The Inco Superstack in Sudbury, Ontario, with a height of 380 meters (1,250 ft), is the tallest chimney in Canada and the Western hemisphere, and the second tallest freestanding chimney in the world after the GRES-2 Power Station in Kazakhstan. It is also the second tallest freestanding structure of any type in Canada, ranking behind the CN Tower but ahead of First Canadian Place. Just 69 metres shorter than the Empire State Building, it is the 33rd tallest freestanding structure in the world. The Superstack sits atop the largest nickel smelting operation in the world at Inco's Copper Cliff processing facility in the city of Greater Sudbury.
It was constructed in 1972 by Inco Limited (now Vale) at an estimated cost of 25 million dollars; from the date of its completion until the GRES-2 chimney was constructed in 1987, it was the world's tallest smokestack. Between the years 1972–75 it was the tallest freestanding structure in Canada. There were once many other shorter stacks beside it. Most have been demolished.
Prior to the construction of the Superstack, the waste gases contributed to severe local ecological damage. Compounded by open coke beds in the early to mid-20th century and logging for fuel, an inevitable near-total loss of native vegetation occurred. Of particular interest to geologists are the now exposed rocky outcrops, which have been permanently stained charcoal black, first by the pollution wafting over the decades from the roasting yards then by the acid rain in a layer which penetrates up to three inches into the once pink-gray granite.
By the 1950s, Sudbury's image as a barren, rocky wasteland was established. It was famously used to train NASA astronauts during the Apollo Project due to it's dead Moon-like appearance and the belief that the huge tailings piles would provide an analog of the Moon's surface. Despite the industry, a growing working class population, and subsequent deforestation of the past century, some pockets of old growth forests have remarkably survived.
The Superstack was built to disperse sulphur gases and other byproducts of the smelting process away from the city of Sudbury. It did this by placing the gases high in the air, where they normally blew right past the city on the prevailing winds. As a result, these gases can be detected in the atmosphere around Greater Sudbury in a 240 kilometers (150 mi) radius of the Inco plant. During the 1970s and 80s, the smoke plume formed a permanent, opaque, cloud-like formation running across the entire horizon as seen from a distance. Periodic inversions would cause the plume to fall into the city.
Construction of the Superstack was followed by an environmental reclamation project which has included rehabilitation of existing landscapes and selected water bodies such as Lake Ramsey. The ambitious regreening plan which has seen over three million new trees planted within the Greater Sudbury area. In 1992, Inco and the city was given an award by the United Nations in honour of its environmental rehabilitation programs.
While the Superstack lowered the ground-level pollution in the city, it has dispersed sulfur dioxide, and nitrogen dioxide gases over a much larger area. Though not the single source of lake acidification, it appears even the heavily industrialized Ohio Valley has contributed to the ecological problem of lakes as far north as northern Ontario. Research from data gleaned up to the late 1980s demonstrated acid rain to have affected the biology of some 7,000 lakes.
Prior to Vale's purchase of Inco, a major construction effort by Inco in the early 1990s dramatically scrubbed waste gases before pumping them up the Superstack. These upgrades were completed in 1994 and emissions from then on have been much reduced.By comparison to the plume prior to installation, the plume now disperses quite rapidly and is often see-through even at the stack site. Despite the 90% reduction in the sulphur dioxide and other gases, carbon dioxide and water vapour are the most visible component and continue to contribute to Vale's image as a pollution source.
It remains to be seen how Inco's new owners, Vale, will continue, considering the Brazilian company's past practices outside of North America. If Vale considers further reductions in emissions with 21st-century technologies, the Superstack might no longer be needed, and could be converted into a communications observation tower.
Emissions reductions and increases in thermal efficiency have reached the point where natural draught is no longer sufficient to draw flue gas up the stack, necessitating the use of induced draught fans and/or reheating of the flue gas using natural gas burners.
In contrast to the reduction of SO2 emissions, Inco's Superstack stands out in North America in its arsenic, nickel and lead emissions to the atmosphere. Using data compiled by the Commission for Environmental Cooperation, Inco alone accounts for 20% of all of the arsenic emitted in North America, 13% of the lead and 30% of the nickel. Although it is not strictly fair to compare a nickel-copper smelter to a lead smelter, by so doing one can get an idea of how poor the containment of lead is at Copper Cliff. In 1998, Inco emitted 146.7 metric tons of lead at Copper Cliff with a smelter production of 238,500 metric tons of nickel-copper matte. The EPA regulations in the United States require a primary lead smelter to limit emissions of lead to 3.0 gm per MT of product. With this emission factor, Copper Cliff would be required to limit emissions of lead to approximately 1.0 MT per year, demonstrating that the actual emission is about 150 times greater than allowed by US regulations for a lead smelter. Even with the 85% reduction postulated by Hatch, Inco would still emit 10 tonnes per year of lead, or four times the amount allowed by the EPA for a lead smelter. 
As a result of the excessive lead emissions from the Inco Superstack, the surrounding community of Copper Cliff was found to have levels of lead in soil tests at a level sufficient to cause harm to young children. 
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