Explosive mixtures containing readily gasified additives

Abstract

A readily gasified additive has been added to AN (ammonium nitrate) based bulk explosives (ANFO, emulsions, and combinations thereof) with positive results, due to the time, temperature, and pressure conditions generated in a blasthole. The preferred final explosive mixture is primarily comprised of AN, fuel oil, and a carbonaceous additive material that is characterized as being readily gasified as indicated by its rapid release of volatile materials and char formation, the high internal porosity and surface area of the base material and its char, and the uniform distribution of catalytic mineral matter throughout the material. It is understood that other readily gasified additives may be similarly effective.

Description

FIELD OF THE INVENTION

The present invention relates generally the field of explosive compositions and methods of formulating same. More particularly, the present invention relates to the incorporation of a non-explosive, carbonaceous additive into explosives, with the additive behaving as more than a diluent or filler, but rather imparting beneficial properties to a blast.

BACKGROUND OF THE INVENTION

Blasting operations in mining conventionally use ANFO (a stoichiometric mix of Ammonium Nitrate and Fuel Oil), AN- (ammonium nitrate) based emulsions, or mixes thereof, as their bulk explosives. Although ANFO and emulsions represent the vast majority of all explosives utilized globally, they have technical limitations, including limited control over velocities of detonation (VOD), potential for severe ground vibrations, periodic generation of toxic gases (NOx), etc. It should also be noted that these explosive materials represent a significant portion of operating expenses, and are subject to supply shortages and sporadic pricing.

A need exists in the art, therefore, for other explosive mixtures which are as effective as ANFO, or more effective than ANFO, in their explosive effect. The present invention imparts enhanced control over VOD, decreased ground vibrations, and reduced NOx formation. It also provides for mixtures that are less costly than the present explosive materials, and are less subject to erratic price fluctuations of the explosives' chemical ingredients.

SUMMARY OF THE INVENTION

The present invention provides an AN-based explosive mixture, for example an ANFO- and/or emulsion-based explosive mixture, which has been modified with the incorporation of a readily gasified carbonaceous additive material. The present mixture is characterized by lower velocity of detonation, provides performance results similar to traditional explosives (specifically cast and fragmentation), has the ability to reduce NOx formation, and is less expensive and less subject to pricing fluctuations than presently available explosive compositions.

More specifically, the present invention comprises an explosive mixture comprising a nitrate oxidizing agent, for example, 30-95% ammonium nitrate; 0-10%, more usually less than 8%, for example 2-8% of a liquid fuel; and 5-80% of a carbonaceous additive material. More specifically, the additive material of the preferred embodiment is a unique coal that is more reactive compared to other coals as indicated by (1) its rapid release of volatiles and char formation, (2) its high internal porosity and surface area, and (3) the presence of finely-dispersed catalytic mineral matter. The mixture may optionally also contain materials including water, suspending/emulsifying agents, inorganic oxidizing agents, bulking agents, among other materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 is horizontal tube furnace used for visualization of coal devolatilization;

FIG. 2(a) compares the DR surface area of the C*P additive and its char (after devolatilization in a horizontal tube furnace, HTF), as well as in a drop tube furnace (DTF), and FIG. 2(b) shows that the N₂ (mesopore) surface area of coal was 35.4 m²/g, and, as noted previously, significantly higher than normally observed for other, similar coals;

FIG. 3 illustrates the modal distribution of various minerals in this coal sample, and the data is summarized in Table 3;

FIG. 4 illustrates the association of the identified minerals with carbon matrix of the coal;

FIG. 5 shows the cumulative size distribution of illite grains in the coal samples. This clearly shows that the catalytic clays are finely and intimately dispersed in the carbon matrix, which is expected to enhance the reactivity.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to multi-component explosive mixtures which are useful in blasting operations such as, but not limited to, mining operations and the like. However, it will be understood that these explosive mixtures can be applied to other blasting operations. The mixture is loaded and the shot is initiated according to conventional blasting methods.

More specifically, the present invention provides an explosive mixture consisting of an oxidizing agent, a liquid fuel, and a carbonaceous additive. The mixture may optionally also include materials including water, suspending/emulsifying agents, inorganic oxidizing agents, bulking agents, among other materials.

Each component of the present mixture is individually not an explosive. As such, each component can be handled, shipped and stored as oxidizers, flammable materials, or other suitable classes which are less hazardous than explosives. Such classifications are shipped more safely and less expensively than explosives. However, it will be understood that these components, when mixed, form an explosive mixture which may be used in blasting operations such as, but not limited to, mining operations and the like.

The first component of the mixture of the present invention is an oxidizing compound. The oxidizing compound can be selected from oxidizing agents conventionally used in the manufacture of explosive compositions to achieve desired properties. General examples of oxidizing agents are anionic oxides of nitrogen, sulfur, and carbon (such as nitrate, sulfate, and carbonate), permanganates, chromates, diatomic oxygen (O₂), ozone, and peroxides. The preferred oxidizing agent is ammonium nitrate and is present at concentrations up to 94% by weight. As used herein, all units of percent (%) refer to percent by weight (% w/w, wt %).

The second component of the mixture is a liquid fuel component, and may or may not be present, as another fuel may replace it. Thus, the second component may be present in an amount of 0-10%, for example 2-8%. In the preferred embodiment, the second component is a flammable or combustible liquid. Such liquids include, but are not limited to, hydrocarbon-based liquid fuels, and blends thereof, including petroleum-derived products (e.g., fuel oils, diesel, kerosene, gasoline, alkanes, and alkenes), oxygen-containing liquid fuels such as alcohols, glycols, ethers, ketones, (e.g., methanol, ethanol, n-propanol, iso-propanol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, glycerin, polyglycols, and polyethers). Conventional ANFO generally comprises a mixture of 94% ammonium nitrate and 6% #2 fuel oil (diesel fuel). The concentration of the second component in the present mixture may decrease as the carbon and hydrogen atoms in the second component chemically necessary for an optimum blast may be replaced in the current mixture with volatile, flammable materials released from the third component. Application of a lesser volume of the liquid second component as described reduces the cost of the present explosive mixture compared with presently available explosive compositions and may have other beneficial effects, including but not limited to reduction of velocity of detonation and reduction of carbon monoxide generation.

The third component in the disclosed mixture is a coal with unique properties, defined in greater detail below. Briefly, the additive material of the preferred embodiment is a unique coal that is more reactive compared to other coals as indicated by (1) its rapid release of volatiles and rapid char formation, (2) its high internal porosity and surface area, including the rapid growth in surface area during pre-combustion stage, allowing for increased reactivity, and (3) the presence of finely-dispersed catalytic mineral matter moieties. Thus, both physical and chemical properties of the third component contribute to its uniquely high reactivity. The relationship between these properties and reactivity has been discussed extensively.^1,2,3,4 Specific details of these characteristics are described further below.

Devolatilization and Char Formation:

The reactivity of coal can be attributed to two processes on the basis of large differences in time scale, namely devolatilization and char formation. The nature of the char formed after release of volatile matter has a strong influence on its reactivity, which is of particular significance.

Referring to FIG. 1, coal particles were devolatilized in a horizontal tube furnace in a nitrogen atmosphere at 1273K. The coal particle was kept in the cold zone followed by inserting into the hot zone at the specified test temperature. Nitrogen (99.9% purity, 2.5 l/min) was purged through the furnace during the tests.

A high resolution CCD camera was used to monitor the dynamic changes of the sample. This coal indicated a more rapid release of gases compared to other coals. Under the same prescribed conditions, the total time for completion of devolatilization for other coals of this type (high volatile bituminous coal, >31% VM) is on the order of 60 seconds. For this material, however, the total time required for devolatilization is on the order of 30 seconds, thus clearly demonstrating the uniquely high devolatilization rate of this coal. Although not highly indicated in combustion reactivity, devolatilization rate is of fundamental importance in gasification. That is, the rapid devolatilization provides for rapid char formation, which is in turn responsible for the uniquely high reactivity of the char.

Porosimetry:

Porosimetry is the study of the porous structure of materials. Properties such as pore diameter, pore volume, and surface area can be quantified. Low-pressure gas adsorption measurements were conducted on a Micromeritics ASAP-2020 porosimeter and surface area analyzer. 1-2 grams of each sample (60-mesh split) were analyzed using nitrogen and carbon dioxide gases to obtain information about mesopore (2-50 nm) and micropore (<2 nm) structures, respectively. The following parameters were calculated from the nitrogen absorption analysis: Brunauer-Emmett-Teller (BET) surface area; and Bopp-Jancso-Heinzinger (BJH) mesopore volume (adsorption branch). From the carbon dioxide adsorption analysis, the following parameters were calculated: Dubinin-Radushkevich (D-R) micropore surface area; D-R monolayer capacity; and Dubinin-Astakhov (D-A) micropore volumes. Based on N₂ and CO₂ adsorption, mesopore and micropore size (width) distributions were also generated.

Table 1 contains the results of the porosimetry on a suite of coals. The sample of interest was analyzed initially (C*P 1) and compared to literature. Due to its uniquely high BET mesopore surface area, it was re-analyzed for confirmation (C*P 2) with a suite of coals to confirm its highly-mesoporous nature. An additional sample of a so-called “Cannel” coal (CAN) was later added to the sample set for comparison. The row containing the BET surface area data is bold, as the data clearly indicates a meaningfully unique property of the “C*P” material.

TABLE 1
Results of porosimetry on C*P, PRB, CC#5, CC#6, and CAN.
	C*P 1	C*P 2	PRB	CC#5	CC#6	CAN
Mesopores
BET Surface	48.00	40.88	7.29	24.24	17.08	0.84
Area (m2/g)
BJH Volume	0.054	0.047	0.019	0.030	0.023	0.0034
(cm3/g)
Adsorption	5.21	5.31	11.29	5.52	6.06	17.16
Ave Pore
Width (nm)
Micropores
D-R Surface	115.11	107.55	162.05	115.65	117.28	40.80
Area (m2/g)
D-R	25.20	23.54	35.47	25.32	25.67	8.93
Monolayer
Capacity
(cm3/g)
D-A Volume	0.055	0.055	0.065	0.056	0.054	0.028
(cm3/g)
Adsorption	1.36	1.36	1.45	1.35	1.36	1.12
Ave Pore
Width (nm)

In addition to the data, a set of Seelyville coals (high volatile bituminous, hvb) from Sullivan, Knox, and Gibson Counties in Indiana were analyzed and displayed BET (N₂) surface areas in the range of 1.8-22.9 m²/g.⁵ Interestingly, they note that the samples with the largest specific surface areas and largest mesopore volumes occur at the shallowest depths. This is certainly consistent with the source of the C*P coal. Additionally, an article specifically related to the porous nature of coals reports a range of BET (N₂) surface areas of 2.9-22.5 m²/g.⁶ The sample set contains 23 coals spanning a variety of coal ranks (lignite A & B; subbituminous C; hvB, hvC, my, & lv bituminous). A study detailing the porous nature of a set of Turkish coals noted a “highest” BET (N₂) surface area of 34 m²/g in their set; the corresponding values for the remainder of the samples were less than 7 m²/g.⁷ It has been indicated elsewhere that the reactivity of chars is proportional to their porosity and surface area in the mesopore range.^8,9

BET surface areas greater than that of C*P have been reported, but have not been found for “base” coals. Rather, they are reported for isolated coal lithotypes (e.g. vitrain, 115.4 m²/g)¹⁰ and chars (e.g. 132.3 m²/g).¹ Vitrains are typically the most mesoporous lithotype, and, when creating char from coal, the pore structure is completely altered, as controlled by the coals unique melting behavior. High mesoporosities have also been reported for synthesized carbonaceous materials (e.g. “Starbons®,” 180 m²/g).¹¹ It can be appreciated that isolating a selected lithotype, generating consistent chars, and synthesizing materials are all energy-intensive processes, and thus more expensive/less practical for this application than the claimed additive material.

In addition to the porosity characteristics described above, characterization of the base coal and its char was examined in-depth as part of a larger body of research. FIG. 2(a) compares the DR surface area of the C*P additive and its char (after devolatilization in a horizontal tube furnace, HTF), as well as in a drop tube furnace (DTF). This figure shows an unusually large CO₂ surface area of the coal sample (>100 m²/g), which implies the presence of high percentage of micropores (<2 μm). After devolatilization in the drop tube furnace at 1673K, the DR CO₂ surface area (119.5 m²/g) of coal changes to 123.3 m²/g, indicating a small change in surface, suggesting that at the higher temperatures of the DTF, the proportion of micropores did not change significantly. FIG. 2(b) shows that the N₂ (mesopore) surface area of coal was 35.4 m²/g, and, as noted previously, significantly higher than normally observed for other, similar coals. Unlike CO₂ surface area, the N₂ surface area of the char decreased to 1.1 m²/gm after devolatilization in the horizontal tube furnace and increased to 57.8 m²/gm after devolatilization in the DTF. Generally, the N₂ surface area of the char increases after devolatilization. It appears that unexpected lower surface area after devolatilization in the HTF may be related to modification of pore distribution to the micropore size range. Insufficient penetration of N₂ molecules into the char pores may also be contributing to unexpectedly lower values of N₂ surface area of the HTF char sample.

Mineral Matter:

SIROQUANT Analysis: Samples were burned at low temperature (120° C.) using radio-frequency oxygen plasma ashing. Under this condition, minerals are known to undergo minimum alteration. The XRD spectra of this low temperature coal ash was obtained by using a Philips PW1050 goniometer that employs CoKα radiation at 45 kV and 30 mA, with step scans from 3-90° 2θ, a step interval of 0.04° and 10 sec count time per step. SIROQUANT™ software, developed by CSIRO, Australia, was later used to quantify the minerals in the ash, which uses the full-profile Rietveld method for curve fitting.

Table 2 provides SIROQUANT analysis of the coal sample and shows that it contains high quartz and pyrite. More significantly, this coal contains 29.2% of K containing clays (illite and muscovite). The presence of 4.8% chamosite is notable as this phase is not commonly reported in coals, but is expected to enhance reactivity.

TABLE 1
SIROQUANT analysis of low temperature ash specimen of coal.
		wt.
Mineral		% in
Matter	Chemical formula	ash
Quartz	SiO2	25.6
Muscovite	KAl2(Si3Al)O10(OH,F)2	8.2
Illite	(K,H3O)(Al,Mg,Fe)2(Al,Si)4O10[(OH)2,H2O]	21.0
Kaolinite	Al2Si2O5(OH)4	24.5
Chamosite	(Fe+2,Mg,Fe+3)5Al(Si3Al)O10(OH,O)8	4.8
Montmo-	(0.5Ca,Na)0.7(Al,Mg,Fe)4[(Si,Al)8O20](OH)4•nH2O)	3.2
rillonite
Gypsum	CaSO4•2H2O	1.4
Bassanite	CaSO4•0.5H2O	0.4
Coquimbite	Fe2(SO4)3•9H2O	2.1
Pyrite	FeS2	7.9
Marcasite	FeS2	0.8

Mineral Liberation Analyzer (MLA): Coal samples were prepared (using an epoxy resin), polished, and coated with carbon. The samples were analyzed with a Mineral Liberation Analyzer (MLA) using the XBSE technique, Extended BSE Liberation Analysis. XBSE implements area X-ray analysis to analyze ore samples containing phases with sufficient BSE contrast to ensure effective segmentation. The high resolution of BSE imaging for grain boundary definition and the speed of single X-ray mineral identification make this method ideal for a great majority of mineralogical samples. The MLA provides modal mineralogy, relative abundance, grain size distribution and MLA images.

FIG. 3 illustrates the modal distribution of various minerals in this coal sample, and the data is summarized in Table 3. Table 3 indicates that the tested coal contains a high concentration of quartz and clays. This analysis is consistent with SIROQUANT analysis such that amount of illite and smectite clays is also high. It should be noted that differences in the phase identification by SIROQUANT and MLA are related to differences in the microscopic and bulk analysis approach and sample preparation. MLA uses raw coal samples while SIROQUANT uses mineral rich low temperature ash specimens. For example, smectite was not reported in SIROQUANT analysis.

FIG. 4 illustrates the association of the identified minerals with carbon matrix of the coal. FIG. 5 shows the cumulative size distribution of illite grains in the coal samples. This clearly shows that the catalytic clays are finely and intimately dispersed in the carbon matrix, which is expected to enhance the reactivity.

TABLE 2
Relative abundance of minerals of coal based on MLA technique.
Mineral
Reference	Formula	wt. %
Pyrite	FeS2	7.91
Calcite	CaCO3	0.02
Quartz	SiO2	26.46
Illite	(K,H3O)(Al,Mg,Fe)2(Al,Si)4O10[(OH)2,H2O]	16.71
Smectite	(Na,Ca)Al4(Si,Al)8O20(OH)42(H2O)	10.35
Siderite	FeCO3	0.22
Kaolinite	Al2Si2O5(OH)4	38.16
Rutile	TiO2	0.16
Other	SiO2	0.01
Total		100.00

Preferred specimens of the third component may be found in the Friendsville Coal Seam at the base of the Mattoon Geological Formation and within the McLeansboro Group of the Missourian Series of the Pennsylvanian Geologic System of sedimentary rock of Southeastern Illinois. However, alternate additive materials achieving the same effect as described herein may also be used. That is, and of particular significance, other carbonaceous materials that can be efficiently gasified under the time, temperature, and pressure conditions of a typical ANFO or emulsion blast may be similarly efficacious. This may include certain other carbonaceous materials, and combinations thereof, including specific samples of coal, coke, lignite, biomass, waste, and the like. The unique, individual physical and chemical characteristics of these materials will define their gasification efficiency, as the properties of naturally occurring products vary greatly depending on species, origin, age, composition, etc. As a notable example of this difference, depending on conditions, the gasification efficiency of different coals can vary by an order of magnitude.¹²

The particle size of the additive material to be incorporated into the mixture may range from a fine powder to a large chunk, preferably below ½ inch. Blast characteristics can be altered by particle size within the mixture, in that smaller particles are more rapidly combusted than larger particles due to higher exposed surface areas. The viscosity of emulsions and other properties of the explosive mixture can also be altered by selection of particle size. For example, when a very fine particle size is added to an emulsion, it acts as a viscosity modifier, increasing the viscosity of the product considerably. Particle size is of consequence in this invention. Whereas small particles (powders, etc.) react readily due to their relatively large external surface area, they do not serve to displace the other ingredients in a solid mixture. Rather, they merely fill the interstitial voids. A key, unique property of the third component described herein is the ability to add a large enough particle such that it displaces the other ingredients while maintaining high reactivity, a consequence of its large internal surface area and high degree of porosity. The particle size of the additive material in the preferred embodiment typically ranges between approximately 0.025 and 0.5 inch. In other words, greater than 95% of the material will pass through a ½ inch screen but will not pass through a 28-mesh screen (i.e., >95%+28-mesh/−½ inch). As such, the chips of the third component have the size to displace the other ingredients in a solid mixture while maintaining the high reactivity associated with fine materials. The increase in reactivity with an increase in active surface area is a well-understood relationship. Thus, inclusion of the third component effectively depletes the required volume of the other components. It is worth noting that the description above is specific to the preferred embodiment, but does not seek to exclude the efficacy of other particle sizes, i.e. <0.025 inch and >0.5 inch (−28-mesh and +½ inch), for other applications or to achieve other properties of an explosive mixture.

The third component can be blended in concentrations up to 80%, more specifically 5% to 50%, of the total weight of the mixture. As has been observed, the blend ratio of the third component can be altered to vary the VOD of the explosive mixture to better match the estimated natural resonant frequency of the material being blasted.

The mechanism of performance of this unique third component is worthy of further discussion. As previously mentioned, ANFO is composed of a stoichiometric blend of ammonium nitrate (94%) and fuel oil (diesel fuel; 6%). In blasting, the term “stoichiometric” blend is commonly referred to as a “zero oxygen balance” blend, which is defined as the point at which a mixture has sufficient oxygen to completely oxidize all fuels it contains without excess oxygen to react with nitrogen to produce nitrogen oxides. As the system is moved away from its “zero oxygen balance” (stoichiometric) state, energy is lost and by-product gases (CO, NOx) are formed. For example:¹³

“Stoichiometric” ANFO (˜94.5% AN+˜5.5% FO):

3NH₄NO₃+(CH₂)→7H₂O+CO₂+3N₂ 0.93 kcal/g

“Over-fueled” ANFO (92.0% AN+8.0% FO):

2NH₄NO₃+(CH₂)→5H₂O+CO+2N₂ 0.81 kcal/g

“Under-fueled” ANFO (96.6% AN+3.4% FO):

5NH₄NO₃+(CH₂)→11H₂O+CO₂+4N₂+2NO 0.60 kcal/g

This sheds light on the role of the third component, and other hydrocarbon fuel additives, in an explosive mixture. Clearly, adding more hydrocarbon fuel to the system beyond its zero oxygen balance quantity decreases the per unit energy, as illustrated above. Based on its composition, approximately 10% of the third component is all that is required to satisfy the hydrogen (H) and carbon (C) requirements to provide the appropriate stoichiometry, and that is in the complete absence of fuel oil, i.e. 0% FO. Thus, this “combustion” model is inconsistent with the ability to add more than 25% of the third component to ANFO without critically compromising the performance of the explosive mixture. If the third component is behaving as a “hydrocarbon fuel,” the results observed by incorporating it should be even more clearly observed by simply over-fueling the ANFO blend, considering the increased combustibility of fuel oil compared to that of coal. In addition, the brilliant post-detonation fireball of mixtures containing a high concentration of the additive (25% to 50+%) is not easily explained. These fireballs are observed with mixtures including the third component with 6% fuel oil relative to AN (not with 4% fuel oil), but the composition of the additive more than satisfies the H and C requirements for combustion. And although only observed as “heat-waves” with visual imaging, the muck pile often appears to emanate flames for minutes after the shot when recorded and viewed with a thermal imaging (infrared) camera. These observations are not consistent with a model that consists simply of the addition of more “hydrocarbon fuel” to an explosive mixture.

Rather than the additive undergoing typical hydrocarbon combustion (“CH₂”+3/2O₂→CO₂+H₂O), the conditions and results support a “gasification” model, wherein the carbon (as char derived from the third component) reacts with water at high temperature and pressure to produce a mixture of hydrogen and carbon monoxide, referred to as “syngas” when produced commercially (C+H₂O→H₂+CO). Exploited industrially for over a century, the gasification process is well-established, and details for its role in explosives are provided herein.

In a general sense, blastholes can be considered “in situ gasifiers,” as they contain the requisite components and conditions to efficiently produce syngas. These include:

• • 1. A carbonaceous material (third component in mixture);
  • 2. Water (typically >10% in the third component, also a product of ANFO combustion);
  • 3. Heat (˜1200° C. for ANFO blasts); and
  • 4. High pressure (10⁵-10⁶ psi for ANFO blasts).

Although such incredible pressures are not strictly necessary for gasification, they typically result in more efficient gasification. Relatively low pressures and slow heating rates encountered in the lab help to explain why establishing significant differences in behavior between the third component and other, comparable materials was difficult. That is, suitable laboratory thermal analyses (TGA, TG-MS, Direct Insertion Probe/DIP-MS, DSC, distillations) are generally performed at atmospheric pressure or under reduced pressure (vacuum), as is conventional or necessary.

As part of the gasification model, it is important to clearly communicate that additional oxygen (O₂) to maintain proper stoichiometry is not necessary as it is with combustion. Instead, the heat, pressure, and water from the detonation are sufficient to effect gasification, a process entirely independent of oxygen balance. And although all carbonaceous materials are capable of gasifying, the third component described herein is exceptionally well-suited, based on its physical and chemical properties. Specifically:

• • Surface Area—The mesopore surface area of the base coal is uniquely high, and the devolatilization process is accompanied by a large growth of D-R CO₂ (micropore) surface area. Active surface area is proportional to reactivity.
  • Rapid Devolatilization—Visual observation of devolatilization and SEM analysis showed that volatiles are released at a much faster rate compared to other coals. As previously noted, although not highly indicated in combustion kinetics, devolatilization rate is of fundamental importance in gasification. That is, rapid devolatilization leads to rapid char formation, allowing for rapid gasification.
  • Mineral Matter—The abundance and fine dispersion of potassium (K) and iron (Fe) containing clays, illite and chamosite, within the carbon matter is expected to have a catalytic effect, thus contributing to increased reactivity.
  • High vitrinite content—After devolatilization, vitrinite is known to form porous, thin carbon-wall char particles, providing substantially higher surface areas, and correspondingly high rates of reactivity.
  • Disordered Carbon—The high proportion of disordered carbon structure favors high reactivity.

As mentioned prior, correlation is difficult to confirm in lab due to temperature, pressure, and reaction-time limitations. Put simply, it is not feasible to recreate blast conditions in a lab. However, the heat and pressure appears sufficient for efficient gasification in a blasthole. As such, the production of hot hydrogen gas readily reacts when mixed with atmospheric oxygen, resulting in the post-detonation fireball, as well as generating the clean-burning flames often seen, via thermal video imaging, emanating from the muckpile. Irrespective of the fireball, additional pressure is generated when the carbon char, a condensed phase of matter, reacts with water to generate CO and H₂ gases.

Not only is the gasification process well-established, the liberation of H₂ from H₂O has an analogy in explosives that has been exploited for decades, namely in “aluminized” products. The addition of aluminum to an explosive formulation can greatly increase the energy, with a maximum being reached at 18-25% Al. This, too, is independent of AN to FO oxygen balance.¹⁴

The oxidation of aluminum is highly exothermic, as outlined below:

2Al(s)+1½O₂(g)→Al₂O₃(s)ΔH_c=−1590 kJ

More pertinent to this invention, aluminum will also react with water, a product of combustion, to liberate hydrogen gas. This is particularly relevant in oxygen deficient environments, including ANFO blends that are properly oxygen balanced (i.e., no excess O₂):

2Al(s)+3H₂O(g)→3H₂(g)+Al₂O₃(s)ΔH_c=−866 kJ

This, too, is an exothermic process, but of greater significance herein is the production of hydrogen gas, which, at the high temperatures of a blast, is likely to combust when exposed to atmospheric oxygen. With respect to the role of water in explosives, it has also been stated that, due to this chemistry, water is effectively a very powerful explosive ingredient in explosives containing aluminum.¹⁵

Although the liberation of hydrogen via gasification is an endothermic process for carbonaceous materials (vs. exothermic for Al), there is much additional thermal energy in a blast to allow for the generation of H₂ and CO gases from the third component and water, thus creating additional gas pressure. That is, excess thermal energy in the blast is initially consumed, but is subsequently reintroduced as additional work via gas generation and hydrogen combustion.

The analogy between the third component and aluminum as it relates to hydrogen generation is strengthened by the observation of the post-detonation fireballs. Although other volatile hydrocarbon could be responsible for this combustion event, hydrogen is most probable due to its wide range in flammability limits and low combustibility (ignition) energy required. Lower and Upper Flammability Limits (LFL-UFL, %) for hydrogen are included below, with other flammable materials for comparison:

• • Hydrogen: 4-75% Δ=71
  • Methane: 5-17% Δ=12%
  • Propane: 2-11% Δ=9
  • Gasoline: 1-6% Δ=5

Several additional field observations are consistent with the gasification mechanism. A reduction in performance and heat generation is observed when the third component was dried, indicating the role of inherent moisture. Thus, it would appear that product moisture from the ANFO combustion is insufficient, or is insufficiently coupled with the carbonaceous additive to effect adequate gasification. Intimate water contact has been connected to efficient gasification.¹⁶

In the test series described as the “Example,” H₂ and CO were detected on shots containing the third component additive. Although generation of CO may not seem favorable in this application, it helps to ensure a shift away from the generation of NOx, which has considerably more severe health concerns. It should be noted that the CO dispersed before reaching the breathing zone, the measurements were always below the 400 ppm threshold limit value, and amounts entering the operator's cab of the D-11R dozer were negligible. CO was not detected following the associated standard ANFO blasts, indicating no gasification.

Another practical observation implicating gasification is the “heat” observed on the muckpile after a shot with the present mixture, which is consistent with hydrogen gas burning. Although a standard video camera merely indicates heat radiating from the muck, infrared (IR) video footage clearly shows flames emanating from the muckpile, similar to a bonfire burning. This visually clear flame appears to be a consequence of H₂ gas seeping through the muck and burning upon exposure to atmospheric oxygen.

Example

Explosive mixtures containing varying concentrations of ammonium nitrate, fuel oil, emulsion, and the previously described carbonaceous additive have been successfully tested. As an illustrative example, an independent engineering firm was commissioned to complete a comprehensive study examining the effects of this additive in ANFO relative to standard ANFO (94% ammonium nitrate prill and 6% fuel oil). The field assessment program included six production blasts—three trials with approximately 25% (24%, 28%, & 33%) of the carbonaceous additive and three shots using standard ANFO. The shots alternated between the claimed mixture and standard ANFO to produce pairs of blasts in an attempt to compare similar geologic conditions (strata, depth, etc.) within each pairing. Among other metrics, the claimed mixture and ANFO shots were compared based on the following:

• • Velocity of Detonation (VOD, described in more detail below).
  • Seismic output in the far field (ground vibrations)
  • Temperatures produced by the blast.
  • Material displacement (casting effectiveness).
  • Fragmentation (size distribution of the material on the surface of the blasted overburden).

The field assessment was conducted in an active coal mine. In order to maintain production output, blast conditions sometimes varied within and between the blast-pairs of the claimed mixture and ANFO. Several factors can affect cast performance and fragmentation including blasthole pattern, pattern width, powder factor, stemming, amount of toe, additive characteristics, and the additive-to-ANFO mix ratio. Various combinations of these factors were encountered in the field programs. The factors described above in the second blast-pair (Explosive Mixture vs. Standard ANFO) were the most comparable of the blast-pairs. The blast pattern, number of holes, average borehole depth, BCY/Blast, total explosives, powder factor, and amount of toe were approximately the same.

In general, the following was observed:

• • 1. VOD is the rate at which the detonation/combustion wave travels through an explosive product. It is an important indicator of the overall energy or power of detonation, and in particular for the brisance or shattering effect of an explosive. As expected, the VOD for the claimed mixture was lower than for ANFO, and it correlates well with the seismic energy findings, described in more detail below. The ability to “dial-in” the detonation velocity with the claimed mixture, over a limited but meaningful range, through variation of the concentration of the third component is an important feature of the system and has been studied extensively. The composite VOD averages for each of the claimed mixture and ANFO blasts were as follows:

	Regression Velocity of Detonation (Feet/Second)
	Resistance	Time Domain
	Wire	Reflectometry
Claimed Mixture (Ave)	12,202	11,882
ANFO (Ave)	14,308	14,222*
*Includes data from only one hole

• • 2. Seismic effects can be reduced by the use of the claimed mixture, likely because of the lower velocity of detonation and corresponding reduction in seismic energy density. As noted above, the ability to customize the mixture so that the detonation velocity can be “dialed-in” offers the possibility of minimizing the seismic effects without the use of decked charges or other means. The average seismic energy density for the blasts of the claimed mixture is approximately 15% lower than the ANFO blasts. The difference in average detonation velocity was also approximately 15%, which affirms a correlation between the two. The significance is that the VOD of the claimed mixture can be varied (by the additive concentration) and hence, within limits, seismic energy can be controlled.
  • 3. Thermal imaging data shows peak temperatures in the post-detonation fireball of the claimed mixture of over 1500° Fahrenheit (815° C.). The fireball appears approximately two seconds after initial detonation, and significant heat can be observed emanating from the muck pile for minutes following the blast. It is hypothesized that in the highwall blast, some burning immediately following detonation contributed to the cast energy.
  • 4. The claimed mixture is capable of casting performance equivalent to ANFO. In the second blast-pair, the claimed mixture cast 15,433 cubic yards, while the ANFO displaced 15,925 yards. The difference in percent cast was only 1.4%. Cast distance was also comparable; both had an average length of 202 feet.
  • 5. The claimed mixture produces satisfactory fragmentation. The claimed mixture outperformed ANFO in the second blast-pair; the D50 measurements for Claimed Mixture and Standard ANFO were 9.94″ and 10.80″, respectively. None of the shots containing the additive required any form of secondary blasting for oversized boulders. Blasting with the claimed mixture also resulted in a lower swell factor. The data for this series indicated an average of about 3% less than ANFO. Theoretically, lower bank swell provides better digging characteristics and results in better bucket-fill and loading efficiencies, thus improving truck cycle times.

Subsequent to the comprehensive study summarized above, other positive effects of this mixture have been observed. Notably, both field and laboratory tests consistently indicate a reduction in the generation of orange smoke, attributed to the generation of NOx, than the base explosive formulations without the claimed additive, i.e. standard AN-based explosives.

REFERENCES

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• ² Hodge, E., Roberts, D., Harris, D., Stubington, J., “The Importance of Char Structure in Determining High Temperature, High Pressure Gasification Rates,” Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem., 2009, Vol. 54, pp. 408-409.
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• ⁴ Sharma, A., Kawashima, H., Saito, I., Takanohashi, T., “Structural Characteristics and Gasification Reactivity of Chars Prepared from K₂CO₃ Mixed HyperCoals and Coals,” Energy Fuels, 2009, Vol. 23, pp. 1888-1895.
• ⁵ Mastalerz, M., Drobniak, A., Strapoć, D., Acosta, W. S., Rupp, J., “Variations in Pore Characteristics in High Volatile Bituminous Coals: Implications for Coal Bed Gas Content,” International Journal of Coal Geology, 2008, Vol. 76, pp. 205-216.
• ⁶ Maldonado-Hódar, F. J., Rivera-Utrilla, J., Mastral-Lamarca, A. M., “Influence and Modification of the Porous Texture of Coals during Hydrogenation,” Fuel, 1995, Vol. 74, pp. 823-829.
• ⁷ Senel, I. G., Gürüz, A. G., Yücel, H., Kandas, A. W., Sarofim, A. F., “Characterization of Pore Structure of Turkish Coals,” Energy Fuels, 2001, Vol. 15, pp. 331-338.
• ⁸ Makoto, N., Masaomi, T., Shigeyuki, U., Toshinori, K., C, L., “Influence of Heating Rate during Pyrolysis on Gasification Reactivity of Coal Chars at High Temperatures,” Journal of the Japan Institute of Energy, 2000, Vol. 79, pp. 1078-1087.
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• ¹¹ Budarin, V., Clark, J. H., Luque, R., White, R., “Starbons®: Cooking Up Nanostructured Mesoporous Materials,” Material Matters, 2009, Vol. 4, pp. 19-22.
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• ¹³ Illinois Blasters Training Manual, IL-DNR, 2007, p. 1.
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Claims

1. An explosive mixture comprising:

an oxidizing agent, and

a carbonaceous material.

2. The mixture according to claim 1, wherein the oxidizing agent may be present as, or contained within, a solid or liquid, including solutions, suspensions, or emulsions.

3. The mixture according to claim 2, wherein the oxidizing agent is ammonium nitrate.

4. The mixture according to claim 1, further comprising a liquid fuel component.

5. The mixture according to claim 4, wherein the liquid fuel component is a fuel oil or diesel fuel.

6. The mixture according to claim 4, wherein the amount of the liquid fuel component is 0-10% by weight.

7. The mixture according to claim 1, wherein said carbonaceous material is characterized as being readily gasified, as indicated by its rapid release of volatile materials and rapid char formation, high internal porosity and surface area of the base material and its char, and uniform distribution of catalytic mineral matter throughout the material.

8. The mixture according to claim 1, wherein said carbonaceous material is present in the amount of 5% to 80% by weight.

9. The mixture according to claim 1, wherein said carbonaceous material has a particle size such that it displaces the oxidizing agent in solid mixtures.

10. An explosive mixture comprising:

an oxidizing agent,

a liquid fuel component, and

a carbonaceous material

wherein the oxidizing agent may be present as, or contained within, a solid or liquid, including solutions, suspensions, or emulsions, wherein the liquid fuel component is present up 10% by weight, and wherein the carbonaceous material is coal which is characterized as being readily gasified.

11. The mixture according to claim 10, wherein the oxidizing agent is ammonium nitrate.

12. The mixture according to claim 10, wherein the liquid fuel component is a fuel oil or diesel fuel.

13. The mixture according to claim 10, wherein the carbonaceous material is coal which is characterized as being readily gasified, as indicated by its rapid release of volatile materials and rapid char formation, high internal porosity and surface area of the base material and its char, and uniform distribution of catalytic mineral matter throughout the material.

14. The mixture according to claim 10, wherein said carbonaceous material is present in the amount of 5% to 80% by weight.

15. The mixture according to claim 10, wherein said carbonaceous material has a particle size such that it displaces the oxidizing agent in solid mixtures.

16. An explosive mixture comprising an oxidizing agent, a liquid fuel component, and a carbonaceous material, wherein the oxidizing agent is ammonium nitrate, wherein the liquid fuel component is a fuel oil or diesel fuel, and wherein the carbonaceous material is coal which is characterized as being readily gasified as indicated by its rapid release of volatile materials and rapid char formation, high internal porosity and surface area of the base material and its char, and uniform distribution of catalytic mineral matter throughout the material, and wherein said carbonaceous material has a particle size great enough such that it displaces the oxidizing agent in solid mixtures.