Mine Fill

Mine FillAn introduction to

Chapter 1Introduction Mining is a process to extract valuable minerals from the earths crust. Coal mining in India is dominated with open cast mining which accounts for about 80% of total mining while underground mining is 20%.Underground mining is the way to extract the ores from deep, results in the creation of voids.

Why mine fillEnsuring long term regional stabilityLimiting excavation exposureWaste disposalUnderground mining creates voids which needs to be filled to avoid subsidence and for other mine safety regionsProvides an option of disposing of waste materials in underground rather than on the surface

Schematic diagram showing how fill preserves confinement at the boundary of an excavation and assists in mobilising the shear strength along existing joints and arresting potential failure propagation

Disciplines involved in the conception, design, construction and operation o f mine fill system:Mining engineering OperatingPlanningMineral processing Rock mechanicsSoil mechanicsEnvironmental engineeringCement technologyPozzolan chemistryMineral chemistryIndustrial engineering and Geology

Chapter -2Basic mine fill materialMill tailingsAggregate or rockWater BinderTailingsTailings are the waste produced during mineral processing (separation of valuable mineral & waste)It ranges from clay to sand in particle sizeUse of these processed tailings as a fill material in underground mine voids will provide a good waste disposal technique and reduce the surface impact of miningTailings contain various agents like cyanide, lime, acid, sulfide, arsenic and other heavy metals may as a result of processing , become unstable, implications of which should be fully considered in any form of tailing disposal including mine fill

Sources of Mine Fill TailingsMine development waste rockQuarry produced rock fillVarious smelter slag as bulk filling mediaMine tailings from concentrationsHeavy media plant rejectsDune sandLeach pad residuesSome of the important features of tailing to be used as mine fill

Grain size distribution: This is very important feature as it determines many of the ultimate properties of the fillVoid ratioFlow propertiesPermeability/percolation ratePumpability

Mineralogy: Influences other characteristics such as water retention, strength, settling characteristics and abrasion action.Other properties affected by mineralogy are

Specific gravity (determinant of density of fill)Silica minerals(particularly quartz) as it can be very abrasive and result in high pipeline wear andSulfides which may results in the breakdown of the hydrated cement in the fill over time

Particle shape Tailing oxidation and aggregate grading are also influencing the performance of filling partially

Natural sandNatural surface sands are also used as fill materials, either as a sole source for hydraulic fill or supplement tailings in paste fill.Natural sand deposits are formed by fluvial, glacial or aeolian processes and are often are high in silica with well rounded particles.Sizing between and within deposits can vary widely

Rock and AggregateSources:Waste rock from open cut operationsWaste rock from underground development miningQuarried rocks and coarse gravelsHuge amount of waste rock is generated where an underground mine is developed beneath an open cut operationThe use of rock generally carries a price premium, including extra rehabilitation at the end of mine lifeIt is generally used when other cheaper suitable materials are not availableAlluvial sand can also be used , especially if available close proximity to the mine, but sever ecological damage to river system result from their recoveryMoisture content of aggregate is an important parameter and should be monitored as it can change the water balance of the fillThis may cause problems in terms of transportation, drainage and fill stabilityUniaxial compressibility strength (UCS) is an important parameter for the same WaterImportant constituent of the fill either hydraulic or paste fillPresence of salt in sufficient concentration may affects the fill strength. Laboratory test shows that for both tailings and aggregate, increase in salinity decrease fill strengths.

CementMost widely used cements are hydraulic cements, comprise a fine powder that reacts with water to bind particles together as aggregates by hardening from flowable plastic state to a solidMain constituents of cements are:Carbon, silicate, aluminum, iron (C, S, A, F)The setting and subsequent curing of Portland cement are mainly due to the hydration of calcium silicates.The initial hardening reaction is primarily due toC3S, C2SPozzolans Materials which, though not cementeceous in themselves, contain constituents that will combine with lime at ordinary temperature in presence of water from unstable compounds that exhibits cementing properties Fly ash, Slag, Gypsum along with pozzolans are some of the other components of mine fillAdmixtures are an adhesive substance added to cement are now frequently used to enhance the performance of concrete, mortar and grouts before or after hydration of the mixAccording to ASTM C 125 (2) A material other than water aggregates, hydraulic cement and fiber reinforcement used as an ingredient of concrete or mortar, and added to the material immediately before or during its mixingSome other ingredients such as rheology modifiers, Hydration modifiers and durability enhancers are frequently used in mine fill

Chapter -3Geomechanics of mine fillMine fill is a complex subject encompassing many disciplines such as:

Soil mechanicsConcrete technologyFluid mechanicsProcess engineering Mine backfilling applications and the relevant fill parametersDry fill (DF)Hydraulic fill (HF)Cemented hydraulic fill (CHF)Paste fillComposite fills

Fig (a). Hydraulic fill in a typical open stope, (b). Composite fill in open stopeFig. (a)Fig. (b)Dry fillRelevant features of dry fill areBulk unit weightDry unit weightAngle of reposeAngle of frictionParticle size distribution (P80, P50, P10)Apparent cohesionRelative densityShear strengthArching

Hydraulic fill Relevant features of HF areVoid ratio and porosityRelative densityPermeabilityActive/passive earth pressureEffective stressSaturated , submerged and bulk unit weightShear strengthSeepage, drainage and flow nets or flow pathsPipingQuick conditions, liquefactionArching

Cemented hydraulic fillCemented hydraulic fill is made by adding binders some of the relevant features of CHF are

Void ratio and porosityRelative densityPermeabilityShear strengthArchingBulk saturated, submerged, unit weightsLateral earth pressureSeepage, drainageLiquefactionSlurry rheologyPaste fillPaste fill is made by combining the tailings and binders with a certain amount of water to achieve a thick mud like consistency

Relevant feature of paste fill areSame as for CHF plusPaste rheologyShotcrete It is used to construct fill retaining walls known as fill bulkhead. Knowledge in the following areas is considered necessary to use shotcrete in backfill operations:Cement chemistry and concrete technologyCompressive tensile and flexural strengthsConcrete and shotcrete mix designsReinforcing fibers and slumpGeofabrics-geotextilesIt is used in engineering drainage systems incorporated with the shotcrete bulkheads.Therefore it is an important field for better backfill environment

Phases of backfill materialTailings or backfill are not homogeneous medial like soil it comprises of three different phases i.e., Solid, Liquid and Gas If all these three phases are present in tailings then it is classified as unsaturated tailingsWhen only two phases namely solid and liquid are present it is classified as saturated tailingsSome volumetric relationshipsIn order to arrive at some useful volumetric relationships it is necessary to lump all the solid grains into a solid mass and alll the liquid into a liquid mass and similarly all the gas chambers into a separate gas volume. After this lumping of different phases into separate volumes, the original tailings will be represented by three separate phases.

Where, V= total volume of tailings (with all three phases)Vs = some of the volume of all solid grainsVw = some of the volume of all the water contained between grainsVa = some of the volume of all air between grains and water film

Ratio of the volume of all the space between the mineral grains to the volume of all the mineral grains is called the void ratio (e).It is important property of the fill material as it indicates the amount of space between the solid particles and their close proximity

Ratio of volume of space between the mineral grains to the total volume is also useful property and called Porosity (n)

Degree of saturationRatio between the volume of water filled in the voids to that of the volume of voids is called the degree of saturation (Sr)This is an indication of the extent to which water is present in the voidsFor example if the degree of saturation is 80% this means that 80% of all the pore space is filled with water, if Sr = 0% sample is completely dry while if Sr= 100% fill is fully saturated and all the pores are filled with water

Water content of fillWater content of a fill sample is the ratio between the weight of water present in the sample to the weight of solids, and is given by the following relationship

Moisture content of fillAmount of water present in the tailings is called moisture content of the sample, it is a fraction of total weight of solids and water togetherIt is very important to differentiate water content and moisture content to calculate the weights of water and solids

Moisture content (m) is given by the following relationship

Solids content (Cw)When ratio of the weight of solids to the total weight of the fill is expressed as percentage is called the solid contentIt can be represented as

Example:The wet weight of a fill sample is 225 g and after completely drying in an oven the weight of the sample is 175 g. determine the water content and moisture contentSolutionThe water content = 0.286 or 28.6%Moisture content = .22 or 22.2 %Alternatively the moisture content = .222

water content = 0.286

Saturated fills, slurries and pastesThe weight of water = ..(1) The weight of solids = ..(2)The water content = ..(3)

Chapter-4Fluid Mechanics of Mine FillTwo main aspects of this chapter are:

The delivery of mine fill as a high density slurry from surface to underground , using boreholes and/or pipelines. The transport mechanism can be by pumping or gravity , or some combination of both.The drainage of water through fill placed underground in stopes. Since paste fill has very low permeability and rock fill tends to contain little water, this aspect is of particular interest for hydraulic fill types.Transport and delivery of fill slurriesFill from surface to underground as high-density slurry or paste typically using a combination of boreholes and pipelines, frequently using pumps and nearly always using gravity. The topic of interest here is in the properties of the various high-density mineral suspensions and in particularly their behaviour in pipelines and boreholes.Generally it is necessary to maximise the density of the hydraulic fill slurry or the paste fill while ensuring that it can be reticulated to the limits of the underground mine without the risk of blockages or line breakages.Rheology of Newtonian and non-Newtonian fluids

A fluid is a continuous substance that will deform or flow in response to shear stressFluid will tend to take the shape of the surrounding container.Shear stress is the force acting over an area, and the shear strain will be proportional to the shear stress.For a Newtonian fluid the rate of shear strain is directly proportional to the shear stress. This constant is dynamic viscosity .Water is a classic example of Newtonian fluid a fluid that obeys Newtons law of viscosity.Fig 1 shows the shear stress against shear rate for a range of Newtonian and non-Newtonian fluids. Low density mineral slurries behave as Newtonian fluids, their flow properties being dominated by the water phase.

Hydraulic fill slurry behaviourHydraulic fill slurries are prepared from mineral processing waste streams by partial dewatering and desliming to remove some of the finest size fractions.Modern high density hydraulic fill slurries are mostly designed to have a density in the range of 45%-50%cv (solid by volume).There should be a critical deposit velocity and settling of solids for better placementDurand (1953) defined the critical settling velocity as:VD = FL [2gD(s-1)]0.5

Where g= grvitational constant (m/s2)D= internal pipe diameter (m)S= specific gravity of particles FL = Durand settling velocity parameter (%)

Fig-3. Limiting settling velocity parameters (Durand, 1953)Gilchrist (1988) desctibes four flow regims for hydraulic transportation in horizontal pipes, these are:Homogenous flow: the concentration of the particle is constant across the pipe cross section generally not the case when average p: The concentration of particle is not constant across the pipe cross section. Particles are suspended by turbulence within the flow.Moving bed: The particles move along the pipe invert as a dispersed bed.Stationary bed: A stationary bed of particles remains in contact with the pipe invert. Above this layer the flow can be heterogeneous by siltation or moving bed flow.

By the above assumptions Gilchrist concluded that

Deslimed tailings are transported in a fully suspended heterogeneous regime at velocities greater than the critical deposit velocityAt densities below 2.0 kg/l, the flow regime is usually sliding bed and saltation, andAt densities above e 2.0 kg/l, the flow regime is typically homogenous flow Paste fill behaviourPaste behaves as a non- settling slurry and therefore does not have a critical settling velocity.In this case flow will occur when the driving head exceeds the wall shear stress.If paste has been delivered at too high a pulp density, flow will not occur and the paste could block the borehole and pipelines.

Paste fill flow in pipes and wall shear stress :Shear rate is determined from

Where w = shear rate at wall of the pipe (1/s)V= Fluid velocity (m/s)D= Internal diameter of pipe (m)For a typical paste fill system shear rate will range from 25-80/s at 80m3/hr

Wall shear stress is determined from:

Yield shear stress effect of pulp density

Reticulation design Majority of fill delivery systems utilize gravity as the motive source to deliver high density slurries of pastes via boreholes and pipes to the working.Some mines dont have sufficient driving head to achieve delivery to all parts of the mine and high pressure pumping system are used.Process of reticulation design is to match the delivery volume, slurry densities, pipeline diameter, borehole diameters and friction loses with the static head and/ or pumping head required to achieve delivery.Free fall section is common to both hydraulic and paste fill where excessive velocity could cause extreme wear conditions.

Steward (1988) provide a design steps to be undertaken for fill reticulation design.

These steps are applicable to full flow reticulation design for both fill typesThe steps are:Determine mine fill requirementsDetermine the static pressure head available for delivery throughout the mine lifeDetermine the total pipeline lengths. This may vary for different working areas of the mineDetermine the system frictional loses.Balance the total frictional loses to the static head by variations to pipe diameter, slurry density or, rarely energy dissipation methodsHydraulic fill reticulation designVan der Walt (1988) lists a number of points to consider when designing a fill system.The key generic points are:

The transport velocity of the slurry must be significantly higher than the critical velocity to prevent the slurry from settling outThe transport velocity must be kept as low as possible to minimize friction losses and pipe wearStandard pipe sizes are preferred In vertical columns ,the maximum flow rate is at the point where the frictional losses exactly equal the available potential head Flow rate of slurry through the system is determined by the inlet conditions Maximum working pressure in the system will be found at the bottom of vertical columns and will be determined by the frictional losses in the horizontal columnsBursting discs and collection sumps should be provided at the points of maximum pressure in case blockage in the pipeProvision must be made for the flushing of lines before and after fillingCalculating friction system losses in hydraulic fill systemHigher densities and finer particles are significantly involved in hydraulic fillCook (1993) proposes that high- concentration(settling) slurries be considered as consisting of the following components:Vehicle portion, consisting of the finer settling and non-settling particles and the carrier fluid,Suspended load, those solid particles supported by the yield shear stress within the vehicle portion, and Coarse fraction being those particles supported by inter-particle contact The friction losses in the reticulation system are a function of the wall shear stresses Cooke (1993) gives the following relationship;

Where:w = density of carrier fluid Sv = relative density of slurry vehicleVm= mean velocity of mixture Fv = friction factor for vehicle portion

Friction factor for high density finer slurries can be determined from the diameter and roughness of the pipe, the velocity, apparent viscosity (K), yield shear stress , flow behaviour index (n) and density of the mixtureFor turbulent flow in the rough pipes, the friction factor is

Where fk=0 and fk are calculated form the colebrook white relationship for smooth wall and rough wall Newtonian flow respectively:

Drainage through hydraulic fillDrainage analysisCan be calculated through Darcys law Q = KAH/LWhereQ= flow rate out of the stope (m3/s)K= fill mass permeability (m/s)A= cross sectional area of drawpoint (m2)H/L= hydraulic gradient in the drawpoint (m/m)

Testing and measurementLaboratory scale rheologyYield shear stress and slump testYield shear stress determined by the vane shear viscometerViscosity measurementPipe loop testing

Chapter-5Introduction to Hydraulic FillHydraulic fill is a class of mine fill types that are delivered as high density slurry through boreholes and pipelines to the underground mine voids.The name is derived from the water born delivery method.Hydraulic fill is most commonly prepared by dewatering and desliming mineral processing waste streams and has the following characteristics:Maximum particle size: less than 1mm and most of the finest sizes are removed to ensure not more than 10% by weight of less 10 m are retained to ensure adequate fill permeability.Slurries are made at densities between 40-50%cv (solid by volume).The slurry transport regime is heterogeneous and turbulent at average velocities higher than the critical settling velocity.Hydraulic fill has a permeability in situ in the range of 10-5 -10-6 m/s. excess water used to deliver the solid components to the stope must drain out of the fill, by vertical gravity drainage through the fill, decantation and through engineered drainage facilities at stope access pointsPlaced hydraulic fill has a porosity typically around 50%. At 50% porosity (void ratio= 1.0),the bulk density is one half of the dry solid density; e.g. tailings with a specific gravity of 2.8 will have a dry bulk density of around 1.4t/m2 Mining methodsDescriptionsKey characteristicsCut and fillUncemented hydraulic fill placed in long pours to fill each lift as minedFlat beach angles in the range of 20 (1:30) provide a good working platformMostly uncemented cap placed to provide hard mucking surfaceLong term drainage facilities designed into the base of cut and fill mining areaSuitable for under and overhead methodsRelatively simple barricade built to contain each fillDrift and fillOrebody mined as a series of longer primary stope & secondary pillarsEach drift filled tight to the back to provide support for the removal of adjacent pillar driftCemented fill required to maintain stable side exposure in secondary drift strong enough for self weight of fill plus any surcharge load from the back of overbodyPost pillar- cut and fillLarge plan area mined in lift leaving slender pillarsEach lift filled with uncemented hydraulic fillIt provides a working platform for mining operationsIt provides confinement to the slender pillars, maintaning performance Bench stopingSmall single sublevel stopes mined and post filledEngineered barricades required in all opening at the base of the bench to retain the fill and permit effective drainageCemented fill required in primary benches Flat mucking surface required for extraction of next sublevelWaste rock often dumped into secondary benches for disposal Sublevel open-stopingLarger stopes usually mined over several vertical sublevels and filled at the end of productionEngineered barricades required in all openings on each sublevels to retain fill & permit effective drainageMost drainage will report to lowest levels with only minor amounts higher up in the stope Table: Use of hydraulic fill by mining methodDesign Demand from mining methods Hydraulic fill is used in a number of different applications in a variety of mining methods.

Preparation of hydraulic fillHydraulic fill is mainly slurry based mine filling where a solid waste material like tailings, sand or waste rock is used.Slurry densities are typically 25-35%cw (solid by weight)This includes as well stabilized circuit for slurry transport to the destination pointHydraulic fill plant performs two related functions of dewatering the slurry and removing the finest fraction of the tailings material.The tailings slurry is dewatered to minimise the quantity of water that will be placed underground and must drain out of the fill during and after placementThe slurry density should be between 45-50% cv (solid by volume)This is typically greater than 70%cw (solid by weight) or relative density greater than 1.8Hydraulic fill also removes the finest size fractions to achieve the required permeability targets and so ensure proper drainage

Components of hydraulic fillHydrocyclonesSpiral and rake classifiersDrum filtersElutriation tanksStorage tanks and pachucasDelivery system from preparation site to stope

Fill containment-design and construction of fill barricadesFill barricade is important to retain the fill solids while permitting the excess transport water to drain out of the stopeWall must have the structural capacity to withstand the maximum anticipated lateral pressure that the hydraulic fill will imposeVarious types of barricaded designs have evolved in mining districts, some of them are as followsWaste rock barricade with very limited application in some cut and fill operations with very low lift hights.Timber and permeable hessian barricadeArched impermeable concrete masonry block work up to 1m thick over spans of 4m X 4m, with sealing grout, hatchways and drainage pipes

Placement and drainageHydraulic fill placed into production voids such as stope must be allowed to drain to remove transport waterConsequence of not meeting this may leads to barricade failure, allowing a rush of fluidized fill in to the mine working and cause tragedyEarth pressure /or pore pressure loads applied to retaining barricades must be lower than the design strength of these structuresThe excess transport water with which the hydraulic fill is delivered must be able to drain freely from the fill and from stopeThe excess water should be minimized by : maximizing slurry placement and reducing, diverting or eliminating flushing water delivered to the stope

Hydraulic fill summary of key issuesAdvantages and limitations of hydraulic fill:The risk of inrush and its consequences can be higher in uncommented hydraulic fill compared to cemented hydraulic and paste fill operations if badly designedThe fill placement rate is constrained by drainage rate and account must always be taken of pouring and resting times and the establishment of unsaturated filling conditionsThe desliming process reduces the available tonnage of fill material to be placed underground Surface processing plant is relatively simple and low capital cost but requires effective instrumentation and quality control systemsCement binder is not required in many situations where future exposure is not required, thus subsequently reduce the cost compared to paste fillInadequate collection of drainage water can result in poor roadway condition, damage to vehicles and have major impact on the ventilation system