Most people think that hard coal, extracted from NWR mines several hundred metres below ground, is simply loaded onto railcars once it surfaces and is then delivered straight to customers. The truth, however, is rather different.
A long journey involving sophisticated chemical and technological treatments awaits the coal after it arrives at the surface. During these processes one can hear everyday expressions such as showering, washing and drying.
All NWR mines include preparation plants, where the coal is converted into final products that we, often without realising, encounter almost every day. Let us now take a closer look at the journey taken by coal to its end-consumers.
Extracted coal is always taken to the surface in a skip bucket that can carry as much as 20 tonnes. Next, it is deposited in a bin next to the skip tower. This is where coal starts to make its way through the mine’s facilities, undergoing complex processing before being loaded onto railcars for the final journey to customers.
At the coal preparation plant: A sophisticated journey to the customer
“Coal separation according to the ash content and some other parameters varies between the preparation plants at the individual NWR mines because of the differing technologies at the plants. Nevertheless, the principle is quite similar in all of them,” said OKD processing specialist Marek Špok.
The raw coal in a skip bucket traverses a distance of several hundred metres to the separator, which is the initial stage of coal preparation, with the grain size variation set at up to 400 mm.
Vibratory screens with a 200mm mesh are used in the initial separation to create two basic fractions: up to 200mm and bigger than 200mm. Larger chunks of coal usually contain rock. Due to their higher weight and hardness, any rocks that are present usually crush the larger pieces of coal into smaller sizes.
A separation conveyor is then used for the manual collection of coal residues from rejected rock. The recovered coal is automatically crushed and moved to the 0 - 200 mm fraction, ready for further processing. Should the capacity of the available coal bins prove insufficient to contain all the extracted coal, temporary stockpiling is available.
The waste rock, or “tailings”, is used elsewhere, mainly in the construction of roads, railway corridors, dams and flood protection structures. The tailings are also a suitable and cheap material for landscaping and land reclamation, activities that have transformed landscapes around NWR mines in the Karviná and Frýdek-Místek regions. While the public still tends to retain the image of mining-damaged territory as being nothing but scorched earth, the truth is quite different. Thanks to considerable investments made by NWR in reclamation projects, areas previously affected by mining undergo conversion to self-contained ecosystems that provide habitats for plants and animals. Some sites are turned into leisure areas for people.
Not a grain wasted
At the point of entry into the preparation plant, the incoming coal is wet- or dry-separated by special vibrating separators. For example, at the ČSM and Paskov Mines, coal is separated from rock and sorted into fractions below and above 80mm using a Bretfolt rotary separator (see figure 1). The heavier rocks crush coal and push it out from the separator.
“Larger chunks of coal are then directed to a ‚rough processing line‘, while the smaller ones go into the fine processing line. The size, or the grain size to be more precise, that is used to separate coal differs between the mines in relation to the particular technology employed,” explained Mr Špok.
Taking the example of the ČSM and Paskov Mines further, the rough line processes coal with a grain size above 15mm, while the fine line receives coal with a particle size above 1mm. The Lazy plant processes coal exceeding 30mm on the rough line, and particles above 1mm on the fine line, while the fine lines at the ČSA and Darkov Mines receive coal of grain sizes above 0.5mm and 0.35mm respectively. So, the widely held view that miners use only big lumps of coal, known from our household stoves, and the smaller grains and dust are simply tossed to the winds, is completely wrong. The objective is to use every single grain of coal that is extracted from below the surface. Which is one reason why coal is sometimes called “black gold.”
| Processing plants at OKD mines |
Rock removal
(mm) |
Rough processing
(mm) |
Fine processing
(mm) |
| Darkov (in operation since1987) |
>40 |
1,0 – 40 |
0.35 –1.0 |
| Paskov (in operation since1971) |
>150 |
15 – 150 |
0.5 – 15 |
| ČSM (in operation since1970) |
>100 |
15 – 100 |
1.0 –15 |
| ČSA (in operation since1954) |
>60 |
0.5 – 60 |
|
| Lazy (in operation since1961) |
>200 |
30 – 200 |
1.0 –30 |
How coal is showered and dried
In the preliminary rough processing, coal is separated from tailings using a heavy magnetite liquid, i.e. a suspension of iron ore. The suspension has a specific density, causing coal to float while contaminant rock settles at the bottom. Coal is then removed from the separation device, the magnetite washed off and the remaining water removed on vibrating screens.
Coal sent through the fine processing line is separated in a similar way. A dense magnetite suspension is used in washing and separating dense medium cyclones or in dense medium baths. Although difficult to believe, even the finest coal particles, smaller than 1mm, are further separated into grain size groups, namely up to 0.25mm and 0.25 to 1mm, and processed further.
Coal grains of sizes from 0.25mm to 1mm are separated in spirals and the smallest ones, of sizes up to 0.25 mm, are divided in flotation cells using froth flotation agents (see figure 2). The coal is then dewatered in screen bowl centrifuges and dewatering filters. In simple terms: the flotation agent, i.e. a special type of oil, creates mineralised foam. Each tiny bubble within the foam carries a coal grain, which is carried to a collection trough along with the foam. The clean product coal is then chemically separated from the oil (by a de-foaming agent).
The magnetite, used to create the dense suspension, is subsequently separated from the water using magnetic separators (see figure 3). When it is again mixed with water to the correct ratio, magnetite is reused in the coal preparation processes. The whole process is geared to using all material as efficiently as possible. That is why water is also reused in coal processing. NWR sees two fundamental reasons for reusing water: an environmental one and an economic one.
Water recycling: Benefiting the environment as well as business
The coal preparation process is quite demanding in terms of water consumption. Several cubic metres of water are necessary for each tonne of processed coal. NWR therefore strives to recycle water as much as possible. When further recycling is technically impossible, water passes through a wastewater treatment plant before entering the public sewerage system. Each coal preparation plant has a proprietary water reservoir nearby, fitted with a pumping station, which delivers water to a water treatment facility. This facility contains large covered tanks for washing water for the coal preparation process.
Quality monitoring at every step
Once through the above preparation processes, the coal is stored in washed coal bins. At this stage, the water content level and the coal quality make the coal suitable for distribution to customers. The storage bins are placed over loading points, where coal is filled into railcars for shipment to customers. “As we consider the quality of coal we supply to be our key competitive advantage, we know the precise parameters and quality of the coal delivered to our customers. Sampling stations at the loading points are used to establish the quality of coal, which is then conveyed to the customer. However, sampling does not take place only at the very end of the whole preparation process, but also at every stage in order to ensure that the complete process is proceeding as desired,” said Mr Špok.
The tailings, separated at each stage of the process, pass through the same washing and dewatering process as coal, in order to make them suitable for use in civil engineering or land reclamation.
Lazy facility: A small nut or a bigger one?
Anyone who has ever used hard coal for heating at home will remember the endless discussions over the best size of coal to use. All but one of NWR’s mines sell coal exclusively to large industrial customers.
The exception is Lazy, which along with the ČSA facility comprises part of the Karviná Mine. In its coal preparation process, the Lazy plant produces thermal coal in the “slack” (10 to 30mm), “nut” (30 to 50mm) and “egg” or “lump” (50 to 200mm) sizes for retail customers. Therefore, if you happen to use hard coal from the Ostrava-Karviná coalfield in your home, you can be sure that it was mined at the Lazy facility near the town of Orlová. Lazy operates a coal preparation plant solely for thermal coal. Any coking coal produced at the mine is shipped by rail to the preparation plant at the Paskov Mine.
It should be noted that coal sold to retail customers and for household use is just a small fraction of NWR‘s total production.
Are middlings coal or rock?
Carried to the surface by the skip buckets are three types of materials that have greatly varying qualities: pure coal, rock, and “middlings”. Middlings, as the name may suggest, are rocks interspersed with coal that does not separate away during the mining or transport to the surface. The material is lighter than rock and heavier than coal. Some energy operations are capable of processing and effectively utilising middlings in their production. Middlings thus have their regular consumers. Nonetheless, the coal preparation plants strive to separate the rock from the coal contained in middlings. For example, at Paskov the middlings obtained from coal separation are crushed and washed again to split the two constituents. Only when that fails are the remaining middlings set aside for sale to customers.
The whole coal preparation process in NWR is typical for its high degree of mechanisation and automation. All the distinct stages are controlled by staff from central desks with the use of computers to configure, monitor and check the many steps along the route taken by coal from the skip bucket to the customer.
Marek Síbrt,
marek.sibrt@okd.cz