Lead sulfate dependence

By-Product Recovery. The anode slime contains gold, silver, platinum, palladium, selenium, and teUurium. The sulfur, selenium, and teUurium in the slimes combine with copper and sUver to give precipitates (30). Some arsenic, antimony, and bismuth can also enter the slime, depending on the concentrations in the electrolyte. Other elements that may precipitate in the electrolytic ceUs are lead and tin, which form lead sulfate and Sn(0H)2S04. 

Salts such as silver chloride or lead sulfate which are ordinarily called insoluble do have a definite value of solubility in water. This value can be determined from conductance measurements of their saturated solutions. Since a very small amount of solute is present it must be completely dissociated into ions even in a saturated solution so that the equivalent conductivity, KV, is equal to the equivalent conductivity at infinite dilution which according to Kohlrausch s law is the sum of ionic conductances or ionic mobilities (ionic conductances are often referred to as ionic mobilities on account of the dependence of ionic conductances on the velocities at which ions migrate under the influence of an applied emf) ... 

Nebel and Cramer (I40) show that the addition of a series of lead compounds to carbon at a concentration of ca. 5 wt. % lowers the ignition temperature (raises the combustion rate) of the carbon. Of importance is the finding that the extent of the catalytic effect depends on the particular salt. Lead acetate is the most effective, lowering the ignition temperature 293° lead sulfate is the least effective, lowering the ignition temperature only 39°. Lead pyrophosphate and lead orthophosphate are found not to lower the ignition temperature. 

Lead chromate occurs in nature as crocoite, an orange-red mineral. Synthetically prepared lead chromate and its solid solutions with lead sulfate and lead molybdate represent a hue range from primrose yellow to red. The various hues of chrome yellow, chrome orange, and molybdate orange depend not only on composition but also on crystal structure and particle size. 

The above reactions start at the two surfaces of the plates and form two zones rich in PbS04, which grow towards the core of the plate (Fig. 3.3). Cured pastes are yellow in colour (basic lead sulfates and PbO are hydrated), while sulfated zones are grey. This difference in colouring allows easy determination of the growth rate of the sulfate zones towards the interior of the plate. The rate of movement of the reaction layer (FjuO into the bulk of the cured paste depends on the following parameters ... 

The dependence of the amount of H2SO4 that has reacted with 10 g of 3BS or 4BS paste as a function of soaking time is shown in Fig. 3.10. 4BS pastes react with H2SO4 much more slowly than do 3BS pastes. The smaller surface area and the greater thickness of the crystals in 4BS pastes cause the rate of the reaction of sulfation to slow down within the first 30 min of soaking [11-16]. The structure of the lead sulfate layer that covers the 4BS crystals has been examined [11-14]. First, polyhedral particles with an average size of up to 2 pm are formed on the surface of... 

Table 3.1 [15]. The volume of the particles increases during the oxidation of PbO to p-Pb02, and decreases during the oxidation of PbS04 and basic lead sulfates to Pb02. Thus, the overall volume change of the crystals will depend on the phase composition of the paste.

Another option for secondary smelters is to desulfurize the battery paste prior to smelting. Chemical desulfurization, however, is dependent on physical mixing conditions and temperature. Chemical desulfurization is achieved by adding a concentrated sodium carbonate solution to an agitated mix of battery paste sludge to convert the lead sulfates to lead carbonates. Complete conversion of lead sulfate to lead carbonate eliminates sulfur in the furnace feed material and sulfur dioxide in the exhaust gases. Complete desulfurization is, however, rarely achieved under normal industrial conditions. 

The depth to which lead sulfate penetrates is dependent on the rate of discharge, as well as on the density and surface area of the plate. Paste density is the key factor in providing the macropores which are necessary for the transport of solution and ionic species to and from the reaction sites within the interior of the plate, while surface area provides sites for the current-generating electrochemical reaction. For the same paste density and surface area, the extent to which lead sulfate can penetrate is determined by the discharge rate. 

Except for the electrochemical reaction (2.6), all other reactions depend on the pH of the solution. A number of electrochemical reactions proceed in this system, which form different electrode systems, depending on lead ion valency, solution composition and pH, and electrode potential. These reactions cover a potential range of 2.0 V. Table 2.4 summarises the electrochemical reactions involving Pb, lead oxides, PbS04 and basic lead sulfates, and the equilibrium potentials of the respective electrode systems. The reactions and the equilibrium potentials for the hydrogen and oxygen electrodes are also given in the table. Several chemical reactions in which basic lead sulfates take part are also included in Table 2.4. 

The driving force for an electrochemical reaction to proceed is polarization of the electrode, i.e. the potential difference between the equilibrium potential of the reaction and the electrode potential. The rate of the electrochemical reaction depends on the hindrances that have to be overcome by the reacting particles for the reaction to proceed. The hydrogen reaction on lead proceeds with great hindrances, i.e. at high overpotential. Hence, the competing reaction of lead sulfate reduction to lead proceeds with high coulombic efficiency and kinetic stability. This, in turn, ensures stable performance of the lead—acid battery. 

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