Pub. 7 2017 Issue 1

21 MINING FOCUS rock into a previously mined out pit. The blasted rock is moved off the ore body with explosives and does not require the use of heavy machinery. The cast, or thrown material, is what blasters call the “cast to final” (CTF) benefit. The Vernal Mine operated for several years mining multiple benches with pit widths averaging 330 feet wide with an overall cast to final that was on the order of ~7 percent of the total overburden moved annually. It was however, easily recognized that a benefit on the order of ~15 percent existed for a single blast. The first change made was changing pit widths from 330 feet to 240 feet wide. With the narrower pits the CTF of a single blast went from 15 percent to 20 percent. The 5 percent average cast increase was a success for the low cover bench but was overwhelmed by the total overburden moved annually. When considering both the 70foot high cover and the 40foot low cover benches combined, the cast to final realized was only a 3 percent overall improvement. The next major change was blasting of a taller bench. The Engineering department was able to create an opportunity to test blasting both the high cover and the low cover together. The constraints of the test bench were a 100 foot bench height and a 240 foot pit width. The drill pattern for the test bench was a 22 feet x 22 feet square pattern with 8 3/4 inch drill hole. The holes were drilled vertically and loaded with a 1.25 lbs/BCY explosive powder factor using ANFO as the blasting agent. The shot was a success and a 28 percent CTF benefit was realized. It then became obvious that if both benches were blasted together throughout the entire year, an increase in the CTF from 7 percent to at least 28 percent or higher could be achieved. The 100 foot bench height test created a great learning opportunity for the mining operation. It was discovered that the current overburden drill was only capable of drilling 100 foot holes, and this higher bench produced a jagged overhanging rock that caused the highwall to be unsafe to work beneath. In order to make the highwall safe additional scaling and chain dragging along the highwall edge was performed. Subsequent cast shots were not attempted due to excessively large toe burdens on the order of ~55 feet that were created from scaling activities required to make a safe highwall. It was obvious from testing that cast blasting a single bench would be the most cost effective method to uncover the ore body. The next challenge was going to be justifying a large overburden drill with the ability to drill angled holes in order to blast a safe high wall and minimize the toe burdens. SIMPLOT PHOSPHATES continued on page 22