Leady oxides

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. 

Production of leady oxide. Pure lead ingots are subjected simultaneously to abrasion and surface oxidation in a ball mill, or are melted in a Barton pot and oxidized in an air atmosphere [2]. A 70-85% oxidized lead powder (called leady oxide ) is obtained with a characteristic grain size-distribution. The same leady oxide is used for the production of both positive and negative plates. 

When the leady oxide used for 3BS paste preparation has a degree of oxidation of 80-84%, the acid-to-oxide ratio is low (4%), the soaking step is short and the formation of the plates is conducted in H2SO4 solutions with relative density below... 

Another study [40] examined the effect of adding lOg of natural graphite with a grain size of 520 pm to positive plates in 4.5-Ah VRLA cells. The active material was prepared by mixing 2kg of leady oxide and 0.8 g of polyester fibre with 150 ml of water and then 271ml of 1.200 rel. dens, sulfuric acid. The pastes were applied to lead-caldum tin grids, cured at 80°C for 72 h, and dried in air. The test cells, composed of one positive and two negative plates, were formed and then floated at a... 

Both the methods (Barton pot and ball-mill) produced partially oxidised lead oxide containing between 20% and 40% free lead. Hence, this oxide was called leady oxide . The production time of this oxide was reduced substantially, which gave a strong impetus to the development of the lead—acid battery industry after 1926. Nowadays, these two processes are still the dominating methods for leady oxide production. 

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. 

It is a monoclinic crystaUine substance built of long tbin crystals. It is formed when PbO is mixed with H2SO4 solution during paste preparation, within a very narrow pH interval 8—15 wt% H2S04/leady oxide. Monobasic lead sulfate is obtained also during soaking of tbe cured... 

This lead compound forms prismatic crystals with a length of 1—4 pm and 0.2—08 pm in cross section. Its density is 6.5 g cm. It is poorly soluble in water, 0.0262 g L. 3BS is obtained when leady oxide is mixed with sulfuric acid solution (up to 8 wt % H2S04/Pb0) and constitutes the basic component of the battery paste when the latter is prepared at temperatures below 70 °C. Tribasic lead sulfate exerts an influence on the structure of the lead dioxide active mass and thus on some of the performance characteristics of the battery. 

It comprises prismatic crystals with a length from 10 to 100 pm and diameter from 3 to 15 pm. 4BS is formed when leady oxide is mixed with sulfuric acid solution, H2S04/Pb0 < 6 wt%, at temperature higher than 75 °C, as well as during curing of the paste at high temperatures (>85 °C) in the presence of water steam. The structure of the active mass formed from 4BS ensures long battery cycle life. 

Leady oxide manufacture. Pure lead ingots are subjected to simultaneous grinding and surface oxidation (ball-mill method) or are melted and oxidized in humidified air (Barton pot method). A 60—80% oxidized lead powder (leady oxide) is obtained with a corresponding particle size distribution. 

Tube filling. The tubular plates are filled, under pressure, with a slurry of 3BS or 4BS positive pastes diluted with water, or with a suspension obtained from leady oxide and red lead... 

Over 95% of failed lead—acid batteries are recycled in these pools, yielding secondary lead which is re-used for the manufacture of new lead—acid batteries. The secondary lead is purified to a degree, allowing its utilization in the production of leady oxide and lead alloys. A certain amount of primary lead extracted from lead ores is also added to the lead pool and used in the manufacture of leady oxide. Thanks to the high percentage of recycled secondary lead and the simple technology of manufacture, the lead—acid battery is the cheapest chemical power source available. 

As a rule, national battery standards stipulate only Pb purity grade of 99.99% without specifying the type and amount of allowable impurities. The specific infiuence of additives to and impurities in lead alloys has been in the focus of attention of many researchers [6—12]. Table 4.3 summarises tbe maximum allowable impurity levels for both primary and secondary lead for battery use [10]. Secondary lead comes from recycling batteries after purification. Lead of the purity grade presented in Table 4.3 can be used for the manufacture of leady oxide and lead alloys for both positive and negative grids. 

Barton Pot Method of Leady Oxide Production with Moderate Temperature Oxidation of Lead... 

In 1898, George V. Barton proposed a method that made the production process of lead oxide considerably faster and easier. This method yields partially oxidized lead powder (leady oxide) which ensures high battery performance. 

Schematic of a typical unit for leady oxide production is presented in Fig. 5.4 [12]. The molten lead (450 °C) is fed via a pump into a large reaction pot equipped with rapidly rotating paddle to agitate the lead. The reaction pot is heated to about 470 °C and the molten lead is stirred and pulverised eontinuously. A stream of humidified air oxidizes the lead and earties the lead oxide partieles to the elassifier, where the coarse grains are separated from the fine partieles and then returned to the reaetor. The coarse particles are oxidized, dispersed and returned baek to flie classifier. A eleaning system (dust collector) for the escape of air ensures low emission levels.

Since oxidation of lead is an exothermic reaction, considerable amounts of heat are produced in the reaction pot. The reaction temperature should be controlled carefully as it determines the type of the resulting leady oxide. The processing temperature should be kept below 487 °C for the production of tet-PbO, which is the oxide modification preferred by the battery industry. Since the oxidation must be carried out at temperatures below the polymorphic transition temperature of 487 °C, the reactor pot is kept within the temperature range of 460—470 °C. Under these conditions, small amounts (less than 15%) of orthorhomb-PbO are formed and the subsequent parameters of the battery are virtually unaffected. 

The obtained leady oxide with acceptable small particles and phase composition passes through a series of cyclone separators and a dust collector to remove the lead oxide dust from the air stream, and is then transported to a silo. The entire technological process is monitored by sensors and controlled by a computer. 

The Barton method yields a 70—80% oxidized leady oxide. At high reactor temperatures, the oxide is entirely orthorhombic. At low temperatures, tet-PbO is formed along with ortho-rhomb-PbO. Small amounts of PbO and Pb304 can also be obtained by this method. 

The influence of some of the Barton pot process parameters on the structure and properties of the obtained leady oxide can be summarised as follows ... 

In the production practice, interrelated variation of the different process parameters is applied to obtain the desired leady oxide powder quality. When the leady oxide powder is too coarse to he used directly in battery manufacture, it is subjected to further grinding in a mill. 

About 75% of the total leady oxide production is realised by the Barton pot method. To bring the Barton pot process into operation at the desired reaction temperatures requires some 30 min each day. The required power for oxide manufacture is approximately 65 kW per ton of leady oxide. 

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