In coal preparation practice sharply enough there is a question of dressing of fine grains. It is connected by that eventually the quantity of reserves of coal is promptly reduced, and it is necessary to search for alternative variants of more careful coal preparation, and also to develop techniques of redressing of coal slimes. As a rule, it is the most difficult to concentrate with coal fine grains: the more finely coal, the more difficult a technique of its extraction from not concentrated raw coal. But drives time, technologies do races forward, and that was considered earlier impossible, becomes achievable thanks to scientific progress and engineering thought. What for to lose or leave in a waste a valuable component if he can be extracted and by that to raise efficiency of dressing several times? And before engineers there is a new problem – how to extract a concentrate with peak efficiency, and thus to spend a minimum of efforts. Modern methods allow to extract fine grains with sufficient degree of efficiency. At the same time there are the technologies which considerably are simplifying and reducing the price of this process. Let's consider, to begin with, that themselves fine grains of coal represent, and also what basic techniques apply now in Ukraine and in the world.
In dressing practice thin, or mikronized, it is accepted to name a grain-size category of 0-0,1 (0,2) mm. This class corresponds to a curve bend "the external specific area of a surface of a material – diametr of a material" (pic. 1). The piece "a" where surface Ssp, sharply increases (coal of a class 0 – 100 mcm) it is possible to allocate 100 mm, as specific, coal such name ultrathin. The piece "b" (coal of a class 100 – 200 mm) corresponds to a zone of fine grains. Starting with above told, thin (mikronized) is a polydispersed material less than 100 (75) microns. The second approach is the binding of the top boundary a mikronized material to a joint of scopes of various methods of dressing, for example "gravitation-flotation", "gravitation-flow separation" etc. At dehydration of granular materials to тонкодисперсным carry also suspensions with grains 0,5 – 100 microns.
As a rule to a coal preparation apply gravitational or flotation dressing methods. These methods possess sufficient degree of efficiency, are rather simple in application, them have learnt to apply to dressing of various kinds of coal. However in a case with micronized coal these methods lose the efficiency. Micronized parts are too small that in a due measure to feel on itself influence of the brute forces applied to dressing of larger parts. This fact undoubtedly does fine grains of coal by a specific category of a concentrated material. We will consider processes gravitation and flotation in more details to understand that goes not so in the course of dressing of thin grain by classical methods of a coal preparation.
Let's begin with gravitation. This process is based on division of coal and soil under the influence of force of gravitation. However micron grains slightly feel on themselves influence of gravitational forces as in a heavy liquid hydrodynamic processes proceeding there render on a part considerably a greater influence, than division factors: the part is simply carried away by environment in this or that product, it is simply not enough of it to resist to influence of environment. The part in the size less than 100 microns comes nearer to suspensoid, and, therefore, losses micronized coal with suspensoid also are considerable. Presence in suspension of ultrathin grains also involves such negative phenomenon, as change of density of the suspension. Because of it larger grains cease to be concentrated effectively, and it is one of the main problems of a gravity preparation. Thin grains try to separate in advance, but without preliminary operations to enrich micronized coal with gravitational methods extremely difficultly.
The second coal preparation method is flotation. This method is based on distinction in wettability of a surface of divided minerals. The more strongly a difference, the more possibly that one parts adhere to a surface of a vial and float up with it hill up, and others owing to high hydrophilicity their surfaces will fall to a bottom and form a deposit. Distinction in wettability can be strengthened or lowered by means of the special reagents added in a pulp before or during process of flotation. It is difficult enough process depending on a great number of factors. But thin parts here again behave absolutely differently, than larger grains. For flotation such parts too are imperceptible. Fine grains only select on themselves a reagent because of the big specific surface, but in a foamy product do not pass, because at the expense of the hydrodynamic processes occurring in a pulp, thin parts of coal, as a rule, do not cake on vials with air, they simply bend around them, being carried away with a stream of a pulp and are lost with a waste (pic 2). It is very difficult to operate similar process, even application of the strongest reagents does not solve a problem of subsidence of parts on air vials. Without increase grains flotation process loses the efficiency and also does not approach for application it for dressing of parts of the micron size.
Thus, dressing micronized coal particles by means of classical methods is unproductive, without application of additional receptions on improvement of washability of this grain-size category efficiency of its dressing comes nearer to a zero mark. Let's consider now, what technological solutions for today are capable to solve this problem.
For today the most effective method of dressing micronized coal is the selective flocculation (agglomeration) of coal grains by means of reagents of the various nature. It can be organic connections (polymers of the various nature), oils, pitches etc. Depending on a reagent kind it is possible to allocate 3 basic directions of a selective flocculation:
Flocculation process occurs directly ahead of dressing process in special devices, for example, contact tubs, where the reagent mixes up with an initial pulp in a turbulent mode to provide the best conditions of interaction of a reagent with coal parts. After intensive hashing again formed floccules (agglomerates) arrive on flotation and are successfully allocated in a concentrate. Such solution of a problem is rather effective, however it demands additional capital and working costs: service of additional devices, the big expense of the electric power for maintenance of a turbulent mode of hashing etc.
The purpose of the further research and minings &ndash to simplify dressing process micronized coal by means of combination of processes of a hydraulic transport of a pulp in a hydrotransport network which is a part of any concentrating scheme, and a selective flocculation (aggregation). The pipeline as a matter of fact replaces with itself a contact tub for a preliminary flocculation (agglomeration) before dressing, on an exit from the pipeline we receive coal mix already ready to dressing.