A new mining method

The conventional practice in open cut coal strip mining has been to blast the overburden or coal layers in separate blast events. This is done in order to maximise the benefits from throw blasting and also to limit coal losses. Each blast event usually has a particular design of hole diameters, burdens and spacings, explosive charges and blast timing in order to achieve the required blast outcome for either the overburden or coal. This means that each blast has to be separately cleaned on the surface after excavation of the previous blast. Hole locations then have to be individually surveyed and marked, holes drilled, explosives and initiators loaded and the blast fired. Each of these actions takes considerable time and resources and therefore impacts on mine productivity. The mine has to be cleared for each blast event and every blast presents a new potential source for noise, dust or vibration disturbance to the neighbouring population. Exposed highwalls are also subjected to vibration and damage from each subsequent blast event. Orica has recently developed a revolutionary new mining method that combines the separate blast events into a single blast through the innovative use of the i-konTM electronic initiation system. Multiple overburden and coal strata can now be drilled, loaded with explosives and initiators and blasted in a single cycle. Each layer is blasted with a unique design and achieves its targeted blast result, which is different from that of the other layers. Designs in each layer generally differ in explosive type and powder factor, inter-hole and inter-row delays, direction of initiation and initiation time and position, making this method distinct from conventional through-seam blasting. High throw is achieved in the overburden while coal losses and pulverisation are reduced, leading to increased coal seam recoveries. Specialised blast models and blast design software are used in the design process. The method has been proven in full-scale field trials in the Hunter Valley, NSW, Australia. It is being used commercially in production blasting in Australia and may be applicable at virtually all open cut coal strip mines around the world. ORICA BLAST BASED SERVICES WHITE PAPER 03 ORICA MINING SERVICES A new mining method 04 Introduction The mining methods used in open cut coal strip mining in most major coal producing countries have been well established and fairly consistent over the past few decades. These general mining practices have been described by Kukla et al. (1993) and Aspinall et al. (1993) amongst others. Figure 1 shows a schematic of typical coal strip mining operations. In most cases the overburden and coal cannot be excavated without prior blasting operations. Conventional drill and blast practices are used in both the overburden and coal, with each of these being mined in a separate cycle. Each layer is cleared and prepared, surveyed, drilled, loaded with explosives, initiators and stemming and blasted. Australian coal mine productivity and overall production have shown a gradual increase over the years (Barnett, 2000) as these practices have been refined and as economies of scale, especially from the use of increasingly larger mining equipment, have steadily been introduced. Another factor driving mine productivity has been the use of “throw” or “cast” blasting, whereby a substantial proportion of the overburden material is thrown or cast directly into a final spoil position by the blast. This practice effectively utilises the explosive energy to perform two functions, namely fragmentation of the rock for subsequent excavation as well as the direct excavation of a proportion of the rock without the need for mechanical handling afterwards. Commonly around 30% of overburden material can be moved in this way, leading to a correspondingly reduced amount of waste to be moved by the mine fleet. Overviews of throw blasting practices and the effects of blast variables have been discussed by Sengstock and Kennedy (1995), Singh, Singh and Katkar (1996) and Workman (1998) amongst others. More recently, increases in throw blasting performance using the i-kon electronic initiation system have been described by Brent (2002). Figure 2 (on next page) shows the general sequence of events in throw blasting of overburden and removal of the underlying coal seam. Throw blasting generally employs high explosive powder factors and blast timing that favours forward displacement of the muckpile. The overburden is drilled with large diameter holes, typically 229 311 mm (9 12 1/4 inch), on a correspondingly large pattern. High-energy bulk explosives are used to maximise muckpile throw and fragmentation in the critical areas for excavation; usually the toe and key cut regions. Care is taken to provide an adequate standoff distance to the top of the underlying coal seams so as to minimise pulverisation, dilution and movement of the coal into the void. Restricting coal loss in throw blasting has been an area of active research for many years (see for example Kanchibotla, Cocker and Scott, 1999) ����������������� ��������������� ������������� ������������������� ��������������� ���������� �������������������������������� ������������ �������������� Figure 1. Schematic of typical coal strip mining operations. ORICA BLAST BASED SERVICES WHITE PAPER 05 ORICA MINING SERVICES