Patent Application
20060272756
The present invention relates generally to explosives and processes for manufacturing such explosives.
The explosive hexahydro-1,3,5-trinitro-s-triazine is often referred to as RDX. Two processes have been used commercially to manufacture RDX. The first is direct nitration, which produces type A RDX. In this process, hexamethylenetetramine is reacted with nitric acid at 30° C. or less. The direct nitration method is not widely used today for economic reasons. The second process, known as the Bachmann process, is currently the most common method used to manufacture RDX. It produces type B RDX. In this process, hexamethylenetetramine is reacted with nitric acid in the presence of ammonium nitrate and acetic anhydride.
The primary difference between the two types of RDX is that type A is essentially pure, while type B is contaminated with HMX. However, for practical purposes, both processes yield RDX of essentially identical utility. In both processes, the raw RDX is further purified and the crystal morphology is modified by recrystallization.
RDX is commonly manufactured in a wide range of particle sizes (grades) from 25 microns to 600 microns in diameter by recrystallization. Recrystallized RDX may also be ground, for example in a fluid energy mill, to obtain finer particles ranging from 2 to 25 microns in diameter. However, all RDX commercially produced today consists of orthorhombic crystals with a density in the range of 1.80-1.82 grams/cm3. This form of RDX has been designated as the α polymorph, or RDX(I). The exact crystal density of a given lot of RDX is a function of purity (i.e., HMX content), and the absence or presence of crystal defects and inclusions.
A β polymorph of RDX has been reported in the literature. The stability of β-RDX is unknown and no measurements of physical properties or sensitivity have been reported other than the crystal morphology is dendritic.
RDX is an explosive material and therefore is used in a variety of applications in which controlled explosions are useful. In these applications, it is necessary to initiate the detonation of the RDX, and of course it is important to do so in a safe way.
A slapper detonator is a device that offers a relatively high degree of initiation safety. Slapper detonators function by rapidly discharging voltage through a low inductance circuit. The circuit comprises a high-voltage spark gap switch (typically 500-3,500 volts), a high-voltage low-inductance capacitor (typically 500-3,500 volts and 0.1-0.2 μF), and an exploding-foil initiator (EFI) bridge. The entire circuit inductance is typically 20-50 nH, and sometimes less (1-20 nH). Discharging such a circuit causes a current of several thousand amperes to flow through the EFI bridge, which in turn causes the EFI bridge to explode. The exploding bridge then accelerates a polymeric flyer (typically a thin polyamide film) across a short gap, where it slaps a pellet of a secondary explosive, causing the secondary explosive to detonate.
Many explosives have been detonated in a laboratory setting by slapper detonators, such as HNS, PETN, CL-20, TNT, RDX, HMX, and various formulations made from such explosives. However, such laboratory initiation systems typically function at high voltages with large capacitors and discharge energies of 250 mJ to 1,225 mJ. Such systems are generally unsuitable for use outside the laboratory. To be useful outside the laboratory, experience has shown that it is desirable to significantly reduce the firing voltage and capacitor size (firing energy) of the circuit. While this can be accomplished to some degree by designing the electrical firing circuit to be more efficient, ultimately the minimum firing energy is controlled by the sensitivity of the explosive.
The current state of the art is the low-energy foil initiator (LEFI) These devices typically function with firing energies below 100 mJ. To this end, explosives have been developed that have fine particle size and high surface area, such as HNS-IV, PETN, and CL-20, which can be initiated with less than 100 mJ. However, each of these explosives has significant problems. HNS-IV is difficult to manufacture and purify, and therefore is expensive. PETN has excellent sensitivity and an acceptable price, but has marginal thermal stability for non-laboratory applications. CL-20 is expensive and cannot be recrystallized to a very small particle size. It is therefore just barely sensitive enough for a LEFI application.
There is a need for new explosive materials that can be initiated by LEFI devices, and that overcome at least some of the above-described problems.
One aspect of the present invention is a process for making an explosive. The process comprises dissolving RDX in a volume of a first solvent to form a first solution, and adding a second solvent to the first solution. The second solvent is miscible with the first solvent, but RDX is soluble in the second solvent to an extent no greater than 1 g RDX/100 g of the second solvent. RDX crystals are precipitated and can be recovered.
Another aspect of the invention is an explosive prepared by the above process. The explosive comprises primarily RDX, but can also contain smaller amounts of other materials such as HMX.
Another aspect of the invention is RDX crystals having a crystal density of less than 1.80 g/cm3.
The present invention relates to a novel form of RDX that can be used in the perforation of well casing, among other applications.
The production of the novel form of RDX begins with a particulate RDX composition. This starting composition contains primarily RDX (e.g., at least about 90 wt % RDX on a dry solids basis, and in some embodiments at least about 99 wt % RDX), but it can also contain smaller amounts of other explosive or non-explosive substances, such as HMX. Type B RDX is one suitable starting material.
The RDX is dissolved in a first solvent to form a first solution. RDX should be soluble in this first solvent to an extent of greater than 1 g RDX/100 g solvent. In various embodiments of the invention, the solubility of RDX in the first solvent is greater than 5 g/100 g, 10 g/100 g, or 25 g/100 g. (All solubility figures in this patent are at room temperature unless otherwise stated.) The concentration of RDX in the solution will generally be about 1-50 wt %, although higher or lower concentrations can be used in some situations.
The first solvent will typically be an organic solvent, for example one having about 2-10 carbon atoms. Ketones are one group of suitable solvents. Specific examples of suitable first solvents include acetone, dimethylsulfoxide, and dimethylformamide.
A second solvent is then added to the solution, in order to cause “crash” precipitation of RDX particles. The second solvent is miscible with the first solvent, but RDX is much less soluble in the second solvent than in the first solvent. In various embodiments of the invention, RDX is soluble in the second solvent to an extent no greater than 1 g of RDX/100 g of the second solvent, or in some cases no greater than 0.1 g/100 g. Suitable examples of second solvents include water and various dilute aqueous solutions.
The second solvent can be added in an excess compared to the volume of the first solvent in the solution. For example, the second solvent can be added in a volume that is about 2-10 times greater than the volume of the first solvent. Even more of the second solvent can be used, although it may be economically undesirable in many cases. In contrast, if the amount of the second solvent used is too small, the resulting crystals will not have the desired properties and will not function as an EFI explosive. The solution can be agitated during and/or after the addition of the second solvent.
The addition of the second solvent will cause precipitation of RDX particles. The particles can be recovered, for example by filtration, and then washed and dried. The final RDX composition can be essentially pure RDX, or it can contain smaller amounts of other substances, such as HMX. In contrast to the RDX that has been commercially available in the past, the RDX has a crystal density of less than 1.80 g/cm3. In some cases, the RDX has a crystal density of about 1.65-1.73 g/cm3. In some embodiments of the invention, the RDX has a surface area of greater than about 1.15 m2/g.
The detonation of RDX produced by the above-described process can generally be initiated with less energy than what is required to initiate previously-known RDX compositions. In some embodiments of the invention, detonation of the RDX can be initiated with less than about 100 mJ, or in some cases, less than about 75 mJ.
The RDX composition of the present invention can be used in a variety of applications. For example, it can be used in perforating the casing of subterranean wells, mining, construction blasting, and many other applications that are well known in the explosive industry.
The perforating gun 16 comprises a plurality of shaped charges 20, each of which contains an explosive material. This explosive material can be the RDX produced as described above, alone or in combination with other materials that are suitable for use in an explosive composition. Detonation of the explosive material in the shaped charge 20 can be initiated by a low-energy foil initiator 22. When an electrical signal is sent via a control line from a control device at the surface (not shown in
It should be understood that the arrangement shown in
Specific embodiments of the present invention can be further understood from the following example.
RDX crystals were prepared by crash precipitation. Type B RDX was dissolved in acetone to make a 10% by weight solution. A large excess of deionized water was added to this solution with vigorous stirring to precipitate fine particle size RDX. The precipitated RDX crystals were filtered from the liquid and washed. The resulting RDX was dried at 50-55° C. overnight in a drying oven. The measured BET surface area of the precipitated RDX was in excess of 1.2 m2/g. When examined by light microscope, the RDX crystals appeared to be polycrystalline and orthorhombic. However, when the crystal density of the crash-precipitated RDX was checked by helium pyconometer, the crystal density was found to be 1.69 g/cm3, which is significantly different than the starting material (ca. 1.80-1.82 g/cm3).
The RDX crystals were successfully detonated in a low-energy exploding foil initiator (LEFI) at 72 mJ (1300 volts, 0.085 μF).
The preceding description is not intended to be an exhaustive list of every possible embodiment of the present invention. Persons skilled in the art will recognize that modifications could be made to the embodiments described above which would remain within the scope of the following claims.
