A PROCESS AND APPARATUS FOR ACID MINE DRAINAGE TREATMENT

Abstract

An apparatus for the treatment of acid mine drainage and selective recovery of at least one of metals, critical elements, sulphuric acid and water is disclosed. The apparatus includes at least one electrochemical reactor, at least one catholyte reservoir and at least one anolyte reservoir for containing the acid mine drainage and a buffer, respectively. The reservoirs are in fluid communication with the at least one electrochemical reactor. The apparatus also includes at least one sensor for monitoring a pH of a contents of the reactor; and a power source for supplying an electrical current to the at least one electrochemical reactor. The electrical current is supplied until a predetermined pH is reached for the selective recovery of the at least one of metals, critical elements, sulphuric acid and water. A process for the treatment of acid mine drainage is also disclosed.

Claims

1. An apparatus for treatment of acid mine drainage, said apparatus comprising: at least one electrochemical reactor comprising at least one anode electrode, at least one cathode electrode and a semipermeable membrane for separating the at least one anode electrode and the at least one cathode electrode and for defining an anode chamber containing the at least one anode electrode and a cathode chamber containing the at least one cathode electrode; at least one anolyte reservoir for containing buffer and at least one catholyte reservoir for receiving and containing the acid mine drainage, said at least one anolyte reservoir and said at least one catholyte reservoir being distinct from the at least one electrochemical reactor and in fluid communication with the at least one electrochemical reactor so that said buffer can circulate between the anode chamber and the at least one anolyte chamber and said acid mine drainage can circulate between the cathode chamber and the at least one catholyte reservoir; at least one sensor for monitoring a pH of a contents of the reactor; and a power source for supplying an electrical current to the at least one electrochemical reactor, wherein supply of the electrical current is controlled so that the electrical current is supplied until a predetermined pH is reached for selective recovery of at least one of metals, critical elements, sulphuric acid and water, and wherein the selective recovery of the metals or critical elements includes precipitation of the metals or critical elements from the acid mine drainage and wherein the precipitation is encouraged to occur within the at least one catholyte reservoir to resist precipitation or adherence of precipitant on a surface of the at least one cathode electrode.

2. The apparatus of claim 1, wherein the buffer includes sodium borate buffer solution, water or an acidic buffer.

3. The apparatus of claim 1, wherein the buffer is an acidic buffer and the buffering agent is citric acid, acetic acid, hydrochloric acid or sulphuric acid.

4. The apparatus of claim 1, wherein the metals or critical elements recovered are at least one of aluminium, arsenic, barium, chromium, copper, iron, molybdenum, selenium, lead, cobalt, magnesium, manganese, molybdenum, nickel, zinc and cadmium.

5. The apparatus of claim 1, wherein the critical elements recovered include rare earth elements and yttrium (REY).

6. The apparatus of claim 1, wherein the critical elements recovered include any one or more of cerium, dysprosium, erbium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, terbium, thulium and ytterbium or a hydroxide, oxide, (oxy)hydroxide or sulphate thereof.

7. The apparatus of claim 5, wherein the rare earth elements recovered include any one of rhenium, rubidium, scandium, strontium, tantalum, tellurium, tin, titanium, tungsten, uranium, vanadium and zirconium.

8. The apparatus of claim 1, wherein the semipermeable membrane is an anion exchange membrane.

9. The apparatus of claim 1, wherein the sulphuric acid is recovered from the buffer within the anode chamber of the reactor.

10. The apparatus of claim 1, wherein the water is recovered from the acid mine drainage in the cathode chamber of the reactor.

11. The apparatus of claim 1, wherein the predetermined pH is dependent on the selected metals or critical elements to be recovered and wherein the predetermined pH ranges from between about 2 to about 10.2.

12. The apparatus of claim 1, further comprising one or more additional electrochemical reactors arranged in series for sequential recovery of different metals and/or critical elements.

13. (canceled)

14. (canceled)

15. A process for treatment of acid mine drainage, said process comprising: providing at least one electrochemical reactor comprising at least one anode electrode, at least one cathode electrode and a semipermeable membrane for separating the at least one anode electrode and the at least one cathode electrode and for defining an anode chamber containing the at least one anode electrode and a cathode chamber containing the at least one cathode electrode; providing at least one anolyte reservoir for containing buffer and at least one catholyte reservoir for receiving and containing the acid mine drainage, said at least one anolyte reservoir and said at least one catholyte reservoir being distinct from the at least one electrochemical reactor and in fluid communication with the anode chamber and the cathode chamber of the reactor, respectively; supplying the acid mine drainage to the cathode chamber of the at least one electrochemical reactor from the at least one catholyte reservoir and supplying the buffer to the anode chamber of the at least one electrochemical reactor from the at least one anolyte reservoir; monitoring a pH of contents of the at least one electrochemical reactor; controlling supply of an electrical current to the at least one electrochemical reactor until a desired pH is reached for selective recovery of at least one of metals, critical elements, sulphuric acid and water; and recirculating the acid mine drainage from the cathode chamber back into the at least one catholyte reservoir, wherein the selective recovery of the metals or the critical elements includes precipitating the metals or the critical elements from the acid mine drainage and the pH of the catholyte reservoir is controlled at an elevated pH relative to the acid mine drainage to encourage precipitation to occur within the at least one catholyte reservoir and to resist precipitation or adherence of precipitant on a surface of the at least one cathode electrode.

16. The process of claim 15, wherein the supplying the acid mine drainage includes pumping the acid mine drainage from the at least one catholyte reservoir to the cathode chamber.

17. The process of claim 15, wherein the supplying the buffer includes pumping the buffer from the at least one anolyte reservoir to the anode chamber.

18. The process of claim 15, further comprising recirculating the buffer from the anode chamber back into the at least one anolyte reservoir.

19. The process of claim 15, wherein the process is a batch process.

20. The process of claim 15, wherein the process is a continuous process.

21. The process of claim 15, wherein the controlling the electrical current supplied to the at least one electrochemical reactor includes applying the electrical current within a specified voltage until a desired pH is reached.

22. The process of claim 15, wherein the monitoring the pH includes using at least one sensor to monitor the pH.

23. (canceled)

24. (canceled)

Description

BRIEF DESCRIPTION OF DRAWINGS

[0142] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[0143] FIG. 1 shows an apparatus according to an embodiment of the present invention working in batch mode for use in the treatment of acid mine drainage and selective recovery metals, critical elements, sulphuric acid and water;

[0144] FIG. 2 shows an apparatus according to an embodiment of the present invention working in continuous mode for use in treatment of acid mine drainage and selective recovery metals, critical elements, sulphuric acid and water;

[0145] FIG. 3 shows a process flow diagram according to an embodiment of the present invention;

[0146] FIG. 4 shows a system diagram of a configuration of an example electrochemical system;

[0147] FIGS. 5a and 5b are plots of experimental and modelling data from AMD processed by an electrochemical system according to an embodiment of the present invention. The plots show concentration against pH of contaminants in the liquid phase during electrochemical treatment of acid mine drainage;

[0148] FIGS. 6a and 6b are plots showing a percentage removal of metals from treated acid mine drainage respectively obtained from Texas and Mt Morgan sites;

[0149] FIGS. 7a and 7b are plots respectively showing variations in sludge composition at different pH stages for water from tailings storage facilities within Texas and Mt Morgan closed mines;

[0150] FIGS. 8a and 8b are plots showing a solids composition of rare earth element oxides at varying pH stages obtained from Texas and Mt Morgan derived acid mine drainage, respectively; and

[0151] FIG. 9 is a plot showing solids composition of REYs recovered using electrochemical (ECR) and chemical addition (CaO and NaOH).

DETAILED DESCRIPTION

[0152] FIG. 1 shows an apparatus (100) according to an embodiment of the present disclosure for use in the treatment of acid mine drainage (122) and selective recovery metals, critical elements, sulphuric acid and water.

[0153] The apparatus (100) includes at least one electrochemical reactor (110), at least one catholyte reservoir (120) and at least one anolyte reservoir (130) for containing the acid mine drainage (122) and a 1M sodium borate buffer (132), respectively. The reservoirs (122,132) are in fluid communication with the electrochemical reactor (110), i.e., are connected by tubing (not shown).

[0154] The apparatus (100) further includes a sensor (140) for monitoring a pH of the contents of the reactor (110), and a power source (150) for supplying and controlling an electrical current to the reactor (110).

[0155] The electrical current is supplied to the reactor (110) until a predetermined pH is reached. The predetermined pH will depend on the particular metals and/or critical elements to be recovered from the AMD (122).

[0156] The apparatus further includes an AEM (160) dividing the rector (110) into a cathode chamber (124) and an anode chamber (134).

[0157] A platinum-iridium oxide coated titanium cathode electrode (128) is positioned within the cathode chamber (124) and a stainless-steel anode electrode (138) is positioned within the anode chamber (134).

[0158] The apparatus further includes a pump (170) for pumping the AMD (122) and the buffer (132) through the apparatus (100).

[0159] The cathode chamber includes an inlet (125) for entry of the AMD (122) from the catholyte reservoir (120) and an outlet (126) for egress of the AMD (122) into the catholyte reservoir (120).

[0160] Likewise, the anode chamber (134) includes an inlet (135) for entry of the buffer (132) and an outlet (136) for egress of the buffer (132) back to the anolyte reservoir (130).

[0161] In use, pump (170) is used to supply AMD (122) to the cathode chamber (124) of the reactor (110) from the catholyte reservoir (120) and to supply buffer (132) to the anode chamber (134) of the reactor (110) from the anolyte reservoir (130).

[0162] The AMD (122) is recirculated by pumping the AMD (122) within the cathode chamber (124) back into the catholyte reservoir (120).

[0163] The inlet (125) of the cathode chamber (124) accepts the AMD (122) coming from the catholyte reservoir (120). The AMD (122) coming from the catholyte reservoir (120) will be already pH adjusted and depleted of metals that precipitate at a lower pH than the prevailing pH. The AMD (122) entering the cathode chamber (124) will then have its pH raised by flowing through the cathode chamber (124). Thus, the AMD (122) flowing out of the cathode chamber (124) will have a higher pH than the AMD (122) flowing into the cathode chamber (124). The AMD (122) flowing through the outlet (126) of the cathode chamber (124) is then re-circulated into the catholyte reservoir (120) and mixed with the AMD in the catholyte reservoir (120) to control the pH in the cathode chamber (124) at a set point, which is higher than the pH of the AMD (122) flowing into the catholyte reservoir (120).

[0164] The buffer (132) is recirculated by pumping the buffer (132) within the anode chamber (134) back into the anolyte reservoir (130).

[0165] The power source (150) is used to control the supply of the electrical current to the reactor (110) by applying the electrical current with a specified voltage causing oxidation reactions to occur at the anode (138) and reduction reactions to occur at the cathode (128). The reduction reactions drive metal hydroxides, oxides or sulphates to precipitate out of the AMD (122). The pH of the AMD (122) increases due to the concentration of hydroxide ions in the AMD (122) increasing through the reduction reactions concomitantly with the migration of sulphate anions from the AMD (122) within the cathode chamber (124) through the AEM (160) into the buffer (132) contained within the anode chamber (134).

[0166] The reduction reactions within the AMD (122) encourage the generation of sulphate anions within the AMD (122), which migrate across the AEM (160) from the cathode chamber (124) to the anode chamber (134) to generate sulphuric acid within the buffer (132).

[0167] The pH of the contents of the reactor is continuously or periodically monitored using pH sensor (140) until the desired pH is reached. At the desired pH, selected metals and/or critical elements precipitate out of the AMD (122) forming precipitant (160), which is encouraged to settle within the catholyte reservoir (120).

[0168] At this point, the electrical current supplied from the power source (150) is stopped, and the precipitant (160) can then be collected from the cathode chamber (120) for further processing, if required.

[0169] Further, sulphuric acid formed within the buffer (132) from the migration of sulphate anions from the AMD (122) into the buffer (132) can be recovered.

[0170] Yet further, electrochemical removal of metals, critical elements and sulphate from the AMD (122) effectively treats the AMD to enable the recovery of treated water.

[0171] Once the pH sensor (140) reports that it has reached a desired pH for the solution, the power source (150) is isolated or stopped, and metals that become solid at that pH precipitate.

[0172] Over time metals and/or critical elements may accumulate within the cathode chamber (124) or directly adhere to the surface of the cathode electrode (128) and flushing of the cathode chamber may be required.

[0173] When flushing is required, the current supply to the reactor is stopped and either fresh AMD (122) or sulphuric acid solution is flushed through the cathode chamber (124) of the reactor (110) to dissolve any metals or critical elements deposited on the surface of the cathode electrode (128) or within the cathode chamber (124).

[0174] FIG. 2 shows an apparatus (200) according to another embodiment of the present disclosure for use in treatment of acid mine drainage (222) and selective recovery metals, critical elements, sulphuric acid and water.

[0175] The apparatus (200) includes a first electrochemical reactor (210) in fluid communication with a first catholyte reservoir (220) and at least a second electrochemical reactor (212) in fluid communication with a second catholyte reservoir (221), wherein the first and second catholyte reservoirs (220, 221) are in fluid communication with each other and are for containing the acid mine drainage (222).

[0176] The apparatus (200) also includes an anolyte reservoir (230) for containing a buffer (232), the anolyte reservoir (230) being in fluid communication with the first reactor (210).

[0177] The anolyte reservoir (230) is in fluid communication with the anode chamber (215) of the first reactor (210) and the anode chamber (215) of the first reactor (210) is in fluid communication with the anode chamber (217) of the second reactor (212).

[0178] The apparatus (200) also includes a first sensor (240) for monitoring the pH of the contents of the first reactor (210) and a second sensor (241) for monitoring the pH of the contents of the second reactor (212), and a power source (250, not shown) for supplying an electrical current to the first (210) and second reactors (212).

[0179] Each of the first (210) and second (212) reactors include an AEM (260, 262) dividing each reactor into a cathode chamber (214, 216) and an anode chamber (215, 217).

[0180] The anode chamber (215) of the first reactor (210) is in fluid communication with the anode chamber (217) of the second reactor (212). Further, the anolyte reservoir (230) is in fluid communication with the anode chamber (215) of the first reactor (210).

[0181] The cathode chamber (214) of the first reactor (210) includes an inlet for entry of the AMD (222) from the first catholyte reservoir (220) and an outlet for egress of the AMD (222) into the second catholyte reservoir (221).

[0182] Likewise, the anode chamber (215) of the first reactor (210) includes an inlet for entry of the buffer (232) from the anolyte reservoir (230) and an outlet for egress of the buffer (232) into the anode chamber (217) of the second reactor (212).

[0183] A pump (not shown) is used to pump the AMD (222) and the buffer (232) throughout the apparatus (200).

[0184] In the apparatus shown in FIG. 2, a filter (240) is used to filter the AMD (222) supplied from the first catholyte reservoir (220) before it enters the cathode chamber (214) of the first reactor (210).

[0185] As shown, precipitation (260) of the metals and critical elements is encouraged to settle within the first and second catholyte reservoirs (220, 221).

[0186] Dryers (250a, 250b) are included in the apparatus to dry the precipitated metals and or critical elements (260) recovered from the AMD (222) before they are conveyed to a transportation vehicle (270).

[0187] Further, a mixer (280) is included for maintaining a substantially homogenous solution of the recovered water and/or sulphuric acid-buffer solution and to ensure the pH of discharged water is suitable for environmental release, that is, having a near-neutral pH.

[0188] In use, a pump (not shown) is again used to pump AMD (222) from the first catholyte reservoir (220), through filter 240 and into the cathode chamber (214) of the first reactor (210).

[0189] The AMD (222) within the cathode chamber (214) of the first reactor (210) is then recirculated and pumped back into the first catholyte reservoir (220). Precipitant (260) formed from the AMD (222) is first encouraged to settle within the first catholyte reservoir (220).

[0190] The remaining liquid phase of the AMD (222) is then pumped from the first catholyte reservoir (220) to the second catholyte reservoir (221) before being pumped into the cathode chamber (216) of the second reactor (212).

[0191] The AMD (222) within the cathode chamber (216) of the second reactor (212) is then recirculated and pumped back into the second catholyte reservoir (221). Precipitant (260) formed from the AMD is encouraged to settle within the second catholyte reservoir (221).

[0192] The remaining liquid phase of the AMD (222) is then pumped from the second catholyte reservoir (221) into a mixer (280) before being released as clean treated water or transported for further processing.

[0193] Further, the pump (not shown) is used to supply buffer (232) to the anode chamber (215) of the first reactor (210) from the anolyte reservoir (230). The buffer (232) is then pumped into the anode chamber 217 of the second reactor (212) before being removed from the apparatus for further processing to recover sulphuric acid.

[0194] The pH of the contents of both the first (210) and second (212) reactors are continuously or periodically monitored throughout the process.

[0195] A power source (250; not shown) is used to control the supply of electrical current to the first reactor (210) and the second reactor (212) until a desired pH is met.

[0196] The predetermined pH of the contents of the first reactor (210) is different to the predetermined pH of the contents of the second reactor (212).

[0197] The predetermined pH of the AMD within the cathode chamber (214) of the first reactor (210) would typically be around 4.2 to precipitate a range of metals and the predetermined pH of the AMD within the cathode chamber (216) of the second reactor (212) would be around 10, to precipitate remaining metals and elements. At pH 4 aluminium, iron, arsenic, barium, chromium, copper, and lead are preferentially precipitated out from the AMD (222) and at pH 10 magnesium, manganese, and sulphur are preferentially precipitated out from the AMD (222). This results in the sequential selective recovery of different metals and/or critical elements. Other elements such as cadmium, cobalt, nickel, and zinc precipitate across the pH range. The pH can be controlled in smaller increments, resulting in preferential recovery of other elements at given pH values. For example, rare earths typically precipitate at a pH around 7.

[0198] Once the predetermined pH of the contents is reached for each reactor respectively, the electrical current supplied to each reactor is independently controlled so as to maintain the predetermined pH of the contents within the first and second reactors (210, 212) at their predetermined pH.

[0199] Controlling the electrical current supplied to at least one electrochemical reactor is automated in that the pH sensor is associated with a controller for monitoring and collecting data output from the pH sensor. The controller isolates the power supply when the desired pH is reached.

[0200] The controller collects data indicative of pH values output from the pH sensor and processes and compares the data to a predetermined pH value. Based on the result of the comparison the controller determines whether the data is substantially the same as the predetermined pH value or not. In response to the data being the same as the predetermined pH value, the controller isolates the power supply supplying the electrical current to the electrochemical reactor. In response to the data varying from the predetermined pH value, the controller resupplies the electrical current to the electrochemical reactor.

[0201] In some embodiments, the pH and flow in the chambers can be controlled automatically by pH indicator controls, transmitters, flow controls, and voltage indicators, and can remotely monitored. These controls are depicted in FIG. 4.

[0202] For the embodiment shown in FIG. 2, in the event that metals and/or critical elements accumulate within the cathode chambers (214,215) or directly adhere to the surface of the cathode electrode of the first and/or second reactors (210,212), flushing of the cathode chambers (214,215) may be required.

[0203] When flushing, the current supply to each reactor is stopped and either fresh AMD (122) or sulphuric acid solution is flushed through the cathode chamber (124) of the first reactor (110), and into the cathode chamber (214) of the second reactor (212) to dissolve any metals or critical elements deposited on the surface of the cathode electrodes or within the cathode chambers (214, 215).

[0204] FIG. 3 shows a process flow diagram of the embodiment of the present invention as shown in FIG. 2.

Example

[0205] The following example is provided to demonstrate how the present invention can be used to treat AMD for critical element recovery. The example shows the efficient precipitation of solid critical elements from AMD using a chemical-free approach. The example is not to be considered limiting on the scope and ambit of the present invention as hereinbefore described.

Methods

Modelling Methods

[0206] Both electrochemical and chemical AMD treatment relies on solubility mechanisms to precipitate the metals from the water. Through varying the charge balance of the system, by removing sulphate ions (electrochemically) or adding cations sodium and calcium (chemically) the pH of the AMD increases, which induces the precipitation of metal (oxy)hydroxides and sulphates. Solubility theory is well understood and a variety of modelling platforms are available to simulate the experiments through the evaluation of ion pairing and acid-base reactions using laws of mass-action, ionic strength using chemical activity correction factors, pH using a charge and/or mass balance, and saturation using a saturation index (SI). In the modelling here, precipitation kinetics were not considered and an equilibrium approach was assumed.

[0207] Simulations using a solubility software program, PHREEQC (Version 3), were performed to understand saturation and precipitation of high concentration metals during the different experiments. The initial solution compositions in PHREEQC were defined based on the ICP-OES analysis performed to characterize the real AMD samples. The components included: Al, Ca, Cu, Fe(II), Fe(III), Mg, Mn, Na, SO.sub.4.sup.2 and Zn. These components were included as they were in relatively high concentrations (>0.8 mM) in the samples tested and were included in the database which was used (WATEQ4F).

[0208] The equilibrium phases included in the model using the EQUILIBRIUM PHASES block were determined by running a simulation with none included to determine the possible mineral phases above saturation (considered conservatively to be a saturation index greater than 0). Then a second simulation including all the potential mineral phases previous identified was performed to evaluate which ones actually formed according to the model. Non-forming mineral phases were removed from future simulations.

[0209] For each AMD source and each method of pH amendment, a series of simulations were performed where sulphate was removed stepwise (to reflect the chemistry of the cathode solution in the electrochemical experiments) or NaOH or CaO was added stepwise (to reflect the chemical addition experiments which represent the main existing methods of treating AMD). This was done using a REACTION block. The resulting mineral phase concentrations and aqueous concentrations are the model output. The model was used to describe the processes in both the 2-stage and multistage experiments. However, there are two key limitations: a) the PHREEQC database in use (WATEQ4F) is limited to certain components and precipitation products and b) metals in low concentration (<0.8 mM) were not included.

Experimental Methods

Overview

[0210] Two types of experiments were performed. Both corresponded to the system configuration shown in FIG. 4. The first consisted of three treatments comparing the efficacy and sludge quality between electrochemical AMD treatment compared with chemical dosing with NaOH and slaked lime. This was performed in two pH stages. The first stage elevated the pH to 4.2 and the second to 10.2. After each stage several types settling tests were performed. The second type of experiment was multistage tests only using the electrochemical reactor (FIG. 4). The multistage tests increased the pH in smaller increments of 0.5 or 1. The smaller increments provide refined data on the nature of the precipitation relating to pH.

Electrochemical Reactor Description

[0211] For both types of experiments the electrochemical reactor consisted of two self-manufactured chambers separated by rubber gaskets, a stainless steel cathode, a platinum-iridium oxide coated titanium electrode anode (Magneto Special Anodes B V, Netherlands), and an AEM (Membranes International IC., USA, AEM-7001) with effective area of 32 cm.sup.2. The areas of the chambers were 8 cm high, 4 cm wide and 1.2 cm thick. A pump (Watson Marlow Sci 323) was used to supply 85 mL min.sup.−1 flow rate (90 RPM) through the reactor with anolyte and catholyte individually recirculated to external reservoirs. The reservoirs were vented to maintain atmospheric pressure. An external power source (Elektro-Automatik GmbH & Co. KG, EA-PS 3016-10B) was used to supply an external current. The pH was measured using an Endress+Hauser Orbisint CPS11D glass electrode.

Analytical Methods

[0212] For the liquid analysis, the composition of the AMD was analysed by inductively coupled plasma optical emission spectroscopy (ICP-OES) for major metals and inductively coupled plasma mass spectrometry (ICP-MS) for trace metals prior to use. ICP-MS samples were unfiltered and digestion was performed (USEPA SW846-3005, nitric/hydrochloric acid digestion). The process followed APHA 3125; USEPA SW846-6020 and was performed at Analytical Laboratory Services (ALS), Brisbane, Australia using their method ALS QWI-EN/EG020. Trace Hg was also analysed for using flow injection mercury system (FIMS) following AS 3550, APHA 3112 Hg-B, which was performed at Analytical Services Laboratory, Brisbane, Australia.

[0213] ICP-OES of the liquids was conducted at the Analytical Services Laboratory, The University of Queensland, Brisbane, Australia (Perkin Elmer Optima 7300DV, Waltham, Mass., USA) after nitric acid digestion for total and soluble cation concentrations.

[0214] For the dried precipitation products, ICP-OES and ICP-MS was performed at Queensland University of Technology's Central Analytical Research Facility (CARF) using a Perkin Elmer Optima 8300 DV Inductively Coupled Plasma Optical Emission Spectrometer and Agilent 8800 Inductively Coupled Plasma Mass Spectrometer, respectively.

2-Stage Tests

[0215] In the electrochemical treatments the catholyte was the field collected AMD (unfiltered, but let settle, stored under refrigeration until 24 h prior to experiments). The anolyte was 1 M sodium borate buffer solution. Stage 1 operated until the pH of the AMD (catholyte) reached 4.2.

[0216] Once the solution reached pH 4.2, the current was turned off. A 20 mL sample was collected for total suspended solids (TSS) analysis. The AMD then underwent a settling rate test (SRT). The SRT was performed by pouring the total AMD (catholyte) solution into a 1 L measuring cylinder. The sludge height was recorded every 3 min for the first 30 min then every 30 min for the next 2 hours and finally at 24 h. After the settling rate test, a 10 mL sample of the liquid fraction was taken for ICP-OES analysis and the liquid fraction of the solution was decanted back into a suitable bottle for the next stage of electrochemical treatment. Ten mL of the separated sludge was centrifuged (Eppendorf Centrifuge 5810) for 5 min at 3200 rcf with the sludge height measured afterwards reflecting the theoretical minimal sludge volume. The sludge volume index was determined by the volume in mL occupied by 1 g of a suspension after 30 min of settling, see Equation 1 below.

SVI (mL g.sup.−1)=settled sludge volume at 30 min (mL L.sup.−1)*1000/total suspended solids (TSS)(mg L.sup.−1)  (1)

[0217] Drying time is an important consideration in full-scale AMD treatment. To test the differences in drying time, 50 mL of each sludge was weighed, they were simultaneously dried in an oven at 60-70° C. and weighed regularly until the weight recorded a constant value. Linear regression using Microsoft Excel 2016 was performed during the 70° C. period (23.5 h until the end of drying time) to identify any differences in sludge drying time by comparing the 95% confidence intervals of the slope parameter. The dried solids from the sludge were analysed for their bulk chemical composition using ICP-OES and ICP-MS. Total suspended solids (TSS) were performed according to Standard Methods (Eaton et al., 1998).

[0218] The next stage of the electrochemical treatment was identical to the first, except the pH was now elevated to the maximum it would reach in the electrochemical reactor, around 10.2. After the pH reached −10.2, the current was again turned off and the same methods as above were repeated. When the pH stagnated in the second stage, it was assumed to be because of hydroxide ions dominating ionic migration (Thompson Brewster et al. 2017).

[0219] To compare the electrochemical treatment performance with commonly used chemical precipitation, the same experiments were performed, except rather than using electrochemical sulphate removal for pH adjustment, chemical addition with NaOH (Merck Pty Ltd, pellets for analysis, CAS-No: 1310-73-2) and with slaked lime (Alfa Aesar, reagent grade, CAS-No: 1305-78-8) were performed with the AMD contained in a beaker on a magnetic stirrer. In the same order as for the electrochemical experiments, the pH was adjusted to 4.2 then the same series of settling and sludge tests were performed. Then the solid and liquid phases separated and the liquid was raised again to pH 10.2, and the settling and sludge tests performed. The lime was prepared by mixing 1 part CaO with 9 parts water and stirring at a high rate on a magnetic stirrer for at least 10 minutes prior to use.

Multistage Tests

[0220] The multistage tests were performed similar to the electrochemical tests, but stopped at pH 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10 and the final value possible (˜10.2). 1 L of Texas Silver Mine AMD was used in one experiment and 0.5 L Mt Morgan Gold Mine AMD was used in another. The anolyte was 200 mL 0.5 M sodium borate buffer solution, which was replaced if necessary when the anolyte pH fell below 7. After each pH increment was reached, the reactor was stopped, emptied and the liquid left to settle for at least 1 h in a beaker. After this, 10 mL or 4 mL samples were taken from Texas Silver Mine and Mt Morgan Gold Mine experiments, respectively. The liquid was decanted from the sludge. The liquid was used in the next stage. The remaining sludge was dried at 65° C. with the mass of sludge and solids percentage evaluated. The dried solids from the sludge were analysed for their bulk chemical composition using ICP-OES and ICP-MS. The sodium borate buffer solution was made from 61.83 g boric acid (Sigma-Aldrich, ReagentPlus®99.5%, CAS-No: 10043-35-3) and 10 g sodium hydroxide added into 1600 mL MilliQ water, stirred and made up to 2 L with MilliQ water.

Results

Modelling Results and Aqueous Phase Removal

Precipitation of Solids

[0221] As follows solubility theory, different metals oxides, hydroxides and sulphates become saturated at different pHs. Modelling of the major precipitants was performed, which involved removing sulphate (as would happen in the electrochemical treatment) or adding lime or NaOH (as would happen during the chemical addition treatment). All results show that as the pH increases the initial precipitation of Al occurs firstly as jurbanite (AlOHSO.sub.4) and then as diaspore (AlOOH). Buffering occurs at pH 4 while diaspore is forming. Once Al is completely depleted from the aqueous phase, the pH rises again until a second plateau. The second buffering stage occurs at ˜pH 10, where brucite (Mg(OH).sub.2) formation absorbs additional alkalinity produced through treatment. Consistent results occur between the three types of treatment indicating this is a trend common to AMD containing high concentrations of Al and Mg. The modelling and experimental results illustrate that not all metals are removed from solution evenly with increasing pH, and these differences can be used to exploit the targeted composition of the solid precipitants. It also illustrates that the trend in removal for the major metals is independent of the type of treatment—chemical or electrochemical.

Removal of Metals from the Liquid Phase

[0222] FIGS. 5a and 5b shows that the electrochemical system removes Fe, Al, Mg and SO.sub.4, and levels of Na and Ca remain constant. Referring to FIG. 5a, the model of Mt Morgan does not validly model SO.sub.4, Mg and Al. Epsomite (MgSO.sub.4) was included in the PHREEQC model, but it does not reach saturation in the model. However, it is clearly forming experimentally based on the discrepancy between the experimental and model results for Mg and S. As the concentration of Mg is over 6 times higher in Mt Morgan compared to Texas (see FIG. 5b), the discrepancy is exaggerated there.

[0223] It is also possible that for both Mt Morgan and Texas, jurbanite (AlOHSO.sub.4) is not initially saturated, as the model predicts, and that explains the slower than predicted removal of Al during the experiment at the lower pH values.

[0224] Modelling of the NaOH and lime chemical treatment was also performed across the whole pH range. In comparison to the NaOH and lime treatment, modelling illustrates that the liquid composition of AMD treated by the electrochemical cell is the least contaminated of the three methods overall. NaOH addition is not expected to reduce the sulphate concentrations, and to increase the Na concentration. However, NaOH treatment does remove Fe, Al and Mg successfully. Lime addition displays similar removal efficacy compared to ECR treatment. However, towards the high end of the pH range Ca concentrations in the liquid phase increase.

Water Disposal Characteristics

Water Treatment Compared to Guidelines

[0225] Tables 1 provides data for the final water quality of the AMD compared to two potential downstream uses as set out in the Australian and New Zealand Environment and Conservation Council (ANZECC) guidelines.

[0226] For both Texas and Mt Morgan AMD all three treatment options removed nearly all listed contaminants to the required guidelines, except for sulphate. The exceptions were both during lime treatment, in which there was insufficient removal of Al in Texas and Pb in Mt Morgan. For Texas, the lime treatment removed 23-26% more sulphate compared to electrochemical treatment. For Mt Morgan, ECR treatment removed the most sulphate by approximately 14-25% compared to lime. In both cases, NaOH treatment was ineffective at removing sulphate. Lime and electrochemical treatment was similar overall. However, lime treatment had two exceptions in meeting the discharge guidelines.

[0227] Some elements are potentially relevant to the ANZECC (2000) guidelines, but are not included in the table (see Table 1 footnotes). The ICP-MS results of the solid precipitate illustrates at least partial, if not full removal of arsenic, beryllium, molybdenum, selenium, uranium and vanadium was achieved. Mercury was below the detection limit of the ICP-MS in the solids and fluoride was not measured.

TABLE-US-00001 TABLE 1 Treated water quality, a comparison of electrochemically treated (ECR), sodium hydroxide chemical dosing (NaOH) and lime dosing (lime). ANCECC 2000 guidelines Texas Mt Morgan Stock Recreational Element Original ECR NaOH Lime Original ECR NaOH Lime water* purposes** Al 442.5 0.58 0.27 5.1 2317 0.55 0.03 0.74 5 0.2 B 0.69 0.75 0.62 0.56 0.00 2.69*** 0.00 0.00 5 1 Cd 0.1 0.00 0.00 0.00 0.13 0.00 0.00 0.00 0.01 0.005 Ca 368.3 367.4 377.2 489.2 364.4 234.2 354.3 442.4 1000 Not listed Cr 0.29 0.00 0.00 0.00 0.02 0.00 0.00 0.00 1 0.05 Co 3.0 0.01 0.05 0.02 5.16 0.00 0.01 0.06 1 Not listed Cu 9.0 0.15 0.45 0.36 65.00 0.03 0.05 0.07 0.4 1 Fe 323.6 0.34 0.00 0.09 66.26 0.00 0.00 0.00 Not 0.3 sufficienly toxic Pb 1.3 0.01 0.06 0.05 6.18 0.11 0.07 0.12 0.1 0.05 Mg 714.8 317.2 245.2 7.35 4564 1022 2148 1188 2000 Not listed Mn 63.4 0.93 0.70 0.12 244.7 0.84 0.18 0.18 Not 0.1 sufficiently toxic Ni 4.66 0.01 0.00 0.00 2.07 0.00 0.00 0.00 1 0.1 Zn 105.9 0.28 0.03 0.12 55.10 0.00 0.00 0.00 20 5 SO4 9391 4195 8597 3099 29547 4962 27938 5769 1000 400 *ANZECC 2000 Table 4.3.2 also includes the elements arsenic (Texas initially over), beryllium (both within limit initially), fluoride (not measured by ICP-MS), mercury (both within limit initially), molybdenum (both within limit initially), selenium (both over limit initially), uranium (both within limit initially), and vanadium (not listed for stock water). **ANCECC 2000 Table 5.2.3 also includes the elements arsenic (both initially over), beryllium (not listed), fluoride (not measured by ICP-MS), mercury (both within limit initially), molybdenum (not listed), selenium (both over limit initially), uranium (not listed), and vanadium (not listed). ***Boron increase due to the experimental choice of anolyte (sodium borate) and will not be present during continuous operation.

Precipitated Solids

[0228] Removal of Metals from Solution

[0229] The multistage experiments illustrate that all the metals fall into three classes of removal when treated using the ECR: low pH, high pH and continuously removed. FIGS. 6a and 6b shows that Al, Ba, Cr, Cu, Fe and Pb precipitate at low pH. Mg and Mn at high pH and Cd, Co, Ni and Zn are removed continuously. The results from Texas AMD clearly show this result. However, for Mt Morgan nearly all metals are shown to precipitate at relatively low pH. This could be explained by the very high concentrations of SO.sub.4.sup.2− and Mg in Mt Morgan AMD (29 000 mg SO.sub.4.sup.2− L.sup.−1, 4500 mg Mg L.sup.−1) which requires a large amount of current to be added (alkalinity removed through SO.sub.4.sup.2− migration) before precipitating in amounts significant enough to increase the pH.

[0230] This order of sequential precipitation is further supported by the two stage experiments where Al, As, Ba, Cr, Cu, Fe, Mo, Se, Pb are mostly removed during the first stage; Co, Mg, Mn, Mo, Ni, Zn are largely removed during the second stage and Cd is removed in both stages relatively equally.

[0231] FIGS. 6a and 6b show the percentage removal of metals from acid mine drainage through electrochemically induced precipitation. Pollutants removed at low pH are indicated by square markers (blue), those removed at high pH by circle markers (green) and those constantly removed by triangle markers (red). The three classes of results and not clearly seen in the Mt Morgan results due to the very high concentrations of SO.sub.4.sup.2− and Mg, dominating the results (29 000 mg SO.sub.4.sup.2− L.sup.−1, 4500 mg Mg L.sup.−1).

Sludge Composition

[0232] FIG. 7a and FIG. 7b shows the variation in sludge composition at the different pH stages for Texas and Mt Morgan, respectively. These graphs clearly illustrate the possibility of producing solid products with targeted composition dependent on the pH stage. Differences in staged composition are largely dependent on the initial composition of the AMD. The general trends between the results in FIGS. 7a and 7b as well as the modelling above support the selective precipitation of Fe (pH <4), Al (pH 4-6) then Mg and Mn (pH >7) as the highest concentration metals in the solid product. This data illustrates that the experimental results follow closely the solubility models and associated theory for the higher concentration metals.

Recovery of Rare Earth Elements

[0233] The percentage of rare earth elements and yttrium (REYs) in the solids are shown in FIGS. 8a and 8b. The highest concentration REOs are Yttrium (Y), Neodymium (Nd), Cerium (Ce), Gadolinium (Gd), Dysprosium (Dy) and Samarium (Sm). Also detected in lower concentrations were Erbium (Er), Europium (Eu), Holmium (Ho), Lutetium (Lu), Praseodymium (Pr), Terbium (Tb), Thulium (Tm) and Ytterbium (Yb). FIGS. 8a and 8b illustrate the maximum concentrations of REYs occur between a specific pH of 5-7. The prevalence of REY precipitation appears consistent between the two types of AMD.

[0234] FIGS. 8a and 8b show the solids composition of rare earth element oxides at varying pH stages. The gap between the presented REYs and the total percentage is comprised of Erbium (Er), Europium (Eu), Holmium (Ho), Lutetium (Lu), Praseodymium (Pr), Terbium (Tb), Thulium (Tm) and Ytterbium (Yb).

Comparison to Chemical Dosing

Major Metal Composition

[0235] In the dried solids, metals during the chemical experiments were found in lower concentrations compared to metals in the electrochemical experiments. This effect was due to the additional Ca and Na, which was precipitating during the chemical experiments, effectively ‘diluting’ the solid product. Our results support a finding that an electricity driven process can up concentrate higher levels of metals compared to chemical addition processes.

Rare Earth Element Recovery

[0236] FIG. 9 compares the REY concentration in electrochemically generated solids and chemically generated solids. Stage 2 (S2, between pH 4-10) is where the majority of REYs precipitate (see FIGS. 8a and 8b). Electrochemically generated solids have higher REYs percentage composition compared to chemically generated solids. This is due to the solids from the chemical addition treatments also containing a significant mass of the elements that were added (Ca and Na), effectively ‘diluting’ the solids.

[0237] FIG. 9 shows the solids composition of REYs using electrochemical (ECR) and chemical (CaO and NaOH) addition. All values are for S2 (refers to stage 2, pH 4-10) as this was the pH range where the majority of REYs were shown to precipitate.

Sludge Characteristics

[0238] The theoretical minimal sludge volume per litre of AMD was at least halved for electrochemically generated sludge for both samples. The electrochemically produced sludge was between 2 and 20 times smaller in volume compared to NaOH chemical addition. Centrifugation was also observed to effectively separate water from solid metal precipitates.

[0239] For all 4 permutations of the experiment the electrochemically generated sludge had the lowest SVI of those measured (measurements were not possible for Mt Morgan CaO stage 2 and Texas stage 2 NaOH due to sampling errors). There was one exception for Texas CaO stage 1, which had a particularly low SVI, corresponding to a very fast settling sludge.

[0240] For Mt Morgan, stage 1 and stage 2 electrochemically generated sludge (after settling, but before centrifugation) had the lowest percentage of solids by weight in the sludge. For Texas, NaOH had the lowest percentage of solids for both stages and CaO had the highest for both stages.

[0241] There were no significant differences in the time it took the different sludges to dry. For Mt Morgan there was only 1 sample with sufficient sludge produced for comparison. However, this dried faster than the other NaOH and lime samples for Mt Morgan. This provides some evidence that the electrochemically generated sludge may dry faster, but it requires further investigation to confirm.

[0242] In the present specification and claims (if any), the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers.

[0243] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[0244] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.

CITATIONS

[0245] Thompson Brewster, E., Jermakka, J., Freguia, S. & Batstone, D. J. (2017) Modelling recovery of ammonium from urine by electro-concentration in a 3-chamber cell. Water Research, 124, 210-218, doi: 10.1016/j.watres.2017.07.043. [0246] Eaton, A., Clesceri, L., Greenberg, A. & Franson, M. (1998) Standard Methods for the Examination of Water and Wastewater (20.sup.th edition), American Public Health Association, Washington, D.C.