WO2021077156A1

WO2021077156A1 - Dewatering process

Info

Publication number: WO2021077156A1
Application number: PCT/AU2020/051123
Authority: WO
Inventors: Firuz Zare; Negareh Ghasemi; Keith Hoffman; Mansour Edraki
Original assignee: The University Of Queensland
Priority date: 2019-10-21
Filing date: 2020-10-19
Publication date: 2021-04-29

Abstract

Disclosed herein is a method for dewatering mineral tailings comprising a mineral component with water bound within or thereto, the method comprising subjecting at least a portion of the mineral tailings to a pulsed electric field with a frequency of from about 10 kHz up to about 1 MHz and a peak electric field strength of from about 5 V/cm up to about 500 V/cm to liberate the water bound within or to the mineral component of the mineral tailings. Also disclosed herein is an apparatus for dewatering mineral tailings comprising a mineral component with water bound within or thereto, the system comprising: two spaced apart electrodes configured to apply a pulsed electric field to mineral tailings located therebetween; and an electric signal generator configured to generate the pulsed electric field at a frequency of from about 1 kHz up to about 1 MHz and at sufficient voltage to provide the pulsed electric field with a peak electric field strength of from about 5 V/cm up to about 500 V/cm.

Description

DEWATERING PROCESS

Field

[0001] The invention relates to methods and/or systems for dewatering mineral tailings that comprises a mineral component with water bound within or thereto.

Background

[0002] Typically, during mineral processing, an ore is comminuted and subjected to various processes to separate the overburden, host rocks and/or gangue components. The resultant upgraded ore may then be comminuted further and treated chemically to extract the valuable components. Once the valuable components have been removed the depleted ore, generally a fine particulate material in the form of a slurry of tailings, is dewatered and disposed of.

[0003] Originally mineral tailings were disposed of directly into water ways such as rivers or the ocean. However, this approach to tailings disposal is no longer tenable since mineral tailings are often toxic and such disposal contaminates waterways, threatening human health, damaging the ecosystem and killing wildlife. Given this, it has become common practice to store mineral tailings in dams, ponds, or purpose-built above ground structures, or by dry stacking the mineral tailings.

[0004] The use of tailing storage facilities also has significant shortcomings. As discussed above, prior to storage in tailing dams, the mineral tailings are generally dewatered to reduce storage volume. This dewatering process requires expensive plant equipment, such as gravity thickeners, and may require the use of toxic chemicals to encourage flocculation and settling of mineral solids in the tailings. Further, despite first being dewatered, the volume of mineral tailings is still significant and as such, tailing storage facilities require a large land area. This creates additional problems particularly in terms of risk management and land rehabilitation at end of life of the mine site.

[0005] As commented above, tailing dams, in the case of valley-fill operations, are a point of considerable risk. There have been several major incidents in which embankments of the dams have burst, causing tailings to flood surrounding areas with highly toxic waste, and which in some cases, have tragically lead to loss of human life. [0006] An alternative means of storage is to further dewater the mineral tailings so that they can be dry stacked. Dry stacking involves dewatering the mineral tailings to sufficient solids content that the mineral tailings are effectively in the form of a solid cake which can then be stacked in a storage area. This is viable in dry climates and water sensitive areas, where water entrainment in the deposited fine tailings hinders effective recycling of tailings decant water.

[0007] Dry stacking requires less storage space than tailings dams since dry stacked tailings have increased solid content and thus comparatively reduced volume, has a lower risk management profile than tailings dams since dry stacked tailings are less flowable, and allows water to be recovered from the mineral tailings and reused in the process (which is an important consideration in mine sites with limited or restricted access to water). However, dry stacking also has significant disadvantages over the use of tailing dams. Specifically, to achieve the higher levels of dewatering to form the solid cake, additional energy intensive dewatering processes such as vacuum filters, pressure filters, or gravity belt thickeners are required which add considerable capital cost and ongoing operating expenses. In most cases, the current technologies cannot cope with daily throughput demands of mining operations. Dry stacking is not viable in wet climates.

[0008] Given the shortcomings above, there is significant interest in developing better and/or alternative methods for dewatering of mineral tailings whether for storage in a tailings dam or via dry stacking. There is also a need for in-situ recovery of water from deposited tailings in water stressed areas. It is an object of the invention to address at least one of the aforementioned problems of the prior art.

Summary of Invention

[0009] In a first aspect of the invention, there is provided a method for dewatering mineral tailings comprising a mineral component with water bound within or thereto, the method comprising: subjecting at least a portion of the mineral tailings to a pulsed electric field with a frequency of from about 1 kHz up to about 1 MHz and a peak electric field strength of from about 5 V/cm up to about 500 V/cm to liberate the water bound within or to the mineral component of the mineral tailings.

[0010] By “bound” it is meant that water is attracted to or is interacting with and/or adsorbed to surfaces of the mineral components of the mineral tailing (typically in particulate form). That is, “bound water” is not “free water”, wherein “free water” refers to water molecules that do not interact with the mineral components of the mineral tailings and, for example, substantially interact with only other water molecules (for example via hydrogen bonding) and/or with charged ions in solution.

[0011] In an embodiment, the frequency is from about 2.5 kHz. Preferably, the frequency is from about 5 kHz. More preferably, the frequency is from about 7.5 kHz. Most preferably, the frequency is from about 10 kHz. Additionally, or alternatively, in an embodiment, the frequency is up to about 750 kHz. Preferably, the frequency is up to about 500 kHz. More preferably, the frequency is up to about 250 kHz. Most preferably, the frequency is up to about 100 kHz.

[0012] In an embodiment, the peak electric field strength is from about 6 V/cm. Preferably, the peak electric field strength is from about 7 V/cm. More preferably, the peak electric field strength is from about 8 V/cm. Most preferably, the peak electric field strength is from about 10 V/cm. Additionally, or alternatively, in an embodiment, the peak electric field strength is up to about 300 V/cm. Preferably, the peak electric field strength is up to about 200 V/cm. More preferably, the peak electric field strength is up to about 100 V/cm. Most preferably, the peak electric field strength is up to about 50 V/cm.

[0013] In an embodiment, the pulsed electric field has a duty cycle of from about 5% to about 90%. Preferably, the duty cycle is from about 10%. More preferably, the duty cycle is from about 15%. Most preferably, the duty cycle is from about 20%. Additionally, or alternatively, it is preferred that the duty cycle is up to about 80%. More preferably, the duty cycle is up to about 70%. Most preferably, the duty cycle is up to about 60%.

[0014] In an embodiment, the water is electrostatically bound and/or adsorbed to the mineral component of the mineral tailings.

[0015] In an embodiment, the method further comprises separating the liberated water from the mineral tailings.

[0016] In an embodiment, the mineral component comprises a tectosilicate mineral, for example quartz, and/or a phyllosilicate mineral, for example serpentine, kaolinite and montmorillonite, ferromagnesian mineral, for example olivine, and carbonate mineral, for example calcite. [0017] In an embodiment, the mineral component comprises one or more metal oxides, metal hydroxides, mixed metal oxides, or mixed metal hydroxides; wherein the metal oxides, metal hydroxides, mixed metal oxides, or mixed metal hydroxides comprise a metal or metals selected from the group consisting of: Al, Ba, Ca, Cr, K, Fe, Mg, Mn, Na, Ti, V, or Zr.

[0018] In an embodiment, the mineral component comprises one or more of metal sulfides, pyrite (FeS₂) in particular, in association with quartz, ferromagnesian minerals such as chlorite, carbonates such as dolomite, aluminosilicate minerals such as feldspars, and phyllosilicates such as clay minerals.

[0019] In an embodiment, the mineral component of the mineral tailings is a particulate material with a D90 particle size of 500 μm or less. In one form, the D90 particle size is 475 μm or less. In another form, the D90 particle size is 450 μm or less. In still another form, the D90 particle size is 425 μm or less.

[0020] In an embodiment, the mineral component of the mineral tailings is a particulate material with a D50 particle size of 100 μm or less. Preferably, the D50 particle size is 90 μm or less. More preferably, the D50 particle size is 80 μm or less. Most preferably, the D50 particle size is 75 μm or less.

[0021] In an embodiment, the mineral component of the mineral tailings is a particulate material with a D10 particle size of 5 μm or less. Preferably, the D10 particle size is 4 μm or less. More preferably, the D10 particle size is 3 μm or less. Most preferably, the D10 particle size is 2 μm or less.

[0022] In an embodiment, the pulsed electric field is generated using a unipolar voltage signal.

[0023] In an embodiment, the pulsed electric field is a continuous and pulsed electric field.

[0024] In an embodiment, prior to the step of subjecting at least the portion of the mineral tailings to the pulsed electric field, the method comprises locating at least the portion of the mineral tailings between two electrodes, e.g. which are preferably in the form of two electrode plates. [0025] In an embodiment, the step of subjecting at least the portion of the mineral tailings to the pulsed electric field further comprises passing an electric current through at least the portion of the mineral tailings. The electric current waveform and amplitude depend on the conductivity of the tailings, the dimension of the chamber and size of the electrodes.

[0026] In forms of the above embodiment in which the method comprises locating at least the portion of the mineral tailings between two electrodes (such as between two electrode plates), the step of passing the electric current through at least the portion of the mineral tailings further comprises passing the electric current between the two electrodes.

[0027] In a second aspect of the invention, there is provided an apparatus for dewatering mineral tailings comprising a mineral component with water bound within or thereto, the system comprising: two spaced apart electrodes configured to apply a pulsed electric field to mineral tailings located therebetween; and an electric signal generator configured to generate the pulsed electric field at a frequency of from about 1 kHz up to about 1 MHz and at sufficient voltage to provide the pulsed electric field with a peak electric field strength of from about 5 V/cm up to about 500 V/cm.

[0028] In an embodiment, the frequency is from about 2.5 kHz. Preferably, the frequency is from about 5 kHz. More preferably, the frequency is from about 7.5 kHz. Most preferably, the frequency is from about 10 kHz. Additionally, or alternatively, in an embodiment, the frequency is up to about 750 kHz. Preferably, the frequency is up to about 500 kHz. More preferably, the frequency is up to about 250 kHz. Most preferably, the frequency is up to about 100 kHz.

[0029] In an embodiment, the peak electric field strength is from about 6 V/cm. Preferably, the peak electric field strength is from about 7 V/cm. More preferably, the peak electric field strength is from about 8 V/cm. Most preferably, the peak electric field strength is from about 10 V/cm. Additionally, or alternatively, in an embodiment, the peak electric field strength is up to about 300 V/cm. Preferably, the peak electric field strength is up to about 200 V/cm. More preferably, the peak electric field strength is up to about 100 V/cm. Most preferably, the peak electric field strength is up to about 50 V/cm.

[0030] In an embodiment, the pulsed electric field has a duty cycle of from about 5% to about 90%. Preferably, the duty cycle is from about 10%. More preferably, the duty cycle is from about 15%. Most preferably, the duty cycle is from about 20%. Additionally, or alternatively, it is preferred that the duty cycle is up to about 80%. More preferably, the duty cycle is up to about 70%. Most preferably, the duty cycle is up to about 60%.

[0031] In an embodiment, the electric signal generator is configured to generate the pulsed electric field using a unipolar voltage signal.

[0032] In an embodiment, the electric signal generator is configured to generate a continuous pulsed electric field.

[0033] In an embodiment, the electric signal generator is configured to pass an electric current between the two spaced apart electrodes.

[0034] In a third aspect of the invention, there is provided the use of an apparatus according to the second aspect and/or embodiments thereof to dewater mineral tailings.

[0035] In forms of the first, second, or third aspects (and/or embodiments thereof), the mineral tailings are in the form of a slurry or sludge.

Brief Description of Drawings

[0036] Figure 1 is a schematic illustration of a static mineral tailings dewatering system according to one embodiment of the invention.

[0037] Figure 2 is a schematic illustration of a dynamic mineral tailings dewatering system according to another embodiment of the invention.

[0038] Figure 3 is a circuit diagram of a high frequency converter with a design based on an AC-DC diode rectifier with an adjustable auto-transformer to control DC voltage.

[0039] Figure 4 is a circuit diagram of an 34092B gate drive.

Description of Embodiments

[0040] The present invention relates to a method and apparatus for dewatering mineral tailings that comprises the application of a pulsed electric field to the mineral tailings to release water bound within the mineral tailings as free water which free water can be readily separated from the solid mineral component of the mineral tailings.

[0041] Without wishing to be bound by theory, the inventors are of the view that the application of a pulsed electric field with a frequency of from about 1 kHz up to about 1 MHz and a peak electric field strength of from about 1 V/cm up to about 500 V/cm causes high frequency resonance within the mineral tailings which liberates water bound within the mineral tailings, and in particular electrostatically bound water which may for example be bound in the form of a double layer around the surface of particulate minerals in the mineral tailings.

[0042] The present invention finds use for in-situ dewatering processes or dewatering at various treatment stages in a dewatering process and may be used in place of an existing dewatering process or as an additional unit process prior to, during, or after an existing dewatering process. In one example, the method and apparatus of the invention is used instead of a traditional gravity separator, thus reducing capital expense. In another example, the method and apparatus of the invention is used as an additional dewatering step after gravity separation to further thicken the underflow from a gravity thickener prior to storage. In still a further example, the method and apparatus of the invention is integrated into an existing unit process, and may for example, be integrated into a gravity thickener to treat the thickened tailings prior to discharge as underflow from the gravity thickener. Furthermore, this additional treatment step may be useful in place of, or prior to, other existing thickening unit process such as vacuum filtration, pressure filtration, or gravity belt thickening.

[0043] Figure 1 is an illustrative embodiment of a static mineral tailings dewatering system 100 according to one embodiment of the invention. The system 100 comprises an electric signal generator 102, and chamber 104 arranged between electrodes 106 and 108. Electric signal generator 102 is configured to generate an electric signal 110 at a frequency of from about 10 kHz up to about 100,000 kHz such that a pulsed electric field with an electric field strength of from about 10 V/cm up to about 50 V/cm is generated across chamber 104 and current flows between electrodes 106 and 108 when chamber 104 contains mineral tailings 112, such as in the form of a slime. The electric current waveform and amplitude depend on the conductivity of the tailings, the dimension of the chamber and size of the plates. As generally discussed above, the inventors are of the view that the pulsed electric field causes high frequency resonance within the mineral tailings which liberates water bound within the mineral tailings. The bound water is thus converted to free water and can be readily removed from the system 100 by means known to those skilled in the art, e.g. decanted or otherwise pumped from an outlet of the system 100. The remaining mineral tailings are at higher solids content and thus are more easily and cost effectively stored or disposed of.

[0044] A typical treatment time for an in-situ dewatering process is from about 5 minutes up to about 1 hour, subject to the nature of the material being dewatered and the dewatering parameters being used.

[0045] The static system 100 may be adapted for batch or semi batch operation. The static system 100 may include a chamber 104 for receiving the and retaining the mineral tailings for treatment. Alternatively, the chamber 104 may instead be a space defined by the separation of the two electrodes 106 and 108. In this case, the electrodes may be lowered or submerged into the mineral tailings for localised treatment of those mineral tailings.

[0046] Figure 2 is an illustrative embodiment of a dynamic mineral tailings dewatering system according to another embodiment of the invention for a plug flow type dewatering processes. The system 200 comprises an electric signal generator 202, and flow chamber 204 with electrodes 206 and 208 arranged in opposite walls of flow chamber 204. Flow chamber 204 includes: an inlet 210 through which mineral tailings are pumped via pump 212, a free water outlet 214, and a dewatered mineral tailings outlet 216. During operation, electric signal generator 202 is configured to generate an electric signal 218 at a frequency of from about 10 kHz up to about 100,000 kHz such that a pulsed electric field with an electric field strength of from about 10 V/cm up to about 50 V/cm is generated across flow chamber 204 and current flows between electrodes 206 and 208 as mineral tailings is pumped therethrough. As above, the application of the pulsed electric field frees bound water from the mineral tailings. Freed water is discharged through free water outlet 214 and dewatered mineral tailings are discharged through outlet 216.

[0047] It is contemplated that the methods, systems, and apparatus can be applied to dewater a variety of different mineral tailings, a non-limiting list of examples includes: copper tailings, coal tailings, and red mud. Examples

Exemplary compositions of the tailings used in the experiment are provided below:

[0048] Composition 1 - Copper tailings

[0049] Mineral phases were dominated by K-feldspar (KAIS_i3O₈), magnetite (Fe₃O₄), quartz (SiO₂), pyrite (FeS₂), calcite (CaCO₃) and biotite (K(Mg,Fe)₃(AIS_i3O₁₀)(OH)₂), as major phases (> 5% wt.), and chlorite ((Mg3,Fe₂)Al(AlS_i3)O₁₀(OH)₈), plagioclase (Na,Ca)(Al,Si)₄O₈, garnet (Ca,Mg,Fe,Mn)3(Al,Fe,Cr,V,Zr,Ti)₂(SiO₄)₃ and barite (BaSO₄) as minor phases (1 - 5% wt.). The median particle size was < 75 μm, and approximately 85% of particles were < 150 μm. All particles were < 425 μm.

[0050] Composition 2 - Coal tailings

[0051] The coal samples was dominated by clay minerals with major concentrations of Kaolinite [(Al₂SiO₅)(OH)₄], quartz (SiO₂), interlayered illite-smectite, Smectite (Na_0.3Al₂(Si,Al)₄O₁₀(OH)₂·2H₂O, with minor (Illite/muscovite, [KAl₃S_i3O₁₀(OH)₂], Gypsum (CaSO₄·2H₂O). Up to 52 % clay (<2μm), and 76 % silt (0.002 -0.02mm).

[0052] Composition 3 - Red mud (Bauxite residue)

[0053] The mineralogy of red mud sample was dominated by Hematite (Fe₂O₃), followed by smaller concentrations of Hydrocalumite [Ca₂Al(OH)₆, [Cl_1-x(OH)_x]·3H₂O] , sodalite [Na₈Al₆Si₆O₂₄(SO₄)], followed by Quartz (SiO₂), gibbsite Al(OH)₃, boehmite AIO(OH), rutile TiO₂, anatase TiO₂, calcite CaCO₃, halite NaCl, , Makatite, Na₂Si₄O₈(OH)₂.4H₂O. Red mud has up to 70% clay size fraction (<2 μm).

Experimental setup

[0054] For the purpose of the experiments, two large cuvettes with width 2.5 cm, length 2.5 cm, height 11 cm, and volume 65ml were used and to apply high voltage pulsed power signals to the tailings with plate electrodes having width of 2.5 cm and a length of 10.5 cm (see Figure 1). [0055] The electric signal generator is a high frequency converter with a design based on an AC-DC diode rectifier with an adjustable auto-transformer to control DC voltage. The DC voltage is supplied to a single phase inverter based on GaN semiconductor technology with an ultra-fast switching frequency and transient. A complete Half bridge power circuit with High Efficiency 600V 70mΩ X-GaN transistor is utilised for DC-AC inverter topology. The circuit diagram is shown in the Figure 3.

[0056] GaN switching devices require special PCB boards with very low loop inductances and gate drives. Given this, a AN 34092B gate drive (see Figure 4) was used which (i) supports high switching frequency ( ~4MHz), (ii) achieved safe operation by negative voltage source and active miller clamp, and (iii) facilitated gate drive design with high precision gate current source. High / Fow side Isolated DC-DC modules are used to provide the necessary bias of the gate drivers for high and low side semiconductor switches. The isolation of these modules is greater than 3000V.

[0057] Control signaling is generated by an advanced micro-controller TMS320F28379D, connected to a PC with Matlab/Simulink software to program Pulse Width Modulation strategy, generating unipolar signal with adjustable frequency and duty cycle.

[0058] Several electrical pulses with different amplitudes and duty cycles to stimulate the tailing samples inside the cuvette. In particular, unipolar pulses with duty cycles of 20%, 50%, and 60% and with the amplitude of the voltage changing from 10V to 60V were applied to the tailings samples in the cuvette. The cuvette was oriented in horizontal position to allow for the separated water (runny water) to run out of the cuvette.

Results

[0059] The above set up was used to treat the mineral tailings discussed above with an initial moisture content of about 45 wt%.

[0060] From trial runs, it was found that wider pulses (60%) with larger amplitude are more effective. Given this, the mineral tailing was subjected to a 10 minute dewatering process using pulses with 60% duty cycle and 60 V amplitude at 20 kHz. [0061] After subjecting the mineral tailings to this dewatering process and decanting the liberated water, the moisture content of the mineral tailing dropped to ~30 wt%. Thus, the process was found to mobilise 30% of the initial water.

[0062] Although the invention has been described in connection with aspects and preferred embodiments thereof, it should be understood that various modifications, additions and alterations may be made to the invention by one skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for dewatering mineral tailings comprising a mineral component with water bound within or thereto, the method comprising: subjecting at least a portion of the mineral tailings to a pulsed electric field with a frequency of from about 10 kHz up to about 1 MHz and a peak electric field strength of from about 5 V/cm up to about 500 V/cm to liberate the water bound within or to the mineral component of the mineral tailings.

2. The method of claim 1, wherein the water is electrostatically bound to the mineral component of the mineral tailings.

3. The method of claim 1 or 2, wherein the method further comprises separating the liberated water from the mineral tailings.

4. The method of any one of the preceding claims, wherein the mineral component comprises an aluminosilicate material.

5. The method of any one of the preceding claims, wherein the mineral component comprises one or more metal oxides, metal hydroxides, mixed metal oxides, or mixed metal hydroxides; wherein the metal oxides, metal hydroxides, mixed metal oxides, or mixed metal hydroxides comprise a metal or metals selected from the group consisting of: Al, Ba, Ca, Cr, K, Fe, Mg, Mn, Na, Ti, V, or Zr.

6. The method of any one of the preceding claims, wherein the mineral component of the mineral tailings has a D90 particle size of 500 μm or less.

7. The method of any one of the preceding claims, wherein the mineral component of the mineral tailings has a D50 particle size of 100 μm or less.

8. The method of any one of the preceding claims, wherein the mineral component of the mineral tailings has a D10 particle size of 4 μm or less.

9. The method of any one of the preceding claims, wherein the pulsed electric field is generated using a unipolar voltage signal.

10. The method of any one of the preceding claims, wherein the electric field is a continuous electric field.

11. The method of any one of the preceding claims, wherein the step of subjecting at least the portion of the mineral tailings to the pulsed electric field further comprises passing an electric current through at least the portion of the mineral tailings.

12. The method of any one of the preceding claims, wherein prior to the step of subjecting at least the portion of the mineral tailings to the pulsed electric field, the method comprises locating at least the portion of the mineral tailings between two electrodes.

13. An apparatus for dewatering mineral tailings comprising a mineral component with water bound within or thereto, the system comprising: two spaced apart electrodes configured to apply a pulsed electric field to mineral tailings located therebetween; and an electric signal generator configured to generate the pulsed electric field at a frequency of from about 1 kHz up to about 1 MHz and at sufficient voltage to provide the pulsed electric field with a peak electric field strength of from about 5 V/cm up to about 500 V/cm.

14. The apparatus of claim 13, wherein the electric signal generator is configured to generate the pulsed electric field using a unipolar voltage signal.

15. The apparatus of claim 13 or 14, wherein electric signal generator is configured to generate a continuous pulsed electric field.

16. The apparatus of any one of claims 13 to 15, wherein the electric signal generator is configured to pass an electric current between the two spaced apart electrodes.

17. The apparatus of any one of claims 13 to 16 wherein the frequency is from about 10 kHz up to about 500 kHz.

18. The apparatus of any one of claims 13 to 17 wherein the field strength is from about 10 V/cm up to about 50 V/cm.

19. The apparatus of any one of claims 13 to 18, wherein the pulsed electric field has a duty cycle of from about 5% to about 90%.

20. Use of an apparatus according to any one of claims 13 to 19 to dewater mineral tailings.