Chemistry of inorganic compounds – With additive – For stabilizing crystal size or shape
Reexamination Certificate
2001-02-06
2003-06-03
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
With additive
For stabilizing crystal size or shape
C023S300000, C023S30200R, C149S046000, C423S396000
Reexamination Certificate
active
06572833
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to ammonium nitrate bodies produced by spraying a liquid mixture of ammonium nitrate and water that is highly concentrated in ammonium nitrate (herein “AN”) in to an atmosphere where the AN rapidly crystallises and water is removed by evaporation. The bodies may be prills (i.e. dense round particles resulting from crystallisation of free-falling droplets of mixture) or agglomerates (i.e. less dense associations of the component AN crystallites, irregular in shape and dimensions) or granules (i.e. generally round particles grown by spraying the mixture on to tumbling seed material such as small AN prill so that layers of AN crystals become encrusted on the seed material).
The invention is particularly concerned with ammonium nitrate bodies for use in explosives products, but is not restricted to such bodies. In particular, the invention may also extend to such bodies for use in fertiliser products.
DESCRIPTION OF THE PRIOR ART
Descriptions of technologies for the production of AN prills and pan-granulated AN bodies are available in the patent and other literature. Reference may usefully be had to EP-A-0320153 (acoustic frequency vibratory prilling to produce a substantially mono-sized prill), U.S. Pat. No. 5,354,520 (intensive prilling process) and U.S. Pat. No. 5,395,559 (pan-granulation to fatten mono-sized small prills). The preferred size for AN prills used in explosives products is about 2-3 mm; the preferred size of granulated products is about 3-4 mm and for these a 1 mm seed prill would be suitable.
AN prill intended for use as oxidiser in explosives products is required to have a sufficient porosity to allow absorption of from 6 to 12% by weight of fuel oil and yet be a particulate free-flowing product which can be poured or augered into bore-holes. This product is so-called ANFO. Such a prill is also a very satisfactory material for blending with emulsion explosive. To achieve such porosity, the AN/water mixture should contain up to, at most, around 97.5% by weight of AN, the preferred range being 92-97.5%, more preferably 94-96% by weight AN. The water content of the AN/water mixture also influences the density and friability of the AN prill. Thus, for instance, a fertiliser grade AN prill which is produced from an AN/water mixture containing at least 98% by weight AN is hard, dense and of low porosity. Other characteristics of an explosives grade AN prill that influence friability and stability towards atmospheric changes (e.g. temperature cycling and humidity) are the micro-crystalline structure and the residual moisture content of the prill. These characteristics are important also in other forms of AN bodies used as solid oxidiser in explosives products, e.g. pan-granulated AN particles, although they would not possess the hardness or low friability desired in an explosive grade AN prill. It is known to incorporate crystal-habit modifiers in AN/water mixtures used as feed in the described processes to encourage or cause the AN to crystallise as small crystals of preferred conformation. As a general rule, smaller crystals of lower aspect ratio are a feature of stronger, less friable AN bodies, especially prills. U.S. Pat. No. 5,597,977 describes the use of poly-styrene sulphonate as an ingredient in AN/water mixtures used to produce AN bodies.
Aside from residual water, minor impurities and minor amounts of deliberately added processing aids and product improvers, the AN bodies intended for use as explosives' oxidant will preferably consist of AN. It is, however, known to substitute some of the AN by alkali- and/or alkaline metal nitrates in explosives' oxidisers, and references herein to AN should be construed as AN optionally with a proportion of the ammonium nitrate substituted by one or more such other nitrate oxidant. Thus, references to AN proportions in the feed mixtures may for practical purposes be read as proportions of total nitrate oxidant.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a process for the production of ammonium nitrate bodies in particulate form wherein an ammonium nitrate/water liquid mixture containing at least 92% by weight ammonium nitrate, optionally with a proportion of the ammonium nitrate substituted by alkali- and/or alkaline earth metal nitrate, and a small amount of poly-styrene sulphonate crystal-habit modifier is sprayed as droplets in to an atmosphere in which cooling effects crystallisation of the ammonium nitrate and in which the crystallised particles are dried to remove moisture characterised in that the liquid mixture also contains ammonium sulphate, the concentration of poly-styrene sulphonate in the liquid mixture is at least 0.01% by weight, and the concentration of ammonium sulphate in the liquid mixture is greater than that of the poly-styrene sulphonate and is at least 0.04% by weight.
The present invention further provides a process for producing AN bodies by prilling, pan-granulation or other means of production of AN particles composed of aggregates of AN crystals in which an AN/water mixture as immediately hereinbefore described is sprayed with cooling to effect crystallisation of the AN followed by drying in a stream of gas, especially air, to remove substantially all free moisture, preferably to a residual water content of at most 0.1% water by weight in the dried product. The process type is preferably prilling under conditions of acoustic frequency vibratory prilling such that substantially mono-sized prills are produced, with drying effected in one or a train of two or more rotary driers under co- or counter current drying gas, e.g. air, flow, and the process is preferably in accordance with the immediately preceding paragraphs.
The present invention also provides AN bodies, especially AN prills, produced by a process as described in either of the two immediately preceding paragraphs.
The invention also extends to the use of a synergistic combination of poly-styrene sulphonate and ammonium sulphate as crystal-habit modifier in the production of ammonium nitrate prill, especially explosive-grade ammonium nitrate prill.
The AN/water mixture preferably contains at most about 97.5%, more preferably 94-97%, even more preferably 95-97%, by weight of AN. However, the AN/water mixture may contain more AN if the AN bodies are to be used in fertiliser products.
Preferably, the concentration of poly-styrene sulphonate is at least 0.02% by weight of the mixture.
The mixture further includes a quantity of ammonium sulphate which is greater than the quantity of poly-styrene sulphonate present, preferably at a concentration in the mixture of, say, 0.06 to 0.15% by weight, more preferably 0.08 to 0.13% by weight.
The poly-styrene sulphonate may be considered to be present as its ammonium salt, given the preponderance of ammonium ions present in the mixture and the normal practice of pH adjustment to a value of around 5.5 by ammonia addition. Poly-styrene sulphonate exists as a range of molecules, not merely because as a polymer there will be a range of chain lengths determined by the polymerisation process and any fractionation, but also because the degree of sulphonation of polymer chains can vary widely. Thus, at one end of the spectrum of possible molecules is poly (vinyl benzene sulphonate) and descending from that best characterised species are a range of sulphonated poly-styrenes. These should preferably be sulphonated to a level of at least 50% relative to poly (vinyl benzene sulphonate), more preferably at least 75%, and most desirably at least 90%. We have produced sulphonated poly-styrenes with more than 90% degree of sulphonation from a range of poly-styrenes with number average molecular weights from 40,000 to 200,000; all were effective additives. Higher molecular weight materials are more viscous and this can make production and handling more difficult. Low molecular weight material has been produced by styrene polymerisation followed immediately by sulphonation without any intermediate separation or purification stag
Brues Michael
Cooper John
Hsu Noel
Peddie Ronald O.
Greenlee Winner and Sullivan P.C.
Langel Wayne A.
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