Using ferrous sulfate vs SMB as a precipitant details?

[orvi]

Advice is worth what you pay for it. But I use AR and SMB.

And these processes give off NOx (the brown gas coming off the reaction) which can cause permanent lung damage, thus a chemical fume hood is necessary.

My Nitric is 67-70% technical grade. Some say put in the HCL and add the Nitric slowly. That is not what I do, and at this point, I have refined over 100oz of Au to 99.9%. The first step is to inquartation where you lower the Au concentration by adding silver. Pour that into the shot and dissolve in dilute (35%) Nitric acid. This yields a brown powder/solid. From experience and XFR analysis the gold is 97-98% pure.

AR dissolves low carat or high carat gold. More than 10k and less than 18k doesn't work well.

But once you have the brown powder, I rinse it and dissolve it in AR. My Aqua Regia AR formula is 4 parts HCl to 1 part NHO3 (67-70%). To go further the amount of AR is normally 150ml max per 31.1gr of Au. This breaks down to 4.82ml/gr.

I refine 3oz at a time in a 5000ml beaker. I will start by adding 100ml of HCL and 25ml of NHO3. And go up from there. I also heat it to approx 380 degrees C. But this takes hours... in the process, I add 4-5ml of Sulfuric Acid to drop out the lead.
When the gold is completely dissolved the liquid will be yellow and there is normally a powder in the bottom. (Lead and other base metals). From here I rinse and filter through a Buckner funnel until the liquid is completely clear. Here is the purpose behind a 5000ml beaker for 3toz of gold. By my formula, it only requires 450ml of AR. But in the rinse process, 1500 to 2000ml of H2O gets added.

Some will disagree with this, but I add Urea a pitch at a time to the liquid until it stops reacting to burn off the unused Nitric. If done properly it takes 1gr of Sodium Meta Bisulfite for every gr of gold in solution. I hate to have too much SMB personally. I filter and rinse.

This gold is 98-99% pure. I repeat the AR process and the difference is that the liquid has little to no solids precipitate and it is a beautiful color of orange. The gold that precipitates on the second go-round is 99.9 - 100% pure.

Another good point of reference is that it takes roughly 1.22ml of 70% Nitric acid combined with 1.22ml of H20 per gram of metal to dissolve the base metals from inquarted gold.

well written. caution and patience is sometimes better than losing just a few % of the yield, when you are working with multiple oz in early days i experienced some runaway AR dissolutions but luckily I was able to quickly place the beaker in the bucket, so no big damage was done... from then, I fill the beaker just to the 30-40 % of volume and start with only half of nitric required for the rxn.
i just add, from my experience, that rose and pink 18k gold dissolves fairly well in AR (warm/hot), due to low silver content (generally when silver is kept below 10%). it produce white AgCl ppt, but AR slowly eat the gold trough this, leaving AgCl "corpses" of granules or flakes dissolved. i found out that this process is speeded up extraordinarily when using ultrasound bath to disintegrate the AgCl layer

personally, im used to recover mostly electro-scrapp (pins, fingers-platings, etc.), so i go straightly to AR (nitric acid is determining reagent for me from point of obtaining here, HCl is easy-obtainable), dissolve everything, then reduce remains of nitric using sulphamic acid while solution is still warm (nice chemistry behind - decomposition is quick and practically quantitative, forming nitrous oxide. from my point of wiew, urea dont perform anywhere as well as sulphamic acid). adding until it stop fizzing, then cool it down to ambient temperature, filter through Buchner and precipitate with sodium sulfite or SMB. most of the times, i have no idea how much gold i should expect, so generally i add it spoon at a time with stirring, until i can smell the SO2 gas from distance, and than add just half spoonful "for sure"
without adding deNOxing agent, its very common you find no ppt in the morning, because residual nitric kill all the SO2 and gold goes back into the solution
gold mud is settled overnight, most of the liquid decanted. gold is then washed 2-3 times with hot HCl (sometimes i encounter minute dissolution of gold back into the solution, so i always keep all acid washings, use it as starting acid for next batch) until the washings lose color, and next with water, than dryied in a beaker... and thats it if small quantity is produced (under half-gram), after washing i sweep the beaker with paper towel and directly burn it in a crucible and melt it XRF assays are in range of 99.7 to 99.94 % Au, rest being solely Ag. done numerous times over years

i never trusted the purity of technical ferrous sulfate (which i could access here), so i never used it in larger scale. SMB or sodium sulfite are abundant chemicals for me, so i stick to this route, its efficient, no waste metal is added...