4metals,
Everyone does things differently. I know that everything in your procedure will work. However, in several areas, I prefer to do things in another way.
10-14 - Although I have produced and converted silver chloride many 100s of times, I have learned to not produce it unless it is absolutely unavoidable to do so - e.g., the AgCl that is produced when directly dissolving karat gold in AR. Unless the AgCl is processed immediately, and not aged, it is often difficult to get a 100% conversion. If you have several 100 oz of aged AgCl, it can also filter and rinse very slowly. It can become very pasty and it tends to pack tightly in a vacuum filter.
I much prefer to use copper (buss bar is best) to cement out the silver. It only takes 2 steps (cementation, filtering), whereas AgCl takes 4 (precipitation, filtering, conversion, filtration). Copper cementation removes 100% of the silver. You knows it's finished when a drop of HCl or salt water produces no white cloud. Even though cementation only produces 99% silver, at best, that is fine if you're using the silver for inquartation. With aged AgCl, I doubt if the purity is any better, considering the rinsing problems.
BTW, what is the frying pan method?
15-18 - I'm a huge believer in using the HCl and HNO3 separately, rather than premixing them. Generally, the gold powder is first covered with an excess of HCl. The HNO3 is then added in small increments. When an increment stops fizzing, more is added. When a small addition produces no reaction, the gold is dissolved - this assumes that an excess of HCl was added to start with. After doing this a few times, knowing when you hit the endpoint becomes second nature. If, when learning, you overshoot the nitric, just boil it down as normal.
This method eliminates the need to boil the solution down or to add urea. I don't like urea at all for this application. If you add too much, it will form a precipitate. I also felt that I got purer gold when I didn't use urea.
A very slight excess of nitric acid will cause no problems when you use SMB to drop the gold. The SMB will first react with the excess nitric and then it will drop the gold with no problems. If there is much more than a slight excess of nitric, this process won't work well, since it will take a ton of SMB to kill the nitric. I actually prefer using sodium sulfite. It seems to stink less than when using SMB.
16 - Instead of using ice, I prefer to add 3 volumes of water to the AR. This precipitates out the greater bulk of AgCl that was dissolved in the strong AR. It actually might be best to dilute AND use ice. I also prefer dropping the gold from a more dilute solution.
20 - To be safe rather than sorry, I always added a little sulfuric to the aqua regia before filtering. It won't hurt anything.
Everyone does things differently. I know that everything in your procedure will work. However, in several areas, I prefer to do things in another way.
10-14 - Although I have produced and converted silver chloride many 100s of times, I have learned to not produce it unless it is absolutely unavoidable to do so - e.g., the AgCl that is produced when directly dissolving karat gold in AR. Unless the AgCl is processed immediately, and not aged, it is often difficult to get a 100% conversion. If you have several 100 oz of aged AgCl, it can also filter and rinse very slowly. It can become very pasty and it tends to pack tightly in a vacuum filter.
I much prefer to use copper (buss bar is best) to cement out the silver. It only takes 2 steps (cementation, filtering), whereas AgCl takes 4 (precipitation, filtering, conversion, filtration). Copper cementation removes 100% of the silver. You knows it's finished when a drop of HCl or salt water produces no white cloud. Even though cementation only produces 99% silver, at best, that is fine if you're using the silver for inquartation. With aged AgCl, I doubt if the purity is any better, considering the rinsing problems.
BTW, what is the frying pan method?
15-18 - I'm a huge believer in using the HCl and HNO3 separately, rather than premixing them. Generally, the gold powder is first covered with an excess of HCl. The HNO3 is then added in small increments. When an increment stops fizzing, more is added. When a small addition produces no reaction, the gold is dissolved - this assumes that an excess of HCl was added to start with. After doing this a few times, knowing when you hit the endpoint becomes second nature. If, when learning, you overshoot the nitric, just boil it down as normal.
This method eliminates the need to boil the solution down or to add urea. I don't like urea at all for this application. If you add too much, it will form a precipitate. I also felt that I got purer gold when I didn't use urea.
A very slight excess of nitric acid will cause no problems when you use SMB to drop the gold. The SMB will first react with the excess nitric and then it will drop the gold with no problems. If there is much more than a slight excess of nitric, this process won't work well, since it will take a ton of SMB to kill the nitric. I actually prefer using sodium sulfite. It seems to stink less than when using SMB.
16 - Instead of using ice, I prefer to add 3 volumes of water to the AR. This precipitates out the greater bulk of AgCl that was dissolved in the strong AR. It actually might be best to dilute AND use ice. I also prefer dropping the gold from a more dilute solution.
20 - To be safe rather than sorry, I always added a little sulfuric to the aqua regia before filtering. It won't hurt anything.
