Mucking efficiency in open pit blasting

CIM Bulletin, Vol. 2, No. 6, 2007

P. Segarra, J.A. Sanchidrián, J.J. Montoro, and L.M. López

Mining operations are under constant pressure when it comes to optimization and cost reduction. Drilling and blasting, though by themselves comprise a minor percentage (about 15%) of mining costs, have a strong influence on the downstream operation costs, as they are responsible for the fragmentation of the blasted rock. Strong emphasis has been put on the fragmentation of the muckpile (e.g., the mine-tomill approach), despite the difficulties for measuring it, in order to establish its relation with downstream processes. One such process is the digging and loading of the muckpile. An alternative analysis is followed in the present work, focusing on directly monitoring the excavator’s efficiency as a quality parameter of the blasting operation. Such an indicator, though influenced by external factorskills, is a measurement of the rock movement and fragmentation achieved by the blast; together with hauling they encompass about 60% of mining costs (ore processing costs excluded). A literature survey shows this topic as challenging due to the lack of models to guide drilling and blasting towards a specific mucking productivity. This is contrary to what happens with fragmentation, where models exist that relate drilling and blasting parameters with the fragment size distribution characteristics. Powder factor and delay between rows appear to be the key parameters, although there are some contradictory indications regarding the influence of the powder factor. The excavators’ efficiency, given as the mucking rate and bucket payload, is investigated in this paper in terms of the influence of the blasting parameters on it. Quantitative data are given for 11 blasts in an open pit iron ore mine. The predominant rocks in the blocks, itabirite and hematite, are described by point-load strength and density measurements from about 40 samples collected in the bench levels where the blasts took place. Blasting parameters, drilling, charging, and timing were carefully monitored in all blasts. ANFO was used in three blasts, ANFO/emulsion blend in one blast, and watergel in seven, with a range of powder factor between 0.98 and 1.45 kg/m3. The explosive performance is assessed from in situ VOD measurements. The explosive energy has been rated as heat of explosion and useful work to expansion pressures of 100 and 20 MPa, considering both full and partial reaction. The non-ideal energy delivery has been obtained from the VOD measurements. Rope shovels and front-end loaders were used to dig and load the blasted rock into trucks. Their work is assessed by in situ measurements of the time elapsed from the moment in which the excavator dumps the first bucket until the 240 t truck is filled, and of the number of buckets required. From those, the mucking rate (bank cubic metres mucked per hour) and the mean bucket load (bank cubic metres loaded in each bucket operation) are determined. The data measured correspond to samples of 6% to 22% of the total muckpile mass. The mean bucket load is independent of the blasting parameters, whereas it is sensitive to variations in the dipper size. Besides the relation with the mean bucket load, the mucking rate has been found to be positively correlated with the explosive energy; other blasting parameters also correlate with the mucking rate, though their influence can be explained in terms of their usual variation in blast design practices driven by explosive energy variations. No significant correlation of the mucking rate with the rock properties has been found. The analysis of the data, combined with other published data, suggest a non-monotonic relation of the mucking rate with the powder factor, or amount of explosive per unit volume of rock. An acceptable explanation of the experimental mucking rates can be obtained with quadratic or with bell-shaped functions, if powder factor is given in the form of explosive energy loaded above grade per unit rock volume (energy powder factor) rather than in the classical form of explosive mass per unit volume. Of the differentexplosive energy ratings, the useful work in a partial detonation regime appears to be the best suited to group the mucking rate data, especially when the cut-off pressure is 100 MPa. Mucking rates increase with the energy powder factor above grade towards a maximum, beyond which an additional increase of the powder factor does not result in improved mucking rates.