Some thoughts on boiling.
In thinking back, there are certain things I boiled. Also, Harold has mentioned that he boiled certain things, although I can't remember what.
As a general rule, I never boil anything in which any values are dissolved, since there could easily be losses in the vapors. On the other hand, if I am leaching out base metals away from the values, I think it's usually safe to boil.
For example, I used to run a lot of 10K class rings in 4 liter beakers. I found that, in reality, most were closer to 9K, and I could leach out the silver and base metals with nitric acid. This was easier and faster than inquarting or dissolving directly in AR. However, this nitric dissolving went slowly. To speed this up, I used a fairly strong nitric, about 2/3, by volume, and boiled the solution. After a couple of hours, I had a fairly pure gold sponge, still in the perfect shape of the original class rings - I could still read the words and see the emblems on the brown ring. When I probed the gold rings with a stir rod, they fell apart. If I felt any solid portions, I continued the nitric. Then, I poured off the nitric, rinsed a couple of times, and easily dissolved the gold sponge in AR. I'm sure I lost a little silver in the vapors but, it wasn't much and, at that time, silver was low and I wasn't that concerned about it.
Another place I often boiled, especially with small quantities, was when dissolving Kovar away from gold plate and gold braze on IC package lids or when dissolving the base metals away from the gold on entire all-gold CPU packages. Kovar dissolves slowly and boiling really speeds it up and insures 100 dissolution.
I also have boiled when leaching gold-filled with nitric.
In any case, when I did boil, I always covered the container. With beakers, I used watch glasses. If I had no watch glasses, non-porous acidproof and heatproof saucers from the kitchen are of similar shape and they worked almost as well. When boiling, I first covered the material with water, heated it below the boiling point, and then added the nitric in increments, small ones at first. After it got going well, I increased the temperature. This, along with not running too much material at one time, helps prevent boilovers.
This whole thing about boiling or not boiling depends on the situation (and, sometimes, the equipment available). In some cases, it's OK and even desirable. In other cases, you should never do it. It reminds me of the rule, "Never add water to acids. Always add acids to water." With very concentrated acids, such as concentrated sulfuric, you must ALWAYS follow this rule. If you add the water to the acid, it will immediately heat up at the point of entry, turn to steam, and splatter acid in your face. Even when you properly add the sulfuric to the water and make up a 50/50 sulfuric solution, it will get close to the boiling point. However, with less concentrated acids, such as 70% nitric or 32% hydrochloric, it is perfectly safe to add the water to the acid, as anyone doing this for awhile knows. All of these things require knowledge, though, and you should definitely follow the general rules until you have enough experience to know what you are doing.
In thinking back, there are certain things I boiled. Also, Harold has mentioned that he boiled certain things, although I can't remember what.
As a general rule, I never boil anything in which any values are dissolved, since there could easily be losses in the vapors. On the other hand, if I am leaching out base metals away from the values, I think it's usually safe to boil.
For example, I used to run a lot of 10K class rings in 4 liter beakers. I found that, in reality, most were closer to 9K, and I could leach out the silver and base metals with nitric acid. This was easier and faster than inquarting or dissolving directly in AR. However, this nitric dissolving went slowly. To speed this up, I used a fairly strong nitric, about 2/3, by volume, and boiled the solution. After a couple of hours, I had a fairly pure gold sponge, still in the perfect shape of the original class rings - I could still read the words and see the emblems on the brown ring. When I probed the gold rings with a stir rod, they fell apart. If I felt any solid portions, I continued the nitric. Then, I poured off the nitric, rinsed a couple of times, and easily dissolved the gold sponge in AR. I'm sure I lost a little silver in the vapors but, it wasn't much and, at that time, silver was low and I wasn't that concerned about it.
Another place I often boiled, especially with small quantities, was when dissolving Kovar away from gold plate and gold braze on IC package lids or when dissolving the base metals away from the gold on entire all-gold CPU packages. Kovar dissolves slowly and boiling really speeds it up and insures 100 dissolution.
I also have boiled when leaching gold-filled with nitric.
In any case, when I did boil, I always covered the container. With beakers, I used watch glasses. If I had no watch glasses, non-porous acidproof and heatproof saucers from the kitchen are of similar shape and they worked almost as well. When boiling, I first covered the material with water, heated it below the boiling point, and then added the nitric in increments, small ones at first. After it got going well, I increased the temperature. This, along with not running too much material at one time, helps prevent boilovers.
This whole thing about boiling or not boiling depends on the situation (and, sometimes, the equipment available). In some cases, it's OK and even desirable. In other cases, you should never do it. It reminds me of the rule, "Never add water to acids. Always add acids to water." With very concentrated acids, such as concentrated sulfuric, you must ALWAYS follow this rule. If you add the water to the acid, it will immediately heat up at the point of entry, turn to steam, and splatter acid in your face. Even when you properly add the sulfuric to the water and make up a 50/50 sulfuric solution, it will get close to the boiling point. However, with less concentrated acids, such as 70% nitric or 32% hydrochloric, it is perfectly safe to add the water to the acid, as anyone doing this for awhile knows. All of these things require knowledge, though, and you should definitely follow the general rules until you have enough experience to know what you are doing.
