Pasted plates, lead oxides

In the lead accumulator the electrode plates are lead grids, filled initially with a paste of lead oxide PbO. They are placed in an electrolyte of dilute sulfuric acid. One electrode plate is marked plus , the other minus . The battery is charged with dc current from a rectifier. In this process the plus plate is connected to the positive pole of the rectifier, the minus plate to its negative pole. On the plus plate the lead oxide is changed to lead dioxide PbOj, and on the minus plate metallic lead is formed. Be-... 

Conventional lead-acid batteries contain a positive electrode (Pb02 plate) and a negative electrode (Pb plate) immersed in a sulfuric acid electrolyte and having a separator interposed between each plate. Such electrodes are typically made by applying a paste containing lead oxides and lead sulfates to the surface of a battery plate and electrochemically forming the paste into an active material. 

The flat (or pasted) plate cell, pioneered in the 1880s, has a positive plate made from a lead alloy lattice grid into which a paste of lead oxide and... 

PIa.tes, Plates are the part of the cell that ultimately become the battery electrodes. The plates consist of an electrically conductive grid pasted with a lead oxide—lead sulfate paste which is the precursor to the electrode active materials which participate in the electrochemical charge—discharge reactions. 

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. 

The cast grids are made into battery anode and cathode plates by the application of a lead oxide paste of 70 percent lead oxide (PbO) and 30 percent metallic lead. Lead ingots are tumbled in a ball mill with airproducing lead oxide and fine lead dust (referred to as leady oxide ). Leady oxide particulates are entrained in the mill exhaust air, which is treated sequentially by a cyclone separator and fabric filter. The used fabric filter bags are shipped to a RCRA-permitled commercially operated ha2ardous waste landfill located in Colorado. The leady oxide production process does not produce wastewater. 

The production of tubular positive plates is in principle similar to that of pasted plates. A number of manufacturers use the same gray oxide as the basic filling substance. Sometimes the share or red lead or minium (Pb304) is increased above 25 or even to 100wt.%. The latter is more economic when the manufacturer runs his own minium plant then the expense of the chemical oxidation of lead oxide (PbO) to minium (Pb304) may be compensated by reduced formation cost. Furthermore, curing is not required, because of the high oxidation state, and the battery starts with full capacity when formed. 

Different methods are in use for plate filling. The material can be filled as a powder with the aid of vibrators. Other techniques use a slurry of lead oxide or even a paste, as described above [27]. 

The hrst working lead cell, manufactured in 1859 by a French scientist, Gaston Plante, consisted of two lead plates separated by a strip of cloth, coiled, and inserted into a jar with sulfuric acid. A surface layer of lead dioxide was produced by electrochemical reactions in the first charge cycle. Later developments led to electrodes made by pasting a mass of lead oxides and sulfuric oxide into grids of lead-antimony alloy. 

Curing is the process of exposing plates pasted positive and negative to a regime of (a) controlled time (minimum 32h), (b) temperature (30-35°C), and (c) relative humidity (>90%). This process converts the free lead into lead oxide, using oxygen from the surrounding air. The plates are allowed to cure for a minimum of 32 h. Care is also taken to ensure that the maximum temperature of the plate does not exceed 60°C. The cured plates are then parted. 

This chapter reviews the effects of additives in the positive active-material of the lead acid battery. Common materials found in the oxide and the positive-plate paste, such as lead oxides, basie lead sulfates, and lead earbonate, are not included. Additives and impurities that derive from grid eorrosion or from the reaction of the plate with the electrolyte are also beyond the seope of this chapter. 

Particles of lead dioxide in lead monoxide, such as those formed in a ball-mill, can be formed by treating the oxide with ozone before paste mixing [49]. The use of persulfate [50-53] and peroxides [54] to effect the partial conversion of lead oxide in the paste to lead dioxide has also been proposed. A proprietary process for treating the surfaces of unformed plates with ozone gas produced a thin coating of lead dioxide, which enhanced formation [55,56]. Much lower quantities of lead dioxide are needed with this approach than when red lead is added to the plate, and the normal battery paste mix can be used. Dipping or spraying the plate with a persulfate solution has also been adopted to oxidize the surface PbO to conductive Pb02 [57]. 

In 1881, Camille Faure coated the lead plates with a paste of red lead oxide, sulfuric acid and water, and then charged them to form Pb and Pb02 active masses. The specific energy of the battery increased to 8 Wh kg at 10 hours discharge rate [12]. 

This lead compound exists in two polymorphic forms tetragonal (P-PbO) and orthorhombic (a-PbO). The solubility of the two forms in water at 25 °C is 0.0504 g for a-PbO and 0.1065 g for 3-PbO [6]. Lead oxide forms lead hydroxides, 3Pb0-H20 and 5PbO H2O [7,8]. Lead oxide is hydrated forming Pb(OH)2, a compound of amphoteric nature. It dissociates to HPb02 and Pb(OH) ions. In the battery industry, lead oxide is obtained by partial thermal oxidation of lead and is ealled leady oxide , as it eontains between 73% and 85% PbO, the remaining part being non-oxidized lead. The basie eonstituent of leady oxide is tet-PbO, but orthorhombie PbO is also present, up to 5—6%. Leady oxide is used for tbe preparation of the pastes for lead—aeid battery plate production. 

The above investigations indicate that the physical and chemical properties of the lead oxide used as precursor material for the production of battery plates, though the lead oxide is only a starting compound for a number of chemical processes (paste preparation and plate formation, whereby Pb02 and Pb are formed), exert an influence on the energetic and capacity performance parameters of lead-acid cells and batteries. Hence, it is essential to produce leady oxides with optimum and stable physico-chemical properties which would guarantee high battery performance. 

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. 

The rate of lead oxidation is influenced also by the moisture content of the paste prior to curing. The amount of water in the paste pores should be reduced so as to open the pores for access of oxygen from the air needed to oxidize the residual free lead in the paste. The rate of water evaporation from the paste pores depends on the RH in the curing chamber. This dependence is illustrated in Fig. 8.17 for plate curing at 25 °C and various relative humidities [14]. 

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