The physical and chemical specialty of lead powder

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The color of lead powder

Lead powder is a kind of two-component powder material which contains lead oxide in its surface layer and lead in its inside part. Its colors are determined by the content of lead oxide, particle structure and size dimension, which range from grey black, grey green to yellow. The lead powder with high oxidation degree is close to yellow; while the lead powder with low oxidation degree is close to black and it is usually appears to be grey green or yellow green. As the oxidation degree of the lead powder made by different lead powder machines will not be totally same, their colors will also be a slightly different. For example, the lead powder made by Shimadzu lead powder machine often appears to be oyster while the Barton Lead powders mostly appears to be dark yellow.

The bulk density of lead powder

Bulk density is also called as apparent density which is the density of lead powder natural accumulation. The measured bulk density of lead powder is a comprehensive measurement of its particle size and oxidation degree. Under normal conditions, the bulk density will be smaller due to the smaller particle and higher oxidation degree. Besides, the bulk density is also related with the preparation method of lead powder.

The size distribution of lead powder

The size distribution of lead powder refers to the proportion of the lead powders in all kinds of different particle size in the total lead powder and its relationship between the distribution state and oxidation.

The technology which measures the particle size distribution includes: sedimentation analysis and nephelometry which can be used in various viscosity medium. However, the most widely used method is screen analysis. This method uses a full set of wire mesh sifter that has different standards of mesh number, which begin with its finest evelets and arrange orderly one above the other. The full set of sifter is fixed on the hertz oscillator, in which the powder/acetone suspended solids are poured into the top of the sifter and then begin the vibration. After proper time of vibration, remove all of the sifters and dry the powders in each sifter and weigh them so as to measure the percentage rate of each sifter powder.

In the production practice, we often use the above simple sieve analysis method to qualitative control the lead powder particle size. Usually, the lead powder produced by the lead powder machine is often judged by exceeding the proportional numbers of 300 meshes. In general, the powder amounts which exceed 300 meshes (the grain size is 43μm) should be more than 55 percents, the powder amounts which not exceed 100 meshes should be less than 7 percents and the large particle powder amounts which not exceed 42 meshes should be less than 3 percents. If more homogeneous particle size is required, the lead powder should be further grinding in the hammer mill.

The oxidation degree of lead powder

The weight percentage rate of lead oxide in lead powder is called oxidation degree.

The electrolyte uptake of lead powder

Electrolyte uptake refers to the water milliliter number absorbed by each kilogram of lead powder or the water grams absorbed by every 100 grams of lead powder. The electrolyte uptake is related with the porosity or aperture ratio of the lead powder, which reflects the water absorbing capacity and acid absorption ability of lead powder in the process of paste production.

Electrolyte uptake is often controlled in the range of 95-130ml/kg. As the density of water is 1, the electrolyte uptake can be expressed as 9.5 percents to 13 percents which means the water absorption is 9.5-13g in every 100 grams of lead powder.

The acid absorption value of lead powder

The reaction extent between lead powder and sulfuric acid is called acid absorption value.  Adding a certain amount of lead powder into the dilute sulfuric acid with the density of 1:100, placing it in the thermostatic bath at 25 degrees Celsius and fully vibrate it. Filtering it with the filter paper and take out part of the filtrate in which titrate it with 1mol/dm3 NaOH. Calculating the sulfuric acid grams absorbed by 1g of lead powder according to the differential concentration of NaOH before and after the reaction and uses this value to measure the acid absorption degree. Usually the acid absorption of lead powder is stipulated to be 0.1-0.3g. Generally speaking, the bigger acid absorption value is, the higher oxidation degree of the lead powder will be; on the contrary, the oxidation degree of lead powder will be lower due to the smaller acid absorption value.

The morphological structure of lead powder

The particle state of lead powder can be generally divided into two categories which are petaliform and cone structure. It is considered that the morphological structure of lead powder is related to the lead powder machine model. The lead powder produced by screen lead powder machine has petaliform shape, coarse particle and thin oxide layer; while the lead powder produced by winnowing lead powder machine has cone shape, fine particle and thick oxide layer. The relationship between the morphological structure of lead powder and electrical property has not been agreed yet. Most people think that the lead powder will not have obvious influence on the battery performance, therefore, the result needed to be further studied.

The experience of production practice has proved that the lead oxide features will have much larger influence on the work performance of battery positive pole than on the negative plate. For example, the electrical characteristic of battery positive plate obviously relies on the phase composition and particle size distribution of lead oxide.