Tetrabasic lead sulfate pastes

Sulfate salts have been introduced into the positive paste of VRLA cells during the paste-mixing process. In a recent study [65], sodium sulfate was added to positive pastes at 0, 0.01, 0.05, 1, and 2M levels (note, the authors did not explain how the molarity levels were calculated). The plates were cured at 85°C to form tetrabasic lead sulfate and a-PbO. The tetrabasic lead sulfate crystals had sizes of... 

Tetrabasic Lead Sulfate Pastes, 4Pb0 PbS04 (4BS)... 

A sample scheme of the successive technological procedures involved in the process of 3BS paste preparation is presented in Fig. 6.32. An example of a formulation for 3BS paste preparation could be LO (78% PbO) — 500 kg, H2O — 65 L, H2SO4 (1.4 g cm ) — 39 L, fibres — 0.35 kg. For the conversion of the 3BS paste into 4BS during the plate curing procedure, some 6—7 kg of tetrabasic lead sulfate nucleants should also be added to the initial paste mix. 

Preparation of tetrabasic lead sulfate (4BS) pastes for positive plates. 

Tribasic and tetrabasic lead sulfate crystals (3Pb0PbS04-H20 and 4Pb0PbS04-xH20) contain water and are partly hydrated (x in the 4BS formula is a variable with a value between 0.18 and 0.16, i.e. 4BS crystals are superficially hydrated) [17]. PbO in the CL2 layer is hydrated, too. It can be expected that bonding between 3BS or 4BS particles of the paste and of the corrosion layer will occur by an interaction between their hydrated layers. 

Tetrabasic lead sulfate crystals are big in size (10—100 pm in length and 1—10 pm in diameter). The processes of sulfation of 4BS crystals have been studied by many scientists and the obtained results are summarised in [9,14—17]. Conjectures about the processes that occur on soaking 4BS pastes are made on grounds of the obtained phases across the 4BS crystal cross-section as a result of soaking (see Fig. 9.25). 

Curing the paste at higher temperature for extended hours will produce a high percentage of tetrabasic lead sulfate and result in a plate with lower energy density. At the end of curing, the free lead content of the paste should be lower than 2%. Table 7.2 lists the typical curing profiles. 

The use of pre-sulfated paste materials containing basic lead sulfate, e.g., tribasic and tetrabasic lead sulfates (3PbO x PbS04 x H2O and 4PbO x PbS04) made in dry form prior to forming the paste has also been proposed to improve the efficiency of the paste (13). 

Paste Mixing. The active materials for both positive and negative plates are made from the identical base materials. Lead oxide, fibers, water, and a dilute solution of sulfuric acid are combined in an agitated batch mixer or reactor to form a pastelike mixture of lead sulfates, the normal, tribasic, and tetrabasic sulfates, plus PbO, water, and free lead. The positive and negative pastes differ only in additives to the base mixture. Organic expanders, barium sulfate [7727-43-7] BaSO carbon, and occasionally mineral oil are added to the negative paste. Red lead [1314-41 -6] or minium, Pb O, is sometimes added to the positive mix. The paste for both electrodes is characterized by cube weight or density, penetration, and raw plate density. 

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... 

---