Patent Grant
8202411
Not applicable.
1. Technical Field
The present invention generally relates to the field of electrowinning, also referred to as electrorefining or electroextraction, and more specifically to apparatus and processes for electrolytically removing metals from conductive liquids and facilitating recovery of such metals. Still more specifically, the invention relates to such apparatus and processes which provide for particulate cathode bed churning that is uncoupled to the flow of catholyte.
2. Description of Related Art
In an electrowinning process, a target metal ion is electrolytically removed from the solution phase of a conductive liquid and is deposited as a solid metal that can be recovered and potentially sold, either directly or after further purification. This approach to metal extraction is commonly used for the recovery and purification of several metals in the mining industry. Some of the metals that have been previously targeted for electrolytic recovery include copper, iron, lead and zinc. According to the U.S. Census Bureau's Economic Census, Mining Industry Series, in 2002 the electrowinning of copper from leaching operations accounted for more than 30% of copper ore mining production.
Some of the advantages of existing electrowinning methods include low operating costs, no chemical reagents required, solid product in low volume, high value form, reduced disposal issues and harmless byproducts (e.g., oxygen), potential for selectivity, and modular, scalable technology. These advantages are in many cases offset by common drawbacks such as medium to high capital cost, possible side reactions, and electrode fouling, corrosion, or other undesirable chemical reaction. Many existing systems suffer from awkward product removal, in some cases requiring equipment disassembly. Another common drawback of existing electrowinning technologies is low efficiency when processing dilute sources of metals. Most existing methods cannot be satisfactorily scaled up for processing large volumes of liquid and increased space velocities.
Conventional electrolytic technologies include standard plate and frame (2-dimensional) electro-deposition cells. These electrolytic systems are typically either planar or annular in design and operate at only moderate current densities, thus requiring large electrodes with correspondingly larger capital investments. Additionally, such cells generally suffer from large Ohmic losses due to significant interelectrode distances. Conventional plate and frame electrolytic technologies are not suitable for recovery of metals from dilute streams containing less than 1000 ppm metal.
Another type of technology in use today for electrowinning employs porous packed beds which provide large electrode areas (3-dimensional) and operate at higher current densities than typical 2-dimensional systems. Packed beds tend to become occluded by metal deposition, however, and are subject to shorting by interelectrode dendritic growth. Fluidized bed electrodes have also been extensively investigated. These electrolytic systems typically operate at much higher current densities than simple plate cells, and allow, to some extent, for electrode particle growth. Fluidized bed electrolytic systems still suffer from energy-intensive fluidization and dendrite growth leading to bed coagulation and process instability.
Spouted electrode technology (SET) comprises a modification of the fluidized bed technology, in which a jet or stream of electrolyte is introduced into the draft tube of an electrode forming a spout. The stream of electrolyte entrains and transports electrode particles in the particle bed up the draft tube, after which the particles are released onto the top of the particle bed and ultimately move to the bottom of the bed to be picked up again by the electrolyte stream. The electrode particles are in constant motion moving through the draft tube and into and through the particle bed during electrolysis. SET provides a number of improvements that overcome many of the limitations of previous systems, by providing a churning packed bed that resists dendrite growth, accommodates cathode particle growth, allows cathode bed removal/replacement without cell disassembly, and require less fluid transport energy than a fully fluidized bed. Although SET has been studied for many years, it has until recently been hampered by scale-up issues due to the typically annular design and unsuitability at metal concentrations below a few thousand parts per million.
U.S. Pat. No. 5,635,051 discloses a zinc electrowinning process using a mobile bed of particles. Motion of the bed is achieved by imposing a flow on the electrolyte solution in such a manner as to create a levitation region (a spout) in the cell distinct from, and preferably adjacent to, the moving packed bed. Favorable results in terms of production rate, current efficiency and energy consumption are said to be achieved by using a unique combination of design parameters and operating conditions achieved by selected ranges for particle size, current density, particle bed thickness and acid content of the electrolyte.
U.S. Pat. No. 6,298,996 discloses a spouted electrode and its use in electrowinning, in which cathode and anode chambers are separated by an inexpensive microporous membrane that prevents the cell from short-circuiting. The feed solution is jetted upwards into the cathode particle bed and fluidizes the central particles up through a spout tube. At the top of the cell the fluid velocity drops allowing the particles to fall back onto the bed. As with other conventional SET designs, the feed fluid and cathode bed particles are coupled in motion and flow path resulting in moderate removal rates and moderate control of selectivity. A prior art SET configuration, in which fluid and cathode bed flows are coupled, is shown in
Most existing spouted electrode designs (both annular and planar) rely on the use of an electrolyte jet to churn the cathode bed. The linking or coupling of cathode spouting to electrolyte motion imposes certain inherent limitations on the technology. For instance, since a high flow rate is typically required to achieve spouting, operation parameter flexibility is limited. For heavy materials, such as plated metals, this can waste large amounts of energy to jet the electrolytic fluid fast enough to cause spouting. Also, most electrolyte passes through the spout and bypasses the bed so that excessive amounts of fluid transport occur and most fluid is simply recirculated without being treated. In many cases the result is low per pass removal rates, necessitating batch mode operation and increasing energy demands for pumping. Since most industrial metal recovery applications are more conducive to flow-through treatment, batch mode operation can be especially disadvantageous. The problem usually grows worse at lower metal concentrations where less bed motion is needed and higher electrolyte flow rates in the bed would improve efficiencies by reducing mass transport limitations. At metal concentrations of less than about 2000 ppm, the inherent problems of jetted SET are often glaringly apparent. Therefore, jetted SET is traditionally applied to only relatively concentrated solutions (e.g., 5000+ ppm), in which the above-mentioned limitations are less noticeable and higher operation current densities can be employed to offset the increased pumping demands.
Although there has been considerable advancement in the art of electrowinning, there is continuing interest in reducing process complexity, increasing separation efficiency and providing greater energy efficiency, in order to lower costs and boost productivity in such industries as metal mining, refining and recycling and for conserving natural resources and preventing pollution.
In accordance with certain embodiments of the invention, a spouted bed reactor is provided which comprises a particle bed, an inlet for a flowing fluid stream; and a particle bed churning device configured for spouting particle bed particles in said reactor independently of the flow of the fluid stream. In certain embodiments the reactor is configured for electrowinning a heavy metal from a fluid stream, wherein the particle bed comprises electrically conductive cathode particles and the fluid stream comprises an electrically conductive solution comprising the heavy metal. In some embodiments, the electrowinning reactor comprises at least one electrolytic cell, with each cell comprising an anolyte chamber comprising an anode and configured for containing an anolyte, a catholyte chamber comprising a current collector and configured for containing a particulate cathode bed and a flowing stream of the heavy metal-containing fluid, and an ion-permeable membrane separating the anolyte and catholyte chambers.
In accordance with certain embodiments, the cathode particles are magnetic and the particle bed churning device comprises a magnetic conveyor assembly comprising a plurality of magnets attached to a conveyor belt or the particle bed churning device comprises an array of electromagnets. In accordance with certain other embodiments, the particle bed churning device comprises a conveyor belt or auger screw assembly.
Also provided in accordance with certain embodiments of the invention is an electrowinning system comprising at least one spouted bed reactor wherein each of the reactors comprises: an anolyte chamber comprising an anode and configured for containing an anolyte, a catholyte chamber comprising a current collector and configured for containing a particulate cathode bed and a flowing stream of an electrically conductive heavy metal-containing fluid, and an ion-permeable membrane separating the anolyte and catholyte chambers, an inlet for an electrically conductive metal-containing fluid stream; and a particle bed churning device configured for spouting particle bed particles in the catholyte chamber reactor independently of the flow of the fluid stream.
Further provided in accordance with certain embodiments of the invention is an electrowinning process comprising: (a) providing an electrowinning system as described above; (b) flowing an electrically conductive anolyte solution through the anolyte chamber; (c) flowing a catholyte solution through a cathode particle bed in the catholyte chamber, wherein the catholyte solution comprises an electrically conductive fluid containing at least one heavy metal salt dissolved therein; (d) establishing a predetermined voltage and current across the electrolytic cell sufficient to effect reduction of a selected heavy metal at the particle bed and cause an oxidation reaction at the anode; and (e) spouting the cathode particles, wherein such “spouting” is independent of the flow of the catholyte solution of step (c). Thus the particles are not required to be levitated and carried by the flow of the catholyte solution in order to redistribute or chum the particle bed. Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices or methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, and by referring to the accompanying drawings.
Certain terms are used throughout the following description and claims to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function.
In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”.
“Spouting” refers to the facilitated movement or transport of particles from one region of a particulate bed of a reactor to another region of the particulate bed away from the first region, to redistribute or churn the particulate bed. Spouting of the particles deters sticking and clumping of particles as may occur during operation of the reactor. Spouting of a particle bed is preferably distinct from any process of removing spent particles and returning fresh particles to the bed.
The term “mechanically spouted” when referring to a particle bed, means that the particle bed is spouted by any mechanical means including, but not limited to, a conveyor belt, auger screw, or magnetic spouting mechanism. The mechanism may include transporting particles through a spout, channel, or tube from a first point (e.g., the spout inlet) to a second point (e.g., the spout outlet).
The term “magnetically spouted,” when referring to a particle bed, means magnetically churned or redistributed by being magnetically attracted to a magnet and moved through a spout or channel from a first point (e.g., the spout inlet) to a second point (e.g., the spout outlet), and released by ceasing the magnetic attraction. Magnetic spouting may be accomplished by attracting particles to a series of permanent magnets and then moving the magnets with attached particles, or it may be accomplished by attracting the particles to electromagnets and moving the particles to successive electromagnets by coordinated energizing and de-energizing of neighboring electromagnets, for example.
The term “magnetic bead” or “magnetic particle” refers to a bead or particle that is magnetically attracted to a magnet.
The term “heavy metal” as used in the context of this disclosure refers to metals with densities greater than 4 g/cm3. Heavy metals include, but are not limited to, chromium, manganese, iron, cobalt, nickel, copper zinc, gallium, germanium, arsenic, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, antimony, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, thallium, lead, and bismuth.
The term “couple” or “couples” is intended to mean either an indirect or direct electrical, magnetic or other mechanical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical, magnetic or other mechanical connection, or through an indirect electrical, magnetic or other mechanical connection via other devices and connections.
Apparatus and methods for mechanically, magnetically or electromagnetically spouting particle beds are described herein in the context of electrowinning of metals from fluids. Embodiments of the apparatus define inlets and outlets for the fluid feed stream which are distinct from the inlets and outlets for the spouted bed particles. Embodiments of the electrowinning methods provide spouted bed particle flow that is not dependent on the flow of the feed stream. Thus it is not a requirement for the flowing feed stream to levitate or hydraulically spout the cathode bed particles. This allows the velocity and residence time of the fluid feed stream on (or thorough) the particle bed to be controlled independently of the rate of spouting (or transport) of bed particles. Optimization of the velocity and residence time of a dilute metal-containing solution enhances recovery of the metal in many instances and improves the efficiency of an electrowinning process.
Electrolytic Cell for Magnetically Recirculating a Particulate Cathode Bed
Referring to
The anode 115 may be configured as two parts divided by a mechanical support, as shown, or as a single structure in the absence of a mechanical support. Microporous membrane separator 124 separates the anode and cathode chambers. The ion-permeable separator membrane may be any suitable microporous polymer or clay filled polymer or a cation exchange membrane, for example.
The cathode current collector plate 111, which may be made of stainless steel or another suitable corrosion-resistant material (e.g., titanium, zirconium, graphite, Hastalloy, Inconel), may also serve as the cathode conveying surface for magnetic coupling to the magnetic conveyor drive (like that shown in
In a marked departure from other spouted electrolytic cells that rely on high velocity fluid flow to spout the cathode beads by levitating the beads, and to thereby churn the cathode bed, a representative MSET cell 10 instead employs a mechanical magnetic conveyer assembly 30 for bed churning. The magnetic conveyer assembly 30 is located adjacent to the cathode chamber 14. As explained in more detail in the section subtitled “Process,” the MSET cell provides that the bed churning operation is uncoupled from the transport of electrolyte and will allow for optimal (lower) fluid flow velocities to be used, and also allows for greatly reduced fluid pumping costs, among other advantages.
Magnetic conveyer assembly 30 is shown in
The magnets may be used singly or stacked, as necessary, to obtain the desired magnet power. The magnets 36, or stacks of magnets, are spaced apart from each other a sufficient distance along the length of belt 32 to avoid detrimental competition between neighboring magnets for attracting cathode beads. When the system is in use, interference between closely-spaced magnets will tend to pull cathode beads in the reverse direction by the next oncoming magnet, causing fewer beads to exit the top of the spout. For example, three stacked magnets spaced at least 5 inches apart on the belt is optimal in an exemplary 36 in.×12 in.×3 in. system, as illustrated in
As shown in
Although permanent magnets may be used for many applications, there may be instances in which electromagnets are used instead, as illustrated in
Bench-scale MSET Electrolytic Cell
An exemplary bench-scale MSET electrolytic cell 400 is illustrated, disassembled, in
When cell 400 is assembled, a suitable gasket material is used to seal the edges of catholyte chamber 414 and anolyte chamber 416. One such gasket material is ¼ inch wide closed cell PVC foam. Cell 400 has a microporous membrane layer 424 (similar to membrane 24 in
In another model of an MSET cell, the magnets were doubled in width compared to an earlier cell, and fits the same original sized window, which was just wide enough to accommodate the wider magnets. The double width magnetic drive was also mounted on a chassis similar to that shown in
As discussed in the “Process” section, below, as a result of this modification of the magnetic drive, the cathode churning or “spouting” rate was doubled compared to the previous version by doubling the width of the magnets to 1.5 inches, to prevent agglomeration of beads at higher metal concentrations (>500 mg/L). Doubling the width of the magnets did increase the volume of beads carried up the recirculation channel by nearly two fold compared to the first version of the electrolytic cell. The polyethylene flaps that act as the cathode bead check valve at the top of the recirculation channel were also redesigned so as to prevent them from inverting under the increased bead load weight, allowing the undesirable back-flow of beads down the spout tube. Spacing between the magnets can also be increased to ensure that the timing of cathode bead release (from the magnet drive) is out of phase with the opening of the check valve.
In another MSET cell design, the treatment capacity of the MSET cell of
The representative embodiments shown in
Magnetic Cathode Particles (MSET Cathode)
A suitable cathode material that is electrically conductive, magnetic, and corrosion resistant is employed as the cathode particles' substrate. A readily available magnetically attractive bead material comprising zinc plated, 2 mm diameter #7 steel shot (Precision Reloading, Inc.) is one such substrate material. Spherical beads are generally satisfactory for magnetically recirculating, while other shapes such as cylindrical beads (e.g., cut wire) do not flow as well in the MSET electrolytic system. Cathode bead surfaces are prepared in a way that minimizes their corrosion by metal/H2SO4 solutions. In various tests, the zinc plated steel shot were used directly, although care had to be taken to not corrode the zinc coating off of the shot bead in acidic solutions. The bare steel shot could be plated directly with various non-corrosive metals by electroless deposition and thermal annealing. Electroless copper coating was used to create cathode particles that are more stable to corrosion in acid solution. For some of the tests, the electroless plating solution was designed to work for the steel shot from Ballistic Products Inc., at room temperature for convenient bench-scale processing. Shot that has no small holes leading to a hollow center is a generally preferred substrate. Shots that have small holes are more difficult to seal with an initial copper coating.
One representative method for preparing cathode particles is as follows:
1) Prepare Reagents for Copper Plating Solution
2) Making the Copper Plating Solution
3) Activating Shot Surface
4) Copper Deposition onto Shot
5) Anneal the Shot
6) Clean Carbon “Soot” from Copper Surface
A representative two-cell model of a MSET treatment system 500 is shown in
Additional features of this model may include a “smart” control/alert system for remote operation/oversight, and an automated cathode particle addition/removal/recovery system (not shown) which will allow long-term continuous operation of the system. The system may be configured for both 110V ac power and solar powered operation, for remote site operation. Monitoring and control circuitry may also be included in this system. Some of the potential benefits of a MSET treatment system such as this include the significant reduction in fluid circulation costs and much greater operation flexibility, while retaining the advantages of high operation current densities and efficiencies at low metal concentrations; compact and low capital cells; no need for solution pretreatment; easy and compact contaminant capture with simple removal; and modular and readily automated equipment.
Depending on the anticipated end-use application and location, further modifications may be included in the MSET system if desired. For instance, the system may be weatherproofed and animal-proofed for outdoor use, and extra care taken to ensure that the components of the magnetic conveyor drive are corrosion resistant and all electrical connections are water resistant and that the selected drive motors and pumps are suitable for outdoor use.
MSET-Acid Mine Drainage Treatment Platform
An exemplary MSET-AMD treatment system platform 1000 shown in
The system control module 1030 contains an electronics system for automated control and user interface. System monitoring sensors include appropriately located feed flow meter, cell voltage monitoring, magnetic drive motor speed sensor, pH sensors, temperature sensors, power supply control, and safety shut-down interlock interface. For example, a commercial CPU and A/D board may be used for computer control of the system. The computer is appropriately programmed with system software written in C++ and stored in its non-volatile memory. Circuitries of all other boards (frequency/voltage converter, voltage amplifier, 12V and 5V sources, power control interface, user interface, LCD and keypad) are suitably prepared using known techniques. In
The safety shut-down interlock module (power interlock module) 1060 functions as the power distribution module to each component of the electrolytic cells. The interlock module contains solid state relay circuits that can be triggered by the system control module and leak sensors to shut down the power distribution to the electrolytic cells, magnetic conveyor drive motor, and feed pump in the event of leaking or other system failures. This module allows for manual or automated power control. Commercially sold control systems such as a programmable logic controller (PLC) will provide this functionality.
Solar panels (like those schematically illustrated in
Process
Electro-Refining Process—Single Stage for Batch Mode Metal Removal.
An example of a batch mode electrowinning process for recovering a single target metal employs an electrolytic cell as described above, connected to a fluid circulation system for recirculating a metal-contaminated aqueous feed streams, as illustrated in the fluid flow diagram shown in
A separate reservoir 670 is used for circulation of the anolyte in the anode chamber 612, via lines 672 and 674 anode solution pump 676 and anolyte pump 676. The electrolyte used as the anolyte typically comprises any suitable sulfate, or phosphate, or a combinations of any of those. The anolyte flow rate is maintained at a suitable rate to purge evolved oxygen from the flow chamber and maintain the fluid level over the anode and membrane surfaces. Preferably the cathode and anode solutions are isolated from each other (instead of being fed from the same pump and reservoir), as this was found to decrease the cell current passed during operation by about a factor of ten compared to circulation of an anolyte that is not isolated from the catholyte.
MSET cell 610 and magnetic drive assembly 630 are similar to those shown in
A high-current power supply, such as 120 VAC power or an alternative off-grid source, e.g., Kepco ATE 55-20M, is used in voltage-control or current-control mode for the electrolytic cell power. Cell potentials are typically in the range of 0.5-12 V. Electrolysis commences and continues by continued application of the voltage and circulation of electrolytes, until the operation is stopped. The system may be set to automatically cease operation when a desired low level of metal in the catholyte is achieved. Digital multi-meters (not shown) can be used for monitoring cell voltage and current. A small amount of anolyte makeup water from the feed stream or another source may be added during operation of the system, if necessary. During operation of the system, cathode fluid samples may be collected periodically and the cell voltage and current recorded and adjusted, if desired. Digital in-line flow meter 667, 675 and an external hand-held ultrasound Doppler flow velocity (not shown) are used to meter the flow of the catholyte and anolyte, respectively. For production of reduced-metal oxides an air line (connected to an air pump or a source of compressed air) may be employed for supplying additional O2 to the catholyte.
The magnetic conveyor belt 632 is operated at a rate sufficient to move successive portions of the magnetic cathode particles from the bottom of the bed, up through the spout tube, and out the top of the tube, thereby returning the particles to the top of the bed. Referring briefly to the exploded view of a reactor (electrolytic cell) 100 shown in
Finally, the captured, reduced metal or metal oxide product deposited on the cathode beads is recovered using one or more recovery method that is suitable for chosen application. Some known recovery methods that may be used include settling tanks, particle filtration, cathode recovery, and magnetic separation. Additionally, if significant amounts of hydrogen or arsine gas are generated, suitable gas recovery methods may be implemented. When hydrogen is produced it can be combusted or used as a fuel, depending on the application, location, regulations and other specific requirements. A convenient, passive method for solids separation is settling. This is the common approach used in large water treatment facilities for mass (flocculated) solids removal. The solids are later recovered and fluid is either recycled back into the process or discharged. Most other solids removal methods are active. Particle filtration is useful in either a production/separations process (i.e., a filter press, bag filter, drum filter, conveyor filter) or where there is only a small amount of solids to remove before discharge. Magnetic separation is typically used in industry to remove magnetic products continuously from a process stream containing magnetic particles such as ferrous metals (Fe, Co, Ni) or ferrites. Cathode recovery may be necessary if the cathode substrate beads grow too large with deposited metal. The plated beads may be exchanged for fresh ones or the deposited metal can be re-oxidized into a concentrated liquid solution for further refinement by conventional processes. At the end of batch-mode processing, the concentration of target metal in tank 602 is significantly reduced, in many instances to less than 0.5 mg/L.
Electro-Refining Process—Single-stage Catholyte Flow Through.
A representative single-stage catholyte flow through (one pass) process for removal of a single metal is carried out essentially as described above with respect to the fluid flow diagram shown in
Electro-Refining Process—Multi-stage (Multi-Metal) Catholyte Flow Through.
For some applications, multiple electrolytic stages are used to sequentially remove individual metals selectively from a mixed-metal feed stream in order of more positive to more negative reduction potentials. A modified version of an above-described process is employed, which has multiple electrolytic cell stages, each having its current density separately optimized for a respective target metal. System 700, having a pair of electrolytic cells 710a, 710b with the cathode feed fluid flow connected in series, is illustrated in
By decoupling catholyte flow and cathode particle “spouting,” slower flow rates (cathode chamber space velocity) are made possible, to permit enhanced recovery of low concentration metals. In some applications, the electrolytic cells are operated at lower current densities and at lower voltages compared to typical electrolytic cells used for electro-refining. The metal-containing feed streams may be of a wider variety of electrolyte types and strengths than has usually been possible with other electrowinning systems. In the system of
A major feature for a refining application is the selectivity of metal recovery. The most common and problematic contaminant to be encountered is typically iron. Several experiments were conducted to demonstrate selectivity of this technology for recovering copper from feed streams containing high iron concentrations as shown in
Electro-Refining Process—Multi-Stage (Single Metal) Catholyte Flow Through.
Another variation of a multi-stage process employs a system similar to that shown in
Although the foregoing multi-stage processes focus on representative two-electrolytic cell systems, it should be understood that additional electrolytic cells and stages may also be employed, each having its current density and other operating parameters optimized for electrolytic removal of a targeted metal. Likewise, the capacity of a process described herein may be increased by operating, in parallel, one or more additional multi-stage series of MSET cells. It should be appreciated that various features of the apparatus and processes described herein may be employed separately or in combinations other than just the representative embodiments shown.
As an aid in implementing a MSET metal recovery process, a list of exemplary target metals and their known reduction potentials (relative to the standard hydrogen electrode, SHE) are given in Table 1, for reference.
For a given set of MSET operating conditions, the relative electrowinning efficiency for each metal closely follows the relative reduction potentials making it possible for selective electro-refining from mixed sources. The effect of pH also has an influence on electrowinning efficiency and final metal concentration of the effluent in addition to the feed flow rate and current density as mentioned above. These effects are illustrated in
The copper removal rate for the example of
The new magnetically spouted electrolytic technology has made possible for the first time the parameterization of the process variables, allowing significant advances to be made in electrowinning rate, efficiency and selectivity. These advances have provided the performance necessary to design simple, low-cost, flow-through metal recovery and refining systems for direct capture of valuable metals from a variety of aqueous sources too dilute to be economically viable with currently available technology.
Since the metal removal rate in a flow-through treatment system is paramount, a series of experiments were conducted to determine how to best increase metal removal rates by adjusting the flow rate and cell current density. The range of operating conditions examined is given in Table 2 and Table 3. The range of flow rates encompassed an acceptable range for a pilot-scale system, such as that described above and illustrated in
Copper Removal
Copper removal performance from 0.1 M H2SO4 (pH 0.7, 200 mg/L Cu) indicated that the rate of copper removal is diffusion limited at the low concentrations of interest. This was clearly shown by the concentration, volume, and flow rate dependencies of copper removal rate. Ex. No. 1 (66.6 mg/L initial Cu conc.) and Ex. No. 4 (191.3 mg/L initial Cu conc.) both required nearly the same amount of time (about 20 min) to reduce the 7 L feed solution concentration to <4 mg/L at the same flow rate and cell voltage (
For Ex. No. 1 (Cu) in Table 2 the copper concentration was reduced to about 0.4 mg/L after 20 min and to 0.09 mg/L after 60 min of run time. This is lower than the copper concentration measured in the municipal tap water in the building (˜0.1-0.2 mg/L) and is much lower than the action limit for copper in drinking water (1.3 mg/L). This clearly demonstrates the potential for water polishing at low metals concentrations by this electrolytic technology.
The most significant results for copper removal were obtained from Ex. No. 10 (Cu) in Table 2 (
Nickel Removal
Nickel removal was investigated first under the same conditions used above for copper (0.1 M H2SO4, pH 0.7) and then at lower acid concentration (0.01 M H2SO4, pH 1.7) with low and high flow. Overall, the higher pH resulted in higher initial nickel removal rates while low feed flow of the catholyte allowed nickel removal to occur (high feed flow minimized nickel removal).
In Table 2, Ex. 6 (Ni) showed a 35.8% reduction of its initial concentration (7.5 Lpm, 185.8 mg/L Ni, 0.1 M H2SO4 feed solution),
Ex. 11 (Ni) (solid circles in
Zinc Removal
Zinc removal was investigated under the best conditions used above for nickel (0.01 M H2SO4, pH 1.7, 6.7 Lpm feed flow). Two tests, Ex. 13 and Ex. 14 in Table 2, provided very similar results in that the initial zinc concentration was rapidly reduced during the first 10-20 min followed by little change in the concentration. The results for Ex. 14 are shown in
pH Conditioning
During the initial studies of MSET removal of Ni, Zn, and Mn from 0.01 M H2SO4 solutions the pH was shifted relatively rapidly to pH neutral. This occurred within 20-30 min due to rapid consumption of acid protons for hydrogen production. Longer treatment time was required to neutralize 0.1 M H2SO4. Neutralization of the test solutions from pH 1.7 occurred fairly rapidly indicating that higher pH levels more often encountered (pH 2-3) will be easily neutralized by this electrolytic process. It was observed several times that, after the acid protons were removed and the feed solution was neutralized, the cathode beads can become the most reactive part of the system resulting in re-dissolution of metals or precipitation of metal oxides/hydroxides. Accordingly, the treatment times in most situations is kept shorter than the pH neutralization times. When the MSET system is intended for use in treating acid mine drainage (AMD), for example, neutralization of mine drainage pH is desirable, so that no post-conditioning of the effluent is necessary.
Faradaic Efficiencies
Very good Faradaic efficiencies were obtained for copper removal with >80% being an achievable goal at a copper concentration below 200 ppm. This is an improvement over the Faradaic efficiencies typically obtained with the prior fluid-spouted electrolytic cell. When Faradaic efficiencies were greater than 100% an electroless deposition process was thought to be active. Faradaic efficiencies for nickel and zinc were poor (on the order of 12-20%) due to the slow removal rates from solution. Lowering the feed flow rate and raising the pH helped to increase deposition rates/efficiencies for these metals. This may indicate that the co-generation of hydrogen inhibits the deposition of these metals at lower flow rates. Higher flow rates are necessary to overcome mass transport limitations created by a high gas bubble load on the cathode surfaces.
The large difference in metal removal efficiencies between metals with positive and negative reduction potentials will allow a system to be designed with high selectivity in metal removal. The direct benefit is partial or complete refinement of metals being recovered by this electrolytic system, thus increasing the sale value of metals recovered. This was demonstrated for the case of recovering copper from a solution containing both copper and iron shown in
During the course of carrying out the foregoing studies it was discovered that when the anode and cathode solutions were isolated from one another, a partial electrical short from the system was eliminated, and greatly reduced the current density (at constant voltage) at the cathode by about 10 times. This was found to be important for scale-up reactor process design, since the partial electrical short had not been previously identified and its contribution to the current density would change with changing reactor size and with the number of reactors in series. The greatly reduced current density allowed for a new process/production capability of this technology to be discovered, i.e., the selective production of reduced metal oxides by partial reduction of target metals in the presence of dissolved oxygen. This has not been previously described with respect to spouted cathode reactor technologies. Two of the metal oxides the system produced directly from solution are cuprous oxide (Cu2O) and iron ferrite (Fe3O4). Both of those materials are of great commercial value and this new production method has the potential to significantly reduce their production costs compared to current methods.
In the studies summarized in Table 3, the anolyte fluid and the catholyte fluid were separate.
As discussed above with respect to Table 1, the electrolytic reduction potentials of metal ions has the greatest influence on their electrolytic deposition/removal from waste streams. Secondary, yet important effects are reaction kinetics, or reduction/oxidation rates, at electrode surfaces. Competing reduction/oxidation processes are also important in some circumstances such as processing iron group metals. Two important reference potentials are indicated in italics in Table 1 for proton reduction resulting in hydrogen production (defined as 0.0 V) and water reduction that results in production of hydrogen and hydroxide (−0.8277 V). The competition between metal reduction and hydrogen production/re-oxidation becomes less favorable as potential becomes more negative. Hydrogen reduction can be shifted more negative by the use of additives that increase the reduction overpotential or making the process kinetically less favorable. At more negative potentials water reduction occurs and water itself will oxidize metals with more negative reduction potentials. For all of these reasons, reducing metals with more negative potentials in acidic, aqueous media is challenging.
The removal of copper from relatively dilute sulfuric acid solutions was initially examined with a MSET system similar to that shown in
The rate of copper removal was determined from the analysis of copper remaining in the feed solution. Analysis of dissolved copper was conducted by colorometric analysis using the CuVer2™ color indicator reagent purchased from HACH, Inc. Sample preparation was conducted by using the indicator reagent as instructed and visible spectroscopic measurements were made using a Hewlett Packard Model 8453 UV-Vis Spectrophotometer. A 1 cm pathlength solution cell was suitable for detecting copper in the concentration range of ˜0.1 to 10 mg/L. A linear five-point calibration curve of concentration versus absorbance was first made with prepared copper standard solutions (0, 0.5, 1, 2, 5, mg/L).
Further experiments were first conducted under constant voltage control with the catholyte and anolyte solutions isolated from each other as in
Copper removal was also examined in single-pass flow-through operating mode. For these experiments a second reservoir was added above the cell (see
In the experiments described in Table 3, the Faradaic efficiencies were calculated in excess of 100% by one to two orders of magnitude indicating that electroless deposition or reduction processes were occurring. The current response of the reactor may indicate that the initial current applied serves to initiate electroless copper reduction. The application of an applied voltage is still required to perpetuate the copper reduction processes as indicated in the constant current experiments. The terminal reductant that would have to account for the large amount of charge not measured in the applied cell current is not immediately obvious. The reductant could be hydrogen produced at the cathode, although producing it from the acid would also require passage of charge. The more likely explanation is the exposed steel of the cathode particles' core behaving as a sacrificial anode for copper reduction. This activity is similar to the commercial cementation process used for copper precipitation from solution. The end result, however, is that very low current densities will promote copper removal from water. The solids produced in all of the copper experiments were dark red-black to brown powders that were flushed from the reactor (no cathode removal necessary for product recovery). X-ray diffraction analysis of about 1.5 grams of solids recovered from experiment number “18” in Table 3 (1.4 V, 13 L/min) showed approximately a 1:1 mixture of copper metal and Cu2O powder,
The multi-gram quantities of Cu2O production has not been reported before for a spouting electrolyte technology. Previously, the production of copper metal was nearly exclusive at higher current densities (>45 mA/cm2). Cu2O appears to be produced by incomplete reduction of Cu2+in the presence of water and/or an oxygen-containing gas through a complex series of partial reductions and/or oxidations. For copper metal recovery, the current density threshold (at 0.8 V) appears to be near 1.16 mA/cm2 geometric cathode area (˜15.5 μA/cm2 absolute cathode area calculated by assuming close-packed 2 mm spheres using the Kepler conjecture). At this current density the amount of copper versus cuprous oxide formed is about 1:1. At higher current densities (at 0.8 V) near 45-80 mA/cm2 geometric area copper metal is almost exclusively produced (previous work). These results indicate that the production of pure Cu2O should be promoted at even lower current densities in the presence of additional oxygen injected into the solution.
Copper removal rate from solution was approximately 25% greater than the previous configuration (of
Initial rates of copper removal at three different flow rates at 0.83 V (constant voltage, 12 L) increased with increasing flow rate, but removal rates were almost constant with number of feed solution turnovers. In
Copper removal under constant current control (1.16 mA/cm2, 13 L/min) was essentially linear with concentration and time after the initial startup period (0-1 min) (Ex. 23 in Table 3). As shown in
The removal of nickel was initially intended, but the degradation of the prepared copper coating on the cathode beads, which appeared to spall off, resulted in some dissolution of iron from the steel shot during these experiments. The result was mixtures of 200 mg/L nickel (as Ni2+) and approximately 200-500 mg/L iron (as Fe3+). What was found in these experiments was the surprising production of iron ferrite during the first several minutes of operation, followed by partial removal of nickel, most likely as nickel metal.
Nickel(II) sulfate hexahydrate was dissolved in 0.01 N H2SO4 to make nominally 200 mg/L nickel feed solutions at pH 2. Feed solution volume was 12 L. The feed solution was put into the cathode feed reservoir (see
A system configured as shown in
The rate of nickel removal was determined from the analysis of nickel remaining in the feed solution. This analysis was very problematic in the presence of dissolved iron. Four approaches were attempted, three involved iron oxidation or precipitation, pH adjustment and complexation with a color indicator. The final preparation and analysis method that worked was particle filtration of the raw samples, which were then analyzed directly for the Ni2+ (H2O)6 species. The Ni2+ d→d transition absorbance band at −0.395 nm does not overlap with those of Fe2+ or Cu2+ and has minimal interference from Fe3+ as baseline offset. Visible spectroscopic measurements were made using a Hewlett Packard Model 8453 UV-Vis Spectrophotometer. A 30 cm pathlength solution cell was suitable for detecting nickel in the concentration range of −0.5 to 1800 mg/L. A linear four-point linear calibration curve of concentration versus absorbance was first made with prepared copper standard solutions (0, 10, 56.1, 112.25, 224.5 mg/L).
In general, nickel was removed slowly relative to copper at the low voltages and current densities tested (2-3 V, 0.4-0.6 mA/cm2). Nickel reduction seemed to be disrupted by dissolution of the iron cathode substrate or reduction of dissolved of iron to solid iron products that fouled the cathode surface. A clear example of nickel removal from solution is shown in
During the first 10-20 minutes of each test a significant amount (several grams) of dark/black fine solid was generated in the feed solution. It was determined by X-ray diffraction and elemental analysis (by energy dispersive X-ray analysis, EDX) that this solid is iron ferrite (Fe3O4). Its persistent production was selective for iron ferrite, an inverse spinel-structured oxide, in multi-gram quantities from a mixture of nickel and iron. This is the most surprising result of the program since Fe3O4 is a highly refractory material that is normally produced synthetically by solid state reaction of oxides or carbonate precursors at high temperatures (>1000° C.) or under extreme hydrothermal conditions around 600° C. The X-ray analysis in
The removal of zinc from sulfuric acid solutions was examined because zinc is a very reactive metal to acid solutions and represents a significant challenge for electrowinning without the use of additives (ammonium salts, glucose, surfactants) to assist in zinc reduction. Zinc(II) sulfate pentahydrate was dissolved in 0.01 NH2SO4 to make nominally 200 mg/L zinc feed solutions at pH 2. Feed solution volume was 12 L. The feed solution was put into the cathode feed reservoir. Zinc plated #7 steel shot (purchased) was used as the cathode substrate (−645 cm3).
A system configured as shown in
The rate of zinc removal was determined from the analysis of zinc remaining in the feed solution. Analysis of dissolved zinc was conducted by colorometric analysis using the ZincoVer™ color indicator reagent set purchased from HACH, Inc. Sample preparation was conducted by immediately filtering zinc particles off of the samples (to prevent re-dissolution) and using the indicator reagents as instructed. Visible spectroscopic measurements were made using a DR890 HACH meter (accuracy was confirmed with a Hewlett Packard Model 8453 UV-Vis Spectrophotometer). A linear four-point calibration curve of actual concentration versus the DR890 HACH meter concentration response was first made with standard zinc solutions.
At low current densities near 4.65 mA/cm2 (regardless of voltage) the removal of zinc did not occur. In fact the acid corrosion of the cathode's zinc coating occurred as indicated by an increase in solution concentration of zinc and corrosion/tarnishing of the cathode beads. At higher current density, 11.6 mA/cm2, there was no net gain or loss of zinc from solution at 448 mg/L (trial G14-23). Corrosion rate of the cathode's zinc coating was suppressed, but the current density was not great enough to drive reduction of zinc from solution at a greater rate than corrosion.
At the highest current densities tested, 58.1 and 69.8 mA/cm2, the removal of zinc did occur. A representative test, shown in
At slightly higher current density, 69.8 mA/cm2, but lower feed flow rate, 7 L/min, the removal of zinc was slower (Faradaic efficiency of 3.1%). The lower flow rate appears to be reducing the removal efficiency on a feed-volume turnover basis indicating that the space velocity through the cathode bed is an important parameter for a reactive/corrosive metal like zinc. The zinc plated cathode beads remained shiny throughout this experiment with only a little zinc lost to solution during startup. At lower feed flow there was much less zinc-like powder flushed from the reactor than at high flow (13 L/min).
At the higher current densities used for zinc removal there were significant pH shifts measured for the feed solution and anolyte solution. For Ex. 33 (Zn) the feed solution pH increased from 2.5 to 4.7 during the 30 min experiment. At the same time the anolyte solution pH decreased from 2.3 to 1.9. The pH shift of the feed solution is due to electrolytic reduction of acid protons to hydrogen gas and reduction of dissolved oxygen to hydroxide, both processes reduce the acid proton concentration. The sulfate anions left behind will transport through the membrane separating cathode from anode since this membrane (DARAMIC™) is not very selective to ions. The pH shift of the anolyte solution is made possible by the sulfate left over from acid reduction at the cathode passed through the membrane separator to balance charge with acid protons generated at the anode.
Consuming acid protons is a useful capability for treating (neutralizing) acid mine drainage at the same time as recovering metal products. The hydrogen generated by acid reduction can also be captured for use in a fuel cell to help generate power for the system or combusted to provide heat or steam for power generation.
The removal of arsenic from sulfuric acid solution was examined because it is such a toxic hazard (upwards of 10-20 mg/L) in groundwater and aquifers near ore deposits such as copper and gold and found in many geothermal brines. Arsenic(III) acid (from As2O3 dissolved in nitric acid as an atomic absorption standard) was diluted into 0.01 N H2SO4 to make 10 mg/L arsenic feed solutions at pH 2. Feed solution volume was 7 L. The feed solution was put into the cathode feed reservoir Zinc plated #7 steel shot (purchased) was used as the cathode substrate (−645 cm3).
A system configured as shown in
The rate of arsenic removal was determined from the analysis of arsenic remaining in the feed solution. Analysis of dissolved arsenic was conducted by semi-quantitative colorimetric analysis using the arsenic test kit (based on the well known Marsh test) purchased from HACH, Inc. The error bars for concentration measurements between 400 and 2000 μg/L are near±250 μg/L with a practical detection limit around 100 μg/L. The reduced arsenic forms expected are either elemental As or arsine, AsH3. Elemental arsenic is a semi-metal with electrical resistivity about 20 times that of copper, therefore electrical passivation of the cathode is not expected if it is deposited as a metal. Arsine is a highly toxic gas that can escape from solution and will require proper containment/capture controls if this is the primary reduced form. In an exemplary test, arsenic was depleted from a 10 mg/L solution (0.01 NH2SO4) down to about 125 μg/L concentration in 30 min under constant current control, as shown in
As noted above, the dramatic change in current density at a given cell voltage encountered in these studies was a result of changing the reactor's fluid flow configuration. This difference, in comparison to previous work, clearly demonstrated that the metal removal rate is a function of current density while the voltage difference between anode and cathode is less important as a process parameter.
The influence of feed flow rate on removal rate for copper and iron was negligible at the lower current densities examined, which allows for lower space velocities to be used to increase the feed solution residence time in the electrolysis cell. Higher space velocities will be generally needed for zinc and nickel recovery. Residence time and space velocity are important parameters in the design of a MSET system for a particular end use application.
Large-scale dynamic flow electrolysis reactors have electrode areas that are typically too great for meaningful use of reference electrodes due to inhomogeneities in solution concentrations, electric field strengths, resistances, changing electrode surfaces, competing processes, and so forth. Rather, the dominant electrolytic processes in large membrane reactors appear to determine the “pinning” potentials at the electrodes while the current density (electron transfer rate) determines the rate of reaction at the electrodes. (For example, the oxidation of water at the anode producing O2 and H+ can potentially pin the anode potential at the oxidation potential of water, roughly 1.2 V vs NHE.) There still exists a critical relationship between cell current, cell voltage, metal concentration, and cathode potential that must be balanced to promote the desired reactions and process rates.
The new magnetically spouted electrolytic technology makes possible the parameterization of the above-mentioned variables, allowing significant advances to be made in electrowinning rate, efficiency and selectivity. These advances have provided the performance necessary to design a simple, low-cost, flow-through metal recovery and refining system for direct capture of valuable metals from a variety of aqueous sources too dilute to be economically viable with currently available technology. A fully developed MSET electro-refining system will have the capability to sequentially removal of metals selectively from mixed-metal feed streams in order of more positive to more negative reduction potentials. This is expected to be accomplished by a flow-through system with multiple electrolytic cell stages, each with its own flow velocity and current density optimized for each target metal.
Optimal operating conditions for electrodeposition of individual target metals, particularly those providing selective metal removal of a selected metal from binary and tertiary mixtures of metals can be determined. The removal of copper and silver are relatively straight forward, as their reduction potentials are more positive than that of hydrogen, as can be seen in Table 1. Therefore, copper's electrodeposition is determined primarily by diffusion kinetics of the metal ion to the cathode surface, cathode potential, and cell current density. In the electrolytic cell, improving electrodeposition rate of copper translates to higher flow rates (producing more turbulent mixing of the cathode particles) and higher current density. The removal of metals with reduction potentials more negative than that of hydrogen (e.g., iron, lead, nickel, zinc) becomes more difficult as the reduction potential becomes more negative due to competing hydrogen production and parasitic corrosion of these metals by protons in acid solution. This is why zinc metal corrodes in sulfuric acid, producing hydrogen gas. The situation is even more difficult when a metal's reduction potential is more negative than water, such as manganese, for example, which can also react and be corroded by water as well as acid protons. The end result is that electrodeposition of metals with negative reduction potentials is determined by many factors including diffusion kinetics of the metal ion and protons, pH, current density, and selectivity of electron transfer kinetics for each reductant species present. It is expected that as the pH is raised, the metal removal rate will increase. Related to acid concentration is electrolyte strength. Feed solutions containing low acid concentrations and low metal concentrations are more electrically resistive resulting in higher operating voltages and greater power consumption. This issue can be greatly reduced in membrane reactors by keeping the anode solution electrolyte concentration high (e.g., 0.1 N H2SO4). This concentration provides a high enough density of acid proton charge carriers through the membrane to minimize charge transport resistance between anode and cathode. The electrolyte strength at the cathode, however, can still have a strong influence on the rate of the desired electrolytic processes. In the electrolytic cell, improving electrodeposition rates for metals with negative reduction potentials translates to properly balancing flow rate (intermediate or low), with current density (higher) and pH level or pH gradients near the surface of the cathode particles.
Optimization of metal removal rates, a major issue in a flow-through treatment system, is obtained by adjusting the flow rate and cell current density in the new electrolytic cell. For instance, the initial range of operating conditions may be as shown in Table 4, representing a range of flow rates that encompass an acceptable range for a pilot-scale flow-through treatment system suitable for use in the magnetically spouted electrolytic treatment of acid mine drainage, for instance. The initial metal concentration in the test feed solution is 200 ppm, and the feed solution also contains sulfuric acid, H2SO4, and has a pH of 2-4, to simulate the pH associated with acid leach solutions, acid mine drainage, and acid rock runoff.
Initially, screening of these operating parameters is made at either end of the flow rate range while using the minimum current density. For example, the best of these two flow rates are then used to examine metal removal at the highest current density. These results are evaluated to determine the trade-off between flow rate and current density. From those results, an optimal current density is estimated and tested. Selective metal removal for recovery of enriched metals and increased value from low grade ore and mixed waste streams is just one of several potential applications of the new electrolytic cell. Similar optimization procedures are employed for treating binary and tertiary mixed-metal feed streams Metal ions with more positive reduction potentials are expected to be removed first followed by removal of metal ions with more negative reduction potentials, in the approximate order Listed in Table 1.
Industrial Applications of MSET Systems and Processes
Various embodiments of the above-described MSET systems and processes are potentially useful in a number of industries, including but not limited to, the mining industry, metal finishing industry, semiconductor/electronics industry, environmental remediation industry and naval/commercial shipping industry.
Mining. The mining industry represents about 5% of the United States' gross domestic product. Today, nearly 47,000 pounds of materials are mined for each person in the US annually to maintain the current standard of living. The ever-mounting pressures from resource demand, foreign competition, rising energy costs, and tightening environmental impact regulations make it essential to develop new, innovative processing methods that can increase efficiency, competitiveness, and productivity while being environmentally responsible.
The mining industry utilizes heap-leach and solvent extraction processes (e.g., Merrill-Crowe Process or in-situ mining) for recovery of a number of metals. The traditional Merrill-Crow type process utilizes cyanide solutions for ore extraction followed by addition of zinc powder to precipitate the desired value metal. Other metal extraction products now exist that replace cyanide as the chemical complexing agent for solvent extraction and in-situ mining, however, the grade (purity) of extracted metal solution is typically low and high leach concentrations (>1%, 10,000 ppm) are necessary to make this process economically viable. For example, iron, lead, arsenic, and cobalt (and added zinc) are often problematic impurities in silver and copper extractions. Conventional electrowinning technologies can increase extracted metal purity, however, they are economically limited to high metal-concentration solutions and they typically leave behind a large amount of metal value which is lost to toxic mine drainage waste and runoff that must be treated. A MSET electro-refining system will potentially provide better control over product purity, and recovery of a much greater yield (estimated 10-80% greater) of metal from lower concentration leach solutions. For use in the magnetically spouted electrolytic treatment of acid mine drainage, an exemplary feed solution further contains sulfuric acid, H2SO4, and has a pH of 2-4, to simulate the pH associated with leach solutions, acid mine drainage, and acid rock runoff.
Some of the new MSET systems and processes will be of potentially great benefit to smaller-scale mining operations around high quality ore deposits. Many smaller ore deposits (i.e., those yielding at least 100 million pounds annually), as in the case of copper, are not utilized due to the large capital costs that must be recovered for the typically massive operations. By using an appropriate MSET system and process, small copper oxide deposits can now potentially be mined to increase their total yield, increase purity, and reduce effluent metal concentrations. The same is true for mining of silver, cobalt, nickel and other valuable metals.
Other metal sources that can potentially employ one or more of the new MSET systems and processes include mining sites with very large volumes of runoff, waste effluent, legacy wastes and contamination. One such example is the Berkeley Mine Pit in Butte, Mont. This mining pit is considered to be the largest contaminated body of water in the US with over 36 billion gallons of acidified mine drainage containing toxic levels of valuable metals. Table 5 lists the primary contaminants that are found in the Berkeley Mine Pit's water. One MSET process suitable for use in an acid mine drainage application employs a system 900, schematically illustrated in
Using a MSET-AMD treatment/recovery system platform such as that shown in
For some end-use applications, in which reliability is a major issue, it may be more appropriate to use a propane-powered generator for intermittent charging of the system battery bank, rather than relying entirely on solar power. The power buffer time may also be reduced, thus reducing the number of batteries (e.g., from 8 to 4), and at the same time reducing the overall weight of the treatment platform. This type of a system makes possible a small footprint, lowered maintenance requirements, and lower construction cost than a large bank of solar panels, and offers the possibility of providing power at regular, predictable intervals including night time, stormy days, and dark underground locations.
Geothermal Fluids. The direct utilization of geothermal brines as a low-concentration metal source is also possible with some of the new MSET systems and processes. Geothermal fluid compositions vary dramatically as they reflect the local mineralogy and temperature of the source making metal and mineral extraction potential very site-specific. Higher temperature geothermal sources can carry up to several ppm arsenic. One of the needs for practical geothermal power utilization is the ability to meet discharge criteria for spent brines, especially for direct-use geothermal heating systems. A suitable MSET system and process can potentially meet this need. One example of a geothermal fluid application is the CalEnergy Mineral Recovery Project for the geothermal power complex on the Salton Sea in southern California's Imperial Valley. That facility currently utilizes existing solvent extraction, ion exchange, and electrowinning technologies to recover high-grade zinc. Such a zinc recovery process could potentially be simplified with direct capture or could potentially be made more energy efficient for electrowinning by employing a suitable MSET system and process.
Metal Finishing. Some MSET systems and processes will be of potential use in the metal finishing industry, and may benefit by gaining the ability to recover and recycle metals and to reduce metal waste discharge. Spent plating and washing fluids are generally too dilute or impure for direct recycle back into the plating process. Recovering metals from the spent fluids in a pure, solid form in a single step is possible with a suitable MSET system and process which will minimize toxic waste discharge from plating activities.
Semiconductor and Electronics Fabrication and Recycling. Discharge from semiconductor and electronics fabrication plants has become a significant issue due to the toxicity of the metals present (e.g., cadmium, tin, chromium, nickel, lead, silver, copper and others). These waste streams are created primarily during metal plating, etching, and cleaning steps during printed circuit board and lithographic fabrication processes. Recovery of these metals from waste streams by using a suitable MSET system and process will potentially help reduce the cost burden of waste disposal from this industry. Similarly, the recyclable metal components of discarded electronic components and batteries may be solubilized and then recovered by a MSET system and process, for reuse.
Environmental Remediation Industry. Environmental remediation, especially for the cleanup of acid mine drainage (AMD) and legacy wastes at numerous DOE sites, is another application that is well-suited for many of the new MSET metal removal/recovery systems and processes. Significant heavy metal contamination of waterways largely arises from decontamination or decommissioning of nuclear and industrial sites and results from runoff associated activities in both metal and coal mining, chemical processing of ores and AMD. Table 6 summarizes the main contaminant metals found in AMD and at contaminated DOE sites and their typical concentration ranges. The U.S. mining industry spends over $1 million per day treating AMD at active mines while losing tons of valuable metals because conventional electrowinning technology is generally unsuitable for the dilute streams created. Suitable MSET systems and processes have the potential to efficiently treat these waste streams and at the same time recover metal value that can help to offset the remediation costs.
Naval/Commercial Shipping Industry. Suitable MSET systems and processes are used for mitigation of heavy metal contamination in water for problematic issues such as copper, lead, zinc, and nickel leaching into water circulation systems, metal loading in greywater, and release of metals from paint during hull cleaning of naval and commercial ships.
Manufacture of Specialty Metal-based Materials. Manufacturing of specialty metal-based materials will also potentially benefit from some of the newly found capabilities of suitable MSET systems and processes for producing reduced metal oxides directly from metal solutions. One example, which is described above, is cuprous oxide (Cu2O), which is recovered in pure powder form by settling, filtration or centrifugal separation. Today, the bulk production of Cu2O is primarily accomplished by electrolytic oxidation of metallic copper or by caustic chemical reduction of copper sulfate. Use of a suitable MSET system and process eliminates the need for refined copper as a feed stock and eliminates the use of additional chemical processing steps, as it produces Cu2O in powder form directly from acidic copper sulfate (or sulfate-based heap-leach extraction fluids). Cuprous oxide is widely used as an anti-fungal agent in agriculture and horticulture, and it is the typical active ingredient in anti-fouling marine paints and coatings.
Another example of a valuable metal oxide that may be advantageously produced directly from metal solutions using a suitable MSET system and process is iron ferrite (magnetite), Fe3O4. The production of spinel-structured oxides such as magnetite from solution at room temperature is extremely significant in that these types of materials are typically made in bulk by high temperature (600-1200° C.) solid state synthesis methods or rigorous hydrothermal conditions from metal oxide and carbonate precursors. The products must then be ground to small particles and powders for use. The selective production of iron ferrite (magnetite), Fe3O4, as a pure, crystalline powder was demonstrated repeatedly from a mixed stream containing nickel and iron sulfates. A suitable MSET system and process eliminates the high temperature production energy cost and greatly reduces the amount of energy and time necessary for grinding the product. Iron ferrite is a technologically important material for magnetic recording media, magnetic cores in electric power transformers and chokes, magnetic shielding, ferrofluids, biomedical imaging, and it is also an important feedstock for manufacture of color pigments. Additional common applications of ferrites, and other spinel-structured oxides, are catalysts, battery electrodes, and refractory ceramics.
The new magnetically spouted electrode technology (MSET) disclosed herein decouples the feed fluid and cathode bed particle motion and flow paths providing greater control over the feed fluid flow dynamics while retaining the advantages of a spouted cathode bed. Many embodiments of the MSET make it possible to achieve a marked increase in removal efficiency and rate at low metal concentrations (<300 ppm), thereby making possible a practical flow-through operation instead of batch-mode. Metal concentrations in the discharged effluent can even be reduced to the μg/L (ppb) range. The performance improvement is illustrated in
In some embodiments, the copper removal rate of a MSET system is 10 lb/day per square foot of geometric cathode area (49 kg/day/m2). A MSET electro-refining plant system having 200 ft2 geometric cathode area is generally suitable for recovering 1 ton copper per day from a relatively dilute (<1000 ppm) aqueous source. Power consumption cost for the new magnetically spouted cell performance shown here (including cell current, magnetic drive, and fluid pumping) are typically better than that of other electrowinning systems.
Another potential benefit of many embodiments of the new MSET is greater control of metal reduction/removal selectivity from mixed sources. Other important potential benefits are reduced power consumption for fluid circulation (can be operated on gravity feed) and reduced mechanical wear and stress on the electrolytic cell by slower moving cathode particles. Thus, many embodiments of the new MSET technology offer a low cost electrowinning approach that provides the necessary performance for energy efficient and selective metal reclamation and refining, even at very low concentrations.
The anticipated public benefits from successful development of the proposed high-efficiency electro-refining system are economic and environmental. Benefits include very low-cost and selective (high purity) metal recovery from liquid feed streams and the profitable utilization of metals that are currently discharged as waste from mining operations. Production of a pure or near-pure metal from low-grade ore extractions and mixed-metal sources in a single process will also reduce the cost and complexity of metal refining. This, in turn, leads to lower supplier costs to manufacturing and consumer goods as well as making the metal refining industry more profitable and competitive.
Magnetically-spouted electrolytic technology (MSET) makes possible a variety of compact, reagent-free systems that require no feed stream pre-treatment. In many instances, metals and reduced metal oxides are recovered in a purified, solid form that can be easily recovered for direct sale or use. Selective metal recovery from multi-component feed streams is also made possible with many of the MSET systems. A major capability of the many MSET systems and processes is efficient (>80%) removal of metals from low concentration (<500 mg/L) feed-streams. Typically, economic recovery of metals at low concentrations is not possible in conventional electrowinning technologies. In many instances, effluent metal concentrations can be reduced enough with a MSET system and process to meet waste-water discharge and even drinking water standards, which is typically done by non-electrolytic methods such as precipitation, ion exchange, filtration and the like.
Continuous flow-through MSET electrowinning/refining systems that eliminate processing steps and greatly reduce energy consumption in the production of specialty metal-based products are also made possible. The electrowinning efficiency provided by this technology can potentially boost product recovery from low concentration or low-grade heap-leach and Crowe-type extraction fluids.
Existing metal removal processes using carbon felt cylinder technologies are typically useful for liquid streams containing metal (e.g., Cu) in the concentration range of log C0-1.0 to 2.0 ppm. For packed bed technologies, the range is also about log C0-1.0 to 2.0 ppm. With fluidized bed (removal on carbon) technologies, the concentration range is about log C0 0.0 to 3.0 ppm. The conventional scraped rotating cylinder technology generally offers a metal removal concentration range of log C0 0.5 to 3.5 ppm. For mesh in inert fluidized bed technologies, the metal recovery range is typically about log C0 1.5 to 3.0 ppm. In the case of conventional rotating drum electrowinning processes, the range is usually about log C0 3.5 to 5.0 ppm. Metal removal processes employing a sparged parallel plate technology typically operate in the concentration range of about log C0 3.0 to 4.5 ppm, and conventional parallel plate methods are generally applicable to liquid streams containing a metal concentration of about log C0 3.75 to 5.0 ppm. By contrast, many embodiments of the present electrowinning method are applicable to liquid streams ranging in metal concentration from log C0 1.0 to 3.5 ppm.
In many instances, removal rates with MSET systems are as much as 25% greater at 100 times lower power consumption, compared to most existing systems. Still another potential advantage of MSET is that many of the new MSET systems and processes provide the conditions needed to observe more complex reduction chemistry and interactions/reactions between metal ions and oxygen dissolved in solution producing reduced metal oxides that are specialty metal-based products of commercial value and technological importance. Discovering new process complexities results in additional capabilities for controlling system performance and product output.
Although magnetic conveyer assemblies and processes are emphasized in the foregoing description, it should be understood that other suitable mechanical or electromagnetic devices could be used similarly for churning a particle bed independently of the flow of a fluid. In additional embodiments, mechanical devices such as conveyer belts and auger screw devices are configured for churning a particle bed to eliminate reliance on the flow of catholyte for transporting and churning a cathode particle bed. In another alternative embodiment, an electromagnetic assembly is used to move cathode particles in such a way that the cathode bed is mixed. One of these alternative electrowinning systems and processes that decouple cathode particle churning from the catholyte fluid flow may be preferred in certain applications.
Auger Screw Spouting Mechanism
In an alternative embodiment, an electrowinning apparatus is equipped with an auger screw mechanism 1230 which “spouts” the cathode particles.
Conveyor Drive Spouting Mechanism
Another embodiment of an electrowinning apparatus is equipped with a conveyor drive spouting mechanism 1330 which transports cathode particles to chum the cathode bed.
Referring now to
Electromagnet Spouting Mechanism
Another embodiment of an electrowinning apparatus is equipped with an electromagnetic spouting mechanism which transports cathode particles in order to churn the cathode bed.
A potential advantage of using stationary electromagnets in some situations is that this approach eliminates moving mechanical parts from the magnetic spouting mechanism. Another potential advantage of stationary electromagnets in some situations is that they need not be arranged in a strictly linear fashion. Rather they may be offset to effect cathode particle transport laterally as well as vertically, or at any desired angle. For some applications, it may be preferred to position an electromagnet to the side of electromagnet K above the draft tube, in order to transport the cathode particles to that side of the draft tube where the particles are then released back to the cathode bed without having a mechanical check valve mechanism on the top of the draft tube. This approach potentially provides more reliable and more efficient spouting in some embodiments, and eliminates moving parts within the reactor.
Continuous Electrowinning Reactor Design and Process
Referring now to
For continuous operation, the cathode particles are fed into the first electrified cathode bed chamber 1614a, transported up draft tube 1618a by spouting mechanism 1630a. The curved arrows in
Cathode particles can be used in either batch or continuous operating mode. For batch operation, cathode particles are loaded into a reactor for a period of operation time during which they enlarge with a deposited metal product. After the particles grow to fill the head space above a particle bed (i.e., the reactor is full) the reactor is shut down and the enlarged particles are drained from the reactor. New cathode particles are then loaded into the reactor and the batch processing is restarted.
For continuous operation either (1) the reduced metal product must not adhere to the cathode particles so that the product may be flushed from the reactor or (2) the cathode particles must be cycled through cathodes and or reactors in series to increase the residence time of the particles so that they may grow large enough with deposited metals to produce a marketable quantity of product. For dilute metal-containing liquid streams targeted by embodiments of this technology, the cathode particle residence time required for economic metal production is long enough (e.g., usually in the range of days to weeks) that the particles require several passes through a continuous reactor or reactor system having several reactor cathodes in series. One advantage to using a continuous reactor process with series flow of cathode particles is that the reactors need not be shut down to remove product or add cathode particles. Another advantage is the continuous output of product over time from a single reactor system.
In a reactor that has a spout tube asymmetrically positioned on one side of the cathode (as in
While the preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. The electrowinning processes described herein are considered representative of other mechanically, magnetically or electromagnetically spouted bed processes which provide mutual independence of feed stream flow and spouted bed particle flow, provide for controlling residence time of a feed stream independently of spouted bed particles, and define inlets and outlets for a feed stream independent of inlets and outlets for spouted bed particles. Accordingly, the scope of protection is not limited by the representative description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference in their entirety, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.
Ag
Cu
As + 2H2O
H2
Fe
Pb
Sn
Ni
Cd
Cr
Zn
H2 +2OH−
Mn
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. EP-D-04-022 awarded by the U.S. Environmental Protection Agency and Grant No. DE-FG02-05ER84320 awarded by the U.S. Department of Energy.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3219318 | Hershler | Aug 1961 | A |
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