Process of drying sulfide ores in direct contact with hot drying gases
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process of drying sulfide ores in direct contact with hot drying gases, wherein the sulfide ores are processed with production of SO 2  -containing gases, sulfuric acid is produced from the SO 2  -containing gases in a contact process plant, and the drying gases are heated up with surplus heat from the contact process plant. 
     2. Discussion of Prior Art 
     Sulfide ores or ore concentrates, such as copper, nickel, lead, tin and iron ores, and optional admixtures, such as sand, lime, and metal concentrates recovered from slags, must sometimes be dried before their metallurgical processing. The metallurgical processing may succeed a roasting treatment or the roasting may be effected at the same time as the metallurgical processing. The SO 2  -containing gases thus produced are processed in contact process plants to produce sulfuric acid. Particularly in processes in which the roasting and metallurgical processing are performed at the same time, the material must be dried to a very low water content in order to ensure favorable ignition behavior of the ores. On the other hand, the ignitability of the ores must be taken into account during the drying because an ignition during drying must be prevented. This is ensured in that the inlet temperature of the drying gases is properly selected in consideration of the oxygen content of the gases and of the flow pattern of the gas and solids. Drying is generally effected with hot flue gases, which are produced by a combustion of solid fuels. Because a considerable quantity of heat is needed for the evaporation of the water, primary energy is required in a large quantity. 
     From &#34;Metall&#34;, Vol. 37, No. 4, April 1983, page 299 it is also known to use hot air for a drying of copper ores and to heat up said air with surplus heat from the contact process plant. That practice saves a corresponding quantity of primary energy. But that saved quantity of primary energy must be replaced by high-temperature surplus heat, which becomes available in the contact process plant during the processing of gases having a high SO 2  content and which could otherwise by used to produce high-pressure steam. 
     It is an object of the invention to minimize the use of high-grade heat for drying. 
     SUMMARY OF INVENTION 
     This object is accomplished in accordance with the invention in that hot dry tail gases from the contact process plant are used as drying gases. 
     The term &#34;tail gas&#34; describes the dry exhaust gases which are discharged from the SO 3  absorber succeeding the last contacting tray of the contact process plant and are substantially free of SO 2  and SO 3 . These gases generally become available at a temperature of 60° to 80° C. although their temperature may rise above 140° C. if the absorption is effected at elevated temperatures. Said gases have only a relatively low content of residual oxygen. The tail gas is heated up by an indirect heat exchange with surplus heat form the contact process plant. 
     In accordance with a preferred further feature, the tail gases are supplied to the drying equipment at a rate which corresponds to the specific gas flow rating of the drying equipment, the tail gas which is used is heated with surplus heat from the contact process plant to such a temperature that heat in the quantity required for an evaporation of the water is supplied to the drying equipment, and the humid drying gases leaving the drying equipment are at a temperature which is only so much higher than the dew point temperature of the humid drying gases that there is no condensation in the succeeding equipment. The &#34;specific gas flow rating&#34; of the drying equipment is the highest permissible volumetric gas rate per cross-sectional area of gas flow in the drying equipment employed, also in consideration of the permissible loading of the gaseous effluent with entrained solids and in consideration of the energy required for gas transport. Even in the succeeding equipment, such as dust collectors, the temperature of the humid gases leaving the drying equipment must not drop below the dew point temperature. On the other hand, the outlet temperature should be as low as possible because this will minimize the heat losses which are due to the gaseous effluent and the solids being discharged. 
     The term &#34;surplus heat form the contact process plant&#34; describes the heat which is not required to heat SO 2  -containing gases in order to maintain a thermally self-sufficient operation in the contact process plant and which must be removed from the contact process system. The rate at which surplus heat becomes available depends on the SO 2  concentration and on the rate of contact process gas. If the ore has a relatively high sulfur content, the surplus heat which becomes available in the contact process plant used in the integrated process complex may be sufficient for a complete drying of the ore which is supplied, even if the SO 2  concentration is relatively low and amounts, e.g., to 8 to 10 vol.% SO 2 . The higher the SO 2  concentration of the contact process gases, the higher will be the rate of surplus heat. The surplus heat may be removed from the contact process system at a temperature between about 150° and 700° C. and its lower temperature limit is substantially determined by the dew point temperature of the SO 3  -containing gases. The utilization of heat can be greatly improved in that drying is effected with dry gases which are supplied at a relatively high rate and are at a temperature which is substantially below the highest temperature that is permissible in view of the igniting behavior of the ores. 
     A further preferred feature resides in that the tail gas supplied to the drying equipment is preheated with surplus heat from the contact process plant and is subsequently indirectly heated to the required inlet temperature for the drying equipment. That practice, which requires an additional heat exchanger, is adopted if adequate surplus heat is not available from the contact process system. The indirect heating ensures an optimum preservation of the advantage afforded by the use of perfectly dry tail gases. 
     In accordance with a further feature the tail gas supplied to the drying equipment is preheated with surplus heat from the contact process plant and is subsequently heated by a direct admixing of hot flue gases to the inlet temperature for the drying equipment, the mixed gases are supplied to the drying equipment at a volumetric rate which corresponds to the specific gas flow rating of the drying equipment, and the inlet temperature is so adjusted that the quantity of heat required for an evaporation of the water is supplied to the drying equipment and the outlet temperature of the humid drying gases leaving the drying equipment is sufficiently higher than the dew point temperature of the drying gases. In that case the requirements described hereinbefore are also properly met. 
     The flue gases are directly admixed when they are at their highest temperature because the rate at which flue gases are admixed can be minimized in that case so that the rate at which water is introduced is low too. Flue gases are produced by the combustion of fossil fuels and inevitably contain water. The dry tail gases from the contact process plant are supplied instead of undried secondary air to the combustion chamber in which the flue gases are produced. That procedure is adopted if the surplus heat from the contact process plant is not sufficient for the drying of the ores and permits the advantages afforded by the drying with dry tail gases to be substantially preserved. 
     In accordance with a further preferred feature the outlet temperature of the humid drying gases leaving the drying equipment is 30° to 70° C. above the dew point temperature of the humid gaseous effluent. In that case the heat supplied by the drying gases is utilized to a high degree and a temperature drop below the dew point temperature is reliably avoided also in the succeeding equipment. 
     Drying is preferably carried out in equipment which has a high specific gas flow rating and can be operated with a low temperature difference between the inlet and outlet temperature of the gases. Such equipment may consist, e.g., of a suspension dryer, a fluidized bed dryer or a rotary drum dryer. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will be explained more in detail with reference to drawings and examples. 
     FIG. 1 is a flow scheme illustrating the smelting of a copper ore concentrate in combination with the production of sulfuric acid in a process in which the concentrate is dried in a rotary drum dryer. 
     FIG. 2 is a flow scheme showing a fluidized bed dryer. 
     FIG. 3 is a flow scheme showing a combination of a rotary drum dryer and a suspension dryer. 
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Referring to the drawings, a mixture of ore concentrate and admixtures is charged by a feeder 1 into the dryer 2, 2a, 2b, 2c. Hot tail gas is supplied to the dryer via a line 3. The waterladen exhaust gas leaving the dryer is delivered in line 4 to a gas purifier 5 and from the latter in line 6 to a chimney. The dried mixed solids 7a collected in the gas purifier 5 are supplied in line 8 to the copper smelter 9. A smelting is effected in a suspension smelting furnace, which is succeeded by a converter supplied with commercially pure oxygen. The resulting gases have a high SO 2  content and are supplied in line 10 to the gas purifier 11. Air is admixed to the purified gases in order to adjust the required ratio of O 2  to SO 2 . The gases are then supplied in line 12 to the contact process plant 13 for producing sulfuric acid. The tail gas leaves the contact process plant 13 in line 14. A branch stream of the tail gas is recycled in line 15 to the contact process plant 13 and is heated up therein. The tail gas which has been heated up is supplied in line 3 to the dryer 2. If the surplus heat of the contact process plant 13 is not sufficient to heat, the branch stream of tail gas to the inlet temperature required for the drying equipment, fuel oil in line 16 and primary air in line 17 is supplied to the combustion chamber 18 for the production of flue gases. 
     FIG. 1 shows a rotary drum dryer 2 having a solids discharge port 7 and a gas outlet line 4 as separate elements. 
     FIG. 2 shows a fluidized bed dryer 2a, in which all solids together with the gaseous effluent are supplied in line 4a to the bag house 5. All solids which have been collected are delivered via line 7a to line 8. 
     FIG. 3 shows a rotary drum dryer and a suspension dryer used in combination. From the short rotary drum 2, the predried solids and the gas enter the cage mill 2a, in which the drying is continued and coarse particles are ground. Solids and gas then enter the suspension dryer 2b for a final drying. Solids and gas are supplied in line 4a to a cyclone separator 5a, in which a large part of the solids is collected and delivered via line 7 to line 8. The gas and the remaining solids are supplied in line 4b to the dust-collecting electrostatic precipitator 5. 
     In the examples, a copper concentrate was used which had the following composition based on dry ore: 
     Copper: 25% 
     Iron: 32% 
     Sulfur: 34.5% 
     The concentrate had a moisture content of 8%. 
     The admixtures used consisted of sand having a water content of 8% and of a slag concentrate derive from slags and having a water content of 12%. 
     On a dry basis, 51,800 kg copper concentrate, 7,900 kg sand and 2,600 kg slag concentrate were mixed per hour. The feeding temperature was 20° C. 
     The resulting mixture having a water content of 8.1% was supplied to the dryer at a rate of 67,800 kg/h. 
     The contact process plant operated in the integrated process complex produced sulfuric acid at a rate of 938,000 kg/h H 2  SO 4  of 100% concentration. 
     
         __________________________________________________________________________
                  Examples 1 to 3 (FIG. 1)                                
                                 Examples 4 to 6 (FIG. 2)                 
                                                Examples 7 to 9 (FIG. 3)  
                  Example                                                 
   Energy-        1        3     4        6     7        9                
Item                                                                      
   determining    Flue                                                    
                      2    Tail gas +                                     
                                 Flue                                     
                                     5    Tail gas +                      
                                                Flue                      
                                                    8    Tail gas +       
No.                                                                       
   parameters     gas Tail gas                                            
                           flue gas                                       
                                 gas Tail gas                             
                                          flue gas                        
                                                gas Tail                  
                                                         flue             
__________________________________________________________________________
                                                         gas              
 8 Solids rate                                                            
            t/h   62.5                                                    
                      62.5 62.5  62.5                                     
                                     62.5 62.5  62.5                      
                                                    62.5 62.5             
   H.sub.2 O - residual                                                   
            % H.sub.2 O                                                   
                  0.3 0.3  0.3   0.3 0.3  0.3   0.3 0.3  0.3              
   moisture                                                               
   Temperature                                                            
            °C.                                                    
                  100 80   90    100 95   95    75  75   75               
18 Gas rate (as                                                           
            sm.sup.3 /h                                                   
                  38,330                                                  
                      39,960                                              
                           40,600                                         
                                 57,140                                   
                                     59,310                               
                                          59,000                          
                                                55,100                    
                                                    59,100                
                                                         57,600           
   dry gas)                                                               
   H.sub.2 O content                                                      
            gH.sub.2 O/sm.sup.3                                           
                  39  0    8     34  0    2     33  0    5                
            of dry gas                                                    
   Gas temperature                                                        
            °C.                                                    
                  450 408  423   320 314  315   300 286  300              
 4 Gas rate (as                                                           
            sm.sup.3 /h                                                   
                  47,260                                                  
                      46,860                                              
                           47,430                                         
                                 66,596                                   
                                     66,280                               
                                          66,160                          
                                                64,300                    
                                                    66,000                
                                                         65,000           
   dry gas)                                                               
   H.sub.2 O content                                                      
            gH.sub.2 O/sm.sup.3                                           
                  179 134  140   127 90   93    130 91   98               
            of dry gas                                                    
   Dew point tem-                                                         
            °C.                                                    
                  64  53   54    52  46   47    53  46   48               
   perature                                                               
   Gas temperature                                                        
            °C.                                                    
                  120 100  110   100 95   95    80  80   80               
16 Fuel oil rate                                                          
            kg/h  580 --   170   610 --   52    550 --   157              
17 Combustion air                                                         
            sm.sup.3 /h                                                   
                  38,330                                                  
                      --   5,230 57,625                                   
                                     --   1,610 55,550                    
                                                    --   4,740            
   rate (as dry                                                           
   air)                                                                   
   H.sub.2 O content                                                      
            gH.sub.2 O/sm.sup.3                                           
                  20  --   20    20       20    20  --   20               
            of dry gas                                                    
   Air temperature                                                        
            °C.                                                    
                  20  --   20    20       20    20  --   20               
 3 Tail gas rate                                                          
            sm.sup.3 /h                                                   
                  --  39,961                                              
                           40,603                                         
                                 --  59,310                               
                                          57,500                          
                                                --  59,123                
                                                         53,000           
   (as dry gas)                                                           
   H.sub.2 O content                                                      
            gH.sub.2 O/sm.sup.3                                           
                  --  0    0     --  0    0     --  0    0                
            of dry gas                                                    
   Tail gas tem-                                                          
            °C.                                                    
                  --  450  405   --  320  289   --  300  247              
   perature                                                               
12 SO.sub.2 concen-                                                       
            vol. % SO.sub.2                                               
                  10.5                                                    
                      11.2 10.5  10.5                                     
                                     11.8 10.5  9.0 10.8 9.0              
   tration                                                                
   Gas rate (as                                                           
            sm.sup.3 /h                                                   
                  84,120                                                  
                      79,000                                              
                           84,120                                         
                                 84,120                                   
                                     74,610                               
                                          84,120                          
                                                97,552                    
                                                    81,926                
                                                         97,552           
   dry gas)                                                               
14 Tail gas rate                                                          
            sm.sup.3 /h                                                   
                  75,540                                                  
                      61,830                                              
                           75,540                                         
                                 75,540                                   
                                     61,380                               
                                          75,540                          
                                                84,315                    
                                                    68,670                
                                                         84,315           
   (as dry gas)                                                           
   Tail gas °C.                                                    
                  70  70   70    70  70   70    70  70   70               
   temperature                                                            
__________________________________________________________________________
 
    
     The advantages afforded by the invention reside in that the consumption of expensive primary energy is avoided or reduced and the consumption of high-grade surplus heat from the contact process plant is greatly reduced by the use of the low-grade heat contained in the tail gases.