Silica: From Quartz to Pharmaceutical-Grade Silicon Dioxide
At a glance
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Chemical name |
Silicon dioxide |
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Chemical formula |
SiO₂ |
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Appearance |
White crystalline or finely divided powder, depending on the form |
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Water solubility |
Practically insoluble |
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Natural mineral |
Quartz; silica also occurs in sand, sandstone, granite, and many other rocks |
Silicon dioxide is one of the most abundant mineral compounds on Earth. Its best-known crystalline form is quartz, although silica occurs throughout sand, sandstone, granite, and many other rocks. Humans have used it for tools, ceramics, glass, jewellery, electronics, and fibre-optic communications.
Pharmaceutical silica may be purified from high-purity quartz or silica sand, or it may be precipitated from a purified silicate solution. Both routes yield material with the overall composition SiO₂, but crystalline quartz and finely divided amorphous or precipitated silica can differ substantially in structure, surface area, particle shape, and handling properties.
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Mineral and technology connection: From sand to glass, clocks, and computer chips Quartz sand is the principal raw material for glass. Quartz also exhibits the piezoelectric effect: mechanical pressure can generate an electrical response, and an electrical signal can make the crystal vibrate. This property helps quartz keep accurate time in watches and makes it useful in sensors, microphones, and ultrasound equipment. Silicon makes up almost 28% of Earth’s crust by weight, yet elemental silicon is rare in nature. Producing semiconductor silicon requires removing oxygen from silica and purifying the silicon to extremely high levels, sometimes beyond 99.9999999%. |
Route one: Purification from quartz or silica sand
Quartz-bearing rock or silica sand may contain clay, feldspar, iron oxides, carbonate minerals, heavy minerals, organic matter, and moisture. The material is crushed or screened and then processed to remove these impurities.
Possible steps include washing, size classification, magnetic separation, flotation, acid leaching, filtration, and controlled drying. The exact sequence depends on the deposit and the required purity.
Throughout this route, the material remains SiO₂. Processing improves purity and controls particle size and physical characteristics without changing the basic chemical composition.
Route two: Chemically precipitated silica
Silicon dioxide can also be prepared from sodium silicate, often called water glass, which is itself manufactured from purified silica and sodium compounds. Acidification destabilizes the soluble silicate and produces finely divided hydrated silica that can be washed and dried toward the required SiO₂ material.
A simplified overall representation is:
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Na₂SiO₃ + 2HCl → SiO₂↓ + 2NaCl + H₂O |
The exact precipitation chemistry is more complex because freshly formed silica commonly contains water and hydroxyl groups. pH, temperature, concentration, addition rate, mixing, and ageing time strongly influence surface area, porosity, aggregate structure, and particle size.
Washing, drying, milling, and testing
The precipitated silica is separated and washed thoroughly to remove sodium chloride, excess acid, and other soluble residues. Controlled drying produces the required moisture and surface characteristics, after which the powder may be milled or classified.
Testing may include identity, silicon dioxide content, moisture or loss on drying, particle size, surface area, soluble salts, heavy metals, and other elemental impurities. The required tests depend on whether the material is crystalline, amorphous, colloidal, or precipitated silica.