Sulfuric cell
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First time electrolyzer de-plating
The following comments are not meant to detract from what you are doing, I'm merely trying to give you a better understanding of what factors are important when using the cell verses which ones are more trivial. Please accept them in this light.
I admire your attention to detail as far as the temperature and electrical parameters of the stripping cell reaction is concerned, but frankly a few degrees Celsius or Fahrenheit and/or minor dc volts or amps variations of your cell will not go to drastically affect the final outcome. If it makes you feel better about getting the reaction 'just right', you can continuously monitor your cell temperature, amps, and volts parameters, but all of it will vary with each and every unique type of scrap you are running.
Think of the gold plated part(s) you are stripping as another unknown (perhaps the biggest) and you will see where I'm coming from. In your quote above you mentioned a particular weight of 'board edge pins, DB sockets, cable connectors, etc...', but you do not specify the manufacturer, year of manufacture, application, base metal, etc. These factors all will affect the yields as well as the results of the observations you are charting, maybe even more so than the temperature, voltage, and amperage. In fact the physical characteristics of the gold plated part may be the biggest contributing factor to the variations you measure and for this reason your values will always be different for each and every unique type of scrap.
In short the generic terms like cold, hot, and 12 v 10 A battery charger are more than accurate enough to get a cell operational and producing gold. If you need a more accurate assessment of the temperatures, then let me suggest these operating conditions for the hobbyist cell set up:
1. Wear goggles, protective gloves, and a good acid resistant apron. Start with your sulfuric acid at room temperature (20C). The sulfuric acid should be of 95% concentration or higher.
2. Use a standard 'manual' automotive charger with a built in analog amp meter (one with a moving needle) that is capable of delivering 12v DC @ 10 amps on a continuous basis. The automatic variety of chargers (with led bar graph amp meters) should not be used. The automatic types contain electronic circuits used to sense the battery condition and control the charging of batteries while protecting them and you from harm. These extra circuits don't 'play nice' (new technical term) with the cell set up.
3. Attach a piece of lead to the negative lead of the charger and hang it from one side of your cell set up.
4. Attach the positive lead (copper alligator clip) of the charger to the part to be stripped, or to a copper basket arranged to hold smaller items in bulk. The size of your cell and consequently the basket size will limit the mass of small items you can strip in one run. Be sure the negative lead has an uninterrupted path to the items to be stripped. Imagine a line of sight (LOS) between the part(s) to be stripped and the negative lead. If this LOS is broken by the copper metal from the front edge of the basket, the basket will act as a shield (aka: farady shield) to the pins and slow the strip time and heat the acid. Once the gold is stripped, remove the part as soon as possible.
I have noticed that the longer a stripped (ie: gold is all removed) part stays in the electrolyte with voltage applied, the more the cell will heat. The energy of the charger is no longer being used to strip gold, instead it is being conducted to the negative lead and generating heat very quickly once the gold is removed. In a perfect world we could calculate the gold yielded from a stripped part by the amount of time required to strip the part in the cell and recording the amount of amps used. In the real world the electrons are conducted along various side paths in addition to being used to strip the gold and we as humans don't remove the part at the exact moment all of the gold is stripped, so the real number of amps consumed is a sum of all of the losses, plus the amps required to strip the gold.
5. Rinse the stripped part in water (or baking soda water) over a container sized as needed for your cell set up. I like to use a spray bottle of straight water, it's quick, conserves water, and the rinse water is always clean that touches the part(s). The rinse pot eventually accumulates a layer of black gold powder on the bottom after extended use followed by a settling period. This black powder is combined with the cell yields.
6. Dab the rinsed part onto a dry paper towel. I like paper towels because they can be burnt to recover any traces of gold in them. One paper towel goes a long way if folded properly and allowed to dry between cell runs. This step is very important to the life of your copper clips and basket. Be sure your clip/basket has most of the water removed from it before placing it back in the cell.
7. Repeat steps 4 through 6 until the cell gets hot to the touch. If you are using a thermometer to determine when your cell is hot, stop when the cell reaches 90C. A cell gets hot for several reasons:
A. Normal stripping of gold.
B. Leaving stripped parts in the electrolyte with the power applied after they have been stripped.
C. Adding water to the cell. Either via air, intentionally, or unintentionally.
D. Your cell is too small for the batch sizes you are running.
E. Shorting of the positive and negative electrodes.
F. Higher amperage flowing per second in a given volume of sulfuric acid.
G. Other factors
Notes:
A. Diluted sulfuric acid, especially when 'hot' (ie > 60C) will eat copper (and many other metals) quickly. Dilution can occur via rinse water trapped on the surface of the anode lead or from absorption through the air.
B. If you rinse your copper clips or basket and do not remove the excess rinse water from the copper before adding it back into the sulfuric acid cell, you are in effect placing your copper into diluted sulfuric acid and will experience a much shorter life cycle for these items. Drying your leads/basket between 'dip and rinse' cycles will greatly extend the life of these parts.
C. Parts must be fully exposed to the sulfuric acid in order for them to strip. Sounds obvious, but a large mass of pins will mechanically shield one another from exposure to the sulfuric acid and the pins on the bottom and inside of the pile will not strip completely or at all.
D. A cell is full and should be cleaned out when the sulfuric acid is saturated with black powder to the point where the powder short circuits the negative and positive leads of the cell when in normal operation. This shorting can occur on the bottom (most common) or on top of the electrolyte as floating blobs of gold and bubbles. Try to sink any floaters with a glass rod to prevent the build up of top floaters. Proper placement of the positive and negative leads can extend the time required between clean outs. The leads should be mounted so that they do not rest directly on the bottom of the cell. Let everything settle for 24 hours before you clean out the cell. You may find the sulfuric acid clears (straw colored to green brown) after the cell settles. This upper layer of acid can be carefully siphoned off for use in the next cell without any further treatment. The black sludge is treated as described in my Black Powder from the Cell post.
You may want to clean out your cell sooner if you want to determine yield data for the scrap in question or you are out of material to process and want to store the cell.
E. The greater the flow of amps in a cell, the greater the number of gold plated parts the cell can strip in the same amount of time. This heats the cell faster as well. The larger the volume of acid in the cell the longer it takes to heat up.
I'm interested in you CCPWM results, but I'm going to predict it will take longer to strip the same parts than with pure DC. The off cycles do not supply the electrons that do the work of stripping the gold atoms off of the part(s). Less electrons in a given time equals less gold stripped in that time.
Steve
Black Powder from Cell
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When your cell is full you should:
1. Let all the black powder settle
2. Pour off the bulk of the concentrated sulfuric acid. Don't worry about the small amount of residual black powder that is in the acid that is poured off as you can get it on the next batch.
3. The remaining acid with the bulk of the black powder in it should be slowly added (let it cool before adding more) to five or six times it's volume of water.
4. Stir this very well and allow to settle again.
5. Siphon off the colored solution. Repeat this process until the wash is no longer colored.
6. Test a few drops of the rinse water with a drop or two of 3% unscented clear household ammonia, if the rinse water turns blue when the ammonium hydroxide is added, copper is still present and more rinsing is required.
7. Dissolve the black powder with AR or HCl-Cl and proceed as typical for these processes.
The concentrated acid that was poured off is used in your next cell run as is. Hot concentrated acid is very dangerous, so exercise extreme caution when handling it. Never add water to concentrated sulfuric acid/powder mixture, add the acid/powder slowly to the water instead.
Steve
Harold's wash procedure
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Lately I've been swamped with people asking me how to wash the gold powder so here's a quick tutorial on how I handle it. This process is a variation of Harold's wash procedure.
1. After precipitating the gold with SMB, test for gold in solution with stannous chloride, if present (purple swab) add more SMB until test is negative.
2. Let the gold powder settle to the bottom of the beaker over night.
3. Siphon or gently pour off the used SMB solution into an old milk jug labeled Used SMB. Save in your stock pot or properly dispose of.
4. The gold powder stays in the same beaker you precipitated it in.
5. Add just enough water to cover the gold powder and boil for 5 minutes (Removes undissolved and water soluble salts)
6. Let settle and pour or siphon off the water into the stock pot.
7. Repeat three times.
8. Add just enough muriatic acid to cover the gold powder.
9. Boil until the acid is no longer discolored by the process and pour or siphon off into the stock pot after testing with stannous. If gold is present keep in separate beaker to precipitate later.
10. Repeat the water rinse as above.
11. If you suspect silver is present as a contamination, repeat the process with hot 3% household ammonium hydroxide (unscented- clear). Put these rinses in a separate container and add muriatic acid until no more white silver chloride precipitates. You have added enough HCl when there are no more white fumes coming out of the liquid. Any karat, dental, cpu lid, or gold filled scrap will have silver as a contaminate.
12. Finish up the rinses with three more hot water rinses. Put these rinses in with the ammonium hydroxide rinses.
13. Gently heat the beaker containing the gold powder while swirling the beaker. As the gold dries it will begin to clump and move around the beaker freely. Don't allow the gold to stick to the beaker.
14. When the gold is 100% dry it should be a nice light tan color, if not redissolve it and precipitate again with SMB. Repeating the above cleaning process after precipitation. Good clean gold settles very quickly when precipitated and clumps into nice balls when dried. No gold should be stuck to the beaker when you are done.
15. When completed transfer the gold to a properly prepared melting dish and melt into a button.
Steve
