Lime in Steelmaking: What the Melt Shop Needs from Its Lime Supplier

Of all the industries that buy lime, steel is the most demanding and the least forgiving. A melt shop charges lime by the tonne into a furnace at well over a thousand degrees, and everything the lime brings with it — the impurities, the moisture, the fines — goes into the heat. A good lime makes a fluid, reactive slag that pulls silica, phosphorus and sulphur out of the metal. A poor one makes slag that is slow to form, hard to work and expensive in energy and in refractory.

This article is for the purchase engineer or melt-shop metallurgist writing or reviewing a lime specification. It describes what the furnace needs and why, in terms that can be put on an order and checked on a certificate. It does not give plant-specific limits; those belong to the plant.

What the lime is doing in the furnace

Steelmaking is oxidation: carbon, silicon, manganese and phosphorus are burnt out of the hot metal, and the oxides have to go somewhere. Lime — calcium oxide — gives them somewhere to go. It combines with silica to form a calcium-silicate slag, and a basic slag (one with more lime than silica) is what holds phosphorus and sulphur away from the metal. The ratio of basic to acidic oxides in the slag — its basicity — is set by how much lime is charged against how much silica is present, and that is the first reason consistency matters: the operator sets lime additions on an assumed available-lime content, and a lot that is lower than assumed makes a slag that is less basic than planned.

The five things the furnace is asking for

Available lime, not total calcium

Total CaO counts calcium that is still locked up as carbonate (unburnt core) or already combined with silica in the lime itself. Neither fluxes anything. The figure the furnace is paying for is available lime — the calcium oxide that will actually react — and that is the number to put on the specification, with the test method named. The certificate of analysis article explains how it is reported.

Low silica, low sulphur

Silica in the lime consumes lime to flux itself before it does any useful work, so every percent of SiO₂ in the lime is lime the plant paid for and did not get. Sulphur in the lime goes straight into the slag–metal balance the plant is trying to shift the other way. Both are set as maxima on the specification and reported on every certificate. The level of each is largely decided by the limestone the kiln burns, which is why a manufacturer that controls its stone can hold them.

Reactivity matched to the process

A basic oxygen furnace wants lime that dissolves into the slag quickly, in the few minutes the blow allows; too slow and the slag is late, too fast and the lime can decrepitate into dust before it is in the bath. An electric arc furnace with a longer heat can use a somewhat less reactive lime. Reactivity is measured by a slaking test — the temperature rise, or the time to reach a set temperature, when lime is dropped into water — and the result depends on the method, so the specification must name the method. The reactivity article covers the tests.

Size, and few fines

Lime is charged in a lump band chosen for the charging system and the furnace: large enough not to be blown out with the off-gas or lost to the dust system, small enough to dissolve in time. Deshwali Minerals grades quick lime in 0–3 mm, 0–10 mm and 10–50 mm fractions; which band suits a given furnace is the plant's call. The specification should state the band and the maximum fraction outside it, because fines are the lime the furnace never sees.

Consistency, lot after lot

A melt shop tunes its additions to a number and runs that number for months. A supplier whose lime averages well but swings from lot to lot costs more in operation than one whose lime sits a little lower and never moves. Ask a prospective supplier for a run of certificates, not one; look at the spread, not the mean.

Writing the specification

A usable steel-plant lime specification states, for the grade ordered:

• available lime (minimum) and the test method;
• SiO₂, S and, where it matters, MgO and loss on ignition (maxima);
• reactivity (the figure and the method);
• the size band and the permitted fraction outside it;
• moisture (maximum), because wet lime has already started to slake;
• the certificate of analysis required with each lot, and the retained-sample arrangement for a retest.

Deshwali Minerals publishes the parameters it states for quick lime; the figures a plant needs are agreed on the order, and the certificate reports each lot against them.

Dolomite and dolomitic lime in the melt shop

Many basic-oxygen shops add a magnesium source to saturate the slag with MgO and reduce attack on the magnesia refractory lining. That source is raw dolomite or calcined dolomitic lime, charged alongside the high-calcium flux. Deshwali Minerals supplies dolomite graded to the size the plant specifies; the dolomite article describes the material.

Receiving lime at a steel plant

Lime that has stood in a wet yard is not the lime that was certified. Covered storage, a short time between delivery and charging, and a gate check of the vehicle against the e-way bill and the lot against its certificate are the receiving practices that keep the specification meaningful. The packing, storage and transport article sets them out.

Sources

• Bureau of Indian Standards, IS 1514:1990 — Methods of sampling and test for quicklime and hydrated lime: BIS standards portal
• ASTM C25 (chemical analysis) and ASTM C110 (physical testing) of limestone, quicklime and hydrated lime: ASTM International
• Deshwali Minerals — lime in steelmaking and the quick lime specification