Patent Publication Number: US-11035023-B2

Title: Reactor systems for recovering metals, and related methods

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/690,717, filed Aug. 30, 2017, now U.S. Pat. No. 10,378,081, issued Aug. 13, 2019 which is a continuation of U.S. patent application Ser. No. 14/845,101, filed Sep. 3, 2015, now U.S. Pat. No. 9,777,346, issued Oct. 3, 2017, the disclosure of each of which is hereby incorporated herein in its entirety by this reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under Contract Number DE-AC07-05-1D14517 awarded by the United States Department of Energy. The government has certain rights in the invention. 
    
    
     FIELD 
     Embodiments of the disclosure relate generally to methods of recovering metals from electronic waste, and to related systems for recovering such metals. More particularly, embodiments of the disclosure relate to sequentially recovering base metals from electronic waste, followed by recovering precious metals, such as silver, gold, and palladium therefrom. 
     BACKGROUND 
     Electronic devices such as smart phones, tablets, desktop computers, laptop computers, televisions, and other electronics contain a significant amount of valuable metals. End-of-life electronic devices (also referred to as electronic waste, or so-called “e-waste”) contain metals including base metals (e.g., zinc, tin, lead, nickel, and copper) and precious metals (e.g., silver, gold, palladium) in electronic circuity of such devices. For example, electronic devices may include circuitry connected by copper traces, nickel leads and terminals, nickel plating on conductive traces, solder (which may include tin, silver, copper, and combinations thereof) used to form electrical connections between conductive components, surface contacts coated with gold, and capacitors including silver and palladium. The electronic devices also include rare earth magnets formed from rare earth elements (e.g., neodymium, yttrium, samarium, etc.) within circuity, speakers, displays, storage devices (e.g., hard disk drives), vibrating components of telephones, and other electronic components of the device. 
     Currently, the metals contained in electronic waste are not sufficiently recovered prior to disposing the electronic waste. In some instances, the electronic waste is landfilled or combusted (e.g., incinerated) without recovering a significant portion of the metals therein. Landfilling the electronic waste has the potential to contaminate soil and underground water. The combustion process may release toxic compounds (e.g., lead) into the atmosphere. 
     Methods of recycling the metals in the electronic waste have been proposed. Such methods include leaching the electronic waste with a strong acid which includes dissolving a majority, if not all, of the metals in the electronic waste in a single leaching solution (leachate). However, it is difficult to recover substantially pure metals of each of the metals dissolved in the leachate. In addition, the leaching process produces hydroxide ions that increase a pH of the leachate and consume a significant amount of the acid therein. To compensate for the increase in pH, additional fresh acid is added to the leachate, undesirably consuming a significant amount of the acid. 
     Accordingly, there is a continuing need for methods of recovering substantially pure metals from electronic waste without consuming large amounts of acid while recovering substantially pure elemental metals. 
     BRIEF SUMMARY 
     In accordance with one embodiment described herein, a method of recovering metals from electronic waste comprises providing a powder comprising electronic waste in at least a first reactor and a second reactor, providing an electrolyte comprising at least ferric ions in an electrochemical cell in fluid communication with the first reactor and the second reactor, contacting the powder within the first reactor with the electrolyte to dissolve at least one base metal from the powder into the electrolyte and reduce at least some of the ferric ions to ferrous ions, contacting the powder within the second reactor with the electrolyte to dissolve at least one base metal from the powder into the electrolyte, the powder in the second reactor comprising a higher weight percent of the at least one base metal than the powder in the first reactor, and oxidizing the ferrous ions at an anode of the electrochemical cell to regenerate the ferric ions. 
     In additional embodiments, a reactor system for recovering metals from electronic waste comprises a plurality of reactors substantially filled with a powder comprising electronic waste, a first electrochemical cell comprising a first electrolyte including ferrous ions, ferric ions, and at least one of hydrochloric acid and sulfuric acid in fluid communication with at least some of the plurality of reactors, and a vessel including a leachate, the vessel configured to be in fluid communication with each of the plurality of reactors. 
     In further embodiments, another method of recovering metals from electronic waste comprises providing a first reactor including powderized electronic waste, a second reactor including powderized electronic waste, and a third reactor including powderized electronic waste, providing a first electrolyte comprising iron ions in a first electrochemical cell, passing the first electrolyte from the first electrochemical cell to the first reactor, from the first reactor to the second reactor, and from the second reactor to the first electrochemical cell, reducing at least one of dissolved silver, dissolved copper, dissolved nickel, dissolved lead, dissolved tin, and dissolved zinc from within the first electrolyte at a cathode of the first electrochemical cell, providing a vessel including a leachate, placing the first reactor in fluid communication with the vessel, placing the third reactor in fluid communication with the first electrochemical cell, contacting the powder in the first reactor with the leachate to dissolve at least one of silver and palladium within the powder in the leachate, and passing the first electrolyte from the first electrochemical cell to the second reactor, from the second reactor to the third reactor, and from the third reactor to the first electrochemical cell. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a simplified flow diagram of a method of recovering metals from electronic waste, in accordance with embodiments of the disclosure; 
         FIG. 2  is simplified schematic illustrating a reaction system, in accordance with embodiments of the disclosure; 
         FIG. 3  is a simplified schematic illustrating an electrochemical cell in fluid communication with a plurality of reactors configured in series, in accordance with embodiments of the disclosure; and 
         FIG. 4  is a simplified schematic illustrating another electrochemical cell in fluid communication with a reactor, in accordance with embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Methods of recovering rare earth elements, base metals, and precious metals from electronic waste (e.g., cell phones, printed circuit boards (PCBs), laptop computers, desktop computers, televisions, etc.) are described, as are reactor systems for recovering such materials from electronic waste. After initial shredding of the electronic waste, the magnetic portions may be separated from the non-magnetic portions thereof. The magnetic portions include rare earth-based magnets and ferrous components (e.g., steel) while the non-magnetic portions include base metals and precious metals. The non-magnetic portions are reduced to a powder and the base and precious metals are recovered by sequential electrochemical and leaching processes. The base metals and precious metals are recovered in elemental form without consuming large amounts of chemicals, such as leachates or oxidants. The reactor system is configured to selectively place one or more reactors of a plurality of reactors in fluid communication with one or more chemistries formulated for recovering a particular metal found in the electronic waste. Accordingly, the methods and systems of the disclosure may increase the recovery efficiency of metals from electronic waste, increase purity of the recovered metals, and consume fewer chemicals than other methods of recovering metals from electronic waste. 
     The following description provides specific details, such as material types, dimensions, and processing conditions in order to provide a thorough description of embodiments of the disclosure. However, a person of ordinary skill in the art will understand that the embodiments of the disclosure may be practiced without employing these specific details. Indeed, the embodiments of the disclosure may be practiced in conjunction with conventional fabrication techniques employed in the industry. In addition, the description provided below does not form a complete process flow, apparatus, or reaction system for recovering metals from electronic waste. The structures described below do not form a complete apparatus or a complete reaction system. Only those process acts and structures necessary to understand the embodiments of the disclosure are described in detail below. Additional acts to recover metals from electronic waste may be performed by conventional recovery techniques. Also note, any drawings accompanying the present application are for illustrative purposes only, and are thus not drawn to scale. Additionally, elements common between figures may retain the same numerical designation. 
       FIG. 1  is a simplified flow diagram illustrating a method of recovering metals from electronic waste, in accordance with embodiments of the disclosure. The method may include an electronic waste shredding process  100  that includes shredding the electronic waste into smaller portions; a magnetic separation process  102  that includes separating magnetic portions of the shredded electronic waste from non-magnetic portions thereof; a magnetic component recovery process  104  (shown in broken lines); and a non-magnetic component recovery process  106  (shown in broken lines). The magnetic component recovery process  104  may include an anaerobic leaching process  108  including exposing the magnetic components to a sulfuric acid leachate in a leaching vessel while purging oxygen from the leaching vessel; a rare earth element precipitation process  110  including precipitating dissolved rare earth elements as double salts from the leachate; and a rare earth element recovery process  111  including recovering precipitated double salts of the rare earth elements and converting the double salts to hydroxides of the rare earth elements. The non-magnetic component recovery process  106  may include a size reduction process  112  (e.g., a milling process) including reducing a size of the non-magnetic components to a powder; a base metal recovery process  114  including recovering at least one of zinc, tin, lead, nickel, copper, and, optionally, silver from the powder and depositing the at least one of zinc, tin, lead, nickel, copper, and, optionally, silver on a cathode of an electrochemical cell; an optional silver recovery process  116  including leaching silver from the powder and recovering the silver from the leachate; a gold recovery process  118  including leaching gold from the powder and depositing the gold on a cathode of an electrochemical cell; and a palladium recovery process  120  including leaching palladium from the powder and recovering the palladium from the leachate. 
     The electronic waste shredding process  100  includes mechanically shredding or otherwise cutting electronic waste (e.g., cell phones, printed circuit boards (PCBs), laptop computers, desktop computers, televisions, etc.) into smaller pieces. The electronic waste shredding process  100  may include substantially detaching magnetic materials of the electronic waste from the non-magnetic materials thereof. During the electronic waste shredding process  100 , the magnetic materials may attach to steel or other ferrous materials of the electronic waste. In some embodiments, the electronic waste is shredded in an e-waste shredder or an e-scrap shredder. 
     Magnetic components in the electronic waste may include rare earth-based magnets and magnetic materials located in hard disk drives (HDDs), loudspeakers, displays, batteries, and in vibrating assemblies of the electronic devices. The magnetic separation process  102  includes separating the magnetic materials from the non-magnetic materials of the shredded electronic waste. Methods of separating magnetic materials from non-magnetic materials are known in the art and therefore, are not described in detail herein. By way of non-limiting example, the shredded electronic waste may be placed onto a lower conveyor belt configured to pass under an upper conveyor belt disposed around a stationary magnetic assembly. The magnetic materials may attach to the upper conveyor belt as the lower conveyor belt passes under the magnetic assembly. As the upper conveyor belt rotates, the magnetic materials may fall outside of the magnetic field of the magnetic assembly and may fall from the upper conveyor belt. Accordingly, the magnetic material may be separated from the non-magnetic material. 
     The magnetic component recovery process  104  includes dissolving the rare earth elements located in the magnetic materials of the electronic waste in the anaerobic leaching process  108 , precipitating the dissolved rare earth elements in the rare earth element precipitation process  110 , and recovering the rare earth elements in the rare earth element recovery process  111 . The anaerobic leaching process  108  includes exposing the rare earth elements to a leachate and dissolving the rare earth elements in the leachate. The leachate may include sulfuric acid, nitric acid, hydrochloric acid, or perchloric acid. In some embodiments, the leachate includes sulfuric acid at about 1 molar (1 M). In some embodiments, a pH of the leachate may be maintained at about 0.5 or lower, such as by adding additional acid to the leachate. 
     During the leaching process, the rare earth elements may react with water and protons from the leachate to dissolve the rare earth elements, while steel in the magnetic materials may undesirably react with oxygen, as shown in the following reactions for a rare earth magnet alloy of neodymium (e.g., Nd 2 Fe 14 B):
 
Nd 2 Fe 14 B(s)+3H 2 O(aq)+34H + →2Nd +3 (aq)+14Fe +2 (aq)+H 3 BO 3 (aq)+18.5H 2 (g)  (1)
 
2Fe+O 2 +4H + 4H + →2H 2 O  (2)
 
As indicated in reaction 2 above, oxygen may react with the steel present in the magnetic material. In some embodiments, the anaerobic leaching process  108  includes purging oxygen from the vessel in which the magnetic components are leached to facilitate leaching of the rare earth elements (e.g., reaction 1) and reduce leaching of the steel (e.g., reaction 2). By way of non-limiting example, a reaction vessel in which the anaerobic leaching process  108  takes place may be purged of oxygen to reduce the amount of steel leaching.
 
     The rare earth element precipitation process  110  may include increasing the pH of the leachate to precipitate the rare earth elements dissolved therein. In some embodiments, the rare earth elements precipitate as double salts. The pH of the leachate may be increased by, for example, adding one or more of sodium hydroxide (NaOH), potassium hydroxide (KOH), or ammonium hydroxide (NH 4 OH) to the leachate. In some embodiments, the pH of the leachate may be increased to a pH of between about 2.0 and about 2.5. In some embodiments, the pH of the leachate is increased by adding, for example, 5 M NaOH to the leachate. In other embodiments, double salts of the rare earth elements may be precipitated by adding potassium sulfate, sodium sulfate, or a combination thereof to the leachate. In yet other embodiments, the rare earth elements may be precipitated as an oxalate or a fluoride. Methods of precipitating rare earth elements are known in the art and, therefore, are not described in detail herein. 
     The rare earth element recovery process  111  may include recovering the precipitated double salts of the rare earth elements formed during the rare earth element precipitation process  110  and exposing the rare earth element double salts to a hydroxide (e.g., NaOH) to form a rare earth element hydroxide (e.g., Nd(OH) 3 ). 
     The non-magnetic component recovery process  106  includes reducing a size of the non-magnetic components into a powder in the size reduction process  112 . The size reduction process  112  may include milling (e.g., ball milling) the non-magnetic materials to a powder having a particle size below about 1 mm. In some embodiments, the electronic waste is milled to a powder having a size between about 0.1 mm and about 1.0 mm, such as between about 0.1 mm and about 0.5 mm or between about 0.5 mm and about 1.0 mm, although the size of the powder is not so limited. The powder may include base metals and precious metals from the electronic waste as well as glass and other non-magnetic materials. 
     Referring to  FIG. 2 , a reactor system  200  for the base metal recovery process  114 , the silver recovery process  116 , the gold recovery process  118 , and the palladium recovery process  120  is illustrated. The reactor system  200  includes a plurality of reactors (e.g., a first reactor  201 , a second reactor  202 , a third reactor  203 , a fourth reactor  204 , and a fifth reactor  205 , etc.) that are configured to be in fluid communication with one or more of a first electrochemical cell  210  for the base metal recovery process  114 , a first leaching vessel  230  for the silver recovery process  116 , a second electrochemical cell  250  for the gold recovery process  118 , a second leaching vessel  270  for the palladium recovery process  120 , and a rinse solution tank  290  for rinsing any of the reactors of the plurality of reactors. Pumps  212 ,  232 ,  252 ,  272 ,  292  may be connected to the first electrochemical cell  210 , the first leaching vessel  230 , the second electrochemical cell  250 , the second leaching vessel  270 , and the rinse solution tank  290 , respectively, to pump solutions therefrom to each of the plurality of reactors, depending on the configuration of the reactor system  200 . 
     Although five reactors are illustrated in  FIG. 2 , in some embodiments, the plurality of reactors may include more or less reactors (e.g., three, four, six, seven, etc.) than the number illustrated. Each of the first reactor  201 , the second reactor  202 , the third reactor  203 , the fourth reactor  204 , and the fifth reactor  205  may be loaded with powder  222  formed during the size reduction process  112 , as described above. Each of the plurality of reactors may be of substantially the same design, construction, and materials. 
     The reactor system  200  may be configured such that each of the plurality of reactors is configured to be exposed to different chemistries formulated to recover the metals from the powders  222  disposed within the reactors. The reactor system  200  may be configured such that each of the plurality of reactors may be selectively placed in fluid communication with each of the first electrochemical cell  210 , the first leaching vessel  230 , the second electrochemical cell  250 , the second leaching vessel  270 , and the rinse solution tank  290 . For example, as illustrated in  FIG. 2 , the reactor system  200  may include one or more valves configured to selectively place each of the plurality of reactors in fluid communication with one or more of the first electrochemical cell  210 , the first leaching vessel  230 , the second electrochemical cell  250 , the second leaching vessel  270 , and the rinse solution tank  290 . The reactor system  200  may further be configured to selectively place one or more reactors in series behind one or more other reactors. For example, as will be described herein, the reactor system  200  may be configured to selectively place at least a first reactor in fluid communication with, for example, at least a second reactor and the first electrochemical cell  210 . 
     The reactor system  200  may also be configured to selectively isolate each of the plurality of reactors from each other and from one or more of the first electrochemical cell  210 , the first leaching vessel  230 , the second electrochemical cell  250 , the second leaching vessel  270 , and the rinse solution tank  290 . For example, each reactor may be configured to be isolated from the reactor system  200  for loading and unloading the powder  222  therefrom while the other reactors of the plurality of reactors remain in fluid communication with other portions of the reactor system  200 . Accordingly, at any one time, one or more of the reactors of the plurality of reactors may be in fluid communication with the first electrochemical cell  210 , while one or more other reactors of the plurality of reactors is in fluid communication with the first leaching vessel  230 , one or more reactors is in fluid communication with the second electrochemical cell  250 , and one or more other reactors is in fluid communication with the second leaching vessel  270 . Yet another reactor may be offline (e.g., isolated from fluid communication from other portions of the reactor system  200 ), so that powders  222  disposed therein may be removed and replaced with fresh powder  222  including base metals and precious metals to be recovered therefrom. 
     With reference again to  FIG. 1 , the base metal recovery process  114  includes exposing the powder  222  ( FIG. 2 ) in at least one of the reactors of the plurality of reactors to an electrolyte comprising an oxidizer, dissolving at least one base metal within the powder  222  with the electrolyte, and recovering the at least one base metal at a cathode of the first electrochemical cell  210  ( FIG. 2 ). 
     Referring to  FIG. 3 , a first electrochemical reaction system  300  is illustrated. The first electrochemical reaction system  300  includes the first electrochemical cell  210  in fluid communication with at least the first reactor  201  and at least the second reactor  202 . The first electrochemical cell  210  may include an electrolyte  214 , a cathode  216 , and an anode  218 . The anode  218  may comprise a carbon material, such as a carbon felt. The cathode  216  may comprise stainless steel (e.g.,  304  stainless steel), nickel, or other suitable materials that are not substantially corroded by the chemistry of the electrolyte  214 . The anode  218  and the cathode  216  may be connected to a power supply  224  configured for applying a current between the anode  218  and the cathode  216 . Although the anode  218  and the cathode  216  are illustrated as being in communication with the electrolyte  214  in the same chamber, in some embodiments, the anode  218  and the cathode  216  may be isolated from each other. For example, in some embodiments, an ionic membrane may isolate the anode  218  and the cathode  216  from each other. 
     The electrolyte  214  may include ferrous ions (Fe 2+ ) and ferric ions (Fe 2+ ) dissolved in one of hydrochloric acid or sulfuric acid. The electrolyte  214  may be formed by dissolving ferric chloride (FeCl 3 ), ferrous chloride (FeCl 2 ), or a combination thereof in one of hydrochloric acid or sulfuric acid. Where the electrolyte  214  comprises hydrochloric acid, a concentration of the hydrochloric acid may be between about 0.1 M and about 1.0 M hydrochloric acid, such as between about 0.1 M and about 0.5 M, or between about 0.5 M and about 1.0 M hydrochloric acid. In embodiments where the electrolyte  214  comprises sulfuric acid, a concentration of the sulfuric acid may be between about 0.05 M and about 2.0 M sulfuric acid, such as between about 0.05 M and about 1.0 M, or between about 1.0 M and about 2.0 M sulfuric acid. 
     The pump  212  may be in fluid communication with the first electrochemical cell  210  and may be configured to pump the electrolyte  214  from the first electrochemical cell  210  to the first reactor  201 . With continued reference to  FIG. 3 , the first reactor  201  and the second reactor  202  may be connected in series and, therefore, the electrolyte  214  may pass from the first reactor  201  to the second reactor  202 , and from the second reactor  202  to the first electrochemical cell  210 . 
     In the first reactor  201 , ferric ions in the electrolyte  214  may oxidize and dissolve the base metals located in the powder  222 . Oxidation of the base metals may reduce the ferric ions to ferrous ions. For example, zinc, tin, lead (if present in the electronic waste), nickel, copper, and optionally, silver may be dissolved, according to the following reactions, respectively:
 
2Fe 3+ (aq)+Zn(s)→Fe 2+ (aq)+Zn 2+ (aq)  (3),
 
2Fe 3+ (aq)+Sn(s)→Fe 2+ (aq)+Sn 2+ (aq)  (4),
 
2Fe 3+ (aq)+Pb(s)→Fe 2+ (aq)+Pb 2+ (aq)  (5),
 
2Fe 3+ (aq)+Ni(s)→Fe 2+ (aq)+Ni 2+ (aq)  (6),
 
2Fe 3+ (aq)+Cu(s)→Fe 2+ (aq)+Cu 2+ (aq)  (7),
 
Fe 3+ (aq)+Ag(s)→Fe 2+ (aq)+Ag + (aq)  (8)
 
The silver may be dissolved when the electrolyte  214  includes sulfuric acid and may not be dissolved when the electrolyte  214  includes hydrochloric acid. In some embodiments, the electrolyte  214  is formulated such that precious metals in the powder  222  (e.g., silver, gold, and palladium) do not dissolve therein, but rather, remain as solids in the powder  222 . Thus, in some embodiments, the electrolyte  214  may be formulated to dissolve base metals without substantially dissolving the precious metals.
 
     In some embodiments, the base metals dissolve in order of lowest reduction potential to highest reduction potential (i.e., less noble metals (e.g., zinc, tin, and lead) dissolve prior to more noble metals (e.g., nickel, copper, and silver)), according to the galvanic series (also referred to in the art as the “electropotential series”). By way of non-limiting example, if copper in the powder  222  is contacted by the electrolyte  214  and dissolves, the dissolved copper may be reduced into elemental form upon contacting an undissolved metal having a lower reduction potential (e.g., zinc, tin, or lead) while the metal with a lower reduction potential is dissolved in the electrolyte  214 . For example, dissolved copper may react with undissolved tin, such as in the following galvanic reaction:
 
Cu 2+ (aq)+Sn(s)→Cu(s)+Sn +2 (aq)  (9).
 
Accordingly, the metals in the powder  222  may be dissolved in the electrolyte  214  according to their relative reduction potentials.
 
     As described above, the first electrochemical reaction system  300  may be configured such that the electrolyte  214  passes from the first reactor  201  to the second reactor  202 . Zinc, tin, lead, nickel, copper, and optionally, silver within the powder  222  of the second reactor  202  may dissolve in the electrolyte  214  as described above with reference to the first reactor  201 . 
     After contacting the powder  222  in the second reactor  202 , the electrolyte  214  may flow to the first electrochemical cell  210  where it is brought into contact with the cathode  216 . The dissolved metals (e.g., Zn 2+ , Sn 2+ , Pb 2+ , Ni 2+ , Ag + ) may be reduced (e.g., electrowon) at the cathode  216  and deposited in elemental form thereon, according to the following reactions:
 
Zn 2+ (aq)+2 e   − →Zn(s)  (10),
 
Sn 2+ (aq)+2 e   − →Sn(s)  (11),
 
Pb 2+ (aq)+2 e   − →Pb(s)  (12),
 
Ni 2+ (aq)+2 e   − →Ni(s)  (13),
 
Cu 2+ (aq)+2 e   − →Cu(s)  (14),
 
Ag + (aq)+ e   − →Ag(s)  (15).
 
     In some embodiments, zinc, tin, lead, and nickel may deposit on the cathode  216  prior to copper (or silver) being deposited on the cathode  216 . For example, as described above, the base metals may dissolve into the electrolyte  214  based on their respective reduction potentials. Thus, if zinc, tin, lead, and nickel dissolve from the powders  222  prior to dissolution of copper, the zinc, tin, lead, and nickel may be deposited at the cathode  216  prior to deposition of copper at the cathode  216 . In some embodiments, zinc, tin, lead, and nickel are dissolved together and deposit on the cathode  216  together, while copper is deposited in substantially pure elemental form, without a substantial amount of zinc, tin, lead, or nickel. Accordingly, the base metals may be recovered sequentially at the cathode  216  based on the order in which they were dissolved. 
     The ferrous ions may be oxidized to ferric ions at the anode  218  where the ferric ions are regenerated, as shown in the reaction below:
 
Fe 2+ (aq)→Fe 3+ (aq)+ e   −   (16).
 
     The first electrochemical cell  210  may be operated at a current density of between about 5 mA/cm 2  and about 200 A/cm 2 , such as between about 5 mA/cm 2  and about 50 mA/cm 2 , between about 50 mA/cm 2  and about 100 mA/cm 2 , or between about 100 mA/cm 2  and about 200 mA/cm 2 . In some embodiments, the current density is between about 100 mA/cm 2  and about 200 mA/cm 2 . 
     The first electrochemical reaction system  300  may be operated at a wide range of temperatures and pressures. The first electrochemical reaction system  300  may be operated at, for example, between about a freezing point of the electrolyte  214  and about a boiling point of the electrolyte  214  (each of which may depend on, for example, the molarity of the hydrochloric acid or sulfuric acid in the electrolyte  214 ). In some embodiments, the first electrochemical reaction system  300  is operated at ambient temperature (e.g., between about 20° C. and about 25° C., such as about 22° C.). The pressure of the first electrochemical reaction system  300  may be sufficient for the electrolyte  214  to flow from the first electrochemical cell  210 , through each of the first reactor  201  and the second reactor  202  and back to the first electrochemical cell  210 . Accordingly, the pressure of the first electrochemical cell  210  may depend on pressure losses through the powder  222  in the first reactor  201  and the second reactor  202 . 
     As described above, the pump  212  may be in fluid communication with the first electrochemical cell  210  and may be configured to pump the electrolyte  214 , including the regenerated oxidizer (e.g., ferric ions), to the first reactor  201 . Thus, the first electrochemical reaction system  300  may be configured such that the electrolyte  214  passes from the first electrochemical cell  210  to the first reactor  201 , from the first reactor  201  to the second reactor  202 , and from the second reactor  202  to the first electrochemical cell  210 . 
     Accordingly, base metals from electronic waste may be deposited on the cathode  216  of the first electrochemical cell  210  in an electrowinning process. In some embodiments, the base metals may be deposited as a group (e.g., one or more of zinc, tin, lead, nickel, and copper) and may be collected at the cathode  216  as an alloy. The alloy may include bronze (which may include tin and copper and trace amounts of silver), an alloy of copper and nickel (e.g., constantan or cupronickel), or other alloys including two or more of zinc, tin, lead, nickel, copper, and silver. In other embodiments, the base metals may be deposited on the cathode  216  in sequential order (e.g., from less noble to more noble) based on the order in which the base metals are dissolved in the reactors. By way of example, tin, copper, and silver may be recovered in substantially pure form. The metals recovered at the cathode  216  may be substantially pure. In some embodiments, each of the metals recovered at the cathode  216  may have a purity between about 80 percent and about 100 percent, such as between about 80 percent and about 85 percent, between about 85 percent and about 90 percent, between about 90 percent and about 95 percent, or between about 95 percent and about 100 percent. 
     Although  FIG. 3  illustrates the first reactor  201  and the second reactor  202 , the first electrochemical reaction system  300  may include any of the plurality of reactors, depending on the particular configuration of the reactor system  200  ( FIG. 2 ). 
     Advantageously, the first electrochemical reaction system  300  does not consume the oxidizer of the electrolyte  214  (e.g., the ferric or ferrous ions) or the acid of the electrolyte  214 . Rather, as described above, the oxidizer in the electrolyte  214  is regenerated in the electrochemical cell  210 . Thus, the first electrochemical reaction system  300  is configured to recover base metals without consuming large amounts of oxidizer or acid, as in some conventional recycling or electrochemical recycling operations. 
     In some embodiments, configuring the first electrochemical reaction system  300  with the second reactor  202  in series behind the first reactor  201  may improve the efficiency of the electrorecycling process compared to an electrorecycling process including only one reactor. As the electrolyte  214  cycles through the first reactor  201 , the rate of dissolution (and hence, the rate of recovery) of the base metals decreases over time. For example, not all of the ferric ions are reduced in the first reactor  201 , resulting in less dissolved metals in the electrolyte  214 . The ferric ions that do not oxidize the metals in the powder  222  return to the cathode  216  and are reduced to ferrous ions at the cathode  216 , which are regenerated to ferric ions at the anode  218 , reducing the efficiency of the first electrochemical cell  210 . Accordingly, as described above, the first reactor  201  and the second reactor  202  may be placed in series. In some embodiments, the powder  222  in the first reactor  201  may have previously been exposed to the electrolyte  214  and may, therefore, include a lower weight percent of the base metals than the powder  222  in the second reactor  202 . The powder  222  in the second reactor  202  may include a higher amount (e.g., a higher concentration) of base metals. In some embodiments, the powder  222  in the second reactor  202  may be fresh powder (e.g., may not have previously been exposed to the electrolyte  214 ). 
     Accordingly, the electrolyte  214  from the first electrochemical cell  210  may pass through the first reactor  201  having a lower weight percent of the base metals than the powder  222  in the second reactor  202 . Therefore, the freshly regenerated electrolyte  214  from the first electrochemical cell  210  comprising a higher concentration of oxidant (e.g., Fe 3+ ) may contact powders  222  having a lower weight percent of base metals in the first reactor  201  prior to contacting powders  222  having a higher weight percent of base metals in the second reactor  202 . The higher weight percent of base metals in the powder  222  in the second reactor  202  may substantially completely reduce the ferric oxidizer to ferrous ions. Accordingly, the efficiency of the first electrochemical reaction system  300  may be improved by contacting a powder  222  including a lower weight percent of base metals in the first reactor  201  prior to contacting a powder  222  including a higher weight percent of base metals in the second reactor  202 , which may be accomplished by passing the electrolyte  214  through the first reactor  201  and second reactor  202  in series. 
     After a predetermined period of time, substantially all of the base metals in the first reactor  201  may have been recovered at the cathode  216  of the first electrochemical cell  210 . In some embodiments, the first reactor  201  may be taken out of fluid communication with the first electrochemical cell  210 , such as by opening and closing one or more valves of the reactor system  200  ( FIG. 2 ) and, as discussed below, precious metals may be recovered therefrom, such as in the silver recovery process  116 , the gold recovery process  118 , and the palladium recovery process  120 . The reactor system  200  may be configured to place, for example, the third reactor  203  in fluid communication with the first electrochemical cell  210 , such as by opening and closing one or more valves of the reactor system  200 . In some embodiments, the reactor system  200  is configured such that the third reactor  203  is placed in fluid communication with the first electrochemical cell  210  in series behind the second reactor  202 . Accordingly, the reactor system  200  may be configured such that the electrolyte  214  flows from the first electrochemical cell  210  to the second reactor  202 , from the second reactor  202  to the third reactor  203 , and from the third reactor  203  to the first electrochemical cell  210 . Since the second reactor  202  has previously been contacted with the electrolyte  214 , the third reactor  203  may include a higher weight percent of base metals (e.g., such as in freshly loaded powder  222 ) than the second reactor  202 . The process of recovering the base metals from the second reactor  202  and the third reactor  203  may continue until the base metals are substantially completely recovered from the second reactor  202 , at which point, the second reactor  202  is taken out of fluid communication with the first electrochemical cell  210  and another new reactor (e.g., the fourth reactor  204 ) may be placed in series behind the third reactor  203  as the cycle continues. Thus, a reactor having a higher weight percent of base metals may be located in series behind a reactor including a powder having at least some of the base metals removed therefrom. 
     With reference again to  FIG. 1 , after recovering the base metals in the base metal recovery process  114 , silver may be recovered from the powder  222  ( FIG. 2 ) in the silver recovery process  116  (e.g., such as in embodiments where the electrolyte  214  includes hydrochloric acid). The silver recovery process  116  may include recovering silver from the powder  222  from which substantially all of the base metals have been recovered. For example, silver may be recovered from the powder  222  in the first reactor  201  after the base metals have been recovered during the base metal recovery process  114 . In some embodiments, the powder  222  in the first reactor  201  may be rinsed by flowing, for example, water from the rinse solution tank  290  ( FIG. 2 ) through the first reactor  201 . The water used to rinse the electrolyte  214  from the first reactor  201  may flow back to the first electrochemical cell  210 . 
     In embodiments in which the electrolyte  214  ( FIG. 3 ) includes hydrochloric acid, the silver in the powder  222  may be oxidized by the ferric ions and the oxidized silver ions (e.g., Ag + ) may react with the chloride ions from the hydrochloric acid to form insoluble silver chloride (AgCl). During the silver recovery process  116 , the silver chloride in the first reactor  201  may be exposed to a leachate formulated to dissolve the silver from the silver chloride. The leachate may include a thiosulfate (e.g., S 2 O 3   2− ), such as ammonium thiosulfate ((NH 4 ) 2 S 2 O 3 ), calcium thiosulfate (CaS 2 O 3 ), potassium thiosulfate (K 2 S 2 O 3 ), sodium thiosulfate (Na 2 S 2 O 3 ), or combinations thereof. In some embodiments, the leachate includes sodium thiosulfate at a concentration of between about 0.1 M and about 2.0 M, such as between about 0.1 M and about 0.5 M, between about 0.5 M and about 1.0 M, or between about 1.0 M and about 2.0 M. 
     The leachate may be contained in the first leaching vessel  230 . The reactor system  200  may be configured to place the first reactor  201  in fluid communication with the first leaching vessel  230 , such as by opening and closing one or more valves. The pump  232  may be configured to pump the leachate from the first leaching vessel  230  to the first reactor  201 . After passing through the first reactor  201 , the leachate may be directed back to the first leaching vessel  230 . 
     The thiosulfate in the leachate may react with the silver chloride to form silver thiosulfate ([Ag(S 2 O 3 ) 2 ) 3− ]), according to the following reaction.
 
AgCl(s)+2S 2 O 3   2− (aq)→[(Ag(S 2 O 3 ) 2 ) 3− ](aq)+Cl − (aq)  (17).
 
The leachate may be cycled through the first reactor  201  until the leachate is saturated with silver thiosulfate. In some embodiments, the first leaching vessel  230  may include enough leachate to leach silver from a plurality of reactors.
 
     After the leachate in the first leaching vessel  230  is saturated with silver thiosulfate, the silver from the silver thiosulfate may be recovered. In some embodiments, the silver is reduced by cementation wherein a metal that is more electropositive than silver is added to the solution to precipitate the silver. By way of non-limiting example, the silver thiosulfate may be exposed to one or more of zinc, aluminum, and iron to form pure silver by reduction. In some embodiments, zinc powder is added to first leaching vessel  230  to reduce the silver according to the following reaction:
 
2[Ag(S 2 O 3 ) 2 ) 3− ](aq)+Zn(s)→2Ag(s)+Zn 2+ (aq)+4(S 2 O 3 ) 2− (aq)  (18).
 
Where the precipitating agent is aluminum or iron, the reduction of silver is substantially the same.
 
     The pure silver may be recovered by passing the solution including the silver through a filter and recovering the solids that do not pass through the filter, by settling, or by other methods of solid-liquid separation, as known in the art. In some embodiments, the recovered silver has a purity greater than 95 percent, such as between about 95 percent and about 97 percent, between about 97 percent and about 99 percent, or greater than 99 percent. 
     In some embodiments, additional leachate may be added to the first leaching vessel  230  after the precipitated silver is recovered. 
     In other embodiments, where the electrolyte  214  of the base metal recovery process  114  includes sulfuric acid, the silver may dissolve in the electrolyte  214 . In some such embodiments, the silver may be recovered at the cathode  216  of the first electrochemical cell  210  in substantially pure form after recovering zinc, tin, lead, nickel, and copper, or as an alloy with one or more of zinc, tin, lead, nickel, and copper. 
     The first reactor  201  may be rinsed after the silver is recovered from the powder  222  therein. The first reactor  201  may be rinsed by flowing, for example, water from the rinse solution tank  290  through the first reactor  201 . The water used to rinse the first reactor  201  may be returned to the first leaching vessel  230 . 
     After recovering the base metals in the base metal recovery process  114  and silver during the silver recovery process  116 , gold may be recovered from the powder  222 . The reactor system  200  ( FIG. 2 ) may be configured to place the first reactor  201  in fluid communication with the second electrochemical cell  250 . Referring to  FIG. 4 , a second electrochemical reaction system  400  for recovering gold is illustrated. The second electrochemical reaction system  400  includes the second electrochemical cell  250 , which may comprise an electrolyte  234  including iodide, a cathode  236  and an anode  238 . The anode  238  and the cathode  236  may be connected to a power supply  242  configured for applying a current between the anode  238  and the cathode  236 . 
     In some embodiments, the electrolyte  234  may include iodide ions (I − ) and triiodide ions (I 3   − ). The iodide ions may be formed by dissolving potassium iodide (KI), sodium iodide (NaI), another iodide salt, or combinations thereof in water or another solvent. The pH of the electrolyte  234  may be maintained at approximately 7.0. The electrolyte  234  may include between about 0.05 M and about 1.0 M potassium iodide, such as between about 0.05 M and about 0.2 M, between about 0.2 M and about 0.5 M, or between about 0.5 M and about 1.0 M. 
     The pump  252  may be configured to pump the electrolyte  234  from the second electrochemical cell  250  to the first reactor  201 . The powder  222  in the first reactor  201  may be contacted with the electrolyte  234 . The electrolyte  234  may be formulated to dissolve the gold in the powder  222 , forming gold complexes, according to the following reactions:
 
Au(s)+2I − (aq)→AuI 2   − (aq)+ e   −   (19),
 
Au(s)+4I − (aq)→AuI 4   − (aq)+3 e   −   (20),
 
2Au(s)+I 3   − (aq)+I − (aq)→AuI 2   − (aq)  (21).
 
     The dissolved gold complexes may pass from the first reactor  201  to the second electrochemical cell  250 , where the gold complexes come into contact with the cathode  236 . At the cathode  236 , the gold complexes may be reduced (e.g., electrowon) and deposited as gold on the cathode  236 , according to the following reactions:
 
AuI 2   − (aq)+ e   − →Au(s)+2I − (aq)  (22),
 
AuI 4   − (aq)+2 e   − →AuI 2 (aq)+2I − (aq)  (23),
 
AuI 4   − (aq)+3 e   − →Au(s)+4I − (aq)  (24).
 
     In some embodiments, the recovered gold has a purity greater than 95 percent, such as between about 95 percent and about 97 percent, between about 97 percent and about 99 percent, or greater than 99 percent. 
     At the anode  238 , the iodide ions in the electrolyte  234  may be oxidized to regenerate the triiodide ions, according to the following reaction:
 
3I − (aq)→I 3   − (aq)+2 e   −   (25).
 
The iodide ions may pass from the second electrochemical cell  250  to the first reactor  201  where the iodide ions contact the gold in the powder  222  and form gold complexes, as the cycle repeats. Accordingly, the iodide may dissolve the gold in the powder  222 , the dissolved gold may be recovered at the cathode  236 , and the iodide may be regenerated at the anode  238 .
 
     In other embodiments, the gold recovery process  118  may include exposing the powder  222  in, for example, the first reactor  201  to a leachate formulated for leaching gold. Leachates for leaching gold may include hypochlorite (e.g., bleach), bromine- or bromide-containing solutions, and cyanide solutions, as known in the art for leaching gold. In some such embodiments, the reactor system  200  may include a leaching vessel including the leachate formulated for leaching gold rather than the second electrochemical cell  250 . The gold dissolved in the leachate may be recovered by adding, for example, sodium metabisulphite, zinc metal, oxalic acid, ferrosulfate, or sodium nitrate to the leachate to precipitate the gold, as known in the art for precipitating gold. 
     In some embodiments, the powder  222  in the first reactor  201  may be rinsed by flowing, for example, water from the rinse solution tank  290  through the first reactor  201 . The water used to rinse the first reactor  201  may be flowed back to the second electrochemical cell  250 . 
     Although  FIG. 4  illustrates that the reactor of the second electrochemical reaction system  400  includes the first reactor  201 , the reactor of the second electrochemical reaction system  400  may include any of the reactors of the reactor system  200  ( FIG. 2 ). For example, the reactor may include any of the reactors from which base metals and silver have been recovered from the powders  222  during the base metal recovery process  114  and the silver recovery process  116 , respectively. 
     Referring to  FIG. 1 , after the base metals, silver, and gold have been recovered from the powder  222  in the first reactor  201 , palladium may be recovered in the palladium recovery process  120 . The reactor system  200  ( FIG. 2 ) may be configured to place the first reactor  201  in fluid communication with the second leaching vessel  270 . 
     The palladium may be recovered by contacting the powder  222  with a leachate formulated to leach palladium from the powder  222 . The leachate may be contained within the second leaching vessel  270 . The leachate may include any chemistry that is known to leach palladium. In some embodiments, the leachate includes hydrogen peroxide and hydrochloric acid, such as about 2 M hydrochloric acid with about 3 volume percent hydrogen peroxide, although other concentrations of hydrochloric acid and hydrogen peroxide may be used. 
     The pump  272  may be configured to pump the leachate from the second leaching vessel  270  to the first reactor  201 . Palladium in the powder  222  is dissolved (e.g., leached) into the leachate according to the following reaction:
 
Pd(s)+4HCl(aq)+H 2 O 2 (aq)→H 2 PdCl 4 (aq)+2H 2 O(aq)  (26).
 
The leachate may flow from the first reactor  201  back to the second leaching vessel  270 . The leachate may be cycled through the first reactor  201  until the leachate is saturated with dissolved palladium. In some embodiments, the second leaching vessel  270  includes enough leachate to leach palladium from a plurality of reactors.
 
     After the leachate in the second leaching vessel  270  is saturated with dissolved palladium, the dissolved palladium may be recovered. In some embodiments, the palladium is reduced by cementation wherein a metal that is more electropositive than palladium is added to the solution to reduce the palladium. By way of non-limiting example, the dissolved palladium is exposed to one or more of zinc, aluminum, and tin. In some embodiments, zinc powder may be added to the second leaching vessel  270  to precipitate the palladium (e.g., by reduction), according to the following reaction:
 
PdCl 4   2− (aq)+Zn(s)→Pd(s)+Zn 2+ (aq)+4Cl − (aq)  (27).
 
     The precipitated palladium may be recovered by passing the solution including palladium through a filter and recovering solids that do not pass through the filter, by settling, or by other methods of solid-liquid separation, as known in the art. In some embodiments, the recovered palladium has a purity greater than 95 percent, such as between about 95 percent and about 97 percent, between about 97 percent and about 99 percent, or greater than 99 percent. 
     In some embodiments, additional leachate may be added to the second leaching vessel  270  after the precipitated palladium is recovered. 
     Although the silver recovery process  116 , the gold recovery process  118 , and the palladium recovery process  120  have been described with reference to the first reactor  201 , it is understood that the silver recovery process  116 , the gold recovery process  118 , and the palladium recovery process  120  may include any of the plurality of reactors. For example, the reactor system  200  may be configured such that any one reactor or group of reactors of the plurality of reactors may be in fluid communication with one or more of the first electrochemical cell  210 , the first leaching vessel  230 , the second electrochemical cell  250 , the second leaching vessel  270 , and the rinse solution tank  290  at any particular time. By way of non-limiting example, in some embodiments, the third reactor  203  and the fourth reactor  204  may be in fluid communication with the first electrochemical cell  210  while the second reactor  202  is in fluid communication with the first leaching vessel  230 , and the first reactor  201  is in fluid communication with the second electrochemical cell  250 . The fifth reactor  205  may be in fluid communication with the second leaching vessel  270  or may be isolated from the reactor system  200  to be loaded with fresh powder  222 . Accordingly, the reactor system  200  may be configured to recover base metals from at least some reactors while simultaneously recovering silver, gold, or palladium from one or more other reactors. 
     While embodiments of the disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the disclosure is not limited to the particular forms disclosed. Rather, the disclosure encompasses all modifications, variations, combinations, and alternatives falling within the scope of the disclosure as defined by the following appended claims and their legal equivalents.