Page 216 - Handbook of Battery Materials

6.4 PbO 2 as Active Material in Lead–Acid Batteries 185

Gray oxide can be produced by a milling process, which, strictly speaking, does
not mill the material. A rotating drum is filled with solid balls or ingots of lead.
Flakes are shared, crushed, and at the same time partly oxidized by an airstream
that flows through the drum. Temperature and airflow rate are used to control the
process to achieve the desired powder. At the end the oxidized material is carried
away by an airstream and classified. Particles that are too coarse are fed back into
mill.
Another process, the Barton process, is based on molten lead. The core of such
a device is the ‘Barton reactor,’ a heated pot that is partly filled with molten lead. It
is continuously refilled by a fine stream of molten lead. Fine droplets of lead are
produced by a fast rotating paddle that is partly immersed under the surface of the
molten lead within the ‘Barton reactor.’ The surface of each droplet is transformed
by oxidation into a shell of PbO by an airstream that simultaneously carries away
the oxidized particles if they are small enough; otherwise, they fall back into the
melt and the process is repeated. Thus the airstream acts as a classifier for particle
size.
A short description of both processes is given in Ref. [15]. Nowadays, the Barton
process is preferred for a number of reasons: it can more easily be installed in
small units and it can be controlled faster [16]. (In a mill it takes a number of hours
for the material to run through the drum, so controlling actions are slow.)
Paste mixing means the addition of sulfuric acid and water. The result is a fairly
stiff paste with a density between 1.1 and 1.4 g cm −3 containing 8–12 wt% of lead
sulfate. The water content of this mix determines the porosity of the active material
achievable later (cf. ‘curing’ below). In the paste, a mixture of lead sulfate and
basic lead sulfate is formed (cf. Table 6.1). In the usual mixing process between
◦
room temperature and 50 C, tribasic lead sulfate is formed. The generation of the
tetrabasic modification (4PbO·PbSO 4 ) is favored at temperatures above 70 C [17].
◦
To a certain extent, the formation of the tetrabasic variant is desired, because
4PbO·PbSO 4 forms fairly large crystals when transformed into lead dioxide (PbO 2 ).
This results in a mechanically stable active material, but there are disadvantages
because it is more difficult to transform this material into lead dioxide, that is, the
formation process (see below) is more expensive (and takes longer) and the initial
capacity is slightly reduced (cf., e.g., Ref. [18]. For ‘long-life batteries’ (Bell systems
cell), a special process has been developed to produce pure tetrabasic material
[19].
Sometimes red lead or minium (Pb 3 O 4 ) is added to the paste. The addition is
usually 5–10 wt%, and is mainly made for easier formation of the final compound
lead dioxide (PbO 2 ) (cf. ‘curing’ and ‘formation,’ below).
A general problem in open mixing machines is that the exothermic process
PbO + H 2 SO 4 → PbSO 4 + H 2 O (6.30)
−1
causes a temperature increase of the mix ( H =−173 kJ mol ). In mixing
machines the evaporation of water is often used for cooling, but then water is
lost and the composition of the mix changes. For this reason a certain surplus
of water is scheduled in the paste recipe. However, evaporation rate depends