Sourse: "Joint-Stock Company, scientifically research centre "GCN"" — Moskow — 2007.
Now more and more actual there is a question of economy of fuel. Roasting of clinker is energetic manufacture. The expense of fuel on manufacture 1 ò of clinker on a wet way makes — 200 kg of conditional fuel.
- On dry — 130 kg of conditional fuel
At first sight the question decision can be reduced to refusal of a wet way of manufacture and to translate all factories for a dry way. But to please the dry way cannot forget about quality made clinker. High quality of clinker depends on many factors and first of all from quality and raw materials kind, i.e. from its chemical and mineralogical structure. It is known that the best clinker and cements on their basis turn out from firm raw materials, i.e. from limestone half- and fine-grained structure. But stocks of high-quality limestones for cement manufacture are rather limited, while in Russia there are huge stocks soft high-quality carbonat raw materials is powed a chalk with a good chemical compound and high reactionary ability.
In Russia on cretaceous deposits factories of a wet way of manufacture which make quality cement were under construction. It is Sebrjakovsky, Ulyanovsk and other factories.
Quality of clinker is provided not only a subtlety of a grinding of a mix, but still its homogenization. By manufacture of a raw flour with use of limestone of special complexities with homogenization does not arise.
At an estimation of processing of dry raw mixes the most part of researchers ascertains that the powder raw mix guarantees its best homogenization and as consequence reliable operation of furnaces and reception high-quality of clinker. However it is noticed on dry mixes with limestone use that at storage of a raw mix and in cyclonic heat exchangers there is an agglomeration of particles of a material. This phenomenon has formed the basis for use offer lumpy torments.
Still in the seventies the last century it has been noticed that at a dry way of manufacture there is an agglomeration phenomenon at preparation, storage, hashing and roasting of a raw mix.
Researches of this phenomenon have been spent by Timashev on mixes with use of limestone also similar to researches the marl, a chalk and clay have been subjected.
Agglomeration is a process of formation pieces from thin materials as a result of heating arising in the course of a friction.
As a rule the raw flour prepares from the materials containing tens of minerals which are inevitably characterized by micro-heterogeneity structure and a structure. Each raw mix is poly-disperse system with the size of particles from 3-5 À0 to 200 microns. Depending on granular structure and the form of particles distances between them and a surface of contacts in system vary.
Reception of a homogeneous mix of diverse disperse materials becomes complicated also that such material is the active system capable to self-regulation of the properties.
The aspiration of energetically sated disperse system to reduction of a specific surface does inevitable agglomeration of powders at crushing, classifications, storage and other processes. Agglomeration occurs selectively according to features of properties processed materials. The sizes, the form, density and durability of formed agglomerates depend on the sizes and autohesion properties of particles, and also on conditions of technological process.
Autohesion is the special case of adhesion designating interaction of homogeneous phases, defined by forces of an intermolecular attraction and amplifying if particles electrical are charged.
At powders there can be agglomerates in the size from several micrometers to several centimetres they are astable, have low density, but nevertheless interaction and in them other character, than interaction between agglomerates have coupling of particles. Uniformity of a mix, stability of processes of hashing and a dosage of powders is as a result broken.
We have faced this phenomenon in the course of preparation of laboratory raw mixes by hashing of preliminary crushed components. The thin-dispersion chalk in a raw mix formed agglomerates in the size to 1,5 mm.
Agglomeration of particles in dry raw mixes occurs both in the course of a grinding, and at storage of a raw flour.
At crushing simultaneously with dispersion there is the agglomeration of homogeneous particles caused by presence of free communications, arising at mechanical influence on particles. As a result at achievement of a certain limit the specific surface of a powder remains stable, and at long crushing probably its some decrease because of formation in the course of agglomeration of the molekular-dense structures steady against external influences.
At a dry way of manufacture the raw flour after a grinding moves in the mixing silos representing cylindrical ferro-concrete or metal tanks in which the raw flour carefully mixes up, its structure is averaged and corrected. Carefully correct and to mix a dry raw flour in silos it is more difficult, than slam in pools.
At storage of a raw mix in free strew a condition coupling of particles because of a roughness of their surface and forces of a friction is insignificant. However here again there can be a strong communication between separate grains at contact of the most active sites.
In formation autohesion contacts (an attraction of homogeneous particles) take part molecular (van-der-vaalsovy), electric, mechanical and Kulons forces. The size molecular components autohesions forces depends on the form and the sizes of particles, and so backlashes between them. Electric forces is formation as a result of donor-acceptorn from processes with formation of the electric layer caused by electric heterogeneity of a surface of adjoining particles. Kulons forces, in difference from electric, are defined by preliminary (natural) charge of particles, instead of those processes which occur in a contact zone. Mechanical forces are provided with gearing of particles.
From a material of particles van-der-vaalsovyh and electric forces, and the size depends on them plastic properties - the area and durability of contacts. The form of particles influences both durability of contacts. And on their number.
In Timashev's works have been defined autohesions characteristics of components of a cement raw mix on which it is possible to estimate propensity of materials to agglomeration. So for clays and chalks these indicators in 4-5 times above, than for limestone. autohesions characteristics powder materials should be considered at designing and operation of the corresponding process equipment.
Fluidity of raw mixes has direct dependence from autohesions characteristics of materials of its components and their ability to adsorb a moisture from air. It is noticed that in the presence of a moisture durability autohesions contacts increases at corresponding decrease in mobility of powders. It is connected as with increase in the area of contacts owing to formation of bridges of a liquid between particles, and with occurrence of the capillary forces which are pulling together particles at the expense of negative hydrostatic pressure.
Mobility of raw mixes can be varied by decrease in their humidity and integration grinding to a certain limit.
For reception of homogeneous mixes it is necessary to interfere with formation autohesions contacts and to destroy them in case of occurrence. This problem dares, as a rule by aeration powder materials. However infringement of uniformity of their chemical compound is thus possible, especially at essential distinctions of grains in the sizes and density. Hashing conditions improve with increase of speed of blown air.
It is thin disperse mixes are less mobile, than large as with reduction of the size of particles the number of contacts in which forces autohesion both friction operate increases, and resistance powder a material to shift destruction raises.
Undesirable agglomeration powder materials can be suppressed by application of the surface-active substances (SUPERFICIALLY ACTIVE SUBSTANCES) adsorbed on a surface of contact of particles and reducing their free energy. The additives of SUPERFICIALLY ACTIVE SUBSTANCES raising fluidity of a mix. Can be as hydro-fills and waterproof. Advantage of the last consists that they interfere with adsorption of steams of water and reduce capillary to a component autohesions forces.
All above told in a greater degree concerns raw mixes on the basis of the limestone which sizes of particles can be regulated a subtlety of a grinding and to hold at level 40-50 microns. For raw mixes on a basis of powdery chalks which size of particles approximately on 80 % makes 3-9 microns, i.e. at 5-10 time is more thin, the situation becomes complicated. As it has been told, the chalk, by the nature has the big ability to autohesion to contacts. Mixes on chalks have smaller mobility. Destruction of formed agglomerates of a chalk in a raw mix by aeration is ineffective, because of higher durability autohesions contacts and the big expenses of energy for aeration, than for mixes on the basis of limestone.
Direct consequence of strengthening autohesion abilities of raw mixes on the basis of a chalk is both their agglomeration in a suspension, and formation of makes on stove walls heat exchangers. Selective extraction from a raw mix of a material with raised autohesion ability conducts to change of its chemical compound and uniformity infringement.
Use of SUPERFICIALLY ACTIVE SUBSTANCES will demand additional researches. About quantity of SUPERFICIALLY ACTIVE SUBSTANCES can make 0,5 % from quantity of a raw mix that for factory productivity of 1 million tons of clinker in a year will make 7,5 thousands tons, i.e. some echelons in year, plus additional capital investments on branch building on storage, transportation and dispensing of SUPERFICIALLY ACTIVE SUBSTANCES. Thus it is necessary to consider that substances used as SUPERFICIALLY ACTIVE SUBSTANCES are aggressive and expensive.
Recognising that spontaneous agglomeration of powders is inevitable it is necessary to consider it at a technology choice preparations a raw mix.
In Joint-Stock Company "FACE DOWNWARDS" Hyprocement-science has been made experiment on studying of dependence of distribution of agglomerates of a chalk in raw mixes from hashing conditions. 3 raw mixes from identical raw materials (a chalk of a deposit from the Voronezh region, loam and pirit candle ends) have been with that end in view prepared. Components have been preliminary ground in identical conditions and three raw mixes are made of them. The first mix was mixed on shafts within 2 hours, the second in the same conditions mixed up 4 hours, the third mix after hashing on shafts within 2 hours was mixed with ethyl spirit that modulates a wet way of a grinding.
From skilled mixes have been selected on 4 dot tests on the chemical analysis. Results of the analysis are presented in table 1:
Table 1
The chemical analysis of dot tests of skilled raw mixes. The analysis of the received data shows that the mix 3 has more uniform chemical compound.
This experiment visually (in macro — form) confirms results of researches received by Timashev and other researchers at level of studying of the phenomenon of agglomeration, autohesion and durability measurements autohesions contacts of materials for cement manufacture.
We consider that owing to natural factors chalks, namely their high propensity to autohesion and high natural humidity to prepare raw mixes on a dry way it is impossible. Mixes on the basis of a chalk prepared on a dry way will be non-uniform and quality of clinker will be very low.
At the decision of a problem of use of a chalk it is represented expedient so to organize manufacture process that the raw mix was prepared on a wet way which provides high degree of homogenization, absence of agglomerates and high quality of clinker. At a wet way of preparation of a raw mix of an expense of the electric power on a grinding much more low, than at a dry grinding, updating and transportation of raw slam becomes simpler.
For decrease in the expense of fuel on roasting clinker it is possible to use a moist way, with use of press filters which will allow to lower humidity of a mix arriving on roasting.