Mineral Facts and Problems: 1960 Edition Page: 18
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MINERAL FACTS AND PROBLEMS, ANNIVERSARY EDITION
Bauxite specifications vary with plant and
ultimate use. Table 1 gives the chemical analy-
ses of bauxites used by the metallurgical, abra-
sive, chemical, and refractory industries. Na-
tional stockpile specifications for all grades
except chemical have been established (3).2
TABLE 1.--Chemical analysis of typical bauxites by grade 1
Grade A1203 SiOg Fe2Os Tio
Metallurgical--... 5055-- 0-15.-.. 5-30_......
Abrasive uM.... Min. 55 - a Max. 5-_ _ Max. 6_ Min. 2.5
Chemical......... Min. 55-58. Max. 5-12:_ Max. 2....
Refractory ......- Min. 59-61. Max. 1.5-5.5_ Max. 2... Max. 2.5
1 Most of the remainder represents water of crystallization.
Red mud, the residue that results after alu-
mina is extracted from bauxite by the Bayer
process, consists largely of a complex sodium
aluminum silicate and iron oxides. In the
combination process the red mud is the feed
to a subsequent lime-soda sinter step. The
residue from the combination process is called
brown mud or sinter mud and contains cal-
cium silicates and iron oxides but little so-
dium aluminum silicate.
GEOLOGY AND EXPLORATION (3)
The commercial production of alumina is
derived almost entirely from the hydrated ox-
ides of aluminum contained in bauxite. Po-
tential nonbauxitic sources are the aluminum
sulfates such as alunite formed from hydro-
thermal solutions; the aluminum silicates such
as the feldspars formed by igneous intrusions;
the aluminum silicates such as the andalusite-
sillimanite series formed in schist belts by
metamorphism; and the hydrated aluminum
silicates such as clay formed in beds by the
processes of weathering and sedimentation.
Bauxite is formed by the weathering of
alumina-bearing materials such as feldspars or
clays. Ideal conditions, most likely to occur
in wet tropical or subtropical climates, are
nearly continual rainfall, constant warm tem-
peratures, and relatively good drainage over
a long period of time (1, 16). The formation
of hydrated aluminum silicates or hydrated
aluminum oxides is strongly influenced by the
degree of acidity or alkalinity of the weather-
ing solutions. The Arkansas bauxite deposits
were formed by weathering of nepheline syenite
intrusions during the period between the for-
mation of the Midway marine clays and the
deposition of the Saline formation of the Wil-
2 Italicized numbers in parentheses refer to items in the
bibliography at end of chapter.
cox group of continental sediments. Some later
transported deposits were formed in the un-
consolidated clays and sands near the syenite
Bauxite forms on a wide variety of rocks.
European and Jamaican bauxites are fre-
quently found in the sinkholes on the erosion
surfaces of limestones. Indian bauxites are
associated with basalts, and the bauxites of
Western Africa and the Guianas in South
America are associated with the weathering
of schistose rocks. Many of the largest de-
posits are of the Pliocene or later ages and
are covered by little, if any, overburden.
Prospecting is done along the unconforma-
ble surfaces on which the weathering interval
was suitable for the formation of bauxite.
There is no rapid method of chemical analy-
sis for alumina, but differential thermal meth-
ods have been developed for quick approxi-
mations of the kaolinite and gibbsite content
Since bauxite forms by weathering, the ore
body, which may be buried by normal geologi-
cal processes, is usually nearly horizontal and
close to the surface. Initial drilling is done
at 400- to 4,000-foot intervals and development
drilling at 50- to 200-foot intervals, depending
on the depth and continuity of the deposit.
Open-pit mining accounts for 80 percent of
the bauxite produced in the United States,
also accounts for most of the bauxite produced
in the rest of the world except Europe. In
Arkansas, stripping is done by draglines, shov-
els, and carryalls. Stripping ratios of as much
as 10 feet of overburden to 1 foot of ore are
used and a ratio of 15 to 1 is considered fea-
sible. Several pits in Arkansas have been
mined to depths of more than 100 feet, and
about 200 feet is estimated to be the present
economic limit for large ore bodies. The pits
stand well for unconsolidated rocks, but slump-
ing does occur. Bauxite deposits in Jamaica
lie at the surface, and stripping is limited to
vegetation and topsoil (11). In Surinam, as
much as 80 feet is stripped by draglines, hy-
draulic giants, or dredges (22).
Underground mining accounts for more than
80 percent of French bauxite production and
is widely used in other parts of Europe in-
cluding the U.S.S.R. European deposits fre-
quently are in steeply folded limestone beds,
and the ore-shoot pattern is typical of sink-
hole deposition. Room-and-pillar or shrink-
age stoping is used, depending on size, dip of
the ore body, and condition of the back.
The flat-lying underground deposits of the
United States are worked by room-and-pillar
or longwall methods. The unconsolidated lig-
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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/m1/26/: accessed October 20, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.