USRE23017E

USRE23017E - Treatment of alkali metal salts

Info

Publication number: USRE23017E
Authority: US
Publication date: 1948-07-27

Description

July 27, 1948- J. R, BATES TREATMENT OF ALKALI ME'II'AL SLTS original Filed oct. 18, 1944 2 Sheets-Sheet l w, .mi

IN'vENToR www R. n A N MM Jy J. R; BATES TREATMENT 0F ALKALI' METAL sALTs July 27, 1948.

'2 Sheets-Sheet 2 Original Filed Oct. 18, 1944 L/UHN R. BA T55 ATTORNEY AReimied July 21,i 194s John R. Bates,l Swarthmore, Pa., assignor to Hodry Process Corporation, Wilmington, Del., a corporation of Delaware Original No. 2,375,761,"(1ated May 15, V1945, Serial No. 559,226, October 18, 1944.

Application for reissue May 11, 1946, Serial No. 669,013

Claims.

-The present application is a. continuation-inpart of each of my applications Ser. No. 319,446, filed February 17, 1940 and Ser.- No. 358,707, filed September 27, 1940, both of the aforesaid applications being now abandoned.

This invention relates to methods of maintaining the desirable properties of heat transfer salts, with special reference to alkali metal salts of the oxyacids of nitrogen, and to apparatus for such methods.

Heat transfer salts which have low melting points are desirable for controlling temperatures in chemical and other equipment. Where the temperature is to be maintained between say '100 and 900 F. in the zone controlled by the salt, it is desirableI for the salt to have a subv stantially lower melting point, so that it may be melted by the steam available, which ordinarily is at 400 F. or lower. Thus, when a plant is shut down and cooled, it is desirable to be able to melt the salt in the lines with steam in tracers. My associates and I have found that mixed nitrates and nitritesof one or more of the alkali metals are particularly suited to such uses, and upon the basis of our ndings the first large plants employing such temperature Acontrolling media have been installed. As a result of these installations, we have found that in use the properties of the salts change, resulting in an increased melting point, in the formation of deposits which in some instances partially plug some of the lines and in all cases eil'ect a reduction of the heat transfer efficiency of the systems, and in the deposition of solid phase from the compositions.

One object of the invention is to provide method and apparatus for controlling the composition of liquids. Another object is to control the melting point of a temperature regulating salt mixture of nitrites and nitrates ina manner to maintain the mixture in readily ilowable condition during use. Another object is to adjust continuously or from time to time the comapparent from the detailed description whichv follows:

Referring to the drawings,

Flgs.-2 and 3 are modified forms of a portion of the apparatus shown in Fig. 1.

Fig. 4 is a modied form of the present invention employing recirculation through the re ducer. l The present invention is particularly adaptabie to commercial operations where a body of the salt mixture is circulated in heat exchange relationl with apparatus which is used in carrying out various physical and chemical reactions and provides an eflicient and relatively inexpensive arrangement for readiusting the composition of the mixture while it is in use in maintaining an apparatus at the vdesired temperature. A small portion of the mixture, depending upon the rate of rise of its melting point and the melting point at which it is desired to maintain the mixture, is tapped from the temperature means where it is further treated in a. mannerV to obtain some further reduction or to remove components contained therein which might deleteriously affect the ilow of the mixture through the salt circuit or give to the mixture other undesirable properties such as increasing its corrosive action. In the second chamber alkali produced during use of the salt in the indirect heat exchange system and produced during reduction is neutralized with nitric acid or with an oxide of nitrogen. Preferably salt from the loutlet of the reducer is recirculated to the inlet thereof independently of the neutralizing zone or second chamber and of the heat exn change system. By operating in this manner the alkali concentration of the recirculated salt mixture is higher than the alkali concentration of the salt mixture in the heat exchange system. However, 'the total alkali which it is necessary to neutralize is less than ii recirculation around the reducer is not employed.

The salt may be withdrawn at a rate and so treated that the alkali content of the salt in the zone of use is maintained at below about 5% alkali (calculated as NaOH) and preferably between about 0.2% and 2%. Withdrawn salt is subjected to reduction to convert nitrate to ni- Fig. 1 is a semi-diagrammatic view of one form trite at a. rate to maintain the per cent nitrate in the zone of use between about 10% and about The salt may be reduced by contacting it y with a reducing gas in 3 a recirculatory system in which a higher alkali contentfis maintained than V in the zone of use.

In Fig. 1, A indicates any commercial apparatus which is used to carry out an industrial operation and wherein it is desired, during the operation, to control the temperature of various parts of the apparatus. Any known arrangement for circulating the salt mixture in heat exchange relation with the apparatus may be used, for example, it may be passed through conduits which are positioned in heat exchange relation with the apparatus or the apparatus may be contained in a bath of salt mixture. A circulating system generally indicated at C is provided for continuously circulating the salt mixture and the circuit may be provided with the customary heat exchanger ill for adjusting the temperature of the mixture as well as with surge tank il and pump i2 for controlling the circulation.

When the salt mixture is used as a heat exchange medium its composition gradually changes and its melting point usually increases due to increase in the nitrate and decrease in the nitrite, which may be caused in some systems by oxidation from exposure to air which enters the circulating 'system through unavoidable leaks occurring therein as the apparatus parts expand and contract. After the meltingy point of the mixture has increased so much that it is difficult to circulate the mixture at the temperatures obtaining in the industrial apparatus and circulating equipment, it becomes necessary to replace the mixture with a new composition of desired melting point or to readjust the concentration of the nitrate and nitrite in the used mixture to obtain the desired melting point.

According to this invention, the melting point of the salt mixture in the circulating system is not permittedto change to the extent that any diiliculty exists in maintaining the mixture in a readily iiowable condition. This is effected by continuous or intermittent reduction of nitrates to nitrites, as by tapping a small portion of the mixture from the system and treating this portion while the main body of the salt is used in. controlling the temperature of the apparatus and then returning the diverted portion to the main body of salt after it has been treated to increase the nitrite concentration and decrease the nitrate concentration, thus maintaining a balance between these components the same as, or substantially the same as, that of the original mixture, or in a predetermined range adjacent thereto. During the treating operation, the portion withdrawn from the main body of salt may be treated so that its nitrite and nitrate proportions are increased and decreased, respectively, to a controlled or desired degree so that when it is admixed with the main body of the salt `the concentration of these components in the main body of the mixture may be accurately controlled.

Again referring to Fig. 1, a valved branch I3 extends from the main salt circulating system and is in communication with a chamber R. at a point adjacent its top so that a predetermined amount of the liquid may be tapped from the system for passage intermittently or continuously downwardly through the chamber to be contacted by a reducing medium for changing nitrates to nitrites. Any known or desired reducing medium may be used, for example, hydrogen, hydrocarbons, a hydrocarbon-hydrogen mixture, ammonia gas, etc. As is indicated in the iigure, hydrogen is preferred as the reducing agent and it is supplied to the chamber R at a point ad- Jacent its bottom through the valved line Il and passes upwardly through a gas dispersion means which is preferablya finely perforated metal plate ii, but which may be any perforated or porous material capable of iinely dispersing the hydrogen. For the purpose of accelerating the reduction in the chamber R, a packing ii is prvided and is retained therein above the dispersion plate I5 as by a iinemesh screen Il. Any desired packing material may be used but preferably one which will withstand corrosion. Stainless steel, nickel or Monel metal give very good results and the most elllcient reduction occurs when the metal packing is in the form of short, fiat, helical turnings, since packing in this form provides great surface contact and permits the hydrogen bubbles to ilow through the packing without material coagulation or channelling.

Before returning the reduced salt to the cir-- culating system it is preferably passed from the chamber R to the chamber N for treatment in removing the alkali existing in the salt at this time or at least removing a sufficient amount that the flowability oi the salt in the circulatingrv system will not be adversely aiected. The charnber N may be constructed generally like the chamber R but best results have been obtained tralizing agent, such as oxides of nitrogen is admitted at the bottom of the chamber as by line Nto pass through a. dispersion element 2U similar to that of chamberfR. The chamber N may, if desired, contain a. suitable packing material to increase surface contact between the iluids as they pass countercurrently through the chamber, but sufilcient excess alkali may be removed readily without resorting to this expedient. From the chamber N the salt is returned through the line 2| to the circulating system C where it is mixed with the main body of salt. The salt supply lines to and removal lines from the treating vcases R and N are arranged so as to maintain the cases substantially full lof salt during the treating operations. l

, In Fig. 2 is shown a modified form of reduction chamber which may be substituted for the chamber R in Fig. l. This chamber, generally indicated at Rx is constructed in a manner which adapts it to use a iloating or movable hydrogen dispersion plate, for example, one constructed of unglazed porcelain or other porous material which if rigidly secured might be broken or damaged durlng use due to temperature changes in the chamber and the resulting unequal contraction and expansion between the chamber walls and plate# The chamber Rr is provided with suitable packing 2l which is held in place by the screen 29 and an annular well 3U is provided at the bottom of the chamber/and is in communication therewith to receive salt from the chamber through the screen 29. A dispersionr plate 3| is disposed below the screen 29 and is provided with an annular ange 32 which extends downwardly into the well 3U forming a liquid seal with the salt therein. Hydrogen is supplied below plate 3| by line 33 and will pass through the dispersion plate 3| when the back pressure of the hydrogen on the leg 34 'of the seal is less than the pressure of the salt on the leg 35 of the seal. The screen 29 functions additionally to limit the extent of upward movement of the plate Il.

In carrying out the invention, a salt mixture is passed through the circulating system C in order to control the temperature oi' the industrial apparatus A which may be in the range of about 400 to 1000* F. i'or example. A vpredetermined amount olV the mixture, depending upon the percent rise of nitratesduring circulation, is, withdrawn from the circulating system through the branch il and admitted to the chamber R at a temperature between '750 and 950 F. and prei erably. in order to obtain the most eillcient reduction, will be admitted at a temperature between 800 and 900 F. II the salt mixture is used to control temperatures below 750 or 800 F. it wili be necessary for the best reduction to raise the temperature of the portion withdrawn from the circulating system before it is supplied to the chamber R. As the hydrogen' from the line Il ifs dispersed upwardly into the chamber R it reacts with the downwardly flowing salt mixture and reduces a portion or all of the nitrate and forms additional nitrite as one product of the reaction,

which is eventually returned with the mixture to the circulating system, and water, the other prod,- uct o! the reaction, which is passed from the chamber R through the linev 22 in theiiorm of steam. The reduced mixture is then through the line I8 to the cham-ber N at substantially the same temperature at which the reduc-f tion is carried out or at a temperature which4 .will effect the total or any desired partial removal of the .alkali content of the reduced mixture when it is reacted with oxides of nitrogen. Either NO or NO2 alone or equi-molal mixtures may be used for removing a desired portion oi the alkali. When NO alone is used the concentration of nitrites in the mixture is increased vsince the alkali changes to nitrite and some of the nitrate present in the mixture is also reduced to nitrite which lowers the nitrate concentration. When a mixture oi NO and NO2 in equal proportions is' used. only the alkali is changed to nitrite and the nitrite concentration of the mixture is therefore increased while the nitrate 'concentration remains unchanged. When NO: Ai's used alone, the alkali changes to nitrite Aand nitrate and the bal'- ance of these components ot the mixture remains substantially unchanged. As the neutralizing reaction is carried out in the chamber N, water, which is the other product of the reaction. is removed through the line 2l. Some metals suitable for equipment for circulating the mixture can withstand up to 5% of alkali without being adversely affected and this amount can be tolerated in the circulating system without adversely.

.aecting the nowability of the mixture. Accordingly at times the neutralizing chamber N may be by-passed during the treating operation and the reduced mixturev returned directly to thel circulating system. However, in order to prevent thel building up in the circulating system oi' too large a percentage oi' alkali, a portion .of the alkali formed in the reduction chamber R is preferably removed, as in chamber N, before returning the mixture to the circulating system. The neutralization step is preferably regulated so that at least .2% and up to 2% or somewhat more4 of alkali remains in the mixture when it is returned to the circulating system. since, as heretofore mentioned, such amounts of alkali do not ai'fect the circulation oi' the mixture adversely and in addition it has been found that small amounts of alkali increase the rate of reduction when the salt ls passed to the chamber R.

Fig. 3 shows a modiiied iormof apparatus and method oleii'ecting reduction oi the salt portion from the main circulating system. In 'this form of the invention, the salt mixture from the circulating system is sprayed into the chamber above the packing to form thin lms o! salt thereon and the hydrogen is supplied at the bottom in an undispersed state so that the salt nlms on the packing are completely surrounded by a continuous phase or hydrogen. The chamber Rz, like chamber R of Fig. l, contains a packing 40 which is adapted to provide the greatest degree oi surface contact for the fluids and is held in the chamber by means of 'a screen l'i. Salt is supplied to a reservoir 42 through the line Il from thesalt circuit and will be forced to pass from the reservoir through line 4l and be admitted near the top of the chamber Ra where it is dispersed into the packing by a spray 45. Hydrogen is 'supplied to the chamber Rz from the line 48 at a point above the level of the salt in the reservoir 42 and, passing through the screen l. flows upwardly in xcontinuous phase through the packing to react with l the salt mixture and reduce the l nitrate concentration.

In the ow sheet designated as Fig. 4 a plant salt'circuit is shown including serially in a recirculating cycle a surge tank 5|, a controlled zone lla heat exchanger 03 and a piunp 5l for returning salt to the surge tank. Suitable salt lines .interconnect the equipment in the circuit for the conveyance of the salt. It is to be understood this circuit is merely illustrative oi a zone l in which the salt is used and forms no part of the vent 50.

` present invention, and that it may be changed in into the lower end of the treater, whereby the alkaliin the salt is neutralized, and in the case of the nitrogen oxides, by which term it is intended to include their acids, converted into an alkali metal salt of an oxyacld of nitrogen. The treating material for the neutralization may be introduced into the treater through the line 5l, and waste gas may pass out of the treater through may, if desired, be eifected by treatment vwith other acid oxides, as for instance, with silica. alumina, or silica alumina gel. In case solid acid oxides are employed the oxides may beplaced as a packing material in the treater 5I through vwhich the salt is ilowed. The solid acid oxides alone may be employed or they may be employed in combination with oxy acids of'nitrogen introduced'thwugh une u. j` It should 'be noted that' the molten material may carry suspended solids.

It is preferred that the treater It .be operated eitherlcontinuously or intermittently at a rate to maintain the alkali in the plant salt circuit, at less than about 5% (calculated as NaOH) of the total salt, and generally for most etlicient operation at between about 0.2% and 2%. systems it is undesirable for the alkali content to be allowed to rise above 2% tor any extended time, due to the tendency to deposit scaleand -to deposit in the cooler portions. o! the system. though under some operating conditions the content may be maintained `at above about 2% and The reduction of the alkali content,

In manyv maintained above about 750.

amori 4 7 Y below 5% without disadvantage resulting therefrom. The most desirable upper limit of alkali will vof course dependl upon the specific conditions to which the salt is subjected in the sone in which it is employed, in particular the temperatures to a which the salt is allowed to drop. In a system in which thev alkali has .been allowed tn build up above 2% and in which it is desired to maintain the alkali within the preferred range, the neutralizer is operated at a rate to lower the alkali l0 content to within the preferred range.

Por conservation of equipment, it is desirable that the `pressure vdrop between ,the points at which the lines 55 and Il are connected be su!- ncient to maintain circulation through the neutralizar I8.

Salt is withdrawn from the plant circuitand lay-pass, being conveyed, preferably from the plant circuit, to a reducer l! by a line Il. In the reducer il. the salt is reacted with hydrogen, and for most rapid reduction the temperature is The reducer Il is located in a recirculating system in which the salt is conveyed by a line 64 from the reducer to pump i! and from the latter to the former by g5 line When alkali-metal nitrates and nitrites are contacted in molten state with hydrogen, some of the nitrate is reduced to nitrite. While this reduction is the principal reaction occurring there is some free alkali formed. The alkali 30 here formed is neutralized to maintain the alkali in the zone of use atdesirably low value.

It has been found desirable to recirculate the salt as described in the reducer, whereby a high alkali contentl may be built up and 35 maintained in the reducer. This high alkali.y content decreases the production of alkali relatively to the amount of reduction occurring. Accordingly, the alkali content in the .recirculating reducer system is built' up to and maintained ,4o

vat a concentration above the concentration ot alkali in the zone where the salt is used, and preferably at a concentration above 1%. By this means it is possible to save considerable initial investment in the neutraliser due to the smaller 4.'.

. size required, and to save considerable in operating expense due to more emcient consumption of neutralizing material.

' Salt is drawn oil' from the reduction system at a rate to compensate for the feed to this syso0 tem, and is returned to the plant circuit preferably by feeding it through line 61 to the neutralizer. Introduction of this high alkali salt directly into the neutraliser with the sale bypassed from thev circuit results in high efficiency, since it ss has been found in the neutralization of these molten salts the rate of neutralization increases with increasing concentration of alkali. f

In the type of reducer shown in Fig. 4 apackling material is dipped into a body of the molten gq salt to be reduced. The packing is then withdrawn from the body of salt, and, while stillhot and carrying a iilm of molten salt, is exposed to contact with an atmosphere containing hydrogen superposed on the body of salt whereby re- 05 ductiony is eifected. The dipping and exposing are repeated until the reduction has been eiIected to the desired extent. Any desired packing material may be employed which is not attacked by the salt being treated such as Raschig rings or 70 steel turnings. v

The reducer 6I shown involves an inclined drum il, carried by hollow trunnions It, posi-p tioned axially thereof. Thel trunnions are supported by bearings ll. A partition 1| is posi- 75 tioned in the drum adjacent the lower end wall and is provided with a plurality of check valves 12, on the lip-stream side ox' the partition. the number oi' valves being sumcient that at least one valve is always above the liquidlevel. The drum is filled with packing between the partition Il and the upper end wall. Screens 13 are provided to cover the check valves 12 and thereby prevent the packing fromr interfering with the operation of the valves. A receiver 1I, for salt from the drum, communicates with the drum by a sleeve in engagement with the lower trunnion. The receiver Il functions also as a chamber orzone for the introduction of hydrogen into the lower trunnion. A level of molten salt may be maintained in the receiver N, whereby it functions as a surge tank for the recirculating reducer system. A similar receiver 'Il for hydrogen communicates by a sleeve with the upper trunnion. A salt-ino troducing tube 1l is .provided` which extends through the receiver 1i and into the upper trunnion. The salt from the receiver` ll ilows through the line 6l to pump 6l from which it is conducted by line. II to introducing tube 1l. Hydrogen is introduced by a line il into the receiver 14 from which it ilows through the drum. Reduction of the mixed nitrate-nitrite salt, re-

suits in the production of water vapor, which,

in mixture with some hydrogen, iiows out of the drum through receiver 'Il and line 1l. If desired the mixed hydrogen and water vapor may be passed through a heat exchanger 1I where the water is condensed rand, separated from the hold the salt eat increasingly higher levels toward the upper end of the drum, so that the packing close tothe axis of the drum will be 'coated with a"film of salt upon rotation. Alternatively the valve l0 may be closed, and

the salt withdrawn from the plant circuit by line li at a rate suillcient that the amount ci' neutralized and reduced salt` fed into the circuit is sumcient to maintain the alkali in the circuit within the indicated limits. In this case the per cent of nitrate reduced to nitrite in the salt fed to the reducing system will not be as great as in case a portion of the salt is withdrawn directly to the neutralizer. The reducer is operated either with or without recycling salt thereto at a rate to maintain the per cent of nitrate inthe salt in the plant circuit in the range above stated, i. e., between about 10% and about 50% nitrate. l In a typical plant which involves the use ,ol

magnitude of minute leaks of materials, such as hydrocarbons, oxygen and carbon dioxide into the salt system. In the plant referred to, the

rise in melting point has been found to average Vabout 4"v F. per month, though for thereasons stated this will vary widely in diiierent plants.

In about ten months of use the freezing point i will have risen about 40 F. irom the original 288 F., but the determination is indenite since when the salt is cooled to 400 F. a solid phase commences to appear. To maintain the total alkali (calculated as NaOH) in this system constant, thereby avoiding eitherincrease or decrease, about 434,000 pounds of salt is passed from the plant circuit per month to the neutralizer, and reacted with concentrated nitric acid, fed at a rate below stated, which maintains the alkali at about 0.5% in the plant circuit. Ii lowering the alkali content is desired, say due t a rise of the alkali above about 2%, the rate oi flow oi both the salt and acid may be increased over that necessary to maintain a fixed alkali content. Salt is withdrawn from the circuit and passed to the reducer which is operated at a rate to maintain the nitrate ion concentration in the plant circuit preferably below about 50%. Thus, when the nitrate ion rises to 47.5%

of the total salt, it is passed to the reducer cir- V cuit at a rate of 40,500 pounds per month. -The salt in the reducer circuit is circulated at a rate of about '730,000 pounds per month. By maintaining the rate of the hydrogen i'eed suiilcient to hold the nitrate ion concentration in the reducer circuit at about 10.5%, `enough alkali is formed to maintain the alkali concentration at about 2.3% when the plant circuit is maintained at about 0.5%. Reduction at such a concentration of alkali effects a considerable lowering in the weight of alkali formed per weight oi' salt reduced over reduction at a lower alkali concentration, such as is maintained inthe plant circuit. The amount of salt in the reduction circuit is maintained about constant by passing about 36,500 pounds of salt from the reducer to the plant circuit by way of the neutralizer. With the alkali concentrations stated, the average alkali content of salt fed to the neutralizer is 0.64% which being higher than in the plant circuit allows the requisite weight of alkali to be reduced with smaller equipment than if all salt for neutralization were drawn )from the plant circuit. Nitric acid of 91.4% concentration is fed at a rate oi 2,710 pounds per month, whereby the salt from the neutralizer has an alkali content of about 0.2% and there is an excess of about 5% nitric acid.

I claim as my invention:

l. In a process of treating a salt mixture consisting principally of alkali metal nitrate and nitrite which has been maintained in circulation in an indirect heat exchange system and in which the melting point has increased as a result of such use, the-steps comprising contacting said molten mixture with a reducing agent under such conditions that the melting vpoint of the mixture is decreased, and returning the molten mixture for indirect heat exchange.

2. In a process of treating a. salt mixture consisting principally of alkali metal ni-trate and nitrite` which has been maintained in circulation in an indirect heat exchange system and in which the melting point has increased as a result of such use, the steps comprisingcontacting said molten mixture with hydrogen under such l.conditions that the melting point oi the mixture is decreased, and returning the molten mixture for indirect heat exchange. i

3. In a. process of treating asalt mixture consisting principallyof alkali metal nitrate and nitrite which has been maintained in circulation in an indirect heat exchange system and in which the melting point has increased as a result of such use, the steps comprising contacting said molten mixture with a hydrocarbon under such conditions that the melting point of the mixture is decreased, and returning the molten mixture for indirect heat exchange.

4. In a process of treating a salt mixture consisting principally of alkali metal nitrate and nitrite which has been maintained in circulation in an indirect heat exchange system and in which the melting point has increased as a result of such use, the steps comprising contacting said molten mixture with ammonia under such conditions that the melting point oi the mixture isdecreased, and'returning the molten mixture for indirect heat exchange.

5. In a, process of treating a salt mixture con-- sisting principally of alkali metal nitrate and nitrite which has-been maintained in circulation in an indirect heat transfer system and in which the alkalinity and melting point has increased as a result of such use, the steps comprising contacting said molten mixture with a reducing agent under such conditions that the melting point of themixture is decreased, further contacting said molten mixture with an oxide of nitrogen until the alkalinity has been decreased and returning the molten mixture, for indirect heat transfer.

6. In a process of treating a salt mixture consisting principally of alkali metal nitrate and nitrite which has been maintained in circulation in an indirect heat exchange system and in which the melting point has increased as a result of such use, the steps'comprising withdrawing salt mixture from said system, maintaining a recirculating stream of said Withdrawn salt mixture, passing said recirculating stream through a reducing zone in contact with hydrogen in which zone nitrate is reduced to nitrite, whereby the alkali concentration of salt mixture in saidstream is maintained at above the alkali concentration of the salt mixture in the system, withdrawing salt mixture from said recirculating stream and introducing it into an alkali neutralization zone, neutralizing alkali in said neutralization zone by contacting salt mixture therein with a reagent selected from the group consisting of nitric acid vapors and an oxide oi nitrogen until the alkalinity hasbeen decreased, returning salt mixture from said neutralizing zone to said system, and maintaining said salt mixture in molten state in said contact zone and in said neutralizing zone, said recirculating stream being independent of sail heat exchange system and said alkali neutralizing zone.

7. `In a process of treating a salt mixture consisting principally of alkali metal nitrate and nitrite which has been maintained in circulation in an indirect heat exchange system and in which the melting point has increased as a result of such use, the steps comprising withdrawing salt vmixture from said system, passing said withdrawn salt mixture through a Contact zone in contact with hydrogen, passing salt mixture from said contact zone into an alkali neutralizing zone,

neutralizing alkali in the salt mixture in the neuthan the alkali concentration of salt mixture inV said system, returning salt from said neutralizing lone to said'system, and maintaining said salt mixture in molten state in said contact zone and in said neutralizing sone.

8. In a process of treating a salt mixture-conlisting principally ot alkali metal nitrate and nitrite which has been maintained in circulation in an indirect heat transfer system and in which the alkalinity and melting point has increased as a result oi such use. the steps comprising contacting said molten mixture with a reducing agent under such conditions that the melting point of the mixture is decreased, further contactingsaid molten mixture with a reagent selected from the group consisting .of nitric acid vapors and an oxide of nitrogen until the a1- kalinity has been deereased"and returning the n molten mixture for indirect heat transfer. V

9. In a process for the production of an alkali metal nitrite, the step which comprises contacting molten sait comprising an alkali metal nitrate with hydrogen at reduction conditions including temperatures in the range of 750 to 950 F. to convert at least a portion oi' said alkali metal nitrate to the corresponding alkali metal nitrite.

10. The process of claim 9 in which the alkali metal nitrate is sodium nitrate.

l1. The process of claim 9 in which ythe alkali metal nitrate is potassium nitrate.

12. In a process for the production of an alkali metal nitrite, the step which comprises contacting molten salt comprising an alkali metal nitrate with a gaseous composition comprising principally hydrogen at reduction conditions including temperatures in the range ot 800 to 900 F. to convert at least a portion of said alkali metal nitrate to the corresponding alkali metal nitrite, the hydrogen oi said gaseous composition being the only material present in said gaseous composition in an amount eiIective to react with a substantial amount oi' said molten salt.

13. The process of converting an alkali metal I 750 F. until atleast a portion ci' said alkali metal nitrate is reduced to the corresponding alkali metal nitrite and free alkali is concomitantly formed in the resulting reduced molten salt, and

contacting said reduced molten salt containing said free alkali with an oxide of nitrogen to decrease the free alkali content of said reduced molten salt.

14. The process of converting an alkali metal nitrate to the corresponding alkali metal nitrite, which comprises reacting molten salt comprising an alkali metal nitrate with hydrogen under reducing conditions including temperatures above 750 F. until at least a portion of said alkali metal nitrate is reduced to' the corresponding alkali metal nitrite and free alkali is concomitantly formed in the resulting reduced molten salt, the free alkali content of said reduced molten salt being more than 2 per cent. and contacting said reduced molten salt containing said free alkali with an oxide of nitrogen' to decrease the free alkali content of said reduced molten salt to less than 2 per cent.

15. The process of converting an alkali metal nitrate to the corresponding alkali metal nitrite, which comprises passing molten salt comprising an alkali metal nitrate in one direction through a plurality of contacting stages and maintaining the temperature of said molten salt in each stage within the range of 750 to 950 F., passing a stream of hydrogen in the opposite direction through said plurality of contacting stages in intimate contact with said molten salt, tosconvert at least a substantial portion of said alkali metal nitrate to the corresponding alkali metal nitrite.

JOHN R. BATES.

REFERENCES CITED The followingA references are of record in the le of this patent:

, UNITED STATES PATENTS Number Name Date 2,294,374 Bates Sept. l, 1942