Method for forming basalt fibers with improved tensile strength

A method of improving the tensile strength of drawn fibers produced from molten basalt rock. Strength is increased by reducing the ferric iron content of the final fibers below that which would be present in the fibers if drawn under normal atmospheric and operational conditions. This is accomplished by either adding a reducing agent to the melt, by drawing the fibers in an inert or reducing atmosphere, or by a combination of both methods.

BACKGROUND OF THE INVENTION 
This invention relates to the production of fibers from basalt rock. The 
general technology necessary to produce such fibers is known, and utilizes 
available equipment and processes. A discussion of basalt, including a 
review of its chemical composition, microstructure, and physical 
characteristics, is provided in U.S. Pat. No. 3,557,575, which is hereby 
incorporated into this disclosure by reference. Similarly, a detailed 
discussion of the manner by which basalt can be drawn as fibers is 
provided in U.S. Pat. No. 4,008,094, which also is incorporated by 
reference. An apparatus for producing fibers of ceramic materials is 
disclosed in U.S. Pat. No. 3,066,504, which is further incorporated by 
reference. 
The general technology for drawing fibers from basalt is well known and 
recognized in the literature. A review of this literature is set out in an 
article titled "Mineral Fiber from Basalt--Potential New U.S. Industry?" 
by R. A. V. Raff in the periodical Engineering and Mining Journal 
published in February, 1974. The present disclosure relates to the 
discovery of an improved process by which the tensile strength of such 
fibers can be increased beyond those values expected in the use of the 
prior art processes. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Basalt rock contains compounds of iron, including substantial components of 
both ferrous oxide, FeO and ferric oxide, Fe.sub.2 O.sub.3. Typical 
samples of basalt rock contain about two percent of ferric oxide and nine 
to twelve percent of ferrous oxide. However, when the basalt rock is 
melted under normal process conditions in an electric furnace and 
subsequently drawn through a platinum die, substantial portions of the 
ferrous oxide are oxidized to produce an increase in the ratio of ferric 
oxide to ferrous oxide over that ratio which is present in the initial 
rock. We have found that by controlling this rate of oxidation so as to 
minimize the ratio of ferric oxide to ferrous oxide in the resulting 
fibers (or conversely, to increase the ratio of ferrous oxide to ferric 
oxide) the resulting fiber will demonstrate desirable increased tensile 
strength. 
The improvement in tensile strength in the fiber may be accomplished in 
several different ways. Reduction of ferric iron as it is formed can be 
accomplished in an induction furnace by adding sugar, graphite or other 
carbon sources to the melt. Carbon can also be introduced by utilizing a 
graphite crucible or a crucible having a graphite lining as the vessel in 
which heat is used to melt the pellets of basalt rock. The carbon source 
materials in the melt form carbon monoxide or carbon dioxide when heated. 
Both substances are gaseous, and either reducing or inert agents are mixed 
intimately with the melt to provide the reducing or inert atmosphere 
necessary to counter normal oxidation of the ferrous iron components. 
During production of the fibers, it is normal that additional oxidation of 
the ferrous compounds present in the melt occurs. Prevention or reduction 
of this oxidation by use of an inert atmosphere such as nitrogen, or a 
reducing atmosphere, such as carbon monoxide, allows fibers of greatly 
increased strength to be prepared.

EXAMPLE 1 
A sample of basalt rock having an initial content of 2.1% ferric oxide and 
11.5% ferrous oxide was melted under normal ceramic conditions and drawn 
to produce fibers of identical diameter, the fibers being drawn under air 
and under a nitrogen atmosphere, which was utilized to prevent oxidation 
of the ferrous iron. These tests were repeated several times and are 
summarized in the following table 1. Samples 1 and 2 were drawn under air, 
allowing oxidation of the ferrous iron without control. Sample 3 was drawn 
under a nitrogen atmosphere to prevent oxidation of the ferrous iron. The 
substantial improvements in tensile strength are readily observable. 
TABLE 1 
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Tensile 
Temp. of Strength 
Sample % FeO % Fe.sub.2 O.sub.3 
Drawing .degree. C. 
GPa psi 
______________________________________ 
1 5.7 8.5 1250.degree. 
1.72 249,000 
1325.degree. 
1.93 280,000 
1370.degree. 
2.09 303,000 
2 7.1 7.0 1250.degree. 
2.14 310,000 
6.8 7.3 1325.degree. 
2.42 350,000 
3 9.8 4.0 1250.degree. 
2.84 412,000 
9.5 4.3 1325.degree. 
3.07 445,000 
9.2 4.7 1370.degree. 
3.17 460,000 
______________________________________ 
EXAMPLE 2 
Tests have shown that the amount of ferrous iron in the final drawn fibers 
can be increased by placing a carbon rod in the molten basalt during the 
fiber forming operation, or by adding two percent starch to the melt. 
Taking a typical basalt fiber formed in air, the resulting percentage of 
ferrous oxide in the fiber was measured at 5.7%. A fiber formed by the 
identical process with the addition of 2% starch to the melt was found to 
have a ferrous oxide content of 8.5%. A fiber formed by the identical 
process, but having a carbon rod placed in the melt was found to have a 
ferrous oxide content of 10.7%. Our experiments show that the tensile 
strength of the resulting fibers increases with an increase in the 
percentage of the ferrous oxide content in the fiber. 
EXAMPLE 3 
Basalt fibers were made from four additional basalts, each showing an 
increase in tensile strength with an increase in the percentage of ferrous 
iron. The chemical analysis of the basalts are as follows: 
TABLE 2 
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X-6 K-9048 K-9017 O-2 
______________________________________ 
SiO.sub.2 49.10 50.48 53.61 50.50 
Al.sub.2 O.sub.3 
13.80 5.18 5.14 16.00 
TiO.sub.2 3.16 1.69 1.84 2.17 
Fe.sub.2 O.sub.3 
2.10 3.20 3.31 2.96 
FeO 11.50 7.51 8.34 10.22 
MnO 0.21 0.19 0.18 -- 
CaO 9.43 10.62 8.43 10.00 
MgO 5.25 6.49 4.98 4.30 
K.sub.2 O 1.26 0.80 1.14 0.35 
(Na.sub.2) 3.09 2.62 2.73 3.20 
P.sub.2 O.sub.5 
0.68 0.33 0.35 
______________________________________ 
The above basalts were melted and drawn with varying ratios of ferrous 
oxide to ferric oxide in the final fibers as reported in the following 
table. 
TABLE 3 
______________________________________ 
Tensile Strength 
% FeO % Fe.sub.2 O.sub.3 
Temp. GPa psi 
______________________________________ 
X-6 9.1 5.2 1300.degree. 
3.22 467,000 
8.3 6.1 1300 2.98 432,000 
6.8 7.3 1325 2.42 350,000 
5.7 8.5 1325 1.93 280,000 
K-9048 6.5 4.6 1300.degree. 
3.11 450,000 
5.5 5.4 1300 2.40 348,000 
K-9017 7.8 3.9 1350 3.16 458,000 
7.3 4.5 .350 2.06 298,000 
O-2 8.6 4.6 1300.degree. 
3.13 454,000 
5.5 8.1 1300 2.40 347,000 
______________________________________ 
As can be seen in Table 3, the resulting tensile strength of the fibers 
increases with an increase in its ferrous oxide content. 
This discovery of a method of producing fibers of greater tensile strength 
than expected from basalt rock is believed to possibly open new industrial 
applications to a readily available raw product. 
In general, staple fibers from available minerals have substantial 
industrial values if their mechanical properties are adequate. Basalt is 
basically an inexhaustible natural resource. Only electric power is 
required to melt and draw the fiber and such processes are ecologically 
nonpolluting. This method is believed to be capable of extending the use 
of basalt fibers into reinforcing applications where these fibers have not 
found acceptance because of tensile strength limitations.