Modern inorganic chemistry

142 THE ELEMENTS OF GROUP III

From the sodium aluminate solution, aluminium hydroxide is

precipitated by passing in carbon dioxide:

2[A1(OH) 4 (H 2 O) 2 ] - + H 2 O + C0 2 -»2[Al(OH) 3 (H 2 0) 3 ]i + CO^ ~

Alternatively, the solution is 'seeded' with a little previously prepared

aluminium hydroxide:

[A1(OH) 4 (H 2 0) 2 ]- + H 2 O ^ [Al(OH) 3 (H 2 0) 3 ]i + OH~.

The pure oxide is then obtained by heating the precipitated

hydroxide:

2A1(OH) 3 (H 2 O) 3 -> A1 2 O 3 + 6H 2 O

The pure oxide is dissolved in molten cryolite in an iron bath lined

with graphite which acts as the cathode (see Figure 7.1). The anode

Carbon anodes- Carbon cathode

Solid crust of ^

electrolyte

Molten Nv

aluminium g

h

• .^
  ^

^Ul

-_ _ _:: ':":"•_"- --~-~-~-~---<-f-i^2 -~ - -.-_ -

f.-_-ii i in ii-i-i-i------------- -.- -.-.-. _

-:-_.:

_ ,

7^II ^

;::tn

_- ~- "

/Cast iron

bath

""Molt6n

electrolyte

(alumina in

cryolite)

Figure 7.1. Extraction of aluminium

consists of carbon rods suspended in the molten electrolyte. A low
voltage must be used to avoid decomposition of the cryolite, and
a very high current density is employed. (The proportion of the
cost of this process for electric power is high; hence it is usually
carried out where electric power is cheap and plentiful.) Molten
aluminium collects on the floor of the graphite-lined bath and is
run off at intervals, fresh alumina being added as required. The
temperature of the bath (1100-1200 K) is maintained by the passage
of the current. Oxygen is evolved at the anode, which is slowly
attacked to form oxides of carbon and a little carbon tetrafluoride,
CF 4 , may also be formed by slight electrolysis of the cryolite. A
promising alternative to graphite for the bath lining is silicon nitride,
Si 3 N 4 which is very resistant to molten aluminium and cryolite. It
is a non-conductor, and hence resistant cathodes made of titanium
diborate (TiB 2 ) are used.
In a newer process, in which purification of the oxide is of much