Aluminium Design and Construction John Dwight

The main requirements for the production of aluminium are thus
bauxite, coal and cheap electricity. These are never all found in one
place. The location usually preferred for a smelter is near a dedicated
hydroelectric power plant, which may be thousands of kilometres from
the alumina source and from markets for the ingot, as, for example,
with the Kitemat smelter in British Columbia, Canada. Sometimes a
smelter is designed for strategic reasons to run on coal-fired electricity.
The new 50 000 tonne/yr smelter at Richards Bay in Natal, South Africa,
which relies on coal-fired electricity, has a total of nearly 600 pots contained
in four ‘potrooms’ each 900 m long×30 m wide.
2.1.2 Secondary metal
Not all the metal going into aluminium products comes from ingot, an
important ingredient being ‘secondary metal’, i.e. scrap. This is partly
supplied by scrap merchants, and partly comes from process scrap
generated in the rolling mill or extrusion plant. The composition of
such scrap is important, the best scrap being pure aluminium or a low
alloy. Melted-down airframe material is less convenient, as it contains
a relatively large amount of copper or zinc, making it less suitable for
making non-aeronautical alloys.
2.2 FLAT PRODUCTS
2.2.1 Rolling mill practice
Aluminium plate and sheet are manufactured in conventional rolling
mills, the main difference from steel being the lower temperatures
involved.
Alloyed metal is produced by melting a mixture of ingot and scrap,
to which are added metered quantities of ‘hardeners’ (small aluminium
ingots containing a high concentration of alloying ingredient). Rolling
slabs are then produced by vertical continuous casting in a long length,
from which individual slabs are cut. Each slab is skimmed on both
faces and slowly heated in a furnace, from which it enters the hot-line.
This might typically comprise a hot mill followed by a three-stand
tandem set-up. The material emerging therefrom, called ‘hot-mill strip’,
can be shipped directly as plate or reduced further in a cold mill to
produce sheet. The immediate output from the cold mill is coiled strip,
which is decoiled and cut into sheets. Plate and sheet widths thus produced
are typically about 1.5 m, although greater sizes are possible.
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.