HIGH CONCENTRATION FILL TECHNOLOGY – A NEW AREA ON ASH UTILIZATION By Dr. C. N. Ghosh, Chief Scientist Central institute of mining & fuel research ( Council of Scientific and Industrial Research ) Dhanbad

INTRODUCTION About 100 Thermal Power Stations in India consume about 400 Mt. of coal(35-50% ash) and generate about 150 Million Ton of Coal Ash per annum. About 3000 Million Ton of coal is locked up in pillars in different coalfields in India and could not be extracted due to non- availability of filling material. Increased demand of coal production means increased depth of working by faster mechanized extraction techniques and decreased % extraction (about 30%) as coal pillars are left for strata control

Conventional hydraulic stowing demands classified material (coarser size) for improved stowing performance Reduced Cut – off – grade and increased ash production (80% F.A+20% B.A) results in generation of finer waste Paste fill technology facilitate high filling rate suitable for mechanized operations and provides an avenue for utilization of total tailings, Ash etc. Share of mine filling in ash utilization is a meager 5-10%, the use of ash in mines can produce a quantum jump in ash utilisation and would help to achieve the aim of 100% ash utilisation INTRODUCTION

DEFINATION OF PASTE Paste flow Slump Test: a) Small Slump b) Large Slump First used at the Grund Mine in Germany during the 1980’s, The term “Paste” refers to a class of backfill that has an appearance akin to “toothpaste” and have low water content such that the mixture has a consistency as measured by ASTM slump cone test from slightly greater than zero up to nearly 305mm (12 inches), high densities( > 75% Cw) low permeability, generally consisting about 15% minus 20 micron meter (625 mesh) fines content. OR According to Golder Associates “A Paste can be defined as a mixture of solid and water that has little or no bleed of water when Idle”

PASTE TECHNOLOGY OFFERS: Low permeability with little or no water bleed, hence binder and leachate washout is reduced. Reduce mine dewatering costs. Enhanced filling rate, hence mining cycle time is reduced. Higher Strength with lesser binder as in-place segregation and dilution of constituent particles is less Much lighter/simple barricading arrangement Hurdles : High initial capital investment Strict control (PLC) and greater level of engineering design Relatively new technology and many technical aspect are not fully understood. Paste Fill vis-à-vis Conventional fill

PASTE FILL MIX CONSTITUENT Water Pond Ash (80% Fly Ash + 20% Bottom Ash) Binder (OPC, blast furnace slag etc.) > 75% (by weight ) Expected Slump Paste Fill

DESIGN OF PASTE FILL SYSTEM void

Paste Fill Plant has Three Essential Functional Components De-Watering Mixing Pumping, transportation and deposition De Watering Thickeners and Filters High Density Thickeners Cyclones Centrifuges Tank Dewatering Tailspiner - special centrifugal case Fluidization PASTE BACKFILL PREPERATION

Mixer Continual Mixing plant Continual paste production plant ASH FROM PLANT

PASTE BACKFILL PREPERATION Batch paste production plant Batch Mixing Plant ASH FROM PLANT

Transportation of high concentration ash slurry via pipeline distribution network to underground void is facilitated by means of gravity or piston pumps. PUMPING,TRANSPORTATION & DEPOSITION Different Configuration of Transportation Gravity System Gravity & Pumping System Pumping & Gravity System

The pumpability of paste fill depends mainly on its rheological properties (viscosity, yield stress) and type of pump used. High concentration low settling paste fail to obey Newton’s law of viscosity and exhibit Non-Newtonian behavior. The rheological behavior of Non-Newtonian fluid is obtained by testing with viscometer PUMPING,TRANSPORTATION & DEPOSITION

(  ) ()() ( y)( y) General Rheological Model Equation  =  y + k  n Where  = Shear Stress (Pa)  y = Yield Stress (Pa)  = Shear Rate (sec -1 ) k = Consistency Index n = Flow behavior Index The Bingham plastic and Power Law Model are most popular because of their simplicity and the wide range of fluids they can describe PUMPING,TRANSPORTATION & DEPOSITION

A Full scale pumping loop test on paste of desired consistency is necessary to determine the pressure gradient, head loss for various combination of pipe diameter, flow rate and throughput. Results of loop test will determine operating pressure, choice of pipe diameter and flow rate. In general pipe diameter of paste lies between 100 – 200mm, flow velocity varies between 0.1 – 1 m/sec PUMPING,TRANSPORTATION & DEPOSITION

Pumping of high concentration slurry is done by means of positive displacement pumps It discharges fixed quantity of fluid with each revolution of pump roter, hence its capacity remains unaffected by changes in pressure it must develop Envirotech Pump systems (Geho), Putzmeister, Schwing, Abel are some of the companies manufacturing high pressure (165 bar), high discharge rate ( m 3 /hr) piston pumps

LABORATORY STUDIES For the first time in India high concentration ash stowing was proposed to be undertaken at the underground mines of Madhuband Colliery, Barora Area, BCCL using ash of Chandrapura Thermal Power Station (DVC). Laboratory study to determine the suitability CTPS ash, barricade design, optimum concentration, compressibility etc was carried out at CIMFR, Dhanbad Physical characteristics: Specific gravity:2.15 Bulk density:0.83 t/m 3 Percentage Void:61.40%

Grain Size Distribution Auto – oxidation Characteristics Proximate Analysis Moisture percentage: Ash percentage: Volatile Mater percentage: Unburnt Carbon percentage: Crossing Point & Ignition Point Temperature CPT & IPT : Not Reached till C LABORATORY STUDIES Size ( microns) % Retained

Gallery Model study was carried out to optimize slurry concentration, barricade design, drainage rate and quantity etc. Slurry of different concentrations (50%, 55%, 57%, 58% & 59% by weight) was prepared and poured in to the model, following was observed: Cw < 55% : behaved as dilute slurry 55% <= Cw <= 58% : behaved as a paste/ semi paste Cw > 58% : found to be un-pourable Drainage rate with optimized barricade configuration of single layer bamboo matting and single layer hessian cloth was found to be good LABORATORY STUDIES

Compressibility Characteristics L/D = 0.8 Loading Rate = 5 tonne Material = Air Dried Ash Sample (CTPS) At 100 kg/cm 2 ( ~ 400m depth) Compressibility: Pond Ash = % Sand = 7.5 % LABORATORY STUDIES

CONCLUSIONS & RECOMMENDATIONS To determine the suitability of any material to be used as paste fill following investigations are recommended Determination of particle size Particle shape, mineralogy Relation between slump vs. water content vs. pressure drop Material Sp. Gr., Porosity, Permeability, strength test (UCS) De-watering test (Thickening or filteration) Generate frictional data from pump loop test State- of–the –art PLC control is essential as slight change in moisture content may lead to wide variation in viscosity and pressure drop. Study of rheological properties of the mix is imperative

Laboratory study on CTPS Ash indicates its suitability for use in underground coal mines. Model test indicated that ash slurry up to a concentration of 58% (by weight) can be used for stowing Compressibility of pond ash was found to be slightly more than that the desired value at 100 kg/cm 2 Coordinated endeavor by both Power Plant and Mining officials are required to implement this technology in India. Effort should be directed not only to install new paste fill plant but also to modify the existing stowing plant to meet the same end CONCLUSIONS & RECOMMENDATIONS

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