Environmental Geochemistry of Gold
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Published: Tue, 12 Sep 2017
Gold is a chemical element with the symbol (Au) and atomic number 79. In its pure state, it is a bright, moderately reddish yellow, dense, soft malleable and ductile metal. Pure gold has a density of 19,300kg/m3, though native gold typically has a density of 15,00kg/m3 (Marsden et al 2006). Because gold is inert at ambient temperature and pressure, there are very few naturally occurring compounds of the metal. The purity of gold (sometimes referred to as “fitness”) is expressed in karats. Pure gold is 24 karat (Boyle 1987). Gold is one of the native earth metals that is of very high value in our world today.
The current price of gold per Ounce is approximately 1220usd (GFMS, Thomson Reuters, 2016). Gold plays a vital role in the economic development of a country. Gold, the noblest of metals, has been used by man for more than 5000 years (Nelson et al, 2014). Its malleability, softness and resistance to tarnish, led to its earliest uses in art and currency. It is the metal choice for jewellery. Some of the countries where gold is mined are South Africa, USA, Canada, China, Australia, Indonesia, Peru, Russia etc. The average concentration of gold in the earth’s crust is 0.005 g/t, which is much lower than most other metals, for example, copper (50% g/t) and silver (0.07 g/t)(Boyle, R. W.1979).
Gold is commonly associated with other metals like iron, lead, zinc, copper and silver. Minerals most associated with gold are pyrite, marcasite and pyrrhotiteetc. Gold is the most noble of metals, and its geochemistry is conditioned principally by this fact Minor concentrations of gold occurs in most natural substances. In seawater, for example, there is approximately 0.012 parts per billion (ppb) of gold, and in fresh water it is slightly higher at 0.02ppb (Boyle, R. W.1979). Its average concentration in the earth’s crust or lithosphere is approximately 5ppb, and in sedimentary rocks it may have concentrations of up to 2100ppb or 2.1 parts per million (ppm). At these concentrations 20 or 30 tons of rock must be processed to extract a single ounce of precious gold. As a result, gold can only be mined profitably where it is highly concentrated by natural chemical and physical processes. Gold occurs in many different rock types and geological environments and economic deposits are mainly of two types: lode deposits (primary) and placer deposits (secondary)(BGS 2010). Gold is extracted by open cast or underground mining method, which depends on the geological settings of a deposit. Gold in its raw state is displayed in the diagrams below.
- PROPERTIES OF GOLD
- Physical Properties
Gold is both ductile and malleable. Ductile meaning it can be drawn into wire forms. Malleable meaning capable of being hammered into thin sheets. Gold is quite soft. It can be scratched by a penny. Its melting point is 1,064. 760C (1,948.570F) and its boiling point is about 2,7000C (4,9000F). Its density is 19.3 grams per cubic centimeter. Gold leaf is translucent and the transmitted light is greenish blue colour gold metal reflects yellow and red, leaving the greenish colour (Boyle R. W. 1979).
- Chemical Properties
Gold is known to be a noble metal. It is chemically inactive. This is because it is highly resistance to react with other elements. The oxidation state of gold ranges from -1 to +5. The +1 and +3 (Au (I) AND Au (III)) are the most common oxidation state of gold. It is resistant to the effect of most acids and therefore does not corrode easily. Apart from acids, gold does not combine directly with oxygen). Gold has one stable isotope, 197 Au. Aqua regia, a mixture of nitric and hydrochloric acids, has the ability to dissolve gold (Young and Derek, 2009).
- USES OF GOLD
Gold has been used to make jewellery and ornamental objects for thousands of years. Gold nuggets found in a stream are easy to work and were probably one of the first metals used by humans. Today most gold that is newly recycled is used in the manufacture of jewellery. About 78% of gold consumed each year is used in the manufacture of jewellery. Special properties of gold make it good suitable for manufacturing of jewellery. These include very high lustre and its yellow colour. Pure gold is known as 24 karat gold. Pure gold is too soft, craftsmen learned that alloying gold with other metals such as copper, silver and platinum increase its durability.
Gold is also used for medical purposes, to treat small number of medical conditions. Radioactive gold is used in diagnosis. Gold as an extraordinary ductile and malleable metal, it is still cosmic demand in jewellery, medical applications, because of its unique physical and chemical properties (Spitzer and Rodnei, 2004). According to (Renner et al. 2000), gold and its alloys are used for conductor material and low-voltage contacts for communication and information transfer equipment, where even very thin oxide layers would cause interruptions or failures in signal transfer. Below are the examples of processed gold.
- EXTRACTION OF GOLD
Gold panning, sluicing and mercury processes have been extensively used in placer mining to recover gold from primary sources. The primary sources of gold are alluvial gravel and auriferous quartz. Gold extraction may require comminution, mineral resources, pyro-metallurgical and hydrometallurgical process to be performed on the ore. Mechanical separation, pyro-metallurgical, hydrometallurgical and bio-hydrometallurgical technologies have been extensively used to recover gold from secondary sources (Fleming, 1992). Mining gold from year to year is always an increase, in which the more developed, ranging from mining system that uses simple system to a more modern processing system. Gold mining was first carried out in alluvial areas. Processing system is also very simple to do that is by panning using a tray, which then developed with gold mining wave table, and followed by gravity separation method using the amalgamation process with the capture of mercury.
- GOLD PROCESSING
After the extraction of gold from its ore, it is further passed through comminution (crushing / milling). This comminution process is divided into stages: Refractory ore processing, crushing and milling.
Once the size of the ore is reduced, the process is then performed by separating the mineral concentration process gold from mineral impurities, consisting of several kinds of processing techniques: process gravity concentration, amalgamation process, flotation process, leaching process.
From the stage of the process, will then be obtained concentrate processing ores containing various metals. And the concentrate will be processed in order to obtain the value of gold in high purity chemical process. And a few stages of the treatment process through a process: smelting, parting and aqua regia.
After the treatment of the processed gold by the above processes, it is finally made into different forms, for example gold bars, jewelries, gold equipment etc. Below is a flow chart showing the processing of gold.
- GOLD DISCARDING
Recent research has proven Gold and other precious metals for example silver, palladium and platinum are discarded mostly through e-waste in our environments today. (Husman, 2004) revealed that in mobile phones, three quarter of the environmental impacts result from gold and palladium content. According to (Husman et al, 2007). Gold accounts for 16% of the total environmental weight of materials contained in waste IT and telecommunication equipment (excluding monitors). UNEP (2005) reported that 20 to 50 million tonnes of small waste electrical and electronic equipment (sWEEE) are generated worldwide every year. Report shows that these discarded metals are being recycled and reused.
- GOLD WASTE AND POLLUTION
As gold is being extracted from mines, gold waste are concentrated in the rock piles, tailings and waste rocks. The waste from gold mines constitutes the largest single source of waste and pollution. As at 1997, South Africa produced an estimated 468 million tons of mineral waste per annum (DWAF, 2001). Gold mining waste was estimated to account for 221 million tons of 47% of mineral waste produced in South Africa, making it the largest, single source of waste and pollution (DWAF, 2001). There are more than 270 tailings in the Witwatersrand Basin, covering approximately covering 400 km in surface area (AngloGold Ashanti, 2004). These dams are mostly unlined and many are not vegetated, providing a source of extensive dust, as well as soil and water (surface and groundwater) pollution (Anglo Gold Ashanti, 2004).
Historically impoundment on land was the preferred option for tailings disposal. The environmental implication of this disposal option include contamination of streams and rivers by acid mine drainage (AMD), contamination of streams due to surface run off from the impoundment area, air and water contamination due to wind erosion of dried-out tailings. The exposure of sulphide bearing rock materials to oxygen and water produces Acid mine drainage. Mine waste problems are numerous, but the most difficult problem to handle is the acid mine drainage (AMD) that emanates from both surface and underground workings, waste and development rock, tailing piles and ponds (Durkin and Hermann, 1994).
Gold mining has been reported to be a dangerous activity as the heavy metals, mainly Hg, Pb and As are released into the environment. There were studies based on the assessment of the above stated heavy metals and their presence in soil and water in Kenya, South Africa and other parts of Africa. These metals were selected because of their toxicity nature in similar mining environments.
According to the United States Agency for Toxic substances and Disease Registry (ATSDR, 1999) Hg, Pb and As are in the top list of hazardous substances. As and Pb are major metals in gold – sulphide deposits, where they occur as minerals mainly in arsenopyrite (FeAsS) and galena (PbS), respectively. Under natural conditions, they are relatively stable. However, gold extraction from its ore breaks down these minerals due to exposure to oxygen and water, thereby making it acidic. Mercury as a pollutant in mining is released during gold processing, when mercury is used to amalgamate gold so as to recover it from its ore. Cadmium is also another toxic metal, occupying position seven in the list of hazardous substances, generally occurring as an isometric trace element in sphalerite. It is estimated that about 1.32kg of Hg is lost for every 1kg of (Au) produced (Harada et al., 1997). As, Pb and Hg are all known as poisonous metals which when taken in by humans causes lots of diseases.
- GOLD STANDARDS
Gold standard is referred to as monetary system in which the standard economic unit of account is based on a fixed quantity of gold. Three types of gold standard can be distinguished: specie, exchange, and bullion. Most nations abandoned the gold standard as the basis of their monetary systems at some point in the 20th century, although many hold substantial gold reserves. An estimated total of 174,100 tonnes of gold have been mined in human history, according to (GFMS Thomson Reuters, 2012). This is roughly equivalent to 5.6 billion troy ounces or, in terms of volume, about 9,261 cubic metres (327,000 cu ft), or cube 21 metres (69 ft) on a side. There are also other varying estimates of the total volume of gold mined.
Gold is defined as a chemical element with the symbol (Au) and atomic number 79. In its pure state, it is a bright, moderately reddish yellow, dense, soft malleable and ductile metal. Gold is one of the native earth metals that is of very high value in our world today. The current price of gold per Ounce is approximately 1220usd (GFMS, Thomson Reuters, 2016). It has different properties, which are physical and chemical properties. Physical properties which consist of the malleable and ductile nature of gold, density of gold and also its melting and boiling point. Chemical properties which is its resistance to corrosion and it is chemically inactive. Gold has been reported to be used for the production of jewelries, medals and monetary exchange.
Gold can be recovered or extracted from primary source in placer mining by the use of panning, sluicing and mercury or cyanide amalgamation. In secondary sources the use of hydrometallurgy, pyrometallurgy and bio-hydrometallurgy has been applied to recover gold from ore. After the extraction of gold from its ore, it’s passed through crushing and and milling process. This is done so as to reduce the size before conducting other processes, which involves leaching, flotation, gravity concentration and amalgamation to separate the gold concentration from its impurities. And finally through a few stages of the treatment process which involves smelting, parting and aqua regia. The processed gold after the above treatment is then produced into various forms for example jewelries, medals etc. Discarded gold from e-waste and other sources is being recycled into new forms. Generally mine water waste, rock piles and tailings from mining has led to the pollution of our environment today. The most common problem concerned with mining is Acid mine drainage. (AMD), occurs when rock bearing sulphide materials are exposed to oxygen and water. The extraction of gold from its ore also causes the release of toxic substances for example lead, mercury, arsenic etc. These substances, when introduced into our environment and spread through water bodies, it causes adverse effect to human’s health. Gold standard is known to be a monetary system, in which the standard economic unit of account is based on a fixed quantity of gold. Three types have been distinguished: specie, bullion and exchange.
Research has shown that gold is chemically inactive, which is because of its high resistance to react with other metals. It is proven that in its natural state, it occurs with other metals and minerals. The extraction of gold from its ore causes the exposure of this minerals and heavy metals in our environment. Heavy metals e.g. Lead (Pb), Arsenic (AS), Mercury (Hg). And sulphide minerals which are mostly common with ores, when exposed to oxygen and water produces Acid mine drainage (AMD). Mercury is introduced by humans in the extraction of gold to amalgamate with gold making it easier to recover from ore. These heavy metals are toxic substances, which when released into water bodies in our environment cause severe hazard to humans.
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Figure 5. Simplified gold processing flow chart (after “Processing flow chart of gold”, 2011)
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