Processes during paste preparation
Phase composition of the paste. It depends on H2SO4/LO ratio (LO is the oxidized lead powder), temperature, additives and time of mixing. It has been established that the paste is a non-equilibrium system consisting of crystalline basic lead sulfates and oxides, and amorphous sulfate-containing components. Depending on the temperature of preparation pastes with the following compositions are obtained:
At to < 60oC
- H2SO4/LO ratio up to 12%. The paste contains 3PbO.PbSO4.H2O(3BS) + tet-PbO + orthorhombic-PbO + Pb. Maximum content of 3BS is obtained at 10% H2SO4/LO.
Over 8% H2SO4/LO ratio, mostly PbO.PbSO4 (1BS) is formed.
At to > 70oC
- H2SO4/LO ratio up to 7%. The paste contains 4PbO.PbSO4(4BS) + tet-PbO + orthorhombic-PbO + Pb. Maximum content of 4BS is obtained at 6.5% H2SO4/LO. At beginning of mixing 3BS and orthorhombic-PbO are formed first. Then (3BS) + tet-PbO + orthorhombic-PbO react and 4BS is formed. 4BS nucleation is the slowest process. It depends strongly on temperature. In the presence of surface active additive(s) (expander(s)) 4BS and orthorhombic-PbO are not formed at all.
- H2SO4/LO ratio between 7% and 12%. The paste contains 3BS + 1BS + tet-PbO + orthorhomb-PbO.
With time of stirring 3BS crystals grow up to 2-4 μm in size and 4BS ones reach sizes of up to 20-50 μm.
Technological parameters. Paste density is determined by the ratio liquid (H2SO4 solution+H2O) : LO. The total pore volume of the paste (dried) depends on paste density while the average pore radius and the consistency are determined by the phase composition of the paste.
Semi-suspension technology for preparation of 4BS pastes. It has been developed based on an Eirich Evactherm® paste mixer. The basic principle of this new technology is that 4BS crystals with dimensions between 20 and 30 μm are formed first from a semi-suspension at a temperature higher than 90oC and then the excess water is removed from the semi-suspension under vacuum until the desired paste density is obtained. During the vacuum treatment the temperature of the paste decreases and after that small 4BS and PbO crystals are formed. During the paste formation procedure, the large 4BS crystals build up the PbO2 skeleton of PAM, whereas the small crystals form the energetic PbO2 structure, which participates in the charge-discharge processes on battery cycling. It has been found, that 4BS particles comprise crystal and amorphous zones. The crystal zones contain water molecules part of which can be easily be removed from the particles under vacuum treatment and curing as a result of which the crystallinity of the 4BS particles decreases. Another part of the bound water remains in the 4BS particles after curing of the pastes and can leave them only on heating up to 250oC. The ability of water to leave the particles depends on the density of the semi-suspension used for paste preparation. The higher the H2O content in the semi-suspension the easier it leaves the 4BS crystals on vacuum treatment. Experimental tests have shown that the best battery performance is obtained when the paste is prepared under the following conditions: degree of lead oxidation in the leady oxide (LO) 85%, H2SO4/LO ratio 5-6%, liquid content (H2SO4+H2O) in the semi-suspension 240-260 ml/kg LO, temperature of the semi-suspension equal to or higher than 90oC, duration of paste mixing about 15 min. The new semi-suspension technology of 4BS paste preparation facilitates the formation of stable PAM structure that ensures high capacity and long cycle life of the positive plates of lead-acid batteries.
Pastes containing 4BS+Pb3O4. Positive plates manufactured with 4BS pastes exhibit high stability of the active mass structure thus ensuring long cycle life of the plates during battery operation. Since 4BS crystals are large in size, their oxidation to PbO2 at formation is strongly impeded; hence, the plates have low initial capacity. The present technology suggests the preparation of 4BS pastes in the presence of Pb3O4. Three types of pastes are proposed depending on the method of Pb3O4 introduction: (i) a mixture of 4BS and Pb3O4 used as initial material for paste preparation; (ii) 4BS obtained in the presence of Pb3O4; and (iii) Pb3O4 pre-decomposed with H2SO4 to PbO2+PbSO4 and then used for paste preparation. The processes that occur during paste preparation were followed and its structure and crystal morphology were determined. By testing batteries manufactured with plates prepared by the above methods, their initial capacity was measured to be above 100% vs. the rated capacity and the battery life was considerably longer than that of batteries produced with classical pastes generally used in the battery industry.
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- M. Matrakova, D. Pavlov,Thermal analysis of lead-acid battery pastes and active materials, Proceedings of the 6th International conference LABAT’2005, 13-16 June 2005, Varna, Bulgaria, p. 129
N. Kapkov, A new technology for production of 4PbO.PbSO4 lead-acid battery pastes
Keywords: paste preparation for lead-acid battery plates, lead-acid battery paste composition, leady oxide, LO, H2SO4/LO ratio, 3PbO.PbSO4.H2O, 3BS, 4PbO.PbSO4, 4BS, semi-suspension technology for paste preparation, vacuum treatment, (4BS+Pb3O4) containing pastes