How is gold produced?

gold processing

Gold processing and ore preparation for use in various products.

For thousands of years, the word gold has meant something of beauty or value. These images are derived from two properties of gold, its color and chemical stability. The color of gold is due to the electronic structure of the gold atom, which absorbs electromagnetic radiation with wavelengths less than 5600 angstroms but reflects wavelengths greater than 5600 angstroms - the wavelength of yellow light. The chemical stability of gold depends on the relative instability of the compounds it forms with oxygen and water - a property that allows gold to be purified from less noble metals by oxidizing other metals and then separating them from the molten gold as slag. However, gold readily dissolves in a number of solvents, including oxidizing solutions of hydrochloric acid and dilute solutions of sodium cyanide. Gold dissolves easily in these solvents due to the formation of very stable complex ions.

Gold (Au) melts at 1064°C (1947°F). Its relatively high density (19.3 grams per cubic centimeter) made it recoverable by alluvial mining and gravity concentration techniques. With the face-centered cubic crystal structure, it has the softness or flexibility to form complex structures without sophisticated metal machining equipment. This in turn has led to its application, since ancient times, to the manufacture of jewelry and decorative items.

history

The history of gold stretches back at least 6,000 years, and the earliest identifiable and factually dated finds were made in Egypt and Mesopotamia c. 4000 BC The first major find was located on the Bulgarian shores of the Black Sea near the present-day city of Varna. By 3000 BC, gold rings were used as a means of payment. Until the time of Christ, Egypt remained the center of gold production. However, gold was also found in India, Ireland, Gaul and the Iberian Peninsula. With the exception of coins, almost all uses of the metal were decorative—for example, for weapons, goblets, jewelry, and figurines.

Egyptian wall reliefs from 2300 BC show gold in various stages of refining and mechanical work. During these ancient times, gold was mined from placers - that is, elemental gold particles found in river sands. The gold was concentrated by washing lighter river sands with water, leaving behind dense gold particles, which could then be further concentrated by dissolution. By 2000 B.C., a salt purification process for gold and silver alloys was developed to remove the silver. Mining of alluvial deposits and, later, knot or vein deposits required fracking before the gold could be extracted, and this consumed huge amounts of manpower. By AD 100, up to 40,000 slaves were employed in gold mining in Spain. The advent of Christianity somewhat dampened the demand for gold until about the 10th century. The amalgamation technique, alloying with mercury to improve gold recovery, was discovered around this time.

The colonization of South and Central America that began during the 16th century led to gold being mined and refined in the New World before it was transported to Europe; However, American mines were a greater source of silver than gold. During the early to mid-18th century, large deposits of gold were discovered in Brazil and on the eastern slopes of the Ural Mountains in Russia. The main placer deposits were found in Siberia in 1840, and gold was discovered in California in 1848. The largest gold discovery in history was in the Witwatersrand, South Africa. Discovered in 1886, it produced 25 percent of the world's gold by 1899 and 40 percent by 1985. The discovery of the Witwatersrand deposit coincided with the discovery of the cyanide process, which made it possible to recover no longer found gold values in both gravitational concentration and consolidation . With E.B. Miller's process for refining impure gold with chlorine gas (patented in Britain in 1867) and Emile Woolwell's process for electrorefining (introduced in Hamburg, Germany, in 1878) made it routinely possible to achieve higher puritys than allowed by refining Fires.

ores

The main ores of gold contain gold in its original form and are exogenous (formed on Earth's surface) and endogenous (formed within the Earth). The most famous external raw material is alluvial gold. Alluvial gold refers to gold found in riverbeds, streams, and floodplains. It is always the element gold and is usually made up of very fine particles. Alluvial gold deposits are formed through the weathering actions of wind, rain, and temperature change on gold-bearing rocks. They were the most common type in ancient times. Alginate exogenous gold can also exist as oxidized ore bodies formed under a process called secondary enrichment, in which other metallic elements and sulfides are gradually leached out, leaving behind gold and insoluble metal oxides as surface deposits.

Intrinsic gold ores include vein and vein deposits of elemental gold in quartzite or mixtures of quartzite and various iron sulfide minerals, particularly pyrite (FeS2) and pyrrhotite (Fe1-XS). Where gold is present in sulfide ore bodies, the gold, though still elemental in form, is so finely dispersed that concentration by methods such as those applied to alluvial gold is impossible.

Native gold is the most common of all gold minerals, accounting for about 80 percent of the metal in the Earth's crust. They are sometimes found as nuggets up to 12 mm (0.5 in) in diameter, and in rare cases nuggets of genuine gold weighing up to 50 kg have been found - the largest weighing 92 kg. Genuine gold always contains about 0.1 to 4 percent silver. Electrum is an alloy of gold and silver that contains from 20 to 45 percent silver. It varies from pale yellow to silvery white and is usually associated with silver sulfide mineral deposits.

Gold also forms metals with the element tellurium; The most common are calaverite (AuTe2) and sylvanite (AuAgTe4). Metals other than gold are rare enough to have little economic importance.

extraction and refining

merge

Elemental gold (and silver too) is soluble in mercury, so when particles of the metal come into contact with a surface of fresh mercury, they are hydrated and melt, forming an alloy called amalgam. This phenomenon is exploited to recover and concentrate fine particles of gold or silver.

Amalgamation is achieved by passing a slurry of ore over mercury-coated copper plates, by mixing a slurry of ore and mercury in a cylindrical or conical vessel called an amalgam barrel, or by grinding the ore in a ball, rod, or pebble mill to free gold of the metal mold and then add the mercury to the mill and continue grinding until the gold has dissolved into the mercury. The denser amalgam is then separated from the now barren ore in the mill dump. After filtering and washing to remove impurities, the amalgam is heated in a sealed retort to distill the mercury, which is recovered for reuse.

Although amalgamation is still widely practiced in gold recovery, the real risks of mercury poisoning both from operators and the environment have limited its application and forced the use of carefully designed equipment to ensure no contamination.

cyanosis

More gold is recovered by cyanide than any other process. In the cyanidation process, metallic gold is oxidized and dissolved in an alkaline cyanide solution. The oxidizing agent used is atmospheric oxygen, which in the presence of an aqueous solution of sodium cyanide leads to the dissolution of gold and the formation of sodium cyanurate and sodium hydroxide, according to the so-called Elsner reaction:

When the dissolution of gold is complete, the gold-bearing solution is separated from the solids.

With ores with a high gold content (i.e. more than 20 grams of gold per ton of ore), the cyanidation process is carried out by vat leachate, which involves holding a slurry of ore and solvent for several hours in large motorized tanks. To extract gold from low-grade ores, heap filtration is practiced. The huge piles described above are sprayed with a dilute solution of sodium cyanide, and this seeps down through the piled ore, dissolving the gold.

Huge amounts of solutions and solids are bound to a filtration circuit in a vessel, due to the very low concentrations of gold in the ores. In order to eliminate the huge capital costs associated with purchasing and installing solids/liquids separation equipment, technologies have been developed that avoid the entire separation process. One of these is the addition of granular activated carbon to the ore slurry during or upon completion of the gold melting. The dissolved gold is easily adsorbed onto the carbon, and thus removed from the solution, and the granular carbon is separated from the now-bare ore by running the slurry through a sieve. The gold is then leached from the carbon particles by a strong solution of sodium cyanide and sodium hydroxide, and recovered from the solution by electroextraction directly on steel wool or by the Merrill Crow process. In the latter process, the gold-bearing solution is deoxygenated and passed through a filter press, where the gold is displaced from the solution by reduction with zinc metal powder.

extraction from refractory ores

Many gold-bearing ores and concentrates are not easily amenable to blue, due to the presence of substances that consume the cyanide reagent before it can dissolve the gold, preferably absorb the gold as it dissolves (a phenomenon called pre-stealing), or completely surround the gold particles in such a way as to prevent the access of the cyanide leaching solution. Referred to as refractory, these ores often contain the sulfide minerals pyrite, pyrrhotite, or arsenibirite. Gold can be liberated from these ores or concentrates by treating them with different oxidation processes. The most common method is roasting the gold-bearing minerals at temperatures from 450° to 750°C (840° to 1380°F) to destroy the interfering sulfides. Oxidation can also be accomplished by the use of high pressure reactors called autoclaves, in which the metals in the aqueous slurry are treated at high temperature and pressure with oxygen-carrying gases. After oxidation is complete, bluing, as described above, is used to melt and extract the gold.


A large percentage of gold is recovered from refractory ores, and great skill is required in designing and operating such facilities.

refining

Gold extracted by amalgamation or bluish contains a variety of impurities, including zinc, copper, silver and iron. Two methods are commonly used for purification: the Miller process and the Folwell process. The Miller process is based on the fact that almost all impurities in gold combine with gaseous chlorine more readily than gold at temperatures equal to or greater than the melting point of gold. So the impure gold melts and gaseous chlorine blows into the resulting liquid. The impurities form chloride compounds that separate in a layer on the surface of the molten gold.


Miller's process is quick and simple, but it only yields gold that is 99.5 percent pure. The Wohlwill process increases purity to approximately 99.99% by electrolysis. In this process, an infusion of impure gold is lowered into an electrolyte solution of hydrochloric acid and gold chloride. Under the action of electric current, the casting acts as a positively charged electrode, or anode. The anode dissolves, and the impurities pass into the solution or pass to the bottom of the electrorefining tank as an insoluble viscous substance. The gold migrates under the action of the electric field to a negatively charged electrode called the cathode, where it is restored to an extremely pure metallic state.


Although the Wohlwill process produces high purity gold, it requires the producer to hold a large stock of gold (mainly for the electrolyte), and this is very expensive. Processes based on direct chemical refining and recovery from solution as elemental gold can greatly speed up gold processing and virtually eliminate costly stockpiles during the process.