Sustainable development in practice of mine closure

Category Economic geology & mineral exploration
Group GSI.IR
Location 20th WORLD MINING CONGRESS 2005
Author Jan Palarski*
Holding Date 14 February 2006
 
ABSTRACT
 
Mining industry is the important contributor to the Polish economy. The mineral sector creates many opportunities including: jobs, development of infrastructure and services in mining regions, transfer of technology and research results. However, the operational live of mines is finite. Due to the exhaustion of reserves and high operational costs, mines need to be closed.
This paper considers many aspects of mine closure policy process. Mine closure plans should include site closure problems as well as economic, environmental, social and employee matters. The paper provides information on technical, social, ecological and economic aspects of mine closure in Poland. Some of these are related to physical stability of rock mass, water management and chemical stability, tailings and waste disposal facilities, post closure land use and environmental monitoring.
Key words: Sustainable development, mine closure, surface damages, filling and grouting process
 
 

INTRODUCTION
 
Over the last fifteen years Polish mining industry has been put under tremendous pressure to improve its social, economic and environmental performance.
 
Since 1989 Polish mining sector has closed 32 coal and 9 lead and zinc, cooper, barite and salt underground mines. In addition, many sand and limestone quarries and borehole mines (salt, sulphur, oil and gas) were closed in this period. In years 2003-2004 total or partial closure processes have been carried out in following mines:
·            7 underground hard coal mines,
·            2 underground lead and zinc mines,
·            2 underground and 2 borehole salt mines,
·            1 underground copper mine,
·            15 oil and gas mines,
·            ca. 250 open cast mines of different minerals.
 
Mining companies decided to undertake new programs of internal reform which aim at achieving a serious change in the mineral sector. This initiative in coal mining industry was called “Polish Coal Mining Restructuring Program”. In order to improve the competitiveness of the sector, the program focused on people, technology process and environment. The key elements of the restructuring program were changes to existing legislation, policies and working practices. The objective was to convert the mining industry into a fully commercial business. To achieve this, significant increase of productivity and the cost reductions were needed. The program included a number of elements:
·            Restructuring of management process,
·            Closing of inefficient or uneconomical mines,
·            Reduction of workforce and production capacity,
·            Continuing advances in underground mining technology,
·            Protection of environment and public health and safety.
 
Coal plays a significant economic role in Poland. Hard coal
produced in country is mainly from underground mines in the Upper Silesian Basin (96%). Coal from this region is produced by three state controlled companies that operate 36 mines and by two independent mines, Table 1.

 

 

Factor

 

1.

Number of longwalls

144

2.

Mining depth  (average)

720 m

3.

Panel width (average)

210 m

4.

Panel length (average)

850 m

5.

Cutting height (average)

2.38 m

6.

Number of gate entries

2

7.

Depth of cut

0.8m

8.

Average production

2851 tonnes per day

 
 
Table 1: Longwall specification (2003)
 
Around 20 Mt of hard coal production is exported to Europe. Approximately 80%  (about 80 Mt) of total hard coal production is currently utilized by power stations (electricity and heat production), steel and cement industry and for domestic consumption. Coal is used to generate 96% of total Polish electricity. The current mining practice in Poland is dominated by longwalls operating in seams from 1.5m to 4.5m high at depth from 300m to 1200m.
 
Currently, one of the most important aspects of Polish coal mining industry is a proper preparation of mine closure plan and its implementation. The paper will provide information on technical, social, ecological and economic aspects of mine closure according to the principles of sustainable development.
 
SUSTAINABLE DEVELOPMENT FOR THE MINING
 
The widely accepted definition of sustainable development is the one used by the World Commission on Environment and Development (1987): “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. Mining has a key role to play in assisting the mineral sector to make an essential contribution to sustainable development of mankind. Sustainable developments means on one hand stability and on the other hand change. Mining needs to demonstrate continuous improvement of its economic, social and environmental contribution to sustainable development. The stability refers to the fundamental processes of the economy, environment and culture, but change means necessity of permanent innovation.  It is often questioned if mining can be sustainable, because it extracts finite resources. Thus, mining reduces the potential for future generations and it does not contribute to sustainable development. This is a narrow interpretation of the ideas of sustainability. Mining contributes in a wider sense to the particular principles of sustainable development. First of all, mining should maximize economic, ecological, and socio-cultural benefits from mineral extraction and contribute to the improvement of local development.
 
One of the most important aspects of mining management is the proper preparation of mine closure plan and its implementation. Generally closure planning should start during the pre-feasibility phase of a mining project, and should be supplemented and improved during mine life to ensure a successful final closure. Every closure plan considers the long term social effects and physical, chemical and biological impacts on the ecosystem.
 
Mine closure is much more than cessation of mineral production and removal of infrastructure – decommissioning. The mine closure also includes rehabilitation of site, which means the return of disturbed land to a stable, productive and self-sustaining condition. In addition, the socio-economic issues of closure and the impact on mining communities and the local economic development should be addressed. Not long ago when mines stopped operating, they were simply abandoned and flooded. The traditional approach towards underground mine closure focused on surface protection, sealing and/or filling shafts, adits, underground workings and where needed, location of pump stations and monitoring systems. Today, responsible mine closure additionally involves removing unwanted equipment and infrastructure, securing waste dumps and impoundments, protecting ground water, neutralization of hazardous wastes and rehabilitation of land, Figure 1.
 
Polish coal mines are being closed on the basis of the closure plan elaborated earlier and according to the program of solving the socio-economic problems. The plan includes the rules of safe closing of the underground workings as well as demolition or restoration of surface infrastructures. It also includes the reports on gas and water hazards, on mining damages and the information on after use of the site. The rehabilitation plans and monitoring systems prepared as part of the environmental impact assessment are tested and verified.
 
MINE CLOSURE
 
During the restructuring process of Polish coal mines, in the period of 1989-2003, 24 collieries were closed and 16 others were combined into 8 mines. The location of both operating and closed mines is presented in Figure 2. It shows clearly that the closed mines are situated in the Northern part of Silesia Coal Basin and in Lower Silesia Coal Basin. In those regions it was necessary to fill or seal about 3000 km of underground workings and 92 km of shafts. In addition 2300 buildings and many surface structures were demolished. The mining damages were removed in about 4000 buildings and infrastructure and over 1100 hectares of mined-out area was rehabilitated.
 
Since 1993 the number of miners working in the coal mines has declined from about 312,000 to about 128,000 in 2004.
 
Mining in each abandoned colliery in Silesia was carried out in at least several seams. A dozens of mines has operated at the depth below 20m. In addition, in this region at the depth of less than 300m underground mining of zinc and lead ores was carried out on the area of the coal mines. All the collieries were operating under the built up areas, including urban districts, industries, roads, motorways, railways, pipelines, rivers, etc. Apart from that methane occurs, the coal has a tendency to spontaneous combustion and still huge amounts of water – usually over 5000 m3/day/mine – are running there, Figure 3.
 
Those complicated geological conditions, existing infrastructure on the surface and vast areas of mining activity require the use of special mine closure methods, removal of mining damages, water pumping as well as monitoring. When the mine is abandoned and closed, the greatest attention is paid to the following issues:
·            minimisation of the surface damages and creation of the depressions and sinkholes over abandoned workings,
·            removal of all unwanted plants and equipments,
·            secure waste impoundments and dumps,
·            use of proper methods for inset protection, filling the shafts and construction of clay seals or plugs during the general shafts filling,
·            use of proper mine ventilation during the removal of unwanted infrastructure, sealing and filling operations,
·            designing a mine water scheme which will totally satisfy the requirements of water capture, pumping, treatment and effluent disposal or detoxifying,
·            site restoration and monitoring.
 
abandoned mines Influence onto the surface
 
Old and shallow workings pose great danger for the buildings, infrastructure and the people. Over the workings of up to 100 m deep, there are sinkholes and depressions created on the surface. However, there are some large surface subsidence depressions coming from  workings deeper than the above ones. To eliminate the surface effects and to minimize the danger of gas outlet and underground fire, all shallow workings in old mines should be filled with backfill materials. Not all the roadways can be reached from other underground workings, so there is a need to do the boreholes from the surface. They are situated in such a way the best filling of the voids can be achieved and sudden inrush of water can be avoided,  otherwise, cavings around the bore-holes can be expected. If possible, before filling the void an exact estimation of its volume should be done by means of TV camera put through the bore-hole down on the line. The camera survey can determine areas of roof fall and the distance from the hole to various objects as mine entries, coal pillars, voids and fractures. Void filling is done through the bore-holes of 120 ¸ 200mm diameter; using the following methods (see Figure 4):
·            gravity placement method of dry waste material (crushed rock, surface sand, gravel, slag and ashes) – usually the material is transported through a well from surface directly into a void;
·            pneumatic transport of the dry fly-ash with the capacity up to 30t/h, while the pressure is about 0,25 MPa;
·            pneumatic transport of the fly-ash mixed with water at the outlet of the quantity of up to 20 l/min.;
·            supply by gravity or by means of the pump of the ash-water mixture or mixture of ash (up to 70% of the dry mass), tailing (up to 25%), cement (up to 10%), and calcium chloride (2%) - concentration of the mixture is to be decided each time according to the state and conditions of the void filling (mixture concentration < 75% by mass).
 
The last solution is most effective and is most often used in Polish collieries. The slurry of fill material is gravity fed or is pumped  down through a well into the underground voids until the well will not accept any additional mixture. Slurry injection under pressure allows gaining increased distribution of the fill material within mine workings and caving areas. Properly chosen quality and quantity of fill mixture, appropriate borehole spacing, and injection pressure allow achieving high fill factor and adequate distribution of the fill mixture within mine workings and caving areas. By means of geophysical techniques the fill factor of underground voids can be controlled. After the fill process is stopped, it is important to ensure that the wells don’t have a pathway. All fill wells should be plugged and sealed with grouting mixture or cement from the bottom to the surface.
 
During backfilling, many samples of mixture are collected for  testing of slump, viscosity, compressive strength, elastic modulus, hydraulic conductivity, swelling strain and chemical parameters (pH and trace elements leached from fill mixture). The best results are achieved by filling of voids and tunnels or by grouting of caving areas directly from underground workings. In this case a broad assortment of mixture can be used for filling and grouting processes. These mixtures may include coal combustion by-product, tailing, sludge, sand and binder. For filling operation a special distribution plant with gravitational transport is used, Figure 5.
 
Figure 6 presents an example of the bore-holes location along the railways running over the workings driven in years 1870-1880. Up to 5 m high voids are situated at the depth of 47 m up to 74 m. To protect the railways, viaduct and a part of the highway, the fill of the workings on 16000 m2 areas had to be done. During the period of 14 months about 23,000 t of the fly ash-cement-water mixture of concentration 69.5% by mass was gravitationally supplied into the underground voids. It fully protected the surface in this region. The experiments proved that to fill 1 m3 of void about 1.3-1.8 m3 of the mixture depending on the cracks and the concentration of slurry is needed.
 
Shaft filling
 
The abandoned shafts are filled or they are left without any filling, for example to be used for pumping water. Before filling the shaft all the equipment, guides, cables, ropes, pipes or ladders must be removed. Shaft insets must be prepared for constructing the barricades, fill dams or plugs, Figure 7. Hardcore in the form of broken rock is recommended for shaft inset locations and sumps. The remaining part of the shaft can be filled with mining waste, coal combustion by-products as well as with demolition materials (crashed brick, concrete). When water or gas flows into the shaft it is necessary to prepare clay seals; their thickness and depth depend on geological conditions near the shaft, technical state of its support, material used for its filling and seal construction. Table 2 presents basic requirements for the shaft filling materials and  construction of the seals.

1.

Construction material

-

Clay, bentonite, concrete

2.

Permability

m/s

£ 10-9

3.

Plasticity index

%

10¸20

4.

Lenght of clay seal

m

5¸50 (average 15 m)

Dam

1.

Construction material

-

Concrete, brick, timber sand, fly ash, roof fall rock.

2.

Length of dam

m

0,5 * diameter, minimum 5 m

3.

Strength of grouting rock mass at the interface of seal or dam

MPa

³ 10

 
Table 2: Properties of fill and construction material
 
Capping represents the final operation of mine closure. The shafts are protected by the reinforced – concrete plate of the diameter that is twice as big as the diameter of the shaft. There is a 600 mm diameter concrete or steel pipe for later filling and a gas vent in the shaft capping.
 
Pumping and Disposal of Mine Water
 
Almost all the abandoned and operating mines in Silesia are connected either by workings, cracks or fractures in boundary pillars and barriers. Accumulated mine water in abandoned collieries could cause an unacceptable risk to adjacent operating mines due to high static pressures developing on barriers and the risk of breaking through strata. Operating mines are protected by various strategically located pumping stations.
 
To simplify the drainage system and minimize the risk the abandoned mines were grouped into 4 systems regarding the hydraulic connections among them and protecting mines in operation as well as reducing the costs of pumping, Figure 8.
 
The quality of mine waters depends on chemical processes that take place in the strata, gob area and workings especially in those filled with the fly-ash and in the strata containing pyrite. The products of pyrite oxidation that dissolve in water cause the increase of SO2 ions and the appearance of acid reaction. In general, in Polish collieries there are mine waters containing huge amount of sodium chloride (salt) up to 130 g/l. After mine closure, there is a visible tendency of chlorides’ decrease in the mine waters, that may be a result of great water inflow from the overburden to carbon strata through cracks and workings where the partial salt washing had already taken place. In regions where backfilled workings with fly-ash become flooded after mine closure, there is an increase of water alkalinity (pH rise), sulphate penetration into the saline waters and a decrease of dangerous barium ions that are precipitated in the waters containing sulphates and remained in the rock mass, Figure 9.

 
 
 
 

Figure 4: Void filling with the use of: a) gravity placement, b) pneumatic transport: 1 – fly ash tank, 2 – air compressor, 3 – hopper; c) pneumatic transport with water added at the outlet: 1 – water tank, 2 – fly ash tank, 3 – pump, 4 – air compressor, 5 – hopper; d) mixture prepared at the inlet: 1 – cement silo, 2 – tailing silo, 3 – fly ash silo, 4 – water tank, 5 – mixture tank.

 

Figure 5: Void filling with the use of gravitational distribution plant: 1 – cement silo, 2 – water silo, 3 – fly ash silo, 4 – mixture tank

 

 

 

 

 

Figure 7: Scheme of a shaft filling

 

 

Figure 6: Example of the bore-holes location along the railways running over the workings driven in years 1870-1880

 


 

CONCLUSIONS
 
1)Polish coal mining industry faces a number of challenges. To address the economic challenges, a major reorganisation of industry takes place and it aims at full commercialization.
2)First of all, mining should maximize economic, ecological, and socio-cultural benefits from mineral extraction and contribute to the improvement  of local development.
3)Mining restructuring caused non-profitable mines closure and introduction of the more effective mining technologies that are useful for the environment.
4)Closed mines cause a huge danger for the surface when improper fill methods of abandoned workings and shafts are introduced. Moreover, special methods of dewatering of abandoned mines must be introduced to prevent working mines from flooding. In Poland, for the closure of coal mines, the ways of safe filli

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