how to extract gold from gold alloy ?

Answer 1

While possible, this is a very expensive chemical process.

Answer 2

Only you and god know what you want to do, but it will depend a lot on the specific allow. If it is a small jewel made of simple gold-silver alloy you may treat it with hot nitric acid to dissolve silver, then melt and cast the resulting solid to form a bullion. (VERY HAZARDOUS PROCEDURE)

If it is a more complex alloy, you may want to cupellate it (ALSO VERY HAZARDOUS)
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By the way, mercury will form amalgam with many metals, not only gold. Amalgamation will make no good for purifying gold. (HAZARDOUS, HAZARDOUS HAZARDOUS!!!)

Cyanadation and other methods are reserved for those who know what they are doing.

In all cases it is difficult, hazardous, expensive and requires a great amount of knowledge and/or experience. If you are not a qualified chemist or alike, please don't even think on attempting it.

Sorry if I don't fulfill your expectations, but believe me, that's why gold is so expensive.

Answer 3

This wiki article explains it:

http://en.wikipedia.org/wiki/Gold_extraction

But to extract gold from gold alloy, you can use Mercury because mercury for a amalgam with gold

Au + Hg -> AuHg (gold amalgam).

Following link has some explanation on gold amalgamation:

http://www.pbs.org/weta/roughscience/series3/treasure/extract.html