WO2009023915A1

WO2009023915A1 - A method of improving hydraulic performance of clay

Info

Publication number: WO2009023915A1
Application number: PCT/AU2008/001212
Authority: WO
Inventors: Will Peter Gates
Original assignee: Elco Solutions Pty Ltd
Priority date: 2007-08-20
Filing date: 2008-08-19
Publication date: 2009-02-26
Also published as: CL2008002455A1; AR067984A1; NZ583373A; PE20090857A1; ZA201001177B; EP2181220A1; BRPI0815569A2; AU2008288689A1; CA2696845A1; AU2008288689B2

Abstract

A method of improving hydraulic performance of clay comprising its modification by the addition to the clay of a silica component containing free silica or a siliceous material. The clay is advantageously a swelling sodium bentonite clay with smectite content greater than 70 wt%. The modified clay may be employed in geosynthetic clay liners used for the containment of strong alkaline solutions as well as other barrier systems.

Description

A METHOD OF IMPROVING HYDRAULIC PERFORMANCE OF CLAY

FIELD OF THE INVENTION

This invention relates to a method of improving hydraulic performance of clay. The clay may be modified natural clay or clay-based natural material, useful for application in geosynthetic clay liners or other barrier systems, to be applied to alkaline (high pH) liquid waste management. Methods of producing suitable modified clays for this purpose, and the modified clays themselves, are also encompassed by the invention.

BACKGROUND TO THE INVENTION

Stringent regulation and legislation due to public demand for improved waste containment systems that minimise the environmental risks associated with the processing and storage of waste solutions, liquids, liquors, leachates and wastewaters have caused the mineral processing industry to seek improved methods to contain, temporarily or otherwise, such wastes. Strongly alkaline waste liquids, such as those resulting from the processing of mineral ores, particularly bauxitic ores, constitute a significant threat to surface and groundwaters. Such wastes are regulated by various laws and, as such, their containment and subsequent processing and disposal pose serious engineering considerations.

Until recently, such liquors have been stored in evaporation ponds or are constituents of mineral tailings dams, where the hydraulic barrier consists of at least 900mm of compacted clay, often borrowed locally from soils. Because of the costs associated with procuring suitable materials and obtaining adequate compaction, compacted clay liner systems often have infiltration rates and hydraulic conductivities that are excessive for the containment of industrial waste water. Typically, hydraulic conductivity values of less than 10^'9 m/sec are required of compacted clay liners. These limitations, and more stringent regulations, make compacted clay liners obsolete in the more stringently regulated waste management industry.

Alternative barrier systems, such as those utilising geosynthetic clay liners (GCL) are commonly replacing compacted clay liners to achieve the required hydraulic conductivity to behave adequately as hydraulic barriers. Many GCL products are available, containing bentonites from different sources or which have undergone different pre-treatment. In a preferred GCL product, about 10mm of a beneficiated swelling sodium bentonite is contained between two sheets of geotextile. Sodium beneficiation of the bentonite enhances its wetting, dispersion and gel formation necessary for proper hydraulic barrier function, and enables a 10 mm thick GCL to perform as well or better than up to 900 mm of compacted clay liner as a hydraulic barrier to many aqueous solutions under a variety of conditions. However, current GCLs have lower than desired hydraulic performance to alkaline (high pH) solutions, more especially strongly alkaline solutions which may be produced by some industries and for which containment is required.

The problems associated with containing strongly alkaline solutions within a clay-based liner system may result from the combined effect of (1 ) high ionic strength and (2) strongly reactive conditions which can result in the dissolution and loss of the smectite component of the bentonite. Reaction of high ionic strength aqueous solutions (l>0.5M) with clays causes (i) colloidal clays to flocculate, (ii) collapse of gel structure and (iii) an increase in the connectivity of interparticulate pores, thereby destroying the hydraulic barrier properties of the bentonite. In the case of swelling bentonites, the smectite component is the reactive ingredient necessary for barrier function, so its loss due to dissolution will result in degradation of any hydraulic barrier capability.

SUMMARY OF THE INVENTION It is an object of the present invention to improve the hydraulic performance of clays or natural clayey materials, including bentonite or bentonite containing smectite, in alkaline solutions.

In the case of bentonite clays, it is a further object of the present invention to protect the active, or swelling component of the bentonite, e.g., the smectite clay mineral montmorillonite, from dissolution caused by alkaline solutions, liquids, leachates, liquors or wastewaters, particularly strongly alkaline solutions liquids, leachates, liquors or wastewaters, or those of high pH. With these objects in view, the present invention provides a method of improving hydraulic performance of clay, such as a bentonite or other clay material, comprising its modification by the addition to the clay of a silica component containing free silica or a siliceous material. The free silica or siliceous material may be added in dry form, for example as a powder, or aqueous form, for example as a stable colloidal suspension.

The material used as a source of free silica, not native to the bentonite, that is purposefully added to the clay in accordance with the method, is preferably the powdered form of activated or fumed silica (SiO₂), although any siliceous mineral, powdered or otherwise, such as opal CT, cristobalite, tridamite, quartz or other silica polytypes, siliceous material such as silica glass, volcanic ash, flyash, diatomaceous earth or other forms of natural or man made SiO₂, including organic sources of silica such as rice hulls or derivatives thereof, is suitable. The silica should be added to the clay at a rate between 1 and 50 wt%, preferably between 5% and 10 wt% of the mass of the clay to achieve improved hydraulic performance and barrier resistance to alkaline solutions, especially strong alkaline solutions.

Of particular concern is the hydraulic performance of natural bentonites, beneficiated bentonites or swelling sodium bentonites used in barrier technologies - in alkaline, especially strongly alkaline solutions, liquids, liquors, leachates or wastewaters. The hydraulic performance of swelling bentonites, such as sodium bentonites, to strongly alkaline solutions (solutions of high pH) liquors or leachates, such as those constituting wastes common in the mineral processing industries, or as are known to occur during the degradation of cement structures, may be enhanced by the addition of free silica or siliceous materials.

That is, free silica mineral species are thermodynamically highly reactive under alkaline conditions, and can be the source of silica for new mineral phases (Taubald et al., 2000; Claret et al., 2002; Ramirez et al., 2002; Sanchez et al., 2006). The invention disclosed here is that these new mineral phases, if forming within the pore space of high swelling bentonites with low permeability (<10^'1° m/s) to the high pH liquid, can minimise the detrimental effects of ionic strength through precipitation and pore filling by new mineral phases, thereby enhancing the hydraulic performance of the bentonite. Thus, the addition of free silica to bentonites is deemed advantageous for hydraulic barrier quality to liquids of high pH.

Swelling bentonites, such as swelling sodium bentonites or other beneficiated bentonites treated in accordance with the method preferably contain >70 wt% smectite which is preferably saturated with at least 80 wt % sodium (Na). The sodium bentonite or Na smectite clay may be in its natural sodium form or beneficiated to create a sodium form. Organic polymers, organic cations or any organic ionic and non-ionic materials known to improve clay dispersion and gel formation may be contained within the modified smectite clay, especially sodium smectite clay, but these are expected to be adversely affected by reaction with strongly alkaline pH.

The swelling sodium bentonites may be themselves processed to a preferably powdered form, a granulated form or a pelletised form suitable for efficient encapsulation within a geosynthetic clay liner, or be used in their natural unprocessed state in compacted clay liners or other barrier systems. Further, the swelling sodium bentonites may be in a dry form or variously hydrated with water or other suitable solvents to contain water at a controlled saturation involving partial pressures of water vapour less than 1 atmosphere, preferably fully saturated by water, but also may be in an uncontrolled and unsaturated state, as may be expected in many field applications.

The swelling sodium bentonites may be alternatively activated or beneficiated to contain any suitable exchange cation such as, but not limited to H, Li, Na, K, Mg, Ca, Rb, Sr, Ba and Cs as well as transition metals and lanthanides. Improving the hydraulic performance of the clay - as represented by parameters such as lowered permeability or hydraulic conductivity to permeating solutions - may be achieved, at least in part, by protecting the active ingredient of the clay, advantageously swelling sodium bentonite, from dissolution in alkaline solution. Such an active ingredient may be a smectite selected from the swelling 2:1 layer silicate group consisting of montmorillonite, beidellite, nontronite (chloropal), saponite, hectorite, stevensite and synthetic analogs thereof. Montmorillonite is of special interest. Other suitable clays for treatment, that is modification, in accordance with the method include other swelling clay minerals such as vermiculites and non-smectite clay minerals such as kaolin, illite, palygorskite (attapulgite) and sepiolite, or synthetic analogs thereof, and natural or synthetic layer double hydroxides or hydrotalcites. Again, these clays are preferably in the natural, though possibly beneficiated, form.

Such protection from dissolution may be achieved whether the modified clay is used alone or as a component of a geosynthetic clay liner or material used as a component of compacted clay liners for containment of liquors, groundwaters or leachates. Use of the modified clay in applications other than geosynthetic clay liners is also possible. Non limiting examples are barrier systems including bunds or encasement structures made from compacted clay, slit trenches and cut-off walls formed from slurries of the modified clay, or the modified clay could be incorporated in other manufactured clay-based composite barriers.

The addition of free silica, or siliceous materials may be in sufficient quantity to achieve a target SiO₂ or free SiO₂ analysis at which acceptable hydraulic performance of the clay is achieved. Addition at a rate of at least 1 wt% and as much as 50 wt%, but preferably 5 - 10 wt% SiO₂ by weight of the clay, provides considerable protection against dissolution of the active ingredient of bentonite, thus providing a greater potential lifetime of a geosynthetic clay liner (GCL) to alkaline solutions, particularly strongly alkaline solutions or solutions of high pH. Non-limiting examples of such solutions include aqueous solutions having pH greater than or equal to 9 such as mine waste leachates, mineral ore processing liquors, light metal (e.g Mg, Al, Ti, Ni and so on) including Bayer and other Bauxite processing liquors, alkaline abattoir wastewaters and food processing wastewaters, as well as leachates from cement-based waste storage impoundments often used in radioactive waste containment (Landais and Aranyossy, 2004).

Without wishing to be bound by any theory, the present Applicant postulates that the hydraulic performance and smectite protection mechanism is related to the dissolution of native colloidal (< 0.2 micron) free silica, which improves the swelling performance of bentonite which contains native free silica. Strongly alkaline solutions are considered to be largely non-compatible with most Na-bentonites: the high salinity is expected to cause detrimental changes to the gel structure of the hydrated bentonite and cause substantial increases in void ratio (Figure 1 ). High ionic strength induced flocculation and opening of pore volume within the bentonite results in large increases in saturate hydraulic conductivity, or k, by as much as 40 to 100 times (arrow labelled 1 in Figure 1 ).

However, the Applicant has found that bentonite incompatibility to strongly alkaline permeants may be minimal if the bentonite contains native colloidal free silica and displays the properties given in Tables 1-3 (see below) and/or has additional free silica, in the form of activated silica, added. The high solution pH has an opposing effect to the high salinity on the void ratio of hydrated bentonite (arrow labelled 2 in Figure 1). High pH results in (i) dispersion of bentonite particles, and (particularly when the initial k is <10^"10 m/s), also (ii) pore clogging due to precipitation of new mineral phases as evidenced in our experiments. Such new siliceous precipitates may include, but are not limited to, hydrous and hydroxylated aluminosilicate minerals such as sodium aluminium silicate and/or zeolite-like minerals and/or hydrous carbonate precipitates such as alumohydocalcite and/or more crystalline aluminosilicate minerals such as vuagnatite and philipsite, all which form at elevated pH, in association with smectite surfaces or otherwise. Together, such reactions may serve to enhance or maintain barrier performance to these liquors.

In a further aspect, the present invention provides a method for production of clay liners, such as, geosynthetic clay liners, containing, as the predominant active ingredient, swelling sodium bentonite, in a preferably powdered form, as a preferably natural or beneficiated material, comprising adding free silica or siliceous material in an appropriate concentration to the swelling sodium bentonite and intimately mixing the free silica or siliceous material and bentonite to form a modified clay mixture.

The modified clay mixture may either be incorporated into a geosynthetic clay liner during its manufacture or the modified clay mixture is added subsequent to the manufacture of the geosynthetic clay liner. The clay liner incorporating the modified clay need not be a geosynthetic clay liner. Non limiting examples of its use are compacted clay liners, compacted clay encasements, polymer-bentonite composite liners, or as a clay suspension or slurry used to form cut-off walls or slit trenches for sub-surface containment of wastewaters, processing wastes, leachates and groundwaters. The modified clay may be used in hydraulic barrier systems including those formed from compacted bentonite or from hydrated clay or bentonite slurries. Such barrier systems may include sealants composed of bentonite-cement mix or polymer-bentonite mix.

An amount of free silica or siliceous material may be added to the swelling bentonite clay at a rate related to the analysis of raw bentonite clay. The silica component may be added to, and mixed with, the bentonite component of the GCL. In this case, free silica or siliceous material is added in desired proportion, as above described, directly to dry sodium bentonite and intimately mixed with the sodium bentonite, prior to incorporation of the mixture into a geosynthetic clay liner during manufacture of the GCL.

Alternatively, the silica component, for example in the form of aqueous silica, may be added directly to the pre-manufactured geosynthetic clay liner. The free silica or siliceous material may be added separately from, but immediately after, the addition of dry sodium bentonite to the geosynthetic clay liner (GCL), for example, being applied at, or incorporated within, the surface of the GCL furthest from initial contact with alkaline solutions. The surface may be composed of synthetic woven geotextiles, non-woven geotextiles, or composite geotextiles which contain the sodium bentonite or mixture of bentonite with free silica or siliceous material. The modified clay mixture may itself be applied, at or incorporated within, the surface of the GCL furthest from initial contact with alkaline solution.

Alternatively, the free silica or siliceous material may be added in desired proportion, as above described, directly to, and incorporated within, the surface of the GCL that has initial contact with alkaline solutions. This surface may also be comprised of synthetic materials, such as a woven geotextile, nonwoven geotextile, or composite geotextile which can hold or otherwise contain the desired proportion of added free silica or siliceous material and bentonite. The modified clay mixture may itself be applied at, or incorporated within, the surface of the GCL initially contacting alkaline solution. The free silica may be alternatively added as a stable colloidal suspension in aqueous solution to a post-manufactured geosynthetic clay liner. Addition may occur immediately subsequent to installation on site, but prior to the addition of geomembranes or covering with a standard drainage layer. The colloidal suspension may be sprayed directly onto the surface of the geosynthetic clay liner that will have initial direct contact with the alkaline solutions. Addition of the colloidal silica suspension serves additionally to establish pre-hydration of the geosynthetic clay liner to optimise the hydraulic properties of the geosynthetic clay liner prior to contact with alkaline solutions, especially strongly alkaline solutions.

In a further aspect of the invention, suitable for raw clays having a free silica analysis above 5-10 wt%, the present invention provides a method of producing a clay barrier to alkaline solution comprising (a) analysing free silica content of the clay; (b) comparing the determined free silica content of the clay with a target value associated with alkaline resistance; (c) contacting the clay, with acceptable free silica analysis above 5-10 wt% silica, with an alkaline solution; and (d) employing the contacted clay, with acceptable free silica analysis, as a barrier to alkaline solutions. The clay produced by the method may be used in a geosynthetic clay liner (GCL).

DETAILED DESCRIPTION OF THE INVENTION

The methods and clay products of the present invention may be more fully understood from the following description including non-limiting embodiments and examples of the various aspects of the invention.

The following embodiment of the present invention relates to a method of manufacture of a clay, for example a bentonite clay, for use in a geosynthetic clay liner (GCL) in which the clay is modified by addition of free silica. In this case, free silica is added either to the bentonite component of the GCL; to the geotextile components of the GCL; or by addition of aqueous silica directly to the GCL during manufacture, or subsequent to GCL manufacture.

The bentonite clay used as raw material, whether or not with prior beneficiation, is composed predominantly of substantially water-free swelling sodium smectite. As described below, while siliceous impurities such as colloidal opal CT, cristobalite, tridamite and quartz may be present in small percentages, it is generally recognized that bentonites suitable for barrier systems such as GCLs should be composed of >70 wt% smectite and preferably >90% smectite. Lower percentages of the smectite component may be acceptable if >90% of the smectite present has an average fundamental particle size of <0.2 microns. If the bentonite is not natively in a sodium (Na) form, it should be beneficiated by suitable means, for example as described in Murray (1995).

The components of a typical bentonite suitable for the embodiments of this invention are listed in Tables 1 , 2 and 3.

Table 1. Particle size distribution and montmorillonite content of a bentonite suitable for providing good barrier performance to strongly alkaline solutions.

Table 2. Mineralogy of a bentonite suitable for providing good barrier performance to strongly alkaline solutions.

Most of the bentonite (>60 wt % of the bulk) is composed of particles less than 0.5 microns in size (See Tables 1 and 2) and 97% of the <0.2 micron fraction is montmorillonite. Montmorillonite is the predominant component of the bentonite in all particle size fractions < 2 micron, and the finer fractions (< 0.5 micron) are essentially pure montmorillonite (See Table 2). The small amount of non- montmorillonite mineral present in the <0.2 micron fraction is represented by native free silica (Siθ₂) in the form of opaline silicates. It may be noted that the SiO₂ content of both the bulk and finer fractions are elevated (by about 5 - 10% of SiO₂) relative to a bentonite having no free silica (See Table 3). The chemical analysis provided in Table 3 represents a Na form of bentonite having >80% of its exchange complex saturated by Na.

Table 3. Major oxides for the bulk and <0.2 micron fractions of a bentonite suitable for providing good barrier performance to strongly alkaline solutions.

Example 1

The following example provides an indication of the reactivity of various mineral components in bentonite with strongly alkaline pH liquids. A bentonite having the mineralogy, particle size distribution and chemical composition as described in Tables 1 to 3, as well as the same bentonite to which was added 10 wt% powdered activated silica, was reacted at room temperature for six (6) months with 1 M NaOH solution. Portions of the reacted material were either (i) dried as is, (ii) filtered, (iii) centrifuged or (iv) washed by centrifugation. Dried solids were studied by X-ray diffraction (XRD), infrared spectroscopy and thermal gravimetric analysis (TGA). Elemental analysis of the reaction solution after 6 weeks was measured using optical emission spectroscopy. SOLIDS ANALYSIS

Figure 2 provides XRD traces (Cu Ka radiation) of reaction products, after 6 weeks and 6 months contacting with 1 M NaOH, of bentonite containing native colloidal free silica (lower trace of each set) and bentonite containing 10% activated silica (upper trace of each set), showing loss of free silica and the formation of hydrous aluminosilicate and carbonate minerals. S=smectite, Q=quartz, O=opal, F=feldspar, SAS=sodium aluminium silicate, T=trona, V=vuagnatite, AHC=alumohydrocalcite. Results from XRD (Figure 2, Table 4) for reaction of Na bentonite with 1 M NaOH for 6 weeks and 6 months can be summarized as follows:

1. Reflection for opal-CT (indexed as cristobalite) at 22 °2Θ (4.04 A) loses >60% of its original intensity after 6 weeks and nearly 80% of its orginal intensity after 6 months.

2. Reflections for quartz at 20.8, 24.4, 26.6, 36.5, 50.1 and 60 °2Θ (4.27, 3.35, 2.46, 1.82 and 1.54 A respectively) lose about 8-10% of their original intensity after 6 weeks and 20% of original intensity after 6 months.

3. Reflections for montmorillonite d(00l) first five orders near 7.1 , 14.2, 21.6, 28.8 and 35.7 °2Θ (12.6, 6.23, 4.11 , 3.09 and 2.51 A, respectively) lose about 60% of their original intensity on exposure to a strongly alkaline solution for as long as 6 months.

4. Reflections for montmorillonite d(hkl) at 19.7, 34.9 and 61.0 °2Θ (4.49, 2.57 and 1.50 A, respectively) lose about 10% of their original intensity after 6 months reaction.

5. Following 6 weeks reaction reflections associated with sodium aluminium silicate hydrate (24.48 °2Θ, 3.63 A) as well as the carbonate evaporate trona (29.24, 30.00 °2Θ, 3.05 and 2.97 A, respectively), are observed.

6. Following 6 months reaction further reflections associated with the hydrous aluminosilicate vuagnatite (29.94 and 35.56 °2Θ; 2.98 and 2.52 A, respectively) and alumohydrocalcite (27.56 and 34.90 °2Θ, 3.23 and 2.57 A) precipitates, are observed.

Table 4. Changes to integrated intensity of reflections for mineral components after reaction with 1 M NaOH.

SAS = sodium aluminium silicate; AHC = alumohydrocalcite

Results from XRD (Figure 2, Table 4) for reaction of Na bentonite having 10 wt% powdered activated silica with 1 M NaOH for 6 weeks and 6 months can be summarized as follows:

7. Addition of 10 wt % free silica as activated silica observably increased the reflection at 15.5 A (6.22 °2Θ) associated with a 2 hydrate smectite component

8. Addition of 10 wt% free silica as activated silica observably reduced the loss of smectite reflections - the d(00l) reflections lose about 50% of their original intensity, but broaden and shift to lower 2Θ and the d(hkl) lose about 20% of their original intensity.

9. Addition of 10 wt% free silica as activated silica observably decreases the loss of the reflections associated with native opal CT after 6 months.

10. Addition of 10 wt% free silica as activated silica observably increases the precipitation of hydrous aluminosilicates and carbonates after 6 weeks. Figure 3 provides Infrared spectra of bentonite (lower trace of each pair) and bentonite with 10 wt% free silica added (upper trace of each pair) following 6 weeks reaction with 1 M NaOH. Fig 3a - details presence of absorbance bands near 3475, 3378, 3246 and a broad band near 2950 - cm^"1, associated with hydrous aluminosilicate phases in addition to the smectite band near 3630 cm^"1. Fig 3b - details appearance of absorbance bands near 1430, 1475 and 852 cm^"1 as carbonates, shift of main band from 1030 to 1046 cm^"1, loss of absorbance bands near 780 - 800 cm^"1 and protection of absorbance bands associated with smectite near 848, 880 and 915 cm^"1 with addition of 10% activated silica. Results from Infrared Spectroscopy (Figure 3, Table 5) for reaction with 1

M NaOH for about 6 weeks indicate the formation of a hydrous new phase. The results can be summarized as follows:

1. After 6 weeks reaction with 1M NaOH, broad overlapping bands near 3420, 3250 and 3030 cm^"1 associated with water adsorbed to the smectite surfaces substantially disappear or shift to lower wavenumber, centred near 2990 cm^"1.

2. Following 6 months reaction, the adsorbed O-H stretching bands reveal a substantial shift to higher wavenumbers 3026 cm^"1, but also several unique bands form near 2848, 3246, 3378 and 3575 cm^"1, indicate the development of structural OH bands associated with hydrous mineral phases.

3. Changes to quartz are observable as a shift in the absorbencies due to Si-O stretching band from 1032 to 1046 cm^"1 and decrease in the doublet at 802 and 787 cm^"1.

2. Changes to the smectite component of bentonite, observed as decreases in OH bending bands e (848, 881 and 916 cm^"1) associated with octahedral coordinated cations, indicate the loss of smectite structural integrity.

5. Addition of 10 wt% free silica as activated silica observably decreased the loss of absorbance bands associated with smectite and for interlayer cationic water. 6. A band near 1460 cm^"1 is observed in the sample reacted for 6 weeks, that shifted from ~1490 cm^"1 in the original material. This band resolves to two bands near 1475 and 1430 cm^"1 following 6 month reaction, and is accompanied by a band at 852 cm^"1, all consistent with the formation of carbonate minerals. Washing, by filtration or centrifugation, indicated the new phases associated with these bands were soluble in water.

Table 5. Details of changes to the IR spectra of bentonite when reacted in 1 M NaOH.

Results from thermal gravimetric analysis (Table 6 and Figure 4 which illustrates thermal gravimetric analysis of bentonite (Fig. 4a) and bentonite including 5 wt% free silica as activated silica (Fig 4b) and the same materials following 6 weeks reaction with 1 M NaOH (Figs 4a.1 and 4b.1 ) detailing changes to the temperatures at which dehydroxylation events occur) indicate that reaction of bentonite for 6 weeks, with and without 5 wt% free silica as activated silica, to strongly alkaline solutions indicate the formation of a hydrous new phase. The results can be summarized as follows.

1. Formation of a hydrous new phase having a dehydroxylation event centred near 605 ⁰C for bentonite, and 570 ⁰C for bentonite with 5 wt% added free silica, after reaction with NaOH. 2. The dehydroxylation temperature of the new phase is intermediate between the two dehydroxylation events associated with the parent bentonite (-490 and -660 ⁰C).

3. Addition of free silica as activated silica decreases the dehydroxylation temperature of the new phase.

Table 6. Temperatures of the dehydroxylation events.

Example 2 The following example provides an indication of the hydraulic performance of a bentonite containing native colloidal free silica when subjected to strongly alkaline and saline solutions. A fluid (ore processing leachate) reported to have the following properties: density of about 1.04 g/cm³; initial pH of 12.4; with dominant anions and cations of Na (12700 ppm), Cl (1070 ppm), Al (3360 ppm) and S (460 ppm) was used. The measurements were conducted on 4.7 mm thick, 76 mm diameter powdered sodium bentonite material extracted from an unused, geosynthetic clay liner (GCL) as received from the factory to establish hydraulic performance of the sodium bentonite clay when contacted with the strongly alkaline fluid or solution. Figure 5 shows the hydraulic conductivity (k value) measured as a function of hydrostatic head of alkaline leachate for the bentonite. Also shown is an estimated k value (6 x 10^~10 m/s) for a hydraulic head of 800 mm, which would be typical for a worse-case scenario for the entire 800 mm of a standard drainage layer being fully saturated. Example 3

Geosynthetic clay liner samples containing the bentonite with properties described in Tables 1-3, and also geosynthetic clay liner samples containing the same bentonite plus 10 wt% silica as activated (fumed) silica, were permeated with deionised water, with water containing 0.1 M alkali chloride salt and with 1 M NaOH containing 0.1 M alkali chloride salt. Procedures outlined in ASTM D5887 for permeability and ASTM D5890 for free swell were followed. Results are summarised in Table 7.

1. Addition of 10 wt% silica to the geosynthetic clay liner yielded similar hydraulic performance to deionised water, water containing 0.1 M dissolved chloride salts or to 1 M Na OH solutions containing 0.1 M dissolved salts, as measured by the saturated hydraulic conductivity.

2. Addition of 10 wt% SiO₂ as activated silica improved the swelling of the geosynthetic clay liner in deionised water, water containing 0.1 M dissolved salts and 1 M NaOH solutions containing 0.1 M dissolved salts, as indicated by an increase in GCL thickness.

Table 7. Performance of geosynthetic clay liner to various liquids

Example 4

The following example provides further indication of the hydraulic performance of a bentonite containing native colloidal free silica when subjected to strongly alkaline and saline solutions. The same geosynthetic clay liners used in Example 3 that had been permeated with water containing 0.1 M dissolved alkali chloride salts and 1 M NaOH containing 0.1 M dissolved salts were examined by scanning electron microscopy (SEM). Specimens were flash frozen in liquid nitrogen, cut polished and examined after freeze drying. Results are shown in Figures 6, 7 and 8 which respectively provide: Fig 6: Backscattered electron micrographs of geosynthetic clay liner

(having no added silica) permeated with water containing 0.1 M dissolved salts (Fig 6a) and with 1 M NaOH containing 0.1 M dissolved salts (Fig 6b).

Fig 7: Backscattered electron micrographs of a geosynthetic clay liner (bentonite only) reacted with 1 M NaOH containing 0.1 M dissolved salts and EDS spectra of the regions circled indicating chemistry of montmorillonite (Fig 7a) and of a silica rich phase (Fig 7b).

Fig 8: Backscattered electron micrographs depicting formation of aluminosilicate phases (web-like material in Fig 8b) in a geosynthetic clay liner containing bentonite plus 10 wt% SiO₂ as activated silica (larger angular particles in Fig 8a) reacted with 1 M NaOH + 0.1 M dissolved salts. Results can be summarised as follows.

1. reaction of the geosynthetic clay liner containing only bentonite with 1 M NaOH (containing 0.1 M dissolved salts) resulted in a contraction of clay fabric (Figure 6). 2. reaction of the geosynthetic clay liner containing only bentonite with

1 M NaOH (containing 0.1 M dissolved salts) resulted in formation of silica rich phases not observed in the sample reacted in 0.1 M dissolved salts.

3. reaction of the geosynthetic clay liner containing bentonite plus 10 wt% SiO₂ with 1 M NaOH (containing 0.1 M dissolved salts) resulted in the development of a web-like aluminosilicate precipitate which potentially can fill pores. Example 5

The bentonite with properties as described above in Tables 1 - 3, with and without 10 wt% activated SiO₂, as well as a bentonite with similar chemistry, but without having native colloidal opaline silica, and that bentonite with 10 wt% SiO₂ added as activated silica, were reacted with an ore processing leachate having similar properties as described in Example 2, for 6 weeks. 50 grams of bentonite or bentonite mixture were reacted with 200 ml of liquor. For both bentonites in which 10 wt% SiO₂ was added as activated silica, the resulting mixture produced a thick slurry with little excess or free water, indicating enhanced swelling and gel formation of the bentonite to the higher ionic strength solution when activated silica was added. Results from X-ray diffraction can be summarised as follows:

1. Reaction of bentonite 1 (which contained native colloidal free silica) with alkaline mineral ore processing leachate resulted in precipitation of alumohydrocalcite (27.56 and 34.90 °2Θ, 3.23 and 2.57 A), as well as sodium aluminium silicate hydrate (24.48 °2Θ, 3.63 A) and the carbonate evaporate trona (29.24, 30.00 °2Θ, 3.05 and 2.97 A, respectively).

2. Reaction of bentonite 1 , with 10% added silica as fumed silica, resulted in precipitation of hydrous aluminosilicate vuagnatite (29.94 and 35.56 °2Θ; 2.98 and 2.52 A, respectively), alumohydrocalcite (27.56 and 34.90 °2Θ, 3.23 and 2.57 A), as well as sodium aluminium silicate hydrate (24.48 °2Θ, 3.63 A) and the carbonate evaporate trona (29.24, 30.00 °2Θ, 3.05 and 2.97 A, respectively).

3. Reaction of bentonite 2 (which had no native colloidal free silica) with alkaline mineral ore processing leachate did not result in precipitation of alumohydrocalcite, but the carbonate evaporate trona (29.24, 30.00 °2Θ, 3.05 and 2.97 A, respectively) was observed.

4. Reaction of bentonite 2, with 10% added silica as fumed silica, resulted in precipitation of alumohydrocalcite (27.56 and 34.90 °2Θ, 3.23 and 2.57 A), sodium aluminium silicate hydrate (24.48 °2Θ, 3.63 A), as well as the carbonate evaporate trona (29.24, 30.00 °2Θ, 3.05 and 2.97 A, respectively). Taken together, the above experiments detailed in the examples and their associated data demonstrate that: a) strongly alkaline (high pH) liquids cause preferential dissolution of any native colloidal free silica in the bentonite. The native opal CT (cristobalite) phase, as well as colloidal quartz, in the bentonite reacted with 1 M NaOH. Further, reaction with 1 M NaOH resulted in the nearly complete removal of the opalline silica phase as observed in XRD and considerably reduced the absorbance bands for Si-O, associated with quartz, opal or glass observed in the IR data (Example 1). b) addition of free silica in the form of siliceous minerals or materials enhances the protection of the bentonite against strongly alkaline (high pH) liquids. The XRD, IR and SEM results indicate that reaction of bentonite with 1 M NaOH results in partial dissolution of the smectite component, but that 10 wt % added silica substantially protects the smectite from dissolution, and promotes the formation of new mineral phases (Example 1). IR results further indicate that addition of 10 wt% additional free silica (as activated silica) provides protection for the bentonite against dissolution as, for example, the intensity of the band near 880 cm^"1, which is less affected by 1 M NaOH when 10 wt% activated silica is added to the bentonite. c) XRD data provides evidence that the dissolution of native silica, and the addition of 10 wt% Siθ₂ as activated silica enhances the swelling of the bentonite. These results are observed as a reduction in the ratio (R) of d(001 )/d(02,11 ) bands for bentonite containing 5 wt% activated silica (R=O.97) compared to bentonite without free silica added (R=1.36) and an increase in the d(001 ) spacing to 15.5 A following 6 weeks or 6 months reaction when 10 wt% silica is added (Example 1 ). d) The above reactions result in pore clogging via enhanced swelling of the bentonite. The loss of higher order (00I) reflections (e.g d(002)) in the XRD patterns following reaction of the bentonite containing native free silica (indexed as cristobalite) indicate that significant disruption of ordered layer stacking within the bentonite occurred, due to dispersion and/or loss of ordered stacking domains associated with dissolution of silica. The value of the ratio R (d(opal)/d(smectite)) for the original bentonite was 1.84, but dissolution of the opal reduced that ratio by half when no free silica was added. Colloidal opalline silicas are known to be intimately associated with crystalline layers of bentonites, involved in cementation of stacking domains, and their presence in close association with montmorillonite is probably a limiting factor to the swell index (-24 ml/ 2 g) of the bentonite used (properties listed in Tables 1-3). The swelling index of bentonites which contain native colloidal free silicates is increased upon the removal of the native free silicates and importantly, reaction with 1 M NaOH results in a swell index as high as the original material (Example 3). Thus, the removal of the cementing silica through dissolution in strongly alkaline solutions, enhances the dispersion and swelling of the bentonite. e) The reactions of free silica with alkaline solution can result in pore clogging via precipitation of new mineral phases, probably poorly crystalline sodium silicates, within the pore spaces of the bentonite, particularly for (i) when the bentonite has >90% of its fundamental particle size < 0.2 μm, as for the bentonites used here, and (ii) when the saturated hydraulic conductivity of the bentonite to water is < 1x10^"10 m/s. IR spectra and TGA analysis (example 1 ) and SEM imaging (Example 4) all provide evidence for the presence of a poorly crystalline and/or amorphous (glass-like) phase, which is mostly opaque to XRD, in the reacted products. These results are observed as the presence of a dehydroxylation mass loss centered between 570 and 605 ⁰C in the TGA plots. They are also observed as the presence of a broad band centred near 3000 cm^"1, some of which was removed by washing with deionised water (pH 6.8). Increased aging improved crystallinity of the new phases formed. Electron microscopy with associated spectroscopic analyses provides evidence mineral phases formed are enriched in silica and aluminium, but have different chemistry than smectite. f) Barrier performance of sodium bentonite containing colloidal free silica is not adversely affected by reaction with strongly alkaline (high pH) liquids, particularly 1 M NaOH (Example 3), and the hydraulic performance is enhanced when exposed to ore processing liquors (Example 2). g) Barrier performance of sodium bentonite containing free silica to strongly alkaline pH solutions, is not adversely affectedly by the addition of 10 wt% SiO₂ as activated silica powder (Example 3). h) Addition of 10 wt% activated silica to bentonite enhanced smectite swelling when exposed to strongly alkaline solutions: the swelling index was the same as for the original betonite in deionised water (Example 3). Modifications and variations to the method of improving hydraulic performance of a clay with modification by free silica or siliceous material, the modified clay and method for its production, as described above, may be apparent to the skilled reader of this disclosure. Such modifications and variations are within the scope of the present invention.

REFERENCES

ASTM D5887-Appendix X2 - 'Standard Test Method for Measurement of Index Flux Through Saturated Geosynthetic Clay Liner Specimens Using a

Flexible Wall Permeameter - Calculation of Hydraulic Conductivity Using

Index Flux Test Data' ASTM D5890 - 'Standard Test Method for Swell Index of Clay Mineral

Component of Geosynthetic Clay Liners' Claret F (2002) Experimental investigation of the interaction of clays with high-pH solutions: A case study from the Callovo-Oxfordian formation, Haute Marne underground laboratory (France). Clays and Clay Minerals 50: 633-646 Landais P, Aranyossy JF (eds) (2004) Clays in natural and engineered barriers for radioactive waste confinement (ANDRA meeting, Reims, 2002). Applied Clay Science 26: 1-539

Murray, HH (1995), Clays in Industry and Environment, pp 49-55 In Churchman,

GJ, Fitzpatrick, RW and Eggleton, RA (Eds.) Clays controlling the

Environment, CSIRO Publishing, Melbourne.

Ramirez S, Cuevas J, Vigil R, Leguey, S (2002) Hydrothermal alteration of "La Serrata" bentonite (Almerfa, Spain) by alkaline solutions. Applied Clay

Science 21 : 257-269 Sanchez L, Cuevas, J, Ramirez S, Riuiz De Leon D, Fernandez R, Vigi DeIa Villa

R, Leguey S (2006) Reactions of FEBEX bentonite in hyperalkaline conditions resembling the cement-bentonite interface. Applied Clay Science 33:125-141.

Taubald H, Bauer A, Schafer T, Geckeis H, Satir M (2000) Experimental investigation of the effect of high-pH solutions on the Opalinus shale and the Hammerschmiede smectite. Clay Minerals 35 515-524.

Claims

CLAIMS:

1. A method of improving hydraulic performance of clay comprising its modification by the addition to the clay of a silica component containing free silica or a siliceous material.

2. The method of claim 1 wherein the clay is a bentonite.

3. The method of claim 1 or 2 wherein the free silica or siliceous material is added in dry form.

4. The method of claim 1 or 2 wherein the free silica or siliceous material is added in aqueous form, for example in the form of a stable colloidal suspension.

5. The method of claim 3 wherein the free silica is added in the powdered form of activated silica.

6. The method of claim 3 wherein the siliceous material is selected from the group consisting of opal CT, cristobalite, tridamite, quartz or their other polytypes, silica glass, volcanic ash, flyash, diatomaceous earth and organic sources of silica.

7. The method of any one of the preceding claims wherein silica (SiO₂) is added at a rate between 1 and 50 wt% of the mass of the clay.

8. The method of claim 7 wherein silica is added at a rate between 5 and 10 wt% of the mass of the clay.

9. The method of any one of claims 2 to 8 wherein said bentonite is a swelling bentonite.

10. The method of claim 9 wherein said swelling bentonite contains greater than 70 wt% smectite.

11. The method of claim 10 wherein said smectite is saturated with at least 80 wt% sodium.

12. The method of claim 11 wherein said clay is in its natural sodium form or beneficiated to create a sodium form.

13. The method of any one of the preceding claims wherein the clay is processed into a powdered form, a granulated form or a pelletised form suitable for encapsulation within a geosynthetic clay liner.

14. The method of claim 9 wherein said swelling bentonite is in dry form.

15. The method of claim 14 wherein said swelling bentonite is hydrated.

16. The method of claim 15 wherein the swelling bentonite is partly saturated by water.

17. The method of claim 16 wherein the swelling bentonite is fully saturated by water,

18. The method of any one of claims 9 to 17 wherein said bentonite is activated or beneficiated to contain an exchange cation.

19. The method of claim 18 wherein said exchange cation is selected from the group consisting of H, Li, Na, K₁ Mg, Ca, Rb, Sr, Ba and Cs, transition metals and lanthanides.

20. The method of any one of the preceding claims wherein an active ingredient of the clay is protected from dissolution in alkaline solution.

21. The method of claim 20 wherein the active ingredient is smectite selected from the swelling 2:1 layer silicate group consisting of montmorillonite, beidellite, nontronite (chloropal), saponite, hectorite, stevensite and synthetic analogs thereof.

22. The method of claim 21 wherein said active ingredient is montmorillonite.

23. The method of claim 1 wherein the clay includes a swelling clay mineral selected from the group consisting of vermiculites and non-smectite clay minerals including kaolin, illite, palygorskite (attapulgite) and sepiolite, and layer double hydroxides (hydrotalcites) or synthetic analogs thereof.

24. A modified clay produced by the method of any one of the preceding claims.

25. Use of the modified clay of claim 24 in a geosynthetic clay liner.

26. Use according to claim 25 in which the geosynthetic clay liner is used for containment of an alkaline solution.

27. Use according to claim 26 wherein the alkaline solution has a pH greater than or equal to 9.

28. Use according to claim 27 wherein the alkaline solution is selected from the group consisting of mine waste leachates, mineral ore processing liquors, light metal processing liquors, alkaline abattoir wastewaters, food processing wastewaters, alkaline groundwater and surface waters and leachates from cemented radioactive waste containment systems

29. A method for the production of clay liners containing, as the predominant active ingredient, swelling sodium bentonite, comprising adding free silica or siliceous material in an appropriate concentration to the swelling sodium bentonite; and intimately mixing the free silica or siliceous material and bentonite to form a modified clay mixture.

30. The method of claim 29 wherein the modified clay mixture is incorporated into a geosynthetic clay liner during its manufacture.

31. The method of claim 29 wherein the modified clay mixture is added subsequent to the manufacture of the geosynthetic clay liner.

32. The method of any one of claims 29 to 31 wherein the amount of free silica or siliceous material added to the swelling sodium bentonite clay is at a rate related to the silica analysis of raw swelling bentonite clay.

33. The method of any one of claims 29 to 32 for containing alkaline solutions wherein modified clay mixture containing free silica or siliceous material, is applied at or incorporated within, a surface of the liner furthest from initial contact with strongly alkaline solutions.

34. The method of any one of claims 29 to 33 for containing alkaline solutions wherein modified clay mixture, free silica or siliceous material is applied at, or incorporated within, the surface of the liner initially contacting alkaline solution.

35. The method of claim 33 or 34 wherein the surface is comprised of synthetic materials selected from the group consisting of synthetic woven geotextiles, non-woven geotextiles and composite geotextiles.

36. The method of claim 29 wherein free silica is added as a stable colloidal suspension in aqueous solution to a post - manufactured geosynthetic clay liner.

37. The method of claim 36 wherein the colloidal suspension is sprayed directly onto the surface of the geosynthetic clay liner that will have initial contact with alkaline solutions.

38. The method of claim 36 or 37 wherein addition of colloidal suspension of free silica pre-hydrates the geosynthetic clay liner.

39. A clay liner produced by the method of any one of claims 29 to 38.

40. A method of producing a clay barrier to alkaline solution comprising: (a) analysing free silica content of the clay;

(b) comparing the determined free silica content of the clay with a target value associated with alkaline resistance;

(c) contacting the clay, with acceptable free silica analysis above 5-10 wt% silica, with an alkaline solution; and

(d) employing the contacted clay, with acceptable free silica analysis, as a barrier to alkaline solutions.

41. Use of the clay produced by the method of claim 40 in a geosynthetic clay liner.

42. Use of the modified clay of claim 24 or 40 in a barrier system including those formed from compacted bentonite or from hydrated clay or bentonite slurries.

43. Use of the modified clay of claim 24 in slit trenches, bunds, encasements and other non geosynthetic clay based hydraulic barriers including sealants.

44. Use of the modified clay of claim 43 wherein the sealant is a polymer- bentonite mix or bentonite - cement mix.