Somewhere there is another thread on this... I talked to my girlfriend who is a chemistry professor when this discussion came up last time. I still use carbide part of the time and was curious. Anyway what I gathered without experiment is the following:
1) As stated in all the previous posts, used carbide will almost certainly continue reacting and generating acetylene for a while(the water in the air is enough for this). The dangers are obvious.
2) Calcium hydroxide in solution is very basic, hence a small amount will help improve acidic soils. This also means that getting it on gear, when wet, is generally a bad idea. I've never gotten a chemical burn from wet spent carbide but I expect getting it in your eyes or anywhere else with sensitive tissue (nose, mouth, lungs) would be bad.
3) The process used to make carbide results in trace amounts of heavy metals being mixed with the carbide. This is a good reason for throwing away(i.e. sending to a land fill) carbide instead of just dumping it at the entry or in your garden.
Anyway, just what I found/learned. I would be really interested in seeing a good wet chemistry test on spent carbide.
Of course in the end whether you use alkaline batteries, rechargable batteries, carbide, candles or anything else the amount of gas you spend getting to the cave probably means a lot more.
James Hunter