PRECIOUS AND BASE METAL RECOVERY SYSTEMS

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PRECIOUS AND BASE METAL RECOVERY SYSTEMS
EXPERIENCIA EN SISTEMAS DE RECUPERACION DE
METALES PRECIOSOS
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WWW.scotiacorp.COM
Largest importer of activated carbon in usa
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CMP 2008 Presentation on Activated Carbon
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No Matter what your wifes told you, size does
count

• I used this same statement 8 years ago when I
  gave the first presentation and it still applies.
• When you look at carbon you have to look at it
  as a material that will breakdown sooner than
  later. It is a friable (but still abrasive)
  material that is so porous that it is just
  waiting to break.
• So it is quite important to know the size of the
  material you are putting into your circuit and
  what kind of size distribution you have in the
  active circuit

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Carbon size
Depending on the circuit equipment, the standard
size distribution you get can be useful
information or some of the worst information you
could use. Why? If you are using square mesh
screens, the information is useful, if you used
wedge wire screens (which most of the high
production plants use) the information means
nothing. The carbon used in gold adsorption is
made from coconut shell that has been charred and
then activated. By nature, it is platey, that
is, it may be long and wide, but it is usually
thin. That aspect of carbon completely changes
how the material is sized on wedge wire
screens. I have tested carbon using lab wedge
wire screen (specially fabricated) and found a
carbon that had 0 passing 12 mesh (square mesh),
passing 50 1.68 mm, 40 1.0 mm and 18 -0.85
mm. Why is this important? If you have wedge
wire screens in your plant, you initially
attrition your carbon (if youre smart anyway),
then send you new carbon over your sizing screen.
If you use the carbon from above and you used
1.0 mm screen, you only get to use 82 of your
carbon.
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Carbon size
But now you have to worry, how much of the carbon
can you keep in the circuit. I have always
favored the largest carbon I could find to use in
a CIP or CIL. Some would say that the larger
size carbon decreases the adsorption kinetics.
Theoretically that is true and you might even be
able to measure it in the lab, but in the plant
youll never see a difference. So, in my opinion,
bigger is better. The larger the starting size,
the longer you get to use your carbon and the
better chance to get to limit your loses of gold
through carbon that passes through the safety
screens. What else can affect the life in the
carbon and have an effect on the size? The
carbon hardness. If the carbon is not
sufficiently hard, the probably of the carbon
breaking goes up. There are portions of the
world, where a higher CTC carbon (60 CTC) is
favored over lower CTC carbon (45 to 50 CTC).
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Carbon hardness
Lets look at the facts. The bulk density of a
60 CTC carbon ranges from 0.48 to 0.49 g/cc and
a 45 CTC carbon ranges from 0.51 to 0.52 g/cc.
This has to be, because to get to the higher CTC
number (which is also referred to as its
activity by some) you have to convert substrate
carbon (the particles back bone) into pore space.
Therefore there is more surface area and more
spaced to adsorb the carbon tetrachloride. So
you have removed from 3.9 to 7.8 of the carbon
structure. Which of these material do you expect
to be stronger? That is a loaded question,
because all carbon is not equal and youll have
to compared the strength from carbon that come
from the same source (same company) to get a good
comparison. But if you look a carbon from the
same source youll find the lower CTC is always
harder.
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Carbon hardness
Now lets talk about the standard hardness test.
The standard test is the ball pan hardness test,
so what? Well the ASTM 3802-79(2005) test more
or less takes a weighed sample of carbon, adds it
to an 80 mesh screen and add 15 -½ steel balls
and 15 - 3/8 steel balls to the screen and lets
it rip. This test doesnt show you the
resistance to breaking into particles that flow
through your screens, it give you an idea of how
hard it is. But, which information do you really
want? Many companies have developed their own
hardness tests, which most likely should be
called resistance to breakage tests. These tests
work for them and I would recommend you look at
them yourself. This information brings us around
full circle once more to the discussion of our
friable carbon. Its going to break, but you
dont have to go out of your way to break it.
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Keeping your carbon in the circuit
So whats a mother to do? Look at you carbon
handling, Im sure your design engineer has set
your system to move the carbon in a 10 w/w
slurry (actually a 20 v/v slurry). Are you
cheating, I know the operators want to transfer
that carbon as quickly as possible and its oh so
easy to turn down the dilution water So whats
happening, well with even the best (softest)
pumping system, you are essentially taking your
grandmothers best china, putting into a box and
drop kicking it. The carbon particles are
smashing into each other, they are hitting the
pipe walls, elbows and impeller blades. All of
which causes that friable carbon to break.
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So what, we break a little carbon you said it was
friable
Im glad you asked. If your carbon breaks when
you strip it or regenerate it, you collect it
when you size your carbon. You lose about 2
ounces per ton of the value when you sent it out
for treatment, so you just toss away anything
less than 2 oz/t (cost 1500 per ton of carbon).
If you have 100 turnover (which is doing and
outstanding job) in an 80 ton CIL, your only
losing 120,000 per year. Come on, thats the
cost of doing business right? Well, yes and
no
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The rest of the story
Paul Harvey
Remember, 100 is doing a great job, most of us
miss that target. Additionally, I sure you were
assuming you were getting less than 2 oz/T (60
g/t) carbon. Well when Ive tried to collect all
of my carbon fines and treat them, the best I did
was 65 weight recovery. Theres loss that
happen by ashing the carbon the kiln (it happens,
sometimes) and carbon that is too fine to be
caught by the screens and goes to tails. So of
that carbon doesnt come from the strip side of
the process. It comes from the adsorption side,
and if your carbon has a loading gradient from
150 to 6 oz/T (fairly common) and 25 of the
unrecoverable loss come from that side of the
circuit, thats only 7 tons (out of 8o) , so
what? Well you see, thats 7 tons at 78 ounces
per ton or 410,000. Now thats a cost of doing
business. This is from a circuit that we fought
hard to have the hardest carbon, the best pumps
and highest consciousness of the frailty of our
carbon. What happen if this is in a large CIL
complex that uses 800 tons of carbon (which it
was), Now thats a target of opportunity. So take
it a step further and look at using softer
carbon, is this a chance you want to take?
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Other stuff with activated carbon whats hot?
In Nevada, its mercury. An activated carbon
(actually sulfur impregnated activated carbon) is
being used by most properties as the MACT
(Maximum Achievable Control Technology) method of
removal. These system can easily handle
extremely large air volumes (over 13,000 cfm) in
a single unit, but do handle up to that level
significantly more economically than wet scrubber
units.
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Impregnated Carbon
Hows it work? The sulfur reacts with the
gaseous mercury to mercurous sulfide, which is
highly insoluble and consider non
hazardous. Great, I gotta get me some. Well
hold on thar cowboy (that Nevada talk) not so
fast. The gas stream does contain gaseous
mercury and you sometime find some of that
condensing on the carbon prior to arriving in the
vicinity of the sulfur. This situation has
produced carbon that fails the TCLP (said,
tee-clip) test. So you have a great method for
removing mercury to form another waste stream
from the mine.
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Impregnated Carbon
This problem is being looked into, but as yet we
dont have a definitive answer. If youre not
from Nevada, why should you care Well, all I
can say is, Be scared, be very scared because
if your ore contains any mercury, these type of
regulations are coming your way. I doubt youll
find a regulator that will say, Heck, its only
a little mercury, whats that amongst friends?