PENOX GmbH. Figure 1. 1 st Edition TBLS+ Technology. by David Hardy & Ian Klein, PENOX GmbH. Page 1

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1 TBLS+ - Tetra Basic Lead Sulphate A Successffull and Maturiing Technollogy by David Hardy & Ian Klein, PENOX GmbH The formation of a homogenous tetra-basic sulphate crystal structure in the positive plates of lead-acid batteries is well known to offer significant electrochemical improvements in battery performance in respect of improved cycling and battery life. A number of methods for controlling the tetra-basic lead sulphate crystals are also well known in the literature. Simply heating the pasted plates during curing to more than 80 degc, for example during a short steam cure will generate a significant tetra-basic crystal structure although with a wide ranging crystal size distribution, sizes of more than 50 microns being common. Therefore see figure 1 showing big sized tetra basic lead sulphate crystals formed after a steam cure step using no seeding crystals or any other additives. Figure 1 Page 1

2 Another method, a so called in-situ method, uses the control of the temperature-time profile in the paste mixer to generate a tetra-basic lead sulphate structure. One disadvantage of this method is to control the crystal size, if not a special type of mixer is used. One significant issue in creating the tetra-basic structure is to control the crystal size distribution. Tetra-basic crystals, in comparison to tri-basic crystals, are much harder to create. As the lead sulphate crystals develop there may be a limited number of developing crystal seed sites due to a higher activation energy compared to tri-basic and hence it is common to produce a relatively wide distribution across a plate with a number of very large crystals. These larger tetra-basic crystals lead to problems in the later formation / charging of the plates as evidenced by an increased specific energy consumption and extended formation times when compared to tri-basic active materials. The idea of seeding the tetra-basic lead sulphate crystals as a means to control the growth of these crystals is a well known technology. The principle is to add a powdered tetra-basic lead sulphate crystal to the paste in the paste mixer. In this way a number of crystal seed sites are created. This leads to a large number of relatively even tetra-basic lead sulphate crystals. This technology is described in several patents. The company Penox has long been a producer of lead oxides and also tri- and tetra-basic lead sulphates for the plastic additives market. A development programme was started inhouse some 4 years ago to seed tetra-basic crystals in positive plates. This has lead to the development of a specific patented tetra-basic product named TBLS+. Penox is now aware of a number of battery companies that are applying the product TBLS+ or similar additives in their standard production process to control the crystal size. During the last years Penox has worked with many battery companies world-wide in applying this technology and this presentation describes some of the lessons learnt in applying this very powerful seeding technology going from a research and development product to a standard additive for many battery producers. Page 2

3 Early Developments Getting the Tetra-Basic Particle Size and Handling Right The first trials carried out used tetra-basic lead sulphate as a powder with approx. 4 to 5 micron particle size, showed some benefits with an improvement in the homogeneity of the crystal size distribution. The main means of generating the crystals has in general been the heating of the pasted plates to more than 80 degc in a steam cure process. It has been found that only a relatively short treatment of 1 to 2 hours is required to fully develop the crystal structure. However many plate samples showed evidence of larger crystals as analysed using Scanning Electron Micrographs and in an effort to improve the distribution and minimise these larger crystals with more than 50 micron length a more finely milled tetra-basic lead sulphate powder with a particle size of approx. 2 to 3 microns was used. Figure 2 is showing a tetra basic lead sulphate structure formed after a steam cure step using 1 % of TTBLS powder with a particle size of approx. 5 micron as a seeding crystal. It can be seen that there are again some bigger crystals in the active material. Figure 2 Page 3

4 Again there were some improvements in the crystal distribution across the plates but not as much as expected. The following figure 3 shows a tetra basic lead sulphate structure again formed after a steam cure step using 1 % of TTBLS powder with a particle size of 2 to 3 micron as a seeding crystal with a few bigger sized crystals only. Figure 3 In addition there were growing handling problems with such a fine dust, usually added directly into the paste mixer. The generation of fine lead based dusts can lead to problems with the blood lead levels of the workers and also it is possible that a significant fraction of the seeding material is lost to the paste mixer exhaust system. Building on Penox experience from the milling of organic powder pigments, a wet milled slurry of tetra basic lead sulphate in water was tested. The wet milling process allows a very fine controlled milling of the tetra basic crystals with particle sizes below one micron with none of the dusting/hygiene issues of a powder. It was noticed that there was a strong tendency of these finely milled particles to re-agglomerate after a time. Again, experience from pigment processing indicated that a special type of silicic acid could be used to prevent the re-agglomeration and this resulted in the Penox patent for TBLS+. Page 4

5 Graph 1 demonstrates the different particle size distribution curves of the above mentioned types of tetra basic lead sulphate. The particle size curves are measured by using a laser diffraction equipment % ,1 0,6 1, Std. TTBLS Finely milled TTBLS TBLS+ Micron Graph 1 TBLS+ - Early Customer Trials and Production Process Development Customer trials with TBLS+ very quickly showed benefits in the curing and formation in SLI plate production related to process time reductions and energy savings. The TBLS+ was added as approx. 1%, of the lead oxide content in the batch with a small correction for the water addition in the paster, because the TBLS+ is a water based slurry containing approx. 40% solids. The final crystal size of the plates can be controlled easily by the amount of TBLS+ added to the paste batch as shown in the following pages (figures 4, 5 and 6). Where figure 4 gives an idea about the tetra basic lead sulphate structure formed after a steam cure step using 0.5 % of TBLS+ performes very well for industrial batteries). as a seeding crystal (the concentration that Page 5

6 Figure 4 Figure 5 shows again a tetra basic lead sulphate structure formed after a steam cure step using 1.5 % of TBLS+ as seeding crystal. In most cases 1 to 1.5% TBLS+ are used for SLI batteries. Figure 5 Page 6

7 Finally figure 6 is showing a tetra basic lead sulphate structure formed after a steam cure step using 3 % of TBLS+ as seeding crystal. Figure 6 These trials showed for example that using a 1 hour steam cure for gravity casted grids a very homogeneous tetra-basic crystal structure could be achieved. Following the steam cure a short finishing cure was applied to reduce the free lead content before starting the final drying. The excellent tetra basic crystal structure with no crystals bigger than 15 microns and a good paste porosity, measured by mercury vapour penetration as shown in graph 2 for a Mercury porosimetry method result for the intrusion volume after 5 sec. Graph 2 Page 7

8 lead to an almost 40% formation time reduction when compared to plates with larger uncontrolled tetra-basic crystals. These early trials supported the concept of the continuous cure process effectively passing pasted plates through a temperature-humidity profiled tunnel in a continuous process. A patented technology developed by the German company Muenstermann that enables the battery producer to cure and dry their plates within 4 hours. Since Penox produces the tetra-basic lead sulphate used to produce TBLS+, several production developments were introduced. Commercial tetra-basic lead sulphates as produced in a wet batch process at high temperatures using litharge (PbO), water and sulphuric acid are never 100% tetra-basic and may drop down to 60 or 80% with the remainder being a mix of un-reacted PbO and tri-basic lead sulphate. During the wet milling production process for TBLS+ it was noticed that the tetra-basic content could decrease. This was found to be due to some un-reacted PbO inside the un-milled tetra-basic lead sulphate being exposed during the milling process. In addition, the importance of a good ph control was identified to ensure a long and stable shelf-life for TBLS+. TBLS+ - Large Scale Trials and Establishment as a Standard Additive Over the past 3 years more then 30 full-scale plant trials for starter and industrial batteries have been carried out across Europe, South America, United States and Asia. As a result of these many trials there have been a number of areas where improvements in the application of tetra-basic seeding have been made. Paste Mixer TBLS+ is best added after the oxide and water but before the acid addition. Adding the TBLS+ slurry directly onto a dry oxide can lead to an inhomogeneous distribution and therefore needs some additional mixing time to eliminate potential inhomogeneities. Adding the TBLS+ after the water and oxide have been mixed leads to a well dispersed mix within one or two minutes Page 8

9 One area to watch for is over-filling of the paste mixer and general poor paste mixing. It has been noted in some cases that efforts to improve the productivity have lead to increased batch sizes in the mixer leading to some mixing problems that finally can result in greater batch-to-batch variations of the paste plasticity and the paste density values. Also it has been noticed that very often in the paste recipe the acid-oxide ratio has been reduced to minimise the possibility of temperature spikes in the paster. These temperature spikes can lead to uncontrolled tetra-basic crystal formation. With TBLS+ this is no longer such a problem and it is common to increase acid-oxide ratios from 6 or 7 litres per 100kg lead oxide up to 8 or 10 litres per 100kg lead oxide giving higher porosity values for the paste and obviously reduces the work in the formation section. But these changes have to fit into the electrical characteristics of the battery produced. Curing and Drying First plant trials with TBLS+ are usually without any changes to the existing process. This establishes a base case or reference for further developments. A number of issues have been identified with many current curing processes. With large curing chambers, the pallets holding the plates can take a full day to load. During this period a general loading temperature-humidity profile, in most cases the same as for the curing operation, is used. However, due to the opening and closing of the doors and the movements of the pallets the actual conditions inside the chamber can vary significantly. The actual temperature-humidity profile experienced by the first pallets are very different to the last pallet loaded. Likewise it can be observed that the free lead in the paste can start an exothermic reaction and can sometimes dry out the plates especially if the more reactive mill oxides are used. During trials, Penox uses small data-loggers to check the actual temperature-humidity at several points across the chamber and usually within a stack of plates. In many cases it is noticed that the conditions in the chambers are very variable. This is often due to poor air circulation within the chamber. Penox has worked with a number of chambers and has developed a specific TBLS+ profile together with the Italian manufacturers CAM srl. Page 9

10 TBLS+ treated plates in general are better able to cope with the above mentioned problems in the curing chamber. However, it has been found that a short steam cure of 1 to 2 hours with more then 80 degc on the plates leads to a very well developed tetra-basic crystal structure and helps to improve the adherence of the paste on the grid, especially if the more corrosion resistant lead alloys are in use. Thereafter, the plates are less sensitive to the secondary curing that was implemented to speed up the free lead reduction. It has also found to be important to have a good air circulation during the final drying to ensure the reaction of the small parts of residual free lead. The following two graphs are showing what savings are possible for the cure/dry operation. Graph 3 compares the different ways of curing and drying of pasted plates. Graph Big Chamber - 3bas. Big Chamber - 4bas. Big Chamber (with CAM Concure Chamber - Process 4bas. Steaming Lead reduction ,5 Curing Drying ,5 The following graph 4 on the next page is giving a summary about possible savings in percent by changing the curing/drying process as mentioned before. Page 10

11 % 90,0 80,0 70,0 60,0 50,0 40,0 30,0 20,0 10,0 0,0 Savings in % Concure Process Big Chamber (with extra... Big Chamber - 4bas. Plates Big Chamber - 3bas. Plates CAM Chamber - 4bas. Plates Graph 4 Formation TBLS+ has been used in a number of different formation processes like air cooled; water bath; acid recirculation. In general TBLS+ has shown very positive effects due to the very much improved porosity of the plates. In general, TBLS+ provides much deeper pores allowing a much better contact between the acid and the active material. Thanks to the smaller sized tetra basic crystals the active material does not need so many resting periods during the formation to allow the active material to relax. This was more important for the large sized tetra basic crystals to prevent a reduction in the paste-grid adhesion. Due to the improved formation characteristics of the positive plates treated with TBLS+, it has been noticed that some focus needs to be applied to the negative plate. Especially during formation or charging, the negative plate can be fully charged well before the positive, leading to heat and gassing in the final stages of the formation process. It can be summarized that with the use of TBLS+ it is possible to achieve significant savings in time and energy as shown next page in graph 5 and 6 for a 60 Ah OEM battery. Page 11

12 st 1 Edition TBLS+ Technology Ford OEM Battery, 60Ah, LB3 60 Ah, LB Ah / Hours Time Resting Time Ah Std 4 bas. 52 7, TBLS+ 30, ,5 TBLS+ optimised 17, Graph 5 Savings for Battery type 60Ah, LB3 % 70,0 60,0 50,0 40,0 30,0 20,0 10,0 0,0 Time Energy optimised Std 4 bas. TBLS+ TBLS+ Graph 6 The best results are achieved using a formation process that was optimised for the small sized tetra basic plates, in the graph 5 and 6 named optimised TBLS+. TBLS+ - Generating Plate Mass Reductions of 4 to 5% A major outcome in the development of TBLS+ has been the need for a very close cooperation with the customer. Some benefits such as improved curing and formation can be achieved relatively quickly. However, by working through the battery production process starting at the paste production and ending at the formation process can lead to additional and very significant savings. This requires a very open and cooperative relationship, which Penox has been fortunate enough to enjoy with several customers. A number of processing issues in the battery production process have been identified above. Many of these issues can be resolved or significantly improved with some limited effort. Page 12

13 Once a good level of confidence has been established in the application of TBLS+ the next step is to look at reducing the quantity of the active mass on the positive plate. The significantly improved porosity of the plates means that much more of the active mass is taking part in the electrochemical process. The active mass is reduced by small changes in the paste recipe to reduce the paste density for example from 4.2 kg/litre to 4.0 kg/litre by changing the acid/oxide ratio as previously mentioned. The combined effect of a homogenous crystal structure with an uniform and high porosity with good penetration of the acid into the active material provides an excellent positive plate for the lead-acid battery. However, this work on improving the positive plate, traditionally one of the major causes of battery failure, has indicated that the limiting factor in battery life could now be the negative plate. Further work in this area has shown that adding TBLS+ to the negative plates can significantly improve the porosity and permit a reduction in the plate active mass. The following slides will show some results. At first graph 7 is coming up with the Mercury-porosity measurement results of tri- and tetra basic cured negative active material, where the intrusion volume of the tetra basic cured paste is 4 times more than for the tri basic cured material. Graph 7 Page 13

14 PENOX GmbH 1st Edition TBLS+ Technology The following figure 7 is showing the crystal structure of a tetra basic cured negative plate. Figure 7 Graph 8 shows the XRD patterns of negative active material with and without TBLS+. Graph 8 Presentation Ian Klein LABAT May 2008 Page 14

15 But it must be stated that for the formation of tetra basic lead sulphate crystals there is a need for some changes to the recipe of the negative active material especially for the expander mixture and it has to be applied steam for a period of 3 hours to get a 100% conversion into tetra basic crystals. Not only does the application of TBLS+ permit active mass reductions to be made in both plates but with the improved performance of the plates a re-evaluation of the basic battery design is possible. One Penox customer has been able to reconfigure the numbers of positive and negative plates per cell leading to not only a lighter battery but with savings in grids as well as paste. With the lead price at generally high levels this development is probably one of the most significant means to reduce the costs of battery production as can be seen on the following graph 9. Savings in % % Std. design Std. design with TBLS+ Re-design with TBLS+ Savings in k /Mio Batteries k Lead price 1,200 /ton Lead price 1,800 /ton Lead price 2,300 /ton 0 Std. design Std. design Re-design with with TBLS+ TBLS+ Graph 9 Page 15

16 Remarks: All SEM pictures shown in this paper have a magnitude of 1,000x. The XRD and Mercury Porosity Measurement were analysed by H.C. Starck GmbH, Goslar Germany. References: Bode Lead-Acid Batteries Schwinhorst, Nitsche Method for Producing, Maturing and Drying Negative and Positive Plates for Lead Accumulators (WO 2006/128621) Nitsche, Klein Additive for Producing a Positive Active Material for Lead-Acid Storage Batteries, a Method for its Production and a Method for its use (EP ) Nitsche, Lahme Curing of Positive Plates (EP ) Boden, Labovitz Battery Paste Additive and Method for Producing Battery Plates (WO 2005/094501) Meyer Paste Curing Additive (WO 2006/034466) Page 16