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LEAD - CALCIUM- ALUMINUM ALLOYS SAFETY PRECAUTIONS
Lead pot explosions can occur when casting calcium lead alloys with aluminum. The aluminum is very important in the lead - calcium alloys to control calcium losses. With proper handling, the aluminum alloy can be used safely. This report will explain how a lead pot fire/explosion can occur and the methods to prevent such explosions.
1. Theory of Lead Pot Fire/Explosion with Aluminum
Aluminum is present in lead - calcium alloys to reduce dross formation by forming a thin oxide skin on the molten lead surface. Metallic aluminum particles can be explosive. Three (3) ingredients are required for the explosion to occur:
OXYGEN: |
Red lead oxide (PB3O4) formed by improper drossing. |
FUEL: |
Metallic aluminum particles formed by precipitation in improperly drossed melt pots. |
IGNITOR: |
Heat in excess of 1,200°F or an open flame. |
If these conditions are present, the powdered aluminum is a powerful fuel which is extremely explosive.
2. Mechanism for the Explosion
a) OXYGEN
The oxygen for the explosion can come from an improperly drossed lead pot. Newly formed dross is a mixture of lead alloy and lead oxide (PbO). This dross contains very little oxygen and will not support an explosion. If the dross is allowed to build up in the pot over several days or weeks, the lead oxide will oxidize to red lead oxide (Pb3O4). Red lead oxide dross is rich in oxygen and appears very red, brittle and crusty. This oxygen-rich, red oxide dross can supply all the oxygen required for rapid reaction with aluminum. Proper drossing will prevent build-up of the red lead oxide dross.
b) METALLIC ALUMINUM PARTICLES
The aluminum forms as a thin oxide skin on the molten lead surface. Lead can still oxidize in the melt pot and is mixed with the aluminum. The dross, a mixture of lead alloy, lead oxide and small aluminum particles, floats on the surface of the pot lead and is slightly cooler. This temperature differential will increase the dross layer thickness and will cause more aluminum to precipitate out of the lead. When initially formed, the aluminum particles are surrounded by lead or lead oxide and are quite inert. We are not aware of any danger of an explosion with newly formed dross and the dross can be handled as normal dross.
The danger of explosions occurs when the dross is allowed to remain in the pot too long and the aluminum particles become surrounded by red oxide. This dangerous dross can also develop in cold spots in the melt pot or in difficult to dross areas of the pot. The build-up of dross causes more aluminum to segregate out of the pot metal.
Proper drossing will prevent the build-up of aluminum particles in the dross.
c) IGNITOR
If the oxygen source (red lead) and metallic aluminum particles are present as described above, many things can ignite the explosions. The hot dross on a melt pot can be ignited by open flames from a torch, hammering with a pry bar, even excessive heat (1,200°
F) from the melt pot’s burners or burner exhaust.
Cold, red oxide dross is not an explosion hazard and can be handled like other drosses. Using an open flame to remove dross build-up will recreate the dangerous situation.
3. Proper Dross Removal/Prevention Procedures with Lead - Calcium + Aluminum Alloys.
a) The addition of too many pigs at one time will lower the melt pot temperature. Lower temperatures (below 900°
F) will cause aluminum to accumulate in the dross.
b) Low lead levels in the melt pot require longer recovery times to
reach the recommended temperature.
c) Proper burner and control maintenance will prevent changes in working temperatures.
d) Lead - calcium + aluminum alloys require a pot temperature of 900°
- 950°
F. Lower temperatures cause excessive amounts of aluminum to quickly collect on the surface and mix with the dross. Higher temperatures generally will not affect dross accumulation since the aluminum reduces calcium and lead oxidation.
e) All trimmings, re-melt grids, pigs, etc. have lead oxide on their outside surface. The rate of addition should be regulated to reduce dross formation.
f) Any type of agitation, open shaft pumps, vibration, excessive drossing, splashing of scrap, etc., will increase dross accumulation.
g) Depending on the condition of the melt pot, a minimum of two dross removals should be made per working shift. Complete dross removal means scraping out all sides, the corners and cold spots in the melt pot.
h) Do a complete dross removal before furnace shut-down and not after a furnace start-up.
I) Do not stir dross back into the molten lead. This will cause pumps and lines to clog with dross.
j) Do not scrape all the dross into a large pile for removal. This just generates more dross.
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