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CONTROL OF FURNACE POT DROSS
When casting low antimony lead the temperatures of the pot, feedline and ladle should be controlled as close as possible as overheating and under-heating can cause dross. The drossing of the pot by the operator must be controlled. The pot should never be drossed when it is below operating temperature (usually 850°F/454°C minimum and 900°F/482°C maximum) or you run the risk of removing grain refiners. A pot should be drossed at the beginning of a shift and very little thereafter. The pot level should not be allowed to vary drastically as this causes dross build-up on the sides of the pot, the pump, feedlines, etc. It also causes wide temperature variations when the operator adds pigs too fast in order to fill the pot. Over agitation of the lead in the pot will cause dross. Therefore, you should be using the latest design in lead pumps available. Wirtz lead pumps are designed to keep dross to a minimum. Of course, overflow lead systems will cause excessive dross.
Some calcium alloys may dross more rapidly than antimony alloys and therefore different temperature perimeters are necessary for the lead pot. The furnace temperatures and lead levels in the pot are very important. The following temperature information is applicable for Calcium or Calcium Tin alloys. The temperature should be between 750°F (399°C) and 800°F (427°C). Below 700°F (371°C) it is more difficult for calcium to go into solution and above 800°F (427°C) more dross will be generated.
For all Calcium Aluminum alloys it is necessary to use higher temperatures to keep the aluminum from coming out of solution. The lead pot should be operated between 900°F (482°C) and 950°F (510°C).
The operator should dross at the beginning of the shift and not again during the shift.
Lead levels for calcium/calcium tin alloys should be maintained closer than with antimony to avoid dross on the sides of the pot and to maintain temperatures. Therefore, one good control method is to weld two marks on the pot to show maximum and minimum levels of lead. Install a clip board with a calcium control sheet (see attached). Each time the operator adds a pig he marks it on the sheet at the noted time of day. After each 7 pigs of 100 pounds (45.5 kg) are added to the pot, he should add one pig of 25 pounds (11.4 kg) of 1% calcium alloy. The pot should be checked by quality control every 4 hours and corrected, until confidence is achieved and then once every 8 hours. The calcium level should be maintained at .085% minimum and .10 or 11% maximum. If quality control checks show the calcium level is not within these limits, change to one 25 pound (11.4 kg) pig for every 6 or 8 regular pigs. If the system is working properly, quality control will never have to add calcium because the system is in balance and the calcium level is always correct.
This system will also help the operator because he will not be adjusting the machine for soft or stiff grids during the shift. Chilling of the pot or addition of large amounts of scrap will throw the system out of balance and the pot should be checked, corrected and started again.
Pot size can have a major effect on the total operation. Larger pots will change levels more slowly and maintain more uniform temperatures when adding lead. They are easier to dross and maintain because there is more room. Larger pots will usually generate less dross than small pots.
As noted there are certain basics to be adhered to with any lead system. The temperatures and the pot levels must be controlled. Agitation should be kept to a minimum. Drossing should be controlled.
CALCIUM CONTROL SHEET FOR LEAD POTS
| Time |
Pot Drossed |
No. of 100 lb (45.5 kg) Ca Alloy Pigs Added |
No. of 25 lb (11.4 kg) 1% Ca Rich Pigs Added |
Quality Pot Temp. |
Control Check Ca Level |
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