DRILL BIT MONITORING AND REPLACEMENT OPTIMIZATION IN OPEN-PIT MINES

Madencilik - Mining, 2021, 60(2), 83-87 www.mining.org.tr Original Research / Orijinal Araştırma Drill bit monitoring and replacement optimization in open-pit mines Açık ocak madenleri̇nde delme operasyonunda kullanılan deli̇ci̇ uçların opti̇mum deği̇şti̇rme zamanının tayi̇ni̇ Ömer Faruk UĞURLUa,* a Istanbul University - Cerrahpaşa, Faculty of Engineering, Mining Engineering Department, Istanbul, TURKEY Geliş - Received: 25 Aralık - December 2020 ▪ Kabul - Accepted: 18 Mart - March 2021 A B S T R A C T Since 2012, low commodity prices have forced many mining companies to suspend or cease operations. To remain in business, some mine managers are exploring strategies to reduce operational costs. Given its importance as a cost element, increasing bench drilling efficiency and performance in open-pit mines has the potential to generate considerable savings. Efficiency and performance gains can be realized by monitoring the drilling operation, analyzing monitoring data with statistical tools and optimizing operational variables. Finding the best configuration of controllable drilling parameters would help to increase penetration rate and optimize drill bit replacement time so that fewer drill bits are consumed. In this paper, the optimal replacement time of a tricone drill bit is formulated as a cost minimization problem and solved by a genetic algorithm (GA). To demonstrate the proposed approach, the effects of controllable variables on drilling performance are experimentally quantified by statistical methods and used for optimization. Results show that the proposed approach can be used to determine the optimal replacement time for drill bits in open-pit mines. Keywords: Open-pit mining, Drilling operation, Design of experiment, Genetic algorithm, Optimization. Introduction Rotary drilling is the most extensively used technique for drilling operations, ranging from surface blast hole mining to deep drilling. The rotary drilling technique is based on two distinct motions-axial thrust and rotational torque-provided by a hydraulic or electric rotary head. Axial thrust is needed to push the bit into the rock to break one unit volume of rock. Rotational torque is a force acting on a drill rig to rotate a drill bit through the rock formation. The tricone bits use the thrust and torque to spall the rock (Ghosh et al. 2016). Sufficient weight on the drill bit is necessary to accomplish the drilling operation. Weight on the bit includes the dead weight of the drilling rig (i.e., the rotary head, drill rods and cables) and the pulldown force. A feed system that generates adequate pulldown force is used to move the rotary head up and down (Atlas Copco, 2012). Drill holes must also be cleaned during drilling by removing cuttings between the wall of the hole and drill rod with compressed air (Ghosh et al., 2016). The air is also used for cooling to protect the bearings. Insufficient air pressure is among the primary reasons for drill bit wear and shorter bearing life. On the other hand, excessive air causes dust and noise problems, shortens bit life and increases operational costs (Fiscor, 2011). Therefore, the operational parameters of a drilling machine such as rotation speed, pulldown force and bailing air pressure have a profound effect on rock fragmentation success. Rate of Penetration (ROP) is assumed as an effective way to measure drilling operation because it directly shows the capacity of the production (Kricak et al., 2015). Even though ROP is directly affected by the properties of rock formation, it is difficult to model the precise association between them in relation to non-linearity, complexity and deviousness (Taheri et al., 2016). Furthermore, operational parameters are adjusted for rock characteristics. The investigators showed, depending upon the hardness of the rock, that increasing weight on the bit helps to increase ROP. For the soft type of formations, bit weight can cause a slight rise in ROP because the teeth of the bit will bury into the formation and increased torque can hardly change ROP. Moreover, rotation speed must be chosen carefully to achieve the desired ROP. High rotation speed increases ROP when the bit is new. However, the bit is worn the effect of rotation speed is decreasing dramatically. For hard rock formations, weight on the bit is crucial to increase ROP until a certain point because it reduces the life of the bit which affects the drilling rate (Irawan et al., 2012). *Corresponding author/Sorumlu yazar: • https://orcid.org/0000-0002-5817-3268 https://doi.org/10.30797/madencilik.847142 83 Ö.F. Uğurlu / Scientific Mining Journal, 2021, 60(2), 83-87 It is important to note that before having a high rotation speed in order to achieve the desired ROP, the drill bit should be allowed to move into the rock with a slow rotation speed. As a result, it is not only difficult but also infeasible to develop a model that take into account all parameters which have a direct impact on ROP (Taheri et al., 2016). The complexity of drilling operation increases particularly by geological condition (Hatherly et al., 2015). Hence, the drilling environment is generally assumed to be homogenous. Cutting tools are considered the most expensive tools during a drilling operation (Plinninger et al., 2002), accounting for an estimated 21% of total drilling costs (Tail et al., 2010). The main reason for tool consumption is bit deterioration associated with the interaction between the rock and the bit. As the worn bit penetrates into the ground, ROP decreases. On the other hand, if the bit is changed before its beneficial life, the cost of drilling increases unnecessarily (Tail et al., 2010). As a result, a trade-off can be seen between drill bit wear and drilling cost (Ugurlu and Kumral, 2020a). The mechanism of drill bit wear depends on rock characteristics and equipment reliability (Ugurlu and Kumral, 2020b). Moreover, operational parameters also have a huge impact on drill bit wear. Immoderate pull down force can cause over stress on drill bits and it might even break the teeth of the bits. Besides, both an immoderate rotation speed and a lack of bailing pressure are two of the main reasons for bit wear. Optimization of operational parameters minimizes operational costs while maximizing the sustainability of drill bits (Eren and Ozbayoglu, 2010). Drill bit manufacturers and testing laboratories can provide predicted drill bit replacement times. However, the manufacturer’s recommendations are general and do not consider mine- and equipment-specific characteristics (Motahhari et al., 2009). During field operations, drill bits are changed when the drilling operator detects comparatively high vibration (Ghosh et al., 2016). An alternative to both approaches is to use drill bit monitoring and optimize drilling parameters to calculate drill bit replacement time. Statistical methods can be used to find optimum parameters such that longer drill bit life and (...truncated)