Coal Mining: The value of the black rock and its by-products
By Calvin Manika
Coal mining, the flagship of Hwange’s local economy in Zimbabwe is known for the black rock which is used locally and exported to other countries in the SADC region. Despite, calls for the ban of the use of coal amidst climate change issues, the black rock produces by-products of high value which have their markets.
More than 200 minerals have been identified in coal and its low temperature ash. Coal miners say minerals in coal are also a potentially valuable source of some critical metals. Along the coal processing structure many valuable by-products are made,
Coal is a complex geologic material composed mainly of organic matter and mineral matter, experts say, the latter including minerals, poorly crystalline mineraloids, and elements associated with non-mineral inorganics.
“Minerals in coal are often regarded as a nuisance being responsible for most of the problems arising during coal utilization, on the other hand, the minerals are also seen as a potentially valuable source of critical metals and may also, in some cases, have a beneficial effect in coal gasification and liquefaction,” said Gary Mpofu, an expert in coal mining.
Coal, which is valued for its energy potential, contains water, minerals, oil, gas, rock fragments, and fossils. Mpofu says minerals in coal play a significant role in affecting the utilisation of coal, from mining to grinding to combustion and technological problems, from coal cleaning to waste disposal, and from the environmental impacts to human health consequences.
“The minerals reduce the economic value of the coal they are in by diluting the energy content and add expense when removed by coal cleaning processes, whose primary purpose is to remove the minerals prior to combustion or coke production. But coke production is growing in Hwange especially among Chinese investors,” said Mpofu.
At least every year, a coke battery is established in Hwange, giving demand for coal among the coke production companies and other users.
“When coal is heated in the absence of air, it produces gas which is called coal gas. It produces liquid which is called coal tar. A solid residue is also produced which is called coke. Coal tar is used as a base for coatings and paint, in roofing and paving, and as a binder in asphalt products. Both coal tar and coal-tar pitch contain many chemical compounds, including carcinogens such as benzene,” added Mpofu.
Benzene is in demand as a raw material for the chemical industry. It is the main product obtained by the pressure refining. Coal miners say, the remainder serves as an additive for motor fuels or is further treated to yield refined products.
Although the occurrence of minerals in coal is usually regarded as having deleterious effects in coal utilization, findings by this reporter shows that, minerals in coal have some important beneficial aspects. Coal and coal ash are attracting increasing attention as potential sources of critical elements, such as rare earth elements all of which are in demand in the semi-conductor industry and the production of advanced materials.
“In several countries, rare earth element concentrations are sufficiently high in the coal ashes to make extraction an economically viable option. High-Al coals have also attracted much attention in recent years in China, using it for Al extraction,” said Petros Kasirori, a geologist.
Some of the products from heated coal are boiler slag, bottom ash, and fly ash. Over the years, the products have been used in the power plants and beyond, having a fair market value in the mining and construction industry. The Mining Vision Magazine established that, boiler slag is produced in coal fired power plants that use wet-bottom boilers. It forms from melted minerals left over from coal combustion.
“This molten ash is quenched with water to form a solid, black, glassy waste product, called slag. Boiler slag is commonly re-purposed as a useful combustion byproduct. The slag material can be ground into particles which are used as roofing granules and blasting grit,” said Kasirori.
“Bottom ash is produced in dry-bottom boilers, which are the most common types of boilers in coal-fired power plants. Bottom ash consists of the melted minerals left over from coal combustion. The ash settles on the walls and at the base of the boiler as a gray, sandy material. The ash is periodically removed and disposed of in landfills or re-purposed as a useful coal-combustion byproduct.
Fly ash includes ash, dust, soot, and cinders material produced from coal combustion. It consists of the mineral matter in the coal which did not combust in the furnace and did not settle to the bottom of the boiler as bottom slag or bottom ash.
“Fly ash is used in many ways but mostly, to make concrete and grout; for use in structural fills; for waste stabilization (thickening, solidifying); in cement production; and in mining applications such as mine fills, where permitted,” said Kasirori.