PERCHLORATES The present major use of perchlorate salts is as oxidizers in solid propellants. The pottassium salt was first used and quickly followed by now most important salt --ammonium perchlorate. Lithium perchlorate, which has the highest weight percent oxygen, has been tested as an oxidizer in solid propellants, but has not found favor with propellant manufacturers. All the important perchlorates are produced by a double decomposition reaction with sodium perchlorate: NaCIO4 + MX -> MCIO4 + NaX
Generall The cells may also be arranged for continuous operation, ie., in series. The concentrated sodium chlorate solution enters the first cell, flows from cell to cell, and leaves the last cell essentially depleted of sodium chlorate. The advantage of the series process is that the individual cells can be regulated with respect to temperature and current density for the most economical production of sodium perchlorate. The anodes are suspended in the tank through a cover parallel to the sides of the tank and the cooling coils. The sides of the tank and cooling coils act as the cathode. The electrical connection is made to the anode above the cover. The hydrogen formed in the cell can be vented to the atmosphere through a stack at the end of the cell. The main variation from one commercial cell to another has been the type of anode used. Most commercial cells are equipped with platinum anodes. The cost bas been decreased in some cases by using platinum on tantalum or copper. The only real substitute for platinum that has proved of any real value is lead dioxide. It is reported that one manufacturer of ammonium perchlorate uses lead dioxide anodes in the sodium perchlorate cell. When lead dioxide anodes are used in a perchlorate cell, stainless steel or nickel cathodes are used. Mild steel cathodes cannot be used because the lead dioxide anodes are poisoned by the chromate ions present in the electrolyte to inhibit corrosion of the mild steel.
|Typical operating conditions for a commercial sodium perchlorate cell|
|Temperature||35 to 45@C|
|Feed rate||At least 2 gal/min|
|pH||6.0 to 6.8|
|Feed to Cell|| Sodium chlorate 400 gpl|
Sodium perchlorate 400 gpl
Sodium dichromate 5 gpl
|Cathode current density||2 amps/sq in. (31 amps/sq dm)|
|Cell voltage||6.5 to 7.0 volts|
|Power consumption||1.36 to 1.60 kwhab d-c|
|Sodium dichromate concentration||2.5 to 5.0 gpl|
|Calcium and magnesium||As low as possible/td>|
|Final sodium chlorate concentration||As high as impurity removal in recovery will permit|
Temperature affects all important dependent variables in sodium perchlorate cells, and the optimum temperature must be arrived at through compromise. For example, with an increase in temperature, the current efficiency is reduced, cell voltage decreases, platinum loss increases, solubility of perchlorate increases, and the equilibrium chloride concentration increases. The quantitative effect of electrolyte temperature on current efficiency at a current density of 0.34 amperes per square centimeter is small up to 60'C at high sodium chlorate concentration. Sodium perchlorate cell operating temperature is controlled by the method of heat removal (coils in cell) and the voltage drop across the cell solution. Wider anode-cathode spacing results in an extra heat load that must be removed to obtain low cell temperatures. Schumacher has indicated increased platinum consumption with an increase in temperature from 40 to 65C. The feed solution to the cell, depending on the method of isolation of the sodium chlorate, contains sodium chlorate, sodium dichromate, sodium perchlorate, and traces of chloride, sulfate, calcium, and possibly magnesium ions.
The literature contains a number of references to the effects of current density on current efficiency. A review of the reports suggests that two amperes per square inch is a good choice. At high sodium chlorate concentrations and at temperatures below 50*C, current efficiency is practically independent of current density in the range of 1.0 to 2.5 amperes per square inch (15.5 to 39 amps/sq dm). In some Cell designs, the upper limit of current density is apparently determined by the anode electrical connections and the cooling capacity of the cells. Anode current densities of 2.6 to 4.5 amperes per square inch (400 to 700 amps/sq cm) have been used in Europe. Higher platinum losses at higher current density have been reported by Schumachee and by Wranglen." Cathode current density is generally determined by cell design. This is particularly true when the cell body is used as the cathode. When individual cathodes are installed in a perchlorate cell, the cathode current density is generally the same as the anode current density. Literature references on the study of cathode current density are limited. The voltage drop across the perchlorate cell depends on: (1) anode and cathode material (2) cathode-anode spacing (3) concentration of reagents in the cell (4) cell temperature (5) current density on the anode and the cathode Because of the high anodic potential essential for the formation of perchlorate, the voltage drop across the cell is relatively high. The voltage across the cell increase near the end of a batch process when the sodium chlorate concentration is low. Under these conditions, ozone is found in the gases from the cell. The literature reports involving lead dioxide anodes in sodium perchlorate cells generally give a lower voltage drop than the 5.0 to 6.0 volts reported for laboratory cells using platinum. The reason for the lower voltage using lead dioxide anodes appears to be the lower current density and the higher operating temperature employed.
Almost without exception, all of the investigators of the electrochemical production of sodium perchlorate agree that the current efficiency is very low at low chlorate concentrations. There does not appear to be agreement in the literature on the sodium chlorate concentration at which the current efficiency starts to decrease. This is probably true because of the effect of temperature, current density, and pH, as well as the chlorate concentration, on the current efficiency. The final concentration of sodium chlorate in the cell effluent depends, in part, on the method of isolation of the sodium perchlorate. In general, the higher the concentration of sodium chlorate in the cell effluent, the higher the current efficiency for a batch process. The effluent from a sodium perchlorate cell varies from 600 to 1,000 grams per liter sodium perchlorate, 5 to 50 grams per liter sodium chlorate. and 2 to 5 grams per liter sodium dichromate, depending on the cell design, operating conditions, and method of subsequent treatment of the sodium perchlorate solution. Sodium perchlorate can be isolated from the cell effiuent as either the hydrate or the anhydrous form. In some cases, the cell effluent can be used without isolation of the sodium perchlorate. This approach will be discussed later. Depending on the concentration of the sodium perchlorate in the solution, it is either isolated by cooling, or the solution is further concentrated by evaporation, followed by cooling. Sodium perchlorate forms the monohydrate when crystallized below about 52'C, the exact temperature depending on the amount of sodium chlorate present in the solution. Above this temperature, it crystallizes in the anhydrous form. If an evaporator is part of the isolation system, the salt is generally isolated in the anhydrous form. If no evaporator is used, the salt is isolated as the monohydrate. In either case, because of the high solubuity of sodium perchlorate, the mother liquor from the isolation of the crystals contains a high concentration of sodium perchlorate. This mother liquor, after enrichment with sodium chlorate, is used as feed to the sodium perchlorate cells. Because of the high solubility of sodium perchlorate, its isolation is avoided when it is used as an intermediate to produce other perchlorates. For example, the sodium chlorate can be destroyed by chemical treatment and the dichromate removed as insoluble chromic hydroxide. This leaves a relatively pure solution of sodium perchlorate for conversion to other salts. The manufacture of other perchlorate salts takes advantage of their lower solubilities. Potassium perchlorate is prepared by the double decomposition reaction of sodium perchlorate and potassium chloride. NaCIO3 + KCl -> KClO4 + NaCl (5) Either the purified sodium perchlorate cell solution or sodium perchlorate crystals dissolved in water is treated with potassium chloride. The relatively insoluble potassium perchlorate crystallizes and is separated by centrifuging. If it is necessary to control crystal size and size distribution, the solution is first heated and then cooled. Ammonium perchlorate is prepared by reactions similar to those used to form potassium perchlorate. NaClO4 + NH4Cl - NH4Cl04 + NaCl (6) The feasibility of the process lies in the mutual solubility relationship between ammonium perchlorate and sodium chloride, which permits the reaction products to be separated by fractional crystallization. The solubility of sodium chloride varies only slightly with temperature, while that of ammonium perchlorate is temperature-dependent. Thus, on cooling, ammonium perchlorate can be recovered. The mother liquor, on evaporation, deposits a crop of sodium chloride crystals which are filtered hot. The filtrate, rich in ammonium perchlorate, is then recycled. The preparation of other perchlorate salts is similar in principle to those already described. Lithium perchlorate may be produced by electrolysis of lithium chlorate or lithium chloride. On a laboratory scale, other perchlorates are generally prepared by reaction of either ammonium perchlorate or perchloric acid with the desired metal oxides, hydroxides, or carbonates. References 1. BENNETT, C. W., AND MACIK, E. L., Trans. Am. Electrochem. Soc., 29, 323 (1916). la. BROVO, J. B., AND DELANO, P. H., U.S. Patent 3,020,124 (Febniary 6, 1962). 2. Chem. & Eng. News, page 21, April 27, 1959. 3. DODGEN, J. E., "Design Study on an Alternate Method for Production of Ammonium Perchlorate," TMR No. 190, Naval Propellant Plant, Indianhead, Maryland, July 17, 1961. 4. HAMPEL, C. A., AND LEPPLA, P. W., Trans. Electrochem. Soc., 92, 55 (1947). 5. KARR, E. H., U.S. Patent 2,772@ (November 27,1956). 6. Knibbs, N. V. S., and Palfreeman, H., Transl. Fared. Soc., 16, 402 (1921) 7. Mochalov, K. N., Trans. Butlerov Inst,. Chem. Technol. Kazan, 1, 21 (1934) 8. Pernert, J. C., US Patent 2,392,769 (January 15, 1946) 9. Ryan, J. R., US Patent 2,392,769 (January 8, 1946) 10. Schumacher, J. C., ACS Monograph No.146, "Perchlorates, Their Properties, Manufacture and Uses," New York, Reinhold Publishing Corp., 1960 11. Schumache,r J. C., Trans. Electrochem. Soc., 92, 45 (1947) 12. Schumacher, J. C. and Stern, D. R., Chem. Eng. Progress, 53, 428 (1957) 13. Schumacher, J. C. and Stern, D. R., and Graham, P. R., J. Electrochem. Soc., 105, 151 (1958) 14. Wranglen, D. G., Teknisk Tidskrift, Stockholm, May 13, 1960.
Thomas W. Clapper
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