Note: Descriptions are shown in the official language in which they were submitted.
B~CKGROUND OF THE INVENTION
This invention relates to a method ~or obtaining
high purity silica from sand and sllica concentrates.
High purity silica with relatively low impurity
levels, typically containing less than 1 ppm of iron, is one
of the most important raw materials used by the semiconductor
and by the optical fibre industries. Such high purity silica
or quartz usually has, heretofore, been obtained by expensive
and high energy-consuming melting and vapour phase processes.
PRIOR ART
The removal of iron contamination from sand by leach-
ing in mineral acids, such as hydrochloric or sulphuric acids,
is described in the prior art. U.S. Patent No~ 1,983,270, for
example, teaches the leaching of sand by dilute hydrochloric or
sulphuric acid, then heating and evaporating the slurry to
increase the acid concentration~ The process of U.S. Patent
No. 1,983,27~ applies mechanical rubbing or attrition to the
sand particles prior to acid leaching, in order to increase the
surface of the sand accessible to leaching. U.S. Patent No.
4,042,671 describes a container, a method of layering of sand
and pebbles, and ways of recycling the leaching acid in a process
for the removal of iron compounds from sand.
There are known methods of silica purification wherein
silica is first completely digested and subsequently precipitated
in a purified form. One such methGd is described for example in
U.S. Patent No. 3,808,309 in which amorphous silica is dissolved
in aqueous alkaline solution and an acid is added to form silicic
acid which is then extracted with an organic hydrogen bonding
agent and then polymerized in the presence of a fluoride catalyst.
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It is also well known, and i~ used in various glass
technology processes, that hydrogen fluoride, in substantial
concentration, completely dissolves silica or silica-bearing
compounds to form fluosilicic acid.
U.S. Patent No. 3,968,197 describes a process in
which sodium silicofluoride, obtained by prior reaction with
hydrogen fluoride, is reacted wi-th ammonia to precipitate very
pure silica gel and sodium fluoride.
Removal of impurities, specifically of iron oxides,
from the surface of sand by leaching in a solution containing
sulphuric acid and sodium chloride in approximately equal quanti-
ties, is described in U.S. Patent No. 2,306,021. In a preferred
embodiment of this process about one percent hydrofluoric acid
is added to the above leaching solu-tion, for what is described
as soft~ning of the silica pore walls, and enhancing the leach-
ing action of the above reagents.
In a process for the purification of sand, U.S. Patent
No. 2,381,843 teaches the soaking of silica-bearing minerals in
a solution containing titanous sulphate and about 2 lbs of
hydrogen fluoride per ton of sand, then washing. In a similar
process for the removal of ferruginous coating, U.S. Patent
No. 2,583,280 teaches the treatment of sand in a solution
containing less than 0.1% hydrofluoric acid together with
sulphurous acid and/or sodium hydrosulphite. In anoth~er process
described in U.S. Patent 2,769,540, sulphuric acid leaching is
followed by several flotation steps utilizing h~drofluoric
acid and organic amine conditioners for cleaning, and separating
~and from feldspar. In all the above described sand cleaning
processes, hydrofluoric acid is used only in small quantities
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and in conjunction ~ith a principal leaching reagent to enhance
the purifying action of the latter.
U.S. Patent NoO 2,952,516 teaches the purification
of sand by the leaching action of hot hydrochloric and fluo-
silicic acids, either added together, or applied in separate
but subsequent leaching steps. The fluosilicic acid utilized
in this process is prepared by acidifying alkali metal fluoride
solution or by hydrolysing silicon tetrafluoride with hydro-
chloric acid.
lQ In most of the above processes, the purified sand
product still contained several hundred parts per million iron
oxide. This level of impurity, however, is too high for the
current industrial processes utilizing high purity silica, which
for the purposes of the present specification, is considered as
such, when the main impurities present are only iron oxide and
alumina in quantities less than 2 ppm and 50 ppm, respectively.
OBJECT OF THE INVE2iITION
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Thus, it is an object of the pxesent invention to
provide a process for the production of high puri-ty silica, as
herein defined, by relatively inexpensive chemical means.
SUMMARY OF TElE INVENTION
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A relatively inexpensive method has now been found
whereby iron and other impurities such as feldspar, and other
alumina and calcia-bearing compounds, can be removed from
silica and quartz concentrates, not only from the-surface of
the particles as taught by the prior art, but also impurities
that are occluded, trapped or are strongly adherent to silica
are made accessible to separation.
By one aspect of this invention there is provided
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a process Eor producing high purity silica containing less -than
2 ppm iron oxide and 50 ppm alumina, from par-ticulate siliceous
materials containing adhering and occluded mineral impurities,
comprising:
treating said particulate material with an aqueous
solution containing between about 3% and about 20~ by weiyht
hydrofluoric acid at a temperature above about 50C.
DESCRIPTION OF DRAWINGS
Figure 1 is a schematic flowsheet of the process of
the present invention.
DETAILED DESCRIPTION_OF PREFERRED EMBODIMENTS
The sand, quartz, or other siliceous minerals to be
purified, according to the present invention, have generally
already been subjected to beneficiation, and are found as
tailings from some other ore separation proces.s. Prior treat-
ment may include size separation, wet or dry screening, magnetic
separation, flotation, or a combination of any of these separa-
tion proeess steps. It is usually advantageous to have the sand
or silica concentrate in a -50 Tyler mesh size, but the particle
size is in no way critical to the present process. It is how-
ever, important for achieving bes-t results, to restrict the
ferric oxide content of the siliceous material to less than
150 ppm and the alumina content to below 1% on a dry weight
pereent basis. The sand, quartz or silica concentrate is -then
subjeeted to dilute hydrofluoric acid treatment with agitation,
at a temperature above 50C and below the boiling point of the
solution, for a period of several hours. The strength of the
acid can vary between 3 and 20 percent. Hydrofluoric acid
concentrations below 3 percent would be very slow acting,
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resulting in a commercially impracticable process. The use of
an acid concentration above 20 percent would, on the other hand,
lead to a considerable loss of high purity silica by dissolution
also to increased corrosion of e~uipment, and to health hazards
to operating personnel. The duration of the treatment, the acid
concentration and temperature are adjusted between the specified
limits, according to the mineral nature and size of the sand or
the silica concentrate. I:E metallic containers and stirrers
are used it is advantageous that they be lined or coated with
some dilute acid resistant polymer, rubber or plastic material.
It is well known, as mentioned hereinabove, that
hydrofluoric acid attacks sand, feldspar and other alumina and
calcia-bearing siliceous materials. It has been found, surpris-
ingly, that in the hydrofluoric acid concentration and tempera-
ture ranges described by the present process, the dissolution
rate of silica by the acid is considerably slower than the rate
of dissolution of the impurities. As a result, the feldspar,
alumina and calcia-bearing siliceous materials and iron will go
into solution, or appear as slime. Another effect is that -the
iron oxide impurity can be leached not only from the surface of
the particles but also in an occluded state. The various
alumina and calcia-bearing materials that are strongly adherent
to the sand particles are thus separated by the preEerential
action o~ the acid on these impurities and are rendered access-
ible to the acid before the silica is substantially dissolved.
After the dilute hydrofluoric acid treatment has
been completed the solid impurities are separated by desliming,
flotation or similar physical separation methods, and the,
dissolved impurities are washed off with water. The high purity
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silica obtained by the present process usually contains less
than 1 ppm iron oxide.
The following examples show the improvement resul-ting
from the application oP the process described hereinabove.
E mple 1
40 lbs. of silica, obtained as tailings from a
previous flotation separation and then subjected to magnetic
separation, was treated by 58 lbs. of 14.5%HF solu-tion in a
polyethylene lined vessel, with continuous stirring. The
temperature of the solution was controlled at 63C.
The impurities present in the silica tailings are
listed below:
Fe23 41 ppm
A123 0.23% by wt.
Na2O 0.101% by wt.
K2O 0.061% by wt.
CaO 0.025% by wt.
After 4 hours treatment the iron oxide content of
the silica was reduced to less than 1 ppm and the alumina
content to less than 30 ppm. The treatment was continued for
another 18 hours, without substantial further reductions in
the impurity levels. At the end of the total 22 hours'
period of hydroPluoric acid treatment, 30 lbs. of high purity
silica was obtained with iron and alumina contents below 1 ppm
and 28 ppm respectively. The -200 Tyler mesh slimes separated,
contained 6 ppm iron oxide and over 42 ppm alumina.
Example 2
40 lbs. of silica tailings, with a composition
described in Example 1, was agitated in a 9.1 wt. % hydrofluoric
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acid solution with the t~mperature maintained at 62C. After
7 hours treatment 36.5 ]b. of high purity silica was obtained
with Eerric oxide and alumina conten-ts at 1 ppm and 37 ppm
respectively. The -200 Tyler mesh slimes were found to contain
102 ppm ferric oxide and 0.067% alumina.
