Iron, Steel & Metallurgy

The amount of lime is calculated from the acidic oxide load entering the slag, not from the basicity target. The sources are listed in order: silicon in the hot metal (each 1% Si → 21.4 kg SiO₂/t), sand and dirt on the scrap, gangue from ore or pellets, SiO₂ and Al₂O₃ from ferroalloys, MgO from lining wear. Until that total exists, a target B2 has no numerical meaning.

The calculation has two steps. First find the CaO the target basicity demands (CaO = B2 × ΣSiO₂), then divide it by the available CaO of the lime. With lime at 92% available CaO, 68.5 kg of CaO needs 74.5 kg of product; if available CaO has drifted to 86%, the same result costs 80 kg. Those extra 6 kg arrive straight from the certificate, with no operator error involved.

The chain starts in ore preparation, not in the melt shop. Sinter mix basicity and pellet fluxing are set on the mining and ore beneficiation side; high-basicity sinter lowers the acidic oxide load reaching the converter from the outset. Confining the charge calculation to the melt shop means ignoring the cheapest source of basicity you have.

There is an order to reading a certificate: available CaO first, then loss on ignition, then sulphur, then reactivity, and size distribution last. Total CaO alone is misleading; overburnt lime looks chemically clean but does not dissolve in the slag and contributes nothing to basicity despite being present on the charge sheet.

Reactivity is measured by the EN 459-2 wet slaking test and expressed as t60: the time for water at 20 °C to reach 60 °C. Soft-burnt lime comes in under three minutes; anything beyond six minutes is hard-burnt and dissolves late in the melt shop. This test reads the calibration of the lime kiln, not the chemistry — it is usually why two plants behave very differently on the same CaO figure.

Lime chemistry comes from its feedstock. The SiO₂, Al₂O₃ and sulphur profile of the natural calcium carbonate fed to the kiln sets the impurity profile of the lime that leaves it; when the quarry or the seam changes, expect the lime analysis to shift too. Tracking each source rather than each batch is usually enough at incoming inspection — but once silo residence exceeds three to four weeks, loss on ignition should be re-measured.

Sulphur in blast furnace hot metal typically runs 0.020–0.040%. Because the converter is an oxidising vessel it cannot remove sulphur effectively; expecting it to only grows slag volume and iron loss. Sulphur is therefore removed in the torpedo car or the charging ladle, with a KR-type stirrer or by co-injection.

A typical co-injection recipe is 0.4–0.8 kg/t of magnesium with 3–6 kg/t of fine lime. Magnesium drops sulphur fast; the lime binds the sulphide in a stable form, prevents reversion and keeps the slag skimmable. Lime alone needs 5–10 kg/t and a longer treatment; the choice between them is a comparison between the magnesium price and the time window available on the ladle.

Particle size decides the outcome here: 0–1 mm, d50 of 40–100 µm, and a product that does not cake in pneumatic conveying. Lime that has picked up moisture plugs the lance and the transport line, so injection lime needs its own closed, dry silo. Target sulphur is generally below 0.005%, and below 0.002% for IF and line pipe grades.

Most of the lime is charged in the first third of the blow. Early lime stops the acidic primary slag formed by silicon oxidation from attacking the lining; lime charged late has no time to dissolve and remains as a solid island at the end of the blow. Charging timing changes the result as much as lime quality does.

Dephosphorisation demands three conditions at once: 15–25% FeO in the slag, B2 ≥ 3.0 and a relatively low temperature. Above 1650 °C the equilibrium reverses and phosphorus returns from the slag to the metal. End-of-blow temperature is therefore not a separate variable alongside the phosphorus target but a direct part of it.

The phosphorus partition ratio Lp = (%P)/[%P] sits between 50 and 150 in a well-run converter. If it is low, the first thing to question is not the lime charge but whether the slag is genuinely liquid. Undissolved lime raises basicity on paper without taking phosphorus; that gap between measurement and reality is the real reason behind most decisions to charge more lime.

Target basicity in the EAF is lower than in the converter; B2 typically runs 1.8–2.4. The reason is physical rather than chemical: to cover the arc, foaming slag has to stay viscous and fluid at the same time. Above 2.5 the slag dries out, the foam collapses and the arc is exposed; panel heat load, electrode consumption and noise all rise together.

What holds the foam up is the solid second-phase particles inside the slag. Running just above MgO saturation (8–14% MgO) generates those particles; staying below saturation both weakens the foam and pulls MgO out of the lining. On DRI/HBI charges, Al₂O₃ and SiO₂ both rise, so B4 = (CaO+MgO)/(SiO₂+Al₂O₃) should be tracked; on those charges B2 alone reads systematically optimistic.

The dust and gas leaving the furnace are another point of contact for lime. Capturing acidic components upstream of the baghouse and removing SO₂ from the sinter plant stack belong to flue gas treatment; the fineness and reactivity required of that sorbent are a completely different specification from slag lime, and the two do not share a silo.

Converter slag carried over into the ladle during tapping is the single biggest obstacle in front of a white slag. Every kilogram of FeO-bearing slag that gets across is consumed by aluminium in the ladle, raises alloy cost and delays desulphurisation. Slag stoppers, thickness measurement and the practice of cutting the tap early matter as much here as lime quality.

White slag is built in the CaO–Al₂O₃–SiO₂ system with FeO+MnO below 1% and basicity in the 2.5–4.0 band. Under those conditions the sulphur partition ratio Ls climbs to 300–1000; the same quantity is 2–8 in the converter. The difference comes from lowering oxygen activity, not from basicity — in a ladle that has not been aluminium-killed, sulphur will not come down whatever the basicity.

Lime entering the ladle must be low in sulphur, low in loss on ignition and dry. Damp lime or hydrate is a hydrogen source in liquid steel; on heavy sections even a 1 ppm rise in hydrogen enlarges the risk of flaking. Calcium aluminate based synthetic slags are not a replacement for lime but a companion to it, used to speed up how fast the lime melts in.

The life of an MgO-C lining depends largely on whether the slag is saturated in MgO. Unsaturated slag dissolves the lining; saturated slag leaves a protective layer on its surface. Taking slag MgO from 6% to 10% in the same furnace produces a visible change in refractory consumption per heat without touching the brick recipe at all.

In the converter this principle becomes slag splashing practice: the slag left after tapping is thickened with dolomitic lime and splashed onto the lining with a nitrogen blow, where it freezes. Well-executed splashing lifts converter campaign life from a few thousand heats into the tens of thousands; the critical variables are the MgO and FeO content of the remaining slag and the blowing time.

But raising MgO without limit does not work either. Far above saturation the slag becomes viscous, metal–slag mixing during the blow deteriorates, phosphorus removal falls and slag retention problems begin. The right practice is to stay just above the saturation curve and move the target as temperature and FeO move — a fixed MgO target is as misleading as a fixed lime dose.

Steel plant slag is not waste; it is a product that has to be managed. Because free CaO and MgO in converter slag hydrate and expand in contact with water, the slag must be weathered for three to six months or steam-stabilised before it goes into road base or concrete. Skip that step and material laid as aggregate heaves months later and lifts the pavement above it.

Granulated blast furnace slag, once ground, is a binder that substitutes for cement and creates value directly on the construction and building materials side. Converter slag cannot follow the same route because of its free lime; treating the two as one line item is wrong both technically and commercially, and in most plants it hides half the revenue slag could earn.

Acidic effluent from pickling lines, cooling water blowdown and mill scale sludge are neutralised with lime on the water and wastewater treatment side; the product used there is not melt shop lime but directly dosable hydrate. If you want a lime balance built from your own slag analyses and charge records, you can reach us through the contact page.