Brief Study of Coal Mines Planning and Designing
Ideal conditions for Opencast Mining
1. Thickness of coal seams - > 5m
2. Flat gradient - < 1 in 6
3. Strike length - > 1 km
3. No.of seams - Less the better, with
higher thickness
5. Coal without stone bands
6. Free from surface structures / features.
7. Preferably non-forest land.
8. Availability of adequate place for dumping.
9. Stripping ratio depending on quality of the coal
Basis for the initial study is Geological Report
◾There will be some overlap between various studies/reports.
◾ Every step deals with Mining Technology, equipment, production scheduling, capital cost, revenue cost, internal rate of return, dispatches etc. However the degree of accuracy of data on the above aspects increases with each step.
◾ Generally After F.R. approval, there is need to go in for long term planning, medium term planning (about 5 years) and S.T.P. Short-term planning (6 months – one year).
◾These reports should adapt circular analysis approach by considering various aspects like additional information available while operating the mine, slippages occurred, reserves, pit-slopes, pit-limits, pit scheduling to control faster rate of extraction from coal inventory which otherwise may lead to reduction in coal production in subsequent years or more equipment for higher OB removal to maintain targeted production.
Steps for Designing of Open Cast
A. Study of deposit exploration (GR)
B. Final pit limits.
C. Development sequence.
D. Design of Access Ramp and Haul Roads
E. Pit design
F. Annual production capacity – system availability – Mining technology
G. Main Mining Equipment – Matching to Mining Plan
H. Optimizing Dumping strategy and balancing
I. Year-wise / Stage – Wise plans
J. Financial viability (Capital & Revenue)
K. Coal Washing
L. Environment Stipulations
A. Study of GR on the Deposit Exploration
Salient Information
◾ Extent of deposit i.e. strike and dip rise widths.
◾Geology – Geological succession, sequence of beds, drilling, logging, no. of seams, sequence of coal seams, description.
◾Inter seam partings.
◾Structure – No. of faults, Throws, dip of seams and beds
◾Reserves – Quality wise/seam wise/depth-wise etc. based on Block model, Gridded seam model & polygonal Model
◾ O.B. volumes & S.R. – Sector-wise, depth-wise etc.
◾Coal seam analysis, proximate and ultimate , Ash etc. & OB strata hardness etc.
◾Drilling details (Lithology)
◾Physiography
◾Climate and Vegetation
◾Preliminary EMP information
B. Final Pit Limit
The criteria for delineation of boundaries of an Opencast Project
▪️ Rise side boundary - Usually incrop of the bottom most seam
▪️Lateral sides - Usually by Geological disturbances like faults, surface structures / features like nalah, river etc or limitation of the strike length of the deposits.
▪️ Dip side - Depends on the economical stripping ratio, in absence of natural constraints
▪️ Haul analysis Lead, lift increase cost - Decides mode of transport
▪️ Dump area -100m from mine boundary
▪️ Dump limits - Dump area - 50 m from internal access road.
▪️ Dump area - 80-100 m. from mine infrastructure
▪️ Mineable Reserves - various losses, batter, barrier etc. dilution etc (50 Cm against Roof, floor)
▪️ Maximum depth - increases haul cost (30-40% of total cost at 1 1/2km lead)
Estimation of Coal, OB & Stripping Ratio
Rough assessment is given below
◾Assuming Pit slopes of 450, the Quarry floor and Quarry surface are delineated
◾ Volume of Coal & OB = Average area x Average depth - (A)
◾Average area = (Quarry surface area + Quarry floor area) / 2
◾Average depth = (Minimum depth + maximum depth) / 2
◾Volume of Coal=(Quarry floor area x Cumulative thickness of all coal seams) - (B)
◾ Coal in tonnes = Volume of coal x Specific gravity – (C)
◾ OB Volume = Coal + OB Volume (A) – Coal Volume (B) - (D)
◾ Stripping ratio = OB Volume (D) / Coal in tones (C)
For accurate S.R,the volumes of OB and coal are to be made seam wise
The boundaries are firmed up based on the economic stripping ratio.
Economic stripping ratio varies with:
◾Average sales realization i.e. higher the quality of coal, higher the realization.
◾ Capital investment requirements
◾ Operating cost.
◾Also surface constraints
The following thumb rules may be of some Help (with outsourcing OB removal):
Projects are likely to be economically viable upto a maximum SR indicated against the grade of coal
Grade Stripping ratio
(Cum/T)
F 6
E 7
D 8
C 10
C. Development Sequence
Opening the deposit - Access trench
Box cut - To accommodate all equipment
Mine phases - Depends on the shape of the property
➖Progress of Benches
➖Coal production schedules
➖OB excavation schedules
Note:- Maximize inventory of coal in the initial years
◾Differ OB stripping requirements as much as possible – Average SR versus natural SR
◾Income generated in the first 5 to 10 years but not remote economics will either make or break the project.
◾ Profits for in the future have practically no impact on the project NPV.
Each successive phase will be less profitable and the ultimate limit will be loss even after ploughing in earlier profits.
D. Design of Access Ramp & Haul roads
Location of Access ramp is guided by the following:
1) Generally located at the minimum depth of incrop of bottom most seam – So as to reduce initial waste stripping (unproductive work)
2) Should facilitate maximizing of internal dumping and sectional working.
3) Should be near the External dump / Coal yard / Mine service facilities etc.
4) Main haul road upto the pit limit should be planned in the beginning only.
5) Since haul costs constitute about more than 40% and tyre costs about 10% of the total mining costs,priority for design,construction and maintenance should be given.
6) Gradient, width, drainage, curves, super-elevation, base, sub base, type and thickness of top dressing should be like that of national highways of heavy traffic
E. Pit design
General design parameters and planning data
◾Density of seams, total coal column thickness
◾ Thickness of partings
◾ Gradient
◾ Geological disturbances
◾Strike lengths, shape of property
◾ Geological Reserves
◾Surface constraints/adjacent habitat
◾ Dump yards availability with lead and lift
◾Mining system, optimum mining concept
◾ Depth of operation, mineable reserves after various losses, OB volumes, SR
◾Spoil characteristics
◾Work practices in the Area
◾ Diggability characteristics
As per Regulation 98 of CMR 1957,
In alluvial soil
◾Sides shall be sloped at 450 or
Benched – height not more than 1.5m
Width – not less than height
In hard ground
◾Sides adequately benched/sloped so as to prevent danger from fall of sides
Coal
◾Sides shall be sloped at 450 or
Benched – height not more than 3m
However, exemption can be sought from Inspectorate.
Overall slope of a pit depends on several factors of the pit slope:
a) Geological disturbances like faults etc.
b) Hydrological condition of strata
c) Orientation of slip planes
d) Nature of strata – i.e. hardness, material consolidation etc.
e) Depth of workings
f) Design of haul roads & ramps in the highwall
g) Stratigraphy – thickness, spacing of the clay bands or other weak layers
In general for OC mines upto a depth of 200m, overall slope angle of 45° is permitted
For Deeper mines flatter highwall slopes will be necessary from safety point of view ( for 35T dumpers and above )
For Deeper mines flatter highwall slopes will be necessary from safety point of view ( dumpers below 35T capacity )
Typical High Wall Layout
Working benches
1) General
Width - 40 – 45m
Height - Generally equal to height of the boom or in some cases upto 3m above the boom height.
In case of Backhoe – digging height of machine
Bench slope - 560 to 700
2) Dragline:
Width of the cut - 60m
Height of bench - maximum digging depth
3) In order to even out the yearly OB removal quantities and the
economics sometimes alternative workings and non-working benches( around 25m wide )are proposed
F. Fixing annual production capacity
After defining the boundaries, the annual production capacity will be fixed based on the following criteria :
1) The available mineable coal reserves
2) Geometry of the deposit - Strike & dip rise lengths
3) Structure of the deposit - Thickness of seams, partings, faults, gradient etc.
4) Linkage - Basket, Captive use etc.
5) HEMM configuration - Capacity of HEMM.
6) Surface structures - Limits the material to be blasted.
G. Excavator machine productivity
Based on
◾Dipper cycle time
◾Waiting time for truck spotting
◾Bad blast factor
◾Truck loading time etc.
Annual capacity
◾Equipment available hours
◾Utilization percentage
◾Proper fragmentation,less throw off material,bench height,bench slopes ,avoiding toes and secondary blasting,improves shovel efficiency
◾Optimum fragmentation-cost of drilling,blasting vis-à-vis cost of dozing, hauling, loading, dumping.
G. Main mining equipment
1)Shovel dumper technology
This is the most commonly used technology in Opencast mines.
◾ Basically two variants – Rope shovels and Hydraulic shovels.
◾ Shovel used for excavation & Dumper for transport of material.
◾Bucket capacities vary from 0.9 Cum to 40 Cum with matching Trucks/Dumpers of 16 Cum to more than 240 T.
◾Can be deployed for removal of varying thickness of materials.
◾Harder materials require blasting.
◾ Can be deployed for removal of steep & thin seams (hydraulic shovels).
2)Draglines
A dragline bucket system consists of a large bucket which is suspended from a boom with wire ropes.
◾ Draglines are deployed wherever there is scope for side casting overburden above coal seam into the de-coaled area.
◾It is very cost effective technology & can be deployed in flatter seams.
3)In pit crusher conveyor technology
◾ The drilled & blasted OB is loaded by shovels and transported by dumpers to Crushers. The crushed OB is then transported by a series of conveyors into a spreader for dumping.
◾Can be used in steep seams where laying of transport roads for the trucks is difficult.
◾While S.R indicates volumes,cut off ratio indicates depth and reflects in dumper haulage cost,which increase with depth and may become prohibitive where in
alternative haulage system may have to thought off.
◾Ideally suited where material has to be transported over a large distance & lift.
4)Continuous Miner Technology - Bucket Wheel Excavators
◾Presently being used in Naveyali, Lignite Opencast mines, having soft strata.
◾The buck wheel excavates the material without blasting which is transported by a series of belt conveyor to spreader.
5)Rock breaker technology
◾Rock breaker technology can be applied for breaking of rock/coal/ore even about 500 kg/sq.cm.
◾It can be used wherever it is not possible to go for blasting – near vicinity of structures and habitations.
6)Surface Miner Technology
◾Used for selective mining.
◾Larger strike length of about 600 m – 1000 m and widths of around 300 m are ideally suited for surface miner.
◾Does not need drilling and blasting. The machine cuts the coal & loads into trucks for onward transportation to Surface
Other common mining equipment to all the technologies
◾Drills
◾Dozers ,Graders & Compactors
◾Water Sprinklers
◾Pumps etc.
◾Electrical equipment
◾Crushers & Conveyors for Coal/OB
