The Walrus and the Carpenter
Were walking close at hand:
They wept like anything to see
Such quantities of sand.
Through the Looking Glass
(Lewis Carroll, 1871)
Over the years I’ve written about common sand used for construction. This article addresses silica sand or industrial sand, which is composed mostly of quartz (silica).
Silica sand is used for industrial processes like abrasives, engine traction, fillers, filtration, foundry sand, glass manufacturing, golf courses, metallurgy, roofing granules, etc.
Many folks view construction sand as an infinite, available resource. But construction sand isn’t so readily available as it must meet specifications for the intended use.
Silica sand also has to meet more rigorous specs than construction sand. In almost every case, deposits suitable for use as construction sand cannot economically be processed into silica sand.
This is because many young geologic deposits used for construction sand, such as modern river alluvium or terrace deposits, consist of a variety of minerals including quartz, feldspar and other rock particles. It commonly is too expensive to separate the other minerals to make pure silica sand.
Silica sand derives from old geologic deposits that, over geologic time, have been subjected to repeated erosion cycles that break down and remove the softer particles, leaving only pure quartz grains.
One of the most effective geologic agents for removing impurities is the wind, and older aeolian deposits are some of the best, most frequently utilised sources of silica sand.
Industrial sand deposits mined in the USA vary greatly in chemistry, location and physical characteristics. In general terms, the highest quality industrial sand deposits are in the Midwest.
Nevertheless, there are high quality silica sand deposits throughout much of the US. This is fortunate because recently there has been an escalation in demand for silica sand. Unless you’ve been hiding under a rock, you know this increased demand is for sand used in a process called hydraulic fracturing (fracking) to develop natural gas from shale.
The process involves injecting a mixture of water, silica sand (commonly known as proppant or “frac sand”) and chemicals into rock formations at high pressure to fracture the rock and prop open the newly-formed fractures (see Figure 1). This increases the flow rate of natural gas or oil to the well.
Frac sand has some of the most rigorous specs among industrial sand applications. Strong, smooth, round silica sand grains of specific sizes are used to maximise permeability. The structural integrity of silica sand provides the required crush resistance for the high pressures in wells.
The nearly spherical shape of the grains contributes to its strength, as well as the permeability of the fractures held open by the proppant.
You might wonder how much demand in the US has increased. At the start of the 21st century, silica sand production was about 30 million tonnes per year and evenly divided between glassmaking, foundry sand and all other uses. Frac sand production was trivial. Today annual production is about 110 million tonnes – nearly four times as great as at the turn of the century, and nearly two-thirds of the production is for frac sand.
For comparison, Australia produces slightly more than half a million tonnes of silica sand per year. I think it’s safe to say that the US produces “such quantities of sand”, indeed!