Preparation of leady oxide for lead–acid battery by cementation reaction
Introduction
Many ingenious ways to make lead oxide have been explored and evaluated, especially with respect to leady oxide for lead–acid battery manufacture, and, as a result, nearly 50 different oxide-manufacturing procedures have been evaluated during the past 95 years. Over the past two decades or so, three basic ways of making battery leady oxide have been established as a proven commercial success, viz., (i) Shimadzu ball mill, (ii) Hardinge cone ball mill, (iii) Barton-pot. Today, nearly all battery leady oxide is made in either a ball mill or a Barton-pot [1], [2], [3], [4]. Other processes for the production of lead dust, for example, spraying molten lead under gas pressure or ultrasonics, are not in use at this time. The purely chemical processes, which include the reduction of plumbous salts with zinc, the decomposition of lead carbonate, preparations based on lead amalgam or lead–sodium alloy, as well as lead sulfate and lead dioxide, and the precipitated lead oxyhydrates or hydroxides, have not yet become acceptable for battery use. Lead dust produced by processes that require additional chemicals is less economical than the ball-mill or Barton-pot processes [5].
In the Barton-pot approach to making battery oxide, lead is melted, forced into a spray of droplets, and then oxidized by air at a regulated temperature(330°C to 400°C). Any accumulated bulk molten lead is broken up again into droplets by a revolving paddle that directs the lead against a fixed baffle arrangement attached to the side of the pot. By careful control of the pot temperature, paddle rotation speed, rate of air flow and size of operating load, battery oxide of the desired polymorph and particle-size distribution can be obtained. Some manufacturers, however, pass the Baron-pot oxide through an additional milling stage, which aims to produce finer particles with greater reactivity. The ball-mill process, which is the alternative means for preparing battery oxide, involves tumbling lead balls, cylinders, billets or entire ingots in a rotating steel drum through which a stream of air is passed. The heat generated by friction between the lead pieces is sufficient to start oxide formation. This reaction generates more heat and thus allows the lead particles that are rubbed off by the abrasion to be converted to leady oxide of the required composition. As with the Barton-pot method, the relative amounts of the oxide constituents can be controlled by manipulation of the operational parameters which govern the oxide-making process, namely: mill temperature, mill speed, flow rate and temperature of the air stream, amount of mill charge [1], [2], [6].
The ball-mill drum is usually cooled either with water or by forced air flow and, as a result, the oxidation takes places at a much lower temperature than in a Barton-pot. This difference in operation causes corresponding differences in the relative amounts of free-lead, α-PbO and β-PbO that are produced by the two processes. In general, the conversion of lead to oxide is lower for the ball-mill process, i.e., the material has a higher free-lead content than the Barton-pot variety. The transition temperature at which α-PbO converts to β-PbO is around 480°C; therefore, the latter will be formed in appreciable amounts only when using the Baron-pot method [6]. Moreover, Barton-pot and ball-mill oxides also exhibit dissimilarities in particle size and morphology. The free-lead particles are elongated (or flaky) in ball-mill oxide, but roughly spherical in Barton-pot material. The greater surface area of the free-lead particles in ball-mill material increases the rate of secondary oxidation during storage. Ball-mill oxide provides plates with greater mechanical strength and higher initial electrical capacity. The Barton-pot process produces the larger oxide particles. As a consequence, Barton-pot oxide is less prone to aggregation and, therefore, is easier to handle.
Barton-pot oxide particles are usually round or spherical in shape, and are all less than 60 μm in size with a mean diameter of 3 to 4 μm. By contrast, ball-mill oxide particles are flat and non-uniform in shape, and are smaller in size [7]. Thus, the surface area and acid absorption of ball-mill oxide are much higher than those of Baron-pot oxide. Therefore, the active material utilization of ball-mill oxide is greater than that of Barton-pot oxide. On the other hand, the active-material utilization of Barton-pot and ball-mill oxides is relativelylow (around 35–40%) composed with that of the active material in other batteries, e.g., nickel/metal–hydride and lithium-ion. This is due to the regulated particle morphology and limited surface area of the leady oxides. Lam et al. [8] have reported that active-material utilization can be increased to 50% by an improvement in electrical conductivity caused by the addition of a proprietary particulate that consists of glass-flakes coated with a thin (<5 μm) layer of tin dioxide.
The objective of this study is to prepare a leady oxide that has much superior physical and chemical characteristics. This has been achieved through the oxidation and ball milling of sponge lead that has a dendrite phase prepared by a purely chemical cementation reaction in acidic solution.
Section snippets
Preparation of leady oxide
A 1.0-wt.% HCl solution containing Pb+2 and Cl− was heated to 90(±5)°C, and then sponge lead was prepared via a cementation reaction by using a pure aluminum or a magnesium rod as the reductant. The sponge lead was cleaned with warm distilled water. The cleaned sponge lead was placed in a crucible (diameter 60 mm, height 55 mm), and then subsequently oxidized in air in an electric furnace at 300°C or 400°C. Finally, the oxidized sponge lead was converted in leady oxide by ball milling for 1 h.
Characterization of leady oxide
Morphology of sponge lead
The sponge lead was prepared for morphology observation by washing with distilled water, cleaning with ethyl alcohol, and then drying in a vacuum desiccator at 75°C for 24 h. The morphology of leady oxide prepared in a 1.0 wt.% HCl solution is presented in Fig. 1. The macroscopic morphology observed at a low-magnification of 100× is shown in Fig. 1(a). The obtained sponge lead consists of a dendritic phase of leaf shape and growth with a preferred orientation. The microscopic morphology for the
Conclusions
In this study, the physical and chemical characteristics of leady oxide powders prepared by a cementation reaction in HCl aqueous solution using pure aluminum or magnesium as a reducing agent have been investigated. The results show that the powders have better physical and chemical characteristics than conventional Barton-pot or ball-mill leady oxide. The powders have a flat-plate shape and a dendritic structure. The specific surface area and acid absorption capability, which is critical for
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