Wednesday, March 27, 2013

Refining to 99.95% Gold

by Steve Looser

Before using this information and education site you should agree that you alone are responsible for your own health and safety and that you will not hold the author of this site or any contributing authors and publishers responsible for any of your decisions or actions resulting from any information contained herein. You agree to do your research and accept full responsibility for your actions. If you agree to the statement above then continue reading and enjoy, if not we advise you to leave now or continue reading for learning only. I strongly recommend that you take a laboratory safety study course or lab safety class before proceeding with processes described in this book and do not work alone. I also suggest that you start with simple and basic refining methods before you attempt some of the more complicated ones. Also be easy on yourself, perfecting your skill comes with practice.

Preparing Scrap Gold for Refining
When precious metal bearing material is received from a customer it is weighed and copies of the gross weight received with the customer name and date, an order number, processing instructions and lot number are produced. Samples are taken and the material is sealed in a drum with a metal seal and identification. Processes for sampling vary depending on the material. Several samples are taken from the material. Samples are given to the customer, some are sent out for an independent assay analysis and some for your in house assay. In the case of raw metal it is melted homogenized and poured into and ingot so drilling's can be taken as samples. If it is solutions you will need to draw liquid samples. If it is sweeps that must be incinerated it will have to go through a lengthy process that is described below. Before dissolving gold in aqua regia, (1 part nitric acid & 3 parts hydrochloric acid) make sure it is free from waxes, oils plastics, tar, any organic materials, sulfides, chemicals, etc. "Sweeps" material is often full of crushable materials like old broken crucibles, clay and glass as well as combustible materials like polishing dust, paper, waxes, common garbage and sometimes even food. Incinerating is often done first before any of the separating of the materials mentioned above.
The incinerator is designed to allow air to pass over the material so as to completely oxidize all of the organic material and convert it to a carbon less ash. During the incineration the material is turned mechanically in some types of barrel incinerators and in others the incinerator doors are opened and the material is raked to expose it to oxygen. These incinerators are exhausted into smoke and dust collection systems. Back in the day they called them Bag Houses. After it has finished roasting to ash it is allowed to cool to room temperature and then it can be crushed into a fine powder in a crusher or ball mill. After removal of the material from the ball mill or other type of crusher a magnet is then used to remove steel and other magnetic materials. It is then screened to remove certain materials that will not crush. After screening to a uniform size it can be thoroughly mixed so that multiple samples can be taken for assays. Analysis on this dust is normally done using the fire assay method.
2 Preparing scrap gold for refining
Silver will not dissolve in aqua regia except in very small quantities as a dissolved chloride. When silver is introduced into aqua regia it quickly forms a hard impermeable layer of silver chloride, (AgCl) on the surface and stops reacting with the acid. The same thing occurs when trying to dissolve gold if the silver percentage in the gold reaches near 15% or more. It's a common procedure to reduce the silver percentage in your gold alloy to 12% before refining by melting it and stirring copper into the melt. Old school called this "Inquarting" because they added 4 times the weight to the gold using copper or silver. Inquarting silver was often used making the gold 25% or less of the alloy and then it was dissolved in nitric acid to leave the pure gold remaining as a brown residue in the bottom of the beaker. But the term has evolved to mean adding copper or any base metal to gold scrap to make the gold one fourth of the total weight so that the silver will sluff off during the digestion with aqua regia.
This Video Below Is One Of A Series Of Fantastic Training Videos That I Highly Recommend

Gold Refining Complete Process for the Home Refiner  Subscribe to Streetips YouTube Channel for More Videos

In gold refining you will find silver to be your enemy but save your silver sludges for the end of the year and process them near tax time because they will be your friend. When tax time rolls around each year for large jewelry manufacturers and other precious metal industries the settlement for their floor sweeps and polishing dusts can often pay their taxes. The same goes for a refinery with their silver chloride sludges and their slags from smelting

3 Preparing scrap gold for refining

The gold is melted in zirconia crucible using a hydrogen-oxygen torch. The crucible must be capable of withstanding the high temperature produced by the torch without cracking. Wesgo Platinum Crucibles are what we used to use at The Phoenix Smelting and Refining Co.
If the gold is clean and free of foreign material such as gold received as pendants, rings, and chains, it can be added directly into the aqua regia for dissolving but usually it is melted and poured into a tank of cold water to create shot. It's good to do this for a couple of reasons. One is that it gives you a uniform and homogeneous alloy that will dissolve at an even rate. Two is that you can introduce copper and zinc into the melt to effectively lower the silver content in the alloy. If the content of silver is too high in the alloy it forms a barrier to the acid and slows or stops the acid from dissolving the gold. When melting gold scrap and alloying gold a carbon rod is used to stir the molten metal.
*Also remember to always preheat your carbon stir rod before sticking it in molten metal unless you want instant dental caps and a gold faceplate. Graphite always seems to attract moisture out of the air and if you don't dry them prior to placing them into a melt they will cause the metal to explode.
I did it once with a long rod in a blast furnace and ate silver. It fused silver in my clothing and shoes and there were pieces of silver stuck in the cracks between my teeth. I'm Lucky I was wearing safety glasses. *Always preheat your carbon rod before touching it to molten metal. Stirring of the molten metal must be done very thoroughly using circular motions and contacting all areas for the molten metal for several minutes.

4 Preparing scrap gold for refining
Borax or boric acid is used as flux. The addition of a light sprinkle of flux to the molten metal cleans the surface of the metal and helps remove oxides. The molten flux forms a glass like substance called slag and will contain impurities as well as some of the original melt in the form of tiny or even microscopic particles. This is why you never dispose of your slag. If the metal is to be poured into water, any excess slag which might pour out with the metal into the water should be removed with a carbon stir rod. The rod is swept across the surface of the molten metal and all around the inner edges of the crucible with a twirling motion which creates a slag ball on the tip. Slag removal is more important when pouring shot than when pouring bar although it's always a good thing to do. If you are pouring scrap into bars it is not necessary to remove slag before pouring as it is easily tapped off after the bar cools, whereas with shot it would be very difficult to remove slag.

5 Preparing scrap gold for refining and pouring shot
Your shot tank should be filled with cold water to 1.5 inches from the top of the tank. When the metal is molted the crucible is held with crucible tongs balancing it on the edge of the shot tank and tilted very slowly. The metal should have poured slow enough to cause the stream of molted metal to break into individual droplets, breaking just below the water line. If the metal is too cold it will solidify before the pouring is completed. If the metal is too hot or poured too fast it can explode into powder and dust in the water. It's helpful to have an assistant play a torch flame over the molten metal while pouring. a hydrogen flame with the oxygen turned on low to give a feather like reducing flame works well for this.
Once the pouring is completed the water is emptied from the tank and the tank is tilted over a clean container and the shot is washed out of the tank by spraying with water. If the shot is to be weighed it must be dried thoroughly. You can do this by putting it in ceramic dishes and placing it in a drying kiln at a low temperature that will not discolor the shot. You will only want to heat it just over the boiling point of water around 250°C will do but make sure all the water is eliminated.
You don't want to calculate an alloy charge with wet metal.

6 Dissolving Gold
Before you begin handling the acids and chemicals be sure to put on your PPE, personal protective equipment such as a rubber apron, rubber boots, a pair of splash proof goggles or a face shield, and rubber gloves. Also you should have a 5 gallon pail of neutralizing material handy that you pre make by mixing sand with sodium carbonate. If you have a spill you cover it with this. You should have a good water supply and plenty of paper towels available.
After the gold bench filings and scrap are prepared for refining the metal is placed in a heavy beaker on a hot plate. Hydrochloric acid is added to the beaker amounting to 4 to 5 times the weight of the gold. Nitric acid is added very slowly to the hydrochloric acid amounting to 1.5 times the weight of the gold. This acid solution is Aqua Regia and dissolves the gold. The reaction of the dissolving gold will generate heat and it is only necessary to turn on the hot plate at the lowest setting after the reaction has slowed down an hour after starting. It is important to do all of your work with acids under a good fume hood or ventilation system working at all times with backup electrical power in case of a power outage. At times the gold scrap or filings can react violently with the acid and generate too many fumes. This is why you should only add half of the nitric acid in the beginning and wait for the initial reaction to subside before adding the remainder. Aqua regia is 3 parts HCL and 1 part HN03. An example would be 600mls HCL and 200mls HN03 but my suggestion is to place your 600ml HCL in the beaker first then carefully and slowly add your gold scrap in shot or sheet form to the acid and then only add 100ml of the HNO3 VERY SLOWLY to the HCL and let it react until there is no more reaction before adding the remaining 100mls. You must always be cautious of a possible boil over of your acids causing a potential loss or injury as well as damage to your equipment. The amount of acid is added in small portions to control the rate of reaction is based on the size of the batch being dissolved and the capacity of the acid scrubber equipment to handle the amount of gases being generated.

The average small size lot we processed was between 5 and 10 troy ounces and took up to 3 hours to dissolve. The scrap needed occasional stirring and the use of a plastic stir rod to break up the silver chlorides which held to the metal. Most gold scrap has silver as part of the alloy and this silver converts to silver chloride when under attack by aqua regia forming a cake or crust like layer on the gold. Before you begin to dissolve your scrap you can melt zinc and copper into your gold to lower the silver content ratio in your scrap then pour it into shot. Then when you dissolve your scrap the silver chloride will easily fall off the gold and settle to the bottom of the beaker. You want the silver to be no more than 10-12% of the alloy you are going to dissolve. By having your silver content in the gold alloy low enough the silver will fall away from the gold as it is dissolving and form a powder in the bottom of the beaker. It's a good idea to have a stirrer hotplate to speed the dissolving process but not necessary. A large lot is sometimes processed overnight. When all of the gold is dissolved you should feel no pieces or particles with your stir rod and there will be no more bubbling and fuming.
Now it is time to let the solution cool and pour it off into a plastic tank to chill by adding cold distilled or deionized water to double its volume. Add about 20 milliliters of sulphuric acid to the cooling aqua aqua regia to precipitate lead. After you double the volume you then set up your filter funnel with its adapter and filtrate bottle. Note* you can pay a lot of money for laboratory vacuum flasks or you can use a 6 gallon water bottle, the same kind home brewers use for beer making.
The filtration system is made up of a 240cm Buchner porcelain filter funnel and a 5 gallon glass filter bottle, a rubber filter adapter to fit the funnel to the bottle or flask vacuum tight and a Tygon plastic hose which connects from the filter flask to a vacuum pump. All of these items can be purchased at a laboratory supply company. The adaptors and filter flasks can be found at Amazon and large filter flasks up to 20 liters can be very expensive at laboratory supply companies so we just used 5 gallon water bottles. The adaptors fit into the neck of the flask or bottle and the funnel fits through the adaptor into the flask. A vacuum is drawn and a filter paper is laid down over perforations in the funnels plate wetting it with a stream of water from a wash bottle while laying it down. Then the gold solution is slowly poured into the funnel filling it close to the top. The first 500mls can be emptied from the flask and re-filtered to help the paper be more retentive by clogging the pores in the paper. This helps to make a clearer filtrate. Filter all of the gold solution this way and wash all of the undissolved silver chloride and other insoluble material onto the filter paper also. When all of the solution has passed through the filter, wash the remaining insolubles by pouring a dilute 1% nitric acid solution over it and allowing the vacuum to pull it through the paper. Repeat this about 5 times until the solution exiting the stem of the funnel is clear. That is the way you filter rinse. The small amount of nitric acid is necessary to prevent the silver chloride from dispersing, (called peptization) into fine particles that will pass through the filter paper and contaminating the filtrate.


After filtering turn off the vacuum and carefully remove the funnel. Then remove the paper from the funnel and place it in a bucket to save along with other chlorides for processing at a later date when the value makes it profitable to spend time on it. Also take a wash bottle and wash away the chlorides adhering to the funnel into the same bucket. Then take the funnel to a wash basin and wash it thoroughly with a sponge and powdered cleanser before placing it in a safe place.

Note* I want to mention here the "Old School Refining Method

After dissolving gold scrap and filtering out the chlorides we used to evaporate the filtrate to a crystal and rehydrate with HCL repeating this 3 or more times to a crystal until no brown nitric oxide fumes appeared. We used Coors Evaporation Dishes:

Doing this eliminated the use of urea to sequester the HNO3 and allowed for easier precipitation of the gold with Ferrous Sulfate without the interference of the nitric acid.
You then pour the filtrate into a clean container such as a 5 gallon plastic bucket and dilute it to double the volume with water. Now you are ready to add urea to chelate the nitric acid. When you are adding and stirring in the urea you will get a fizzing reaction. You will want to continue adding urea until the fizzing stops or at least diminishes. When the fizzing slows or stops it's time to precipitate your gold using one of the following precipitation reagents,
Ferrous Sulfate, (my favorite), Sulfur Dioxide Gas, Sodium Sulfite or Sodium Meta Bisulfite. You will need to add about 1.5 times the weight of the gold to precipitate all of the gold. If the filtrate foams while adding the reagent you should dilute it more and dissolve the reagent in water before adding it to the filtrate. The gold will begin to precipitate as a light color, then turn dark brown.
When the precipitate turns dark brown the precipitation is complete but it doesn't have to turn dark to be complete. When you think the precipitation is complete let it stand about 30 minutes then filter it through a Whatman # 2, 40 or 52 filter paper. You can test the clear filtrate to see if there is any gold left in it with a solution of stannous chloride by placing a couple of drops of the filtrate on a spot plate and taking an eye dropper and dropping 1 drop of stannous chloride into the filtrate. It it turns purple the precipitation is complete. Another way is to take a 100ml sample of your filtrate and add more sodium sulfite to it letting it set overnight.

Filter rinsing greatly speeds the rinsing process over trying to rinse and let settle before decanting or pumping the solution off of the settled contents. You just add your precipitate into the Buchner funnel filter and begin pulling water and rinse solutions through it letting the vacuum speed the process. It can literally save you hours or even days on big projects.
Filter rinsing removes trace impurities which may still be in your gold. Pass all of the solution through the filter paper and wash all the precipitate onto the paper. Some of the precipitate will stick to the container and can be removed with a nylon scouring pad which can be easily rinsed clean using a wash bottle. When all to the precipitate is on the paper a step by step rinsing and washing procedure is done as follows.

1. Filter wash and rinse the precipitate with distilled water allowing the vacuum to draw the water through. Repeat this a minimum of 5 times.

2. If a Whatman #52 glazed hardened filter paper is used, you can remove the gold from the paper and place it in a beaker. Then cover the precipitate with hot 1:1 nitric acid and water. Stir it occasionally and allow it to stand for at least 5 to 10 minutes.

3. Filter rinse the precipitate 5 more times with distilled or deionized water.

4. Cut the vacuum and cover precipitate with hot 1:1 HCL / water and let stand for 5 minutes. Then turn on vacuum and pull the solution through filter paper.

5. Filter rinse the precipitate 5 times with hot distilled water containing approximately10 to 30 ml per liter HCL.

6. Allow the vacuum to continue running to dry or semi- dry the precipitate.
Then sprinkle a very small covering of borax or boric acid on the surface of the precipitate. Gather up the paper and precipitate and place it in a zirconium oxide crucible for melting. The crucible must be new or one that has only been used to melt 999+ fine gold. Take a camel hair brush and brush all of the precipitate which adheres to the edge of the porcelain funnel into the crucible

7. Melt the gold with a hydrogen-oxygen torch. Bring the metal to a white glowingtemperature and turn off the hydrogen, blowing pure oxygen on the metal until it cools to a solid. This will sometimes produce discolorations and what could be oxides of trace metal impurities like copper. Don't worry and think that you have failed to produce pure gold. I have melted the most popular 9999 fine gold coins down in new crucibles and seen this occur. Nevertheless repeat this melting and blowing pure oxygen over the molten metal until all of this discoloration no longer appears. If you have a clean crucible and done everything correctly up to this point you shouldn't have any problem doing this final cleaning of your gold. If it will not clean up then assay it and it will probably be at least 999 Fine. Like I said I have melted the most popular 9999 Fine

Gold coins and still found this. Hey that tells you that I get my stuff pretty darn clean don't it. I want to mention also that if you did your last filtration's of your precipitate on a glazed hardened filter paper then you can brush your precipitate off of it into your crucible and re-use the paper for more filtration's instead of burning it. Here is another method you might prefer For these procedures you may want to use reagent grade acids for your final acid washes and rinses. I am not sure if it really helps to use reagent rather than industrial grade or technical grade chemicals. You will spend a whole lot less money if you stick with industrial grade. I don't think you want to get too cheap and use farm grade though. If you have the time and money to go through a lot of testing and lab work you might find that you can save a lot of money and use the cheapest chemicals for certain applications. I used to go to a company just outside of Modesto, California by the name of Snowden Chemical. They supplied the wine industry in that area with bottled SO2, (pressurized sulfur dioxide gas) which precipitates gold nicely after chelating the nitric acid with Urea prills, Urea can be farm grade. Sodium Meta Bisulfite or Sodium Sulfite is producing SO2 as soon as it enters aqua regia so any of these will work well.

1• The gold is dissolved in aqua regia then diluted in cold water and a small amount of sulfuric acid is added to precipitate lead. Then it is filtered.

2• Potassium nitrite is added to the filtrate solution using about 2 to 5 grams per ounce of the gold you expect to recover. This is to chelate trace platinum group metals.

3• The pH of the solution is adjusted to 1.5 using Potassium Hydroxide.

4• Then the gold is precipitated using sodium or potassium sulphite, sodium metabisulphite or sulfur dioxide gas....By the way, it is not okay to breath sulfur dioxide because it will eventually cause you to have allergies to sulphites and then you're really screwed because you won't be able to enjoy wine or dried fruit again without getting a sore throat from the sulphites. SO2 and or sodium sulfite is used in the wine industry to stop the fermentation before bottling and on dried fruit to stop bacteria growth during the drying process or to speed the drying.

5• The precipitate is transferred to the filter and filter rinsed as before using distilled or deionized water. Wash it thoroughly on the paper covering the gold precipitate and letting the vacuum pull the water through it five times or more. Use a Whatman® #40 or #52 filter paper that you can get at a laboratory supply. If you don't have a vacuum filter funnel arrangement then you will have to be much more patient and double up on the rinsing.

6• Remove the precipitate from the paper and place it in a beaker on a stirrer hotplate and covered with 1:1 HNO3 / distilled water and allowed to simmer just below boiling on a low heat and stir for about 1 hour.

7• Then the precipitate is filter rinsed with distilled water 10 times. You should shop around to find the lower cost filter papers. I always used Whatman to be safe but I see people using coffee filters and claiming to get a result of 999 gold. I really didn't know at the time and just used Whatman papers to be on the safe side but  pick the least expensive filter papers on the market that get the job done and won't contaminate.

8• Place the precipitate back into a beaker on a stirrer hotplate and cover with 50/50, (1:1) solution of ammonium hydroxide and distilled water and add 2.5 grams per liter of EDTA, (ethylenediamine tetra acetic acid) for 1 hour cold and stir. It's important that you don't breath that ammonia hydroxide because it causes chronic damage to the thyroid glands with prolonged exposure. Ever wonder why some housekeepers have a weight problem? It's their exposure to ammonia cleaners every day that's messing up their thyroids. Sorry housekeepers, you need to be careful also.

9• Filter rinse the precipitate 10 times more with distilled water.

10• Return the precipitate to a beaker and this time cover with a 1:1 solution of HCL/distilled water and let it leach out trace contaminants at near boiling for another hour or so.

11• Then the precipitate is filter rinsed 10 more times with hot deionized or distilled water containing approximately 10 ml per liter HCL. Now I hope that you realize you've messed around with this gold long enough and you know deep down inside yourself that it's pure! You have done all that you can other than repeating the process. If you want to produce 9999 or 99999 you have to employ the Wohlwill process
 of electroplating the gold
through a gold chloride and hydrochloric acid bath.
12• Allow it to dry on the paper and sprinkle a little boric acid on it then gather it into you clean crucible and melt it. After pouring your bar wash it in hydrochloric acid, rinse it, dry it, weigh it, and stamp it for identification. Buy yourself a little numbers and letters stamp set at the hardware store if you don't have one. The gold that you pour should assay at 9995+Fine.
If you have an electric kiln you may want to gather the gold withthe filter paper and place it in a ceramic dish to dry it and burn off the paper to ash before you melt it. You will find that some crucibles work much better than others. I had good results using a quartz silica crucible for small pours because they didn't crack as often as some other types. But zirconia cruciblesare by far the best type you can buy in my opinion for the big melts. What you really are looking to do here is build yourself a good reputation as a refiner who knows his shit so don't ever try to cut corners and keep your end product consistent. Then after you have refined some jewelers scrap and sold a few of your bars you will start to get word of mouth advertising and people will come to you to refine their gold or buy yours. Take you time, make sure everything is clean practice safety always and do it right. It will pay. Refine 100 Kg Gold to 9995 Fine Daily. Visit: GOLD AND SILVER REFINING SYSTEMS
12• Allow it to dry on the paper and sprinkle a little boric acid on it then gather it into you clean crucible and melt it. After pouring your bar wash it in hydrochloric acid, rinse it, dry it, weigh it, and stamp it for identification. Buy yourself a little numbers and letters stamp set at the hardware store if you don't have one. The gold that you pour should assay at 9995+Fine.

If you have an electric kiln you may want to gather the gold withthe filter paper and place it in a ceramic dish to dry it and burn off the paper to ash before you melt it. You will find that some crucibles work much better than others. I had good results using a quartz silica crucible for small pours because they didn't crack as often as some other types. But zirconia crucibles
are by far the best type you can buy in my opinion for the big melts. What you really are looking to do here is build yourself a good reputation as a refiner who knows his shit so don't ever try to cut corners and keep your end product consistent. Then after you have refined some jewelers scrap and sold a few of your bars you will start to get word of mouth advertising and people will come to you to refine their gold or buy yours. Take you time, make sure everything is clean practice safety always and do it right. It will pay. Refine 100 Kg Gold to 9995 Fine Daily. Visit: GOLD AND SILVER REFINING SYSTEMS
12-13 Gold Refining to 9995+ Fine Gold

The method below was originally posted on pg.48 and I moved it to this more appropriate post.

This method produced 999.54 fine gold Dissolve scrap in aqua regia and precipitate with sodium sulfite. Filter wash the precipitate with deionized or distilled water by pouring the water over the precipitate in the funnel and letting the water pull through completely before adding more water. Repeat this about 20 times. Next pour hot nitric acid 1: 3 water over the precipitate and observe it lighten in color. Next repeat rinse procedure with water as stated above. Then wash with weak HCL and again as indicated above with water. Gather to dry in kiln and burn off filter paper at 1100° F and then place in a clean crucible and melt with hydrogen oxygen torch. Play the hydrogen over the bar as it is being poured and cooling. Drop a pinch of ammonium chloride on the bar immediately after pouring while still glowing red. To further complicate things if you are totally anal you can use the process below. 100 ounce Refining procedure Dissolve scrap with 1 gallon nitric acid and 3 gallons hydrochloric acid. After it's finished dissolving carefully add urea until fuming subsides. Filter on a #40 or #52 Whatman filter paper with pulp. Stir in 100 grams ammonium sulfate and double the

volume of the filtrate with deionized water. Let stand for 2 hours then filter again with a #40 Whatman filter paper and pulp. Rinse paper once with 1% nitric acid. Add more urea to the filtrate. Very carefully stir in a solution of 1 liter deionized water and 200 grams sodium nitrite for complexing platinum groups. Then carefully add 1 oz. ammonium sulfite for every ounce of 99.9% gold you will precipitate. Let it settle for 2 hours. Then filter the gold with a #52 Whatman filter paper. Rinse with deionized water. Dry in the filter with the vacuum pulling all the water out until

dry. Take the gold off the filter and place it in a glass beaker and cover it with a 1:1 HCL and deionized water. Stir for 3 hours on stirrer hot plate or let stand for 24 hours. Pour off and rinse thoroughly with deionized water ,( check pH) then cover with a 1:1 solution of ammonium hydroxide, (see footnote) and deionized water with 2-3 grams of EDTA. Stir for 3 hours or let stand for 24 hours. Gather the precipitate into the filter funnel and vacuum filter dry it then gather it and place in porcelain drying dish in an electric kiln. Set the temperature and 1100° F and roast it until all the filter paper is disintegrated. Remove the gold sponge and gather into a Zirconium crucible. Melt with a hydrogen oxygen torch using a rosebud torch tip. Pour into a preheated cast iron mold or graphite mold and sprinkle a pinch of ammonium chloride on the red hot bar immediately after pouring. Your bar should receive an assay result of .999 to .999.5 Fine Gold.

If either of these methods doesn't get you what you want in terms of purity even after taking the metal through the process 2 times. You have got contamination entering your process somewhere like on your crucibles, stir rods, molds etc. Too further refine the gold to 999.9 fine and higher you will need to set up Wohlwill gold cells. An excellent summary of this process can be found her
I would also like to mention that the methods described above only work well for gold scrap containing 25% gold or higher. If you are refining gold that is 25% or less it is best to part it with HNO3, (nitric acid) first and then after washing the remaining residue and rinsing with distilled water you then take that gold and re-dissolve it in aqua regia and follow the process described above. Finally I highly advise not to use the aqua regia refining methods described above on circuit boards or other low grade gold bearing materials that contain high amounts of lead, and other base metals. It is near impossible from my experience to recover the gold. The gold will become trapped in solution. That is why large operations use various types of leaching on low grade scraps and ores such as weak cyanide solutions with a buffered pH of 10.5 I have leached gold plated and gold filled scrap with hot/very weak cyanide solutions with the addition of hydrogen peroxide 35% to introduce the oxygen necessary to strip the gold. This is the same procedure jewelers use that is called "Bombing" and it strips a tiny layer of gold off the jewelry to leave a bright and shiny surface. We used to do this with gold charms after casting in quantities of 50 ounces or more. They were placed in a casserole in a deep wash basin
and covered with hot water then a sodium cyanide egg was dropped into the hot water and allowed to dissolve after pouring one ounce of 35% hydrogen peroxide into the casserole and the glass lid was quickly placed on top and held tight. Almost as soon as you get the lid on it goes poof bang and shoots the hot liquid out under the lid even though you try to hold it shut. That's why they call it Bombing, because it goes Boom! All this must be done with heavy rubber gloves , face mask and apron of course. The gold from leaching and bombing solutions can be recovered by vacuuming out the oxygen then stirring in zinc powder which will precipitate the gold. The gold/zinc is then is rinsed free of the cyanide and the excess zinc can be dissolved away with HCL. Then the remaining gold can be refined to 999 or higher with the aqua regia method.
I processed one batch of 37 pounds of gold filled jewelry with cyanide and recovered about 3.5 ounces of 999 Fine Gold. There are many good books on cyanide extraction of gold.                 
This assay of 999.54 parts per thousand 
                                    from methods described above.

Note: Do not breath Ammonia Hydroxide.

You should do all of this under a fume hood with face

shield, rubber apron, rubber gloves and rubber boots.

Always remember that you are handling corrosive and toxic

chemicals, hydrochloric acid is corrosive and nitric acid is

toxic and corrosive, people have been seriously injured when not

following safety operating procedures and wearing all appropriate

personal protective equipment , (PPE).

See More by clicking the link below.

Steve Looser
Phoenix Consulting
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