Cement production for heat resistant concrete - AR

Contents

Paper

1 Analysis of existing binding material production technologies

1.1 Characteristics of the produced binder

1.2 Characteristics of raw materials for binder production

1.3 Selection and justification of binding material production technology

1.4 New in the production of binder

2. Technological part

2.1 Operating mode of the enterprise

2.2 Enterprise Performance Calculation

2.3 Calculation of crude mixture composition

2.4 Calculation of the enterprise's demand for raw materials

2.5 Selection of process equipment

2.6 Calculation of raw materials and finished products warehouses

2.7 Development of binding material production technology

3. Production and quality control

4. Occupational safety at the enterprise

Conclusion

List of literature

1 Analysis of existing binding material production technologies

1.1 Characteristics of the produced binder

Alumina cement is a high-strength binder that quickly hardens in water and air, obtained by firing a mixture of materials rich in alumina with lime or limestone before melting or sintering and then finely grinding the calcined product. Unlike Portland cement, the clinker of which consists mainly of calcium silicates, alumina cement is obtained from slag (melt) or clinker containing mainly low-base calcium aluminates. The most important mineral of alumina cement is calcium monoaluminate (CaOA1203). The cement usually contains minerals 12CAO 7A1203 (5CAO 3A1203), CaO A1203

In the Russian Federation, as a result of independent studies conducted by a group of scientists, several methods for producing alumina cement were developed and the physical and chemical processes of its production and hardening were studied. The results of these works made it possible to organize the production of alumina cement by blast furnace smelting and to rationally use it in many areas of the construction industry. Alumina cement is also used as an essential component in the production of several types of expanding cements.

The chemical composition of alumina cements is diverse and depends on the composition of raw materials and production technology. The content of the most important oxides is characterized by large fluctuations,%: Si02 - 5-10, A1203 - 35-50, Fe203 - 5-15 (including iron oxide), CaO - 35-45. In addition, it usually contains 1.5-2.5% TiO2, 0.5-1.5% MgO, about 1% S03, 0.5-1% R20.

Chemical-mineralogical composition of the produced cement. Calcium monoaluminate (CA) contains 64.5% A1203 and 35.5% CaO, its melting point is 1973 K. It has the ability to form solid solutions. When synthesized by sintering in an oxidizing medium, it is able to involve oxides of iron, manganese, ferrite and calcium chromites, etc., in the crystal lattice. It is believed that the high rate of hardening of calcium monoaluminate is due to the irregular coordination of calcium atoms with oxygen atoms, wherein aluminum and oxygen atoms form a deformed type of structure characteristic of A14 tetrahedra.

The single calcium dialuminate (CA2) contained 78.4% AI2C3 and 21.6% CaO. Its composition is more precisely characterized by the formula C3A5.

It melts incongruently at 1843 K to form a melt and A1203. The existence of CA2 in two modifications was established, and an unstable modification can be formed with extremely rapid cooling, so it could not be found in alumina cements.

Single calcium CA6 hexaluminate contains 90.65% A1203 and 9.35% CaO. This is a little-studied compound found in a fused corundum. Bicalcium C2S silicate is also present in the alumina cement. Calcium alumosilicate gelenite (C2AS) containing 37.2% A1203, 21.9% Si02 and 40.9% CaO has a great influence on cement quality. Gelenite in crystalline form does not have hydraulic activity, so that a significant part of alumina does not form hydraulically active calcium aluminates, but is bound in a practically inert compound.

Alumina cement usually contains ferrous compounds, since they are present in the initial alumina raw material component.

The experience of production and the results of extensive studies have shown that it is advisable to either reduce the content of iron oxides in the raw material charge, or completely free cement from iron, as is the case with blast furnace smelting of high-alumina slag and cast iron. This is due to the fact that calcium alumoferrites bind a certain amount of alumina and thereby remove it from the most hydraulically active compounds - calcium aluminates, which slightly reduces the quality of cement. In addition, iron oxide is also able to bind alumina to the hydraulically inert ferrous spinel FeO A1203. She can participate also in education poorly hydraulic FeO Si02 connection 6CaO 4A1203.

The magnesium oxide may form a compound as a 6CaO4Al203MgOSi02. With a large content, a hydraulically inert magnesium spinel occurs, periclase and ocermanite (2CAO MgO 2SiO) also appear. Small amounts of magnesium oxide slightly lower the melting point and viscosity of the slag. The magnesium oxide content of the cement is considered to be below 2%. Titanium dioxide is almost always contained in raw materials. It has been established that it forms mainly perovskite (CaOTiO2). The presence of this compound in an amount of up to 3-4% positively affects the process. In feedstocks, there are typically such small impurities which generally adversely affect the quality of the cement. These are alkalis, phosphoric anhydride (about 1%), chromium oxides, sulfur and its compounds, etc.

Special properties include:

1. Rapid build-up in early life;

2. When hardening concrete on alumina cement, a large amount of heat is generated, which allows these concretes to be used at negative temperatures up to 10 degrees without heating;

3. Alumina cement has an increased density of cement stone, which determines the greater stability of concrete against all types of aggressive liquids and gases compared to concrete on Portland cement;

4. Alumina cement is more flame resistant and thermally stable than Portland cement. In the mixture with refractory aggregates: chamotte, chromite ore, magnesite, etc., alumina cement can be used to produce hydraulically hardening refractory solutions and concretes.

1.2 Characteristics of raw materials for binder production

Raw materials. The most important alumina-containing raw material component in the production of alumina cement is the relatively low prevalence of bauxite. This is a scarce feedstock used mainly for the production of metallic aluminum. Bauxite contains alumina hydrates in the form of boehmite, hydrargillite and rarely diaspora with impurities of silica, iron, magnesium, titanium oxides, etc. For example, boxites in some deposits contain boehmite and diaspora, as well as iron in the form of hematite and silica in the form of quartz or opal. The alumina content of the bauxites can reach 70%.

The quality of bauxite is characterized by the content of A1203 and the quality coefficient - the ratio of the amount of alumina by weight to the corresponding amount of iron oxide. For the production of alumina cement, bauxites are used, mainly of grades B2, B-3 and B-7 with a quality coefficient of 7, 5 and 5.6, respectively; the alumina content in them should be at least 46 and 30%. Bauxite grade B-1 with quality factor 9 containing at least 49% alumina is also used. Bauxite deposits are available in a number of regions of the Russian Federation.

Although the amount of iron oxide is not regulated by GOST, it can be seen from the foregoing that it is extremely important for the technology for the production of alumina cement. In the Ural bauxites used in us, the iron oxide content reaches 28%. Recently, dumping slags of aluminum thermal production of ferroalloys, as well as slags of secondary remelting of aluminum and its alloys, have begun to be used.

Pure alumina of various grades is used to obtain high-alumina and especially pure high-alumina cements. Limestone and, in some cases, burnt lime serve as a lime component. There are patents for the complex production of alumina cement and sulfuric acid, cement and phosphorus. In these cases, instead of lime, gypsum, phosphorites, etc., are used. In smelting reduction, the component of the feed stock is also coke, which, first of all, requires the lowest possible content of silicic acid in the ash part.

Unlike Portland cement, in the production of alumina cement, it is difficult to choose a universal method for calculating the expected mineralogical composition of a melt or clinker. This is due to the fact that the mineralogical composition of clinker of alumina cement depends on the method of production - melting or sintering, the nature of the firing medium - oxidizing or reducing, crystallization conditions (depending on the nature of cooling), the content of iron oxide in the raw material mixture and iron-containing compounds formed after firing and cooling, the type and composition of the obtained solid solutions, etc.

Additive - scrap of high-clay products (bricks for lining furnaces).

1.3 Selection and justification of binding material production technology

Production methods. As noted above, there are two fundamentally different methods for the production of alumina cement - melting the charge and sintering. When choosing a particular method, it is necessary to take into account a number of factors and, above all, the chemical composition of bauxite of a certain grade, and in particular the content of silicic acid and iron oxide in it. Based on experimental studies, sintering and melting temperatures and the interval between them are determined, as well as the quality of the resulting melt or clinker. Technical and economic analysis reveals which method of production in these conditions is more rational. At the same time, the presence and cost of electricity, the quality of coke, etc., are taken into account.

Melting. Alumina cement can be produced by melting in watercolor furnaces (water-cooled cans). Bauxite, limestone and coke are loaded in the upper part of the furnace in the specified design ratio. Air heated in recuperators is blown through lances; the melt formed at the bottom of the furnace at 1773-1873 K is discharged through the blade; metal iron melt is discharged from furnace separately. Experiments were conducted on the use of oxygen-enriched air for these furnaces. Their productivity reached 50 t/day at a specific fuel consumption of about 500 kg per 1 t of melt.

This production requires high quality bauxites with a low silica content, since the reduction of silica to silicon and the production of simultaneously siliceous iron or ferrosilicon occur at high temperatures, which are difficult to create in these furnaces. The melt (slag) is cooled in special molds and chilled in crushers and then finely ground in multi-chamber pipe mills.

In France and England, open-hearth flame furnaces are used, equipped with a vertical pipe through which raw materials are supplied to the furnace. Furnaces operate on pulverized fuel with hot blast. Slag is produced at 1823-1873 K. Productivity reaches 70 tons/day. There is a method of electrofusion of alumina cement, in which the product is not contaminated with silicic acid contained in coke ash, since ferrosilicon is simultaneously melted.

There is experience in the use of arc furnaces operating mainly on alternating current. To intensify the melting process, the raw materials were pre-dried, crushed and briquetted or granulated after thorough mixing. To avoid emissions from the furnace, which are due to the rapid release of water and carbon dioxide from the raw material, bauxite is previously calcined and limestone calcined. The productivity of furnaces reaches 30-40 tons/day. Electricity consumption is about 4320-5040 MJ per 1 ton of product. And these electric furnaces melt high-quality alumina cement from high-silica bauxite.

Due to the high temperature in such an electric furnace reaching 2273 K and the use of coke in the charge, the silica of the charge is reduced to silicon and ferrosilicon is formed as a result of interaction with metal iron. For example, when using bauxite containing 15-17% Si02, its amount in cement (melt) is reduced to 6-8%. The amount of ferrosilicon with 13-15% Si is about 35% by weight of cement. The specific consumption of electricity is very high, reaching 900010,800 MJ per 1 ton of cement.

The disadvantage of this method is the limited limit of silica reducibility due to the formation of significant amounts of calcium carbide, which increases with increasing temperature.

Alumina cement in the USA is produced by combining the process of weak sintering of the charge in a rotary furnace with its subsequent melting in a bath furnace. Views are expressed on the possibility of melting in rotary furnaces, but this method is not used in industry.

Of great importance is the method of blast furnace smelting of cast iron and high-clay slag, successfully developed by our scientists. Abroad, it is called the "Russian method of producing alumina cement." The organization of the production of this cement in the Russian Federation was preceded by extensive experimental studies, which made it possible to establish the rational composition of the blast furnace charge, melting conditions and, in particular, the cooling regime of the melted slag. Studies of the construction and technical properties of the obtained cement and the technical and economic indicators of its production and use testified to the effectiveness of this method. Since 1936, it began to be used at the Pasha cement metallurgical, and then at the All-Rhne Sinechikhinsky plants.

Ferruginous bauxite, limestone, coke and metal scrap are loaded into a conventional blast furnace, from which high-clay slag is periodically discharged on the upper cell, and on the lower one - special types of cast iron containing impurities of titanium, copper and other substances coming from bauxite and scrap. The temperature of the slag is 1873-1973 K. Although with this technology the product (high-alumina slag) does not contain iron at all, since it completely passed into cast iron, it is somewhat enriched with silica due to coke ash. The slag exit to 1 t of cast iron is much higher, than at usual melting of cast iron from iron ores.

Experimental studies conducted by the Ural Research and Design Institute of Building Materials in Chelyabinsk showed the possibility of producing melted alumina and high-alumina slags (cements) by the method of aluminothermy. G.I. Zoldat, A.A. Kondrashenkov managed to reduce the content of silicon dioxide in metallurgical slags and thereby enrich them with alumina. Reduction of silica in this process occurs by reaction

3Si02 + 4 A1 — 3Si + 2A1203.

Thermite mixture consisting of iron ore and aluminium is introduced into molten blast furnace slag when it is discharged from furnace or into slag ladle. A high heat generation reaction occurs and the slag temperature rises to 2273 K and above. When 12-33% of the termite mixture (based on the weight of slag) is introduced, silicon passes into ferrosilicon and a metal ferrosilicoaluminium melt is deposited on the bottom. In blast furnace slag, as a result of reduction, the content of silicon dioxide from 36.04% is reduced to 6.48%, and alumina is increased from 13.07 to 58.79%. Slag samples in crushed form are alumina cements, which differ, however, from the usual reduced strength at the initial hardening time.

Sintering. The study of the sintering process of alumina cement was given great attention in our time, because due to the relatively low temperatures, usually about 1473-1673 K, it can be carried out in roasting units widely used in the industry.

The method of sintering in rotary and other furnaces during oxidative and reducing firing was thoroughly and deeply investigated, but was not introduced into production for a number of reasons. This is, in particular, a small interval between the sintering and melting temperatures, which leads to the appearance of rings and pads in the furnace, as well as the need to use high-quality low-silica and low-iron bauxites necessary for the manufacture of metallic aluminum. Experimental studies of Yuzhgiprocement revealed the possibility of producing alumina cement on an agglomeration tape (sintering grid).

According to the UZTMK218 sintering grating installation project with a useful area of ​ ​ 18 m2 with a specific reduction of 0.46 tons from 1 m2 of the useful area, the grid productivity will be 65 thousand tons per year.

A feature of this process is the rapid firing of the clinker. At a belt speed of 0.6 m/min, the firing rate along the section of the charge layer is 1-2 cm/min. The duration of firing depending on the thickness of the charge layer is 13-20 minutes.

Hot clinker coming off the belt is fractionated by means of scattering, at that fractions less than 15 mm are returned to the belt in the form of bedding for raw charge. The amount of return is 20-30%, the specific heat consumption is 1100-1200 kcal/kg clinker, the thermal power of the tape is 9106 kcal/h.

The cooling rate of the melt (slag) is of great importance, since it significantly affects its crystal structure, which greatly affects the quality of the cement. As is known, the rapid cooling of hot melts (e.g. blast furnace slags) to prevent their crystallization usually significantly increases their hydraulic activity as an additive to cement, as well as the ability to harden on their own. It was assumed that high-alumina melts in a glassy state, quickly cooled, would have higher binding properties.

However, it turned out that the construction and technical properties characteristic of alumina cements and, first of all, high initial strength are manifested only in uniformly crystallized, i.e., slowly cooled cements. It has been found that the vitreous phase of calcium aluminates almost completely loses its high activity. Crystalline formation of calcium silicates and aluminates having binding properties can be considered to lose them if they are in vitreous state.

It would seem that high-earth melts (slags) should undergo slow and uniform cooling in order to crystallize more fully and evenly. However, in such a process, along with calcium aluminates, calcium gelenite, a compound that in a crystalline state is inert and acquires hydraulic activity only in the form of a vitreous phase, will also crystallize. Therefore, it has become necessary to find a combined cooling method in which conditions are created for solidification of gelenite in the form of glass during crystallization of calcium aluminates. This method is proposed by NIIcement. The physicochemical basis of it is as follows. In the CaO system - A1203 - Si02 there is a gelenite stability field, in which the compositions of alumina cement produced in our country are usually located.

The equilibrium crystallization of such melts leads to the appearance primarily of gelenite, crystallizing at 1683-1793 K. After that, calcium aluminates also crystallize at lower temperatures in the form of solid solutions. Therefore, it has been proposed to provide conditions under which the melt rapidly passes the temperature range by rapid cooling. This prevents the crystallization of gelenite and the formation of active aluminosilicate glass, followed as the crystallization temperature of calcium aluminates decreases. The degree of cooling during granulation should be extremely accurate so that the transition of calcium aluminates to the vitreous phase does not occur, which is unacceptable.

Experiments have shown that some time after release from the domna, the melt should not undergo aqueous, but steam-air granulation. To do this, the granulation unit was placed at a distance from the domna cell. With this method, it was possible to significantly improve the quality of alumina cement, bring the content of Si02 in it to 11-13%. Examination of the obtained slags under a microscope showed that the surface of the resulting granules with a size of 20-30 mm consists of glass, and inside they contain well crystallized calcium aluminates and eutectic germinations of calcium monoaluminate and bicalcium silicate. The grinding capacity of the rapidly cooled slag is dramatically improved and the productivity of the cement mills is accordingly increased. Tests of experimental cements showed that their strength increases by about 1.5-2 times compared to the strength of cements obtained from slow cooling melts.

The mineralogical composition of alumina cements is very diverse and, as can be seen from the above, is determined by many production factors. Often, the layers of the same melt sample have a different mineralogical composition. For example, in conventional melt cooling in molds, the surface immediately adjacent to its walls has a glassy structure due to faster cooling. The inside of the material is completely crystallized. Therefore, it is practically impossible to determine the phase composition of alumina cement by a calculated method from the data of chemical analysis. It is installed usually using the petrographic or X-ray diffraction method.

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