Abstract:
There is disclosed an environmentally friendly method for recovering gold, silver, copper and iron from valuable metal-contained plasma-molten slag. At first, plasma is used to burn the used printed circuit boards, thus producing the slag. Then, the slag is grinded. Then, leaching, crystallization, precipitation, replacement and electric winning are conducted to recover gold, silver, copper and iron.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an environmentally friendly method for recovering gold, silver, copper and iron from valuable metal contained plasma-molten slag via plasma burning, grinding, leaching, crystallization, precipitation, replacement and electric winning. 
       DESCRIPTION OF THE RELATED ARTS 
       [0002]    Used printed circuit boards contain valuable metals such as silver, gold, copper and iron. If they are disposed of without recovering the valuable metals contained, it will be a hazard to the environment and loss of resources. Conventionally, the used printed circuit boards can be directly crushed before the valuable metals are recovered as disclosed in Taiwanese Patent Publication Nos. 247281 and 36904 for example. However, it consumes excessive energy because they contain a lot of elastic resin that is difficultly crushed to debris. Moreover, there are problems related to production of undesired dust and noise and wearing of machines. 
         [0003]    Alternatively, the conventional burning method can be used for pretreatment. However, it takes much time and therefore expenses a lot of energy to burn the used printed circuit boards for low combustion efficiency. 
         [0004]    As disclosed in Taiwanese Patent Publication No. 1268184, the used printed circuit boards are heated to a temperature higher than 200 degrees Celsius to melt solder on the used printed circuit boards so that electronic parts can be removed from the used printed circuit boards. Then, the electronic parts are submerged in a solvent and dissolved so that the valuable metals can be recovered. 
         [0005]    Therefore, the present invention is intended to obviate or at least alleviate the problems encountered in prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    It is the primary objective of the present invention to provide an environmentally friendly method for recovering valuable metals from used printed circuit boards. 
         [0007]    To achieve the foregoing objective, in the method, plasma is used to burn the used printed circuit boards, thus providing slag. The slag is grinded to debris smaller than 2 mm. A sieve with meshes of 0.149 mm is used to screen the debris into two fractions, the larger and the smaller ones. A magnet is used to separate ferromagnetic debris from non-ferromagnetic debris so that the ferromagnetic debris can be provided to a steel-making factory. The non-ferromagnetic large debris, which is rich in copper, can be provided to a copper refinery. The non-ferromagnetic small debris, which still contains valuable metals, can be further treated for recovering gold, silver and copper. 18N sulfuric acid is used to leach the debris. The solid/liquid ratio is retained at 10 g/50 ml, and the operational temperature is kept at 70 degrees Celsius so that 90.56% of the copper is released from the non-ferromagnetic small debris to the sulfuric acid after 1 hour. The copper-contained primary leaching solution is subsequently separated from the primary sulfuric acid-leached residue and retained at 27 degrees Celsius for 48 hours for crystallization. 58.28% of the copper is precipitated and recovered in the form of copper sulfate crystals. The copper remained in the crystallization filtrate is replaced with iron powder, which is used as replacement reagent. 100% of the copper can be recovered as copper powder from the filtrate when iron is added at 100 times of the theoretical amount. 18N sulfuric acid is used to leach the primary sulfuric acid-leached residue again. The solid/liquid ratio is retained at 50 g/50 ml, and the temperature is kept at 70 degrees Celsius so that 100% of the copper is released from the primary sulfuric acid-leached residue to the sulfuric acid after 2 hours. The secondary copper-contained leaching solution is then separated from the secondary sulfuric acid-leached residue. The copper in the solution is again replaced with iron powder so that copper is completely recovered when iron is added at 150 times of the theoretical amount. 8N nitric acid is used to leach the secondary sulfuric acid-leached residue, wherein the solid/liquid ratio is retained at 1 g/50 ml, and the temperature is kept at 70 degrees Celsius so that 100% of the silver is released to the nitric acid from the residue after 4 hours. Then the optimal silver-contained leaching solution is separated from the nitric acid-leached residue and using ammonia solution to adjust the pH value to 10. Subsequently 12N hydrochloric acid is provided for precipitation reaction. The ratio of the hydrochloric acid to the silver-containing leaching solution is 1:4 so that 100% of the silver is recovered in the form of silver chloride. The nitric acid-leached residue is then treated with 100% aqua liquid for recovering gold. The ratio of the residue to the aqua liquid is 0.5 g/50 ml, and the temperature is kept at 70 degrees Celsius so that 100% of the gold is released to the aqua liquid from the nitric acid-leached residue after 4 hours, thus providing optimal gold-contained leaching solution. The solution is treated with zinc powder, as the replacement reagent, to recover about 99.43% of the gold from the optimal gold-contained leaching solution. 
         [0008]    Other objectives, advantages and features of the present invention will become apparent from the following description referring to the attached drawings. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0009]    The present invention will be described via detailed illustration of the preferred embodiment referring to the drawing. 
           [0010]      FIG. 1  is a flow chart of a method for recovering valuable metals from valuable metal-contained plasma-molten slag according to the preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]    Referring to  FIG. 1 , it reveals a method for recovering valuable metals from valuable metal-contained plasma-caused slag according to the preferred embodiment of the present invention. At  1 , the slag is collected. 
         [0012]    At  2 , the slag is screened with a sieve with meshes of 0.149 mm. Thus, large debris  21  larger than 0.149 mm in size is separated from small debris  22  smaller than 0.149 mm in size. 
         [0013]    At  3 , the large debris  21  is tested with a magnet so that ferromagnetic debris  31  is separated from non-ferromagnetic debris  32 . The ferromagnetic debris  31  can be provided to a steel-making factory. The non-ferromagnetic debris  32  is rich in copper and can be provided to a copper refinery. 
         [0014]    At  4 , the small debris  22  is tested with a magnet so that ferromagnetic debris  41  is separated from non-ferromagnetic debris  42 . The ferromagnetic debris  41  can be provided to a steel-making factory. 
         [0015]    At  5 , primary sulfuric acid leaching occurs. The non-ferromagnetic debris  42  is leached with 18N sulfuric acid. During the leaching, the solid/liquid ratio is retained at 10 g/50 ml, and the temperature is kept at 70 degrees Celsius. After 1 hour of leaching, about 90.56% of the copper is released from the non-ferromagnetic debris  42  to the sulfuric acid. Primary copper-contained leaching solution  51  is separated from primary sulfuric acid-leached residue  52 . 
         [0016]    At  6 , the copper-contained leaching solution  51  is retained at 27 degrees Celsius for 48 hours for crystallization. Thus, 58.28% of the copper is recovered in the form of copper sulfate crystals  61 . The copper sulfate crystals  61  are separated from optimal crystallization filtrate  62 . 
         [0017]    At  7 , the copper in the optimal crystallization filtrate  62  is replaced with iron powder that is used as replacement reagent  7 . When the amount of the iron reaches 100 times of the theoretical value, 100% of the copper is recovered from the optical crystallization filtrate  62 . Copper powder  71  is recovered. 
         [0018]    At  8 , secondary sulfuric acid leaching occurs. The sulfuric acid-leached residue  52  is leached with 18N sulfuric acid again. During the leaching, the solid/liquid ratio is retained at 50 g/50 ml, and the temperature is kept at 70 degrees Celsius. After 2 hours of leaching, about 100% of the copper is released from the sulfuric acid-leached residue  52  to the sulfuric acid. Then, secondary copper-contained leaching solution  81  is separated from secondary sulfuric acid-leached residue  82 . 
         [0019]    At  9 , the copper in the secondary copper-contained leaching solution  81  is replaced with iron powder that is used as replacement reagent  9 . When the amount of the iron reaches 150 times of the theoretical value, 100% of the copper is recovered from the secondary copper-contained leaching solution  81 . Copper powder  91  is recovered. 
         [0020]    At  10 , the sulfuric acid-leached residue  82  is leached with 8N nitric acid. During the leaching, the solid/liquid ratio is retained at 1 g/50 ml, and the temperature is kept at 70 degrees Celsius. After 4 hours of leaching, about 100% of the silver is released to the nitric acid from the sulfuric acid-leached residue  82 . Optimal silver-contained leaching solution  101  is separated from nitric acid-leached residue  102 . 
         [0021]    At  11 , the pH of the optimal silver-contained leaching solution  101  is adjusted with ammonia solution. 
         [0022]    At  12 , 12N hydrochloric acid is used as precipitating agent. During the precipitation, the ratio of the hydrochloric acid to the silver-contained leaching solution  101  is 1:4. Thus, 100% of the silver is recovered in the form of silver chloride  121 . 
         [0023]    At  13 , the nitric acid-leached residue  102  is leached with 100% aqua liquid. During the leaching, the ratio of the nitric acid-leached residue  102  to the 100% aqua liquid is 0.5 g/50 ml. The temperature is 70 degrees Celsius. After 4 hours, 100% of the gold is released to the aqua liquid from the nitric acid-leached residue  102 , thus providing optimal gold-contained leaching solution  131 . 
         [0024]    At  14 , the gold in the optimal gold-contained leaching solution  131  is replaced with zinc powder that is used as replacement agent. Thus, 99.43% of the gold is recovered from the optimal gold-contained leaching solution  131 . Gold  141  is collected. 
         [0025]    The method of the present invention exhibits several advantages. Firstly, gold, silver, copper and iron are rapidly recovered from used printed circuit boards. Therefore, the valuable metals can be processed in refineries and reused to reduce the waste of metals. Secondly, hazardous materials produced during the recovering of the valuable metals are reduced. Therefore, the hazard to the environment and human bodies is reduced. 
         [0026]    The present invention has been described via the detailed illustration of the preferred embodiment. Those skilled in the art can derive variations from the preferred embodiment without departing from the scope of the present invention. Therefore, the preferred embodiment shall not limit the scope of the present invention defined in the claims. 
         [0027]    The present invention has been described via the detailed illustration of the preferred embodiment. Those skilled in the art can derive variations from the preferred embodiment without departing from the scope of the present invention. Therefore, the preferred embodiment shall not limit the scope of the present invention defined in the claims.