produced. HF reacts with silicon oxide (SiO 2 ) in the fluorapatite to form SiF 4
gas, as shown in Reaction 10.2.
Ca 10 F 2 ðPO 4 Þ 6 þ 7H 2 SO 4 þ 3H 2 O ! 3CaH 4 ðPO 4 Þ 2 :H 2 O þ 7CaSO 4 þ 2HF "
Fluorapatite
ð10:1Þ
SiO 2 þ 4HF ! SiF 4 þ 2H 2 O
ð10:2Þ
In the aqueous scrubber, SiF 4 readily reacts with water, forming
fluorosilicic acid (H 2 SiF 6 ), as shown in Reaction 10.3. Fluorosilicic acid is
highly soluble in water and is readily absorbed by plants.
1
3SiF 4 þ 2H 2 O ! SiO 2 þ 2H 2 SiF 6
ð10:3Þ
10.3.3 MANUFACTURE OF ALUMINUM
Manufacture of aluminum is carried out almost exclusively by the Hall–
Herouet process, where alumina (Al 2 O 3 ) is dissolved in molton cryolite and
reduced electrolytically. The electrolytic cell contains a carbon lining, serving
as both the cathode and the container for the melt. Reaction 10.4 shows the
process.
ð10:4Þ
As shown in Reaction 10.4, CO and CO 2 are the two gases emitted in the
process: there is no emission of any F-containing substance. In reality,
however, a number of other substances are emitted in the process. This is
because several catalysts, including CaF 2 , AlF 3 , and cryolite, are used in the
eletrolysis of alumina, and as these are heated at high temperatures, some will
escape from the cells, contaminating the surrounding atmosphere. In addition
to CO and CO 2 , several other gases, including SO 2 , SiF 4 , HF, COS, CS 2 , He,
and water vapor, are also produced in the electrolysis cells, and are emitted into
the surrounding air. Moreover, a large number of particulates are also emitted,
including Al 2 O 3 , carbon (C), cryolite, AlF 3 , CaF 2 , Fe 2 O 3 , and chiolite
(Na 5 Al 3 F 14 ).
10.3.4 MANUFACTURE OF STEEL
In the manufacture of steel, calcium fluoride (CaF 2 ) is used as a flux (a
substance that promotes fusion of metals) in open-hearth smelters to increase
fluidity of the slag and enhance the removal of impurities, such as P and sulfur
(S), from the melts. F compounds emitted from this operation include HF and
CaF 2 .
1
Environmental Fluoride
153
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Ref: 4365 MING-HO YU Chap-010 Page: 153 149-170
gas, as shown in Reaction 10.2.
Ca 10 F 2 ðPO 4 Þ 6 þ 7H 2 SO 4 þ 3H 2 O ! 3CaH 4 ðPO 4 Þ 2 :H 2 O þ 7CaSO 4 þ 2HF "
Fluorapatite
ð10:1Þ
SiO 2 þ 4HF ! SiF 4 þ 2H 2 O
ð10:2Þ
In the aqueous scrubber, SiF 4 readily reacts with water, forming
fluorosilicic acid (H 2 SiF 6 ), as shown in Reaction 10.3. Fluorosilicic acid is
highly soluble in water and is readily absorbed by plants.
1
3SiF 4 þ 2H 2 O ! SiO 2 þ 2H 2 SiF 6
ð10:3Þ
10.3.3 MANUFACTURE OF ALUMINUM
Manufacture of aluminum is carried out almost exclusively by the Hall–
Herouet process, where alumina (Al 2 O 3 ) is dissolved in molton cryolite and
reduced electrolytically. The electrolytic cell contains a carbon lining, serving
as both the cathode and the container for the melt. Reaction 10.4 shows the
process.
ð10:4Þ
As shown in Reaction 10.4, CO and CO 2 are the two gases emitted in the
process: there is no emission of any F-containing substance. In reality,
however, a number of other substances are emitted in the process. This is
because several catalysts, including CaF 2 , AlF 3 , and cryolite, are used in the
eletrolysis of alumina, and as these are heated at high temperatures, some will
escape from the cells, contaminating the surrounding atmosphere. In addition
to CO and CO 2 , several other gases, including SO 2 , SiF 4 , HF, COS, CS 2 , He,
and water vapor, are also produced in the electrolysis cells, and are emitted into
the surrounding air. Moreover, a large number of particulates are also emitted,
including Al 2 O 3 , carbon (C), cryolite, AlF 3 , CaF 2 , Fe 2 O 3 , and chiolite
(Na 5 Al 3 F 14 ).
10.3.4 MANUFACTURE OF STEEL
In the manufacture of steel, calcium fluoride (CaF 2 ) is used as a flux (a
substance that promotes fusion of metals) in open-hearth smelters to increase
fluidity of the slag and enhance the removal of impurities, such as P and sulfur
(S), from the melts. F compounds emitted from this operation include HF and
CaF 2 .
1
Environmental Fluoride
153
[16:52 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-010.3d]
Ref: 4365 MING-HO YU Chap-010 Page: 153 149-170