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A Basic Guide To Automotive Battery Paste Mixing
Successful battery paste mixing combines several process steps.
Mixing the paste – (example only as each battery manufacture will have their own proprietary formula)
In the paste mixing process, powdered lead oxide is mixed with sulfuric acid, water, and various additives to produce a paste.
Lead oxide is a metallic lead resulting from limited oxidation by air. The product consists of 63 to 80% litharge and about 18-36% free metallic lead in a fine powdered state. Oxide made by Barton Pot method will generally have about 18 to 28% free metallic lead. Oxide made by the Ball Mill process will typically have about 23 to 36% free metallic lead.
The oxide particle size impacts on battery performance; thus, the particle size will likely be a compromise. Battery plates made from larger particle size will have longer life but lower capacity than plates from oxide of smaller particle size. The particle size used to meet typical design needs has a 175 acid absorption.
The powered lead oxide is turned into paste by adding water and sulfuric acid in a paste mixer. It is difficult to provide a formula for all plate performance designs. The following are examples of frequently used paste formula:
| Positive Paste: Lead oxide 100kg Floc desired quantity* Water 8.8 liters Sulfuric acid 1400 density 9.6 liters *The addition of floc increases the paste to grid holding strength and can reduce paste shedding. Floc can be added dry or as a slurry mix. The water used in the slurry is considered as a portion of the total water addition. Frequently used floc is about .0625 inch (1.5mm) in length and 3.0 denier. |
Negative paste: 1) inorganic additive, mainly barium sulfate 2) an organic additive, such as lignin 3) lamp black (soot) |
When mixing the paste, first release the oxide into the mixer. Vibrate to dislodge all oxide from its storage container. Follow the oxide by adding the expander, floc, and then the water. Adding the water as rapidly as possible is the preferred approach. Mix these items for a minimum of 4 minutes to form an “initial paste”.
To the “initial paste”, slowly add the sulfuric acid and distribute it as equally and uniformly as possible throughout the paste. Using a multi-nozzle dispersion technique adds to uniformity.
The acid adding may take 12 to 15 minutes. The addition rate will vary with the cooling capacity of the paste mixer. After adding the total acid quantity, continue mixing for at least 4 minutes-or until the paste temperature reaches about 110F (44C).
The paste mixer’s shear action is important. High shear has been found to be more beneficial than low shear. Mixers having mullers and wheels have a high shear. Those with mullers and plows have lower shear. Vertical mullers in mixers have lower shear. Paddle-pony-mixers have the lowest shear.
The paste formula given here are suitable for the vast majority of automotive starter battery plates. However, they are provided only as a general formula. Alternate formula can be developed to achieve specific design objectives. For example, more concentrated sulfuric acid can be used, while adjusting the volume of water.
Caution: Because the mixing of paste creates heat, sufficient mixer cooling is important when varying the formula. 140F (60C) is considered a critical temperature, and it should not be exceeded during the mixing operation. Above this temperature, the paste loses too much water and tetrabasic crystals begin to form. Excessive water loss unfavorably influences the paste density, paste workability, theoretical capacity, and porosity.
Mixing of negative paste into positive paste should be avoided. Positive paste can be “contaminated” by negative paste.
All “contaminated” positive paste should be scrapped or used as “remix” in negative paste. “Remix” into negative paste should not exceed 10% of the oxide batch weight, and it should not be dried or lumpy material. It does not soften with water. If dried or lumpy, scrap the “remix”. “Remix” addition to negative paste should be done immediately after the acid has been added while the paste mix is still hot before its cooling phase.
If only one paste mixer and one pasting machine are to be used, scheduling of positive followed by negative plate making is appropriate when it is necessary to paste both plate types in a given day.
Preventing “contamination” requires disciplined cleaning of the paste mixer, pasting machine, and tools. This is a must at the end of each day, or if the paste formula are changed during the day. Cleaning involves a disciplined pattern.
To prevent paste from dropping onto cleaned equipment, it is recommended that cleaning begin at the top and move downward-go from the mixer, to the cone feeder, to the paster Hopper, to the pasting belt, and finally to the under carriage of the paster. If a cone feeder is not used, be certain that the paste trolley used to transport paste to the pasting machine is properly cleaned when changing form negative to positive paste.
To ensure the paste mix is constant from batch to batch, generally two measurements are used-cube weight and penetration.
Cube weight is measured by packing mixed paste into a definite volume cup, and then weighing the filled cup in grams. Such a cup is called a “cube cup”.
Suggestions: Ensure that the “cube cup” is clean before using. When packing the “cube cup” over fill it, hit its bottom on a flat surface to remove voids, and the drag a flat device across the top surface to level the filled cup before weighing.
Pliability of the paste can be measured by dropping a pointed “penetrometer” into the paste from about 6” (152mm) above its surface.
Typical readings, for pasting on optaMAC 610 belt type paster are:
|
For Positive Mix: Cubic Weight: |
For Negative Mix: Cubic Weight: |
If the paste cube weights have a tendency to exceed specified values, the amount of acid used may be increased. If the weights are low, the amount of acid may be decreased.
To reduce paste pellets sticking to the pasting belt during application to grids, the paste should be on the drier side. If the “initial paste” mix is wet and sticky before the acid addition, the final paste will be a creamy consistency which will stick easily to the pasting machine belt and cause pellets to pull out. Final paste will have a consistency described as stiff mortar or cement. The paste should have a “crisp-crunchy” feel to the hand, and it should shear well in the pasting machine’s hopper during application.
Caution: If your paste is too wet in the hopper; the paste can be pulled back up into the hopper in a “sucking” fashion. When this occurs, grid panel fill-out can be incomplete in a random pattern across the grid panel. At no time should a paste mix be adjusted by the addition of oxide. The mix may appear to be acceptable, but the resulting hydroset, or cured, plated will be scrap.