Rub baking soda on with cloth . Repeat.
Pretty much the same way as removing gold and other precious metal from the ground. It is refined or taken to a smelter refinery where in most instances heat is a major factor and then chemicals such as sulfuric acids which are used to separate the mater and extract and separate the mixtures of material that make up the component being refined. The exact refining process is more dependent on the exact material being refined and used in the refining process. Since different components from different obsolete circuitry is comprised of various materials and that much of this make-up is of a hazardous nature and the disposal of the waste and by-product is regulated by the government entities like the EPA, this is a good place to start your detailed research on the process.