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Method: Salvaging lead from sealed lead-acid batteries

Why

Lead is a useful material. Relatively inexpensive, fairly heavy, fairly corrosion-resistant, easy to melt, easy to cast, absorbing x-ray and gamma-ray pretty well, generally being a nice useful metal to have around. It is mildly toxic; so don't eat the batteries. Lead was around since ancient times, and the mankind is still here, so it is likely not as bad as the brusselocrats want us to believe. Damn you, RoHS.

A common underestimated source of lead is in the sealed lead-acid (aka valve-regulated) batteries. Such batteries have failry limited lifetime (just a few years) and in critical applications (e.g. UPS power supplies) they tend to be replaced periodically. A well-positioned IT technician can therefore have a steady supply of dead batteries.

How

The battery is composed from several series-connected cells, each one having about 2 volts. A 12-volt battery, common in the UPS world, therefore contains 6 cells. The cells are hermetically closed and the whole battery looks like a plastic box with a pair of terminals.

The individual cells aren't "really" hermetic; that would be a recipe for an explosion, as the cells can release gas under certain conditions. The overpressure has to be vented. For that reason, each cell typically has a valve - a rubber cap over a hole. The valves are hidden under a faceplate, which has a small hole for venting the gases from underneath it. The entire battery is otherwise sealed shut, the plastic case being welded together.

Discharge the batteries before disassembling. They are able of giving out quite a lot of

amps and could cause spark-damage on tools, quite some frightening, and possible secondary damage (e.g. a technician dropping or breaking something due to being startled, a fire, or spilling the acid).

Valves

The valves, once accessed by removing the faceplate, can be temporarily removed and a distilled water can be added to the cells; in some cases the batteries fail due to depletion of electrolyte. It's a waste to discard them when a few drops of water could buy them some more life.

Opening the case

The most difficult part of the operation is opening the case of the battery. It is composed from two parts of plastic (the bottom, with the cell chambers, and the lid), which are glued or ultrasound-welded or otherwise irreversibly connected together.

Wear safety glasses and maybe a pair of rubber gloves from now. The batteries contain sulfuric acid inside and may spit out droplets or you could touch the absorbent soaked with the acid. While not fatal, it is somewhat unpleasant on hands and very bad in eyes. The acid should be immobilized in an absorbent layer, but we do not really know the state of the battery before opening it.

A heated blade instrument is best for opening of the case. Guide the cut along the seam. Possibly cut off parts of the lid, or use a longer blade, in order to reach deeper into the case, as even the internal walls between the cells are bonded to the lid. Prying the lid apart piece by piece is also a variant but takes a lot of sweat. Another possibility is cutting the lid off piece by piece with a tool. A hacksaw or a circular saw is also an option, though dispersion of droplets of acid is more probable that way.

The cut should be led optimally below the bottom of the valves and above the tops of the cells, so neither will interfere with the further cutting and prying apart. That height is roughly at the position of the  seam between the battery top and bottom.

The cells are connected together in series, with cast rods/shapes of lead. These pieces protrude above the top sides of the cells, and may interfere with cutting. Be aware of their presence, don't be surprised. They can be cut off with a screwdriver and a blow of a hammer, if they turn out to be a problem.


Valve-regulated battery

Partially removed lid, a cut through the plastic (badly located too high) shown.

Valves shown

Valves shown, part of the lid removed, a second cut shown (still too high)

Detail of a valve, look inside to the battery

Case, part of the top removed, broken-away part between the second and third cut line

Part of the case top removed

Most of the top removed, bottom of the valve plate shown, note the broken seams between the cells

Exposed top, note the torn-off (instead of hot-cut) seams between the cells, the parallel connections between the plates, and the serial connections between the cells

Cells

The cells consist of sandwiched anode and cathode plates. In a common lead-acid battery the positive electrode (cathode) is a lead frame filled with blocks of PbO2 and the negative electrode (anode) is a lead frame filled with blocks of spongy lead. In a discharged battery, both are coated with a layer of PbSO4. There are usually several plates per cell, with several anode and cathode plates connected in parallel and separated with a material soaked with sulfuric acid. Often the material is glass-fiber mat/wool.

Carefully pull the cells out of their chambers. Slightly spread them apart, remove the absorbent with electrolyte, separate the cathode and anode plates. Remove remains of absorbent from the plates by gentle scrubbing. Wash the plates under running water, possibly using a soft brush to remove the remains of the absorber.

Keep and handle the cathode and anode plates separately. They are made of different materials and can have different uses.


Partially pulled-out cell

Partially pulled-out cell

Three cells out, three in

Single cell, somewhat separated

Single cell, different angle

Single cell, with cathodes, anodes, and electrolyte-soaked mat separated

Salvaged cathode and anode plates, remains of absorbent on cathodes

Resulting materials

Cathodes

The cathodes are lead-frame boards with cells filled with a rust-colored powdery material. This is the PbO2. It is often present as a bulk material, so it can not be washed off the underlying lead surface; do not expect it to be a thin layer. (Feel free to try on a small piece, but don't be surprised if you find lead oxide all through to the other side.) The material tends to be released from the electrode surfaces and make everything it comes in touch with somewhat dirty and poisonous.

The PbO2 can be retrieved from the plates and used in various ways; it finds use in pyrotechnics as an oxidizer in some compositions, or in electrochemistry as material for electrodes. Unmodified cathodes could be used in e.g. production of chlorine or electroplating copper and zinc or other methods using sulfuric acid or sulfate electrolytes. The large area of spongy electrodes is an advantage here.

The dark red PbO2 can be converted to yellow PbO by calcination with e.g. a torch. This was tried in an attempt to melt the lead underneath the oxide layer, and finding that there is no lead underneath, that the material is bulk oxide. Later literature research confirmed this.


Cathode detail

Cathode detail

Anodes

The anodes are lead-frame boards, similar to the cathodes, but filled with greyish spongy-looking material. This is what we are after - the material is lead, or a lead-antimony alloy.

The lead plates can be used as-is, e.g. in electrochemistry as electrodes or as plates for radiation shielding. Another possibility is remelting the metal and casting it into the desired shape, e.g. bulk shielding plates, weights, bullets or shot, or balls or cylinders used as media for a ball mill. The material should be cleaned before casting, e.g. by letting the molten metal settle and then collecting the oxides and other impurities as scum from the surface.


Anode detail

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