Patent Application
20110290386
The present invention refers to the development of a low density granular Oxidizing Agent whose ingredients do not segregate and that when mixed with combustible elements, is transformed into a low density granular Blasting Agent suitable for using in open pit and underground mining applications in order to control rock fragmentation and damage.
The most used explosives in the mining industry contain ammonium nitrate (AN) as their main oxidizing ingredient due to its low cost, safe handling and ease of transportation and storage.
Mixtures of prilled ammonium nitrate and fuel oil, commonly named ANFO, are widely used in open pit and underground blasting operations. ANFO, in turn, has been mixed with other industrial explosives such as “explosive emulsions” (to produce the so called Heavy ANFOs) in order to modify the performance of the resulting mixture and adapt it to the operation requirements.
Damage control in blasting environments requires in many operational instances the use of explosives developing lower energies than the one liberated by the above-mentioned industrial products, being density reduction by the addition of a diluent agent one of the most accepted practices for the purpose of reducing the energy.
The density of an explosive mixture plays a very important role in the energy and pressure delivered during detonation and consequently, in the results of rock fragmentation and/or wall control; reason why a great effort in time and resources has been placed to reduce the density of explosive mixtures, particularly mixtures comprising ANFO prills.
A typical option to reduce density of prilled ANFO is to mix it with diluents such as sawdust, bagasse, silica or plastic microbubbles, volcanic rock, rice hulls, expanded polystyrene and a series of similar products. There is a limit below which it is not convenient to dilute a mixture containing ANFO due to the risks of detonation failure; said limit shall depend in part on the type of diluent used. All these mixtures suffer as well from a serious problem: the unavoidable segregation of the diluent ingredients while they are being mixed and/or loaded into the hole, as a result of the difference in specific weights between ANFO and the diluent itself, which only increases the risk of detonation failures.
Some examples of low-density explosive mixtures developed for damage control are mentioned below.
Patent 246 457 to Gotz et al., published in East Germany on Jun. 10, 1987, introduces an explosive mixture comprising 10% to 80% by volume of ammonium nitrate treated with 0% to 1% by volume of diesel oil and 90% to 10% by volume of foamed polystyrene.
U.S. Pat. No. 4,957,569 issued on Sep. 8, 1990 in the name of Waldock, introduces an explosive emulsion that comprises various salts in the oil-dispersed phase, to which low density diluents such as expanded polystyrene, sawdust, perlite and vermiculite, have been added.
U.S. Pat. No. 6,955,731 B2 issued in the name of WALDOCK refers to mixtures of an emulsion and ANFO known in the art as Heavy ANFOs, to which rice hulls have been added in order to reduce density of the mixture and increase sensitivity of the explosive.
Patent 1,601,972, published in Russia on Jul. 9, 1995, introduces an explosive mixture that contains ammonium nitrate and polystyrene, the latter being expanded in a hot aqueous solution of ethylene glycol as an inherent part of the manufacturing process.
U.S. Pat. No. 6,425,965 B1 of Jul. 30, 2002 of G. Silva, includes a novel concept by which a reactive nature is conferred to a diluent agent without losing its low density characteristics, thus allowing its use as a reactive diluent agent in the typical explosives employed in blasting operations, particularly of ANFO in prilled form.
The present invention may be summarized as the development of a low density granular Oxidizing Agent (PAN) to which explosive properties are conferred through the simple mixture thereof with combustible ingredients, thereby producing a low density granular Blasting Agent (PANFO) having many applications in the mining industry.
The source of the oxidizing ingredient consists in a concentrated aqueous solution of ammonium nitrate, which is absorbed by expanded perlite granules, an inert mineral of a very low density and with a high liquid absorption and retention capacity.
In a first manufacturing stage, the expanded perlite is soaked in the ammonium nitrate solution until ensuring that all its pores are completely saturated with said solution. Precipitation of the ammonium nitrate crystals in the perlite shall start as a result of the solution cooling below its saturation temperature and by the evaporation process of water from the solution. The granular product that we have named PAN shall be obtained upon precipitation of said crystals within the pores and on the surface of perlite and when residual moisture has been removed.
Thus, PAN consists of an oxidizing salt mainly contained within the pores of inert material granules. As such, for handling, storage and transportation purposes, PAN should be classified as an Oxidizing Agent, in the same way as prilled ammonium nitrate (AN) used for manufacturing ANFO.
PAN granules may conveniently be treated with typical combustible ingredients such as gas oil, fuel oil, mineral or vegetable oils and others, to originate the product we have named PANFO, a low density granular explosive product that for regulatory purposes should be classified as a Blasting Agent, in the same way as prilled ANFO.
The reactive nature and low density of PANFO makes it an ideal agent to be used by itself, in blasting operations requiring low energy products, or else as a diluent in mixtures with ANFO and explosive emulsions in order to reduce density and detonation pressure in the resulting mixture, both these parameters having the greatest importance for fragmentation and damage control.
When used as a diluent agent of ANFO, the reactive nature of PANFO allows dilution of the mixture to values that are not possible with other diluents without running the risk of detonation failure, whether the latter is a result of requiring a higher degree of dilution, the unavoidable segregation of the selected diluent or of a combination of the above two. In other words, the use of PANFO as a diluent added to ANFO allows working in a density range starting from ANFO's nominal density (850 kg/m3) to the density of PANFO itself (250-300 kg/m3) without any risk of generating detonation failures due to an excessive dilution and/or segregation of the components.
The main purpose of the present invention is the development of PANFO, a low density granular explosive that for handling, storing and transportation purposes should be classified as a Blasting Agent. The manufacture of PANFO would comprise two stages, an initial stage where two ingredients are mixed to form PAN, followed by a second stage that would comprise the addition to PAN, preferably at the moment of unloading it into the blastholes, of a third ingredient which would give origin to PANFO.
The three preferred basic components of PANFO consist then in expanded perlite particles, of an oxidizing ammonium nitrate aqueous solution and of a liquid combustible of the fuel oil type. Perlite is an inert material obtained from siliceous rocks, its more distinctive characteristic with respect to similar volcanic rocks being its capacity of being expanded when subjected to high temperatures, whereby it will become an extremely light and absorbent product. Owing to its high absorption capacity, perlite is commonly used to control spills (oil, water), deodorize liquid effluents and in its granular form, as a carrier for pesticides, herbicides and similar liquid substances.
In the present invention, expanded perlite is used in granular form, with a preferred particle size between 2 mm and 100 mm with densities going between 50 kg/m3 and 350 kg/m3, its preferred density being about 100 kg/cm3, because at said value perlite preserves an adequate liquid absorption and retention capacity while maintaining sufficient mechanic strength.
From the various Oxidizing Agents, ammonium nitrate is the most used in the explosives industry for its cost, safety and availability. The ammonium nitrate aqueous solution typically used during prill manufacture is a 96% concentrated solution with a saturation temperature of 125° C. However, solutions at a lower concentration and saturation temperature are transferred and used for different purposes, such as the case of solutions having 83% by weight ammonium nitrate with a saturation temperature that fluctuates about 65° C. This last concentration has proved convenient in PAN manufacture due to its relatively high ammonium nitrate contents and to its low saturation temperature that makes handling thereof easier during operations. In the present invention, it is convenient to work with an oversaturated oxidizing solution at high temperatures, so as to ensure the existence of sufficient ammonium nitrate crystals to fill all the pores of the expanded perlite, increase crystallization percentage by cooling and as a result, reduce percentage of the crystallization generated by evaporation of water from the solution.
The absorption of the oxidizing solution in perlite occurs practically upon contact. Crystallization of the ammonium nitrate salts within perlite porous spaces shall occur as a result of the natural cooling process and the evaporation of water from the solution. The more concentrated the solution, more crystals will be precipitated by cooling and less water will be necessary to evaporate in order to complete crystallization and drying. The ammonium nitrate crystals thus formed will remain trapped within perlite porosities and in a thin surface layer without any possibility of being segregated unless physical destruction of the assembly is induced. Perlite acts then as a carrier for ammonium nitrate salts giving rise to PAN, a granular product whose final density shall depend to a great extent on the porosity degree of the perlite used.
To reduce crystallization of ammonium nitrate on perlite surface and prevent the generation of a thicker coating layer that increases PAN density, it is convenient to facilitate drainage of the excess solution, by excess being understood that amount that exceeds the retention capacity of perlite granules. To this effect it is recommended to work with an oversaturated solution at a temperature several degrees above the saturation temperature that corresponds to concentration of the solution. Said oversaturated solution may or may not have additional dissolved salts such as sodium nitrate and calcium nitrate. Once the solution has been absorbed, NA crystals have precipitated and the granular mixture has been dried, density will be increased from the approximately 100 kg/m3 which correspond to the untreated perlite to about 250-320 kg/m3 corresponding to PAN.
Due to the inert composition of perlite, PAN should be classified for storage, handling and transport purposes as an Oxidizing Agent. Furthermore, after adding the combustible ingredient that would give origin to PANFO, this should be classified in turn as a Blasting Agent, in the same way as standard prilled ANFO.
Due to fewer storage, handling and transportation restrictions associated with PAN, it is convenient that the addition of combustible required to transform it into PANFO, be conducted moments before it is downloaded into the blastholes, in a similar fashion as with the ammonium nitrate prills used to produce ANFO.
There are two ways of reducing PAN or PANFO density, namely: using a less porous perlite or else using a more diluted ammonium nitrate solution such that not sufficient crystals form within the perlite pores. However, the use of oversaturated solutions is preferred so that crystallization is mainly produced by a cooling action and not by evaporation, thus reducing drying times and making the manufacturing stage easier.
Liquid combustibles typically used in industry, such as fuel oil or gas oil are recommended; however any other liquid combustible that may be absorbed by PAN or dissolved in the ammonium nitrate aqueous solution would also be adequate. For example, it has experimentally been confirmed that dissolution of sugar in the oxidizing solution of ammonium nitrate would give rise to precipitation of both crystals in perlite. Although feasible, said alternative is not advantageous because in so doing we will end up working with a Blasting Agent instead of with an Oxidizing Agent such as PAN.
Addition of the combustible agent to PAN must be carried out at a weight ratio so to produce a resulting PANFO balanced in oxygen. For that purpose approximately 6% of the weight corresponding to precipitated ammonium nitrate crystals must be absorbed through the pores. Due to the high absorption capacity of liquid combustible characterizing perlite, part of said percentage may not be in an intimate contact with the ammonium nitrate crystals, thus tending to promote formation of toxic nitrogen gases. If possible, it is recommended to carry out trials with the available products in order to determine the fuel percentage that will prevent a development of said highly toxic gases. If it is not possible to carry out trials, it is recommended to increase combustible contents to 6% of the total weight, that is the weight of PAN and not only of the nitrate crystals; this will result in a PANFO that is slightly richer in combustible but without the capacity of generating the undesirable nitrogen gases.
It is convenient to proceed with a drying stage of PAN, not only to ensure crystal precipitation but also to remove residual moisture that may later affect PANFO initiation sensitivity and performance. Of the several types of dryers, the most efficient for treating said granular product are fluidized bed dryers that due to the excellent contact between air and PAN particles offers the best possible energetic exchange for moisture removal. However, PANFO irregular grain size distribution may reduce their efficiency and make them susceptible of generating an uneven drying. Belt dryers with countercurrent hot air flows are another valid alternative.
Neither PAN nor PANFO have resistance to the damaging effects of water dissolving the ammonium nitrate crystals and desensitizing the product. However, there are ways of conferring a certain water resistance to the product, the most practical being mixing PANFO with an explosive emulsion at a ratio that ensures an adequate protection. Said ratio shall be a function of the degree of protection sought, and will have to be established in the field. Furthermore, the addition of certain patented products based on guar gums and other powdery components, such as that used in WR-ANFO (registered trademark) that consists of standard ANFO mixed with guar gum), would also provide a certain degree of protection from water damage.
If necessary, PANFO may be mixed with high explosive fines to provide higher initiation sensitivity and a performance that complies with user's requirements. Said compounds include pentaerythrol tetraamine (PETN), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), trinitrotoluene (TNT), nitroguanidine, cyclotetramethylene tetranitramine (HMX), as well as the typical products known in the mining industry, particularly prilled ANFO and explosive emulsions.
An example of a two-stage manufacturing process of PANFO will be described next. Manufacturing of PAN followed by drying, and Manufacturing of PANFO as the final result For the first stage, a granular expanded perlite having a 3 mm to 5 mm average particle size and a density of 0.1 g/cm3, was mixed with a 96% ammonium nitrate concentrated solution at 125° C. saturation temperature. Once the solution was absorbed by the perlite, the excess of saturated ammonium nitrate solution was drained. The resulting wet PAN granules were dried using a countercurrent hot air flow belt dryer. The final bulk density of PAN was measured at 0.32 g/cm3.
In the second stage, PAN was treated by adding fuel oil in an amount equivalent to a 6% weight basis of the ammonium nitrate contained within PAN, resulting in PANFO having a density of 0.37 g/cm3. The mixing process of PAN and fuel oil was done right before loading it into the blastholes.
The velocity of detonation (VOD) was measured, resulting in a value of 1,937 m/sec. Comparing this result to the 3.500 to 5.500 m/sec VOD values produced by typical commercial explosives used in the industry, a substantial and highly convenient lower value is obtained.
In addition to the example previously described, a number of experimental tests were conducted, whose results clearly show the potential the product has for optimizing blasting operations in the mining industry.
The following table summarizes VOD testing performed on PANFO prepared using 100 kg/m3 expanded perlite (0.1 g/cm3), ammonium nitrate (AN) solution and approximately 6% to 10% fuel oil, at various densities, diameters and initiation conditions.
In reference to the above experimental results, the following comments are relevant:
1. Velocity of Detonation (VOD) records fall within a 1500 to 1900 m/sec range value, considerably lower than those generated by commercial explosives commonly used in industry, (most typically between 3.500 to 5.500 m/sec).
2. Tests #13, 14 and 15, with a 6% fuel oil content (corresponding to an oxygen-balanced mixture), showed a better performance than when a 10% fuel oil content was used, as in Tests #7 and 8. Although these are preliminary results, they indicate that it would not be necessary to use a greater percentage of fuel oil to compensate for its eventual absorption within the perlite.
3. It is possible to detonate PANFO in 50 mm diameters at a density of 0.20 g/cm3 using a 0.10 g/cm3 perlite. However, in Test #5, the observed velocity of detonation (VOD), both its numeric value (1230 m/sec) and its depicted record, indicated a marginal detonation front propagation. For this reason, to ensure a proper detonation propagation it is deemed convenient to manufacture PANFO at densities greater than 0.25 g/cm3, at least when it is manufactured with a perlite having a density greater than 0.10 g/cm3.
4. According to Test #4, the fuel oil used as the combustible ingredient may be replaced by sugars dissolved in the ammonium nitrate solution. However, this implies premixing the ingredients until obtaining a product (PAN+Sugar) that would classified as a Blasting Agent and not as an Oxidizing Agent (PAN), which would hinder logistics with regard to handling, transportation and storage thereof.
5. According to Test #6, ammonium nitrate may be replaced by urea, however, the VOD record obtained indicated a partial and delayed detonation of the mixture in spite of having been conducted in a 100 mm diameter at 0.36 g/cm3 density. This means that when mixed with said saline solution, the resulting product has low sensitivity to initiation and is prone to a detonation failure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 278-2009 | Feb 2009 | CL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CL09/00015 | 9/21/2009 | WO | 00 | 8/4/2011 |
