Acid slags

Fireclay refractories are used in kilns, ladles, and heat regenerators, acid—slag-resistant apphcations, boilers, blast furnaces, and rotary kilns. They are generally inexpensive. 

Reducing Atm Carbon Stability to Acid Slags Basic Slags Molten Metals... 

Equation (10.4) relies on a knowledge of both the sulphide and oxide activities. However, S was not yet included in their database for oxides. They therefore utilised the approach of Reddy and Blander (1987, 1989) to relate o,a s lo Ihe S content and sulphide capacities could then be predicted through calculation of oa o alone. The approach yields the following expressions for sulphide capacities for basic and acid slags ... 

Contain >93% Si02, <3.5% CaO, <2% A1203, <1.-5% Fe203 have high corrosion resistance to acidic slags poor resistance to high temperatures and thermal shocks... 

Properties High heat resistance, great strength, high thermal conductivity, high resistance to attack by acids and acid slags, porosity about 25%, low permeability. 

The involatile products of these reactions form an acidic slag (SiOi-FeO SiOi-MnO, etc.), which forms a separate lighter phase on top of the molten iron. These exothermic reactions occurring during the blow are sufficient to raise the temperature of the converted metal from about 1,350 to around 1,600°C or higher, adequate to keep the residual iron molten. This temperature rise is necessary for handling since the melting point of the iron phase rises as the concentration of impurities is decreased by the conversion process. 

Whether an acidic or a basic slag will be needed is a factor that must be considered when a furnace is constructed so that its refractory linings will be compatible with the slag. Silica bricks would deteriorate quickly in the presence of basic slag, and magnesia or lime bricks would dissolve in acid slag. 

Originally, the process was operated with a molten acid slag, which only permitted the removal of Si, Mn and C. Small amounts of limestone were often added towards the end of the refining process to produce a boil , caused by the calcination of the limestone and the evolution of carbon dioxide, but the amounts were insufficient to affect significantly the chemistry of the slag. 

The basic slags clearly degrade silica-forming materials more readily than the acidic slags. In a locally low partial pressure of oxygen, metal silicides may form (Ferber and Tennery, 1984). 

Basic refractories react with acidic slags. Therefore, they are not useful in acidic enviroiunents. They are used under basic conditions and are based on MgO. Examples are magnesite, dolomite, chrome-magnesite, magnesite-chrome, alumina, and mullite. 

Generally, acid slags react with basic bricks and basic slags with acid bricks. But there are exceptions to this general rule. For instance, if iron oxide is present in a slag, both acid and basic bricks will be attacked. The attack... 

In olivenes, FeO is an impurity. This impurity tends to reduce the refractoriness of forsterite bricks. The iron content is adjusted to get a refractoriness of 1750°C. The RUL value minimum is 1550°C. Thermal expansion is low, which results in a moderately good spalling resistance. Fosterite is resistant to both acid and basic slags. Its resistance to acid slag is not as much as that of silica, and its resistance to basic slag is not as much as that of chrome-magnesite. 

These refractory materials are also resistant to slags that are rich in silica (called acid slags) and are often used as containment vessels for them. However, they are readily attacked by slags composed of a high proportion of CaO and/or MgO (basic slags), and contact with these oxide materials should be avoided. 

Basic slag Basic slag Acid slag Cryolite... 

The first fundamental principle is that acid refractories tend to resist acid slags better than basic slags and, conversely, basic refractories tend to resist basic slags better than acid slags. The definitions of acidity and basicity in room-temperature solution chemistry and of refractory chemistry at elevated temperature have a key difference. 

This leads to the observation that acid refractories are more compatible with acid slags, that is, acid materials experience less corrosion loss against acid slags as compared to basic slags. In a like manner, basic refractories are more compatible with basic slags than with acid slags. 

---