Viscosity of Slags

Viscosity of Slags

slagSlag is a multi component systemIn steel making it consists of acidic oxides such as SiO2 and P2O5and Basic oxides such as FeO, CaO, MgO etc.Cations are surounded by maximum number of oxygen in a three dimensional crystalline structure, number being the coordination numberCations of basic oxides viz Fe2+, Ca2+, Mg 2+ have radii smaller than cations of Al2O3, SiO2 etc.Structure of an oxide ratio of radii of cations/anions

Structure of an oxideBasic oxides octahedral, Acidic Oxides TetrahedralIn silica each atom of silicon is bonded with four oxygen atomsEach atom of oxygen is bonded with 2 silicon atoms

Structure of Silica

The elemental tetrahedral of silica are joined at the vertices to give the hexagonal network in three dimensions During melting the crystalline network of silica is broken by thermal agitation

Network former and breaker oxidesBonding between cations and anions in acidic oxides is strongThese form complex ions such as SiO44 -, In slags they tend to form hexagonal networkThese oxides are therefore called network formers or acidsThey can accept one or several oxygen ionsBasic oxides dissociate and form simple ions like Ca2+ and O2- All basic oxides are donors of oxygen ionsThese oxides are called network breakers since they destroy the hexagonal network of silica by reacting with it

Structure of slagMost slags are silicates. Pure silica has very high viscosity at melting pointsAddition of basic oxides decreases the viscosity by breaking the hexagonal network of silicaConsider the addition of CaO to silicaCalcium oxide dissociates to Ca2+ and O2- Each O2- ion causes two of SiO44 tetrahedra to seperateThe Ca2+ ion is accommodated in the interstices of the silicate structure

Breaking down of silica networkIncreasing addition of Cao causes further break downThe number of vertices destroyed depends on the composition of the binary oxide & the ratio of O/Si in the melt

Viscosity0 the viscosity, A an empirical constant, E the activation energy, T the temperature and R the gas constantViscosity controls the fluidity of the slag inversely relatedViscosity of slag is a function of temperature, composition and percent solid present in slagThe decrease in viscosity is greater with alkaline oxides like Na2O and Fluorides like NaF2 since Na has one charge, each tetrahedron of silica will have Na ion attached to oxygen ionAlumina act as a network breaker in an acidic slag and network former in basic slag

Continued ...Presence of solid particles in slag increase the viscosity of slag is the volume fraction of solids in the slagIf volume fraction between 5-10% viscosity of slag increases by 114-130%As the network is progressively broken down the activation energy for viscous flow decrease from a high value of about 140 Kcal/mole for pure SiO2 to about 30 Kcal/mole at the orthosilicate compositionViscosity of slag decreases as the temperature inceases