Ammonium nitrate products and method for preparing the same
10774014 ยท 2020-09-15
Assignee
Inventors
Cpc classification
C01F7/02
CHEMISTRY; METALLURGY
International classification
Abstract
The invention refers to a free-flowing ammonium nitrate (AN) product which comprises a mixture of AN particles and beads or granules of activated alumina, a process for preparing the same and the use of said beads or granules as free-flowing additive for AN particles.
Claims
1. An ammonium nitrate (AN) product comprising a mixture of: i) AN particles; and ii) beads or granules consisting of activated alumina (AA), wherein over 95% by weight of the beads or granules of AA have a particle size distribution ranging from 1.0 to 5.0 mm.
2. The AN product according to claim 1 consisting essentially of a mixture of: i) AN particles; and ii) beads or granules of AA.
3. The AN product according to claim 1, wherein the AN particles are coated with an organic anticaking agent.
4. The AN product according to claim 1, wherein the AN particles are technical grade AN particles.
5. The AN product according to claim 1, wherein the AA is present in an amount ranging from about 0.01 wt % to about 2 wt %.
6. The AN product according to claim 1, wherein the AA is present in an amount ranging from about 0.1 wt % to about 1.0 wt %.
7. The AN product according to claim 1, wherein the beads or granules of AA have a specific surface area ranging from about 100 to about 500 m.sup.2g.sup.1.
8. The AN product according to claim 1, wherein the beads or granules of AA have a specific surface area ranging from about 250 to about 400 m.sup.2g.sup.1.
9. The AN product according to claim 1, wherein over 95% by weight of the beads or granules of AA have a particle size distribution ranging from 1.5 to 3.0 mm.
10. The AN product according to claim 1, wherein the AN product is suitable as a raw material for explosives manufacture.
11. The AN product according to claim 1, wherein the AN product is suitable as oxidizer in explosives.
12. An explosive comprising the AN product as defined in claim 1.
13. A method for preparation of an ammonium nitrate (AN) product of claim 1 comprising mechanically mixing AN particles with beads or granules consisting of activated alumina (AA), wherein over 95% by weight of the beads or granules of AA have a particle size distribution ranging from 1.0 to 5.0 mm.
14. The method according to claim 13, wherein the beads or granules of activated alumina are mixed with the AN particles after the AN particles have been dried, cooled down and coated with an organic anticaking agent, and before being bagged in Protective Bags.
15. The method according to claim 13, wherein the AN particles are technical grade AN particles, preferably porous technical grade AN particles.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
DETAILED DESCRIPTION OF THE INVENTION
(2) As used herein, the term about means a slight variation of the value specified, preferably within 10 percent of the value specified. Nevertheless, the term about can mean a higher tolerance of variation depending on for instance the experimental technique used. Said variations of a specified value are understood by the skilled person and are within the context of the present invention. Further, to provide a more concise description, some of the quantitative expressions given herein are not qualified with the term about. It is understood that, whether the term about is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including equivalents and approximations due to the experimental and/or measurement conditions for such given value.
(3) Concentrations, amounts, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. As an illustration, a numerical range of about 1 micron to about 5 microns should be interpreted to include not only the explicitly recited values of about 1 micron to about 5 microns, but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3.5, and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting only one numerical value.
(4) After extensive research, the inventor found that surface morphology of the AN particles had a main influence on final coating operations (e.g. with anticaking agents) as well as on its flowing behavior. AN particles that have been submitted to drying processes, typically porous AN particles produced from prilling AN solutions containing water (1-10 wt %), normally exhibit bulges and deformations on surface structure linked to saturated solution transfer towards surface through the pore network. These morphological deviations from round and smooth particles have a major impact on the loss of the mass-flow behavior of the bulk product, and thus in the apparition of arching phenomena that controls their discharge flow.
(5) However, the inventor also found that if surface of the AN particle is fully dry, the effect of surface morphology could be cushioned. In this sense, it was evidenced that freshly produced porous AN showed good flowing behavior. Porous AN production process typically involves a final drying step, prior to cooling, sieving and anticaking coating operations, in which the AN particles surface is fully dried. This drying process can be fulfilled, leaving just some residual non-dryable moisture, normally ranging <0.1 wt %, ascribed to diffusion restricted inner particle positions.
(6) Nevertheless, the good flowing behavior is lost as soon as the product was submitted to humid ambient (fixed time at relative humidity higher than critical relative humidity of AN) enough just to cause almost negligible (by standard analysis methods such as Karl-Fischer) moisture uptakes, assumed to take place at some extension on the external surface of the AN particles. Then, the inventor envisaged that if the surface of the AN particles was protected against moisture uptake, this is, if surface was maintained dry during the storage time until final use, mass-flowing behavior might be ensured.
(7) Despite AN product is normally bagged in Protective Bags (PB) such as Flexible Intermediate Bulk Containers (FIBCs), the performance of these bags as moisture barrier is not totally effective. There exists a transport of water vapour through plastic layers obeying Fick's law, which relates mass transfer rate M (mol s.sup.1) to a concentration gradient, which acts as the driving force, and in a finite difference form is described as:
(8)
(9) where D (m.sup.2 s.sup.1) is the diffusion coefficient of the barrier material, A is the area (m.sup.2), and P.sub.w (mol m.sup.3) is the water vapour partial pressure difference across the increment in length, the thickness of the barrier layer, AX (m).
(10) Inside the PB, the water vapour partial pressure will be defined by the Critical Relative Humidity (CRH) of the AN, i.e., by the water vapour adsorption equilibrium dependence on temperature. If ambient air RH, and thus outer water vapour partial pressure, is higher, a partial pressure gradient between the two sides (in & out) will be generated, acting as the driving force for moisture transfer into the PB, which is then adsorbed on the AN product maintaining the equilibrium condition at the inside. Then, if storage conditions are unfavorable, this is, in the open and for humid locations, an effective moisture transfer through the PB layer(s) takes place.
(11) Thus, the addition of moisture traps that could account for the moisture transferred through the PB layer(s) was considered by the inventor as a useful strategy to extend the mass-flowing behavior of the AN product.
(12) The use of internal additives, melt additives, such as MgN was not considered due to the negative effect it might have on the characteristics of the porous AN, such as decreased porosity due to strong drying requirements.
(13) Then, the mixing of the finished AN product with moisture traps was considered. The inventor envisaged that the moisture trap should have a number of properties as described below. It should be chemically compatible with the AN. It should function as preferential site for the adsorption of moisture transferred through PB layer(s), this function enabled by materials capable to decrease the RH well below the CRH of the AN. Moreover, it should be particulated, having particle size distribution and particle density similar to the ones of the AN, to prevent segregation processes, and it should be preferably white colored to prevent aspect heterogeneities. The moisture trap should also have high resistance to attrition and crushing. In addition, it should preferably enable chemisorption process, resulting in practically irreversible adsorption of water, instead of reversible physisorption or capillary adsorption that could induce redistribution processes, ascribed to temperature variations, of trapped moisture towards the AN. The moisture trap should show high adsorption capacity, in order to extend the storage time for which AN product will retain its mass-flowing behavior for minimum doses of the additive. This minimization of the additive dose is a critical requirement since AN products for explosive manufacturing require high purity, typically >99.0%. In this sense, the dose of additives, if not showing oxidizing characteristic, results in almost proportional decrease of final explosive specific energy. Another critical requirement is the specific cost that the additive will imply in the AN product. In any case, the additive dose should lead to minimum impact on both AN product characteristics and final explosive performance.
(14) According to these requirements moisture trap screening was initiated. Typical moisture traps such as calcium chloride, activated carbons and alkaline (earth) oxides and hydroxides have not been considered due to incompatibility with AN. Drierite and anhydrous magnesium sulphate are chemically compatible, non-disintegrating, non-wetting, and economical to use. However, commercial products show non-spherical irregular shapes and water trapping capacity is relatively low. Zeolitic nature molecular sieves present outstanding adsorption capacity at low RHs, following a Langmuir-type adsorption isotherm, but in contrast, granule structuration requires incorporation of colored binders, some of which could have chemical incompatibilities with AN and, in addition, product price is not competitive.
(15) Experimental tests were also conducted. Particular attention was paid to moisture traps agents previously proposed for AN such as the use of AN particles containing partially hydrated MgN. AN containing partially hydrated MgN has relatively low adsorption capacity. Anyhow, it has little effect on AN product characteristics as raw material for explosive manufacture, allowing higher doses which overcome this low adsorption capacity. However, it was found that as moisture content increases the hardness of this moisture trap decreases substantially resulting in very fragile product, whose collapse has a negative impact on product flowability.
(16) To the best knowledge of the inventor, beads or granules of AA have not been suggested as protection agent against moisture uptake in the AN industry. However, beads or granules of AA seem to meet most of the requirements initially sought and were also evaluated in this moisture trap screening. It was observed that relatively low doses of beads or granules of AA provided products having improved flowing behavior.
(17) The present invention has therefore developed a method of preparation of free-flowing AN products consisting in the use of beads or granules of AA as the most suitable flow aid to finish the AN particles that may be obtained in a standard prilling or granulation manufacturing process, just before packaging in PBs. Advantageously, the addition of beads or granules of AA to AN particles helps inhibiting the surface of as prepared AN particles to get moisturized due to water vapour permeation through PB layer(s) for a certain storage time, i.e. until adsorption capacity of the dosed alumina at the CRH of the AN is fulfilled.
(18) The product of the invention represents a suitable replacement to AN particles currently commercialized or known in the state of the art. Preferably, the product according to the invention is a non-powder AN product comprising or consist of a mixture of AN particles, such as prills or granules, and beads or granules of AA. The product of the invention is especially advantageous with porous AN prills.
(19) For the purpose of the present invention, AN particles may be prepared by conventional techniques well-known for a person skilled in the art. Beads or granules of AA are preferably ad-mixed subsequent to coating the AN particles with surface active anti-caking agent, such as those of organic nature commonly used in the industry, and before packing operations. Examples of anti-caking agents are those commercialized by ArrMaz under the trademark name GALORYL (e.g. GALORYL AT, GALORYL ATH, GALORYL ATH H) and by Kao Corporation under the trademark name SK FERT; these agents typically consist of an oily base containing fatty amines, an oily base containing fatty amines and other fatty derivatives or a mixture of amines dispersed in mineral oil and wax.
(20) In a preferred embodiment, the AN product of the invention is compatible with the requirements for use in explosives. With this in mind, technical grade ammonium nitrate products (commonly abbreviated as TGAN or TAN) manufactured as prills or granules are preferred. TAN products as prills or granules, comprising all AN products produced by means of prilling or granulation processes, are classified as dense (or high density) or porous (or low density), depending on their apparent density. Apparent density (pore density) of dense TAN prills or granules is in the range 1.05-0.90 g cm.sup.3. while it ranges 0.90-0.60 g cm.sup.3 for porous TAN, the latter type being preferred in the present invention.
(21) AA is used for a wide range of adsorbent and catalysts applications. It is typically manufactured by calcination of boehmite or gibbsite at 400-600 C., resulting in a highly porous structured material of aluminum oxide. AA shows affinity for polar molecule (e.g. water) adsorption. The water adsorption capacity per unit weight of AA is directly proportional to its specific surface and depends on RH. For the purpose of this invention, AA is used in the form of granules or beads, which are commercially available even at particle size distributions that match the typical particle size distribution (PSD) of prilled porous AN. Preferably, the beads or granules of AA are of a large specific surface area (100-500 m.sup.2g.sup.1) and show water adsorption capacities ranging 15-25 wt % at RH equal to 50%. In a more particular embodiment, the beads or granules of AA have a specific surface area ranging from about 250 to about 400 m.sup.2g.sup.1. In a more particular embodiment, the beads or granules of AA have a particle size distribution of 1.0-5.0 mm (>95 wt %), more particularly 1.5-5.0 mm (>95 wt %) or 1.5-3.0 mm (>95 wt %). Moreover, bulk density of highly porous beads or granules of AA ranges 700-850 kgm.sup.3 which is also within prilled porous AN range. Additionally, beads or granules of AA have high resistance to attrition and crushing, if compared for instance to porous AN particles themselves, and they do not swell nor soften when adsorb water.
(22) Very low amounts of beads or granules of AA have been proven to significantly improve the flowability behavior. In an embodiment of the invention, the beads or granules of AA are present in the product of the invention in an amount ranging from about 0.01 wt % to about 2.0 wt %, and more particularly from about 0.1 wt % or 0.2 wt % to about 1.0 wt % or 1.5 wt %. In particular embodiments, the amount is about 0.1 wt %, about 0.2 wt %, about 0.3 wt %, about 0.4 wt %, about 0.5 wt %, about 0.6 wt %, about 0.7 wt %, about 0.8 wt %, about 0.9 wt %, about 1.0 wt %, about 1.1 wt %, about 1.2 wt %, about 1.3 wt %, about 1.4 wt %, or about 1.5 wt %. Preferably, the amount ranges from about 0.5 wt % to about 1.0 wt %. Of course, values above these ranges are also contemplated in the present invention, but less preferred from an economic point of view.
(23) The AN product of the invention is useful as raw material for the manufacture of explosives, and in particular it is suitable as oxidizer component in explosive compositions. In a particular embodiment the AN product of the invention consists essentially of a mixture of i) AN particles and ii) beads or granules of AA. In a more particular embodiment the AN product of the invention consists of a mixture of i) AN particles and ii) beads or granules of AA. In addition, the present invention relates to an explosive comprising the AN product of the invention.
EXAMPLES
(24) Two porous prilled ammonium nitrate products, namely PPAN A and PPAN B, were tested.
(25) The definitions related to the tests are as follows:
(26) Flowability Test:
(27) A 1000 g sample of the porous AN is placed inside a hopper, which is then submitted to fixed vibration conditions (acceleration of 6.0 ms.sup.2 for 1 min) in order to mimic real operation compaction process. Once vibration stops, the sample is allowed a defined time at rest before opening the unloading valve. The time needed for the sample to fully discharge is recorded. If the sample requires more than 300 s to discharge, the test is concluded.
(28) Two types of discharge flow have been evidenced. On one hand, mass-flow, which describes continuous flow of PPAN particles ranging 5 to approx. 8 s for full discharge. On the other hand, controlled-flow, for which cohesive arching episodes lead to a discontinuous flow at the beginning of the discharge operation. Once a critical discharge time (discharge amount) has been reached the arch fully collapses and the the mass-flowing behavior is recovered. In the limit, the discontinuous flow can be so slow that test result is considered as absence of flow (if discharge time higher than 5 min).
(29) Moisture Content:
(30) The moisture content of the PPAN samples was established as the mean of three Karl-Fischer measurements.
(31) Sample Aging Simulation:
(32) The dry fresh sample is submitted to aging process in order to simulate moisture uptake during real storage and transport stages. The process consists in bagging the fresh dry sample in low-density polyethylene (LDPE) zip bags (2 L capacity and 50 m thickness). After, the bagged sample is submitted to humid ambient conditions. Due to several facts this simulation fastens up the real moisturizing process, as it takes place in the FIBCs. On one hand, the specific area of the PB layer, per AN product unit mass, is much higher in these small bags than in FIBCs. In addition, the moisture vapor transmission rate of LDPE, i.e. the diffusion coefficient, is higher than the ones for real FIBC materials PP and HDPE. Moreover the thickness is lower for these zip bags which present just one layer.
(33) Friability
(34) The sample is sieved to remove fines (<1.00 mm). Approximately 100 g of this sieved sample is feed to a cyclone operated with dry air at 170 kPa. Sample collected from the bottom outlet of the cyclone is further sieved through 1.00 mm screen. The amount of fines generated is measured and expressed as wt % of the initial sample weight.
(35) Crushing Strength
(36) The sample is screened in between 2.00 and 3.00 mm, and the obtained product is subsequently submitted, particle by particle, to a crushing strength (CS) test by means of an electronic compression force gauge. The average compression force (in N) at the crushing point is calculated from a total of 20 measurements.
Example 1
(37) This example shows the moisturizing process due to water vapour permeation through PB layer(s). Freshly produced porous AN B having a moisture content equal to 0.03 wt % was bagged in two FIBCs, 1000 kg capacity, and leaved isolated in the open. Two layer FIBCs, external laminated PP layer (160 gm.sup.2) and internal HDPE layer (100 m thick), were used.
(38) The FIBCs were stored in the open for 2 and 6 months, respectively. Time after which, using an appropriate sampling device, samples were picked from different internal positions at the FIBCs as described in
(39) TABLE-US-00001 TABLE 1 Moisture content as a function of sampling point within the FIBC and storage time Moisture content Sampling position After 2 months After 6 months x.sub.1y.sub.1z.sub.1 0.04 0.07 x.sub.1y.sub.1z.sub.2 0.04 0.08 x.sub.1y.sub.1z.sub.3 0.04 0.05 x.sub.2y.sub.1z.sub.2 0.03 0.04 x.sub.2y.sub.1z.sub.3 0.04 0.08
(40) Results evidenced that there exist some permeation through FIBCs layers whose extension depends on storage time. As a result of this permeation process, a gradient of moisture content, increasing from central positions towards positions near the layers of the FIBC, was generated.
Example 2
(41) This example shows the effect of mixing varying doses of AA beads (1.5-3.0 mm) with fresh dry porous AN A and B products under simulated packaging conditions. The resulting flowing characteristics together with moisture uptake data over the AN products are shown in Tables 2 and 3.
(42) TABLE-US-00002 TABLE 2 Moisture content (h) on the porous AN and discharge time for 20 min at rest (t), after submission of porous AN A-AA bead compositions to different aging times. Aging ambient conditions (15-25 C. and 50 < RH < 95%). AA bead As After After dose, dried 2 days 6 days 14 days 30 days wt % h, % t, s h, % t, s h, % t, s h, % t, s h, % t, s 0.04 4 0.05 0.06 0.11 0.12 0.1 0.04 4 0.05 18 0.07 0.12 0.13 0.2 0.04 5 0.05 5 0.06 0.09 0.11 0.5 0.04 4 0.04 4 0.05 5 0.07 0.10 1.0 0.04 5 0.04 5 0.04 5 0.05 250 0.07
(43) TABLE-US-00003 TABLE 3 Moisture content (h) on the porous AN and discharge time for 20 min at rest (t), after submission of porous AN B-AA bead compositions to different aging times. Aging ambient conditions (5-7 C. and RH >80%) AA bead As After After dose, dried 4 days 9 days 18 days 30 days wt % h, % t, s h, % t, s h, % t, s h, % t, s h, % t, s 0.04 4 0.05 0.06 0.07 0.09 0.5 0.04 4 0.04 5 0.04 5 0.06 145 0.07 1.0 0.04 5 0.04 4 0.04 5 0.05 5 0.06
(44) Results evidence that the addition of AA beads allowed preventing the bagged AN products to get moisturized due to permeation through PB layer up to a certain extent, i.e. to a certain amount of permeated water vapour, depending on the dose of the specific AA beads used. In contrast to residual non-dryable moisture, this permeated water vapour would have the ability to adsorb in any surface position of the porous AN, including external surface where inter-particle contact takes place. In the same trend, it has been evidenced that slight increase of moisture content due to permeation, even values equal to 0.01%, result in a noticeable impact on flowing behaviour of these porous AN products.
Example 3
(45) This example shows the disadvantage of using materials that despite showing moisture trap behaviour as preferential sites for water adsorption are or become fragile enough to collapse in real handling operations. AN containing partially hydrated MgN is an interesting moisture trap additive since its dosage has little effect on AN product characteristics as raw material for explosive manufacture. Moreover, for moisture contents below full hydration of the MgN, this additive is harder than porous AN. However, moisture content exceeding the MgN hydration value, results in a dramatic drop on the hardness. Table 4 shows the effect of moisture content on the hardness (expressed as friability and crushing strength) of a dense AN containing around 2300 ppm of Mg as MgN, this is, hexahydration achieved for approximately for a moisture content equal to 1.0 wt %.
(46) TABLE-US-00004 TABLE 4 Moisture content (h) of the dense AN containing MgN and its effect on crushing strength (CS) and friability (F) h, wt % F, % CS, N 0.6 0.0 9.8 1.0 0.0 8.3 2.1 2.8 0.4 3.0 7.4 0.5
(47) The addition of 5 wt % of crushed AN containing MgN having a moisture content equal to a 2.1 wt % resulted in the total suppression of the discharge flow of as dried porous AN A and B products.
(48) In contrast, it has been found that AA beads used in previous examples have high resistance to attrition and crushing. Moreover, AA beads maintain hardness after adsorbing water. For example, a crushing strength value equal to 26.7 and 16.2 N was established for fresh AA beads and AA beads after overnight immersion in water, respectively. Friability was equal to 0% in both cases. This ensures no practical crushing of AA bead additive in normal applications for product of the invention.