Letter 3003

Dissolving gold deposit for analysis 

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Hi. I'm trying to determine the quantity of gold deposited on our product. I'm using an ICP to do so. I've played around with two sample prep techniques. The first is to semi-selectively dissolve the gold from our product with cyanide stripper. I then take the gold laden stripper solution and put it into a 20%HCl, 5%HNO3 matrix since I would prefer to deal with an acid matrix to ensure compatibility with the other samples run on the instrument. The dilution of the stripper solution in the acid matrix is of course performed in a hood to contain any hydrogen cyanide liberated in the process. My problem with this technique is that it seems to result in the formation of a suspended solid. The numbers I'm getting indicate to me that the precipitate is a problem (either it's a gold salt or it absorbs gold). The second procedure I've tried is to dissolve the entire part in aqua regia. This procedure seems to work ok but I don't like the fact that I have to dissolve all that extra metal and complicate the matrix.

JD Kronicz
Hanover, PA

1. This will work with copper alloys: Cut open the part so that the base metal is exposed. Use nitric acid to dissolve away the part. What you have left (after careful rinsing) is the gold shell. Then dissolve this in aqua regia. This is a very thin foil so don't lose any while rinsing. 2. This works best when the deposit is particularly thick: It is the usual procedure of one gold bath vendor that I know to have their customers plate a copper or brass coupon (small disk or rectangular piece) with over 1000 microinches, just the gold, not the underplate. Then they cut the coupon in half and do a cross section and Knoop hardness on one half. The other half goes as I detailed above, first nitric acid, then rinse, then aqua regia. The test is for impurities. You don't actually look for the amount of gold deposited using this technique. The vendor will do one free test a month for the customer. 3. Just measure the thickness using cross section or XRF. It's an easier method than anything else I know of for estimating the gold deposit.

Bill Vins microwave & cable assemblies Mesa (what a place-a), Arizona

Thanks for your response Bill. I am familiar with the technique you referred to. The problem is (as you alluded to) when you're talking about actual product and much thinner gold the foil is very hard to handle. I am interested in quantitative gold and not impurities. With impurities it is ok to lose a little gold as all you are interested in is the ratio of impurity to gold. The xray technique is fine for thickness but I'm looking for usage on the part. We do routinely perform thickness by XRF to check functional areas of the part. JDK

JD Kronicz
- Hanover, PA

JD, If you can strip the part using aqua regia, using care to minimize attack on the base metal, the gold can be precipitated form the aqua regia as follows: 1) Boil aqua regia down to incipient dryness- till only a few mls. of liquid remain. 2) Reconstitute using 100-200 mls. deionized water. 3) Adjust pH to 2.5 and bring solution to boil. 4) Add 10% sodium nitrite a few mls. at a time. The gold will precipitate first as a fine powder and with further boiling will begin to agglomerate. When adding sodium nitrite no longer produces any precipitate, filter out the gold, wash with HCl then DI water and oven dry. Determine the gold content gravimetrically. The precipitate will be 99.99% pure, so if your deposit is a Type 1 or 2, the alloying elements will not be present in your precipitate. Good luck.

Keith Wicklund CEF-SE
Minneapolis, MN

Another method for precipitating the gold from aqua regia which doesn't involve boiling away the nitric acid and generating toxic nitrogen oxides is as follows:

After dissolving your gold or base metal/gold into the aqua regia, add granulated or prilled urea to the acid/gold complex until the fizzing stops. This will destroy the nitric acid without boiling. The fizzing isn't violent nor are any disagreeable fumes released. A slight excess of urea at this point is ok. Next, slowly add a solution of sodium bisulfite (sodium metabisulfite in water) to the HCL/gold/urea mixture. When you start to smell sulfur dioxide being released, that is a good indication that all of the gold is reduced. Any further addition of sodium bisulfite will begin to reduce copper chloride to cupric chloride, which will precipitate along with your gold. This can discolor the gold, release clouds of noxious fumes when the gold is melted and generally screw up your assay. The gold will precipitate over the course of several hours as a chocolate brown powder which melts to the yellow color we all know and love. Add a few ml of isopropyl alcohol of other ! non-acid labile surfactant to free the finer particles which are floating atop the surface tension. After the solution becomes a transparent green, decant off the remaining HCl/copper chloride solution, cover the gold with concentrated HCl and swirl. This will dissolve out any precipitated cupric chloride. Wash the gold several times in distilled water until the water shows neutral to litmus. You can substitute concentrated ammonium hydroxide solution for the HCL wash since it too will dissolve cupric chloride with the added feature that cupric chloride will turn aqua ammonia blue. Repeat the ammonia rinse until no more blue is seen, and then do it once more. One caution here: If there is any possibility that silver salts have been precipitated along with the gold, do not use ammonia. Ammonia will form explosive acetylides such as silver fulminate, especially with insoluble silver chloride or silver oxide. When these dry, they will violently detonate if you even look a! t them too hard. This hazard can be completely avoided if you filter your acid after the urea step since this would trap any insoluble silver chloride and keep it out of your gold. I know this post is long, but the beauty of this process is in the details. I will post a second, even easier method in a followup post so that I won't choke the server with this one.

Frank

Frank Roberts
KLRU-TV - Austin, Texas

As promised, here is another simpler method for precipitation of gold from aqua regia:

After dissolving your gold/base metal in the aqua regia, allow the acid to cool and then add an excess of sulfurous (not sulfuric) acid to the pot. This both destroys the excess nitric acid and reduces the gold in one step. Unlike the urea/sodium bisulfite method, no cupric chloride will be precipitated, only your gold. Since sulfurous acid is a water solution of sulfur dioxide gas, you can also bubble SO2 gas through your acid, although this method takes a few hours of slow bubbling to precipitate all of the dissolved gold.

Frank

Frank Roberts
KLRU-TV - Austin, Texas

Hi,

Langford and Parker, Analysis of Electroplating and Related Solutions indicates that sulfuric acid, with careful additions of hydrogen peroxide, will precipitate the gold from a cyanide solution. Other reducing agents listed are ferrous sulphate, oxalic acid, and potassium nitrite. Regards, Tom

Tom Pullizzi Platronica.com Falls Township, PA

 

How to make luster gold powder form real gold?

Mahendra Bir Singh
Bir Singh Auto Center - Kathmandu, Nepal

What doing if the gold came from electronic scrap and the concentration is to poor (from contact ) ? Did some first thing to do ?

Eric Tordeur
- Belgium

Thank you Frank Roberts.

Your method Recovering Gold from Aqua Regia worked well for me. However, I found at first with high quantity of copper from circuit boards I had problems with precipitation. I solved this problem by first dissolving most of the copper etc with nitric acid and pouring it off before Aqua Regia process. I now have this nitric copper etc solution. Can you suggest the best way to recover silver and copper from this?

Tony Plummer
- Harare, Zimbabwe

Dear Keith .W
I read your suggestion for determining the gravimatically in reply to JD. You have not mentioned that, what will have happened to silver chloride which get settled in gold chloride solution (aqua regia solution)?
Does silver chloride remain as it is after boiling the aqua regia? Kindly reply for this.

Rajesh
- Vellore, TamilNadu, India

Dear Sir,

I want to refine gold from Scrape and I use aqua regia reaction to desolved gold. now for preciptation I use SODIUM METABISULFITE solution (300 Grams per Liter) and added as 1 Gram Gold = 3 ml of SODIUM METABISULFITE solution.

I find some amount of preciptation but it desolved again with the solution.

How I can recover my gold from this solution?

MAX PANCHAL
GOLD SMITH - INDIA

Talk about precipitating gold from the solution but nowhere I read the easy way to determine if any or all gold is precipitated out. I use a little tin ( from leadfree tin solder for water pipes) dissolved in Hydrocloric acid. Put a drop of this on absorbent paper and then add a drop of the solution where the gold is from precipitated. If any gold remains in the solution the drop turns black. When all gold is precipitated the color stays neutral. This detects 4 parts in a million.

Alexander Baldal
- Philipsburg, Sint Maarten Netherlanfds Antilles

I AM IN THE MIDDLE OF CONFUSION WHEN I MAKE MIXTURE OF AQUA REGIA, I REALIZE THAT OUR NITRIC ACID SUPPLIER SUPPLY ME WITH ONLY 68% CONCENTRATED NITRIC ACID SO WHAT WILL BE THE RATIO OF MIXING IT WITH hcl I WONDER AND I HAVE BEEN GETTING PINK COLOUR MIXTURE WHEN I MIX THE NITRIC AND HCL WITH RATIO OF 3 TO 1 (3 HCL TO 1 NITRIC)NEEDED ADVISE AND ALSO THE WAY TO NEUTRALIZE NITRIC BEFORE EXTRACTING THEM,
MY VERY MUCH THANK TO WHO EVER CAB HELP AND ADVISE ME ON THIS MATTER
JIMI LIM

Jimi Lim
Singapore

Hi I am working in gold refining plant.we dissolve 12.5 kilos of gold in approx.50lt. aqua regia.after filtration of silverchloride we bring the ph to 7 to destroy the excess nitric acid by adding soda ash solution.we precipitate the gold using sodium metabisulphite solution.can anybody give me full chemical reaction taking place in the whole process. thanks,

Vijay Pillai
chemist - UAE

Hai,
I'm from Indonesia need help.
Note : Nitric acid 68%, HCL 30% (sold in the store)
I use Aqua regia (450 ml. HCL, 150 ml. Nitric Acid, 200 grams scraps of metals).
I wait one night (the color of solution is yellow-brown), then tomorrow I added 450 grams sodium bicarbonate and some of lime to neutralize the acid. The reaction is :
Bottom precipitate is metals, middle precipitate is orange-brown mud.

I pour off the container with filter. I get the clear solution. The orange-brown mud in the filter when dried is change to brown. The metals color is cloudy brown-grey.

1. How to neutralize the solution (HCL, Nitric Acid, Sodium Bicarbonate) Ph 1.
2. How to find out the brown mud if is indicate of gold.
3. How to cleans the metals cloudy brown-grey.

I'm very thank's for any response because I'm really confusing.

Boboy

Boy Raynolds
hobbyist - Jakarta, Indonesia

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