CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from South African provisional patent application number 2019/05828 filed on 4 September 2019, which is incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to backfilling. Particularly, although not exclusively, it relates to backfilling in mining applications. It provides a backfilling process and a composition for admixture with a backfill material.

BACKGROUND TO THE INVENTION

Backfilling in mining applications refers to a process of continuously filling the worked out void behind an advancing face. The material used for backfilling may include mine tailings that remain after separation of the valuable fraction of a mineral ore from the uneconomic fraction (gangue). Tailings are typically generated through crushing and grinding operations and may have a high proportion of particles with dimensions of a fraction of a millimeter.

Backfilling serves several functions in underground mines. It is increasingly used to reduce the surface impact from the wastes produced by the mining operations. Other important objectives of backfilling are to improve ground stability and reduce ore dilution. It can be used as ground support for pillars and walls, to help prevent caving and roof falls in orebodies that are hosted In poor rock. Filling of open stope voids maintains stability of the adjacent working areas and reduces the risk of local or regional ground failure.

Backfilling can also enhance pillar recovery for improved productivity. The blasting of pillars adjacent to the backfill enables higher recovery of ore reserves. This is important because the volume of orebody locked up in pillars can be similar to the volume of orebody that will be mined over two decades. In stoping mining methods, backfilling allows stable vertical fill exposures to be created as the pillars between slopes are removed. The backfill acts as a support and prevents heading collapse and problems with subsidence. When used for this purpose the backfill material may include a binder, usually cement. Backfilling can also offer environmental benefits because a high percentage of the total tailings produced by an underground mine can be placed back underground.

Hydraulic backfills are any kind of backfill carried by water through pipelines. Hydraulic backfills are the particular area of application of the present invention. The backfill material is pumped to a stope, drift or other backfill site or zone as an aqueous slurry of tailing fines. The tailings are mixed on the surface with water and the binder, and then piped down a decline, shaft or surface borehole into the backfill site. The solid particles are sluiced with the water so that they do not have a chance to settle until they reach the backfill site.

The proportion of water required to hold the tailings in suspension to enable pumping is significantly higher than would be found on surface fills. The reason for this is that the slurry must be pumpable over long distances; for example, 2 km to 11 km from where it is prepared. In some cases the quantity of water in one cubic metre (1 000 litres) of the slurry may range from 580 to 630 litres, with the tailings making up most of fhe balance (420 to 370 litres respectively).

When the backfill is inside fhe stope, the high proportion of water combined with the uncontrolled nature of the tailing solids can have a number of undesirable side effects. As soon as the backfill is in place the water may begin to bleed to the top of the backfill. The backfill is thus susceptible to shrinkage and subsidence when dewatering, and to a high degree of compaction or deflection before offering significant resistance to compression. Furthermore, the bleed water may form a hydraulic head of supernatant liquid water above the backfill. On account of its inherently unstable nature, the presence of this water can give rise to a danger that it may burst the paddock built to contain it, potentially causing fatalities, mining halts and severe financial implications for the mine.

The excess supernatant water, often now at great depth, may have to be pumped out of the void created between the hanging wail and the top surface of the backfill plug and directed to settling dams underground. In some circumstances the water may have to be pumped back to the backfill processing site at ground level, which as mentioned may be up to 11 km away from the site of the backfill. This can entail significant cost. New backfill material may also then have to be pumped back into the void created by the removal of the supernatant fluid, otherwise the backfill will not be able to serve adequately as a support for the hanging wall above. Cost savings could be achieved if the formation of the supernatant fluid and the subsidence of the backfill could be reduced. About 20% of a cubic metre of backfill may be expected to bleed off to form the supernatant fluid, leaving only about 80% of the original cubic metre filling the backfilled cavity. The cost of refilling the pumped-out 20% void with backfill material can be estimated with reference to an exemplary backfilling cost in the region of ZAR 300 per cubic metre. Based on this cost, the water that bleeds off may represent a cost of about ZAR 60 per cubic metre of backfilled stope. It follows that cost savings of the order of ZAR 60 / m ³ might be obtainable if the water in the backfill could be bound up such that a supernatant fluid does not form.

The water bleeding from the settling backfill material can also run out of a backfilled stope through fractures, cracks and fissures in the side walls or footwalls holding the backfill. In some cases, this water can drain into production stopes and give rise to concerns relating to safety and stability.

Certain mines have unique problems in this regard, which are associated with the particular type of mining method which they use. They mine iarge ore bodies cailed drifts, which are long tunnels that are mined horizontally to a certain depth. A bulkhead or retaining wall is then built and the mined-out area is backfilled. However, as a result of damage sustained by the rock during blasting operations, bed separation can occur within the adjacent rock. Thin cavities in rock walls surrounding the backfill may be formed by separation along bedding planes due to differential lowering of strata over mine workings. This can cause the drift to spring leaks. The entire backfilling process may then need to be halted so that the entire backfill can be pumped out and replaced. All the backfill in the ranges may need to be flushed out with wafer, the leaks in the drift identified and plugged, and the backfill operation restarted by first pre-flushing the entire range with water and then pumping backfill material back Into the ranges. Even after that has been accomplished, a risk will remain that not ail of the leaks will have been plugged and that the replacement process may have to be repeated. it will be appreciated that the replacement of an entire backfill can entail significant costs because of the Iarge volumes involved. For example:

• approximately 190 cubic metres of backfill may have to be flushed out at a cost of about ZAR 57 000 (ZAR 300 per m ³ );

• approximately 190 000 litres of additional water may be needed for a pre-flush to lubricate the backfill pipe range; and • approximately 190 000 litres of water may be needed for a post-flush to clean the range of any remaining residue on the sidewalls of the pipe.

In addition, there is a significant opportunity cost that may be lost while mining activities in the drifts adjacent to the backfill site are halted until the backfill refilling process has been completed. in summary, a need exists for backfill materials capable of reducing wafer bleeding. A further need exists for backfill materials that are capable of at least partially sealing rock fractures. The need for such materials is significant because backfilling volumes in a single mine can reach as high as 160 000 m ³ per month.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a process for backfilling which includes a step of pumping a slurry to a backfill site and allowing it to set therein, the slurry comprising: water; an aggregate; and a backfill admixture composition comprising a sealing component.

For purposes of this specification the sealing component will be understood broadly and shall include a substance capable of reducing bleeding of water from a backfill material by a mechanism selected from the group consisting of water-absorption, water- adsorption, gelling, water-binding, water-holding, water-repellence, water-resistance, water reduction, flocculation, and acceleration.

The sealing component may comprise an accelerator. The accelerator may comprise a curing accelerator such as a setting and hardening agent for activating hydration of the backfill binder. The accelerator may comprise a shotcrefe accelerator. The backfill admixture composition may accordingly comprise a shotcrete accelerator. The backfill admixture component may further comprise a pumping aiding component.

The slurry may further comprise a backfill binder. The binder may comprise a cementitious substance.

The backfill admixture composition may comprise components selected from the group consisting of a flocculating component, an adhesion promoting component, and a preservative component.

The aggregate may comprise tailings derived from a mine.

According to a further aspect of the invention there is provided a process for backfilling which includes a step of pumping a slurry to a backfill site and allowing it to set therein, the slurry comprising: water; an aggregate; and a backfill admixture composition comprising a shotcrete accelerator.

The slurry may include a backfill binder. The binder may comprise a cementitious binder.

The aggregate may comprise tailings derived from a mine.

The backfill admixture composition may further comprise components selected from the group consisting of a pumping aiding component, a sealing component, a flocculating component, an adhesion promoting component, and a preservative component.

According to a further aspect of the invention there is provided a process for providing backfill for a mine comprising: providing water; providing tailings from the mine; and providing a backfill admixture composition as described in this specification.

The process may further comprise mixing the water, the tailings and the backfill admixture composition in predetermined proportions to form a filling material to backfill the mine. The water, the tailings and the backfill admixture composition may be provided in first to third predetermined amounts respectively.

The process for providing a backfill may include providing a backfill binder in a predetermined amount and mixing it with the water, the tailings and the backfill admixture composition.

The process may include employing the resultant filling material underground. The process may include employing the resultant filling material aboveground.

According to a further aspect of the invention there is provided a process for transporting mine waste residue comprising: generating a pumping mixture by: providing water: providing tailings derived from the mine; providing a backfill admixture composition as described in this specification; and mixing said water, said tailings and said backfill admixture composition; and pumping said pumping mixture through a pipe.

The water, the tailings and the backfill admixture composition may be provided in first to third predetermined amounts respectively.

The process may include providing a backfill binder in a predetermined amount and mixing it with the water, the tailings and the backfill admixture composition. The mixing may be performed in a predetermined manner under predetermined conditions.

According to a further aspect of the invention there is provided a backfill admixture composition comprising a sealing component. The sealing component may comprise an accelerator. The accelerator may include a setting and hardening agent for activating hydration of the backfill binder. The accelerator may comprise a shotcrete accelerator.

The backfill admixture composition may accordingly comprise a shotcrete accelerator. The backfill admixture composition may comprise a shotcrete accelerator as sealing component.

The backfill admixture component may further comprise a pumping aiding component. The backfill admixture composition may comprise components selected from the group consisting of a flocculating component, an adhesion promoting component, and a preservative component.

According to a further aspect of the invention there is provided a pumpable slurry for use in forming a backfill in an underground cavity, the slurry comprising water, an aggregate, and a backfill admixture composition as described in this specification. The aggregate may comprise tailings derived from a mine. The water, the tailings and the backfill admixture composition may be present in the slurry in first to third predetermined amounts respectively.

The pumpable slurry may include a backfill binder. The backfill binder may comprise a cementitious binder. The binder may be present in a predetermined amount.

The invention extends to a filling material comprising water, an aggregate, and a backfill admixture composition as described in this specification. The aggregate may comprise tailings derived from a mine. The filling material may include a backfill binder.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and modes of performing the invention will now be described by way of example only.

One exemplary embodiment of the invention provides a backfill admixture composition for promoting absorption or binding of water in the backfili materiai, and sealing and water-tightness of underground cavities containing the resultant backfill. Pumpable slurries and backfill materials which include the backfill admixture composition may be suitable for use in a variety of applications. These include but are not limited to mining and tunnelling applications.

The invention also provides a process for backfilling. The process may involve pumping a slurry to a backfill site or zone such as an underground mine stope and allowing it to set therein. The slurry may comprise water. It may comprise an aggregate it may comprise a binder. It may comprise a backfill admixture composition as described in this specification.

The aggregate may comprise tailings derived from a mine The backfill admixture composition may include components selected from the group consisting of a pumping aiding component, a sealing component, a flocculating component, an adhesion promoting component, and a preservative component.

Examples of the components of the backfill admixture composition will now be discussed.

The pumping aiding component may include a constituent selected from the group consisting of rheology modifiers, viscosity modifiers, flow aids, water-soluble synthetic and natural organic compounds including guar-based gels and gelling agents, lignosulphonates, cellulose ether, polyethylene oxide, alginates, organic water-soluble flocculants, emulsions of organic materials, fine clays, kaolin, diatomaceous earth, and calcined pozzolans. The rheology modifiers may be selected for their capability to provide plastic flow of the slurry rather than elastic deformation of the slurry in response to an applied force. in exemplary variants of the process and composition, the pumping aiding component may include either one or both of a guar-based compound and a lignosulphonate.

The pumping aiding component may be present in a proportion of up to about 35% by weight of the composition. In an exemplary embodiment, the pumping aiding component may comprise approximately 30% by weight of the composition.

The pumping aiding component may be selected based on its capability to control pumpability and flow of the slurry.

The sealing component may include a constituent selected from the group consisting of water- absorbing agents, water-adsorbing agents, gelling agents, water-binding agents, water-holding agents, water-repellent agents, water-resistant agents, water reducing agents, flocculants, accelerators, polysaccharide polymers, superabsorbent (hydrogel-forming) polymers, soaps, surfactants (e.g. block co-polymer surfactants), fatty acids, conventional water reducers, methyl siliconates, and sodium silicate. The superabsorbent polymers may be derived from solution polymerization.

In exemplary variants of the process and composition, the sealing component may include a modified polysaccharide polymer. In other exemplary variants of the process and composition, the sealing component may include an accelerator. The accelerator may comprise a curing accelerator. The accelerator may comprise a shotcrete accelerator. Without limitation thereto, the shotcrete accelerator may comprise a wetcrete accelerator. Without limitation thereto, the shotcrete accelerator may be selected from the group consisting of: BASF® MasterRoc SA 160; SIKA® Sigunit L-72 AF; the CHRYSO® Jet range of products (including CHRYSO® Jet 50 AF and CHRYSO® Jet 25 AF); the EUCON® Sureshot range of products (including EUCON® Sureshot AF and EUCON® Sureshot AF2LV); and analogues of the aforegoing products.

The accelerator may comprise a low alkali accelerator. It may comprise a sparingly caustic accelerator.

The accelerator may be provided as an aqueous solution containing inorganic salts such as, but not limited to, aluminium sulphate. It may include diethanolamine. It may include an acid. The acid may be an organic acid selected from the group consisting of oxalic acid and formic acid. The acid may be an inorganic acid selected from the group consisting of phosphoric acid and hydrofluoric acid.

The sealing component may, instead or in addition, comprise a product selected from the CHRYSO® Tera range of admixture compositions. It may, for example, comprise CHRYSO® Tera 100.

The sealing component may be configured to stiffen (gel) the backfill slurry. It may be configured to delay onset of the stiffening or gelling until a required period of time has elapsed following ejection of the slurry into the cavity to be backfilled. The dosage of sealing component per cubic metre of slurry may be selected to provide the required period of delay. For example, for compositions which include CHRYSO® Tera 100, a dosage of 2 litres ot that product may be applied per cubic metre, to provide a delay of approximately 45 minutes before stiffening or gelling of the backfill slurry commences. Higher dosages may be selected If faster gelling is required. For example, for faster gelling, 3 litres of the sealing component may be employed per cubic metre of slurry. Delays in gelling are advantageous in circumstances where, for example, a long cavity may require backfilling. In some cases, for example, a cavity may extend for 120 metres. For such cavities a hose could be positioned with its mouth in a middle region of the cavity so that the backfill composition could be dispensed from the hose in both directions, thereby to fill 60 metres of the cavity on each side. In such circumstances it will be appreciated that a delayed onset of the gelling of the slurry would be advantageous to permit extremities of the cavity to be filled before the onset of the gelling process.

The sealing component may be present in the composition in a proportion of from about 5% to about 15% by weight of the composition.

The sealing component may be selected based on its capability to:

• reduce bleeding of water from a backfill;

• reduce permeability of fissures and cracks in substrates and surrounding formations that define backfilled voids, e.g. rock walls, footwalls, hanging walls, benches and other underground wall formations;

• confer, upon a backfill material, properties of swelling, expansion and/or plugging;

• impart and promote water repellence, water absorption and/or sealing properties of backfill and substrate material; and/or

• reduce moisture migration by capillary action within the backfill and substrate material.

The flocculating component may include a constituent selected from the group consisting of maleic anhydride copolymer, sodium salt of styrene, polyacrylamide, and poly(methacrylic acid). in exemplary variants of the process and composition, the flocculating component may include polyacrylamide.

The flocculating component may be present in a proportion of from about 3% to about 7% by weight of the composition.

The flocculating component may be selected based on its capability to reduce destructive bleeding of water from the backfill fo a more gradual and/or near-zero seepage, both in the cured and/or activated polymer and in the substrate near the surfaces of the backfill.

The adhesion promoting component may include a constituent selected from the group consisting of polyacrylamide, acrylic compounds, polyvinyl chlorides, polyvinyl acetates, and butadiene- styrene co-polymers it may include a surface tension relieving agent.

In exemplary variants of the process and composition, the adhesion promoting component may include an acrylic compound. The adhesion promoting component may be present in a proportion ot from about 3% to about 7% by weight of the composition.

The adhesion promoting component may be selected based on its capability to contribute to bond strength and/or adhesion strength of the composition in relation to the substrate.

The preservative component may include a constituent selected from the group consisting of bactericides, fungicides, and insecticides. The bactericides may be selected from the group consisting of sodium benzoate, polyhalogenated phenols, benzalkonium chloride, and copper compounds. The fungicides may be selected from the group consisting of polyhalogenated phenols, copper acetoarsenite, and copper arsenate. The insecticides may include an emulsion of Dieldrin. The insecticides may include an insecticide for controlling termites.

In exemplary variants of the process and composition, the preservative component may include sodium benzoate.

The preservative component may be present in a proportion of from about 0.1% to about 1% by weight of the composition.

The preservative component may be selected based on its capability to control organisms which might otherwise contribute to the degradation of polymer-based emulsions and suspensions within the composition, caused by the presence of organism colonies present on or within the substrate and/or the cured or activated polymers. In use of the backfill admixture composition, the preservative component may extend the useful life of the backfill material.

The balance of the backfill admixture composition may include an aqueous fluid, typically water. It will be appreciated that the composition water may, in use, be augmented by additional water provided as dilution water and/or seepage water.

The backfill admixture composition may be substantially devoid of volatile components, apart from the water.

The proportions of the backfill admixture composition may be selected such that they confer on the composition a liquid consistency instead of a paste-like consistency. Those skilled in the art will appreciate that the constituents listed in each of the above-mentioned component categories may, in use, amplify each other to form up. Therefore, at least one of the components may include a plurality of the constituents listed for that component.

Further features of the invention provide for the backfill admixture composition to include at least one of a foaming agent, a lubricant, a superpiasticizer (such as sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate or a polycarboxylate ether), and an inhibitor and/or an activator suitable for changing the characteristics of the polymer thereby to enhance control over the direction of flow and penetration of the backfill material.

Certain embodiments of the backfill admixture composition may comprise a polymer-based emulsion or suspension of polymer emulsoids, colloidally dispersed in a solution of water. Such embodiments may be referred to as polymer-based grouts for admixture with a backfill material.

The backfill admixture composition may be provided as an aqueous solution.

In use, the backfill admixfure composition may be suitable to promote at least one property selected from the group consisting of flow, adhesion, water-repellence, water-absorption, water- binding, sealing, and curing of or by a backfill material.

The backfill admixture composition may be formulated to have properties of low alkalinity and low toxicity and may be adapted to provide self-curing or self-activation of the backfill material.

The backfill admixture composition may be formulated to have a specific gravity in the region of 1 ,38 (±0,03) kg/dm ³ . it may be formulated to have a dynamic viscosity at 20 ^º C of less than about

250 MPa/s.

The backfill admixture composition may be formulated with a pH of from about 3 to about 5. The pH of the final backfill material may be expected to be in the region of pH 7 on account of additive water.

The backfill admixture composition may be provided in a water reduceable, pumpable liquid form. Design requirements for the backfill admixture composition may apply. For example, the composition should not reduce the strength of the backfill below targeted design strengths specified by the rock mechanics for a given application site. By way of example only, and without limitation thereto, specified design strengths may vary from 0.5 MPa to 1.2 MPa after 28 days, depending on the site in certain applications where the backfill material is required to function in a support system, design strengths as high as 16 MPa in 28 days may be specified.

It may also be a design requirement to restrict the bleed after application to a predefined liquid head, e.g. to no more than a few millimetres of liquid head.

In use of the process and composition described herein, compliance with design requirements can be assessed through routine experimentation using trial mixes. The dosage of the admixture composition may thus be targeted to meet specific backfilling and compliance requirements. The routine trials should be conducted using the materials and conditions that will be experienced in use.

In use, the backfill admixture composition may be blended down or diffused into the tailings it may be injected in a fluid state info the tailings or entrained in them. It may then be transferred (by means of high-pressure pumping) to the backfill site or zone it may then be expected to diffuse into the substrate and react under the influence of moisture and/or pressure to cause a catalytic reaction. This may act to seal off fissures and small cracks at the application zone and reduce seepage of water from the backfill.

The catalytic reaction may be expected to generate a non-soluble crystalline formation throughout the pores and capillary tracts of the substrate, as well as cracks. This may create a long-term or near-permanent sealing of the substrate and limit the penetration of water and other liquids from any direction, even under high hydrostatic pressure. Activation may also be achieved through agitation and/or pressure when the polymer is subjected to shear action as it passes through orifices or fissures.

The backfill admixture composition may be formulated to set up and self-cure as a water- absorbing and/or water-binding, sealing substance having the capability to provide at least a partial level of water tightness. The resultant substance may be an insoluble or sparingly soluble, chemically inert, flexible or slightly flexible solid adapted to maintain adhesion to various substrates despite continued blasting, mining-induced subsidence and stress redistribution. It may be adapted to withstand high formation pressures.

The backfill admixture composition may be formulated to exhibit low viscosity and very small particle size, to permit deep penetration into water-bearing fractures.

The backfill admixture composition may be formulated to be non-toxic and not exothermic, and to have limited or no reaction either to temperature or climatic conditions.

The backfill binder that is included in the slurry pumped to the backfill site may comprise a settable reinforcing material such as a cementitious substance. The binder may be selected from the group consisting of Portland cement, ground granulated blast furnace slag (GGBS), pulverized fuel ash (fly ash), microsilica, pozzolans, polymers, and binding agents. The binder may include lime for activation.

The backfill site or zone may define an underground cavity or void. The void may be a void created by mining an orebody.

Instead, the backfill site or zone may define an above-ground cavity or void.

The backfill site may be selected from the group consisting of stopes, drifts, adits, mine shafts, inclines, galleries, bords, vugs, decline tunnels, gullies, underground passageways, underground voids, worked out areas, mined out zones, ramps, benches, caves, quarries, wells, trenches, building foundations, boreholes, tunnels, sinkholes, geologically-formed subterranean formations, bored subterranean formations, excavated subterranean formations, and lava tubes.

The tailings may be derived from the treatment of ore-bearing rocks. The tailings may be derived from a mine mining at least one of gold, silver, copper, zinc, uranium, platinum, palladium, nickel, beryllium, cobait, chromium, gallium, indium, lead, lithium, magnesium, manganese, molybdenum, aluminum, barium, antimony, bismuth, tantalum, titanium, tungsten, vanadium, zinc, iron, diamonds, sapphires, opals, emeralds, rubies, graphite, aiexandrltes, aquamarines, spinels, topaz, cadmium, potash, molybdenum, rare earth elements and platinum group metals.

The tailings may include gangue. The tailings may comprise mine waste. The tailings may include fines. For example, gold ore treatment slimes may comprise fines having a random size distribution, the majority being below 75 microns, and water recovered from the mineral processing plant.

The backfill admixture composition described herein may be able to hold, absorb, bind, gel or otherwise lock up water bleeding from backfill material, so that the development of a supernatant fluid portion is hindered. This may permit important cost savings. An illustrative example of a cost model is presented below. No undertaking is given as to the accuracy of the specific figures presented, The stated costs will vary depending upon unpredictable economic factors including fluctuating input costs, inflation and exchange rates; nevertheless, the model provides guidance on the way in which cost savings may be achieved.

For brevity the disclosed backfill admixture composition is referred to in the model as “SuperGel 265”.

Exemplary Cost Model for Backfilling a 10 000 m ³ Drift

The following model is premised on a typical supernatant bleed of about 30% of the mass of a fill.

Cost of a standard backfilling operation not using SuperGei 265

• Initial fill of 10 000 m ³ @ ZAR 300/m ³ = ZAR 3 000 000

• Refill after bleeding of supernatant water (30% of 10 000 m ³ i.e. 3 000 m ³ ) = ZAR 900 000

• Last fill (30% of 3000 m ³ i.e. 900 m ³ ) = ZAR 270 000

• Total cost: ZAR 4 170 000

Cost of a backfilling operation using SuperGei 265

(The additional cost of the SuperGei 265 admixture is based on a price of ZAR 53 per cubic metre and a dose of two litres of the admixture per cubic metre of backfill material. Two litres may be expected to give one hour for the backfill material to flow into the stopes and may eliminate about 20% of bleed.)

• Initial fill of 10 000 m ³ @ ZAR 300/m ³ plus ZAR 53/m ³ = ZAR 3 530 000

• Refill after bleeding of supernatant water (10% of 10 000 m ³ i.e. 1 000 m ³ ) = ZAR 353 000

• Last fill (10% of 1000 m ³ ) = ZAR 35 300

• Total cost: ZAR 3 918 300 In standard backfilling operations that do not use SuperGel 265, a putative backfill volume of 42 000 m ³ per month in a mine would be sufficient to fill a drift having a volume less than 29 400 m ³ (70% of 42 000). The total cost would be in the region of ZAR 12 600 000. Water run off or bleeding would be 12 600 000 litres which would need to be pumped out.

The described model predicts an increased efficiency of about 28% on the use of backfill· Thus, if the above operation were to be carried out with the addition of SuperGel 265, the same volume of 42 000 m ³ would be sufficient to fill a significantly larger drift (of the order of 37 800 m ³ ) using similar quantities of tailings, binder, admixture and surface and underground labour.

Thus, when using SuperGel 265 the total calculated cost for filling a drift of 37 800 m ³ would be ZAR 14 826 000 according to the model. By contrast, the total calculated cost for filling a 37 800 m ³ drift with backfill material that does not include SuperGel 265 would be ZAR 16 200 000. This figure is calculated by acknowledging that, based on a 30% bleed component, 54 000 m ³ of standard backfill material would be required to give a nett fill of 37 800 m ³ . Therefore, even with the additional cost of the SuperGel 265 there would be still be a nett saving of around ZAR 1 400 000.

For final tight filling, a dosage of six (6) litres/m ³ of SuperGel 265 can be admixed instead of two litres/m ³ for the previous fills. To facilitate the tight fill, a storage tank and peristaltic pump can be installed underground instead of requiring pumping from the surface. The cost of a tight fill may be expected to be about ZAR 441 /m ³ .

In addition to the cost savings illustrated above, the disclosed backfilling process may have other benefits. For example, additional savings may result from the reduced need to pump out water from the void between the hanging wall and the top of the backfill, and less water may need to be pumped to the surface. Using the figures of the cost model, 2 800 000 fewer litres of water needs to be pumped out when using the SuperGel 265 admixture. This is calculated as the difference between 3900 m ³ in the standard operation and 1 100 m ³ if the admixture is used.

Sealing of fractures and fissures in rock walls surrounding the backfill maferiai may also be promoted, so the disclosed backfilling process may reduce delays and inefficiencies caused by water running from the backfill material into production stopes, and may inhibit water leakages from drifts and longholes into haulages and working areas. Wastage of backfill material caused by leaking of the material from fractured drifts and !ongboies may also be reduced.

The disclosed process may reduce the need to remove and replace entire backfills, thereby saving costs. It may also mitigate opportunity costs that may be lost during delays in getting back into drifts to mine, while re-filling of backfill zones is carried out.

A backfill formed according to the invention may be of value in situations where it will be required to support a roof which would otherwise tend to collapse. By supporting the roof, the backfill can take up mining induced stresses so that adjacent pillars are in a less stressed condition and can be removed under normal procedures.

The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention set forth in any accompanying claims.

Finally, throughout the specification and any accompanying claims, unless the context requires otherwise:

• the word “comprise” or variations such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers;

• the word “bleed” or variations such as “bleeding” will be understood to include reference to processes selected from the group consisting of bleeding, exudation, sweating and seepage (typically of an aqueous fluid such as water);

• the phrase “low alkali” will be understood to include reference to a composition that is substantially devoid of caustic alkali substances, i.e. a composition wherein the concentration of such substances approaches zero; for example, wherein the concentration of alkali metals does not exceed about 2% Na ₂ O-equivalents; and

• the phrase "shotcrete accelerator” will be understood to include reference to a curing accelerator formulated and marketed for use as an accelerator for shotcreting applications.