NITRATO DE AMONIO DE ALTA DENSIDAD Y EMULSION GASIFICADA: MEZCLA EXPLOSIVA EN LAS MINAS DEL PERU

HDAN and Gasified Emulsion Blends Improving Blasting at Peruvian Mines

Romel Villanueva L., Department of Mining Engineering, National University of Trujillo, Perú

Abstract

Peru does not produce ammonium nitrate; nevertheless it is a large consumer of both, porous prill (PP) and high-density (HD) varieties. Explosives cost is an important preoccupation in Peru, due to fluctuations of the imported ANFO grade Ammonium Nitrate (AN). The use of Gasified Emulsion (GE) -manufactured with much cheaper HDAN- has become an alternative to reduce such costs. However, some problems were reported in the application of pure GE, including explosive percolation in highly fractured rock and lack of sensitivity by the effect of hydrostatic pressure in very deep holes. It was, hence, proposed to have an explosive based on GE too, but more robust and capable to keep the shape of the explosive column into the blast hole, by minimizing the leakage into the cracks. This could be achieved by adding limited amounts of lower priced HDAN, at the same time of providing gas pressure benefits. Other alternative is to add a low-density material, such as polystyrene prills, into the GE, although it reduces the explosive energy.The chemical gassing controls the density of GE, thus it can be obtained a wide range of densities.

To improve the rigidity without affecting sensitivity, a blend of 25% HDAN and 75% GE was prepared. This balance increases the effective energy per unit of mass contained in the blast hole, keeping, at the same time, good sensitivity thanks to the bubbles (hot points) which formation depends on the dosage of sodium nitrite, reacting with the emulsion. The HDAN prill is nonporous, therefore it plays no role in the sensitivity, but contributes to the mix with two important things: higher physical rigidity and higher nitrogen contentto increase the volume of the gases.This blasting agent blend was named GE-HDAN. It has been tested at the largest Peruvian mines, in blast holes ranging from 6 m (19.7 ft) in depth and 0.13 m (5 in) in diameter to 17 m (55.8 ft) in depth and 0.31 m (12.25 in) in diameter. Range of density used was 0.9 to 1.17 g/cm3 (56.2 to 73.0 lb/ft3). Numerous VOD (Velocity of Detonation) tests, fragmentation analysis and non-fumes blasts have demonstrated an excellent detonation performance and VOD in the order of 5700 m/s (18700 ft/s), always greater than Heavy ANFOs. GE-HDAN energy delivery, sensitivity and water resistance encourage to its massive use, in addition to application of low cost HDAN. In consequence, an economic benefit is also reported with savings for the users.

Copyright © 2016 International Society of Explosives Engineers

2016G - HDAN and Gasified Emulsion Blends Improving Blasting at Peruvian Mines 1 of 10

Introduction – Background At three of the largest Peruvian open pit mines, the two of Southern Peru Copper and Yanacocha Gold, engineers have searched for long time, an explosive blend allowable to use in a width range of densities and energy outputs, without a cost variation. In the heavy ANFO blends, any change in percentage of its components, determine changes in density and energy delivery but also in the blend cost, since imported Porous Prill Ammonium Nitrate (PPAN) may have significant price differences between one lot to another, in function of the offer and demand. However, by the side of the emulsion, manufactured from High Density Ammonium Nitrate (HDAN), a more stable pricing level can be expected.

The Gasified Explosive Emulsion (GE), hence, appeared as a simple solution for this task, due to its no necessity of PPAN on its composition, in addition to its width range of densities that can be obtained in the field and its high detonation performance thanks to its homogeneity.

Nevertheless, when the explosive column is compressed by the stemming load, a contamination zone is created at the top of the column and, also, when the rock is highly fractured, it results in a losing of explosive though the fractures and, therefore, a reduction of the column height and the effective powder factor (Villanueva, 2015).

To prevent those inconveniences, trials were made by adding expanded polystyrene pearls to the GE, intended to provide it with higher viscosity and better superficial tension to reduce the stemming penetration. Figure 2-b shows the typical appearance of GE and polystyrene blend 60/40. Results in both aspects were, in fact, acceptable however there was an important energy lost in the explosive –proportional to the pearls content- thus its application should be limited to soft rocks, where energy demands were not too big.

As a consequence of previous field investigations at Colombian Coal mines, usage of High-Density Ammonium Nitrate (HDAN) appeared as a reasonable solution (Wilches, Gonzales, & Proulx, 2015). When adding a quantity of HDAN to the GE –approximately 20% to 30% in mass- the detonation behavior is superior to that of the Heavy ANFO’s and explosive lost by the rock fractures is much less than the pure GE. Based on these preliminary results, an aggressive trials program was organized, in several mining operations at the same time, intended to obtain field results in different working environments and various types of rock. Only variations in the density, by adjusting the gassing additive, were practiced during the trials, trying to determine an ideal explosive density for a particular rock characteristics and a pre-calculated blastability index.

First results were captured at Yanacocha gold mine, in holes of 0.27 m (10 5/8 in) in diameter and 11 m (36.09 ft) in depth and almost immediately after, at Cuajone and Toquepala, giant Southern Peru Copper mines, in holes of 0.31 m (12.25 in) in diameter and 16 m (52.49 ft) in depth. Each one of the three operations represented a case study, because their own particularities; nevertheless, in all of them, technical benefits and global cost savings were appreciated, as illustrated in this document.

Figure 1 shows the above-mentioned mines location. Yanacocha is located at the North of the country, in the Department of Cajamarca, while Cuajone and Toquepala –as the name of the owning Company indicates- are located in the South, in the Departments of Moquegua and Tacna, respectively.



The Explosive Blend The HDAN is added to and mixed with the GE, to provide it with more stemming penetration resistance and less fluidity to prevent its spill or filtration through the rock fractures. After many field trials and laboratory experiments, it has been determined that the blend that offers the best combined performance, including useful energy output, velocity of detonation, no fumes generation and operational flexibility is the one containing 75% of GE and 25% of HDAN. This blend has been named as GE-HDAN 75/25 (Gasified Emulsion & High Density Ammonium Nitrate).

The GE-HDAN 75/25 blend is based on GE, as the primary ingredient. The chemical sensitization, by gassing the blend, is responsible for the explosive adequate initiation and it finally contributes to the fragmentation performance, at the time of giving to it the appropriate density and other detonation features. The HDAN added to the GE accomplish with two important roles: Physically, it provides the explosive column with grater stiffness within the blast hole, so it becomes more resistant to the stemming compression and penetration. Secondly, the no-inert ammonium nitrate mass contributes with additional chemical energy, which is finally released as an extra pressure detonation gas, contributing to the separation of the rock fragments. That makes the detonation processes, definitely, more efficient.

Figure 1. Yanacocha, Cuajone and Toquepala mines location



Gassing the explosive emulsion When a little amount of sodium nitrite enters in contact with the explosive emulsion, a chemical reaction takes place, producing nitrogen gas small bubbles, which remain trapped into the matrix emulsion (Tyler & Taylor, 2008). This mechanism increases the volume and, hence, the density is reduced from its initial estate (without gassing) to its final estate (when the gassing process has finished). Figure 2-b shows the bubbles and Figure 3 shows the gassing curve. The gassing bubbles provide the necessary sensitivity to initiate the detonation. Content of sodium nitrite is around 0.4%, although value may vary from 0.2% to 0.6%, depending on the final product density intended to obtain. Figure 4 shows the control of the open cup density in the field.

Figure 2. Appearance of the gasified explosives blends.a) GE and Polystyrene 60/40. b) GE and HDAN 75/25.

Figure 3. Density of Gasified Emulsion Figure 4. Density control in the field



Energy and Velocity of Detonation Theoretical GE-HDAN 75/25 blend Absolute Energy output, calculated from its thermodynamic reaction equation, is 2.7 MJ/Kg (1141.9 BTU/lb) and its Relative Weight Strength (RWS) –by comparison with the 3.8 MJ/Kg (1613 BTU/lb) of ANFO- is 70.8%. Since the GE-HDAN blend is suitable to have different density values, by mean of the chemical gassing process, it is more reliable and useful to plot the Relative Bulk Strength (RBS), as shown in the Figure 5.

Many Velocity of Detonation (VOD) measurements of the GE-HDAN blends were achieved in the field, for different blast holes diameters and lengths, as well as various sampling cup densities. Following is a graphic that shows a typical VOD, with a maximum VOD value of 5680 m/s (18635 ft/s) for an GE-HDAN 75/25 with cup density of 1.10 g/cm3(68.7 lb/ft3), in a 0.31 m (12.25 in) diameter and 16 m (52.5 ft) depth blast hole, loaded with a 9 m (29.5 ft) confined charge height.

Figure 5. RBS of GE-HDAN 75/25

Figure 6. VOD of the GE-HDAN 75/25



It becomes a common practice to calculate the detonation pressure (PD) of an explosive from the unreacted explosive density and velocity of detonation (Cooper, 1996). It is proposed that the detonation pressure calculated can be used to approximate the shock energy content of the explosive, for comparison of shock energies of non-ideal blasting agents.

The conversion of an explosive chemical potential energy into useful energy or work, in unconfined or lightly confined conditions, depends on the explosive type, the efficiency of the chemical reaction between the oxidizers and fuel components and the oxygen balance. Ideal explosive formulations, such as emulsions, typically convert higher percentage of chemical energy into high-pressure gases than a lower performance explosive such as ANFO. This conversion efficiency is related to the amount of useful work an explosive can perform during rock blasting (Fleetwood, Villaescusa, & Eloranta, 2012).

The following table shows a theoretical comparison (Esen, 2008) between the most used Heavy ANFO HA 40/60 and the proposed GE-HDAN 75/25 blend. It can be seen that the Heavy ANFO Absolute Weight Strength is much higher than GE-HDAN resulting one. However, other properties such as Velocity of Detonation, Pressure of Detonation and Useful Energy are higher for the GE-HDAN.

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Useful Shock Energy

Table 1. Useful Shock Energy - Comparison



Application at Southern and Yanacocha mines Before application of the GE-HDAN blends, common explosive agents used in those mines were standard Heavy ANFO’s (HA) and the Gasified Heavy ANFO (GHA 70/30), with 70% Emulsion and 30% ANFO. Table 2 shows typical drilling parameters, both at Yanacocha and Southern Peru, before introduction of the GE-HDAN blends.

In the case of the Southern Peru Copper mines (Cuajone and Toquepala), research was focused on the technical-economical comparison of two explosives: the conventional GHA 70/30 and the new proposed GE-HDAN 75/25. Figure 7 shows the explosive columns for both explosives.

Table 2. Drill and Blast Parameters

Figure 7. Explosive column - Cuajone copper mine



Historically, the best performance of the GHA 70/30 used to be obtained with a cup-density of 1.19 g/cm3 (74.3 lb/ft3). By other hand, after several test-error trials with the GE-HDAN 75/25, it was possible to reduce density down to 1.13 g/cm3 (70.5 lb/ft3). Then and keeping the good fragmentation it was possible to increase the stemming, reducing, in consequence, the explosive column height. This is translated in a less explosive consumption per hole and / or rock mass (powder factor) and, consequently, a lower blasting cost. Note that GHA is based on ANFO (PPAN and diesel fuel oil), which incur in more costly materials.

In the case of Yanacocha the explosive column scheme is presented in the Figure 8. The standard explosive was a Heavy ANFO (HA 40/60), with a density of 1.20 g/cm3 (74.9 lb/ft3). This was fully replaced by the GE-HDAN 75/25, keeping the same explosive column height but reducing the cup density to 1.10 g/cm3 (68.7 lb/ft3). Fragmentation results were measured to be the same but explosives consumption dropped down.

Results and benefits Fragmentation results at Cuajone mine, shown in the Figure 9, were measured using the “Split On Line” software. A digital camera was placed at the Shovel 4 during the month of April 2015, when the usage of the GE-HDAN 75/25 had already been standardized in all sectors of the mine. The granulometry

Figure 8. Explosive Column – Yanacocha mine



index P80 dropped from 0.3 m (11.7 in) to 0.23 m (9.2 in), which also hardly impacted in the crushing process. Keeping the granulometry index P80 around of 0.27 m (10.5 in) one can reduce the explosives consumption by increasing the Burden/Spacing pattern. It is the real goal of the GE-HDAN blend. Figure 10 shows the remarkable reduction of the powder factor with the GE-HDAN blend, from 0.45 Kg/TM (1.9 lb/yd3) during first quarter of the year to 0.35 Kg/TM (1.5 lb/yd3) in July and August. Where the average of rock density is 2.5 g/cm3 (156.1 lb/ft3).

In addition, the HDAN acquisition cost in Peru is less than the PPAN. Therefore, it is also valued by the users the impact in the unitary cost of the explosive agent blend, since the GE-HDAN 75/25 is, in average, 10% cheaper than the Heavy ANFO blends.

Conclusion and Future Work It has been demonstrated that pure gasified emulsion can be improved when a minor quantity of high-density ammonium nitrate is added. The best proportion of HDAN is between 20% and 30%. Thus the application of GE-HDAN blends has been successful both technically and economically, due to reducing the consumptions but maintaining the granulometry index, the mines have obtained a savings of 10%. The probability of nitrous fumes is very low.

Figure 9. Muck pile granulometry in Cuajone copper mine

Figure 10. Powder Factor in Cuajone Copper mine



It has begun testing laboratory to measure and compare the most used explosive blends in the Peruvian mining. One useful method of examining the available work energy of an explosive is through cylinder testing for determination of the ratio of chemical energy converted into useful work energy.

Parallel to this work we have been researching in how to improve the explosive emulsion formulation, allowing using until 50% of HDAN without risk of fumes. At this manner, the explosive blend GE-HDAN 50/50 will be more economical and technically suitable (Villanueva, 2015). In addition, it is being carried out a compressibility study of the GE-HDAN (series of proportions) and its influence on the VOD.

Acknowledgements I wish to express my sincere thanks to the Blasting Engineers Jesús Cruces and Ricardo Puerta, from Cuajone Mine and Yorhinio León, from Yanacocha Mine, for giving me the opportunity to see, monitor and analyze the experimental blasts on their respective operations.

I remain in a great thankful debt with the Explosives Company EXSA who, among to be the first to make trials and introduced the massive application of the HDAN at Peruvian mines, provided me with the chance to use and test it in different mines within the Peruvian territory.I am very grateful to Ronald Añazco.

Also, my thanks to the Mining Engineering Department of the National University of Trujillo. Finally, I would like to thank Engineer Juan Carlos Vásquez for his global advisory on all our research.

ReferencesTyler, D. A., & Taylor, M. W. (2008). Patent No. WO 2008/026124. USA.

Wilches, W., Gonzales, J. C., & Proulx, R. (2015). HDAN Emulsion Blend Implementation Results in Open Pit Coal Mine. The International Society of Explosives Engineers Annual Conference (pp. 1-2). New Orleans: ISEE.

Cooper, P. (1996). Explosives Engineering. New York: Wiley-VCH.

Esen, S. (2008). A non-ideal detonation model for evaluating the performance of explosives in rock blasting,. In Rock Mech. Rock Engng. (Vol. 41, pp. 460-497).

Fleetwood, K. G., Villaescusa, E., & Eloranta, J. (2012). Comparison of the Non-Ideal Shock Energies of Sensitised and Unsensitised Bulk ANFO-Emulsion Blends in Intermediate Blasthole Diameters. International Society of Explosives Engineers, (pp. 7-10). Columbia.

Villanueva, R. B. (2015). XIII SIPERVOR. Open Pit Blasting Using Gasified Emulsion Based Explosives: A Brief Overview of the Concepts and Practices ant Peruvian Mines. Lima: UNI.



NITRATO DE AMONIO DE ALTA DENSIDAD Y EMULSION GASIFICADA: MEZCLA EXPLOSIVA EN LAS MINAS DEL PERU

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