AU2014101152A4 - Saline resistant geosynthetic clay liner - Google Patents

Abstract

A geosynthetic clay liner for use in creating a hydraulic barrier is described. The liner comprises at least one flexible carrier layer, at least one flexible layer of bentonite disposed on top, and at least one flexible cover sheet disposed on top of the bentonite layer. The bentonite has properties that cause the bentonite to swell when contacted with a high-ionic or saline liquid, forming a hydraulic barrier that is resistant to high-ionic liquids. ~r-4 . .. . . . .. . . . .. . . . . . . . . .. . . . . . . . . .

Description

1 SALINE RESISTANT GEOSYNTHETIC CLAY LINER FIELD OF THE INVENTION [0001] The present invention relates to a geosynthetic clay liner for use in containment applications, in particular, a geosynthetic clay liner having resistance to saline and sodic conditions. BACKGROUND TO THE INVENTION [0002] It is known that mineral and energy exploration and processing industries produce vast volumes of liquid wastes. Large volumes of these liquid wastes or process liquids are typically held in waste containment structures. The nature of process liquids is such that they pose a hazard to surrounding ground and ground water. Environmental regulations therefore typically require that waste containment structures be constructed with appropriate measures in place to minimise seepage of the liquid into the surrounding groundwater. Similar issues and regulatory requirements also apply to other waste and liquid containment structures, such as landfills. [0003] Various means can be used to prevent seepage from a waste containment structure, typically in the form of a type of liner surrounding the containment structure and forming a barrier between the waste and the surrounding ground. A fundamental requirement of the liner is that it maintains a hydraulic conductivity lower than the design value for the projected contaminating lifespan of the waste containment facility. [0004] The liner may take a variety of forms and can include a thick layer of compacted clay, built around the structure to provide hydraulic protection. However, this approach can often be uneconomical, at least in part due to scarcity of suitable and economical clayey soils. These difficulties with traditional clay liners have led to adoption of composite liner systems comprised of a 2 geomembrane (GM) primary barrier and one or more low permeability mineral layers. A geosynthetic clay liner (GCL) is one such composite liner system, comprising a layer of bentonite or other clay having low hydraulic conductivity, sandwiched and confined between two or more layers of a geotextile material. When the GCL comes into contact with very low ionic-strength liquid such as potable water, the clay is hydrated to create a containment barrier of low permeability by the swelling action of the clay filling up pore space and constricting water flow paths. GCLs have been found to be highly effective in many applications, including landfills, mining and in providing liquid containment in reservoirs, irrigation canals and the like. [0005] Whilst GCLs have been successful in many applications, the efficacy of their application in aggressive environments, such as saline and saline-sodic environments, is largely unsubstantiated. That is, whilst GCLs provide suitable protection and containment of low salinity or low ionic strength liquids, interaction of GCLs with saline or other high ionic strength liquids generally results in increased potential for hydraulic incompatibility, enabling liquid to escape from the containment structure. High ionic strength liquids remove water from the surface of the clay, causing the clay to behave plastically. This is a particular problem in the containment of some mining and exploration process waters, such as coal seam gas (CSG) process waters, since these process waters have saline and saline-sodic chemistries. Incompatibility of process waters having saline and saline-sodic chemistries with GCLs presents a significant threat to groundwater resources and ecosystems surrounding the containment facility. [0006] There is therefore an existing need for a hydraulic barrier system capable of providing an effective barrier against transmission of liquid under aggressive geochemical conditions, such as saline or saline-sodic conditions, where the high ionic strength of leachate has negative effects on the hydraulic conductivity of known clay-based barrier systems and liners. The resulting hydraulic barrier system would be capable of substantially preventing or mitigating fluid loss from a containment structure. The resulting hydraulic barrier system would also be capable of providing resistance to leachates substantially 3 for the duration of the projected contaminating or operational lifespan of the containment facility in which the hydraulic barrier system is employed. It is an object of the present invention to provide a hydraulic barrier that fulfils these needs. SUMMARY OF THE INVENTION [0007] With the aforementioned object in mind, according to a first aspect of the present invention, there is provided a geosynthetic clay liner comprising at least one flexible carrier sheet layer, at least one flexible layer of bentonite disposed on top of at least one flexible carrier sheet layer, and at least one flexible cover sheet layer disposed on top of at least one layer of bentonite, wherein the layer of bentonite has properties that cause the bentonite to swell when contacted with a saline liquid to form a hydraulic barrier. [0008] Advantageously and unexpectedly, saline water hydrates the bentonite, causing the bentonite to swell and form a substantially impermeable layer having saturated hydraulic conductivity (k) values of a magnitude whereby liquid is substantially prevented from passing through the bentonite layer. This contrasts to known GCLs where the bentonite layer is unable to swell sufficiently in saline conditions and therefore fails to create an effective hydraulic barrier. [0009] In a preferred embodiment, the flexible bentonite layer has liquid permeability or saturated hydraulic conductivity of less than about 1 x 10-10 m/s when contacted with a saline liquid. Even more preferably, the flexible bentonite layer has liquid permeability or saturated hydraulic conductivity of less than about 5 x 1011 m/s when contacted with saline liquid. [0010] The flexible layer of bentonite is preferably provided in a powder or granular form, or a combination of both of these. Bentonite powder is applied to the first flexible layer in quantities sufficient to meet the specific requirements of the particular containment facility. In a preferred embodiment, the bentonite forms 4 at least a 5mm layer between the first and second flexible layers in the assembled GCL. Preferably, the bentonite is applied at about between 3.5 - 5.5 kg/M 2 . [0011] The bentonite is preferably a sodium bentonite. In one embodiment, the bentonite is specifically beneficiated and/or modified for resistance to saline sodic conditions prior to containment between the first and second flexible layers. In a preferred embodiment, one or more additives are added to the bentonite, said one or more additives having qualities which assist in maintaining the bentonite in a swollen state under saline-sodic conditions. Desirably, the one or more additives act to enhance bentonite swelling when combined with the bentonite and produce a thick, impervious layer when contacted with saline liquid. The one or more additives are preferably polymer based. The bentonite and additives are milled and blended to ensure even amalgamation. [0012] The bentonite preferably has a mineralogical composition consisting of a major proportion of smectite and minor proportions of one or more of kaolin, quartz and anastase. The bentonite desirably has a range of major and trace element compositions, including a series of oxides at varying percentages, with silica content being present in dominant percentages. [0013] The flexible carrier and cover layers may be comprised of one or more suitable materials and may be made from the same or different material. It is preferred that the flexible carrier and cover layers are fabricated from a material having sufficient permeability to permit sufficient liquid to reach and hydrate the bentonite. Optionally, one or more of flexible carrier and/or cover layers can be fabricated from a low permeability material. The GCL may optionally include additional flexible layers intermediate of the cover and carrier flexible layers and/or further flexible layers of bentonite spaced therebetween. The resulting GCL can be conveniently provided as a roll of the composite structure, which assists in transportation and ease of installation.

5 [0014] It is preferred that the resulting GCL has a saturated hydraulic conductivity of less than about 1 x 1010 m/s, more preferably less than about 5 x 10-11 m/s. Optionally, the flexible bentonite layer can be pre-hydrated, for example, with deonised water, prior to utilisation of the GCL in a containment facility. BRIEF DESCRIPTION OF THE DRAWINGS [0015] The present invention will now be described more fully with reference to the accompanying drawings in which: [0016] Figure 1 is a sectional view of a geosynthetic clay liner in accordance with the present invention; and [0017] Figure 2 is a sectional view of the geosynthetic clay liner of Figure 1. DESCRIPTION OF PREFERRED EMBODIMENT [0018] Referring initially to Figure 1 there is shown a geosynthetic clay liner 10 comprising multiple layers which together serve to create a hydraulic barrier to provide containment of a liquid in a containment structure. The geosynthetic clay liner 10 of the present invention is particularly suited for creating a hydraulic barrier to provide containment of leachates, including high ionic strength leachates such as saline, saline-sodic and/or high/low pH leachates and prevent leaching into surrounding environment. The geosynthetic clay liner 10 of the present invention finds particular application in the handling, intermittent storage and disposal of saline or saline-sodic brines, such as from coal seam gas (CSG) production. [0019] As shown in Figures 1 and 2, the geosynthetic clay liner 10 includes a flexible carrier sheet 12, at least one flexible layer of bentonite 14 and a flexible cover sheet 16. Each layer 12, 14, 16 is substantially the same size and surface 6 area so as to create a composite 'sandwich' when the respective layers are placed together in the final geosynthetic clay liner 10 product. [0020] The flexible carrier sheet 12 and flexible cover sheet 16 can be comprised of one or more flexible materials and can be comprised of the same material as each other or be substantially interchangeable. In one embodiment, the flexible carrier and cover sheets 12, 16 are fabricated from a durable geotextile, typically made from polypropylene. Optionally and depending on site requirements, the flexible carrier and/or cover sheets 12, 16 can be fabricated from a low permeability material. The flexible carrier and cover sheets 12, 16 are strengthened by needle-punching during their fabrication so as to improve their tensile strength and fibre entanglement. The flexible carrier layer 12, bentonite layer 14 and cover layer 16 are needle punched together to provide internal shear and peel strength to the resulting geosynthetic clay liner 10. The flexible carrier layer 12 may be chemically or physically enhanced or altered to increase adherence of the flexible bentonite layer 14 or to increase adherence to underlying materials during use. The various layers of the geosynthetic clay liner 10 can be attached by any suitable means, such as by stitching or needle punching the layers together. [0021] The flexible layer of bentonite 14 is provided by evenly distributing a layer of high quality bentonite powder or granules on top of the flexible carrier layer 12. In a preferred embodiment, a bentonite powder is used, desirably having 100% of bentonite particles smaller than 150pm and preferably more than 50% of bentonite particles smaller than 0.5pm. [0022] The bentonite is selected for its properties to withstand the detrimental effects of high ionic strength liquids, such as saline-sodic leachates, including increase in hydraulic conductivity. The bentonite desirably has a fine natural grain size or is subjected to treatment, such as milling, to reduce the average grain size. The bentonite is selected for properties of having very low permeability.

7 [0023] The bentonite is desirably specifically beneficiated and/or modified for resistance to high ionic strength liquids. One or more additives and/or reagents are added to the bentonite, these one or more additives having qualities which assist in maintaining the bentonite in a swollen state under saline or saline-sodic conditions, thereby creating an effective hydraulic barrier to leachate. At least one additive imparts improved swelling qualities to the bentonite when combined, so that the bentonite creates a thick, impervious layer upon coming into contact with saline liquid. The additive is desirably polymer based. Multiple additives can be added to the bentonite before incorporating into the geosynthetic clay liner 10 to impart hydraulic barrier qualities appropriate for the particular application. [0024] In a preferred embodiment, the bentonite is a high quality sodium bentonite powder having a major proportion of smectite and minor proportions of one or more of kaolin, quartz and/or anastase. The bentonite includes a range of major and trace element components. Typical ranges of these components from bentonite samples sourced from two geographically separate locations are outlined in Table 1. The bentonite advantageously has qualities of low hydraulic permeability, though has low swell properties. In known GCL's, only bentonites exhibiting high swell properties provide low hydraulic conductivity, therefore it is an unexpected and advantageous effect that the bentonite utilised in the present invention has capacity to create a hydraulic barrier in the presence of saline-sodic conditions. Component Measurement Location 1 Location 2 Average Range Average Range SiO 2 % 58-62 59-63 A1 2 0 3 % 18-22 19-23 MgO % 3-6 3-6 Fe 2 0 3 % 3-7 4-6 CaO % 0 0.05-0.03 Na 2 0 % 1 -2 1 -2

K

2 0 % 0.5-2 0.5-2 TiO 2 % 0.2-1 0.2-1

P

2 0 5 % 0.01 0.01 MnO % 0.01 0.01

SO

3 % 0.01 - 0.2 0.01 - 0.2 ZnO ppm 5-20 9-20 CuO ppm 20-45 15-30 8 SrO ppm 20-50 45-75 ZrO 2 ppm 200 - 240 200 - 275 NiO ppm 25-50 20-55 Rb 2 0 ppm 55-80 55-70 BaO ppm 1-160 25-165

V

2 0 5 ppm 115-205 100-170 Cr 2 0 3 ppm 125-155 130-180 La 2 0 3 ppm 9-65 5-60 CeO 2 ppm 30-40 20-30 PbO ppm 10-20 10-20

Y

2 0 3 ppm 15-55 10-30 Coo ppm 5-20 4-25 Ga 2 0 3 ppm 20-35 25-40

U

3 0 8 ppm 20-30 25-30 ThO 2 ppm 0-2 0-10 As 2 0 5 ppm 0-2 0 SnO 2 ppm 2-6 3-10 CI ppm 1650-1855 1700-2340 Table 1: Chemical Composition of Typical Bentonite [0025] The bentonite powder is applied to the flexible carrier sheet 12 at density of about 3.5 - 5.5 kg/M 2 . The flexible cover sheet 16 is placed on top of flexible bentonite layer 14 and secured thereon by suitable means, to form a composite sandwich consisting of layers of geofabric and bentonite. [0026] If decreased permeability is desired from the geosynthetic clay liner, additional layers of geosynthetic materials and/or bentonite can be added. Referring to Figure 2, there is shown a further embodiment of the geosynthetic clay liner 10, having a flexible carrier layer 12 the flexible bentonite layer 14, the flexible cover layer 16 and an additional flexible outer layer 18. The additional flexible outer layer 18 is a sheet or sheets of geofabric or polymer material which may be located adjacent to the top cover layer 16, the bottom carrier layer 12, or additional flexible outer layers 18 can be provided adjacent to both the cover and carrier layers 16 and 12, providing provision is made to permit hydration of the bentonite layer 10 to occur to ensure bentonite swelling occurs. The result is a flexible geosynthetic clay liner 10 with improved permeability characteristics, able to remain impermeable to high ionic strength liquids for an extended period of time, desirably for at least the projected contaminating lifespan of the containment facility in which the geosynthetic clay liner 10 is being used.

9 [0027] In one embodiment of the present invention, the flexible bentonite layer 14 is pre-hydrated prior to installation in the proposed containment facility. Pre hydration can be conducted by application of very low ionic strength water, including deionised or potable water. In this embodiment, pre-hydrated bentonite as described above is covered with at least one geotextile layer on each side to prevent layers sticking together. Pre-hydration of the bentonite layer can be beneficial in imparting additional saline resistance to the geosynthetic clay liner 10, which may find particular utility in hyper saline environments or applications where enhanced saline resistance is required. [0028] In use, the geosynthetic clay liner 10 of the present invention is appropriately positioned and installed as required, depending on the proposed containment facility and the material the containment facility is intended to contain. The one or more flexible layers of bentonite 14, 22 can be pre-hydrated prior to active use of the geosynthetic clay liner 10. Pre-hydration of the bentonite with substantially non-saline liquid, such as deionised or potable water, can cause the bentonite to rapidly hydrate, creating a hydraulic containment barrier prior to contact of high ionic leachate with the geosynthetic clay liner 10. [0029] Pre-hydrated or un-hydrated geotextile clay liner 10 of the present invention can be installed as per known installation techniques, such as by delivering the geotextile clay liner 10 to site in a roll and unrolling the liner 10 over the area requiring a hydraulic barrier. Example: Resistance of Bentonite to High Saline-Sodic Conditions [0030] Four control bentonites (Cl to C4) and two bentonites that each underwent a beneficiation and modification process to impart properties to enable the bentonite to maintain a swollen state under saline-sodic conditions (B1, B2), were tested for their ability to withstand saline-sodic leachates. The bentonites were assessed on parameters of standard methods for fluid loss, swell index and saturated hydraulic conductivity. All tests were carried out using test leachates 10 that mimic potentially aggressive environments, that is, potentially incompatible leachates such as brine and seawater. The specific leachates used in the testing were: artificial seawater of 0.3M, 1 M and 2M NaCI respectively; and coal seam gas (CSG) surrogate brine consisting of 20% soluble sodium salts (2:1:1 NaCI:NaHCO 3 :Na2CO 3 ). The brine had pH in the range of 9.1 - 9.2. Swell Index and Fluid Loss [0031] Swell index tests were conducted by dropping a sample of bentonite powder directly into 90mL of the test saline leachate, then topping up with the leachate. Fluid loss tests were conducted by reacting the bentonite samples with the test leachates. All measurements were made gravimetrically and converted to volume based on the specific gravity of the leachate. [0032] Results of swell index and fluid loss from the four control bentonite products are shown in Table 1. All samples had low swell index (SI) and high fluid loss (FL) when reacted with the 20% saline-sodic brine. The samples also returned calculated saturated hydraulic conductivity (Kcaic) values of 8 x 10-10 m/s or greater, which is equivalent to a constant head of water moving 25mm or more per year. On the basis of these results, it would be expected that a GCL incorporating any of the control bentonites in a saline-sodic environment, such as containment of coal seam gas brines, would not provide sufficient containment for any period of time in excess of one year. Bentonite SI (mL/2g) FL (mL) Filter cake Kcaic (m/s) (mm) C1 4.0 103 4.0 8.2 x 10-10 C2 4.0 118 3.7 8.2 x 10-10 C3 5.0 97 5.9 1.3 x 10-9 C4 5.5 106 4.3 8.7 x 10-10 Table 1: Testing results of control bentonites with 20% saline-sodic CSG brine surrogate. [0033] Results of the test bentonites B1 and B2, reacted with the same saline sodic brine and artificial seawaters as the controls are given in Table 2. SI values for both B1 and B2 in 20% brine are double that of the control bentonites. FL 11 values of B1 and B2 are approximately 2.5x and 5x less respectively. Kcaic values are similarly lower than in the control group. These results indicate that B1 has some capacity to withstand strongly saline-sodic leachates, but has better capacity for performance at lower ionic strengths. B2 has excellent performance capacity in strongly saline and saline-sodic leachates. Bentonite Leachate SI (mL/2g) FL (mL) Filter cake Kcaic (m/s) (mm) B1 0.3M NaCI 8 17 2.4 7.6 x 10-11 s.d. (n=3) - 0.5 - 1.7 x 10-12 B1 | 20% brine 8.5 40 4.7 3.2 x 10-10 s.d. (n=8) - 3 0.3 1.4 x 10- 11 B2 | 2M NaCI 8 23 4.6 2.1 x 10-10 s.d (n=4) - 1 0.6 3.7 x 10-11 B2 1 20% brine 9 22 4.5 1.7 x 10-10 s.d (n=4) - 2 0.4 2.3 x 10-11 Table 2: Test bentonite sample results on 0.3M NaCI and 20% saline-sodic brine surrogate Triaxial Saturated Hydraulic Conductivity [0034] Saturated hydraulic conductivity tests, using flexible-walled triaxial permeameters, were performed on the test bentonites B1 in synthetic seawater (~0.6 M) and 1 M NaCI, and on B2 in 2M NaCI and 20% saline-sodic brine. Some tests were conducted by directly hydrating the dry bentonite product with the test leachate. Dry bentonite powder was lightly packed into the triaxial cell with carrier and cover geotextiles to mimic a GCL. [0035] Results of the triaxial hydraulic conductivity (ktriaxial) tests are shown in Table 3. These results demonstrate that both B1 and B2 have low ktriaxial values in both saline and saline-sodic leachates, indicating suitability for use in containing fluids in aggressive geochemical environments. Prehydration with deionised water has a weakly positive effect on ktriaxial values. This indicates that B1 and B2 are able to hydrate effectively even in high ionic strength leachates.

12 Bentonite Leachate Hydration method ktraxial (m/s) B1 Seawater No pre-hydration 3.1 x 10-11 Pre-hydrated with deionised 1.1 x 10-11 water 1 M NaCI No pre-hydration 1.9 x 10-11 Pre-hydrated with deionised 1.1 x 10-11 water B2 2M NaCI No pre-hydration 3.3 x 10 Pre-hydrated with deionised 2.0 x 10-11 water 20% brine No pre-hydration 7.3 x 10 Pre-hydrated with deionised 5.1 x 10-12 water Table 3: Triaxial hydraulic conductivity tests on bentonite samples [0036] The test bentonite products B1 and B2 have demonstrated remarkable hydraulic performance properties in presence of saline-sodic leachates compared to control bentonites. These results indicate that products B1 and B2 are resistant to the detrimental effects of the high ionic strength of saline-sodic leachates and are suitable for use in GCLs to be used in containing liquid in aggressive geochemical environments.

Claims (5)

1. A geosynthetic clay liner comprising at least one flexible carrier sheet layer, at least one flexible layer of bentonite disposed on top of the at least one flexible carrier sheet layer, and at least one flexible cover sheet disposed on top of the at least one layer of bentonite, wherein the layer of bentonite has properties that cause the bentonite to swell when contacted with a saline liquid to form a hydraulic barrier.

2. The geosynthetic clay liner of claim 1, wherein the geosynthetic clay liner has a saturated hydraulic conductivity of less than about 1 x 10-10 m/s, more preferably 5 x 10-11 m/s.

3. The geosynthetic clay liner of claim 1 or 2, wherein the bentonite has a mineralogical composition consisting of a major proportion of smectite and minor proportions of one or more of kaolin, quartz and anastase.

4. The geosynthetic clay liner of any one of claims 1 to 3, wherein the bentonite layer includes one or more additives, said one or more additives imparting qualities to maintain the bentonite in a swollen state under saline-sodic conditions.

5. The geosynthetic clay liner of any one of claims 1 to 4, wherein the bentonite is pre-hydrated prior to installation of the geosynthetic clay liner. GEOFABRICS AUSTRALASIA PTY LTD WATERMARK PATENT AND TRADE MARKS ATTORNEYS UIP1461AUOO