The chemistry of steel slag and its recovery.

The melting process, in brief

Steel is produced mainly via two routes: from iron ore (blast furnace + basic oxygen furnace, BOF) or from scrap (electric arc furnace, EAF). In both cases the heart of the process is melting: the metal charge is heated above 1,500-1,600 °C until it becomes a liquid bath. To this bath are added fluxes, first and foremost lime (CaO), which play a precise and far from secondary chemical role.

The molten metal is not pure: it contains impurities (silicon, manganese, phosphorus, sulfur) and oxides. To obtain quality steel you must remove these impurities, and this is precisely where slag comes into play.

Why slag forms

Slag is the oxide phase that forms above the metal bath: being lighter, it floats and separates from the metal. It is not an accidental by-product, but a deliberate process tool, with three functions:

• Collecting impurities: the oxides that form (and elements like phosphorus and sulfur) migrate into the slag, "cleaning" the metal.
• Protecting the bath: it acts as a blanket that limits oxidation and heat loss.
• Controlling the reactions: its composition (the "basicity", i.e. the ratio of basic oxides like CaO to acidic ones like SiO₂) governs the ability to capture phosphorus and sulfur.

Chemically, slag is a system of molten oxides: mainly CaO, SiO₂, FeO, MgO, Al₂O₃, MnO. Typical reactions are oxidations and the formation of silicates and aluminates, plus desulfurization (schematically CaO + S → CaS captured by the slag).

Black slag and white slag: two different chemistries

In industrial practice, two main slags are distinguished, produced in two different phases and with very different compositions — and problems.

Black slag (from melting/oxidation)

It's the slag that forms during oxidizing melting in the furnace. It's rich in iron oxides (FeO) and other metal oxides, has a dark color and almost always contains trapped metallic fraction (droplets of steel left imprisoned). It's precisely this metal, and the recoverable oxides, that make it a valuable material if treated correctly.

White slag (from ladle refining)

It's the slag from the refining/reduction phase in the ladle, where the final steel composition is adjusted and desulfurization occurs. It's rich in lime (CaO) and low in FeO, light in color. It has an important technical peculiarity: it tends to spontaneously pulverize ("dusting") as it cools, due to the transformation of dicalcium silicate (from β to γ form) which involves a volume expansion and the shattering of the material. This instability makes it hard to handle but potentially valuable, because it's rich in CaO and useful compounds.

The chemistry of recovery

Recovering slag means working on two fronts:

• Recovery of the metallic fraction: separating the trapped iron/steel to return it to the production cycle — direct, immediate value.
• Valorization of the oxides: the oxide matrix, once stabilized, can become raw material for cement and binders, construction aggregates and, in some cases, agricultural amendments — replacing virgin raw materials.

The technical crux is the thermal and chemical control of the treatment: managing temperatures and atmosphere (oxidizing or reducing) to favor the right reactions, stabilize the unstable white slag, and avoid wasting value. It's a problem of high-temperature process chemistry, where it matters to understand the phase diagrams of the oxides, reaction kinetics and heat management.

This is exactly the domain of our controlled reactors for slag treatment: they bring into a thermally precise, controlled environment the chemistry that, in a heap or pit, would occur uncontrolled and wasted.