Blends Of Ammonium Nitrate and Emulsion Explosives are Gaining Widespread Popularity in Today’s Blasting Industry.
By Anthony Konya and Dr. Calvin J. Konya
Blends of ammonium nitrate and emulsion explosives are gaining widespread popularity in today’s blasting industry because of the water resistance, energy, ease of loading and economical price; however, these explosives do have major set-backs including increased fumes and the large increase in critical diameter of the product.
|Figure 1 - Emulsion, 70/30 Blend, 50/50 Blend, 30/70 Heavy ANFO, ANFO (from left to right).|
These set-backs can be accommodated with a proper understanding of the explosive and allow the mine to maintain control over there drilling and blasting program. It should be noted, that from strictly an economic standpoint blends, where no water is present, will cost more per ton of ore produced than an ANFO and should not be substituted unless a detailed economic study has been completed considering both the direct and indirect cost of blends. This article will discuss both the blends (greater percentage of emulsion) and Heavy ANFO (greater percentage of ANFO).
Heavy Emulsion Blends
A blend is a mixture of emulsion and ammonium nitrate that is cheaper than emulsion yet is still water resistant. A typical blend will have over 50 percent of its total mixture be emulsion by weight with the remainder composing of ammonium nitrate or ANFO.
These blends can be pumped into blastholes forcing the water upward out of the blastholes. The water resistance increases with the increasing emulsion content, however the explosive power per pound will generally decrease as the emulsion content is increased.
This is because emulsion contains water that does not contribute energy to the detonation, whereas the ammonium nitrate is directly contributing energy to the detonation. Blends that have a high percentage of emulsion are generally very difficult to distinguish from a normal emulsion (Figures 2 & 3 on page 60).
These blends are also notorious for producing high levels of fumes, specifically visible nitrous oxides (NOx). Currently, from studies conducted by NIOSH it is known that any form of blend or heavy ANFO will produce greater amounts of fumes than straight emulsion or straight ANFO, it is believed this may stem from detonation parameters. However, a major problem with emulsion blends is that typically ammonium nitrate is used, not ANFO. Without the use of ANFO, one of two situations must occur:
1. The emulsion must be over-oiled, this can lead to problems with the emulsion and creation of both carbon monoxide (CO) and nitrous oxides (NOx).
2. The emulsion is not over-oiled, leading to improper oxygen balance and creation of large amount of nitrous oxides (NOx).
|Figure 2 – Straight Emulsion.|
|Figure 3 – 70 percent emulsion and 30 percent ammonium nitrate blend.|
In addition, either of these situations can lead to a decrease in explosive energy and additional variability in the blasting. Worse yet, many times hard-fertilizer grade AN prills are used to reduce cost of production further increasing the problems with this product.
In one case the authors investigated where a 70/30 blend was being used with hard prills, the entire pit would be smoked out for hours after each blast from large amounts of visible NOx. In order to improve this situation, ensure that if a blend is being used for water resistance, ANFO is fueled properly and being mixed with the emulsion.
Furthermore, in some places in the world these blends are being sold mixing hard fertilizer prill AN with emulsion, yet selling the hard prill AN for the price of an ANFO. This dramatically increases the cost of the product, for example let’s assume that ANFO sells for $0.30 per pound and hard-prill sells for $0.18 per pound. In a 70/30 blend, a mine would be overcharged $0.036 per lb. At one large aggregate operation this overcharge equates to more than $150,000 per year and in many large metals mines can result in over $750,000 per year overcharge.
The 50/50 category of blends is those that contain approximately even amount, by weight, of emulsion and ammonium nitrate. These blends are generally water resistance with a proper emulsion but can break down if water is flowing through the blasthole.
This blend is generally augured into the borehole, requiring the need for dewatering, pumping, and has increased explosive energy cost. These blends become much easier to recognize, as the prills can be seen but the mixture is still retaining the look of the emulsion.
The 50/50 blend, when improperly made can create large problems for the mine especially with the increase of blasting fumes (CO, NOx). It is critical for this blend that both the emulsion and ammonium nitrate be sensitized. This means that the emulsion should either be gassed or contain microspheres and a proper ANFO with 6 percent oil should be used for maximum energy release.
|Figure 4 – 50 percent emulsion and 50 percent ANFO blend.|
|Figure 5 – 30 percent emulsion 70 percent ANFO Heavy ANFO.|
A Heavy ANFO is similar to a blend, yet it contains more ANFO than emulsion; the case that will be presented is a 30 percent emulsion and 70 percent ANFO. A Heavy ANFO will have more energy per pound than a blend or normal ANFO. It should always be made from a properly oiled ANFO mixed with an emulsion and is augured into the borehole. A Heavy ANFO is distinguishable from a 50/50 blend because it begins to look like prills with an emulsion coating.
A Heavy ANFO should also have the emulsion sensitized to decrease the amount of fumes generated from the blasting. While chemical gassing can be used, microspheres will give a more reliable and consistent detonation. In some parts of the world, companies are charged for a sensitized emulsion and given an un-sensitized emulsion in a Heavy ANFO.
For example, let’s assume that an un-sensitized emulsion sells for $0.40 per lb. and a sensitized emulsion sells for $0.65 per lb. In a 30/70 Heavy ANFO this would be an overcharge of $0.075 per lb. This can equate to more than $300,000 at larger aggregate operations and more than $1,500,000 at metal mines.
When ammonium nitrate prills are added to emulsions, the prills should not contain large quantities of crushed prill or fine prill dust. Water resistance decreases as prill size decreases. Large quantities of AN dust can also change the characteristics of the explosive and also result in rapid aging and breakdown of the emulsion.
Blends and Heavy ANFOs are fantastic options for many mining operations looking to optimize their blasting or reduce their blasting costs. However, many of the costs associated with blends are not accurately identified and go hidden in feasibility studies including the large increase of blasting fumes and critical diameter of the product.
With stringent EPA regulations, many mines can face problems with large increases to blasting fumes produced exceeding permit models. In addition, “visible NOx” can cause concerns of neighbors and damage to the reputation of the mine in the local community. Finally, this increase in critical diameter of the product can result in an increasing number of misfires.
These blends and heavy ANFO can also have confusing terminology that can lead to a mine operator being overcharged on product, costing hundreds of thousands per year. In order to avoid this and other problems a general knowledge of blends, ANFO and emulsion is necessary. It is best practice to have an experienced explosive engineer help in the development of blend usage at a mine.