OA17680A - Methods for producing explosive ANFO and heavy ANFO compositions. - Google Patents

Abstract

The invention relates to methods for producing explosive compositions formed by ammonium nitrate and fuel, also known as ANFO (Ammonium nitrate/Fuel Oil), and to the explosive compositions formed by bulk emulsion and ANFO, also known as heavy ANFO. The invention especially comprises two types of method for using high-density ammonium nitrate in the production of said explosive compositions.

Description

The présent invention is related to methods for making explosive compositions comprising ammonium nitrate and fuel, also known as ANFO (ammonium nitrate / fuel oil) and the explosive compositions comprising bulk émulsion and ANFO, also known as heavy ANFO. In particular, the présent invention comprises two types of method to the use of ammonium nitrate of high density in the development of these explosive compositions.

Description of the state of the art

ANFO type explosives (ammonium nitrate / fuel oil) are a mixture of ammonium nitrate with fuel, and are used as blast agents in industry and mining. Generally, ANFO type explosives consist of 94% of ammonium nitrate and 6% of fuel, and hâve a bulk density between 0.8 and 0.9 kg / L. The ammonium nitrate particles used for ANFO type explosives are porous and spherical, since the air cavities enclosed in the micropore structure of the particles provide a greater number of initiation points with high sensitivity to détonation, also called hot spots. These points are adiabatically compressed as a resuit of mechanical action and transfer the initiation energy through the load. The fuel is absorbed by the ammonium nitrate particles to produce a free-flowing particulate mixture which can be detonated. Other additives may be added to this mixture to modify the properties of the ANFO type explosive, such as guar gum and polyisobutylene to enhance water résistance type.

The détonation velocity of the ANFO type explosive is within the range of 2500 to 3500 m/s due to its volumétrie density and flame température, which are lower than those of other industrial explosives. The sensitivity of these materials to détonation is smaller than that of other types of explosives such as émulsion and dynamite; therefore, manipulation of ANFO type explosives has a lower risk. The ANFO performance dépends on the quality of the ammonium nitrate particles used and the effîciency in the mixture of those with the fuel.

Heavy ANFO type explosives consist of a mixture of bulk émulsion and ANFO. The use of such émulsion as a coating material créâtes a water résistant barrier around the particles of ANFO, eliminating the problem of its low water résistance. This waterproof layer also improves other characteristics of ANFO, increasing its density, détonation velocity, sensitivity to initiation and intensity of shockwave. Heavy ANFO is generally prepared in a mixer truck in which, first, the ANFO is prepared, and then mixed with the émulsion. The bulk émulsion used in the préparation of heavy ANFO can be gasified or not gasified. The heavy ANFO System allows great flexibility in relation to bulk émulsion ANFO. This relationship or ratio can be optimized depending on the spécifie requirements of the blast.

As it has been mentioned, the ANFO and heavy ANFO type explosive normally require porous ammonium nitrate particles. However, the présent invention proposes two methods for making heavy ANFO using ammonium nitrate of high density, technical grade or fertilizer, as a substitute for porous ammonium nitrate in the manufacturing process of ANFO. The first consists on the heat treatment of ammonium nitrate of high density modifying its crystal structure and obtaining therefore a réduction in density and an increase in the fuel absorption capacity. Therefore, enabling the use of ammonium nitrate heat treated to produce ANFO and heavy ANFO. The second is the use of ammonium nitrate of high density for making ANFO, modifying the explosive composition to use less fuel than regular ANFO type explosives. And then, the use of aerated bulk émulsion with the ANFO type explosive composition to obtain heavy ANFO.

Regarding the first proposed method, there are is a precedent in the US Patent 5,240,524 which aims to reduce the density of ammonium nitrate increasing its pore space through treatment with a gasifying agent. The cited invention uses a liquid medium which is a solvent of ammonium nitrate and a gasifying agent such as sodium nitrate, to penetrate cracks or pre-existing fissures in the particle and cause a chemical reaction with formation of gaseous products that increase size of the cracks and expel liquid outwardly. However, the présent application is a method for treating ammonium nitrate which differs completely from what was proposed by the US patent 5,240,524, because it includes only the variation of the crystal structure of ammonium nitrate by heating the particles by conduction, without chemical reactions or the use of liquid media or gasifying agents. Consequently, the characteristics of the porous spaces also differ since the first case has increased the size of cracks and fissures while in the présent invention a reorganization of the molécules of ammonium nitrate is produced.

Another method used to reduce the density of ammonium nitrate is found in US patent 5,540,793 which consists of a method for manufacturing ammonium nitrate using encapsulated microspheres which reduce the density of the product. The invention extends to ANFO and heavy ANFO type explosive compositions made from ammonium nitrate of reduced density. This method differs from the présent invention as it adds physical agents such as microspheres and must be done during the manufacture of ammonium nitrate. Additionally, the density of ammonium nitrate particles can reach values lower than those of the présent invention, between 0.54 and 0.67 kg/L.

Likewise, the document US 5,409,556 consists in preparing heavy ANFO type explosive compositions with a reduced density by adding expanded grains such as corn, rice or wheat, up to 10 percent of the composition. This composition can be prepared using porous or high density ammonium nitrate. The cited invention differs from the présent invention since it entails the addition of raw materials such as expanded grains, and the density réduction of the heavy ANFO type explosive composition, instead of reducing density of ammonium nitrate.

Additionally, the document US 4,853,050 is known, which is related to a method for producing an explosive composition comprising particulate ammonium nitrate, where its water résistance is improved by a coating of asphalt; as well as the method for preparing such ANFO/ explosive émulsion mixtures using ammonium nitrate with improved characteristics. The treatment made to ammonium nitrate proposed in this invention aims to improve the water résistance, and not to modify the crystalline structure of ammonium nitrate. Even as opposed to the présent invention it results in an increased density of the ammonium nitrate treated.

Furthermore, the document US 4,084,995 relates to the préparation of a granulated explosive composition based on ammonium nitrate and calcium nitrate, which are heated, with a liquid phase which includes fuel and water at températures where the salts are combined, and then cooled with mechanical stirring. The resulting explosive composition shows an increased sensitivity to the detonators and a lower critical diameter, characteristics dépendent on the physical state of the mixture resulting from the treatment received, and not on the chemical composition. However, the goal of the treatment made to the ammonium nitrate differs from the présent invention since it is not intended to modify its crystal structure but to combine ammonium nitrate and calcium nitrate to obtain a mixture with a particular physical state.

Regarding the second proposed method, it has been preceded in US Patent 2006/0243362, in which Houston et. al. used ammonium nitrate of similar features to those presented herein for high density ammonium nitrate to produce a hydrogel type explosive composition. However, the System in which this raw material is used, and the only one in which tests hâve been done according to the patent, completely differs from heavy ANFO based on gasified émulsion and ANFO which is proposed in this invention.

Also, in the state of the art, it is known the US Patent 5,458,707 which uses an émulsion gasified type explosive composition for the manufacturing of heavy ANFO. However, the invention aims to modify the composition of the gasifiable émulsion based on a PIBSA, including an ester of orthophosphoric acid and a co-emulsifier to prevent dégradation of the émulsion in the interaction with the porous ammonium nitrate in the préparation of heavy ANFO. Instead, the présent invention does not change the characteristics of the gasifiable émulsion as it used ammonium nitrate of high density or heat treated ammonium nitrate to replace porous ammonium nitrate.

Summary of the Invention

The présent invention includes two methods for the production of heavy ANFO using ammonium nitrate of high density.

The first method involves heat treatment of ammonium nitrate of high density by loading the ammonium nitrate of high density in a tank; the entry of ammonium nitrate of high density in a 5 heating device at room température; heating the ammonium nitrate of high density within said heating device; removal of the treated ammonium nitrate from the heating device at a température less than 60 ° C and sieving the treated ammonium nitrate, so that the heat treated ammonium nitrate is obtained.

Heat treated ammonium nitrate is used for the préparation of ANFO at a température below 30 ° C 10 by loading it in a tank; fuel loading into a separate tank; feeding heat treated ammonium nitrate into a mixing chamber while the fuel is fed into the same chamber; and mixing the heat treated ammonium nitrate and fuel, obtaining ANFO.

The heat treated ammonium nitrate is also used for the manufacture of heavy ANFO by feeding from a hopper to a mixing chamber and injecting the fuel in the feeding pipe of heat treated 15 ammonium nitrate, obtaining ANFO; loading bulk émulsion gasified or not gasified in the hopper;

feeding bulk émulsion in the same chamber as the ANFO; and the mixture of bulk émulsion and ANFO, obtaining heavy ANFO.

The second proposed method in the présent invention is the use of ammonium nitrate of high density, with a density of 0.95 kg/L to 1.00 kg/L, for the préparation of heavy ANFO through loading 20 ammonium nitrate of high density in a hopper, feeding ammonium nitrate of high density in the mixing chamber and injecting fuel into the feeding tube of ammonium nitrate in a fuel proportion: ammonium nitrate of high density from 2:98 to 6:94; loading bulk émulsion into a hopper and the gasified of said bulk émulsion; feeding said émulsion gasified in the same mixing chamber of ANFO in a gasified bulk émulsion ratio: ANFO from 20:80 to 70.30; and the mixture of both components 25 getting heavy ANFO.

Description of figures

Figure 1 shows the process and related components therein used for heat treatment of the ammonium nitrate of high density.

Figure 2 shows the process and related components therein for the manufacture of ANFO.

Figure 3 shows the process and related components used in the manufacture of heavy ANFO.

Detailed description of the Invention

The first embodiment of the présent invention is a method for modifying the crystal structure of ammonium nitrate of high density, preferably technical grade or fertilizer, creating pores in the particles. As a resuit, the density of the ammonium nitrate is reduced while the optimal characteristics of hardness of the particles is maintained and the fuel absorption capacity of the 5 same increases. A process for heat treatment of ammonium nitrate of high density, preferably technical grade or fertilizer, which is illustrated in Figure 1 is proposed.

The first step is to load the ammonium nitrate of high density (1.f), which normally has the properties listed in Tables 1 and 2, in a tank (1a). Then, the ammonium nitrate is sieved (1b) to remove the fines. This step is performed in order to avoid a réduction in the efficiency of the heating 10 process due to variability in the particle size of the ammonium nitrate. If it has less than 5% of fines, there is no need to sift it before entering the heating device (1c).

Table 1. Différences between ammonium nitrate technical grade and Ammonium Nitrate of Fertilizer Grade.

Parameter	Unit	Ammonium nitrate of high density
Technical Grade	Fertilizer Grade
Purity (NH4NO3)	%	>98.5	>97.5
Insoluble	%	0	<2
Calcium (CaO)	%	0	<2
Magnésium (MgO)	%	0	<2
Phosphorus (P2O5)	%	0	<3.5
Potassium (K2O)	%	0	<2

In the next step, the ammonium nitrate of high density (1 .g) enters the heating device (1c) at room température. The heating process is the indirect heat transfer by conduction between the walls of the device and the particles of ammonium nitrate of high density. The purpose of the heat treatment is to increase the température of the particles to generate changes in the crystal structure of the ammonium nitrate avoiding to affect its moisture content.

The change sought in the crystal structure occurs due to the phase transition of ammonium nitrate from Phase IV to Phase III; and it is associated with an increase of volume and réduction of density due to the rearrangement of the molécules. These changes in the crystal structure of ammonium nitrate are known and usually occur during storage of the raw material due to température cycling. However, modification of the physical properties accompanying the phase transition is considered unfavorable because it affects the desired properties of ammonium nitrate, reducing the hardness, flowability, and causing crusting. Together, these changes hâve resulted in a reduced sensitivity to initiation and détonation.

The présent invention proposes an application of physical change caused by the phase transition of ammonium nitrate of high density. Therefore, the température and time of treatment of the ammonium nitrate of high density in the heating device (1.c.) and the température of ammonium nitrate when it exits the heating device (1.h.), are critical variables in the proposed method. The température at which this transition phase starts is 32.3 ° C and continues until 51 ° C, after which a bridge that prevents the transition to Phase III is created, changing directly from Phase IV to II, which may affect the génération of pores and considerably reduce the hardness of ammonium nitrate. Therefore, the température of the heating device (1c) should fluctuate between 50 °C and 150 °C; and the température of ammonium nitrate when it exits the heating device (1.h) should not exceed 60 ° C, and preferably should be between 40 and 60 ° C. The treatment time of ammonium nitrate of high density is directly related to the température of the ammonium nitrate treated when it exits the heating device (1.h). Normally, this treatment time can vary between 1 and 10 minutes, and preferably should be between 3 and 4 minutes.

Other variables that may influence the efficiency of the heat treatment are related to the characteristics of the particles of ammonium nitrate of high density, being the variation in the density and fuel absorption capacity less obvious in those particles that initially hâve a greater hardness, less porous space or cracks, and présent inorganic additives. On the other hand, particles of ammonium nitrate of high density with less hardness would be more affected by the treatment causing increased fines génération due to the fragility of the particles, and causing problems in the préparation of ANFO and heavy ANFO. Preferably the ammonium nitrate of high density should hâve a bulk density between 0.95 and 1.00 kg/L, an absorption capacity of fuel less than or equal to 4% and a hardness of 0.4 to 0.6 kg/particle. Also, both the heat treatment and the methods used for the préparation of ANFO and heavy ANFO should generate the least friction and possible damages in orderto avoid génération offine since heat treated ammonium nitrate particles are weaker.

The next step is the sieving (1d) to the outlet of the heating device (1c) to remove the fines which were generated in the previous stage. The product obtained is heat treated ammonium nitrate (1.i), which has a bulk density between 0.80 and 0.90 kg/L, an absorption capacity of fuel between 5 and 8%, and a hardness of 0.2 to 0.5 kg/particle. This sieving step (1.d) is required to remove potential 5 problems associated with small particles of ammonium nitrate for subséquent use in the manufacture of ANFO and heavy ANFO.

Table 2. Properties of Ammonium Nitrate of High Density, Porous Ammonium Nitrate and Heat Treated Ammonium Nitrate

Parameter	Unit	Ammonium Nitrate of High Density	Porous Ammonium Nitrate	Heat Treated Ammonium Nitrate
Bulk density	kg/L	0.95-100	0.72 - 0.82	0.80 - 0.90
Absorption capacity of oil	%	<4	7-14	5-8
Hardness	kg/particle	0.4-0.6	0.35 - 0.55	0.2 - 0.5

Heat treated ammonium nitrate (1.i) can be used instead of porous ammonium nitrate in the manufacture of explosives ANFO and heavy ANFO type. For the préparation of ANFO, heat treated ammonium nitrate must be at température lower than 30 ° C and preferably between room température and 30 °C. The greater decrease in density and increased absorption of oil, the heat treated ammonium nitrate according to the présent invention shows better results in the élaboration of ANFO and heavy ANFO.

The manufacturing process of ANFO is performed according to the flowchart shown in Figure 2. The process includes the mixture (2.c) of heat treated ammonium nitrate (2.e) with fuel (2.f) in a ratio of 95: 5 or 94: 6, and preferably in a ratio 94: 6. Fuel (2-f) used in the manufacturing process of ANFO may be biofuel, biodiesel, diesel, minerai oil, residual oil, among others.

The manufacturing process of heavy ANFO is performed according to the flowchart shown in Figure 3, preferably in mixing trucks in mining operations. The first step of the process is the load of heat treated ammonium nitrate (3.g) in a hopper (3.b). Then the fuel (3.f) contained in the hopper (3.a) is injected into the supply pipe of heat treated ammonium nitrate, obtaining ANFO (3.i). This mixture is prepared in proportions of 98:2 to 94:6, and preferably 97:3. Fuel (3f) used in the process of élaboration of the heavy ANFO can be biofuel, biodiesel, diesel, minerai oil, residual oil, among others. Then the bulk émulsion (3.h) contained in a hopper (3.c) is gasified and then is mixed with the ANFO (3.i), obtaining a heavy ANFO (3.j). The ratio for mixing the bulk émulsion (3.h) with ANFO (3.i) is preferably among the following: 20:80, 30:70, 40:60, 50:50, 60:40 and 70:30. The bulk émulsion used for this préparation may be gasified or non-gasified. The final product (3.j) is charged into the drills (3-e) in mining operations.

The second embodiment of this invention is the use of ammonium nitrate of high density, preferably of technical grade or fertilizer, as a substitute for porous ammonium nitrate in the préparation of ANFO for the production of heavy ANFO. To achieve this goal, the ANFO type explosive composition is modified to use less fuel than regular ANFO type explosive, and the bulk émulsion is gasified for the development of heavy ANFO.

The development of heavy ANFO is performed as illustrated in Figure 3. This process is performed preferably in a truck mixer located in the different mining units. The process includes feeding ammonium nitrate of high density (3.g), with a bulk density between 0.95 and 1.00 kilograms per liter (kg/L) and an fuel absorption capacity of less than or equal to 4%, in an hopper (3b). Ammonium nitrate of high density used in this préparation has preferably the characteristics described in Tables 1 and 2.

The fuel (3.f) into the hopper (3.a) is injected to the feed pipe ammonium nitrate high density, obtaining ANFO (3.i). The ratio of this mixture can range from 98: 2 to 94: 6 and preferably 97: 3. The proportion of ammonium nitrate and fuel is determined based on the absorption capacity of ammonium nitrate fuel density and the high oxygen balance is to be achieved. Fuel (3.g) used for the production of heavy ANFO may include biofuel, biodiesel, diesel, minerai oil and residual oil.

Bulk émulsion (3h) in the hopper (3.c) must be gasified. In this process, a gasifying agent chemically reacts with the oxidizer sait phase under appropriate pH conditions to produce a fine dispersion of bubbles of nitrogen gas through the émulsion. The gasifying agents used generally include a variety of gas-generating materials, which are known in the art. Preferably, the gasifying agent is a nitrite, specifically an alkali métal nitrite, and more specifically sodium nitrite. Chemical gassing of these compositions is achieved through the reaction between nitrite and ammonium ion with the nitrogen gas génération. The gasifying agent can be mixed with a secondary chemical compound, which is used as a catalyst to increase the rate at which the reaction occurs, such as sodium thiocyanate orthiourea and others. The reaction of chemical gassing using gasifying agents such as nitrite can be accelerated, for example by reducing the pH of the gasifying solution. Preferably, the pH should be lowered to values between 1 and 5, and more specifically from 2 to 4. Suitable acids to lower the pH of the oxidizing solution include sulfuric acid, nitric acid, acetic acid, and others known in the art.

Subsequently, the gasified bulk émulsion (3.h) is mixed with ANFO (3.i), obtaining heavy ANFO (3.j). The ratio for mixing gasified bulk émulsion (3.h) with ANFO (3.j) can be between the following, 20:80, 30:70, 40:60, 50:50, 60:40 and 70 30. And, preferably between 60:40 and 70:30. The final product (3.j) is used for the loading of drill (3-e) in mining operations.

Claims

Claims (14)

1. A method for heat treating the ammonium nitrate of high density, because the method comprises:

a) Load nitrate high density in a tank; where the ammonium nitrate of high density has a bulk density of 0.95 to 1.00 kg/L, an absorption capacity of fuel less than 4% and a hardness of 0.4 to 0.6 kg/particle.

b) Enter the ammonium nitrate of high density in a heating device, where the ammonium nitrate is at room température at the inlet;

c) Heat the ammonium nitrate of high density in the heating device; where the température of the heating device is between 50 and 150 ° C, and the treatment time is from 1 to 10 minutes.

d) Remove the treated ammonium nitrate heating device; where the outlet température is between 40 and 60 ° C; and

e) Sift the ammonium nitrate; obtaining heat treated ammonium nitrate, where the heat treated ammonium nitrate has a bulk density of 0.80 to 0.90 kg/L, a fuel absorption capacity of 5-8% and a hardness of 0.2-0.5 kg/particle.

2. The method according to claim 1, characterize because the nitrate of high density is fertilizer grade ortechnical grade.

3. The method according to claim 1, characterized because the ammonium nitrate of high density is sieved prior to entering the heating device.

4. A method for preparing ANFO characterized by comprising:

a. Load heat treated ammonium nitrate in a tank where the température of heat treated ammonium nitrate is between environmental température and 30 °C; where the heat treated ammonium nitrate has a bulk density of 0.80 to 0.90 kg/L, a fuel absorption capacity of 5-8% and a hardness of 0.2-0.5 kg/particle.

b. Load fuel in a separate tank;

c. Feed heat treated ammonium nitrate in a mixing chamber while fuel is fed into the same mixing chamber; and

d. Mixing heat treated ammonium nitrate and fuel, obtaining ANFO.

5. The method according to claim 4, characterized because the fuel is biofuel, biodiesel, diesel, minerai oil or residual oil.

6. The method according to claim 4, the mixing ratio of the heat treated nitrate ammonium respect to fuel is from 95:5 to 94:6.

7. A method for preparing heavy ANFO, characterized because the method comprises:

a. Load heat treated ammonium nitrate in a hopper; where the heat treated ammonium nitrate has a bulk density of 0.80 to 0.90 kg/L, a fuel absorption capacity of 5-8% and a hardness of 0.2-0.5 kg/particle.

b. Feed heat treated ammonium nitrate in a mixing chamber while fuel is fed into the supply pipe of the heat treated ammonium nitrate, obtaining ANFO;

c. Loading bulk émulsion into a hopper, where the mixing ratio of bulk émulsion relative to ANFO is from 20:80 to 70:30, and where the bulk émulsion may be aerated or gasified;

d. Feed the bulk émulsion in the same mixing chamber as the ANFO; and

e. Mixthe bulk émulsion and the ANFO, getting heavy ANFO.

8. The method according to claim 7, characterized because the fuel is biofuel, biodiesel, diesel, minerai oil or residual oil.

9. The method according to claim 7 characterized because the mixing ratio of fuel relative to the heat-treated ammonium nitrate is 2:98 to 6:94.

10. A method according to claim 7, characterized because the mixing ratio of bulk émulsion relative to ANFO is from 20:80 to 70:30.

11. A method for preparing heavy ANFO, characterized because the method comprises:

a. Load ammonium nitrate of high density in a hopper; where the ammonium nitrate of high density has a bulk density of 0.95 to 1.00 kg/L and a fuel absorption capacity of less than 4%.

b. Feeding the ammonium nitrate of high density into a mixing chamber while fuel is fed into the supply pipe ofthe heat treated ammonium nitrate, obtaining ANFO;

c. Loading the bulk émulsion into a hopper, where the bulk émulsion is gasified; and where the ratio of mixing the bulk émulsion to gasified ANFO is from 20:80 to 70:30

d. Feeding the gasified bulk émulsion into the same mixing chamber as the ANFO; and

e. Mixing the bulk émulsion and the ANFO, getting heavy ANFO.

12. The method according to claim 11, characterized because the fuel is biofuel, biodiesel, diesel, minerai oil or residual oil.

13. The method according to claim 11 characterized because the ratio of mixing the fuel respect to 5 the ammonium nitrate of high density is from 2:98 to 6:94, or 3:97.

14. A method according to claim 11, characterized because the ratio of mixing the bulk émulsion to gasified ANFO is from 20:80 to 70:30.