Method for recovering heat from molten slag and an apparatus therefor
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for recovering heat from molten metallurgical slag using a heat recovery equipment comprising as main component elements a primary heat-exchanger of rotary cylindrical type having a crushing or atomizing device for crushing or atomizing the molten slag with air jet, and a secondary heat-exchanger for recovering high temperature blast from hot slag granules discharged from the primary heat-exchanger. 
     2. Description of Prior Art 
     Generally, in the smelting process of ores and in the refining process of crude metals, a large amount of high temperature molten slag is produced, and the heat brought by the slag out of the system is very large. However, the molten slag is a high-temperature product, it changes considerably in its nature during its cooling process from the liquid state to the solid state so that it is very difficult to handle, and it has very low industrial value. For the above reasons and others, the molten slag discharged from a metallurgical furnace is once cooled in a water-granulation equipment or in a dry pit, and only part of it is utilized as ballast and building material but the greatest part is wasted. 
     Therefore, to date, the slag has never been utilized from the aspect of its heat recovery, and an enormous amount of heat energy has been left unutilized. 
     Meanwhile, in recent years, &#34;save resources&#34; and &#34;save energy&#34; have been strongly and loudly advocated world wide, and the recovery of exhausted heat from the molten slag has been one of the most important and urgent problems to be solved. 
     Particularly, in the field of ferro-nickel production, where a rotary kiln (calcining the ores) and an electric furnace (for reduction melting of the ores) are mostly used, a large amount of molten slag is produced from the electric furnace, reaching as large as 30-40 ton, per ton of nickel content in the ferro-nickel produced. Further, as the molten slag is at a temperature as high as 1500° to 1600° C., it occupies almost about 50% of the heat consumption in the electric furnace, which corresponds to about 15 × 10 6  kcal per ton of Ni, equivalent to 1500 l of heavy oil. 
     In recent years, the world nickel resources are being rapidly exhausted, and along this, the grade of the nickel ores has been lowering steadily. As the nickel ore grade becomes lower, the amount of slag produced per ton of nickel becomes much increased, and this tendency is increasing. Therefore, the importance of the recovery of heat from the molten slag has been increasing. 
     To date, various techniques for recovering slag heat have been proposed and published in many patent specifications and scientific literature. 
     Among them, for example, one method comprises casting the molten slag into a caterpillar type continuous slag casting machine or a slag truck in a closed chamber and bringing the air in and out of the closed chamber to obtain a hot blast. Another method comprises granulating the molten slag in a closed chamber by means of water vapor, air jet and a rotary drum, and introducing air into the chamber to obtain a hot air blast, and still another method comprises casting the molten slag into slag lumps, charging the slag lumps in a shaft tower, and introducing air into the tower to obtain a hot air blast. 
     All of the above conventional methods have considerable limitations in their commercial practice as they have defects in that the equipment requires a large scale structure and is not satisfactory in heat resistance, the heat recovery rate is low (about 35 to 40%), and the temperature of the recovered hot air is low. 
     SUMMARY OF THE INVENTION 
     Therefore, one of the objects of the present invention is to provide a method for recovering heat from a molten slag, which is free from the above defects of the conventional methods, and applicable with great advantage not only to the field of ferro-nickel production but also to other fields such as steel making, where an enormous amount of slag is produced, production of other ferro-alloys and non-ferrous metals. 
     The features of the present invention are that molten slag discharged from a metallurgical furnace and flowing down from a slag trough is atomized by air jet into a primary heat-exchanger of rotary cylindrical type, and hot slag granules thus obtained and air are caused to pass in a co-current therethrough so as to exchange the heat of the slag to the air flow to obtain a primary hot blast, then the hot slag granules discharge from the primary heat-exchanger are supplied to a secondary heat-exchanger connected to the primary heat-exchanger where the hot slag granules are brought into contact with a counter flow of cold air supplied from outside the system, or with a counter flow of a mixture of the above cold air and a part of the primary hot blast exhausted from the primary heat-exchanger to effect heat-exchange and to recover a secondary hot blast. 
     In the present invention, in order to achieve effective heat exchange with the molten slag, the slag becomes solidified. However, it is still at high temperatures and in order to increase the surface area of the solidifying slag as much as possible, it is necessary to divide the slag as finely as possible. For this purpose, the molten slag stream discharged continuously from a metallurgical furnace is atomized finely by blowing a compressed air jet thereto from an air nozzle provided at the lower side at the front end of a slag trough, and the 1100° divided slag granules thus obtained are introduced by the air atomization into a primary heat-exchanger of rotary cylindrical type where a preliminary heat exchange is performed with the air jet from the nozzle and then the hot slag granules discharged from the primary heat exchanger is introduced into a counter-current type secondary heat-exchanger separate from the primary heat-exchanger to recover continuously the hot blast from the hot slag granules. In this case, it is desirable to maintain the hot slag granules after the atomizing at a temperature below its melting point, for example, within a range of from 1100 to 1300° C. in case of the molten slag produced during ferro-nickel refining, so as to prevent the granules from adhering to each other or adhering to the inside wall of the equipments. 
     It may be considered to be feasible to perform the atomizing of the molten slag and the heat-exchange simultaneously only by the primary heat-exchanger. However, this procedure would have a defect in that it is necessary to discharge the slag while it is still at high temperatures after the heat recovery in order to increase the temperature of the recovered hot air blast because the slag granules and the air are in a co-current flow in the primary heat-exchanger, as clearly understood from common knowledge of the chemical engineering, and thus the heat recovery efficiency is lowered. 
     On the other hand, in order to increase the heat recovery efficiency by lowering the temperature of the discharged slag, it is necessary to lower the temperature of the recovered hot air blast, so that the application of the hot blast is considerably limited. 
     Therefore, the provision of the two separate heat-exchangers as in the present invention is most advantageous because the temperature of the hot blast can be maintained at a higher level and the heat efficiency can be increased considerably. 
     The present invention is considered to be most advantageous when it is considered that most of the modern metallurgical furnaces discharge the molten slag only intermittently, because according to the present invention, it is possible to obtain a hot blast continuously from the heat-exchanger and supply the hot blast directly to its specific destination such as a rotary dryer and a rotary kiln burner in accordance with the operational conditions. 
     The amount and temperature of the primary hot air blast exhaust from the primary heat-exchanger depend almost on the ratio of the air used for atomizing the molten slag to the molten slag, and the control of the temperature of the hot slag granules discharged from the primary heat-exchanger and the amount of air coming in through openings of the primary heat-exhanger, and the amount and temperature of the finally recovered secondary hot air blast and heat recovery efficiency depend on the control of the amount of the hot blast exhausted from the primary heat-exchanger, the amount of cold air coming into the secondary heat-exchanger from outside the system and the temperature of the cooled slag discharged from the secondary heat-exchanger. Therefore, the primary and secondary heat-exchangers are operated under such conditions as to obtain an optimum amount and temperature of the recovered secondary hot blast for its specific application with the highest possible heat efficiency. 
     The connection between the primary heat-exchanger and the secondary heat-exchanger, namely the transfer of the hot slag after atomizing therebetween is preferably performed continuously by a heat-resistant conveyor designed so as to prevent the heat loss, or performed discontinuously by storing the hot slag granules temporarily in a heat-resistant container. In the case where the molten slag is discharged intermittently from an electric furnace, it is desirable to provide on the upper portion of the secondary heat-exchanger a heat-resistant storing receptacle, for instance a slag bin, having a capacity matching to the interval of the molten slag discharge and the maximum storing amount of the hot slag granules discharged from the primary heat-exchanger, and a device for discharging a constant amount of the hot slag granules stored in the receptacle so as to supply the hot slag granules continuously to the secondary heat-exchanger. 
     Further, in order to facilitate the removal of the slag adhering to the inside wall of the primary heat-exhanger in a plate-like or ring-like form, it is effective, if the primary heat-exchanger is designed so as to have a top-cut conical shape with its wider end forming the discharge end or to cool the outer shell of the cylindrical heat-exchanger by water sprays or by air blowings. In this case, cool water, hot water or water vapor may be mixed with the atomizing air jet, or may be blown into the primary heat-exchanger independently from the atomizing air jet. The procedures, however, should be avoided when the recovered hot blast is intended to be used for drying and calcining of ores or to be used as heat source for burners. If necessary, it is desirable to provide a crushing device at the discharging end of the primary heat-exchanger for finely crushing the hot slag granules obtained in the primary heat-exchanger so as to obtain an optimum slag particle size (for example not larger than 20 mm) for the subsequent transfer and treatment in the secondary heat-exchanger. 
     Regarding the secondary heat-exchanger, a conventional type of heat-exchanger such as a fluidized bed type, a stoker type and a pebble-heater type may be used other than the rotary cylindrical type depending on the condition of the hot slag granules after the atomizing but, it is desirable that they are of heat insulating structure and are designed so as to flow the hot slag granules in counter current against the air flow, thereby obtaining a high-temperature hot blast with a high degree of heat efficiency. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     The present invention will be described in more detail with reference to the attached drawings. 
     FIG. 1 shows one example of the apparatus used in the present invention. 
     FIG. 2 is a graph showing the relation between the heat recovery efficiency and the temperature of the secondary hot air blast. 
     FIG. 3 is a graph showing the relation between the heat recovery efficiency and the ratio of the slag atomizing air to the slag. 
    
    
     In FIG. 1, the molten slag stream 2 dropping down from the trough 1 is atomized into floating hot slag granules 6 by the air jet 3 blown aslantly upward from the bottom side of the trough 1, and these granules 6 solidify at least on the outer circumferential portion during their floating in the primary heat-exchanger 4 of rotary cylindrical type, and drop on the bottom of the primary heat-exchanger 4 where they remain as hot granules 7, and move down due to the so-called kiln action gathering into the lower portion of the hood 5. The hot slag granules 7 thus obtained are supplied continuously to the secondary heat-exhanger 11, for example a rotary cylindrical type separately arranged from the primary heat-exchanger 4 through a chute, or they are once stored in a slag bin 8 and then supplied to the secondary heat-exchanger 11 continuously with a controlled flow rate. 
     Meanwhile, the air jet used for atomizing the molten slag in the primary heat-exchanger 4 together with air coming in the cylinder from openings, flow in a co-current to the hot slag granules, exchanging part of the slag heat to form a primary hot blast 9. 
     The cool air 10 introduced from outside the system into the secondary heat-exchanger 11 is mixed with at least part of the primary hot air blast 9 exhausted from the primary heat-exchanger 4 to obtain a hot air blast at a controlled temperature and in a controlled flow rate, which is blown into the secondary heat-exchanger 11 through the blowing-in opening 14 at the lower portion of the secondary heat-exchanger 11 in countercurrent to the hot slag granules 7 so as to fully cool the hot slag granules 7. After the cooling the hot blast is recovered as a secondary recovered hot blast 15 which is directed to its final destination. The cooled slag granules 13, after enough heat-exchange, are discharged from the lower portion of the hood 12 provided at the discharge end of the secondary heat-exchanger 11. 
     Although not shown in FIG. 1, a blower or exhaust fun of optimum design are provided in the air flow paths. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be understood more clearly from the following embodiments. 
     EXAMPLE 1 
     The primary hot air blast exhausted from the primary heat-exchanger of rotary cylindrical type is mixed with a predetermined amount of cool air from outside the system to adjust the blast temperature and amount, thereby varying the temperature of the secondary hot air blast recovered from the secondary heat-exchanger of rotary cylindrical type. Variations in the heat recovery efficiency in this embodiment are shown in Table 3 and FIG. 2. 
     
                       Table 1                                                     
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Molten Slag                                                               
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Electric Furnace                                                          
                7500 KVA (Ferro-nickel                                    
                     refining furnace)                                    
Slag Flow Rate  60 t/h (average)                                          
Slag Temperature                                                          
                1500°-1600° C.                              
Treating Time   30- 35 min.                                               
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                       Table 2                                                     
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Equipment                                                                 
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Primary       2.5m (3m) diameter × 7m length                        
              No lining                                                   
Heat-         Inclination : 4/100                                         
Exchanger     Rotation Rate: 15 rpm                                       
Secondary     2.5m diameter × 10m length                            
              Heat insulation of 100 mm castable                          
              lining with a lifter                                        
Heat-                                                                     
              Inclination : 4/100                                         
Exchanger     Rotation Rate: 3 rpm                                        
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                                           Table 3                                 
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Relation between the temperature of the                                   
recovered secondary hot air blast and                                     
the heat recovery efficiency                                              
                   1   2   3   4                                          
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Primary                                                                   
      Atomizing air jet                                                   
      (Nm.sup.3 /H)                                                       
                   14,400                                                 
                       14,400                                             
                           14,400                                         
                               14,400                                     
Heat- Primary blast (Nm.sup.3 /H)                                         
                   16,800                                                 
                       16,800                                             
                           16,800                                         
                               16,800                                     
      Primary blast temp. (° C.)                                   
                   250 250 250 250                                        
exchange                                                                  
      Hot slag temp. (° C.)                                        
                   1,200                                                  
                       1,200                                              
                           1,200                                          
                               1,200                                      
Secondary                                                                 
      Secondary blast                                                     
      (Nm.sup.3 /H)                                                       
                   58,000                                                 
                       72,200                                             
                           94,200                                         
                               115,200                                    
Heat- Secondary blast                                                     
      temp. (° C.)                                                 
                   860 735 600 510                                        
exchange                                                                  
      Cooled slag temp. (° C.)                                     
                   200 150 100 70                                         
      Heat Recovery (%)                                                   
                   56  59  62  63                                         
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     As shown in Table 3 and FIG. 2, the temperature of the secondary hot air blast has an almost linear relation with the heat recovery efficiency, and when the temperature of the secondary hot air blast is at 860° C., the heat recovery efficiency is 56%, and when the temperature is at 510° C., the efficiency is 63%. 
     EXAMPLE 2 
     Under the same condition of the slag tapping as in Example 1, the amount of the slag atomizing air, namely the ratio of the atomizing air/slag is changed. The heat-exchange efficiency (namely transfer ratio of the heat amount contained in the slag entering the heat-exchanger to the hot air blast) in each of the primary and secondary heat-exchangers, and the heat recovery efficiency, etc., are shown in Table 4, and the relation between the ratio of the atomizing air/slag and the heat recovery efficiency is shown in FIG. 3. 
     As understood from the data shown in Table 4 and in FIG. 3, the most advantageous ratio of the atomizing air to the slag for obtaining a desired heat recovery efficiency is from 200 to 280 Nm 3  /t. When the amount of the atomizing air is too small, the atomizing of the slag is incomplete so that the molten slag drops down from the top end of the slag trough and solidifies resulting in an increased proportion of lump slag in the hot slag granules, hence lowering of the heat recovery efficiency in the secondary heat-exchanger. On the other hand, when the amount of the atomizing air is excessive, the slag is atomized into excessively fine particles so that radiation and heat transfer to the surface of the shell of the primary heat-exchanger increase, resulting in lowering of the heat recovery efficiency. 
     EXAMPLE 3 
     This example shows the saving of the dryer fuel when part of the recovered hot air blast exhausted from the secondary heat-exchanger is utilized for drying nickel ores in a rotary dryer. 
     As shown in Table 6, when the temperature of the hot air blast is changed within a range of from 450° to 740° C., while the amount of the hot air blast introduced into the rotary dryer is maintained constant at 24,300 Nm 3  /H, more saving of the fuel can be achieved by a higher temperature of the hot air blast, and when the amount of the hot air blast is changed within a range of from 15,000 to 27,700 Nm 3  /H, while the temperature of the hot air blast is maintained constant at 670° C., more saving of the fuel can be achieved by a larger amount of the hot air blast. 
     
                                           Table 4                                 
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Results obtained by changing the amount                                   
of atomizing air jet (atomizing air/slag)                                 
              Experiments No.                                             
              1   2   3   4   5   6                                       
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Primary                                                                   
      Ratio                                                               
      of atomizing                                                        
              120 160 200 240 280 320                                     
      air jet/slag                                                        
      (Nm.sup.3 /t)                                                       
      Primary hot                                                         
      blast (Nm.sup.3 /H)                                                 
              16,000                                                      
                  16,200                                                  
                      16,500                                              
                          16,800                                          
                              17,000                                      
                                  19,200                                  
Heat- Temp. of red                                                        
      hot slag (° C.)                                              
              1,250                                                       
                  1,240                                                   
                      1,220                                               
                          1,200                                           
                              1,170                                       
                                  1,130                                   
      Primary hot                                                         
      blast temp.                                                         
              170 200 230 250 270 250                                     
      (° C.)                                                       
      Primary heat-                                                       
      exchange rate                                                       
              2.9 3.4 4.0 4.4 4.8 5.1                                     
Exchange                                                                  
      (%)                                                                 
Secondary                                                                 
      Amount of cool                                                      
      air (Nm.sup.3 /H)                                                   
              58,200                                                      
                  58,200                                                  
                      56,400                                              
                          54,900                                          
                              52,900                                      
                                  52,000                                  
      Secondary hot                                                       
      blast (Nm.sup.3 /H)                                                 
              74,200                                                      
                  74,400                                                  
                      72,900                                              
                          71,700                                          
                              69,900                                      
                                  68,800                                  
      Ratio of                                                            
Heat- secondary hot                                                       
      blast slag                                                          
      (Nm.sup.3 /t)                                                       
              1,237                                                       
                  1,240                                                   
                      1,215                                               
                          1,195                                           
                              1,165                                       
                                  1,147                                   
      Secondary hot                                                       
      blast temp.                                                         
              600 650 680 700 700 700                                     
      (° C.)                                                       
      Cooled slag                                                         
              200 160 120 100 100 100                                     
      temp. (° C.)                                                 
      Secondary heat-                                                     
      exchange rate                                                       
              66.9                                                        
                  73.3                                                    
                      76.3                                                
                          78.6                                            
                              78.3                                        
                                  78.0                                    
Exchange                                                                  
      (%)                                                                 
      Heat recovery                                                       
              48.5                                                        
                  53.0                                                    
                      54.5                                                
                          55.3                                            
                              54.0                                        
                                  53.1                                    
      (%)                                                                 
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                       Table 5                                                     
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Rotary Dryer                                                              
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Treating Capacity Wet ore 50 t/H. Dewatering 4.8t/H                       
3 m diameter × 30 m length                                          
No lining, With lifter along the whole length                             
Inclination 3/100                                                         
Rotation Rate : 1.5 rpm                                                   
C grade Heavy Oil Burner                                                  
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                                           Table 6                                 
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Saving of Fuel                                                            
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            Hot Air Blast                                                 
                     Fuel (Heavy Oil)                                     
            Amount                                                        
                 Temp.                                                    
                     Amount                                               
                           Saved   Saving                                 
            (Nm.sup.3 /H)                                                 
                 (° C.)                                            
                     used (l/H)                                           
                           Amount (l/H)                                   
                                   Rate (%)                               
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No recovered hot                                                          
air blast is used                                                         
            --   --  609   --      --                                     
Recovered                                                                 
      Constant                                                            
            24,300                                                        
                 450 263   346     56.8                                   
      Amount of                                                           
            24,300                                                        
                 535 183   426     70.0                                   
hot   hot air                                                             
            24,300                                                        
                 650  84   525     86.2                                   
      blast 24,300                                                        
                 740  0    609     100                                    
air   used                                                                
      Constant                                                            
            15,000                                                        
                 670 274   335     55.0                                   
blast Temp. of                                                            
            19,500                                                        
                 670 174   435     71.4                                   
      hot air                                                             
            23,600                                                        
                 670  83   526     86.4                                   
is    blast 27,700                                                        
                 670  0    609     100                                    
used  used                                                                
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  What is claimed is: