Basic lead sulfate paste

The active mass obtained from pastes containing no H2SO4 (0%) has very low capacity, irrespective of the temperature of paste preparation. That is why battery manufacturers use basic lead sulfate pastes, and not lead oxide ones, for the production of positive plates. 

Technological Applicability of Basic Lead Sulfate Pastes in the Battery Industry... 

The pore volume and surface area distribution vs. pore radius for PAM formed from pastes with various phase compositions and densities are given in Fig. 10.20 [23]. The curves for chemically prepared Pb02 are also provided for comparison. When the PAM is obtained by formation of basic lead sulfate pastes, the pore volume grows gradually within a wide range of radii (0.05 to 1.2 pm). For chemically prepared Pb02, the total pore volume attains its maximum value within a very narrow range of radii. A steep rise in the surface area curve for... 

PAM prepared from basic lead sulfate pastes is observed for pore radii smaller than 0.5—0.1 pm. In the case of chemical Ph02, this rise starts at radii ten times smaller (0.01 pm). Both the pore volume and the surface area are strongly dependent on the phase composition and density of the paste. These experiments suggest that the porosity of the PAM can be controlled through the phase composition and the density of the initial paste. 

Paste mixing means the addition of sulfuric acid and water. The result is a fairly stiff paste with a density between 1.1 and 1.4gcm 3 containing 8-12wt% of lead sulfate. The water content of thus 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 1). In the usual mixing process between room temperature and 50 °C, tribasic lead sulfate is formed. The generation of the tetrabasic... 

Later in the formation process, the electrochemical reactions cause H2SO4 to be extracted from the plates and, consequently, the add concentration increases. At the end of the formation process, the add concentration is higher than that at the beginning of formation. This increase is due to the extraction from the plates of the acid used in the production of basic lead sulfates during paste preparation and in sulfation of the paste during plate soaking. 

The above reactions start at the two surfaces of the plates and form two zones rich in PbS04, which grow towards the core of the plate (Fig. 3.3). Cured pastes are yellow in colour (basic lead sulfates and PbO are hydrated), while sulfated zones are grey. This difference in colouring allows easy determination of the growth rate of the sulfate zones towards the interior of the plate. The rate of movement of the reaction layer (FjuO into the bulk of the cured paste depends on the following parameters ... 

Table 3.1 [15]. The volume of the particles increases during the oxidation of PbO to p-Pb02, and decreases during the oxidation of PbS04 and basic lead sulfates to Pb02. Thus, the overall volume change of the crystals will depend on the phase composition of the paste.

In positive-plate manufacture, 3BS and 4BS phases are never used alone. They are always in combination with PbO, which improves the connection between the basic lead sulfate crystals and hence facilitates the formation of a mechanically strong porous mass or skeleton. The ratio between the basic lead sulfates and the PbO in the pastes exerts an influence on the initial capacity and the cycle-life performance of the battery, namely the higher the PbO content in the paste, the lower is the initial capacity of the positive plates (Fig. 3.33). 

The paste for negative battery plates after curing consists of 3BS, PbO and small amounts of Pb. During formation, the basic lead sulfate is partially reduced to Pb and PbS04 forms (first formation stage). When the whole amount of basic lead sulfates is consumed, the electrode potential rises by absolute value and PbS04 is reduced to Pb (formation stage 2). 

Plate curing. Pallets with plates are placed in a high humidity chamber and left to cure at 35 °C for 48—72 h. During the curing process, lead in the paste is oxidized, the basic lead sulfates recrystallize and the plates are then dried to moisture content <0.5%. 

The type of basic lead sulfate in the paste affects the structure of the active materials formed, especially of the lead dioxide active mass. And the active mass structure in turn determines the charge/discharge cycling performance of the battery. Thus, control of the a-PbO p-PbO ratio... 

Lander [1] has established that the paste is composed of basic lead sulfates, non-reacted lead oxides, hydrated lead oxides, free lead particles and basic lead carbonates. Some key thermodynamic data for basic lead sulfates, lead oxides and lead hydrates (hydroxides) are presented in Appendix 1 [2]. 

Three types of reactions take place during paste preparation (1) oxidation of lead, (2) formation of basic lead sulfates and (3) formation of hydrates. The thermal effects of the different reactions are calculated on the basis of thermodynamic data. The heat effects of the respective reactions of formation of basic lead sulfates and hydrates are given in Table 6.4. 

To sum up, the performance parameters of the battery (capacity and cycle life) are greatly pre-determined by the type of paste used for plate manufacture, i.e. by the type of basic lead sulfate(s) it contains. The type and amount of basic lead sulfates in the paste influence both the initial capacity and the cycle life performance of the battery. Therefore, it is essential to know very well the processes that take place during paste preparation. The technological procedures of paste preparation should be conducted under strict control of the specified technological parameters. 

One of its major advantages is that it allows the reaction between lead oxide and H2SO4 to proceed in a semi-suspension state (i.e. at densities between 3.20 and 3.50 g cm ). This method has been developed in our laboratory [34,35]. On completion of the crystallization process of basic lead sulfate, the semi-suspension can be concentrated through removal of the excess water (by evaporation under vacuum) until a paste of a desired density is obtained. 

The semi-suspension has a much lower viscosity than that of the paste. This would allow the chemical reaction between H2SO4 and PbO to proceed uniformly throughout the whole mixer volume. This, in turn, would yield a homogeneous paste. Secondly, the ion transport between PbO in the powder and the growing basic lead sulfate crystals is much faster in the semi-suspension than in the paste and hence the chemical reaction is facilitated, which reduces the time for preparation of high-quality pastes. 

When the paste is ready, it is poured into an intermediate paste feeder (hopper) from where it is charged periodically (in batches) into the pasting machine mounted under the hopper. The paste is stirred in the hopper to maintain its homogeneity and to allow recrystallization of basic lead sulfates to reach an advanced stage. 

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