CA1112843A - Process for converting ammonium sulfite to ammonium sulfate - Google Patents

Abstract

Abstract A process for producing ammonium sulfate from ammonium sulfite by oxidation of an ammonium sulfite solution having a pH of between about 6 and about 7.5, at a temperature between about 190°F, and below about 300°F.
and a pressure between about 200 psig and about 300 psig, with oxygen or oxygen containing gas.

Description

PROCESS ~OR CON~r~RTIN(l A~ONIII~
SULFITE TO ~`l~IONIIl~l SUL~ATE

Background Of The Invention The present invent;on relates to a nel~ and improved process for producing ammonium sulfate from ammonium sul-fite.
~lore particularly, the present invention relates to a direct oxidation process for oxidizing ~queous solutions of ammonium sulfite, formed by absorption of sulfur dioxîde in a(iueous ammonia solutions, to produce a solution of ammonium sulfate, from which the sulfate may then be recovered as a crystallize~
product.
In many chemical and metallurgical processes such as, for example, in the recovery of molybdenum by roasting molybdenum disulfide ore, sulfur dioxide gas is generated and must be collected or recovered, not only for pollution and ecological reasons, but also because the product can be utilized for producing valuable by-products. In conventional processes, i this`gas is remo~ed from the flue gas produced in the ore roasting operation by scru~bin~ the flue gas with an aqueous ammonia solution, The sulfur dioxide is recovered in the orm of an ammonium sulfite liquor, and sufficient a~monia is utilized to produce an absorber discharge solution containing essentially ammonium bisulfite~
Efforts have been made to recover ammonium sulfate from ammonium sulfite absorber discharge solutions. Among the known processes are the direct oxidation o~ the sulfite to tlle sulfate by means o an oxygen bearing gas such as air. The ammonium sulfate is then recovered as a crystallized product. In U.S.
Pat No. 2,810,627, issue~ Oct. 22, 1957, to H F Johnstone et al. or "Recovery of Sulfur Dioxide From Gases and Production

-2-of Ammonium Sulfate," an ammonillm sulfite--ammonium bisulfite solution is oxidized with air at a temperature of about 86C. and at atmospheric pressure. At the same time, sulfur dioxide is produced which is stripped from the solution and recovered or utilized in other processes. In U.S. Pat. No.

3,330,620, issued July ll, 1967, to A. Vian-Ortuno et al.
for "Method For Oxidizing Ammonium Sulfite To Ammonium Sulfate," an aqueous ammonium sulfite solution is oxidized by contacting it with an oxygen containing gas in the presence of a dispersion of a water immiscihle nitrogen containing organic base, such as the various amines and pyridic bases.
The oxidation reaction is carried out at a te~perature o-f between 40C. and 140C., and at an absolute pressure o~ bet~een 1 and 11 atmospheres. The ammonium sulfate thus produced is recovered as a crystallized product. Broadly speaking, the oxidation reaction of ammonium sulfite with oxy~en to produce ammonium sulfate is old and well known, See U.S. Pat. No.
1,106,919, issued Aug. 11, 1914 to C. Bosch for "Producing Ammonium Sulfate."

Objects Of The Invention The principal object of the present invention is to provide an improved process for the production of ammonium sulfate by dir~ct oxidation of an ammonium sulfite liquor with oxygen or an oxygen containing gas such as air, Another object of the invention is to provide a process of the foregoing character in which the production of elemental sulfur is minimized and in which substantially ; one mole Or sulfate is produced for each mole of sulfite in the liquor, A further object of the invention is to provide a new and improved process for production of ammonium sulfate by direct air oxidation of ammonium sulfite without the use of catalysts, reaction p~omoting agents, or the like.
- 15 Still another object of the invention is to con~rert ammonium sulfite in an aqueous liquor substantially completcly to ammonium sulfate, in a sinmple and economical manner.
Other objects and advantages of the present invention will become apparent as the following descriptlon proceeds.

SUM~ARY OF T~IE INVENTION
The process of the present invention involves the direct oxidation of ammonium sulfite liquor with air or other oxygen bearing gas. The oxidation reaction is carried out with the sulfite liquor at a p~ of between about 6 and about 7.5, and a specific gravity of about 14Be' to about 26Be'. However, th~ process is not restricted to this exact specifîc gravity range. A lower or higher range may be used, but the range listed is most conductive for the process. It is also possible to make a similar reaction take place, using a lower feed liquor pH, and a higher oxygen over pressure, with the consequential formation of sulfuric acid and ammonium sulfite ((NH4)2SO3). With the addition of ammonia to the system, ammonium sulfate can be produced.
The liquor is maintained under agitation and at a temperature of about ~ 190F. (88C) and below about 300F. (150C), and preferably between about 220F. (104C) and about 275F.
(135C). The system is maintained under a pressure of about 200 psig (1.379 ~ 106 N/m2) to about 300 psig (2.068 X 106 N/m2). The process is continuous, with ammonium sulfate rich liquor being withdrawn for cyrstallization of the ammonium sulfate. The conversion to the desired product is in excess of 99%. The ammonium sulfate is useful for a variety of applications including extensive use of a fertilizer, either alone or in an admixture with other fertilizing agents such as ferrous sulfate.
In one particular aspect the present invention provides in a process for the production of ammonium sulfate comprising the steps of washing a waste gas containing sulfur oxides with water and aqueous ammonia at substantia~ly atmospher;c pressure to absorb sulfur oxides therein and form an aqueous solution having a pH in the range of 5.5 to 6.5 comprising jl/ - -5--ammonium sulfit~, ammonium bisulfite, and ammoni-ll,l s~llfate, the improvement which comprises ad~usting the p~ of said solution to a value in the range of between about 6.6 and 7.5 by the addition of aqueous ammonifl in a first reaction zone effecting conversion of ammonium bisulfite in said solution to ammonium sulfite; thereafter increasing the pressure of said solution and continuously supplying said solution to a pressurized reaction zone maintained at a pressure in the range of 200 to 300 psig; continuously intimately contacting said solution at said pressure and pH
and~at~a temperature in the range of 190F to 275F with oxygen~containing gas in a high liquid-gas interface relationship effecting oxidation of ammonium sulfite and substantially complete conversion of ammonium sulfite in - said solution to ammonium sulfate with the formation of solid ammonium sulfate without the formation of by-product free sulfur; continuously discharging oxygen depleted gas `~
from said pressurized reaction zone into said first reaction zone; continuously withdrawing an aqueous solution of ammonium sulfate together with solid ammonium sulfate from said pressurized reaction zone; and recovering solid ammonium sulfate as the product of the process.
DRAWINGS
Figure 1 is a schematic flow diagram illustrating the process of the present invention.
Figure 2 is a vertical section view showing an autoclave with an oxygen dispersing shaft and agitator therein.

j1~ J~ Sa-..
'~ ' r~ ~ 3 Dcscription Of I'hc Invention Referring to ~IG. 1 of the dralling, gases colltaining sulur dioxide produced by the roasti,ng oE sulfide orcs, such as molybdenum disul:fide ores, are removed from a roaster 10, through conduits 11, by means of an appropriate suction fan 12, and solid parti.cles are removed from the gas in a cyclone separator 14. The roaster gases are cooled in a heat cxchanger 15 and passed through a venturi scrilblcr 16 and separator 1~ to an ammonia absorbtion tower 19. In the absor~er, sul-fur di.oxide is re]novecl from the gas stream by contact, in three stages, with water and aqueous ammoni~. In the absorber 19, as shown, water is fed through a water line 20 to the'upper or third stage 21, while 29% aqueous ammonia is fed, through a feed,line 22~ to the second stage 24 and the first stage 25 of the absorber 19. Spent gases~ from which the sulfur dioxide has been removed, are pulled out of the top of the absorber 19 through a conduit 26, a separator and demister 29, and then through a conduit 30, by a fan 28.
The wet gas then flo~s through a wet electrostatic precipitator 31 and thence are dischargecl to the atmosphere throu~h an exhaust stack 32.
In the absorbèr 19, the sulfur dioxide reacts with thc ammoniacal solution to ~roduce an absorber discharge li~uor which has a pH of about 5.5 to 6.5 and specific gravity : 25 of about 1.16 to about 1.20 ~about 19 to 23~e'). The liquor includes ammonium sulfite, ammonium bisulfite and some ammonium sulfate, in approximately the following proportions:
~+
. NH ElS~3~ 5~+
(NH ) SO4-----~ -- 5~+

The above ratios can be changed by adding more or less ammonia to the scrubbing liquor.
The absorber discharge liquor is collected in a doctoring tank 34, to which water and additional aqueous ammonia are added in order to adjust the pH of the solution to between about 6 and about 7.5, and,preferably between 7.0 and 7.~, thereby producing a liquor in which all the contained sulfur is substantially in the form of ammonium sulite. The ammonium sulfite solution thus produced is then treated in accordance with the present invention by reaction with oxygen, utilizing compressed air as the oxygen bearing gas, to produce an ammonium sulfate solution.
For producing ammonium sulfate, the ammonium sulfite absorber discharge liquor is pumped by a pump 35, from the doctoring tank 34 to a filter 36. Solids from the filter are recycled, while the liquor is pumped through a conduit 38, to the first of a series of three pressure oxidation vessels or autoclaves 39, 40, 41. In each of the pressure vessels or autoclaves 39, 40, 41, the liquor is agitated ~y an agitator 44, and is simultaneously contacted with compressed air, oxygen or other oxygen bearing gas. The flow of air is countercurren*
so that oxygen rich air flows into reactor 41, fr~m there into reactor 40, and finally into reactor 39. The reaction ideally would be two-thirds con~ersion in reactor 39, two-thirds conversion of the remainder in reactor 40,' and finally nearly complete conversion in reactor 41~ No definite reaction cycle is required, ho~ever. Alternatively, the air, oxygen or oxygen bearing gas may be fed to each vessel through a feed line 45 from a'compressed gas storage tank 46. The compressed air storage tank 46 is supplied with compressed air from an air compressor 48 having an air inlet line 49.

lZ~3~3 In each autoclave 39, 40, ~1, an effort is made to maximize the gas-liquid interface by utilizing a high speed agitator 44, ~hich transmitts a p~ripheral velocity of about 175 in/sec. (445 cm./sec.~ to the agitator head.
The pressure in each vessel îs maintained at about 300 p5ig.
~2.068 X 10 N/m2), and the vessels are maintained at a temperature of above about 200F. and below 300F. (~0 and 150C.), and preferably between 220 an~T 275F. (104 and ]35C.).
The governing reaction in the process is represented by the following equation:
(NH4)2SO3~02 (Air) 300 psi 2~N~4)2SO4 ~ 65 Kcal/mol.

Below about 200~F. (90C.~ the oxidation reaction does not proceed~ while at or abo~e about 300F. (150C.~ the partia]
pressures of oxygen, nitrogen, sulfur dioxide, ammonia and lS water vapor aTe sufficient to prohibit the addition of sufficient air at an operating pressure of 300 psi (2.06~ X 10S N/m2~, with the result that the ammonium sulfite decomposes to produce ammonium bisulfite and ammonia according to the following TeaCtiOn: -(NH4)2SO3 - ~ NH4HS03 ~ NH
and the ammonium bisulfite reacts with further ammonium sulfite to produce ammonium sulfate, water and sulfur, according to the equation:
2NH4HSo3 + (~l~4)2 3 > 2(~TH4)2S~ ~ S + H2O
The oxidation reaction from ammonium sulfite to ammonium sulfate is exothermic and, as indicated above, the heat of reaction is about 65 kilocalories per mole. It has been observe that, at a four molar concentration of the feed liquor, the reaction will be well self-sustaining and, in fact, excess heat ~2a43 energy must be rcmoved from the reac~ion vessels. Each of the vessels is accordingly pro~ided with a mcans for coolin~ the vessel. Ammoniu3n sulfate liquol is discharged from the thi7d rcaction vessel 41 through a filtcr 50 and directed throug}l an cx~it conduit 51 to a crystallizer or feed storage. Oxygen d~plgted air, and any other dissociated gases a-rc withdraw from the first reactor vessel 39 in the series, through a line 52, and may be aspiratccl through the liquicl in the doctoring tank 34 before discharge to the atrnosphere The purpose of this is to remove c,O2 or NH3 gases, whicheYer the casc m~y be. Al~ernatively, instead of utilizing countercurrent air flow, -~resh air, under ~ressure, may be allowed to enter each autoclave separatel~ ith each autoclave being vented scparately.
r:ach autoclave would, ho~ever, be maintained at a pressllre of between 200 and 300 psi an~ a reaction temperature of bet~;Qen 2~0F. and 275F.
An illustrative pressure vessel or autoclave with a high speed agitator for thoroughly intermixing oxygen with the ammonium sulfite liquor is shol~n in FIG. 2. The autoclave or pressure vessel 60 there shown is illustrative of the type of pressure vessel 39, 40, 41, utilized in the above-described process, The autoclave or pressure vessel 60 com-prises a tan~ or vessel 61 having a removable cover 62 secured thcreto by any appropriate means such as bolts 64, and a suitable water cooling system (not shol~n). A sealing gas~et 65~ is provided bet~een the co~er and the flanged rim 66, of the ~Tessel 61. Ammonium sul-fite liquor 68 is introduced into the interior o-f the vcssel 61 to a desired level 69 through an inlet line 70 which extends through the side wall 61 and terminates flush with the interior wall surface. Liquor is discharged frelll the interior of the vessel through an outlet line 72 extendi.ng throug}l the wall of the vessel 61 which determines the level 69 of the liquor. This is commonly referred to as a cascade system.
For axitating thc hody of liquid 68, a high speed agitator 75 is provided. The agitator comprises a vertical hollow agitator shaft 76 rotatably supForted in a bearing 78 in t}-e cover 62 and drivingly coupled to a power source such as a motor 79 mounted on the cover or vessel. At its lowcr end adjacent the bottom o-f the vessel 61, the agitator sh~lft 76 is provided ~ith four paddles 80, of a design suitablc for - providing a high shear. Oxygen to be reacted with the ammonium sulfite liquor in the vessel 61 is introduced into the vessel through an appropriate inlet conduit 81 which extends through the cover 62. In or.der to introduce the oxygen below the surface of the liquor, and to maximize the gas liquid interace, the hollo~J agitator shaft 76 is providcd with a plurality of transverse inlet ports 82 above the licLuid level ; 69, opening into a longitudinal bore 84, extending through the length o the shaf~ 76 to the impeller end where the longitudinal ~ bore 84 contacts the liquid through transverse outlet ports 85.
; As the agitator is rotated at a high speed by the motor 79, gas from above the liquid levcl 69 is pulled into the inlet ports 82 and downwardly through the agitator shaft passage 84 and out~ardly through the transverse outlet ports 85 by centrifugal force. As thc gas is pullcd outward].y into the path of the impellor7 the gas particles are -finely divided and dispersed throughout the liquor, in the form of minute droplets, thereby ma~imizing the gas-liquid inter-face. At the same time7 some gas is pulled into the licLuor at the upper level 69 thereof due to the rapid 34~

stirring action of the agitator. The gas above the liquid level is maintained under a pressure of 200 to 300 psi in order to assure a significant partial pressure of oxygen, to promote the oxidation reaction~ and to oxidize the sulfite to sulfate and minimize the formation of sulfur. Baffles 86 equally spaced around the interior of the vessel further increase the turbulence of the liquid and thereby enhance the liquid-gas mixing effect.
Table 1 presents twenty-one illustrative examples o~
the present invention. Each example is carried out in a series of three autoclave reactors, as described above, under the tempera-ture and pressure conditions stated. As sho~ in Table 1, the autoclave reactor pressures were varied from 200 to 300 psig ~1.379 X 106 N/m2 to 2.068 X 106 N/m2) and the temperatures in the reactors were varied from 190 to 370F. (88 to 188~C.).
The solution feed rate through the system varied from 4.7 to 10 liters per hour while the air draw off rate raried from 9.5 to 30 liters per minute. The pH of the feed was varied from 5.7 to 7.5, while the specific gra~ity of the feed was varied from 14 to 26Bel. The product conversion percentage in each reactor and the percent residual oxygen was determined for most examples and is presented in Table 1.

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r. ~ o~ o r-l -~ ~ r~ ~ r.l r - - - - ~ - - - c~ ~ u 2~3 Reerring to Table 1, examples 1 through 8 demonstrate the effect of variation in pressure from 200 to 300 psig. It can bc noted that, at the higher pressures, the ~ercen. conversion was increased and the percent residual oxygen decreased. Exampl~s 9 through 12 demonstrate the effect of using reaction temperatures of 300F and above. In each oE these examples, sulfur ~as formed, and in examples 9 and 12 the amount of sulfur formed ~as sufficient to preclude computation of the percent conversion to am~lonium sulfate. These examples demonstrate that the temperature must be maintained below 300F. The remaining examples demonstrate that maximum results are achieved ~hen the temperature in the autocla~e reactors does not exceed about Examples 15 and 16 demonstrate that a lo~ p~l also results in the production of sulfur. The density of the solution should range betwecn 1.16 and 1.19 (1923~e') and should be in balance with the system. If too higil a density is used, the con~erted solution may salt out or precipitate~ and if too low a density is used more energy is required -for crystallization.
The prescnt invention as herein descri~ed provides an improved process for the production of ammonium sulfate by direct oxidation of an ammonium sulfite liquor with oYygen or an oxygen containing gas such as air. In this process, the production of elemental sulEur is minimized and substantially one mole of sulfate is produced for each mole of sulfite in the liquor. Ammonium sulfate is I)roduced by direct air oxidation of ammonillm sul-~ite in an aqueous liquor, ~ithout the usc of l'i -~t284~ ç

catalysts, reacti.on promoting agents, or the like, sllb-stantially completely to ammonium sulfate, in a simple and economical manner.
l~hile a certain il.lustrative process embodying the present invention has been described above in considera~le detail, ic should be wlderstood that there is no intention to limit the invention to the specific form disclosed. On the contrary, the intention is to cover all modificacions, alternat;.ves, equivalents and uses falling within the spirit and scope of the inventi.on as expressed in the appended cla;.ms, `W~ ;.J~I ~a~ involl~ien:

Claims (5)

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

1. In a process for the production of ammonium sulfate compris-ing the steps of washing a waste gas containing sulfur oxides with water and aqueous ammonia at substantially atmospheric pressure to absorb sulfur oxides therein and form an aqueous solution having a pH in the range of 5.5 to 6.5 comprising ammonium sulfite, ammonium bisulfite, and ammonium sulfate, the improvement which comprises adjusting the pH of said solution to a value in the range of between about 6.6 and 7.5 by the addition of aqueous ammonia in a first re-action zone effecting conversion of ammonium bisulfite in said so-lution to ammonium sulfite; thereafter increasing the pressure of said solution and continuously supplying said solution to a pres-surized reaction zone maintained at a pressure in the range of 200 to 300 psig; continuously intimately contacting said solution at said pressure and pH and at a temperature in the range of 190°F to 275°F with oxygen-containing gas in a high liquid-gas interface re-lationship effecting oxidation of ammonium sulfite and substantially complete conversion of ammonium sulfite in said solution to ammonium sulfate with the formation of solid ammonium sulfate without the formation of by-product free sulfur; continuously discharging oxygen depleted gas from said pressurized reaction zone into said first reaction zone; continuously withdrawing an aqueous solution of ammo-nium sulfate together with solid ammonium sulfate from said pressur-ized reaction zone; and recovering solid ammonium sulfate as the product of the process.

2. A process as claimed in Claim 1 wherein the reaction is carried out at a pressure of 300 psig.

3. A process according to Claim 1 wherein the temperature is maintained within the range of 220 to 275°F.

4. A process according to Claim 1 wherein the pH is maintained within the range of 7.0 to 7.2.

5. A process as claimed in Claim 1 wherein the depleted oxygen bearing gas issuing from the pressure reaction vessels is passed through the ammonium sulfite solution during the pH adjustment thereof before being discharged to the atmosphere.