RU2341476C1 - Method of baking binding materials - Google Patents

Abstract

FIELD: physics; heating.

SUBSTANCE: proposed method of baking binding materials involves grinding raw components and solid fuel, batching them, mixing, moistening, pelletisation the mixture thereby obtaining granules and subsequent burning of the granules in a layer on conveyer grid using a bedding layer, separating the layer of granules and the grate-bars of the grid. The pelletisation process is carried out in three stages. At the first stage not more than 40% of the raw mixture is moulded into "nucleating seeds" with particle size of 4-6 mm. At the second stage on the surface of the "nucleating seeds", 35-40% of the raw mixture is deposited, pre-mixed with solid fuel. At the third stage the remaining 20-25% raw mixture is deposited on a nucleus, comprising the "nucleating seed" and the fuel-raw mixture coating on its surface.

EFFECT: lower fuel expenditure, faster batch pelletisation process, lower maximum temperature level of released gases, reduced heat loss.

4 cl, 2 tbl

Description

The invention relates to a technology of firing on a conveyor belt of cementitious materials, for example, Portland cement clinker, lime, other types of clinker on limestone raw materials.

A known method of firing cement clinker on a conveyor (agglomeration) grate, including grinding raw materials and solid fuels, dosing and mixing, moistening and pelletizing the mixture, followed by firing in a layer on a conveyor grate using a layer of “litter” of clinker, thickness 50-60 mm separating the layer of granules and grid grates. The purpose of the litter is to protect the grate of the grate from melting under the influence of elevated temperatures at which clinker is burned. After firing, the bedding is separated from the product on a screen. (Valberg G.S. Obtaining cement clinker on an agglomeration grate. - M.: Stroyizdat, 1957. p. 82 [1]

The disadvantage of this method is the low productivity of firing, since up to 50% of the clinker obtained, especially from the upper part of the layer, must return to re-firing due to lack of heat during firing of the upper horizons of the layer.

There is a method of firing according to the patent of the Russian Federation 2049749 (Ufimtsev V.M., Goldengorin I.L. A method for producing cement clinker on a conveyor grill [2]), which provides for the deposition on the surface of a core containing 70-80% of the raw mix and 100% fuel , the remainder of 20-30% of the mixture in the form of a "white shell" on the surface of the granules.

The disadvantage of this method are:

- significant, up to 20%, loss of charge fuel (coke) with mechanical underburning is due to the short duration of a single pellet in the high temperature zone. Therefore, coke, especially its large particles in the core of the granule, does not have time to burn out completely.

- low efficiency of the granulation process, which determines the significant size of the used granulators, disk, drum, because they combine the formation of "embryos" and granulation itself - layer-by-layer rolling onto the surface of the embryo mixture. The amount of charge rolled onto the surface of the embryo in one cycle (one revolution of the plate or drum of the granulator) is proportional to the surface of the granule, which quadratically depends on its diameter. Therefore, the formation of embryos and their enlargement to a size of 3-5 mm is considered a stage limiting the performance of granulation;

- increased temperature of the exhaust gases of the process and, as a result, overheating of the grid grates, reducing their service life. Providing normal conditions for their operation is possible by increasing the height of the layer of the bedding by reducing the layer of the charge, which is irrational, because grill productivity will decrease.

In addition, to ensure grinding of limestone, it is necessary to dry it in a special device, for example, in a drying drum, which complicates and increases the cost of technology.

The technical problems solved in the invention are:

- reduction of loss of charge fuel in the form of carbon residues in the composition of the product;

- acceleration of the granulation process of the mixture (mixture of raw materials and fuel);

- reduction of the maximum temperature of the exhaust gas;

- reduce heat costs associated with drying limestone.

These tasks are solved:

Reducing the loss of charge fuel is achieved by creating intra-feed pellets of the "white core", consisting solely of a raw mixture and not containing charge fuel.

The granulation process is accelerated by molding 4–6 mm “seeds” from 35–40% of the raw mix, onto the surface of which, using a granulator, charge fuel and the remainder of the raw mix in the amount of 60–65% are applied in one or two steps. Moreover, the size of the granules reaches a value of 6-8 mm.

Reducing the temperature of the exhaust gases is ensured by using limestone crushed stone with a grain size of 10-20 mm as a bedding.

The reduction of costs associated with the drying of limestone is achieved by adding the bedding extracted from the calcination products to the limestone component of the raw material mixture.

The effectiveness of the proposed method was tested using a laboratory plate granulator with a plate diameter of 0.7 m and a semi-industrial sintering plant with a diameter of the working chamber 200 and its height 400 mm

The powdered components of the raw material mixture and finely divided coke were metered and mixed in a drum mixer and then granulated to a size of 6-8 mm. Cement clinker and limestone with a size of 10-20 mm were used as a bedding. Layer thickness - 50 mm. The raw cement mixture had a saturation coefficient of 0.90, a silicate module of 2.5 and an aluminate module of 1.2. The height of the charge layer in the working chamber was 300 mm, the proportion of coke in the charge was 12% of the mass of raw materials. In the experiments, gas ignition of the charge was used. The quality of the product was evaluated by the content of unbound lime (CaO _sv ) in it, which for this technology should not exceed 1.5-2.0%. Table 1 below shows the conditions and results of comparative experiments. The distribution of fuel inside the charge granules was varied, comparing the amount of unburned carbon (C) in the experiments according to the analogue, prototype and the claimed method. The "white core" in experiments 3-6 was made manually.

Table 1 The results of the experiments by analogue, prototype and the claimed method No. Share of raw materials / share of coke,%, * Clinker,% Note core black obl white obl. CaO _St. FROM one 100/100 - - 1.84 1,6 [1], analog 2 75/100 - 25/0 1.37 0.95 [2] prototype 3 30/0 40/100 30/0 1.17 0.41 according to the claimed method four 40/0 40/100 20/0 1.21 0.27 according to the claimed method 5 45/0 40/100 15/0 1,53 0.32 according to the claimed method 6 40/0 30/100 30/0 1.05 0.40 according to the claimed method * - from the total mass of the component.

From the data of Table 1 it follows that the largest amount of carbon and CaO _sv. observed in experiments on the analog mode (experiment 1) and prototype (experiment 2).

When using two and three-layer (core and one or two shells) charge granules, it is possible to significantly reduce the loss of coke, in comparison with the analogue and prototype.

According to the calculated data for granules with an average diameter of 7 mm, a core of 5 mm in size should account for approximately 40% of the mass of raw materials. When reducing the consumption of raw materials per embryo to 30% (experiment 3), the initial embryo size decreases by 25%, which subsequently slows down the production of granules of optimal size. In the case of using granules, the core of which has an excessively large mass, but not containing fuel, a heat deficiency in the center of the granules is inevitable. Under such conditions, clinker formation within the granule is not completed (experiment 5). With an increase in the proportion of the outer, “white shell” up to 30% of the mass of raw materials (experiment 6), the CaO content of _sv decreases, i.e. clinker quality improves, but at the same time, the amount of carbon in the clinker slightly increases. Therefore, to increase the share of the outer “white shell” over 25% is irrational.

The optimal ratio of components in the charge granule:

• raw core - 40%;

• a mixture of 35-40% of raw materials and 100% of fuel - “black shell”;

• 20-25% residual fraction of raw materials - “white shell” on the surface of the granules.

In production conditions, the embryos are obtained in a special device by the molding method. Then they come in the form of cylinders to a disk or drum granulator, where in the process of rolling the cylindrical shape changes to spheroid and subsequent rolling (drawing) on their surface of black and white shells.

Table 2 shows the data comparing the results of experiments using clinker and limestone bedding.

From the data presented in it it follows that the yield of the product, clinker, and its quality (CaO content of _sv ) in the experiments on the prototype and the claimed method are close to normal, but in the experiment on the analogue they are insufficient (the clinker yield is only 65%, and the CaO content _St. - as much as possible.

table 2 The results of the experiments by analogue, prototype and the claimed method No. clinker laid Max. T _{exhaust gas} , ° C Note % yield CaO _St. view layer height, mm one 65 1.84 clinker fifty 520 [1], analog 2 95 1.37 clinker fifty 720 [2] prototype 3 95 1.32 clinker 75 560 [2] prototype four 95 1.25 limestone fifty 480 according to the claim. way 5 95 1.21 limestone thirty 610 according to the claim. way 6 95 1.29 limestone 60 410 according to the claim. way

The permissible temperature level, 500-600 ° C, was recorded in experiments 1.3-6, but in experiment 3 it was achieved by increasing the height of the clinker bed layer by 1.5 times. In the case of using limestone bedding, the maximum temperature level of the exhaust gases is close to normal with a bedding height of only 30 mm. However, reducing this indicator below 50 mm is impractical for technical reasons: the smaller the layer height and the larger the size of the limestone grains in the litter, the more difficult it is to maintain a given level of litter height on the grid grates.

The decrease in the fraction of carbon residues in the clinker, obtained in the presence of a white core in the charge granules, is probably due to the conditions created in this case for accelerated burnout of the fuel concentrated in the "black shell".

The increase in carbon residues in the clinker with an increase in the share of the “white shell” over 25% of the mass of raw materials can be explained by the delay in ignition of the fuel in view of its screening by a layer of limestone on the surface of the granules. In this case, the ignition of coke is delayed, which reduces the duration of the period of its burning inside a single granule.

The decrease in temperature of the exhaust gases when using limestone crushed stone as a bed should be explained by the heat consumption for the decomposition of calcium carbonate, the main mineral in the limestone. This process is initiated when limestone is heated above 700 ° C. In the case of more than 700 ° C temperature, an endothermic reaction of its decomposition takes place, which significantly reduces the temperature of the exhaust gases on the grid grates. After being used as bedding, partially calcined limestone is isolated from the calcined products and added to crushed limestone, which is supplied to the grinding as a raw material component.

The introduction of partially calcined limestone in the form of a bedding in the composition of the raw material will reduce the cost of drying the raw material and increase the productivity of the granulation process. This is due to the high hygroscopicity of lime, which interacts with moisture, forming calcium dihydrate in the form of so-called "Fluff", particles with a size of 20-5 microns. It is known that fluff is an effective plasticizing additive that increases the yield of "commodity" granules and increases their strength (Production of pellets. Sulimenko EI Tutorial. M .: Metallurgy. 1988. p.14 [3]).

The use of two-stage granulation: preparation of a raw core with a diameter of about 5 mm and subsequent rolling on it of a black and white shell sequentially allows you to:

- to increase the productivity of the process in drum and disk granulators by at least 30-40%;

- optimize the combustion conditions of the charge fuel in the pellet, which reduces the loss of coke with clinker by 2-4 times and improves the quality of the product.

For the preparation of nuclei, it is advisable to use mechanical devices — a belt press with a perforated forming lattice or hole rolls or other devices also having a perforated working surface. The indicated devices are simple in design and are characterized by high productivity (Artificial porous aggregates and lightweight concrete based on them: Reference manual / S.G. Vasilkov, S.P. Onatsky, M.P. Elinson and others; Ed. Yu. P. Gorlova. - M.: Stroyizdat, 1987. - p. 253 [4]).

The use of limestone bedding improves the operating conditions of the grid-irons of the conveyor grate, the most vulnerable part of this device, which will increase its utilization rate by approximately 5% and the process productivity by 5-10%. The latter is achieved by reducing the height of the bedding layer on the conveyor and, accordingly, increasing the height of the charge layer on it.

The introduction of partially calcined limestone in the form of a litter in the composition of the raw material will reduce the cost of drying the raw material by 30-50% and increase the productivity of granulation by 20-40%.

Claims (4)

1. A method of firing binders, including grinding raw materials and solid fuels, dosing, mixing, moistening, pelletizing the mixture to obtain granules and subsequent firing of the granules in a layer on a conveyor grill using a “bedding” layer separating the layer of granules and grid grates, characterized in that the pelletization process is carried out in three stages, at the first of which no more than 40% of the raw mix is formed into “embryos” with a size of 4-6 mm, at the second stage 35-40% of the raw mix is applied to the surface of the embryo, mixed with solid fuel, in the third stage, the remaining 20-25% of the raw material mixture is applied to the core, including the “germ” and the fuel-raw shell on its surface. 2. The method according to claim 1, characterized in that to obtain the "nuclei" use mechanical devices - a belt press, or hole rolls, or other devices having a perforated forming surface. 3. The method according to claim 1, characterized in that limestone with a grain size of 10-20 mm is used as a "bedding". 4. The method according to claim 3, characterized in that the limestone after use in the composition of the "bedding" is added to the limestone raw material supplied to the grinding.