Non-Chemical Sulfuric/Electrolytic cell question

[bswartzwelder]

I would like to construct a sulfuric/electrolytic cell using a pyrex dish like the ones sold by Lasersteve. I would use a cathode (-) made from a sheet of lead placed in the bottom of the dish with one end protruding up and over the narrow end of the dish. Then make the anode (+) from copper mesh whereby it is shaped like a "U" with the top edges folded over the long sides of the dish. Obviously there would need to be a space between the copper and lead both on the bottom of the cell and on the end where the lead exits over the side of the dish. Would this work, or would the reaction be concentrated near the end where the lead exits the dish? Also, would the copper being shaped like a "U" act like a Faraday Cage and hinder the reaction? Is there an optimal distance between the mesh and lead?

 

[NobleMetalWorks]

I would like to construct a sulfuric/electrolytic cell using a pyrex dish like the ones sold by Lasersteve. I would use a cathode (-) made from a sheet of lead placed in the bottom of the dish with one end protruding up and over the narrow end of the dish.

The cathode should be slightly longer than the anode, and should be placed opposite the anode. If you are using a pyrex dish like Lazer Steve has on his site, you would put your lead cathode on one side of the length of the dish, with the anode on the opposite side of the length of the dish, facing each other across the width of the dish. You don't want to run the lead cathode under the anode, when the gold precipitates it will fall onto the lead cathode, plus you will not be increasing efficiency. The electrical current might also be affected by the material that collects on top of the cathode under the anode.

Then make the anode (+) from copper mesh whereby it is shaped like a "U" with the top edges folded over the long sides of the dish.

Just so long as the copper anode can contain the material you are stripping the gold plate from. A sulfuric stripping cell doesn't work in the same way electrolytic cells normally work. You are not attracting the metal ions to the cathode to be plated, but instead parting the gold from the base metal. This is done by the creation of persulfuric acid in the area of the anode. So long as electrical current can be generated, and the anode is able to create persulfuric acid at the anode, the shape of the anode is not important past being able to hold the material. Keep in mind that you don't want to process a lot of material all at one time so keep your anode relatively small. It's better to do several batches rather than one giant one.

Obviously there would need to be a space between the copper and lead both on the bottom of the cell and on the end where the lead exits over the side of the dish. Would this work, or would the reaction be concentrated near the end where the lead exits the dish?

You are only completing the circuit to create persulfuric acid at the anode, in a small dish the distance between anode and cathode is not going to matter so much, so long as the circuit can be completed. The reaction does not take place at the cathode, other than the creation of hydrogen. The reaction, for our purposes, is created at the anode in the creation of persulfuric acid. It is the persulfuric acid that dissolves the gold into solution, and as it does so oxidizes the base metal under the gold which reduces the efficiency of the electricity from anode to cathode, blocks it in other works. Once all the base metal is oxidized, the electrical current is broken and no more persulfuric acid will be created at the anode, neither will hydrogen be created at the cathode. You can see this in the drop in your amp meter to 0. So although the bubbles form at the cathode, it's what you cannot see at the anode that is the desired reaction.

Also, would the copper being shaped like a "U" act like a Faraday Cage and hinder the reaction? Is there an optimal distance between the mesh and lead?

So long as a circuit can be created from anode to cathode without interference of foreign materials in solution (clean electrolyte solution) the shape of the anode shouldn't matter so much. It's the persulfuric acid that is created at the anode, not the hydrogen created at the cathode, that matters.

A few other things:

Make sure you are taking all the necessary safety precautions, sulfuric acid is nasty stuff.

Use highly concentrated sulfuric acid, 98% is best

Don't leave your sulfuric acid uncovered, it will collect moisture from the air and become dilute

Never add water to acid, specially sulfuric acid, but instead acid into water and slowly at that.

Make sure that either your cathode or anode are somehow insulated from each other so as not to create an electrical current around the rim of the dish. The can be done by mounting it on rubber, or some other non-conductive material. When the hydrogen evolves and the bubbles pop on the surface, it tends to deposit small amounts of sulfuric acid around the rim of the dish. If this conducts electricity from the anode to cathode, it could cause issues. Also, by keeping your solution below the top of the dish will prevent some of the sulfuric acid from accumulating around the rim. Also be aware that some sulfuric acid might be on your work space surface.

Hope this helps

Scott

 

[bswartzwelder]

Thank you, Scott. I purchased a bottle of 98% sulfuric acid some time ago, but then got cold feet and was afraid to use it. With the shipping costs, it was extremely expensive, so I didn't want to use it for any lesser purpose like collecting lead from AR solution. I did purchase Rooto at my local True Value hardware store for about $16.99 plus tax for a half gallon bottle. It says it is made of 93% sulfuric acid, but doesn't say what the other 7% is. I suspect it may be added water. At 93% strength, it's effectiveness in an electrolytic cell would probably be questionable, at best. The 93% will be used instead of the 98% stuff to scavenge the lead from my AR.

One of my biggest concerns was that I had read that the gold plating (on the items being deplated) will not deplate where items touch each other. This means stirring the items very gently with a glass stirring rod. The "U" shaped idea for the basket was to add strength so the basket will not collapse under the action of stirring or the weight of the items inside it. But, then again, I know enough to not mash down while stirring.

I did have a basic understanding of how the cell works. Your explanation was very good and easy to understand. I still will want to do a dry run or two (perhaps using water) to get the mechanics down before I attempt to do it using acid. Once finished with the cell, returning the used acid to the bottle without spilling it may present a challenge.

The forum is really amazing. Some time ago, someone mentioned an electrolytic cell where the gold is inside a rotating drum. It really sounded like a good idea until you thought about the 98% sulfuric acid being carried around the inside AND outside of the drum with each rotation. Not to mention the sulfuric acid which would try to seep out where the axle through the drum exits the container which holds the acid.

 

[NobleMetalWorks]

Thank you, Scott. I purchased a bottle of 98% sulfuric acid some time ago, but then got cold feet and was afraid to use it. With the shipping costs, it was extremely expensive, so I didn't want to use it for any lesser purpose like collecting lead from AR solution.

You can re-use your sulfuric acid if you filter it instead of diluting the whole amount in water. You can do this by filtering through a charmin plug made of fiberglass, or even a fused quartz felt in a buchner funnel if you want to get fancy and expensive. Then you can take the filter media and dissolve the values in AR, leaving behind the fiberglass or quartz felt.

I did purchase Rooto at my local True Value hardware store for about $16.99 plus tax for a half gallon bottle. It says it is made of 93% sulfuric acid, but doesn't say what the other 7% is. I suspect it may be added water. At 93% strength, it's effectiveness in an electrolytic cell would probably be questionable, at best. The 93% will be used instead of the 98% stuff to scavenge the lead from my AR.

A lot of times, drain cleaners have inhibitors in them to prevent them from dissolving metal pipes. These inhibitors may have unknown negative side affects in a electrolytic cell. If you read the contents, you might find something that refers to this fact. In any case, you don't want to use sulfuric acid with an inhibitor in your electrolytic cell unless you know what it is, know the affects, and they are positive.

One of my biggest concerns was that I had read that the gold plating (on the items being deplated) will not deplate where items touch each other. This means stirring the items very gently with a glass stirring rod. The "U" shaped idea for the basket was to add strength so the basket will not collapse under the action of stirring or the weight of the items inside it. But, then again, I know enough to not mash down while stirring.

Once my cell is operating, an electricity is put to the anode/cathode, I don't touch it or by extension touch it with any tool. I wait until the amp meter reads zero, turn off the cell, remove the anode basket from the cell allowing it to drain before placing it in a second pyrex dish and then moving the material around. I do it in this way because stirring while the cell is operating is just plain dangerous, and stirring while the anode basket is in the cell could cause splashing, which you want to obviously avoid.

I did have a basic understanding of how the cell works. Your explanation was very good and easy to understand. I still will want to do a dry run or two (perhaps using water) to get the mechanics down before I attempt to do it using acid. Once finished with the cell, returning the used acid to the bottle without spilling it may present a challenge.

Sulfuric acid is a viscous oily liquid, or at least that's the only way I can think to describe it. It's difficult to filter, and wants to stick to everything like kyro. Personally I try to deal with it while it's warm, it makes it easier for me to handle and filter, but I am by no means suggesting anyone heat it up to filter or pour it, this is just what I found works for me and the way I handle the acid. I am using glass battery jars for my own cells, so after I am finished I retain the sulfuric acid in the battery jar, and store it in a larger plastic container so if it spills it's still contained. But by doing this I transfer the acid less, and loose less in the process. I also filter from one battery jar into another so I don't use beakers, and don't loose sulfuric acid sticking to the sides of beakers. Plus the battery jars were made specifically for this type of acid, and are thick and strong so I feel safe using them. Pyrex has dishes that have glass lids, one specifically has a seal made of some plastic material that seems resistant to acids, although I am not sure on this point and you might want to check before taking my word for it. But those containers seem they should work.

The forum is really amazing. Some time ago, someone mentioned an electrolytic cell where the gold is inside a rotating drum. It really sounded like a good idea until you thought about the 98% sulfuric acid being carried around the inside AND outside of the drum with each rotation. Not to mention the sulfuric acid which would try to seep out where the axle through the drum exits the container which holds the acid.

I agree, the forum is amazing and made up of brilliant individuals from all different types of backgrounds. It makes for a very interesting and engaging place. And even better, people are overly generous with their processes, knowledge and designs and willing to help, freely giving of themselves without expectations. Other people have said this, and I say it whenever possible, this place is probably the single most important repository of information anywhere on the internet, and probably most libraries and colleges.

Scott

Edited; I had stated voltage to zero when I meant amps to zero, I corrected where I made this mistake in the post above.

 

[bswartzwelder]

At the price I paid for the concentrated sulfuric acid, I would never consider diluting any more than is absolutely necessary, unless the bottle dropped and broke on my driveway. I can see diluting the few drops which may not make it through the filter, what's on the sides of the dish after I empty the used solution back into its container, and the washing of the copper basket. In reality, all those operations are washing and not diluting.

Once again, you have taught this old dog something new. I did know that filter papers won't stand up long to highly concentrated sulfuric acid. The fiberglass plug sounds like a great idea.

I have read (and re-read) the ROOTO label numerous times, but have found nothing listed except sulfuric acid as the contents. If it helps, the bar code on the jug is: 0 37704 10796 2 with the spaces as shown. The front panel lists it as ROOTO, then underneath, it says: Professional Drain Opener.

You mentioned you wait until the voltage reaches zero, then turn off your cell. In an electrolytic cell, it was my understanding the voltage remains fairly constant and the amperage goes to zero after the gold has been de-plated?

Again, thanks for sharing your expertise.
Bert

 

[Harold_V]

You mentioned you wait until the voltage reaches zero, then turn off your cell. In an electrolytic cell, it was my understanding the voltage remains fairly constant and the amperage goes to zero after the gold has been de-plated?

Voltage has little reason to change---but amperage is a function of the presence of gold. You are correct.

I visited a government facility that was engaged in the recovery of gold, way back in '75. Their method was to strip until amperage fell to near zero, then to increase to 28 volts and run until amperage fell once again. At that point, it was assumed the pieces were fully stripped. Items should be dumped and reset in the basket, to ensure surfaces were not isolated.

Harold

Error corrected. Thanks to FrugalRefiner

 

[goldsilverpro]

The voltage actually climbs as the amps go to zero. The product of V X A remains the same until you alter the setting.

I visited a government facility that was engaged in the recovery of gold, way back in '75. Their method was to strip until amperage fell to near zero, then to increase to 28 volts and run until amperage fell once again. At that point, it was assumed the pieces were fully stripped. Items should be dumped and reset in the basket, to ensure surfaces were not isolated.

That's exactly how I always ran it. At the end, I increased the voltage to the maximum that my equipment was capable of and waited until the amps dropped to zero.

 

[bswartzwelder]

Thank you, guys. I read one of Harold's older posts on the electrolytic cell (some time ago) and the discussion of the persulfuric acid production, so NobleMetalWorks had a very refreshing review. Strange how little things you thought you knew jump back out at you.

As far as the voltage dropping off to zero (or near zero), I thought I was having another not so little brane f**t and was going to have to go and reread everything on electrolytic cells. For anyone not intimately familiar with electricity and how it acts, volage, amperage, resistance, and wattage can get very confusing.

GSP, you say the voltage actually climbs as the amps go to zero. In most instances, I would tend to agree completely with that because most power supplies like battery chargers are just not that well regulated. They are there to dump amperage into a load until the load gets filled up. The exception would be if someone was using a good quality precision power supply. In that case, the voltage could be set to a predetermined value and will be held at that value pretty much no matter what happens to the amperage (load). In most cases, the V X A remains the same is correct, but there are some real life conditions which can alter the equation slightly. The internal resistance of the cell changes from fully discharged to fully charged. That's where most of the V X A changes occur. However, the battery (and the electrolytic cell) can, and do change temperature as power is being pumped into them. Again, V X A, except the A portion which isn't actively deplating gold is producing a usually unwanted commodity, heat.

A dead battery can no longer produce the current which is needed because its charge has drained off, raising the internal resistance of the battery. The battery voltage in this case drops off as well. A typical battery charger like the ones used in the cell we are discussing have a predetermined output voltage designed into them. This design voltage keeps the charger output voltage within the safe limits of the battery and prevents overcharging. If the battery is in good condition and has just been discharged, then, once the charger is connected to the load (the dead battery), the charger voltage drops because the chargers are not actually capable of maintaining their highest voltage throughout the power curve from the starting charge of a dead battery to the fully charged condition. As the battery becomes fully charged, the voltage of the battery rises as does the output voltage of the charger. In many instances, the windings of the transformer in the battery charger limit two things. They limit the highest current available (determined by the resistance of the winding material Cu or Al) to charge the battery and they limit the highest voltage (determined by the number of turns between the primary and secondary windings)the charger can produce to prevent overcharging.

If the battery has been drained for quite a while, it may not be capable of accepting a charge. At that time, the charger voltage will go to the highest voltage possible and stay there. I have witnessed batteries where at first they would not accept more than just a few amps. As time went by, the current of the charger would very slowly increase until it reached a "terminal" value where the battery could not accept a higher current, and yet the internal resistance of the battery would not go low enough to stop the charging process. The internal resistance of the battery and the resistance of the windings of the charger transformer buck each other and balance each other out. At that point, you just have to knuckle down and buy a new battery.

GSP, while I knew that with the proper power supply you could raise the voltage in the end (assuming it wasn't already maxed out), I never really thought much about it. Doing it your way should ensure you get the last few atoms of gold off your items being deplated. Good tip! I thought the process was done the first time the current reached zero.

 

[g_axelsson]

The voltage actually climbs as the amps go to zero. The product of V X A remains the same until you alter the setting.

The red part is just so wrong. The V x A is effect and to keep effect constant the voltage has to go up as the amperage drops.

You actually disprove yourself in the next sentence...

At the end, I increased the voltage to the maximum that my equipment was capable of and waited until the amps dropped to zero.

If VxA should be constant then the amperage couldn't drop if the voltage is at max.

The only situation you could get a constant power effect is if your supply could regulate to deliver constant effect but that is very rare and I doubt it would be a smart thing to use in reverse plating.
Simpler power supplies as an unregulated battery charger has an internal impedance (resistance) that drops the voltage as more amperage is delivered, but that never delivers constant power.
Regulated power supplies usually delivers a constant voltage from zero ampere up to a limit and then the voltage drops while the amperage is constant at the maximum limit. This isn't either delivering constant effect.

I'm sure that you actually meant something else, just didn't explain it clearly enough for me to understand.

My guess is that you are talking about a regulated power supply where you can set the current and voltage limits individually.
If you start with a cell that can draw more amperage at set voltage than the current limit then the voltage will rise as the plating disappears, but the current is constant in this phase. When the voltage has reached it's limit the voltage will stay constant and the amperage starts to drop down until the plating is gone and the current is almost at zero.

Göran

 

[goldsilverpro]

The voltage actually climbs as the amps go to zero. The product of V X A remains the same until you alter the setting.

The red part is just so wrong. The V x A is effect and to keep effect constant the voltage has to go up as the amperage drops.

You actually disprove yourself in the next sentence...

At the end, I increased the voltage to the maximum that my equipment was capable of and waited until the amps dropped to zero.

If VxA should be constant then the amperage couldn't drop if the voltage is at max.

The only situation you could get a constant power effect is if your supply could regulate to deliver constant effect but that is very rare and I doubt it would be a smart thing to use in reverse plating.
Simpler power supplies as an unregulated battery charger has an internal impedance (resistance) that drops the voltage as more amperage is delivered, but that never delivers constant power.
Regulated power supplies usually delivers a constant voltage from zero ampere up to a limit and then the voltage drops while the amperage is constant at the maximum limit. This isn't either delivering constant effect.

I'm sure that you actually meant something else, just didn't explain it clearly enough for me to understand.

My guess is that you are talking about a regulated power supply where you can set the current and voltage limits individually.
If you start with a cell that can draw more amperage at set voltage than the current limit then the voltage will rise as the plating disappears, but the current is constant in this phase. When the voltage has reached it's limit the voltage will stay constant and the amperage starts to drop down until the plating is gone and the current is almost at zero.

Göran

I stand behind what I said. You didn't consider the passivation effect of the base metal.

In production, I have never used (and never will) a PS where the A and V could be set individually. I much prefer the one knob, two meter units. When you adjust the knob, you are adjusting a power setting - V X A. As the current flow decreases, the voltage increases in an attempt to maintain this power setting. The gold doesn't "dissolve" evenly for several reasons - the gold thickness is not the same over the part and the current applied is not the same over the entire part.

As the thinner gold and/or the gold in the high current density areas is removed and some base metal is exposed, the current starts dropping. This is due to passivation (oxidation) of the surface of the base metal provided by the anodic current. When the gold is totally gone, under ideal conditions, zero current will flow on the passivated surface, even with reasonably high voltage (at least 28V, according to Harold - I believe it). The passivation layer can be broken by too high a temperature, too high a voltage, or if the sulfuric is too weak. Certain contamination could also play a part. Once all the gold is gone, the parts are completely passivated, and the effective resistance becomes infinite. The set V X A relationship no longer holds true. You can definitely get 0 amps with 28 volts.

In the 70's, I ran a 50 gallon sulfuric stripper, all day, 5 days a week, for several years. The approx. 3/16" mild steel tank was used as the cathode. I harvested the gold every week, or so, and used the same solution for at least 2 years (the humidity is quite low in L.A.), at which time there was no noticeable attack on the steel. The rectifier was 250 amps and I'm thinking the max. voltage was 20. I wasn't foolish enough to run the parts piled up in a metal mesh basket. For larger parts, such as all gold side-braze CPU packages, I mounted them on racks with piano wire spring contacts and ran about 300 parts on a rack. For all-metal parts, such as pins or lids, I used a plastic rotating plating barrel with dangler contacts.

I turned the rheostat all the way off and put the parts in the tank. I then slowly turned up the voltage until I had about 200 amps max, to keep from blowing the fuse. At this point, I had about 2 volts. After about 2 or 3 minutes of stripping gold, some of the base metal (usually kovar or a copper alloy) became exposed and passivated. The amps dropped and the volts increased. I then turned up the voltage until I again had 200 amps. I repeated this several times over the next few minutes. At some point, the voltage would be maxed at 20V and the amps were less than 200A. I then let it run until the amps were zero or very close to it. The total cycle time ranged from 5 to 20 minutes.

 

[Palladium]

One of the best technical descriptions i have seen in such a simple explanation.
Simplicity !

 

[bswartzwelder]

GSP, I believe Goran was saying (and you as well) was that you are actually adjusting the current and not the power. You did that to keep the output from blowing the 200 amp fuses. The power supply was outputting the current level which you set it to.

The voltage, in this case was a function of (dependent on) several things. The resistance across the cell and the physical limitations of your power supply being the most important. The temperature (of many variables), operating frequency of the power supply, and many other things all enter into the operation of the power supply, but have less of an effect on its output.

As gold is stripped away and the base metal underneath passivates, the resistance of the cell increases. As the resistance increases, the current MUST drop (unless you are using a constant current power supply). The resistance goes up, the current goes down, and the power supply is able to supply a higher voltage at the reduced current. However, using the V X A equation does not apply completely as it does not take into account the internal resistance (impedance) of the power supply or many of the other variables.

Also, if V X A stayed a constant, as current approaches zero, the voltage would, by necessity have to approach infinity to satisfy the V X A remaining equal. We all know it's not possible for the power supply output voltage to approach infinity. It is limited by the design of the power supply (even the big ones with lots of mojo).

The actual limitations of any power supply can be shown graphically. Set the X-axis equal to amperage and the Y-axis equal to voltage. Put a dot on the X-axis at a point which corresponds to the short circuit current the power supply is able to deliver. Put another dot on the Y-axis at a point which corresponds to the maximum open circuit voltage. You can reverse the current and voltage if you wish, the graph will still be the same. Draw a line through those two points and for ANY output current, you can determine the output voltage or vice versa. This line actually represents the internal resistance of your power supply.

edited to hopefully be more accurate.

 

[Harold_V]

When the gold is totally gone, under ideal conditions, zero current will flow on the passivated surface, even with reasonably high voltage (at least 28V, according to Harold - I believe it).

When I observed the operation in question, it was with notebook in hand, as I had no understanding at all. One of the things I recorded was that the specific gravity of their electrolyte was 1.68. The people operating the stripping cell had an extreme amount of experience, as they had recovered a reported 35,000 troy ounces of gold the previous year. This was an operation by the US Government.

Something to consider. If the base metal was transferred in the process of stripping gold, copper baskets would not work. They do. That's a pretty good sign that amperage falls to zero, or nearly so, otherwise you'd lose the basket, or the base metals would be dissolved. Neither scenario plays out in practice.

The issue about not stripping base metals from over heating was addressed by the use of a water cooled stripping cell. They operated the cell on a full time basis (normal working hours, not round the clock). I was informed that the amperage would, indeed, fall to zero, or very nearly so, when the gold was stripped. They stripped ONLY copper based alloys, never ferrous based.

Harold

 

[Harold_V]

GSP, I believe Goran was saying (and you as well) was that you are actually adjusting the current and not the power.

You have little control over the amperage, aside from the limits provided by the power supply on the top end. In essence, if you happen to have a power supply capable of delivering 1,000 amps, it will deliver only that which it can, at a given voltage. No more, no less (unless you have a direct short). If you wish to limit the amount of amperage, you can reduce the voltage, that I'll give you. It was made abundantly clear to me by those who operated the stripping cell I observed that you control the voltage. Nothing more. A constant amperage power supply is NOT a good fit for a stripping cell, which should be obvious by now.

Harold

edit:
Upon rereading, it's obvious to me that bswartzwelder is confusing the stripping cell operation by introducing watts. That's where you're getting tripped up. You keep trying to maintain a given wattage. That's not what happens. Wattage falls off to zero. If one sets the open circuit voltage to 28, for example, when amperage falls to zero, the job is done.

 

[bswartzwelder]

All,
In my original post, all I was interested in were the mechanics of a sulfuric stripping cell and whether shaping the copper mesh into a "U" shape would act as a Faraday Cage" which would shield the gold and prevent de-plating.

Scott had originally posted the voltage goes to zero. It was my understanding the current went to zero as the items were de-plated. Harold confirmed it is the current which approaches zero and Scott edited his post to correct this.

Harold, GSP, Scott, and others are the true experts in the sulfuric/electrolytic cell operation. I do not have the profound knowledge of the chemistry or the mechanics of what is going on. I think I know the basics, thanks to this forum.

Harold stated "Voltage has little reason to change---but amperage is a function of the presence of gold.". GSP stated "The voltage actually climbs as the amps go to zero. The product of V X A remains the same until you alter the setting.". This was the first mention of V X A which in a purely DC circuit is power (also known as wattage). Goran and I have backgrounds in electronics. Goran stated "The product of V X A remains the same until you alter the setting is just so wrong.". For the product of V X A to remain the same, as current approaches zero, the voltage would have to approach infinity. This is impossible and is limited to the upper design voltage of the power supply.

I think everyone agrees what is going on inside the cell is a dynamic cause and effect reaction. The electrical current causes persulfuric acid to be produced in the area of the anode. The persulfuric acid causes the gold to de-plate. The area where the gold has been de-plated exposes base metal which oxidizes (passivates) almost immediately. This passivated metal which is now exposed to the acid has an oxide coating which has a very high resistance when compared to the resistance of the metal still plated with gold. The overall resistance of the cell rises, if no adjustment is made to the control knob, the current must drop off. As long as the power supply is not of a "constant voltage" design, the voltage now being produced will go up some. But it will not rise as much as the current drops and the overall power consumed by the cell falls. GSP states: "You can definitely get 0 amps with 28 volts.". Using the V X A equation, Power = 0 Amps X 28 Volts = 0 Power or 0 Watts, however you care to define it.

One thing that could be confusing is the way electrical terms are used almost interchangeably. I know that I am guilty of making this mistake at times. In his first post, GSP states "At the end, I increased the voltage to the maximum...". In his next post, he says: "When you adjust the knob, you are adjusting a power setting." "As the current flow decreases, the voltage increases in an attempt to maintain this power setting." Later, in the same post, GSP states: "I turned up the voltage until I had 200 amps". As is usually the case, the current is dependent on the voltage. GSP then states that he turned up the voltage several times until he got 200 amps. The way I read this, he clearly adjusted the amps by raising the voltage until his power supply could no longer produce any higher voltage. The power supply was at its maximum design voltage.

If you short circuit a power supply, its output voltage approaches zero. Unless it has current limiting capabilities, the short circuit current is a direct function of the resistance "seen" by the power supply. In this case, all the interconnecting wiring and their connections. Since you are trying to maximize the power out of your power supply, you try to avoid losses. Losses are caused by high (higher than normal) resistance. Therefore, you keep your leads as physically short as possible, use the largest size wiring/buss bars that are economical, and keep the number of connections to a minimum. Connections must also be kept clean to prevent unwanted heat build up. While working for Westinghouse Electric Corporation, we would check every circuit breaker for good connections across the contacts by applying up to 2000 amps across the contacts and reading the millivolt drops across the same contacts. The engineers had calculated acceptable resistance based on keeping the current constant and reading the voltage loss across the contacts. In other parts of the plant, we used up to 40,000 amps for a different type of test. You could actually touch the buss bar with 40,000 amps flowing through it and not even get the slightest tingle. The voltage was probably only about 1.5 volts maximum, but I don't remember anyone actually measuring it at that point.

 

[g_axelsson]

This is a reply to the post made by GSP. Before I posted it bswartzwelder and Harold made some posts so there are a few details that I'm repeating that's already been discussed by them but I decided to leave it in anyhow.

The voltage actually climbs as the amps go to zero. The product of V X A remains the same until you alter the setting.

The red part is just so wrong. The V x A is effect and to keep effect constant the voltage has to go up as the amperage drops.

You actually disprove yourself in the next sentence...

At the end, I increased the voltage to the maximum that my equipment was capable of and waited until the amps dropped to zero.

If VxA should be constant then the amperage couldn't drop if the voltage is at max.

The only situation you could get a constant power effect is if your supply could regulate to deliver constant effect but that is very rare and I doubt it would be a smart thing to use in reverse plating.
Simpler power supplies as an unregulated battery charger has an internal impedance (resistance) that drops the voltage as more amperage is delivered, but that never delivers constant power.
Regulated power supplies usually delivers a constant voltage from zero ampere up to a limit and then the voltage drops while the amperage is constant at the maximum limit. This isn't either delivering constant effect.

I'm sure that you actually meant something else, just didn't explain it clearly enough for me to understand.

My guess is that you are talking about a regulated power supply where you can set the current and voltage limits individually.
If you start with a cell that can draw more amperage at set voltage than the current limit then the voltage will rise as the plating disappears, but the current is constant in this phase. When the voltage has reached it's limit the voltage will stay constant and the amperage starts to drop down until the plating is gone and the current is almost at zero.

Göran

I stand behind what I said. You didn't consider the passivation effect of the base metal.

In production, I have never used (and never will) a PS where the A and V could be set individually. I much prefer the one knob, two meter units. When you adjust the knob, you are adjusting a power setting - V X A. As the current flow decreases, the voltage increases in an attempt to maintain this power setting. The gold doesn't "dissolve" evenly for several reasons - the gold thickness is not the same over the part and the current applied is not the same over the entire part.

<snip! removed a lot interesting stuff>...

I turned the rheostat all the way off and put the parts in the tank. I then slowly turned up the voltage until I had about 200 amps max, to keep from blowing the fuse. At this point, I had about 2 volts. After about 2 or 3 minutes of stripping gold, some of the base metal (usually kovar or a copper alloy) became exposed and passivated. The amps dropped and the volts increased. I then turned up the voltage until I again had 200 amps. I repeated this several times over the next few minutes. At some point, the voltage would be maxed at 20V and the amps were less than 200A. I then let it run until the amps were zero or very close to it. The total cycle time ranged from 5 to 20 minutes.

I also stand by what I have said. I didn't consider passivisation as I don't have to. I'm looking at the system as two parts, a power supply and a variable load. What makes the load to change is of no concern when designing/choosing the power supply. Only the demands it puts on the power supply.

In your more detailed example it's obvious that the V x A (effect) isn't constant over the whole deplating process, you adjust the voltage all the time, and after the voltage hits it's maximum the current drops while the voltage keeps constant.

I can agree that in a simple power supply without any active regulation you approximate constant V x A over small intervals, just as a slight curve in the road over small distances are almost straight, but the further from the starting point you get the worse this approximation gets.

But I don't understand your dislike of a power supply with dual settings. Your power supply always have a current limit, whether it is a fuse, a weak component or in the optimal case, an electronic current limiter. Look at a power supply as an unlimited voltage and current supply and then add in two circuits, one to lower the current if the voltage is too high and one to lower the voltage if the current is too high. Only one circuit will work at a time, either controlling the current or the voltage. In your example above you were doing the same function as current limit would have done, adjusting the voltage so that the current stays within the 200A limit.

Wouldn't it have been practical to use a power supply with dual settings? Turn down the voltage to zero, add the parts, check that the current limit is set to 200A, turn up the voltage to max. The current limiter will pull down the voltage so the current never goes over 200A, in the beginning it will start at 2V and then after a few minutes when the base metal starts to be exposed it will automatically increase the voltage to maintain the maximum current of 200A. When the voltage has reached 20V it will not increase above that, now the current starts to drop from 200A down to virtually zero ampere and then the process is finished.
With two settings you don't need to adjust the voltage all the time, just turn off the voltage while changing parts and you wouldn't blow any fuses even if you accidentally short circuited the cell.

Even with a two adjustment supply you can't set the current and the amperage that goes to the cell, that is decided by the resistance in the circuit (the sum of the resistance in the power supply, leads and the cell). The settings is MAXIMUM values and the regulation sees that it never goes over the set values. When the load is high (low resistance) the current is at it's maximum while the voltage is regulated and when the load is low (high resistance) the voltage is at maximum and the current is regulated.

With the advances in electronics over time, a larger percent of new power supplies are switched supplies with built in current limit. Some times you can set it as in a lab aggregate and sometimes it is hidden and called "short circuit proof", just limiting the maximum current drawn.

I realize that we are looking at this from two different aspects, you as the refiner with a lot of practical experience and knowledge about refining. Me as an physicist and electronic designer with a lot of experience designing and working with all kind of power supplies over the last 30 years but with only refining experience as a hobby.

Below is a reply to the later postings.

Bswartzwelder is correct, we are both trained in electricity and electronics so when we see V x A we automatically think of it as effect (watts) as that is what it is. So if it was bswartzwelder and me starting to talk watt or GSP that started talking V x A first is not a big deal.

Harold, I have never seen a power supply where the current could be adjusted but not the voltage. In certain circuits it fills an important function, I've designed them myself sometimes, but as a general power supply it wouldn't be a good idea.

I hope I'm not sounding too disrespectful, this discussion have brought the response of two of the giants on which shoulders I'm standing on. Without the knowledge I've gained from Harold and GSP I would never have gotten so far in my own education in refining, I'm hoping I soon can show a 50g gold button as the result.

Göran

 

[Harold_V]

Harold, I have never seen a power supply where the current could be adjusted but not the voltage. In certain circuits it fills an important function, I've designed them myself sometimes, but as a general power supply it wouldn't be a good idea.

Nor have I. That's why I made this statement, above: "You have little control over the amperage, aside from the limits provided by the power supply on the top end. In essence, if you happen to have a power supply capable of delivering 1,000 amps, it will deliver only that which it can, at a given voltage. No more, no less (unless you have a direct short). If you wish to limit the amount of amperage, you can reduce the voltage, that I'll give you. It was made abundantly clear to me by those who operated the stripping cell I observed that you control the voltage. Nothing more."

I hope I'm not sounding too disrespectful, this discussion have brought the response of two of the giants on which shoulders I'm standing on. Without the knowledge I've gained from Harold and GSP I would never have gotten so far in my own education in refining, I'm hoping I soon can show a 50g gold button as the result.

A good, healthy discussion is never disrespectful. So long as people remain calm and present what they know, everyone benefits.
My problem is I know just enough about electricity to be dangerous. It has never been a study for me (save for one year in high school)---but I have considerable experience in wiring--as I've wired three shops (three phase delta), including installing the service and wiring the panels, and CT cans. I've also wired two houses.

I hope to learn from these discussions, and I'm waiting patiently to see that 50g button of yours.
Make certain you wash the resulting gold well before melting. If you do not, it will be obvious. 8)

Harold

Edit:
I don't recall if I made mention, but in the operation I observed, I mentioned a setting of 28 volts. That was the open circuit voltage that was selected. Actual voltage at the cell most certainly would have been lower, at least while the cell was removing gold. How far down I can not say.