Coal Age

SEP 2018

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22 September 2018 blasting continued ever, this theory had a major problem — rock and explosives have similar veloci- ty components, but the density of rock is normally two to five times that of the explosive. The impedance could never be equal and would rarely be above 40%- 50% with minimal difference between ANFO and water gels. When this issue was raised, a very smart salesman coined the term "veloc- ity matching" and stated that the den- sity was causing all the problems in the impedance mismatch and that looking at the velocity alone was a better ap- proach. From a technical basis, this has no value, even if shock breakage was truly the mechanism for rock breakage. But, this now gave the powder salesman a tool to easily sell higher velocity, more expensive water gels, emulsions, and dynamites instead of ANFO. This led to greater profits, but was terrible for the industry as it dramatically increased costs for no reason. Shock Breakage Let's look at a few concrete examples to show how this has no bearing. First, let's discuss shock breakage and some very simple principles associated with it. To begin, let's look at a decoupled charge (such as packaged emulsion) in a borehole. Now the theory of imped- ance mismatch states that mediums with different impedances will have different coefficients of transmission for the shock wave. When the explosive fires, it is in direct contact with air, therefore the shock wave of the explo- sive would have to move into the air. The problem is air has a dramatically lower acoustical impedance than the explosive, so more than 99% of the shockwave is lost already going from the explosive into the air. Let's assume that the less than 1% remainder could break rock, if it reached the free face. After it goes into the air, it then has to go into the rock, again having a reduc- tion in total shock wave pressure by about 99%. This leaves almost no shock- wave pressure starting in the rock. Ad- ditionally, the shock-wave pressure also reduces exponentially as it trav- els through the rock. In the end of this example, less than 0.01% of the total shock-wave pressure would reach the free face, which would put it well under the tensile strength of the rock. What does this mean? If the shockwave theo- ry was correct a decoupled charge could never be used in a borehole. Let's look at another example, in the past, it was stated that when presplitting the collision of shock waves between boreholes is what would cause the pre- split. This was put into the DuPont Blast- ing Handbook, which was then bought by the International Society of Explosive Engineers, and even in the 17th edi- tion of the handbook, still exists today. DuPont knew this was wrong in the 1970s and readily admitted to it, seek- ing to find authors to rewrite the section on presplitting. How do we know this to be false? A good starting point is cap scatter. Throughout history (with the excep- tion of very modern electronic caps), caps typically fired within 10% of their rated firing time, meaning a cap rated to fire at 100 milliseconds (ms) would fire between 90 ms and 110 ms. Now when these caps where used with pre- splitting, the scatter time would cause each borehole to fire at a different mo- ment in time. What if these boreholes fired just one millisecond apart — in this time, the shockwave from the first borehole would have moved about 150 feet, leading to no interaction of shock waves between boreholes. According to the theory, this would cause no presplit to form, yet presplitting has been done for decades and continues to be done with nonelectric caps. In addition to this, the authors have tested numer- ous methods of presplitting, including Precision Presplitting (should not work under this theory do to decoupling) and presplitting with propellants (no shock wave created) and all have worked per- fectly, in many cases better than with high explosives. Finally, let's look at a very real exam- ple of how the impedance theory and velocity matching miserably failed in full-scale. Back in the 1960s, ANFO was becoming a big deal as it was extremely cheap and, by using coal fines and fertil- izer grade prill, mines could make their own blasting agents. ANFO was invent- ed by Bob Akre in Indiana and quickly made its way to the Minnesota Taconite (iron ore) Range. Taconite is one of the densest and highest velocity rocks on Earth. According to the velocity match- ing theory, ANFO would not be able to break it and an expensive slurry or dynamite would have to be used. This was one example given by explo- sive companies — except the taconite miners were laughing at the explosive industry as they shot millions of pounds of ANFO each day and it broke their taconites perfectly. So why did these theories persist? Well it was one of the best sales tools that had been discovered in the explosive in- dustry. However, in the long run, tech- nical information beat down the sales — after costing mines millions of dollars. Today, almost no one discusses velocity matching and those who do are the last die-hard proponents that made their ca- reers using it as a sales tool. Today, the selling of more expensive products is left for other methods. Instead, a new sales tool has emerged that distorts technical data as a way to sell much more expen- sive electronic caps. Electronic Caps Now to preface this, both authors recom- mend and believe electronic caps have many benefits from the reduced cap scatter. In certain vibration situations, they are extremely good tools that can give a site a lot of options with blasting programs. However, the following points are not true about electronic caps: • By reducing cap scatter, fragmenta- tion will improve. This is only par- tially true because while accuracy is important, selecting the proper time delays for hole firing is the ma- jor import factor. • By reducing cap scatter, ground vibration will improve. This again has no technical backing as none of these phenomena are correlated to cap scatter. In fact, in many cases, just substituting electronic caps for nonelectric caps causes an increase in ground vibration. • By reducing cap scatter, the 8-ms rule no longer applies and blasters can now use a 4-ms or 1-ms rule for delaying of charge weights. This has been attempted and has been challenged in numerous court cases

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