MINERAL PROCESSING SITEby Eren Caner ORHAN |
This site is visited times since 15 October
1997
Coal is cleaned by gravity separation to reduce ash and pyrite content. These methods include jigging, tabling, and heavy-media separation. The fines (400 mm) are treated by flotation. To liberate ash and sulfur-bearing components, the run-of-mine coal is sometimes ground to 600 mm or finer, and the whole mass subjected to flotation without recourse to primary gravity separation. The natural hydrophbicity of freshly ground coal is an asset which minimizes the use of collectors. A small quantity of MIBC or pine oil (50-100 gr per tonne of mineral) acts as both collector and frother.
In the cleaning of fine coal, floation has the highest potential since 1920 and it is applied to the -0.5 mm fraction of charcoal and anthracite. As the clean coal surface is hydrophobic, in other words as the coal is naturally hydrophobic, the separation of coal and other minerals.
The natural floatability of coal is related to its chemical and petrographic structure and coalification degree. The increase in the coalification degree causes the increase of carbon, however the ratio of volatile components like oxygen and hydrogen decreases. The highest natural floatability is observed in the coal contatining %89 C (dry, ashless). The contact angle measured for this coal is found to be 600. The natural floatabilityof the coal with more or less than %89 C decreases.
The lignite whose coalification degree is small shows hydrophilic characteristics. In addition to this, as the moisture content of lignite is high, the flotation is applied to the charcoal and anthracite.
On extended storage, or if the coal seam has been in contact with air for geological time periods, coal loses its hydrophobicity and is referred to as “weathered coal”. In such cases, MIBC, low pH, and the addition of hydrocarbon oils such as kerosene or fuel oil make coal matter floatable. The kerosene and fuel oil are obtained from petrol, where as creosol is obtained from coal tar. Mostly used alcohols are metyl iso butyl carbynol (MIBC), amyl, hexyl, heptyl and octyl alcohols. The amounts of oils and alcohols for 1 ton of coal are 50-250 gr/ton and 250-1000 gr/ton respectively.
It is possible to float coal by addition of only an inorganic electrolyte. By this method the time required for flotation is shortened by %30 and concentrate with less water can be obtained.
Sometimes, pyrite is first removed by conventional sulfide mineral flotation using xanthates as collectors. This is followed by the flotation of coal matter and the rejection of ash-forming materials such as silica, silicates and carbonaceous shale.
Unlike other minerals, coal exhibits a high degree of variation with respect to origin, age, rank, moisture content, degree of weathering, physical and chemical structure, and nature of gangue. As a result, it is difficult to develop a unified flotation strategy from sample to sample. Even samples of the same origin undergo surface chemical changes to different extents on prolonged storage.
Coal can be floated in a wide pH range, however the optimum floatability is in at neutal conditions. The optimum recovery obtained with oil type collectors is at pH 7.5.
In the coal flotation, although the pulp density is desired to be %14-18, it can change in the range %5-25. The concentrate obtained from the flotation unit is directly sent to the filters.
Before the usage of fully mechanized coal mining, the larger coal used to be taken and coal finer than 25 mm were left was driven out as tailings. However, today, factors like need of energy, high cost of coal prices caused new methods to be applied to gain finer coal.
Another reason of coal flotation is the environmental aspect. Today, because of the environmental laws, the coal can’t be driven out before a washer unit. So, for about 15 years new methods and technologies are being developed in the coal-cleaning unit.
In the flotation, there are three main characteristics of slime control:
In the first item, the particles covered with ions repel each other forming dispersion and in the third item the decrease in surface energy is due to the decrease in surface area causing flocculation. The main effect of slimes in the flotation is that the particles cause flocculation. Thus, the flocculated particles precipitate and a decrease in the recovery will exist.
The slimes covering the mineral can either prevent the flotation of that mineral or can be collected at the froth phase.
The coverage of minerals with slime can be explanined as the oppositely signed zeta potentials of the large and finer particles. That is, the mineral can not carry same signed electrical charge with slime. To prevent this situation, some electrolytes are used to change the sign of the charge of mineral or slime (calgon, sodium silicate). The prevention can also be provided by the pH adjustment.
This situation creates some difficulties in oxide flotation. Generally, the slime and the minerals to be concentrated are in the similar minerological structure and this will make the selective flotation difficult causing more collector use. Therefore, if the coverage with slime can not be prevented by pH adjustment, use of colloids, use of other potential determining ions; the slime should be separated from the pulp and then should be sent to the flotation.
Eren Caner
Orhan
Hacettepe University
Ankara, Turkey
