The Boliden Mineral Aitik mine has worked to increase the through-put in the primary AG mills together with Swebrec. A raised specific charge from 0.9 to 1.3 kg/m3 raised the throughput by nearly 7%. This was implemented in 2009. For two rounds Swebrec has mapped joints from 3D images to determine the rock mass factor and combined Split Desktop measurements with sieving of muck-pile samples to get the sieving curve of the muck pile, based on the Swebrec function. For a 3rd round such curves were fed into a crusher model and results compared with Online data. The crusher product can affect the mills by three mechanisms; a harder blasting that i) produces more fines that may pass the mill intact, ii) gives a finer crusher product which takes a shorter time to grind iii) increases the amount of larger stones that pass the crusher intact but have an increased grindability. All three must contribute to the increased mill through-put.

A series of detailed small-scale tests have been made to investigate the use of short delays to promote better fragmentation caused by shock wave interactions. The block design had a size of 650/660 × 205 × 300 mm (L × W × H) and two rows with five Ø 10-mm blastholes in each row. The spacing (S) and burden (B) were 110 and 70 mm, respectively, giving an S/B ratio of 1.6. The results showed no distinct differences or high improvements of the fragmentation when the delays were in the time range of interactions compared with no shock wave interactions. The decrease of x 50 (mean size) was around 20 % at a delay time ~1.1 ms/m burden compared with longer delays like 2 ms/m. A statistical analysis of the results has been made to evaluate the minimum at short delays and it is not significant

Sublevel caving (SLC) is a highly productive mining method with the major disadvantages of irrepressible ore loss and dilution. The confined blasting situation of SLC rings is commonly regarded to have a significant impact upon the material flow characteristics and hence on the overall performance. The initial conditions for the ore flow after blasting are unknown, as both the blasted geometry and the fragmentation itself are normally hidden. However, when opening a new drawpoint and in hang-up situations an inspection of the actual conditions is feasible. For this purpose, a remotely operated 3D photogrammetry system was custom built and used to reconstruct the ring front and/or cavity as a geo-referenced mesh model with colour information. Various blasting situations have been observed and this allowed a deeper study of the i) broken geometry and height, ii) over- and underbreak and their effects on subsequently blasted rings, iii) interaction effects between adjacent holes/rings, iv) drilling accuracy based on identified boreholes and v) mapping of geological structures and their influence on the blast result. The capabilities of the 3D image acquisition system to evaluate the blasting results are demonstrated with an example, in which a series of blasts were surveyed. Monitoring under hang-up situations, has then revealed the actual effects from confined blasting and gravity flow related issues. The 3D photogrammetry system is now increasingly being used to study hang-ups. By revealing and quantifying hitherto inaccessible information, the system has proven to be a valuable tool in increasing the understanding of both SLC breakage and flow.

Blasting in SLC (sublevel caving) takes place in a semi-confined situation. Blasted material swells while the caved material compacts, and also, to a lesser extent, fills parts of the void volume of the production drift. Several analytical and empirical models have been developed in the past. However, understanding of the interaction of semi-confined blasting conditions, SLC blast design and rock mass characteristics on rock breaking performance is rudimentary. Instrumentation of the blasted burden with various sensors and study of the dynamics of burden movement against confinement is therefore important. Such measurements are very scarce and thus different systems have been tested both in laboratory and in the field. Main focus was thereby the development of measuring equipment that could be scaled up to full-scale SLC blasting and installed behind the rings. In the field tests, blastholes were drilled in a pillar, parallel to a cross cut drift in the LKAB Kiruna SLC mine. The burden constraint was achieved either by filling the drift of 7.0 x 5.2 m (width x height) with rock masses or reinforcing the drift wall. The tests were instrumented with different redundant sensors, which were installed in holes drilled from a parallel cross cut. The most promising one was a concept, which combines the initial movement recorded by an accelerometer designed to minimize zero-shift with the final displacement recorded by a fibre photoelectric sensor (‘fibre-optic zebra gauge’). For the filled drift wall blasting test the maximum velocity varied between 16 - 32 m/s with a compaction in the range of 4 - 5 %. A gap was verified to exist between the intact pillar and the blasted material. By comparison, the velocity for the unconfined situation yielded a considerably increased velocity (42 - 47 m/s). In addition, ‘Blo-Up’, a component within the Hybrid Stress Blast Model (HSBM), was used to model the blasting results. The ob-jective was to show that reasonable predictions of fragmentation and burden movement under confined conditions can be made. An unconfined reference experiment was used to calibrate Blo-Up and forward predictions of the confined case were made. In both cases, Blo-Up accurately reproduced the burden velocity and displacement observed in the experiments. The suggested measurement concept could be used for further measurements of confined burden movement in filled drift wall blasting tests and under controlled situations. As mentioned the final objective is instrumentation of the burden in SLC production rings.

Small scale-tests have been performed on magnetic mortar blocks to investigate the influence of delay times, specific charge and firing pattern on fragmentation both when the burden is free and when confined by debris simulating the confined conditions of a SLC-blast ring and to study the burden behavior during blasting by implementing dynamic methods of measurements. The dimensions of the test blocks were 660 ×270 ×210 mm (L ×W ×H). Two different blast hole patterns, 2 rows with 5 holes with a burden of 70 mm (S/B ×1.6) and 3 rows with 7 holes with a burden of 58.3 mm (S/B ×1.4). A complete analysis regarding the influence of different delay times ranging from 0 to 4.1 ms/m of burden, different specific charges and different firing patterns on fragmentation has been made, based upon 22 blocks.