When In Doubt, Cement It Out
This is advice I often give to new members who find they've created a bit of a mess, and they're wondering how to recover their values (gold, silver, PGMs, etc.).
Perhaps they've followed a process they've seen on YouTube that left out some important details. Perhaps they've used too much oxidizer. Perhaps they've dissolve a bunch of ewaste into a toxic mix of predominantly base metals with a small amount of values. Whatever the cause, "cementation" is a way to recover.
But cementation is not just for recovering from a mess. It is also the principle used in the stock pot. Solutions that have traces of values remaining in them are best treated in the stock pot to recover whatever may remain after the bulk of the values have been recovered through other methods.
It is also a good method for precipitating PGMs. PGM salts, particularly platinum(IV) salts, are very hazardous. While PGMs can be precipitated as relatively insoluble salts, they are still hazardous and must be further processed to recover them as metals, and some of the metal salts will still remain in solution. Cementing them out of solution recovers them as metals, and does so completely.
In simple terms, a solid piece of metal is put into a solution that already has a metal dissolved in it. The solid metal goes into solution and the dissolved metal precipitates, or "cements", out. This is also known as a "replacement reaction", as the solid metal replaces the metal that was in solution. The precipitate often looks like wet cement due to its color and texture as it settles to the bottom of the container. That simple reaction is the subject of this post.
Testing
The first question to ask is whether there are values in the solution.
As an example, someone may have put a bunch of gold plated circuit boards into some acids, watched their gold disappear, and so they "know" they have gold in their solution. But, although their gold may have dissolved into the acids, it may have already "cemented" back out onto base metals in and on the circuit boards. Or it may never have dissolved, but been coated by other metals as the various, more reactive, metals went into and out of solution on metals that remained on the circuit boards.
The first thing to do is to test the solution. If you're pursuing gold or PGMs, stannous chloride is most often used. To test for palladium, use dimethylglyoxime (DMG). If your target is silver, salt or HCl can be used to detect its presence. All these tests are discussed in the book in my signature line below, as well as extensive posts on the forum that can be found using the search function.
Cementation
Cementation is a process where a solid, elemental metal is used to replace a dissolved, ionic metal that is in a solution. The metals we pursue are usually known as noble metals. The name comes from the resistance of these metals to being oxidized and dissolved. This is all based on the number of electrons in their various orbits, and their locations in the various "shells", but that is far beyond the scope of this post. It is sufficient to understand that various metals are more or less resistant to oxidation, and we can use these properties to our advantage.
These varying resistances to oxidation are summarized in the reactivity series of metals. The list below is my own compilation of the many charts I've reviewed. For this post, it contains only the more common metals.
Aluminum
Zinc
Chromium
Iron
Cadmium
Nickel
Tin
Lead
Copper
Silver
Mercury
Palladium
Platinum
Gold
You may see charts that vary slightly from the list above. Many metals can exist in more than one oxidation state, and those varying oxidation states can move a metal up or down the list. Mine is a "generalized" list that serves the purpose of this post.
When viewing a reactivity chart, the more reactive elements (primarily metals) are usually at the top of the chart, indicating they are more reactive (higher reactivity), and the less reactive metals are usually toward the bottom of the chart (lower reactivity). In general, a more reactive metal can be used to "replace" a metal that is lower on the chart (see notes below in "Things To Keep In Mind").
It can occur unintentionally during a process as described in the example above where a bunch of circuit boards have been put into acid and metals have dissolved, then cemented back out of solution in an uncontrolled manner. It can also be used intentionally to recover values that have been dissolved.
A Cementation Tank
The simplest way to cement values out of a solution is to simply put a piece of solid metal into the solution. But the reaction will be slow and may not recover all the values. The solution needs to circulate so the dissolved values come into close proximity to the solid metal. Circulation also helps prevent the solid metal from just developing a plated surface that will stop further reaction, leaving values in solution.
4metals was kind enough to share his design for a simple, but very effective cementation rig in a post in the Fuzz Button Interconnects, need some advice thread. I've reproduced his diagram below, but it's worth reading the complete, original thread.
My setup is slightly different. I'm sure whatever 4metals used was many times larger than my little system. My modifications suit my much smaller needs.
My PVC pipe is the full height of my container, while 4metals' is shorter and the top sits below the liquid level. My pipe for this build is 2" diameter PVC. I drill large holes in both the bottom and the top of my pipe to allow the solution to flow in at the bottom and out at the top, where his solution simply flows out of the top of the pipe.
The holes at the bottom are a bit above the bottom of the pipe so it doesn't tend to draw in any cement as the solution flows in. The holes at the top are at various levels a little higher than where I put the copper. This allows the solution to flow out regardless of how full the container is, as long as it is at least as high as the lowest holes.
I put a small hole near the bottom of the pipe that I can put my air hose through. It's just big enough for the hose to make a snug fit that keeps it in place.
I also put a piece of perforated plastic just above the lower set of holes. I use a short piece of the same 2" diameter PVC with a section cut out. I can squeeze it together and push it into the bottom of the main pipe, then push it up the pipe so it is just above the lower holes. The tension of this "retainer" holds it in place, but allows it to be removed for cleaning. The round disk of perforated plastic then sits on top of the PVC retainer. The perforated plastic is sold in craft stores as "plastic canvas". It is easily cut with a pair of scissors. This keeps me from having to drill holes in the chunks of copper to hang them inside the pipe. I can just drop a couple of chunks into the pipe and I'm ready to go. Note: I use these chunks of copper because I have many pounds of them. Using pieces of copper pipe that have been cut open and flattened will work faster due to the increased surface area.
I use a lid on my small container to contain the spray. Note: While the lid is silver and might look like metal in the pictures, it is plastic. I glue a short piece of a smaller diameter, 1-1/2" PVC pipe to the center of the lid of my container. It fits inside my main pipe and helps keep the main pipe centered in the container. I put small, matching holes through both pieces of pipe so I can assemble the cap and pipe, and put a small strip of PVC through both of them to hold them together, so I can put it all together before screwing the cap on when I run the rig. I cut the strip along the length of a piece of pipe. I round the edges a bit and heat the ends a bit so I can bend them down a bit to make assembly easier. This is a little fancier than it needs to be, but I enjoy the build.
I drill a small hole in the lid for the air hose. I drill another larger hole and glue a short piece of small diameter CPVC pipe to it to act similar to a reflux condenser. Any spray that tries to escape the container settles onto the inside of the pipe and drips back into the container while allowing the air from the pump to escape.
Here are all the parts laid out before it's put together.
And this is the full rig put together.
That's it! To put it together, fit the retainer ring into the main pipe and push it up till it's just above the lower set of holes. Put the air hose through the lid and then through the bottom of the pipe. I cut a taper on the end of the hose so it will fit into the pipe a little easier. Drop in the perforated disk and add some copper, or hang it inside if you haven't built a perforated disk like mine. Align the main pipe with the smaller section of pipe glued to the lid and put the strip of PVC through the holes to hold them together. Lower the lid and PVC pipe into the jar, screw the lid on, and turn on the air pump.
4metals was kind enough to provide a picture of a 50 gallon cementation tank used by one of his customers.
For you video fans, here's a quick video showing me putting it all together.
You can watch a larger view of the video on YouTube by clicking on the title of the video above.
The air bubbles rise through the pipe, carrying a constant, fresh flow of solution and helping to dislodge the cemented values from the copper. Let it run. Test a sample of the solution from time to time. When the solution tests negative, turn off the pump and remove the lid, pipe, and any remaining copper. Allow the cement to settle and decant or syphon off the barren solution. You can wash the cement in the container, or pour it off to an appropriately sized container for washing.
Why Copper?
Many people talk about using iron, zinc, aluminum, or other metals for cementing. While they will certainly work, and they have their place in certain situations, they will also cement everything below them on the reactivity series. The advantage of using copper is that it will only cement silver, gold, PGMs, and mercury if it happens to be in your solution. Any other metals can be cemented out with iron when you treat the solution for waste. This is discussed fully in the book in my signature line and on the forum. Search for "stock pot".
Things To Keep In Mind
For cementation to work, the metal being used to cement out the values must be soluble in the solution. Looking at the reactivity list, it might be tempting to try to use silver to cement out gold or PGMs. But silver is not soluble in a chloride solution, so it won't work for gold. Copper will work in both nitrate and chloride solutions, so it can be used to recover all of our target metals from either type of solution.
There also needs to be a significant difference in the reactivities of the metals. Silver and the other precious metals are far less reactive than copper, so it works well. Tin is only slightly more reactive than lead, so it won't work well to cement lead out of a solution.
Keep all of the copper under the surface of the solution. If it extends above the solution, it can react with the oxygen in the air, and bits of copper oxides can flake off and join the cement, contaminating it. While cementation is generally considered a recovery process, so the cement will probably be refined further, it's better to keep out as much contamination as possible.
Circulation is necessary. As mentioned above, the solution must be kept in motion. It brings fresh solution into proximity with the copper, and the air bubbles help to dislodge the cement, preventing the copper from being plated/coated over stopping the reaction.
Some people report difficulties when their solutions are highly concentrated. They can be prone to forming an adherent coating on the copper, slowing or stopping the cementing process. In these cases, diluting the solution can help stop this problem, allowing the cemented metal to fall more easily from the copper.
Keep your air pump ABOVE the tank. Electricity can go out. The pump can fail. The hose can burst or come off the pump. If any of those happen, and the pump isn't above the container, the liquid can rush up the air hose and over the top creating a syphon that can cause your solution to flow out and possibly be lost.
I haven't been able to refine much since we moved, so I may have forgotten some important points. Please feel free to add to this thread as needed.
Dave
This is advice I often give to new members who find they've created a bit of a mess, and they're wondering how to recover their values (gold, silver, PGMs, etc.).
Perhaps they've followed a process they've seen on YouTube that left out some important details. Perhaps they've used too much oxidizer. Perhaps they've dissolve a bunch of ewaste into a toxic mix of predominantly base metals with a small amount of values. Whatever the cause, "cementation" is a way to recover.
But cementation is not just for recovering from a mess. It is also the principle used in the stock pot. Solutions that have traces of values remaining in them are best treated in the stock pot to recover whatever may remain after the bulk of the values have been recovered through other methods.
It is also a good method for precipitating PGMs. PGM salts, particularly platinum(IV) salts, are very hazardous. While PGMs can be precipitated as relatively insoluble salts, they are still hazardous and must be further processed to recover them as metals, and some of the metal salts will still remain in solution. Cementing them out of solution recovers them as metals, and does so completely.
In simple terms, a solid piece of metal is put into a solution that already has a metal dissolved in it. The solid metal goes into solution and the dissolved metal precipitates, or "cements", out. This is also known as a "replacement reaction", as the solid metal replaces the metal that was in solution. The precipitate often looks like wet cement due to its color and texture as it settles to the bottom of the container. That simple reaction is the subject of this post.
Testing
The first question to ask is whether there are values in the solution.
As an example, someone may have put a bunch of gold plated circuit boards into some acids, watched their gold disappear, and so they "know" they have gold in their solution. But, although their gold may have dissolved into the acids, it may have already "cemented" back out onto base metals in and on the circuit boards. Or it may never have dissolved, but been coated by other metals as the various, more reactive, metals went into and out of solution on metals that remained on the circuit boards.
The first thing to do is to test the solution. If you're pursuing gold or PGMs, stannous chloride is most often used. To test for palladium, use dimethylglyoxime (DMG). If your target is silver, salt or HCl can be used to detect its presence. All these tests are discussed in the book in my signature line below, as well as extensive posts on the forum that can be found using the search function.
Cementation
Cementation is a process where a solid, elemental metal is used to replace a dissolved, ionic metal that is in a solution. The metals we pursue are usually known as noble metals. The name comes from the resistance of these metals to being oxidized and dissolved. This is all based on the number of electrons in their various orbits, and their locations in the various "shells", but that is far beyond the scope of this post. It is sufficient to understand that various metals are more or less resistant to oxidation, and we can use these properties to our advantage.
These varying resistances to oxidation are summarized in the reactivity series of metals. The list below is my own compilation of the many charts I've reviewed. For this post, it contains only the more common metals.
Aluminum
Zinc
Chromium
Iron
Cadmium
Nickel
Tin
Lead
Copper
Silver
Mercury
Palladium
Platinum
Gold
You may see charts that vary slightly from the list above. Many metals can exist in more than one oxidation state, and those varying oxidation states can move a metal up or down the list. Mine is a "generalized" list that serves the purpose of this post.
When viewing a reactivity chart, the more reactive elements (primarily metals) are usually at the top of the chart, indicating they are more reactive (higher reactivity), and the less reactive metals are usually toward the bottom of the chart (lower reactivity). In general, a more reactive metal can be used to "replace" a metal that is lower on the chart (see notes below in "Things To Keep In Mind").
It can occur unintentionally during a process as described in the example above where a bunch of circuit boards have been put into acid and metals have dissolved, then cemented back out of solution in an uncontrolled manner. It can also be used intentionally to recover values that have been dissolved.
A Cementation Tank
The simplest way to cement values out of a solution is to simply put a piece of solid metal into the solution. But the reaction will be slow and may not recover all the values. The solution needs to circulate so the dissolved values come into close proximity to the solid metal. Circulation also helps prevent the solid metal from just developing a plated surface that will stop further reaction, leaving values in solution.
4metals was kind enough to share his design for a simple, but very effective cementation rig in a post in the Fuzz Button Interconnects, need some advice thread. I've reproduced his diagram below, but it's worth reading the complete, original thread.
My setup is slightly different. I'm sure whatever 4metals used was many times larger than my little system. My modifications suit my much smaller needs.
My PVC pipe is the full height of my container, while 4metals' is shorter and the top sits below the liquid level. My pipe for this build is 2" diameter PVC. I drill large holes in both the bottom and the top of my pipe to allow the solution to flow in at the bottom and out at the top, where his solution simply flows out of the top of the pipe.
The holes at the bottom are a bit above the bottom of the pipe so it doesn't tend to draw in any cement as the solution flows in. The holes at the top are at various levels a little higher than where I put the copper. This allows the solution to flow out regardless of how full the container is, as long as it is at least as high as the lowest holes.
I put a small hole near the bottom of the pipe that I can put my air hose through. It's just big enough for the hose to make a snug fit that keeps it in place.
I also put a piece of perforated plastic just above the lower set of holes. I use a short piece of the same 2" diameter PVC with a section cut out. I can squeeze it together and push it into the bottom of the main pipe, then push it up the pipe so it is just above the lower holes. The tension of this "retainer" holds it in place, but allows it to be removed for cleaning. The round disk of perforated plastic then sits on top of the PVC retainer. The perforated plastic is sold in craft stores as "plastic canvas". It is easily cut with a pair of scissors. This keeps me from having to drill holes in the chunks of copper to hang them inside the pipe. I can just drop a couple of chunks into the pipe and I'm ready to go. Note: I use these chunks of copper because I have many pounds of them. Using pieces of copper pipe that have been cut open and flattened will work faster due to the increased surface area.
I use a lid on my small container to contain the spray. Note: While the lid is silver and might look like metal in the pictures, it is plastic. I glue a short piece of a smaller diameter, 1-1/2" PVC pipe to the center of the lid of my container. It fits inside my main pipe and helps keep the main pipe centered in the container. I put small, matching holes through both pieces of pipe so I can assemble the cap and pipe, and put a small strip of PVC through both of them to hold them together, so I can put it all together before screwing the cap on when I run the rig. I cut the strip along the length of a piece of pipe. I round the edges a bit and heat the ends a bit so I can bend them down a bit to make assembly easier. This is a little fancier than it needs to be, but I enjoy the build.
I drill a small hole in the lid for the air hose. I drill another larger hole and glue a short piece of small diameter CPVC pipe to it to act similar to a reflux condenser. Any spray that tries to escape the container settles onto the inside of the pipe and drips back into the container while allowing the air from the pump to escape.
Here are all the parts laid out before it's put together.
And this is the full rig put together.
That's it! To put it together, fit the retainer ring into the main pipe and push it up till it's just above the lower set of holes. Put the air hose through the lid and then through the bottom of the pipe. I cut a taper on the end of the hose so it will fit into the pipe a little easier. Drop in the perforated disk and add some copper, or hang it inside if you haven't built a perforated disk like mine. Align the main pipe with the smaller section of pipe glued to the lid and put the strip of PVC through the holes to hold them together. Lower the lid and PVC pipe into the jar, screw the lid on, and turn on the air pump.
4metals was kind enough to provide a picture of a 50 gallon cementation tank used by one of his customers.
For you video fans, here's a quick video showing me putting it all together.
You can watch a larger view of the video on YouTube by clicking on the title of the video above.
The air bubbles rise through the pipe, carrying a constant, fresh flow of solution and helping to dislodge the cemented values from the copper. Let it run. Test a sample of the solution from time to time. When the solution tests negative, turn off the pump and remove the lid, pipe, and any remaining copper. Allow the cement to settle and decant or syphon off the barren solution. You can wash the cement in the container, or pour it off to an appropriately sized container for washing.
Why Copper?
Many people talk about using iron, zinc, aluminum, or other metals for cementing. While they will certainly work, and they have their place in certain situations, they will also cement everything below them on the reactivity series. The advantage of using copper is that it will only cement silver, gold, PGMs, and mercury if it happens to be in your solution. Any other metals can be cemented out with iron when you treat the solution for waste. This is discussed fully in the book in my signature line and on the forum. Search for "stock pot".
Things To Keep In Mind
For cementation to work, the metal being used to cement out the values must be soluble in the solution. Looking at the reactivity list, it might be tempting to try to use silver to cement out gold or PGMs. But silver is not soluble in a chloride solution, so it won't work for gold. Copper will work in both nitrate and chloride solutions, so it can be used to recover all of our target metals from either type of solution.
There also needs to be a significant difference in the reactivities of the metals. Silver and the other precious metals are far less reactive than copper, so it works well. Tin is only slightly more reactive than lead, so it won't work well to cement lead out of a solution.
Keep all of the copper under the surface of the solution. If it extends above the solution, it can react with the oxygen in the air, and bits of copper oxides can flake off and join the cement, contaminating it. While cementation is generally considered a recovery process, so the cement will probably be refined further, it's better to keep out as much contamination as possible.
Circulation is necessary. As mentioned above, the solution must be kept in motion. It brings fresh solution into proximity with the copper, and the air bubbles help to dislodge the cement, preventing the copper from being plated/coated over stopping the reaction.
Some people report difficulties when their solutions are highly concentrated. They can be prone to forming an adherent coating on the copper, slowing or stopping the cementing process. In these cases, diluting the solution can help stop this problem, allowing the cemented metal to fall more easily from the copper.
Keep your air pump ABOVE the tank. Electricity can go out. The pump can fail. The hose can burst or come off the pump. If any of those happen, and the pump isn't above the container, the liquid can rush up the air hose and over the top creating a syphon that can cause your solution to flow out and possibly be lost.
I haven't been able to refine much since we moved, so I may have forgotten some important points. Please feel free to add to this thread as needed.
Dave
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easy demonstration is eg. "amalgamated aluminium foil" experiment, where you soak rolled balls of aluminium foil in solution of Hg2+ ions, disrupting the crystal structure of the aluminium metal and ending in reaction of otherwise unreactive Al.
