Leady oxide production

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. 

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. 

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

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.

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. 

After the above brief description of the two methods of leady oxide production the question arises logically as to which of the two processes is better. Table 5.1 compares the characteristics of the leady oxides produced by the Barton pot and ball mill processes [17]. 

The three basic process parameters that are most often used to compare the two methods of leady oxide production can be summarised as follows ... 

A comparison between the reactivity and productivity of the two leady oxide production processes is presented graphically in Figure 5.8. 

Especially important for proper operation of the battery are the impurities contained in the metal used for leady oxide manufacture. Lead for the battery industry is derived from ores mined in different parts of the world (primary lead) or is obtained by recycling of used up batteries that have reached their end of fife (secondary lead). The recycling process is very often performed at the battery manufacturers facilities. Purity standards have been adopted for the lead to be used for leady oxide production. These standards specify different maximum allowable amounts of impurities for flooded and valve-regulated lead-acid battery applications. Table 5.2 presents typical purity specifications for lead for making leady oxide for flooded batteries. 

The acid absorption of the leady oxide is determined by a method similar to that applied for determining the water absorption, but in this case sulfuric acid with 1.10 relative density is used instead of water. Depending on the method of leady oxide production and on the equipment used, the acid absorption value varies. Figure 5.10 presents the acid absorption (in mg H2SO4 per g oxide) as a function of the BET surface area for leady oxides produced by the Barton pot and ball mill methods [21]. 

Environmental and Health Hazard Issues Related to Leady Oxide Production... 

Leady oxide manufacture is potentially very hazardous. It is therefore vital to take adequate measures to minimise, or even eliminate altogether, possible lead dust emissions out to the working environment and thus exposure of the personnel which would cause health problems. This is achieved by the use of a closed (isolated) leady oxide production equipment and transport pipes from the storage silos to the paste mixing unit. Special attention should be paid to file manufacture of tubular battery plates. Until recently, tubular plates were filled with a dry mixture of leady oxide and red lead powders. 

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. 

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. 

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. 

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. 

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. 

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. 

The productivity of the ball mill and the characteristics of the leady oxide obtained depend on... 

The energy consumption for operation of the whole ball mill system is about 200 kW per ton of leady oxide. This is about three times the energy consumed by a Barton pot production unit of the same capacity [16]. 

Red lead blended with leady oxide in the paste for the production of positive battery plates has two beneficial effects (1) shortens the time for positive plate formation, which is, a rule, longer than the formation time of the negatives and (2) increases the initial capacity of the batteries. 

Leady oxide consists of lead oxide and free metallic lead. When produced by the ball mill method, the degree of oxidation of 65—75% is achieved, while the Barton process yields leady oxide powder with 70—80 wt% PbO content. When the leady oxide contains more than 30 wt% free lead, the material becomes very reactive. It is readily oxidized in humid air and the heat produced by this exothermic reaction may lead to combustion and damage the production equipment. Hence, effective control of the percent content of imoxidized lead in the leady oxide is very important, both during the production process and during the subsequent handling and storage of the leady oxide. 

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. 

There are literature data reporting of the praetiee of some battery manufacturers to introduce these additives into the paste through the leady oxide, hy using lead powders containing certain amounts of these hinders [31,32], Thus for example, the maximum allowable content of Bi in the lead for LO production is up to 0.05%. 

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