Production of Ammonium Nitrate by Continuous Vacuum Crystallization

Abstract

ILOT plant production of ammonium nitrate by continuous vacuum crystallization has been described by Miller and Sae-P man (3). Their work led to the construction of a 525-ton-perday vacuum crystallizer plant by the Tennessee Valley Authority at Wilson Dam, Ala., to provide a safer and more economical process than the graining process that had been in use. Operation of the plant was tested on a limited production basis in 1949, and full scale operation was started in 1950, at which time graining operations were discontinued. This paper describes the plant, which is the fist of its kind to utilize continuous vacuum crystallization for large scale production of ammonium nitrate, A flow diagram of the plant is given in Figure 1. The steps in the operation of the plant are: neutralization of nitric acid with ammonia, purification and concentration of the ammonium nitrate solution, cryst,allization, and drying and bagging of the product. A.I.S.I. Type 430 stainless steel is the standard material of construction throughout the plant.All the equipment, with the exception of the settling tanks and the storage tank, is housed. A cross-sectional view of the building is shown in Figure 2. The portion of the building that houses the crystallizers is insulated and is maintained at a temperature of about 105 O F., or slightly above crystallizer temperature. Routine control of the crystallizers is maintained from a central control room kept at normal working temperatures. NeutralizationReaction of nitric acid with anhydrous ammonia, both of which are produced at Wilson Dam by TVA, is carried out in four neutralizers operated in parallel.Each neutralizer consists of a covered, cylindrical tank 10 feet in diameter and 10 feet high. Ammonia gas enters near the bottom through a "wagon-wheel'' sparger, and 42.5% nitric acid enters just above the ammonia. The rate a t which the acid is fed to the system is adjusted manually, and the rate a t which the ammonia , E i n t F d l r c , = r l i a roriilated NEUTRALIZER tion to 6.4 through automatic control described by Harvey and Ewald (1). The heat of reaction maintains the solution a t boiling in the neutralizers, and the result is the evaporation of about 605 pounds of water per ton of ammonium nitrate in the solution. The ammonium nitrate solution at this point in the process contains about 56% IVHJK-03. Two-stage instead of single-stage neutralization is used because the latter results in unstable automatic control and in a significant loss of ammonia. The neutralizers operate a t atmospheric pressure and are protected by water seals against overpressure or vacuum. Ammonium nitrate entrained in the neutralizer vapor and the liberated heat are recovered in the concentration step of the process.PurificationThe neutralized solution contains about 0.03% of precipitated hydroxides of iron and aluminum, which, if allowed to accumulate, would adversely affect the crystallization of ammonium nitrate, The impurities are separated from the solution as a sludge, which collects in the settling tan...

Fertilizer Technology, Production of Diammonium Phosphate by Continuous Vacuum Crystallization

“…The suspension density W'as maintained at about 30% crystals by volume, by regulating the rate of removal of crystals The crystallizer was operated with a circulating suspension, as in the case of ammonium nitrate (5). However, the fine salt separator in the crystallizer was not used.…”

Crystallization of Ammonium Nitrate from Nonaqueous Solvents

“…Its extremely high solubility in water generally precludes preparation through simple crystallization as a viable option, and most commercial processes resort to prilling or related granulation processes, − requiring subsequent drying and/or processing steps. Recrystallized ammonium nitrate displays poor crystal form, and even a special vacuum crystallization process was not competitive with prilling techniques due to its formation of only small crystals. In addition, although no discrete hydrates of ammonium nitrate are known, the salt is highly hygroscopic, compromising its effectiveness as an explosive and leading to agglomeration of particles .…”