Patent Publication Number: US-2016222487-A1

Title: Recovery of metals

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present invention relates to the recovery of valuable metals from waste electronics. In particular, the present invention relates to the recovery of metals such as tin from printed circuit boards. This is a US national stage application of PCT/GB2014/052801 filed Sep. 16, 2014, claiming priority to GB 1316443.9 filed Sep. 16, 2013, the entire disclosures of which are expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the recovery of valuable metals from waste electronics. In particular, the present invention relates to the recovery of metals such as tin from printed circuit boards. 
     BACKGROUND OF THE INVENTION 
     In recent years, there has been concern about the growing volume of end-of-life electronics. A large amount of electronic waste is consigned to landfill without any attempt being made to recycle the constituent materials. This represents both a source of pollution and a waste of valuable resources. With growing pressure to avoid land-filling this electronic waste material, there is a need to develop a method for recycling the material and to enable recovery of the valuable metals therein. 
     During the manufacture of printed circuit boards, there is commonly a step of stripping a solder mask from the board. U.S. Pat. No. 5,244,539 discloses such a method for manufacturing a printed circuit board. The method relies upon the use of a metal-dissolving liquid to perform the stripping. The metal dissolving liquid is used to solubilise the tin and to provide a bright finish to the copper on the board. Nitric acid is commonly used in the metal dissolving liquid. The use of various additives to control the extent and quality of the process are well known (see, for example, U.S. Pat. No. 5,505,872). In these prior art documents the intention is to strip a solder layer from the surface of the board; the methods are not conducted for sufficient time to dissolve metals forming part of the board. Nor are these methods carried out when components are present on the board, due to the risk of component damage. 
     EP 0 023 729 discloses a method for de-tinning tin plate. The method is applied to old cans, tin plate, lacquered tin sheet waste and the like. The method uses an oxidising solution and a source of ferric ions. The method exploits a low temperature exchange reaction between iron and tin. There is no teaching to use this method to recover metals from electronic waste, nor of precipitating out dissolved metals from the de-tinning solution. 
     WO2013/104895 describes a method for recovering metal, such as tin, from electronic waste, such as printed circuit boards, using a recovery solution comprising nitric acid and ferric chloride. Metals are separated from the recovery solution by increasing the pH. While this method is capable of recovering different metals from electronic waste, approximately 30-40 wt % of the metals dissolved from the electronic waste form a sludge. Accordingly, the yield of the method is not high. In addition, the recovered tin precipitate typically contains high levels of iron, which decreases the value. The method is also difficult to scale up in view of the high levels of precipitates that form prior to raising the pH. 
     Accordingly, there is a desire for a recycling process to prevent the present loss of valuable metals from electronic waste into landfill. Moreover, there is a desire for a process that will overcome, or at least mitigate, some or all of the problems associated with the methods of the prior art or at least provide a useful or optimised alternative. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention provides a method for recovering metal from electronic waste, the method comprising; 
     contacting electronic waste with a recovery solution to dissolve metals from the electronic waste into the recovery solution, wherein the recovery solution comprises nitric acid and ferric nitrate; and raising the pH of the recovery solution to precipitate at least some of said metals therefrom. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a flow chart of the method steps of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present disclosure will now be further described. In the following passages different aspects of the disclosure are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. 
     The method of the present invention is for the recovery of metals from electronic waste. Electronic waste, or e-waste, is a term in the art used to describe any discarded electrical or electronic devices. Electronic waste will contain useful metals from the electronic connections and wiring required for the device to have functioned. Associated with this waste there will also be waste in the form of casing or wiring supports. Electronic waste can take many forms including, by way of example, computers, mobile phones and other household electrical devices. Electronic waste will contain a wide variety of different materials. For example, cathode ray tube monitors and televisions may contain solder, lead, cadmium and beryllium. 
     Preferably, the electronic waste comprises or consists of circuit board waste. That is, preferably the waste includes circuit boards and/or printed circuit boards, or broken fragments thereof. Such circuit board waste preferably comprises the electronic components originally soldered to a printed circuit board, since these can be readily recovered using the method of the present invention. A printed circuit board will necessarily contain metal, in particular in the form of wiring and contacts, and may further include solder applied to the contacts. 
     The present inventors have discovered that the use of the recovery solution disclosed herein allows for the treatment of bulk electronic waste. Advantageously, the use of the recovery solution comprising nitric acid and ferric nitrate allows for the dissolution of the metals commonly used in the electronic waste. Moreover, increasing the pH allows for the precipitation of at least some of the dissolved metals from the recovery solution. These metals can then be separated, refined and reused. In comparison to metal recovery methods known in the art, the level of sludge formation is particularly low. 
     The combination of nitric acid and ferric nitrate is highly oxidising. The inventors have surprisingly found that the use of such a highly oxidising recovery solution reduces the amount of metals precipitated from the recovery solution on contact with the electronic waste. In other words, the metals remain dissolved in the recovery solution prior to increasing the pH. This enables the metals to be selectively recovered from the recovery solution by selective pH precipitation, and in high yield. 
     This effect is particularly advantageous when the electronic waste comprises a large number of different metals. For example, when the electronic waste comprises silver and/or lead, the silver and lead remain in solution prior to raising the pH. This is in contrast to methods using ferric chloride-containing or ferric sulphate-containing recovery solutions, which typically result in precipitation of silver and lead salts on contact between the recovery solution and electronic waste. Such precipitation may be particularly problematic when the method is carried out on a large scale. 
     Compared with methods using ferric chloride or ferric sulphate recovery solutions, the method of the present invention is capable of selectively recovering tin in higher yield and with higher purity. In particular, when the electronic waste comprises tin and iron, the method of the present invention is capable of recovering a tin precipitate containing very low levels of iron. This is particularly advantageous in view of the higher sale price of tin compared to iron. The tin precipitate may also contain only minimal levels of other metals, typically substantially no other metals 
     The recovery solution may be re-used merely by the addition of nitric acid, which may serve to oxidise ferrous ions back to ferric ions. This results in significant cost benefits of the method. In addition, the method is environmentally friendly and non-complex. Due to the highly oxidising nature of the recovery solution, more than 10 tonnes of circuit board waste may be processed using only one ton of recovery solution. 
     The precipitated metal may be in the form of, for example, an oxide or a hydroxide. The precipitate may be recovered from the recovery solution, for example, by filtration and/or centrifuge. The pH may be raised, for example, by the addition of NaOH. 
     The step of contacting the electronic waste with a recovery solution typically comprises immersing the electronic waste in the recovery solution. Immersion may result in a high level of contact between the recovery solution and the electronic waste, thereby resulting in the recovery of high levels of metal from the electronic waste. The electronic waste is preferably immersed until substantially all of the metal in the waste is dissolved. This increases the yield of the method. The recovery solution and the immersed electronic waste may be agitated, for example by the use of ultrasonic agitation. Such agitation may increase the contact of the recovery solution with the electronic waste, thereby increasing the efficiency and yield of the method. 
     The inventors have further discovered that the recovery solution can be applied without needing to pre-grind the electronic waste. Indeed, preferably there is no grinding step and the electronic waste is not in a powder form. It will be appreciated that the electronic waste may be damaged or fragmented, for example, if recovered from a skip or landfill. However, the present method allows for the treatment of such waste and even the separation and recovery of electronic components therefrom. 
     In one embodiment it may be advantageous to homogenise the electronic waste with a first fragmentation step. This fragmentation step preferably is conducted to ensure a uniform waste for treatment, without completely destroying the electronic components that may be present. For example, when the waste is primarily circuit board waste, the waste is preferably fragmented to have a longest diameter of from 1 to 10 cm, more preferably from 2 to 8 cm and most preferably from 3 to 7 cm. 
     The present inventors have discovered that the method of the present invention allows for the recovery of metals from electronic components and also for the recovery of discreet electronic components. These components may be separated from the waste by the dissolution of their metal contacts or the solder holding them to the waste, or by shearing. Moreover, because there is no need to grind the waste, it is easy to coarsely filter and separate the treatment mixture. Due to the different relative sizes, without a pre-ground board it is possible to separately recover (1) any non-metal waste, such as the plastic from printing circuit boards; (2) discrete electronic components, such as transistors or resistors; (3) any metal precipitated from the solution; and (4) the recovery solution with dissolved metals therein. Additionally, since the residue from the above (1) is not a fine powder, it may be more readily handled and processed. Preferably the above materials (1) and/or (2) are recycled as appropriate using known recycling techniques. 
     In one embodiment electronic components are separated from the waste by using a standard shearing technique. One advantage of removing electronic components in this way rather than by dissolution of their metal contacts or solder is that damage may be less likely to occur to the components. For example, metals such as, for example, gold, are not lost from the components when the components are contacted with the recovery solution. Another advantage of the use of shearing is that it may avoid an increase in the metals content (including undesirable metals) of the recovery solution, which can lower throughput and use up nitric acid. Since the undesirable metals content is reduced, a higher value metal cake may be obtained on precipitation. Furthermore, reducing the metals content may serve to reduce the occurrence of a run-away exothermic reaction that may occur at excessive metals content. In an alternative embodiment, the occurrence of such a run-away exothermic reaction may be reduced by employing automated temperature control and/or cooling. 
     The recovery solution comprises ferric nitrate. The recovery solution preferably comprises from 0.5 to 12 wt % ferric nitrate, more preferably from 2 to 6 wt %, even more preferably from 1 to 3.5 wt %, still even more preferably about 3 wt %. The ferric ions function to accelerate dissolution of tin and copper alloys. Increased levels of ferric nitrate increase the oxidising power of the recovery solution, thereby increasing the amount of metals that remain in solution prior to raising the pH. However, increased levels of ferric nitrate may result in the recovery of precipitates containing high levels of iron. 
     The recovery solution comprises nitric acid. The recovery solution preferably comprises from 10 to 60 wt % nitric acid, more preferably from 15 to 40 wt %, even more preferably from 5 to 30 wt %, still even more preferably about 20 wt %. This concentration of the solution is particularly effective at dissolving the metals commonly present in electronic waste. In particular, the nitric acid functions to dissolve metals, notably silver and tin from solder as well as copper, lead, iron and other minor metals. Any exposed gold may be liberated by undermining but not dissolved. Gold may be recovered by further treatment steps after solder removal. Nitric acid may also function to re-dissolve any ferrous oxide precipitates, thereby regenerating the ferric nitrate in the recovery solution. This may increase the lifetime of the recovery solution. In addition, the iron content of the recovered precipitates may be reduced. 
     The recovery solution may comprise chloride ions. Chloride ions may function to accelerate dissolving of the metals in the recovery solution. In addition, the presence of chloride ions may allow all metals to be kept in solution. Chloride ions may be in the form of, for example, HCl, NaCl, KCl and/or NH 4 Cl. However, when the electronic waste comprises metals such as, for example, silver and lead, the amount of chloride ions present in the recovery solution is typically kept low in order to avoid precipitation of chloride salts such as, for example, lead chloride or silver chloride. In this regard, the recovery solution preferably comprises less than 5 wt % chloride ions, more preferably less than 3 wt % chloride ions, even more preferably from 0.5 to 2 wt % chloride ions, still more preferably about 1.5 wt % chloride ions 
     Preferably, the recovery solution further comprises sulphamate ions. Nitric acid may be reduced by metallic species in the electronic waste to nitrous acid. Sulphamate ions are capable of reacting with nitrous acid to convert it to sulphuric acid. Accordingly, the evolution of toxic NO x  fumes and the simultaneous temperature rise can be avoided. Sulphamate ions are preferably in the form of ammonium sulphamate, which is easy to handle and readily available. Preferably, the recovery solution comprises from 1 to 10 wt % ammonium sulphamate, more preferably from 2 to 6 wt %, even more preferably about 4 wt %. 
     Preferably, the recovery solution further comprises a copper corrosion inhibitor. Avoiding attack of copper is particularly advantageous when the electronic waste comprises circuit board waste. Circuit boards typically comprise copper boards, which may be recovered from the recovery solution once the solder has been dissolved therefrom. The copper corrosion inhibitor is preferably an azole compound such as, for example, benzotriazole, imidazole, pyrazole, triazole and derivatives thereof. Benzotriazole is a particularly effective copper corrosion inhibitor, is low cost, and is compatible with the other components of the recovery solution. In addition, benzotriazole may also form an inhibiting layer on active metals (i.e. metals that may react to form NO such as, for example, copper, aluminium, zinc and magnesium) present in the electronic waste to prevent them dissolving during processing. Preferably, the recovery solution comprises up to 1 wt % benzotriazole, more preferably from 0.01 to 0.75 wt %, even more preferably from 0.01 to 0.5 wt % benzotriazole, still even more preferably about 0.05 wt %. 
     Preferably the method further comprises subjecting the recovery solution and electronic waste to agitation with an oxygen-containing gas, preferably air. Such agitation increases the contact between the recovery solution and the electronic waste, thereby increasing the efficiency of the method. It also serves to maintain all of the dissolved metals in a high oxidation state prior to increasing the pH. As discussed above, this is particularly important when the electronic waste contains many different types of metal. Such agitation also serves to re-oxidise ferrous ions back to ferric ions. Accordingly, the oxidising power of the recovery solution is maintained. In addition, the iron content of the recovered precipitates may be reduced. Air agitation may be carried out, for example, using a sparge pipe connected to an air pump. 
     The electronic waste is preferably immersed for at least 30 minutes in the recovery solution, more preferably from 30 minutes to 24 hours, even more preferably from 2 to 12 hours. Longer treatments may be suitable, but process efficiency favours a faster processing time. Lower grade boards may typically be processed in from 2 to 4 hours, more typically about 3 hours. Higher grade boards may typically be processed in from 5 to 20 hours, more typically about 8 hours. Preferably the treatment is conducted in a batch-wise manner. 
     Preferably, the method further comprises adding chloride ions to the recovery solution prior to raising the pH. This may allow the precipitation and recovery of metals such as silver and lead in the form of chlorides. It is preferable to add the chloride ions to the recovery solution in a separate step rather than by incorporating large amounts of chloride ions into the recovery solution from the start, so that precipitation of metals such as silver and lead may be carried out in a controlled manner. 
     The pH is preferably raised to from 3 to 6, more preferably from 3 to 5, even more preferably from 3.8 to 4. When the electronic waste comprises tin, such pH values may result in precipitation of a tin oxide-rich precipitate, from which tin may be recovered. In view of the highly oxidising nature of the recovery solution, when iron is present in the electronic waste, the iron tends to remain in solution at such pH values. Accordingly, the amount of iron present in the tin oxide-rich precipitate is low. 
     In a preferred embodiment, the method further comprises:
         (i) raising the pH to from 3 to 6;   (ii) raising the pH to greater than 6; and   (iii) lowering the pH to from 3 to 6,       

     wherein precipitate formed by steps (i), (ii) and (iii) is recovered from the recovery solution before the next step. 
     Such a method is particularly advantageous when the electronic waste comprises both iron and tin. As discussed above, step (i) may result in a tin oxide-rich precipitate containing only low levels of iron. In step (ii), raising the pH to 6, preferably from 6.5 to 7.5, more preferably to about 7, may result in the precipitation of an iron-rich precipitate. Accordingly, lowering the pH in step (iii) to from 3 to 6, preferably from 3 to 5, more preferably from 3.8 to 4, may result in a tin-oxide rich precipitate which contains very low levels of iron, typically substantially no iron. This is particularly beneficial in view of the higher price of tin relative to iron. Furthermore, reducing the amount of iron in the precipitate enables the precipitate to be dried to a greater extent, typically to contain less than 5wt % water, more typically about 3 wt % water. This may be advantageous with regard to transportation and/or processing of the precipitate. 
     Preferably, contacting the electronic waste with the recovery solution is carried out in a revolving drum. The use of a revolving drum increases the contact between the electronic waste and the recovery solution, and may ensure that the recovery solution comes into contact with all parts of the electronic waste. Accordingly, the efficiency and yield of the method is increased. Furthermore, when the electronic waste comprises circuit board waste, the use of a revolving drum may help to knock off components from the board, which may be recovered from the recovery solution for recycling or reuse. 
     Preferably, the method further comprises continuous filtration of the recovery solution. This may help to keep the solution clean and free of deleterious material during the method. In addition, when combined with air agitation, continuous filtration may help to keep the dissolved metals in a high oxidation state. Continuous filtration may be carried out, for example, using a membrane press. 
     The method may be a batch process or a continuous process. Preferably, the method is a batch process and the recovery solution is regenerated with nitric acid between batches. As discussed above, nitric acid may re-oxidise ferrous ions in the recovery solution back to ferric ions. 
     The electronic waste preferably comprises circuit-board waste. The method is particularly effective at recovering metals from circuit-board waste. This is because, inter alia, it is particularly effective at recovering the metals typically contained in circuit boards, in particular circuit board solders, for example solders containing tin, lead and silver. In addition, circuit board waste typically comprises multiple types of metal. 
     The electronic waste is preferably contacted with a recovery solution having a pH of less than 1. More preferably, the pH of the solution will be from −1 to 1 and most preferably about 0. The use of these harsh conditions allows for the full dissolution of metals from the electronic waste into the recovery solution. 
     Preferably, the method further comprises removing any electronic waste residue from the recovery solution before raising the pH. This may allow the precipitated metals to be recovered from the recovery solution more easily. 
     The electronic waste is typically not provided in the form of a powder. 
     The step of raising the pH is preferably conducted stepwise to allow for the separate precipitation and recovery of different metals. This provides a cost effective method of separating out the different metals recovered. 
     The present inventors have discovered that the method is suitable for the recovery of one or more of lead, gold, silver, copper, zinc and tin from electronic waste. Obviously, the metals that can be recovered will depend upon the electronic waste and the metals contained therein. Preferably the method is used for at least the recovery of tin from electronic waste. Tin is a common component in solders and will commonly be present in electronic waste. Preferably the method is used for recovery of metals from electronic waste containing lead-free solders. The method is particularly suitable for the recovery of tin, copper and silver, which are typical components of lead free solders. Preferably the method further comprises a step of subjecting the precipitated metals to a conventional recovery treatment to obtain one or more substantially pure metals therefrom. 
     The process of the present invention may be carried out by a standalone unit. Alternatively, the process may be carried out by an “add-on” unit to a standard waste PCB processing unit. 
     The method may further comprise recovering the precipitated metals from the recovery solution, preferably by filtration or centrifugation. The recovered precipitated metals may be dried, preferably using a rotary drying oven. Such drying enhances the metal concentration in the final metal cake, which may result in a more valuable product. The use of a rotary drying oven may result in the final metal cake comprising less than 5 wt % water, more typically about 3 wt % water. 
     The method may further comprise subjecting the precipitated metals to further treatment to recover one or more substantially pure metals. The further treatment may comprise, for example, smelting and/or electrowinning. 
     The method may comprise additionally recovering and sorting electronic components separated from the electronic waste by the dissolution of metal from the electronic waste. Such electrical components may be re-used or recycled as appropriate using known techniques. 
     In a preferred embodiment of the method: 
     the metals recovered comprise tin; 
     the recovery solution and electronic waste are subjected to air agitation; 
     the pH is raised to from 3 to 6; and 
     the recovery solution comprises:
         i. from 1 to 3.5 wt % ferric nitrate;   ii. from 15 to 40 wt % nitric acid;   iii. from 0.5 to 2 wt % chloride ions;   iv. from 1 to 10 wt % ammonium sulphamate; and   v. from 0.01 to 0.5 wt % benzotriazole.       

     The invention will now be discussed further with reference to the Figure, provided purely by way of example, in which: 
       FIG. 1  shows a flow chart of the method steps of the present invention. The reference numerals refer to the following: (1) electronic waste, (2) recovery solution, (3) immersion, (4) agitation, (5) separation, (6) any initial precipitate, (7) waste residue (plastics), (8) electronic components, (9) recovery solution and dissolved metals, (10) increase pH, (11) precipitated metals, and (12) spent recovery solution. 
     EXAMPLES 
     The invention will be described with reference to the following examples which are provided by way of example and are not limiting. 
     Example 1 
     The following aqueous recovery solution was prepared (in weight %): 
     20% nitric acid 
     1.5% hydrochloric acid 
     6% ferric nitrate 
     4% ammonium sulphamate 
     0.1% benzotriazole 
     4.285 Kg of mixed circuit boards were stripped of electronic components before being immersed in 500 ml of the recovery solution in a reactor vessel. The solution was operated in static mode using continuous filtration and employed air agitation when the solution darkened. The solution was occasionally replenished by adding 40 ml nitric acid and 5 g ferric nitrate. The final volume of recovery solution was 900 ml. 
     After approximately 3 hours, the recovery solution was treated with HCl to precipitate lead and some silver from the recovery solution as insoluble chlorides. The pH was then increased step-wise to selectively recover metals dissolved in the recovery solution. The compositions of the precipitates recovered at each stage are set out below in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Chemical compositions of precipitates. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 % Cu 
                 % Pb 
                 % Zn 
                 % Fe 
                 % Sn 
                 Ag (mg/kg) 
               
               
                   
               
               
                 Precipitate formed when 
                 0.094 
                 72.71 
                 0.006 
                 0.45 
                 0.36 
                 158.0 
               
               
                 900 ml solution treated 
               
               
                 with HCl 
               
            
           
           
               
            
               
                 After lead removal, solution divided into 500 ml sample and a 400 ml sample: 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 500 ml taken to pH 7 
                 0.032 
                 1.86 
                 0.23 
                 8.39 
                 39.85 
                 105.2 
               
               
                 Results: dry ppt 
               
               
                 400 ml taken to about pH 
                 0.047 
                 2.67 
                 0.13 
                 1.81 
                 53.28 
                 163.2 
               
               
                 4 to recover 
               
               
                 predominately tin ppt 
               
               
                 After tin removal, 
                 0.15 
                 12.63 
                 1.00 
                 46.52 
                 0.36 
                 47.90 
               
               
                 resultant solution taken 
               
               
                 to pH 7 to give ppt 
               
               
                 Wash water treated at 
                 0.20 
                 4.38 
                 0.018 
                 0.11 
                 60.60 
                 1954 
               
               
                 about pH 4 
               
               
                   
               
               
                   
                 Cu mg/l 
                 Pb mg/l 
                 Zn mg/l 
                 Fe mg/l 
                 Sn mg/l 
                 Ag mg/l 
               
               
                   
               
               
                 Final wash water after all 
                 3.04 
                 0.05 
                 0.54 
                 0.10 
                 0.14 
                 0.03 
               
               
                 metals removed 
               
               
                   
               
            
           
         
       
     
     Example 2 
     The following aqueous recovery solution was prepared (in weight %): 
     20% nitric acid 
     1.5% hydrochloric acid 
     3% ferric nitrate 
     4% ammonium sulphamate 
     0.1% benzotriazole 
     3.555 Kg of mixed circuit boards were stripped of electronic components before being immersed in 500 ml of the recovery solution in a reactor vessel. The solution was operated in static mode using continuous filtration. The solution was occasionally replenished by adding 40 ml of 50% v/v nitric acid. 
     After approximately 3 hours, the recovery solution was treated with HCl to precipitate lead and some silver from the recovery solution as insoluble chlorides. The pH was then increased step-wise to selectively recover metals dissolved in the recovery solution. The compositions of the precipitates recovered at each stage are set out below in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Chemical compositions of precipitates. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 % Cu 
                 % Pb 
                 % Zn 
                 % Fe 
                 % Sn 
                 Ag (mg/kg) 
               
               
                   
               
               
                 Precipitate formed when 
                 0.001 
                 78.69 
                 0.003 
                 0.005 
                 0.53 
                 60.55 
               
               
                 solution treated with HCl 
               
               
                 Precipitate formed when 
                 0.024 
                 7.01 
                 0.30 
                 3.21 
                 46.69 
                 83.59 
               
               
                 solution taken to pH 7 
               
               
                 Wash water pH 7 
                 0.59 
                 21.70 
                 0.40 
                 2.88 
                 41.90 
                 2879 
               
               
                 precipitate 
               
               
                   
               
               
                   
                 Cu mg/l 
                 Pb mg/l 
                 Zn mg/l 
                 Fe mg/l 
                 Sn mg/l 
                 Ag mg/l 
               
               
                   
               
               
                 Final pH 7 solution after 
                 0.84 
                 0.36 
                 0.07 
                 0.36 
                 0.35 
                 0.01 
               
               
                 metals removed 
               
               
                   
               
            
           
         
       
     
     The results demonstrate that the efficiency of the solution was not effected by reducing the ferric nitrate content to 3%. Furthermore, the solution only needed to be replenished by adding nitric acid. 
     It was also found that the solution could be treated to about pH 5 to recover a higher tin concentrate prior to pH 7 treatment to remove iron. 
     The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.