Some problems in the coal production.
Topics for Exam
Mining (Book 2) Level A2
Surface mining
Surface mining recovers a higher proportion of the coal deposit than underground method and is economic only when the coal seam is close to the surface. There are several types of surface mining, but only three of them are the most common: open-pit mining, strip mining, quarrying.
Open-pit mining refers to a method of extracting rock of minerals from the earth by their removal from an open pit or borrow.
Strip mining is the practice of mining a seam of mineral by first removing a strip of overlaying soil and rock (the overburden). After the mineral has been removed, an adjacent strip is uncovered and its waste material is put in the excavation of the first strip. The method is most commonly used to mine coal, especially lignite. A strip to the overburden next to the previously mined strip is usually drilled. The drill holes are filled with explosives and blasted. The overburden is then removed using large earthmoving equipment.
Quarrying is the extraction of stone blocks of specific colour, size, shape and quality- an operation requiring special and expensive production procedures.
Surface mining offers distinct advantages, for example it makes possible the recovery of deposit which for physical reasons cannot be mined underground, provides safety working conditions, and it is generally cheaper.
Subsurface mine.
Most coal seams are too deep underground for opencast mining and thus this type of mining is called underground mining or subsurface mining. Underground mining currently accounts for about 60% of world coal production. There are four major underground methods: longwall mining, continuous mining, blast mining and shortwall mining.
Longwall mining accounts for about 50% of underground production. It is a machine with a rotating drum that moves mechanically back-and-forth across a wide coal seam. The loosened coal falls onto a pan line that takes the coal to the conveyor belt for removal from the work area. Longwall systems allow a 60-100% coal recovery rate where the surrounding geology allow their use.
Continuous mining utilize a machine with a large rotating steel drum equipped with tungsten carbide teeth that scrape coal from the seam. Operating in a “room and pillar” system it can mine as much as five tons of coal a minute. Continuous miners account for about 45% of underground coal production, and also utilize conveyors to transport the removed coal from the seam.
|
|
|
Blast mining uses explosives such as dynamite to break up the coal seam, after which the coal is gathered and loaded onto shuttle cars or conveyors for removl to a central loading area.
Shortwall mining – a method that accounts for less than 1% of deep coal production. Shortwall involves the use of a continuous mining machines with moveable roof supports, similar to longwall.
Blasting
Blasting and drilling are closely linked in mining and excavation today. Like drilling, blasting methods have been progressively improved. New explosives and detonating equipment have entered the market, making blasting more predictable and accurate in most restricted areas.
In underground and surface mining, the main purpose of rock blasting is to fragment and heap the excavated rock to make loading and haulage as easy as possible. The geological and mechanical conditions of the rock and some limitations (populated areas, industrial plants, hospitals) should determine the final blasting procedure, the blasting geometry, the type of explosive chosen and detonators. In a remote area, blasting may be restricted by the limited availability of explosives and blasting equipment.
Blasting is effected by the following variables:
*rock geological and mechanical properties;
* environmental and safety aspects
* properties and availability of explosives
* blasting geometry
* detonator availability
The rock conditions present the factors that cannot be altered. Blasting safety can sometimes be enhanced by removing people, or isolating delicate equipment and structures. The surroundings should always be taken carefully into account. Explosives, ignition and blasting geometry are changeable factors.
Places in a Surface mine.
Open pit mining is a temporal use of the land surface. During the mining process there will be a time when mining ceases. Usually good mine design and good mining practice require that attention be paid to the post-mining use of the land formerly used by a mine.
The height of the bench is controlled by the selected loading equipment, required degree of selectivity, economics and overall design. For economic reasons bench should be rather high to make drilling and blasting operation cheaper per ton of material. The loading equipment will finally determine the maximum height of the bench and also the minimum to some extent. Operating benches are generally 35 to 40 m minimum width to give sufficient working room for loading and hauling equipment. But the bench widths left behind, here called berm, are typically between 5 and 15 m. The width of the berm is designed to be as small as possible, consistent with access requirements and the ability of the berm to catch and retain falling rock. If the rock is not homogenous, but contains structural discontinuities, local (bench) failures like plane or wedge failures might occur. These can be predicted as a result of the slope stability study and the width of the berm can be selected to keep falling debris. At the crest of the berm there shall be a safety wall to help catch falling rock and to direct rainwater to drain along the berm instead of into the pit.
|
|
|
Loading and haulage.
The need for mechanized underground mining and development operations has led to the ever-growing use of trackless equipment. There are multiple trackless choices available in loading and haulage today. Front-end loaders and Load-Haul-Dump (LHD) machines are the most used alternatives. The LHD have proven to be the most successful alternative, especially in underground mining. The LHDs have the capability to climb steep terrains, move fast over long distances and their outer dimensions are favorable to underground mining operations. LHD technology has increased the productivity and mechanization level in different applications. Front-end loaders have been more popular in surface excavation. In addition, the constructing industry, in large excavation has used more front-end loaders than LHD. However, LHD technology gives the advantage of using same loading units at different excavation sites in underground construction, and thus makes it more versatile and powerful in these applications.
|
|
|
Hoisting.
Raising the coal to the surface was a big problem. In the first shaft mines, coal was loaded into baskets that were carried on the back of men and women. Sometimes they were put on wooden sledges or trams that were then pushed through the main haulage roadway to the shaft bottom to be hung on hoisting ropes or chains. Sledges were pulled first by men and later by animals, including mules, horses, oxen, and even dogs and goats.
The introduction of a steam engine to hoist coal was a major turning point for the industry. In 1840, in Britain, the first cage was used to hoist the loaded car, later the advances in coal-mining techniques were rapid. Compressed-air locomotives, which appeared in the 1880s were expensive to operate. Electric locomotives, introduced in 1887, rapidly became popular, but mules and horses were still working in some mines as late a the 1940s.
Nowadays there are two basic types of hoists: drum hoists and friction hoists. Some mines use high-angle conveyors or hydraulic pipelines for vertical transport.
Technological advancements have made coal mining today more productive than it has ever been. To keep up with technology and to extract coal as efficiently as possible mining personnel must be highly skilled and well trained in the use of complex equipment.
Ground control.
Rock bolting is one of the most common methods of rock reinforcement. Rock bolting is also an inexpensive and uncomplicated way of limiting rock deformation and holding rock pieces together. The main idea of rock bolting is to increase the ability of the rock to support itself and resist stress.
Bolting is normally done during or immediately after excavation, but it can also be done before excavation, especially when large rock masses need to be bolted. The amount of bolting, the frequency and type of bolt depend on the rock conditions, purpose, and lifespan of the excavated space. Bolt can be divided into categories according to the way they behave in the rock, grouted bolts and anchored bolts. They are also classed as temporary or permanent.
|
|
|
Anchored bolts are meant for temporary rock support. The bolt has an expanding anchor in its end. After insertion in the hole, it is rotated or pressed against the bottom of the hole. This expands the wedged bolt end, anchoring it firmly against the sides of the hole.
Personnel Transport.
Miners have a choice between many types of personnel transport systems.
For taking miners straight up and down the mine elevator or lift cage is used. For transport around the mine most of the time miners use tracks. Monorail is a great form of transportation. Monorails can fit three people in each cabin. Chairlifts is a new and very safe method for transporting many people for long distances. Shuttle jeeps and rough terrain vehicles are also very popular, and it is very important to mention that they are safe because they are equipped with roll over protection systems and falling object protection systems.
One very common type of transport is rough terrain vehicles. Personnel transport systems vary by capacity, some of them are great for transporting large numbers of people, others are better-suited for transporting only a few people at a time. Many transport systems will not work in certain mines, because there is not enough room for them.
Some problems in the coal production.
Historically, coal mining has been a very dangerous activity. Open cut hazards, underground mining, roof collapses and gas explosions. The chemistry of burning or combustion involves the chemical reaction of the carbon of coal with oxygen gas. Thousands of miners die from mining accidents each year, especially in the process of coal mining and hard rock mining. Mining accidents can have a variety of causes, including: leaks of poisonous gases such as methane, explosive natural gases called firedamp, dust explosions, collapsing of mine stopes, flooding, malfunctioning mining equipment, general mechanical errors from improperly used equipment.
Modern coal mines have rigorous safety procedures, health and safety standards and worker education and training. Improvements in mining methods longwall mining, hazardous gas monitoring (such as safety-lamps or modern electronic gas monitors) gas drainage, and ventilation have reduced many of the risks of rock falls, explosions and unhealthy air quality. Strip mining is relatively inexpensive and safe to the miners. However, strip mining upsets the ecology and the environment. It is important to mention that mining industry is very important industry, but still there are a lot of problems that must be solved.
Ventilation.
Ventilation of the mine is vitally important task. A sufficient amount of air must be provided to keep the lower levels of mine ventilated. Air quality in mine workings must be kept at acceptable health standard. The air in mine is regularly contaminated by smoke from blasting and exhaust from diesel powered machines.
The mine needs a ventilation system, and usually it is induced ventilation with high-pressure fans on surface, blowing fresh air down through the intake shaft. There is a network of ventilation ducts in the mine. To regulate the direction of airflow for its needed distribution into the ground workings, special devices are installed. They are ventilation doors, ventilation dam (stopping) to control the airflow, passing through active work areas. Then an exhaust airway channels the polluted air away from underground workings. It is very important to provide enough of fresh air directly to the stope, and prevent the loss of fresh air by shortest way between intake and exhaust airways. The duty of special ventilation department is the control of mine atmosphere, inspection and maintenance of ventilation devices, isolation of ventilation ducts against air loss.
Дата добавления: 2019-01-14; просмотров: 299; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!