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
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.
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).
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.
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.
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.
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…
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? Paul Harvey
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.
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.
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?