Pan,
Mercury tends to react with materials which are otherwise considered inert, such as aluminimum (eg low temp heat exchangers) and gold (in certain catalysts). Mercury is also toxic, especially so if present in organic compounds.
I don't have any experience in refining, but my company produces a fairly high Hg content crude, approximately 800 PPB, or 0,8 PPM. Some refineries will buy it (at a reduced price, usually 0.2-0.25 cent/barrel), other will not.... I guess it simply depends on the materials used in the refinery.
Trond