Dust binding process

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In mining, a dust-binding process is a measure that is used to combat deposited explosive coal dust. In this process, highly hygroscopic salts are used to bind dust. The process was introduced in the German hard coal mining industry in the 1960s .

Basics

During the extraction of hard coal and by tectonic forces, the coal is sometimes broken up into the finest coal particles, some of which are smaller than five microns . Due to their small size, they are airworthy and are distributed through the conveyor belt in the mine workings, where they are deposited. Since coal dust is hydrophobic , the dust particles float on water and therefore remain airworthy. Under certain conditions, this fine coal dust can lead to a coal dust explosion. To prevent this, the carbon particles are covered or sprayed with hygroscopic calcium and magnesium salts. These salts can be applied in the form of powder, salt flakes or solution. So that the coal dust also sinks and is covered by the salt, a wetting agent is added to the salts , which lowers the surface tension . How effectively the dust can be bound by the dust binding agent depends on the type of coal, the grain size, the debris and the amount of dust. Up to 200 grams of dust can be bound daily per square meter. In addition to the existing and deposited dust, the process also binds the airborne dust from the weather. The effectiveness and duration of action of the salts on airworthy coal dust were investigated in the 1960s. Two processes have been developed, the powder process and the paste process. The disadvantage of both processes is the aggressiveness of the salts. Objects made of steel , in particular the route extension and the conveyor systems, are attacked in a highly corrosive manner. The salts can cause great damage to electrical systems.

The powder process

With the powder method, calcium chloride powder is distributed in the mine structures to be treated . The calcium chloride is fine-grained and mixed with a wetting agent. It is conveyed into the pit in sacks , each of the sacks weighs 30 kilograms. There are also large containers with a content of 800 kilograms. In order to be able to distribute the powder in the pit to be treated, it is distributed with a small blower. It is sucked out of the sack with a compressed air nozzle. The powder is then blown into the pit using an application hose. Due to its large surface, the powder adheres well to the joints and also well to the roof . The salt takes the bettors contained moisture very quickly. It is thereby quickly brought into solution. The salt is applied to the sole with a shovel. The disadvantage of this process is the high level of layering required to apply the salt. The salt is also questionable in terms of industrial hygiene and is a nuisance for the miners .

The paste process

In the paste process, an aqueous solution is created from water and calcium chloride or magnesium chloride . These solutions have a salt content of 28 percent for calcium chloride and 20 percent for magnesium chloride. A wetting agent is added to the solutions. These salt pastes can bind up to 1.6 times the amount of coal dust. The solution is mixed for days and was pumped to a central pumping station in the early years by means of converted trolleys . From there, the ready-mixed solution was pumped via the pumping station to the respective mine structures. Today the entire pit is supplied via a central, surface pumping station. For this purpose, the dust binding paste is brought to the site via pipes and hoses. The high-pressure lines used have a nominal diameter of up to 80 millimeters. Due to the static pressure that arises in the manhole , operating points 1.5 kilometers away can be supplied without a pump. Automatic high-pressure pumps that generate an operating pressure of up to 300 bar are interposed to supply remote operating points . This makes it possible to convey the paste over several kilometers. The special trolleys are superfluous for the central supply from above ground. In the areas to be treated, the paste is sprayed onto the joints and roofs with the aid of a membrane pump and a spray device. In this process, the sole is either treated with the salt solution or sprinkled with calcium chloride flakes. When the process is used at greater depths , the solutions are enriched to a degree of saturation of a maximum of 33 percent. This is necessary because the relative humidity decreases at greater depths. Although the solution mixed in this concentration costs a little more, the dust-binding effect of the paste also lasts up to 80 percent longer.

Individual evidence

  1. ↑ a b c d e f g h i j k l m Walter Bischoff , Heinz Bramann, Westfälische Berggewerkschaftskasse Bochum: Das kleine Bergbaulexikon. 7th edition, Verlag Glückauf GmbH, Essen 1988, ISBN 3-7739-0501-7 .
  2. ↑ a b c d e f g h i j k Ernst-Ulrich Reuther: Textbook of mining science. First volume, 12th edition, VGE Verlag GmbH, Essen 2010, ISBN 978-3-86797-076-1 .
  3. ↑ Walter Hermülheim: Pit safety assessment of hard coal mines in emerging countries. In: Hossein H. Tudeshi (Ed.) AMS Online GmbH: Advanced Mining Solutions. 2011, No. 3, p. 25.
  4. ↑ a b c d e European Coal and Steel Community (Hrsg.): Technical dust control in mining. Luxembourg 1967, pp. 45-47.
  5. ↑ a b c d e f g h i M. Schnier: The operational developments in the application of the dust binding process in the Federal Republic of Germany. In: Commission of the European Communities (Ed.): Information conference "Flammable dusts". Luxemburg 1981, ISBN 92-825-2967-3 , pp. 161-165.