A funny way to recycle dead batteries (part 2)

In the previous part we’ve discussed a bit of phosphate chemistry and how I started construction of the “reactor” vessel. So I got two flanges welded at the ends of a segment of steel pipe.

Next, top end bottom end-caps have to be machined:

The bottom end-cap is provisioned with 1/2" pipe thread to allow drainage of the bath (if required in the future). For now it has to be plugged. I could not use standard brass plugs, as they would corrode under the conditions in the bath, so here is the steel plug I’m machining:

And with addition with couple of pipe pieces for legs and a seal between the end-cap and the bottom flange, here how it looks assembled:

Now it’s time to think about heating. Remember, we have to almost-boil the mixture to achieve the desired reaction rate. For that I’m attaching a pair of heating elements to the pipe with screw/band clamps.

It was possible to distribute them around the pipe symmetrically but decided to place both heaters on one side. The asymmetry should facilitate convection and better mixing of the bath contents. After attaching and electrically connecting the heaters, the whole pipe was insulated with mineral wool.

Notice another vertical plastic pipe in the background? — That’s for pre-washing / degreasing in alkaline solution. The solution (in the 3L jar nearby) is what came from washing of the batteries, of course.

The top cap has 3 features: a hook in the center to hang the part, a port for a thermometer and a connection for reverse condenser. The thermometer port consists of the parts that we’ve already seen in the first post. Here how they looked like freshly machined:

Their intent is to squeeze a teflon washer (not shown) onto the thermometer body to seal and fix it in place. After coating here is an assembly with the third part acting as a plug when the thermometer is not installed:

And here is the top cap assembled:

So what is the vertical thin black pipe on top? — That would be the reverse condenser. The boiling bath produces quite nasty and corrosive fumes that are not welcome in my lab. It is not possible to completely seal the reaction volume, as in this case the pressure will rise until something ruptures. So the vessel needs to be vented to somewhere outside (with the help of my ventilation system), but the fumes are too hot and too corrosive to be safely guided to the ventilation channel either. Also with the fumes the water and some other juicy stuff escapes thus depleting the bath. Both problems are solved with the reverse condenser. It is just a piece of vertical steel pipe that cools the fumes until they condense and the resulting liquid drops back to the vessel. The heat is then radiated by the pipe to the surrounding air. I’ve placed a plastic insert at the bottom of the condenser to act as a heat break between the condenser and the hot reactor body. After the most of the nasty stuff is condensed and drained back, it is safe co vent the residual gases with a standard detachable pneumatic hose:

The black color of the pipe comes, of course, from the phosphate coating that I have practiced on it.

At this stage it was possible to start actual experimentation. Here is the test part before and after the bath:

I was clearly on the way to something useful, but there still were challenges to overcome. One purely technical thing was the way of holding the part in the reactor. The bolt-and-wire method left non-coated spots under the bolt’s head on the part’s outer face and it is not acceptable for the production. So I designed and machined a special collet-like contraption that grabs the part from inside:

The experimentation with bath composition and exposure time gave various results and it also took some time for the mix to “settle”. I assume that minor impurities has been quickly spent on the first several trials and also during this time the steel surfaces of the vessel have been passivated enough to stop influencing bath composition. Overall I have achieved satisfactory coatings over the original “mill scale” surface of hot-rolled steel, but on a clean shiny surface the coating was still too thin (for my taste).

In the absence of mounting bolts the top cap does not have electrical contact with the vessel body and this allowed me to move into the next stage of experiments — a galvanically-assisted process. The idea is that passing electric current through the part-solution interface changes redox dynamics and ion concentrations, therefore different properties of the coating may be achieved. And this was the case indeed.

What I found was that applying positive potentials to the part produces fast-growing thick coating (which seems good), but adhesion of the coating to the surface and its mechanical toughness are not great. What I assume happens is that positive potential on the part depletes the boundary layer from positive hydrogen ions and therefore decreases acidity of the solution near the part. The decreased acidity (higher pH) leads to decrease in solubility of the salt mixture and the insoluble phosphates start to crush out of the solution onto the part’s surface. This explains bad adhesion of the coating which is now less of a conversion-type coating but more of a sediment-type.

When applying negative potential the effect is thin coating but with a very good adhesion. This clearly corresponds to the case of etching the part’s surface in the conditions of increased acidity and produces true conversion-type coating. The increased acidity, however, leads not only to etching of the metal, but also to dissolution of the coating as soon as it is created (whence the thin layer).

With those observations the logical thing to proceed with was application of AC current which I have sourced from a regular iron-core transformer, pulled out of a dead UPS (just like in this series of posts). On negative half-waves the current would assist in etching the metal surface and on positive — compensate for dissolution of the achieved coating by crushing out the just-dissolved-material back. This approach gave good results indeed, but even stronger and thicker coatings were achieved by applying an asymmetrical AC (or AC / DC mix, if you wish).

One more challenge was in that the coating was still somewhat porous and on drying the parts developed reddish spots due to further oxidation of metal under the coating on contact with air. To prevent this I needed a way to passivate the iron under the phosphate layer. A simple and effective way of doing it turned out to be just a quick wash in dichromate solution. Now that the part was passive and covered in a dense layer of phosphates, the last step is rubbing it with a cloth, soaked in linseed oil. The oil displaces residual water and on a gentle heat polymerises inside the pores, increasing the rust resistance even more.

The end result is a nice-looking thick even grey coating that is hard, electrically insulating, heat resistant to about 500°C and resistant to corrosive environments. The coating also covers the inside surfaces of the part.

The produced parts were subsequently used in the spot welder project where the coating’s great properties came handy.

And just in case you’ve got interested with that fume-hood on photos, here is the article about it.