Process for converting ammonium imidodisulfate, ammonium sulfamate and ammonium dithionate to sulfur dioxide and ammonia
The invention concerns a process for converting ammonium imidodisulfate (IMDS) and/or ammonium sulfamate (SMA), in the optional presence of ammonium dithionate (DTN), to sulfur dioxide and ammonia. 
     These salts are recovered, for example, from the simultaneous absorption of nitrogen oxides and sulfur dioxide present in combustion gas, with aqueous solutions comprising ammonium sulfite and an iron chelate, for example the iron chelate of ethylenediaminotetracetic acid. These salts have no important use and their discharge represents an important loss of reactants and creates a pollution problem. 
     It has been already proposed to convert these salts to ammonium sulfate by oxidation and hydrolysis, but the latter material has no substantial market. 
     It would be more beneficial to recover ammonia from these salts and to recycle it to the step of SO 2  and nitrogen oxide absorption. 
     It has been found that these salts can be converted to a gas mixture comprising SO 2  and NH 3  by contacting them with a molten mixture of ammonium bisulfate and sulfate. 
     The SO 2  and NH 3  containing gas mixture may be treated thereafter by any known means to recover, separately or in admixture, the constituents in the form of NH 3 , SO 2  or sulfur. 
     The process comprises contacting at least one of these salts with molten ammonium sulfate and/or bisulfate, at a temperature of 280° to 420° C., in the presence of a sulfur-containing reducing agent, such as sulfur, hydrogen sulfide, ammonium thiosulfate and/or ammonium polysulfide, and/or a catalyst comprising Mo, W, Cu, Fe, Co, Cr, Mn and/or Ni. 
     In the selected temperature range, equilibrium between ammonium sulfate and ammonium bisulfate occurs. It is thus possible, without disadvantage, to prepare the bath from ammonium sulfate, ammonium bisulfate or a mixture of both. 
     In the presence of the above reducing agents, the decomposition of the above salts takes place mainly according to the following reactions: ##STR1## 
     It is easy to deduce the stoichiometrical proportion from the above relations, when using an ammonium polysulfide. 
     It has thus been found that sulfur and nitrogen of these salts are converted respectively to SO 2  and NH 3 . 
     When the ammonium sulfate/bisulfate bath contains a Mo, W, Cu, Fe, Co, Cr, Mn and/or Ni salt, in the absence of reducing agents, the salt decomposition takes place essentially according to the reactions: ##STR2## 
     It is easy to deduce the stoichiometrical proportion from the above relations, when operating with an ammonium polysufide. 
     In that case, nitrogen of the salts is converted to molecular nitrogen and ammonia, while sulfur is converted to SO 2 . 
     When operating in the simultaneous presence of a reducing agent and a catalyst, the salt decomposition takes place according to several of the above reactions in variable proportions depending on the temperature. As a rule, the reactions of reduction with S, H 2  S and (NH 4 ) 2  S 2  O 3  are preponderant at the lower temperatures, about 280° to 350° C., while the decomposition reactions prevail at temperatures above 350° C. It has also been found that the above catalysts substantially increase the reduction rate, particularly when the reduction agent is H 2  S. 
     The process of the invention may be operated at a temperature of from 280° to 420° C., preferably 300° to 380° C. When operating only with a catalyst, it is desirable to proceed in the temperature range of from 350° to 380° C., thereby obtaining high reaction rates. At temperatures lower than 280° C., the reaction rates are low and ammonia is retained in the bath of molten salt by ammonium bisulfate which is converted to ammonium sulfate, which may result in the solidification of the bath. At temperatures above 420° C., ammonium bisulfate decomposes at a too high rate, so that the bath volatilizes. 
     The process is usually operated at a pressure close to atmospheric pressure. A higher pressure may be used, if necessary; however too high pressures may inhibit the ammonia release and result in solidification of the bath. The useful pressure range is from 0.5 to 10 atm., preferably 1 to 5 atm. 
     The process is usually operated by injecting the salts, either individually or in admixture, as such or in aqueous solution, into a reactor containing an ammonium sulfate/bisulfate bath at a temperature of 280° to 420° C. 
     The charge of salts may also comprise, without disadvantage, other ammonium salts, such as ammonium sulfites and sulfates. By adjusting the operating conditions, they are also decomposed to SO 2  and NH 3 . 
     When using a catalyst, the latter may be introduced into the bath as powdered metal, oxide, sulfide or salt such as, for example, Mo, W, Cu, Fe, Co, Cr, Mn and/or Ni sulfate, nitrate or acetate, in a preferred proportion of 0.1 to 10% b.w. of metal. Continuous supply of catalyst is not necessary. 
     When operating without catalyst, the reducing agent is supplied continuously or by fractions in admixture with the feed charge or separately in at least stoichiometrical reaction proportions. When using both a reducing agent and a catalyst, the proportion of reducing agent may be reduced, if desired. 
     Temporary accumulation of the salts to be decomposed, in the ammonium sulfate/bisulfate bath, due for example to a temperature decrease or a too low proportion of reducing agent, does not alter the overall running of the plant. It results in an increase of the bath volume, which can be brought back to its normal value by adjusting the operating conditions. The temporary use of an excess of reducing agent or a too high temperature may result in a decrease of the bath volume due to ammonium sulfate decomposition. The normal volume of the bath may be attained by decreasing the temperature and/or the feed rate of the reducing agent. 
     The gas discharged from the reactor comprises SO 2  and NH 3 , possibly also H 2  O and/or N 2  and, from time to time, low amounts of H 2  S and S from the unconverted reducing agents. When the reactor is heated by direct contact of the salt bath with hot gas from a burner, the latter gas is also present in the effluent gas. 
     The gas mixture may be treated thereafter by any known method to recover, preferably separately, NH 3  and a valuable sulfur compound such as So 2 , H 2  SO 4  or even sulfur. Reference is made to the process of the French Pat. No. 2,199,475 according to which the SO 2  and NH 3  containing gas mixture is admixed with a H 2  and/or CO containing reducing gas and contacted first with a solid catalyst, so as to convert a portion of SO 2  to H 2  S, and then a selective solvent to convert therein H 2  S and SO 2  to elemental sulfur. Ammonia may be recovered thereafter. 
    
    
     The following, non limitative examples, illustrate the invention: 
     EXAMPLE 1 
     400 g of ammonium hydrogen sulfate are introduced into an electrically heated quartz reactor which is thereafter heated to 350° C. 50 cc/hour of an aqueous solution of 1 mole/liter ammonium imidodisulfate and 2 moles/liter ammonium dithionate is pumped into the bath of molten material. 7 g/hour of liquid sulfur is also pumped into the bath. Stirring of the bath is obtained by bubbling therethrough 100 liters of nitrogen per hour and the temperature is maintained at 350° C. 
     Ammonia is analysed in the effluent gas from the reactor by absorption in a sulfuric acid solution which is thereafter analysed back with a titrated sodium hydroxide solution. So 2  is analysed by absorption in a sodium hydroxide solution which is titrated with iodine. 
     The analysis of the gas gives the following results with respect to time. 
     
                       TABLE I                                                     
______________________________________                                    
            SO.sub.2 flow rate                                            
                          NH.sub.3 flow rate                              
Time in hours                                                             
            moles/hour    moles/hour                                      
______________________________________                                    
1           0.385         0.330                                           
2           0.410         0.355                                           
4           0.390         0.352                                           
______________________________________                                    
 
    
     The theoretical flow rates of SO 2  and NH 3  corresponding to the sulfur reduction of ammonium imidodisulfate and dithionate are respectively 0.4 and 0.35 mole/hour. 
     The level of the ammonium sulfate bath does not vary in the course of time and the analysis of the bath at the end of the experiment shows that no accumulation of IMDS and DTN has taken place. 
     It is thus found that the decomposition of IMDS and DTN to SO 2  and NH 3  is practically quantitative. 
     EXAMPLE 2 
     Example 1 is repeated, except that sulfur is replaced with other reducing agents or a catalyst is introduced into the sulfate bath. The following table summarizes the operating conditions and the average SO 2  and NH 3  hourly feed rates, as determined in the effluent gas for 4 hour-experiments. The theoretical rates, also stated, relate to the reduction of the salts, when using reducing agents, or to their decomposition, when using a catalyst (according to the above stated reactions). 
     The results of experiments 1 to 3 show that, in the selected operating conditions, the imidodisulfate and the dithionate are effectively reduced by the reducing compounds: S, (NH 4 ) 2  S 2  O 3  and H 2  S. 
     The results of experiments 4 to 7 relate to the embodiment of the process which is operated in the presence of a catalyst and without reducing agent. They show that the quantitative decomposition of ammonium imidodisulfate and dithionate may be obtained without decomposing the ammonium sulfate of the bath by judiciously selecting the operating temperature. 
     The results of experiments 8 to 10 show that the simultaneous use of a catalyst (Cu) and a reducing agent (H 2  S) increases the imidodisulfate reduction rate and results in partial reduction of the ammonium bisulfate of the bath. A reduction of the H 2  S feed rate suppresses the parasitic reduction of ammonium bisulfate, while maintaining the quantitative reduction of imidodisulfate. 
     
                                           TABLE II                                
__________________________________________________________________________
Salt Injection                                                            
(moles/h)                                                                 
Expe-                                                                     
    Ammon-                                                                
          Ammon-                     SO.sub.2 Feed Rate                   
                                              NH.sub.3 Feed Rate          
ri- ium   ium                        moles/h  moles/h                     
ment                                                                      
    imidodi-                                                              
          dithio-                T bath                                   
                                     Theore-                              
                                          Mea-                            
                                              Theore-                     
                                                   Mea-                   
No. sulfate                                                               
          nate Reducing Agent                                             
                           Catalyst                                       
                                 °C.                               
                                     tical                                
                                          sured                           
                                              tical                       
                                                   sured                  
                                                       Remarks            
__________________________________________________________________________
1   0.05  0.1  Sulfur 7 g/h                                               
                           None  350 0.4  0.395                           
                                              0.35 0.345                  
                                                       The effluent       
                                                       contains sulfur.   
2   0.05  0.1  (NH.sub.4).sub.2 S.sub.2 O.sub.3                           
                      25 g/h                                              
                           None  350 0.569.sup.(1)                        
                                          0.585                           
                                              0.688.sup.(1)               
                                                   0.680                  
                                                       The effluent       
                                                       contains sulfur.   
3   0.05  0.1  H.sub.2 S                                                  
                      3 Nl/h                                              
                           None  350 0.367                                
                                          0.35                            
                                              0.35 0.33                   
                                                       The effluent       
                                                       contains H.sub.2   
                                                       S.                 
4   0.05  0.1  None        CoSO.sub.4 -20 g                               
                                 380 0.300                                
                                          0.285                           
                                              0.217                       
                                                   0.195                  
5   0.05  0.1  None        Mo.sub.2 O.sub.3 -20 g                         
                                 365 0.300                                
                                          0.310                           
                                              0.217                       
                                                   0.225                  
6   0.05  0.1  None        Mo.sub.2 O.sub.3 -20 g                         
                                 380 0.300                                
                                          0.450                           
                                              0.217                       
                                                   0.275                  
                                                       the volume         
                                                       of the bath        
                                                       decreases.         
7   0.05  0.1  None        Mo.sub.2 O.sub.3 -20 g                         
                                 340 0.300                                
                                          0.180                           
                                              0.217                       
                                                   0.174                  
                                                       The volume         
                                                       of the bath        
                                                       increases.         
8   0.15  0    H.sub.2 S                                                  
                      5 Nl/h                                              
                           None  345 0.400.sup.(2)                        
                                          0.415                           
                                              0.450.sup.(2)               
                                                   0.455                  
9   0.15  0    H.sub.2 S                                                  
                      5 Nl/h                                              
                           CuSO.sub.4 -25 g                               
                                 345 0.400.sup.(2)                        
                                          0.650                           
                                              0.450.sup.(2)               
                                                   0.510                  
                                                       The volume         
                                                       of the bath        
                                                       decreases.         
10  0.15  0    H.sub.2 S                                                  
                      2.4 Nl/h                                            
                           CuSO.sub.4 -25 g                               
                                 345 0.400.sup.(2)                        
                                          0.405                           
                                              0.450.sup.(2)               
                                                   0.420                  
11  0     0.2  S      7 g/h                                               
                           None  355 0.500                                
                                          0.510                           
                                              0.400                       
                                                   0.395                  
__________________________________________________________________________
 .sup.(1) The excess of (NH.sub.4).sub.2 S.sub.2 O.sub.3 with respect to  
 the stoichiometrical proportion for reducing the imidodisulfate and the  
 dithionate is supposed to decompose according to the reaction :          
 (NH.sub.4).sub.2 S.sub.2 O.sub.3 →S + SO.sub.2 + 2 NH.sub.3 +     
 H.sub.2 O.                                                               
 .sup.(2) According to the reaction of H.sub.2 S with ammonium            
 imidodisulfate and dithionate.                                           
 
    
     EXAMPLE 3 
     400 g of ammonium hydrogen sulfate are introduced into an electrically heated quartz reactor which is thereafter heated to 350° C. 50 cc/hour of an aqueous solution of 2 moles/liter of ammonium sulfamate is pumped into the bath. 3.5 g/hour of liquid sulfur is also pumped thereinto. Stirring is obtained by bubbling 100 liters nitrogen per hour through the bath and the temperature is maintained at 350° C. 
     The effluent gas has been analysed in the course of time and the following results have been obtained: 
     
                       TABLE III                                                   
______________________________________                                    
            SO.sub.2 flow rate                                            
                          NH.sub.3 flow rate                              
Time in hours                                                             
            moles/hour    moles/hour                                      
______________________________________                                    
1           0.130         0.175                                           
2           0.152         0.195                                           
4           0.145         0.210                                           
______________________________________                                    
 
    
     The theoretical SO 2  and NH 3  flow rates corresponding to the sulfur reduction of the injected ammonium sulfamate are respectively 0.15 and 0.2 mole/hour. 
     The level of the ammonium sulfate bath does not vary in the course of time and the analysis thereof, at the end of the experiment, shows that no sulfamate has accumulated. 
     EXAMPLE 4 
     Example 3 is repeated, except that sulfur is replaced with other reducing agents, or a catalyst is introduced into the sulfate bath. The following Table summarizes the operating conditions and the average flow rates of SO 2  and NH 3  in the effluent gas over 4 hours. 
     The theoretical rates, as shown in the Table, relate to the reduction of the same salts, when using reducing agents, or to their decomposition, when using a catalyst (according to the above reactions). 
     The results of experiments 1 to 3 show that, in the above operating conditions, sulfamate is effectively reduced by the reducing compounds: S, (NH 4 ) 2  S 2  O 3  and H 2  S. 
     Experiments No. 4 and 5 show that the quantitative decomposition of the sulfamate may also be obtained without reducing agent, in the presence of a catalyst. 
     
                                           TABLE IV                                
__________________________________________________________________________
Expe-                                                                     
    Ammonium                     SO.sub.2 Feed Rate                       
                                            NH.sub.3 Feed Rate            
ri- Sulfamate                    moles/h    moles/h                       
ment                                                                      
    Injected                 T bath                                       
                                 Theore-    Theore-                       
No. (moles/h)                                                             
          Reducing Agent                                                  
                      Catalyst                                            
                             °C.                                   
                                 tical                                    
                                      Measured                            
                                            tical                         
                                                 Measured                 
                                                       Remarks            
__________________________________________________________________________
1   0.1   sulfur 3.5 g/h                                                  
                      None   350 0.15 0.144 0.2  0.197 The effluent       
                                                       contains sulfur.   
2   0.15  (NH.sub.4).sub.2 S.sub.2 O.sub.3 20 g/h                         
                      None   350 0.375.sup.(1)                            
                                      0.38  0.6.sup.(1)                   
                                                 0.58  The effluent       
                                                       contains sulfur.   
3   0.15  H.sub.2 S 2.3 Nl/h                                              
                      None   345 0.2  0.19  0.3  0.29  The effluent       
                                                       contains H.sub.2   
                                                       S                  
4   0.15  None        Mo.sub.2 O.sub.3 20 g                               
                             370 0.15 0.16  0.2  0.21                     
5   0.15  None        CuSO.sub.4 25 g                                     
                             380 0.15 0.14  0.2  0.2                      
6   0.15  H.sub.2 S 1.4 Nl/h                                              
                      CuSO.sub.4 25 g                                     
                             345 0.2.sup.(2)                              
                                      0.21  0.3.sup.(2)                   
                                                 0.31                     
__________________________________________________________________________
 .sup.(1) The excess of (NH.sub.4).sub.2 S.sub.2 O.sub.3 with respect to  
 the stoichiometrical proportion for reducing ammonium sulfamate is       
 supposed to decompose according to the reaction: (NH.sub.4).sub.2 S.sub.2
 O.sub.3 →S + SO.sub.2 + 2 NH.sub.3 + H.sub.2 0.                   
 .sup.(2) According to the reaction of H.sub.2 S with ammonium sulfamate.