Influence of charge arrangement geometry and short-delay blasting on the crushing indices in compression blasting

Abstract

Iron ore specimens, 120  i00  200 mm in size, were broken by blasting into polydis- : perse crushed ore with a distention coefficient of K, = 1.4. The boreholes were arranged in rows. There were three holes in each row with a line of leas= resistance of 28 mm and a proximity factor of m = 1.2. The weigh= of charge in each borehole was 1.3 g of PETN. The charges in each row were detonated together, but with the delay of 5-6 m between the rows. The delay was obtained by means of a device based on closed contacts by a falling weigh=. In allwe detonated 19 blocks of rows with 1-6 in each. The displacement of the MCM, the volume of the zone of compaction thus formed, the displacement of the rock broken down by the first row, and the vol,--p of the zone of distention for each blast were dete~mlned from the movements of metal reference marks fixed in the MCM, as well as from gamma radiographs obtained after the blast. The values of K, in the zones of compaction and distention were determined by means of calibration g=--,- radiographs. Our experiments revealed that after displacement, the MCM in the zone of compaction reaches limiting packing whenever KI ~ 1.3. To obtain limiting packing of the compressing material in the zone of compaction, the number of charges must not exceed four (when K~ = 1.4 for the compressible material before the blast). The maximum displacements in the MCM are due to the action of the firs= row of charges; we therefore performed exPeriments to find the optimum relative line of leas= resistance (w/d) and charge proximity factor (m) in the first row. The experiments were performed with constant specific charges on models 300  275  245 mm in size made of equivalent material (2:1 sand--cement mixture), which were blasted in a box 750  280  250 --,in ~Ize made of i0 mm =hick sheet iron. The explosives in one row comprised 3.9 g of PETN. In all we performed 65 experimental blasts. Table 1 lists the averaged blasting parameters modeling the linear dimensions of the prototype on a scale of i in 50; for this we see that, despite the variation of =he llne of least resistance from 6.5 to 4.5 cm and =he distance between blast holes in one row from 5.5 to 8.0 cm, the area of the cells were constant, averaging 36 cm 2. This permitted us to vary the proximity factor of the charges between 0.85 and 1.78 and to reduce the line of least resistance from 6.5 to 4.5 cm while preserving constant yield of broken rock per blast hole and constant specific expenditure of explosive energy. In all the experiments the compressible medium was crushed sand-cement concrete with a fragment size of 1-25 ram. The coefficients of distention of the compressible medium before the blast were taken to be 1.3, 1.4, and 1.6, as found by the volu~tric method for each blast. As the criterion of the degree of crushingwe used the grain-slze composition of the crushed rock, which was compared in terms of the mean fragment diameter day, the degree of crushing i, and the yield of fragments over 40 mm in size, 8. The depth of the zone of compaction was determined by means of the movements of metal Plates fixed in the compressible material. At the same time the apparatus in Fig. la was used to record the times of motion of the rock at its contact with the compressible medium during the detonation of the firs= row of charges. The apparatus consisted of an explosion box i which contained the blasting model 2 with the firs=