CA1154232A - Nitric acid reconstitution - Google Patents

Abstract

NITRIC ACID RECONSTITUTION
ABSTRACT OF THE DISCLOSURE

There is described a process for the recon-stitution of NOX gases to nitric acid comprising the steps of:
1. contacting the NOX gases in counter-current relationship in one or more packed columns with cooled 50 to 60 percent nitric acid to remove as nitric acid a major portion of the originally introduced NOX values and provide an acid solution leaving the column having a temperature below about 180°F;
2. compressing the residual gases from step 1 to from about 2 to about 6 atmospheres absolute;
and 3. contacting the compressed gases from step 2 with from about 50 to about 60 percent nitric acid in a packed absorption column in counter-current relationship to remove substantially all of the remaining NOX values from the gas stream.

Description

Field of l:he Inventlon:
The present invention relates to a process -for recovering and reconstituting nitric acid in a nitric acid process for the e~traction of alumina values from clay and more specifically to an irnpro~ed method for reconstituting NOx gases produced in such a process.
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L~ O , ~ h ` ~ C n ~ i 7 : .
In order to provide an economical'y useful nitric acid extraction process for alumina, the nitric acid must be recovered from the total process in sufficient quantity as to provide a high percentage of acid recirculation. It is known in the art that substantial nitric acid may be recovered by direct condel-sation of HN03 in the decomposition $tep of such process. Such recovery recycles about 67% of the nitric acid used in the process. ~lowever, substantial amounts o~ the acid exits other stacJes of the decomposition in the form of NOx gases.
The recovery of nitric acid solutions from nitrous gases produced by the catalytic combustion of . ammonia in air is a well-~nown art that is practic~ed commercially around the world. The basic process comprises contacting the ammonia oxidation yases at a pressure of 3 to 6 atmospheres absol~te, or even higher, in a bubble cap-tray absorption columll con-taining of the order of a hundred trays in counter-current relationship with a supply of water introduced at the top of the column. Variations of the technoloqy are concerned substantially with design of -the bubble cap trays, oxidation of the ammonia under pressure, or oxidation at ;bout atmospheric pressure to reduce catalyst consumption followed by the compression of the cool gases, the recovery and re-use of the heat produced in the ammonia oxidation reaction, and . . . ' ~..

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especially recently methods of reducing the approxi matel~ 1,000 ppm NOx concentration in the tail gas before releasing this gas to the atmosphere.
rrhe chemistry o~ the conversion of NOx gases to nitric acid in solution is yenerally consi.dered as consisting of ~ overall reactions which ser~e to de~ine the mass balance between the liquid and vapor streams. Reaction 1: 3No~(g? ~ H20(1)~ a~
2HNO3(aq) ~ NO(g~ which oacurs principally in the liquid phase and reaction 2: 2NO(g) ~ 02;(g) - 2NQ~(g) which occurs substantially in the gas phase. The'rate of reaction 1 is thought to depend primarily upon the rate of absorption o~ NO2 into the liquid~stream~which~
depends~upon the partial pressure~o~ NO2~i~n ~he gas~ ~
stream~and is thus slowed down by the presence of lar~e quantities of inert~gase;s such as N2, and~concen-trations of NO in the gas phase~which t~nd to drive~
reaction 1 in the~reverse~direction~ Once the NO2~has een absorbed the reacti~ons in the liquid phase appea~r to proceed at satis~ac~ory velocities. '~he rate o reaction 2~is proportiol1al ~o the product o$ the s~qu~ar~;
o~ the~pa~rtial pre~ss~ure oE NO and the part;ial pressure~
f 2 and~can~be quite slow in ~the presence o~lar~e amounts of~inert, gases,s;uch;as N2. In the~ammoni~a oxidation process f'or makitlg nitric acid the'feed gases rom the;~oxidizer~comprise on the order of 70 volume percen~ N2~and the~proportion~of N2 increases as the~
NOx gases~are absorbed from the gaseous stream.
Additiona1~N2;is added~with air~to prov1de~some oxygen in the tail gas to;drive reaction 2 toward~completiol1.
Thus, altho'ugh nitric acid has been reaovered ~rom ammonia;oxidation gase6 at about atmospheric~pressure using 2 or 3 absorption towers in series it has been found more economi~al to compress the gases~to 3 to 6 ;
atmospheres absolute so as to increase the partial pressure of the reac~ting gases sufEiciently to pe~rmit ,. ;

.. ' ~L5~ 32 carrying out the reconstitution in a single tall column.
On the other hand NOX gases produced by the thermal decom-position of aluminum nitrate material in properly constuc-ted, indirectly-heated decomposers contain little or no inert gases. A typical composition of such a gas, before any air in-leakage, is about 25 volume percent f (NO2 + NO), about 12 1/2 volume percent 2 and about 62 1/2 volume percent water vapor.
Since in the absorption column water vapor is absorbed in the liquid stream much more rapidly than NO2 the concentrations of the reacting gases increase during passage through the absorption column so that the same or even higher rates of Reactions 1 and 2 may be achieved at near atmospheric pres-sure as can be achieved with ammonia oxidation gases at elevated pressures.
Since both Reactions 1 and 2 are highly exothermic and the easily reversible Reaction 1 can begin converting HNO3 from the acid solution to NO2 in the yas at temperatures as low as 150 to 180F, depending upon the concentration of HNO3 in the liquid and of NO in the gas phase, the removal of heat from the absorption column is of major importance. It is known in the art to remove this heat either by placing water-cooled cooling coils in the liquid layer maintained on the upper side of the bubble cap trays or to withdraw a por-tion of the liquid from each of a number of trays in the column, pass the liquor through individual heat exchangers, and return it to the column after cooling. Plants handliny ammonia oxidation gas typically provide sufficient cooling to the column by one or the other means so that -the strong acid exiting the column is cooler than about 120F, or even lower depending upon the strength of the nitric acid that is being manufactured.

In contrast to the recovery of nitric acid - - . ?

from ammonia oxidation gases there has been very little - need around the world to recover nitric acid from eoncertrated NOX streams such as that described above for the decomposition of aluminum nitrate materials~
Aeeording to the invention, a process for the reconstitution of NQX gases to nitrie aeid eomprises:
a. contacting the NOX gases in a range from about atmospherie to about 50 inches of water negative pressure in countereurrent relationship in one or more paeked eolumns with 50 to 60 pereent nitrie acid solution cooled suffieiently to provide aeid solution exiting said one or more eolumns at a temperature below about 180F to remove as nitrie aeid a major portion of the originally introdueed NOX values;
b. eompressing the residual gases from step a to at least about 2 atmosphere absolute;
c. contaeting the compressed gases from step b, in a pressurized eolumn with from about 50 to about 60 nitrie aeid to remove the residual NOX values from the gas; and d. reeovering the nitrie aeid as it accumulates.
For example, the NOX recovery process of the present invention comprises eontacting NOX gases eontaining at most relatively small proportions of inert diluant gases such as N2 in eounter-eurrent relation with a cooled 50-60% nitrie acid solution in~one or more packed eolumns operating at about or slightly below atmospheric pressure to remove as nitric acid a major portion, preferably 90% or more, of the originally-introduced NOX values, eompressing the residual depleted gases to 2 to 6 or more atmospheres absolute and eontaeting the eompressed gases in eounter-eurrent relation with the nitrie acid solution in a paeked tower to strip substantially all of the remaining nitric acid values from the gas stream.

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Detailed Description:
According to the present invention, the reconstitution of such NOX gases to nitric acid is most efficiently performed as follows:
NOX gases 7 produced by thermal decompositon of aluminum nitrate materials and consisting essentially of NOX, 2' and water vapor and possibly small amounts of N2 from air in-leakage, are partially cooled and absorbed in recircuIating cooled 50 to 60~ nitric acid solution from a common receiver tank in one or more packed absorption towers. Nitric acid gas is contacted in a first packed absorption tower operating around atmospheric pressure to 5-50 inches water column negative pressure in counter-current relationship to an amount of cooled nitric acid solution sufficient to . .

keep the temperature of the acid solu-tion leaviny the column at below about 180F, the unabsorbed gases ex.itiny the tower are co~pressed to at least about ~0 psig and preferably within a range of about ~0 to about 100 psig and contacted in a second, pressurized packed absorption colulan in counter-current relationship ~ith a quantity of the cooled 50-60% nitric acid solution sufficient to maintain the temperature of the liquor leaving the secorld coluMn below about 150~F, preferably below about 130F, and residual gases ~rom the second tower are passed through a small absorber in countercurrent relationship to a flow of a small amount o~ water to absorb excess HC1 gases and then passed to suitable tai]. gas NOX recovery or destruction means before venting to the atmosphere~
Before contacting the gases in the said second, pressuri~ed packed absorption colllmn the NO~ gases are blended with sufficient air to provide about 2-10 percent or more O~ in the tail gas. This air may be introduced at any convenient location. upstream of the second absorption column and is pre~erably introduced upstream of the first packed absorption tower.
The nitric acid solution is maintained at about the acid concentration required for ex'craction, i.e~ within the range of about S0 to 60~ acid, usually about 54 to 58%, and is supplied from one or ~nore surge tanks, as may be desired, through heat exchangers to the individual packed columns at rates to each columll such that the acid solution leaving the colurnn is less than a~out 180F in temperature, prererably less tharl about 150-Fo Liquid draining from the to~;1ers is collected in the surge tank for recirculation and the excess is drawn off as product acid for use, for instance, for the di~estion of alumina frorn calcined clay for the manufacture of aluminum nitrate in a ,_ ~ . . ~

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process for the recovery of metallurgical grade alumina from clay. More specifically, accordin$ to a pre~erred method, concentrated NX gases ~rom aluminum nitrate decomposers are mixed with hot make-up NOx qases from an NH3 oxidizer, pass through a waste heat boiler, wherein re-usable heat is extracted, and then blended with vent gases, and with air introduced through a flow control.valve that is responsive to an o~ygen meter.
The mixed gases pass in to an open spray tos~er where they are counter-currently contacted with acid to remove a portion of the contained heat, water vapor, and N02 and then pass sequentially through one or more packed towers, in which they are counter-currently - contacted with acid which has been cooled in a heat exchanger, and ti-en compressed in a compressor to 2 to 6 or more atmospher~s absolute pressure be~ore passing through another packed tower in counter-current contact with cooled acid. The vapors are then passed throuclll a bubble cap tower where they are contacted counter-currently with water to absorb I~Cl values that may be present and the remaining gases pass throuc3h an absorber for stripping out any residual NOx before . exhausting to the a~mosphere. Nitric acid solutio.
draining from all of the towers is colleeted il~ a tank, in which the acid concentration is control}.ed to ~elow 60~ or preferably below about 58~, by means of the addit.i.on of relatively strong acid from the aoresaid acid and heat recovery operations.
The reaction towers are packed absorption columns wherein the packing may be any desired cornmercially available packing material which preferahly has a large void volume per unit of surface area such as is true of Raschig rings. The large void volurhe of, for instance, Raschig rings minimizes;
the velocity o flow of the gas through the packing, thereby providing gas residence time or the relatively ,.
.

~ 2,32 slow Reaction 2 to proceed~ This reaction tlme would have to be provided by increasing the height of the tower if packin~ with a lower vold volume were provided. S~ch rings also simultaneously provide;a large gas-liquid contact area which in well known manner facilitates the absorption of N02 into the liquid and desorption of the reaction product NO from the li~uid.
Void volumes for a number of packing material~ available arè in "Chemical Engineers Handbook" Fi~th Edition edited by Perry and Chilton -McGraw-Hill Publishing Company, New York,~N.Y.;~Section 18: Gas-Liqu~id Contacts. This section also discusses the relative efficiencies~ o~ various packing materials for a~sorption of gases in~to liquids and pres~ent~s methods of estima~ing absorption rates, h~eat-trans~er rates, pressure drops, etc. Data also may be obtained from packing manu~acturers and ~rom other ~ell known pu~lish~ing sour~ces. ~
~ s mentioned hereinabove, plate-type a~sorption columns g~enerally;are used for absorp~io~n of NOx from gas:es~produced by~oxidation o NH~3~wit~h~air ~o obtain the~maximum~possible void ~olume, and ga~s residence~time wh~erein Reaction 2 mcty proceed substanti~ally to~comple~t~ion. For the concentrated gases addressed;herein, however, an even more important requirement~is the~abstraction of sensible~heat~ from~
the gas~phase produ~ed therein by the exothermic Reaction 2,~ whereby the temperature rise of~the~gas ;~
phase, with~the attendant rap~id decrease in the rate of Reaction 2, is mini~,lzed. Packed to~ers are much more efficient for the removal or this sensible heat than tray-type towers, and~in addition eva~oration of water and acid from the myriads of small droplets dispersed in the gas phase further assists in minimizing the gas temperature rise thereby permittiny use of much smaller , ,. :

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eq~ipment than would be possible with tray-towers.
The following e~amples are intended to better describe and more clearly point out the advantages and preferred manipulative steps of eacll of the steps of the process of the instar,t invention.
The following modified technique might be applied in the processes of the following examples as :EQ1.10WS:
Concentrated NOx gases from ANW decomposers are mixed ~ith hot make-up NOx gases from an NH3-oxidizer, pass thro~gh a waste heat boiler, wherein re-usable heat is extracted~ and then blended with vent gases and air~ The mixed gases pass in to an open spray tower where they are counter-currently contacted witl acid to remove a portion of the contained heat, water vapor, and N02 and then pass sequentially through two packed towers, in which they are counter-currently contacted with acid which has been cooled, and then compressed in a ccmpLessor to 2 to 6 or more atmospheres absolute pressure before passing through a third packed tower in counter~current contact with cooled acidO The gases are then passed through a bubble cap tower where they are contacted counter-currently with water to absorb HCl values that may be present and the rernaining gases pass through a NOx absorber or stripping out any residual NOx before exhausting to the atmosphere. Nitric acid solution draining from all four of the towers is collected in a tank in which the acid concentration is controlled to below ~0%, or preferabIy below about 58%, by means of the addition of relatively strong acid Example 1 Feed gas having an estimated rate (in pound-moIs per hour) and temperature shown in Column 2 of Ta~le 1 is mixed with makeup gas from an atmospheric .

l~4,)~2 pressure Nll3-oxidation ~nit at a rate shown in Column 3, Table 1 and passed through a waste-heat boiler in which heat is extracted and is blende(1 with air from Column ~, Table 1 to produce an assumed column feed gas as given in Column 5~ Table 1. To better illustrate the simplicity of the absorption process it is assumed that the gas is cooled only to about 250F and that no water is condensed in the heat exchanger, although such is not common practice in the nitric-~acid-from-ammonia industry. It is also thought that, in many instancesj heat exchangers or cooling ancl dewatering the gas may be more ex~ensive than liquid-acid-to-water heat exchangers for removing the same quantity of heat.
';ince the column feed gases will reconstitute to about 64% nitric acid, an unnecessarily-high concen-tration that would increase unduly the difficult~ o~
absorption and reconstitutionJ about 191 tons per hour of about 54~ nitric acid solution are ed to the product holding tank. Such addition alone maintains the concentration in the product tank at about 5~9gO~ a satisfactorily low value. In addition about ~.4 tons of dilution water are added to reduce the mean concen-tration in the tank to about 55~ nitric acid.
Example-2 -The column feed ~as from Example 1 is introduced to a first packed tower, which comprises 160 square feet of internal cross section area f about 14.3 feet inside diarneter, and is packed with 2~ nch metal Raschig rings. The to~er is fed with about 4565 GPM of acid solution which is cooled to about 100F and which is distributed over the packing and drains throuc~h the packing in counter-cu rent relation to the rising feed gas. The liquid absorbs nitric acid and water (and a litt]e NO2) from the feed ga5 and d~ains from the z colwlln at a rate o about ~810 GPM at a temperature of about 175F. At the gas and liquid ratex existing at the base of the packing the pres~ure drop is about 1~5 inches water column per foot of pack.ing, just below flooding conditions, but the absorption rate of NO2, and particularly E~2Or is so rapid that the ~as volume decreases to about l/2 of the initial volume within about a ~oot of effective packing height, whereby the pressure drop is reduced to well below that required ~or ~looding.
At a level in the column corresponding to about 12-l/2 feet of fully-effective packing over 90%
of the ~eed NOX values have been absorbed and the gas flow rate and composition is that given in Column 2, . Table 2u Bo~h reactions ~l) and ~2) are continuing but at reduced ratesO
~ t a column height equal to about 2~ feet of fully-effective packing the gas rate and compositi.on is that given in Column 3r Table 2.

~ample 3 _ The gas from the 12-l/2 ~t. effective ~acking height of Example 2 is compressed to 3.0 atmospheres absolute and fed into a packed absorption column. The packed column has.an inside cross-sectional area of S3.6 square ~eet, inside diameter about 8.26 feet r and is fed with about 1200 ~PM of 55% acid cooled to about 114F which absorbs acid, heat and water vaFor from the gas during counter-current contact in the packing so that the liquid dralning from.the column comprises about 1213 GPM at a temperature of about 130~. The column is packed with 2-inch Raschig rings.
Because of the high partial pressures of bo~h NO and 2 in the compressed gas, NO is oxidized to ~ y Reac~ion 2 at a rate o nearly S pound moles NO
per hour per cubic foot of void volumel with an ~12-attendant high rate of heat release to the ~as~ Since the rate of Reaction 2, which is very important in this column, is inhibited by high gas temperatures it is desirable to use a liq-lid seal of cool acid in the coMpressor, and to make ti~e compressor to column connection relatively short so that oxidation occurs mainly within the coluMn where the gas is cooled by contact with the liquid and liquid sprayn The quantities of N0x remaining, including minor amounts of N203 and N204, and HN03 vapors, at column heights equivalent to the listed packing height in feet of fully effective packing are given in Table 3.
If the system feed gas contains HCl in excess of the small amounts soluble in 55% nitric acid, the off-gas from the selected effective packing height o the column is passed throucJh a ~ist eliminator to another column wherein it is contacted in counter-current relation with 200 to 300 pounds per hour o water, as needed to keep the concentration o~ the wea]c acid exiting below about 25 wt.% total acid, whereby the HCl and HN03 vapors are absorbed along with rninor amounts of the contained N0x values. The absorption, as HN03, of the N0x values is minimiæed by limiting the number of trays and the gas residence time to the minimum values required for proper design for absorblng the very-easily-absorbed HCl. It is known to treat the HCl-contair,ing liquid with 020ne to convert the HCl to C12 gas, which is removed and absorbed in caustic liquor, and return the HCl-free acid to the process.
The tail gas exits the columll thro~gh a mist eliminator, passing a ('2 concentration sensor, to an N0x stripping unit which is preferably a proprietary Pura Siv M unit manufactured for sale to the industry by Union Carbide Corporation, New York, New York, that is known to strip the N0x concentration to S0 ppm or ..... , . . . :

1 3w less and permit recycling of the recovered N0x to the reconstitution system (~SP 3,473,893, 11ardison).
It is a]so known to catalytically reduce the N0x to N2 with ammonia or methane or to absorb the N0x in nitric acid solutions from which all residual N0x has been stripped by treatment with hot air in a stripping column. Acid offered for commercial sale usually has been bleached, that is, stripped in SUC11 -manner .
The stripped tail gas is then exhausted to the atmosphere through, if desiredr power recovery means well known to the industr~0 Example 4 . .
The gas frvm the 24 foot effective packing height of Example 2, listed in CO1Umn 3 0~ Table 2, is compressed to 6.0 atmospheres absolute pressure and contacted in a 12.5 square foot inside area, 4 foot insi~e diameter, column packed with 2 inch Raschic3 rings and supplied as in Example 3 with about 280 GPM
of 55~ acid solution at about 10~~ which, ar~ter absorbing heat, nitric acid and water vapor, exits the column at about 130Fo The N0x and HN03 contents o~
the residual gas at various effective packing heights are listed in Table 4. The gas exiting the column through the mist eliminator is treated as described in Example 3.

Example 5 Examples 2-4 show t~e use of a single atmospheric-pressure absorption column ahea~r of the high-pressure column. r~his example shows the use of a short, large-diameter atmospheric-pressure column for removi~g the bulk o the water from the gases followed by absorption in a taller, smaller-diameter atmospheric pressure column to complete absorption o~ 90% or more ... .. . .

of the NOX in the column feed gas (Table 1, Column 5).
The column feed gas (Table 1, Column 5) is absorbed in a first column of 160 square feet cross-section area packed with 6-1/2 feet effective depth of 2 inch Raschig rings sup-plied with about 4585 GPM of acid liquor cooled to about 100F, yielding a liquid effluent of about 4810 GPM at about 162F.
Residual gas exiting at the top of this column is fed to an atmospheric pressure column which comprises a 7 ft. inside diameter column packed with 2-inch Raschig rings supplied through a sprayer with about 845 GPM of acid cooled to about 100F.
At a column height corresponding to about 15 feet of effective packing height the gas composition and heat con-tent are essentially the same as those obtained in Example 2 for 12-1/2 feet of effective packing (Table 2, Column 2).
Upon comparison of the total volumes of effective packing it is seen that the 2-column combination saves about 400 cubic feet of effective packing volume over the single, large dia-meter column of Example 2.
Similarly it is found that at an effective packing height of about 47 feet in the 7-foot diameter column the gas conditions essentially match those of Table 2, Column 3 for the 24-foot packing height of Example 2. The savings for this variation is about 980 cu. ft. of effective packing volume.

, .

~ATE .- E~OUND MOLS/EIOUR

COMPONE~T FEED GAS MAKE UP BLEED AIR COLUi~ Ei'EhD
NO 817:.3 60.7 - ~ ~ 169~.,S~
-~ N(~2 817. 3 : - _ 1525. 8 :~ 2 81~7~. 3~ ~ 40, 9 67. 8 5~71~. 8 H~O ~ 86. 6 ~10~. 6 2. 7~ 41~89. 9 ~:~ N2 : 0 ~ 452. 9 256. 6 7~09. 5 Temperature:, F ~ :: 600: ~ 1 boo 77 ~; 2~s0 : ~
P r e s s u r e, : ~
Atm. Abs. ~ :O.95 1 1 : ~0.948 T~BL~ 2 ~: : :FLOW -: POUNr~ MO~SfHOUR

GAS ~~ 12~ 2 FT. ~ rr~
:: COMPON13NT :: PA:CKING :: P~ G: : ~: :
NO ~ 2 7 . 5 ~ 4 3 N02:: ; ~ 27. 3: H 10.~
~ N203 ~ : n.21 ~ o.os ~ ~ ~
N~204 ~ 0. 24 ~ 0.~1~7

2 ~ 6 6 . 0 ~ 9 9 ~ 29~o ~ ~ ~ 21 : HN03 ~ 3.:1 ~ 20 3 N2 ~ ~ ~709, 5, ; 70~9~. 5 .
' ,;

.:
.

. ., , : ~-.:: :
c .

RATE - POUND MOLS/HOUR IN GAS

EFFECTIVE LOSS, %
PACKING CONC.OF NOX FEED '$
Ft. NX HNO3NOX, PPM(COLUMN 5, TABLE 1) 19.5 6.12 0.83 7700 0.41 23 5.28 0.83 6700 0.36 33 4.03 0.82 5100 0.29 02 Concentration 8.4 to 8.3 Volume %.

FLOW - POUND MOLS/HOUR
EFFECTIVE
PACKING LOSSI %
HEIGHT C'ONC. OF NOX FEED
Ft. NX HNO3NOX~ PPM(COLUMN 5, TABLE 1) 9 5.53 0.31 7050 0.34

3.29 0.29 ~200 0.21 2.47 0.29 3200 0.16 3~ 1.7~ 0.28 2250 0.12 -$~

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the reconstitution of NOX gases to nitric acid comprising the steps of:
a. contacting the NOX gases in a range from about atmospheric to about 50 inches of water negative pressure in countercurrent relationship in one or more packed columns with 50 to 60 percent nitric acid solution cooled sufficiently to provide acid solution exiting said one or more columns at a temperature below about 180°F to remove as nitric acid a major portion of the originally introduced NOX values;
b. compressing the residual gases from step a to at least about 2 atmosphere absolute;
c. contacting the compressed gases from step b in a pressurized column with from about 50 to about 60%
nitric acid to remove the residual NOX values from the gas; and d. recovering the nitric acid as it accumulates.

2. The process of claim 1 wherein said NOX gases are blended with sufficient air to provide between about 2 and about 10 percent oxygen in the tail gas prior to contacting with nitric acid in step c.

3. The process of claim 1 wherein the compression of step b is within the range of from about 2 to about 6 atmosphere absolute

4. The process of claim 1 wherein the contacting nitric acid of step c is cooled sufficiently that the temperature of the acid exiting the column is below about 150°F.

5. The process of claim 1 wherein step a is operated to remove as nitric acid at least about 90% of the originally introduced NOX values.

6. A process for the reconstitution of NOX gases produced by the thermal decomposition of aluminum nitrate to nitric acid comprising the steps:
a. contacting the NOX gases in a range from about atmospheric to about 50 inches of water negative pressure in countercurrent relationship in one or more packed columns with 50 to 60 percent nitric acid solution cooled sufficiently to provide acid solution exiting said one or more columns at a temperature below about 180°F to remove as nitric acid a major portion of the originally introduced NOX values;
b. compressing the residual gases from step a to at least about 2 atmosphere absolute;
c. contacting the compressed gases from step b in a pressurized column with from about 50 to about 60%
nitric acid to remove the residual NOX values from the gas and d. recovering the nitric acid as it accumulates.