EFFECT OF DUST RETURN ON THE CLINKER FORMATION PROCESS

N. S. DUBOVSKAYA, V. M. CHURINA

(Presented by the scientific seminar of the Department of Silicate Technology)

The problem of capturing dust from the waste gases of rotary kilns has now been solved through the use of electrostatic precipitators for these purposes. The problem of utilizing this dust, in order to be resolved, requires specification for each individual cement plant. The characteristics of the raw-material base and the thermal and hydraulic regime of the kiln determine the composition of the dust entrainment and its particle-size distribution.

For continuous use of the dust by returning it to the kiln, combining it with the raw slurry, it is necessary to study the effect of dust on the clinker-formation process and on the structure of the finished clinker. In the Silicates Laboratory of the Tomsk Polytechnic Institute, a preliminary investigation was carried out of the firing processes of raw mixtures containing an addition of dust, as applied to the conditions of the Yashkino cement plant. The purpose of the studies performed was to determine the material composition of the dust, the fundamental influence of dust on the processes during firing, and to establish the limiting dosage of dust in the slurry at which the quality of the clinker remains satisfactory.

In setting up the investigations, the authors used a set of modern physicochemical methods for analyzing silicate materials: the composition of the dust was determined by chemical [1, 2], thermal [3], X-ray [4, 5], and petrographic [6] methods; the composition of the firing product at various temperatures was studied by chemical and petrographic methods, by determining free lime and minerals—silicates.

The chemical compositions of the materials and mixtures used in the work, obtained by adding different amounts of dust to the plant slurry, are given in Table 1.

Comparing the compositions of the slurry, dust, and clinker, it may be said that the dust lies between the slurry and the clinker. This means that the dust contains fired slurry, partly in an unburned and partly in a burned state; the presence of unburned slurry is indicated by the large loss on ignition (l.o.i.) of the dust.

According to the well-known theory of clinker firing developed by V. N. Jung [7], dehydration of the clay component and decarbonization of the limestone component occur in the slurry being fired, which accounts for the loss on ignition. The presence of the latter in the dust in an amount of 25.6% permits the conclusion that the dust contains a considerable quantity of products from the “cold” zones of the kiln. The presence in the dust, although in small—

amounts, of silicate minerals makes it reasonable to conclude that the dust contains products from the “hotter” zones of the kiln. Assumptions concerning the presence in the dust of products from all zones of the kiln are fully confirmed by the results of petrographic and thermal analysis.

Table 1

Chemical composition of the materials used and of sludge mixtures with added dust

Note: The chemical compositions of the sludge–dust mixtures were obtained by calculation.

Examining the dust in immersion preparations at a magnification of 480×, it was possible to detect in it about 65% calcium carbonate in the form of shapeless particles ranging in size from 2–3 microns to 0.05 mm; about 15% metakaolinite, which composes nonpolarizing grains of size 0.1 mm; and grains of dicalcium silicate, alite, and belite.

Fig. 1. Thermogram of the dust studied: 1—dust, 2—synthetic goethite [8], 3—pure limestone

By the DTA method, residues of clay substances that had not decomposed during firing and a considerable amount of calcium carbonate were found in the dust. The thermogram of the dust is shown in Fig. 1.

The endothermic effect at 330° is characteristic of iron oxide hydrate in the form of goethite and is due to loss of water [8]. The characteristic curve of synthetic goethite is given in the same figure. The sharp exotherm at a temperature of about 475° is caused by crystallization of the dehydrated residue into hematite. A carbonate endoeffect is clearly expressed on the curve.

The composition of the dust suggests that adding it to the raw sludge will entail changes in some clinker-formation processes; the influence of the dust should appear in the processes of thermal decomposition of the constituent parts of the sludge and in the assimilation of lime by sesquioxides and silica.

In this connection, the reactivity of sludge with added dust can be characterized by the content of free and mineral-bound ...

lime. In order to select the dosage of dust into the slurry, mixtures with dust contents of 5, 10, 15, and 25% were investigated. Experimental specimens for firing were prepared by pressing from a mixture moistened to 14–16%. They were dried under ordinary conditions and fired. The heating rate of the furnace and the holding time at maximum temperatures were close to firing conditions under plant conditions. Cooling of the fired clinker was rapid.

In the firing products, the content of free lime and silicate minerals was determined by the chemical method [9]. The results of the determinations are presented in Fig. 2.

The addition of dust to the slurry activates the latter throughout the entire firing process. Already at temperatures of 900, 1000, and 1100° the carbonate component of the mixture is decarbonated to a greater degree, since at the indicated temperatures considerably more free lime is detected in the firing products of mixtures containing dust. Mineral formation is also accelerated. The total silicate content in all fired mixtures containing dust is higher than in the original slurry. Apparently, the addition to the slurry of a partially fired material, which the dust is, promotes thermal processes in its constituent parts. And the formation of clinker minerals proceeds under more favorable conditions.

Fig. 2. Activity of mixtures during firing: 1—slurry; 2—slurry + 10% dust; 3—slurry + 25% dust; 4—slurry + 5% dust

As preliminary investigations show, the dust returned to the kiln at the Yashkino cement plant should not worsen the course of the processes during firing. Most likely, they will proceed more actively and more completely. The plant can be recommended to introduce dust into the slurry in an amount of 20%, which practically corresponds to the amount of dust collected at the kilns.

LITERATURE

1. A. I. Ponomarev. Methods of Chemical Analysis of Minerals and Rocks. Moscow, 1951.

2. Giprocement. Control of Cement Production, parts 1, 2. Moscow, 1952.

3. L. G. Berg, A. V. Nikolaev. Thermography. Publishing House of the Academy of Sciences of the USSR, 1947.

4. A. S. Ushanskii et al. X-ray Diffraction. Mashgiz, 1951.

5. V. I. Mikheev. X-ray Determinative Guide to Minerals. Gostekhoizdat, 1957.

6. V. I. Chernov, N. G. Lin. A Practical Guide to the Petrographic Study of Igneous Rocks under the Microscope. Gosvuzizdat, 1963.

7. V. N. Yung. Fundamentals of the Technology of Binding Materials. Promstroiizdat, 1951.

8. N. S. Kurnakov, E. Ya. Rode. Journal of Inorganic Chemistry, 169, 1928, 57–80.

9. Yu. M. Butt. Practicum in the Technology of Binding Materials. Promstroiizdat, 1953.