Cyanates anyone?

Okey, so I had four of these 5L jugs of cyanide water where I've extracted the gold, sitting for a couple of years.
(I electroplated most of it and used zinc powder on the rest)

Now I wanted to make sure I got all of it out so I can dispose of it, so I zinc'ed them again and went to filter it a few days later... in two of the jugs I noticed they were full of white precipitate in the bottom so I tested the pH and it was ~6.5-7! I'm guessing it's zinc cyanide which is a whice solid...
I did not test the pH before putting the zinc powder in but I guess it was this low to start with, but _why_ did two of the jugs have pH 6.5 and the other two pH 12? I don't see how I would have treated them differently...

Added a little bit of NaOH but it did nothing to the pH so I'm guessing the zinc cyanide is eating it up and I need to enough NaOH to 'dissolve' it before the pH starts to rise?
If so - are there any dangers or caveats to this? What does the zinc cyanide covert to and do I need to add more zinc powder once the pH is high again - or is the cyanide destroyed by this low pH (which might have been low for a couple of years)?

Any help appreciated before I foul something up
Thanks

//Greg

You can search the forum for cyanide destruction.

Take a sample and put it in a test tube or a small beaker before playing with a jug full of cyanide.

I think the reason behind the pH being low is lack of NaOH, you either didn't stir well on the remaining two jugs before or something else is up.

If I remember correctly, zinc cyanates will precipitate after bleaching.