why SMB can get gold back

who can tell me why SMB can get gold back,as specific as possible ?
thanks in advance

It has been covered several times on the forum already. I'm working right now so I can't look up one of the threads for you. But I'm sure that using the search function on the forum should give you what your looking for

sodium metabisulfite (SMB) is dissolved and decomposed by acid leaving salt (sodium) and sulfur dioxide gas (SO2). SO2 gas is what precipitates the gold. this is a very simple reaction and is one of the first and most important reactions you will learn in refining.

have you read C.M. Hoke's book "refining precious metal waste" yet?

View attachment cm Hokes.pdf

is there any other precepitation gold easy to access....??? :?:

There are several chemicals or metals that can precipitate gold from solution or displace it from solution, each have different properties and and can be chosen for use for different reasons, copperas or ferrous sulfate is another one you can easily make with dilute sulfuric acid and iron.
Study Hokes and the forum for answers to this question and millions of other questions you have not even thought to ask yet.