By MARK HAMILTON / PEBBLED SERIES
Mining is actually a simple process made voluminous by the very low density of the sought-after minerals. A mine with one gram of gold in every ton of earth would be sufficiently rich to take a look at. Getting at that gram is the process I will describe. Mining engineers might well wince at my description, but it is a way to understand for people like me who are not mining engineers.
First, you remove the overburden, the top layer of what we might call soil. In this overburden there is the layer in which plants grow so it is collected in an area to be redistributed upon closing the mine site. The preparation for closure begins with the very first activity.
With the overburden removed, the exposed rock will be blasted. The one blast per day is very carefully situated depending on the expected mineralization and the planned contours of the pit.
Now you have some big rocks to deal with. These rocks are loaded onto trucks that bring them to the crusher. The crusher does what you might expect, it crushes the big rocks to rocks about the size of a brick, which are transported to the mill. It’s called the SAG mill, which stands for semi-autonomous grinding. Autonomous grinding would use the rocks themselves to grind. Semi-autonomous grinding means you will put some steel balls in there to better control the size of the particles. The various sizes and quantities of the steel balls determine the size of the end particle. For the Pebble mine operation, the end particles would be about like sand.
This next part is quite surprising. The particles are poured into a huge vat of water, think of a large above ground swimming pool. Here they are essentially washed with a petroleum-based fluid whose primary utility is to attach long-carbon chains onto the minerals. These chains make the minerals hydro-phobic (they don’t like water). So, the process literally blows bubbles into the tank. The minerals with their long carbon chains cling to the bubbles and float. The process is called “flotation,” so that’s easy to remember. The froth at the top of the tank is swept off (think of a bar tender pouring a glass of draft beer and swiping the excess froth off the top).
The surprise to me was the similarity to washing clothes. At your home you will fill a tub of water, add some type of petrochemical detergent that will attach a long-polymer chain to the dirt you want removed. In this case the process is somewhat reversed, in that it will make the dirt hydrophilic (mix with, dissolve in, or be wetted by water). Wash away the water and the dirt goes with it. In neither processes do the bubbles do any cleansing, they are simply a bridge to the mineral or the dirt.
All of the material that did not float (the overwhelming amount of the crushed material) constitutes the “tailings”. These materials are not acidic (for the most part inert to slightly alkaline) and are deposited in the tailings-facility. The description of the tailings facility will be presented later. It is an important element in the understanding of the process and the discrediting of some of the most virulent false notions about the mining process.
Once the process has gathered the minerals attached to the bubbles, they are dried and ground further to a consistency about like talc. Then they go to a second flotation cycle. Here, most processes add cyanide to help with the extraction of gold. This procedure is used by the overwhelming number of mining processes and is safe and reliable. In the case of the Pebble project, the decision was made to not add cyanide because of all the misinformation campaigns about it. People know that cyanide is a poison and that was quickly jumped on to bolster the false claims of a dangerous and toxic process. Rather than fight the misinformation, Pebble decided to not use this technique and accepted foregoing the recovery of more than 10 percent of the gold.
That was a bold move, and not one that I would have chosen, since the practice is very common and has been used by mines all over the world and in Alaska for several decades with no incidences. It demonstrates the power of misinformation. The undeniable evidence of decades of demonstrated safety and reliability was overcome by scare tactics. I don’t believe the choice paid any dividends, and certainly not enough to give up 10 percent of the gold recovery.
The tailings left from this stage are handled differently than the tailings from the first floatation. These are called “PAG tailings” which stands for “Potentially Acid Generating.” These need special caution; and are stored separately from the bulk tailings of the first flotation process. These, approximately 12% of the tailings, will be stored in a sealed and lined containment area covered with about five feet of water to ensure they have no access to oxygen, which could cause them over some time to be acidic. These tailings will be monitored throughout the mine life in a facility very close to the mine pit and ultimately discharged into the pit upon closure of the mine, eliminating the need to monitor them forever.
After the second flotation, the minerals will be dried and concentrated for shipment to the available smelters. Sadly, there are no smelters available on American soil, so they will be sent overseas. Although an issue for national security, currently the air quality issues associated with smelter operations make it extremely difficult to imagine the permitting of additional smelters in the United States.
The “Pebbled” series at Must Read Alaska is authored by Mark Hamilton. After 31 years of service to this nation, Hamilton retired as a Major General with the U. S. Army in July of 1998. He served for 12 years as President of University of Alaska, and is now President Emeritus. He worked for the Pebble Partnership for three years before retiring.