CC BY
19
No 3,2004 г.
8"
УДК 666.3.015,4:548.517
ANALYSIS OF CRYSTALLIZATION PROCESSES DURING FIRING OF CERAMIC TILE MADE FROM COAL OUTPUT WASTE
V.T. Stanevich, M.K. Kuderin, Sh.K. Torpischev,
Pavlodar State University after S. Toraigyrov
Ембастуз бассейшшц колиралуыныц цалдъщ жыныстан ту¡м тын к,ыш черепицаньщ icyudipyi жэне цургатуы цалыптастыру кезшде цурылымдъщ курылыстьщ сурсаупарына мацала арналган.
К,алдьщ жьщыстыц жату терецдт мен литологиялыц munmepinen байланысуы органикальщ тутцыргыш заттардыц кинетикасы кушп Kemyi жэне балк,ыгыштъщ тгеруч туралы жумыста сурацтар царастырылган.
Статья посвящена вопросам структурообразования при формовании, сушке и обжиге керамической черепицы из вскрышных пород угледобычи Экибастузского бассейна.
В работе рассмотрены вопросы изменения плавкости и кинетики выгорания органических веществ в зависимости от лито логических типов и глубины залегания вскрышных пород.
The article is about the problems of structure formation during shaping, during andfiring of ceramic tile made from coal mining rock waste in Ekibastuz coal basin.
In this article there are shown the problems of viscosity and kinetics changing in the powder of organic materials in depend on lithological types and depth of coal mining rock placing.
Firing of ceramic tile is a main limit influencing the formation of crystallization structure, physicomechanical qualities and the quality of finished articles.
Coal output wastes according to their mineral composition belong to kaolin-hydromicaceous materials and differ from traditional clays by having organic substance. The processes of phase transformation during firing are mostly studied for the systems on clay basis, and the changes of phase content of coal output wastes at different firing temperatures are not studied well enough.
For the purpose of optimizing the firing regime and studying the process of minerogenesis investigations on defining meltability characteristics were held in conditions of intensive
burning out of organic substances, and also differential-thermal analysis, petrographycal, diffract-metrical analyses of the samples fired at different temperatures were made [1].
On heating microscope MHO-2 there are set the temperatures corresponding the deformation of samples, their maximum shrinkage and swelling-up during which the samples make a semi-sphere and turn into liquid-meltable condition.
The analysis of the data acquired shows that the beginning of deformation of samples from "Stepnoi" pit argillites is seen at 1000-1080 °C with increasing of the temperature mentioned from horizon +150 m to the horizon + 50 m. Deformation of "Maikubenskiy" pit samples from argillites begins at 1050 °C.
The beginning of samples caking is noted at 1240-1250 °C, caking interval made 210-230 °C. According to caking temperature 1240-1250 °C the rocks being studied can be attributed to the group of average-temperature caking. At 1450-1470 °C samples swelling-up is seen. When studying meltability characteristics, softening temperature at which the sample turns into semi-sphere can be considered as an indicator of rock refractoriness, so as samples melting is not performed at this temperature. Argillites belong to the group of high-heat materials with the refractoriness indicator of 1500-1580 °C. The temperature of 1550-1590 °C corresponds to liquid-meltable condition of argillites.
Analyzing meltability characteristics of argillites from "Stepnoi" and "Maikubenskiy" pits it is essential to point out that the most changes in samples structure happen during their thermal processing with temperature interval corresponding the deformation with maximum shrinkage and maximum swelling-up, when caking processes are identical for all the bedding horizons.
On the grounds of above-mentioned it is possible to draw a conclusion about expediency of firing the articles made of argillites at maximum temperature up to 975-1025 °C in conditions of organic substances burning out with minerals formation that encourage ceramic tile formation.
Firing the samples from coal output wastes of optimal fractional composition was performed up to maximum temperature of 950,975 and 1000 °C with firing interval of 100 degrees/hour and at the time minerals formation processes were studied (table 1).
In the samples fired at temperature of 450 °C quartz, feldspars, ferric oxides and hydroxides and clayey minerals remain unchanged. The samples have black coloring in the middle and dark-red coloring on the edges that is an evidence of organic substance beginning to burn out. Radiograms prove the presence of reflexes of quartz, feldspars and clayey minerals, presented by kaolin, hydromica and montmorillon [2].
When firing at temperature up to 550-800 °C kaolin reflexes disappear from the radiograms that signify clayey minerals dehydration and kaolin turning into metakaolin. Under microscope quartz partial melted granules in samples edge area are seen. Samples get more dense structure in light edge area. In central part samples have dark area with not burnt out organic substance.
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At 900 °C firing temperature clayey minerals amorphization takes place and liquid phase appear. On radiograms there are reflexes of quartz, feldspar and hematite.
At 950 °C samples represent a fired iile with burnt out area, but there is a small central area of a dark color with not burnt out organic substance. Quartz and feldspar granules are partial melted. Quartz reflexes (d/n=0.425; 0.334; 0.228; 0.212 hm) and feldspar reflexes (d/n=0.366; 0.181; 0.154 hm) remain on radiograms. Reflexes of hematite (d/n=0.27; 0.22 hm) and crystoballite (d/n=0.251 hm) appear.
Rising of the firing temperature up to 975 °C leads to further tile structure tightening, accompanied by liquid phase quantity increase. Total burning out of organic structure takes place and black core disappears. Partial melted granules of quartz and feldspar are seen, mullite crystals are present in light-brown areas. Reflexes of quartz, feldspar, hematite, mullite and crystoballite are recorded on radiograms.
Table 1
Temperature intervals of physicochemical processes taking place while filing the samples
from coal output wastes
Main physicochemical changes Temperature, °C
"Stepnoi" pit argillite "Maikubenskiy " pit argillite
Horizon +50 Horizon +100 Horizon +150
Low-temperature water exudation 90-100 90-190 90-200 100-190
Ignition and burning of inflammable substance volatile components 250-380 250-380 270-390 290-400
Average-temperature constitutional water exudation 540-620 480-630 520-630 530-640
Ignition and burning of organic substance 430-700 450-780 450-790 480-800
High-temperature water exudation 760-800 720-810 740-800 670-730
Organic substance burning out 850-950 910-950 920-975 850-950
Liquid phase formation 860 910 930 840
New formations crystallization with the participation of liquid phase 960-990 970-990 980-1000 960-990
When firing the samples at temperature up to 1000 °C clayey minerals full amorphization happens, accompanied by further increase of liquid phase and tile tightening with samples
linear sizes lessening. Samples edge area has light-brown coloring but the middle of a dark color remains not faded. This thing happens because of edge area intensive caking and in the central area organic substance fails to burn out because of limited access of oxygen. Radiograms keep reflexes of quartz, feldspar, hematite, mullite and crystoballite.
Petrographycal, diffract-metrical studies of samples from coal output wastes of different bedding horizons showed that beginning with the temperature of 950 °C intensive amorphization of clayey minerals with liquid phase quantity increase takes place. At firing temperature rising up to 975 °C organic substances full burning out and black core disappearance occur. The process of clayey minerals recrystallization comes to its end with new hematite, mullite, crystoballite crystalline phases formation. The appearance of these new formations in the samples structure provides optimal physicomechanical indices of finished articles from coal output wastes.
When heating up the articles up to 700-800 °C due to organic substances burning out the quantity of releasing gaseous components from coal output wastes is 10-15% more than that from clays. At high temperature rising speed in furnace during firing, deformation and considerable changes in tile size take place. Temperature rising speed while firing tiles from coal output wastes should not make more than 100 degrees/hour.
At the stage of caking the articles the speed of firing temperature rising depends on duration of carbon burning out from coal output wastes. For the purpose of defining the maximum temperature and isothermal soaking at maximum temperature there were studied the changes of physicomechanical qualities of ceramic tile made from coal output wastes, fired in three regimes. Maximum firing temperature was changed from 975 °C (the temperature of not full burning out of organic substances) to 975 °C (the temperature of maximum strength of samples and full carbon burning out) and further up to 1000 °C (the temperature of strength decreasing). Isothermal soaking duration at maximum firing temperature was changed from minimal possible one, taking into account temperature gradient by furnace section (2 hours), to maximum one while firing tiles of clay (6 hours).
Formation of crystallization structure of coal output ceramic materials during firing proceeds in conditions of constant changing composition of gaseous medium inside the semi-finished product with organic substances burning out. For figuring out the peculiarities of ceramic tile crystallization structure-formation the kinetics of burning out of organic substance from coal output wastes of different metamorphism stages with 10-15 % content of organic substances was studied.
Duration of carbon burning out process while firing the ceramic materials from coal output wastes increases with the increase of thickness and density of a semi-finished product. That is why for the purpose of studying the kinetics of carbon disks-samples with 15 mm diameter and 10mm depth were used. Fractional composition of coal output wastes correspond«! to earlier limited compositions, optimal by structure-mechanical and drying qualities.
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Investigation of dependence of carbon burning out speed on samples firing temperature and time was carried out by thermo-gravimetric method on the plant the kind of A.V. Shlykov's construction. For mass-losses excluding at the expense of physicomechanical processes in mineral constituent with liberation of gaseous products and burning out of organic constituent gaseous components, the samples were preliminarily fired in a coal stocking at 800 °C during 5 hours. Calcinated samples were fired at heating speed of 3 degrees/minute with additional air supply into the furnace (0.05 m/sec), continuously recording mass changes.
The results of the experiment demonstrate that the maximum speed of mass-losses begins in the interval from 600 till 700 °C and makes 2-3 % per hour. With the increase of firing temperature up to 1000 °C mass-losses speed reduces to 0.3 % per hour. A such temperature reduction of mass-losses intensity is noted, that can be explained by formation of considerable amount of liquid phase in samples, which changes porosity and hampers diffusion processes. At temperature of600-700 °C mass-losses are connected with dehydration of clayey minerals, which content is 35-50 %, and with carbon combustion processes.
When firing up to 600-700 °C thermal expansion of samples is noted, at 800°C and more samples tightening is recorded. At temperature of 1000 °C maximum shrinkage of samples from coal output wastes with organic substances content of 10 and 15 % is registered that comprises 3 and 3.8 % accordingly.
With the aim of defining the optimal temperature of carbon full burning out the samples were fired to the maximum temperature of 850, 900, 950, 975 and 1000 °C with soaking during 2 hours. Temperature rising speed in the furnace comprised 100 degrees/hour. The fired samples were visually studied in cross-section. When firing according to regime, accepted for the articles made of traditional clayey raw materials, zoning is formed in the samples (table 2).
In the samples with the organic substances content of 10 %, fired at 850 °C edge light burnt out and central dark not burnt out areas are distinguished. In edge area some not burnt out particles of organic substances are noted. In the samples fired at temperature of 900-975 °C the sizes of black core considerably lessened, light area expands to the center and finish of organic substances burning out process is observed. While isothermal soaking during 2 hours and firing up to 1000°C insignificant samples deformation and black core are noted. As a result of outer layers caking the diffusion of gases to the central part of the samples and organic substances burning out become difficult. In the samples with the content of organic substances of 15 % kinetics of organic substances burning out is identical, but for the argillite of horizon +50 m the temperature interval change of full burning out of organic substances with an insignificant shift to the area of much higher temperatures is a characteristic feature that is supported by the results of meltability characteristics investigations: the beginning
of deformation of samples of argillite of horizon +50 m is observed at 1080 °C, and argillites of+100 m and +150 m horizons from "Maikubenskiy" pit - at 1000-1050 °C.
On the grounds of the carried out investigations on studying the kinetics of organic substances burning out it is possible to draw a conclusion that the most burning out of organics at lessening the black core occurs in the temperature interval of 950-975 °C.
Remanent black core of the samples fired at 950-975 °C does not influence the performance characteristics of the articles. Regularities of strength changes of the samples fired to maximum temperature of 950-1000 °C are studied with their alternate freezing and defrosting.
Table 2
Content of not burnt out organic substances in the samples fired at different temperatures.
Name of the raw material Firing temperature, °C Content of not burnt out black core, %
"Stepnoi" pit
Argillite 950 10-25
Horizon +100 m 975 0-10
1000 15-20
"Maikubenskiy" pit 950 10-25
Argillite 975 0-10
1000 15-20
Note: the content of not burnt out "black" core is given in percents on the area of samples cross-section.
For the samples, fired at 950-975 °C with water saturation their strength reduction for 18-20 % is a characteristic feature. When testing the samples for cold-resistance after 100 cycles of alternate freezing and defrosting strength reduction makes 4-6 %.
The researches on identifying the water-absorption were done, as a result of which it was defined that during firing the samples of argillite with the content of organic substances of 10 % to maximum temperature of 950 °C water-resistant ceramic tile with water-absorption of 8.9-10.5 % is formed, when firing at 975 °C water-absorption makes 7-10 %, the tile is also water-resistant. When rising the temperature of firing to 1000 °C water-absorption decreases to 6.4-8.6 %.
During firing the samples of argillite of +50 m horizon containing 15 % of organic substances a tile with water-absorption of 9.8-13.6 % and insignificant water-resistance when firing up to 950 °C is formed.
So, as a result of carried out investigations the processes of formation of crystallization structure of samples of argillite during their firing are studied, and the kinetics of organic substances burning out for the purpose of thermal processing regimes optimization is investigated [3].
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LITERATURE
1. Guide on study and evaluation of passing solid minerals and components at coal field and combustible shales exploration. - Moscow: Nauka, 1987. 65 p.
2. Methods of mineralogical explorations: Reference book under Editorship of A.N. Ginsburg. - Moscow: Nedra, 1985. - 480 p.
3. ТУ (specifications) 21 УССР (USSR) 463-89 '-Ceramic Tile" Specifications. - Standards publishing, 1989. - 12 p.
