Ammonium nitrate crystalline forms

Physical and Chemical Properties. Ammonium nitrate is a white, crystalline salt, df = 1.725, that is highly soluble in water, as shown in Table 3 (7). Although it is very hygroscopic, it does not form hydrates. This hygroscopic nature compHcates its usage in explosives, and until about 1940, was a serious impediment to its extensive use in fertilizers. The soHd salt picks up water from air when the vapor pressure of water exceeds the vapor pressure of a saturated aqueous ammonium nitrate solution (see Table 4). 

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

A specific polymorph may be absolutely essential for a crystalline product, for example, one polymorph may have a more desirable color or greater hardness or disperse in water more easily than another polymorph. Often, one polymorphic form is more stable than another (for example, at 80°C the orthorhombic I form of ammonium nitrate is more stable than the trigonal form) at conditions to which a product is exposed. An interesting approach to... 

An unusual feature of the KA-process is that the reaction is conducted at 60-80 °C. Solutions of nitric acid in acetic anhydride are known to be prone to dangerous fume off at temperatures above ambient. However, a saturated solution of ammonium nitrate in fuming nitric acid can be added to warmed acetic anhydride without such danger. In fact, these reactions are commonly conducted at 60-80 °C as a matter of safety by preventing a build-up of unreacted starting material. The hexamine used in these reactions is in the form of the dinitrate salt, which is formed as a crystalline salt on addition of a saturated aqueous solution of hexamine to concentrated nitric acid below 15 °C. The use of hexamine dinitrate in this process reduces the amount of nitric acid needed for the nitrolysis. 

Ammonium nitrate (AN NH4NO3) is a white, crystalline material, the crystal structure of which varies with temperature.melting point is 442 K and its heat of fusion is 71.4 kj kg . Though the mass fraction of oxygen of AN is 0.5996, it is highly hygroscopic and absorbs moisture from the atmosphere to form liquid AN acid. This limits its application in propellants and pyrolants. However, AN is widely used as an oxidizer of explosives such as slurry explosives and ANFO (ammonium nitrate fuel oil) explosives. 

The physicochemical properties of explosives are fundamentally equivalent to those of propellants. Explosives are also made of energetic materials such as nitropolymers and composite materials composed of crystalline particles and polymeric materials. TNT, RDX, and HMX are typical energetic crystalline materials used as explosives. Furthermore, when ammonium nitrate (AN) particles are mixed with an oil, an energetic explosive named ANFO (ammonium nitrate fuel oil) is formed. AN with water is also an explosive, named slurry explosive, used in industrial and civil engineering. A difference between the materials used as explosives and propellants is not readily evident. Propellants can be detonated when they are subjected to excess heat energy or mechanical shock. Explosives can be deflagrated steadily without a detonation wave when they are gently heated without mechanical shock. 

Diammino-silver Nitrate, [Ag(NII3)2]N03.—When aqueous ammonia is added to a solution of silver nitrate until the precipitate first formed redissolves and the solution is evaporated, a crystalline substance separates of composition [Ag(NH3)2]N03. The compound can also be formed by dissolving silver oxide in an anunoniaeal solution of ammonium nitrate, or by dissolving silver nitrate in benzonitrile and passing ammonia gas into the solution.2... 

Hydroxo - chloro - tetrammino - platinic Chloride, [Pt(NH3)4 (0H)C1]C12, is precipitated in crystalline form by the addition of ammonium chloride to a solution of the nitrate. 

In some countries (Great Britain) ammonium nitrate is used with an admixture of triphenylmethane dye, Acid Magenta which inhibits the transformation of one crystalline form into the other at 32°C (Vol. II, p. 454). 

Ammonium nitrate exists in the form of crystals, melting at 169.6°C. It occurs under five crystalline modifications distinguished by the transition temperatures ... 

Crystalline form of ammonium nitrate Temperature °C Change of volume cm3/g Thermal effect cal/g... 

AMMONIUM NITRATE. [CAS 6484-52-2], NH4NO3, formula weight 80.05, colorless crystalline solid, occurs in two forms ... 

Ammonium nitrate (NH4N03), a colorless crystalline solid, occurs in two forms (1) a-ammonium nitrate (tetragonal crystals, stable between -16°C and 32°C melting point 169.9°C density 1.66) and (2) (3-ammonium nitrate (rhombic or monoclinic crystals, stable between 32°C and 84°C with decomposition occurring above 210°C density 1.725). 

B. The curve exhibits sharp breaks.—Two possible causes, namely, a change of polymorphic form or a change of hydration,1 will give rise to a sudden break in the curve. The former case is illustrated by ammonium nitrate, which is capable of existing m no fewer than four crystalline forms. Of these the /3-rhombic passes into the a-rhombic variety at about 32° C.2 At this temperature a break occurs m the solubility curve.3... 

Among the agents preventing the transformation of crystalline forms three methods are of great importance (in addition to keeping the ammonium nitrate diy) ... 

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