4 - Gold precipitation with Sodium Metabisulfite. Now that we have our Gold solution (Chloroauric acid - HAuCl4) filtered and clean of any solid sediments it is time to get the gold back out of the solution and precipitate it to it's metallic form.
There are many ways to precipitate the gold, here we will concentrate on using Sodium Metabisulfite (SMB). Without getting too much into the chemistry, when adding SMB (powder) to neutral/acidic solutions it releases SO2 gas that in turn dissolves in the solution and reacts with the dissolved gold and perform the actual precipitation of the gold. Add 1 gram of SMB for each gram of gold that is dissolved in the solution, using access SMB will not bother the process, but will defiantly bother you as this gas (SO2) is real nasty. Cover the vessel with wet cloth once SMB is added and mixed in the solution. Once mixed and covered, the gold will immediately precipitate and form brown almost black suspension, allow it to settle over night.
5 - Washing and drying the gold.
Now that the gold have settled to the bottom of the flask we can move on to the cleaning phase, that we do by washing the gold powder with water and Hydrochloric acid (32%), washing is a very important part of the refining process and should not be taken lightly if high purity is desired. These steps will eliminate most (almost all) of any other metals contaminants (copper, nickel, silver etc…). A – Now that the gold have settled, swab the liquid with a Q-tip and test it with stannous chloride to confirm that all of the gold really did got out of the solution. If negative move forward, if positive add more SMB and wait few more hours.
B - Prepare the filter rig with a waste solution container and the same filter that had been used before in earlier stages. Decant as much liquid as possible, carefully, leaving the gold powder in the beaker. The filter is there to catch any tiny gold particles that slip out and will be recovered from the filter at a later date with the rest of the filters. C – Tap water washes: add plenty of water, swirl and mix thoroughly to let the water reach all of the powder. Allow the powder to settle for a few minutes (that's up to you to decide how much) and slowly decant or siphon to the filter. Repeat tap water washes two (2) more times. D – Hydrochloric Acid (HCL) washes: Once done with tap water washes, add just enough HCL to cover the gold powder, mix and swirl the powder in the acid and let it stand for 5 minutes. Decant and repeat one more time. The third wash will be boiled, add a bit more acid the usual and place on to hotplate with medium/low heat, let the liquid reach a gentle boil, do not let it go to hot or it will splatter your powder all over the walls of the beaker and possibly out of it. Keep it boiling for 5 minutes, then remove from heat and allow to cool back to room temperature. Once cool down decant the acid. Note 1: A slight change of color (usually yellow) of the acid indicates metal contaminants being dissolved in the acid. Note 2: Every now and then HCL washes may also dissolve minute amounts of gold, so you should test all acid washes with Stannous Chloride test solution, you can keep the acid washes separate from the other washes to be treated later if positive for gold. E – Distilled water washes: Before melting, gold powder should be dried. In order to dry it well we should first wash it well with distilled water. Drying gold powder with tap water can turn up to be quite a pain since minerals that left behind will stick your powder to the flask. Add plenty of D. water and mix thoroughly, wash the flask walls as well. Decant and repeat. Repeat D. water wash 3-5 times, do not skip this step. You can do one wash with short boiling if desired. F – Drying: Once done with D. water washes, decant as much water as possible without disturbing the gold powder and place the flask on top of hotplate on low heat. Let the water evaporate slowly for at least 30 minutes. Heating too fast may cause vapor explosions scattering your gold all over. Once the powder contains only moisture without any water pooling, crank up the heat to medium/high until the gold powder is completely dry. Once dry you should be able to knock the powder off its place simply by tapping or shaking the flask.
6 - Melting of pure 24K gold. Now that our gold powder is clean and dry it's time to melt it into a button, pour ingot or even casting grains. Note: If a re-refining is desired, incinerate the powder (red hot) and repeat all steps from step 3 of this tutorial. A- Transfer gold powder to a watch glass or other vessel. Weight your powder and compare it to your initial calculation of yield, don't forget to log any differences. B – Transfer the powder to a preconditioned and seasoned (if needed) melting dish. You can learn all about preconditioning and seasoning of melting dish right here. Melting dish should be glazed properly with borax.
C – Prepare your torch, melting dish, stainless steel tweezers and a stainless steel bowl of cold water. D – Start melting the powder slowly, heat it evenly starting from the sides working your way to the center, see attached video. E – When full melting is achieved, turn of the torch and allow the button to solidify for few seconds, while still red hot, grab it with tweezers and dunk it in the cold water. You may not get it right the first time or even the second time, not letting the gold to solidify may result a deformation and maybe even loss of some gold, waiting too long and it might get stuck in the glazing, one most fine tune the solidification period of the molten gold. If you mess up, just melt and try again. F – Your pure gold button is almost ready, it probably holds on the surface traces of the borax (liquid glass) glazing on it and we shall remove it with a dilute 10% solution of sulfuric acid (H2SO4). Place the button in the solution and heat for 10-15 minutes, when done, all of the glass will be gone. Wash well with water. And now you are d-o-n-e ! Congrats.