Module: 5 Lecture: 24 FLUORINE

Module:5
Lecture:24 Fluorine
Dr. N. K. Patel
Module: 5
Lecture: 24
FLUORINE
INTRODUCTION
Fluorine is denoted by F, is the lightest halogen atom among others
which having atomic number 9 and mostly found in compounds. As it is more
electronegative element having extreme reactivity, it is handled with great
care. Fluorine is a pale yellow gas composed of diatomic molecules which
having single stable isotope, fluorine-19.
Fluorine is the 13thmost abundant element comprising 600 – 700ppm by
mass in Earth's crust while found rarely in stars as compared to other light
elements.
The industrial production of fluorine is carried out from following three
important minerals:
 Fluorite
 Fluorapatite
 Cryolite
In 1530, fluorite was first described, in the context of smelting. The name
derived from the Latin verb “fluo”, it means “flow”, as fluorite was added to
metal ores to lower their melting points. Henri Moissan was carried out the
isolation of fluorine by electrolysis in 1886. The main use of elemental fluorine
was uranium enrichment.
Hydrofluoric acid is the key intermediate for the fluorochemical industry
as the production of elemental fluorine is difficulty and rarely converted to
free fluorine. The fluorides of low charged metals are ionic salts and high
charged metals are volatile molecular compounds. Generally inorganic
fluorides are used in steel making and aluminium refining while organic
fluorine compounds are used in refrigerant gases due to its high chemical
and thermal stability. Traditional chlorofluorocarbons are widely banned.
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Lecture:24 Fluorine
Dr. N. K. Patel
MANUFACTURE
Raw materials
The main raw material for fluorine production is high purity anhydrous
hydrofluoric acid. Typical specifications for hydrofluoric acid are
Assay
HF2, min
SO2, max
H2SiF6, max
H2O2, max
Non-volatile acid (as H2SO4),
max
Wt %
99.95
0.005
0.001
0.02
0.01
Potassium bifluoride, KF, HF is used as a raw material to charge the cells
initially and for makeup when cells are rebuilt. A newly charged cell required
about 1400kg KF.HF. Overall consumption of KF.HF per kg of fluorine
generated is small. Specifications of commercial-grade flake potassium
bifluoride are
Assay
KF.HF2, min
Cl, max
K2SiF6, max
SO4, max
Fe, max
Pb, max
H2O, max
Wt %
99.3
0.01
0.50
0.01
0.02
0.005
0.1
Manufacture process
The generation of fluorine on an industrial scale is a very complex
operation. The anhydrous hydrogen fluoride was first stored in bulk and
charged to a holding tank from which it was continuously fed to the cells
followed by vapourizer where it was heated. Electrolyte for the cells was
prepared by mixing KF.HF with HF to form KF.2HF.
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Lecture:24 Fluorine
Dr. N. K. Patel
Makeup
water
Vaporizer
Refrigeration
tank
HF
storage tank
HF
Surge
tank
Flue
gases
Flue
gases
Cooling
media
Cooling
media
Scrubber
Vent
Air
Stack
Water
Compressor
Vent to
compressor
Vent
F2 header
Air
Burner
Propane
H2 header
Water
To
atmosphere
Cooler
Electrolytic
cell
To F2
header
To H2
header
Water
Vent
Water
Main
feeder
Power
transformer
Light transformers
Coolers
Distillation
Flue
gases
Flue
gases
Cooler
Rectifiers
NaF
towers
Surge
tank
Makeup
water
Surge
tank
Fluorine
storage tank
Rectifiers
HF
condensers
Electric
heat
Recovered
HF storage
To
HF storage
Water
Fluorine
Air
preheater
Air
Electrolytic
cell
Water
Figure: Manufacture of Fluorine by electrolysis of KHF2
Block diagram of manufacturing process
Diagram with process equipment
Animation
The newly charged cells were started up at a low current, which was
gradually increased at a conditioning station separate from the cell
operating position until full current was obtained at normal voltages. After
conditioning cell was connected in series using ca 12V provided for each cell
by a low voltage, 6000A d-c rectifier. Hydrogen fluoride content was
maintained between 40 – 42% by continuous additions. The electorate level
must be set and the seal between the fluorine and hydrogen compartments
were maintained by controlling the certain electorate level below the cell
head. The cells were operated at 95 – 1050C and cooled with water at 750C
via surge tank.
The gases leaving from the cell contain hydrogen, fluorine and 10% HF.
Hydrogen and fluorine stream were separately joined through header of
each cell for further processing in the plant. Entrained electrolyte in the
product gas streams were removed by demisters and filters. Then, the gas
streams were cooled to -1100C in cooler to reduce the HF concentration to
approximately 3mole%. The condensed HF was recycled, and stored while
hydrogen stream was scrubbed with caustic and vented or burned.
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Lecture:24 Fluorine
Dr. N. K. Patel
Sodium towers or further cooling were used to freeze out the HF, if less
than 3% HF levels were required. The concentration of HF was reduced to less
than 0.2mol%. Compression or exhausters were normally required in both
fluorine and hydrogen streams at some point in the system beyond the HF
condensers.
Material of construction and corrosion
Corrosion occurs due to the chemical reaction with environment which
results into degradation of the properties of a material.
Corrosion rates of some of metal at different temperature is tabulated
in the following table
Table - Corrosion of metals at various temperatures
Material
Nickel
Monel
Inconel
Copper
Aluminium
Magnesium
Iron
Steel
0.27% carbon
Stainless, 310
Temperature, 0C
200
300
400
0.018
0.013
0.96
4.1
0
0
0
0
0.23 0.61
0.051
0
0.23
0.79
0.38
14.2
500
0.129
0.051
1.6
3.0
0.33
600
0.74
15
4.3
25
0.46
700
0.86
3.8
13
74
295
503
Due to the stability of the nickel fluoride film, nickel or monel should be
used for critical applications. Polytetrafluoroethylene (PTFE) is resistant to the
gas under nominally static conditions. But the PTFE must be free of impurities,
foreign materials that could initiate ignition with fluorine. As fluoropolymers
are more susceptible to ignition than the metals so it should be used as little
as possible in fluorine handling.
The equipment lines and fittings used for fluorine handling must be
tight, dry and thoroughly cleaned of all foreign matter before use. Foreign
matter such as organics, which are not removed could burn with fluorine and
initiates the metal equipment. After cleaning, the dry nitrogen should be
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Lecture:24 Fluorine
Dr. N. K. Patel
filled. The corrosion resistance of the materials used with fluorine depends on
the passivation of the system which is a pickling operation used to remove
the traces of foreign matter. The dry nitrogen in the system is slowly replaced
with gaseous fluorine.
Equipment
The design of compressors and blowers for gaseous fluorine service
may vary from multistage centrifugal compressors to diaphragm and piston
types. Using bourdon-type gauges or pressure transducers, pressure is
measured. If parts are contact with fluorine then stainless steel or monel
construction is recommended. For all fluorine temperature-measuring
equipment standard thermocouples are used. While for high temperature
service, nickel-shielded thermocouples can be used. Dilute mixtures such as
10 or 20% F2 in N2 are less hazardous than pure fluorine, but still the
precautions and procedures should be taken.
Health and safety
Fluorine is the most reactive element and dangerous material but may
be handled safely using proper precautions. Safety glasses, neoprene coat,
boots and clean neoprene gloves should be worn to afford overall body
protection. This is effective against both fluorine as well as the hydrofluoric
acid. Hydrofluoric acid is produced from the reaction "moisture in the air.
Ace shields made of conventional materials or highly fluorinated
polymers should be worn during the handling of equipment containing
fluorine under pressure. A mask having a self-contained air supply should
always be available. Shields should be provided for valves as it is a
susceptible area for fluorine fires. High pressure binders and valves should be
located with proper protection.
Toxicity
Fluorine has a sharp, penetrating odour detectable at levels well below
the TLV. Fluorine is corrosive in nature and also irritating to the skin. Even at
low concentrations, irritates the respiratory tract while at high concentrations
fluorine inhalation may result in severe lung congestion. Due to the over
exposure of fluorine leads to irritation or burns of the eyes, skins and
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Lecture:24 Fluorine
Dr. N. K. Patel
respiratory tract. The owing emergency exposure limits (EEL) for humans have
been suggested: 15.0ppm for 10min; for 30min; and 7.5 ppm for 60min.
An eye is the most sensitive area to fluorine. Chronic toxicity not occurs
due to the corrosive effects and discomfort associated with inhalation of
fluorine. As the metabolic fate of fluorine is not clear, it does not seem that
much is converted to fluoride ion in the body.
Burns
When pure fluorine gas is contacted with skin the skin burn occurs
which is comparable to thermal burns and differs considerably from those
produced by hydrogen fluoride. Fluorine burns heal much more rapidly than
hydrofluoric acid burns.
Disposal
For disposal of fluorine, it is converted to gaseous perfluorocarbons or
fluoride salts. As the long atmospheric lifetimes of gaseous perfluorocarbons,
disposal by conversion to fluoride salt is more preferred. For that the following
methods are recommended:
 Scrubbing with caustic solution
 Reaction with solid disposal agents such as an alumina, limestone, lime,
and soda lime
 Reaction with superheated steam
In a caustic scrubbing system caustic potash is preferred because of
the higher solubility of the resulting potassium fluoride. Adequate contact
and residence time should be provided in the scrubbing tower for the
neutralization of the intermediate oxygen difluoride, OF2.
Pollution Control
Waste water treatment
During liquid HF electrolysis, no waste process water forms.
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Effluent
During liquid HF electrolysis, no water borne process effluent is formed.
Engineering aspects
Materials used in the fluorine plant are nickel, monel, aluminum
magnesium, copper, brass, steel and carbon steel. For gaseous service at
temperatures below 2000C, copper, brass and steel are used and for
elevated temperature monel and nickel are used. The case where there is
any danger of temperature build-up only nickel or monel should be used. All
equipment, lines and fitting are checked for any leakage and thoroughly
cleaned of all foreign matter before use. The system is also checked for any
leakage at its working pressure. System should be flushed with a non-aqueous
degreasing solvent like methylene chloride. The system should be filled with
dry nitrogen after cleaning.
Material of construction
Passivation of the system affects the corrosion resistance of the all
materials used with fluorine. Generally carbon steel is most commonly used
material for pipe and fittings in gaseous fluorine service at ambient
temperature and 2.86Ma (415psi) pressures.
While carbon steel or bronze-body gate valves are generally used in
gaseous fluorine service at low pressure. Monel plug valves and plugs are
used for moderate pressure service below 500kPa. PTFE polymer is generally
used for valve-stem packing and it must maintain leak-tight. Bellow-type
valves having monel or stainless steel bellows are recommended for high
pressure services, but not for ball valves
PROPERTIES
 Molecular formula
 Molecular weight
 Appearance
 Odour
 Boiling point
 Melting point
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: 18.99gm/mole
: Pale yellow gas or bright yellow liquid
or transparent liquid
: Pungent irritating odour
: -188.120C
: -219.620C
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Lecture:24 Fluorine
 Density
 Heat of vaporization
 Solubility
Dr. N. K. Patel
: 1.505gm/cm3
: 6.51kJ/mole
: Soluble in water
USES
Fluorine
 Elemental fluorine is also used to produce sulfur tetrafluoride, SF6, by the
reaction of sulfur and fluorine under controlled conditions
 In the preparation of polymer sulfur for adhesive or coatings
 Used in the production of fluorinated organics for the manufacture of
perfluorinated materials
 For plasma etching, flat panel display and MEMS (micro electromechanical) fabrication
 To make sulfur hexafluoride
 In producing isotopically fractionated uranium (from UF6)
 In the manufacture of uranium hexafluoride (UF6)
 In the production of sulfur hexafluoride, SF6
 Its high dielectric strength, inertness, thermal stability and ease of
handling have led to increased use as an electrically insulating
medium, permitting reductions in size, weight, and cost of high voltage
electrical switch gear, breakers, and substations
 Fluorine-18 emits positrons and has a relatively large half-life. This makes
it ideal for use in positron emission topography
Fluorine compounds
General anaesthetics are derived from compounds of fluorine
Anti-reflective coatings contain compounds of fluorine
In refrigeration and air-conditioning systems
Sodium fluoride, stannous fluoride and sodium MFP are added to
toothpastes to prevent dental cavities. Now a day’s these are regularly
added to water as well
 Sulfur hexafluoride is used as a dielectric medium in the electrical
industry
 Uranium hexafluoride is used in the gaseous diffusion process for the
separation and enrichment of uranium-235, which exists in low
concentration in natural uranium




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Lecture:24 Fluorine
Dr. N. K. Patel
 Sulfur tetrafluoride is used to produce fluorochemical intermediates in
the pharmaceutical and herbicide industry
 Tungsten hexafluoride (WF6) and rhenium hexafluoride (ReF6)
respectively are used in the chemical vapour deposition industry to
produce metal coatings
 Nitrogen trifluoride (NF6) is used as a fluorine source in the high power
hydrogen fluoride-deuterium fluoride (HF/DF) chemical laser and in the
production of microelectronic silicon-based components
 The superior chemical and thermal stability of perfluorocarbons has led
to used to high temperature lubrication, thermal testing of electronic
components, and as specialty fluids for vacuum pumps, liquid seals,
and hydraulic applications
 As the perfluorinated material has high solubility of oxygen, it has been
used as synthetic blood substitutes
Hydrofluoric acid
 To etch the glass, generally in light bulbs
 In one step of the production of halons such as Freon
 To obtain pure uranium from uranium hexafluoride
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