Chemistry · Chemistry of the elements · Grade 9

Silicon and silicates

How are window glass, cement and solar panels made from sand?

The situation

At the Quvasoy glass factory in the Fergana Valley, window glass is cast from ordinary sand; at the Akhangaran cement plant limestone and clay are fired in a kiln to make cement; the potters of Rishton make earthenware bowls from clay. The basis of the solar panel on your roof is also ultra-pure silicon. All of these are silicon — the most abundant element in the Earth's crust after oxygen — and its compounds. How do you get glass, cement and a plate that gives electric current from a handful of sand?

Topic

Silicon, silicates and the silicate industry. Kuvasoy glass factory and a solar panel: from sand to window glass, cement, brick and pure silicon

Key ideas

Silicon — a semiconductor
Silicates and glass
Cement and ceramics

Formulas

SiO_{2} + 2 C \overset{}{\to} Si + 2 CO

Industrial production of silicon: reduction of quartz sand with coke at ≈ 2000 °C (technical-grade silicon ≈ 98–99 %).

Na_{2} SiO_{3} + 2 HCl \overset{}{\to} H_{2} SiO_{3} ↓ + 2 NaCl

Preparing silicic acid: when acid is added to “liquid glass”, a gel forms; in ionic form $SiO_{3}^{2 -} + 2 H^{+} \overset{}{\to} H_{2} SiO_{3} ↓$ .

6 SiO_{2} + Na_{2} CO_{3} + CaCO_{3} \overset{}{\to} Na_{2} O \cdot CaO \cdot 6 SiO_{2} + 2 CO_{2} ↑

Making window glass (≈ 1500 °C): sand, soda and limestone.

CaCO_{3} \overset{}{\to} CaO + CO_{2} ↑

The basis of cement clinker is CaO, formed when limestone is burnt; the clinker is fired at ≈ 1450 °C.

λ_{\max} = \frac{h c}{E_{g}} = \frac{1240 eV·nm}{1.12 eV} \approx 1107 nm

A silicon solar cell converts light with a wavelength shorter than ≈ 1107 nm into electricity.

Theory

Silicon: position in the periodic table, structure and properties

Silicon (Si) — period 3, group IV A (14), Z = 14: electron shells 2 · 8 · 4, outer shell $3 s^{2} 3 p^{2}$ . In its compounds the oxidation state is mainly +4 (SiO₂, silicates) and −4 (in silicides, for example Mg₂Si). Silicon makes up ≈ 27.7 % of the Earth's crust by mass — second after oxygen — but does not occur free in nature: it is found as silica SiO₂ (quartz, sand) and as silicates. Crystalline silicon is dark grey, with a metallic lustre, hard and brittle, with a density of 2.33 g/cm³, melting at 1414 °C; its crystal lattice resembles that of diamond (each atom has 4 neighbours, Si–Si 235 pm). Silicon is a semiconductor: its band gap is 1.12 eV, and its conductivity increases when it is heated or illuminated. In the laboratory: $SiO_{2} + 2 Mg \overset{}{\to} Si + 2 MgO$ ; in industry: $SiO_{2} + 2 C \overset{}{\to} Si + 2 CO$ . Under ordinary conditions silicon is inert; on heating it burns: $Si + O_{2} \overset{}{\to} SiO_{2}$ ; it dissolves in alkali solution, releasing hydrogen: $Si + 2 NaOH + H_{2} O \overset{}{\to} Na_{2} SiO_{3} + 2 H_{2} ↑$ ; with magnesium it gives a silicide, from which silane is obtained: $Mg_{2} Si + 4 HCl \overset{}{\to} 2 MgCl_{2} + SiH_{4} ↑$ (SiH₄ ignites spontaneously in air).

Pure silicon and solar power

For a solar cell the silicon must be very pure: at least 99.9999 % (“6N”), and for microchips 99.9999999 % or purer. The route: quartz sand → technical-grade silicon in an electric arc furnace (≈ 98–99 %) → trichlorosilane with HCl ( $Si + 3 HCl \overset{}{\to} SiHCl_{3} + H_{2}$ , ≈ 300 °C) → purification by distillation → reduction with hydrogen ( $SiHCl_{3} + H_{2} \overset{}{\to} Si + 3 HCl$ , ≈ 1100 °C) — polysilicon → a single-crystal ingot pulled from the melt with a seed crystal → thin wafers. A photon of light knocks an electron out of a bond, forming an “electron–hole” pair; the p–n junction separates them — and a current appears in the external circuit. The energy of the photon must be greater than 1.12 eV: light with a wavelength shorter than ≈ 1107 nm works. The power of the panel is proportional to the light flux: $P = η G S$ (in the model $η$ = 20 %, $S$ = 0.1 m²; with the midday sun $G \approx 1000$ W/m²).

Silicon(IV) oxide, silicic acid and silicates (lab work 2)

SiO₂ is a solid with a very high melting point (quartz, sand, opal): each Si atom forms a tetrahedron with 4 O atoms, and the tetrahedra are joined at their corners into a three-dimensional network. SiO₂ is insoluble in water and does not react with acids (except hydrofluoric acid: $SiO_{2} + 4 HF \overset{}{\to} SiF_{4} ↑ + 2 H_{2} O$ — this is how glass is etched), but it is an acidic oxide: fused with alkali or soda it gives silicates: $SiO_{2} + 2 NaOH \overset{}{\to} Na_{2} SiO_{3} + H_{2} O$ . A solution of Na₂SiO₃ and K₂SiO₃ is liquid glass (glue, a fireproof impregnation). If acid is added to it, the weak silicic acid separates as a gel: $Na_{2} SiO_{3} + 2 HCl \overset{}{\to} H_{2} SiO_{3} ↓ + 2 NaCl$ ; even CO₂ displaces it: $Na_{2} SiO_{3} + CO_{2} + H_{2} O \overset{}{\to} H_{2} SiO_{3} ↓ + Na_{2} CO_{3}$ . The dried gel is silica gel (a desiccant). Natural silicates (lab work): quartz SiO₂, feldspar $KAlSi_{3} O_{8}$ = K₂O·Al₂O₃·6SiO₂, mica, kaolin $Al_{2} Si_{2} O_{5} (OH)_{4}$ = Al₂O₃·2SiO₂·2H₂O, talc, asbestos; granite is a mixture of quartz, feldspar and mica. Writing a silicate formula as oxides makes it easier to understand the composition.

Glass (lab work 3)

Ordinary window glass ≈ Na₂O·CaO·6SiO₂: a mixture of sand, soda and limestone is melted at ≈ 1500 °C: $Na_{2} CO_{3} + SiO_{2} \overset{}{\to} Na_{2} SiO_{3} + CO_{2} ↑$ , $CaCO_{3} + SiO_{2} \overset{}{\to} CaSiO_{3} + CO_{2} ↑$ . Glass is an amorphous substance: it has no sharp melting point and softens gradually when heated. The float process (A. Pilkington, the 1950s): molten glass is poured onto a bath of liquid tin and spreads flat on its surface — giving a smooth sheet of uniform thickness, which is then cooled slowly in a tunnel kiln. The Quvasoy “Kvarts” glass factory in the Fergana region has worked since 1975; it makes sheet glass by the float process. Types: soda-lime (windows, containers); potash (hard to melt); lead — crystal glass (brilliant, heavy); borosilicate (B₂O₃ added — heat-resistant laboratory glassware); quartz glass (pure SiO₂); coloured: CoO — blue, Cr₂O₃ — green, iron oxides — green-brown. Glass-ceramics are glass with TiO₂ or ZrO₂ added and crystallised under control: very strong and heat-resistant (cooker hobs).

Cement, concrete and ceramics

Portland cement: limestone (≈ 3/4) and clay (or clay-like soil) are ground and fired in a rotary kiln at ≈ 1450 °C — granules of clinker form: $CaCO_{3} \overset{}{\to} CaO + CO_{2} ↑$ , $3 CaO + SiO_{2} \overset{}{\to} Ca_{3} SiO_{5}$ . The clinker is ground finely with ≈ 5 % gypsum. When mixed with water the silicates are hydrated: $2 Ca_{3} SiO_{5} + 6 H_{2} O \overset{}{\to} Ca_{3} Si_{2} O_{7} \cdot 3 H_{2} O + 3 Ca (OH)_{2}$ — fibres of calcium hydrosilicate interlock and set like stone; the strength keeps increasing for several months (28 days is the standard value). Each tonne of clinker releases ≈ 0.5 t of CO₂ from the limestone. Concrete = cement + sand + gravel + water. Ceramics: clay (mainly kaolinite) is mixed with water, shaped, dried and fired: brick at ≈ 900–1000 °C, porcelain at ≈ 1300–1400 °C. The pottery of Rishton (Fergana region) is famous for its blue and turquoise patterns; the traditional “ishqor” glaze uses alkaline substances obtained from plant ash.

University: the cement plant, lab work and the model

Clinker minerals (shorthand: C = CaO, S = SiO₂, A = Al₂O₃, F = Fe₂O₃): alite C₃S (50–70 %) — sets quickly and gives early strength; belite C₂S (15–30 %) — slow, but hardens for a long time; C₃A (5–10 %) — reacts very quickly and releases much heat (gypsum slows it down); C₄AF (5–15 %). They are calculated from the oxide composition using the Bogue equations: $C_{3} S = 4.071 C - 7.600 S - 6.718 A - 1.430 F - 2.852 S O_{3}$ and so on; the lime saturation factor $LSF = C / (2.8 S + 1.2 A + 0.65 F)$ ≈ 0.92–0.98. Laboratory: on the Vicat apparatus the normal consistency of cement paste is the water/cement ratio at which the plunger stops 6 ± 1 mm above the base plate (usually 0.24–0.30); the setting time of gypsum is the moment when the needle stops short of the bottom of the paste (start of setting) and when it hardly penetrates the surface (end of setting); for ordinary-setting gypsum it is ≈ 6 to 30 minutes. Safety: cement and lime dust burn the eyes and skin — wear goggles, gloves and a mask; asbestos dust is carcinogenic. Model: the Bogue equations and the heats of hydration (C₃S 502, C₂S 260, C₃A 867, C₄AF 419 J/g) are from reference tables; the strength curve and the Vicat and gypsum tests are schematic models fitted to typical values; the diorama is a scaled-down model.

Did you know?

Silicon makes up about 27.7 % of the Earth's crust by mass — second after oxygen; in nature it occurs only in compounds — quartz and silicates.
Portland cement was patented in 1824 by the English bricklayer Joseph Aspdin; he took the name from the resemblance of the set cement to the building stone of the Isle of Portland.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Carbon and its compounds — Grade 9
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Metals and alloys — Grade 9
Alkali metals and soda — Grade 9
Aluminium and its compounds — Grade 9
Copper, silver, gold and zinc — Grade 9