US2930689A - Production of alkali metals - Google Patents

Description

March 29., 1960 s. G. MGGRIFF 2,930,689

PRODUCTION oF ALKALI METALS Filed sept. e, 1955 STUART G. MCGR IFF INVENTOR.

MAM]

attentes Pnonuc'rioN or ALKALr ivrn'rALs Application September 6, 1955, Serial No. 532,514

7 Claims. (Cl. 75-6) This invention relates to new and useful improvements in the production of alkali metals and more particularly to an improved process for preparing sodium or potassium by carbothermic reduction of the alkali metal carbonate.

` The rst recognized production of sodium and potassium metalswas by Sir Humphrey Davey. In November 1807, Davey announced the discovery of sodium and potassium metals which he had been able to produce by electrolysis of potassium or sodium salts. Shortly after Daveys announcement, Gay-Lussac and Thenard announced to the Institute of Science in Paris that they had decomposed potash and soda by treating them with iron at a high temperature. A few weeks later, in the spring of i808, Curaudau told the Institute of Science that he had succeeded in metallizing potash and sodium by strongly heating them with charcoal. The Curaudau process was further developed by Deville in France into a commercial process and for a large number of years was the principal `source of sodium metal. Devilles method produced all the sodium that was needed for the production of aluminum (which was the principal market for sodium) until the sodium industry was upset by the introduction of the Hall electrolytic process for the preparation of aluminum in the last ten years of the 19th century. After the introduction of the Hall process for the making of aluminum, considerable experimenting was done in the search for ways of making sodium cheap enough to render competitive the production of aluminum by sodium reduction. The Castner cell for electrolyzing fused NaOH replaced the carbothermic retorts and became the principal commercial source of sodium metal. rl`his process, however, was not able to produce cheap enough sodium to compete in the production of aluminum. Some thirty years later the Downs cell replaced the Castner cell for the production of sodium and made it possible to use cheap salt as a source of sodium. This process is the present source of substantially all of the sodium which is manufactured today.

A few years ago interest in the production of sodium by sium (known as the Hansgirg process) used at Permanete, v

California, during World War II and which suggested possibilities for a similar process for the preparation vof,

sodium. Further experimental work which has been carried out since the end of World War Il, together with economic evaluations of the various possible processes for making sodium, have indicated that a chemical reduction. 70 process for the manufacture of sodium could be competi tive with the electrolytic process and thereis some denitef bon source.

Patented Mar. 29j, ldt) indication that the competitive advantage would be with the chemical reduction methods if carried out on a suiciently large scale.

In the carbothermic reduction of sodium compounds such as sodium hydroxide and sodium carbonate, one of the principal problems which has faced the commercialization of this method has been the finding of satisfactory materials of construction for containing the molten reactants at very high temperatures. The carbothermic reduction of sodium carbonate requires a reaction tempera ture of about 2000-2100 F. When this heat is supplied through the walls of a furnace by external heating there' occur large heat losses as well as a rapid deterioration of the furnace walls due to the much higher temperature which must be maintained on the outer surfaceof the furA nace Walls to produce the required reaction temperature within the furnace. It would therefore be desirable to provide a method of carbothermically reducing sodium compounds which does not require the transfer of large quantities of heat through the walls of a furnace reactor and which does not face the problem of extraordinarily large heat losses in supplying heat to the reactants.

It is therefore one object of this invention to provide a new and improved process for the carbothermic reduction of alkali metal compounds which does not require the transfer of large quantities of heat through furnace reactor walls. f

Another object of this invention is to provide an improved process for the carbothermic reduction of alkali metal compounds in which the reactants are heated di-f rectly by the combustion gases of the heating medium.

Another object of this invention is to provide a new and improved process for the carbothermic reduction of sodium carbonate utilizing `a submerged burner which transfers heat directly from the combustion gases to the` molten reactants and which provides the force for circulating the reactants in the reactor.

Other objects of this invention will become apparent from time to time throughout the specification and claims as hereinafter related.

This invention comprises a new and improved method for the carbothermic reduction of alkali metal compounds utilizing direct heat transfer from combustion gases, which will be described more fully hereinafter and the novelty v of which will be particularly pointed out and distinctly claimed.

In the accompanying drawing, to be taken as part of the specification there is shown a preferred embodiment of this invention in which drawing the figure is a somewhat diagrammatic View in central section of a reactor for carrying out the method which constitutes this invention.

The carbothermic reduction of sodium or potassium carbonate can be carried out at temperatures of 2000 2100 F. using coke as the reducing agent and at somewhat lower temperatures using hydrocarbons as the car. The temperature can be varied somewhat4 but 20004100 F. has been found to be a satisfactory rej action temperature range. ln this process the transfer of t heat through metal reactor walls presents some considerable difficulty since relatively large heat transfer surfacesv are required for a given sodium production. This inven- A tion is based upon the discovery that the heat required for carrying out this reaction can be supplied without transferring heat through other metal or furnace reactor walls by using a circulating mass of sodium or potassium carbo- 3 nate to serve as a heat transfer fluid and reactant combined. The sodium or potassium carbonate can be heated in one part of a reactor to a temperatureabove the ref quired reaction temperature and then circulated into`a second zone where it will Contact the reductant. When a 'i' submerged burner is used the gaseous combustionv prodi'J 3 A ucts of the submerged burner will act as a pump to pr duce the required circulation. if direct firing other than submerged combustion is used stack gases can be used to produce the required pumping action.

In the accompanying drawing there is shown the reactor generally designated ll in which this process is carried out. The reactor l is of closed metal construction and may, if desired, be lined with a suitable refractory. The reactor 1 is divided by vertical walls and 3 and a horizontally extending baille linto a series of interconnected compartments and passageways. The vertical wall i?. and the outer Wall S of the reactor l deline a heating zone or compartment 6 in which there is positioned a submerged burner 7 having an inlet pipe connection S for providing an air and fuel mixture for the burner 7. Directly above the heating compartment 6 there is positioned an outlet stack pipe 9 for discharge of combustion gases from the reactor. The upper end of the wall 2 provides a Weir 1li for controlling flow `of the molten alkali metal carbonate.

At the lower end of the vertical wall 2 there is provided an l opening or y aperture il for recirculation of the molten carbonate to the heating compartment 6. The vertical wall 3 and horizontal bame l together with the upper wall 12 and outer wall 13 ol the reactor delizie a reaction compartment 14. The reactor is provided with an outlet passageway or conduit 5.5 opening from the compartment 14 for discharge of the sodium Vapor which is formed inV this process. Through the upper wall 12 of the reactor there areprovided a pair of feed tubes or conduits 16 and 17 through which the carbonaceous reductant and the alkali metal carbonate respectively are fed. At the outer end of the baille 4 there is provided a passageway 18 which permits recirculation of the molten alkali metal carbonate through the lower part of the reactor which constitutes a passageway i9 leading back to the heater compartment 6 through the aperture 11. At the lowest part of the bottom wall of the reactor 1 in the portion constituting the passageway i9 there is provided a slag draw-olf passageway 2l) which is sealed as indicated at 21 andkwhich provides a convenient point for removal of slag periodically from the reactor.

In the operation of this process in this apparatus the reactor is charged with a substantial quantity of sodium carbonate and external heat is supplied to heat the entire mass of carbonate to a temperature just above its melting point. Alternatively, the sodium carbonate may be heated externally and fed in the molten state to the reactor. After the reactor is charged with molten carbonate the submerged burner 7 is ignited and supplies all of the heat requirements of the process. The molten sodium carbonate has a density of about 117 lbs./ cu. ft. and a viscosity of less than l() centipoises at reaction temperature. The molten sodium carbonate in the heating compartment 6 is heated by direct contact with the combustion gases from the burner 7 to a temperature of about 2150-2250" F. and is pumped by the gas bubbles in the melt over the edge of the Weir 10 into the reaction compartment 14. In the reaction compartment 14 the carbonaceous reductant, which may be coke, charcoal or hydrocarbon materials, is introduced through the inlet lo for reaction with the heated sodium carbonate. The carbon reacts with the sodium carbonate to form a mixture or sodium vapor and carbon monoxide which is withdrawn through the outlet passageway 15 and transported through suitable external conduits (not shown) to a condenser where the sodium metal is recovered. During this reaction in the reaction compartment 14 the molten sodium carbonate is reduced in temperature to about 200G-2h50 F. Additional sodiumcarbonate is supplied in the reaction compartment through the inlet'conduit 17 to replace the carbonate Whichyhas been decomposed in the reaction. The molten sodium .carbonate at about 2050 F. passes throughthe passage 19 and returns 4to the heating compartment 6 where itis reheated forfurther reaction. When powdered colte is used as the reductant it has been found that some of the coke may be entrained with the molten sodium carbonate and carried into the recirculation passageway 19. kThe upward slope of the baille 4 provides for a re- Vturn of the sodium and carbon monoxide vapors which result from a continuation of the reaction between the coke and the molten sodium carbonate in the passageway 19.

When the molten sodium carbonate in the heating zone 6 is heated to supply carbonate at 2250" F. to the reaction zone 14 and the carbonate leaving the reaction zone is at 2G59 F., sodium is produced at the rate of about 0.78 lb./cu. ft. of molten carbonate circulated through the reaction zone. The heat requirement for this reduction is about 11,300 B.t.u./lb. sodium produced. If the inlet temperature to the reaction zone ld is reduced to 2150" F. and the outlet temperature remains at 2050" F. the production of sodium at a given circulation rate is cut in half.

The operation of the process in the apparatus as described above is a substantially ideal operation and some modiiications have been required as a result of experimental evidence which has been obtained. First of all, it has been found that not all of the gas bubbles of the combustion gases entrained in the fused salt in the heating compartment 6 are completely de-entrained and pass up the stack 9. Some of these gas bubbles carry over into the reaction compartment 14. The water and carbon dioxide which are present in these bubbles are reduced by the carbonaceous reductant in the reaction compartment 14 and result in a slightly increased consumption of carbon and a slight contamination of the product with nitrogen and hydrogen. The extent of this contamination however has not been found to present a serious problem.

There is also a tendency of some entrainment of the fused sodium carbonate into the stack gases due to the velocity of the combustion gases leaving the heater compartment 6. This loss of fused salt in minimized by maintaining relatively large disengaging spaces about the fused salt and by keeping the gas velocities low. For example, a gas rate of 3.8 lb. mols /hr./sq. ft. produced a mean bubble rise in the heater compartment 6 of about 8 and fused salt entrainment was not observed. For a reactor to produce 1GO lbs/hr. of sodium the surface area required to give a combustion gas rate of 3.8 lb. mols/hr./ s'q. ft. is about 20 sq. ft. Further experimental data has proved, however, that higher combustion gas rates may be used without appreciable entrainment of the fused salts and thus permit the use of smaller burner compartments.

' It is also possible to reclaim the molten fused salt which has been entrained with the stack gases by passing those gases through an external separator or collector unit.

In preliminary experiments which were carried out bubbling various inert gases through molten sodium carbonate it was found that a substantial vapozation of the carbonate occurred. However, it was also found that the vaporization of molten sodium carbonate is suppressed by an atmosphere of carbon dioxide. It is therefore desirable in the operation of this process to maintain a substantial partial pressure of carbon dioxide over the molten carbonate to suppress Vaporization.

it has also been found that this process is operated with monoxide which is a by-product of the production of sodium.

When this process is operated as above described there are a number of substantial advantages which result. Firstly, Athere are no heat transfer surfaces required for the transmission of heat to the molten carbonate and the requirement of scarce high temperature alloys is avoided.

Secondly, the capital costs on the apparatus for carrying out this process are substantially lower than with externally iired reactors because of the reduction in cost of the materials of construction. Thirdly, the maintenance costs in the operation of this process are substantially lower since shut downs are much less frequent. Fourthly, the operating costs are lower since less attention is required than with externally red reactors and the firing control is less critical. Furthermore, high heat utilization can be obtained readily in this process since the stack gas temperature approaches very closely the temperature of the fused salt melt.

While this process has been described with principal emphasis upon the use of a submerged burner it should be noted that it would be possible to carry out this process in a reverberatory or other type furnace in which the combustion gases would heat the molten salt to a temperature above that required for reaction and the heated salt would be circulated to -another compartment for reaction with the carbonaceous reductant. The stack gases in such a unit could be used to produce the required pumping action for circulating the molten salt. It should also be noted that While this process has been described with particular emphasis upon the use of coke as the reductant that other carbonaceous reductants could be used. In particular, methane or other gaseous or liquid hydrocarbons can be used in the reaction compartment as the reductants. When hydrocarbons are used as the reductant in this process they are cracked at the high temperature to carbon and hydrogen. The carbon which is produced is in an extremely divided state and will permit the reaction to take place at a somewhat lower temperature.

Having thus described a preferred embodiment of this invention and what is considered to be the best mode of carrying out this invention l wish it understood that within the scope ofthe appended claims this invention may be practiced otherwise than as specifically described.

What is desired to be secured and claimed by Letters Patent of the United States is:

1. A method of producing an alkali metal by reaction of an alkali metal carbonate and a carbonaceous reductant which comprises heating an excess of alkali metal carbonate at least to reaction temperature in a first zone by passing hot combustion gases into contact with said carbonate, circulating the resultant heated carbonate substantially free from said combustion gases to a second zone, adding a carbonaceous reductant to said heated carbonate in said second zone for reaction therewith whereby alkali metal vapor and carbon monoxide are produced, withdrawing the resulting alkali metal vapor and carbon monoxide from said second zone and condensing and recovering said alkali metal, and recycling unreactcd carbonate from said second zone to said rst 2011.

2. A method in accordance with claim 1 in which said alkali metal carbonate Vis sodium carbonate and sodium is recovered.

3. A method in accordance with claim 1 in which said alkali metal carbonate is heated by burning a fuel and passing the resulting hot combustion gases, containing carbon dioxide in an amount suicient to maintain a partial pressure of'carbon dioxide over the resulting molten alkali metal carbonate suicient to retard vaporization of said carbonate, into contact with said carbonate to heat it.

4. A method of producing an alkali metal by reaction of an alkali metal carbonate and a carbonaceous reductant which comprises heating a body of alkali metal carbonate at least to reaction temperature in a rst zone by producing combustion gases in a lower portion of said carbonate, whereby said alkali metal carbonate is heated and the combustion gases rise upwardly through the resulting molten carbonate and pump said heated carbonate to a second zone, adding a carbonaceous reductant to the heated carbonate in said second zone for reaction therewith whereby alkali metal'vapor and carbon monoxide are produced, withdrawing the alkali metal vapor and carbon monoxide from said second zone and condensing and recovering said alkali metal, and recycling unreacted carbonate from said second zone to said rst zone.

5. A method in accordance with claim 4 in which said alkali metal carbonate is sodium carbonate and said alkali metal that is recovered is sodium.

6. A process comprising circulating a body of molten alkali metal carbonate between two zones separated from one another so that gas in either of said zones does not communicate with gas in the other of said zones, heating said carbonate at least to reaction temperature while in said lirst zone by directly contacting said carbonate with hot combustion gases, adding a carbonaceous reductant in less than a stoichiometric amount to said carbonate in said second zone to react with heated alkali metal carbonate therein whereby alkali metal vapor and carbon monoxide are produced, recovering said alkali metal, and adding additional alkali metal carbonate to said trst zone in an amount substantially equivalent to the amount removed from said circulating body upon reaction with said said carbonaceous reductant in said second zone.

7. A process in accordance with claim 6 in which said alkali metal carbonate is sodium carbonate and said alkali metal that is recovered is sodium.

References Cited in the le of this patent UNITED STATES PATENTS 350,574 Wainwright Oct. 12, 1886 380,775 Thowless Apr. 10, 1888 1,837,935 Ylla-Conte Dec. 22. 1931

Claims (1)

1. A METHOD OF PRODUCING AN ALKALI METAL BY REACTION OF AN ALKALI METAL CARBONATE AND A CARBONACEOUS REDUCTANT WHICH COMPRISES HEATING AN EXCESS OF ALKALI METAL CARBONATE AT LEAST TO REACTION TEMPERATURE IN A FRIST ZONE BY PASSING HOT COMBUSTION GASES INTO CONTACT WITH SAID CARBONATE, CIRCULATING THE RESULTANT HEATED CARBONATE SUBSTANTIALLY FREE FROM SAID COMBUSTION GASES TO A SECOND ZONE, ADDING A CARBONACEOUS REDUCTANT TO SAID HEATED CARBONATE IN SAID SECOND ZONE FOR REACTION THEREWITH WHEREBY ALKALI METAL VAPOR AND CARBON MONOXIDE ARE PRODUCED, WITHDRAWING THE RESULTING ALKALI METAL VAPOR AND CARBON MONOXIDE FROM SAID SECOND ZONE AND CONDENSING AND RECOVERING SAID ALKALI METAL, AND RECYCLING UNREACTED CARBONATE FROM SAID SECOND ZONE TO SAID FIRST ZONE.

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