Man's physical universe

xanabras

METALLURGY 635

flotation. Many improved types of concentrating machinery have

made it possible to reopen abandoned mines, and to run them at a

profit. (2) After concentration, many ores need to be roasted — that

is, heated in an excess of air — to convert compounds of sulfur and

arsenic into oxides. (3) The metal oxides so formed must then be

reduced. Reduction may be accomplished by the use of:

a) Carbon. Coke is the most common reducing agent. When coke

is used, a fiiix must always be added to combine with the foreign

materials (the gangue) in

the ore to form a crude form of glass (the

slag), which floats on top of the molten metal and, incidentally, protects

it against oxidation.

It will be pointed out in the next Section

that lime and silica are the principal components of glass. If the

gangue is siliceous, limestone is added as a flux, and vice versa.

b) Hydrogen. Hydrogen is used in the reduction of tungsten oxide.

c) More active metals. Aluminum also serves as a powerful reducing

agent in cases where carbon will not serve. The thermite process

may be used to obtain chromium. It consists of igniting a mixture

of chromium oxide and aluminum powder:

CrzOg + 2A1 —>- AI2O3 + 2 Cr

chromium oxide aluminum aluminum oxide chromium

Welding of iron is sometimes accomplished by the thermit process.

A mixture of iron oxide and aluminum is placed in and around the

joint to be welded, and the mixture is ignited. The reaction produces

superheated molten iron, which welds the two pieces of iron.

d) The electric current. Very active metals may be prepared by

using the electric current as the source of the electrons which must

be added to metals in the reduction process.

Sodium, potassium, magnesium, and aluminum are among the

metals that are separated by the electrolysis of their molten compounds.

(4) After reduction, the crude metal must usually be freed from

small quantities of other metals by refining processes.

This involves a

remelting with a proper flux, distillation, or electrolysis.

The light metals are less easily reduced than the heavier ones.

a general rule, they are separated from their compounds, after concentration,

by electrolytic processes.

Advances in Steel Manufacture Have Enabled Man to Produce Improved

Machines.

The iron obtained from the blast furnace is called pig iron. It

contains several per cent each of carbon and silicon, besides other

As

More magazines by this user
Similar magazines