Monday, November 10, 2014

How to Mine Silver

One of my best friends here in Guatemala, Chris, works as a project manager at a silver mine.  For the last year we’ve known him, he has wanted to take us out to the mine so we can see the “other half” of his life.  He finally made it happen (for the first 3 of our friends at least) on Saturday.

As we got closer to the village where Chris lives 5 days of the week, he asked us to lock our doors.  A lot of the locals are in opposition to the mine, for many reasons.  From what Chris told us yesterday, a lot of their unrest is based upon ignorance.  They are scared of pollution and contamination from the mining chemicals.  What many of them probably don’t know is that Chris’s mine is one of maybe only 5 in the world that uses a new filtration system which enables them to use water on something of a “closed circuit”—meaning that there is no contaminated water being discharged into local streams.  Some of the locals are also against a US company coming in and making so much money on their resources.  Yet Chris explained the other side of that too; his company employs almost 1,000 Guatemalans, and more importantly, they train those local workers for high level positions so they gain the expertise to undertake projects like this efficiently and safely.  Furthermore, the company pays Guatemalan taxes and the local communities receive royalties from the silver mined.  The company is also actively re-foresting over-farmed patches of hillside and working to plant coffee (and a portion of those profits go back to local communities).  At the mine, they give free tours to local community members and explain these things and more, and with new knowledge, they gain more support.

Nevertheless.  There’s still a lot of resentment.  We passed multiple anti-mine signs, and Chris regaled us with stories of riots and close calls from his first year of working in the village.  (My pictures of the signs are blurry because I took them on the way home, as the sun was starting to go down, so lighting wasn't right through the moving car windows).  

"More than 98% say no to the chemical mining of metals.
Welcome Pacific towns in resistance."

"Water and life are worth more than silver and gold."

Aside from a kid throwing his orange peel at our truck, though, we didn’t have any trouble.

We arrived at the mine, and Chris showed us to his office where we suited up (in rubber boots, a belt containing a self-rescuer in case of fire and a battery for our head lamps, a hardhat, head lamp, reflective vests, and safety glasses) and listened to a safety talk before going underground.      

Helping each other get ready...those belts are tricky.

Ready to go underground! 
I’m not sure what I pictured the mining process to be.  I had a vague notion that I would not find men with pick axes chipping the shiny stuff out of the walls.  But beyond that?  I hadn’t even tried to imagine how it would be done.  So at the end of the day when Chris asked us what we’d learned, I kept wishing “everything” was an appropriate answer. 

Chris started by showing us maps of the mine—first 2D views of the mine both horizontally and looking down vertically, and then a 3D version on his computer, complete with projections of development in the next 5 years. 

2D maps of the mine

The mine is designed with long main ramps that slope down, and coming off them are little fingers of earth called stopes.  So in order to mine the silver, they dig a tunnel at the top and bottom of the stope, then drill small holes from the top (60ft deep) in which they put explosives.  They blow up the entire stope, reducing it to a pile of loose rock within the mountain.  Then a truck goes in at the bottom tunnel of the stope and loads up the loose rock, taking it to the surface to be processed.

One of the drills used to drill the bore holes for explosives.

Chris highlighting one of the holes for
the explosives.  

A stope that has already been blown.
(Couldn't get any closer than this to gaze over the gaping hole)

Hauled out of the mine on huge trucks.  Those tires are taller than me.

The rock then goes through a mill which reduces it from gravel-sized to a fine, baby powder consistency. 
(The mill is a giant rotating tube with heavy metal balls in it.  The tube rotates at the perfect speed so that the balls ride halfway up, then drop from the top, crushing the rock below them…over and over and over again). 

From there, the fine powder is mixed with chemicals and water.  The chemicals make the metals stick to air bubbles.  So the most concentrated silver floats immediately, and the bubbles are skimmed off the top of the water.  They then travel through several vats and immediately into the concentrate room. 

 (Couldn't get a great picture of that, sorry.  The vat
 was through a grate right below my feet.)

For the stuff that doesn’t immediately float, a bit more chemicals are mixed in, and that makes the lead (and the silver sticking to the lead) float, and then that is skimmed off and goes to concentrate. 

Skimming off the lead bubbles
If it doesn’t float there, chemicals are added which neutralize the old chemicals and make the zinc (and the silver sticking to it) float, and the same process is followed.

Now skimming off the zinc bubbles!
Then, the concentrate goes through a special filter which more or less squeezes out all of the water (like wringing out a wash cloth), and then drops the dry powder, ready to be shipped. 

The filter
It is then loaded onto a conveyor belt and deposited into 1 or 2 ton bags.  Those bags are loaded onto containers, which then travel by truck to the port where they will be shipped all over the world to be smelted into usable silver. 

Loading onto the conveyor belt

Filling a 1 ton bag

1 ton bags already loaded onto a container, ready to ship

Going back a step, the sediment that didn’t make it to the concentrate room—“the stuff that didn’t float”—goes to a separate, bigger, filter, which also squeezes all of the water (and with it the chemical reagents, which then get neutralized or recycled with the filtered water back into the plant) out of it.  What is left feels like clay, and is essentially just dirt—containing no chemicals and no silver.  

Larger filter.  In the process of opening (water has been
squeezed out) and dropping a sheet of pressed dirt.

Sediment coming out on the conveyor belt.

"No chemicals...and no SILVER!"

It is transported via conveyor belt either to be deposited back onto the hillside where it will be compressed, covered with topsoil, and re-vegetated, or to be made into concrete to fill in the empty stopes in the mine (so they’re not left with a hollow hillside ready to collapse).

Conveyor belts heading towards the hillside

Conveyor belts taking the sediment
to make concrete.

It’s all a really cool process, and it was an intensely interesting day.  It was also fun to drive around the mine with Chris pointing out, “That was my project.  I designed that.  I contracted that building.  I did all the calculations to make that work.  None of these buildings were here when I got here.”  Definitely an experience I never dreamed of, but one I’m very glad I had!

Up on the hill looking down at the whole mining operation.

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