be of correct size and under certain conditions operated satisfactorily.

Up to this time and during these runs wet briquettes were used. The charges were dropped into the reaction chamber under atmospheric pressure, yet in all cases evacuation rates were more rapid than those previously obtained. The

106 THE REFRACTORY OR "EIKELESS COOKER" METHOD OF PKODUCING MAGNESIUM

ultimate vacuum obtained was consider- until the vacuum is broken. The production ably improved. No dificulty was experi- efficiency of 41.5 per cent on the silicon enced with slagging of either the cement basis was obtained after about 150 min. or Carborundum brick. under vacuum. Although times up to 4 hr.

Wet versus Dry Briquettes under vacuum gave efficiencies as high

Tests with Wet Briquettes.-The normal charges as previously used were composed of 650 lb. of wet briquettes mixed in the n~olecular ratio of 1.1 MgO in the calcined dolomite to I Si in the 7 5 per cent ferro- silicon. Approximately I j o lb. of water vapor and other volatile matter was con- tained in each charge. The available magnesium per charge \\,as approximately 9 2 lb. After preheating in the convection furnace a t 12j0°F., the briquettes were dumped directly into the reaction zone according to the revised method, the temperature of the furnace being 2650°F. In all, 16 charges were made. All were the same except that conditions of the condenser and upper chamber were varied in attempting to control the form of magnesium condensation and sodium sepa- ration. Fig. 9 shows a chart of the rate of n~agnesiun~ condensation and silicon ef- ficiency as a function of time, using wet briquettes. Briefly, the curve indicates

as 60 per cent, a study of the heat input to the charge showed that much more than 40 per cent efficiency should have been obtained in the shorter period. The reason for the lower efficiency, therefore, was difficult to understand.

During these runs i t was only when poor magnesium efficiencies were obtained. leaving unreactcd ferrosilicon in the charge, that difficulty in removing the spent briquettes was experienced. Under such conditions the silicon would burn when the vacuum was broken, generating excessively high temperatures, which tcnd- ed to fuse the briquettes together. Effi- ciencies higher than 50 per cent gave no trouble.

Diffusion of magnesium through the bottom alloy seal when the metal con- densed on the bottom closure was also a minor problem. It was eliminated by proper adjustment of the spring holding the seal plates together. Natural gas was introduced in one a t t e m ~ t to eliminate

that magnesium condensation starts about this trouble, but was of doubtful success, 10 min. after charging, reaches a maximum although i t did result in a slightly higher after 40 min. and then decreases slowly magnesium production per charge.

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108 THE REFRACTORY OR ' L F ~ ~ ~ ~ ~ ~ ~ COOKER" METHOD OF PRODUCING MAGNESIUM

in. holes and made in three sections (Fig. 11) was used in the next test. By lowering this core into the furnace, then immediately dumping the charge around it, the tube was not exposed to oxidation until the vacuum was broken. I n the tests on coring the charge consisted of 5 2 5 Ib. of wet briquettes similar to those used previously, the charge now containing approximately 7 4 lb. of magnesium.

The results proved the soundness of the theory. Magnesium production was well started a t the end of 10 min. The maximum rate of condensation of 111,ooo B.t.u. per hour was attained a t the end of 30 min., as compared with the previous maximum without the core of 87,000 B.t.u. per hour a t the end of 40 min. Furthermore, the silicon efficiency was increased from the previous value of 41.5 per cent without the core to 5 5 per cent with the core a t the end of 150 min. (Fig. 12).

Since the question arose as to whether or not some increase in over-all efficiency might not have occurred because there was no necessity for transferring heat to the briquettes formerly occupying the space now taken up by the core, a sub- sequent test was made to check this possibility. The core pipe was replaced by one having twice as many holes but exactly similar otherwise. The results showed that magnesium production began a t the same time as with the former pipe core, but the maximum production rate ob- tained was 139,ooo B.t.u. per hour. Total production was also tremendously increased. At the end of 150 min., the efficiency was 7 0 per cent, an increase of 63 per cent over operations when no core was used and 27 per cent above that with the former steel core. Not only was i t obvious that the briquettes formerly contained in the core could have no effect, but the cause of previous low

F ~ G . I I .--DRILLED 3-INCH-DIAMETER PIPE was eliminated. USED AS CORE FOR MAGNESIUM VENTING. Subsequently, larger cores were tried.

E. G. DE CORIOLIS 1°9

Dry briquettes were also used, with an Repeatedly different amounts of heat per increase in efficiency, but still not equal unit time had to be abstracted from the to that obtained with the wet briquettes. magnesium vapor by the condenser, and Figs. 13 and 14, corrected for differences under such conditions control was difficult.

Time. minutes FIG. 12.-RATE OF MAGNESIUM CONDENSATION 4ND SILICON EFFICIENCY AS A FUNCTIOK O F TIME.

Charge of wet briquettes using 3-inch-diameter pipe core with seventy M-inch holes.

in charge size, summarize all results obtained in the coring tests. No further rise in efficiency was noted when using larger coring.

Condenser Tests

Although primary attention throughout the tests was paid to increasing efficiency and total capacity of the furnace, the condenser problem, while secondary, re- mained extremely troublesome. Nor was it completely solved a t the conclusion of the tests.

While the quality of magnesium pro- duced and the location of most of it on the condenser were quite satisfactory in numerous tests, there was frequently a deposition of the metal admixed with sodium on the colder part of the condenser. At times this was caused by the deposition of calcium salts on the inside of the con- denser from the boiling of hard water.

Two major factors contributed to the condenser difficulties. In the period of de- velopment just ended remarkable changes

The other difficulty arose from the fact that production began so soon after the dumping of the charge. Because of this, magnesium condensation began a t a pressure of 0.4 in. Hg and a considerable amountwas condensed before the pressure had dropped to 0.08 in., 15 min. after the start of production. Under such con- ditions the velocity of gases past the condenser was so great that the magnesium vapor was carried past the part of the condenser on which i t would have con- densed under diffusion-flow conditions. This gave rise to the admixture of sodium with magnesium on the upper part of the condenser, thus causing both a loss in production and a fire hazard.

Various bailles were used to force the magnesium to condense on the proper part of the condenser, but although this worked very well a t first another condition soon developed that prevented the successful application of the principle. Magnesia and other foreign matter had gradually accumulated in the channels connecting

in production rates were induced. the heat-storage chamber with the evacua-

I i I ,o 4 G b ab ,oo 0 I$ I.1 11

Time.minutes Frc. 13.-RATE OF MAGNESIUM PRODUCTION AS A FUNCTION OF TIME.

I . Charge, wet briquettes using 3 - in . dia. pipe core with x.$o J6-in. holes. 2. Charge, wet briquettes using 3 - in . dia. pipe core with jo %-in. holes. 3 . Charge, dry briquettes using 3 - i n . dia. pipe core with 140 >i-i~i . holes. 4 . Charge, wet briquettes using n o core. j. Charge, dry briquettes using no core.

tion system, finally entirely closing them. From this time on all evacuation had to be accomplished through the condenser chamber. This slowed the rate of evacua- tion, but, more important, i t caused a high-pressure condition a t the condenser when baffles were used. This condition gave rise to a very undesirable, crystalline structure of the magnesium.

During this period the condensed mag- nesium was of such a form that part of i t very often fell from the condenser into the reaction zone. When this occurred the magnesium revaporized a t extremely high rates, thus making control of the condenser almost impossible. Furthermore, the vaporization was so rapid that local pressures as high as one half an atmosphere were generated. This is a sufficiently high pressure to produce molten magnesium and to cause it to come into contact with the Carborundum refractory. At times i t was necessary to break vacuum while the magnesium was still in the reaction retort. At these times local temperaturcs as high as 3ooo°F. were generated in the retort by the combusLion of the magnesium. I t was these occurrences that eventually weakened a small section of the reaction retort just a t the top of the charges. Finally, when the thirty-sixth charge was made, the briquettes broke through the weak section and made further opera- tion impossible.

Although compelled to discontiilue the tests, a total of 34 completed runs assures that a number of facts were definitely established. Correlation of results of heat- test records showcd that the heat available for the reaction was ample, about 77 per cent of the total used going into magnesium production.

I n the design of the furnace i t had been thought that the problem of heat transfer from the refractory to the briquettes would be a difficult one. Operation of the

furnace proved, however, that a factor of major importance might materially alter this assumption. When under vacuum and producing magnesium. the magnesium vapor particles apparently transfer heat a t an enormous rate compared with ordinary radiation and conduction. I t seems probable that the problem of heat transfer to the briqucttes is not such a major problem as was first thought. If vertical or large-diameter horizontal retorts are employed, the problem may be only secondary to that of properly venting the charge. Venting is highly important. This incidentally leads to the question of whether or not the long, small-diameter retort used in the Pidgeon method is really concerned with heat transfer, or with the venting of the mag- nesium produced. When using a horizontal charge as in this process, there is always a space above the briquettes, which can serve as a channel for the vapor to escape. Hence, the distance the vapor must dif- fuse is less than the diameter of the retort.

Very few difficulties were experienced with vacuum. Only if excessive dust accumulates during pouring, as with the dry briquettes, can the accumulated dust slow down the rate of evacuation. In general, all vacuum seals, closures, and other parts were satisfactory.

The work has demonstrated that pure, solid structures of good magnesium can be deposited a t pressures as high as 0.06 in. Hg if the t empe ra tu r~ are held ap- proximately right. Sufficient data have been accumulated to make i t possible to design a new condenser that urould pro- vide the necessary characteristics.

Certain conditions cause some slagging and corrosion on the inside of the retort. If the briquettes are allowed to remain in coiltact with silicon carbide a t tem- pcraturcs in excess of 2600°F. while in contact with air, there is a tendency to melt the briquettes and make them flow over the refractory. The iron and silicon

IKER" METHOD OF PRODUCING MAGNESIUM

in the briquettes oxidize and form a com- pound that fuses in air a t about z600°F. Little difficulty of this kind was noticed with the wet briquettes, howevcr. This caused no trouble with the type of bottom seal later used. The only severe corrosion occurred when magnesium dropped from the condenser and melted, as noted previously. The spent briquettes run freely out of the discharge bung when the production efficiency is 60 per cent or higher.

The work carried out to date indicates that the time required to heat the furnace from the lower temperature to the higher, zzooO to z700°F., is approximately 34 hr. Taking the reaction time for wet bri- quettcs as 236 hr., the total cycle would be 33d hr. With slightly more effective venting, i t is reasonable to expect an increase in efficiency of 5 per cent, from 70 to 75 per cent, so that with a total production of IOO lb. of magnesium per charge, the hourly production would be 31 lb. While this is somewhat less than originally contemplated, i t is nevertheless quite satisfactory. This time is figured on the elimination of the preliminary degassing operation within the holding chamber, although degassing within the reaction chamber does have an effect upon the deposition of sodium on the condenser along with the magnesium.

Although planned as a part of the original program, no tests were run using excessive amounts of ferrosilicon or with fluorspar as a catalyst. Since a strong possibility existed that either of these methods of increasing produc- tion might cause excessive slagging and ruin the muffle, their use was postponed in favor of other Iines of inquiry. Their use offers a possibility for future investigation.

Although i t is not possible to design the details of a large commercial furnace,

or to estimate accurately its production capacity and cost of operation without further information concerning the factors in question, i t is possible to give the fundamental features of design that must be incorporated into such a furnace and make a rough estimate of the economics of the process.

Assuming that the difficulties that still remain unsolved can be corrected in the future, i t should be practical to construct a furnace capable of producing between 500 and 1000 lb. of magnesium per charge.

I t is conservative to expect that, when using wet briquettes of equimolecular mix, a silicon efficiency of 75 per cent can easily be attained. This would require 8300 lb. of wet charge containing 23 pcr cent water to produce 1000 lb. of mag- nesium. From previous operations, i t is conservative to estimate that an efficiency of 75 per cent could be obtained in 4 hr. under vacuum, and that the furnace could be reheated to reaction temperatures in 2 hr., utilizing a t least IOOOOF. pre- heated air. The production capacity of the furnace, therefore, would be 1000 lb. of magnesium in 6 hr., or 4000 Ib. per 24-hr. day. This furnace, producing 1000

lb. per charge, would therefore be roughly equivalent in capacity to 40 alloy retorts of the type now used in the Pidgeon process.

Obviously, i t is not possible to place too much confidence in estimated costs. However, i t is felt that the estimates that have been made are conservative, and that they indicate that the refractory- chamber method shows the potentiality of surviving competition with other pro- cesses for production of magnesium.

The process was developed by Dr. J. J. Turin, now Ensign, U.S.N.R., with the collaboration of Jack Huebler. Design of the unit installed a t Luckey, Ohio, was by A. W. Peters.