WO1996026158A1

WO1996026158A1 - Manufacturing method for porous ammonium nitrate

Info

Publication number: WO1996026158A1
Application number: PCT/US1996/002081
Authority: WO
Inventors: Witold Pagowski; Ryszard Scierzynski; Mariusz Wasilewski; Bohdan Subocz; Daniel Buczkowski
Original assignee: Laroche Industries, Inc.
Priority date: 1995-02-21
Filing date: 1996-02-14
Publication date: 1996-08-29
Also published as: PL176297B1; PL307388A1; CA2213479A1; AU4925596A; MX9706336A

Abstract

A method of producing explosive grade porous ammonium nitrate from any type of prilled or granulated nonporous substrate, including those containing additives used to modify physical and mechanical properties. The inventive method comprises wetting the substrate with an aqueous solution of ammonium nitrate in quantities equivalent to 0.2-3 % of actual water and heating the substrate up to a temperature sufficient to ensure that the phase III->II polymorphic transition has occurred. After the thermal treatment, the porous product can be mixed with oil directly, or dried and cooled using standard engineering procedures. The water content of the substrate throughout the actual III->II transition period must be maintained at least at 120 % of the quantity required to fully hydrate all the hydratable additives present in the substrate but not less than 0.2 % of the weight of the substrate.

Description

MANUFACTURING METHOD FOR POROD8 AMMONIUM NITRATE

FIELD OF THE INVENTION

This application discloses a manufacturing method for producing porous grade ammonium nitrate suitable for use in explosive formulations from a nonporous agricultural grade substrate. The substrate may be prilled or granular and may contain additives used to modify the physical and mechanical properties of the substrate.

DESCRIPTION OF RELATED ART Ammonium nitrate, also known as "AN" is produced and marketed almost predominantly in three particle types: low density prills, high density prills and high density granules. These products are produced by different manufacturing processes, and possess different physical and mechanical properties. The low density material is targeted at different markets than the markets targeted by high density material. For example, low density processes are designed to produce porous prills, which will typically absorb fuel oil in quantities greater than 6%. Such mixtures of ammonium nitrate and fuel oil ("ANFO") can be readily detonated with a high explosive charge, and are, therefore, used worldwide as effective blasting agents. By comparison high density ammonium nitrate will only absorb fuel oil in quantities of about 1% and, therefore, is considered to be nonporous. Consequently, except for small quantities of a small diameter high surface area material, high density ammonium nitrate is relatively unimportant in the explosive industry, but is marketed extensively as an important nitrogen fertilizer. Hence high density ammonium nitrate is often referred to by the term "Agricultural grade AN." Nonetheless the production capacity for high density ammonium nitrate grossly outweighs that for low density AN. Consequently high density AN is more readily available than low density AN.

Low density ammonium nitrate is produced by prilling AN melts containing 4-6% water in prilling towers. The process design is such that prills at the base of the prilling tower retain most of the water. The prills are cooled and carefully dried in equipment specifically designed to maintain the porous structure of the prills without causing physical breakdown of the product. Typical low density AN contains less than 0.2% moisture, has bulk densities¹ in the range 0.72- 0.80 grams/cc, and crush strengths normally less than normally less than 25 kg/sq cm. Ammonium nitrate, including low density material is hygroscopic and is routinely treated with an anticaking agent. Such anticaking agents are usually long chain aliphatic amines, salts of methyl naphthalene sulphonate, mixtures of surfactants, or dry-parting agents. In the case of low density AN, the actual choice of anticaking agent varies according to the ultimate explosive formulation desired.

By comparison, high density AN is prilled (or granulated) from melts containing as little as 0.1% water. Since prilling towers in high density operations are shorter than in low density plants, and since the product at the bottom of the tower contains as little as 0.1% moisture, the high density prills can be cooled and dispatched to storage without the complex drying and cooling equipment necessary in low density processes. A high density plant is therefore less complicated and more economical to build than its low density counterpart. High density prills usually have moisture contents less than 0.15%, loose bulk densities in the range of about 0.85 to about 1.05 g/cc, and crush strengths usually in excess of 35 kg/sq cm. For reasons explained later, about 0.1-3% of selected specific inorganic additives are routinely added to the AN melt before actual prilling. Such additives increase crush strength, and reduce the propensity to cake during storage and shipping. Additionally, it is common practice to coat high density ammonium nitrate with 300-2000 ppm anticaking agent.

Ammonium nitrate exists in one of five crystalline modifications between temperatures -18 to +169.6^βC. Transitions between these modifications, which occur at specific temperatures, are reversible and as indicated below are accompanied by meaningful changes in crystal volume.

A-ιmιr-τ.----m Nitrate Crystalline Modifications

Crystal Phase Change Crvstal Transition Transition Volum Terno. -°C1 Canαe .% Melt->I Liquid to cubic crystals 169.6 -

I->II Cubic to tetragonal 125.2 -2.1

II->III Tetragonal to rhombic 84.1 +1.3

III->IV Rhombic to pseudo 32.3 -3.6 tetragonal

IV->V Pseudo tetragonal to -18.0 +2.9 tetragonal

From a product handling point of view, unmodified prills can fracture when subjected to the repeated volume changes associated with cycling through polymorphic transition temperatures. Such fractures generate fines, which accelerate pick up of moisture and promote caking. Avoidance of the II->III->IV transition sequence in favour of phase II->IV transition is important from the product volume change perspective in the design of prill tower operations, while the IV->III transition cycle is of prime concern from its storage and shipping point of view. This latter transition involves a 3.6% change in volume and occurs at 32.3°C, a not unknown temperature in North America.

The amount of water present is known to play an important role in the initiation of IV->III transitions. It has been demonstrated that this transition does not occur until the temperature has reached about 50°C for ammonium nitrate containing less than about 0.08% water.

Consequently common industrial practice in high density prills manufacturing plants is to incorporate specific types of inorganic additives in the AN melt prior to prilling. The most commonly used additives not only increase the prill hardness, but also bind, what otherwise would be free water, into the crystal structure as water of hydration

(crystallization) . The net result is that the ammonium nitrate itself behaves as though no free (i.e unbound) water is present until the additive exhausts its capacity for water of hydration (crystallization) . Such additives are thus known as "hydratable additives." Exemplary hydratable additives include magnesium nitrate, calcium nitrate, aluminum sulfate and aluminum nitrate.

The IV->III polymorphic transition for ammonium nitrate containing 1.22% Magnesium Nitrate (.33% expressed as MgO) occurs at about 52°C at moisture contents of about 0.4%, and at about 40°C for a moisture content of about 0.75%. Other types of additives create centres of nucleation in the melt droplets, which result in the formation of many small crystals within a prill rather than several large ones. This characteristic increases the hardness and mechanical strength of the prills, so that the prills are less prone to shattering. The use of additives in high density AN prills contributes to their higher resistance to attrition and caking during storage and shipping. Granular AN is larger, more dense, and, therefore, harder than high density prills and can be stored and shipped without the use of additives, other than anticaking agents applied to the surface. As indicated above, low density prills contain 4-6% water at the base of the prill tower. The commonly used additives commonly used in high density production interfere with the subsequent drying process and are rarely used in low density operations.

Thus from a comparative product quality and use point of view, low density porous ammonium nitrate prills, which readily absorb fuel oil, are well suited for use in ANFO type blasting agents, but are relatively friable and prone to caking. High density products which can be produced in less complex processes, have superior storage and handling properties, but lack the porosity necessary for explosive applications.

A preferred substrate would have mechanical properties closer to those of high density prills or granules, and could be used in ANFO type applications. Such a substrate would comprise a high density substrate with suitably improved porosity without a significant reduction in bulk density and friability. The preferred substrate would provide a more robust substrate which is better suited for storage and transportation. Moreover, the provision of such a preferred substrate would also eliminate the need for specialized low density manufacturing facilities, thereby creating the potential for locating small scale agricultural to porous grade AN conversion plants at actual mining sites. Such small scale plants would use readily available agricultural grade ammonium nitrate as feedstock.

Researchers have studied ways to suitably modify high density prills for use in explosives for several decades. Indeed industrial practice in Poland has been dependent on small scale mining site agricultural-to-explosive grade AN product conversion facilities since the 1970's. The literature indicates two main approaches to converting high density AN to porous grade AN. One approach involves the treatment of the substrate with water under temperature conditions which promote the IV->III->IV polymorphic transition sequence. Another approach is to add pore forming additives to the high density AN melt prior to prilling.

U.S. Patent 3,804,929 and German Patent 50,820, for example, describe processes for preparing porous grade ammonium nitrate, which involve wetting agricultural grade substrate with water, and then drying it in hot air. French Patent 85,899 involves repeated heating and cooling of agricultural grade ammonium nitrate. All such processes taught in these patents have significant limitations.

For example in the drying processes, it is necessary to maintain the substrate at very high or low temperatures for long periods of time to complete conversion of the crystalline form. Furthermore the processes do not guarantee the formation of products with the necessary oil absorption from a single production cycle. This necessitates two and even three or more repeated wetting and drying cycles, with a very real possibility that the substrate will crumble during the course of the drying processes. The repeated heating and cooling process requires a very long time to complete the conversion, involves high energy consumption, and produces a product with such poor physical stability that its oil absorption capacity rapidly deteriorates.

Pagowski et ε_l . , Polish Patent 95331 (1978) describes a process comprising adding up to 1% of water to ammonium nitrate prills which contain 0.02-5% inorganic salts of oxyacids, and heating the prills above 32.3°C (the IV->III polymorphic transition temperature) under conditions designed to prevent loss of water during the time when the temperature is above 32.3°C. The patent teaches that, if necessary, the product can be cycled through the 32.3°C temperature several times without physical breakdown of the prills. The examples cited include the use of ammonium nitrate containing ammonium sulfate, and a mixture of ammonium phosphate and ammonium sulfate as internal additives, to yield products with oil retention capacities of 6- 8%.

The process described by Pagowski et al. in Polish Patent 95331 was commercialized in Poland in the 1970's and has been used successfully since then to convert all types of agricultural AN to explosive grade porous product in small 1-5 tph conversion plants located at mining sites. In the case of very hard prills such as those containing calcium and magnesium salts, it has been found necessary to repeat the thermal treatment several times to obtain a porous product with the required oil absorption capacity. The same recycle requirement was experienced in Russia which employed the Pagowski et a_L. process to convert prills containing calcium and magnesium nitrates since 1988.

A very similar process is described in U.S. Patent 5,078,813 issued in 1989. That patent describes the preparation of porous prills from wetted agricultural grade substrates stabilized with additives consisting of hydratable salts of magnesium, calcium etc. and oxyacids such as nitrates, phosphates, etc. The wetted substrate is processed several times through the temperature corresponding to the IV->III->IV polymorphic transition point by heating and cooling in order to change the crystal morphology. The experimental data presented confirm that it is necessary to repeat the thermal treatment several times to obtain a product with sufficient porosity for the required oil absorption capacity. Furthermore, the data indicates that relatively long reaction times are necessary to complete even one IV->III->IV cycle for agricultural grade AN containing magnesium nitrate additive.

Several Polish patents involve the use of surfactants to enhance the effectiveness of water on the porosity of agricultural ammonium nitrate. Pagowski et al. , Polish Patent 116,297, teaches a process involving the moistening of agricultural grade AN prills with water, and during a subsequent drying stage, while the prills are above 32.3°C, subjecting them to short bursts of water or aqueous solutions of surfactants or dyes, such that the temperature drops below 32.3°C. The drying is continued until the substrate reaches the required moisture content.

Use of surfactants in the moistening stage is further taught in Pagowski et al. , Polish Patent

155,218 issued in 1992. Agricultural grade prills are first treated with 0.1% of a melt comprising a mixture of an aliphatic amine and caprolactam, subsequently treated with 0.1% water, and then cycled between 40 and 20°C. The resultant product has an oil absorption of 8% as compared to 3% for an identical substrate exposed to just 0.2% water.

An altogether different approach is taught by Olevsky et al. in the 1994 Russian Patent No. 1,616,048. A solution containing urea, sodium carbonate, and a surfactant is metered into the 99% AN melt feeds to the prill tower in a conventional high density plant. Decomposition of the additives creates a porous product with the oil absorption necessary for blasting agent applications. This type of process is obviously restricted to conventional high density manufacturing facilities, with the added problem of possible contamination of recycled fines with organic materials.

SUMMARY OF THE INVENTION An object of the present invention is to provide a simple and energy efficient process for successfully producing explosive grade porous ammonium nitrate from all types of nonporous substrate including those containing additives such as magnesium nitrate and ammonium sulfate for modifying the physical and mechanical properties of the substrate.

Another object of the invention is to provide ammonium nitrate products with higher bulk densities than conventional low density, but with sufficient porosity and oil absorption properties for use as an effective blasting agent.

A further object of the invention is to provide a process which can be employed in standard high density prilling and granulation plants without requiring major modification, and be used to convert agricultural grade AN at remote sites without actual ammonium nitrate manufacturing facilities.

The foregoing and other objects of the invention are achieved by providing a process comprising wetting any nonporous ammonium nitrate particle, including the type containing additives used to modify the physical and mechanical properties of the substrate, with an aqueous solution of ammonium nitrate to provide a wetted nonporous ammonium nitrate particle containing 0.2-3% water. The wetted particle is then heated to a temperature above the phase III->II polymorphic transition point temperature of the wetted nonporous ammonium nitrate particle during a phase III- >II polymorphic transition period. The water content of the substrate particle must be maintained at a level of at least 120% of the amount required to fully hydrate all of the hydratable additives in the substrate, and in no case should be less than 0.2% of the weight of the substrate particle, during the actual phase III->II polymorphic transition period. The product from this process can be mixed with fuel oil to make ANFO while still hot or it can be cooled and dried.

Other objects, advantages and further scope of the applicability of this invention will become apparent from the following detailed description. It should nevertheless be well understood that while this description discloses the preferred embodiments of the invention, it is given only by way of illustration, since changes and or modifications within the spirit and scope of the appended claims will become apparent from the description. We have unexpectedly discovered that the foregoing objects and advantages can be achieved by wetting any nonporous ammonium nitrate particle including the type containing additives used to modify the physical and mechanical properties of the substrate with about 0.2-3% water in the form of an aqueous solution of ammonium nitrate and heating it to a temperature above the phase III->II polymorphic transition temperature. While the heat provides the driving force necessary for the required changes in the crystal structure of the substrate, the presence of free water is required to provide the physical expansion necessary for pore formation. Consequently, process conditions are designed to ensure that the water content of the substrate remains at least equal to 120% of the amount required to fully hydrate all additives in the substrate, but not less than about 0.2% of the weight of the substrate at any moment of time during the phase III->II polymorphic transition period. A range of heat transfer procedures, well understood by designers of chemical processes, can be used to accomplish the actual III->II transition. The product from this process can be mixed with fuel oil while still hot thus negating the need for additional equipment and further processing. Alternatively the product can be cooled and dried using standard techniques, before addition of the fuel oil.

An important aspect of the process of this invention is that it can be employed to convert all types of agricultural grade substrate into porous product with the physical characteristics necessary for practical use in ANFO type formulations. As indicated previously, high density ammonium nitrate prills may contain about 0.1-3% inorganic additives. The actual choice of additives varies from producer to producer but usually involves selection between inorganic nitrates, phosphates, polyphosphates, sulfates, or compounds of boron, or mixtures thereof. Additives can be categorized as hydratable or nonhydratable, with magnesium nitrate, calcium nitrate, and aluminum sulfate typifying the hydratable additives, and ammonium sulfate, ammonium phosphate and boron compounds the nonhydratable additives. The additive used and the concentration added affect prill properties such as hardness, friability, the amount of unbound water that can be present, and the temperature of polymorphic transitions. The process of this invention handles all these variables. For example, because of its ability to bind 6 moles of water per mole of additive, prills containing 0.33% Magnesium Nitrate (1.22% expressed as MgO) demonstrate no free water until the total water exceeds 0.89%.

The presence of free water during the actual III>II polymorphic transition is an essential feature of this invention and consequently process conditions are designed to ensure that the water content of the substrate is at least 120% of the hydratable capacity of all additives in the ammonium nitrate but not less than 0.2% of the weight of the substrate throughout this polymorphic transition. Thus in the 0.33% MgO example, the amount of water maintained during the actual phase III->II polymorphic transition must be at least 1.1%, whereas a minimum of 0.2% is required for substrate containing 0.3% ammonium sulfate. We have determined that the substrate becomes sticky and loses its mobility if too much water is added. Consequently the practical upper limit for water is about 3% of the weight of the substrate.

It should be noted that the products from prilled substrate containing nonhydratable additives or granules containing no additive whatsoever using the process of this invention, have the necessary porosity and mechanical strength for use in explosive applications. This is because normally only a single III->II transition is required, as compared to the IV->III->IV processes where several cycles, with the increased probability of substrate fracture, are required to obtain product of comparable quality. Although AN prills without additives can be converted to porous product using IV->III->IV technology, the product is mechanically too weak to be of practical use. Consequently the production of mechanically strong explosive grade porous product from granules containing no additives using the III->II process is particularly noteworthy.

A distinguishing feature of this invention is that an aqueous solution of ammonium nitrate is used to moisturize the substrate rather than straight water. We have discovered that this change reduces the number of substrate particles which fragment, with a consequent marked reduction in the amount of fines generated by the thermal treatment. Indeed, the amount of fines generated by the process of this invention is so steady that it is possible to recycle them as an aqueous stream with constant volume and concentration to wet entering AN particles in a continuous process. Although a wide range of AN recycle concentrations can be used, 30-50% is preferred.

According to this invention, after moistening the AN substrate, it is then heated to 95°C, i.e.. above the III->II polymorphic transition point temperature under the constraints of the moisture regimes outlined above. It is important to recognize that although the substrate must contain at least 120% of the amount of water required to fully hydrate all internal additives and not less than 0.2% of the weight of the substrate, this does not exclude the loss of water from the process. Thus although the operations outlined in Examples 1, 2, and 4 were carried out in a closed rotating drum and heated through the shell to prevent loss of water, alternative ways of heating the substrate through the III->II transition temperature while maintaining appropriate moisture conditions will be obvious to those ordinarily skilled in the design of heat transfer processes. In such cases, it is necessary that relationships between substrate water content and the corresponding time and temperature regimes be established. For example if a continuous fluid process is used, the key process parameters comprise: a) The feed rates of ammonium nitrate substrate and the aqueous ammonium nitrate solution (i.e.. the amount of water) to the bed.

b) The temperature.

c) Fluidizing air flow rate.

d) The duration of time in each stage of the process.

Under these conditions, a product with the required porosity and physical and mechanical characteristics is produced within a few minutes of thermal treatment, and can then be mixed while still hot, directly from the process with fuel oil to make ANFO. This capability is an important factor at mining site installations. Alternatively the product can be dried and cooled for later use, in which case, depending on the moisture content, the sequence of polymorphic transitions would be II->IV if hydratable additives are present. This transition reduces prill volume changes during the cooling process as compared to the II->III->IV sequence in low density operations. Moisture content of the product from this process is typically less than 0.25%, as compared to the 0.5% for conventional products, thereby negating the need and cost of subsequent drying stages.

The technology described in this invention is simple, rapid, and trouble free from a process operation point of view. The new process requires a single thermal cycle to achieve the same quality of product, which reduces the quantity of fines generated as compared to the processes based on the IV->III->IV polymorphic transition concept. It is, therefore, fundamentally more energy efficient.

A more complete understanding of this invention can be obtained from specific examples and methods of the invention. Example 1

Table I lists the properties of an agricultural grade ammonium nitrate product thermally treated by the process of the present invention. The prills contained 1.8% Magnesium nitrate, 0.1% moisture and had an initial oil absorption of 0.5 cc/100 grams. The corresponding polymorphic transition point temperatures for the IV->III and III->II transitions were determined to be 49.7°C and 89°C respectively using DTG analysis techniques. An 80% aqueous solution of ammonium nitrate was prepared by dissolving the required amount of fines, and was used to wet the prills in the amounts required to satisfy the water contents listed in Table I. The prills were then heated to 95°C, i.e.. above the III->II transition point temperature, in a closed rotating drum using conditions which excluded loss of water. This was accomplished by heating through the shell of the drum. After thermal treatment the prills were cooled to 20°C and then tested for water content, oil absorption, bulk density and crush strength.

For comparison purposes, samples of the same substrate were processed using water rather than AN solution to wet the substrate, and then heated to 60°C (i.e.. above the IV->III but below the III->II transition point temperatures) and then cooled to 20°C in the rotating drum, again using conditions which prohibited loss of water. Water content, oil absorption capacity, bulk density and crush strength were determined as before.

Water content of ammonium nitrate was determined by Karl Fischer titration. Bulk densities were determined by weighing a measured volume of product, and crush strength by crushing 20 individual product particles using a Chatillon type gage and averaging the results. The size fraction of prills and granules used in the crush strength test were 1.5 to 2mm and 2 to 3mm fractions respectively. Some particles were so elastic under test that they deformed rather than fracture. These were discarded. Oil absorption capacity, expressed as the number of ccs oil absorbed by lOOg of substrate, was determined as follows:

100+/-1 grams of the porous AN were weighed into a 250ml Erlenmayer flask. Fuel oil was then added in successive 0.5cc quantities, while shaking flask to agitate the sample. The contents were visually observed to assess the progress of oil absorption by the prills. An additional 0.5cc was not made until it was certain that the prills had not adhered as a single layer to the walls of the flask. In the case where such a layer had formed, further addition of oil was stopped, and the prills were periodically agitated by shaking the flask over a 20 minute period. The oil absorption capacity of the sample was determined as the volume of fuel oil absorbed by the prills, without the prills sticking as a distinct layer to the flask walls after a 20 minute period. The test allows only single prills to be attached to the wall. The same procedure, but with a waiting time longer than 20 minutes is used to determine the absolute oil absorption capacity, i.e. maximum volume of oil which can be absorbed by the sample oil absorption capacity of the porous product. This capacity is usually 1- 2cc/100g higher than that obtained using the absorption time of 20 minutes.

Table 1 Comparison of process and properties data of porous AN prills containing Mg(Ν0₃)₂ made by the process of this invention with those made by the IV- > HI- > IV process

Untreated IV->ΠI->IV

Thermal Treatment Substrate Present nvention Process

Water added % - 1 2 1 2

Oil absorption no drying 0.05 3.5 4.5 1.5 0.5 cc/lOOg after 6-7 drying

Bulk density g/cm³ no drying 1.01 0.92 0.85 0.89 .94 after 0.89 drying

Product moisture % no drying 0.12 0.85 1.5 0.73 1.7 after (OS)* drying

Crush strength no drying 55 17 11 kg/cm²

34

Crush strength based on prills 1.5 - 2 mm diameter Example 2 Table 2 contains the results from the thermal treatment of agricultural grade ammonium nitrate prills containing 0.6% ammonium sulfate as the internal (nonhydratable) additive. The sample had an initial moisture content of 0.15%, and an oil absorption of 0.5cc/100g. The prills were wetted with aqueous ammonium nitrate solution in quantities required to satisfy the water contents shown in Table 2, and then heated through the shell of the closed rotating drum to 95°C, i.e. above the III>II transition point temperature. The conditions precluded prill drying.

The drum was then cooled to 20°C and the porous product analysed for oil absorption, and bulk density, using the methods outlined in Example 1.

For comparison purposes a sample of the same wetted agricultural grade AN containing the ammonium sulfate additive was treated by the process using the IV->III->IV polymorphic transition. In this case the prills were heated to 60°C and then cooled to 20°C in the closed rotating drum. The corresponding water content, oil absorption capacities, bulk densities and crush strengths were determined using the procedures described in Example l. Table 2 Comparison of results using the process of this invention with those produced according to the IV- > III- > IV process, for prills containing (NH₄)₂ S0₄ additive

Untreated ιv->πι->ιv

Thermal Treatment Sμtis rate Present Invention Process

Water added % - 0.5 1.0 0.5 1.0

Oil absorption no drying 0 5.5 6.0 3.0 4.0 cc/lOOg after 11 drying

Bulk density g/cm³ no drying 1.04 0.86 0.75 0.86 .76 after 0.7 drying

Product moisture % no drying 0.1 035 0.85 0.48 0.94 after 0.2 drying

Crush strength no drying 41 18 14 kg/cm² after 26 drying

** Crush strength based on prills 1 - 2 mm diameter prills

Example 3

Agricultural grade granules containing 0.05% moisture and no internal additive, were moistened with 0.3% water in the form of a 10% solution of ammonium nitrate and then heated to 95°C in a fluid bed. The porous product obtained by this treatment had a bulk density of 0.84 g/cm³, and a final moisture content of 0.25%. The product absorbed lOccs of fuel oil per lOOg of sample while still hot, and could be mixed with other liquid ingredients used in explosive formulation. Table 3 Properties of Porous AN Granules Produced in Fluid Bed by This Invention

** Crush Strength based on granules 2 - 3 mm diameter.

Example 4

Granular ammonium nitrate without additives was moisturized with sufficient aqueous ammonium nitrate solution to result in a free water content of 2% in the substrate. The sample was then heated to 95°C indirectly through the shell of a closed rotating drum, and cooled to 20°C. The porous product made by this process had a specific bulk density of 0.7 g/cc, a moisture content of 1.7% and an oil absorption capacity of llccs per lOOg of sample.

For comparison purposes a sample of the same granular substrate was first moistened with 2% water, heated to 60°C in the closed rotating drum, and then cooled to 20°C. The porous product made by this treatment had a specific bulk density of 0.7 g/cm³, contained 1.6% moisture and an oil absorption of 6cc/l00g sample. Table 4 Comparison of Properties of AN Granules produced bv this invention with those produced bv the IV- > III- > IV process using large excess of water

Thermal Treatment Untreated Present Invention IV->ΠΪ->ΪV Granules Process

Water added to Granules Nil 2.0 2.0 %

Oil Absorption cc/lOOg 1.0 11 6

Bulk Density g/cm 0.93 0.7 0.7

Crush Strength kg/cm²** 46 6 ?

Moisture Content % 0.05 1.73 1.60

**Crush Strength based on granules 2 - 3 mm diameter.

Claims

WHAT IS CLAIMED IS:

1: A method of making porous ammonium nitrate, having sufficient oil absorption capacity to enable its use as a blasting agent, the method comprising the steps of: wetting nonporous ammonium nitrate particles with a water or aqueous ammonium nitrate solution to provide a wetted nonporous ammonium nitrate particles containing about 0.2-3% w/w% water; heating said wetted nonporous ammonium nitrate particles to a temperature above the phase III->II polymorphic transition temperature of said wetted nonporous ammonium nitrate particles during a phase

III->II polymorphic transition period; and maintaining the water content of said particles at a level not less than 0.2% of the weight of the particles during said phase III->II polymorphic transition period.

2: The method of claim 1 wherein said nonporous ammonium nitrate particles include at least one hydratable additive to modify the physical and mechanical properties thereof.

3: The method of claim 2 wherein the water content of said nonporous ammonium nitrate particles is maintained at a level of at least about 120% of the amount required to fully hydrate said hydratable additive in said particles, but not less than 0.2% of the weight of the ammonium nitrate. 4: The method of making a porous ammonium nitrate in claim 1 wherein the substrate is prilled.

5: The method of making a porous ammonium nitrate in claim 1 wherein the substrate is granular.

6: The method of making porous ammonium nitrate in claim 2 wherein the additive is selected from a group consisting of magnesium nitrate, calcium nitrate and aluminum sulfate.

7: The method of making a porous ammonium nitrate in claim 6 wherein concentration of magnesium nitrate, expressed as magnesium oxide, is 0.1-1.2%.

8: The method of making porous ammonium nitrate of claim 7 wherein the concentration of magnesium nitrate, expressed as magnesium oxide, is 0.2-0.8%.

9: The method of producing porous ammonium nitrate in claim 1 wherein said ammonium nitrate substrate particle includes at least one nonhydratable additive to modify the physical and mechanical properties thereof.

10: The method of producing porous ammonium nitrate in claim 9 wherein the ammonium nitrate contains at least one nonhydratable additive selected from the group consisting of ammonium sulfate, monoammonium phosphate, diammonium phosphate, a compound of boron, and mixtures thereof. 11: The method of making a porous ammonium nitrate as described in claim 10 wherein the concentration of ammonium sulfate is 0-1%.

12: The product from the method described in claim 1 which absorbs and retains at least 5.8% organic liquid.

13: The product from the method described in claim 1 having a bulk density of 0.8-0.95g/cc.

14: The method of making porous grade ammonium nitrate in claim 1, wherein the substrate is coated with 0-2000ppm of an anticaking agent.

15: An explosive grade ammonium nitrate substrate comprising 0-3% of an additive for modifying physical and mechanical properties of the substrate, a bulk density of 0.8-0.95g/cc, and an oil absorption capacity of at least 5.8cc/l00g, and a porosity obtained by moistening said substrate, heating said substrate through the phase III->II transition point temperature, and maintaining the moisture content of the substrate at no less than 120% of the amount required to fully hydrate said additive, but not less than 0.2% of the weight of the said substrate throughout completion of the polymorphic transition.

16: A porous ammonium nitrate particle prepared by the process comprising the steps of:

moistening a substrate containing 0-3% of an additive with an aqueous solution of ammonium nitrate; heating the substrate to a temperature above the phase III->II polymorphic transition temperature of said substrate during a phase III->II polymorphic transition period; and controlling process conditions such that the water content of the substrate is at least 120% of the quantity required to fully hydrate said additive but not less than 0.2% of the weight of the substrate at any time during said phase III->II polymorphic transition period.

17: The method of claim 16 further comprising the step of mixing the substrate with oil while the substrate is still hot from the process.

18: The method of claim 17 further comprising the steps of cooling the substrate and drying the substrate to remove unbound water.