Perchlorate Production

 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 

SNIP


   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. 
  

SNIP

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. 
   

SNIP

   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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