There are many ways to make colloidal gold solutions, besides the Purple of Cassius reduction we use with SnCl2 to test gold in solution.
Most processes use organic compounds,and the chemistry can get really complexed, different methods can give different sizes of colloidal clumps of elemental gold, or colors of the solution which are related to the size of clumps of the gold atoms which form the colloidal gold in solution. these clumps of gold atoms are charged particles of clumps of elemental metal gold atoms which develop an electrical charge positive or negative (how you make it during the process can determine what the charge of the particle will develop)
For example adding the reagent to the gold solution or adding the gold solution to the reagent, one way will make positively charged gold colloid particles, and the other way will make negatively charged gold colloid particles.
Gold colloids have also been made by electric arc of gold rods operating in liquid.
For a ruby red colloidal gold solution a very common method of making it is to use AuCl3 and bring to 100 degrees C, then slowly add sodium citrate, the sodium citrate acts as a reducing agent and a stabilizer to fix the colloids size at about 20nm in diameter of the clusters of gold atoms, which gives the ruby red color, if you add the sodium citrate to gold chloride solution the colloids are negatively charged due to the absorbed citrate ion.
Adding lots of salt to the ruby red solution can convert the red solution towards a blue solution.
If we have tin or stannous chloride in a gold chloride in solution (Purple of Cassius). Colloidal gold we can recover the gold by strong heating with a strong acid like H2SO4, (boiling the piss out of it).this will break the colloid charge and precipitate the gold from solution. Electrical current could also be used but I do not know the details of that procedure.
The above would be my choice of recovery of gold from solution.
Another method of recovery often suggested:
If we just dried it out, it would give us elemental gold and tin oxides which adding HCl could easily reform the colloid again putting the gold back into solution as colloids with the tin again. once dried to recover the gold we need to carefully incinerate the tin oxide. Here is the problem gold and chloride salts the gold becomes volatile, roasting here would not be a good choice in my opinion.
I would after drying the solute, and before roasting, give it a concentrated wash in NaOH solution, to make water soluble NaCl to be able to remove as much chloride as possible before roasting the powders.
