Method for processing a heavy-metal-containing residue from the chemical industry
BACKGROUND OF THE INVENTION 
     The invention relates to a method for processing heavy-metal-containing residues from the chemical industry, in particular hydrogenation residues. 
     A problem of the chemical industry, in particular of the petrochemical industry, is the ecologically safe processing or reutilization of heavy-metal-containing residues as they occur, for instance, when refining heavy oil and at the hydrogenation of heavy oil. A similar problem arises when processing used oil. 
     It is known to burn such residues in a power plant, wherein, however, the heavy metals obtained, which are exhausted with the smoke gases and the ashes, constitute a burden on the environment. Unacceptably high portions of heavy metal occur in the flue gases and in the waste waters. 
     SUMMARY OF THE INVENTION 
     The invention has as its object to provide an economical and at the same time ecologically safe method for processing heavy-metal-containing residues, by which the heavy metals are converted into water-in-soluble dumpable form and the thermal content of the residues simultaneously is rendered available as a source of energy. 
     According to the invention, this object is achieved in that the heavy-metal-containing residues are introduced into a primary-gas reaction zone of a shaft gasifier containing at least one solid bed of carbon-containing material, that, together with the charge substances, i.e., the heavy-metal-containing residues, and/or together with the material forming the solid-bed, slag-forming substances are introduced to absorb and set the heavy metals, and that the lower zone of the solid bed is maintained at a temperature above the slag and ash melting temperatures, the viscosity of the slag being less than 100 poise. 
     Shaft gasifiers for carrying out the method are known. Such gasifiers comprise a vertical shaft, which constitutes the secondary-gas reaction zone, and an approximately horizontal chamber communicating with the lower end of the shaft, which constitutes the primary-gas reaction zone. A burner for gasifying the heavy-metal-containing residues communicates with the primary gas reaction zone of the gasifier. The carbon-containing charge, which is introduced into the secondary-gas reaction zone, forms in the primary-gas reaction zone, a conical pile having a free surface and having its base in the slag bath. During the gasification of the residues, the heavy metals, to a major extent, are melted into the slag and are discharged from the shaft gasifier commonly with the slag. The slag gathering in the shaft gasifier is chilled with water as it is discharged, with water, to provide a glassily solidified granulate containing the noxious heavy-metal components. It has been found that be cause of the glassy structure of the solidified slag, the latter can be dumped without any risk to the environment since leaching of heavy-metal compounds does not occur. 
     For processing vanadium-containing residues, basic slag formers, in particular CaO-containing substances, suitably are additionally introduced into the secondary-gas reaction zone. 
     For processing ferrous residues, coke, in particular metallurgical coke, advantageously is introduced into the secondary-gas reaction zone and an acidic slag containing at least 40% SiO 2  is formed. 
     In order to minimize discharge of heavy metals with the crude gas resulting from gasification, suitably one or more of the following measures for lowering the formation of soot in the primary-gas reaction zone are applied: 
     (a) Supply of an oxygen-containing gas in order to achieve a C/O 2  ratio, equal to λ, of at least 0.45 to 0.8, preferably 0.6; 
     (b) adjustment of a hydrogen/oxygen ratio of at least 0.35 to 0.7, preferably 0.5; 
     (c) adjustment of the dwell time of the primary gas in the primary-gas reaction zone of from 0.2 to 1.5 s, preferably 0.4 to 0.6 s, and 
     (d) adjustment of the dwell time of the secondary gas in the secondary-gas reaction zone of from 1 to 6 s, preferably 2 to 3 s. 
     Advantageously, the slag-forming additives, such as limestone, are used at a granulation size of up to 20 mm. 
     According to a preferred embodiment, the product gas leaving the shaft gasifier is filtered and the filtered dust is supplied to the primary-gas reaction zone. By this measure substantially are of the heavy metals are substantially entirely separated with the slag, an extremely low percentage building up up in the refractory lining of the shaft gasifier. 
     The invention, furthermore, is concerned with the utilization of sewage sludge. The sewage sludge obtained in the process of clarification of waste waters, depending on its origin, often also contains noxious matter, in particular heavy metals, which restrict the utilization or processing of sewage sludge. Indeed, heavy-metal-containing sewage sludge cannot be used as fertilizer, because undesired heavy-metal contaminations of food may occur. Furthermore, the heavy-metal-containing sewage sludge is not readily pyrolyzable or combustible into ash, because the heavy metals are contained in the flue gases and in the ash in water-soluble form, thus constituting a risk to the environment. 
     The invention has as its further object to render the thermal content of sewage sludge utilizable as an energy source, with the noxious matter possibly contained in the sewage sludge, in particular heavy metals, converted into water-insoluble dumpable form. 
     This object is achieved in that, in addition to the aforementioned heavy-metal-containing residues, sewage sludge is supplied to the primary-gas reaction zone as charge substance, the sewage sludge suitably being supplied in an amount of up to 30% of the amount of heavy-metal-containing residues. 
     Preferably, the sewage sludge is mixed with the heavy-metal-containing residue prior to the feeding thereof. 
     It is advantageous, if the sewage sludge is dried to a residual moisture of 50 to 60% at most, prior to its feeding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is going to be explained in more detail by way of the drawings, in which FIGS. 1 and 2 are respectively schematic illustrations of shaft gasifiers according to different embodiments of the invention, as will as by way of two examples. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The shaft gasifier 1 comprises a vertical upper section 2 constituting the secondary-gas reaction zone 3, and at least one (two in the embodiment illustrated) laterally angled lower section 4 constituting the primary-gas reaction zone 5. Into the upper section, the lumpy carbon-containing shaft charge 6, such as, e.g., coke or coal, if desired commonly with separate slag-forming substances, is charged from above via a sluice (not illustrated). The lumpy charge forms a solid bed 8 on the bottom 7 of the shaft gasifier 1, with dumping cones 10 projecting into the primary-gas reaction zones 5 and each having an exposed surface 9. These dumping cones 10 each enter into a slag tub 11 having an overflow weir 12. 
     On each of the laterally angled lower sections 4, at least one burner 13 is arranged, which, preferably, is designed as a cyclone burner and to which the residue 14 to be utilized is supplied as fuel or in addition to fuel. In addition, steam 15 and oxygen 16 are passed into the primary-gas reaction zone 5 via the burner 13. 
     The product gas 18 leaving the upper part of the secondary-gas reaction zone 3 through the gas outlet 17 is conducted to a dry separator or a wet scrubber 19. The dust portions 20 separated from the gas are supplied back to the shaft gasifier 1 via one of the burners. 
     The method according to the invention will now be explained by way of two examples: 
     EXAMPLE 1 
     A heavy-metal-containing residue 14 (vacuum residue) from heavy-oil processing was supplied to the burner 13 in an amount of 300 kg/h and with a temperature of 200° C. This residue had the following analysis (in % by weight): 
     
         ______________________________________                                    
C    H        O      N     S    H.sub.2 O                                 
                                      ash  V                              
______________________________________                                    
85.6 10.5     0.09   0.55  3.05 0.1   0.11 560 ppm                        
______________________________________                                    
 
    
     Furthermore, steam 15 at 18 bar was fed to the burner 13 in an amount of 160 kg/h, which steam had been overheated to 240° C. Oxygen 16 was injected with a temperature of 70° C. and in an amount of 380 m 3  /h under normal conditions (purity 99.9%, balance N). 
     In the secondary-gas reaction zone 3, metallurgical coke in an amount of 137 kg/h with a temperature of 20° C. was used as charge substance 6 and as slag former. Sieve analysis of the metallurgical coke (in %); 
     
         ______________________________________                                    
&gt;40 mm   40-20 mm     20-10 mm   &lt;10 mm                                   
______________________________________                                    
15.2     82.6         0.9        1.3                                      
______________________________________                                    
 
    
     Chemical analysis of the metallurgical coke (% by weight): 
     
         ______________________________________                                    
C    H       O      N     S    H.sub.2 O                                  
                                    ash                                   
______________________________________                                    
82.68                                                                     
     0.22    0.28   0.62  0.53 4.5  11.17 with 600 ppm V                  
______________________________________                                    
 
    
     The primary gas forming in the primary-gas reaction zone had a temperature of 1770° C. and was obtained in an amount of 1099 m 3  /h under normal conditions (wet). Its dwell time in the primary-gas reaction zone was 0.3 seconds. It had the following chemical composition (calculated as free of N) (% by weight): 
     
         ______________________________________                                    
CO.sub.2                                                                  
        H.sub.2                                                           
               CO         CH.sub.4                                        
                               (COS + H.sub.2 S)                          
______________________________________                                    
22.2    26.1   50.7       0.0  1.0                                        
______________________________________                                    
 
    
     The product gas 18 (crude gas or secondary gas) leaving the secondary-gas reaction zone 3 was obtained in a gas amount of 1322 m 3  /h under normal conditions (wet). The gas temperature was 831° C., the dwell time in the secondary gasification zone was 2 seconds. Its chemical composition (calculated as free of N) was the following (% by weight): 
     
         ______________________________________                                    
CO.sub.2                                                                  
        H.sub.2                                                           
               CO         CH.sub.4                                        
                               (COS + H.sub.2 S)                          
______________________________________                                    
15.5    37.0   46.9       0.0  0.6                                        
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     The slag 21 obtained with a temperature of 1500° C. and a viscosity of 80 poise, which ran over the overflow weir 12, flowing out of the primary-gas reaction zone 5, was granulated by means of water under pressure. In the slag, the ash portions of the carbon-containing charge and of the heavy-metal-containing residue are melted in so that the heavy metals that are contained in the ash also are obtained in the slag. The slag solidified in a glassy manner and was obtained in an amount of 15.3 kg/h. 
     Sieve analysis of the slag: 
     
         ______________________________________                                    
&gt;20 mm  20-10 mm      10-5 mm    5-3 mm                                   
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 0.0     0.1           1.2        5.8                                     
______________________________________                                    
3-2 mm  2-1 mm        1-0.5 mm   &lt;0.5 mm                                  
______________________________________                                    
11.2    30.2          32.4       19.1                                     
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     Chemical analysis of the slag (% by weight): 
     
         ______________________________________                                    
Al.sub.2 O.sub.3                                                          
       Fe.sub.2 O.sub.3                                                   
                 FeO     Fe     SiO.sub.2                                 
                                      CaO   MgO                           
______________________________________                                    
25.0   not de-   7.1     0.1    46.0  9.4   3.2                           
       tectable                                                           
______________________________________                                    
C.sub.tot.                                                                
       S.sub.tot.                                                         
                 TiO.sub.2                                                
                         Na.sub.2 O                                       
                                K.sub.2 O                                 
                                      P.sub.2 O.sub.5                     
                                            V                             
______________________________________                                    
0.15   0.08      1.1     0.8     1.6  0.5   0.82                          
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     The dust 20 separated off the crude gas 18, which was obtained in an amount of 2.49 g/m 3  under normal conditions, had an ash content of 11.8%, with 12.4% by weight of V being present in the ash. 
     Hence follows that the amount of vanadium (177.18 g/h) charged into the shaft gasifier 1, i.e. into its primary-gas reaction zone 5 and secondary-gas reaction zone 3, recurred in the slag in an amount of 125.46 g/h, while in the dust of the crude gas no more than 48.17 g/h were contained. The remainder was found as enrichment in the refractory lining of the shaft furnace. 
     With this balance of vanadium it has not been taken into account that the dust, after having left the shaft gasifier 1, is supplied to the primary-gas reaction zone 5, by which measure almost all of the vanadium is present in the slag in the melted state. In an attempt to leach the solifified slag with H 2  O, no V could be detected in the H 2  O. 
     EXAMPLE 2 
     A heavy-metal-containing residue 14 (vacuum residue) from heavy-oil processing was supplied to the burner 13 in an amount of 300 kg/h and with a temperature of 200° C. This residue had the following analysis (in % by weight): 
     
         ______________________________________                                    
C    H        O      N     S    H.sub.2 O                                 
                                      ash  V                              
______________________________________                                    
85.6 10.5     0.09   0.55  3.05 0.1   0.11 560 ppm                        
______________________________________                                    
 
    
     Furthermore, steam 15 at 18 bar was supplied to the burner 13 in an amount of 198 kg/h, which steam had been overheated to 240° C. Oxygen 16 was injected at a temperature of 70° C. and in an amount of 396 m 3  /h under normal conditions (purity 99.9%, balance N). 
     Into the secondary-gas reaction zone 3, metallurgical coke was used as charge, in an amount of 130 kg/h with a temperature of 20° C. 
     Sieve analysis of the metallurgical coke (in %): 
     
         ______________________________________                                    
&gt;40 mm   40-20 mm     20-10 mm   &lt;10 mm                                   
______________________________________                                    
15.2     82.6         0.9        1.3                                      
______________________________________                                    
 
    
     Chemical analysis of the metallurgical coke (% by weight): 
     
         ______________________________________                                    
C    H       O      N     S    H.sub.2 O                                  
                                    ash                                   
______________________________________                                    
82.68                                                                     
     0.22    0.28   0.62  0.53 4.5  11.17 with 600 ppm V                  
______________________________________                                    
 
    
     For the increased formation of basic slag, limestone was introduced commonly with the metallurgical coke, in an amount of 13 kg/h and with a temperature of 20° C. 
     Chemical analysis of the limestone: 
     
         ______________________________________                                    
MgO            CaO     others                                             
______________________________________                                    
0.7%           59.0%   0.3%                                               
______________________________________                                    
 
    
     The primary gas forming in the primary-gas reaction zone had a temperature of 1839° C. and was obtained in an amount of 1152 m 3  /h under normal conditions (wet). Its dwell time in the primary-gas reaction zone was 0.28 seconds. It had the following chemical composition (calculated as free of N) (% by weight): 
     
         ______________________________________                                    
CO.sub.2                                                                  
        H.sub.2                                                           
               CO         CH.sub.4                                        
                               (COS + H.sub.2 S)                          
______________________________________                                    
25.1    24.9   49.0       0.0  1.0                                        
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     The product gas 18 (crude gas or secondary gas) leaving the secondary-gas reaction zone 3 was obtained in a gas amount of 1364 m 3  /h under normal conditions (wet). The gas temperature was 864° C., the dwell time in the secondary gasification zone was 1.9 seconds. Its chemical composition (calculated as free of N) was as follows (% by weight): 
     
         ______________________________________                                    
CO.sub.2                                                                  
        H.sub.2                                                           
               CO         CH.sub.4                                        
                               (COS + H.sub.2 S)                          
______________________________________                                    
18.7    36.9   43.8       0.0  0.6                                        
______________________________________                                    
 
    
     The slag 21 obtained with a temperature of 1300° C. and a viscosity of 30 poise, which ran over the overflow weir 12, flowing out of the primary-gas reaction zone, was granulated by means of water under pressure. The slag solidified in a glassy manner and was obtained in an amount of 22.3 kg/h. 
     Sieve analysis of the slag: 
     
         ______________________________________                                    
&gt;20 mm  20-10 mm      10-5 mm    5-3 mm                                   
______________________________________                                    
 0.0     0.1           1.2        5.8                                     
______________________________________                                    
3-2 mm  2-1 mm        1-0.5 mm   &lt;0.5 mm                                  
______________________________________                                    
11.2    30.2          32.4       19.1                                     
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     Chemical analysis of the slag (% by weight): 
     
         ______________________________________                                    
AL.sub.2 O.sub.3                                                          
       Fe.sub.2 O.sub.3                                                   
                 FeO     Fe     SiO.sub.2                                 
                                      CaO   MgO                           
______________________________________                                    
16.0   not de-   4.6     0.06   29.4  40.2  2.6                           
       tectable                                                           
______________________________________                                    
C.sub.tot.                                                                
       S.sub.tot.                                                         
                 TiO.sub.2                                                
                         Na.sub.2 O                                       
                                K.sub.2 O                                 
                                      P.sub.2 O.sub.5                     
                                            V                             
______________________________________                                    
      .BHorizBrace.                                                       
0.2   not determined              0.7                                     
______________________________________                                    
 
    
     The dust 20 separated off the crude gas 18, which was obtained in an amount of 0.58 g/m 3  under normal conditions, had an ash content of 14.8% with 13.6% by weight of V being present in the ash. 
     Hence follows that the amount of vanadium (176.7 g/h) charged into the shaft gasifier 1, i.e. into its primary-gas reaction zone 5 and secondary-gas reaction zone 3, recurred in the slag in an amount of 156.24 g/h, while in the dust of the crude gas no more than 15.92 g/h were contained. The remainder was found as enrichment in the refractory lining of the shaft furnace. As in Example 1, it has not been taken into account with this balance of vanadium that the dust, after having left the shaft gasifier 1, is supplied to the primary-gas reaction zone 5, by which measure almost all of the vanadium is present in the slag in the melted state. In an attempt to leach the solidified slag with H 2  O, no V could be detected in the H 2  O. 
     By the method according to the invention it is possible to melt V, Fe, Ni, Cr and other heavy metals almost completely into the slag. It is of a particular advantage to prevent the formation of soot in the primary-gas reaction zone, because soot absorbs heavy metals. 
     As is apparent from FIG. 2, according to a further exemplary embodiment, a supply duct 22 enters into the primary-gas reaction zone 5, through which sewage sludge 23 with a residual moisture of at most 50 to 60% is fed. The sewage sludge 23 can can also be mixed with the residues 14 to be utilized prior to the feeding of the same, through a branch duct 24 entering into the duct feeding the residues 14 to be utilized. 
     The sewage sludge 23 is introduced into the primary-gas reaction zone 5 preferably in an amount of up to 30% of the amount of the residues 14 to be utilized. 
     The ash portions of the carbon-containing charge, of the heavy-metal-containing residue and of the sewage sludge have been melted into the glassily solidified slag, and thus are dumpable without any risk to the environment.