Dr. A.K. Williams, Ph.D
San Pedro, Costa Rica
Everyone who has worked with and/or prospected for gold has on at least a few occasions been forced to use acids or bases in one form or another. These chemicals can be indispensable for cleaning concentrates, cleaning mercury, recovering mercury, gold, silver from solutions of the salts and hundreds of other applications, many of which most of us are not even aware. I am going to try to explain some of the characteristics, uses and precautions that everyone should be aware of when using these chemicals.
What they are
First the acids. What is an acid? As we have stated in other pages on this website, an acid is a chemical which when diluted with a little water gives a pH of less than 7. Acids are classified into two major groups. Strong acids and weak acids. Strong acids are generally acids such as sulfuric, hydrochloric, nitric, and phosphoric. Examples of weak acids are acetic, carbonic, citric and ascorbic.
Furthermore, acids are classified as to their composition. There are inorganic acids and there are organic acids. Basically, the word "organic" means, "containing carbon". So, of the above mentioned acids the strong acids are all "inorganic" and the weak acids are all "organic acids. O.K., someone is going to jump right down my throat because of the fact that carbonic acid does contain carbon. However, it is usually considered the exception to the rule. The rule is that most inorganic acids are strong and most organic acids are weak.
Bases are just the opposite of acids. They give a pH of more than 7 in water solution. As above there are strong bases, weak bases, inorganic bases and organic bases. Some of the more useful bases are sodium hydroxide (caustic soda), Potassium hydroxide (lye), Ammonium hydroxide (ammonia) and calcium hydroxide (slaked lime). These are all strong bases. The weak bases have little practical use in mining so we will just ignore them. Just one example of a weak base, sodium bicarbonate (baking soda).
Some Characteristics of Acids and Bases
At this point we are only going to talk about the strong acids and bases that are useful in the prospecting/mining business. Acids and bases come in different strengths. Some acids are "stronger" than others. This "strength" is expressed as "normality". Hooray!! We are not going into that except as a reference as to relative strengths of acids/bases. This will give you an idea of these relative strengths. The higher the normality, the "stronger" the acid.
- Sulfuric acid H2SO4 - 36N
- Hydrochloric acid HCl - 12N
- Nitric acid HNO3 - 16N
- Ammonium Hydroxide - 15N
As you can see from these numbers Concentrated sulfuric acid is three times as strong as hydrochloric acid. It would take three times as much hydrochloric to neutralize X amount of base as it would sulfuric acid.
Precautions and handling
It is not always the strength of the acid or base that determines how it is going to affect you if you get some on you. For example, nitric acid will burn you. It will produce a brown, discolored patch on your skin. This will be superficial (surface) only unless it stayed on you too long. On the other hand, sulfuric can almost instantly cause deep burns which are not superficial. This is because sulfuric acid has a GREAT affinity for water. It will literally attack water wherever it finds it and produces LOTS of heat. This is what causes the burns. Another characteristic of sulfuric acid is that it destroys cotton clothing. Same reaction as with water. You probably will not notice your Blue Jeans until you wash them then you discover that they are now only about one square yard of holes. This affinity of sulfuric acid for water is why you NEVER pour water into concentrated sulfuric acid. The reaction and liberation of heat can be, and usually is, violent. It can be explosive and ruin your whole week by drenching you in hot concentrated sulfuric acid. Pay attention to me, DONíT do this. There is an old crutch to help you remember how to mix any strong acid with water. "Do like you outta, put the acid in the water". NOT the other way around. This is a good rule to follow when diluting any strong acid but PARTICULARLY sulfuric.
Now, a little about strong bases, caustic soda, lye and ammonium hydroxide. Do any of you know of anything that can burn you more seriously than sulfuric acid? No? Well let me tell you what it is. Strong solutions of strong bases. Strong solutions of caustic soda or lye are very deceptive. They donít sting or burn or hurt immediately. They just feel "slick", like you had oil on your hands. Have you ever got clorox on your hands and they feel "slick" and you canít wash it off? The reason you canít wash it off is that bases penetrate skin immediately. Now clorox is not a real strong solution, only 5%. It ainít NOTHING compared to a good strong solution of lye. Have any of you guys ever made homemade soap? You take animal fat and treat it with lye. That is exactly what happens when you get it on you. It converts all the oils and fats in your skin, which partially protected you from the acids, into soap and glycerin. The slick feel. With no protection lye or caustic will penetrate your skin in seconds. And, it kills everything as it goes. No pain, no burns, but in a few hours your skin will simply slide off onto the floor leaving raw, exposed, muscle tissue. It is painful and very slow healing. One other point; Remember, chemicals react dependent on temperature. If you raise the temperature 10 C the rate at which a chemical reacts or burns you doubles. Hot caustic is something you do NOT want on you.
The moral to this story is, use common sense, use plastic gloves and if appropriate, face protection. DONíT GET HURT!!
Salts? How did we get to that? Thought this page was about acids and bases. Well, you just canít talk about acids and bases without discussing salts. Salt! Thatís what we put on French fries etc. Yeah, thatís one salt, sodium chloride. There are thousands of others that you definitely would NOT want to put on the fries. So, what the ----- is a salt? Glad you asked! A salt is the chemical that is formed when an acid reacts chemically with a base. And, some of them react violently so you should know a little about them. I ainít gonna let you get hurt if you will just hang with me. In fact the whole point of this site is to allow you to use chemicals to get your job done without getting into trouble. Now we know what a salt is, more or less. Example; if you take some hydrochloric acid (you really wouldnít want to drink it) and a strong solution of caustic soda (you damn well better NOT drink it) and you start mixing the two you will notice that the solution starts to get hot. Itís reacting. If you take some "pH paper" and add one to the other until the pH is 7or neutral you have made a salt. You are a chemist because you have just reacted an acid with a base to make a salt with the liberation of energy as heat. Now, donít you feel great! You did it, you made a salt. How do you know you did? It still just looks like water with some acid and base in it. The pH is 7, neutral, neither acid nor basic. Why canít I see a change? Because the salt you made has no color. How can I see what I made? Simple, evaporate the water and you will now find a white crystalline substance left behind. This is your salt made from poisonous caustic soda and poisonous hydrochloric acid. NOW, you can take this stuff and PUT IT ON YOUR FRENCH FRIES! You have succeeded in making sodium chloride, common table salt!! Howís that for a practical application of basement chemistry. Iím sure some of you will feel that you have been "took". All that just to learn how to make table salt? Why waste the time? If you understood it then it WASNíT a waste. You can now apply the same reasoning to make any salt you want. If you had used lye instead of caustic you would have produced potassium chloride (salt substitute). The reaction we just made is represented as:
HCL + NaOH ------ŗ NaCl + HOH (H2O).
Hydrochloric acid plus sodium hydroxide yields sodium chloride and Water. This same reasoning will apply to any acid/base reaction. But, the important question is why would I want to make a salt anyway? Iím gonna get to that in my own bungling, basement way. Might even make it the most important thing in your prospecting career. If you are still with me and understand the above, CONGRATULATIONS, you have just passed inorganic chemistry 101. There will be a pop quiz on Friday!
Aqua regia is a very special case. This is a mixture of concentrated acids, which is not only a very strong acid, but is also a very strong oxidizing agent. It is capable of oxidizing gold to gold chloride, which is soluble in water. Yeah, I heard of it but where do I get it? You donít get it, you make it. Aqua regia is nothing more than a mixture containing 3 parts of concentrated hydrochloric acid to 1 part of concentrated nitric acid. Actually this universally used formula is not quite correct. If you want to make it right you mix 9 parts of nitric to 41 parts of hydrochloric but the 3/1 formula is good enough for government work. This is a rather fearsome reagent. If you are going to make/use it always wear plastic gloves. Someday you will thank me for this advice.
Aqua regia dissolves gold and all the platinum metals very rapidly (especially if hot). It will also dissolve damn near anything else in your sample. Silver, iron, magnesium, manganese, aluminum, etc. In fact it will attack any metal. The stuff emits a red vapor that is very irritating. Itís nasty stuff and unless you really have to use it, it is better to do the job some other way.
If you dissolve gold in aqua regia and want to recover it you will find that you must remove the nitric acid by evaporating the solution with heat. This will produce lots of the red vapor. You must not let the solution go completely dry. Stop heating when you still have just a little liquid left. Now you must add more hydrochloric and repeat the procedure. Repeat this evaporation 3 times or until there are no more red fumes coming off. Now add hydrochloric so that you have enough liquid to work with. All the metals will be in solution as their chloride salts. If you dilute this solution with an equal volume of water you will probably get a white cloud of silver chloride which you can filter off. Now to recover youíre gold. Probably the easiest is to cement it down by adding pieces of zinc or aluminum. You will get a black-brown precipitate in the bottom that you can smelt back to nice yellow gold. There are numerous other ways to recover the gold but thatís another page.
Things That You Can Use Acids and Bases For
Any strong acid can be used for cleaning ore or concentrates. However, there are reasons why you should use one or the other but not just any one. I cannot give you a recipe for cleaning gold bearing materials. It depends too much on what is in these materials that is making cleaning necessary. The one that I can tell you NOT to use is nitric. I know, a lot of people use nitric to clean concentrates prior to amalgamating with mercury to recover the gold. They are certainly losing some gold. How much, I canít say. These folks will almost always say, "Im not losing any gold, gold does not dissolve in nitric acid". They are half- right. Gold does not dissolve in pure nitric. Very fine gold will, however, dissolve in solutions of nitric acid containing chlorine. Virtually all soil contains significant quantities of chloride salts. When these salts are put into a strong acid solution, they "dissociate", that is if the salt was zinc chloride it will break into a zinc ion and two chlorine ions. Take it on faith that the chlorine ion can now pick up hydrogen from the water and now becomes HCl, hydrochloric acid. OK, remember how you make Aqua Regia? Three parts hydrochloric and one part nitric. That will dissolve gold rapidly. You made sort of a "weak" Aqua Regia. It is strong enough to dissolve very fine gold.
So, if you need to clean concentrates DONíT use nitric until you have tried all the others first. It is the last resort. Best to try hydrochloric first. Donít use concentrated acid. Use about 1-2 normal. Concentrated is 12 normal, right? Dilute it with water 1/12 and you have one normal hydrochloric acid etc. You should be buying your hydrochloric at the local hardware store as "muriatic acid". This is 6 normal hydrochloric acid. It has already been diluted 1/1 with water. So if you dilute muriatic 1/6 you will have one normal hydrochloric acid. Yes, you can use sulfuric but remember it is sort of nasty to work with. It tends to eat up your cloths etc. It is the cheapest however. You see, the reason you are washing ore etc with acid is that there are salts in the material which interfere with recovering the gold with mercury or other. What you are trying to do is to make a salt of the acid you are washing with that will dissolve in the water so that you can get rid of it. Almost all salts of hydrochloric (chlorides) are soluble in water. Sulfates (salts of sulfuric) are not all soluble. If you donít convert an insoluble salt to a soluble one, you probably havenít helped the situation any. So, try hydrochloric first. The whole idea is to convert insoluble salts into soluble salts (those that will dissolve in water) so that they can poured off and discarded.
Although having nothing to do with acids and bases I think it is in order to discuss the problem of sulfides in your concentrates and ore. Since we are talking about cleaning of ores of unwanted or interfering salts we should consider the case of ores/concentrates that contain sulfides. Sulfides (salts of sulfur) are one of the main reasons why sometimes you just cant get the gold to amalgamate with mercury, dissolve in cyanide, or react to any other extraction method. Sulfides are very resistant to chemical conversion to soluble salts. So, we have to do something different. One more way to skin the cat. Unfortunately we will have to resort to heat. The time-honored way to remove sulfides from ore is simply to get it very hot.
The way to do this on a small scale is simply to get a piece of sheet metal. Old corrugated roofing, a piece of "valley tin", any old kind of thin steel. Find a few rocks or other equal put the metal on them. Now build a roaring fire underneath. Put your ore/concentrates on the metal.
Spread it out so that it will heat rapidly and go back to your camper for a cold brew. When it starts to get hot, if sulfides are the problem, you will start to smell sulfur, the odor of rotten eggs. Does anyone but me remember when you got a rotten egg once in awhile? If not, itís the odor of a catalytic converter that isnít working just right. Heat the ore/concentrates until the odor is no longer noticeable. Let it cool and give it a wash with 1 normal hydrochloric followed by water. Your material should now be pretty clean and amenable to amalgamation or extraction by chemicals. This is a method that has been used forever in areas where lots of sulfur was deposited with the gold. If sulfides are your problem, try it, it works.
Dealing with black sand
Did you know that you can dissolve black sand? Actually, you donít dissolve it you convert the iron oxide to iron chloride and dissolve that so that you can dispose of it leaving your gold behind. You see, making salts is important and to your advantage. This is one of the cases where you will want to uses concentrated acid. Simply take your gold with the black sand and put it in a suitable container. A borosilicate (Pyrex or Kimax etc) glass container such as a "beaker". Add concentrated hydrochloric acid. You will notice that the acid begins to turn yellow. The color is because the iron chloride that you are making is yellow. Now this is sort of a slow reaction so donít get in a hurry. Remember what we said about reactions and temperature? If the reaction is not going fast enough for you, just warm the reaction until it is just hot but donít let it boil. It is also best not to use a large volume of acid. Itís much better to start with a small amount. When you think the reaction has slowed down or stopped, pour off the acid and replace it with fresh. There is a good scientific reason for this but I really donít think you want to open that can of worms. If you heat this reaction be sure you are out side or in a well ventilated space because it is going to produce fumes of hydrogen chloride that are very irritating. You donít want them in the house. Now, I know that you are by now a pretty good basement chemist and I donít have to tell you to wear gloves, do I? Do I??? If you forgot, go to beginning, go directly to beginning, do not pass precautions ---etcóetc. You have failed a pop quiz and will be penalized 10 points and three burned fingers.
I should also point out one other disadvantage of not wearing gloves. Acid or base burns on your hand makes holding a cold beer very difficult and uncomfortable. You definitely donít want that, do you?