US2426148A - Thermal reduction of metals - Google Patents

Description

Patented Aug. 19, 1947 UNITED STATES PATENT OFFICE THERMAL REDUCTION OF METALS Application October 19, 1943, Serial No. 506,828

6 Claims. (01. 266-15) This invention relates to the thermal reduction of metals and particularly to improvements in preparing the charge used in and apparatus used to produce metal by reduction and distilla tion in retorts.

The Government has built several plants to produce magnesium in such retorts. Such retorts are from 8 to 10 inches in diameter and 7 to 22 feet long. The shorter retorts are closed at one end and condenser at the other. The longer retorts have condensers at each end. The retorts are usually of high nickel chromium steel and are heated externally to temperatures between 2170 F. and 2200 F. Such retorts may be divided into three zones, the reaction zone containing the charge, the condenser zone or zones, and the zone intermediate the reaction zone and the condenser zone. The retort is operated under very high vacuum. Calcined dolomite and 75% ferrosilicon ground to fine mesh are briquetted under pressures as high as 20 tons per square inch for the charge. At the above mentioned temperatures no appreciable amount of the material is reacted without briquetting. A refractory target of diameter the same as the inside of the retort is placed between the intermediate and condenser zones and fills the entire retort except for a hole through the target axially with the retort which acts as an orifice for the passage of the vapors to the condenser zone. The area of the orifice is about of that of the retort. Water is sprayed on the retort outside the condenser zone and usually steel sheets are placed inside this zone to receive the magnesium deposit. The silicon takes the oxygen from the magnesia yielding magnesium in the vapor state and silica. It is believed the silica combines with both the lime and magnesia remaining andif conditions are such that the reaction can proceed, the magnesium silicate phase disappears and a dicalcium silicate residue is formed which releases energy to the first reaction. After extensive research we have found that the present art has many factors which depress the reaction and contribute to very slow speed and low yield. In fact the average speed is less than 1% of the maximum theoretical speed and even less than the maximum theoretical speed at 1700 F. The yield averages about 60% of the possible maximum. Because of these facts both investment cost and production cost per pound produced is excessively high.

We have found novel and unexpected beneficial results by using crushed or preferably'sintered and crushed briquettes as disclosed in our copending application Serial No. 504,082, filed Sept. 2'7, 1943.

The use of material prepared in accordance with said application permits thereaction to proceed at a rapid rate at lower temperature. This provides a greater temperature difference between the walls of the retorts and point of reaction, the material has higher thermal conductivity, and the energy can then be supplied more rapidly to the reaction. The use of crushed briquettes gives novel and unexpected beneficial results, but many other factors depress the reaction and contribute to slow speed and low yield in such retorts. The larger the area of the retort walls which can be utilized to absorb heat and deliver it to the point of reaction, the greater is the capacity.

Calcined dolomite is a poor thermal conductor at high temperature. The conventional briquettes in acharge have few contacts with each other. The material reacts at the contacts first changing about half of the metal to mineral reducing thermal conductivity. The path of heat travel is long and devious. We have found that the sintered and crushed briquettes have better thermal conductivity, the contacts are greatly increased, the path is greatly shortened, and the surface provided for reaction is multiplied 1000 times or more.

The heat loss by conduction through the walls of the intermediate zone of the retort caused by using the condenser zone as a condenser is a heavy thermal loss and prevents the use of the intermediate zone for additional charge and prevents the reaction going to completion at least in the end of the charge opposite the condenser and the residue from the reaction is a waste product. Also the radiation loss from the end of the charge to these Walls is a heavy thermal loss. The target placed in the retort reduces the area for fiow of the vapors and produces a serious back pressure to depress the reaction, yet it has been found so beneficial to repel radiant heat from the charge that its use is preferred, yet even with its use the radiation loss is still too great and the calcium silicates are not formed in this part of the charge and the residue is waste.

We have found that a steel or iron spider with sheets radial with the retorts embedded in the charge adds beneficial results in conducting the heat from the walls of the retort into the charge.

We have found that by welding a box on the condenser end of the retort and putting a solid target in the box, the target being sufficiently large to obstruct all radiation from the end of the briquettes, the box being of sufficient size around the target so as not to interfere with the flow of the vapors around the target and by placing the condenser behind the target the walls of the intermediate and condenser zone can be run hot, the amount of materials charged can be greatly increased, back pressure on the reaction is reduced, and production and yield are increased and the metal produced is of substantially higher purity. We have found that the silicon can thus be substantially all utilized in the reaction. We have found that the retort can be operated at 2100 F. and still have the speed of the reaction increased many fold over the conventional method and the life of the retorts greatly extended.

We have further found that thus the temperature of the end of the charge is increased; that calcium silicate is formed; that by using an iron aluminum silicide as the reducing agent compositioned for the dolomite to produce a residue of the composition of Portland cement, that the residue thus made reacted to the calcium and aluminum silicates, and the residue is Portland cement of good grade needing only addition of a small amount of gypsum as a retarder as in cement manufacture; and such residue is then a very substantial credit to the cost of the metal produced.

It is therefore an object of this invention to utilize the materials to'be reacted in the retorts described made in accordance with our aforesaid application.

It is a further object of this invention to utilize reactants prepared in accordance with our aforesaid application using a reducing agent compositioned'to produce a residue which is Portland cement.

It is also an obiectof this invention to provide a metal spider as described to assist in conducting the heat from the walls of the retort into the charge.

It is a further object of this invention to attach a'box to the end of the retort to contain the target and condenser and provide unrestricted area for the passage of the vapors out of the retort onto the condenser.

It is also anobject of this invention to provide said box with a pig mold and means for melting the condenser metal and running it into said mold.

Other objects appear herein.

The lower the pressure of the vapors over the reaction, the faster is the reaction; the lower the temperature at the point of reaction, the greater is the temperature difference which causes the flow of heat, and these and other factors, such as the amount of material which can be simultaneously'reacted, influence speed of the reaction and yield.

By employing the method described 'in said application for preparing the charge, we have removed substantiallyall of the factors depressing the reaction caused by the briquette, and have made a substantial increase in thermal conductivity of the materials.

The full benefits of utilizing this material cannot be realized, however, if the vaporsare bottled into the retort by the target as described, the target actingas a bottleneck, nor can the benefits thereof be realized with the water cooling walls, of the condenser zone. .Ihere is more than 2000 F. temperature difference between the reaction zone and. the condenser zone in the conventional practice. The heat conducted between these zones through the walls of the retort in the conventional practice is many times the amount of heat required to react the entire charge, and this conduction of heat away from the reaction zone interferes'seriously with transmission of heat to the point of reaction. We have found that it is not necessary to construct the box described of anything more than mild steel, and it is practical to insulate this box to prevent loss of heat and to insulate any part of the retort which is not exposed to the means for heating the retort so that substantially all of the retort can be maintained at and above reaction temperature. Under the conventional practice only about of the retort is used for the charge. With the change proposed it is practical to use of the retort for the charge. We have found another benefit in so doing. The part of the retort farthest in the furnace runs at a somewhat higher temperature and the vapors from the reaction in this part of the retort in passing to the end give up part of their heat to the charge on their way to the condenser. We have found that by thus running the entire retort at high temperature, there is little temperature diiference and little heat loss from the end of the charge by radiation to the walls of the retort and the exothermic cement reactions are complete. In this Way we have eliminated friction to the flow of vapors by putting the target out beyond the retort and the target can, with our construction; be made very much thicker so as to be an effective insulator to the flow of heat from the end of the charge on to the condenser.

We'have mentioned putting a spider into the retort to aid in the conduction of heat from the walls into the charge. It has been found that the charge both before and after reaction is an excellent thermal insulator and has a K value at the high temperatures used'of only about onefive thousandths of that of steel. We have found that it is easy to charge and empty the retort and that the spider or a similar construction offers no difficulties. The crushed charge can be delivered into the retort and around the spider manually or by automatic mechanic means. The reaction goes substantially to completion and the residue is a fine powder which is equally easily removed by a suction fan, thus facilitating the quick recharging of the retorts without shock or danger of injury to the retort. Like thermal conductivity into the charge can be obtained by putting pieces of metal in the'charge, or construc'tingthe spider in a different manner or in a variety of ways.

Although the metal produced in the conventional practice is in' crystal form, the crystals themselves are of commercial purity, being contaminated by dusts, carbides and other impurities and requiring remelting and refining operation before being utilized in fabrication or casting. We have found that by placing a pig mold under the condenser in the box described and withdrawing the water from the condenser that little additional heat is required 'to melt the metal and cast it into pig, and this is done while the high vacuum is maintained. upon the retort. The pig is of high purity and requires 'no 'remelting or refining operation and by utilizing the' vacuum, not only are impurities emoved, but "a substantial gain in the amount of material produced is had because with the conventional method from 12 to 20% "of the material is lost by oxidation and mechanical losses due to the uses of slagsffiuxes and other chemicals, none of which are required when the vacuum remelting described is used.

Referring to the accompanying drawings, Fig. 1 is a vertical sectional view through the" furnace and retort, while Fig. 2 is a side elevational'view of one forming a'heat conducting member.

Fig. '3' is a fragmentary sectional View similar to Fig. 101? ani'odifi'ed formiofa radiant shield.

To desc'ribe'a specific formof apparatus heretofore'generallydiscussedin Fig. 1; the retort 10 extends into the furnace i2 through the opening l4. The end It of the retort is enlarged to receive the target I 8, the condenser" 28, and the pig mold 222. The target iliiis so positioned as to reflect the radiant heat from the charge 24, but is spaced from the walls of the retort to provide aeeams 28. Upon the completion of the reaction the water" is removed from the condenser 20, and electrical,

heat is applied through the electrodes 36 tomelt the metal from'the condenser 20 into thepig mold 22. lhe target [8 and the condenserzfl'are carried by the plate 32, which is removed to" charge the retort. p In order to increase the rate of reaction of the charge 24, conducting member 34 may be em-.

bedded in the charge 24 transversely of the retort ID to conduct the heat into the charge. One form" of conducting member is shown in elevation in Fig. 2.

As shown in Fig. 3, theshield consists of forward members 36 and a rearward larger member 38 which together define a tortuous path for the gases flowing to the condenser 46. It will benoted that the radiant heat is completely shielded from the lower temperature end, yet an adequate path is provided for the distilled gases reducing back pressure upon the reaction to a minimum.

By the use of this invention, the production of the conventional retort can be multiplied many times, the wasteful use of the reducing agent is eliminated, the life of the retort is greatly excharges of briquetted material at relatively low tended, the production per unit investment in plants is greatly multiplied, the residue is made a valuable by-product, and all costs are very substantially reduced.

We have referred only to the reaction between calcined dolomite and 75% ferro silicon. Our invention is equally applicable to any magnesium bearing material with silicon or any silicon alloys of any composition or any other reducing agent, such as aluminum and calcium and the alloys and carbides thereof or mixtures of such reducing agents, alloys or carbides. We have described attaching a box to contain the target and condenser and pig mold and means for remelting to the end of an existing retort. If new retorts are to be built it may be practicable to make the box integral with the retort so that it would not have to be attached thereto. Vfhether the box is attached to the retorts already built or is made integral therewith, in either case an opening would be provided for charging and unloading the furnace or retort which would be so constructed as to be vacuum tight when the retort is in operation and we have found that pure copper as a gasket to seal such opening is satisfactory at high temperatures.

It is entirely feasible and there are certain advantages connected therewith of employing the stored up heat in the shield and surrounding wall structure to melt the metal from the condenser into the pig mold in lieu of applying internal heat to the condenser. From this it will appear that we have disclosed both internal and external heat applying means for melting the metal from the condenser following withdrawal of the water.

The use of our specially prepared charge or the use of means to lead the heat into the charge at a great rate will not aid materially in the speeding up of the reaction if the passage between the charge and the condenser is not such as to let a greater amount of vapor to pass without increasing the amount of resistance to the fiow. Thus it is essential that these means he used in conjunction with the wide but tortuous passage we have described, Likewise, the large passage cannot be used to capacity, unless the rate of liberation of metal vapor is high. The

6 metal produced by these combined means is of high purity. Full advantage of this cannot be taken unless the metal is consolidated within the furnace so that the further melting and refining .iseliminated. Likewise the melting down into pigs inside the furnace is superfluous if it is to be subjected to further refining.

We therefor claim:

1. In a retort chamber defining a high temperature zone in which the charge is placed, a plurality of spaced transversely positoned heat conducting members located in said chamber and so positioned as to be embedded within the charge.

2. An improved retort for the reaction of temperatures and pressures to vaporize a metal therefrom comprising a high heat zone in which the charge is located, heat conductive means embedded in the charge to enable the reaction of the entire charge to be carried out at once, a cold zone, means shielding radiant heat from said cold zone, and a large relatively free tortuous path for the vaporized metal extending between said zones, and a condenser for the vapor in the cold zone.

3. An improved retort for the production of metal by vaporization from briquetted charges comprising a zone externally heated in which the charge is located, heat conducting means projecting from the heated retort wall into said charge, a zone not heated externally containing condenser for collecting the vapor rising from the charge, means for shielding the radiant heat from the condenser, a large free tortuous path for the vapor extending between said zones, a mold to receive the condensed metal from the condenser and means associated with said condenser for melting the condensed metal causing it to fiow into the, mold.

l. An improved retort having wall structure defining axially aligned high and low temperature chambers with said low temperature chamber being enlarged at the junction between said chambers, a radiant heat shield located in said enlargement and intercepting the radial heat projection into said low temperature chamber, said shield being spaced from the wall structure of said enlargement to provide a tortuous path for the unobstructed passage of vapors around said shield, and a condenser for said vapors in said low temperature chamber on the opposite side of said shield from said high temperature chamber.

5. An improved retort having wall structure defining axiall aligned high and low temperature chambers with said low temperature chamber being enlarged at the junction between said chambers, a radiant heat shield located in said enlargement and intercepting the radial heat projection into said low temperature chamber, said shield being spaced from the wall structure of said enlargement to provide a tortuous path for the unobstructed passage of vapors around said shield, a condenser for said vapors in said low temperature chamber on the opposite side of said shield from said high temperature chamber, and means located in said low temperature chamber for selectively heating and cooling said condenser.

6. An improved retort having wall structure defining axially aligned high and low tempera.- ture chambers with said low temperature chamber being enlarged at the junction between said chambers, a radiant heat shield located in said enlargement and intercepting the radial heat projection intov saidulowtemperature chamberr. said shield. being spaced from the-wall structure of. said enlargement to provide:- a =tortu0uspath for; the. unobstructed. passage ofvapors-around" said shield, acondensen forsaid-vapors=-imsaic1 10wv temperature chamber 'on the- 0pp0Site-sid-- of said shield from said high temperature-cham her, and means located in said lowvtemperature chamber for colleoting :materialcondensed upon said condenser,

SVEN :E.- FRANCIS" C." GARY,

REFERENCES CITED The following references areofrecord in the. 15 file of this patent:

Number- UNITED ESTATES PATENTS" Name Date Hansgjrg Feb. 2, 1943 Mathieu Aug. 31, 1943 S'elig'er .Aug. 16, 1938 Bakken et al. Aug. 3, 1926 Ha'nsgirg .Oct. 25,1932 Siichy et a1 Oct. 22, 1940 Pidgeon Sept. 21, 1943 Pi'dgeom, Sept. 21, 1943 Gloss- Dec. 21, 1943 Blackwell et a1.1. July 11,1939 MGiew July-18, 1933

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