OK, let's begin with risk analysis. There is a form of self-deception that says that if we don't think about a risk, it will not happen to us. It is a confusion of subjective reality with objective reality that is often seen in children. I trust that none of the forum members are subject to this delusion.
The matrix depicted below shows that risk has two components: effect or damage, and probability or likelihood. The effect of a systemic poison like cyanide is catastrophic; the effect of dilute acid, even mixed with dilute hydrogen peroxide, is marginal. That means it isn't life-threatening or immediately fatal.
The matrix allows us to compare dissimilar hazards, and on the other hand it enables us to see how a comparison of one type of hazard with another can be misleading or deceptive.
RISK CAN BE MITIGATED. Take for example the very dangerous chemical reaction that occurs when oxygen is mixed with gasoline, and it is heated. Surely no responsible person would do such a thing. Probably I should not have written about it, because some impressionable new user will read about it and immediately go out and set himself on fire.
Cowardice is when you spend all your time inventing reasons not to do things for which there is manageable risk.
The risks of the internal combustion engine (gasoline + oxygen) have been completely mitigated, and that has made all the difference.
It is wise to remember the words of Herbert Spencer:
The ultimate result of shielding men from the effects of folly is to fill the world with fools.
Explosion Risk
Definitions:
"Piranha Solution" is a specific mix of concentrated sulfuric acid with hydrogen peroxide at a concentration of at least 35%. There is no such thing at "dilute piranha solution."
"Peroxysulfuric Acid” is formed by the mixture of sulfuric acid and hydrogen peroxide. This acid can be diluted. Therefore when people say “dilute piranha”, they actually mean “dilute peroxysulfuric”.
There are two types of explosion risk: the risk of physical damage, and societal risk.
Societal risk is where you have a loud explosion and your neighbor calls 911. The police arrive, and in the words of Ricky Ricardo, "You got a lot of splaining to do."
A lot of refiners like to maintain a low profile, due to the value of materials and the danger of the reagents used. Nothing raises your profile like an explosion, so this could be a good and sufficient reason to abandon an activity, if the probability is any higher than "unlikely". In any case, one should have a story to tell "authorities" if they inquire.
As for physical damage, there is damage due to blast overpressure, heat, shrapnel, and the effect of corrosive or flamable materials that may be thrown by the blast.
If you read safety manuals on this subject, like Department of the Army Pamphlet 385–64, "Ammunition and Explosives Safety Standards", they are full of quantity/distance tables, where for a given quantity of explosive, a minimal distance to other structures in mandated. They also have information on structures, armor and barricades that might be of some value in setting up a lab. But the q/d tables are of no use for our purposes, as we have no information regarding the explosive power of the materials we are handling. So we need to fall back on common sense.
Assume the explosive blast, if there is one, will be no greater than a powerful firecracker, like an M-80, that is approximately the same size as the potentially explosive material. With this assumption it immediately becomes clear that the risk can be reduced by reducing the size of potentially explosive material handled at any one time.
We can also see that it is wise to avoid glass containers when possible, as they would provide shrapnel in the event of an explosion.
Explosions of piranha solution have been reported in Chemical and Engineering News, volume 33, number 32, page 3336 and volume 38, number 59, page 1960.
Dilute peroxysulfuric acid is not going to explode, but organic peroxides that it forms might. These are formed from solvents such as those containing ketones (e.g. acetone, methyl ethyl ketone, etc.). This risk is probably higher in the waste processing business than in the jewelry business.
Additionally hydrogen gas liberated by the digestion of metals like zinc and aluminum could cause an explosion.
Spillage:
If one starts with concentrated acid, it must be diluted by pouring it into a borosilicate glass (Pyrex) container containing water. Always add acid to water. This is an exothermic reaction, but if the acid is added in portions, temperature should not exceed 80 degrees Celsius. I check with an infrared thermometer. When the mixture is below 35 deg. C. it can be poured into a plastic container for temporary use. Be sure to label it.
Safety equipment includes an open bucket of water nearby, aborbent material, a large box of sodium bicarbonate or a bag of hydrated lime from the hardware store, and materials for spill containment and control—that could be as simple as a bag of kitty litter. Rubber boots, a rubber or vinyl apron or a rainsuit, gloves that can be removed easily, and something that will effectively protect your eyes are needed. The safety goggles or shield need to stay on when you are bent over, as to wipe up a spill.
The problem is the blue waste liquid containing copper sulfate. It still contains acid and peroxide. It might contain organic peroxides. If you put it in a plastic container it might react with something and heat up to a temperature that would breach the container. If you put it in glass, you have a shrapnel hazard. So either way, put the first container into a larger one for safety.
I had one spill during the experiment. I was trying to vacuum filter the copper sulfate solution, and it failed completely due to crystallization. In taking it apart, about 25 ml of the blue liquid spilled onto a counter, with a few drops hitting the floor.
A local dollar store had been selling wash cloths at 6 for $1, so I bought some as shop rags. I dampened several and set them around the work area, so when the spill occured, it was just a matter of grabbing a cloth in my gloved hand and wiping up the spill. The first cloth was thrown into a sink, and then the process was repeated until all visible spills were gone.
What followed next was instructive. A spray bottle of ammonium hydroxide was available—normally used to clean glassware—so I sprayed it around the spill area. It not only neutralized the remaining acid, but it also "developed" the latent copper sulfate that was not visible after the first cleanup. The traces of copper became highly visible when sprayed with ammonia.
Fumes:
I'll address this risk in a subsequent message.
