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Title:
EXTRACTION OF PRECIOUS METALS
Document Type and Number:
WIPO Patent Application WO/2016/004458
Kind Code:
A1
Abstract:
Disclosed is a method for extracting precious metal ions from a precious metal ion laden aqueous phase having a concentration of greater than 0.1 g/L of one or more precious metal ions. The method comprises contacting the precious metal ion laden aqueous phase with an organic extractant phase comprising an extractant in an organic solvent that is substantially immiscible with the aqueous phase in a microfluidic separation device under conditions to transfer at least some of the precious metal ions from the aqueous phase to the organic extractant phase to provide a precious metal ion complex laden organic extractant phase; and recovering the precious metal ion complex laden organic extractant phase from the microfluidic device.

Inventors:
PRIEST CRAIG IAN (AU)
KRIEL FREDERIK HERMANUS (AU)
RALSTON JOHN (AU)
Application Number:
PCT/AU2015/000394
Publication Date:
January 14, 2016
Filing Date:
July 08, 2015
Export Citation:
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Assignee:
UNIV SOUTH AUSTRALIA (AU)
International Classes:
B01D11/04; C22B3/26
Domestic Patent References:
WO2012079130A12012-06-21
WO2010022441A12010-03-04
Foreign References:
CN103060559A2013-04-24
CN103667697A2014-03-26
Other References:
YIN, C-Y. ET AL.: "Microfluidic solvent extraction of platinum and palladium from a chloride leach solution using Alamine 336", MINERALS ENGINEERING, vol. 45, 2013, pages 18 - 21, XP028590306, DOI: doi:10.1016/j.mineng.2013.01.013
CICERI, D. ET AL.: "The use of microfluidic devices in solvent extraction", JOURNAL OF CHEMICAL TECHNOLOGY AND BIOTECHNOLOGY, vol. 89, no. Issue 6, May 2014 (2014-05-01), pages 771 - 786
PRIEST, C. ET AL.: "Microfluidic solvent extraction of metal ions from industrial grade leach solutions: extraction performance and channel aging", JOURNAL OF FLOW CHEMISTRY, vol. 3, no. 3, 2013, pages 76 - 80
KRIEL, F. ET AL.: "Micro iluidic solvent extraction, stripping, and phase disengagement for high-value platinum chloride solutions", CHEMICAL ENGINEERING SCIENCE, vol. 138, 2015, pages 827 - 833
Attorney, Agent or Firm:
MADDERNS PATENT AND TRADE MARK ATTORNEYS (Adelaide 5001, South Australia, AU)
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Claims:
CLAIMS

1. A method for extracting precious metal ions from a precious metal ion laden aqueous phase having a concentration of greater than 0.1 g/L of one or more precious metal ions, the method comprising: contacting the precious metal ion laden aqueous phase with an organic extractant phase comprising an extractant in an organic solvent that is substantially immiscible with the aqueous phase in a microfluidic separation device under conditions to transfer at least some of the precious metal ions from the aqueous phase to the organic extractant phase to provide a precious metal ion complex laden organic extractant phase; and recovering the precious metal ion complex laden organic extractant phase from the microfluidic device.

2. The method according to claim 1, wherein the conditions to transfer at least some of the precious metal ions from the precious metal ion laden aqueous phase to the organic extractant phase comprise an organic phase to aqueous phase flow rate ratio of from about 0.1 to about 7.

3. The method according to claim 2, wherein the conditions to transfer at least some of the precious metal ions from the precious metal ion laden aqueous phase to the organic extractant phase comprise an organic phase to aqueous phase flow rate ratio of about 0.56, about 1.8, about 2 or about 5.7.

4. A method for scrubbing a precious metal ion complex laden organic extractant phase to reduce the content of unwanted metal ion species therein, the method comprising: contacting the precious metal ion complex laden organic extractant phase with an aqueous phase in a microfluidic separation device under conditions to transfer at least some of the unwanted metal ion species from the organic extractant phase to the aqueous phase to provide a scrubbed precious metal ion complex laden organic extractant phase; and recovering the precious metal ion complex laden organic extractant phase from the microfluidic device.

5. The method according to claim 4, wherein the conditions to transfer at least some of the unwanted metal ion species from the precious metal ion laden organic extractant phase to the aqueous phase comprise an organic phase to aqueous phase flow rate ratio of from about 0.1 to about 7.

6. A method for stripping a precious metal from a precious metal ion complex laden organic extractant phase, the method comprising: contacting the precious metal ion complex laden organic extractant phase with a chloride ion laden aqueous phase in a microfluidic separation device under conditions to transfer at least some of precious metal ions from the precious metal ion complex laden organic extractant phase to the aqueous phase to provide a precious metal ion laden aqueous phase; and recovering the precious metal ion laden aqueous phase from the microfluidic device.

7. The method according to claim 6, wherein the conditions to transfer at least some of precious metal ions from the precious metal ion laden organic extractant phase to the aqueous phase comprise an organic phase to aqueous phase flow rate ratio of from about 0.1 to about 7.

8. The method according to any one of the preceding claims, wherein the micro fluidic separation device comprises a microfluidic substrate comprising an aqueous phase microchannel and an organic phase microchannel, each microchannel comprising an inlet, an outlet and a convergent solvent-solvent contact channel between respective inlets and outlets whereby the aqueous phase microchannel and the organic phase microchannel converge at the solvent-solvent contact channel such that the aqueous phase in the aqueous phase microchannel and the organic phase in the organic phase microchannel flow together in contact with one another in the channel, and exit the device through the respective outlets.

9. The method according to any one of the preceding claims, wherein the microfluidic device comprises a plurality of microfluidic substrates.

10. The method according to claim 9, wherein the plurality of microfluidic substrates are fluidly connected in parallel with a single inlet port for the aqueous phase, a single inlet port for the organic phase, a single outlet port for the aqueous phase, and a single outlet port for the organic phase.

1 1. The method according to any one of claims 1 to 9, wherein the microfluidic device comprises a plurality of microfluidic chips fluidly connected in series.

12. The method according to claim 1 1 , wherein the pressure drop within each microfluidic chip in the series is 1/n where n is the number of microfluidic chips connected in series.

13. The method according to any one of the preceding claims, wherein the precious metal is selected from the group consisting of gold, platinum, palladium, ruthenium, rhodium, palladium, osmium, and iridium.

14. The method according to any one of the preceding claims, wherein the precious metal is platinum.

15. The method according to any one of the preceding claims, wherein the precious metal is palladium.

16. The method according to any one of the preceding claims, wherein the precious metal is rhodium.

17. A precious metal or solution of precious metal ions that has been processed according to any of the preceding claims.

Description:
EXTRACTION OF PRECIOUS METALS

PRIORITY DOCUMENT

[0001 ] The present application claims priority from Australian Provisional Patent Application No.

2014902635 titled "EXTRACTION OF PRECIOUS METALS" and filed on 8 July 2014, the content of which is hereby incorporated by reference in its entirety.

INCORPORATION BY REFERENCE

[0002 ] The following publications are referred to in the present application and their contents are hereby incorporated by reference in their entirety:

International Patent Application No. PCT/AU2009/001086 (WO 2010/022441 ) titled "Extraction Processes" in the name of University of South Australia;

International Patent Application No. PCT/AU201 1/001580 (WO 2012/075527) titled "High Throughput Microfluidic Device" in the name of University of South Australia;

United States Patent No. 4,390,366 titled "Process for the Extraction of Precious Metals from Solutions Thereof in the name of Lea et al;

Chen Q., et al., Journal of Microelectromechanical Systems, 16, 1 193 (2007);

Duffy et al, Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane), Anal. Chem., 70 (23), 4974-4984 (1998);

Hibara, A.; Iwayama, S.; Matsuoka, S.; Ueno, M.; ikutani, Y.; Tokeshi, M.; Kitamori, T., Anal. Chem. 2005, 77, 943;

Nama, D.; Anil Kumar, P. G.; Pregosin, P. S., Magnetic Resonance in Chemistry 2005, 43, 246-

250;

Priest, C; Zhou, J. F.; Sedev, R.; Ralston, J.; Aota, A.; Mawatari, K.; Kitamori, T., International Journal of Mineral Processing 201 1 , 98, 168-173;

Priest, C; Zhou, J. F.; Klink, S.; Sedev, R.; Ralston, J., Solutions Containing Nanoparticles. Chemical Engineering & Technology 2012, 35, 1312-13 19;

Shi J., et al., Applied Physics Letters 91 , 1531 14 (2007); and

Yin, C.-Y.; Nikoloski, A. N.; Wang, M., Minerals Engineering 2013, 45, 18-21.

FIELD

[0003 ] The present disclosure relates to processes for extracting precious metal ion complexes from precious metal ion containing solutions or mixtures, processes for scrubbing solutions containing precious metal ions, and/or to processes for stripping precious metals from solutions containing precious metal ions using microfluidic solvent extraction. BACKGROUND

10004] Microfluidic fluid handling systems boast very high surface-to-volume ratios, which can be exploited in a wide variety of applications. These systems also yield laminar flows and the fluid dimensions are small, which allows fast mass transfer via diffusion. Surface tension forces are large relative to fluid mass and viscous forces and can be exploited to control the fluids. A branch of microfluidics has focused on multiphase systems, where flow of more than one fluid is controlled within microfluidic channels. The applications of multiphase microfluidics include emulsification,

encapsulation, droplet reactors, lenses, and extraction of metal ions.

[00051 Although traditionally a small-volume technology, the advantages of microfluidic systems are equally attractive for higher throughput applications, where modest increases in process efficiency can mean a significant economic advantage. We have previously demonstrated microfluidic solvent extraction of copper(Il) from leach solutions derived from CuO ore (Priest et al., 2012; Priest et al., 201 1). These studies showed that stream-based microfluidic solvent extraction is capable of handling particle-laden leach solutions (i.e. loaded with sub-micron silica particles), without forming crud or blocking the fluidic microchip

10006] More recently, Yin et al. (Yin et al., 2013) used microfluidic solvent extraction to extract very low concentrations (< 20 mg/L) of precious metals using 5% Alamine 336 in kerosene. Their results suggest that extraction proceeds extremely quickly (more than 99% recovery of Pt is achieved after only 1 s contact time in the microchip) but the extraction kinetics is not resolved due to lack of data at shorter times.

[0007] There is a need for microfluidic extraction processes that can be used for the extraction of precious metals on a large scale from industrially or commercially relevant solutions or mixtures that tend to have high concentrations and complex mixtures of precious metals.

SUMMARY

[0008] We have found that precious metals, such as platinum, can be extracted from high concentration solutions (i.e. > 0. 1 g/L) by microfluidic solvent extraction over a range of contact times from 0.5 to 16 s. We have also examined the effect of organic/aqueous ratios on the extractions using a tailored microfluidic substrate design. We have also successfully numbered-up microfluidic substrates by connecting a plurality of microfluidic substrates in parallel or series to achieve higher throughputs, thereby allowing the processes developed to be used on industrial or commercial scales. [0009 ] According to a first aspect, there is provided a method for extracting precious metal ions from a precious metal ion laden aqueous phase having a concentration of greater than 0.1 g/L of one or more precious metal ions, the method comprising: contacting the precious metal ion laden aqueous phase with an organic extractant phase comprising an extractant in an organic solvent that is substantially immiscible with the aqueous phase in a microfluidic separation device under conditions to transfer at least some of the precious metal ions from the aqueous phase to the organic extractant phase to provide a precious metal ion complex laden organic extractant phase; and recovering the precious metal ion complex laden organic extractant phase from the microfluidic device.

[0010] In embodiments of the first aspect, the conditions to transfer at least some of the precious metal ions from the precious metal ion laden aqueous phase to the organic extractant phase comprise an organic phase to aqueous phase flow rate ratio of from about 0.1 to about 7, such as about 0.56, about 1.8, about 2 or about 5.7.

[001 1 ] According to a second aspect, there is provided a method for scrubbing a precious metal ion complex laden organic extractant phase to reduce the content of unwanted metal ion species therein, the method comprising: contacting the precious metal ion complex laden organic extractant phase with an aqueous phase in a microfluidic separation device under conditions to transfer at least some of the unwanted metal ion species from the organic extractant phase to the aqueous phase to provide a scrubbed precious metal ion complex laden organic extractant phase; and recovering the precious metal ion complex laden organic extractant phase from the microfluidic device.

[0012] In embodiments of the second aspect, the conditions to transfer at least some of the unwanted metal ion species from the precious metal ion complex laden organic extractant phase to the aqueous phase comprise an organic phase to aqueous phase flow rate ratio of from about 0.1 to about 7, such as about 3.

[0013] According to a third aspect, there is provided a method for stripping a precious metal from a precious metal ion complex laden organic extractant phase, the method comprising: contacting the precious metal ion complex laden organic extractant phase with a chloride ion laden aqueous phase in a microfluidic separation device under conditions to transfer at least some of precious metal ions from the precious metal ion complex laden organic extractant phase to the aqueous phase to provide a precious metal ion laden aqueous phase; and recovering the precious metal ion laden aqueous phase from the microfluidic device.

[0014] In embodiments of the third aspect, the conditions to transfer at least some of precious metal ions from the precious metal ion complex laden organic extractant phase to the aqueous phase comprise an organic phase to aqueous phase flow rate ratio of from about 0.1 to about 7, such as about 1.8. [0015 J In embodiments of the first second and third aspects, the microfluidic separation device comprises a microfluidic substrate (sometimes referred to herein as a "microchip") comprising an aqueous phase microchannel and an organic phase microchannel, each microchannel comprising an inlet, an outlet and a convergent solvent-solvent contact channel between respective inlets and outlets whereby the aqueous phase microchannel and the organic phase microchannel converge at the solvent-solvent contact channel such that the aqueous phase in the aqueous phase microchannel and the organic phase in the organic phase microchannel flow together in contact with one another in the channel, and exit the device through the respective outlets.

[0016] In embodiments, the microfluidic device comprises a plurality of microfluidic substrates fluidly connected in parallel with single inlet and outlet ports for the aqueous phase, a single inlet and outlet ports for the organic phase.

[0017] In other embodiments, the microfluidic device comprises a plurality of microfluidic substrates fluidly connected in series. The pressure drop within each microfluidic substrate in the series may be 1/n where n is the number of microfluidic substrates connected in series. For example, two microfluidic substrates may be connected in series with half the pressure drop within the first microfluidic substrate and the remaining half in the second one.

[0018] In embodiments of the first second and third aspects, the precious metal is selected from the group consisting of gold, platinum, palladium, ruthenium, rhodium, osmium, and iridium. In specific embodiments, the precious metal is platinum. In other specific embodiments, the precious metal is palladium. In still other specific embodiments, the precious metal is rhodium.

[0019] In a fourth aspect, there is provided a precious metal or solution of precious metal ions that has been processed according to any of the first, second or third aspects of the invention.

BRIEF DESCRIPTION OF DRAWINGS

10020] Embodiments of the present disclsoure will be discussed with reference to the accompanying drawings wherein:

[0021 ] Figure 1 (a) is an illustration of the borosilicate glass microfluidic device, with (b) symmetrical channel cross-section, and (c) tuned to control the flow rate ratio;

[0022] Figure 2 is a plot of extraction versus time for three organic phase/aqueous phase flow rate ratios (· R = 0.56;▲ R = 2.1 ;♦ R = 5.7). Exponential decay fitting to the data in the form of [Pt] = ae ~kt + β is shown (curves); [0023] Figure 3 shows plots of (a) Pt concentration in the raffinate versus contact time for a single microfluidic substrate and three microfluidic substrate module showing good agreement. Inset: Image of the three microfluidic substrate module ( = single chip;■ = multichip); and (b) flow rate versus feed pressure for a single microfluidic substrate and multi- microfluidic substrate module, showing a 2.4x increase in throughput for the multi- microfluidic substrate module (A = single chip (y=0.0061x, R 2 =0.966);■= multichip (y=().0145x, R 2 =0.996));

[0024 ] Figure 4 is a plot showing theoretical calculations of throughput limits for inlet/outlet tubing geometries and lengths, "a" and "b" refer to the length of the inlet and outlet tubing, respectively, and "c" is the radius of the tubing. The asymptotes indicated theoretical limits for the throughput achievable using increasing numbers of microfluidic substrates, wherein

- - - a/b;c = 34/41 cm; 125 um

a/b;c = 100/100 cm; 125 um

a/b;c = 10/10 cm: 125 um

- - a/b:c = 30/30 cm; 250 um

- - a/b;c = 34/41 cm; 500 um

Excluding Tube Resistance

k radius = 250 um

* radius = 125um and the inset plot shows experimental results compared with the theory ( -and ■ ■ , respectively);

[0025] Figure 5 is a schematic showing the experimental setup of a two stage extraction process;

[0026] Figure 6 is a plot showing the flow rate of aqueous and organic phases in a single microfluidic substrate and two microfluidic substrates connected in series plotted against the pressure drop over one microfluidic substrate (♦ = single chip (aq),■ = single chip (org), A = series (aq), x = series (orgl or org2);

[0027] Figure 7 is a plot showing the extraction of platinum in a single microfluidic substrate and in two microfluidic substrates connected in series plotted against contact time (contact time in the two microfluidic substrate series does not include the residence time in connecting tubing, i.e. outside of the contact zones) (■ = single chip, · = two chip series);

[ 0028] Figure 8 shows plots for the results for bulk stripping experiments at equilibrium: (a) concentration of Pt in aqueous phase; and (b) percentage of Pt stripped from the organic phase for different organic phase/aqueous phase ratios;

[ 0029] Figure 9 is a plot showing the percentage recovery of Pt (strip efficiency) for different contact times in the microfluidic substrate; [0030 ] Figure 10 is a schematic diagram showing the theoretical cross-section for a R=l .8 microchannel configuration;

[0031 ] Figure 11 is a schematic showing the general microchannel shape that results from the DRlE/wet etch method; and

[0032] Figure 12 is a photomicrograph of a DRIE/wet etched microfluidic substrate seen from above (left) and at a slight angle to show guide structure (right).

DESCRIPTION OF EMBODIMENTS

[0033] Provided herein is a method for extracting a precious metal from a precious metal ion laden aqueous phase having a concentration of greater than 0.1 g/L of one or more precious metal ions. The method comprises contacting the precious metal ion laden aqueous phase with an organic extractant phase comprising an extractant in an organic solvent that is substantially immiscible with the aqueous phase in a microfluidic separation device under conditions to transfer at least some of the precious metal ions from the aqueous phase to the organic extractant phase to provide a precious metal ion complex laden organic extractant phase; and recovering the precious metal ion complex laden organic extractant phase from the microfluidic device.

[0034] As used herein, the term "precious metal", and variants thereof, means one or more of gold and the platinum group metals platinum, palladium, rhodium, iridium, ruthenium, and osmium, the term "precious metal ion" means an ion of a precious metal, and the term "precious metal ion complex" means an organic or inorganic metal ion complex comprising an ion of a precious metal.

[0035] As used herein, the term "microfluidic", and variants thereof, means that the chip, substrate, device or apparatus contains channels for containing one or more fluids that are typically of nanometre to micrometre dimensions or channels of larger dimensions but containing fluid control features that are of nanometre to micrometre dimensions.

[0036] Precious metals, such as platinum, are typically extracted from precious metals containing concentrates or residues using bulk solvent extraction, ion exchange, and/or chromatography. As described, we have developed a process for extracting precious metals on industrial scales using microfluidic extraction. Microfluidic extraction systems boast very high surface-to-volume ratios and increases in process efficiency can provide significant economic advantages.

[0037] The precious metal ion laden aqueous phase can be any aqueous composition containing precious metal ions of interest, such as a concentrates or residues from primary or secondary precious metal refining. The concentration of precious metal ions in the aqueous feedstock may be from about 0.1 g/L to about 60 g/L, such as about 0. 1 g/L, about 0.2 g/L, about 0.3 g/L, about 0.4 g/L, about 0.5 g/L, about 0.6 g/L, about 0.7 g/L, about 0.8 g/L, about 0.9 g/L, about 1 g/L, about 2 g/L, about 3 g/L, about 4 g/L, about 5 g/L, about 6 g/L, about 7 g/L, about 8 g/L, about 9 g/L, about 10 g/L, about 11 g/L, about 12 g/L, about 13 g/L, about 14 g/L, about 15 g/L, about 16 g/L, about 17 g/L, about 18 g/L, about 19 g/L, about 20 g/L, about 21 g/L, about 22 g/L, about 23 g/L, about 24 g/L, about 25 g L, about 26 g/L, about 27 g/L, about 28 g/L, about 29 g/L, about 30 g/L, about 31 g/L, about 32 g/L, about 33 g/L, about 34 g/L, about 35 g/L, about 36 g/L, about 37 g/L, about 38 g/L, about 39 g/L, about 40 g/L, about 41 g/L, about 42 g/L, about 43 g/L, about 44 g/L, about 45 g/L, about 46 g/L, about 47 g/L, about 48 g/L, about 49 g/L, about 50 g/L, about 51 g/L, about 52 g/L, about 53 g/L, about 54 g/L, about 55 g/L, about 56 g/L, about 57 g/L, about 58 g/L, about 59 g/L, or about 60 g/L. In some embodiments, the concentration of precious metal ions in the aqueous feedstock is about 24 g/L.

[0038] The precious metal ion laden aqueous phase may have an appreciable concentration of inorganic anions, such that metals present can exist as the corresponding complex. In some embodiments, the inorganic anion is selected from the group consisting of: halide ion, thiocyanate ion, thiosulfate ion, nitrate ion, and perchlorate ion. In some embodiments, the halide ion is selected from iodide, bromide, chloride, and fluoride. In some specific embodiments, the halide ion is chloride. Thus, the precious metal ion laden aqueous phase can be treated with hydrochloric acid, sodium chloride or lithium chloride in order to provide a desired chloride ion concentration in the aqueous phase. The concentration of chloride ions in the aqueous phase may be from about 1M to about 13.6M.

10039] The organic extractant phase comprises an extractant that binds the precious metal ion of interest to form a precious metal ion complex. Suitable extractants for platinum ions include secondary amines (e.g. Amberlite), trioctyl phosphine oxide (TOPO), methyl isobutyl ketone (MIBI), tributyl phosphate (TBP), tertiary amines (e.g. trioctylamine), and diethyldithiocarbamic acid.

[0040] Suitable extractants for palladium include oximes such as any one of the LIX series (e.g. LIX 63, 64, 64N, 65, 70, 71 and 73 available from General Mills Corporation, Minneapolis, USA), substituted pyridines (e.g. those disclosed in Canadian Patent No 1337738), alkyl sulphides (e.g. di-n-octyl sulphide, di-n-hexyl sulphide, methyl-n-decyl sulphide, tertiary-butyl-decyl sulphide), trioctyl phosphine oxide (TOPO), methyl isobutyl ketone (MIBI), trioctylamine, and diethyldithiocarbamic acid.

[0041 ] Suitable extractants for rhodium include 8-hydroquinoline derivatives (e.g. Kelex 100™ from Sherex Chemical Co., Ohio, USA), tri-alkylphosphine oxides, aliphatic amines, and aromatic amines.

[0042] Suitable extractants for ruthenium include dialkyl-dithiophosphoric acids (e.g. di-(2-ethylhexyl)- dithiophosphoric acid). [0043] Suitable extractants for iridium include tributyl phosphate (TBP).

[0044] The organic extractant phase also comprises an organic solvent that is substantially immiscible with the aqueous phase. Suitable solvents include hydrocarbon and/or aromatic solvents. The solvent may be selected from the group consisting of: toluene, xylene, cumene, decane, kerosene (or individual components therein), ligroin, cyclohexane, methyl cyclohexane, and solvent naptha. Solvent naphtha (e.g. Solvesso 150) and/or refined kerosene (e.g. Shellsol 2046) are particularly suitable.

[0045] The skilled person will appreciate that one or more of the precious metals can be selectively extracted from an aqueous phase comprising different precious metals by adjusting one or more of the pH of the aqueous phase or the organic extractant phase, altering the oxidation state one or more of the precious metals, etc. A process for sequentially and selectively separating platinum group metals from aqueous phases containing them is disclosed in United States Patent No 4,390,366. Based on the information provided herein, the skilled person can adapt the teachings of United States Patent No 4,390,366 to carry out the processing using the microfluidic device(s) and process(es) described herein.

[0046] The microfluidic separation device used to extract the precious metal ions from the precious metal ion laden aqueous phase may be as described herein and/or as described in published international patent application WO 2010/022441 titled "Extraction Processes" and/or as described in published international patent application WO 2012/075527 titled "High Throughput Microfluidic Device" and or using any of the microfluidic separation techniques known in the art. The skilled person will appreciate that a variety of microfluidic substrate designs and/or arrangements could be used and the processes described herein are not limited to any specific designs and/or arrangements. Indeed, the design and/or arrangement of the microfluidic substrate may be determined by the metal to be extracted, the nature or properties of the aqueous phase, the nature or properties of the organic extractant phase, processing parameters such as residence time, contact time, organic phase/aqueous phase ratio, etc.

[0047] We have successfully used a microfluidic separation device which comprises a microfluidic substrate comprising an aqueous phase microchannel and an organic phase microchannel. Each microchannel comprises an inlet, an outlet and a convergent solvent-solvent contact channel between respective inlets and outlets whereby the aqueous phase microchannel and the organic phase

microchannel converge at the solvent-solvent contact channel such that the aqueous phase in the aqueous phase microchannel and the organic phase in the organic phase microchannel flow together in contact with one another in the channel in a pressure driven, co-current laminar flow. The aqueous phase and the organic phase converge at a junction downstream of the inlets and upstream of the solvent-solvent contact channel, pass along the solvent-solvent contact channel and then diverge at a junction downstream of the solvent-solvent contact channel and exit the device through the respective outlets. [0048 ] The microfluidic substrate may be formed from any suitable material. Materials suitable for the manufacture of microfluidic substrates are known in the art and may be chosen based on considerations such as cost, inertness or reactivity toward fluids and other materials that will be in contact with the microfluidic substrates. Non-limiting examples of suitable materials include metal (e.g. stainless steel, copper), silicon, glass, quartz, and polymers. Suitable polymers include polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), other perfluoropolyether (PFPE) based elastomers,

polymethylmethacrylate (PMMA), polycarbonate (PC), high and low density polyethylene (PE), silicone, and the like. These polymers may have additives that change the chemical resistance or physical integrity of the polymer to suit specific aqueous or organic phase solutions. The microfluidic substrate(s) can be any shape in plan view, including circular, square, rectangular, etc.

[0049] The microchannels and/or any other microfluidic features on the microfluidic substrate can be formed on a surface of the microfluidic substrate using any of the techniques for forming fluid microchannel networks that are known in the art. For example, the microchannels can be fabricated using standard photolithographic and etching procedures including soft lithography techniques (e.g. see Shi, et al., 2007; Chen, et al., 2007; or Duffy et al., 1998), such as near-field phase shift lithography, microtransfer molding, solvent-assisted microcontact molding, microcontact printing, and other lithographic microfabrication techniques employed in the semiconductor industry. Direct machining or forming techniques may also be used as suited to the particular device. Such techniques may include hot embossing, cold stamping, injection moulding, direct mechanical milling, laser etching, chemical etching, reactive ion etching, physical and chemical vapour deposition, and plasma sputtering. The junctions in the microfluidic substrate at which the aqueous phase and the organic phase converge and diverge may take any suitable configuration, such as Y-junction, T-junction, etc. The channel arrangement may be side-by- side or top-bottom. The two channels may be prepared by bonding or otherwise sealing the similar or dissimilar materials (eg. borosilicate glass, silica, metal (including stainless steel), polymers

(polymethylmethacrylate, polycarbonate, cyclic olefin copolymer, fluoropolymer, etc.) and ceramics). The flow of the aqueous phase and organic phase may be controlled and stabilised by a guiding geometry. The guiding geometry may be a ridge, valley, channel, groove, discontinuous structure (such as pillars, membrane, or similar porous structure), which prevents mixing or spillage of the aqueous and organic phases.

[0050J Processing parameters that affect the efficiency of extraction of the precious metal ions from the precious metal ion laden aqueous phase to the organic extractant phase include the contact time (i.e. the period of time in which the aqueous phase and the organic phase are in contact with one another) and the flow rate ratio of organic phase to aqueous phase. The contact time is determined, at least in part, by length ("Z,") of the solvent-solvent contact channel. L may be from about 10 mm to about 500 mm. In some embodiments, L is 129 mm. In some other embodiments, L is 155 mm. The contact time ("t ") may be from about 0.5 s to about 16 s may also be longer depending on the channel design. [0051 ] The flow rate ratio of organic phase to aqueous phase ("R") in the microfluidic extraction processes is an important processing parameter and can affect the extraction efficiency. We have developed methods for designing microfluidic substrates that provide organic phase to aqueous phase flow rate ratios of from about R = 0.1 to about R = 7. This is achieved by adjusting the depth and width of the organic phase microchannel relative to the depth and width of the aqueous phase microchannel to provide the desired flow rate ratio, R. This can be achieved by wet etching the two microchannels for different times to achieve the desired microchannel depths and/or profiles. In some embodiments, an organic phase to aqueous phase flow rate ratio of 0.56 is achieved using an organic phase microchannel depth (dorg) of 40μιη and an aqueous phase microchannel depth (d aq ) of 40μηι. In other embodiments, an organic phase to aqueous phase flow rate ratio of 2.0 is achieved using an organic phase microchannel depth (d org ) of 56μιη and an aqueous phase microchannel depth (d aq ) of 40μιη. In still other embodiments, an organic phase to aqueous phase flow rate ratio of 5.7 is achieved using an organic phase microchannel depth (dorg) of 58μηι and an aqueous phase microchannel depth (d aq ) of 30μιη.

[0052 ] We have carried out platinum extractions using a secondary amine in Solvesso 150 with organic phase to aqueous phase flow rate ratios of 0.56, 2.0 and 5.7. Pseudo-first-order kinetics was observed, governed by diffusion of ions to the liquid-liquid interface. The characteristic diffusion time was calculated to be ~ 1 s for PtCl 6 2" . We found that equilibrium was achieved within about 2 s for R = 0.56 and 5.7 and more than 5 s for R = 2.0. A typical organic extractant phase to aqueous phase ratio of about 1.8 may be used in the microfluidic extraction processes described herein.

[0053] We have also found that the extraction processes described herein can be adapted so that they can be used to scrub the precious metal ion complex laden organic extractant phase. Thus, also provided herein is a method for scrubbing a precious metal ion complex laden organic extractant phase to reduce the content of unwanted metal ion species therein. The method comprises contacting the precious metal ion complex laden organic extractant phase with an aqueous phase in a microfluidic separation device under conditions to transfer at least some of the unwanted metal ion species from the precious metal ion complex laden organic extractant phase to the aqueous phase to provide a scrubbed precious metal ion complex laden organic extractant phase; and recovering the scrubbed precious metal ion complex laden organic extractant phase from the microfluidic device.

[0054] Precious metal ion complex laden organic extractant phases are typically scrubbed in order to wash the organic extractant phase prior to stripping, thus removing other metals inadvertently extracted into the organic extractant phase.

[0055 ] The unwanted metal ion species will typically be base metals that are not desired as part of the precious metals refining process. The base metals may be any metals commonly found in combination with precious metals solutions including, for example, copper, nickel, arsenic, cobalt and iron. The unwanted metal ion species could also be a precious metal other than the precious metal of interest.

[0056] Scrubbing may be carried out with an organic phase to aqueous phase flow rate ratio of from about 0. 1 to about 7 and a contact time of from about 0.5 s to about 16 s. In embodiments, the conditions to transfer at least some of the unwanted metal ion species from the precious metal laden organic extractant phase to the aqueous phase comprise an organic phase to aqueous phase ratio of about 3.

[0057] We have also found that the extraction processes described herein can be adapted so that they can be used to strip precious metal ions from the precious metal ion complex laden organic extractant phase. Thus, also provided herein is a method for stripping a precious metal ion from a precious metal ion complex laden organic extractant phase. The method comprises contacting the precious metal ion complex laden organic extractant phase with a chloride ion laden aqueous phase in a microfluidic separation device under conditions to transfer at least some of precious metal ions from the precious metal ion complex laden organic extractant phase to the aqueous phase to provide a precious metal ion laden aqueous phase; and recovering the precious metal ion laden aqueous phase from the microfluidic device.

[0058] The chloride ion laden aqueous phase can be formed using hydrochloric acid, sodium chloride or lithium chloride. The concentration of chloride ions in the chloride ion laden aqueous phase may be from about 0.5M to about 13.6M. Microfluidic solvent extraction may be beneficial in this process due to reduced exposure to concentrated hydrochloric acid than would otherwise occur in bulk stripping processes.

[0059] Stripping may be carried out with an organic phase to aqueous phase flow rate ratio of from about 0.1 to about 7 and a contact time of from about 0.5 s to about 16 s, for example from about 5 s to about 13 s. We have stripped platinum ions from organic extractant phases containing platinum ions and a secondary amine with an organic phase to aqueous phase flow rate ratio of 1.8. Under the conditions used, 58% of Pt was stripped from the organic phase after 13s.

[0060] Advantageously, we have successfully scaled-up the microfluidic device to include a plurality of microfluidic substrates in fluid connection with one another. The viability of microfluidic extraction for higher throughput applications, including for refining of precious metals, depends on the ability to scale- up volumetric throughput while maintaining the microscopic dimensions of the liquid-liquid contact. In the present case, the 'numbering-up' was achieved by parallelization of individual extraction microfluidic substrates. Thus, the microfluidic device may comprise a plurality of microfluidic substrates fluidly connected in parallel with a single inlet port for the aqueous phase, a single inlet port for the organic phase, a single outlet port for the aqueous phase, and a single outlet port for the organic phase. [0061 ] In embodiments, the aqueous phase and organic phase inlet ports are relatively large (e.g. 0.6 mm diameter) and continue through the plurality of microfluidic substrates, so that the hydrodynamic resistance along the ports is negligible compared to that along the microchannels. In this configuration, the flow of organic and aqueous phases is substantially evenly partitioned to all microchannels in the plurality of microfluidic substrates.

[0062 ] The contact time between the organic phase and the aqueous phase may be from about 0.5 s to about 16 s.

[0063 ] The flow rates achieved in these embodiments are higher than those achieved with a single microfluidic substrate. We found that the flow rate increases by a factor of 2.4 for a given feed pressure when comparing a single microfluidic substrate with three microfluidic substrates connected in parallel. Non-linear increases in flow rate as the number of microfluidic substrates is increased is related to the flow resistance caused by increased flow rates in the inlet/outlet tubing. For the microbore tubing we used radii of 125 μηι, the flow rate with three microfluidic substrates connected in parallel increased by a factor of 2.4. Therefore, there is an upper limit of 10 microfluidic substrates, above which adding more microfluidic substrates is not expected to improve the total flow rate significantly. However, a relatively small increase of r tube to 250 μιη resulted in a flow rate increase with three microfluidic substrates connected in parallel of 2.9, i.e. close to the ideal of 3. Furthermore, an increase of r tube to 500 μιη permits up to 1000 microfluidic substrates to be connected in parallel and a flow rate of about 1 L/h.

[0064] We also carried out extractions using two extraction microfluidic substrates in series. The microfluidic substrates were connected to one another in scries and the pressure drop within each microfluidic substrate in the series was I/n where n is the number of microfluidic substrates connected in series. For example, two microfluidic substrates were connected in series with half the pressure drop within the first microfluidic substrate and the remaining half in the second one. This allows for the flow of a continuous stream of aqueous phase through two extraction stages. Thus, two fresh streams of the organic phase were used to extract the single aqueous phase.

EXAMPLES

[0065 ] Example 1 - Extraction and throughput for micro-solvent extraction of platinum using a single microfluidic substrate

[0066] Aqueous platinum solutions (24 g/L) were prepared by dissolving sodium hexachloroplatinate (IV) (Johnson Matthey) in 0.5 M HC1. Pt 4 ' was extracted using 50 - 100 g/L of a secondary amine in Solvesso 150 (ASCC). Bulk solvent extractions were carried out in a 5 mL glass vial by vigorously shaking the organic and aqueous phases together for 20 s and leaving them to separate. Microf luidic solvent extractions were carried out using the conditions described in Priest et al. (Priest, et al. 2012; Priest, et al. 201 1 ) with custom fabricated microfluidic substrates (ANFF-SA, Australia). In the microfluidic substrates, two microchannels (dimensions given in Table 1) merge at a Y-junction to fonn a single solvent-solvent extraction channel that is divided into two by a guide structure which helps stabilise the co-flowing streams. This is where extraction takes place. The contact length in the solvent- solvent extraction channel, L, was 129 mm (single microfluidic substrate) or 155 mm (multi- microfluidic substrate). Phase disengagement occurred at a downstream Y-junction. Different volumetric ratios, R, were achieved by wet etching the two microchannels for different times to achieve the microchannel profiles shown in Figure 1 , as described by Hibara et al. (Hibara, et al., 2005). Flow was driven by pressure pumps (Mitos P-pump, Dolomite) between 400 and 50 kPa. Flow stability in the microfluidic substrate was monitored using optical microscopy (Olympus, BH2-UMA). UV— vis absorption (Ocean Optics QE65000) was used to determine the concentration of Pt 4+ using a Z-Flow Cell (2.5 mm path length, quartz windows). Absorbance was measured at 259 nm for the Pt 4t and used according to Beer's law.

[0067] Platinum extraction was studied for three different organic to aqueous flow rate ratios.

Equilibrium was achieved within ~ 2 s for two of the designs and more than 5 s for the third, in agreement with fast kinetics expected for the ion exchange mechanism.

[0068 ] The microfluidic substrate design is shown in Figure 1 . Key features are the etch depth for each microchannel, d aq and d ors ,, meniscus height, h, and contact length between the two microchannels, L. Based on these dimensions and any given flow rate, the contact time, t, available for extraction can be calculated. Tuning the respective etch depths of the microchannels allows the flow rate ratio, R, to be varied. At the same time, the contact area (A = hL) of the liquid-liquid interface is altered. Table 1 gives actual and calculated A, R, and the surface-to-volume ratio for the three microfluidic substrates, based on ideal (isotropic) wet etching.

10069] Assuming laminar flow, the flow rate ratio, R, can be estimated using the Hagen-Poiseuille equation;

_ 8μ α¾ , _ ^ org h

~ i 1 r 1 r ' a 4 q ~ π 11 τ 1 · o 4 rg ^ or 9 1 '

[0070] where Q is volumetric flow rate, μ is dynamic viscosity, L is the length of the contact zone, r is the effective radius of a cylindrical microchannel, and P is the hydrodynamic pressure drop in the liquid over the length L. Subscripts aq and org refer to the aqueous and organic phases, respectively. Equating the feed pressures gives [0071] As the microchannel cross-section is not cylindrical, we employ an effective radius based on the hydraulic diameter of the microchannels and assume a rectangular cross-section, i.e. , r = ^j^, where x and y are the width and height of the microchannel. This approach yields R = 0.56, 2.05, and 7. 10 for the three microfluidic substrates. As the isotropic etching of the microchannels leads to a rather consistent

2

geometry, we can define x = 2y for sufficiently large etch depths. It follows that r = - y, which is a useful approximation for the effective radii in Eqn. 2. In our work, we achieved R = 0.56, 2.0, and 5.7 for designs 1 , 2, and 3 (see Table 1), in good agreement with the estimated values, despite the sensitivity of R to small differences in etch depths.

[0072] Table 1. Estimated and actual organic/aqueous flow rate ratios, R, and equilibrium contact times, l eqm-

based on D = 1 () ~9 πι /s and L = ( I Q + 2r a ) μιη.

[0073 ] Microfluidic solvent extraction of platinum was carried out using the three different microchannel designs for / from 0.5 to 8 s. The progress of the extraction is shown in Figure 2 for each R and can be described well by fitting an exponential decay curve in the form of [Pt] = ae ~kt + β, where β is the equilibrium concentration of Pt, / is the contact time, k is the pseudo-first-order rate constant, and a is the maximum change in concentration at equilibrium. In Table 1 , the actual and estimated contact times required to achieve equilibrium, t eqm , are given for each microfluidic substrate design. The estimated times are calculated assuming a diffusion length equal to the aqueous stream width, i.e., 10 μηι + 2r a , and a diffusion coefficient, D = 7 x 10 "9 m 2 /s (Nama et aL, 2005). The actual t eqm is in reasonably good agreement for the microfluidic substrate designs 1 (R = 0.56) and 3 (R = 5.70), although the diffusion times appear to underestimate the actual contact times required. The timescales involved are consistent with rapid, diffusion-limited kinetics which are characteristic of the ion exchange mechanism.

[0074] Bulk solvent extractions were carried out in parallel to compare the raffinate concentration of Pt with that achieved in the microfluidic substrate. The raffinate [Pt] after 20 s bulk mixing (i.e. at equilibrium) for R = 0.6, 2.0, and 6.0 was 15.8, 0.9, and 0.14 g/L, respectively, which agrees with the microfluidic solvent extraction results shown in Figure 2. This result is consistent with the expectation that the physical chemistry of the extraction is unchanged by confinement in the microchannel.

[0075] Example 2 - Extraction and throughput for micro-solvent extraction of platinum using a midti- microfluidic substrate module

[0076] The viability of microfluidic solvent extraction for higher throughput applications, including for refining of precious metals, depends on the ability to scale-up volumetric throughput while maintaining the microscopic dimensions of the liquid-liquid contact. So-called 'numbering-up' can be achieved through parallelization of individual extraction microfluidic substrates. Thus, we assembled three microfluidic substrates in parallel with a single inlet for the aqueous phase and another for the organic phase. These inlet ports were large (0.6 mm diameter) and continued through the entire ~ 10 mm thick 'stack' of microfluidic substrates, so that the hydrodynamic resistance along the inlet port was negligible (six orders of magnitude smaller) compared to that along the microchannel itself. In this configuration, even partitioning of the flow to all channels in the module is expected.

[0077] The flow behavior, extraction efficiency, and throughput in the multichip module was determined for contact times between 1.5 and 4.5 s. Due to the spatial separation of the microfluidic substrates within the stack, we were able to use optical microscopy to focus through the upper microfluidic substrates and observe the flow stability and phase disengagement. No visual difference between the flow behavior in the top, middle, and lower microfluidic substrates was observed with respect to the single microfluidic substrate experiments. Figure 3(a) gives the raffinate [PtJ concentration versus contact time for the single microfluidic substrate and multi- microfluidic substrate module. The results are in good agreement over the range of contact times studied, as one would expect for even and stable partitioning of the liquids.

[0078] Figure 3(b) plots the flow rate against the feed pressure from the pumps. Clearly, the flow rates achieved are much higher in the multi- microfluidic substrate module; however, from the slopes of the linear fits, we found that the flow rate increases by a factor of 2.4, rather than 3, for a given feed pressure and switching from a single microfluidic substrate to multi- microfluidic substrate module. This behavior is related to the flow resistance caused by increased flow rates in the inlet/outlet tubing. For a given feed pressure, the relationship between the number of microfluidic substrates in the module, «, and ratio

Qaq {multichip) between the aqueous phase flow rate in the module and single microfluidic substrate, γ

Q aq (single chip)' is given by and in the t

2.5, which is close to the experimental value of 2.4. The choice of inlet/outlet tubing (total length and diameter), is clearly an important engineering consideration for numbering-up. For the microbore tubing we used, there is a practical limit of γ ~ 10 where adding more microfluidic substrates doesn't improve the total flow rate significantly. In contrast, a small increase of r tube to 500 μτη, permits y→ 1000, or ~ 1 L/h, before this limit is reached.

[0079] Thus, higher throughputs were demonstrated using a three- microfluidic substrate microfluidic solvent extraction module. The flow and kinetics were unchanged from a single microfluidic substrate. An expression for the relationship between the number of microfluidic substrates and flowrate is given, which accounts for upstream and downstream pressure drops.

[0080] Using microbore inlet/outlet tubing, it was shown that the maximum throughput limit was reached for a stack of only about lOx the flow in a single microfluidic substrate (Figure 4), which is insufficient for the envisaged application. Nonetheless, a small increase in the diameter of the inlet/outlet tubing makes it possible to achieve throughputs of litres/hour.

[0081 ] To test the coupling of stages, experiments were carried out using two extraction microfluidic substrates in series. The microfluidic substrates were connected to one another in series with half the pressure drop within the first microfluidic substrate and the remaining half in the second one. This allowed for the flow of a continuous stream of aqueous phase through two extraction stages. Thus, two fresh streams of the organic phase were used to extract the single aqueous phase. The experimental setup is illustrated below in Figure 5. Figure 6 shows the flow rates achieved for a range of pressure drops applied to a single microfluidic substrate (the overall pressure drop for the two- microfluidic substrate series is double these values). The coupling of microfluidic substrates in series did not cause any unexpected flow resistance and R is maintained at approximately 2. Unlike the numbering-up example (Figure 4), the tubing connecting the microfluidic substrates does not appear to play a significant role.

[0082] Figure 7 compares the extraction efficiency for different contact times in the single microfluidic substrate and two- microfluidic substrate series. It appears that, at least for R = 2, the results for the two systems overlap but other configurations may provide greater benefit. Benchmarking experiments may be carried out, where only one stream of organic phase passes through both microfluidic substrates. This assessment may differ for different flow rate ratios (see Figure 2).

[00831 Example 3 - Scrubbing of organic solvents [0084] Scrubbing studies were carried out using the borosilicate glass microfluidic substrates that were previously used for extraction in Example 1.

[0085] Scrubbing of the Pt loaded organic phase is used to wash the Pt loaded organic phase prior to stripping, thus removing other metals inadvertently extracted into the organic phase. In the present work, this step was carried out only to evaluate losses of Pt from the loaded organic phase.

[0086] The bulk scrubbing experiments showed a consistent loss of Pt to aqueous phase of below 0.5 g/L Pt, or less than 4% of the total Pt loaded in the organic phase, for all values of R studied (0.125 < R < 2). As one would expect, the loss of Pt from the organic phase was very similar for scrubbing carried out in the microfluidic substrate for contact times varying from 1 s to 8 s.

[0087 ] Example 4 - Stripping of platinum from organic solvents

[0088] Stripping experiments were carried out using the borosilicate glass microfluidic substrates that were previously used for extraction in Example 1.

10089] The stripping of platinum from a secondary amine through CI " ion exchange was carried out using acidic aqueous phases. Microfluidic substrates may be beneficial due to reduced exposure to concentrated HQ. For stripping, a solution of Pt-loaded secondary amine in Solvesso 150 was prepared by vigorously mixing 50 ml of aqueous Pt solution (24 g/L, 0.5M HQ ) with 90 ml of the secondary amine solution (50 - 100 g/L, Solvesso 150). The ratio of 1.8 was used in the extraction step to ensure maximum extraction of Pt (for a single stage extraction) and keep volumetric ratios consistent with the industrial ratio. The concentration of Pt in the aqueous phase was depleted to 0.64 ± 0.01 g/L, so that the concentration of Pt in the organic phase was 12.98 g/L. The resulting Pt-loaded secondary amine solution (organic phase) was used in the following stripping experiments. Bulk stripping experiments were performed by manually shaking the Pt-loaded organic phase with concentrated hydrochloric acid ( 13.6 M) for 30 s. Microfluidics stripping experiments were earned out at the same organic/aqueous (flow) ratio in the standard microfluidic substrates used for the extractions in Example 1. In both cases, the concentration of Pt in the aqueous phase was measured using UV-vis spectroscopy.

[0090] For bulk extractions where the organic/aqueous ratio, R, can be varied easily, a linear correlation between the organic/aqueous ratio and the concentration of Pt present in the aqueous phase after stripping was observed, Figure 8(a). This corresponds to stripping of approximately 60-70% of the Pt from the organic phase, irrespective of the choice of R, Figure 8(b).

[0091 ] For microfluidic stripping experiments, the organ ic/aqueous ratio was fixed at R = 1.8 as required for flow stability in the existing microfluidic substrate design. The results for the microfluidics experiments are shown in Figure 9. As the ion exchange is diffusion controlled, stripping kinetics are expected to be fast (note that extraction occurred within 5 s, in very good agreement with the

characteristic diffusion time for the ions) and, in these experiments, we observe an increase in Pt concentration up to 12 s - significantly longer than the reverse process (extraction). It was suspected that the scatter in the measurements was due to allowing insufficient time to reach steady state in some experiments. Considering the percentage of Pt recovered from the organic phase, 58% is recovered after 13s which is slightly less than that achieved at equilibrium for the bulk stripping experiments, i.e. the microfluidics stripping experiments did not reach equilibrium using the combination of microfluidic substrates and flow rates studied. From these results it is clear that stripping is slower than extraction, which will need to be taken into account when designing integrated microfluidic circuits.

[0092] Example 5 - Fabrication of microfluidic substrates for different flow ratios

[0093] The organic/aqueous ratios of interest to this work are the industrially relevant ratios of 1.8 for extraction, 3 for scrubbing and 2.3 for stripping.

[0094] Typically, the microfluidic substrates used herein are prepared in a one-step wet (hydrofluoric acid) etch, such that the microchannels have the same depth and the width, while variable, cannot be changed to achieve the desired flow ratios. After evaluating several alternative fabrication methods available for achieving these flow ratios in a microfluidic substrate, two methods were identified: A two- step wet etch or a combined plasma and wet etch.

[0095] The first method used is a two stage wet etch process that is able to adjust the dimensions of the aqueous and organic phase channels by independently controlling the etch time. In this case, one microchannel is first etched (by selective photolithography exposure) for a given time, followed by both microchannels being etched together. Ratios of 1.8, 3 and 2.3 can be achieved by etching one microchannel to 60 μιη (larger than the current etch of 40 μιη) and the second microchannel to 32, 24 or 28 μιη, respectively. Illustrated in Figure 10 is an example of the cross-section for R = 1.8, i.e. 32 μιη etch for the second microchannel. However, the other two ratios are as easily achievable due to the significant overlap of the etching profiles near the guide structure.

[0096] The second method is the sequential use of DR1E (deep reactive-ion etching) and wet (HF) etching of borosilicate glass slides to give microchannels of varying sizes and, potentially, a more stable interface. In this case, two microchannels are first DRI etched according to the microchannel width used for the mask (etching is anisotropic; directional). The two microchannels are then joined by a small amount of wet etching, which also determines the size of the guide structure. We observed that a 10 and 30 μι width mask for the microchannels will give rise to different depth primary channels due to the different rate of etching and, therefore, can be used to control the cross-sectional areas of the respective microchannels. The final microchannels differ from purely wet etched (shallow, curved) microchannels to be distinctly trapezoidal shaped with a sharper and, thus, more effective guide structure, as seen in Figures 1 1 and 12.

[0097] A full set of extraction results for various organic/aqueous flow rate ratios from 0.56 to 7 was obtained, using a two-step wet etch of microchannels in glass microfluidic substrates. The results show extraction equilibrium takes several seconds and shorter times, i.e. 1 s, for large excesses of extractant or platinum in the system.

[0098] The organic/aqueous ratios, R, of interest to this work are the industrially relevant ratios of 1.8 for extraction, 3 for scrubbing and 2.3 for stripping. Each of the methods described above can be used to achieve this. Both methods have been tested and were successful, however the wet-wet etch method gave the most freedom in terms of varying R in the resultant microfluidic substrate. Several microfluidic substrates were fabricated, based on different etch times (and therefore final dimensions) for the two microchannels that contact the organic and aqueous phases.

[0099 ] Thus, using the two stage wet-wet etch method, microchannels of various sizes can be fabricated as illustrated in Figure 1. These sizes can range from a 1 : 1 ratio as in Figure 1(b) to very high theoretical ratios when a configuration in Figure 1(c) is employed.

[00100] Using the Hagen-Poiseuille equation is useful to calculate the theoretical flow rate ratio.

This equation is, however, based on a perfect cylindrical tube and hence a large over estimation of the rate ratio is seen when compared with the practical experimental outcomes from this work. We have found that using the theoretical hydraulic radius gives a much better estimation of the rate ratio, as one might expect. For the current mask design, theoretical rate ratios ranging from 0.56 to, for example, 10 could be achieved, which is more than adequate for applications in precious metals refining.

[00101 ] Five microfluidic substrates with ratios ranging from 0.5 to 5.7 were tested and the extraction results are given in Figure 2. As can be seen for the intermediate ratios (red and green data), equilibrium is reached at around 4 to 5 seconds. For R = 0.5, the extractant in the organic phase is fully loaded (there is an excess of platinum in the system) with platinum after only 1 s. For R = 7, there is a large excess of extractant in the system and effectively all of the platinum is extracted to within 2 or 3 s. As is the case for bulk extraction, R ~ 2 appears to be the most suitable, based on the economics of solvent/extractant inventory.

Prophetic Example 5 - Anticipated microfluidic substrate design [00103] New microfluidic substrate designs covering various exit angles at the outlet Y-j unction and microscale geometries that should enhance the Laplace pressure window at the liquid-liquid interface close to the disengagement location can be studied. The designs have exit angles of 30, 60, 90 and 180° and an alternative design which aims to 'bleed' the organic or aqueous phases, thus reducing any instabilities related to hydrodynamic effects.

[00104] It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

[00105] Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

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[001 14] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any fonn of suggestion that such prior art forms part of the common general knowledge.