METHOD FOR CLEANLY EXTRACTING METALLIC SILVER

Abstract

A method for cleanly extracting metallic silver includes: mixing an acidic solution containing Ce.sup.4+ and NO.sub.3.sup.− with a silver-containing material for leaching; after the leaching is completed, carrying out a solid-liquid separation to obtain a leaching solution containing Ce.sup.3+ and Ag.sup.+; and electrolyzing the leaching solution, wherein an oxidation reaction of Ce′ occurs at an anode to realize a regeneration of Ce.sup.4+ and an electrolytic reduction occurs at a cathode to reduce Ag.sup.+ to obtain the metallic silver. Ce.sup.4+ is used as a leaching agent and an intermediate oxidant to implement a cyclic operation of solution leaching and electrolytic regeneration on the silver-containing material. Almost no NO.sub.x and waste liquid are caused by the extraction process, and the invention is clean and environmentally friendly.

Claims

1. A method for cleanly extracting metallic silver, comprising the following steps: (1) mixing an acidic solution containing Ce.sup.4+ and NO.sub.3.sup.− with a silver-containing material to obtain a mixture for a leaching; after the leaching is completed, carrying out a solid-liquid separation on the mixture to obtain a leaching solution; and (2) electrolyzing the leaching solution obtained in step (1), wherein an oxidation reaction of Ce.sup.3+ occurs at an anode to obtain a solution containing Ce.sup.4+ to achieve a regeneration of Ce.sup.4+, and a reduction reaction of Ag.sup.+ occurs at a cathode to obtain the metallic silver.

2. The method according to claim 1, wherein in step (1), a concentration of Ce.sup.4+ in the acidic solution containing Ce.sup.4+ and NO.sub.3.sup.− is at least 0.1 mol/L.

3. The method according to claim 1, wherein in step (1), a concentration of H.sup.+ in the acidic solution containing Ce.sup.4+ and NO.sub.3.sup.− is at most 1 mol/L.

4. The method according to claim 1, wherein an indication of completion of the leaching in step (1) includes a ratio of Ce.sup.4+ to a total amount of Ce in the leaching solution is at most 5% and a concentration of Ag.sup.+ is at least 0.1 mol/L.

5. The method according to claim 1, wherein in step (1), the leaching solution is subjected to a purification treatment before the electrolyzing.

6. The method according to claim 1, wherein in step (2), the leaching solution is electrolyzed by an electrolytic cell with a diaphragm, the electrolytic cell is separated into a cathode zone and an anode zone by the diaphragm, and the leaching solution enters the electrolytic cell from the cathode zone.

7. The method according to claim 1, wherein in step (2), during the electrolyzing, a current density of the anode ranges from 100 A/m.sup.2 to 4000 A/m.sup.2, and a current density of the cathode ranges from 50 A/m.sup.2 to 700 A/m.sup.2.

8. The method according to claim 1, wherein the solution containing Ce.sup.4+ obtained after the electrolysis in step (2) is returned to step (1) for recycling.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] To facilitate understanding of the present application, the present disclosure provides the following embodiments. Those skilled in the art should understand that these embodiments are merely intended to help to understand the present invention and should not be considered as a specific limitation to the present invention.

Embodiment 1

[0034] (1) A solution containing 1 mol/L of Ce(NO.sub.3).sub.4, 0.1 mol/L of AgNO.sub.3, and 0.01 mol/L of HNO.sub.3 is mixed with spent catalysts (containing 0.5% of silver, and with the remainder alumina) for leaching, and the silver is allowed to be leached into the solution. The leaching is completed when a ratio of Ce.sup.4+ to a total amount of Ce in the leaching solution is less than or equal to 5% and a concentration of Ag is less than or equal to 0.1 mol/L. Then, the obtained leaching solution is filtered to remove residual solids and suspensions to obtain a clarified leaching solution.

[0035] (2) The clarified leaching solution obtained in step (1) is added from a cathode zone of an electrolytic cell. The electrolytic cell is separated into inner and outer areas by a plastic frame with a diaphragm. A platinum mesh is placed in the plastic frame as an anode, and a titanium mesh is placed outside the plastic frame as a cathode. The current density of the anode is controlled to be 100 A/m.sup.2, the current density of the cathode is controlled to be 50 A/m.sup.2, and the leaching solution is electrolyzed. Ag.sup.+ is reduced on the titanium mesh to form metallic silver. The electrolyte passes through the diaphragm and undergoes an oxidation reaction at the anode to convert Ce.sup.3+ to Ce.sup.4+, and finally, the electrolyte is transferred to a transfer tank. By controlling the electrolytic current and the electrolyte flow rate, the ratio of Ce.sup.4+ to the total cerium in the solution produced in the anode zone reaches above 70%. The solution in the transfer tank is monitored and analyzed, and the components of the solution are adjusted in time to ensure that the components of the solution meet the requirements that the Ce.sup.4+ concentration is greater than or equal to 0.1 mol/L and the H.sup.+ concentration is less than or equal to 1 mol/L. The regenerated acidic solution containing Ce.sup.4+ and NO.sub.3.sup.− that meets the requirements is returned to the leaching step and is mixed with the silver-containing material again for the leaching operation.

Embodiment 2

[0036] (1) A solution containing 1.6 mol/L of Ce(NO.sub.3).sub.4, 0.4 mol/L of Ce(NO.sub.3).sub.3 and 0.1 mol/L of HNO.sub.3 continuously flows through a pipeline loaded with spent catalysts (containing 15% silver) for leaching, allowing the ratio of Ce.sup.4+ to the total amount of Ce in the solution at the outlet of the pipeline to be less than 1% and the concentration of Ag.sup.+ to be more than 1 mol/L. Then, the obtained leaching solution is filtered to remove residual solids and suspensions, and a clarified leaching solution is obtained.

[0037] (2) The electrolysis is carried out in the same electrolytic cell as in embodiment 1. The current density of the anode is controlled to be 500 A/m.sup.2, the current density of the cathode is controlled to be 250 A/m.sup.2, and the leaching solution is electrolyzed. The metallic silver is obtained at the cathode. The ratio of Ce.sup.4+ in the total cerium in the solution produced in the anode zone reaches 80% by controlling the electrolytic current and the solution flow rate. The anolyte is transferred to a transfer tank. The solution in the transfer tank is monitored and analyzed, and the compositions of the solution are adjusted in time to ensure that the components of the solution meet the requirements that the Ce.sup.4+ concentration is stable at 1.6 mol/L and the H.sup.+ concentration is stable at 0.1 mol/L. The regenerated acidic solution containing Ce.sup.4+ and NO.sub.3.sup.− that meets the requirements is returned to the leaching step and mixed with the silver-containing material again for the leaching operation.

Embodiment 3

[0038] (1) A solution containing 1.0 mol/L of Ce(NO.sub.3).sub.4, 0.5 mol/L of Ce(NO.sub.3).sub.3, 1.5 mol/L of AnNO.sub.3, and 1 mol/L of HNO.sub.3 is mixed with spent catalysts containing 25% silver for leaching. The leaching is completed when a ratio of Ce.sup.4+ to a total amount of Ce in the leaching solution is less than or equal to 5% and a concentration of Ag.sup.+ is less than or equal to 0.1 mol/L. Then, the obtained leaching solution is filtered to remove residual solids and suspensions to obtain a clarified leaching solution.

[0039] (2) The clarified leaching solution obtained in step (1) is added from a cathode zone of an electrolytic cell. The electrolytic cell is separated into inner and outer areas by a plastic frame with a diaphragm. A platinum-plated titanium mesh is placed outside the plastic frame as an anode, and the silver plate is placed in the plastic frame as a cathode. The current density of the anode is controlled to be 2000 A/m.sup.2, the current density of the cathode is controlled to be 500 A/m.sup.2, and the leaching solution is electrolyzed. Ag.sup.+ is reduced to metallic silver at the cathode. The electrolyte undergoes an oxidation reaction at the anode to convert Ce.sup.3+ to Ce.sup.4+, and then flows from the overflow port of the electrolytic cell to the transfer tank. By controlling the electrolytic current and the solution flow rate, the concentration of Ce.sup.4+ in the solution produced in the anode zone increases to 1.3 mol/L. The solution in the transfer tank is monitored and analyzed. If the concentration of the solution meets the requirements that the Ce.sup.4+ concentration is stable at 1.3 mol/L and the H.sup.+ concentration is less than or equal to 1 mol/L, then the solution is directly returned to the leaching step for recycling. If the requirements are not met, the electrolyte flow rate is adjusted or the corresponding components are added in time to make the components in the solution meet the requirements, and then the obtained acidic solution containing Ce.sup.4+ is returned to the leaching step for recycling.

Embodiment 4

[0040] (1) A solution containing 1.4 mol/L of Ce(NO.sub.3).sub.4, 1 mol/L of AgNO.sub.3, and 0.001 mol/L of HNO.sub.3 is mixed with silver-copper alloy shavings containing 90% silver for leaching. The leaching is completed when a ratio of Ce.sup.4+ to a total amount of Ce in the leaching solution is less than or equal to 5% and a concentration of Ag.sup.+ is less than or equal to 0.1 mol/L. Then, the obtained leaching solution is filtered to remove residual solids and suspensions. According to the purity requirements of electrolytic silver products, when the copper content in the leaching solution is greater than or equal to 35 g/L, a purification step should be added to extract or adsorb the copper in the leaching solution to reduce the copper concentration in the leaching solution to less than 20 g/L. Fine filtration is performed after the removal of impurities to obtain a clarified leaching solution.

[0041] (2) The obtained clarified leaching solution is added from a cathode zone of an electrolytic cell. The electrolytic cell is separated into a plurality of cathode zones and anode zones with a porous porcelain plate and the electrolytic cell is provided with a solution stirring device and a heat exchange device. A platinum mesh is used as an anode and a silver mesh is used as a cathode. The current density of the anode is controlled to be 4000 A/m.sup.2, the current density of the cathode is controlled to be 700 A/m.sup.2, and the leaching solution is electrolyzed. Ag.sup.+ is reduced to metallic silver at the cathode. After analysis, the copper content in the silver product at the cathode is less than 0.01%, which meets the expected requirements. The electrolyte undergoes an oxidation reaction at the anode to convert Ce.sup.3+ to Ce.sup.4+. Then, the electrolyte is transferred from the overflow port of the anode zone to the transfer tank. The solution flow rate is controlled so that the concentration of Ce.sup.4+ in the solution of the transfer tank is greater than or equal to 1 mol/L and the concentration of H.sup.+ is less than or equal to 0.1 mol/L, and then the solution is directly returned to the leaching step for recycling. If the solution in the transfer tank does not meet the requirements after analysis, then the components of the solution are adjusted in time to meet the requirements and the obtained solution is returned to the leaching step for recycling.

[0042] The applicant declares that in the present disclosure, the above embodiments are used to describe the technical process of the present invention, but the present invention is not limited to the above-mentioned technical process. That is, it does not mean that the present invention must rely on the above specific technical process to be implemented. Those skilled in the art should understand that any improvement to the present invention, selection of specific steps of the present invention, and other situations all fall within the scope of the present invention.