With the Silver Chloride (AgCl) method, Silver can be refined in a batch-wise process to a very high degree of purity (999-9999) with relatively “simple tools” and easy to source chemicals.
A disadvantage of this process is the large amounts (relatively) of waste solutions produced.
Silver scrap is first dissolved into solution using a 50:50 mix of Nitric Acid 67% and Distilled water to form Silver Nitrate (AgNO3) solution.
Most of the other base metals in the alloy with the Silver will dissolve as well, with a few exceptions. One worthy of note is Tin (Sn). It will dissolve but immediately turn to Metastannic Acid (H2SnO3), a nasty, pasty solid that is hard to filter.
Most of the precious metals will remain untouched except Palladium, which is soluble in Nitric acid and Platinum, which will also dissolve in case the Silver content within the alloy is above 80% and Platinum content is below 10%.
However, it is worth mentioning that in most cases, both Platinum and Palladium alloyed with Silver will not dissolve quantitatively.
Gold and Rhodium are often plated over Silver items. These metals will not dissolve by Nitric acid and remain as foils, hollow shells, or even powder.
In this Tutorial and the video demonstration below, we will be using scrap Sterling Silver as a case study.
Steps of the process:
- Dissolution + Filtration
- Precipitation of AgCl
- Washing cycles
Scrap Sterling Silver will usually contain all sorts of organic residues, depending on the type of the scrap. Scrap jewelry, for example, will have skin oil, hair, pieces of paper and whatnot.
Flatware and Judaica items may contain wax and even food leftovers.
One example worth mentioning is knife handles which are usually filled with wax or Lead for balance.
Therefore, Silver Scrap must be incinerated or melted before dissolution and filled heavy parts (with wax or Lead) are removed.
Organic materials will react with the acid and create gelatinous substances that produce nothing but trouble while filtering later on in the process. Therefore incineration is a must.
Burning off the organic material could be done in an open furnace or with a simple Oxy/Gas torch. The metal scrap should be heated to red hot for several minutes.
Dissolution + Filtration
When the scrap Silver is ready, place it in a reaction vessel of your choice. The setup could be anything from a simple open mouth Griffin Beaker covered with a watch glass and up to a high-end fancy lab reactor + condensation/reflux + scrubbing system.
The reaction is the same regardless of the setup, but there could be a dramatic difference in efficiency, reaction time and emissions.
The dissolution of the Silver and other metals is achieved by reacting it with a dilute nitric acid solution: One part 67-72% HNO3 / one part distilled water. AKA 50/50 Nitric or appx. 43% Nitric.
As a rule of thumb, it takes about 1.2ml of concentrated Nitric acid (68-72%) and 1.2ml distilled water to dissolve 1 gram of Sterling Silver alloy. The higher the base metals content, the more acid will be needed. Of course, other factors also influence acid consumption, mainly refluxing NO2 fumes.
In the video demonstration below, a Flat Bottom Boiling flask was used and fitted with a Spiral Condenser and an Addition Funnel. Choosing a closed system has a very positive effect on the process in terms of acid consumption, i.e., lowering it by 10-30% depending on specific setup efficiency.
Pre-calculated amount of boiling hot distilled water is added to cover the Silver scrap in the reaction vessel and is kept at near-boiling temp’ by using a Hot Plate or Heating Mantle.
Prepare the calculated amount of needed Nitric Acid and slowly add it to the hot water.
With an Addition Funnel setup, you can adjust it to drip slowly and let it do its thing, or add periodically by larger increments.
With a simpler setup, you would need to add the nitric acid manually.
In any case, you should be very careful and avoid overdoing the nitric, or you will undoubtedly face a boil-over, which may result in severe injury and loss of values.
The point is to use just enough acid to dissolve the Silver. Nearing the end of the dissolution process, watch closely and look for undissolved metal. If you notice metallic pieces that do not react upon addition of more nitric acid, then it is most likely made of Stainless Steel or some other Nitric impervious metal and there’s no point trying to dissolve it.
When the dissolution process is over, let everything cool back to room temp’ and allow the powdered solids to settle, it usually takes several hours.
Prepare a glass or plastic funnel and a CLEAN receiving vessel (Griffin beaker in the video).
Flute a suitable filter paper (see a demo in the video below) and wet it using distilled water only.
Filter the top portion of the solution (supernatant) first as it is clear and free of any floating particulates. It will run very fast through the filter. Leave the junk in the bottom of the reaction vessel to be filtered last, as it will quickly clog the filter.
Remember to wash the filter and the solid residues thoroughly with distilled water to drain all of the Silver Nitrate solution.
Important note - 1: the solution level in the receiving beaker should not exceed ½ - ⅔ of its height.
Important note - 2: Keep the sludge that remains in the filter as it contains Gold and Rhodium plating leftovers.
If you suspect some Palladium was dissolved in the solution, it will be the perfect time to add a few drops of DMG solution.
If nothing happens, go ahead with the next step.
However, if you see a fluffy orange precipitate, keep adding DMG solution until no more precipitate is formed. Filter the solution again and move on to the next step.
Precipitation of AgCl
To precipitate Silver Chloride (AgCl) we should add a chloride (Cl-) source to the Silver Nitrate (AgNO3) solution.
Due to electronegativity differences (a subject I will not bore you with here), the Silver cations “prefer” the Chloride anions over the Nitrate anions, thus combining to form AgCl.
Since AgCl has very low solubility in water and acidic solutions, the result is a chunky white solid precipitate that looks somewhat similar to Cottage Cheese.
Note: If Lead is also present in the solution, it will precipitate as solid PbCl2 alongside the Silver Chloride. However, it will dissolve in boiling hot water.
Top candidates for precipitation are Sodium/Potassium chloride or Hydrochloric acid.
Sodium chloride is by far the cheapest and most convenient choice. You should use simple salt without anti-caking agents (sodium/potassium ferrocyanide). It is sometimes called rock salt or cooking salt. The only ingredient should be NaCl and the grains are usually quite large.
The reaction between Silver Nitrate and Sodium Chloride is a classic double replacement:
AgNO3 + NaCl ==> AgCl + NaNO3
Set the beaker containing the Silver nitrate (AgNO3) solution on a magnetic stirrer plate and add a stir bar. For more significant amounts (over 1lb expected Silver metal), an overhead stirring system would sure come in handy.
For every 1 gram of expected Silver, prepare 0.55 gram dry NaCl.
Before adding NaCl to the Silver nitrate, the salt is first dissolved in a separate beaker using boiling hot water (tap water is fine) and stirred vigorously until the solution is saturated. From time to time, the salt solution may be cloudy. In that case, filter it first.
In any case, do not add dry salt to the Silver nitrate solution.
While stirring the Silver Nitrate solution, add the sodium chloride solution and watch the precipitation. Keep in mind that the process will emit a bit of NO2 (brown gas) due to side reactions of Chloride ions with free nitric acid. Thus the precipitation should be done under the fume hood or in a well-ventilated workspace.
After adding the calculated amount of NaCl and stirring it in thoroughly for several minutes, the solution is allowed to sit for a minute or two. The solution is now tested to see if there’s any more Silver Nitrate by simply adding a few more drops of NaCl solution. If more AgCl precipitates, simply keep adding until no more AgCl precipitate.
It is important not to add any more NaCl than needed. The solubility of AgCl in an acidic medium rises with Cl- ions concentration due to the common ion effect.
This step - the washing, determines the purity of the Silver. The more times it is washed, the better the purity.
The original leach solution, which now contains nitrates and chlorides of mostly Copper and Nickel (and other base metals depending on the source material), is carefully decanted as much as possible to a waste solution bucket and the AgCl is left in the beaker.
Next, the AgCl powder is covered with plenty of boiling tap water and vigorously stirred for several minutes. The powder is then allowed to settle and again slowly decanted.
This wash process is repeated several times.
Keep in mind: A thorough and violent stirring action is imperative and is the key to thoroughly washing all of the powder.
The first two or three washes are combined with the original leach solution in the waste bucket to be treated and disposed of properly. By the way, when large amounts of Silver is refined, it is well worth testing the waste solution with DMG/SnCl2 for the presence of Palladium and Platinum. If the presence of PGM’s is detected, those metals are cemented out with a Copper bus-bar.
The consecutive washes are kept separately as the base metals concentration in those wash solutions will be pretty low and could be reused as the 1st - 3rd washes on the next batch.
From the 3rd/4th wash onward, the wash water is slightly acidified with concentrated Sulfuric or Nitric acid (preferably the latter). About 10-15 ml per liter of wash water (appx. 38-57 ml per gal).
The action of the acid in the wash is twofold: 1st - As the pH rises due to dilution of water, metals oxides/hydroxides (solids) may form and contaminate the AgCl. Acidification ensures it will remain soluble and be washed away. 2nd - Prevent peptization, the acidified water facilitates the coagulation of AgCl particles and as a result, a faster, almost immediate settling of the powder is achieved.
The hotter the wash water, the better. Using hot water serves two purposes; 1st, it helps the AgCl powder settle much faster. 2nd, it will dissolve Lead Chloride, which may have been precipitated with the Silver Chloride (assuming the source material contained any Lead).
It might be problematic to use so much hot water for larger quantities. The washes are done cold and acidified in such cases until the wash runs clear. When cold, settling will take a while. In any case, at least 1-3 hot water washes are needed to remove Lead Chloride.
Washing is repeated until the wash solution is free of any color and passes the base metals presence test and the AgCl powder is pure white. Usually, 8-12 washes are needed.
Testing the wash solution
In most cases, eyesight can’t trust as the wash solution may look colorless, but that doesn’t mean there are no more base metals ions left in the solution.
However, Copper and Nickel, which usually accompany Silver alloys, provide an excellent color indication once the solution is made basic with Ammonia solution.
Sodium Hydroxide solution can also be used, though the colors produced are not as pronounced as with Ammonia.
Pull a 10-50 ml sample from the wash solution and put it in a white glazed porcelain dish or a clear test tube with snow-white background.
Hold the dish under bright white light and slowly bring pH up with Ammonia or NaOH.
Blue is the indicative color. The lighter it is, the better. If the color is deep, more washes are needed. The aim is to have a colorless test, though a light blue could pass assay of 999 fine Silver.
Now that the Silver Chloride has been thoroughly washed and determined by testing it is clean enough, it is time to convert it to Silver Oxide using granulated Sodium Hydroxide.
The following reaction applies:
AgCl + NaOH = AgOH + NaCl
Decomposition of 2 AgOH = Ag2O + H20
The AgCl is covered with water + some excess, about 2-6” depending on the vessel dimensions, the wider it is the lower the excess.
While vigorously stirring the AgCl and water (either with magnetic stirrer or an overhead), NaOH prills are slowly added. Do not add all of the calculated amount at once. NaOH reaction with water is very exothermic and can cause a boil over.
The AgCl powder will gradually change color toward light/dark gray and then to dark brown/black. It should be quite black at the end. After the final addition, stirring is continued for appx. 10-20 minutes. If necessary (indicated by color) more stirring time is allowed and more NaOH is added.
(Some refiners actually choose to slowly add the AgCl sludge to a concentrated and well stirred NaOH solution instead)
Of course, from the addition of NaOH, the solution in the reaction vessel will get very hot and can even reach to a boil.
Warning - Hot caustic solutions are very dangerous and all safety precaution are required.
The resulting Silver Oxide particles are very fluffy but due to the heat of the solution, it will settle rapidly.
From the reaction above we can deduct that a 1:1 molar ratio is required to complete the conversion. But in reality, to proceed with the process in a timely manner an 1:3-3.5 (AgCl:NaOH) molar ratio is preferable.
This translates to the following weight proportion: 100g: 83.5-97.5g (AgCl:NaOH).
With the low prices of NaOH, this excess cost is quite negligible and besides - the leftover caustic solution can be used later to neutralize waste acid solutions.
The “Smear” Test - testing for conversion completion
The Silver Chloride particles react with NaOH to produce Silver Oxide, this happens at the surface of the particle.
The newly formed Silver Oxide layer has a tendency to encapsulate the remaining AgCl inside the particle. Therefore, the need for a vigorous and continuous stirring is required.
To determine if all of the AgCl had been converted to Ag2O, grab a sample of the powder and place in a regular Griffin beaker.
Now use the curved side of a plastic spoon to smush and smear some of the powder against the wall of the beaker.
If you can see a white smear left on the wall, that surely means that not all of the Chloride had been converted to Oxide and more stirring (and maybe more NaOH) is needed.
See a demo of this test in the video tutorial below.
Once determined the conversion had been completed, reduction of Silver Oxide to Silver metal is commenced. The solution after the conversion step is not replaced as the excess NaOH is useful.
A very effective and cheap reducing agent is Dextrose powder.
A little goes a long way and in weight proportion, It takes approximately 3rd of the expected silver metal weight.
The Dextrose powder is slowly added with a spoon to the vigorously stirred Ag2O/water mixture.
The reduction reaction is instant and a change in color of the solution and texture of the powder can be noticed, it usually turns to a strange silky green/gray appearance.
Also, if a transparent beaker is used for the reduction, a silver mirror will slowly appear on the inside of the walls. As the powder mixture is stirred, it will slowly erode the mirror coat to form shiny Silver flakes.
Each addition of Dextrose is allowed to react for several minutes while stirred as this reaction also produces a lot of heat and a choking Hydrogen gas (though not toxic).
The end of the reduction usually happens suddenly, the Silver metal powder will turn very light gray and will settle very fast when stirring is stopped. After settling time of as little as 20-30 seconds, the supernatant should be free of particulates and be very clear, though colorful.
The color of the supernatant will vary from light orange to deep blood red and is most likely due to a caramelization of the sugar given the sweet smell accompanying this process.
Once reduction is done, the light gray Silver powder is washed 5-8 times with plenty of boiling hot water (tap is OK). It is very important to wash away the majority of the solution from the Silver powder as it might interfere with the melting later.
An important note - If too much Copper Hydroxide (Cu(OH)2) is left in the solution prior to reduction it will also be reduced. Presumably to form solid cuprous oxide and perhaps partially to elemental Copper, both has an Iron rust looking appearance.
If a noticeable amount of reddish/orange powder is visible, the Silver powder could be washed several times with hot dilute hydrochloric or sulfuric acid.
After the final washes of the pure Silver powder, the majority of the water is decanted off and the powder is carefully moved to a shallow glass or ceramic cooking pan.
The pan is placed on top of a hot plate and set at low/medium heat to slowly dry out. Do not use too much heat as local boiling of the remaining water may erupt from within the powder and shoot it out of the pan.
Now that Silver powder is dry (or only slightly damp, it is time to melt it.
If you have washed the powder well before drying, you should not expect any particular problems, however leftover sugar from the previous step may form Carbon powder upon heating and will require fluxing with plenty of borax and soda ash. It is best to just wash well and avoid this situation altogether.
Melt the pure Silver powder in a new and clean melting dish with just a touch of borax line the walls of the crucible. Next pour shot or make bars.
Silver could be melted with an Oxy/fuel torch, but bear in mind it likes to absorb much Oxygen once melted and tend to spit it out upon slow cooling (like casting bars), also the could be significant evaporation due to the uneven nature of the flame.
The best way to melt and cast Silver is under an inert gas environment such as CO2 or Argon. Melting in and electric/induction furnace using Graphite crucible will provide sufficient protection from the Oxygen in the air.
If shot are made, then just pour into water. However, if bars are needed, the mold should of course be hot enough to accommodate the drastic temperature change as well as being covered by a blanket of reducing flame, i.e. only fuel.