Open pit gold mining and its environmental impacts

Open pit gold mining and its environmental impacts

By Asoc. Ecol. Costa Rican - Friends of the Earth

Open pit mining removes the surface layer or overburden from the earth to make extensive deposits of low-quality ore accessible. Modern excavation equipment, conveyor belts, large machinery, the use of new inputs and distribution pipes allow today to remove entire mountains in a matter of hours, making it profitable to extract less than one gram of gold per ton of material removed. .

1.1. What is open pit mining

1.1.1. Definition and Characterization of Open Pit Mining
and its Impacts

Basically, the type of mining that would be developed in the gold exploitations that Costa Rica intends to implement is that of "open-pit mining by cyanide leaching." Due to the implications of this type of mining, we will try to be exhaustive in its description.

Open pit mining is an industrial activity with high environmental, social and cultural impact. It is also an unsustainable industrial activity by definition, to the extent that the exploitation of the resource implies its depletion.

The technical innovations that mining has undergone since the second half of this century have radically modified the activity, so that it has gone from the exploitation of high quality underground veins to the exploitation - in open pit mines - of minerals of lower quality disseminated in large deposits.

Open pit mining removes the surface layer or overburden from the earth to make extensive deposits of low-quality ore accessible. Modern excavation equipment, conveyor belts, large machinery, the use of new inputs and distribution pipes allow today to remove entire mountains in a matter of hours, making it profitable to extract less than one gram of gold per ton of material removed. .

There is consensus in the literature on the subject in the sense that no industrial activity is as environmentally, socially and culturally aggressive as open pit mining (MCA).

Open pit mining uses, intensively, large amounts of cyanide, a very toxic substance, which makes it possible to recover the gold from the rest of the removed material. To carry out this entire process, the reservoir must cover large areas and be close to the surface. As part of the process, gigantic craters are dug, which can reach more than 150 hectares in size and more than 500 meters deep.

Vaughan (1989) considers that "in environmental and social terms, no industrial activity is more devastating than surface mining" (open pit).

According to Kussmaul (1989), the environmental impact caused by any mining activity is related to four main factors:

1. Size of the farm, which refers to the volume of production of the farm, which has as a consequence a certain dimension of activities and production of waste and wastewater.

2. Location, which refers to the site where the exploitation is carried out, the nearby populations and the nature of the local topography.

3. Exploitation methods, which depend on the type of deposits to be exploited and which are directly related to the nature and extent of the impact. Three main methods are used:

to. Open pit mining (or surface mining),
b. Underground mining,
c. Mining by washing and dredging.

4. Characteristics of the minerals and their benefit, which refers to the fact that the nature of the mineral determines the treatment to undergo. Minerals can be divided into:

to. Non-metallic minerals (such as building materials), which require little physical treatment, such as crushing and grinding, and which do not require any chemical treatment.

b. Metallic minerals, which generally require a high level of processing, as well as the use of many chemical reagents, and which generate large amounts of fine waste.

1.1.2. Impacts of Mining

Mining activities comprise various stages, each of which has particular environmental impacts. In a broad sense, these stages would be the following:

* prospecting and exploration of deposits,
* development and preparation of mines,
* exploitation of mines,
* Treatment of the minerals obtained in the respective facilities with the aim of obtaining marketable products.

Salinas (1993) cites the following individual activities as possible causes of environmental impact during the EXPLORATION phase:

* preparation of access roads,
* topographic and geological mapping,
* assembly of camps and auxiliary facilities,
* geophysical works,
* hydrogeological investigations,
* opening of trenches and survey wells,
* taking samples.

During the EXPLOITATION phase, the impacts that are produced depend on the method used. According to various authors (Vaughan (op. Cit.), Salinas (op. Cit.), Elizondo (1994)), the main environmental impacts caused by open-pit mining (MCA) in its exploitation phase are the following:

* Affecting the surface: CAM devastates the surface, severely modifies the morphology of the terrain, piles up and exposes large amounts of sterile material, causes the destruction of cultivated areas and other surface heritages, can alter water courses and form large gaps for discarded material.

* Affecting the environment in general: the MCA radically transforms the environment, loses its possible scenic attraction and is affected by the noise produced in the different operations, such as in crushing and grinding, in power generation, in the transportation and loading and unloading of minerals and waste material left over from the mine and the mill.

* Air pollution: the air can become contaminated with solid impurities, for example dust and toxic or inert fuels, capable of penetrating to the lungs, coming from different phases of the process. The air can also be contaminated with vapors or gases of cyanides, mercury, sulfur dioxide contained in waste gases, incomplete combustion processes or emanations from non-circulating water pools or ponds with decomposing organic matter.

* Affectation of surface waters: fine solid waste from the exploitation area can lead to an elevation of the sediment layer in the rivers of the area. Poorly constructed or poorly maintained dams and oxidation ponds, or improper handling, storage, or transportation of inputs (such as fuels, lubricants, chemical reagents, and liquid waste) can lead to contamination of surface waters.

* Affecting groundwater or groundwater: water contaminated with used oil, with reagents, with mineral salts from piles or dumps of solid waste products from the treatment processes, as well as rainwater contaminated with the contents of said dumps, or Waters from tailings ponds or dikes, or contaminated process waters, can reach groundwater. In addition, there may be a decrease in the levels of these groundwater when they are a source of fresh water supply for mineral treatment operations.

* Affecting the soils: the MCA implies the elimination of the soil in the exploitation area, and produces a drying out of the soil in the surrounding area, as well as a decrease in agricultural and agricultural yield. It also tends to cause subsidence and the formation of swamps in case the groundwater level rises again. In addition, it causes the disabling of floors by piling up excess material.

* Impact on the flora: the MCA implies the elimination of the vegetation in the area of ​​the mining operations, as well as a partial destruction or a modification of the flora in the surrounding area, due to the alteration of the water table. It can also put pressure on existing forests in the area, which can be destroyed by the exploitation process or by the expectation that it will take place.

* Impact on fauna: fauna is disturbed and / or driven away by noise and air and water pollution, the elevation of the level of sediment in rivers. Additionally, the erosion of sterile waste heaps can particularly affect aquatic life. Poisoning can also occur by residual reagents contained in water from the exploitation area.

* Impact on populations: the MCA can cause conflicts over land use rights, give rise to the uncontrolled emergence of human settlements causing social problems and destroy areas of tourist potential. It can cause a decrease in the performance of the work of fishermen and farmers due to poisoning and changes in the course of rivers due to the elevation of level by sedimentation. On the other hand, the MCA can cause a negative economic impact due to the displacement of other current and / or future local economic activities.

* Changes in the microclimate: CAM can cause changes in the microclimate and can cause a multiplication of pathogens in puddles and areas covered by stagnant water.

* Post-mining scenic impact: MCA leaves deep craters in the landscape. Their disposal can be so expensive that they can prevent exploitation itself.

1.2. Open pit gold mining by cyanide leaching

The growing interest in the exploitation of gold on the part of a wide variety of mining companies stems from both increases in gold prices (an ounce is currently priced at a price close to 395 dollars), which provide a high profit margin, as in the recent creation of cost-effective methods of gold extraction in extremely poor deposits, thanks to cyanide leaching gold extraction technology.

According to the DuPont Corporation (cited by Alberswerth), it is economically viable to extract minerals with only 0.01 ounces of gold for each ton of ore. This technology has come to replace the recovery of gold by mercury amalgamation, an inefficient process in terms of recovery, since it allows only 60% recovery of the mineral, compared to more than 97% in the case of extraction with cyanide. (Amalgamation is the process by which the mineral joins with the substance used, in this case mercury, in order to separate it from the rest of the material.)

According to the Gold Institute (Gold Institute, cited by Young, 1993), gold production by the cyanide leaching process increased from 468,284 ounces in 1979 to 9.4 million ounces in 1991. To reach the level of 1991 production, more than 683 million tons of ore were treated with cyanide.

1.2.1. Cyanide Heap Leach Mining Technology for the Extraction of Minerals by Cyanide Leaching

Mining operations using cyanide heap leach mining technology in open pit mines are comprised of six main elements, which are:

* the source of the mineral (an ore source),
* the platform (the pad) and the heap (the heap),
* the cyanide solution,
* an application and collection system,
* solution storage ponds,
* a plant for the recovery of metals.

Most operations using cyanide leach extraction use open pit mining to obtain the ore. Open pit mining disrupts large areas of land. However, several operations also use previously mined waste material. Ores (rocks that contain the mineral) are crushed and piled up in a heap that is placed on a leach pad.

Piles of crushed material vary in size. A small cluster can be made up of 6,000 tons of ore, while a large cluster can be up to 600,000 tons, reaching hundreds of feet high and hundreds of yards wide. Leach pads can vary in size. They can be approximately one to 50 acres (1 hectare equals 2,471 acres). The size of the platform depends on the size of the operation and the leaching technique. Generally, leach pads have a liner of synthetic and / or natural materials that are used to "try" to prevent seepage. Sometimes operations use double or triple liners. The use of several effective liners is economically viable and advantageous for the environment, since a leaky deck can contaminate water resources with cyanide.

Once the crushed ore is stacked on the leach pad, it is evenly sprayed with a cyanide solution. A sprinkler system disperses the cyanide solution at 0.005 gallons per minute per square foot (typically). For a small cluster (200 feet by 200 feet), this velocity equals 200 gallons per minute. The cyanide solution contains between 0.3 and 5.0 pounds of cyanide per ton of water (between 0.14 and 2.35 kg of cyanide per ton of water), and has an average concentration of 0.05 percent (about 250 milligrams per liter of cyanide free). The cyanide solution leaches (lava and amalgamates) the microscopic gold particles from the ore as it filters through the heap. Leaching cycles last from a few days to a few months, depending on the size of the heap and the quality of the ore. The cyanide solution that contains the gold - called the "pregnant" solution - flows by gravity into a storage reservoir. From the storage reservoir, pumps or lined trenches are used to carry the solution to the metal recovery plant.

The most used methods for recovering the gold contained in the cyanide "pregnant" solution are zinc precipitation (Merrill-Crowe method) and carbon absorption. In the zinc precipitation process, powdered zinc and lead salts are added to the "pregnant" solution. Gold precipitates (separates) from solution while zinc powder combines with cyanide. The precipitate is then melted to recover the gold. The end products of this process are gold ore bullion and a "sterile" cyanide solution (without gold) (barren solution), which is transferred with pumps to a storage reservoir. It also originates waste material (slag material) consisting of impurities, including heavy metals. These slags are normally discharged into a waste material heap.

The preferred alternative for most operations is carbon absorption, especially in smaller operations and in those in which the amounts of silver associated with gold in the "pregnant" solution are lower. In this process, the pregnant solution is driven by pumps through columns of activated carbon. The gold and silver in the solution adhere to the carbon, and the "sterile" solution, which still contains cyanide, is taken to a storage reservoir. The gold and silver are separated from the carbon by a treatment with hot caustic soda. The solution then passes through a cell containing a stainless steel anode and a cathode to plating the metal. Spent coal is reactivated in a furnace so that it can be reused.

In leaching operations, storage reservoirs are used to store the cyanide solution that will then be sprayed on the heap, on the "pregnant" solution leached from the heap, and on the "sterile" solution resulting from gold processing. For environmental and economic reasons, all storage reservoirs have liners to prevent leakage of the cyanide solution.

Cyanide leaching operations can use a "closed" or "open" system for handling the cyanide solution. In an "open" system, the "sterile" solution remaining after gold recovery is treated or diluted to meet applicable water quality standards for cyanide concentrations and then discharged to the environment. In a "closed" system the "sterile" solution is reused or recycled to minimize the need for more cyanide, and to comply with environmental standards that may be applicable at the mining site. Several large operations on federal lands (in the United States) are using the "closed" system.

1.3. Environmental impacts of gold mining by cyanide leaching

Mining operations that use cyanide extraction technology carry high environmental impacts, which in many cases can be classified as environmental disaster.

1.3.1. About the documentation for the topic

The considerable and often even dramatic environmental and social impact of this type of mining is widely documented. Among others, it is recommended to consult the following authors: Alberswerth et al (op.cit.); AMIGRANSA (op.cit.); Bliss & Olson (op.cit.); Bravo (1994); Danuron Dickson (op.cit.); Emberson-Bain (op.cit.); Hartley (1995); Hocker (1989); Knudson (1990); Mineral Policy Center (1988); Mineral Policy Institute (op.cit.); Moody (op.cit.); Panos Institute (1996), Reece (1995); Sartorio de Ponte (op.cit.); U.S. Department of Labor (1981,), Young (1993).

In the case of Costa Rica, the only mine that has operated with open pit techniques has been the Macacona mine, which is why it represents the only case in which environmental and social impacts can be documented. Regarding this case, it is recommended to consult the following documents: ICEA (1989) and Umaña (1990).

1.3.2. On the use of cyanide in mining using leaching

Given the high toxicity and natural reactivity of cyanide, the containment of this substance is a primary concern in mines where leach extraction is used. The detrimental effects of cyanide on fish, wildlife, and humans have been documented.

to. Cyanide toxicity

For plants and animals, cyanide is extremely toxic. Cyanide spills can kill vegetation and impact photosynthesis and reproductive capabilities of plants. As for animals, cyanide can be absorbed through the skin, ingested or breathed in. Airborne concentrations of 200 parts per million (ppm) of hydrogen cyanide are lethal to animals, while concentrations as low as 0.1 milligrams per liter (mg / l) are lethal to sensitive aquatic species. Sublethal concentrations also affect the reproductive systems of both animals and plants.

The lethal doses for humans are, if they are ingested, 1 to 3 mg / kg of body weight, if they are assimilated, 100-300 mg / kg, and 100-300 ppm if they are inhaled. This means that a portion of cyanide smaller than a grain of rice would be enough to kill an adult. Long-term exposure to a sublethal dose may cause headaches, loss of appetite, weakness, nausea, vertigo, and irritation of the eyes and respiratory system. Great care must be taken when handling cyanide to prevent harmful contact by workers. However, according to the industry, there are no cases of human fatalities in mines using cyanide leaching techniques.

Faced with this fact, frequently used as an argument by mining companies, Philip Hocker (op.cit.) Points out: "Limiting our concern about cyanide to the fact that human deaths have not been reported is to fall into what biochemists call the theory of toxicology "the dead in the streets": the attitude according to which, if no corpses are seen, everything is in order. Despite the absence of human corpses, there is evidence that not everything is in order. "

Mining workers often come into contact with cyanide, especially during the preparation of the cyanide solution and the recovery of gold from the solution. Hazards for mining workers are cyanide dust, cyanide vapors (HCN) in the air from the cyanide solution, and contact of the cyanide solution with the skin.

1.3.3. About the impact on wildlife and waters

Although profitable for mining companies, mines using cyanide leaching are time bombs for the environment, as indicated by the extensive study by the National Wildlife Federation of the United States (Alberswerth et al, 1992 ), of which we cite the main concerns below:

* As millions of tons of ore are extracted from open pit mines and treated with millions of gallons of cyanide solution, operations using cyanide leach extraction disrupt wildlife habitats and watersheds hydrographic, and can result in a multitude of risks to health and the environment. These impacts can manifest themselves during various phases of the operation.

* Cyanide ponds seduce wildlife. The death of wild animals, especially birds, attracted by the lure of the water bodies of these ponds has been frequently recorded. The widespread extent of wildlife mortality at facilities that use this process has raised concerns with the U.S. Fisheries and Wildlife Service, despite the fact that techniques exist to prevent the death of wildlife, such as fences and nets that cover leach pads and storage reservoirs to prevent birds and mammals from coming into contact with the poisonous solution.

* After leaching, the processed ore pile still contains traces of the highly toxic cyanide solution, as well as concentrated heavy metals that have precipitated from the ore. Many operations choose to treat cyanide-contaminated waste by flushing the heap with fresh water until the cyanide concentration drops below the maximum allowed level (this level varies between states and countries). Once the cyanide concentration drops to the permitted level, the processed material is typically left in place, compacted, and an effort to ecologically rebuild the site may or may not be made.

* If used ore and waste rock are not completely rinsed or left untreated, Cyanide can continue to leak into the environment. Both cyanide and the heavy metals released by it (among them are arsenic, antimony, cadmium, chromium, lead, nickel, selenium, thallium) and other toxic substances found in the heap and leachates (for example, sulfides), they are a threat to streams, rivers or lakes, to underground water sources, and to fish, wildlife, and plants (cited also by Hartley, 1995).

Other authors draw attention to the following:

* Cyanide solutions used in mining can leach into groundwater (groundwater) (Engelhardt, 1989, cited by Hocker, 1989; Hilliard, 1994).

* Long-term problems from heavy metal leaching from waste heaps from operations using cyanide leach extraction probably exceed the direct impact of cyanide itself (Hocker, 1989).

Even in the United States, current federal and state regulations do not adequately address the impacts of mining using cyanide leach extraction. Despite the large increase in the number of gold mining activities and the known impacts of these activities, regulatory bodies at the federal and state levels have been slow to address these issues.

1.3.4. About accidents inherent to open pit gold mining by cyanide leaching About cyanide releases to the environment

Cyanide used in the leaching process can and does cause environmental damage. The two most common classes of cyanide releases to the environment in leach mining operations result from:

to. Linings (geomembranes placed under heaps and ponds) that allow seepage due to improper design, manufacturing defects, improper installation and / or damage (holes) produced during the operation process.

In his excellent review of liners (geomembranes) used in gold mining, Reece (op.cit.) States: "All liners have leaks. That is the most important thing to understand about geomembranes used in mining that it uses cyanide leaching. The only difference between them is that some have leaked and others will "(italics in original).

b. Solutions that overflow from storage reservoirs. These releases cause damage to plants and animals that come in contact with lethal concentrations of the cyanide solution, and pose a long-term threat to groundwater (groundwater).

Generally, storage reservoirs are designed to withstand large storms and floods. However, they do not always prevent overflows. Heavy metals and cyanide-contaminated water escaping from a storage reservoir cause further damage when they flow directly into natural waterways. The escaping solution may be enough to kill fish and other aquatic life, or to contaminate drinking water resources.

Comprehensive description on open pit mining and the cyanide leaching method, Text prepared by the Costa Rican Ecologist Association - Friends of the Earth (AECO-AT) for the National Front of Opposition to Open Pit Gold Mining, Costa Rica - Extracted from the website -Posted by noalamina. Sent by the NAC & POP (National and Popular News Network)

* Costa Rican Ecologist Association - Friends of the Earth (AECO-AT).

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