Embodiments of the present invention pertain to improvements to electrowinning apparatus and methods of electrowinning which are particularly useful for precious metals recovery, such as for gold and silver recovery processes.

BACKGROUND TO THE INVENTION

Reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in the arts.

During electrowinning processes within the PGM, gold, and silver mining industry, precious metals may form hard deposits on electrowinning cathodes. These “hard deposits” may be very difficult to remove from the electrowinning cathodes after plating even when using a high-pressure sprayer. Using wire mesh cathodes in an electrowinning cell further complicates removal. Since these hard deposits contain valuable precious metals, it is desirable to remove as much of these difficult-to-remove hard deposits from the electrowinning cathodes to maximize downstream recovery.

Industry practice has, heretofore, included attempts to recover hard-deposited precious metals from electrowinning cathodes by employing a “reverse-polarity step”, wherein hard deposit removal from a cathode is facilitated by temporarily configuring the electrowinning cell to polarize the hard-deposited cathode as the anode would typically be, and simultaneously charging the anode as the cathode would typically be during electrowinning, thus effectively “re-plating” hard- deposited precious metals from the cathode onto the anode. This is followed by subsequent re-plating from the anode back to the cathode in a normal electrowinning cell operating configuration and rewashing the cathode with a pressure sprayer.

Alternatively, conventional practice has involved attempts to remove the hard- deposited cathodes by smelting them in their entirety. In this energy-intensive and carbon impactful process, the hard deposit-laden cathode mesh is ultimately destroyed in a furnace (along with its in-situ deposits), and the hard deposits containing valuable precious metals are recovered in their molten state. Aside from being a heat and energy-intensive process, the drawback of melting wire cathodes in a furnace is that the cathodes are used and treated as sacrificial consumables, rather than renewable/re-useable components of the electrowinning process. To ensure purity, the molten metal values must be finely separated from the molten metal wire and this introduces another critical pyro-refinement step to the recovery process.

To this end, there are no known teachings within the prior art that would promote and/or ensure a “soft” cathode deposit during the electrowinning process. Moreover, heretofore, there are no known teachings within the prior art that relate to “softening” and/or “removal” of hard anode/cathode deposits.

OBJECTS OF THE INVENTION

It is an aim that embodiments of the invention provide an electrowinning apparatus and method which overcomes or ameliorates one or more of the disadvantages or problems described above - or, which at least provides a useful alternative to conventional electrowinning apparatus and methods.

It is an aim of some embodiments to eliminate or at least significantly reduce the formation of “hard” deposits containing precious metals (e.g., those containing silver and/or gold) on wire mesh cathodes. It is an aim of some embodiments to provide a simple procedure and/or system to remove hard deposits containing precious metals (e.g., those containing silver and/or gold) from wire mesh or plate type cathodes.

It is an aim of some embodiments to improve metal balance at mine sites by eliminating or at least significantly reducing the amount of silver and/or gold inventory locked up within electrowinning cathodes in the refinery.

Other preferred objects of the present invention will become apparent from the following description.

SUMMARY OF INVENTION

According to embodiments of the invention, a novel electrowinning apparatus and method is disclosed.

A method of electrowinn ing a precious metal may comprise the step 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. The method may further comprise the step of depositing the precious metal onto the cathode by virtue of passing electrical current from the anode to the cathode.

The method may further comprise 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. This may be accomplished, for instance, by virtue of at least temporarily, increasing a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell above 5 A/m ² or 10 A/m ² of the total cathode area, for example to within a range of 40 to 500 A/m ² of the total cathode area (e.g., 40 to 200 A/m ² ); at least temporarily, increasing a free cyanide concentration of (pure) cyanide to ~0.5 wt.% to 4.5 wt.% (e.g., using sodium cyanide or equivalent) within the electrowinning solution and/or maintaining a concentration of sodium cyanide within the electrowinning solution to within a range of 1 -8 wt.% (for example, between 1 and 6 wt.%, or between 1 and 4 wt.%, sodium cyanide without limitation); at least temporarily, adding silver cyanide to the electrowinning solution; performing a reverse polarity sequence (e.g., a single reverse polarity sequence, at least one reverse polarity sequence, a plurality of reverse polarity sequences, and/or a combination of reverse polarity sequences) by at least temporarily changing the charge of the cathode in the electrowinning cell; and/or at least temporarily, energizing an ultrasonic transducer provided to the electrowinning cell. The above steps for 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 may be performed in any combination, as necessary to achieve optimal effect, without limitation.

The method may comprise the step of providing a heat exchanger to increase or decrease a temperature of the electrowinning solution. For example, in some preferred embodiments, the method may comprise the step of controlling the temperature of the electrowinning solution (e.g., using a solution heater or heat exchanger) such that it remains between approximately 60 and 212 degrees Fahrenheit, without limitation. It should be understood that the electrowinning solution should at all times comprise aqueous cyanide (CN ^_ ) having a pH above approximately 9.5 to avoid generation of gaseous hydrogen cyanide (HCN) below the equilibrium range of 9.3-9.5.

A repeated cyclic changing of polarity may be employed to remove a hard deposit. In other words, a plurality of reverse polarity sequences may be employed. In this regard, successive amounts of hard deposits may be able to be removed with each “run” by virtue of a change in polarity. A “run” may be a cycle of reverse polarity (i.e., “reverse polarity sequence”) wherein most of the hard deposit may be electroplated from one or more electrodes onto one or more target cathodes and/or vice versa.

The method may comprise the steps of altering a composition of the electrowinning solution in a separate mixing or storage tank. The method may include the step of delivering at least some of the electrowinning solution from the mixing or storage tank to the electrowinning cell. The method may include 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. This may be achieved, for example by virtue of: at least temporarily, increasing a current density of the electrowinning cell and/or maintaining current density of the electrowinning cell above 5 A/m ² of the total cathode area, for example to within a range of 40 to 500 A/m ² (e.g., 40 to 200 A/m ² ) of the total cathode area; at least temporarily, increasing a concentration of cyanide (e.g., sodium cyanide or equivalent) within the mixing or storage tank and/or maintaining a concentration of sodium cyanide within the mixing or storage tank to within a range of 1 -8% (e.g., 1 -4%); maintaining a concentration of cyanide within the mixing or storage tank which is higher than a concentration of cyanide used in the electrowinning cell; 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; performing a reverse polarity sequence by at least temporarily changing the charge of the cathode in electrowinning cell; and/or at least temporarily, energizing an ultrasonic transducer provided to the electrowinning cell. The above steps for 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 may be performed in any combination, as necessary to achieve optimal effect, without limitation.

Embodiments of the method disclosed herein may comprise the steps of depositing the precious metal on a cathode in an electrowinning cell; 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: 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 (e.g., 40 to 200 A/m ² ) of the total cathode area; maintaining a concentration of cyanide (e.g., sodium cyanide or equivalent) within the cathode wash cell which is higher than a concentration of cyanide used in the electrowinning cell and/or within a range of 1 -8% sodium cyanide (e.g., 1 -4%); 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; performing a reverse polarity sequence by at least temporarily changing the charge of the cathode in the cathode wash cell; and/or at least temporarily, energizing an ultrasonic transducer provided to the cathode wash cell. The above steps for removing the deposited precious metal from the cathode using the cathode wash cell may be performed in any combination, as necessary to achieve optimal effect, without limitation.

Embodiments of an electrowinning circuit for recovering a precious metal from an electrowinning solution containing dissolved metal ions of the precious metal therein, is also disclosed. As suggested in FIG. 1 , the electrowinning circuit may comprise an electrowinning cell having at least one cathode and at least one anode; and an electrical current passing from the anode to the cathode. The electrowinning circuit may further comprise 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. Such means may comprise 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 (e.g., 40 to 200 A/m ² ) of the total cathode area; a pump and/or adjustable control valve configured for at least temporarily increasing a concentration of cyanide (e.g., sodium cyanide or equivalent) within the electrowinning solution and/or configured for maintaining a concentration of cyanide within the electrowinning solution to within a range of 1 -8 wt.% sodium cyanide (e.g., 1 -4 wt.%); a pump and/or adjustable control valve configured for at least temporarily adding silver cyanide to the electrowinning solution; a rectifier configured to reverse polarity of the anode and cathode for performing a reverse polarity sequence and at least temporarily changing the charge of the cathode in the electrowinning cell; and/or an energizable ultrasonic transducer provided to the electrowinning cell configured for mechanically removing the precious metal from the cathode using sound waves. The above features may be provided to the electrowinning circuit in any combination, without limitation.

In some embodiments, the electrowinning circuit may comprise 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 may comprise 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.

According to some embodiments, as suggested in FIG. 2, the electrowinning circuit may comprise 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 may comprise 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, for example, to within a range of 40 to 200 A/m ² , without limitation.

The electrowinning circuit may comprise a pump and/or adjustable control valve configured for: at least temporarily increasing a concentration of cyanide (e.g., sodium cyanide or equivalent) within the mixing or storage tank, maintaining a concentration of cyanide within the mixing or storage tank to within a range of 1 -8 wt.% sodium cyanide (e.g., 1 -4 wt.%), and/or maintaining a concentration of cyanide within the mixing or storage tank to be higher than a concentration of cyanide used in the electrowinning cell. The electrowinning circuit may comprise 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.

The electrowinning circuit may comprise a pump and/or adjustable control valve configured for introducing process water to the mixing or storage tank. The electrowinning circuit may comprise a pump and/or adjustable control valve configured for at least temporarily adding silver cyanide to the electrowinning solution.

The electrowinning circuit may comprise a rectifier configured to reverse polarity of the anode and cathode for performing a reverse polarity sequence and/or configured for at least temporarily changing the charge of the cathode in the electrowinning cell. The electrowinning circuit may comprise an energizable ultrasonic transducer provided to the electrowinning cell configured for mechanically removing the precious metal from the cathode using sound waves. The electrowinning circuit may comprise means for delivering the electrowinning solution from the mixing or storage tank to the electrowinning cell.

According to some embodiments, as suggested by FIG. 3, in addition to an electrowinning cell having at least one cathode and at least one anode, the electrowinning circuit may further comprise a cathode wash cell and means for removing the cathode from the electrowinning cell and placing it within the cathode wash cell. The cathode wash cell may be configured for removing deposited precious metals from the cathode. The cathode wash cell may comprise 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 (e.g., to within a range of 40 to 200 A/m ² ). The cathode wash cell may comprise a pump and/or adjustable control valve configured for maintaining a higher concentration of cyanide (e.g., sodium cyanide or equivalent) within the cathode wash cell than a concentration of cyanide used in the electrowinning cell, and/or which is configured for maintaining a concentration of cyanide within the mixing or storage tank to within a range of 1 -8 wt.% sodium cyanide (e.g. ,1 -4 wt.%). The cathode wash cell may comprise 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. The electrowinning circuit may comprise a pump and/or adjustable control valve configured for at least temporarily adding silver cyanide to the electrowinning solution.

The cathode wash cell may comprise a rectifier configured to at least temporarily reverse polarity of the cathode in the cathode wash cell. The cathode wash cell may comprise an energizable ultrasonic transducer provided to the cathode wash cell configured for mechanically removing the precious metal from the cathode using sound waves.

According to some embodiments, an electrowinning circuit may comprise a heat exchanger to increase or decrease a temperature of the electrowinning solution. For example, in some preferred embodiments, the solution heater or heat exchanger may be configured to control the temperature of the electrowinning solution such that it remains between approximately 60 and 212 degrees Fahrenheit, without limitation. It should be understood that the electrowinning solution should at all times comprise aqueous cyanide (CN ^_ ) having a pH above approximately 9.5 to avoid generation of gaseous hydrogen cyanide (HCN) below the equilibrium range of 9.3-9.5. According to some embodiments, an electrowinning circuit may be configured to perform a repeated cyclic changing of polarity to remove a hard deposit. In other words, the electrowinning circuit may be configured to perform a plurality of reverse polarity sequences or a combination of reverse polarity sequences, without limitation. In this regard, successive amounts of hard deposits may be able to be removed with each “run” by virtue of a change in polarity. A “run” may be a cycle of reverse polarity (i.e., “reverse polarity sequence”) wherein some to all of the hard deposit may be electroplated from one or more electrodes onto one or more target cathodes and/or vice versa followed by a removal of some to all of the deposit from the source or target electrode by a wash sequence.

According to some embodiments, a method of electrowinning a precious metal may comprise 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, wherein the method may be characterized in that it 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: i. 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; ii. maintaining a concentration of free cyanide within the electrowinning solution between approximately 0.5 and 4.5 wt.% and/or maintaining a target concentration of sodium cyanide within the electrowinning solution to within a range of 1 -8 wt.%; iii. 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 negative charge for a second cycle duration, wherein the second cycle duration is substantially less than the first cycle duration; and, iv. spraying, rinsing, or flushing at least one anode and/or cathode within the electrowinning cell with a fluid such as water.

A ratio of the second cycle duration to the first cycle duration may be less than 0.5. For example, a ratio of the second cycle duration to the first cycle duration may be less than 0.33. As another example, a ratio of the second cycle duration to the first cycle duration may be less than 0.25.

In some embodiments, a method of electrowinning a precious metal may comprise 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; wherein the method may be characterized in that it 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: i. 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; ii. maintaining a concentration of free cyanide within the electrowinning solution between approximately 0.5 and 4.5 wt.% and/or maintaining a target concentration of sodium cyanide within the electrowinning solution to within a range of 1 -8 wt.%; iii. 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 negative charge for a second cycle duration; iv. 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 original polarity (i.e., positive charge) of the first electrowinning cycle; and, v. spraying, rinsing, or flushing at least one anode and/or cathode in the electrowinning cell with a fluid such as water. In some embodiments, the first and second cycle durations may be approximately the same. In some embodiments, the second cycle duration may be greater than the first cycle duration. In some embodiments, the second cycle duration may be less than the first cycle duration. In some embodiments, the third cycle duration may be substantially shorter than the first cycle duration and/or shorter than the second cycle duration, without limitation.

A method of electrowinning a precious metal may comprise 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. The method may be characterized in that it 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 two 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 10 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 between approximately 0.5 and 4.5 wt.% and/or maintaining a target concentration of sodium cyanide within the electrowinning solution to within a range of 1 -8 wt.%, without limitation.

The electrowinning cell described herein may comprise a plurality of said anode and cathode collectively forming “electrodes”. Accordingly, in any of the embodiments proposed, the method may further comprise 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. This may be done ex situ or in situ in relation to the tank of the electrowinning cell. If performed in situ, electrowinning solution may be present or absent from the electrowinning cell.

In some embodiments of a method of electrowinning a precious metal the method may be characterised in that it may comprise 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 two 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 between approximately 0.05 and 4.5 wt.% and/or maintaining a target concentration of sodium cyanide within the electrowinning solution to within a range of 0.1 -8%, without limitation.

In any of the proposed embodiments, the method may further comprise 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 anode, or any combination thereof. The step of removing the precious metal from the cathode and/or anode may be performed outside of the electrowinning cell and/or performed inside of the electrowinning cell, without limitation.

In any of the proposed embodiments, the method may further comprise 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. The cyanide may be provided in the form of sodium cyanide, potassium cyanide, copper cyanide, silver cyanide, equivalents thereof, or a combination thereof, without limitation. The method may further comprise the step of replacing the removed anode and/or cathode to the electrowinning cell. The step of subjecting the removed anode and/or cathode to a warm bath of solution may be performed for a period greater than 30 minutes. The step of subjecting the removed anode and/or cathode to a warm bath of solution may be performed for a period less than 5 days. Accordingly, the step of replacing the removed anode and/or cathode to the electrowinning cell may be performed for a substantially longer period of time than an electrowinning cycle (e.g., first cycle duration, second cycle duration, or third cycle duration), without limitation.

In some embodiments, the anode and/or cathode may not necessarily need to be removed from the electrowinning cell, and the above warm bath soak for the anode and/or cathode may be accomplished by draining the electrowinning solution from the tank of the electrowinning cell, and the fresh warm solution mentioned above pumped into the tank of the electrowinning cell to effect cleaning of the anode and/or cathode.

Further features and advantages of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the invention will be described more fully hereinafter with reference to the accompanying figures.

FIG. 1 depicts a first non-limiting embodiment of an apparatus and method according to the invention. In particular, the figure depicts one way of softening precious metal deposits on cathodes within an electrowinning cell and/or producing a soft precious deposit(s) on cathodes, without limitation. In particular, FIG. 1 depicts a process stream with feed from a pregnant solution tank or strip/elution column, wherein the barren solution is sent to a barren tank downstream of the electrowinning cell.

FIG. 2 depicts a second non-limiting embodiment of an apparatus and method according to the invention. In particular, the figure depicts a second way of softening precious metal deposits on cathodes within an electrowinning cell and/or producing a soft precious deposit(s) on cathodes, without limitation. In particular, FIG. 2 depicts a recirculation of barren solution which was been treated with the reagents in a recycle mode. The recycle mode utilizes a mix/storage tank which is separate from the electrowinning cell.

FIG. 3 depicts a third non-limiting embodiment of an apparatus and method according to the invention. In particular, the figure depicts a way in which cathodes with hard deposits may be cleaned hydrometallurgically, rather than using traditional pyrometallurgical (e.g., “smelting”) processes. This cleaning step may be accomplished in a cathode wash cell. The cathode wash cell comprises a vessel which is separate from the electrolysis cell of the electrowinning cell.

FIG. 4 depicts a method according to some non-limiting embodiments, and its steps, with optional steps being identified with dashed boxes. In particular, the method depicted in FIG. 4 suggests one way to continuously soften and/or reduce occurrences of hard cathode deposit formation in an electrowinning cell, during an electrowinning process.

FIG. 5 depicts an alternative method according to some non-limiting embodiments, and its steps, with optional steps being identified with dashed boxes. In particular, the method depicted in FIG. 5 suggests one way to soften and/or remove hard cathode deposits using a cleaning cell, in which one or more cathodes/anodes comprising hard deposits are removed from an electrowinning cell and separately cleaned in a batch process.

Figure 6 depicts an alternative embodiment to the one depicted in FIG. 1 , using a heat exchanger.

Figure 7 depicts an alternative embodiment to the one depicted in FIG. 2, using an optional heat exchanger and/or submersion heater.

Figure 7 depicts an alternative embodiment to the one depicted in FIG. 3, using a an optional submersion heater. DETAILED DESCRIPTION OF THE DRAWINGS

Embodiments of an apparatus and method for controlling hard deposits on cathodes (and/or anodes) during the electrowinning of precious metals are disclosed.

In the method, the step of temporarily increasing the current density may be utilized during the electrowinning process to produce a softer deposit and/or to soften an existing hard deposit. In some embodiments, the current density and/or a duration of change in the current density may vary as function of time, rate of plating, softness of deposits, and/or concentration of precious metals in the electrowinning feed, without limitation.

In addition to this, or in lieu of this, a step of increasing a concentration of cyanide (e.g., sodium cyanide or equivalent) may be utilized during the electrowinning process to achieve a softer deposit and/or to soften an existing hard deposit. In some embodiments, the concentration of cyanide may vary as function of time, rate of plating, softness of deposits, and/or concentration of precious metals in the electrowinning feed, without limitation.

In addition to, or in lieu of the above, the step of introducing a lixiviant additive comprising an organic compound, a surface modifier, a viscosity modifier, a wetting agent, a releasing agent, or a soft metal (e.g., lead) may be utilized during the electrowinning process, to produce a softer deposit and/or to help soften hard precious metal deposits which may have already formed on electrowinning cathodes or anodes, without limitation. The composition of the additive may be adjusted during the electrowinning process. The amount of the additive added to the electrowinning cell (or a cleaning cell) may be varied over time. The additive may be introduced to the electrowinning process periodically. In some embodiments, the amount, duration, and/or composition of the additive introduced may vary as function of concentration of precious metal content in the electrowinning feed, without limitation. Mechanical cleaning means may be optionally provided to an electrowinning cell or cathode wash cell (i.e., “cleaning tank”). The mechanical cleaning means may be periodically or continuously used during the electrowinning process. Such means may include an ultrasonic transducer, without limitation. The ultrasonic transducer may be continuously or periodically energized, without limitation. The ultrasonic transducer may be energized differently as a function of time and/or “pulsed” to provide waves of varying amplitude or intensity, without limitation. Such means may include a sprayer or high-pressure sprayer or water current or water jet to wash the softened deposit from either electrode.

For embodiments incorporating such optional means for mechanical cleaning, an ultrasonic cleaning step may be optionally employed in order to aid soft deposit formation and/or hard deposit removal from cathodes/anodes. The cleaning step may be achieved by activating (e.g., energizing) the mechanical cleaning means.

In some embodiments, reverse polarity switching of the anodes and cathodes may be used with the method steps depicted herein. If polarity switching is employed, the step of reversing the polarity of anodes and cathodes within the electrowinning cell is preferably done intermittently and/or periodically (e.g., between normal operation electrowinning plating cycles).

Embodiments may incorporate one or a combination of process parameters with or without the addition or use of special reagents (i.e., “additives”) in the process to effect soft cathode deposits during electrowinning. Process parameters may include, without limitation: the use of high current density (i.e., greater than 40A/m ² ) in the electrowinning cell; the use of lixiviant having high free cyanide concentrations (e.g., greater than 0.5%, for example between 1 and 8 % by weight sodium cyanide) in the electrowinning cell; the use of one or more lixiviant additives (e.g., a small amount of: one or more soft metals, organic compound(s), wetting agent(s), surface modifier(s), viscosity modifier(s), releasing agent(s)) in the electrowinning cell, the use of increased silver concentrations within the lixiviant by virtue of the addition of Ag(CN)2 or “silver” solution to the electrowinning cell; and/or the introduction of mechanical waves or vibrations to the electrowinning cell for cleaning cathodes during the electrowinning process. Alternative mechanical cleaning method steps are envisaged. Though mechanical cleaning is preferably induced via ultrasonic methods, other mechanical means such as cathode scraping mechanisms, air or fluid spraying, and cathode vibrating/shaking/flexing means may be employed, without limitation.

It should be understood that in some embodiments, the unique and novel and unobvious addition or use of special reagents (i.e. , “additives”) during the process may accompany other broader process parameters including, without limitation: the use of current densities greater than 10 A/m ² in the electrowinning cell; and the use of lixiviant having free cyanide concentrations greater than 0.05% (e.g., 0.1 - 8% by weight as measured by sodium cyanide in the electrowinning cell, without limitation.

Turning now to FIG. 1 , an electrowinning cell is provided. The electrowinning cell comprises a pump for introducing electrowinning feed from an electrowinning feed tank, a rectifier, and an electrolytic cell comprising alternating anodes and cathodes. The electrowinning cell may comprise mechanical cleaning means, such as an ultrasonic transducer for assisting with cathode cleaning wire mesh cathodes. When energized, the ultrasonic transducer produces waves that can assist with removal of electroplated deposits from the wire mesh cathodes. Energizing of such mechanical cleaning means may be done continuously or periodically/intermittently, without limitation. Moreover, amplitude and/or frequency of the ultrasonic waves generated may be adjusted over time. Amplitude and/or frequency of the ultrasonic waves generated may differ during different electrowinning cycles or portions of the electrowinning process, without limitation.

Cathodes used in embodiments may be of any type but are preferably wire mesh, such as stainless-steel wire mesh, without limitation.

An exhaust fan may be used to remove noxious fumes and/or gasses produced by and/or accumulating around the electrowinning cell. Barren solution from the electrowinning cell may be removed and recycled as lixiviant or sent to the adsorption circuit, without limitation.

At the early stages of electrowinning, when cathode/anode surfaces are fresh, clean, and/or new, silver cyanide solution may be temporarily introduced in sufficient concentration to help soften early deposits on the wire mesh cathodes. Cyanide concentration (e.g., sodium cyanide concentration) and/or current density may also remain elevated to promote keeping precious metal deposits soft. The ultrasonic transducer can be energized to discourage hard plating. It is anticipated that introduction of silver solution (e.g., Ag(CN)2) may help weaken or soften initially-deposited precious metal deposits to aid in precious metal deposit removal from the wire cathodes. The addition and elevated concentration of silver solution may be maintained throughout the electrowinning process, or its addition to the electrowinning cell may be gradually reduced to lower silver concentration within the electrowinning cell. At any time during electrowinning, silver solution may be intermittently added to the electrowinning cell (e.g., if it is determined that deposits formations are becoming too hard).

Turning now to FIG. 2, an electrowinning cell may be accompanied by a mix/storage tank as shown. The electrowinn ing cell may operate normally, i.e., consistent with traditional prior art methods, in the sense that electrowinning feed may be pumped to the electrowinning cell from an electrowinning feed tank, and fumes evacuated by an exhaust fan. Barren solution may be removed from the electrowinning cell as depicted and as done for the embodiment shown in FIG. 1 . However, as depicted in FIG. 2, some of the barren solution removed from the electrowinning cell may be transferred to the mix/storage tank, where additional process water, reagent/additive(s), and/or cyanide solution can be introduced to and/or mixed with the barren solution leaving the electrowinning cell via respective pumps and/or control valves. Thus, the composition of the barren solution changes in the mix/storage tank such that it may comprise an increased concentration of reagent/additive(s) and/or cyanide. The resulting solution may be pumped from the mix/storage tank back to the electrowinning cell, and may, as shown, be combined with the electrowinning feed upstream of a feed inlet to the electrowinning cell. A control valve may be employed to the circuit to allow adjustment of the ratio of electrowinning feed to recycled solution combined therewith. The electrowinning cell may comprise an electrolytic cell and a rectifier for adjusting a current applied to spaced electrodes (e.g., anodes and cathodes) which may be energized with a negative (cathode) or positive (anode) charge in alternating fashion. As with other embodiments, the rectifier may be configured for continuously or intermittently increasing current density. As with other embodiments, the rectifier may be configured to enable reverse-polarity sequences.

The mix/storage tank may comprise a pump and/or valve for introducing and/or controlling an amount of process water delivered thereto. The mix/storage tank may comprise a pump and/or valve for introducing and/or controlling an amount of cyanide (e.g., NaCN, KCN, Cu(CN)2, Ag(CN)2, an equivalent thereof, or a combination thereof) delivered thereto. The mix/storage tank may further comprise a pump and/or valve for introducing and/or controlling an amount of a cleaning reagent or additive delivered thereto. The cleaning reagent or additive may comprise a hard electrowinning deposit remover configured for promoting softening and/or removal of electrowinning hard deposits from the wire cathodes/anodes, without limitation. The process water, cleaning reagent/additive, and/or added free cyanide (e.g., by virtue of addition of one or more cyanide species such as NaCN, KCN, etc.) - or, a solution containing one or more of the aforementioned, may be pumped into the electrolytic cell at controlled process conditions which promote releasing of hard deposits and/or softening of the deposits. The deposit-laden anode/cathode may be energized with a charge which is opposite in the electrowinning cell. In this regard, the rectifier may be used to allow the electrowinning cell to undergo a reverse polarity sequence in which a cathode or anode plated with precious metal deposits may be freed of its deposits through electrolysis.

Turning now to FIG. 3, an electrowinning cell as described above may be provided. In the non-limiting exemplary embodiment depicted, the electrowinning cell is provided without mechanical cleaning means, although the same may be provided to the electrowinning cell as an option.

After plating a precious metal on a cathode or anode, a hoist may be used to remove one or more cathodes and/or anodes comprising hard-deposits containing one or more precious metals (i.e., gold and/or silver) and re-locate them to a separate cathode wash cell (i.e., “cleaning tank”), instead of to a smelting operation typically used for destruction and recovery.

The cathode wash cell may comprise a pump and/or valve for introducing and/or controlling an amount of process water delivered thereto. The cathode wash cell may further comprise a pump and/or valve for introducing and/or controlling an amount of a cleaning reagent or additive delivered thereto. The cleaning reagent or additive may comprise a hard electrowinning deposit remover configured for promoting softening and/or removal of electrowinning hard deposits from the wire cathodes/anodes, without limitation. The cathode wash cell may further comprise a rectifier, and an electrolytic cell comprising spaced plates which may be energized with a negative or positive charge in alternating fashion. The process water, the cleaning reagent/additive, and cyanide (e.g., NaCN) - or, a solution containing the same, may be pumped into the electrolytic cell at controlled process conditions which promote releasing of hard deposits and/or softening of the deposits. The laden anode/cathode may be energized with a charge which is opposite of the spaced plates in the electrolytic cell. In this regard, the cathode wash cell may be used as a reverse polarity tank in which a cathode or anode plated with precious metal deposits may be freed of its deposits through electrolysis.

The concentration of cyanide (e.g., NaCN or equivalent) in the cathode wash cell is preferably higher than the cyanide concentration within the upstream electrowinning cell from which the cathodes and/or anodes comprising hard- deposits was taken from. The rectifier may be configured such that current density in the cathode wash cell may be higher than what is used in the upstream electrowinning cell from which the cathodes and/or anodes comprising hard-deposits was taken from.

The combination of higher cyanide concentration and higher current density in the cathode wash cell may present synergistic effects which foster the loosening hard deposits from the cathode(s)/anode(s) therein, and/or softening the plated precious metals. Accordingly, the cathode(s)/anode(s) placed therein may be cleaned and then hoisted from the cathode wash cell and placed back into the upstream electrowinning cell for recycle/reuse - without the need to sacrificially smelt them in order to recover precious metals entrained therein.

The cathode wash cell may, as shown, comprise mechanical cleaning means, such as an ultrasonic transducer for assisting with the cathode cleaning wire mesh cathodes. In a fashion similar to that described above for the embodiment shown in FIG. 1 , when energized, the ultrasonic transducer will produce waves that can assist the removal of hard and/or soft electroplated deposits from the wire mesh cathodes. Energizing of such mechanical cleaning means may be done continuously or periodically/intermittently, without limitation. Moreover, the amplitude and/or frequency of the ultrasonic waves generated may be adjusted (e.g., increased or decreased) over time. Amplitude and/or frequency of the ultrasonic waves generated may differ during different electrowinning cycles or portions of the electrowinning process, without limitation. Other mechanical means for cleaning such as scrapers or mechanical vibration means may be applied, without limitation.

Though not expressly depicted, an exhaust fan may be used to remove noxious fumes and/or gasses produced by and/or accumulating around the cathode wash cell, without limitation. Solution may be removed from the cathode wash cell by a pump, and controlled via a control valve as shown. The solution may be recycled directly to the electrowinning cell, or to an electrowinning feed tank as shown, without limitation. While not expressly depicted in FIG. 3, some or all of the solution from the mix/storage tank may be removed and sent to adsorption for recycling as lixiviant, without limitation. Moreover, while not expressly depicted in FIG. 3, at least some or all of the solution from the cathode wash cell may be combined with the electrowinning feed in a controlled manner upstream of a feed inlet to the electrowinning cell in a manner similar to what is suggested in FIG. 2.

Removed solids (i.e., “soft” deposits) which are dislodged from the anode(s)/cathode(s) in the cathode wash cell may fall to the bottom of the electrolytic cell portion of the cathode wash cell. These fallen solids may be pumped from the bottom of the electrolytic cell portion of the cathode wash cell and sent to another downstream refining process step.

For any of the embodiments discussed or shown herein, a reverse polarity step (e.g., within the electrowinning cell and/or cathode wash cell) may be employed, without limitation. Adding this step may further assist with dislodging hard- deposited materials containing precious metals from a cathode(s)/anode(s).

In some preferred embodiments, current density is maintained above approximately 5 A/m ² of cathode area, and even more particularly, above approximately 15 A/m ² of cathode area. For example, in some preferred embodiments, a high current density regime is used to soften cathode deposits and may be within the range of about 40-500 A/m ² of cathode area, without limitation.

In some preferred embodiments, target free cyanide concentration is maintained at all times above approximately Wo by weight of electrowinning solution, and preferably below approximately 4Wo by weight of electrowinning solution. However, free cyanide concentration may temporarily or continuously be elevated above approximately 0.65% by weight. For example, in some preferred embodiments, target sodium cyanide concentration used to soften cathode deposits may be kept within the range of about 1 -8% by weight of electrowinning solution, without limitation. In some preferred embodiments, 1 -4% or approximately 1.5% to 3% by weight sodium cyanide concentration of the electrowinning solution may be maintained to clean hard-deposits of precious metals from cathodes (e.g., approximately 2% by weight sodium cyanide concentration of the electrowinning solution may be maintained).

In some preferred embodiments, a lixiviant additive may be provided to improve softening of precious metals deposited onto cathodes and/or soften existing hard- deposits. The additive may comprise a soft metal, viscosity modifier, a surface modifier, and/or a release agent, without limitation. The additive may comprise a plurality of soft metals, viscosity modifiers, surface modifiers, and/or release agents, without limitation. The additive may comprise a combination of one or more soft metals, viscosity modifiers, surface modifiers, and/or release agents, without limitation. In some non-limiting embodiments, the additive may comprise 1 -10 ppm lead nitrate, without limitation.

In some preferred embodiments, silver cyanide may be provided to improve softening of deposited precious metals onto cathodes. In some non-limiting embodiments, the application of the silver cyanide may be used during initial plating to effectively “pre-coat” wire mesh cathodes with a thin, soft layer of “silver- heavy” deposits prior to and/or during the plating of gold. In this regard, harder deposits of gold may become easier to dislodge from a cathode(s). In some nonlimiting embodiments, silver cyanide may be added to an electrowinning solution intermittently. In some embodiments, 50-200 ppm of silver cyanide may be present in the electrowinning solution.

In some embodiments, such as the one depicted in FIG. 1 , the addition of silver cyanide to an electrowinning cell may be performed as a function of electrowinning feed composition. For instance, in cases where the electrowinning feed solution comprises gold and/or silver concentrations above 250 ppm, less or no silver cyanide may be added, since cathode deposits are anticipated to be softer and more easily removed. However, in instances where gold and/or silver concentrations in the electrowinning feed solution are measured to be below 100 ppm, harder cathode deposits may be anticipated - thus, necessitating the addition of a temporary addition or boosting of silver cyanide to the electrowinning solution to soften deposits. In some preferred embodiments, a composition of electrowinning feed solution may be monitored over time and periodically measured. A threshold for gold and/or silver concentration may be predetermined or set. If the % gold and/or silver in the electrowinning feed solution is measured to be below (or falls below) the threshold, a harder deposit may be expected and one or more of the following steps may be taken: the current density in the electrowinning cell may be increased, cyanide may be added to the electrowinning solution to increase cyanide concentration of the electrowinning solution in the electrowinning cell, silver cyanide may be added to the electrowinning cell to increase silver cyanide concentration of the electrowinning solution in the electrowinning cell, an amount of additive may be added to the electrowinning solution to help soften deposits or encourage releasability of deposits, and/or a reverse polarity step may be performed at any time during the electrowinning process, particularly in cases where there are hard-deposit occurrences.

Conversely, if the % gold and/or silver in the electrowinning feed solution is measured to be above (or rises above) the threshold, a softer deposit may be expected and one or more of the following steps may be taken: decrease current density in the electrowinning cell, remove cyanide from - or decrease cyanide concentration of the electrowinning solution in the electrowinning cell, remove silver cyanide from - or decrease silver cyanide concentration of the electrowinning solution in the electrowinning cell, remove an amount of additive from - or decrease an amount of an additive added to the electrowinning solution in the electrowinning cell, and/or omit, skip, or do not perform a reverse polarity step during electrowinning.

Turning now to FIG. 3, an electrowinning cell may be provided. An electrowinning feed solution may be provided to the electrowinning cell. A concentration of precious metals in the electrowinning feed solution may be measured (e.g., ppm Au, ppm Ag). Cyanide (e.g., sodium cyanide, potassium cyanide, copper cyanide, silver cyanide, or equivalent) may be added to boost the concentration of free cyanide in the electrowinning solution (e.g., to between 1 and 4 % sodium cyanide weight), in order to prevent the formation of hard cathode deposits. Current density may be maintained above 40 A/m ² of cathode area, but preferably less than 200 A/m ² of cathode area. If further softening of deposits becomes necessary, optional steps may be further employed. For example, silver cyanide solution may be added to the electrowinning feed solution or to the electrowinning cell. An additive as described above may be added to the electrowinning feed solution or to the electrowinning cell. One or more reverse polarity steps may be performed, e.g., by switching the polarity or charge of the anodes and cathodes in the electrowinning cell. Mechanical cleaning steps may be performed, e.g., by virtue of energizing an ultrasonic transducer, high pressure spray, mechanical cathode scraper, or mechanically vibrating one or more cathodes, without limitation. It will be appreciated by those skilled in the art that the above steps may be performed in any useable order, and that any one of the above steps may be performed in combination with other steps, as needed to effect soft cathode deposits.

From the method shown in FIG. 4, it can be further appreciated that for embodiments employing a cathode wash cell (as depicted in FIG. 3), similar steps may be performed. In such cases, some or all of the steps outlined in FIG. 4 which are used in conjunction with an electrowinning cell may be performed in conjunction with a cathode wash cell, without limitation.

EXAMPLE 1

According to one non-limiting embodiment, an electrowinning process may involve providing an electrowinning cell with an electrowinning solution comprising a cyanide concentration which is greater than or equal to 0.6 weight% to produce a soft deposit during electrowinning; and/or providing an electrowinn ing solution comprising a sodium cyanide concentration greater than or equal 1 .0 weight% to an electrowinning cell to soften a hard deposit already formed during electrowinning, without limitation.

Sodium cyanide (or equivalent) may be added to the elution solution to strip the carbon as conventionally done. For example, an AARL circuit may utilize an approximately 5% sodium cyanide (NaCN) pre-soak with 10 BV rinse, which equates to an approximately 0.32% NaCN concentration in the final electrowinning solution. As another example, the AARL circuit may utilize an approximately 5% NaCN pre-soak with 6 BV rinse, which equates to an approximately 0.52% NaCN concentration in the final electrowinning solution.

However, in order to produce softer deposits and/or discourage hard cathode deposit formation according to the invention, preferred embodiments employ the use of higher percent NaCN pre-soak concentrations and/or lesser BV of wash solution, in order to provide a final electrowinning solution having NaCN concentrations that are greater than or equal to 0.65 weight %. Most preferably, final NaCN concentrations are maintained above 1%, for example, between 1 and 4% by weight NaCN, without limitation.

For instance, in some preferred embodiments, a 2% by weight NaCN solution may be employed to soften and remove a hard anode and/or cathode deposit during electrowinning.

An additive may be present in the electrowinning solution, or added during electrowinning to soften cathode deposits and/or promote removal of cathode deposits. For example, a lead-containing reagent may be added to produce a >1 ppm Pb concentration in an electrowinning feed to produce a softer deposit during electrowinning. For example, 5 ppm Pb may be added to 500 ppm of Ag/Au/Cu in the electrowinning feed solution to obtain a soft cathode deposit.

The additive may be delivered to the electrowinning cell periodically, for example, it may be used/introduced to the electrowinning cell upon use of new or clean anodes/cathodes. In this regard, the additive may assist with preconditioning fresh cathode/anode surfaces. Preconditioning of the fresh, clean cathode/anode surfaces using the additive in the lixiviant may help with the formation of weak, soft, plated cathode layers, thus facilitating the removal of subsequent deposits formed thereover. The additive may also be delivered continuously to the electrowinning cell and the amount of it added to the electrowinning cell optionally adjusted over time, for example, as a function of precious metal concentration in the electrowinning feed. In this regard, the small amount of lead present in solution may help soften newly- formed deposits of precious metals on wire cathodes/anodes.

In addition to, or in lieu of the aforementioned lead-based additive, lixiviant comprising various amounts/concentrations of a wetting agent, oxidant, and/or organic may be added to the electrowinning cell solution to produce a soft(er) deposit during electrowinning and/or to soften already-present hard deposits which were formed during the electrowinning process, without limitation.

Current density is normally limited to 5-15 A/m ² for production cells, but according to some embodiments of the present invention, additional measures may be taken to soften deposits and/or loosen hard deposits by increasing the same. For example, in some embodiments, electrowinning may be performed at a current density which is greater than 40 A/m ² of cathode area in order to achieve or encourage the formation of softer cathode deposits. In one particular test, an approximately 40 A/m ² of cathode area current density condition was achieved using a 1 Amp setting on the rectifier. In order to economize process conditions, current density may be kept below 500 A/m ² of cathode area (e.g., less than 200 A/m ² of cathode area), without limitation.

EXAMPLE 2

According to one non-limiting embodiment, an electrowinning process may involve providing an electrowinning cell with an electrowinning solution comprising a cyanide concentration which is greater than or equal to 0.6 weight% for example, an electrowinning solution comprising a free cyanide concentration which is greater than or equal 0.65 weight% and less than or equal to 5.0 weight% (e.g., approximately 1.0 - 2.5 weight% or approximately 2 weight%), without limitation. Sodium cyanide may be used as a matter of preference, although other species of cyanide may be employed in addition to, or in lieu of sodium cyanide. Current density used within the electrowinning cell may substantially exceed the traditional 5-15 A/m ² used for production cells. For example, electrowinning in the electrowinning cell may be performed at a current density which is greater than 40 A/m ² of cathode area, in order to achieve or encourage the formation of softer initial cathode deposits on cathodes. In one particular test, an approximately 40 A/m ² of cathode area current density condition was achieved using a 1 Amp setting on the electrowinning cell rectifier. In order to economize process conditions, current density may be kept below 500 A/m ² of cathode area.

A first electrowinning cycle may be performed using the electrowinning cell. Some of the electrodes within the cell are positively-charged anodes and some of the electrodes are negatively-charged cathodes during the first electrowinning cycle. The first electrowinning cycle may be performed for a first cycle duration lasting, for example, between 60 and 400 minutes. During the first cycle duration, initial plating of precious metals (from the electrowinning solution) onto the cathodes within the cell may be achieved. In some embodiments, the first electrowinning cycle may be around 90 to 180 minutes long (or longer, e.g., 240 minutes), without limitation.

After completion of the first electrowinning cycle and lapsing of first cycle duration, a relatively short reverse polarity cycle may be subsequently performed after the first electrowinning cycle, wherein the charge of the previous anodes and cathodes within the electrowinning cell are at least temporarily switch ed/in verted, and one or more precious metal plated previously negatively-charged cathodes will temporarily become positively-charged “anodes” during the relatively short reverse polarity cycle. This relatively short reverse polarity cycle may be considered to be a second electrowinning cycle, wherein the previously positively-charged anodes from the first electrowinning cycle will temporarily become negatively-charged “cathodes”.

This relatively short reverse polarity cycle (or “second” electrowinning cycle) may be performed between 1 and 60 minutes (i.e., for a second cycle duration), or otherwise for a shorter duration relative to the first cycle duration in this Example. It is anticipated that this relatively short reverse polarity cycle may be substantially less than the first cycle duration. Traditionally, a reverse polarity cycle may take 400 minutes or more to completely re-plate initially deposited precious metals (which remained lodged on or hard-plated to cathodes of the first electrowinning cycle) onto the reversed-polarity anodes of the first electrowinning cycle. However, preferred embodiments may involve a relatively short reverse polarity cycle after the first electrowinning cycle lasting between 1 and 60 minutes, and preferably less than about 30 or 45 minutes, (e.g., 1 -20 minutes, approximately 10 minutes or less, approximately 15 minutes or less, or approximately 20 minutes or less), without limitation. Thus, in some preferred embodiments, the ratio of the second cycle duration to the first cycle duration is preferably between 0.01 and 0.5, more preferably between 0.5 to 0.3, such as 0.1 to 0.25 or approximately 0.2, without limitation. Thus, the second cycle duration of the relatively short reverse polarity cycle is preferably substantially shorter than the first cycle duration. The second cycle duration may be at least twice as short as the first cycle duration. The second cycle duration may be less than 1 /3 ^rd of the first cycle duration. The cycle second cycle duration may be less than 14 of the first cycle duration. The cycle second cycle duration may be less than a fifth of the first cycle duration, of the first electrowinning cycle, without limitation.

The inventors have unexpectedly discovered that at the abovementioned elevated free cyanide concentrations and higher current density regimes, a relatively short reverse polarity cycle following an electrowinning cycle is sufficient enough to substantially soften plated electrodes loaded with precious metals during the electrowinning cycle, thus, allowing for easy removal of precious metal deposits from the electrodes with nothing more than simple flushing of unpressurized or pressurized water. Accordingly, the technical effect provides the advantage of reducing reliance on energy-intensive smelting practices that consume electrodes and allowing for cathode (and/or anode) re-use within the electrowinning cell and/or method of electrowinning. It should be understood that for this Example, the other Examples (1 and 3) mentioned, and for any one or more of the methods discussed herein or claimed below, one or more precious metal removal steps may take place between one, some, or all of the electrowinning cycles. In some embodiments, cleaning of one or more electrodes (cathode and/or anode) may occur between a first and second electrowinning cycle (and/or between a second and third electrowinning cycle, between a third and fourth electrowinning cycle, between a fourth and fifth electrowinning cycle and so on and so forth). The cleaning cycles may include removal of precious metal deposits from one or more electrodes of the electrowinning cell (anode or cathode).

For example, between any two or more successive electrowinning cycles, one or more electrodes may be temporarily hoisted or substantially removed from an electrowinning cell and cleaned (e.g., by virtue of rinsing the one or more electrodes, washing the one or more electrodes, pressure-washing the one or more electrodes, spraying the one or more electrodes, water-jetting the one or more electrodes, mechanically or physically scraping the one or more electrodes, mechanically or physically vibrating or inducing vibrations into the one or more electrodes, generating a mechanical wave using an ultrasonic transducer or sound generator, air blowing the one or more electrodes, mechanically or physically shaking the one or more electrodes, and/or mechanically or physically flexing the one or more electrodes, without limitation). Any of the above steps may be taken in any order, and in any combination, without limitation.

Alternatively, or in addition to, the above ex situ cleaning of the one or more electrodes between electrowinning cycles, the one or more electrodes (e.g., one or more anodes and/or cathodes in the cell) may be cleaned of precious metal deposits in situ, that is, while remaining in the electrowinning cell (full of electrowinning solution or not). In situ cleaning may include one or more of the same steps mentioned above, in any order, and in any combination, without limitation. It is preferred that during any intervening cleaning step between successive electrowinning cycles, both cathodes and anodes are washed after each electrowinning cycle (e.g., including after each reverse polarity cycle). Moreover, optionally, the inventors have discovered the advantage of periodically removing electrodes (anode or cathode) from the electrowinning cell and submersing them in a warm bath of solution comprising 1 -8% sodium cyanide solution (e.g., 1 -4% sodium cyanide). The cyanide solution may comprise any of the cyanide species mentioned herein. The cyanide solution may comprise one or more of the agents or lixiviant additives mentioned herein (e.g., a wetting agent, oxidant, organic compound, surface modifier, viscosity modifier, releasing agent, soft metal (e.g., lead), or any combination thereof). Preferably, the temperature of the warm bath is maintained between approximately 60 and 212 degrees Fahrenheit above a pH value of approximately 9.5. The cyanide solution may comprise sodium cyanide (NaCN), potassium cyanide (KCN), copper cyanide (CU(CN)2), and/or silver cyanide (Ag(CN) 2), without limitation. Doing this completely removes or reduces precious metal buildup on electrodes (anode or cathode) and ensures fresh electrode surfaces for future electrowinning cycles within the electrowinning cell.

EXAMPLE 3

The (second electrowinning) reverse polarity cycle described in Example 2 may alternatively be taken to a full term within the purview of traditional reverse polarity cycle times, such that the second cycle duration is substantially the same duration as the first cycle duration or longer. In other words, in some embodiments, the ratio of the second cycle duration to the first cycle duration may be between 0.7 and 1.5, more preferably between 0.8 to 1.3, such as 0.9 - 1.1 , or 0.2, without limitation. The reverse polarity cycle may be much longer than the first cycle duration of the first electrowinning cycle as well. Thus, the second cycle duration in this example may be substantially similar to - or greater than the first cycle duration. During the first electrowinning cycle, precious metals are initially plated onto cathodes. In the following reverse polarity cycles, much of the deposits originally-deposited onto the negatively-charged cathodes will soften, sludge, and/or deposit onto the (original) anodes which are now negatively-charged during the reverse polarity cycle. A third electrowinning cycle may subsequently follow the reverse polarity cycle. In this third electrowinning cycle, the polarity of anodes and cathodes may yet again, be switched, such that the polarity of the same resets to the same conditions as the first electrowinning cycle. In a first portion of the duration of this third electrowinning cycle, the anodes (i.e., which were cathodes plating during the second electrowinning/reverse polarity cycle) will easily self-sludge. Deposit removal from the now-plated “anodes” of the third electrowinning cycle may further be accomplished via a supplemental spray or rinsing.

It should be understood that at any time between the first, second, and/or third electrowinning cycles, electrowinning solution may be flushed or removed from the electrowinning cell and replaced with new electrowinning solution, and softened deposit or sludge may be washed from the electrodes and removed from the cell.

In this specification, adjectives such as first and second, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Where the context permits, reference to an integer or a component or step (or the like) is not to be interpreted as being limited to only one of that integer, component, or step, but rather could be one or more of that integer, component, or step etc.

Where used herein in this specification, drawings, and in the claims, the term “cathode” means an electrode that is negatively-charged. It should be understood to those skilled in the art, that at various points during electrowinning, polarity of electrodes within the electrowinning cell may be temporarily switched (e.g., in the case of a reverse polarity step), wherein anodes which were previously positively- charged may become negatively charged - in which case, a previous anode may also be construed as a cathode while it is negatively-charged during said reverse polarity step. The term “sodium cyanide” and/or “NaCN” has been used for convenience throughout this specification, drawings, and claims, in order to establish and suggest one possible best mode for practicing embodiments the invention and quantifying target free cyanide concentrations. However, it is anticipated by the inventors that alternative cyanide species may be practiced (in addition to, or in lieu of sodium cyanide (NaCN)) to increase free cyanide concentration. Accordingly, where it is used and appears throughout this specification, drawings, and claims, the term “sodium cyanide” and/or “NaCN” may be interpreted broadly to also include or comprise amounts of potassium cyanide (KCN), copper cyanide (CU(CN)2), silver cyanide (Ag(CN)2), or a combination thereof, without limitation.

Similarly, where used herein and in the appended figures and claims, the broader term “cyanide” used in conjunction with the term “concentration” suggests an increase in free cyanide concentration (such as an increase in free cyanide concentration by approximately ~0.5 to -4.5 wt.%) which may be accomplished by introducing one or more or several cyanide species. While this increase in free cyanide concentration may (in some preferred embodiments) be measured in relation to sodium cyanide concentration (e.g., a 1 -8 wt.% target sodium cyanide concentration), other cyanide species may be used to increase the free cyanide concentration to within approximately -0.5 to -4.5 wt.%.

Cyanide species used to increase free cyanide concentration may comprise any one or more of the following, without limitation: “sodium cyanide”, “potassium cyanide”, “KCN”, “copper cyanide”, “Cu(CN)2”, “silver cyanide”, “Ag(CN)2”, “inorganic cyanide”, or a “cyanide compound” (e.g. cyanide solution comprising one or more of: NaCN, KCN, Cu(CN)2, Ag(CN)2, or equivalent).

The above description of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. The invention is intended to embrace all alternatives, modifications, and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above-described invention.

In this specification, the terms ‘comprises’, ‘comprising’, ‘includes’, ‘including’, or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed. Moreover, it should be understood that while certain method steps and/or apparatus features may be shown or discussed together, it is anticipated by the inventors that one or more features and/or steps may be combined or grouped without employing other mentioned features and/or steps. Additionally, method steps may be performed in sequences other than what is expressly depicted or disclosed herein, such that the order of execution of the steps may be performed in various different orders.

Moreover, it is anticipated that certain steps or featured elements described herein may be optionally omitted, without limitation.