The principles behind a copper cell.
Our purpose in refining copper is primarily to concentrate the precious metals we have collected in copper based bullion anodes. For years I used smelting techniques, which we have discussed in the smelting thread, to separate the metals, which can be oxidized out of the copper. The remaining copper holds the precious metals, which are separated out by the electrolytic copper refining process and at the same time concentrated in the anode slimes.
Since so many of our members pursue refining as a hobby and thus on a small scale, the beauty of the electrolytic copper process is it can be scaled to work in a tank as small as a beaker all the way up to cells the volume of swimming pools. Ironically silver cells behave the same way, once you get the hang of the chemistry in the cell, it works on 1 gallon cells (like Kadriver’s stainless steel bowl cell) up to cells making thousands of ounces a day.
So lets start with the electrolyte.
Copper as copper sulfate 45 – 50 g/L
Sulfuric acid 170 – 200 g/L
The following additives, used in very small quantities, are to level the deposit and affect the grain of the deposit making it denser.
Bone glue, 1 gram for every 25 pounds of anode processed thru the solution
Thiourea, 1 gram for every 20 pounds of anode processed thru the solution
NaCl, 0.04g/L
Operating temperature, 60 C (140 F)
Anode bags 13 ounce polynap fabric. (Available from Anode Products of Schiller Park, IL)
Large copper producers control their organic additives to the point that the use of anode bags is not necessary; the slimes are pulled from the solution by filtration and vacuumed off the bottom of the cell. In my experience, I have always had better luck with anode bags, but the biggest system I have ever designed or worked with produced 200 pounds of pure copper per day. I am a believer in anode bags, most impurities in commercial copper come from carry over from particulate entrapment in the cathode deposit, and the right anode bags will prevent that.
Current density 25 - 28 Amps/sq. ft.
Anode material is the copper collector material we produced from the smelting process with a copper content >98.5%. I should add here that I have had success while the copper content was as low as 96% if the 2 1/2% difference is metals that will not dissolve but will end up in the slimes. When the percentage of precious metals in the bar gets above 4%, aside from being a good problem to have, the anodes can passivate and your efficiency goes bad. The fix is to add pure copper and recast your anodes. Any decent sized refinery will be fire assaying their bars after they are smelted and cast anyway so this is easy to prevent.
Large producers cast anodes which are large flat plates with tabs on top to hang directly on the buss bars but smaller producers use titanium anode hooks which thread into a tapped hole drilled into the rectangular cast anode. These hooks look like this;
Cathodes were classically a high purity copper sheet on which the copper built up to the point that they were removed and washed and cast into pure copper ingots. So the copper sheet is an expendable supply, which you will need to replace routinely. The method of choice used today is a 316 stainless steel plate which has a plastic strip covering the edges so no plating deposits on the edge. When the plating builds up the cathode sheet is removed, rinsed, and the edge covering is removed allowing the easier removal of the deposited copper. Once the copper is peeled off, the strips are replaced and the cathode plate is re-used over and over.
Typically, a small plating unit will have 10 rows of anodes and 9 rows of cathodes. These are arranged so there is about a 2” space between the plates. A unit of this size requires a 300 Amp rectifier and can produce about 200 grams of copper per hour. I mention this size because it is fitting for a refiner who processes small quantities of copper base bullion or one that sees a lot of gold plated copper scrap which can often be melted into anodes and processed (after sampling and assay) in the cell. For some reason, this size cell, producing about 75 pounds of refined copper a week, is popular with small to medium sized refiners who do not specialize in electronic scrap but see it enough to process it in house.
Of course this will work with a beaker containing an anode standing on one side with a battery clip (+ plus connection) connecting to the power supply, and a cathode standing on the opposite side with a battery clip (- negative connection) to the power supply.
So what happens to the impurities that remained in your anode material?
Some of these impurities will end up in the anode slimes. These metals include Gold, Platinum, Palladium, (all platinum group metals) Selenium, Tellurium, Lead, and Tin. Silver does have some ability to dissolve in the electrolyte, but in practice 99% of silver ends up in the slimes. Some copper also ends up in the slimes but rarely more than a few percent. Generating these slimes is the main reason a refiner uses a copper electrolytic cell, but for this thread we are only discussing the process of producing the concentrate, affectionately called slimes, we will not discuss refining the slimes yet. That will have to wait for the completion of the Smelting, Electrolytic copper, Slimes refining trilogy.
The metals that will dissolve in the electrolyte are Arsenic, (Arsenic is actually a metalloid) Bismuth, Cobalt, Iron, Nickel, and Antimony. With magnetic separation and proper treatment in the smelting process, these elements can be kept to a minimum. Fortunately copper plates out at a lower applied potential than these elements so they remain in solution. Eventually, they will require the solution to be changed out to lower their concentrations.
The copper plating baths are easily controlled with simple titrations to control the copper levels (EDTA titration) and the sulfuric acid levels (NaOH titration). While the levelers and grain density additions can be controlled in larger cells with a Hull cell test, smaller cells can even do without these additions by using the proper anode bags. One caution is excessive dendritic growth can short out plates which are closely spaced, so if you cannot tend to your cells daily and plan to run continuously, additives will buy you time between required cathode change outs.
These are the edges of stainless steel cathodes from a clients system, they do not maintain the organics closely (they do it by pounds of anode added) and they do bag their anodes. Their feedstock is from smelted circuit board metallic fractions.
Back in the 1980’s I was smelting prepared jewelers sweeps in a rotary furnace with copper as a collector, the PM’s were recovered in copper cells with the copper recycled back to perform duty as a collector and the PM’s collecting in slimes in the cell room which had about 10 or 12 cells (I can’t remember exactly) which processed all of the bars generated from the sweeps melting. Since all bars were assayed separately we were able to segregate the bars with PGM’s from those without and generate slimes that either had or didn’t have PGM’s in them. That made for easier recovery. The combination of being able to mix prepared and assayed sweeps into rotary melt lots and the ability to place anodes into the proper copper cell made it easier to keep the PGM’s separate. And thus a point that I may have made once before comes up again; a good analytical lab is the best support and avenue to refining success there is.
Our purpose in refining copper is primarily to concentrate the precious metals we have collected in copper based bullion anodes. For years I used smelting techniques, which we have discussed in the smelting thread, to separate the metals, which can be oxidized out of the copper. The remaining copper holds the precious metals, which are separated out by the electrolytic copper refining process and at the same time concentrated in the anode slimes.
Since so many of our members pursue refining as a hobby and thus on a small scale, the beauty of the electrolytic copper process is it can be scaled to work in a tank as small as a beaker all the way up to cells the volume of swimming pools. Ironically silver cells behave the same way, once you get the hang of the chemistry in the cell, it works on 1 gallon cells (like Kadriver’s stainless steel bowl cell) up to cells making thousands of ounces a day.
So lets start with the electrolyte.
Copper as copper sulfate 45 – 50 g/L
Sulfuric acid 170 – 200 g/L
The following additives, used in very small quantities, are to level the deposit and affect the grain of the deposit making it denser.
Bone glue, 1 gram for every 25 pounds of anode processed thru the solution
Thiourea, 1 gram for every 20 pounds of anode processed thru the solution
NaCl, 0.04g/L
Operating temperature, 60 C (140 F)
Anode bags 13 ounce polynap fabric. (Available from Anode Products of Schiller Park, IL)
Large copper producers control their organic additives to the point that the use of anode bags is not necessary; the slimes are pulled from the solution by filtration and vacuumed off the bottom of the cell. In my experience, I have always had better luck with anode bags, but the biggest system I have ever designed or worked with produced 200 pounds of pure copper per day. I am a believer in anode bags, most impurities in commercial copper come from carry over from particulate entrapment in the cathode deposit, and the right anode bags will prevent that.
Current density 25 - 28 Amps/sq. ft.
Anode material is the copper collector material we produced from the smelting process with a copper content >98.5%. I should add here that I have had success while the copper content was as low as 96% if the 2 1/2% difference is metals that will not dissolve but will end up in the slimes. When the percentage of precious metals in the bar gets above 4%, aside from being a good problem to have, the anodes can passivate and your efficiency goes bad. The fix is to add pure copper and recast your anodes. Any decent sized refinery will be fire assaying their bars after they are smelted and cast anyway so this is easy to prevent.
Large producers cast anodes which are large flat plates with tabs on top to hang directly on the buss bars but smaller producers use titanium anode hooks which thread into a tapped hole drilled into the rectangular cast anode. These hooks look like this;
Cathodes were classically a high purity copper sheet on which the copper built up to the point that they were removed and washed and cast into pure copper ingots. So the copper sheet is an expendable supply, which you will need to replace routinely. The method of choice used today is a 316 stainless steel plate which has a plastic strip covering the edges so no plating deposits on the edge. When the plating builds up the cathode sheet is removed, rinsed, and the edge covering is removed allowing the easier removal of the deposited copper. Once the copper is peeled off, the strips are replaced and the cathode plate is re-used over and over.
Typically, a small plating unit will have 10 rows of anodes and 9 rows of cathodes. These are arranged so there is about a 2” space between the plates. A unit of this size requires a 300 Amp rectifier and can produce about 200 grams of copper per hour. I mention this size because it is fitting for a refiner who processes small quantities of copper base bullion or one that sees a lot of gold plated copper scrap which can often be melted into anodes and processed (after sampling and assay) in the cell. For some reason, this size cell, producing about 75 pounds of refined copper a week, is popular with small to medium sized refiners who do not specialize in electronic scrap but see it enough to process it in house.
Of course this will work with a beaker containing an anode standing on one side with a battery clip (+ plus connection) connecting to the power supply, and a cathode standing on the opposite side with a battery clip (- negative connection) to the power supply.
So what happens to the impurities that remained in your anode material?
Some of these impurities will end up in the anode slimes. These metals include Gold, Platinum, Palladium, (all platinum group metals) Selenium, Tellurium, Lead, and Tin. Silver does have some ability to dissolve in the electrolyte, but in practice 99% of silver ends up in the slimes. Some copper also ends up in the slimes but rarely more than a few percent. Generating these slimes is the main reason a refiner uses a copper electrolytic cell, but for this thread we are only discussing the process of producing the concentrate, affectionately called slimes, we will not discuss refining the slimes yet. That will have to wait for the completion of the Smelting, Electrolytic copper, Slimes refining trilogy.
The metals that will dissolve in the electrolyte are Arsenic, (Arsenic is actually a metalloid) Bismuth, Cobalt, Iron, Nickel, and Antimony. With magnetic separation and proper treatment in the smelting process, these elements can be kept to a minimum. Fortunately copper plates out at a lower applied potential than these elements so they remain in solution. Eventually, they will require the solution to be changed out to lower their concentrations.
The copper plating baths are easily controlled with simple titrations to control the copper levels (EDTA titration) and the sulfuric acid levels (NaOH titration). While the levelers and grain density additions can be controlled in larger cells with a Hull cell test, smaller cells can even do without these additions by using the proper anode bags. One caution is excessive dendritic growth can short out plates which are closely spaced, so if you cannot tend to your cells daily and plan to run continuously, additives will buy you time between required cathode change outs.
These are the edges of stainless steel cathodes from a clients system, they do not maintain the organics closely (they do it by pounds of anode added) and they do bag their anodes. Their feedstock is from smelted circuit board metallic fractions.
Back in the 1980’s I was smelting prepared jewelers sweeps in a rotary furnace with copper as a collector, the PM’s were recovered in copper cells with the copper recycled back to perform duty as a collector and the PM’s collecting in slimes in the cell room which had about 10 or 12 cells (I can’t remember exactly) which processed all of the bars generated from the sweeps melting. Since all bars were assayed separately we were able to segregate the bars with PGM’s from those without and generate slimes that either had or didn’t have PGM’s in them. That made for easier recovery. The combination of being able to mix prepared and assayed sweeps into rotary melt lots and the ability to place anodes into the proper copper cell made it easier to keep the PGM’s separate. And thus a point that I may have made once before comes up again; a good analytical lab is the best support and avenue to refining success there is.
