Enter the tapped slag analysis in weight percent to get binary, ternary and quaternary basicity in one go. Add a target B2, the slag mass and your lime’s CaO content to estimate how much lime would close the gap.
Slag basicity compares the basic oxides in a slag with the acidic ones. The same slag gives a different number in each form, so a basicity figure means little until you say which ratio it is. Binary basicity (B2) is the quick shop-floor number. Ternary basicity (B3) adds MgO, which matters when dolomite, magnesian ore or lining wear put magnesia into the slag. Quaternary basicity (B4) also adds Al2O3 to the acid side, which matters when alumina from ore or reductant ash swings from lot to lot. Showing all three side by side makes it obvious when MgO or Al2O3 is moving the slag while B2 stays flat. For what basicity does to melting, viscosity and recovery, read the slag basicity guide.
| Output | Formula (all oxides in wt% of slag) |
|---|---|
| B2, binary basicity | CaO ÷ SiO2 |
| B3, ternary basicity | (CaO + MgO) ÷ SiO2 |
| B4, quaternary basicity | (CaO + MgO) ÷ (SiO2 + Al2O3) |
| Slag reading | Basic if B2 is above 1, acidic if below 1 (within 0.005 of 1 shows as about neutral) |
| Lime to reach target B2 (kg) | (target B2 × SiO2 − CaO) ÷ 100 × slag mass (t) × 1,000 ÷ (CaO in lime ÷ 100) |
The lime line works because basicity is a ratio of masses. Adding lime raises the CaO in the slag without changing its SiO2, so the extra CaO needed is the target ratio times the SiO2 already there, minus the CaO already there, scaled to the slag mass. Dividing by the lime’s CaO fraction turns pure CaO into kilograms of lime. Multiplying the slag mass in tonnes by 1,000 gives the result in kilograms.
That estimate assumes three things: the lime carries no silica, all of its CaO reports to the slag, and the slag’s SiO2 does not change. In alloys where part of the charged silica is reduced into the metal, or where the flux is dolomite rather than lime, use the full balance in how to calculate flux addition for slag basicity, which accounts for silica to metal and the flux’s own silica.
These numbers are illustrative only. They are not from any plant and are not a recommended target. Say a tapped slag analyses 40% CaO, 35% SiO2, 8% MgO and 12% Al2O3.
Notice how far apart the three ratios sit for one slag: B3 is about 20% above B2 because of the MgO, and B4 falls back towards 1 because of the alumina. A target written as “basicity 1.2” could mean any of them.
A hand calculation tells you where the last tap landed. A furnace digital twin such as Furnace Twin computes basicity for every heat from charge and slag chemistry, flags drift with its likely cause, and runs basicity changes as scenarios on a model calibrated to your furnace, so only the best option goes to a trial.
Divide the weight percent of basic oxides in the slag by the acidic oxides. The simplest form is B2 = CaO ÷ SiO2; for a slag with 40% CaO and 35% SiO2, B2 = 1.14. B3 adds MgO to the top and B4 also adds Al2O3 to the bottom.
The same thing as slag basicity: a ratio of basic to acidic oxides in the slag. Above 1 is called a basic slag and below 1 an acidic slag. Always state which form (B2, B3 or B4) the number refers to.
Steelmaking usually quotes binary basicity, CaO ÷ SiO2, and some plants use (CaO + MgO) ÷ SiO2 or (CaO + MgO) ÷ (SiO2 + Al2O3). The formula is the same across processes; the target depends on the process and should come from your own practice.
For a target B2, the extra CaO needed is (target × SiO2 − CaO) ÷ 100 × slag mass. Divide by the CaO fraction of the lime to get the lime mass. This assumes the lime carries no silica and the slag’s SiO2 stays the same.
See it on your own data. Furnace Twin — Ask the furnace, see the proof. Book a 30-minute working session with an engineer.
