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.