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Evaluation of Bench Height Selection for Limestone Resource Optimization A
Case Study
N K Sharma, R Mazumder, D K Panda, A K Dubey
Abstract
The economic viability of the modern day limestone mine and its long term sustainability
is highly dependent upon careful planning and management. Declining trends in availability of an
average ore grades, increasing mining costs and environmental considerations will ensure that
this situation will remain in the foreseeable future. The optimization and strategic planning for
management of a large open pit limestone mine having a life of several years is an enormous and
complex task. Strategic planning for mine optimization is carried out basically to enhance the
quarry life with minimum generation of waste keeping in view the two main objectives (i)
optimal grade of limestone and its production, a mine can produce up to the life of the deposit and
(ii) optimization based on grade constraints keeping the consideration of raw mix based on
cheaper fuel and additives avoiding the imported fuel and costlier additives. Though a number of
techniques have been successfully applied to resolve some important optimization problems, the
problem of determining an optimal ultimate pit which depends upon the selection of optimum
bench height is still very much unresolved.
In this paper, it has been attempted to provide a long term strategic plan for optimization
of limestone mine by preparing ore body models at variable bench height systems of 9m, 10m
and 12m to study the effect of dilution / segregation of various grade due to compositing at
different bench heights. The effect of various ore body models is being compared for
determination of optimum ultimate pit limit of a mine which depends upon blended (optimized)
and final (un optimized) pit and resultant optimum production schedule over the total life of the
mine. Multimine scheduler software which uses LG (Lerch-Grossman) method to determine an
optimal ultimate pit is used to carry out pit optimization for maximization of resources for 9m,
10m and 12m bench model.
The limestone deposit of the studied area belongs to Narji formation of Kurnool Group is
stratiform, stratabound of regular habit associated with variable thickness of calcareous clay at the
top and purple siliceous limestone as inter-burden and also at the bottom of various boreholes.
The ultimate pit limit for all the three models is created with a specified ROM cut-off percentage
of CaO and SiO2, which gives the shape of the mine at the end of its life. The results of ultimate
pit reserve are reported as range of grade tonnage relationship for blended and final pit is
compared for all the three bench models. This allowed to calibrate the model for mining bench
height, realistic dilution / segregation, blending solution and production scheduling at specific cut
off grade for optimized life of mine.
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calcareous clay and limestone with CaO 42444648
4215.00% Blendable grade lst
2 >4015.00% Low grade limestone
ZONE-2
(Calcareous Clay)
and
ZONE-3
(Dark & Light
Grey Limestone)
3 15.00% V. Low grade limestone
3.0 Ore body regularization by different bench height
To estimate mineable reserves it is necessary to consider economic, mining,
environmental and other factors. A convenient way to discuss these factors is to consider the ore
loss and dilution gain that is incurred during the various phases of the mining project. The
concepts of ore loss and dilution gain are illustrated in Fig. 1.
ORE LOSS
INTERNAL DILUTION
EDGE DILUTION
BENCH HEIGHT
ACTUAL ORE
OUTLINE
GEOLOGISTS
INTERPRETED
ORE OUTLINE
ACTUAL ORE
OUTLINE
GEOLOGISTS
INTERPRETED
ORE OUTLINE
Fig. 1: Dilution and ore concepts
During production, ore losses and dilution gains are caused by several interrelated
factors, namely:
irregularities in the outline of the ore zone;
the sampling method and sampling density;
the attitude of the ore body;
the bench height;
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human error; and
ore losses to the dump or heaps.
Many of these factors can be best utilized by simulating mining through scheduler based
on the various bench height systems. Not only does it solve mining, grade control and blending
problems prior to mine development, but also many of the other geological and sampling
uncertainties associated with estimating the in situ resources.
There is a possibility of compositing of samples falling within the height of bench leading
to segregation or dilution of internal or external waste into making material to be categorized
blendable / low / very low grade material. Fig.2 shows the impact of compositing at different
bench height.
Fig. 2: Borehole sample composites of 9m, 10m and 12m bench height system
This effect would impact the blending solution during production and determines the
reserves in the blended pit (production) and final pit (not mined and left out due to no blending
solution) during simulation of production scheduling.
Bore hole composite sample length of 9m, 10m and 12m comprising of litho units of
Calcareous Clay, Light Grey Limestone and Dark Grey Limestone have been tried for bench
height selection for the Limestone Deposit as shown in Table 2.
Table. 2: Composite Sample Length and Percentage Recovery of Different Bench
Height System (Total Sample Length= 1979.50m)
Sample Category 9m Bench
Height
10m Bench
Height
12m Bench
Height
Composite Sample Length for SiO215
Percentage Recovery
197.02
9.69%
203.35
10.01%
218.31
10.74%
4.0 Deposit modeling
To study the effect of dilution / segregation, different ore body models have been made at
different height of mining block or cell dimension equal to bench heights of 9m, 10m and 12m.
The top bench floor, block size details and number of benches for the three ore body models
prepared are given in Table. 3.
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Table. 3: Top bench floor, block size details and number of benches for each ore body model
Ore Body Models Top Bench FloorX & Y Cell
Dimension(m)
Z Cell Dimension /
Bench Height (m)No. of Benches
9m Bench Model
10m Bench Model12m Bench Model
86
9086
100m
100m100m
9m
10m12m
9
97
The 3D geological model has been created from the borehole data where Black cotton
soil (overburden) and Calcareous clay is forming the top of the limestone for all the ore body
models.
For each ore body model bench height system is maintained till the depth of 14 mRL.
The 3D geological model of a 12m bench model with black cotton soil, calcareous clay and dark
and light grey limestone has been shown in Fig. 3.
Fig. 3: 3-D view of a 12m Geological Model of a limestone deposit with Black Cotton
Soil, Calcareous Clay and Dark Grey and Light Grey Limestone Zones.
5.0 Optimization of Ultimate Pit and Scheduling
The 3D geological model prepared for 9m, 10m and 12m bench height system is used for
subsequent pit optimization with the help of Multimine Scheduler software.
Multimine scheduler software which uses LG (Lerch-Grossman) method to determine an
optimal ultimate pit is used to carryout pit optimization of all ore body models. The ultimate pit
limits define what is mineable from a given deposit while satisfying the blending criteria
according to which it identifies which blocks should be mined (blended pit) and which ones
should be left in the ground (final pit).
All the models prepared are being used for optimization of resource by finding the
blending solution using all the grades present in the model except soil. Once the optimal solution
is obtained for the specified cut off grade criteria, the ultimate pit shell is created around the
blended pit. The blended limestone cut-off specification and quantity targets are given in Table.4.
Table. 4: Specification of blended Run of Mine Limestone
Annual Run of Mine Production Target
Quality (%) Quantity(tonnes)
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CaO% SiO2%
45-47 11.5-12.52250000
Long term production strategy plan has been carried out for annual production target and
production schedules has been prepared for 9m, 10m and 12m bench models with minimum
quality variation and an effective optimal in pit blending scheme.
5.1 Impact of Bench Height Selection on Ultimate Pit Shell Limit
The ultimate pit limit gives the shape of the mine at the end of its life. Optimum ultimate
pit design plays a major role in all stages of the life of an open pit: at the feasibility stage when
there is a need to produce a whole-of-life pit design; at the operating phase when pits need to be
developed to respond to change in limestone grade requirement, mining cost, cement making
costs, ore reserves and wall slope; and towards the end of a mines life where the final pit design
may allow the economic termination of a project. Pit limit (excluding safety limits for road,
railway track, rivulet, forest etc.), limiting bench parameter (desired bench upto which mining has
to take place) and other required quality parameters constraints such as (e.g. CaO%, SiO 2%) are
applied to get the required optimal ultimate pit / blended pit.
In this case the impact of variable bench height systems of 9m, 10m and 12m is studied to
determine the most optimum ultimate pit for the limestone deposit in terms of its final shape at
the end of the life of the mine and the total limestone reserve. Fig.4 shows the generalized layout
of the limestone deposit and the shape of the ultimate pit shell simulated for the production
scheduling based on the required blending criteria as per table.4 for 9m, 10m and 12m bench
model at the end of the life of the mine. From the fig. 4 it is observed that the ultimate pit shell of
the 12m bench model has the most favorable operating condition in comparison to ultimate pit
shell of 9m and 10m bench model. The ultimate pit shells of 9m and 10m bench models have
more number of working pits and some of the working pits are small in size which will be
uneconomical for mining operation. However the ultimate pit shell of 12m bench model has less
number of working pits and are wider in size and therefore the mining operation will be more
economical and better manageable than the other models.
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Generalized Layout of the Limestone Deposit 9m Ultimate Pit Shell
10m Ultimate Pit Shell 12m Ultimate Pit Shell
Fig. 4: Generalized layout of the limestone deposit and the ultimate pit shell of the 9m,
10m and 12m bench model
5.2 Impact of Bench Height on resource optimization and blending
The output of the pit optimization shows the reserves of the total rock, ore and chemical -
compositions of the blended pit and final pit for particular models. Here blended / ultimate pit is
the pit obtained by maintaining balance between elements or rock types according to user defined
blending criteria for production and development. Final pit material is the material which is left
out in the mine and could not be utilized in blending as it does not satisfy the blending criteria.
Table.5 shows quality and quantity of blended and final pit (with constraints such as excavation
upto desired benches and ROM quality targets) obtained from blending all grades of limestone of
9m,10m and 12m bench model. The 12m bench model has the highest total blended limestone
reserve of 221.64M.T which is 8.81and 7.12 M.T. higher than the 9m and 10m bench model,
where as the blended ROM grade remains same. The quantity of limestone left in the final pit is
15% and rest 85% of the reserves have been utilized in blending for production in 12m bench
model as compared to the 9m and 10m bench model where the final pit quantity is 18% and 17%
and Blended pit quantity is 82% and 83% respectively (ref. Fig-5)
Table. 5: Quality and Quantity of Blended and Final pit for 9m, 10m and 12m Bench Model
9m Model 10m Model 12m ModelOptimized
Pit Rock
(M.T)
Ore
(M.T)
CaO
(%)
SiO2
(%)
Rock
(M.T)
Ore
(M.T)
CaO
(%)
SiO2
(%)
Rock
(M.T)
Ore
(M.T)
CaO
(%)
SiO2
(%)Blended Pit 219.81 212.83 47.00 12.43 220.62 214.52 47.00 12.38 228.65 221.64 47.00 12.40
Final Pit 47.38 46.90 48.31 11.06 45.68 44.87 48.69 10.98 39.01 38.51 48.49 11.08
Total 267.19 259.73 47.24 12.19 266.30 259.39 47.29 12.13 267.66 260.15 47.22 12.21
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38.51
,15%
46.9,
18%
212.83
,82%
44.87
,17%
214.52
,83%
221.64
,85%
Blended Pit Final Pit
10m bench
model
9m bench
model
12m bench
model
Fig. 5: Distribution of limestone (ore) in blended pit and final pit in 9m, 10m and 12m bench model
It can be inferred that from 9m to 12m bench model, there is an increase in the reserves
of ore in blended pit and decrease in quantity of left out limestone in the final pit with change in
bench height while the total reserves of ore remains same. It is also observed that there is a better
segregation of grade and reserves in the 12m bench model for both cement grade and low grade
limestone than other models which has resulted in optimum utilization of all the grades for
blending on the ROM grade constraints imposed. Fig.6 and 7 shows the distribution of cement
grade (SiO2% 15% including blendable and low
grade limestone) in blended and final pit respectively.
191.71
192.00198.26
46.86 44.8538.51
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
180.00
200.00
Reserves(M.T
)
Blended Pit Final Pit
9m Bench Model 10m Bench Model 12m Bench Model
Fig. 6: Distribution of cement grade limestone in blended and final pit in 9m, 10m and 12m bench
model
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21.12
22.52
23.38
0.03 0.01 0.00
0.00
5.00
10.00
15.00
20.00
25.00
Reserves(M
.T)
Blended Pit Final Pit
9m Bench Model 10m Bench Model 12m Bench Model
Fig. 7: Distribution of low grade limestone in blended and final pit in 9m, 10m and 12m bench model
The distribution of blendable grade, low grade and cement grade limestone in 9m, 10m
and 12m bench model in the blended pit is further evaluated for understanding the segregation ofvarious grades of limestone both in the form of reserves and grade (Table. 6). It is observed that
90.66 M.T of cement grade limestone having CaO%>48 is obtained from 12m bench model
which is 3.93 M.T and 10.23 M.T. higher from the 9m and 10m model having the limestone
reserves of 86.73 M.T and 80.43 M.T respectively. Similarly 107.60 M.T of cement grade
limestone having CaO% >424215
14.82 44.93 16.58 16.07 44.54 16.47 15.35 43.80 16.43
LowGradeLimestone
CaO% 4015
6.30 27.23 30.52 6.45 31.97 28.87 8.02 32.40 28.03
CaO% >42
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Distribution of different grades of limestone in blended pit in 9m, 10m and 12m bench
model is shown in Fig.8. It is observed that, effective dilution gain in terms of grade & tonnage
for low and blendable limestone, vis--vis segregation of CaO% >48 grade limestone in terms of
tonnage is found in 12m bench height model. The overall factors responsible for better achievable
blendability of various grades within the limestone ore body is due to its distribution, dilution &
segregation of both low & high grade limestone for 12m bench height model.
6.30,3%
14.82, 7%
104.98, 49%
86.7
3,
41%
6.45,3%
16.07, 7%
111.57, 53%
80.4
3,
37%
8.02,4%
15.35, 7%
107.60, 48%
90.6
6,
41%
BLEN. GR. LOW GR. CEM. GR.>4248 CaO
10m bench
model
9m bench
model
12m bench
model
Fig. 8: Percentage distribution of different grades of limestone in blended pit in 9m, 10m and 12m
bench model
5.3 Production Scheduling
The blended pit limestone of each bench model is scheduled while targeting quantity andquality objectives like CaO% and SiO2% to find 5 yearly schedules for 9m, 10m and 12m bench
model. Table.7 shows the 5-yearly excavation at the specified blended ROM quality for 9m, 10m
and 12m bench model. It has been observed that 12m bench model sustains for the total life of 99
years which is 5 and 3 years higher in comparison to 9m and 10m bench model having life of 94
and 96 years respectively. The scheduler reports of blended ROM quality for 12m bench models
are shown in Fig.6 indicating sustainable life of the deposit.
Table. 7: Scheduler result for 9m, 10m and 12m bench models (at specified ROM quality)
9m ore body model 10m ore body model 12m ore body model
Year Ore CaO SiO2 Year Ore CaO SiO2 Year Ore CaO SiO2
0-5 11.29 46.24 12.27 0-5 11.38 45.73 12.93 0-5 11.37 46.55 11.876-10 11.22 46.45 12.37 6-10 11.28 45.60 12.81 6-10 11.19 46.54 12.15
11-15 11.31 45.97 12.38 11-15 11.19 46.73 12.05 11-15 11.42 46.92 12.00
16-20 11.30 45.84 12.63 16-20 11.29 46.11 12.49 16-20 11.04 46.03 12.38
21-25 11.32 46.99 11.76 21-25 11.19 45.82 12.61 21-25 11.30 46.36 12.64
26-30 11.10 46.96 11.90 26-30 11.25 46.53 12.49 26-30 11.42 45.64 12.47
31-35 11.26 45.19 13.17 31-35 11.18 46.22 12.09 31-35 11.06 45.99 12.73
36-40 11.35 46.47 12.85 36-40 11.27 47.29 12.18 36-40 11.43 46.48 12.91
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41-45 11.10 46.97 12.31 41-45 11.34 47.26 12.37 41-45 11.32 46.44 12.43
46-50 11.40 46.45 12.50 46-50 11.25 47.64 12.44 46-50 11.18 46.30 12.70
51-55 11.23 47.51 12.19 51-55 11.25 47.54 12.50 51-55 11.29 47.96 11.87
56-60 11.13 47.38 12.67 56-60 11.25 47.16 12.27 56-60 11.15 47.51 12.85
61-65 11.41 47.27 12.65 61-65 11.25 47.53 12.33 61-65 11.40 47.57 12.78
66-70 11.25 47.48 12.75 66-70 11.35 47.29 12.33 66-70 11.13 46.86 12.59
71-75 11.15 46.87 12.81 71-75 11.17 47.25 12.05 71-75 11.35 47.63 12.83
76-80 11.33 47.82 12.83 76-80 11.27 47.79 12.38 76-80 11.10 47.56 12.46
81-85 11.25 48.12 12.54 81-85 11.22 47.60 12.50 81-85 11.40 47.97 12.38
86-90 11.29 48.31 12.31 86-90 11.19 47.70 12.00 86-90 11.19 48.24 11.59
91-94 10.15 48.92 11.23 91-95 11.37 48.15 12.35 91-95 11.07 47.83 11.63
95-96 0 .59 48.65 11.92 96-99 7 .85 47.92 12.89
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
CaO%&
SiO2
%
0-5 6-10 11-15 16-20 21-25 26-30 31-35 36-40 41-45 46-50 51-55 56-60 61-65 66-70 71-75 76-80 81-85 86-90 91-95 96-99
YearCaO% SiO2%
Fig. 6: Scheduler report (5-yearly) for 12m bench model
6.0 Conclusion
Strategic planning for mine optimization is carried out basically to enhance the quarry
life with minimum generation of waste keeping in view the two main objectives (i) optimal gradeof limestone and its production, a mine can produce up to the life of the deposit and (ii)
optimization based on grade constraints keeping the consideration of raw mix based on cheaper
fuel and additives avoiding the imported fuel and costlier additives. To study the effect of dilution
/ segregation of various grades of limestone due to compositing at different bench heights, bench
models at variable bench height systems of 9m, 10m and 12m are prepared. The effect of various
bench height models is being compared for determination of optimum ultimate pit limit of a mine
which depends upon blended (optimized) and final (un optimized) pit and the resultant optimum
production schedule over the total life of the mine. Pit optimization for maximization of resources
is carried out with the help of Multimine scheduler software which uses LG (Lerch-Grossman)
method to determine an optimal ultimate pit for 9m, 10m and 12m bench model.
From the result of pit optimization of 9m, 10m and 12m bench model it is observed that
the 12m bench model generates the ultimate pit shell with lesser number of working pits which
are wider in size and hence will be more economical for mining operations. It is also observed
that due to better dilution and segregation of both low and high grade limestone in 12m bench
model in terms of grade and tonnage leads to more proper blending of different grades of
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limestone which results in the increase of the total limestone reserves in the blended pit with
desired blended ROM limestone with optimum production schedule and enhanced quarry life.
7.0 Acknowledgement
The authors have freely drawn upon completed R&D reports and some unpublished workof NCB. This paper is being published with permission of the Director General, NCB.
8.0 References
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5. I M Glacken et.al, Mining Bench Height Evaluation for Wallabay Resources A Conditional Simulation Case Study, 4 th International Mining Geology
Conference Coolum, dated 14-17 May 2000, PP-195.
6. Integration Road to increased production by Stephen B Chessman, P Geo.Geocom Software International Inc.
7. Pelly Peter, F.D., 1991, Guidelines to the evaluation of selectively mined, openpit gold deposits during the exploration stage of mine creation, MSc.
Dissertation, Rhodes University, Grahamstown, Eastern Cape. South Africa.
8. Raj K Singhal et.al (1988) Computer Application in Mineral Industry. A.ABalkema, Netherland.
9. R Dey, A K Dubey, N K Sharma, D K Panda, M Imran, Multimine Schedulingfor Optimization of Blended Limestone from Four Mine Blocks for Cement
Manufacture 12th NCB International Seminar on Cement and Building
Materials, 15-18 Nov. 2011, New Delhi.
10. R M Jara, Block Size Selection and its Impact on Open Pit Design and MinePlanning The Journal of The South African Institute of Mining and Metallurgy,Vol. 106, March 2006, PP-205-211.
11. R Mazumder, A K Dubey, N K Sharma, D K Panda, M Imran, Coordination ofthe Operation of Four Quarries to Optimize Limestone Blending Cement
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