Quicklime

In the EN 459-2 wet slaking test, 150 g of lime is stirred into 600 mL of water at 20 °C and the time taken to reach 60 °C is recorded; that time is t60. One to two minutes indicates a soft-burnt, highly reactive product; above six minutes points to a hard-burnt, slow one. The test is simple, but the result is sensitive to the size fraction tested — if the fraction is not stated, the number cannot be compared.

Which reactivity is right is decided by the process. Hot metal desulphurisation gives the injected lime an 8–15 minute window in which it must dissolve; a continuous slaker making milk of lime works with 10–20 minutes of residence. A calcium carbide furnace or a batch slaker, by contrast, does not want an over-reactive product: it produces sudden boiling, spitting and a vapour load the equipment was never sized for.

The common mistake is to specify t60 on its own. The same t60 can come with a very different total temperature rise: a product low in available CaO starts fast and stops early, its curve flattening before 45 K. A reactivity assessment means reading t60 and Δt together; demanding only one of the two lets the wrong batch look contractually compliant.

Calcite decomposition reaches equilibrium around 898 °C at one atmosphere; in practice kilns run at 950–1150 °C. If the temperature is low or residence too short, undecomposed CaCO₃ remains in the core of the grain — a raw core. It raises loss on ignition and lowers available CaO: dead weight that is transported, ground and invoiced without contributing anything to the process.

At the other extreme is overburning. A grain held too long at high temperature grows its CaO crystals, closes its pore structure and drops from 1–3 m²/g of specific surface to below 0.5 m²/g. Hard-burnt lime reacts slowly with water and ends up as residue in the slaker. Impurities accelerate the process: SiO₂ and Al₂O₃ in the natural calcium carbonate feed start sintering at lower temperatures and cut reactivity to a degree chemical analysis will not reveal.

Raw cores and overburnt grains can coexist in the same batch; with a wide band such as 0–90 mm passing the same time in the same kiln, that is unavoidable. The practical consequence is simple: narrowing the size band narrows the reactivity band. Asking for 10–40 mm in a specification is not only about handling — it is a demand for consistency within the batch.

Total CaO is the elemental calcium content measured by XRF, and it counts calcium locked in carbonate or sulphate just the same. Available CaO is determined by sucrose extraction and titration and gives the fraction that can actually hydrate and react. The gap between the two is, quite literally, raw cores, carbonated surface and hard-burnt grains.

Dosing must always be calculated on available CaO. A product with 96% total CaO but 88% available CaO is consumed roughly 4.5% faster than one at 92% available to do the same work. If the price per tonne is equal, the real cost is not: the unit of comparison between suppliers is not the tonne but the tonne of available CaO. In most plants this single correction shows up as a visible line in the annual reagent budget.

Stoichiometry starts from the same number: 56 g/mol of CaO becomes 74 g/mol of Ca(OH)₂, so 1 kg of available CaO is equivalent to 1.32 kg of hydrated lime and neutralises 2 moles of H⁺. Leave headroom for storage losses: a surface that hydrates and then carbonates in the silo loses available CaO over months, so a 3–5% safety margin is normal working practice.

Lump lime (10–40 and 40–90 mm) is charged directly into electric arc furnaces and converters, fed to batch slakers, and creates the least dust in bulk handling. Crushed 0–3 mm is the standard feed for continuous slakers and milk of lime lines. Ground powder (≥ 90% below 90 µm) belongs to dry sorbent injection, soil spreading and dry mortar blends.

The wrong fraction penalises you in two different ways. Material that is too fine heats very quickly in the slaker, agglomerates, and hydrates on the outer surface before water reaches the core — this is what is called burning during slaking, and it yields a coarse, low-surface hydrate. Material that is too coarse never slakes to the centre; grit and residue rise, and screened-out material that still carries available CaO goes to waste.

Fraction also decides silo and conveying design. Ground lime does not flow freely and bridges in the cone; fluidising air, a vibrating discharger and the right cone angle have to be in the design. Lump lime flows easily but slakes if it takes up moisture, expands to roughly twice its volume and forms a hard crust welded to the silo wall. In both cases keeping the silo vent filter dry is the cheapest precaution the plant can take.

The reaction CaO + H₂O → Ca(OH)₂ releases about 65 kJ/mol, that is 1150 kJ per kg of CaO. Slaking one tonne of lime liberates 1.15 GJ — enough to take roughly 3.4 m³ of water from 20 °C to boiling. The real job of a slaker design is not to start the reaction but to remove that heat in a controlled way.

For milk of lime the water-to-lime ratio is typically 3.5–5:1; for dry hydrate 0.4–0.6:1, with a target slaking temperature of 80–95 °C. Below that ratio you get local boiling, steam release and coarse crystalline hydrate. Slaking too cold (under 40 °C) produces coarse, low-surface Ca(OH)₂ that settles fast and dulls the dosing curve — more product for the same pH. Slaking temperature is therefore a measured and logged parameter, not an observational detail.

Every slaking leaves an unreacted residue — grit — typically 2–5% with a good lime, separated by hydrocyclone or screening. If the grit figure jumps, check slaking water temperature and feed rate before blaming the lime. Milk of lime made on site does the same job in water and wastewater treatment lines as a purchased hydrate slurry; the difference is that its fineness depends on your slaking conditions — the variable stays on your side of the fence.

In iron, steel and metallurgy lime is the slag former: it takes slag basicity (CaO/SiO₂) into the typical 2.8–4.0 band and binds sulphur and phosphorus out of the metal. But the sulphur distribution ratio depends not only on basicity — oxygen activity in the slag and temperature matter as much. In an oxidising slag, adding more lime past a point no longer lowers sulphur; it only grows slag volume and metal loss.

Reactivity and size act here through dissolution rate. Hard-burnt lime dissolves late in the converter, slag forms late, and slopping and refractory wear increase early in the blow. In hot metal desulphurisation the injected fine lime (0–1 mm) is used blended with magnesium or calcium carbide, and contact time is measured in minutes; in that process reactivity is efficiency, and efficiency is lime consumption.

The impurity line is often more decisive than price. Every extra point of SiO₂ inside the lime demands extra lime to hold the basicity target — one percentage point translates into several kilograms more lime, and the slag that goes with it, per tonne of steel. Low impurity also means less slag, less heat loss and less entrained metal; this is where lime selection enters the cost per heat.

In soil improvement and stabilisation CaO first works as a drying agent. It binds water chemically in hydration, the released heat accelerates evaporation, and the workability of a clay soil changes within hours. In practice 1% CaO lowers moisture content by roughly 0.5–1.5 points; after rain, this first effect is often what saves the site programme.

The second job is slower and delivers the actual strength. Once pore-water pH reaches 12.4, silica and alumina dissolve out of the clay minerals and combine with calcium into calcium silicate and aluminate hydrates. This pozzolanic gain plays out over days and weeks; performance is judged on 7- and 28-day unconfined compressive strength or CBR, and a measurement taken right after mixing is misleading. Dosing starts with the Eades–Grim method (ASTM D6276) and typically lands in the 2–5% range.

In road infrastructure and asphalt work three site variables decide the outcome: mixing fineness, curing time and the compaction window. On lime-treated soil compaction should normally be finished within 2–4 hours of mixing; delay costs compacted density. And where sulphate content exceeds 0.3%, no lime decision should be taken without laboratory confirmation, because of ettringite formation and the heave that follows it.

In flue gas treatment quicklime rarely meets the acid gases directly. Two routes dominate: CaO is fed to an on-site hydrator, converted to Ca(OH)₂ and injected as a dry sorbent; or it is slaked into milk of lime and atomised into the gas stream in a spray dryer absorber. In both routes what the plant stores and hauls is CaO — more active substance per tonne, and a smaller storage volume.

Efficiency is set by the surface of the sorbent, not by its chemistry. In dry injection a hydrate of 20–40 m²/g specific surface captures 70–90% of SO₂ at a Ca/S molar ratio of 2–3; a low-surface hydrate needs a markedly higher ratio for the same result. In a spray dryer absorber water vapour speeds the reaction, so Ca/S drops to 1.2–1.6 — but now the approach temperature, the 10–20 K left to adiabatic saturation, becomes the critical parameter.

Acid gas composition enters the choice as well. HCl and HF are captured distinctly more easily than SO₂; where HCl dominates, high removal is achieved even at low Ca/S, whereas where SO₂ dominates, sorbent quality and reactor temperature come to the front. Dosing should be feedback-controlled from stack measurement rather than fixed: when load shifts, a fixed dose either breaches the limit or wastes sorbent and creates residue.