Soaking and formation of tetrabasic lead sulfate

Soaking and formation (first charge) stages of the Faure process were studied as separate stages from various 4PbO·PbSO4 (4BS) precursors (pure tetrabasic lead sulfate or industrial tetrabasic positive active material). The evolution during soaking and formation of the various well-defined 4BS precursors was followed by means of scanning electron microscopy, X-ray diffraction and wet chemical analysis. The following results arise from this approach. The sulfation stage results in the formation of PbSO4 crystals with two completely different morphologies. The first type of PbSO4 crystals nucleate at the 4BS needle surface and this leads rapidly to large polyhedral particles of 2 μm in diameter. The second type of PbSO4 crystals consist of very small interconnected grains (0.2 μm) that result from the slow transformation of the 4BS needle into PbSO4 via a metasomatic process. Since the 2 μm PbSO4 polyhedral particles resulting from the first sulfation process are more difficult to convert into PbO2,, the PbO2 crystal growth mainly takes place on the second type of PbSO4 small 0.2 μm interconnected crystals. It can be concluded that the 4BS formation route mainly proceeds via the double metasomatic process 4BS → PbSO4 → PbO2 leading to the textural relationship between PbO2 and its precursor 4BS. The double metasomatic process gives rise to the same type of macrotexture at 20 and 55 °C with a rigid skeleton and a high porosity. On the other hand, the thickness of the individual PbO2 grains (i.e. the microtexture) depends on temperature. Very thin interconnected PbO2 crystals are obtained at 20 °C while thicker grains are observed at 55 °C. These textural observations are confirmed by C//20 discharge capacity measurements on tetrabasic-based plates and batteries. The influence of soaking duration is outlined.