Spinning Gold: From Ore to Bullion
For literally thousands of years, the prospect of finding gold has driven men to great lengths. From the days of the mythical (or not) King Solomon’s mines to the present day, the search for gold meant wading in riverbeds to sift gold from the rushing water or digging into the earth to find the “mother lode”. Today, most gold is mined from deep in the earth, since most of the surface gold – known as alluvial gold – has been found. The gold-mining process is intricate and multi-faceted, tying cutting-edge technologies with old-fashioned determination.
Finding Gold: Eureka!
While gold exploration used to be a matter mostly of "boot and hammer" prospecting, gold mining today is largely a matter of technology. First, geologists use geology maps to look for favorable areas to explore. Ore deposits are not easy to find and many of the ones exposed on the surface have already been found. Geologists use the physical and chemical characteristics of the rocks they are looking for to zero in on prospective areas. Once favorable geology is established, remote sensing, airborne and ground geophysics and geochemistry are used to outline targets for drill testing.
Drilling and Engineering: Taking stock
Drilling at these sites brings up rock samples from various locations. These samples are analyzed to determine if any gold exists there, the size of the deposit, and the quality of the gold. Using this information, mining engineers determine if enough gold is under the surface to make the mining worthwhile; the type of mine needed; the physical obstacles to getting to the gold; and what impact a mine would have on the area's wildlife and environment. If the gold is close to the surface, the engineers will design an open-pit mine; if the gold is buried deeply, they will plan an underground mine.
Building a Mine: Be prepared
Before the gold can be mined, an infrastructure must be created. Even if the gold is close to the surface, the simplest open-pit mine can take up to a year to build. In fact, the time between discovering gold and actually bringing it out of the earth can be up to five years. Since mines are often in remote locations, an entire infrastructure – roads, administrative offices, equipment storage areas, even towns, schools and medical facilities – must be built. The plans for the mines must be given the green light by a number of authorities at each level of government. Also, the mining company must put aside money for reclaiming the land once the gold is mined. In all, the preparation process can end up costing hundreds of millions of dollars – before a single ounce of gold is mined.
Mining and Processing: Taking the good from the bad
Ore samples are taken and examined for the metallurgical quality of the gold in order to determine the appropriate processing technique required to remove the gold. The mine site infrastructure includes a processing area where the ore is crushed and undergoes various processes depending on the nature of the associated minerals and then the loose rock is sent to the appropriate processing location. The process for low-grade ore is relatively simple: a cyanide solution is applied to the heap, dissolving the gold, which is then collected. High-grade ore, on the other hand, heads to the grinding mill for a more extensive process. There are several different ore types which require different processes for optimal recovery of the gold. For example:
Oxide ore goes directly to the leaching circuit, where cyanide dissolves the gold.
Refractory ore, which contains carbon, is roasted at 1000 degrees Fahrenheit, burning off the sulphide and carbon, then heads to the leaching circuit.
Sulphide refractory ore, which does not contain carbon, is oxidized in an autoclave in order to separate the sulphide safely and in an environmentally friendly manner, from the ore, which then enters the leaching circuit. In the leaching circuit, the gold is extracted from the solution and deposited onto activated carbon, from which the gold is then chemically stripped. The impure gold is then melted into Dore bars, which are about 90 percent pure gold. These bars are usually shipped to a refinery where they undergo further processing.
Refining: From 90 percent to 99.99 percent pure
The refining process strips out the remaining impurities from the gold, which is either recycled scrap being upgraded or gold destined to become bullion bars. In the first step, crude gold is melted and treated with chloride, converting remaining metals to chlorides that will drift off the gold. The resulting 99.5 percent pure gold is cast into electrodes known as anodes, which are put into an electrolytic cell. After a current is passed through the cell, the end product is 99.99 percent pure gold.
Reclamation: Giving back the land
After a number of years, the gold reserves in a mine will be exhausted. In the old days, a spent mine would be boarded up and abandoned, but nowadays a reclamation project returns the land, as much as possible, to its previous natural state. The project follows the plan submitted to government authorities before the mine was built, which details the strict guidelines the company will follow, during the life of the mine and its closure, to protect wildlife and the surrounding environment. The reclamation project will include such things as planting trees and grass, and returning wildlife to the area.