MICROFLUIDIC DEVICE AND METHOD FOR ANALYSIS OF A PARTICULATE SAMPLE

Abstract

The present invention relates generally to devices able to manipulate, process, treat, sort, measure and/or analyse samples at a micro level, commonly referred to as microfluidic devices. In particular, the present invention relates to a microfluidic device that can be used for the analysis of particulate samples, such as by the leaching at a micro level of a crushed rock particulate sample from a mineral ore body and the subsequent analysis of the leachate. The present invention also relates to a method for the use of a microfluidic device for the analysis of a particulate sample.

Claims

1. A microfluidic device for analysis of a particulate sample, the device including at least one upper sample chamber with a reagent inlet and a sealable upper opening for loading sample in the sample chamber, and at least one lower flow chamber with an analyte outlet, wherein: a) the sample chamber includes a fluid pervious floor upon which, in use, the sample will rest; and b) the flow chamber includes spaced upright members therein, the upright members having upper surfaces, at least a portion of the upright surfaces together forming the fluid pervious floor of the sample chamber, with the spaces between the upright members forming microchannels in fluid communication with the analyte outlet.

2. A device according to claim 1, wherein the upright members are an array of micropillars, in the form of individual columnar members with either a circular, square, rectangular, oval or other suitable cross-section, whereby the spaces between the micropillars form a regular series of microchannels therebetween.

3. A device according to claim 1, wherein the upright members are a random or ordered series of micro-walls or micro-ridges, between which suitable microchannels are formed that permit a continuous flow of liquid therethrough.

4. A microfluidic device according to claim 1, including one upper sample chamber and one flow chamber, at least a portion of the upper surfaces of the upright members in the flow chamber forming the fluid pervious floor of the sample chamber.

5. A microfluidic device according to claim 4, wherein the area of the sample chamber is the same or less than the area of the flow chamber.

6. A microfluidic device according to claim 4, wherein the area of the flow chamber is from about 40 mm.sup.2 to about 100 mm.sup.2.

7. A microfluidic device according to claim 4, wherein the volume of the sample chamber is from about 50 microlitres to about 800 microlitres.

8. A microfluidic device according to claim 1, including multiple upper sample chambers, each with a fluid pervious floor and a reagent inlet, and a single flow chamber, at least a portion of the upper surfaces of the upright members in the flow chamber forming the fluid pervious floors of the sample chambers.

9. A microfluidic device according to claim 8, wherein the total area of all sample chambers is less than the area of the flow chamber.

10. A microfluidic device according to claim 8, wherein the area of the flow chamber is from about 40 mm.sup.2 to about 100 mm.sup.2.

11. A microfluidic device according to claim 8, wherein the total volume of all sample chambers is from about 50 microlitres to about 800 microlitres.

12. A microfluidic device according to claim 1, including multiple upper sample chambers, each with a fluid pervious floor and a reagent inlet, and multiple flow chambers, each with upright members and microchannels, one sample chamber being in fluid communication with one flow chamber, the flow chambers being in fluid communication with the analyte outlet either in series or in parallel.

13. A microfluidic device according to claim 12, wherein the area of one sample chamber is the same as the area of the flow chamber that it is in fluid communication with.

14. A microfluidic device according to claim 12, wherein the total area of all flow chambers is from about 40 mm.sup.2 to about 100 mm.sup.2.

15. A microfluidic device according to claim 12, wherein the total volume of all sample chambers is from about 50 microlitres to about 800 microlitres.

16. A microfluidic device according to claim 1, wherein the upright members are micropillars and the height of the micropillars is between about 1 and 100 micrometres, the size of the micropillars is between about 1 and 100 micrometres, and/or the spacing between the micropillars is between about 1 and 100 micrometres.

17. A microfluidic device according to claim 1, wherein the sealable upper opening of the or each sample chamber is removable.

18. A microfluidic device according to claim 1, including one or more integrated detection devices and/or one or more integrated analysis device.

19. A microfluidic device according to claim 18, wherein the integrated detection devices and integrated analysis devices include optical absorbance, fluorescence, transmission, Raman or emission spectroscopy, or electrochemical sensors, including redox, impedance or conductivity sensors, or the like, or upon refractive index.

20. (canceled)

21. A method of analysing a particulate sample using a microfluidic device, the method including the steps of: a) loading a particulate sample into a sealable upper opening of an upper sample chamber of the device, to rest upon a fluid pervious floor of the sample chamber; b) passing reagent through a reagent inlet in the sample chamber to flow through the device and react with the sample to form an analyte; c) passing analyte and unreacted reagent through the fluid pervious floor into a lower flow chamber of the device, the flow chamber including spaced upright members therein, the upright members having upper surfaces that together form the fluid pervious floor of the sample chamber, with the spaces between the upright members forming microchannels in fluid communication with an analyte outlet in the flow chamber; and d) passing analyte and unreacted reagent through the microchannels and out the analyte outlet for subsequent analysis.

22. A method according to claim 20, wherein the particulate sample is a mineral ore, soil, a chemical, biological material, or a pharmaceutical.

23. A method according to claim 20, wherein the particulate sample is a rock sample from a mineral ore body, the reagent is a leaching reagent, and the subsequent analysis is of the leaching of the ore body, including reaction kinetics monitoring, leaching conditions screening and/or leaching mechanism studies.

24. A method according to claim 23 wherein the particulate sample is a sulphide-bearing mining waste derived from the processing of a pyrite mineral, and the subsequent analysis is reaction conditions screening to predict acid mine drainage formation as an outcome of mineral processing.

25. A method according to claim 20, wherein the particulate sample is a sample with pharmaceutical properties, the reagent simulates a biological environment reagent, and the subsequent analysis is of the pharmaceutical release, including dissolution and release kinetics monitoring and mechanism studies.

26. A method according to claim 20, wherein the particulate sample is a soil sample containing agricultural chemicals, soil contaminant, or naturally present chemical, the reagent simulates environmental events, such as rain, flooding, or irrigation, and the subsequent analysis is of the dissolution or release of the dissolved soil component, including dissolution and release kinetics monitoring and mechanism studies.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0044] Having briefly described the general concepts involved with the present invention, a preferred embodiment will now be described that is in accordance with the present invention. However, it is to be understood that the following description of the drawings and examples is not to limit the generality of the above description.

[0045] In the drawings:

[0046] FIG. 1 is a schematic illustration of the workflow for a preferred embodiment of microfluidic device in accordance with a preferred embodiment of the present invention, showing a single sample chamber and upright members in the form of micropillars (inset: an image of the experimental set-up with inlet and outlet tubing).

[0047] FIGS. 1a and 1b are alternative embodiments of microfluidic devices that are also in accordance with the present invention.

[0048] FIG. 2 shows results from the experimental work with the embodiment of FIG. 1, in particular a comparison of the sulphur (A) and iron (B) release rate from pyrite as a function of time without surface treatment and with surface treatment.

[0049] FIG. 3 also shows results from the experimental work with the embodiment of FIG. 1, in particular aqueous iron and sulphur released from pyrite as a function of pH at room temperature.

[0050] FIG. 4 also shows results from the experimental work with the embodiment of FIG. 1, in particular a 3D graph of the dissolution rate of pyrite as a function of temperature and ferric ion concentration.

[0051] FIG. 5 also shows results from the experimental work with the embodiment of FIG. 1, in particular fitted sulphur (2p) XPS spectra of the pyrite samples treated at different conditions, where the colour scheme is: red, disulphide; green, sulphide; yellow, sulphate; blue, elemental sulphur and purple, polysulfide with dotted line showing the doublet peak of each sulphur species of the same colour.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0052] For a preferred embodiment of a microfluidic device in accordance with the present invention, a robust microfluidic device and method were developed for screening geological phenomena that occurs at the solid/liquid interface of rock ore samples as received. Evaluation of mineral dissolution/leaching for a range of reaction conditions were carried out using real rock samples with species diversity. For the purposes of illustration, screening was for acid mine drainage (AMD) under typical environmental conditions in the field. However, it will be appreciated that the inventive microfluidic device and method can also be applied to the optimization of industrial leach processes at mineral processing plants, and indeed also to non-mineral situations.

[0053] Pyrite (FeS.sub.2) is the most abundant sulphide mineral in the earth's crust and is a primary contributor to AMD and the consequent metal release of sulphide bearing mining wastes. The rate of pyrite oxidation and the resulting acid production is dependent on various environmental factors that are dynamic and often vary substantially between regions. Here, ferric ion concentration, pH, and temperature (which are the common factors affecting the oxidation of pyrite) were examined by loading a single sample chamber in a microfluidic device with a particulate sample in accordance with this embodiment of the present invention.

[0054] A schematic of the overall experimental setup is given in FIG. 1, with examples of suitable microfluidic devices shown in FIG. 1a and FIG. 1b.

[0055] Parallel testing in the microfluidic device offered high-throughput screening capacity. For each experiment, only 50 mg of the rock sample was required. Reagent consumption was approximately 3 mL for screening up to 6 hours of reaction time. Five parallel experiments were conducted, although the inventive method allows for greater parallelization as required. Surface characterization of the sample residue was carried out by X-ray photoelectron spectroscopy (XPS) to correlate the surface chemistry of the reaction residue with the leaching behaviour observed by solution analysis.

Materials and Methods

[0056] Pyrite ore sample (FeS.sub.2) was supplied by Geo Discoveries (NSW, Australia). The phase purity of the pyrite ore was confirmed by quantitative X-ray diffraction and chemical analyses. The ore was then crushed, ground and screened to a particle size range of 38-75 m with particle surface area measured at 0.35 m.sup.2.Math.g.sup.1. In this respect, it is envisaged that a suitable size range for most particulate samples will be from about 2 m to about 1 mm, with a usual range likely to be from about 20 m to about 600 m.

[0057] Acid washing is often used for leaching experiments to remove any surface oxidised layer of ore samples that might be formed during sample preparation. However, the literature is typically silent on detail for tracking any change of leachate chemistry under these different conditions, principally due to it being difficult to access using conventional methods. It will be seen from the below description and discussion that the use of the device and method of the present invention can be advantageous for tracking the evolution of leachate chemistry under different conditions and does show that such an acid wash in fact tends to have no impact on the leach behaviour of the intrinsic mineral of examined samples.

[0058] In this respect, each pyrite ore sample was washed with 3 M HCl solution for several minutes to remove oxidised surface layers possibly formed during sample preparation.

[0059] Following sample preparation, 1 M KOH (AR, 85%) and hydrochloric acid (37%) were used to adjust the pH of a reagent solution. FeCl.sub.3.Math.6H.sub.2O (AR) was used for the preparation of ferric ion concentrations. 0.1 M KCl was used as the background electrolyte to ensure the solution ionic strengths were approximately constant throughout the experiments. All chemicals were purchased from Chem-Supply, Australia. Milli-Q water (18 M.Math.cm resistivity) was used to prepare all solutions.

[0060] In order to form the upper layer of the device, a mass ratio of 10:1 of Sylgard 184 silicone elastomer base and curing agent was mixed thoroughly and poured onto a hydrophobized silicon wafer-based container. The PDMS was cured at 60 C. for 4 h, then peeled off from the silicon master. After this, the sample chamber and reagent inlet port were formed by coring 4 mm and 1.5 mm holes respectively with a biopsy punch.

[0061] The microfluidic device of the embodiment illustrated in FIG. 1 (and also FIG. 1a) was assembled by sealing this thin upper PDMS layer (thickness 8 mm) on a pillar cuvette (the base layer) with pillars having a 6 m gap and 10 m height through plasma bonding. The pillar cuvette was of the type described in Holzner, G.; Kriel, F. H.; Priest, C., Pillar Cuvettes: Capillary-Filled, Microliter Quartz Cuvettes with Microscale Path Lengths for Optical Spectroscopy. Anal. Chem. 2015, 87 (9), 4757-4764, the full content of which is incorporated herein by reference.

[0062] During plasma bonding of the upper PDMS layer with the base layer (the pillar cuvette), care was taken to align the sample chamber and reagent inlet within the area of the pillar cuvette arrangement in the flow chamber.

[0063] After loading ore samples into the sample chamber, the opening of the sample chamber was sealed with a thin layer of PDMS (a removable cover) which allowed introduction to the sample chamber via the reagent inlet and TYGON tubing, together with optical inspection of the sample chamber, as required.

[0064] Reagent flow through the reagent inlet and the sample chamber was driven by a peristatic pump (Gilson, Minipuls) through capillary tubing (0.5 mm inner and 1.58 mm outer diameter) into the sample chamber at 0.650.05 mL/h. The leach solution (analyte) was collected at the analyte outlet through capillary tubing in a glass vial over a period of 1 h for each sample.

[0065] The analyte collected at the analyte outlet was analysed by inductively coupled plasma mass spectrometry (ICP-MS) (Agilent 8800). The average leach rate (mM.Math.m.sup.2.Math.s.sup.1) during the collection period (1 h per measurement) was determined from the ion concentration in the collected analyte and from the sample surface area. Screening experiments were repeated three times with similar results, showing an experimental error within 6%.

[0066] After the leach, the ore samples were rinsed with milli-Q water to remove any residue leachate. The removable PDMS cover was then removed from the sample chamber and the wet samples were quickly transferred into a small plastic vial with rinsing water and stored in the freezer for cold stage (134 C.).

[0067] XPS spectra were collected using a Kratos AXIS Ultra DLD spectrometer. The x-ray was a mono-chromatic aluminum x-ray running at 225 W with a characteristic energy of 1486.6 eV. The area of analysis (Iris aperture) was a 0.3 mm0.7 mm slot; the analysis depth was approximately 15 nm into the surface of the sample. The analysis vacuum was 410.sup.8 Torr. The electron take-off angle was normal to the sample surface. Spectra were interpreted using the software package CasaXPS.

Results and Discussion

[0068] Pyrite dissolution kinetics and surface pre-treatmentThe flow chemistry applied in the microfluidic device of the invention enables the examination of reaction dynamics, particularly in the early stages of leaching experiments, which are difficult to access using conventional methods. FIG. 2 shows the evolution of the leachate chemistry for both dissolved iron and sulphur in pyrite as a function of time (up to 6 hours). A rapid decline in both sulphur and iron in the leachate was observed for the untreated pyrite sample compared to the leached sample in the first two hours, suggesting that the leaching removed the more reactive surface layer that contains both iron and sulphur species. The leachate chemistry (for sulphur and iron respectively) was quite stable and very similar at the later stage of leaching for both untreated and treated samples. This may be attributed to the dissolution of fresh pyrite exposed during the course of leaching. The Fe/S ratio (0.41-0.46) observed at a later stage in the leachate for both pyritic samples is slightly below the stoichiometric dissolution of pyrite (0.5).

[0069] Generally, the result shows that that an acid wash (as a pre-treatment) has no impact on the leach behaviour of intrinsic pyrite mineral. Therefore, for further investigation of AMD formation under various conditions, an acid wash (a pre-treatment) was used as standard sample treatment before running experiments. Further details about the surface chemistry of the outer layer of pyrite before and after leaching will be discussed below. No surface passivation was observed under the conditions examined during the leach time.

[0070] Effect of pHthe effect of pH on the dissolution rate of pyrite was examined in the range of pH 2-10. The leach rates of each element at different pH were calculated from the measured solution total sulphur or iron concentration divided by the sample's total surface area (determined from the mass and specific surface area of the sample) and the collection time for each measurement (1 h). The average value of the leach rate at steady state (after initial removal of the oxidation layer) was then plotted against pH (error bars represent the standard deviation of the obtained leach rates), as shown in FIG. 3.

[0071] FIG. 3 shows that the pH has a significant but very different impact on the leach (release) rate of sulphur and iron from pyrite. The higher the pH, the higher the concentration of sulphur compounds in solution. However, the concentration of iron detected in solution was decreased as the pH increased. When the pH was higher than 7, only trace amounts of iron were detected in solution. The Fe/S ratio calculated from the total iron and sulphur species detected in the solutions declined from 0.46 to 0.008 as the pH increased from 2 to 10, i.e. the pyrite surface did not exhibit the expected stoichiometric dissolution (Fe/S=0.5) at high pH. This is most probably due to the formation of iron precipitate as iron (hydro)oxides on the pyrite surface.

[0072] As the pH went higher, more iron (hydro)oxides grew at the pyrite surface. However, this did not prevent further pyrite oxidation because sulphur species were still released to the solution at an increasing rate, as shown in FIG. 3. The reaction rate obtained is around four orders of magnitude higher than expected, probably due to the continuous flow of fresh leaching solution and the continuous removal of by-products from the pyrite sample.

[0073] The log rate of pyrite dissolution (mol.Math.m.sup.2.Math.s.sup.1) plotted against pH showed a reaction order for pH of 0.04 (with a R square factor of 0.96) in the range of pH 2-10, which vary widely between 0.11 to 0.5. The small reaction order of the present study for H+ indicates that pH has a lesser effect on the observed rate compared to bath-scale experiments, due to the continuous removal of iron precipitates (formed at higher pH) in the microfluidic flow system avoiding possible surface passivation. The sulphur concentration detected in the leachate was applied for the calculation of pyrite dissolution rate instead of iron due to the precipitation of iron species at the pyrite surface during leaching at higher pHs.

[0074] Combinatorial screening (temperature, ferric ion concentration and time)benefiting from the minimization of the reaction system, the microfluidic device of the invention is attractive for the screening of multiple variables of the same or a different nature within a single device. In this study, ferric ion concentration and temperature were chosen as screening parameters for the study of acid mine drainage (AMD). With this in mind, each sample chamber was exposed to different leach conditions, enabling rapid parameter screening (results shown in FIG. 4).

[0075] FIG. 4 shows the screening results for varied ferric ion concentration (0, 5, 10, 20, and 40 mM ferric ion concentration in the fresh leach solution) for three different temperatures (23, 50 and 75 C.). The average pyrite dissolution rates were determined at steady state (observed between 3 to 6 h) based on the sulphur concentration measured in the leachate. The relationship between the leach rate and the two variables (temperature and Fe.sup.3+ concentration) is clearly seen in FIG. 4.

[0076] Increasing either variable increases the leach rate, but using a fraction of the sample, reagent and time. Surface passivation was not observed under the examined conditions, which is likely due to the continuous flow of fresh leach solution. On the basis of the above results, the reaction order of pyrite dissolution rate on Fe.sup.3+ concentration was calculated as 0.720.06, which was as expected. The Arrhenius plot shows the apparent activation energy of the pyrite oxidation reaction by ferric ion solution is around 30.60.7 kJ.Math.mol.sup.1, again being consistent with expected values ranging between 33 to 63 kJ.Math.mol.sup.1.

[0077] Surface analysis by XPSdue to the formation of iron precipitate on the pyrite surface during leaching, analysis of the sulphur signal is practically more important than that of iron. FIG. 5 shows sulphur (2p) spectra of pyrite samples treated at different conditions: untreated, acid washed (3 M HCl) and leached for 6 h at pH 2. For all pyrite samples, the characteristic peak located at 162 eV in the sulphur (2p) spectra agrees with the expected value for pyritic S.sub.2.sup.2. A small S.sup.2 peak at 161 eV was also found on all samples. Signals detected in the range of 164 to 165 eV suggested the presence of elemental sulphur and S.sub.n.sup.2 at the surface of all pyrite samples. On untreated samples, there was a peak at 168 eV, attributed to the presence of sulphate species on the surface. This peak intensity was significantly decreased after the acid wash, suggesting sulphate species were removed from the surface during washing. After leaching for 6 h, the peak at 168 eV was barely observed, indicating a complete dissolution of sulphates from the surface. No traces of sulphites were found in the 166 to 167 eV range. These results are in good agreement with the analysis of the leachate solutions collected.

Implications

[0078] Microfluidic screening of geological phenomena such as leaching offers a rapid approach to investigating natural processes that are environmentally or commercially important. The complex parameter space encountered in these reaction systems demands high throughput multiparameter screening, using minimal sample, reagent and time. The microfluidic approach able to be used by the adoption of the microfluidic device and method of the present invention meets these demands and is shown to report meaningful, time-resolved results for various reaction conditions.

[0079] Mineral samples in particulate form can be directly loaded into the device, without the need for flat, large areas of sample (e.g. polished or embedded in resin). Samples can thus be obtained direct from mine sites and, in many cases, on site screening will be possible. Low-cost testing could precede field trials, which are typically expensive and time-consuming. In addition, the method of the present invention could be used to study a wide range of solid-liquid interactions in flow (adsorption, dissolution, and other surface chemistry phenomena) across many different fields of application, including outside of the AMD and mineral processing work illustrated in these examples.

[0080] In conclusion, it must be appreciated that there may be other variations and modifications to the configurations described herein which are also within the scope of the present invention.