System and method for controlling or removing hard deposits on electrowinning cathodes

Abstract

Electrowinning circuits and methods of electrowinning are disclosed. Embodiments may be characterized in that that they promote the removal of hard cathode deposits from cathodes. Embodiments may also be characterized in that they promote softer deposits of precious metals onto cathodes during the electrowinning process. An advantage of practicing disclosed embodiments is that precious metal deposits on cathodes may be more easily recovered and the cathodes may be re-used in the electrowinning process. Thus, cathode destruction via smelting to recover precious metal deposits can be avoided.

What is claimed is: 
     
         1 . A method of electrowinning a precious metal, the method comprising the steps of:
 providing an electrowinning solution containing dissolved metal ions of the precious metal therein, to an electrowinning cell having at least one cathode and at least one anode; and   depositing the precious metal onto the cathode by virtue of passing electrical current from the anode to the cathode;   
       CHARACTERIZED IN THAT the method further comprises the step of:
 softening the precious metal deposited on the cathode or at least discouraging the formation of hard deposits of the precious metal onto the cathode by virtue of performing at least one of the following steps: 
 i. at least temporarily, increasing a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell to within a range of 40 to 500 A/m 2  of the total cathode area; 
 ii. at least temporarily, increasing a concentration of free cyanide within the electrowinning solution to 0.5 to 4.5 wt. % and/or maintaining a concentration of sodium cyanide within the electrowinning solution to within a range of 1-8 wt. %; 
 iii. at least temporarily, adding silver cyanide to the electrowinning solution; 
 iv. performing at least one reverse polarity sequence by at least temporarily changing the charge of the cathode and anode in the electrowinning cell; 
 v. at least temporarily, energizing an ultrasonic transducer provided to the electrowinning cell. 
 
     
     
         2 . A method of electrowinning a precious metal, the method comprising the steps of:
 providing an electrowinning solution containing dissolved metal ions of the precious metal therein, to an electrowinning cell having at least one cathode and at least one anode; and   depositing the precious metal onto the cathode by virtue of passing electrical current from the anode to the cathode;   
       CHARACTERIZED IN THAT the method further comprises the steps of:
 altering a composition of the electrowinning solution within a separate mixing or storage tank; and 
 softening the precious metal deposited on the cathode or at least discouraging the formation of hard deposits of the precious metal onto the cathode by virtue of performing at least one of the following steps: 
 i. at least temporarily, increasing a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell to within a range of 40 to 500 A/m 2  of the total cathode area; 
 ii. at least temporarily, increasing a concentration of free cyanide within the mixing or storage tank to 0.5 to 4.5 wt. % and/or maintaining a concentration of sodium cyanide within the mixing or storage tank to within a range of 1-8 wt. %; 
 iii. maintaining a concentration of cyanide within the mixing or storage tank which is higher than a concentration of cyanide used in the electrowinning cell; 
 iv. introducing an additive or reagent to the electrowinning solution in the mixing or storage tank; the additive comprising a hard electrowinning deposit remover selected from one or more of the group consisting of: one or more soft metals, one or more wetting agents, one or more surface modifiers, one or more viscosity modifiers, one or more releasing agents, one or more oxidants, one or more organic compounds; 
 v. performing at least one reverse polarity sequence by at least temporarily changing the charge of the cathode and anode in electrowinning cell; 
 vi. at least temporarily, energizing an ultrasonic transducer provided to the electrowinning cell; and 
 delivering at least some of the electrowinning solution from the mixing or storage tank to the electrowinning cell. 
 
     
     
         3 . A method of electrowinning a precious metal, the method comprising the steps of:
 depositing the precious metal on a cathode in an electrowinning cell;   CHARACTERIZED IN THAT the method further comprises the steps of:   subsequently removing the cathode from the electrowinning cell and placing it within a cathode wash cell; and   removing the deposited precious metal from the cathode using the cathode wash cell by virtue of performing at least one of the following steps:   i. maintaining a current density of the cathode wash cell which is above a current density used for the electrowinning cell and/or within a range of 40 to 500 A/m 2  of the total cathode area;   ii. maintaining a concentration of free cyanide within the cathode wash cell which is higher than a concentration of sodium cyanide used in the electrowinning cell and/or within a range of 1-8 wt. %;   iii. introducing an additive or reagent to the cathode wash cell; the additive comprising a hard electrowinning deposit remover selected from one or more of the group consisting of: one or more soft metals, one or more wetting agents, one or more surface modifiers, one or more viscosity modifiers, one or more releasing agents, one or more oxidants, one or more organic compounds;   iv. performing at least one reverse polarity sequence by at least temporarily changing the charge of the cathode in the cathode wash cell;   v. at least temporarily, energizing an ultrasonic transducer provided to the cathode wash cell.   
     
     
         4 . An electrowinning circuit for recovering a precious metal from an electrowinning solution containing dissolved metal ions of the precious metal therein, the electrowinning circuit comprising:
 an electrowinning cell having at least one cathode and at least one anode; and   an electrical current passing from the anode to the cathode;   
       CHARACTERIZED IN THAT the electrowinning circuit further comprises:
 means for softening the precious metal deposited on the cathode or at least discouraging the formation of hard deposits of the precious metal onto the cathode during electrowinning; said means comprising at least one of the following elements: 
 i. a rectifier configured to at least temporarily increase current density and/or configured for maintaining current density of the electrowinning cell within a range of 40 to 500 A/m 2  of the total cathode area; 
 ii. a pump and/or adjustable control valve configured for at least temporarily increasing a concentration of free cyanide within the electrowinning solution to 0.5 to 4.5 wt. % and/or configured for maintaining a concentration of sodium cyanide within the electrowinning solution to within a range of 1-8 wt. %; 
 iii. a pump and/or adjustable control valve configured for at least temporarily adding silver cyanide to the electrowinning solution; 
 iv. a rectifier configured to reverse polarity of the anode and cathode for performing one or more reverse polarity sequences and at least temporarily changing the charge of the cathode and anode in the electrowinning cell; 
 v. an energizable ultrasonic transducer provided to the electrowinning cell configured for mechanically removing the precious metal from the cathode using sound waves. 
 
     
     
         5 . An electrowinning circuit for recovering a precious metal from an electrowinning solution containing dissolved metal ions of the precious metal therein, the electrowinning circuit comprising:
 an electrowinning cell having at least one cathode and at least one anode; and   an electrical current passing from the anode to the cathode;   
       CHARACTERIZED IN THAT the electrowinning circuit further comprises:
 a mixing or storage tank configured for receiving the electrowinning solution from the electrowinning cell and being further configured to aid in the softening the precious metal upon its deposition onto the cathode or at least discourage the formation of hard deposits of the precious metal onto the cathode; the electrowinning circuit further comprising at least one of the following elements: 
 i. a rectifier configured to at least temporarily increase current density and/or configured for maintaining current density of the electrowinning cell to within a range of 40 to 500 A/m 2  of the total cathode area; 
 ii. a pump and/or adjustable control valve configured for: at least temporarily increasing a concentration of free cyanide within the mixing or storage tank to 0.5 to 4.5 wt. %, or, maintaining a concentration of sodium cyanide within the mixing or storage tank to within a range of 1-8 wt. %, and/or maintaining a concentration of free cyanide within the mixing or storage tank to be higher than a concentration of free cyanide used in the electrowinning cell; 
 iii. a pump and/or adjustable control valve configured for introducing an additive or reagent to solution to the mixing or storage tank; the additive or reagent comprising a hard electrowinning deposit remover selected from one or more of the group consisting of: one or more soft metals, one or more wetting agents, one or more surface modifiers, one or more viscosity modifiers, one or more releasing agents, one or more oxidants, one or more organic compounds; 
 iv. a pump and/or adjustable control valve configured for introducing process water to the mixing or storage tank; 
 v. a rectifier configured to reverse polarity of the anode and cathode for performing at least one reverse polarity sequence and/or configured for at least temporarily changing the charge of the cathode and anode in the electrowinning cell; 
 vi. an energizable ultrasonic transducer provided to the electrowinning cell configured for mechanically removing the precious metal from the cathode using sound waves; and 
 means for delivering the electrowinning solution from the mixing or storage tank to the electrowinning cell. 
 
     
     
         6 . An electrowinning circuit for recovering a precious metal from an electrowinning solution containing dissolved metal ions of the precious metal therein, the electrowinning circuit comprising:
 an electrowinning cell having at least one cathode and at least one anode;   
       the cathode being configured to receive deposits of the precious metal thereon; 
       CHARACTERIZED IN THAT the electrowinning circuit further comprises:
 a cathode wash cell; and 
 means for removing the cathode and/or anode from the electrowinning cell and placing it within the cathode wash cell; 
 the cathode wash cell being configured for removing deposited precious metals from the cathode and/or anode and comprising at least one of the following elements: 
 i. a rectifier configured for maintaining a current density of the cathode wash cell above a current density used for the electrowinning cell and/or configured for maintaining current density of the cathode wash cell to within a range of 40 to 500 A/m 2  of the total cathode area; 
 ii. a pump and/or adjustable control valve configured for maintaining a higher concentration of free cyanide within the cathode wash cell than a concentration of free cyanide used in the electrowinning cell, and/or which is configured for maintaining a concentration of sodium cyanide within the mixing or storage tank to within a range of 1-8 wt. %; 
 iii. a pump and/or adjustable control valve configured for introducing an additive or reagent to solution to the cathode wash cell; the additive comprising a hard electrowinning deposit remover selected from one or more of the group consisting of: one or more soft metals, one or more wetting agents, one or more surface modifiers, one or more viscosity modifiers, one or more releasing agents, one or more oxidants, one or more organic compounds; 
 iv. a rectifier configured to at least temporarily reverse polarity of the cathode and/or anode in the cathode wash cell; and 
 v. an energizable ultrasonic transducer provided to the cathode wash cell configured for mechanically removing the precious metal from the cathode and/or anode using sound waves. 
 
     
     
         7 . The method according to  claim 1 or 2 , further comprising the step of:
 maintaining and/or controlling a temperature of the electrowinning solution between approximately 60 and 212 degrees Fahrenheit; or,   increasing or decreasing a temperature of the electrowinning solution using a solution heater or heat exchanger.   
     
     
         8 . The method according to  claim 3 , further comprising the step of:
 maintaining and/or controlling a temperature of the cathode wash cell between approximately 60 and 212 degrees Fahrenheit; or,   increasing or decreasing a temperature of the cathode wash cell using a solution heater or a heat exchanger.   
     
     
         9 . The electrowinning circuit according to  claim 4 or 5 , further comprising a solution heater or heat exchanger configured to:
 increase or decrease a temperature of the electrowinning solution; or,   maintain and/or control a temperature of the electrowinning solution such that it remains between approximately 60 and 212 degrees Fahrenheit.   
     
     
         10 . The electrowinning circuit according to  claim 6  further comprising a solution heater or heat exchanger configured to:
 increase or decrease a temperature of the cathode wash cell; or, 
 maintain and/or control a temperature of the cathode wash cell between approximately 60 and 212 degrees Fahrenheit. 
 
     
     
         11 . A method of electrowinning a precious metal, the method comprising the steps of:
 providing an electrowinning solution containing dissolved metal ions of the precious metal therein, to an electrowinning cell having at least one cathode and at least one anode; and   depositing the precious metal onto the cathode during a first electrowinning cycle for a first cycle duration by virtue of passing electrical current from the anode to the cathode;   
       CHARACTERIZED IN THAT the method further comprises the step of:
 softening the precious metal deposited on the cathode or at least discouraging the formation of hard deposits of the precious metal onto the cathode by virtue of performing the following steps: 
 v. maintaining a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell to within a range of 40 to 500 A/m 2  of the total cathode area; 
 vi. maintaining a concentration of free cyanide within the electrowinning solution to 0.5 to 4.5 wt. % and/or maintaining a concentration of sodium cyanide within the electrowinning solution to within a range of 1-8 wt. %; 
 vii. performing at least one reverse polarity sequence (i.e., “second electrowinning cycle”) by at least temporarily changing the charge of the cathode in the electrowinning cell to a positive charge for a second cycle duration, wherein the second cycle duration is substantially less than the first cycle duration; and, 
 viii. spraying, rinsing, or flushing at least one anode and/or cathode within the electrowinning cell with a fluid such as water. 
 
     
     
         12 . The method of  claim 11 , wherein a ratio of the second cycle duration to the first cycle duration is less than 0.5. 
     
     
         13 . The method of  claim 11 or 12 , wherein a ratio of the second cycle duration to the first cycle duration is less than 0.33. 
     
     
         14 . The method of any one of  claims 11-13 , wherein a ratio of the second cycle duration to the first cycle duration is less than 0.25. 
     
     
         15 . A method of electrowinning a precious metal, the method comprising the steps of:
 providing an electrowinning solution containing dissolved metal ions of the precious metal therein, to an electrowinning cell having at least one cathode and at least one anode; and   depositing the precious metal onto the cathode during a first electrowinning cycle for a first cycle duration by virtue of passing electrical current from the anode to the cathode;   
       CHARACTERIZED IN THAT the method further comprises the step of:
 softening the precious metal deposited on the cathode or at least discouraging the formation of hard deposits of the precious metal onto the cathode by virtue of performing the following steps: 
 vi. maintaining a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell to within a range of 40 to 500 A/m 2  of the total cathode area; 
 vii. maintaining a concentration of free cyanide within the electrowinning solution to 0.5 to 4.5 wt. % and/or maintaining a target concentration of sodium cyanide within the electrowinning solution to within a range of 1-8 wt. %; 
 viii. performing at least one reverse polarity sequence (i.e., “second electrowinning cycle”) by at least temporarily changing the charge of the cathode in the electrowinning cell to a positive charge for a second cycle duration; 
 ix. subsequently performing a third electrowinning cycle after the second electrowinning cycle for a third cycle duration, wherein the charge of the cathode reverts back to its polarity (i.e., negative charge) of the first electrowinning cycle; and, 
 x. spraying, rinsing, or flushing at least one anode and/or cathode in the electrowinning cell with a fluid such as water. 
 
     
     
         16 . The method of  claim 15 , wherein the first and second cycle durations are approximately the same. 
     
     
         17 . The method of  claim 15 , wherein the second cycle duration is greater than the first cycle duration. 
     
     
         18 . The method of any one of  claims 15-17 , wherein the third cycle duration is substantially shorter than the first cycle duration. 
     
     
         19 . The method of any one of  claims 15-18 , wherein the third cycle duration is substantially shorter than the second cycle duration. 
     
     
         20 . The method of any one of  claims 1-19 , further comprising the step of removing portions of the precious metal from the cathode and/or the anode by virtue of rinsing, washing, pressure-washing, spraying, water-jetting, scraping, vibrating, generating a mechanical wave using an ultrasonic transducer or sound generator, air blowing, mechanical shaking, flexing the cathode and/or the anode, or a combination thereof. 
     
     
         21 . The method of  claim 20 , wherein the step of removing the precious metal from the cathode and/or anode is performed outside of the electrowinning cell. 
     
     
         22 . The method of  claim 20 , wherein the step of removing the precious metal from the cathode and/or anode is performed inside of the electrowinning cell. 
     
     
         23 . A method of electrowinning a precious metal, the method comprising the steps of:
 providing an electrowinning solution containing dissolved metal ions of the precious metal therein, to an electrowinning cell having at least one cathode and at least one anode; and   depositing the precious metal onto the cathode by virtue of passing electrical current from the anode to the cathode;   
       CHARACTERIZED IN THAT the method further comprises the step of:
 softening the deposited precious metal deposited on the cathode or at least discouraging the formation of hard deposits of the precious metal onto the cathode by virtue of performing the following steps: 
 i. at least temporarily, increasing a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell to within a range of 5 to 500 A/m 2  of the total cathode area; and 
 ii. at least temporarily, increasing a concentration of free cyanide within the electrowinning solution to 0.5 to 4.5 wt. % and/or maintaining a concentration of sodium cyanide within the electrowinning solution to within a range of 1-8%. 
 
     
     
         24 . The method of  claim 11 or 15 , further comprising the step of removing portions of the precious metal from the cathode and/or anode by virtue of rinsing, washing, pressure-washing, spraying, water-jetting, scraping, vibrating, generating a mechanical wave using an ultrasonic transducer or sound generator, air blowing, mechanical shaking, or flexing the cathode and/or anode. 
     
     
         25 . The method of  claim 24 , wherein the electrowinning cell comprises a plurality of said anode and cathode—collectively forming electrodes of the electrowinning cell, the method further comprising the step of removing portions of the precious metal from all of the electrodes by virtue of rinsing, washing, pressure-washing, spraying, water-jetting, scraping, vibrating, generating a mechanical wave using an ultrasonic transducer or sound generator, air blowing, mechanical shaking, or flexing the electrodes within the electrowinning cell and/or outside of the electrowinning cell. 
     
     
         26 . A method of electrowinning a precious metal, the method comprising the steps of:
 providing an electrowinning solution containing dissolved metal ions of the precious metal therein, to an electrowinning cell having at least one cathode and at least one anode; and   depositing the precious metal onto the cathode by virtue of passing electrical current from the anode to the cathode;   
       CHARACTERIZED IN THAT the method further comprises the step of:
 softening the deposited precious metal deposited on the cathode or at least discouraging the formation of hard deposits of the precious metal onto the cathode by virtue of performing the following steps: 
 i. at least temporarily, increasing a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell to within a range of 40 to 500 A/m 2  of the total cathode area; and 
 ii. at least temporarily, increasing a concentration of free cyanide within the electrowinning solution to 0.05 to 4.5 wt. % and/or maintaining a concentration of sodium cyanide within the electrowinning solution to within a range of 0.1-8%. 
 
     
     
         27 . A method of electrowinning a precious metal according to  any of the preceding claims , further comprising the step of removing the anode and/or cathode, and subjecting the removed anode and/or cathode to a warm bath of solution held between 60 and 212 degrees Fahrenheit at a pH above 9.5, the solution comprising 1-8 wt. % sodium cyanide. 
     
     
         28 . The method of  claim 27 , wherein the cyanide is provided in the form of sodium cyanide, potassium cyanide, copper cyanide, silver cyanide, or a combination thereof. 
     
     
         29 . The method of  claim 27 or 28 , further comprising the step of replacing the removed anode and/or cathode to the electrowinning cell. 
     
     
         30 . The method according to any one of  claims 27-29 , wherein the step of subjecting the removed anode and/or cathode to a warm bath of solution is performed for a period greater than 30 minutes. 
     
     
         31 . The method according to any one of  claims 27-30 , wherein the step of subjecting the removed anode and/or cathode to a warm bath of solution is performed for a period less than 5 days.