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Gold Mining

From RitchieWiki

Mining History
Mining Processes
A gold nugget, picture courtesy of the U.S. Geological Survey
Gold mining dates back to primitive times with rudimentary tools such stone hammers, antler picks, and bone or wooden shovels. The Egyptians, Semites, and Indians all used a technique called fire-setting to break apart quartz veins. Other evidence suggests that placer deposits were worked during the same period using a crude form of panning and sluicing.

Eventually gold mining techniques evolved for both the placer mining of gold in placer deposits and in the lode mining of gold veins located underground. Open-pit mining is the latest method employed in the mining of gold from hard rock. It estimated that in gold mining operations today, approximately three tons of extracted ore yields about one ounce (29 g) of gold.[1]

Contents

[edit] Characteristics of Gold

Gold in its pure form exists as a yellow metal that is solid at room temperature. It is a transitional element represented by the symbol AU and the atomic number 79 on the Periodic Table of Elements.[2]

Gold has many unique characteristics that make it highly desirable to work with. Even though gold is solid at room temperature, it is also very malleable and never tarnishes. It can be beaten down into a thickness of 0.000005 inches (0.000013 cm), and one ounce (29 g) of gold can be drawn out to a length of 62 miles (100 km).[3] Gold is also virtually indestructible and can be melted down and reworked. It was the first metal discovered by humans and used on a wide scale by many ancient civilizations in the creation of amulets, statues, and religious idols, and eventually currency in the form of coins. During the 19th century, gold began to be used as a way of measuring the value of most of the world’s national currencies. This system was abandoned in 1934 when the U.S. stopped minting gold coins and began using the dollar as the principal unit of all national monetary transactions. Since the 1970s gold has mostly been bought and sold with widely fluctuating prices and it has greatly declined as a currency reserve. On the other hand, its use for commercial and industrial processes has increased dramatically.[4]

[edit] History

Gold was the first metal discovered by early hominids during prehistoric times.[5] It was most likely found in pure form as nuggets or flakes in placer deposits by streams and in sand. As communities were formed and human development progressed, gold was considered sacred; it symbolized great wealth and prestige. It was used by many early cultures to make amulets for personal adornment. Later on, gold was used to make religious objects and idols. Other civilizations even equated gold with gods and rulers and gold was even sought in their name. With its brilliance and malleability, gold, throughout history, has been associated with gods, power, immortality, and wealth in many cultures. The Phoenicians, Egyptians, Indians, Hittites, and Chinese are just some of early ancient civilizations that sought gold and worked mines.[6]

[edit] The Greeks

The Greeks were the first civilization to use gold as a form of currency.
Around 700 B.C. the first gold coins started to appear and eventually gold, when measured out, gained monetary value, shaping the concept of money itself—something that was portable, private, and permanent.[7] Before this, gold was simply regarded as a prized commodity. In Ancient Greece, gold was used as a form of currency. The Greeks actively mined gold from state-or privately-owned placer mines by paying a royalty to the state. These mines were located throughout the Mediterranean and Middle Eastern regions. The origins of gold were debated by philosophical greats Aristotle and Plato. It was even suggested that gold was a dense combination of sunlight and water.[8]

[edit] The Romans

The Roman civilization was the first to develop gold mining techniques that were actually advanced and complex. They devised and introduced the use of sluices and the first Long Toms. They also implemented the first form of hydraulic mining by diverting streams and building water wheels. Another technique called roasting was used in the underground mining of gold to separate gold from rock.[9] Gold during the Classic period permitted world economies to grow and acquire wealth. It was traded by the both the Greeks and Romans during this timeframe for spices from India and silk from China. At the height of the Roman Empire, around 98 to 106 A.D., gold and silver coins were the dominant currency.[10]

[edit] Gold in the New World

Many countries explored and conquered new territories in the quest for gold. In the New World, the Spaniards pursued conquests in Mexico and South America in 1550 when they found placer deposits of gold in Colombia. During the 16th century, the Portuguese pursued gold in Brazil. In 1693 large deposits of gold were found in Minas Geraes. Over the next century, the state functioned as the world’s primary resource of the precious metal. The Morro Velho, an underground gold mine in Minas Geraes, was in production for close to 150 years and is still producing gold.[11]

[edit] Gold Rushes of the 19th Century

The ushering in of the 19th century witnessed an unprecedented number of gold rushes on the continents of South America, North America, and Australia. The discovery of gold was typically in payable placer gold and became pivotal in populating areas on each continent. Gold rushes also brought both economic and infrastructure development to the areas in which they were centralized.

The first of these gold rushes occurred in the United States with the Georgia Gold Rush in the Southern Appalachian Mountains in 1829, followed by the California Gold Rush in the Sierra Nevada foothills in 1848 and ending with the gold rush in Nome, Alaska, in 1899. A trail of subsequent gold rushes occurred north of the border in Canada, in the Fraser Canyon region of British Columbia and up through the Rocky Mountains, spreading up to the Yukon with the Klondike Gold Rush in 1896. The Australian Gold Rush starting in 1851 was another significant gold rush globally. [12]

[edit] Development of Gold Mining Methods

Aside from the Roman operations, gold mining during the 19th century consisted of small-scale placer mining of gold found mostly in placer deposits. Individual miners or small bands of miners worked a claim by digging up the pay dirt and removing layers of overburden. The gold would be sourced out from the pay dirt using relatively simple means such as panning, rockers, Long Toms, or sluices. As gold mined from surface-level placer deposits was fast becoming exhausted, miners struggled to find different ways to process greater volumes of pay dirt to justify the increasing cost of gold mining. This led to the use of very evasive and environmentally controversial mining techniques such as hydraulic mining and dredge mining. Gold stores found in placer deposits were eventually exhausted and gold mining moved underground or to open-pit mines. Sub-surface gold mining was very labor-intensive and involved removing greater layers of overburden from the earth. Underground mining involved the mining of quartz and hard rock using hydraulic drills to create holes in which explosives were detonated. Water was then added to the holes, creating a grinding compound to remove dirt and debris. Stamp machines were also used in underground hard rock gold mining to crush large rocks removed from the mines.[13] Gold mining eventually went from being a small-scale surface operation to involving organized camps of miners, extracting gold from underground. Eventually, as more capital was needed to carry out gold mining, companies bought up large tracts of land and hired miners to do the work. Individual mining on a claim became largely a thing of the past.

[edit] Process

Gold is found in a lode or vein inside bedrock or loosened from the bedrock through the process of natural erosion over time, forming a placer deposit. Two primary ways to mine gold is therefore placer mining or hard rock mining, also called lode mining.

[edit] Placer Mining

Placer mining is practiced when gold is found in unconsolidated deposits in sand and gravel. Since gold is dense, it is easily separated. Sand and gravel suspend in flowing water and heavier material like gold will sink to the bottom and can be removed by hand.[14] Gold panning is the simplest method of placer mining, using a pan, water, and a swirling action to separate the gold from gravel, clay, or sand. Sluicing with sluice boxes is another efficient method of recovering gold in placer mining operations and is still in use today. Two other methods that perform a similar function to sluices include rockers and Long Toms. Hydraulic mining is another type of placer mining technique, in which jets of water are directed at sand and gravel banks to wash away material. The broken up material and debris gets washed directly into giant-sized sluice boxes to separate the gold from gravel and sand. Dredge mining is also used to mine gold from placer deposits using buckets or huge shovels to scoop up gold-bearing gravel from the bottom of streams, ponds, lakes, or riverbeds. Modern-day placer mining operations still use some of these methods, but heavy construction machinery is also widely used now in placer mining to dig up the paystreak and transport gold-bearing gravel directly over to a grizzly, an arrangement of metal beams or pipes, mounted on top of a sluice box. The gravel is dumped into the grizzly to separate coarse, larger-sized gravel from the finer gravel that gets washed down into the sluice.[15]

[edit] Lode Mining

A vertical mine shaft to an underground gold mine.
Lode mining involves extracting gold from within a vein ore lode of ore bedrock. Lode mining is therefore the equivalent of mining gold in hard rock. Approximately half the gold mined in the world is deposited inside rock veins. The gold in these veins may be microscopic in size, or can be found in nuggets, sheets, or in gold compounds. Gold can also be mined as a by-product in the production of copper, zinc, or lead. In fact, one third of all gold is produced in this way.[16]

The cost of hard rock mining can only be justified when a significant concentration of gold is found embedded within the ore. On average, gold ore needs to be concentrated by 1,000 times above its natural distribution to become valuable for gold mining.[17] Most of the gold located underground is found deep in the earth in pockets. These pockets get filled with gold, ore, and usually quartz.[18] If enough gold is located in one of these pockets then hard rock mining may be considered.

The recovery and production of gold in an underground mine involves sinking a vertical or decline shaft deep down into the strata of the earth. The shaft is used for transporting miners, mining equipment, and other materials. The vertical shaft serves as the main shaft of the underground mine. Horizontal shafts are extended off the vertical main shaft and this permits specific areas to be mined underground where the gold-bearing ore has been identified. Extracting the ore involves drilling holes into the ore body, filling the drilled holes with explosives, and then blasting off the ore in pieces. The blasted chunks of stope or rock face are then cleaned, releasing the ore. The ore in the underground mine gets transported to the surface to a treatment plant by means of a vertical or horizontal transportation system—usually a network of conveyor belts.[19]

[edit] Open-pit Mining

In recent years, open-pit mining has emerged as the newest method of mining gold.[20] Open-pit mining involves removing layers of overburden strip by strip, forming a deep pit to reach and expose the gold-bearing ore. The pit spirals deeper downwards into the earth with each subsequent strip. In the United States, most gold mining is carried out at the surface level in open-pit mines.[21] An open-pit mine is not dug until geologists look at satellite surveys and conduct geochemistry testing to determine the exact location of a gold ore deposit. Another method used to determine the presence of gold is scanning with metal detectors. Computers are then used to design the actual mine and determine the approximate size and measurement of the pit. Releasing the gold-bearing ore in an open-pit mine is usually accomplished with the drill-and-blast method. Excavators then load the ore onto an ore transport system where it is hauled by heavy trucks to a treatment plant.[22] Samples of the ore are examined to determine both the grade of the ore and its metallurgical properties. Ore is usually classified as low-grade or high-grade. Depending on the grade of the ore, it goes through various processes to extract the gold. Once gold is extracted, it is goes through a purification process. 
Waihi open-pit gold mine in New Zealand

[edit] Gold Extraction

After gold-bearing ore has been mined, it is usually broken up to prepare the gold embedded within it for treatment. This process is known as comminution.[23] Comminution involves a multi-phase process of crushing and milling circuits. Modern technology in gold extraction involves using large mills fed directly with run-of-mine material. The gold is recovered from the ore either by vat leaching or heap leaching.

Heap leaching or cyanide processing is used to extract gold from high-tonnage, low-grade ores. The process involves breaking the low-grade ore into smaller pieces and placing the pieces on a specially designed pad with grooves. A diluted cyanide solution is then repeatedly sprayed over the heaps of gold-bearing ore over a long period of time, sometimes as long as three months.[24] The cyanide percolates through the ore heap to dissolve the gold. This gold solution is then collected. Heap leaching is very cost-effective but recoveries are usually measured in the range of 70 and 85 percent.[25]

High-grade gold-bearing ores are usually delivered to a grinding mill and pulverized into a fine powder. Depending on the metallurgic make-up of the ore, it is treated in one of three different recovery processes. Refractory ores are ores where the gold is locked within a sulphide mineral and isn’t readily available to be treated immediately with a cyanidation process. Refractory ores with carbon and sulphide are roasted at a temperature of 1000 degrees Fahrenheit to burn off the sulfide and carbon. Roasting the ore produces oxide ore. Sulfide refractory ores are ores that contain no carbon. These ores are oxidized in an autoclave to free the gold from the sulfide minerals. Once the ore is turned into oxide ore, it is sent directly for leaching.[26]

Vat leaching is the common practice for high-grade gold-bearing ore. Once the ore has been crushed into a fine powder, it is fed into a thickener and combined with a cyanide and water solution to produce sludge. The liquid sludge is contained in large holding tanks and contains about 70 percent gold at this stage. The next step involves running the sludge through a clarifier to remove any leftover rock or clay. The clarified sludge is then passed through a de-areator tank that removes the bubbles of air and takes the clarification process one step further.[27]

An alternative method used to extract gold from ore is amalgamation, a mercury-based process. When mercury is applied to the gold-bearing ore it picks up on the gold and produces a mixture called an amalgam that is then distilled. The mercury is boiled off to remove it from the amalgam. Since mercury is very toxic, this method of extraction requires a specialized facility, and is therefore a very expensive undertaking.[28]

[edit] Gold Purification

After the gold is extracted from the ore through vat leaching, heap leaching, or amalgamation, it must go through a purification process to eliminate further impurities, including other metals such as silver, zinc, and iron.

Sometimes zinc can be added to the de-areated solution after vat leaching and then drawn under pressure through a filter press. The filter press is used to screen out impurities in the gold. The gold and zinc precipitate onto a heavy canvas cloth. The zinc-gold precipitate condenses into solid form and is cleaned from the heavy cloth while extreme heat is used to burn off the zinc.[29] Zinc can also be used to precipitate silver, leaving a silver and gold mixture that can be used in the processing of other metals.

There are two principal techniques used in the gold purification process. The Miller process uses chlorine gas to reach a purification percentage of about 99.5 percent. Any impurities in the form of other elements present in gold will form chlorides more easily than gold will, at a temperature greater or less than the actual melting point of gold. These gold impurities form compounds that result in a visible layer on the surface of the molten gold.[30]

A more expensive purification method called the Wohlwill process electrolyses the gold to purities of 99.99 percent. A casting of impure gold is placed into an electrolyte solution consisting of hydrochloric acid and gold chlorine. An electric current is used and the casting serves as a positively charged electrode or anode. The anode dissolves and the impurities in the gold pass into solution or fall off to the bottom of a type of electro-refining tank as an insoluble slime. Gold is attracted under the influence of electrical currents to a negatively charged electrode known as a cathode and restored to a pure gold metallic state.[31] Once gold is purified through either one of these two processes, it is smelted and cast into bars that may be sent to other plants for further refining.

[edit] References

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  5. History of Gold in the Early Days. E-Goldprospecting.2008-12-10.
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  7. A Brief History of Gold. Only Gold. 2008-12-10.
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  16. Gold Mining - How Do We Get Gold Out of the Ground. Wow Gold and Power Leveling Service. 2008-12-10.
  17. Gold Mining. BullionVault.com. 2008-12-10.
  18. Mining the Motherlode. Lode vs. Placer Mining. Wells Historical Society. 2008-12-10.
  19. Gold production and mine-site rehabilitation processes. AngloGold Ashanti. Annual Report 2006. 2008-12-10.
  20. Gold Mining in the 21st Century: How Foes Open Pit Gold Mining Really Work? QuoteGold.com. 2008-12-10.
  21. Gold Production and Refining. ResponsibleGold.org. 2008-12-10.
  22. Gold Production and Mine Site Rehabilitation - AngloGold Ashanti 2006 Annual Report
  23. Gold production and mine-site rehabilitation processes. AngloGold Ashanti. Annual Report 2006. 2008-12-10.
  24. Gold production and mine-site rehabilitation processes. AngloGold Ashanti. Annual Report 2006. 2008-12-10.
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