The Inquartation method is one of the most common methods of refining low and medium karat gold jewelry (9K-18K) and can also serve the first step in refining sterling silver.
Inquarting usually doesn’t apply to high karat gold jewelry (21K-24K) since it is low in silver content. A low silver content (less then 8%) will not create passivation of the metal and therefore can be dissolved directly with Aqua Regia. High silver content however, will form a layer of insoluble silver chloride protecting the gold from Aqua Regia.
Inquarting is needed most when refining yellow or green gold jewelry due to their high silver content. Although white and red gold jewelry contains little to no silver, they should still be inquarted due to the high base metal content.
General consensus is that dissolving base metals with gold will add steps to the refining process and consume more chemicals.
When done properly, one can achieve great results and produce high purity gold and silver of up to 99.95%
In this article there are embedded Youtube videos of the entire process, from scrap Gold 14K jewelry to pure 24K Gold. The videos are very important in understanding the process.
Abstract The process
1 - Inquarting low karat gold jewelry with silver (base metal).
2 - Silver digestion with nitric acid.
3 - Gold digestion with Aqua Regia.
4 - Gold precipitation with Sodium Metabisulfite.
5 - Washing and drying the gold.
6 - Melting of pure 24K gold.
Equipment needed for the process
- SAFETY EQUIPMENT: Goggles & Gloves - Stays on you throughout the whole process !!
- 3 Chemical cups
- Waste solution container
- Glass stirring rod
- Filter papers
- Melting dish
- Melting dish holder
- Blow torch
- Carbon stirring rod - optional
- Deep stainless steel bowl
- Stainless steel tweezers
- Hot plate
- Spray bottle (with tap water)
Chemicals needed for the process
- 32% Hydrochloric Acid (HCL)
- 70% Nitric Acid (HNO3)
- Sulfuric Acid (H2SO4) – optional
- Hydrogen Peroxide (H2O2) - optional
- Sodium Metabisulfite (Powder)
- Distilled water
- Stannous Chloride test kit
1 - Inquarting low karat gold jewelry with silver
In the Inquarting process we reduce the gold content in an alloy to 25% (i.e. gold content becomes one quarter of the alloy). We do this by melting the scrap gold jewelry with the appropriate amount of base metal forming a 25% gold content alloy that is equal to 6 karats (6k).
Base metal for Inquartation can be pure silver, sterling silver and even scrap copper.
It is highly recommended to use silver as the base metal because it will consume less nitric acid than copper.
First, you need to know the pure gold content in your scrap jewelry, which can be calculated Here.
Example 1: Lot of 17.5 grams of scrap gold 14K contains 10.2 grams pure gold.
The calculation: 17.5 x 0.583 = 10.2 , 10.2 x 4 = 40.8 , 40.8 – 17.5 = 23.3
Therefore, 23.3 grams of base metal needs to be added to reduce gold content to 25%.
Example 2: Lot of 33.1 grams of scrap gold 10K contains 13.8 grams pure gold.
The calculation: 33.1 x 0.416 = 13.8 , 13.8 x 4 = 55.2 , 55.2 – 33.1 = 22.1
Therefore, 22.1 grams of base metal needs to be added to reduce gold content to 25%.
A - Once the metal's proportions are calculated, place the Scrap gold and base metals in a melting dish that has properly glazed with borax (note: that some types of melting dishes need proper seasoning before the actual melting).
B - Melt the scrap gold and base metal(s) together. A small lot can be stirred well and form a consistent alloy simply by stirring the melting dish containing the molten metals. Larger lot with deep molten metal pools should be stirred with a carbon rod, each stir no longer than one second.
It is very important that the alloy is consistent, i.e. that gold/base metal proportion is equal throughout the alloy.
Tip: Preheat the carbon rod before introducing it to the molten metal. This will prevent molten metal from clinging to the rod.
C - Pour the molten metal slowly, shot by shot in to a deep stainless steel bowl filled with cold water. The "shot" should look like metallic corn flakes. Flakes have a large surface area for the nitric acid to work on.
2 – Silver (base metal) digestion with nitric acid
Lowering gold content to 25% and the balance (75%) being silver/copper/sterling silver opens the possibility of dissolving all base metals and leaving behind only the gold and traces of un-dissolved base metals. Palladium, if present will dissolve by the Nitric Acid as well.
The silver leaching process we present is sometimes called the "50/50" method, since it requires the dilution of 70% nitric acid with an equal amount of distilled water. 3% H2O2 can also be used for dilution.
In theory, less than 2 ml of 70% nitric acid is capable of dissolving 1 gram of pure silver. In reality, you can't determine how much silver, copper and other base metals an alloy is comprised of in an economical manner. In fact, some karat jewelry actually contains so little silver that it can be digested directly with Aqua Regia.
The key is not to use an excess of nitric. Therefore, a rule of thumb is to use 2.5ml of 70% nitric acid for each gram of base metal that needs to be dissolved. This is good middle ground in that if more nitric is needed you just add it and if not, the excess is not too great.
You should also know that any Platinum that might be present in the starting materials and had been alloyed with silver will partially dissolved as well in the nitric acid digestion.
Rhodium (from plating of white gold) will be left with the solids.
A - Place the "shots" in a chemical cup and calculate the needed volume of nitric acid.
Before adding the nitric acid, add an equal amount of distilled water or H2O2 and then start adding the nitric acid in small increments until you reach the respective amount calculated earlier. Ex: For 50ml of calculated nitric you will need 50ml of distilled water. It is a 1:1 ratio.
H2O2 is used to help suppress some of the dangerous NOx fumes.
The liquid level should not exceed more than 1/3 of the chemical cup’s total capacity to prevent spills and foam over.
B - Place the cup on hotplate, turn the heat up to medium low and allow it to reach a gentle boil and maintain it for one hour. Cover the cup with a piece of wet cloth or a watch glass.
The scary RED FUMES are a combination nitrogen oxide and nitrogen dioxide gases. These gases can prove deadly with careless and reckless treatment. Boil under a fume hood or outdoors.
C - When done boiling, allow the solution to settle for a couple of hours.
After settling, filter the solution to another chemical cup to be treated later for its Silver content. Try not to agitate the solids (the gold) too much as you don't want it scattered in the filter. You want the gold to remain in the starting cup as much as possible so decanting or siphoning the liquid is recommended.
In the show case video we left the solution over night to settle in the cold outdoor temperatures, which is why you can see silver nitrate crystal growth.This is not a problem because silver nitrate is soluble in water.
D - Wash the solids with plenty of distilled water at least 5 times. The third wash should be brought to a gentle boil for 5 minutes followed by the next two water washes.
In the video you can see what happens later if the Silver Nitrate is not washed properly. Decant or siphon the water washes to the same cup with the main Silver/Copper Nitrate solution (through the filter of course).
Wash the filter clean with distilled water when done, draining all traces of silver nitrate with the rest of the solution.
Silver/Copper Nitrate solution should not be discarded. Keep it for later recovery and refining of Silver metal.
3 - Gold digestion with Aqua Regia The now cleaned gold is ready to be dissolved with Aqua Regia.
The gold may still have traces of different contaminants (silver chloride and flux leftover from the melting process) that may have passed through the nitric leach and water wash. This is the reason that the gold should be dissolved and filtered.
Aqua Regia (AR) is the combination of Hydrochloric Acid (HCL) with Nitric Acid (HNO3). Together they have the capability to dissolve gold and platinum.
Some AR recipes calls for 4:1 or 3:1 HCL:HNO3 and then neutralize the excess nitric acid (Dnoxx).
With our method you won't need any neutralization, simply because you will not use excess nitric acid.
Calculating 32% HCL volume needed: calculate how much pure gold is in the cup in grams. Multiply by 4, and that is the volume in ml of HCL that should be added.
Example: Lot of 17.5 grams of scrap gold 14K contains 10.2 grams pure gold.
10.2 grams x 4 = 40.8 ml. Therefore, 41ml (round up) of 32% HCL in needed
Note: excess HCL will not harm the process by any means.
Calculating 70% Nitric Acid volume needed: Professional literature suggests that AR solution has a potential of dissolving 1 gram of gold per each 0.95 ml of 70% Nitric Acid added.
You can safely round this number up to 1 ml of nitric acid per each gram of gold you wish to dissolve.
Example: 10.2 grams of gold will need 10.2 ml of 70% nitric acid. it's better to round it down to 10 ml of Nitric Acid.
A – Add calculated amount of 32% Hydrochloric Acid to the gold.
Any white precipitation is due to Silver Nitrate traces. This will not affect the process from carrying on.
B – Start adding the Nitric Acid in small increments of 1-3 ml each time. After each addition wait one minute to see if reaction begins.
Once reaction begins (bubbling and fizzing), stop nitric additions and place the cup on a hotplate at medium heat. Don't forget to cover the cup with a wet cloth or watch glass.
Let the reaction work. When it ceases add one more Nitric increment and observe what happens. If nothing happens it means that all of the gold had been dissolved and you should remove from heat. If fizzing and bubbling continues, let it boil again until the reaction ceases and repeat.
You rarely need the full amount of calculated Nitric Acid to complete the gold dissolution. In any case, do not add more then your calculated amount of Nitric Acid. Any solids that remain and are not reacting to hot AR are not gold and may very well be flux left over or PGM (Platinum Group Metals).
Note: Fluid level should not exceed more than 1/3 of the chemical cup total capacity.
C - Once you believe all of the gold is dissolved (this is where experience comes into play), remove from hotplate and let the solution cool down to room temperature.
D – Optional - Once the gold solution (HAuCl4) is at room temperature, add just a few drops of Sulfuric Acid. Any Lead (Pb) that may have been in the jewelry will precipitate as white Lead Sulfate (PbSO4) crystals. Concentrated Sulfuric Acid (98%) will react exothermically with the gold solution so make sure you add it slowly to prevent splashes.
E – Dilute gold solution 4 times over with tap water and let it settle for 10 minutes.
The dilution will push out (precipitate) any Silver Chloride that might have dissolved in the Aqua Regia in the form of super fine white powder, almost unseen.
F - Prepare a new filter (do not use the same one from step 2) and filter the gold solution to a new chemical cup.
The new cup should be squeaky clean with no scratches on the bottom.
Pour the liquids first and let it drain. Then pour the left over solids to the filter at the end so you don't clog the filter and slow down the filtration. Using a Vacuum filtration system (Amazon link) will reduce filtration time to only a couple of minutes.
Make sure you wash the original cup with a lot of water (tap) so that all of the gold solution will go to the filter.
When done filtering, wash the filter and solids in it with plenty of water to make sure all of the gold is drained down to the new chemical cup. Wash it with a squirt bottle until no more yellow color is on the filter.
Solution should be clear with no solids on the bottom and have rich golden color.
Video tutorial Part 1:
Helping with this article: Harold_V, goldenchild, Barren Realms 007 and gold4mike All members at the Gold Refining Forum