Gold has been refined and melted, now has orange hue.

[ddoc]

Hello, long time reader - first time poster.

I have been processing scrap gold for a few months. This is the first time I have encountered this issue - Explanation:

I typically secure gold filled watches.

I dissolved base metals in Nitric Acid.
Dissolve the gold in AR.
I precipitate the gold using SMB; after neutralizing the AR with Urea - if needed.
Once I have the gold powder I dissolved in AR again.
Once it's precipitated for the second time and cleaned with distilled water.
I rinse again with HCL.
Rinse with distilled water.
Rinse with Nitric Acid.
Final rinse with distilled water.
Melt the powder in a mini-furnace.

This procedure had a been working just fine I was getting shine gold "buttons" that acid test higher than 22K.

This last batch came out with an orange sheen/streaks. I have never encountered this before. Can someone please tell me what is happening and how to correct this?

I am attaching an image - the dark spots are bits of borax I have not removed yet.

Thank you in advance.

 

[Lou]

Could just be iron contamination, but the green borax makes me think nickel/copper.

Try 20% w/v warm sulfuric and see if that doesn't help.

If not, might be something else.

 

[kurtak]

Once it's precipitated for the second time and cleaned with distilled water.
I rinse again with HCL.
Rinse with distilled water.
Rinse with Nitric Acid.
Final rinse with distilled water.
.

Your gold still looks very contaminated

in the above quote from your OP you keep saying "rinse" --- you don't just "rinse" the gold powders - you need to "wash" them

here is the proper procedure

Each of these "washes" need to be brought to a simmer type "boil" for 10 - 15 minutes each wash

Water washes - start with three - after third wash test with PH paper - if acidic - wash again - repeat till PH test is not acidic (usually takes 5 sometimes 6 washes)
decant washes into quart mason jar to let ultra fine Au settle out that pours off when decanting

HCl washes - do this wash as many time as it takes till you see no color change in the HCl - could be as few as 2 or 3 - or as many as 5 or 6
decant these washes into it's own quart mason jar again for ultra fine Au settling - also test these decanted washes with stannous for PGMS (the reason for decanting to its own mason jar) that may have dragged down as traces when dropping your Au - if PGM positive use zinc to recover the PGMs

Water washes - again do this till PH paper test show no acid (usually takes 5 sometimes 6 washes)

Ammonium Hydroxide wash - one wash (hot) this gets ride of any silver chloride - decant into beaker with enough HCl to acidify the ammonium & drop out the silver (as a chloride)

Water washes - as many as it takes to rid the ammonium (PH paper does not show basic positive test) usually takes 5 or 6 - decant these washes into same beaker as first ammonium wash - make sure the ammonium is acidified & not basic

In other words - you want to do your water washes till testing with PH papers shows no more chem from the previous chem treatment & do your HCl wash till it has removed all base metals &/or PGMs (no color change in the HCl) that may have dragged down when dropping your gold --- meaning there is no "set" number for each wash - preform each wash till it has completed its intended task --- it could be just a few washes if it was a small &/or clean gold drop - or it could be a lot of washes if it was a large drop &/or a lot of BM/PGM drag down

I have had to do as many as 7 or 8 washes (each wash) on larger batches &/or the leach source was dirty to start with

Kurt

 

[Lou]

Kurt,

You can use less washes if you don't unnecessarily heat your reagents. Don't boil muriatic and certainly don't boil with ammonia, that defeats the purpose of using these GASES dissolved in water!

If one must use HCl, use one wash of the straight stuff, suck that dry and then rinse with water. HCl will remove AgCl.

 

[Lou]

I don't find boiling/simmering/etc necessary, nor ammonia whatsoever. Needless to use it unless thoroughly contaminated with large quantities of silver. Do not go to rinsing with water straight away. This procedure requires a funnel and good vacuum pump which is what I use because I don't have all day nor the patience to wash and decant and chase the suspended particulates all over.

In the precipitation reactor it can be washed on commercial scale in situ, but for less than multi thousand ounce batches, wash procedure is as follows:

>After precipitation liquors test barren for Au via stannous chloride swab test, the gold is dumped into funnel and rinsed out with 5% w/v HCl solution. The filtrate is allowed to pass through, examined with laser to ensure no turbidity (often helpful to double filter with the gold sponge acting as its own filter aid).
>That liquid is emptied out of the filtering flask. Then, the gold is rinsed with strong/concentrated muriatic to remove any silver chloride as AgCl2- and any cuprous chloride (from overuse of sulfite) as a similar complex. As you can see in the paper below, silver chloride is soluble in strong muriatic.
https://kb.osu.edu/dspace/bitstream/handle/1811/5375/v68n02_100.pdf;jsessionid=21A3063785C0D29F17AE51A324456A44?sequence=1.

Note: This strong muriatic acid wash is allowed to sit without vacuum for 5-10 minutes and gravity filter; this liquid is re-used for another gold digestion. Any copper (I) chloride will also complex with the high chloride concentration and be removed, along with base metal residuals.

>The gold sponge is vacuumed free of acid (that acid saved) and then rinsed off with distilled water; first with vacuum, then with water with until near neutral by gravity so the water has time to penetrate the pores. All of those rinses can be used to rinse the gold out of the reactor until impurities levels build up (usually judge by color). The gold sponge filter cake is finally sucked dry and is good to melt and should be free of the usual suspects (Zn, Ni, Co, Fe, Cd, Sn, Cu, Ag, Pb). One can easily tell when all the muriatic is removed from the gold by putting but a drop of ammonia on the surface of the sponge--any dense white fumes and it is not rinsed well enough. Slight acidity is hardly a concern if induction melting, but there is a risk of gold loss if torch melting buttons from sponge containing free muriatic acid or excess alkali metal chloride. Don't melt gold with salts on it, or silver chloride in it, without some sort of reducer, or your gold volatilizes as Au2Cl6.

FYI, if higher purity gold is desired, then the sponge can be further digested in hot 70% sulfuric.

Ammonia has been the de facto method to remove silver chloride, but it is unnecessary in all but the most extreme circumstances. Heating ammonia water makes it work less well, as seen in this second table:
http://www.saltlakemetals.com/Solubility_Of_Silver_Chloride.htm

I should emphasize that washing with water first is often a bad idea, because the dilution of the dissolution acid (already partially neutralized by sulfite, which is a base) may cause the soluble impurity metals to precipitate as hydroxides.

Speaking of hydroxides; if your solution shows Pd and Pt, a quick coarse separation is to heat the solution to near boiling, neutralize to pH 8, filter and rinse the heck out of the hydroxide cake. Therein will be your palladium and most of the base metals. The platinum is in the filtrate, relatively clean. That can be reduced with zinc turnings or aluminum shavings at pH 1. The hydroxide cake can then be treated with HCl and the palladium recovered any which way.

Lou