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
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
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.
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
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"
- Sulfuric acid H2SO4 - 36N
- Hydrochloric acid HCl - 12N
- Nitric acid HNO3 - 16N
- Ammonium Hydroxide - 15N
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
|Precautions and handling
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, DONT 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.
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 dont 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 cant wash it off? The reason you cant wash it
off is that bases penetrate skin immediately. Now clorox is not a
real strong solution, only 5%. It aint 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.
moral to this story is, use common sense, use plastic gloves and
if appropriate, face protection. DONT GET HURT!!
How did we get to that? Thought this page was about acids and
bases. Well, you just cant talk about acids and bases
without discussing salts. Salt! Thats what we put on French
fries etc. Yeah, thats 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 aint 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 wouldnt 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. Its 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,
dont 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 cant
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!! Hows that for a practical application of basement
chemistry. Im 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 WASNT 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
NaOH ------à NaCl + HOH
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? Im
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!
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 dont 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.
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. Its nasty stuff and unless
you really have to use it, it is better to do the job some other
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 youre 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 thats another page.
|Things That You Can Use Acids and Bases For
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 cant
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.
you need to clean concentrates DONT use nitric until you
have tried all the others first. It is the last resort. Best to
try hydrochloric first. Dont 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 dont convert an insoluble salt to a
soluble one, you probably havent 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.
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.
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.
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, its the odor of a catalytic converter that
isnt 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
know that you can dissolve black sand? Actually, you dont
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
dont 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 dont let it
boil. It is also best not to use a large volume of acid. Its
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 dont 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 dont want them in the
house. Now, I know that you are by now a pretty good basement
chemist and I dont 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 ---etcetc. You have failed a pop quiz and will
be penalized 10 points and three burned fingers.
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 dont
want that, do you?