Ian Klein of Penox explains how the addition of a new additive can speed up this time consuming process
Until recently the curing and drying process in lead-acid battery making has lagged behind all of the advances being made in the process. Now an innovative and patented method which allows curing and drying plates in approximately four hours is available. For the first time, it enables control of the size of crystals as well as their porosity and prevents the shrinking of active material by up to 10%, a problem related to conventional curing and drying chambers.
This article will present the fundamentals as well as the instrumental implementation of this process, which has already been adopted for industrial production by lead acid battery manufacturers.
In the conventional process, the processed and stacked plates are cured and dried in chambers for a period of one up to several days. During this process 3- or/and 4-basic lead sulphates are cured, the free lead is reduced and finally the plates are dried. Controlling the size of crystals and porosity, which would allow adjusting the performance of the electrodes, is not possible with the current processes.
The curing process forms the highly porous structure of active material out of 3- or 4-basic lead sulphates. During formation, the basic lead sulphates will be transformed into lead dioxide at the positive electrode and into lead at the negative electrodes. The performance of the electrodes is basically determined by the characteristics of the lead sulphate structures. During discharging and recharging, the electrodes are subjected to a chemical transformation
Negative electrode: Pb ⇔ PbSO4
Positive electrode: PbO2 ⇔ PbSO4
which concerns, beside the volume ratio, most notably the conductivity. This results in a major change to the battery plate.
Pb and PbO4 are conductors while PbSO4 is an isolator. To allow recharge after deep discharges, the framework has to retain a continuously conducting core in this case. Retaining the conducting core requires controlling of the size of the lead sulphate crystals.
Sulphuric acid, required for the transformation as an electrolyte, must be available in a sufficient quantity, which demands an adequate pore diameter. Controlling of the pore diameter is desirable.
3-basic lead sulphates
3-basic lead sulphates are formed spontaneously at temperatures below approximately 60°C. The growth rate at this temperature is very low. The spontaneous curing of 3-basic lead sulphate takes 16 to 24 hours. The 3-basic crystals present sizes of < 3 µm. The diameter of the pores is approx. 0.4 µm.
4-basic lead sulphates
4-basic lead sulphates are formed spontaneously at temperatures above 70°C, especially above 80°C. As a result, the growth rate is much higher. The spontaneous curing of the 4-basic lead sulphates above 80°C (normal curing in a steam chamber) accounts for 2 to 4 hours. The size of the crystals ranges from 40 to 80 µm. In general, the formation of 4-basic lead sulphates requires much more time and energy.
By applying seeding crystals, red lead (Pb3O4) or 4-basic lead sulphates, the size of the crystals can be reduced to approximately 20 µm. Despite many attempts, the interesting range of crystal sizes between 5 and 15 µm has not been available until now.
TBLS+: controlling crystal size
The engineering consultant Dr Nitsche (http://www.iee-dr-nitsche.de) and the Penox GmbH (http://www.penoxgroup.com) have developed an additive based on 4-basic seeding crystals in the scale of nanotechnology. This additive, marketed under the brand name TBLS+, allows fitting the crystal size to customers’ demands. The amount of TBLS+ added to the paste mixture determines crystal sizes. Active material produced applying TBLS+ exhibits a homogenous structure (Figure 1 to Figure 3), which can not be achieved by spontaneous curing (Figure 4).
3-basic lead sulphate
The lead oxide used for the production of battery plates still contains free lead (25-30%), which is oxidised by atmospheric oxygen during the curing process. The curing of 3-basic lead sulphate takes approximately 16-24 hours. During this time the reduction of free lead occurs with spontaneous exothermic reactions. The generated heat boosts the evaporation of water so that the plates tend to dry too quickly.
The drying of plates depends on a large degree of the reactivity of the lead dust used. Reactive lead oxide is produced by highly efficient lead mills. To avoid the following disadvantages, the oxidation of free lead should occur after the completion of the curing process and not simultaneously to it.
The growth of crystals in stable form should be not interrupted too early. If this process is aborted too early, the 3-basic crystals will only reach sizes of < 1 µm. The result would be a weak cycle performance of the batteries.
In a stack of plates, overheating and the resultant rapid drying lead to the occurrence of grey circles i.e. areas with a very high concentration of free lead. These areas are difficult to form and show a significantly lower porosity.
The active material/grid connection is unsatisfactory in the case of rapid drying of active material. The result is an intensified loss of capacity in battery operation.
A long drying process and insufficient cross-linking of crystals lead to a shrinking of active material.
4-basic lead sulphate
In general, the curing to 4-basic lead sulphate occurs in an environment of steam. As a result, crystal growth can be completed within 2–4 hours (when using TBLS+ within 1 hour). Free lead is preferentially reduced at a temperature of 50 to 60°C and residual moisture of 5 to 7%. However, when using conventional curing and drying chambers, the plates often dry out before the free lead has been reduced. The results are grey circles in the middle of the plates, i.e. areas with a high concentration of free lead, which are difficult to form. In addition, the shrinking of active material occurs together with the appearance of cracks, which leads to a loss of porosity of up to 10%.
A new generation of curing and drying chambers based on the Concure process
Together with the engineering consultant Dr Werner Nitsche, Bernd Münstermann GmbH & Co KG (http://www.muenstermann.com) has designed and developed a special curing chamber which prevents premature drying. The name Concure stands for a new process of curing and drying positive and negative plates in the production of lead accumulators.
Concure has proved to be an important improvement: the design of the installation and the process flow avoid spontaneous and non-controllable process steps in the production of positive and negative plates.
Innovative production facilities
Generally, curing based on the Concure process can either take place in curing chambers or in a continuous flow unit. Both concepts offer advantages but also include restrictions. When curing runs the batch process, in which the plates are first stacked on pallets and then cured in a special chamber, a high degree of freedom exists in regard to curing and the respective recipe. However, preparing the pallets and loading the chambers is time-consuming. On the other hand, a continuous flow unit shortens the loading time of the installation, but provides less flexibility in regard to curing plates. For this reason, continuous installations are usually used if a single product is manufactured in large quantities.
Münstermann combines the advantages of both processes by using appropriate automation. Figure 5 shows the 3-D layout of a complete Concure manufacturing facility. Having passed the stacker, the pasted plates are placed automatically on special pallets in stacks of about 140 pieces (see Figure 6). Each of these pallets is loaded with 32 stacks. Batches of 6 pallets are placed in a stacking magazine.
Now the pallets can be loaded into and out of the curing and drying chamber by a fork-lift truck. A virtually continuous manufacturing process can be achieved by automating this step.
A special unit was designed and developed for an automatic loading and unloading operation. It places the loaded stack magazine automatically inside the chambers and unloads it after the curing and drying process. A second robot handles unloading the pallets. The empty pallets are returned via a transport system behind the chambers and are then ready to be loaded again.
The actual curing and drying process
The formation of 3- or 4-basic lead sulphate crystals occurs in an atmosphere of water vapour with an almost complete exclusion of oxygen. In this way, undesirable dehydration of the plates is prevented. An inflatable gasket between the pallet and the opening acts as a seal. During the steaming process, several chemical processes occur, accounting for the actual quality of the plates. Whether 3- or 4-basic lead sulphates are cured for positive or negative plates, this is controlled by additives. Applying TBLS+ offers the unique possibility to control the crystal size of 4-basic lead sulphates by adjusting the amount of TBLS+ added. As a result, pore diameters can be kept deliberately in a range of approximately 0.4 to 4 µm.
Free lead reduction occurs in a second step. Over a period of 1.5 hours air with a temperature of 50 to 60°C is passed through the plates. Regulating air flow and distribution allows keeping moisture of the plates at 5 to 6%. This value is ideal for an adequate reduction of free lead down to 2% (for a lead content of < 1% the processing time has to be increased to 2 hours).
The last step is the final drying of the plates. Hot air is pressed through the upright standing and closely packed plates. After a drying time of 30 minutes the plates have a residual moisture content of < 0.5%.
Advantages of the Concure process
The Concure process offers several new advantages regarding the quality of the produced lead acid batteries as well as the production process.
Advantages of the 4-basic lead sulphates
The advantages of the 4-basic lead sulphates concerning an improved shelf life have been described in many publications. Well-known battery manufacturers, mainly US-American OEM suppliers, have now been counting on them for a long time. However, 4-basic lead sulphates could not convince European and Asian producers due to economic reasons.
The crystals resulting from a spontaneous curing process have a size of up to 50 µm without the addition of red lead and approximately 20 µm with the addition of red lead.
The increased amount of active material necessary, difficult formation and the higher costs of red lead have until now made the production of lead acid batteries including 4-basic active material too expensive. The combination of TBLS+ and the Concure technology allows controlling crystal size and porosity and prevents the shrinking of active material, which can not be avoided with the conventional curing process. Instead of an increased use of active material, a saving of 5 to 8 % can be achieved without shortening shelf life. Whereas lead acid batteries made with 3-basic cured electrodes exhibit a loss of capacity after several discharges, 4-basic cured electrodes allow the capacity to remain on a high level for a significantly longer period of time.
Advantages concerning the absence of the shrinking of active material
The higher porosity which can be achieved by using TBLS+ in combination with the Concure technology allows saving active material. Plates from the Concure process show visually no cracks as they can be frequently seen at conventionally cured plates.
Without TBLS+ 4-basic lead sulphate crystals have already reached a size of > 20 µm after 1 hour of steaming in the Concure process. This rough crystal structure has an average pore diameter of 2.18 µm. The overall porosity of approximately 55% is close to that of plates containing TBLS+ and being cured and dried in conventional curing chambers. However, the size of the 4-basic crystals can be limited to 8 µm by using TBLS+. The mercury porosimetry shows an average pore diameter of 1.5 µm. However, a porosity of 62% is achieved when using the Concure chamber. At a crystal size of 5 µm and an average pore diameter of 1.3 µm, the increased quantity of mercury absorbed is significant. The porosity of 62% for Concure chambers, determined using mercury porosimetry, is 10% higher than the porosity of 55% achieved with conventional curing chambers. Comparing plates with added TBLS+ cured on the one hand in a conventional chamber and on the other hand in a Concure chamber shows that, at almost the same crystal size and pore diameter, the absence of active material shrinking has a very positive effect to the porosity.
The shrinking of active material is prevented by the rapid extraction of water in the final process step, which follows directly the degradation of free lead. Plates without TBLS+ do not exhibit active material shrinking in the Concure process, either.
The porosity can be improved in this way, but due to the rough crystal structure it cannot reach the level of plates with added TBLS+.
The variations in quality connected with the conventional curing process can be prevented with the Concure process.
The Concure technology enables starting the curing process within 2 hours after pasting in the batch option and within a few minutes in the continuous flow option. A dehydration of the plates due to uncontrollable oxidation of free lead can be avoided.
Constant conditions of temperature, moisture and air circulation inside the Concure unit produce plates with equal quality. The little variations in quality allow reducing the active material used, now assuring compliance with the 6-sigma standards.
New possibilities in the production
The Concure technology offers for the first time the chance to complete the whole curing and drying process within 4 hours. This fact constitutes the base of completely new options in production planning. Pre-assembly and assembly can be directly connected. Modern assembly lines with short re-tooling times can be synchronised with the upstream pasting lines. Furthermore, the described form of automating the curing process offers a high degree of flexibility. Intermediate storage facilities, including conventional curing and drying chambers without automation not longer exist.
The additional flexibility allows reacting to the demands of customers within 24 hours. If the Concure process is combined with an innovative formation process of a German manufacturer including acid circulation, a total production time for SLI batteries of 12 hours for smaller batteries and a maximum of 24 hours for heavy-truck batteries can be achieved. As a result, stocks in finished goods warehouses can be considerably reduced.
The revolutionary reduction of the total lead time for manufacturing batteries has a positive effect on a company’s earnings, while the product quality is improved. The Concure process now for over 3 years has proved its value in the industrial production of battery manufacturers, including OEM suppliers.