Mineral Facts and Problems: 1960 Edition Page: 19
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ALUMINA AND BAUXITE
nitic clays and sandstones of the back are weak
and must be supported by a cap of bauxite
about 2 feet thick. A modified longwall
method,. which retreats by mining diamond-
shaped rooms along a cave line, has been de-
veloped by Reynolds Mining Co. for use in its
Hurricane Creek mine in Arkansas. Using this
method, average recoveries of the ore mined
were about 80 percent, although in soft parts of
the ore body recovery may be as low as 50
Restoration of mined-out open-pit areas to
usefulness is required in Jamaica and is be-
coming increasingly important in other areas.
Subsidence where bauxite was extracted un-
derground is of minor importance today but
may become more serious in the future.
Bauxite is often washed to remove silica or
clay minerals. The high-iron ores of Surinam
are concentrated by jigging or heavy-medium
In submarginal bauxites the iron oxides, clay
minerals, and part of the titanium minerals
are often so finely dispersed through the ore
that physical methods have not produced suc-
During World War II a pilot plant was op-
erated by the Bureau of Mines at Bauxite,
Ark., to test various ore-dressing procedures,
including gravity, flotation, and magnetic
methods (20). Beneficiation by magnetic tech-
niques to eliminate iron is practiced on a lim-
ited scale on bauxite for nonmetallurgical
Although research on the extraction of alu-
mina has continued for many years, virtually
all of the commercially produced alumina is
obtained by a process patented by Karl Bayer
in 1888 (German Patent 43,977) for use on
bauxite. A number of modifications and im-
provements have been made in the process to
adapt it to various types of bauxite. The
process involves a caustic leach at elevated tem-
perature and pressure, followed by separation
of the resulting sodium aluminate solution and
selective precipitation of the alumina. There
are two principal variations of the Bayer proc-
ess: (1) The European Bayer, in which the
approximate conditions of leaching are a pres-
sure of 210 p.s.i., a temperature of 3900 F., a
caustic concentration of 400 grams per liter,
and a digestion time of 2 to 8 hours to effect
solution of the monohydrate mineral boehmite;
and (2) the American Bayer, in which a pres-
sure of about 60 p.s.i., a temperature of about
2900 F., a caustic concentration of 170 grams
per liter, and a digestion time of 1/2 to 1 hour
are used to dissolve the trihydrate mineral
gibbsite (3, 18, 19). The American process is
used for treating trihydrate bauxites, because
it requires less evaporation and lower tem-
peratures and is more adaptable to continuous
In both processes the pregnant solution is
separated from the red mud tailings by coun-
tercurrent decantation and filtration. The liquor
is cooled until it becomes supersaturated, then
seeded with crystals of aluminum trihydrate.
About one-half of the alumina in solution is
precipitated in a 36- to 96-hour period. The
precipitate is then filtered, washed, and cal-
cined at 2,0000 F. to obtain the final product.
Caustic soda is regenerated in the precipita-
tion step and, together with the unprecipitated
alumina, is recycled to the digesters. For
some nonmetal uses the alumina is left in the
trihydrate form or is only partly calcined.
The finely divided residue resulting from
leaching contains Fe203, TiO2, and a complex
sodium aluminum silicate compound. This
compound represents a loss of soda and alu-
mina, and the quantity discarded in the residue
is related to the silica content of the bauxite.
Approximately 1.1 units of alumina and 1.2
units of soda are lost for each unit of silica
in the ore. Bauxite must contain less than 8
percent silica for economic treatment by the
Approximately 4 long dry tons of bauxite
is required to produce 2 short tons of alumina,
which will yield upon electrolysis slightly more
than 1 short ton of aluminum. The materials
other than bauxite required in the Bayer proc-
ess are soda ash; lime for causticizing the soda
ash; and fuel oil, gas, or coal (2).
Crude bauxite from the mines usually is
crushed to about 2-inch size in hammermills,
gyratories, or jaw crushers. Free moisture,
but not the water of crystallization, is re-
moved from the ores by drying when they
are to be transported considerable distances.
Bauxite from Surinam and British Guiana is
dried to less than 3 percent free moisture.
Bauxite from Jamaica and Haiti, after drying,
may still contain as much as 15 percent free
moisture to prevent dusting when the ore is
transported. Further treatment depends upon
the end use. Ores for refractory uses are cal-
cined in rotary kilns at 2,5000 to 2,9000 F.
and for abrasive uses at 1,7000 to 2,4000 F.
to remove water of crystallization.
Bauxite with more than 8 percent silica is
treated by the combination process (3), which
first became commercial as a result of the re-
duced supply of imported high-grade bauxite
during World War II. In this process, baux-
ite containing 12 to 15 percent silica is first
subjected to a Bayer leach. The resulting red
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United States. Bureau of Mines. Mineral Facts and Problems: 1960 Edition, report, 1960; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc38790/m1/27/: accessed February 26, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.