are the most significant.
Placer deposits originate when
chemical and mechanical weathering
liberates minerals from the source rock.
This is followed by transport of the
minerals to the sea by water, wind, and
gravity. Upon reaching the sea, deposits
of economic value form primarily by
hydraulic sorting.  Heavy minerals
.including zircon, are concentrated by
marine currents and wave action.
Mechanical sorting by the sea and wind
fiuther sorts the grains to produce well-
sorted beach deposits.
Technology
Economic concentrations of zircon are
found in association with other heavy
minerals such as ilmenite, monazite, and
rutile.  Heavy-mineral sand deposits
usually are mined by floating cutterhead-
or bucket wheel-dredges that handle up to
2,800 of sand per hour. Sand recovered
by these techniques is sent to a wet mill
and treated by wet-gravity methods, using
spirals, cones, sluices, or jigs to produce
a mixed heavy-mineral concentrate
containing zircon. The mixed concentrate
typically  contains  other  economic
minerals such as the titanium minerals,
ilmenite, leucoxene, and rutile, and often
smaller amounts of the rare-earth mineral
monante.
The mixed heavy-mineral concentrate
is scrubbed, dried, and screened, and the
individual heavy minerals are separated
by   electrostatic,  electromagnetic,
magnetic, and gravity processes. Zircon,
in contrast to ilmenite, rutile, and many
other heavy minerals, is nonconductive
and can be separated, along with
monazite, by electrostatic methods.
Monazite, which is slightly magnetic and
may be slightly higher in specific gravity,
can be separated from zircon by
electromagnet or by gravity concentration
methods.
To obtain premium-grade zircon, the
zircon concentrate from the electrostatic-
electromagnetic circuit is subjected again
to gravity concentration to reduce the
content of aluminum- and titanium-
bearing  minerals.   Certain  zircon
products are leached in an acidic solution


to remove iron oxide and other grain
coatings.
Zircon used in foundry and certain
refractory applications is graded and
sized, and in many cases, ground or
milled to produce zircon flour. Foundry
applications generally use zircon sand and
flour mixtures, which may be treated with
resin coatings and binders.
Zirconia is produced directly from
zircon by either plasma fusion or electric
arc techniques. The plasma method for
zirconia production employs heating a
finely divided zircon above its
dissociation temperature to form small
zirconia crystallites and glassy amorphous
silica.  The hot zirconia and silica
particles are rapidly quenched and the
silica removed by leaching with sodium
hydroxide, leaving insoluble zirconia
crystallites.
In electric arc production, zircon is
heated to temperatures of approximately
2,500f C to produce a dissociation. The
silicon component is vaporized and
recovered as fumed silica, leaving a
residual melt that is air quenched to form
zirconia. A more complex electric arc
method melts a mixture of limestone and
zircon to form calcium zirconate and
tricalcium silicate clinker. Cooling of the
mixture disintegrates the mixture into a
very fine powder composed of tricalcium
silicate and lime and a coarser fraction of
calcium zirconate crystals. The calcium
zirconate is separated from the other
constituents by either air classification or
flotation.  The acid-soluble calcium
zirconate crystals are treated with acids
or other reagents to form zirconia or
zirconium salts. Other compounds of
zirconium, such as the hydrous or
carbonated  oxide,  acetate,  sulfate,
fluoride,  chloride,  and   organic
complexes, are usually prepared from
zirconia or its salts by chemical reactions.
The deBoer-van Arkel iodide process,
first described in 1925, is essentially a
refining process and was commercially
adopted in 1945 as the first practical
method for producing ductile zirconium
metal. Zirconium metal is reacted with
iodine vapor at 2000 C to form zirconium
tetraiodide, leaving most impurities
except hafnium in a solid state. The


gaseous halide diffuses to a heated
filament where, at 1,3000 C, the reaction
is reversed, depositing very high-purity
elemental zirconium on the filament and
regenerating iodine vapor for reuse. The
process yields high-purity metal but is
expensive to operate.  Industrial-scale
plants for producing zirconium metal are
based on the Kroll process in which
zirconium tetrachloride is reduced with
molten magnesium in an inert
atmosphere. The resulting mixture of
zirconium metal sponge and magnesium
chloride is vacuum distilled to remove the
magnesium chloride. The zirconium is
crushed, sized, and compacted to form a
consumable electrode. The electrode is
arc melted in an inert atmosphere to give
a first-melt ingot. The first-melt ingot is
then used as a consumable electrode to
produce a metallurgically homogeneous
second-melt ingot, which is machined to
give a clean surface and readied for
fabrication. Zirconium tetrachloride is
produced by chlorinating zircon sand in a
fluidized bed containing carbon at a
temperature of about 1,1500 C.
Byproducts and Coproducts
Zircon is mined from deposits with a
heavy-mineral grade between 2% and
20%. The associated economic heavy
minerals  are  ilmenite,  leucoxene,
monazite, rutile, and the tin minerals
cassiterite and stannite. Its classification
as a byproduct or a coproduct depends on
the fraction of zircon in relation to the
other minerals. If zircon and the titanium
minerals are expected to account for the
majority of the revenue, it is considered
a coproduct.  If minerals other than
zircon provide the carrying costs of the
operation, then zircon is considered a
byproduct.     Small  quantities  of
baddeleyite are recovered as a byproduct
of copper, phosphate, and vermiculite
mining in the Republic of South Africa.
ANNUAL REVIEW
Issues


Processing of some mineral sands by
certain  methods    results  in  the


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