Evaluation of Bench Height Selection for Limestone Resource Optimization A Case Study. N K Sharma, R Mazumder, D K Panda, A K Dubey

Transcription

1 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 s 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 s 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. 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 s is created with a specified ROM cut-off percentage of CaO and SiO 2, 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 s. This allowed to calibrate the for mining bench height, realistic dilution / segregation, blending solution and production scheduling at specific cut off grade for optimized life of mine. 1

2 1.0 Introduction The production and management of desired ROM grade limestone for maximum life from a large open cast limestone mine is an enormous and complex task. Optimization techniques can be successfully applied to resolve a number of important problems that arise in the planning and management of a mine. These applications include: selection of optimum bench height for ore ing and reserve estimation; the design of optimum ultimate pits; the determination of optimal blends; the determination of optimal production schedules; the determination of optimal operating layouts; and many more. The optimum ultimate pit of a limestone mine is defined as to be that pit which is the result of extracting the volume of material which is obtained by maintaining balance between elements or rock types according to user defined blending criteria whilst satisfying the operational requirement of safe wall slopes. The ultimate pit limit gives the shape of the mine at the end of its life. The production schedule of a mine can be defined as a year-by-year sequence in which blocks should be removed from the mine while targeting quality parameter such as CaO% and SiO 2 % etc. The bench height is the critical function for deciding ore grade composite for the height of the bench applied for mining. This paper deals with evaluating the impact of bench height selection on sustainability of quality and life of mine at feasibility stage to maximize the resource. 2.0 Geological and Mining Setup The Limestone in the study area geologically referred to as an equivalent of Narji Limestone belongs to Jammalamadugu formation of Kurnool Group of rocks of Palnad basin. The Litho units encountered in the subject area and the generalized succession of rock types is as follows. Surface Soil Dark Grey Limestone Light Grey Limestone Purple Siliceous Limestone The deposit has been classified broadly as per lithological units into three zones and are named as ZONE-1, ZONE-2 and ZONE-3 which represents the soil, calcareous clay and limestone (dark and light grey) respectively. Calcareous clay and limestone except soil are further classified into 7 classes based on the range of CaO% and taking SiO 2 % range between <15% and >15% as function of difference within the classes. Where the grades falling CaO >42% and SiO 2 <15% is for cement grade limestone (at CaO 2% interval) and while keeping SiO 2 >15%, the blendable grade limestone is classified for limestone containing >42<48%CaO and low grade limestone containing >40<42% CaO and remaining material / rock units within 2

3 calcareous clay and limestone with CaO <40% as very low grade limestone. The grade categories are coded as 1, 2, 3, 4, 10, 11, 12 and 13. The details are given in Table. 1 for reference. Table. 1: Zone and Grade Classification of various litho units Litho-unit Grade Code Block Cells Soil and Limestone Categories Range of CaO% Range of SiO 2% grades ZONE-1(Soil) Soil (Waste) ZONE-2 10 >42<44 <15.00% (Calcareous Clay) and 11 >44<46 Cement grade Limestone ZONE-3 (Dark & Light Grey Limestone) 12 >46<48 13 >48 1 >42<48 >15.00% Blendable grade lst 2 >40<42 >15.00% Low grade limestone 3 <40 >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. ACTUAL ORE OUTLINE GEOLOGIST S INTERPRETED ORE OUTLINE ORE LOSS INTERNAL DILUTION BENCH HEIGHT EDGE DILUTION 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; 3

4 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= m) Sample Category 9m Bench Height 10m Bench Height 12m Bench Height Composite Sample Length for SiO 2 < Percentage Recovery 90.31% 89.96% 89.27% Composite Sample Length for SiO 2 >15 Percentage Recovery % % % 4.0 Deposit ing To study the effect of dilution / segregation, different ore body s 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 s prepared are given in Table. 3. 4

5 Table. 3: Top bench floor, block size details and number of benches for each ore body Ore Body Models 9m Bench Model 10m Bench Model 12m Bench Model Top Bench Floor X & Y Cell Dimension(m) 100m 100m 100m Z Cell Dimension / Bench Height (m) 9m 10m 12m No. of Benches The 3D geological 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 s. For each ore body bench height system is maintained till the depth of 14 mrl. The 3D geological of a 12m bench 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 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 s. 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 s prepared are being used for optimization of resource by finding the blending solution using all the grades present in the 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) 5

6 CaO% SiO 2 % 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 s 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 mine s 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 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 has the most favorable operating condition in comparison to ultimate pit shell of 9m and 10m bench. The ultimate pit shells of 9m and 10m bench s 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 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 s. 6

7 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 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 s. 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 e xcavation upto desired benches and ROM quality targets) obtained from blending all grades of limestone of 9m,10m and 12m bench. The 12m bench has the highest total blended limestone reserve of M.T which is 8.81and 7.12 M.T. higher than the 9m and 10m bench, 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 as compared to the 9m and 10m bench 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 Optimized Pit Rock (M.T) 9m Model 10m Model 12m Model Ore (M.T) CaO SiO 2 Rock (M.T) Ore (M.T) CaO SiO 2 Rock (M.T) Ore (M.T) CaO SiO 2 Blended Pit Final Pit Total

8 10m bench 38.51, 15% 44.87, 17% 46.9, 18% 9m bench 12m bench , 82% , 83% , 85% Blended Pit Final Pit Fig. 5: Distribution of limestone (ore) in blended pit and final pit in 9m, 10m and 12m bench It can be inferred that from 9m to 12m bench, 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 for both cement grade and low grade limestone than other s 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 (SiO 2 % <15%) and low grade limestone having (SiO 2 % >15% including blendable and low grade limestone) in blended and final pit respectively. 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 8

9 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 The distribution of blendable grade, low grade and cement grade limestone in 9m, 10m and 12m bench in the blended pit is further evaluated for understanding the segregation of various grades of limestone both in the form of reserves and grade (Table. 6). It is observed that M.T of cement grade limestone having CaO%>48 is obtained from 12m bench which is 3.93 M.T and M.T. higher from the 9m and 10m having the limestone reserves of M.T and M.T respectively. Similarly M.T of cement grade limestone having CaO% >42<48 is obtained from 12m bench which is 2.60 M.T. higher from the 9m and 3.97 M.T. less from the 10m. It suggests that there is dilution in terms of tonnage within the cement grade in 10m. Similarly low grade limestone having CaO% <40 and >40<42 and SiO 2 % >15 shows segregation both in the form of reserve and grade in 12m bench. Low grade limestone of 8.02 M.T is obtained from 12m bench having an average grade of 32.4% CaO and 28.03% SiO 2 which is 1.72 M.T and 1.57 M.T. higher from the 9m and 10m. Table. 6: Quality and Quantity of Blendable grade, Low grade and Cement grade Limestone in Blended pit for 9m, 10m and 12m bench Blended Pit Limestone Blendable Grade Limestone Low Grade Limestone Cement Grade Limestone Limestone Model Grades CaO%>42<48; SiO 2% >15 CaO% <40 and >40<42; SiO 2%>15 CaO% >42<48 SiO 2%<15 CaO%>48 SiO 2%<15 Blended Pit (M.T) 9m Model 10m Model 12m Model CaO SiO 2 Blended Pit (M.T) CaO SiO 2 Blended Pit (M.T) CaO SiO T. Cem Gr Lst Total Ore (Blended Pit)

10 Distribution of different grades of limestone in blended pit in 9m, 10m and 12m bench is shown in Fig.8. It is observed that, effective dilution gain in terms of grade & tonnage for low and blendable limestone, vis-à-vis segregation of CaO% >48 grade limestone in terms of tonnage is found in 12m bench height. 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. 10m bench 15.35, 7% 16.07, 7% 14.82, 7% 6.45, 3% 6.30, 3% 8.02, 4% 9m bench 90.66, 41% 80.43, 37% 86.73, 41% , 49% 12m bench , 53% , 48% BLEN. GR. LOW GR. CEM. GR.>42<48 CaO CEM. GR.>48 CaO Fig. 8: Percentage distribution of different grades of limestone in blended pit in 9m, 10m and 12m bench 5.3 Production Scheduling The blended pit limestone of each bench is scheduled while targeting quantity and quality objectives like CaO% and SiO 2 % to find 5 yearly schedules for 9m, 10m and 12m bench. Table.7 shows the 5-yearly excavation at the specified blended ROM quality for 9m, 10m and 12m bench. It has been observed that 12m bench sustains for the total life of 99 years which is 5 and 3 years higher in comparison to 9m and 10m bench having life of 94 and 96 years respectively. The scheduler reports of blended ROM quality for 12m bench s are shown in Fig.6 indicating sustainable life of the deposit. Table. 7: Scheduler result for 9m, 10m and 12m bench s (at specified ROM quality) 9m ore body 10m ore body 12m ore body Year Ore CaO SiO 2 Year Ore CaO SiO 2 Year Ore CaO SiO

11 CaO% & SiO 2 % Conclusion CaO% Year SiO2% Fig. 6: Scheduler report (5-yearly) for 12m bench 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. To study the effect of dilution / segregation of various grades of limestone due to compositing at different bench heights, bench s at variable bench height systems of 9m, 10m and 12m are prepared. The effect of various bench height s 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. From the result of pit optimization of 9m, 10m and 12m bench it is observed that the 12m bench 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 in terms of grade and tonnage leads to more proper blending of different grades of 11

12 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 work of NCB. This paper is being published with permission of the Director General, NCB. 8.0 References 1. A J Simclair, G H Blackwell Applied Mineral Inventory Estimation Combridge University Press, 2002, PP Cement and Concrete Science and Technology Vol-1, Part-II. 3. Dagdelen, K., 2001, Open Pit Optimization Strategies for Improving Economics of Mining Projects Through Mine Planning, 17 International Mining Congress and Exhibition of Turkey. 4. H Kose et.al., Economic Evaluation of Optimum Bench Height in Quarries The Journal of the South African Institute of Mining and Mettallurgy, Feb-2005, PP 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 May 2000, PP 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, open pit 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.A Balkema, Netherland. 9. R Dey, A K Dubey, N K Sharma, D K Panda, M Imran, Multimine Scheduling for Optimization of Blended Limestone from Four Mine Blocks for Cement Manufacture 12 th NCB International Seminar on Cement and Building Materials, Nov. 2011, New Delhi. 10. R M Jara, Block Size Selection and its Impact on Open Pit Design and Mine Planning The Journal of The South African Institute of Mining and Metallurgy, Vol. 106, March 2006, PP R Mazumder, A K Dubey, N K Sharma, D K Panda, M Imran, Coordination of the Operation of Four Quarries to Optimize Limestone Blending Cement International, 1/2013, Vol II PP