As promised here is our process for mercury disposal.  We use it on B5
mostly

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Make sure you wear gloves for this procedure.  All containers and gloves
used should be rinsed with water and the water put in the waste mercury
container for next time.

All solutions containing mercury salts are collected into a labelled
collection jar, "Used Mercury Salts".  We keep this in our grossing room.
The jar is actually a clear glass 3 kg sodium phosphate container.

When the jar is about half full, to each 900 mL (estimate, don't measure)
add 40 grams of sodium carbonate to raise the pH to 8.0 or higher and mix
well.

Leave it for a while, then filter the solution and collect the precipitate
into a plastic jar for disposal.  Put the filter paper in the jar as well.
Seal the jar very well.  We coat the lid with several layers of paraffin wax
until it is about a quarter inch thick.  Rinse the funnel and put the
washings into the waste mercury container.

The filtrate should be clear - if not repeat the process.  We check by
adding a little dissolved sodium carbonate to see if any more precipitate
forms.  If it does not, all mercury salts have been removed and the filtrate
may safely be disposed of down the sink (check your local regulations).

Pack the jar containing the precipitate inside a safety container and seal.

Label with WHMIS label and contact Occupational Health and Safety for
disposal.  We use a service in Vancouver, BC, Canada.  The contact is Rob
Mitchell and the company is Cybertech (250) 523-9688.  I believe they ship
it to a company that recovers the mercury.

Presumably there are similar companies in the US and other countries.

Reference
Hazardous Chemicals in the Histopathology Laboratory,
Crookham, J & Dapson, R,
Anateen, Ltd.

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Bryan Llewellyn