Hardenable or settable material and method for stabilising CSG salts TECHNICAL FIELD

[0001] The present invention relates to a geotechnical stabilising composition. The invention could also be used in some waste disposal, construction and mining applications. The present inventio also relates to a method for treating brines, neutralising acid rock drainage, stabilising or binding CSG salts in. Registered Waste Facilities (RWF) and filling bore holes.

B ACKGROUND ART

[0002] Underground mining of coal may involve the use of longwall mining equipment to remove coal . The longwall mining machine removes coal from a long face. The longwall mining machine has its own hydraulic roof supports which advance with the machine as mining progresses. Once the machine has excavated a cavity in the coal seam, it moves along the coal seam, thereby extending the cavity, known as the goaf, which will collapse causing subsidence of the surface. The process of developing a Long Wall mine requires the construction of drives with roof support. This leaves voids in the mine,

[0003] Underground coal mines may also use the room and pillar mining technique. In this technique, a network of rooms is cut into the coal seams. Pillars of cod are left beliind in order to keep up the roof. In some coal mines, these pillars can constitute up to 40 of the total coal in the coal seam. Once the seam has been exhausted, the pillars can be removed, which will typically resul in. the roof collapsing. This is known as retreat mining. Retreat mining entails significant risk as the collapse of the roof can be somewhat unpredictable.

[0004] Large volumes of coal are also extracted using open cut mining. In open cut mining, a pit of progressively increasing depth and width is formed to remove (lie o erburden and then the coal. When the pit reaches its maximum practical size, shafts or diives may be excavated into the remaining coal i the wall at the bottom of the pit. This removes additional coal from the coal seam. Additional shafts or drives ma be cut to extract additional, coal. Normally, there will be pillars or wdls of coal left between adjacent shafts or drives in order to provide support for the roof and to minimi se the risk of the roof collapsing and burying equipment. The pillars or walls of coal represen sterilised resource. [0005] Hard-rock underground mining invol ves the excavation of shafts or drives and stopes to remove ore. Meshes, roof panels and or rock bolts, and cemented fill may be used to support the roof in hard-rock underground mining. Once an ore body is exhausted, the shafts or drives are simply left in place and the entrance to the mine is normally locked Of collapsed to prevent unauthorised access. Cemented stopes when in place act to stabilise the mining area.

[0006] Extraction of natural gas from coal seams and shale rock is becoming a large industry in several countries. For example, in Australia, extraction of coal seam, gas (which, is also known as "coal bed methane" in some countries) is leading to the development of a large natural gas industry.. Extraction of coal seam gas normally involves drilling a well into the coal seam. Water is pumped out of the coal seam which causes a reduction in pressure within the coal seam. As a result of this reduction in pressure, the coal seam gas can also escape from the coal seam via the well. In some instances, fracturing of the coal in the coal seam takes place to increase the production of coal seam gas. This fracturin of the coal is often referred to as "tracking".

[0007] Production of coal seam gas results in large quantities, often millions of litres, of water being removed from the coal seam and comin to the surface. This water is often quite salty and can constitute a salty water having significant dissolved salt content. Sodium chloride is often the predominant salt present in the salt water, but other salt species may also be present. Other contaminants may also be contained in the salty water, such as organic specie from the coal seam and other chemicals that may have been injected into the coal seam to enhance extraction of coal seam gas or to improve flacking of the coal seam.

[0008] Disposal of salty water extracted during coal seam gas production can be

problematical. Indeed, in most states of Australia, coal seam gas salty water is considered to be a regulated waste. One possible solution that ^' has ^' been proposed for disposing of this salty water includes re-injecting the salty water back into the coal seam or deep underground. Thi has met resistance from environmental groups because the salty wate could escape from where it ha been placed and potentially contaminate underground aquifers. Another proposal to treat the salty water involves passing the brine through a reverse osmosis plant to produce a clea stream of water that can be used in agriculture or disposed of by pumpin to natural waterways, and a concentrated brine stream. The concentrated brine stream may be suitable for disposal at sea. However, most coal seam gas extraction plants are located some distance from the sea, making disposal of the concentrated brine in the sea impractical and environmentally unacceptable, if other contaminants are present in the brine, disposal of the concentrated brine at sea may not be allowed. A furl her possibility for disposal of the salty water involves passing the salty water to ponds or dams and allowing the water in the pond or dams to evaporate to form salt. Again, the location of many coal seam gas extraction plants makes this option impractical. This option requires purchase of significant areas of land upon which to place the ponds or dams. Coal seam, gas plants are also often located in regions of good-quality farming land and there is resistance t devoting farming land to salt formation ponds or dams for disposal of brines. Further, if the salty water is contaminated with other contaminants, the salt that is produced may comprise a contaminated salt that would be considered to be a regulated waste, thereby increasing the cost of further disposal. Recently Regulators have rejected evaporation ponds as an acceptable method of OSG water disposal,

[0009] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

SUMMARY OF INVENTION

[0010] In a first aspect the present invention provides a method for forming a hardenable or settable material comprising (a) treating a water containing dissol ved salts to produce solid salt comprising a mixture containing Nad and Na2C03, and (b) forming a hardenable or settable composition by mixing an hydraulic binder and optionally other additives, the solid salt produced in step (a) and water in an amount to react with the hydraulic binder but not sufficient to fully dissolve the solid salt to form a stabilised salt based material .

[0011] In a second aspect, the presen invention provides a hardenable or settable material comprising a hydraulic binder and particles of solid salt, the particles of solid salt containing NaCl and Na2C03. in this aspect of the present invention, water may be added in an amount to react with the hydraulic binder but not in excess to thereby avoid or minimise dissolution of the solid salt.

[0012] The hardenable or settable material of the present invention can be disposed of by placement in a RWF, The hardenable ^' or settable material, may be used as a geotechnical stabilisation agent and it may be used to fill or support voids in mines. The hardenable or settable material of the present invention may also be used in some construction-related applications. Thus, the present invention should not be considered to be limited to use solely in applications relating to the stabilisation of geological features (whether naturally occurring or man-made). Indeed, the present invention may be used as a substitute for concrete in many applications. [0013] In a third aspect, the present invention provides a method for stabilising void in an underground mine or an open cut mine void, the method comprising (a) treating water containing dissolved salt to produce solid salt comprising a mixture of NaCI and Na2C03, (b) forming a hardenabk or settable composition by mixing an hydraulic binder, the salt produced in step (a) and water in an amount to react with the hydraulic binder but not sufficient to fully dissolve the solid salt to form a geotechnical stabilisation agent, and (c) supplying the

geotechnical stabilisation agent to the void and allowing the geotechnical stabilisation agent to set to form a hard mass to thereby stabilise the void,

[0014] The solid salt used in the present invention contains NaCI and Na2CQ3. In some embodiments, the salt further comprises NaHC03. It has been found that although using salt that comprises a mixture of NaCI and Na2C03 enables the harden able or settable material (which may also be referred to as a cementitious material) to harden and set, improved strength and binding properties are obtained if NaHC03 is also present in the salt mixture. In preferred embodiments, the salt comprises a mixture of NaCI, Na2C03 and NaHC03.

[0015] I other embodiments, the salt may further comprise one or more of KC1, K2C03 HC03 and CaS04. Other additives may also be used, such as setting accelerators,

waterproofing agents, fungicides and the like.

[0016] In some embodiments, the salt may comprise a mixture comprising from 10 to 40% Na2C03 by weight of the total salt mixture, 3 to 90% NaCI by weight of the total salt mixture and from 0 to 30% NaHC03 by weight of the total salt mixture. In other embodiments, the salt, may comprise mixture comprising from 15 to 35% Na2C03 by weight of the total salt mixture, 40 to 85% NaCI by weight of the total salt mixture and from 5 to 25% NaHC03 by weight of the total salt mixture. In another embodiment, the salt may comprise a mixture comprising from 20 to 35% Na2C03 by weight of the total salt mixture, 45 to 70% NaCI by weight of the total salt mixture and from 10 to 25% NaHC03 by weight of the total salt mixture. In a further

embodiment, the salt may comprise a mixture comprisin about 30% Na2C03 by weight of the total salt mixture, about 50% NaCI by weight of the total salt mixture and about 20% NaHC03 by weight of the total salt mixture. Specific ratios of NaCI: Na2C03: NaHC03 may include 85:15:5, or 70:20: 10 or 50:30:20. Other specific ratios failing within the above stated ranges may also be used,

[0017] The salt mixture may also contain other salts. These other salts will normally be present in lesser amounts and need not be described in detail. It will be understood that the exact compositio of the salt mixture will be dependent upon the dissol ved salts present in the salty water from which the solid salt mixture is recovered.

[0018] Water is added to the hardenable or settabie material to cause the hydraulic binder to set. Water is desirably added in an amount such that the hydraulic binder can fully set by reacting with the water. However, use of an excess of water (that is, in excess of that required to react with the hydraulic binder) should be avoided in order to avoid complete or even partial dissolution of the solid salt. In some embodiments, water is added in an amount such that dissolution of the salt is minimised. The hardenable or settabie material may have water present in step (b). The water may be added during batching of the hydraulic binder and the solid salt. Alternatively; the hydraulic binder and solid salt may be batched together and transported as an essentially dry mixtare to a site where the dry components are subsequently mixed with water. In another embodiment, the solid salt is formed- and transported to a remote site for mixing with the hydraulic binder. Water may be added at that site or water may be added at a different site.

[001 ] The water may comprise fres water. In other embodiments, the water is added as part of a brine solution. The brine solution may comprise a concentrated brine. The brine solution may comprise from 20% dissolved sait(s) up to the saturated concentration of dissolved salt(s). The concentrated brine may be formed as part of the process for forming the solid salt. For example, if a reverse osmosis process is used as part of the process to form the solid salt, a concentrated brine will be produced as a product of the reverse osmosis process. Some of that concentrated brine may be used to form the solid salt and some of the concentrated brine may be used by adding it in liquid form to the hardenable or settabie component.

[0020] The brine may contain dissolved NaCl and dissolved Na2C03. The brine may also contain. dissolved NaHC03. It has been surprisingly found that a brine, especially a concentrated brine, containing dissol ed NaCl and dissolved Na2C03 allows for the mixture to set hard. If brine is used instead of water, additional brine may be added (when compared to using fresh water) to the mixture without causing excessive dissolution of the solid salt, it being appreciated that as a brine solution is being used, it has less capacity to dissolve the solid salts as the brine already contains dissolved salt. This may have benefits in. that adding more liquid to the dry components may allow for greater workability of the wet mixture prior to setting. The brine may comprise dissolved NaCl and dissolved Na2C03 and optionally NaHCQ3 present in an. amount of from 20% by weight of the brine up the solubility limit of those salts in water (calculated from the total weight of the dissolved salt components). [0021] In some embodiments, the hardenable or settable material may further comprise particles of aggregate. The particles of aggregate thai may be added to the cementiiious material or the geotechnical stabilising agent may comprise any particulate material that is suitable for use as an aggregate in concrete. In embodiments where the eementitious material or the geotechnical stabilisin agent is used to stabilise a void in a mine, the aggregate will, preferably comprise mine rejects, such as overburden, tailings, and excavated rock. The aggregate ma also comprise waste streams from power stations, such as fly ash, or waste streams from the aluminium industry, such as spent pot lining and anode butts. The aggregate material may also comprise sand, gravel, particles of glass (such, as waste glass or discard glass), a clay material, (such as bentonite) or indeed any other aggregate suitable for use with eementitious binders, and mixtures thereof.

[0022] In a fourt aspect, the present, invention provides geotechnical stabilisation agent comprising solid salt comprising a mixture of MaCI and a2CG3, and an hydraulic binder.

[0023] The solid salt may also include NaBC03.Water is added to the geotechnical stabilisation agent. The water facilitates setting of the hydraulic binde so that the geotechnical stabilisation agent can form a solid mass.

[0024] In desirable embodiments of the present invention, the solidified product is formed of a matrix comprising particles of the solid salt and the set hydraulic binder. Effectively, the hydraulic binder acts to bind togethe the particles of solid salt. As the particles of solid salt are not fully dissolved, the particles of solid salt in the set product may effectively act as an aggregate in the set product,

[0025] In one embodiment, the hardenable or settable material comprises a mixture of from 50 to 99 solid salt and from i to 50% hydraulic binder, by weight, calculated on a dry basis.

[0026] In some embodiments, the hardenable or settable material comprises from 60 to 99% solid salt, preferably from 70 to 98% solid salt, more preferably from 80.to 98% solid salt, even more preferably from 88 to 98% solid salt, yet more preferably from 90 to 98% solid salt, or even about 95% solid salt, calculated on the basis and calculated as a percentage of the total amount of dry ingredients .

[0027] In some embodiments, the hardenable o settable material comprises from 1%; to 40% hydraulic binder, preferably from 2 to 30% hydraulic binder, more preferably from 2 to 20% hydraulic binder, even more preferabl from 2 to 12% hydraulic binder, yet more preferabl from 2 to 10% hydraulic binder, or even about 5 hydraulic binder, calculated on the basis and calculated as a percentage of the total amount of dry ingredients.

[0028] The amount of water that is added to the hardenable or settabk material preferably comprises from .10% to 40% by weight, calculated as weight percent of the total weight of dry solids, more preferably from 15 to 35%, even more preferably from 20 to 33 %, yet more preferably from 23 to 30%. yet eve more preferably from 25 to 30%.

[0029] In some embodiments, the amount of water that, is added is the minimum amount of water required to react: with the hydraulic binder. In this embodiment, dissolution of one or more of the salts present in the salt is largely avoided, preferably completel avoided. In this manner, any deleterious effects arising from dissolution of one or more of the salt compounds on the final cementitiou product is avoided,

[0030] Without -wishing to be bound by theory, the present inventor have postulated that using an amount of water that is sufficient to allow the hydraulic binder to set but is not sufficient to also dissolve all of the solid salt means that particles of solid salt will remain in the set material.. These panicles of solid salt effectively act as an aggregate in the set material, thereby forming solid mass having reasonable to good strength properties. The present inventors have also postulated that adding too little water will result in the hardenable or sellable material being difficult to handle and setting too quickly whilst adding too much water will result in dissolution of too much of the particles of solid salt,

[0031] In some embodiments of the present invention, the salt is formed from a water containing dissolved salt. The water containing dissolved salt ma comprise a water containing dissol ved salt extracted during coal seam gas extraction or during extraction of gas or oil from shale. In other embodiments, the water containing dissolved salt may comprise water extracted during fracking operations. Other waters containing dissolved salt may also be used. The water containing dissolved salt may also comprise a water containing dissolved salt extracted from an underground aquifer. The water containing dissolved salt may contain appreciable quantities of dis olved salt.

[0032] In one embodiment, the water containing dissolved salt is treated using a reverse osmosis process as part of a process to form, a solid salt and water. One example of suitable technology in this regard is available from Carina Water Resources LLC, based in West

Virginia, USA. The process to produce solid salt involves 4 steps, being; 1/ cyclones, to remove solids, 2/ decomposin to remove bacteria and organies, 37 pressure filtration t produce agriculture quality water and wet, coarse salt, which goes to 4/ reverse osmosis, with self- cleaning membranes, which produces a concentrated brine that leads finally, possibly after further evaporation processes and crystallisation, to a solid salt ( a mixture of various sails and contaminants) and potable water or water vapour.

[0033] In other embodiments, the solid salt may be formed by placing the water containing dissolved salt in a solar evaporation ponds or dam and evaporating water to form solid salt or through a mechanical crysta!liser.

[0034] The hydraulic binder used in the present invention may comprise a cementitious binder. Where a cementitious binder is used, the hardenable or sellable material comprises a cementitious material. The cementitious binder may comprise a Portland cement. Other cementitious binders, such as energetically modified cement, Pozzolan-lirae cements, slag-lime cements, supersulfated cements, calcium aluminate cements, calcium sulfoaluminate cements, and magnesia cements may also be used in the present invention.

[0035] As mentioned above, in some embodiments, particles of an aggregate material may be added to the cementitious material or the geotechnical stabilisation agent . The aggregate may be added in an amount from 0% to 70% by weight, calculated as weight percent of the total weight of dry solids. Preferably, the aggregate may be added in an amount of from 0 to 66%, more preferabl from 0 to 50%, even more preferably from 0 to 45%, or in other amounts within that range.

[0036] The aggregate may be sized such that the particle size distribution of the aggregate falls within a predetermined range of particle sizes. The particle size may vary across a wide range. The aggregate may comprise flotation tailings, such as flotation tailings obtained from Washing of coal. In other embodiments, the aggregate may comprise mine reject or tailings solids. Preferably, the aggregate is used in an "as -available" condition, without using milling or grinding to alter the particle size. The economic considerations for avoiding milling or grinding of the aggregate are apparent. In some embodiments, the aggregate particles may be sized such that they have a particle- size of up to 50mm.

[0037] The salt that is added to the hardenable or sellable material may be predominantly sodium chloride. Other salts, including sodium carbonate and (optionally) sodium bicarbonate will also be present, The salt that is added may comprise a solid salt recovered from groundwater extracted during coal seam gas extraction or extraction of gas from shale deposit or extraction of natural gas from other geological features. [0038] In some embodiments of the present invention, the hardenable or set table material is made by batching appropriate quantities of the solid ingredients. The solid salt may contain some moisture and may or may not set. By forming the geotechnical stabilising agent by batching the solid ingredients, the cementitious material or the geotechnical stabilising agent ma be stored for later use,

[0039] In other embodiments, the hardenable or sellable material is made by mixing the ingredients into a slurry. In this embodiment, the slurry is suitably used shortly after being formed. This enables the slurry, for example, to be pumped into a void in a mine or into a RWF or into a borehole and subsequently allowed to set to form a solid, stable mass.

[0040] In some embodiments, the hardenable or sellable material having a relativel low moisture content is placed into storage and subsequently retrieved from storage, mixed with water to form a slurry and sent for final use,

[00 1] In a preferred embodiment of the present invention, the solid salt is formed by treating a water containing dissolved salt extracted from a coal seam gas well, with the water containing dissolved salt being treated at or near the locatio of the coal seam gas well. The solid salt may then be transported, such as by road or rail transport, to a mine located away from the coal seam gas well and the salt may then be batched with the other ingredients to form the geotechnical stabilising agent. The hardenable or sellable material may then be used as a geotechnical technical stabilising agent to stabilise a void in the mine. The geotechnical stabilising agent may be mixed with water to form a slurry and the slurry may be pumped or otherwise delivered into the void in the mine and allowed to set to form a solid mass that stabilise the void.

[0042] The hardenable or sellable material of the present invention suitably comprises a mixture of having a very high content of solid salt and a relatively low content of hydraulic binder. The present inventors have discovered thai such a. mixture Can set to form a hard mass having strength properties that make it suitable for placing in voids in mines or as a stabilised solid i RWFs.

[00431 In some embodiments, the salt comprises a solid salt recovered from a water containing dissolved salt extracted during coal seam gas extraction. Such salt will mainly comprise NaCl, with some NaHC03 and Na2C03 also being present. Mino amounts of other salt species and contaminants ma also be present in the salt.

[0044] In some embodiments, the hardened mass formed from the salt, hydraulic binder, water an optionally aggregate lias a compressive strength of at least 750kPa, more preferably greater than IMpa, even more preferably 1.5 pa or greater. The hardened mass may have a strength of up to 5 Mpa, or a compressive strength of up to 4 Mpa, or a compressive strength of up t 3 Mpa.

[0045] In another aspect, the water containing dissolved salt removed during coal seam gas extraction may be used (either as a liquid or as a solid salt) to contact mine- water having a lo w pH to thereby increase the pH of the mine water. It will be appreciated that the coal seam gas brine contains sodium bicarbonate- and sodium carbonate which can be effective to increase the pH of low pH water. This may be- effective to neutralise the low pH water. Precipitation of one or more dissolved metals in the low pH water may also take place.

[0046] The present inventors have also found that a satisfactory hardenable or settable material can be obtained by mixing a hydraulic binder with brine instead of water. The brine suitably comprises a solution containing dissolved NaCl and Na2C.03 and optionally dissolved NaHC03. Therefore, in a further aspect, the present invention provides a hardenable or settable material comprising a hydraulic binder mixed with a brine containing dissolved NaCl a

Na2C03 and optionally dissolved NaHC03. The hardenable or settable material in this aspect preferably also includes aggregate.

[0047] The brine suitably comprises a concentrated brine. In one embodiment, the brine comprises a mixture of NaCl, Na2C03 and optionally NaHC03, with that mixture being, dissolved in water in an amount of from 20% by weight up to the solubility limit of the mixture in water.

[0048] The ratio of NaCl, Na2C03 and NaHC0 present in the brine may comprise from 1.0 to 40% Na2C03 by weight of the total weight of NaCl, Na2C03 and NaHCOS, 30 to 90% NaCl by weight of the total weight of NaCl,. Na2C03 and NaHC03, and from 0 to 30%

NaHC03 by weight of the total weight of NaCl, Na2C03 and NaHCQ3, In other embodiments, the salt may comprise a mixture comprising from 15 to 35% Na2C03 by weight of the total weight of NaCl, Na2C03 and NaHCC , 40 to 85% NaCl by weight of the total weight of NaCl, Na2C03 and NaHC03and from 5 to 25% NaHC03 by weight of the total weight of NaCl, Na2C03 and NaHC03. In another embodiment, the brine may comprise a dissolved salt mixture comprising from 20 to 35% Na2C03 b weight of the total weight of NaCl, Na2C03 and

NaHC03, 45- to 70% NaCl by weight of the total, weight of NaCl, Na2C03 and NaHC03and from 10 to 25% NaHCOS by weight of the total weight of NaCl, Na2C03 and NaHCOS. In a further embodiment, the brine may comprise a mixture comprising about 30% Na2C03 by weight of the total weight of NaCl, Na2C03 and NaHC03, about 50% NaCl by weight of the total weight of NaCl Na2C03 and NaHiC03and about 20% NaHCG3 b weight of the total weight of NaCl, Na2CQ3 and NaHCCB. Specific ratios of NaCI: Na2C03: NaHC03 may include 85:15:5, or 70:20: 10 or 50:30:20. Other specific ratios falling within the above stated ranges may also be used. The amount of brine added in this aspect of the present invention may range from 15% to 60% by weight based on the total weight of solids, or from 17% to 50% by weight based on the total weight of solids, with amounts of 17%, 20%, 30%, 40 % and 50% providing a satisfactor product. In preferred embodiments, the present invention provides a hardenable or sellable material in the form of a cementitious material or a geotechnical

stabilisin agent that can be used to stabilise voids in a mine, in some embodiments, the present, invention can be used to treat water containing dissolved salt, such as water containing dissolved salt brought to the surface during extraction of coal seam gas. These types of water containing dissol ved salt are frequently classified as a regulated waste, which entails expensive disposal processes. The present invention treats the water containing dissolved salt to form a clean water steam and a solid salt which can then be used in the geotechnical stabilising agent that can be used to stabilise -voids in mines. When .the cementitious material or the geotechnical stabilising agent has set to a solid mass, the salt is effectively encased in a solid matrix, thereby effecting safe disposal of the salt and benefi tting the mine. Preliminary studies by the present inventors have revealed that the coal seam gas industry in the state of Queensland, Australia, could potentially produce 1. ,000,000 tonnes of salt each year and this salt could be effectively disposed of by forming a cementitious material or a. geotechnical stabilising agent, in accordance with the present invention and using that geotechnical stabilising agent to stabilise or fill voids in mines, such as hard rock mines or coal mines. Embodiments of the present in ention c n take a brine that is classified as a regulated waste and treat that brine to form a useful product.

[0049] in preferred embodiments of the presen invention, hardenable or sellable material or the cementitious material or the geotechnical stabilising agent is mixed with water to produce a slurry which can set to form a solid mass. The slurry suitably can be pumped so they can be easily delivered int a void. The slurry suitably has a low beach angle (such as 2 to. 5°) to facilitate -flow into inaccessible areas. The slurry is suitably a non -settling slurry or an slowly settling slurry to minimise risk of pipe blockage during pumping. If any additional additives are included in the slurry, they will ideally not be harmful to the environment. The slurry suitably has a low free water release. The slurry may have a moisture content of less than 30% by weight, calculated on the basis of the wet slurry.

[0050] As the set hardenable or sellable material, cementitious material or the geotechnical stabilisation agent contains significant quantities of solid salt, the salt may be susceptible to dissolutio if i t comes into contact with a large head of water or with running water. Therefore, in some embodiments, it may be desirable to use the cementitious material or the geotechnical stabilisation agent to fill voids in areas that are likely to remain dry. If the geotechnical stabilisation agent is to be used in filling voids: in regions that may be exposed to water having a high head of pressure, or to running water, it may be desirable to encase the geotechnical stabilisation agent in a water resistant coating, such as a polymeric coating, resin or a concrete coating or a bituminous coating. In one embodiment, the geotechnical stabilisation agent may be shotcreted to cover exposed surfaces in concrete, seal voids or coat the cemented salt.

[0051] In a further aspect, the present invention provides a method for forming a hardenable mass comprising (a) treating a water containing dissolved salt to produce solid salt comprising a mixture of NaCl and a2C03, and (b) forming a cementitious composition by mixing an hydraulic binder, the solid salt produced in step (a) and water in an amount to react wit the hydraulic binder but not sufficient to fully dissolve the solid salt to form a hardened mass and allowing the hardenable mass to set.

[0052] The salt mixture may further include NaHC03.

[0053] In some embodiments, the invention may be used to stabilise salt in regulated waste facilities (RWFs).

[0054] The quantities and relative amounts of each component added to the hardenable or settable material may be as described with reference t the quantitie and relative amount of those components present in the cementitiou material or the geotechnical stabilising agent, as described hereinabove.

[0055] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0056] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

Examples;

[0057] The following sample were made and some were subjected to testing. In the following examples, unless otherwise specified, the salt comprised a mixture of NaCl, Na2C03 and NaHC03 , The ratio of each component was selected so that the final salt mixture reflected a typical salt mixture derived from salt water obtained from a coal gas seam extractio process, typically comprising 50% NaCl, 30% Na2C03 and 20% NaHCCB, calculated as a weight percentage calculated on the total weight of the salt mixture. Where othe salt mixtures were used, these are noted:

[0058] The samples produced in examples 1 and 5 were tested for compressive strength. Example 1 exhibited a compressive strength of 1 ,5M a and example 5 exhibited a compressive strength of 3.0Mpa. It is noted that geotechnieal stabilisation agents havin a compressive strength of above 750kPa are generally considered to be suitable for filling voids in some minin operations.

[0059] Some of the examples were failures. The reasons that those examples failed to properly set include insufficient cement added to the mixture, insufficient water added to the mixture, too much water added to the mixture, or improper or insufficient mixing of the components (experimental techniques were refined as the project developed).

[0060] Examples 39, 40, 42 43, 44 and 45 used a concentrated brine in place of water. The concentrated brine essentially comprised a mixture of NaCl, Na2C03 and NaHC03 in a ratio of 70:20:10, This mixture was dissolved in water to its solubility limit in orde to form the concentrated brine. Example 39, 40, 43 and 44 included solid salt. Examples 42 and 45 had no solid salt present. In each instance, a satisfactor set material was obtained. In examples 40 and 42, 30% brine was added. In example 43, 40% brine was added. In example 44, 50% brine was added. This produced a set, product that was estimated to have compression strength of 3MPa (example 42), 2MPa (example 43) and I MPa (example 44). The set product formed using concentrated brine instead of water appears to have a higher early set strength then the set product that is formed using water. Advantageously, u ing brine instead of water also allows extra liquid to be added (when compared to usin water) without detrimentally affecting compressive strength. This has benefit in tha the wet mixture becomes easier to work and pump.

[0061] Examples 42 and 45 used a mixture that comprised cement, concentrated brine and aggregate (sand). Again, the concentrated brine essentially comprised a mixture of NaCl, Na2C03 and !%HCQ3 in a ratio of 70:20: 10, This mixture was dissolved in water to its solubility limit in order to form the concentrated brine. No solid salt was present in these examples. A satisfactory set product was obtained.

[0062] The present inventors have discovered that a hardenable material may be formed by mixing in hydraulic binders (such as a cement) with a salt mixture that include NaCi and Na2C03. Na2CC)3, is commonly found mi ed with ordinary salt (NaCl) that is recovered from treating coal seam gas water (and other salty waters). Thus, embodiments of the present invention provide a method for treating coal seam ga water t recover a salt mixture therefrom, which salt mixture comprises NaCl and Na2C03. The salt mixture will also typically contain NaHC03 and if has been found that a stronger material having better binding properties is obtained when the salt mixture present in the geoteehnical stabilising agent or hardenahle material comprises a mixture of NaCI, Na2C03 and NaHC()3.

[0063] Reference throughout this specification to One embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment.

Furthermore, the particular features, structures, o characteristics may be combined in any suitable manner in one or more combinations.

[0064] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.