REFRACTORIES IN HEAT UNITS
REFRACTORIES AND CARBON CATHODE MATERIALS
FOR THE ALUMINUM INDUSTRY.
CHAPTER 2. REFRACTORIES AND CARBON CATHODE BLOCKS
FOR ELECTROLYTIC PRODUCTION OF ALUMINUM
A. L. Yurkov
Translated from Novye Ogneupory, No. 9, pp. 15 – 21, September, 2005.
2.1. PRINCIPLES OF INDUSTRIAL ALUMINUM
OF AN ALUMINUM ELECTROLYSIS SHOP.
ELECTROLYSIS CELL TYPES
In the commercial manufacture of aluminum, the electro-
lyte performs three major functions: (i) as a current conduc-
tor between the anode and the cathode; (ii) as a solvent for
aluminum oxide, and (iii) as a physical interface between
aluminum formed on the cathode and carbon dioxide formed
on the anode. The electrolyte typically used is cryolite
= 1010°C) with aluminum oxide dissolved in
it and additives (up to 16 – 18%) of aluminum, calcium, and
magnesium fluorides. The additives serve to reduce the melt
ing point to 930 – 960°C.
In the molten state, the electrolyte dissociates into ions:
trivalent aluminum ions throw off a charge on the cathode to
yield metallic aluminum:
+3e ® Al¯, (2.1)
whereas on the carbon anode, oxygen ions become dis
charged and enter into an interaction with anodic carbon:
–4e +C® CO
Molten aluminum and electrolyte are immiscible liquids;
separated by the specific density in an electrolysis cell, alu
minum forms a bottom layer (density 2.3 g/cm
) and the
electrolyte forms a supernatant layer (about 2.1 g/cm
A carbon anode is kept immersed in the electrolyte. The sur-
face of molten aluminum on the bath bottom serves as a cath-
ode. The distance between the surfaces of anode and cathode
(interelectrode spacing) is maintained at a level of 4–6cm.
An operating electrolysis cell should never be emptied com-
pletely, the metal level should be 25 – 35 cm, and the electro-
lyte level, 15 – 25 cm. Over the melt (at the air – electrolyte
interface), a crust of solidified electrolyte is formed; on top
of the crust, a layer of alumina pack is placed. On the inside,
the side walls of the bath shaft are coated with a layer of so
lidified electrolyte (the so-called scull).
The aluminum electrolysis is controlled using several pa
rameters (of these, average values are given): electric power
consumption, 13,000 – 15,000 kW/h per ton metal; alumina
consumption, about 1920 kg per ton metal; carbon anode
consumption, 600 kg per ton metal; current efficiency (per
centage of theoretical value) about 95% for a bath with
self-baking anode and up to 90% for baths with baked ano
des. As a rule, in baths with a long service life these parame
ters tend to decrease.
The electrolysis cells in terms of power consumed are
conventionally divided into: (i) medium-power, up to 100 kA;
(ii) high-power, up to 200 kA, and (iii) superpower electroly
sis cells, 200 – 330 kA. Operating voltage in the
Hall-Heroult process is maintained at a level of 4.1 – 4.5 V,
and the cathode current density, at a level of 0.6 –
. In what follows we give a simplified diagram
for voltage drop in an electrolytic bath according to H. Oye
 (total voltage, 4.2 V; current efficiency, 95%; power con
sumption, 13.2 kW × h per kg aluminum; electrolyte tempe
Refractories and Industrial Ceramics Vol. 46, No. 6, 2005
1083-4877/05/4606-0365 © 2005 Springer Science+Business Media, Inc.
The text of this paper is borrowed from the book A. L. Yurkov,
Refractories and Carbon Cathode Materials for the Aluminum
Industry, Krasnoyarsk (2005).
RUSAL Co., Krasnoyarsk, Russia.