- Selective mining improves quality in oil shale extraction, told German Wirtgen Group company, active now in Estonia, in a press release.
Oil shale is a type of rock consisting of both organic carbon and mineral constituents. Oil shale resources are estimated to amount to some 10 quintillion tonnes worldwide. The amount of oil they would yield exceeds the current total oil reserves by as much as 50 percent.
According to Wirtgen Group, there is some 5 billion tonnes of oil shale occur in Estonia, 1.5 billion tonnes of which can be considered recoverable and are mostly enclosed by limestone deposits. The northern European country alone realizes some 70 percent of the global oil shale production.
Oil shale mining enabled Estonia to secure a high level of independence in energy supply, states Wirtgen Group.
In addition, the mining of oil shale has meanwhile become a major factor influencing oil prices on a global scale. Three methods have to date established themselves in oil shale mining:
· drilling and blasting
· semi-selective mining using rippers,bulldozers and excavators
- surface mining as an entirely selective mining method
A direct comparison between the three methods used in Estonia has shown that the surface mining technology offers clear advantages in terms of the run-of-mine (ROM) material quality. Estonia’s leading oil shale producers, “Kiviöli Keemiatööstuse” (short: Kiviöli) and “Eesti Energia Mining”, therefore rely on the use of the Wirtgen surface miners.
An economical way to self-sufficiency in resources using surface miners
The endeavour to mine oil shale in Estonia has long been characterized by the notion of self-sufficiency in resources. The use of surface miners has now given priority also to the economic efficiency of the mining process. Oil shale mining in Estonia began as early as 1916. Four deposits are currently operated as opencast mines, generating ap-proximately 50 percent of the country’s entire oil shale production.
The mining material is processed in two different ways: Kiviöli supplies refineries for the production of heavy fuel oil, while Eesti Energia Mining supplies power plants for elec-tric power generation. Both companies use a 2500 SM for mining. Comparing conven-tional methods with the surface mining technology clearly shows that selective mining results in tremendous economic benefits for the companies both in heavy fuel oil pro-duction and energy generation.
Selective mining of horizontal oil shale and limestone seams with the 2500 SM. Two of the Wirtgen miners are currently in operation in Estonia.
The 2500 SM is a heavy-duty, high-performance miner equipped with a 2.5 m wide cut-ting drum for material mining. Depending on rock hardness, it achieves cutting perfor-mances of up to 1,400 tonnes per hour – as is the case in coal and oil shale mining. The design of the 2500 SM minimizes the time required for maintenance procedures: vital machine components, such as the cutting drum, offer easy access to allow the quick replacement of wear parts, such as cutting tools. Over 50 miners of type 2500 SM are in operation around the globe.
Wirtgen GmbH has developed the 4200 SM for even higher performance levels: this surface miner is capable of loading heavy-duty trucks of up to 200 tonnes in a mere matter of minutes. Being the biggest Wirtgen surface miner, it is capable of mining up to 12 million tonnes per year in soft rock.
Comparative testing of the mining methods
The use of Wirtgen surface miners in Estonia allowed a direct comparison to be drawn between surface mining and the conventional mining processes of drilling and blasting and semi-selective mining. The comparative test assessed quantitative parameters, such as the ROM, but also included qualitative aspects, such as the calorific value of the mining material.
Method 1: drilling and blasting
In non-selective mining, the material is loosened by means of blasting and then loaded onto trucks by excavators. In the process, the oil shale mingles with the embedded limestone. Prior to processing, the material extracted by blasting needs to be crushed in crushing plants.
Method 2: semi-selective mining
Semi-selective mining does not use drilling and blasting. Oil shale and limestone are extracted and loaded using rippers, bulldozers and excavators. This process also results in a significant degree of mixing between the two materials. Both oil shale and limestone are of a plate-like nature. As a result, crushers are required to achieve the specified particle size.
Method 3: surface mining
Surface miners are equipped with a rotating cutting drum enabling precise adjustment of the cutting depth to within a centimetre to cut the different layers of rock. The highly selective mining process enables separate extraction of the oil shale and limestone layers. The use of primary crushers is eliminated as surface miners cut, crush and load the material in a single working pass.
The test results at a glance
The comparative test was performed in the Kiviöli mine. The deposit encompasses an area of approx. 6 acres. Mining operations began in 2003. Production amounts to some 800,000 tonnes of oil shale per year. The layer of earth covering the material has a thickness ranging from 2 m to 20 m. The oil shale occurs in horizontal layers and seam thicknesses of 0.2 m to 0.6 m. The oil shale seams are partly interspersed with lime-stone deposits. Unconfined compressive strengths of oil shale and limestone range from 15 to 40 MPa and from 40 to 100 MPa respectively. The calorific value of the dif-ferent oil shale seams ranges from 1,200 to 4,500 kCal/kg.
The comparative test was performed by Damian Baranowski, graduate engineer and member of staff of the “Department of Mining Engineering” at the German University of Aachen. The parameters relating to the conventional mining methods were taken from the first years of operation in which drilling and blasting as well as semi-selective mining was used. The results of the surface mining process were collected during the use of a 2500 SM by Kiviöli from 2006.