Patent Publication Number: US-4224058-A

Title: Methods of desulphurizing fluid materials

Description:
This application is a division of our copending application Ser. No. 838,945, filed Oct. 3, 1977 and allowed Jan. 5, 1979, U.S. Pat. No. 4,161,400, which in turn was a continuation-in-part of our then copending application Ser. No. 705,525, filed July 15, 1976, now U.S. Pat. No. 4,084,960, issued Apr. 18, 1978. 
    
    
     This invention relates to methods of desulphurizing fluid materials and particularly to a method of external desulphurizing fluids such as molten iron and steel, stack gases, coal gases, coal liquification products, and the like using rare earth oxides, rare earth fluorocarbonates or rare earth oxyfluorides in an essentially dry process. 
     As we have indicated above this method is adapted to the desulphurization of essentially any fluid material. We shall, however, discuss the method in connection with the two most pressing problems of desulphurization which industry presently faces, i.e. the desulphurization of molten iron and steel baths and the desulphurization of stack gases. 
     External desulphurization of molten iron and steel has been practiced for quite some time. It is a recognized, even necessary practice, in much of the iron and steel produced today. In current practices for desulphurization of iron and steel it is common to add magnesium metal, magcoke, calcium oxide, calcium carbide or mixtures of calcium oxide and calcium carbide as the desulphurizing agent. Unfortunately, there are serious problems, as well as major cost items involved, in the use of all of these materials for desulphurization. Obviously, both CaO and CaC 2  must be stored under dry conditions, since CaO will hydrate and CaC 2  will liberate acetylene on contact with moisture. Magnesium is, of course, highly incendiary and must be carefully stored and handled. There are also further problems associated with the disposal of spent desulphurization slags containing unreacted CaC 2 . 
     We have found that these storage, material handling and disposal problems are markedly reduced by using rare earth oxides in a low oxygen content bath of molten iron or steel. The process is adapted to the desulphurization of pig iron or steel where carbon monoxide, evolved by the reaction, where carbon is used as a deoxidizer, is diluted with an inert gas such as nitrogen or by vacuum degassing the melt in order to reduce the oxygen potential and thereby increase the efficiency of the reaction by reducing the likelihood of forming oxysulfides. The principle may also be used for desulphurizing stack gases from boilers, etc., as we shall discuss in more detail hereafter. 
     In desulphurizing molten iron and steel in the practice of this invention we preferably follow the steps of reacting rare earth oxide, rare earth oxyfluorides, rare earth fluocarbonates and mixtures thereof including bastnasite concentrates in the presence of a deoxidizing agent with the sulphur to be removed to form one of the group consisting of are earth sulphide and are earth oxysulphide and mixtures thereof. 
     Preferably, hot metal is treated in a ladle or transfer car with rare earth oxides, by the simple addition and mixing of the rare earth oxides, by an injection technique in which the rare earth oxides are injected into the molten bath in a carrier gas such as argon or nitrogen or by the use of an &#34;active lining&#34; i.e., a rare earth oxide lining in the vessel. In any case, the chemical reactions involved are: 
     
         2CeO.sub.2 (s)+[C]=Ce.sub.2 O.sub.3 (s)+CO.sub.(g)         ( 1) 
    
     
         RE.sub.2 O.sub.3 (s)+[C]+[S].sub.1w/o =RE.sub.2 O.sub.2 S.sub.(s) +CO.sub.(g)                                               ( 2) and 
    
     
         RE.sub.2 O.sub.2 S.sub.(s) +2[C]+2[S].sub.1w/o =RE.sub.2 S.sub.3(s) +2CO.sub.(g)                                              ( 3) 
    
     The product sulphide or oxysulphide will either be fixed in an `active` lining or removed by flotation and absorbed into the slag cover and vessel lining depending upon the process used for introducing the rare earth oxide. 
     The products of desulphurization of carbon saturated iron with RE oxides is dependent on the partial pressure of CO, pCO, and the Henrian sulphur activity in the metal, h S . Using cerium as the representative rare earth, the following standard free energy changes the equilibrium constants at 1500° C. for different desulphurization reactions can be calculated from thermodynamic data in the literature: 
     
         __________________________________________________________________________
REACTION                  ΔG° cal.                           
                                   K.sub.1773                             
__________________________________________________________________________
2CeO.sub.2(s) + [C] = Ce.sub.2 O.sub.3(s) + CO.sub.(g)                    
                           66000 - 53.16T                                 
                                   pCO = 3041                             
Ce.sub.2 O.sub.3(s) + [C] + [S].sub.1w/o = Ce.sub.2 O.sub.2 S.sub.(s) +   
CO.sub.(g)                 18220 - 26.43T                                 
                                   pCO/h.sub.S = 3395                     
Ce.sub.2 O.sub.2 S.sub.(s) + 2[C] + 2[S].sub.1w/o = Ce.sub.2 S.sub.3(s) + 
2CO.sub.(g)                66180 - 39.86T                                 
                                   p.sup.2 CO/h.sub.S.sup.2 = 3.6         
3/2 Ce.sub.2 O.sub.2 S.sub.(s) + 3[C] + 5/2[S].sub.1w/o = Ce.sub.3        
S.sub.4(s) + 3CO.sub.(g)   127050 - 72.1T                                 
                                   p.sup.3 CO/h.sub.S.sup.5/2 = 1.25      
Ce.sub.2 O.sub.2 S.sub.(s) + 2[C] + [S].sub.1w/o = 2CeS.sub.(s) +         
2CO.sub.(g)                 120,860 - 61.0T                               
                                   p.sup.2 CO/h.sub.S = .027              
C.sub.(s) + 1/2 O.sub.2(g) = CO.sub.(g)                                   
                          -28200 - 20.16T                                 
                                   pCO/p.sup.1/2 O.sub.2 = 7.6 ×    
                                   10.sup.-7                              
1/2S.sub.2(g) = [S].sub.1w/o                                              
                          -31520 + 5.27T                                  
                                   h.sub.S /p.sup.1/2 S.sub.2 = 5.4       
                                   × 10.sup.2                       
__________________________________________________________________________
 
    
     The thermodynamics of desulphurization with lanthanium oxide, La 2  O 3 , are similar although, in this case, LaO 2  is unstable and there will be no conversion corresponding to CeO 2  →Ce 2  O 3 . 
     In the case of desulphurization of gases, such as stack gases, assuming the following gas composition at 1000° C.: 
     
         ______________________________________                                    
Component       Vol.%                                                     
______________________________________                                    
CO.sub.2        16                                                        
CO              40                                                        
H.sub.2         40                                                        
N.sub.2         4                                                         
H.sub.2 S       0.3                                                       
                (200 grains/100 ft.sup.3.)                                
______________________________________                                    
 
    
     This equilibrium gas composition is reversed by point A on the diagram illustrated as FIG. 6 where CO/CO 2  =2.5 and H 2  /H 2  S=133. This point lies within the Ce 2  O 2  S phase field and at constant CO/CO 2  desulphurization with Ce 2  O 3  will take place up to point B. At point B, H 2  /H 2  S≃10 4  and the concentration of H 2  S is 0.004 vol.% (˜3 grains/100 ft. 3 ). Beyond this point, desulphurization is not possible. 
     The basic theory for this invention is supported by the standard free energies of rare earth compounds likely to be involved. Examples of these appear in Table I which follows: 
     
                                           TABLE 1                                 
__________________________________________________________________________
Standard Free Energies of Formation of                                    
Some Rare Earth Compounds: ΔG°=X-YT cal/g.f.w.               
                               Estimated                                  
Reaction         X     Y   Temp.(°K).                              
                               Error(kcal)                                
__________________________________________________________________________
CeO.sub.2(s) = Ce.sub.(1) + O.sub.2(g)                                    
                 259,900                                                  
                       49.5                                               
                           1071-2000                                      
                                 ±3                                    
Ce.sub.2 O.sub.3(s) = 2Ce.sub.(1) + 3/2 O.sub.2(g)                        
                 425,621                                                  
                       66.0                                               
                           1071-2000                                      
                                 ±3                                    
La.sub.2 O.sub.3(s) = 2La.sub.(1) + 3/2 O.sub.2(g)                        
                 428,655                                                  
                       68.0                                               
                           1193-2000                                      
                                 ±3                                    
CeS.sub.(s) = Ce.sub.(1) + 1/2 S.sub.2(g)                                 
                 132,480                                                  
                       24.9                                               
                           1071-2000                                      
                                 ±2                                    
Ce.sub.3 S.sub.4(s) = 3Ce.sub.(1) + 2S.sub.2(g)                           
                 483,180                                                  
                       98.2*                                              
                           1071-2000                                      
                                 ±10                                   
Ce.sub.2 S.sub.3(s) = 2Ce.sub.(1) + 3/2 S.sub.2(g)                        
                 351,160*                                                 
                       76.0*                                              
                           1071-2000                                      
                                 ±10                                   
LaS.sub.(s) = La.sub.(1) + 1/2 S.sub.2(g)                                 
                 123,250                                                  
                       25.3                                               
                           1193-2000                                      
                                 ±6                                    
Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(1) + O.sub.2(g)  O.sub.2(g) + 1/2   
S.sub.2(g)       410,730                                                  
                       65.0                                               
                           1071-2000                                      
                                 ±15                                   
La.sub.2 O.sub.2 S.sub.(s) = 2La.sub.(s) + O.sub.2(g) + 1/2 S.sub.2(g)    
                 407,700*                                                 
                       65.0*                                              
                           1193-2000                                      
                                 ±15                                   
__________________________________________________________________________
 *Estimated                                                               
 
    
     The three phase equilibria at 1273° K. for the Ce--O--S System is set out in Table II as follows: 
     
                                           TABLE II                                
__________________________________________________________________________
Ce-O-S System                                                             
Three Phase Equilibria at 1273° K.                                 
REACTION            ΔG° cal                                  
                             K.sub.1273                                   
__________________________________________________________________________
Ce.sub.2 O.sub.3(s) + 1/2S.sub.2(g) = Ce.sub.2 O.sub.2 S.sub.(s) +        
1/2O.sub.2(g)       14890 - 1.0T                                          
                             (pO.sub.2 /pS.sub.2).sup.1/2  = 4.6 ×  
                             10.sup.-3                                    
Ce.sub.2 O.sub.2 S.sub.(s) + 1/2S.sub.2(g) = 2CeS.sub.(s)                 
                    145770 - 15.2T                                        
                             pO.sub.2 /p.sup.1/2 S.sub.2 = 2.0 ×    
                             10.sup.-22                                   
3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2 S.sub.2(g) = 2Ce.sub.3 S.sub.4(s) +     
30.sub.2(g)         265830 + 1.4T                                         
                             p.sup.3 O.sub.2 /p.sup.5/2 S.sub.2 = 1.1     
                             × 10.sup.-46                           
Ce.sub.2 O.sub.2 S.sub.(s) + S.sub.2(g) = Ce.sub.2 S.sub.3                
                    59570 + 11.0T                                         
                             pO.sub.2 /pS.sub.2 = 2.3 × 10.sup.-13  
Ce.sub.3 S.sub.4(s) =  3CeS.sub.(s) + 1/2S.sub.2(g)                       
                    85740 - 23.5T                                         
                             p.sup.1/2 S.sub.2 = 2.5 × 10.sup.-10   
2Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2S.sub.2(g)               
                    87120 - 31.6T                                         
                             p.sup.1/2 S.sub.2 = 8.9 × 10.sup.-8    
CO.sub.(g) + 1/2O.sub.2(g) = CO.sub.2(g)                                  
                    - 67500 + 20.75T                                      
                             pCO.sub.2 /(pCO . p.sup.1/2 O.sub.2) = 1.1   
                             × 10.sup.7                             
H.sub.2(g) + 1/2S.sub.2(g) = H.sub.2 S.sub.(g)                            
                    - 21580 + 11.80T                                      
                             pH.sub.2 S/(pH.sub.2 . p.sup.1/2 S.sub.2) =  
                             13.4                                         
H.sub.2(g) + 1/2O.sub.2(g) = H.sub.2 O.sub.(g)                            
                    - 58900 + 13.1T                                       
                             pH.sub.2 O/(pH.sub.2 . p.sup.1/2 O.sub.2) =  
                             1.8 × 10.sup.7                         
__________________________________________________________________________
 
    
     Typical calculations of energy changes involved in the systems involved in this invention are as follows: 
     
         ______________________________________                                    
S.sub.2(g) + Ce.sub.2 O.sub.2 S.sub.(s) = Ce.sub.2 S.sub.3(s)             
+ O.sub.2(g)                                                              
Ce.sub.2 S.sub.3(s) = 2Ce.sub.(l) + 3/2 S.sub.2(g) : ΔG° =   
351160 - 76.0T cal                                                        
Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + O.sub.2(g) + 1/2 S.sub.2(g) :  
ΔG° = 410730 - 65.0T cal                                     
Ce.sub.2 O.sub.2 S.sub.(s) + S.sub.2(g) = Ce.sub.2 S.sub.3(s)             
+ O.sub.2(g) : ΔG° = 59570 + 11.0T cal                       
@ 1273° K. ΔG° = 73573 cal and pO.sub.2 /pS.sub.2 =   
2.33 × 10.sup.-13                                                   
______________________________________                                    
Ce.sub.2 O.sub.3(s) + 1/2 S.sub.2(g) = Ce.sub.2 O.sub.2 S                 
+ 1/2 O.sub.2(g)                                                          
Ce.sub.2 O.sub.3(s) = 2Ce.sub.(l) + 3/20.sub.2(g) : ΔG°  =   
425621 - 66.0T cal                                                        
Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + O.sub.2(g) + 1/2 S.sub.2(g) :  
ΔG° = 410730 - 65.0T cal                                     
Ce.sub.2 O.sub.3(s) + 1/2 S.sub.2(g) = Ce.sub.2 O.sub.2 S.sub.(s) + 1/2   
O.sub.2(g) : -ΔG° 14891 - 1.0T cal                           
@ 1273° K. ΔG° = 13618 cal and (pO.sub.2 /pS.sub.2).su
p.1/2 = 4.6 × 10.sup.-3                                             
______________________________________                                    
Ce.sub.2 O.sub.2 S.sub.(s) + 1/2 S.sub.2(g) = 2CeS.sub.(s) + O.sub.2(g)   
Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + 1/2 S.sub.2(g) + O.sub.2(g) :  
ΔG° = 410730 - 65.0T cal                                     
2CeS.sub.(s) = 2Ce.sub.(l) + S.sub.2(g) : ΔG° = 264960 -     
49.8T cal                                                                 
Ce.sub.2 O.sub.2 S.sub.(s) + 1/2 S.sub.2(g) = 2CeS.sub.(s) + O.sub.2(g)   
ΔG° = 145770 - 15.2T cal                                     
@ 1273° K. ΔG° = 126420 cal. and pO.sub.2 /p.sup.1/2  
S.sub.2 = 1.96 × 10.sup.-22                                         
______________________________________                                    
3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2 S.sub.2(g) = 2Ce.sub.3 S.sub.4(s) + 3   
O.sub.2(g)                                                                
2Ce.sub.3 S.sub.4(s) = 6Ce.sub.(l) + 4S.sub.2(g) : ΔG° =     
966360 - 196.4T cal                                                       
3Ce.sub.2 O.sub.2 S.sub.(s) = 6Ce.sub.(l) + 3 O.sub.2(g) + 3/2 S.sub.2(g) 
ΔG° = 1232190 - 195.0T cal                                   
3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2 S.sub.2(g) = 2Ce.sub.3 S.sub.4(s) + 3   
O.sub.2(g) :                                                              
ΔG° = 265830 + 1.4T cal                                      
@ 1273° K. ΔG° = 267612 cal and p.sup.3 O.sub.2       
/p.sup.5/2 S.sub.2 = 1.12 × 10.sup.-46                              
______________________________________                                    
Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2 S.sub.2(g)                       
Ce.sub.3 S.sub.4(s) = 3Ce.sub.(l) + 2S.sub.3(g) : ΔG° =      
48318 - 98.2T cal.                                                        
3CeS.sub.(s) = 3Ce.sub.(l) + 3/2 S.sub.2(g) : ΔG° = 397,440  
- 74.7T cal.                                                              
Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2 S.sub.2(g) : ΔG° =  
85740 - 23.5T cal.                                                        
@ 1273° K. ΔG° = 55824 cal p.sup.1/2 S.sub.2 = 2.6    
× 10.sup.-10                                                        
______________________________________                                    
3Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2 S.sub.2(g)              
2Ce.sub.3 S.sub.4(s) = 6Ce.sub.(l) + 4 S.sub.2(g) : ΔG° =    
966360 - 196.4T cal.                                                      
3Ce.sub.2 S.sub.3(s) = 6Ce.sub.(l) + 9/2 S.sub.2(g) : ΔG° =  
1053480 - 228.0T cal.                                                     
3Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2 S.sub.2(g) :            
ΔG° = 87120 - 31.6T cal.                                     
@ 1273° K. ΔG° = 468893 cal. and p.sup.1/2 S.sub.2 =  
8.9 × 10.sup.-9                                                     
______________________________________                                    
H.sub.2(g) + 1/2 S.sub.2(g) = H.sub.2 S.sub.(g)                           
H.sub.2(g) + 1/2 S.sub.2(g) = H.sub.2 S.sub.(g) : ΔG° =      
-21580 + 11.80T cal.                                                      
@ 1273° K. ΔG° = -6559 and pH.sub.2 S/(pH.sub.2 .     
p.sup.1/2 S.sub.2) = 13.4                                                 
______________________________________                                    
pH.sub.2 /pH.sub.2 S                                                      
                  log pS.sub.2                                            
1                 -2.25                                                   
10.sup.2          -6.25                                                   
10.sup.4          -10.25                                                  
10.sup.6          -14.25                                                  
10.sup.8          -18.25                                                  
10.sup.10         -22.25                                                  
10.sup.12         -26.25                                                  
______________________________________                                    
H.sub.2(g) + 1/2 O.sub.2(g) = H.sub.2 O.sub.(g)                           
H.sub. 2(g) + 1/2 O.sub.2(g) = H.sub.2 O.sub.(g) : ΔG° =     
-58900 + 13.1T cal.                                                       
@ 1273° K. ΔG° = -42223 cal. and (pH.sub.2 /pH.sub.2  
O)                                                                        
p.sup.1/2 O.sub.2 = 5.6 × 10.sup.-8                                 
______________________________________                                    
pH.sub.2 /pH.sub.2 O                                                      
                  log pO.sub.2                                            
10.sup.-4         -6.5                                                    
10.sup.-2         -10.5                                                   
1                 -14.5                                                   
10.sup.2          -18.5                                                   
10.sup.4          -22.5                                                   
10.sup.6          -26.5                                                   
10.sup.8          -30.5                                                   
______________________________________                                    
CO.sub.(g) + 1/2 O.sub.2(g) = CO.sub.2(g)                                 
CO.sub.(g) + 1/2 O.sub.2(g) = CO.sub.2(g) : ΔG° = -67500 +   
20.75T cal.                                                               
@ 1273° K. ΔG° = -41085 and pCO.sub.2 /(pCO .         
p.sup.1/2 O.sub.2) = 1.1 × 10.sup.7                                 
______________________________________                                    
pCO/pCO.sub.2     log pO.sub.2                                            
10.sup.-4         -6.1                                                    
10.sup.-2         -10.1                                                   
1                 -14.1                                                   
10.sup.2          -18.1                                                   
10.sup. 4         -20.1                                                   
10.sup.6          -24.1                                                   
10.sup.8          -30.1                                                   
______________________________________                                    
 
    
    
    
     In the foregoing general description of this invention, certain objects, purposes and advantages have been outlined. Other objects, purposes and advantages of this invention will be apparent, however, from the following description and the accompanying drawings in which: 
     FIG. 1 is a stability diagram showing w/o sulphur as partial pressure of CO; 
     FIG. 2a and 2b show Ce 2  S 3  and Ce 2  O 2  S layers on a pellet of CeO 2  ; 
     FIG. 3 is a graph of the theoretical CeO 2  required for removal of 0.01 w/o S/THM; 
     FIG. 4 is a graph showing the volume of nitrogen required to produce a given partial pressure of CO; 
     FIG. 5 is a graph showing the CeO 2  requirements as a function of partial pressure of CO; and 
     FIG. 6 is a stability diagram for stack gas systems treated according to this invention. 
    
    
     Referring back to the discussion of free energy set out above, it is clear that these free energy changes may be used to determine the fields of stability of Ce 2  O 3 , Ce 2  O 2  S, Ce 2  S 3 , Ce 3  S 4  and CeS in terms of the partial pressure of Co and the Henrian sulphur activity of the melt at 1500° C. The resultant stability diagram is shown in FIG. 1, the boundaries between the phase fields being given by the following relationships: 
     
         ______________________________________                                    
BOUNDARY       EQUATION                                                   
______________________________________                                    
Ce.sub.2 O.sub.3 --Ce.sub.2 O.sub.2 S                                     
               log pCO = log h.sub.S + 3.53                               
Ce.sub.2 O.sub.2 S--Ce.sub.2 S.sub.3                                      
               log pCO = log h.sub.S + 0.28                               
Ce.sub.2 O.sub.2 S--Ce.sub.3 S.sub.4                                      
               log pCO = 0.83 log h.sub.S + 0.03                          
Ce.sub.2 O.sub.2 S--Ces                                                   
               log pCO = 0.5 log h.sub.S - 0.79                           
Ce.sub.2 S.sub.3 --Ce.sub.3 S.sub.4                                       
               log h.sub.S = - 1.47                                       
Ce.sub.3 S.sub.4 --CeS                                                    
               log h.sub.S = - 2.45                                       
______________________________________                                    
 
    
     The phase fields in FIG. 1 are also shown in terms of the Henrian activity of oxygen, h O , and the approximate [w/o S] in the iron melt using an activity coefficient f S  ≃5.5 for graphite saturated conditions. 
     The coordinates of the points B, C, D and E on the diagram are given below: 
     
         ______________________________________                                    
COOR-                                                                     
DINATES  B         C         D       E                                    
______________________________________                                    
pCO atm. 9.8 × 10.sup.-3                                            
                   6.5 × 10.sup.-2                                  
                             1.0     1.0                                  
h.sub.S  3.5 × 10.sup.-3                                            
                   3.4 × 10.sup.-2                                  
                             5.3 × 10.sup.-1                        
                                     2.9 × 10.sup.-4                
Approx.  6.4 × 10.sup.-4                                            
                   6.2 × 10.sup.-3                                  
                             9.6 × 10.sup.-2                        
                                     5.3 × 10.sup.-5                
[w/o S]                                                                   
______________________________________                                    
 
    
     The points B and C represent simultaneous equilibria between the oxysulphide and two sulphides at 1500° C. These univariant points are only a function of temperature. The points E and D represent the minimum sulphur contents or activities at which oxysulphide and Ce 2  S 3  can be formed, respectively, at pCO=1 atm. Thus, carbon saturated hot metal cannot be desulphurized by oxysulphide formation below h S  ≃2.9×10 -4  ([w/o S]≃5.3×10 -5 ) at pCO=1 atm. However, lower sulphur levels may be attained by reducing the partial pressure of CO. 
     The conversion of CeO 2  →Ce 2  O 3  →Ce 2  O 2  S→Ce 2  S 3  is illustrated in FIGS. 2a and 2b which show Ce 2  S 3  and Ce 2  O 2  S layers on a pellet of CeO 2  (which first transformed to Ce 2  O 3 ) on immersion in graphite saturated iron at ˜1600° C., initially containing 0.10 w/o S, for 10 hours. The final sulphur content was ˜0.03 w/o S and the experiment was carried out under argon, where pCO&lt;&lt;1 atm. 
     The conversion of the oxide to oxysulphide and sulphide is mass transfer controlled and, as in conventional external desulphurization with CaC 2 , vigorous stirring will be required for the simple addition process and circulation of hot metal may be required in the `active` lining process. 
     From FIG. 1 it is apparent that the external desulphurization of graphite saturated iron is thermodynamically possible using RE oxides. For example the diagram indicates that hot metal sulphur levels of ˜0.5 ppm (point E) can be achieved by cerium oxide addition even at pCO=1 atm. Desulphurization in this case will take place through the transformation sequence CeO 2  →Ce 2  O 3  →Ce 2  O 2  S which required 2 moles of CeO 2  to remove 1 gm. atom of sulphur. The efficiency of sulphur removal/lb. CeO 2  added can, however, be greatly increased by the formation of sulphides. 1 mole CeO 2  is required per g. atom of sulphur for CeS formation and 2/3 moles CeO 2  for Ce 2  S 3  formation. The theoretical CeO 2  requirements for the removal of 0.01 w/o S/THM for the various desulphurization products are given below and expressed graphically in FIG. 3. 
     
         ______________________________________                                    
         lb CeO.sub.2 /0.01                                               
                    ft.sup.3 CO/lb                                        
PRODUCT  w/o S.THM  CeO.sub.2                                             
                             ft.sup.3 CO/0.01 w/o S.THM                   
______________________________________                                    
Ce.sub.2 O.sub.2 S                                                        
          2.15      2.1      4.5                                          
CeS      1.1        4.2      4.5                                          
Ce.sub.3 S.sub.4                                                          
         0.8        4.2      3.4                                          
Ce.sub.2 S.sub.3                                                          
         0.7        4.2      3.0                                          
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     The volume of carbon monoxide produced in ft 3  CO/lb CeO 2  and ft 3  CO/0.01 w/o S.THM are also given in the above table for each desulphurization product. For efficient desulphurization the partial pressure of carbon monoxide should be sufficiently low to avoid oxysulphide formation. For example, FIG. 1 shows that oxysulphide will not form in a graphite saturated melt until [w/o S]&lt;0.01 when pCO≃0.1 atm. It will form however when [w/o S]≃0.10 at pCO=1 atm. Thus by reducing the pCO in the desulphurization process to 0.1 atm., hot metal can be desulphurized to 0.01 w/o S with a CeO 2  addition of 0.72 lb/0.01 w/o S removed for each ton hot metal. 
     The choice of the method of reducing the partial pressure of carbon monoxide depends on economic and technical considerations. However, in an injection process calculations can be made for the volume of injection gas, say nitrogen, required to produce a given pCO. 
     Thus: 
     
         V.sub.N.sbsb.2 =V.sub.CO (1-pCO)/pCO 
    
     where 
     V CO  is the scf of CO formed/lb CeO 2  added 
     V N .sbsb.2 is the scf of N 2  required/lb CeO 2  added and 
     pCO is the desired partial pressure of CO in atm. 
     The results of these calculations for Ce 2  S 3   formation are shown in FIG. 4, which also shows the [w/o S] in equilibrium with Ce 2  S 3 (s) as a function of pCO. From this figure it is apparent that the volume of N 2  /lb CeO 2  required to form Ce 2  S 3  is excessive and if an injection process were used a balance would have to be struck between sulphide and oxysulphide formation. When, for example, hot metal is to desulphurize from 0.05 to 0.01 w/o S at pCO=0.2 atm., 1[16 scf N 2  /lb CeO 2  would be required for Ce 2  S 3  formation and the sulphur content would drop to 0.02 w/o. The remaining 0.01 w/o S would be removed by oxysulphide formation. From FIG. 3, it can be seen that ˜2 lbs of CeO 2  /THM would be required for Ce 2  S 3  formation and 2 lbs for Ce 2  O 2  S formation giving a total requirement of 4 lbs CeO 2  /THM. 
     Calculations similar to the one above have been used to construct FIG. 5 where the CeO 2  requirements in lbs/THM are shown as a function of pCO. 
     When large volumes of nitrogen are used in an injection process the heat carried away by the nitrogen, as sensible heat, is not large but the increased losses by radiation may be excessive. Injection rates with CaC 2  for example are in the order of 0.1 scf N 2  /lb CaC 2 . 
     Vacuum processing is an alternative method of reducing the partial pressure of carbon monoxide. This is impractical in hot metal external desulphurization but not in steelmaking (see below). 
     Still another alternative approach to external desulphurization using rare earth oxides is the use of active linings which would involve the `gunning` or flame-spraying of HM transfer car linings with rare earth oxides. Here the oxides would transform to oxysulphides during the transfer of hot metal from the blast furnace to the steelmaking plant, and the oxide would be regenerated by atmospheric oxidation when the car was emptied. It is estimated that for a 200 ton transfer car, conversion of a 2 mm layer (˜0.080&#34;) of oxide to oxysulphide would reduce the sulphur content of the hot metal by ˜0.02 w/o S. This process has the following advantages: 
     (1) continuous regeneration of rare earth oxide by atmospheric oxidation when the car is empty, 
     (2) reaction times would be in the order of hours, 
     (3) the absence of a sulphur rich desulphurization slag, and 
     (4) the absence of suspended sulphides in the hot metal. 
     The mechanical integrity and the life of an &#34;active&#34; lining is, of course, critical and some pollution problems may be associated with oxide regeneration by atmospheric oxidation. 
     With regard to steelmaking applications, vacuum desulphurization could be carried out by an &#34;active&#34; lining in the ASEA-SKF process and circulation vacuum degassing processes. 
     In the foregoing specification, we have set out certain preferred practices and embodiments of our invention, however, it will be understood that this invention may be otherwise embodied within the scope of the following claims.