Anatomy of an Open-Pit Mine Blast

Posted by Stop Ajax Mine on February 23rd, 2015 4:14am

Blasting Video

For a fascinating look at open pit mine blasts, Google search “open pit mine blasting video”.  Run time is 4:31 minutes.  To help you understand what is happening in these blasts, Tony Brumell has put together this commentary:

Things to Look For and Understand in Open Pit Mine Blasting

There are many commonalities that exist in most, if not all, big open pit blasts

While the hole sizes and depths may vary, the over-all pattern of holes is pretty consistent. You will easily be able to see this in many of the shots portrayed.

1. These blasts will be overwhelming in scope and aftermath.

2. The holes are on 30 to 60 foot grid patterns. These will vary with the kind  and characteristics of the rock to be shot.

3. Most patterns that will form part of the final pit wall will have what is called a pre-shear line. Look for the sequential release of a single line of holes around the extremity of the larger pattern. These holes are 3 inch diameter and are drilled at intervals of 3 to 8 ft. and only every second hole is loaded with some form of "stick" powder. Most common would be 75% Forcite (Dynamite) not ammonium nitrate and fuel oil (ANFO). When shot, the web between the holes will shear ,forming an even back wall. You may see the running detonation of the "E" cord (fuse) as a line of smoke racing across the ground at roughly 5800 ft. per second. This new back wall is extremely important in that it will reflect the shock wave from the main blast back into the pattern and vastly increase the breakage in the rock.

4. Next will come the main blast. The blasting agent used will be an ANFO type of explosive – ammonium nitrate and fuel oil (diesel) mixture which may have a percentage of aluminum powder added. This will increase the efficacy of the ANFO by up to 30%. This blasting agent, which may come as a wet slurry or a semi dry prill (pellet) form, is an extremely stable blasting agent and can only be detonated with another high speed detonator such as Dynamite. Down hole detonation is accomplished by two or three "pop can" size detonators tied into the down line (E cord )at 15 to 20 foot intervals  or the propagation of the explosive may not be complete.(i.e. only partial detonation may be achieved)

5.  You will notice the grid pattern of holes in the main shot .This grid runs at roughly 25 feet spacing. The hole diameters will range from 6 inches to 16 inches and be 45 feet deep. This pattern and hole dimensions will vary depending on particle size and range required so the blasted rock can be handled efficiently by the big electric shovels and trucks and of course the crusher it will be fed into. If the rock is left too large, secondary blasting may be required or further reduction by massive air hammers may be needed and this is very expensive.

6. The separate lines of holes will detonate progressively across the pattern. This is achieved by the use of time delays which actually slow propagation of the E cord for short time intervals in the order of one to twenty milliseconds. (These delays have been in use for more than fifty years. KGHM will tell you that everything has changed in blasting. Don't believe it .Some of the bells and whistles are new but their function is the same as it was when I worked on blast crew at Texas Gulf Sulphur Kidd Creek Mine in1969.) Each delay is tied into the E cord separately so as to achieve a proper progression of the blast. These delays may decrease the intensity of the blast from that of a "whole" shot to a "rolling thunder" which, from a distance, sounds and feels like driving over a cattle guard at high speed.

7. The larger holes may hold up to a tonne of explosive and may be lined with a long plastic bag if there is a large amount of water in the hole.(This is visible in some of the video.) After the down line with its detonators is installed, the hole is filled with blasting agent up to within ten feet of the top. The rest of the hole is filled or "stemmed" with angular gravel. When the explosion occurs this stemming locks up tight and redirects the blast wave down to increase its efficacy. The pattern of E cord on the surface must be protected or it might be broken and might cause the blast to be arrested at .that point.(You can imagine the difficulties this might create for the blast crew.) The noise and dust hasn't changed much in fifty years and the "shock" wave is the same and can be deadly at a kilometer or more. (Ask me about the raven at Texas Gulf Sulphur Kidd Creek Mine in Timmins.)

8.  Sometimes after a shot you will see yellow or orange smoke at the blast site. This smoke is mainly oxides of nitrogen with a medley of other toxins created in the blast. If you are close to this after the blast "run away!" It can be deadly and you, or say a city of 90,000, should not be exposed to it every day for 23 years. This dust,  laden with all its toxins, can rise to more than a kilometer and will move down wind for many kilometers. (Imagine a light plane flying through it.)

9. These shots can exceed one million tonnes. These really big ones are truly awesome and it should be noted that fugitive fly rock can easily hit an unsuspecting target a kilometer away. At the AJAX site the average daily blast will not be less than a quarter of a million tonnes and will occur every day for over nine thousand days. Some shots will have 300 holes or more with a tonne of explosive in each. Many minerals and chemicals are released in particulate form and a lot of these are toxic, even carcinogenic. After every shot, these gases and dust must clear the pit before the crews can go back to work. Maybe we should evacuate Kamloops also. If there is no wind blowing to bring the dust to town it will simply sit in the pit for hours. Well! We can't afford that, so everyone has to go back to work and we'll deal with the consequences later.(Sometimes many years later.) During an inversion event this particulate may hang over Kamloops for days, slowly descending for you to breathe and enjoy.

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