The manufacture of Perchlorates (II)

A problem not usually encountered in other inorganic salts is that when ammonium perchlorate crystallizes out of an aqueous solution, it invariably contains a small amount of included moisture. This included water presents a drying problem because time must be allowed for some of the included water to reach the surface of the crystal. Equilibrium moisture content Figure 1 illustrates the time dependency of drying ammonium perchlorate. It is noted that the ultimate equilibrium moisture content is related to the initial total moisture content. Table 1. Moisture content of Ammonium Perchlorate crystals. Size Fraction Total Moisture + 48 mesh 0.132% - 48 + 150 mesh 0.085% -150 + 325 mesh 0.064% Composite Sample 0.101% Also, as would be expected, the rate of approach to equilibrium is a function of the drying temperature. There is considerable proof that the moisture content does not approach an equilibrium surface moisture, but that the final equilibrium is determined by the extent of included moisture. It would be expected that the moisture content is proportional to the specific surface area of the crystals. This would certainly apply to a material which contained no internal moisture. However, ammonium perchlorate crystals show the opposite trend (Table 1). Caking, then, results not from storage in a high humidity environment, but from the slow release of moisture from the interior of the crystals. This could be reduced by prolonged drying before packaging, but this would necessitate handling large quantities of ammonium perchlorate at elevated temperatures, thereby increasing the hazards of the drying process. The solution to the problem is partially obtained by packing ammonium per-ehlorate with bags of desiccant to absorb the moisture as it is released. Crystallization operation By far the most critical unit operation in the plant is the crystallization of ammonium per chlorate. Operating problems in centrifuging, drying, screening, and blending can usually be traced to improper control of the crystallizer. For example, a low bed density and low crystallizer circulation rate cause a fine crystal size distribution and, consequently, a high reject rate and overloaded downstream equipment. Propellant manufacturers desire firm, well-rounded, single crystals. Figure 2 is a photomicrograph of ammonium perchlorate crystals. The rounding is the result of mild attrition in the crystallizer; the fractured crystals are produced in centrifuging, drying, and blending. Careful selection of equipment is necessary to minimize crystal fracture. Unfortunately, ammonium perchlorate is a relatively fragile crystal and the drying step must reduce the moisture content to a very low value without agglomeration. Thus, some crystal fracture is tolerated to eliminate crystal agglomeration. Ammonium perchlorate is dimorphous; the transition between the two crystalline forms, orthorhombic and cubic, occurs at 240°C. Deflagration occurs at about 475°C with evolution of light. At higher temperatures, the decomposition is explosive. Although it would appear from these figures that ammonium perchlorate can be dried in complete safety with conventional equipment and with little regard for temperature, the thermal sensitivity is affected by small quantities of impurities. Also, the decomposition appears to be auto-catalytic so that decomposition products tend to accelerate the reaction. For these reasons, the dryer should have low hold-up and a temperature not exceeding 150°C. High velocities of dry air are used to compensate for these unfavorable drying conditions. Potassium Perchlorate Potassium perchlorate is also manufactured by the general double decomposition process. A few process modifications are necessary because of potassium perchlorate's physical and chemical properties. As indicated in Figure 3, the very low solubility of potassium perchlorate in aqueous solution permits almost complete recovery of perchlorate from the mother liquor by refrigeration to 0°C. In practice, potassium perchlorate is prepared by the addition of a hot, concentrated solution of chemical grade TRONA potassium chloride in slight excess to a sodium perchlorate solution. Crystallization is accomplished by batch cooling. The slurry is filtered and the cake washed to remove chlorides. The potassium perchlorate cake is discharged directed to a rotary driver where the total moisture content is reduced to less than 0.05 per cent. The mother liquor containing chloride, perchlorate, chlorate, sodium, and potassium ions is sent to an evaporator for sodium chloride removal and then used in subsequent batches for crystallizer feed make-up. Gradual impurity build-up necessitates discarding the mother liquor. Potassium perchlorate is more stable than ammonium perchlorate. Unless mixed with combustible substances, it does not tend to ignite or explode. Thus, potassium perchlorate may be dried at much higher temperatures. (Decomposition only becomes noticeable at 455°C.) Lithium Perchlorate Lithium perchlorate has not yet been produced in quantities comparable to potassium perchlorate and ammonium perchlorate. A review of its physical properties (Table 2) indicates it is superior to both potassium perchlorate and ammonium perchlorate as a propellant oxidizer in many respects. Table 2 illustrates the superior performance characteristics of lithium perchlorate. On a weight basis, it has a higher oxygen content than ammonium perchlorate, yet lithium perchlorate is more stable. The favorable oxygen content, oxygen balance, and stability are all attractive features of the lithium sale A disadvantage of lithium perchlorate as an oxidizer is that most exhaust products are solids, as are potassium perchlorate exhaust products; thus, performance gains are almost wiped out. There are additional factors to consider. Lithium perchlorate is extremely hygroscopic and exists as a trihydrate at room temperature. Traces of moisture in propellant mixesare intolerable, since moisture affects the aging properties and results in gassing in propellants containing aluminum powders or a polyurethane binder. In the event propellant manufacturers solve their lithium perchlorate problems and shift their present emphasis on ammonium perchlorate to lithium perchlorate, the existing perchlorate plants can be converted with minor modifications. As indicated in Figure 4, by cooling from 80°C to 20° C a crop of lithium perchlorate crystals would be obtained; then sodium chloride could be recovered by concentration, as in the ammonium perchlorate process. A process has been developed for the preparation of lithium perchlorate based on the reaction of ammonium perchlorate and lithium hydroxide. By boiling an aqueous solution of these reactants, ammonia is driven off. The resulting solution of Lithium perchlorate can be crystallized to yield the trihydrate, or the lithium perchlorate can be recovered by evaporating the solution to dryness. The literature (1) indicates that a process involving the anodic oxidation of lithium chloride using platinum anodes might be developed for the manufacture of lithium perchlorate. 1. Izgaryshev, N. A., and Khachaturyan, M. G., Doklady Akad. Nauk., U.S.S.R., 56, 929-32 (1947) and Doklady Akad. Nauk., U.S.S.R., 59, 1125-28 (1948).