Blast Timing

The main principle of blast sequencing is to allow adequate dynamic relief of individual blast holes as the pattern is detonated, but at the same time maintaining confinement and controlling air blast and vibration. To control these aspects of the blast, a delay is built into the firing sequence between adjacent holes in the same row and also between adjacent rows. A rule of thumb for the timing of hole-to-hole delays is given by Equation (4.6) (Konya and Walter, 1990)  

The constant TH (ms/ft) is dependent upon rock type and has a value of 1.2-1.5 ms/ft for compact limestone while for magnesite rock 2.5 ms/ft is used between holes. 

The timing of interrow delays will depend upon the desired shape of the muck pile (blasted rock pile). A rule of thumb for this delay is given by Equation (4.7)  

Since the objective of the blast is to break the rock but to minimize forward movement and mixing of the muck pile, it is important to use an appropriate interrow time delay, which is typically 3-4 ms/ft of burden. 

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Two types of fuse are in common use by the US Army (1) blasting time fuse that has a spiral wrapped outer cover usually colored orange and (2) safety fuse M700 that has a smooth green plastic cover with Length markers of abrasive material (so they can be felt in the dark). These two types of fuse are shown in Fig 2 (Ref 1)... 

Fuse, Blasting, Time Fuse, M700, Loading, Assembly, Packaging Fuse, Firecracker, 30 sec Fuse, Safety Bickford Type Fuse, Time, Plastic-Coated Graphite, Dry Glass, Ground (for ordnance use)... 

Average oxygen blast time [min] 17.47 15.23 Decreased 2.23min... 

Selection of grit size will also depend on several factors the metal to be pretreated, the type of equipment being used, the pressure and angle of blast impact and the blasting time. Grits in the range of 125-315 p-m are suitable, but the optimum size for the work in hand can only really be determined by experiment. 

Opera.tlon, Because of the long residence time of the materials (8—10 h), the blast furnace process can exhibit considerable inertia, and control is usually appHed where the goal is maintaining smooth, stable input conditions. One of the most important aspects of blast furnace control is supply of consistent quaUty raw materials, which is why there is a strong emphasis on quaUty control at coke plants, peUeti2ing plants, and sinter plants (see Quality ASSURANCE/QUALITY control). 

DRI, in peUet/lump or HBI form, can be added to the blast furnace burden to increase furnace productivity and reduce coke requirements. It can be used for short-term increases in blast furnace output when a faciUty is short of hot metal during times of high steel demand, or when one of several blast furnaces is down for a reline. It also can be justified if the increased output is sufficient to allow operation of fewer blast furnaces long-term. 

For gas-fired systems the state-of-the-art is represented by the preheater described in Reference 69. A pebble bed instead of a cored brick matrix is used. The pebbles are made of alumina spheres, 20 mm in diameter. Heat-transfer coefficients 3—4 times greater than for checkerwork matrices are achieved. A prototype device 400 m in volume has been operated for three years at an industrial blast furnace, achieving preheat temperatures of 1670 to 1770 K. 

AppHcation of an adhesion-promoting paint before metal spraying improves the coating. Color-coded paints, which indicate compatibiHty with specific plastics, can be appHed at 20 times the rate of grit blasting, typically at 0.025-mm dry film thickness. The main test and control method is cross-hatch adhesion. Among the most common plastics coated with such paints are polycarbonate, poly(phenylene ether), polystyrene, ABS, poly(vinyl chloride), polyethylene, polyester, and polyetherimide. 

Manufacture and Processing. The largest volume of coal is carbonized in batch coke ovens to produce a hard coke suitable for blast furnaces for the reduction of iron ore. Oven temperatures, as measured in the flues, are between 1250 and 1350° and residence time varies between 17 and 30 h. The gas made in this process is mainly used as fuel and other appHcations in the steel works (see Fuels, synthetic). 

For practical reasons, the blast furnace hearth is divided into two principal zones the bottom and the sidewalls. Each of these zones exhibits unique problems and wear mechanisms. The largest refractory mass is contained within the hearth bottom. The outside diameters of these bottoms can exceed 16 or 17 m and their depth is dependent on whether underhearth cooling is utilized. When cooling is not employed, this refractory depth usually is determined by mathematical models these predict a stabilization isotherm location which defines the limit of dissolution of the carbon by iron. Often, this depth exceeds 3 m of carbon. However, because the stabilization isotherm location is also a function of furnace diameter, often times thermal equiHbrium caimot be achieved without some form of underhearth cooling. 

Worldwide demand for blast furnace coke has decreased over the past decade. Although, as shown in Figure 1, blast furnace hot metal production (pig iron) increased by about 4% from 1980 to 1990, coke production decreased by about 2% over the same time period (3). This discrepancy of increased hot metal and decreased coke production is accounted for by steady improvement in the amounts of coke required to produce pig iron. Increased technical capabihties, although not universally implemented, have allowed for about a 10% decrease in coke rate, ie, coke consumed per pig iron produced, because of better specification of coke quaUty and improvements in blast furnace instmmentation, understanding, and operation methods (4). As more blast furnaces implement injection of coal into blast furnaces, additional reduction in coke rate is expected. In some countries that have aggressively adopted coal injection techniques, coke rates have been lowered by 25% (4). 

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