Silver/Lead removal from solution (as chloride/sulfate).
Prepare a similar size beaker (as your reaction beaker) with a bunch of ice cubes in it. Now pour the bulk of the Gold bearing solution into the iced beaker and wash the remaining Gold solution from the ceramic pieces as per the attached video. Note: Don’t forget to examine the ceramic pieces for missed out gold (see video) Stir the solution and ice cubes well with a glass stirring rod and slowly add 10-15 ml of concentrated Sulfuric acid. Do remember, concentrated Sulfuric acid reacts very vigorously upon contact with water, so any addition should be done slowly and by the drop. The cold temperatures and dilution serve to precipitate Silver Chloride (AgCl). The Sulfuric Acid serve to precipitate Lead sulfate (PbSO4). Both of them are white colored salts and are barely distinguishable. Allow the solids to settle for 30-45 minutes. After that time, while the solution is still cold, start to filter the solution. Make sure the receiving flask is clean and scratches free as this will be the beaker which you will precipitate your Gold at. Filter the bulk of the solution, leaving the majority of the solids at the bottom so not to clog the filter. After most of the solution had gone through the filter, pour the remaining solids to the filter. Allow it to drain completely and then wash the filter and the solids in it with plenty of tap water until the filter is white again the wash water come out clear of color.
Precipitation of Gold Powder. We now have a Gold solution which is free from Silver and Lead. The solution it self now contains dissolved Gold and base metals such as Iron, Cobalt, Nickel, Tin and maybe some Copper. To selectively precipitate the gold, Sodium Metabisulfite (SMB - Na2S2O5) powder is added and stirred well. Via simple redox reaction, the SMB will reduce the Gold ions to its elemental state, i.e. to metal powder. Assuming no excess of nitric acid / nitrate salt is present, precipitation should become apparent in few minutes. A clear sign of precipitation will be a darkening of the solution. In case too much nitric acid or nitrate salt was added, usually brown fumes will be emitted and vigorous bubbling occurs once the SMB is added. In such case, Urea is needed to neutralize the remaining nitric acid. If you have followed this instructional to the letter, there should only be a minimal to no excess of nitric acid / nitrate salt, ensuring the AR is neutralized. It takes about 1 gram of dry SMB powder to precipitate 1 gram of gold metal from a fully neutralized Aqua Regia solution. Now, since we never really know just how much gold exactly there is in a given batch, we will need to add SMB according to a rough estimation with a slight excess. For example: If we have processed a 1 lb batch of Ceramic CPU's and we are expecting about 4-5 grams of Gold, then we should add about 7-10 grams of SMB. It is important to keep in mind, that unlike concentrated Gold Chloride solution, the precipitation in this case is very slow, mainly due to the dilution and the presence of other metals in the solution. The precipitation flask should be allowed to stand for at least 24 hours to ensure all of the gold will settle down to the bottom and agglomerate. As selective as the SMB may be, the precipitated gold is still not clean enough and usually will present it self as black powder rather then the familiar light brown mud. The following day, once the Gold powder had settled, TEST the solution with Stannous Chloride solution to ensure all of the Gold is out of the solution. If positive for gold, add more SMB, stir well and allow the solution to sit once again for 12-24 hours. If negative, slowly decant the barren solution to your acidic solutions waste tank. Be careful not to pour out any of gold powder. Use a filter to catch any stray particles if necessary. Wash the remaining powder several times with plain tap water.
This is basically the end of the Recovery Process. A proceeding refining cycle should be taken if high purity is desirable. From this point on you can treat this powder as high karat Gold and use known refining techniques as described in other sections here in Gold-N-Scrap.
