Mineral Facts and Problems: 1960 Edition Page: 79
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unknown depths. Those of Arizona, however,
consist of more or less horizontal asbestos-
bearing serpentine zones in thin-bedded lime-
stone. Such deposits are generally less
extensive and less persistent than those in mas-
sive serpentine. The amosite and crocidolite
of the Union of South Africa occur in banded
ironstones that are so folded and contorted that
the veins are very irregular (15, 16, 17).
In Vermont, the fiber-bearing rock is re-
moved from an open pit, which is worked on
three 125-foot-high benches. The usual prac-
tice in Arizona is to drive a 5- to 6-foot heading
in barren limestone beneath the fiber zone,
which is later blasted down and hand-cobbed.
The Canadian mines were originally open pits,
but there has been a gradual transition to under-
ground mines using block-caving methods.
The chrysotile of Southern Rhodesia, the Union
of South Africa, and Swaziland is obtained
from underground mines. Amosite is obtained
chiefly from large underground workings, and
blue asbestos from small open pits and shallow
mines. The Russian deposits are worked both
in open pits and underground. Cyprus has
large open-pit workings (3, 11, 12, 15).
Asbestos milling is a complex operation in-
volving primarily the separation of fiber from
rock and classification of the fiber by length.
Asebstos, unlike the metals, is not a single com-
modity, but comprises a series of grades depend-
ing upon the length of individual fibers. The
operator has little control over the proportions
of the various grades, as they are fixed in nature,
but he has the important task of minimizing
fiber damage. He must mill the rock with ats
little fiber breakage as possible, because short
fibers are worth much less than long ones.
Special types of rock disintegrators have been
developed, and reduction is accomplished in
stages with fiber removal by air suction at each
In Arizona, the hand-cobbed rock is reduced
in jaw crushers, and the fiber separated by
screening. That which passes over a 1/2-inch
screen is classed as No. 1; minus-1/2-mesh, plus-
1/4 is No. 2; and minus-1/4-mesh grades 3 and
4. Some mills process the Nbs. 3 and 4 with
hammer mills, screens, and air separators.
Nos. 1 and 2 are classed as spinning grade if
the fibers are not too harsh. Some spinning
fiber is obtainable from No. 3.
In Vermont and Canada the mills are large
and complex. The fiber is classified in many
grades, such as spinning, cement stock, paper
Asbestos has a great variety of uses, which
depend primarily on fiber length. The longer
fibers-Canadian groups 1, 2, and 3, and their
equivalents from other countries-are used for
making textile products such as cloth, yarn,
tape, rovings, etc. The asbestos is spun and
woven in much the same way as cotton, silk,
or wool. Asbestos fabrics are used extensively
for lagging cloth, brakeband linings, clutch
facings, safety clothing, packings, gaskets, and
various other appliances. Low-iron fiber is
used in cable insulation (2, 8).
Shorter fibers such as Canadian group 4, and
to some extent 5 and 6, are used widely for
asbestos-cement products such as roofing shin-
gles and flat and corrugated siding. These
products consist of about 80 percent portland
cement and 20 percent asbestos. Canadian
groups 4 and 5 and African amosite are used
extensively for making 85 percent magnesia
block and pipe insulation. They consist of
about 85 percent basic magnesium carbonate
and 15 percent asbestos. Canadian group 5 is
suitable for asbestos paper and millboard manu-
facture. Such products are made on standard
paper-making machines. An important use of
asbestos paper is for making the so-called air-
cell pipe covering. The shortest fibers are used
for boiler and roofing cements, and as filler in
asphalt floor tile and various other products.
Amosite is used for felted insulation in blan-
ket form for high-temperature service up to
9000 F. A loosely compacted form is applied
as a covering for marine turbines, jet engines,
and similar applications. Amosite is also used
as a constituent of 85-percent-magnesia insula-
tion and light-weight, fire-resistant marine par-
Long-fiber crocidolite (blue asbestos) is
woven into fabrics used for locomotive boiler
lagging (in Great Britain) and for acid-resist-
ant packings and gaskets. The principal use
of the shorter crocidolite fibers is in making
asbestos-cement pipe. Bolivian crocidolite has
been found useful for making gas-mask filters.
Tremolite and anthophyllite are used for
chemical-resistant filters, as welding-rod coat-
ing, and as fillers in various products.
There are virtually no commercial byproducts
at asbestos mines and mills. The fiber com-
prises a small part, ranging commonly from
5 to 7 percent of the rock mined, and the waste
rock consists of pulverized serpentine for which
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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/m1/87/: accessed May 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.