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By-product Loads

Mill Tailings

Mill tailings are, by definition, fine-particle residues of milling operations that are devoid of metal values. Particle-size distribution is one of the essential ways of characterizing tailings. The mining industry distinguishes “sands” and “slimes” as components of residues.

Materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are distinct from overburden. Waste rock or other material that overlies an ore or mineral body.

Mill Tailings
Mill Tailings

Mill Tailings Economics

Losses to tailings is the most important parameter in deciding whether a deposit or milling process is economically viable or not. Early milling operations often did not take adequate steps to make tailings areas environmentally safe after closure. Modern mines, particularly those in jurisdictions with well-developed mining regulations and those operated by responsible mining companies, often include the rehabilitation and proper closure of tailings areas in their costs and activities.

Site selection for tailings disposal has to be based on economic and environmental considerations. Additional, tailings impoundment site has to be close to the mill, for economic reasons and to conform with the following three requirements:

  • Be mineralogically barren
  • Have strong structural geology to bear the weight of the impoundment
  • Have a geomorphology that allows surface waters to bypass the dam or drain through it

Factors influencing the design of tailing impoundment include site characteristics, tailing characteristics, effluent characteristics, mine/mill characteristics

STORAGE AND DISPOSAL

Tailings may be discarded on land, into a watercourse, or in a sizable body of water. In the case of underground mining, at least part of the tailings may have to be pumped back into the mine.

Uses for tailings – backfill excavated space, but usually discarded. If possible, tailings should not be stored underground; it is prudent to dispose of milling tailings on the surface and have them easily available when more efficient extraction processes exist. Storing tailings inactive open-pit mines is obviously impossible. Also, abandoned open-pit quarries far from active mills.

Historically, tailings were disposed of in the most convenient manner, such as in downstream running water or down drains. Because of concerns about these sediments in the water and other issues, tailings ponds came into use. The sustainability challenge in the management of tailings and waste rock is to dispose of the material. Such that it is inert or, if not, stable and contained, to minimize water and energy inputs and the surface footprint of wastes and to move toward finding alternate uses.

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Fuel Loads

Coke (Fuel) for Railroads

Coke for railroads in the first years of steam railway locomotives, coke was the normal fuel. This resulted from an early piece of environmental legislation; any proposed locomotive had to “consume its own smoke”. This was not technically possible to achieve until the firebox arch came into use, but burning coke, with its low smoke emissions, was considered to meet the requirement. This rule was quietly dropped, and cheaper coal became the normal fuel, as railways gained acceptance among the public. The smoke plume produced by a travelling locomotive seems now to be a mark of a steam railway, and so preserved for posterity. Wiki

Coke is a grey, hard, and porous fuel with a high carbon content and few impurities, made by heating coal or oil in the absence of air—a destructive distillation process. It is an important industrial product, used mainly in iron ore smelting, but also as a fuel in stoves and forges when air pollution is a concern.

The unqualified term “coke” usually refers to the product derived from low-ash and low-sulphur bituminous coal by a process called coking. A similar product called petroleum coke, or pet coke, is obtained from crude oil in oil refineries. Coke may also be formed naturally by geologic processes.

Coke for Railroads
Coke for Railroads

PRODUCTION

The industrial production of coke from coal is called coking. The coal is baked in an airless kiln, a “coke furnace” or “coking oven”. It has temperatures as high as 2,000 °C (3,600 °F) but usually around 1,000–1,100 °C (1,800–2,000 °F). This process vaporises or decomposes organic substances in the coal, driving off volatile products. Including water, in the form of coal-gas and coal-tar. The non-volatile residue of the decomposition is mostly carbon. In the form of a hard somewhat glassy solid that cement together the original coal particles and minerals.

Some facilities have “by-product” coking ovens in which the volatile decomposition products are collected. It is purified and separated for use in other industries, as fuel or chemical feed stocks. Otherwise the volatile byproducts are burned to heat the coking ovens. This is an older method, but is still being used for new construction.

Coke as a fuel

Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Additionally, the carbon monoxide produced by its combustion reduces iron oxide (hematite) in the production of the iron product. Coke is commonly used as fuel for blacksmiths.

Coke was used in Australia in the 1960’s and early 1970’s for house heating. It was an incentive for home use in the UK (so as to displace coal). After the 1956 Clean Air Act, which was passed in response to the Great Smog of London in 1952.

Since smoke-producing constituents are driven off during the coking of coal. Coke forms a desirable fuel for stoves and furnaces in which conditions are not suitable for the complete burning of bituminous coal itself. It may be combusted producing little or no smoke, while bituminous coal would produce much smoke. Namely, coke was widely used as a smokeless fuel. It is a substitute for coal in domestic heating following the creation of smokeless zones in the United Kingdom. Also in Highland Park distillery in Orkney roasts malted barley for use in their Scotch whiskey in kilns burning a mixture of coke and peat.

Coke may be used to make synthesis gas, a mixture of carbon monoxide and hydrogen.

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