The Illinois silica sand industry is based on the St. Peter Sandstone.
It centers around Ottawa, Wedron, Troy Grove, and Utica in LaSalle
County and in Oregon in Ogle County. Two principal grades of silica sand
are produced—washed and crude. The value of the silica sand produced in
Illinois in 1963 was about $9,000,000.
_Washed Sand._—Although the St. Peter Sandstone is composed almost
entirely of quartz grains, a small amount of clay is present. For some
uses it is not necessary to remove the clay, for others its elimination
is important and is achieved by washing the sand.
In the mining of silica sand that is to be washed, the sandstone is
first blasted loose from the parent deposit to break it into sand or
pieces of various sizes. Some of the larger pieces may require a second
blasting to disintegrate them.
At some pits the material is loaded mechanically and transported to the
washing plant. At others a powerful stream of water is directed against
the broken sandstone (fig. 14) and the resulting mixture of sand and
water flows to a collecting basin from which it is pumped through large
pipes to the processing mill.
In both types of operation the sand is thoroughly washed at the plants.
After it is washed, the sand is further processed to suit the needs of
its users. Much of it is screened into different size grades.
_Uses of Washed Silica Sand._—The washed silica sand produced in
Illinois has many uses, some of which are briefly mentioned below. The
suitability of the sand for some purposes depends in part on its having
been screened to specified sizes.
The high purity of Illinois washed silica sand makes it suitable for
making glass, which is more than half silica sand. Each year over a
million tons is used for this purpose. The purity of the sand also is of
importance for chemical and metallurgical uses such as the manufacture
of sodium silicate and silicon carbide and in alloying.
The hardness of the sand makes it useful for grinding large sheets of
plate glass to prepare them for polishing and also makes it an effective
abrasive agent for sandblasting. Metal castings in foundries and the
exteriors of buildings are cleaned by this process. Illinois produces
thousands of tons of sand yearly for such abrasive purposes.
Because the coarser grains of the washed silica sand are rounded,
strong, and available in uniform sizes, oil operators use thousands of
tons of it annually in the hydraulic fracturing of oil-bearing strata.
The sand is mixed with oil, other petroleum products, or water and is
forced by powerful pumps into sandstone or limestone formations that
contain oil. The great force thus exerted opens fractures in the rock
strata and pushes the liquid and sand into them. When the pressure is
relieved, the sand grains serve as props to hold the fractures open. The
oil can then flow more easily into the wells and oil production is thus
increased.
The washed sand, because it is clean and does not dissolve in water, is
used to filter impurities from drinking water. Its whiteness makes it a
desirable constituent in plaster, mortar, and precast building panels.
[Illustration: Figure 14—Hydraulic mining of silica sand near
Ottawa.]
Because it is round grained and withstands high temperatures without
melting, large tonnages of the washed silica sand are used to make molds
into which molten metal is poured to make various kinds of castings.
A special type of coarse silica sand from Illinois that is carefully
prepared so that it is always of the same grain size is used throughout
the world as a standard in laboratories that test cement and other
commercial products.
[Illustration: Figure 15—Loading crude silica sand.]
Some silica sand is ground to a fine, white powder. The powder, called
ground quartz, ground silica, silica flour, or potter’s flint, has many
uses. It is an ingredient in paints, potters use it in making pottery
and china, it goes into scouring powders, into molds used for precision
types of metal castings, and into enamels.
_Crude Silica Sand._—The crude silica sand produced from the St. Peter
Sandstone generally is yellow or yellowish white and is not washed
before it is used. It probably originally was white, but iron oxide,
similar to the rust that forms on iron, now coats many of the sand
grains and colors the small amount of clay in the sand. Thousands of
tons of crude silica sand are mined annually (fig. 15). Because it is
highly heat resistant, foundries buy much of it to make the molds used
for castings, especially steel castings, and for automobile engine
blocks, train wheels, and a variety of other metal products. Crude
silica sand also is used around industrial furnaces to seal cracks and
openings to prevent the loss of heat, in certain ceramic products, and
for adjusting the silica content of the raw materials used for making
portland cement.