The heart of a lead-acid battery is its active material. If this is not manufactured properly and accurately, the battery performance, or life, will be irretrievably compromised. As with all the other chemical processes in the manufacturing sequence, once completed, it is practically irreversible. The leady oxide resulting from the oxidation process is the base for both the positive and negative active materials.
Delivering the ideal oxide manufacturing process to ensure the best battery performance and life, is not to be taken lightly. Apart from ensuring consistent process and quality control methods, the final battery characteristics are also influenced by the method of converting lead to the sub-oxide form, or leady oxide as it is called in the industry. The intended reaction is simple enough: lead is oxidised in air to form lead monoxide:
2Pb + O2 = 2PbO
In reality, not all of the lead is converted, and the resultant reaction is best described as a partial conversion, leaving up to a third of the lead unconverted, eg:
3Pb + O2 = 2PbO + Pb
Whilst the reaction seems straightforward enough, our understanding of the process is somewhat limited. There is still a lot more grey in our knowledge than there is of black and white. This analogy goes beyond referring to the colour of the lead sub oxide. It applies to our overall understanding of how the materials behave both during the manufacturing processes, and to the performance of the final product.
The basic problem is there are so many variables within each sub process of the total manufacturing operation, the consequences of small variations in the oxide production conditions are difficult to predict.
The two principal methods most widely adopted by LAB manufacturers are the Shimadzu (ball mill) and the Barton pot processes. Whilst the chemical reaction is the same for both, each of these uses a very different principle to convert lead to leady oxide. This difference imparts a very noticeable divergence to the physical and chemical properties of the product from each process. In turn, this divergence can determine the battery performance characteristics, and therefore its suitability for particular market applications. For this reason, the oxide is sometimes considered as the carrier of a battery’s characteristics or ‘DNA’.
To understand the differences in the properties of the leady oxide produced by both methods, there will be a brief comparison of the two processes in a later edition of the magazine, after separate detailed articles on both have been published.
How ball mills work
This article describes the ball mill process. As outlined earlier, the aim is to make a relatively inert material, lead, into an electrochemical powerhouse of electron storage and delivery.
Lead oxide paste for the manufacture of the active material had replaced the surface charging of lead sheet by the end of the 19th century. This is clearly not easy and requires some energy input to enable the oxidation process to create lead monoxide.
At the end of the 19th century, Genzo Shimadzu manufactured lead-acid batteries in Japan in response to the increasing proliferation of electrical machinery and equipment requirements. This was the beginning of the familiar GS battery brand and by 1917, the Japan Storage Battery Company had been launched. By this time Shimadzu had developed the modern paste type of active material.
This newly automated process required large amounts of powdered lead oxide, which at the time was in short supply, of variable quality, and expensive. It was manufactured by various molten lead processes and consisted of finely divided litharge, red lead, and other forms of the higher oxides. The necessity of purchasing it from outside sources added to the manufacturing costs.
Deciding that obtaining the technology and know-how was not cost-effective, and in any event a better method was required, Shimadzu determined to develop an in-house method for oxide production. After a long period of producing inadequate, coarse lead-oxide powder by experimenting with a ball mill borrowed from the Ceramic Institute of Tokyo, he finally hit on the idea of passing air through the ball mill. This gave a finer more reactive powder and, as a bonus, was collected outside of the mill, which was even better than the commercially available processes at that time. The throughput was far greater than the other available production methods, which Shimadzu (incorrectly) attributed to the increased friction created by removing the lubricating oxide from the system.
Not being a chemist, he did not realise the importance of having a higher partial pressure of oxygen due to the constant airflow through the reaction zone of the mill. In any event it worked, and by 1920 he had established the modern ball mill or Shimadzu process. The manufacturing method was so revolutionary, however, that it took three years to be patented because it was considered inconsistent with common knowledge of the period.
The ball mill principle uses both heat and mechanical energy to produce the now-familiar “leady oxide” universally used by LAB manufacturers around the globe (Fig 2). The uniqueness of the production method is the determining factor in establishing the physical and chemical properties of the finished product.
In simple terms it takes small, centimetre sized chunks of lead, grinds them by rotating in a drum, while passing heated or cooled air through the reacting zone. The surface of the nuggets is partially oxidised to the mono-valent structure, which is brittle. This brittle layer is removed by attrition created by the moving lead nuggets in the rotating ball mill cylinder, to produce a flaky powder of leady oxide.
The grinding process continues until the flakes are small enough to be carried by the air stream into a filter. The filtered oxide is then transported to oxide storage silos. Because the reaction is exothermic, it is necessary to regulate the temperature of the reacting zone, in order to control the chemical and physical properties of the oxide.
Fig 3 illustrates the physical and chemical processes occurring within the ball mill drum during the production of leady oxide. These are several:
1. The chemical oxidation of lead in air, to form the monobasic oxide, PbO
2. The grinding action of the lead chunks to break off the brittle outer layer of lead monoxide
3. The grinding action of the lead chunks that reduces the particle size of the oxide
4. The air draught that carries the finer particles out of the mill and provides a higher partial pressure of oxygen to accelerate the lead monoxide reaction
To understand what is happening, in what is essentially a continuous reaction zone, it will simplify matters to take each of the above processes in turn, even though they are simultaneous and interdependent.
1. Chemical oxidation
The first stage is to look at the thermodynamics, to assess the activation energy and the stability of the lead monoxide product. PbO formation has the lowest energy of formation and is therefore a highly stable compound that can be formed with a small energy input, (Table 1). In other words, ideal for a low temperature production process like the ball mill attrition method.
It also indicates, by the negative value of the formation energy, that the reaction is exothermic and will in fact generate heat. This means that any artificial heating at the process start-up will be replaced at some stage by cooling of the drum or atmosphere to prevent a higher, more equipment damaging temperature being generated in the reaction zone.
To see how this pans out for oxide manufacture, we can look at Fig 5, which is a schematic of the equipment and the principle of the operation. This consists of a feed-in chute for the lead nuggets or pieces, a rotating steel drum that will often contain iron balls to grind the lead feed. An extraction fan to pull the air through a filter for collection in a storage silo is shown in a full equipment diagram in Fig 3.
The basic principle is to turn lead into lead monoxide by heating in air is as follows:
Overall reaction:
2Pb + O2 → 2PbO
According to Detchko Pavlov there are two principal sub-reactions:
At the air/PbO interface:
PbO2+ + 2e– + O2 → PbO + O
At the Pb/PbO interface:
Pb → PbO2+ + 2e–
A more graphic representation of the chemistry is given in Fig 5. This shows the oxidation process dependency on a reaction zone above the main particulate mass in the silo. It is within this mass of particulate oxide and lead pieces that the larger and heavier oxide particles are formed. These larger oxide pieces are the first stage of the reaction’s progress to the final leady oxide product. This brings us to the first stage of the ball mill process, which is the grinding or comminution segment.
2. Grinding of lead nuggets to produce lead monoxide flakes
The addition of lead nuggets to the rotating ball mill creates friction through attrition. This generates heat and initiates the oxidation reactions shown above. As mentioned, the oxide is formed as a brittle layer around the surface of the individual lead nugget. This layer cracks and breaks away as the nugget is tumbled with the rest of the load inside the rotating drum. It is gradually ground into smaller and lighter particles that rise to the top of the moving mass as it travels along the length of the drum towards the exit end.
The driving force for this movement is predominantly the gradient formed by the difference in height between the nugget pile at the entrance of the drum and the remaining nugget mass at the exit (Fig 5). The further along this path the oxide has progressed, the smaller the average particle size due to the longer period of comminution resulting from reaching that position.
In the first stages, when the outside layer is formed, this brittle, oxidised covering is not completely converted to the lead monoxide. There is a gradation of oxygen content from the outside inwards, the higher content contained within the immediate surface mass, reducing to zero at some point, within the lead nugget.
When the layer cracks and breaks up, it produces flakes that contain a variable oxygen concentration that can be written as PbyOx. Where y increases and x decreases the further we move from the surface and into the lead nugget. When the particle breaks off it takes a big piece of unreacted (free) lead with it, from the main body of the lead nugget. This freshly exposed surface starts to oxidise immediately as it is transported down the physical material gradient in the drum towards the exit end.
3. Reduction in leady oxide particle size during its residence time in the rotating ball mill
During its progression, the particle is simultaneously oxidised further and reduced in size by the continuing abrasion of the process. Fig 6 shows a representation of a single particle from the initial flake to the final particle before it is carried out in the airstream. This diagram shows that the amount of free lead decreases as the surface is oxidised, and oxygen diffuses towards the centre of the particle.
During this time the particle is breaking up and the average particle size is decreasing. As it is carried around the inside surface of the rotating drum, it falls off the drum’s internal surface and into the reaction zone. Eventually the average particle size is small enough to be carried in the air stream to exit from the drum into the filter housing for collection.
This residence time, level of attrition, and heat content are the parameters that determine the final chemical makeup, along with the particle size and shape. Points to note are:
• The aspect ratio, i.e. the length and width to thickness ratio
• The distribution of the PbO content in the particle
• The unreacted, or free lead content
• The structural form of the PbO molecule
Because the reaction is exothermic, the heat generated within the leady oxide mass, in contact with the rotating drum, will transfer some of the heat to the drum walls. This raises the temperature, not only of the drum, but also of the connected machinery. For this reason, the drum is usually water-cooled by either an external or internal water spray.
In the case of the internal spray this must be carefully controlled as the H2O will increase the oxidation rate by catalysis. This control is important for two reasons:
1. It determines the rate and extent of the lead oxidation in the reaction zone
2. It is instrumental in producing the structure of the lead monoxide that is formed in the process.
Fortunately, the beta form (massicot) is formed at temperatures above 483°C, and since the process is normally within the 110 to 135°C range, the product is invariably alpha phase, or litharge (Fig 7). However, the more regulated the temperature, and the fewer excursions outside of the set limits, the more uniform the oxide quality.
4. The air draught determines the oxygen partial pressure and the particle size of the finished product.
The air stream is very important. Varying it can increase or decrease the oxidation rate of the lead, and also affect the size of the leady oxide particles that are carried out of the mill, forming the final product. During the process, air is pulled through the mill by the pressure difference between the fan-extracted filter housing, and the lead nugget inlet end of the rotating drum.
Control of this factor is as critical as the other parameters in determining not only the oxidation rate, but also the particle’s physical characteristics. The airstream carries oxygen through the reaction zone and brings higher levels of oxygen to the reacting particles. The reaction is dependent on the partial pressure of oxygen according to the JH van’t Hoff relationship:
Δ G0 = –RTlnKp
Where:
Δ G0 = Gibbs free Energy
R = Gas constant
T = Absolute temperature
Kp = Equilibrium constant [Products]/[reactants]
Using the reaction
2Pb + O2 = 2PbO
Δ G0 = -RTln. [PbO]2/[Pb]2 x [O2]
At constant temperature RT is a constant. Chemical activities of a solid are always unity. The activity of a gas is directly proportional to its partial pressure in a system, i.e. [O2] = pO2. The equation then reduces to:
Δ G0 = -constant x ln. (1/pO2)
The activation energy is related to the partial pressure of oxygen and is a key element in determining the rate of the lead monoxide reaction. The airflow is a crucial factor that made the Shimadzu process the most advanced in its era. The other role of the airflow is to create the pull force for lifting the particle and transporting it out of the drum and into the filter bagging area. From basic fluid mechanics, the airflow is critical in determining the size of the oxide particles that are removed from the mill as well as the rate of oxide production. It has been shown that for irregularly shaped particles (where Stokes’ law does not apply) that:
F = 0.5*C*ρ*C*A*V2
Where:
F = Drag force, N.
A = Reference area, m2.
C = Drag coefficient, unitless.
V = Velocity, m/s.
ρ = Density of fluid (liquid or gas), kg/m3.
It is fairly clear from this equation that the effect of the velocity of the moving fluid is a squared term (V2), which means that it will have a major effect on the drag force F. This in turn means that particles will be removed more quickly as the air velocity increases. The effect of this is that the throughput will increase, but the level of oxidation and the oxide surface area will both decrease due to the reduced residence time in the reaction zone.
From the above analysis, it is clear there are several factors that determine the chemical and physical characteristics, as well as the production speed of the leady oxide made from this process. These, along with one other operation— the rate of feeding lead nuggets into the drum— can be summarised as follows:
• Rate of lead material feed
• Airflow
• Process temperature
• Drum rotation speed
• Atmosphere composition
Rate of lead material feed
The lead material feed is a separate piece of equipment and is independent of the oxidation machinery. There are three principal methods of producing the small chips, or nuggets, to feed into the reacting ball mill. These are:
• Melting and casting into small cylinders (generally a few cubic centimetres in size)
• Extruding a strip then chopping it into similar size chunks
• Slicing the incoming ingots directly into slivers as material lead feed
The purpose is to increase the surface area of the lead feed— to speed up the process— as well as preventing the potential damage of having 25kg lead ingots crashing around inside a rotating drum. The size of the lead pieces and the rate at which they are fed into the reacting site is of critical importance.
Taking the 2Pb + O2 = 2PbO reaction occurring in the rotating drum, increasing the Pb reactant will effectively reduce the concentration (partial pressure) of oxygen in the reaction zone. As already demonstrated, the Gibbs free energy is dependent on the partial pressure of oxygen.
Δ G0 = -constant x ln. (1/pO2)
Any decrease in the O2 reactant will result in a more negative value for RTln.(1/PO2) (Naperian logarithmic values for a fraction). Since the free energy is calculated as the negative value of this, then ΔG0 effectively becomes more positive (minus x minus). The reaction has a lower driving force and consequently slows down. Increasing the cold lead feed also cools down the zone, again reducing reaction rates.
However, decreasing the lead feed rate to increase the relative oxygen concentration will slow down the throughput. A balance has to be struck, and most commercially available equipment operates at a leady oxide production rate of around 10 to 15 tonnes per day.
Airflow
The operating parameters for airflow are clearly linked to those described for the rate of lead feed. A superficial view of the chemistry would suggest that the higher the airflow, with a consequential increase in oxygen content, would be beneficial. From a chemistry angle this is entirely correct.
However, we also have to consider the fluid dynamics discussed above. The speed of airflow also determines the size of the leady oxide particles being transferred to the storage silos. This in turn, affects the reactivity of the product and its response to the subsequent manufacturing processes.
All chemical processes, from paste mixing to formation, and ultimately, to the battery’s performance characteristics are tangibly affected by the leady oxide’s production conditions. From the earlier discussion, we know that the relationship between the particles’ drag coefficient and the force exerted by the moving air will determine the physical characteristics of the leady oxide particles in this oxide sub-component.
How these physical and chemical attributes affect each manufacturing stage, and the consequences for battery performance, will be explained, process by process, in later articles in this series. However, at this stage we know that the standard particle size distribution, gives an acceptable range of values that lead-acid battery manufacturers have adapted to over the last century.
Process temperature
The temperature is a critical factor in controlling the type and rate of conversion of the oxide. Increasing the temperature speeds up the reaction rate. Again, referring to the van ’t Hoff equation, the driving force ΔG0 is related directly to temperature. The higher the temperature (in °K) the lower the energy of formation. Translated into reality it means the reaction speeds up as anticipated. There are practical limits as to how high the temperature can be raised.
As discussed earlier, the crystallography of the desired tetragonal α-PbO structure is thermodynamically stable up to 486°C. Beyond this, we see a conversion to β-PbO, the orthorhombic form, which is less beneficial for the battery’s cycle life. The crystallographic form of PbO is extremely important in determining the performance of the finished battery.
It is known that battery pastes made with tetragonal PbO provide a longer battery cycle life than those made with the orthorhombic form of PbO (Fig 7). Experience has shown that characteristics of the original oxide structure are retained and can influence the finished battery’s performance.
However, control of the operating temperature is as much to do with the preservation of the equipment as it is to do with the chemistry and crystallography of the product. Because the oxidation reaction is exothermic, and there is frictional heat from the grinding action of the lead mass in the drum, these factors combine to raise the temperature of the Pb to PbO2 reaction zone.
In order to maintain optimum conditions, the temperature is regulated to ensure a balance of quality and production rate. It is usually controlled by spraying the external surface of the rotating drum with water. There is another established method that uses an internal water spray directly into the mill atmosphere to cool down the reaction zone.
Drum rotation speed
As would be expected, an increase in the drum rotation speed will increase the extent of oxidation of the incoming lead material. This is due to greater attrition forces that increase the kinetic energy and friction of the rotating material, which generate more heat. It also decreases the particle size, leading to a finer powder with a higher specific surface area; this increases the acid and water absorption characteristics.
Humidity of the incoming air
H2O, in this case, acts as a catalyst to speed up the oxidation of Pb to PbO. Provided that the temperature is controlled, the effect on the nature and crystallography of the reaction products will be negligible. The particle size and chemistry will remain unaffected by the presence of moisture.
However, if the moisture content in the final stored product exceeds 5% there is a danger of further spontaneous oxidation of the leady oxide inside the storage silo. This can be a serious consequence, for example, a 40-tonne silo may have its entire contents rendered unusable and itself be seriously damaged if humidity levels are too high. For this reason, the humidity must be closely monitored to avoid initiating this post-ball mill process reaction.
Properties of the manufactured leady oxide and its effect on battery performance
Table 2 gives the physical and chemical attributes of a typical leady oxide produced by a lead ball mill. Since most of the subsequent processes and manufacturing equipment are based on these characteristics, it is important that they are maintained within the narrowest tolerance band possible.
Perhaps the most noticeable is that of the amount of free lead in the finished material. This is controlled to between 22 and 30% of the total mass. By any standards this is a wide tolerance to work with, particularly since subsequent processes from paste mixing to curing need to reduce this level to less than 4% in the dry, cured plate.
Variations in the free lead content do have processing consequences, particularly for the heat generated in paste mixing and curing. Higher temperatures favour formation of tetra-basic sulphates, whilst lower temperatures will produce more of the tri-basic form.
The tetra basic allotrope favours cycle life, whilst the tri-basic variant gives a better CCA output. This is important for manufacturers of automotive and deep-cycle batteries who need to closely control the performance of their products. The tighter the control over the free lead content of the ball mill oxide, the more consistent the battery performance.
Control of temperature is critical in providing the right level of lead oxidation for the leady oxide, which in turn affects the subsequent processes and, to some extent, the final battery performance characteristics.
This article has explained the chemistry and mechanics of the ball mill methods for manufacturing leady oxide. Leady oxide is the starting point for creating the active material of the lead-acid battery. The important process parameters have been described and the consequences if they stray out of specification.
The details of the equipment operating conditions and settings are mostly recommended by the machine manufacturers. The precise settings and tolerances will be accurately determined in the installation and proving trials of the equipment.
The next widely practised method of lead oxide production is the Barton Pot process. A description of this will be included in the summer edition of BEST as part of the lead-acid battery manufacturing series.
