Flow Behaviour of Pond Ash Slurry and Shrinkage of Pond Ash Stowed Mine Area

Abstract

Backfilling enables complete or around ninety percent extraction of coal from the seam and then filling the mine voids with mill tailings; sand or excavated stones etc. which provides additional ground support by constraining lateral deformation of surrounding coal pillars and roof. However the overall feasibility of the backfill operation with different materials has to be studied in detail. The objective of this project is to evaluate the suitability of using pond-ash as backfill material over fly ash and sand in respect to shrinkage of the stowed area. The shrinkage study is done with the help of numerical modeling in FLAC of a mine KTK-5 where pond-ash stowing is going to be implemented. The physical properties like bulk modulus, shear modulus, cohesion, friction angle etc. were also found out through experiments which are required during the numerical modeling. Moreover an effort has also been made to develop comprehensive flow model using CFD and then use the model for predicting pressure drop, volume fraction etc. A 20m pipe with diameter of 20cm is modeled, through which flow is conducted where modeling and meshing is done using ANSYS Fluent. High viscosity fly and pond ash slurry with different concentrations up to 70% by weight of pond ash is passed and for each concentration different velocities are used and pressure drops is calculated. MDD ranges from 1.07gm/cc to 1.27gm/cc. With increase in compaction energy MDD increases due to the closer packing of pond ash particles and OMC decreases from 38% to 28% approx. which might be due to the increase in moisture content leading to less friction between the particles and promoting compact packing with increase in compaction which in turn decreases voids and increase saturation limit. The settling rate for the sample is found to be around 30% with water-liquid (phase1) and pond-ash (phase2). Velocity of 3.5m/s of the paste are optimum with respect to pressure drop. The FLAC simulation yields factor of safety (FOS) after excavation of one pillar with no fill to be 1.5 whereas with pond ash fill it is 2.7. Similarly FOS after stowing with pond ash in the voids of two pillars was found to be 2.5 whereas without fill it was 1.2. After excavation of two and half pillars FOS was calculated to be 1.9 with pond ash fill and 1.0 without fill.