The non-metallic materials added to furnaces and ladles to form and condition slag, and the carbon added to adjust the steel's carbon content. Lime is the most important by far: an electric arc furnace uses around 30–50 kg per tonne of steel to take phosphorus, sulphur and silica out of the metal and protect the furnace lining.
Also called
Steelmaking fluxes
Quicklime
Burnt lime
Dolomitic lime
Limestone
Dolomite
Fluorspar
Metspar
Synthetic slag
Calcium aluminate
Carburiser
Recarburiser
Calcined petroleum coke
Mould powder
Casting powder
AR
الصهارات والمضافات
ZH
熔剂和添加剂
DE
Flussmittel und Zuschlagstoffe
FR
Fondants et additifs
ES
Fundentes y aditivos
IT
Fondenti e additivi
TR
Flakslar ve katkı maddeleri
RU
Флюсы и добавки
Main formsLump and pebble lime, dolomitic lime, crushed limestone and dolomite, fluorspar lumps and briquettes, carburiser granules, mould powders
Typical sizesLime typically 10–50 mm for furnaces, 0–10 mm for injection; carburisers 0.2–5 mm or 1–10 mm
Main standardsASTM C25 (chemical analysis of lime and limestone), ASTM C110 (physical tests); mostly supplier specifications
Key gradesQuicklime CaO above about 90%; dolomitic lime MgO 30–40%; metspar CaF₂ 60–85%; carburiser fixed C 90–99%
Typical usesEAF and BOF slag making, ladle refining, desulphurisation, sintering and blast furnace, continuous casting
Overview
A flux combines with the oxides and impurities in a melt to form a liquid slag that can be separated from the metal. In steelmaking the slag is basic: burnt lime (CaO) is the main flux, because a lime-rich slag holds phosphorus and sulphur that would otherwise stay in the steel. Dolomitic lime adds magnesia, which saturates the slag so it does not dissolve the magnesia-carbon refractories of the furnace and ladle, and helps the slag foam in an arc furnace. In blast furnaces and sinter plants raw limestone and dolomite do the same job.
Other additives fine-tune the slag. Fluorspar (calcium fluoride) makes it more fluid, though it attacks refractories and its use is being cut back; synthetic slags and calcium aluminate fluxes are used in the ladle for desulphurisation and inclusion control. Mould powders and tundish covering compounds are specialised fluxes for continuous casting that lubricate the mould and protect the steel from air.
Carburisers (recarburisers) are carbon added to raise the carbon content of the steel or to foam the slag: anthracite, calcined petroleum coke, graphite and coke breeze, chosen for fixed carbon, low sulphur and nitrogen and a size that dissolves predictably. Alloying additives — the ferro-alloys and metals such as aluminium — are covered on their own pages.
The GCC's arc furnace plants consume large tonnages of lime and dolomitic lime, much of it burned in the region from local limestone and dolomite in the UAE, Oman and Saudi Arabia; fluorspar, mould powders and most carburisers are imported.
Forms & sizes
Burnt lime (quicklime)
CaO above about 90%, lump or pebble; reactive, so it is stored dry and used fresh
Dolomitic lime
CaO with 30–40% MgO, for slag MgO saturation and refractory protection
Limestone and dolomite
Raw stone for sinter plants, blast furnaces and lime kilns
Fluorspar
Metallurgical grade (metspar) 60–85% CaF₂, lumps or briquettes
Synthetic slag
Pre-melted or sintered calcium aluminate for ladle desulphurisation
Carburisers
Anthracite, calcined petroleum coke and graphite granules, bagged in 25 kg or 1 t bags
Mould and tundish powders
Engineered casting powders matched to steel grade and casting speed
Standards & grades
Most fluxes are bought to the steel plant's own specification; lime and limestone are tested with the ASTM methods below.
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