Before explaining how to perform this process, it's probably be better if we first talk about why use it in the first place.
As you probably know, there are many reducing agents that will reduce ionic gold into the metal, such as: SMB (Sodium MetaBisulfite), copperas (Iron II Sulfate), Sulfur Dioxide Gas (SO2), Hydroquinone, formaldehyde, hydrazine Sulfate and many more… each has its own applications and pro's / con's.
The Oxalic/Oxalate process is very selective for reducing Gold ions and it is used when very high purity (9999 and higher) Gold is desired. But due to the nature of this process which involves changing the acidity of the solution (which may form oxides/hydroxides of base metals and interfere the process), It is used almost exclusively as a re-refining process, sometimes referred to as "polishing" step. When in most cases the Gold will be reduced from an Aqua Regia solution the 1st time with the use of SMB or SO2 Gas, get washed and dissolved again, filtered and then reduced the 2nd with Oxalic Acid.
Important: This is not a Recovery process, this is a final step of Refining.
High purity Gold of 9999 and higher could be made with simple setup (as you can view in the video below) that even a hobbyist refiner could build himself, let alone a proper and well organized laboratory.
Who needs this process and why?
Research and processing labs can utilize this process to produce high purity gold powder to be used either as is or as precursor for preparing gold based catalysts. Considering the prices charged by large vendors for precious metals powders, the cost saved by this process is enormous.
Refiners would rather use this process for producing small amounts of bullion gold, amounts too small which doesn't justify setting up an electrolytic refining cell (Wohlwill process).
The use of Oxalic acid is also called for as a first refining step when large amount of PGM's are in solution with the gold, such in the case of dental gold.
WARNING: The process described here involves highly corrosive and toxic chemicals and should only be done outside or in a fume hood. Using all the safety gear such as Gloves, goggles and a respirator is mandatory.
Gold powder which had been refined once (dissolved and dropped out of solution with SMB) was dissolved again by means of Aqua Regia in the usual manner and filtered through a slow/medium speed Filter paper. This is the main Chloroauric Acid solution we will be working with in this process.
- The Gold solution is placed in a high quality borosilicate beaker or in a boiling flask (with tapered ground joint) and the flask is placed on a hot plate/mantle on medium heat. Make sure the solution level will not exceed two thirds of the flask volume.
- In another beaker, weight out Oxalic acid (dehydrate). The amount of Oxalic acid needed is 1.15 gram for every 1 gram of expected gold metal. This ratio represents a 20% excess in Oxalic acid to ensure all of the gold is reduced in case there's too much nitric acid left in the original AR solution.
- Add the Oxalic acid to the gold solution and turn up the heat of the hotplate/mantle until a gentle boil has reached. A boiling flask with tapered ground joints enables you to connect a condenser (Graham) and an addition funnel (see video below), but if you are boiling the solution in an open mouth beaker, you should cover it with a watch glass. The addition of reagents will be done through the spout with a pipette.
- Now prepare a concentrated Sodium Hydroxide (NaOH) solution in another beaker. Simply add some NaOH prills/powder and dissolve it with warm water until no more is dissolved. Either by an addition funnel or by manually adding it with a pipette, start adding the NaOH solution to the gold solution. Remember you are adding a strong base to a strong acid which produce a violent exothermic reaction, thus the addition must be done SLOWLY and DROPWISE.
You may notice some precipitation of brown powder before you even add any NaOH, but that's just about as far as it will go without pH adjustments. At this point, you may also notice brown Nitrogen Dioxide (NO2 – toxic gas) formed in the reaction vessel which is the result of left over excess Nitric acid from the original Aqua Regia solution. If the gas cloud formed is very thick, it's probably a result of way too much Nitric being used and you might want to consider adding more Oxalic acid and or even start over.
It is pointless to try and determine just how much NaOH is needed, so your main indicator for the reaction completion is the color of the solution. As you add NaOH you will notice more and more brownish/black suspension being formed that will slowly coagulate. Keep adding NaOH solution until you can no longer notice the yellow/orange hue of the solution and the bubbles.
- Once the process is done and all of the gold had precipitated out, the solution would normally be either colorless or with just a faint color of blue/green. If there's a significant coloration to the solution, then you probably should consider repeating the process all together. Remember, this process is all about purity.
At this point, the pH of the now barren mother liquor should fall anywhere from 0-2.
- If you used a boiling flask, it is recommended that you now transfer the powder and the barren solution to an open mouth beaker so it will be easier to wash. It is suggested that you will allow the gold suspension to settle to the bottom first. It takes several hours… Even better, let it to settle overnight.
- The following day, slowly decant the barren waste solution either through a filter or into a settling tank to be discarded later.
Washing cycles are done in the usual manner:
- Cover the gold powder with Hydrochloric acid (HCl 25-31%) and boil for few minutes, let cool and settle, now decant and repeat two more times at least.
- Cover the gold powder with Ammonia solution (NH4OH 10-25%) and boil for few minutes, let cool and settle, now decant and repeat one more time at least.
- Cover the gold powder with boiling hot Distilled Water and swirl it around for few minutes, let cool and settle, now decant and repeat 3-5 times.
You can combine all of the above washes with the barren mother liquor and filter it all at once just in case some gold powder slipped through. Also, keep in mind that sometimes, the HCl washes may dissolve some of the gold. You can precipitate it later with SMB.
Remember you must make sure the combined washes solution is acidic, otherwise you risk forming Ammoniacal gold complexes which can turn explosive. You can test the solution with a simple pH test strips.
- After the last distilled water wash is decanted, place the beaker on a hot plate and set it on low heat until the gold powder is completely dry. The color of the powder should be very light brown.
- Depends on what you want to do, you can now use the gold powder as is or melt your gold in a new and clean crucible.
- Sit back, have a beer and enjoy your success :)