Polarization fluorine cells

Figure 4. Galvanostatic polarization curves for the metal fluoride impregnated carbon in the fluorine cell...

As CXF is the material which contributes to the anode effect, the carbon electrode with CXF formed on it is easily polarized. The elimination of traces of water from the crude bath by the hydrolysis of water with fluorine is important for the stable operation of the fluorine cell. 

The primary use of lithium fluoride is in the ceramic industry. It reduces the firing temperature and improves the resistance to abrasion, acid attack and thermal shocks. It is essential component of the fluorine cell electrolyte. An addition of small amount (1-1.5%) to KHF2 HF electrolyte improves the wettability of the carbon anodes and lowers the tendency of the cell to polarize. Another important use of LiF is in flux compositions containing chlorides, borates and other fluorides. Lithium fluoride windows made from high purity crystals are used for X-ray monochromators, UV, visible or IR regions [18]. 

The electrolyte used in fluorine cells is a molten mixture of the approximate composition KF-2HF, at 80-100°C. The anodes are carbon and the cathodes are usually steel. The fluorine liberated at the anode is expected to leave as gas, but it does not form bubbles on the electrode in the usual way, giving rise to what is known in the fluorine industry as polarization . 

From Eq, (1) it is clear that a model of crystal polarization that is adequate for the description of the piezoelectric and pyroelectric properties of the P-phase of PVDF must include an accurate description of both the dipole moment of the repeat unit and the unit cell volume as functions of temperature and applied mechanical stress or strain. The dipole moment of the repeat unit includes contributions from the intrinsic polarity of chemical bonds (primarily carbon-fluorine) owing to differences in electron affinity, induced dipole moments owing to atomic and electronic polarizability, and attenuation owing to the thermal oscillations of the dipole. Previous modeling efforts have emphasized the importance of one more of these effects electronic polarizability based on continuum dielectric theory" or Lorentz field sums of dipole lattices" static, atomic level modeling of the intrinsic bond polarity" atomic level modeling of bond polarity and electronic and atomic polarizability in the absence of thermal motion. " The unit cell volume is responsive to the effects of temperature and stress and therefore requires a model based on an expression of the free energy of the crystal. 

The other important factor to affect the operational conditions of the cell is the voltage increase between the carbon and copper lead. This problem has been solved individually in industry. For example, a 250 pm thick layer of nickel can be coated onto the upper part of the carbon anode using the atmospheric plasma spraying method.7 This electrode has been operated at 15 to 17 A dm-2 in a 1000 A scale industrial cell for 19 months. The cell voltage was 9.5 V and polarization did not occur with this electrode. Characteristic points of this new carbon electrode are low polarizability and no anode effect, and the concentration of carbon tetrafluoride contaminating the fluorine is below 2 ppm. 

A cell with a capacity of 1 L was made of mild steel. An amorphous carbon rod (diameter 25 mm length 15 cm) was used as anode, the inside wall of the cell as cathode and a platinum wire was used as reference electrode. The anode compartment of the cell was separated from the cathode compartment by a skirt of steel welded to the cell cover. The anode gas was passed through a tube filled with tablets of NaF to absorb anhyd HF gas and then led to a gas sampler. Fluorine was detected with K.I soln. After the starting material was added into the molten KIIF2/HF salt, the electrolyte was pre-electrolyzed at a low current density until NF2 was detected, and then current efficiency of each product and polarization curves by galvanostatic or potential sweep method were determined (Table 1). At optimum conditions the current efficiency of NF3 was 55%. 

At this point an interesting observation can be made. Fluorine atoms are put into anesthetics in order to make the molecules more inert. Instead of entering chemical reactions such molecules only participate in intermolecular associations in the cell membrane, a condition for being used as anesthetics. The most potent anesthetics, however, usually contain a last hydrogen, often referred to as the acidic hydrogen. Chloroform, halothane, methoxyflurane are in this category. Molecules containing this C—H bond are, of course, expected to enter polar interactions. 

Polarization can be controlled to some extent, but it is never fully overcome and this, combined with the anode- thode gap of about 5 cm necessary to avoid reactions between the electrolysis products in undivided cells, causes the cell voltage to be from —8.5 to —13 V for the working current density of 70-200 mA cm (Table 5.2). This compares with the calculated reversible cell voltage of — 2.9 V so it is clear that fluorine manufacture cannot be described as energy-efficient The total energy consumption for a cell with a 95% current efficiency (Ic. with a small amount of back chemical reaction between and Hj)... 

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