DESIGN OF OPEN PIT MINE FOR AJABANOKO IRON ORE DEPOSIT, KOGI STATE, NIGERIA

Abstract

The design and planning of an open pit mine is about the economics of the mine and is governed by geological and engineering characteristics. This study carried out the design of an open pit mine for the Ajabanoko iron ore deposit. Rock samples of iron ore, granite gneiss and biotite- gneiss were collected from the iron ore deposit and tested in the laboratory to determine ore parameters, namely uniaxial compressive strength (UCS), tensile strength, unit weight, friction angle, cohesion, bulk density, and specific gravity. The input data for the bench design included drill hole diameter, bench height, burden and spacing. Compass clinometer and Geographic Positioning System (GPS) were used to determine the dip, dip direction and location during mapping and this served as input data in the slope design of Ajabanoko iron ore deposit. The design of the slope was carried out using stereographic projection technique contained in the DIPS software. Slope stability software, SLOPE/W was used to determine the factor of safety of the designed slope using limit equilibrium technique. Mine design software (SURPAC) was used to determine the ore reserve of the deposit while CSMine software package was used to determine the final pit limit. Furthermore, mine equipment selection was carried out for the deposit using EQUIPSELECTOR program. Various models of five major mine equipment (dump trucks, wheel loaders, crawler excavators, bulldozers and blast hole drilling rigs) were considered for selection. The attributes considered for the equipment selection where appropriate are cost/unit, operating weight, payload capacity, bucket capacity, maximum digging depth and power. In addition, an economic evaluation of the iron ore deposit was carried out using the discounted cash flow micro-economic assessment. The results of the laboratory test carried out shows that iron ore had a uniaxial compressive strength (UCS) of 142.90 MPa, while granite iv gneiss and biotite gneiss had UCS of 115.22 MPa and 73.63 MPa respectively. The tensile strength of the iron ore is 6.23 MPa, granite gneiss is 10.32 MPa while that of biotite gneiss is 5.28 MPa. The slope design determined shows a factor of safety that varies from a lower limit of 2.89 to an upper limit of 3.84, these values are higher than 1, considered as the benchmark for stable slopes. A slope angle of 440 is considered acceptable for the eastern face of the pit. The results obtained for the design of Ajabanoko iron ore deposit indicate that drilling, blasting, loading and haulage costs at diameter of 102 mm and bench height of 15 m is the most desirable at reduced cost/tonne for the mines with values of ₦11.78, ₦19.65, ₦16.55 and ₦112.03 respectively. The drilling, blasting, loading and haulage costs/tonne continue to decrease as the drillhole diameter increases. The powder factor decreases as the drill hole diameter increases; also the powder factor increases as the bench height increases. The ore reserve estimate for the Central ore region is 36,319,193.6 tonnes. The design of Ajabanoko iron ore deposit indicates an annual run- of- mine (ROM) production of 5.021 million tonnes within a lifespan of 7.23 years. The mine equipment selection order for dump truck, wheel loader, excavator, bulldozer and drilling rigs at the proposed Ajabanoko iron ore mine is EUCLID R90, CAT IT 62H, TEREX TC 225 LC, HITACHI 2505 and TEREX SKT-12 respectively. The cut-off grade for the iron ore deposit is 27% and the final pit limit is 113.25 m. The NPV, IRR and the payback period of financial analysis at 100% capacity utilization with values of ₦38,379.18 million, 457.39% and 1.12 years respectively shows that the iron ore project has good economic potential.