Acids, Base and Salts In Metallurgical Terminology

Acids, Base and Salts In Metallurgical Terminology Points : Acids, Base and Salts In Metallurgical Terminology, Definition Definition When an oxide on a non-metal combines with water it forms what we call an acid. Thus sulphur trioxide (SO₃) combines with water to form the well — known sulphuric acid (H₂S0₄) and sulphur trioxide is said to be the anhydride of sulphuric acid. Though not all acids are as corrosive as sulphuric, it is fairly well known that in cases of accident involving acids of this type it is necessary to neutralize the acid with some suitable antidote.

Substances which have this effect are called bases. These are metallic oxides (and hydroxides) which, when they react with an acid, produce water and a chemical compound which we call a salt. Typical examples of these acid-base reactions are-

H₂S0₄ + CaO = CaSO₄ = H₂0
Sulphuric Calcium Oxide Calcium Water
Acid (‘quicklime’) sulphate
HCl + NaOH = NaCl + H₂0
Hydrochloric Sodium Sodium Water
Acid hydroxide Chloride
(‘caustic soda’) (‘common salt’)

When we can generalize in respect of equations like this and say —
Acid + Base = Salt + Water
Similarly, the acid anhydride will often combine with a base, forming a salt, e.g., -

SO₃ + CaO = CaSO₄

This type of reaction occurs quite frequently during the melting of metallic ores.

One of the most common elements in the Earth’s crust is silicon, present in the form of its oxide, silica (Si0₂). Since silicon is a non-metal, its oxide is an acid anhydride, and though all the common forms of silica, such as sand, sandstone and quartz, do not seem to be of a very reactive nature at normal temperatures, they are sufficiently reactive when heated to high temperatures to combine with many of the metallic oxides (which are basic) and produce neutral salts called silicates. Silica occurs entangled with most metallic ores, and although some of it is rejected by mechanical means before the ore is charged to the furnace, some remains and could constitute a difficult problem in that its high melting point of 1780°C would cause a sort of ‘indigestion’ in the furnace. To overcome this a sufficient quantity of a basic flux is added in order to combine with the silica and produce a slag with a melting point low enough to allow it to run form the furnace. The cheapest metallic oxide, and the one in general use, is lime.

Si0₂ + 2CaO = 2CaO.Si0₂
Acid Anhydride Base Salt (Calcium Silicate)

The formula for the slag, calcium silicate, is generally written 2CaO.Si0₂ rather than Ca2SiO₄, sine lime and silica will combine in other proportions. When the formula is written in the former manner, the molecular proportions can be seen at a glance.

On the other hand acid/basic reactions can constitute a problem when they involve similar reactions between slags and furnace linings. Thus, since we do not wish to liquefy our furnace lining, we must make sure that it does not react with the charge or the slag covering it. In short, we must make sure that the furnace lining is of the same chemical nature as the slag, i.e., if the slag contains an excess of silica, and is therefore acid, we must line the furnace with a similar silica-rich refractory, such as silica brick or ganister; whilst if the slag contains an excess of lime or other basic material, we must line the furnace with a basic refractory, such as burnt dolomite (CaO.MgO) or burnt magnetite (MgO). If the chemical nature of the furnace lining is the same as that of the slag, then, clearly, no reaction is likely to take place between them. Since silica and ganister, on the one hand, and dolomite and magnetite, on the other, all, have high softening temperatures, they will also be able to resist the high temperature encountered in many of the metallurgical smelting processes.