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FEEDLINE AND LEAD VALVE CLOGGING, CRACKED GRIDS AS CAUSED BY IMPROPER USE OF LOW ANTIMONY LEAD ALLOY
This information bulletin is written to provide the battery industry with more specific information on the use of low antimony lead.
In talking with several manufacturers and discussing their problems associated with low antimony grid production. It is apparent more detailed information on the characteristics of low antimony lead is needed.
Low antimony lead is an alloy with approximately 2 to 3% antimony and other alloying elements, such as tin, arsenic and a grain refining element.
A. A grain refining element is needed since a lead alloy of less than 4-1/2% antimony has a tendency to form coarse grains upon solidification and this tendency will increase as the antimony content is lowered. Typical grain refining elements are sulfur, copper and selenium. "Additions of controlled amounts of copper and sulfur produce nuclei that float throughout the molten lead and initiate solidification in all areas of the casting. These nuclei produce a uniform, fine- grained rounded structure. Solidifying with the molten metal, these rounded grains roll over one another allowing the remaining liquid to fill all areas."
B. To be most effective, the nucleants must remain small and suspended in the melt. Metal temperatures of 850°F - 950°F (454°C - 510°C) are recommended to keep the nucleants small and suspended. Low metal temperatures may result in the growth of the nucleant particles to a size where they are no longer effective. Further growth of these particles will cause them to float out of the solution. If the grain refiners are allowed to grow together and float out of the lead, several problems will occur.
1. Grids cast from a lead without a sufficient amount of small suspended grain refiners will have cracks. These cracks are usually visible immediately after casting if the grid is bent or rolled around a 2" dia. pipe. In some cases the imperfections in the grain boundary does not cause a break or a rupture that is easily visible to the eye but may show up as a crack after a few days as the grid ages.
2. Feedlines and lead valves may have erratic lead flow or even plug up if there is a separation of the grain refiners.
(1) Wirtz Information Bulletin L-2, page 1
(2) RSR - RSR 275 Alloy Properties
3. Heavy dross on the lead pot. This dross will be different than normal and have a blue purple color.
It is important to utilize the proper operating procedures to control the alloy and eliminate these problems. Generally three areas need close attention when trying to control low antimony lead; the lead pot, feedline, and the lead valve.
The lead pot, whether a floor pot or an elevated pot, should be kept at 850°F to 900°F (454°C - 482°C) whenever molten. It should never be kept at a temperature around the melting point as this could cause separation. Because the lead pot temperature will drop when pigs are added, we recommend an operating temperature of 900°F (482°C). Pot levels should not be allowed to fluctuate and cause large number of pigs to be added at one time, which will lower the pot temperature below the danger point of 850°F (454°C). The lead pot should be between the 850°F - 900°F (454°C - 482°C) for a minimum of 1 hour before removing any dross and then only after the dross is stirred to re-dissolve the nucleants.
Another area of the system that needs attention is the feedline; and, there are two distinct types. The elevated or gravity feedline and the pump feedline. Obviously if a lead separation occurs within the feedline it can cause more problems than the same occurrence in the lead pot, because the lead cannot be stirred to re-dissolve the nucleants.
The temperature in the feedline should be 900°F - 950°F (482°C - 510°C) in both the elevated or pump feedlines. In the case of the floor pot with a pump, the feedline must be drained wherever the system is shut down. To accomplish this, the lead valve should be opened to start the lead flowing and then the pump shut off. The valve should be kept open for 30 seconds to completely drain the feedline. This is of vital importance and an automated system can be installed on Wirtz machines to accomplish this.
With an elevated system the elevated feedline cannot be drained, therefore the lead should not be allowed to freeze under normal procedures. Allowing the feedline to freeze would subject the lead to a slow steady drop in temperature and a slow rise in temperature as it is re-heated again. This could allow the nucleants to come out of solution, since the lead would be in the critical low molten range for a considerable time during cooling and re-heating. To prevent this we recommend keeping the feedline up to the operating temperature all the time.
A typical overnight or weekend shut down would be as follows:
1. Close lead valve at lead pot.
2. Turn on water freeze at the lead valve.
3. Remove valve down spout from ladle.
4. Leave feedline and lead valve at operating temperatures of 900°F - 950°F (482°C - 510°C).
If the feedline must be frozen or if a power outage causes the feedline to freeze it should be heated up only after the lead pot is melted and the lead valve at the pot opened. This allows the expanding lead to extrude into the lead pot and will prevent lead line rupture. It should be then heated up to a 1000°F (538°C) and the lead valve opened and the lead line flushed out. The best procedure is not to freeze an elevated feedline. The cost of keeping the feedline up to temperature is very minimal.
The last area to be concerned about is the lead valve. This is normally by design, the coldest part of the lead delivery system. It is desirable to operate the lead valve at a temperature that will give long life and accurate metering. Therefore, overheating of the valve is not desirable.
Some people report no operating problems using an electric heated lead valve, while others have a problem keeping the lead flowing. The typical report is that the lead flow is difficult to get started and the flow will decrease during operation until the flow is too small to continue casting. Upon heating or cleaning the lead valve, normal operation will continue for several hours and then the flow starts to decrease again. This is the basic description of the grain refining nucleants coming out of the solution at the lead valve. For normal operation to resume the valve must be cleaned to remove the nucleants or heated to re-dissolve the nucleants.
For battery manufacturers experiencing this problem Wirtz recommends that the electric heater in the lead valve be replaced with a gas heated system. Wirtz can supply a kit for changing the lead valve heat to gas including a heat containment can which will allow slightly higher temperature in the lead valve.
In summary low antimony lead has many manufacturing advantages; however, to use the lead, close attention to the lead delivery system is needed. The lead should be kept at a uniform temperature from the lead pot to the ladle. This temperature is usually above 850°F (454°C) for most low antimony leads.
If a cold spot in the system exists or if the general temperature is dropped below the 850°F (459°C) temperature, separation can occur. If selenium is used as a grain refiner, the temperature ranges discussed above may be reduced up to 50°F (10°C).
Recent changes in Wirtz Casting equipment
1. Automatic feedline drain - automatically drains pump feedline whenever lead pump is shut off.
2. Lead valve heat containment can - raises operating temperatures of lead valve and end of feedline to reduce cold spots. Helps keep grain refining nucleants in solution at the lead valve.
3. Ladle drip eliminator - cuts off lead flow at the ladle orifice eliminating the troublesome foil or drip.
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