Handbook of Solvents - George Wypych - ChemTech - Ventech!

634 Victor Cherginets
halogenating agents, usually hydrogen halides or the halogens themselves, are used to eliminate
the impurities: 107-111
2
2HX ↑+ O = H O ↑+ 2X
− −
2
[10.4.55]
2−
−
X ↑+ O = 1/ 2O ↑+ 2X
[10.4.56]
2
2
However, the water formed in reaction [10.4.55] exhibits the properties of Lux acid
owing to process opposite to [10.4.45] and this retards the purification process. The cessation
of the HX effect may lead to partial hydrolysis, because reactions of type of [10.4.55]
are reversible. The shift of [10.4.55] to the left is also promoted by the thermal instability of
HBr and HI.
The effectiveness of the purification of melts based on lithium salts by hydrogen
halides is much lower. Water dissolves in the KCl-LiCl melt in appreciable amounts and is
firmly retained at temperatures up to 400 o C. 30 When dry HCl is passed for 1 h, removal of
H 2O is incomplete.
The effectiveness of the purification by the halogens depends on their oxidation-reduction
potentials and decreases from chlorine to iodine. In addition, the latter can disproportionate:
2
3I ↑+ O = 5I
+ IO
2
− − −
3
[10.4.57]
However, the addition of NH 4X is recommended only for the drying and heat treatment
of the initial components and its use for industrial purification of fused salts is undesirable
because it is much easier to ensure the continuous supply of the corresponding
hydrogen halide in the melt being purified. An additional lack of this purifying agent results
in evolution of gaseous ammonia as one of the finishing products.
It is believed that the most convenient method of the elimination of oxygen-containing
impurities from alkali metal halide melts is carbohalogenation. 112-118
The reduction of O 2- concentration under action of CCl 4,Cl 2, C (C is acetylene carbon
black)+Cl 2 and COCl 2 may be explained by the occurrence of the following reactions:
2−
−
CCl ↑+ 2O= 4Cl
+ CO ↑
4
2
[10.4.58]
2−
−
C + Cl ↑ + O = 2 Cl + CO ↑
[10.4.59]
2
2−
−
COCl ↑+ O 2Cl
+ CO ↑
2
2
[10.4.60]
Thermodynamic analysis of processes [10.4.58]-[10.4.60] showed that the effectiveness
of the purification is in all cases approximately the same, 113 but reaction [10.4.58] has
the advantage that highly toxic reagents (Cl 2, COCl 2) are not used in it. For this reason, CCl 4
has been used in virtually all subsequent studies involving the carbochlorination of chloride
melts. 114-118
It has been suggested that CHBr 3, CBr 4,C 2H 5Br and CHI 3 may be used for the removal
of oxygen-containing impurities from bromide and iodide melts. 119 A significant advantage
of carbohalogenation is that it leads to an appreciable decrease in the concentration of transition
metal cations in the melt being purified. However, in the purification of melts by halo-