home Business Technological diagram for the production of Portland cement using the dry method. Wet production method

Technological diagram for the production of Portland cement using the dry method. Wet production method

No matter what steps progress takes in various branches of production and technology, the leading position in construction consistently belongs to the well-known cement. And although cement production is a labor-intensive, energy-intensive and costly process, the return on investment for cement plants is very high.

In order to reduce costs, these enterprises, as a rule, are located in the same place where raw materials are mined.

Main methods of cement production

The basis for cement production is a calcined mass called “clinker”. The composition of clinker can be varied, so we will talk about it later.

The entire technological process of cement production can be divided into two main stages:

obtaining clinker is the most expensive and labor-intensive process;
crushing clinker to obtain a powdery mass.

Self clinker production is divided into four more stages:

the raw materials from which clinker will be prepared are mined and delivered to the processing site;
raw materials are crushed;
the raw material mixture is prepared in the required proportions;
the finished mixture is fired under high temperatures.

1.1 Methods for producing cement are divided into three main groups:

wet;
dry;
combined.

The choice of one of them depends on the thermal power of the enterprise and the quality of the raw materials.

1.2 Wet method

The main advantage of this method is that, no matter what the clinker is made from, no matter how heterogeneous the raw material is initially taken, the wet method allows you to accurately select the desired composition of the sludge.

Sludge, in this case, is a liquid, jelly-like mass containing up to 40% water. Its composition is adjusted in special pools and then fired in rotary kilns at temperatures above 1000ºC.

Wet method requires greater consumption of thermal energy, but allows the production of cement of the highest quality, due to the absence of the influence of sludge heterogeneity on the final product.

2 Dry method

The dry method requires that any raw material be processed without the use of water. In this case, clay, limestone and other components are crushed, then ground to dust and mixed using air supply in closed boxes.

When producing cement using the dry method, ready-made raw materials enter the kiln for firing, Moreover, it does not have water vapor. Consequently, after heat treatment, we obtain ready-made cement that does not require crushing.

The dry method significantly reduces the cost of time, thermal energy and other resources. It is very profitable and efficient with high sludge homogeneity.

2.1 Combined

Production can be based on a wet method and supplemented with a dry one, or a dry method supplemented with a wet one.

In cases where the basis is a wet method, the raw materials, after mixing, are dehydrated using special dryers with filters and sent to the oven almost dry. This allows you to reduce the cost of thermal energy, as it significantly reduces evaporation during the firing process. If clinker production is based on a dry method, the finished mixture is granulated with the addition of water.

In both cases, the clinker enters the kiln with a humidity of 10 to 18%.

2.2 Clinker-free production method

In addition to the traditional methods listed above, cement production can occur using a clinker-free method. In this case, the raw material is blast furnace or hydraulic slag, which is combined with additional components and activators. The output is a slag-alkaline mixture, which is crushed and ground to the desired consistency.

Clinker-free cement production technology has the following positive qualities:

the final product is resistant to any environmental conditions;
thermal energy and other energy costs are significantly reduced;
waste from the metallurgical industry is used as raw material for quality production cement, which has a positive effect on the cleanliness of the environment;
makes it possible to produce the final product with different properties and in different colors without changing the production method.

2.3 Cement production (video)

2.4 Equipment for cement production

Since the entire production process is divided into stages that are inherently very different from each other, equipment for producing cement requires multi-profile equipment. It can be divided into the following subgroups:

equipment for the extraction and transportation of raw materials;
for crushing and storage;
kilns;
machines for grinding and mixing clinker;
machines for packaging ready-made cement.

Since cement production is done in different ways and The raw materials used are different, the equipment in factories can also be different.

Lately, private mini cement production plants have become very popular. Sometimes it is even made at home, but we will talk about that later.

The thing is that the equipment for such plants is not very expensive, they can be installed in relatively small areas, and pay for themselves amazingly quickly.

In addition, assembly, disassembly and transportation production line does not cause any difficulties. Therefore, a private plant can be installed at any unpromising raw material deposit, and, once it is exhausted, transported to another location. This option will free the manufacturer from the task of transporting raw materials, which will allow you to save significantly.

2.5 What does the production line consist of?

Screw crushers. Designed for coarse crushing and grinding of raw materials.
Hammer crushers.
Screens or vibrating sieve. Needed for sifting crushed material.
Material supply device for the first stage.
Transporters. They perform the function of supplying raw materials to the next stage.
Sorting machine.
Threshing and threshing-dosing machines.
Mill with millstones.
Sludge mixing machine.
Rotary drum kiln.
Drying unit.
Refrigeration unit.
Clinker mill.
Bucket elevator with feed augers.
Weighing and packaging equipment.

3 Making quality cement at home

In some cases, when you need to make a concrete screed, but have to travel far for cement, craftsmen undertake to make cement at home. Let us note right away that the process of making cement at home is a very labor-intensive process and requires serious equipment and skills.

Don't expect that you will get a quality product the first time. Before you get real cement, you will have to ruin dozens of kilograms of material.

For the production of Portland cement, hard and soft rocks are used; in this case, both the first and the second may include clay and lime components of the raw material mixture. Soft clay components include clay, loess, and hard ones include clay marl, clay shale. Among soft calcareous components, chalk is used, and among hard ones, limestone.

Soft components are successfully crushed in grinders, while hard components can only be ground in mills. Therefore, the technological scheme of grinding raw materials with the wet method, they are chosen depending on their physical and mechanical properties. There are three options for technological schemes:

· two soft materials - clay and chalk are crushed in grinders;

· two solid materials - clay marl and limestone are crushed in mills;

· one material is soft - clay is crushed in mash; the other is solid - limestone is crushed in a mill.

At domestic factories, the most common production scheme for Portland cement is soft (clay) and hard (limestone) raw materials. It consists of the following operations (Fig. 2. 1.):

The initial technological operation for producing clinker is grinding raw materials.

The need to grind raw materials to a very fine state is determined by the conditions for the formation of clinker of homogeneous composition from two or more raw materials. The chemical interaction of materials during firing occurs first in the solid state.

Rice. 2.1.

This is a type of chemical reaction when a new substance is formed as a result of the exchange of atoms and molecules of two substances in contact with each other. The possibility of such an exchange appears at high temperatures, when atoms and molecules begin to vibrate with great strength. In this case, the formation of new substances occurs on the surface of the grains of the starting materials in contact with each other. Consequently, the larger the surface of these grains and the smaller the cross-section of the grain, the more complete the reaction of the formation of new substances will occur.

Pieces of initial raw materials often measure several tens of centimeters in size. With existing grinding technology, it is possible to obtain material from such pieces in the form of the smallest grains only in several steps. First, the pieces are subjected to coarse grinding - crushing, and then fine grinding.

Depending on the properties of the starting materials in the cement industry, fine grinding is carried out in mills and mashers in the presence of a large amount of water. Mills are used for grinding hard materials(limestone, shales), and chatterers - for materials that easily dissolve in water (chalk, clay).

From the mash, the clay slurry is pumped into a mill, where the limestone is crushed. By grinding the two components together, a more uniform raw material sludge is obtained.

Limestone and clay sludge are fed into the raw mill in a strictly defined ratio corresponding to the chemical composition of the clinker. However, even with the most careful dosage, it is not possible to obtain sludge of the required chemical composition from the mill. The reason for this is mainly due to fluctuations in the characteristics of raw materials within the deposit.

To obtain sludge of a strictly specified chemical composition, it is adjusted in special pools. To do this, sludge with a known low or known high titer (calcium carbonate CaCO3 content) is prepared in one or more mills, and this sludge is added in a certain proportion to the correction sludge pool.

The sludge prepared in this way, which is a creamy mass with a water content of up to 40%, is pumped into the furnace supply tank, from where it is evenly drained into the furnace.

For burning clinker using the wet production method, only rotary kilns are used. They are a steel drum up to 150---185 m long and 3.6--5 m in diameter, lined inside with refractory bricks; The productivity of such kilns reaches 1000-2000 tons of clinker per day.

The furnace drum is installed with an inclination of 3--4°. Sludge is loaded from the side of the raised end of the furnace, and fuel in the form coal dust, gas or fuel oil is blown into the furnace from the opposite side. As a result of the rotation of the inclined drum, the materials contained in it continuously move towards the peeled end. In the area of fuel combustion, the highest temperatures develop - up to 1500 ° C, which is necessary for the interaction of calcium oxide formed during the decomposition of CaCO3 with clay oxides and the production of clinker.

Flue gases move along the entire furnace drum towards the material being fired. When encountering cold materials along the way, the flue gases heat them up and cool them down. As a result, starting from the firing zone, the temperature along the kiln decreases from 1500 to 150-200 ° C.

From the kiln, the clinker enters the refrigerator, where it is cooled by cold air moving towards it. The cooled clinker is sent to the warehouse for storage. Stored is aging (up to 2-3 weeks) in order to extinguish free lime in clinker with moisture from the air and thereby prevent uneven changes in the volume of cement during its hardening.

A highly organized technological process for producing clinker ensures a minimum content of free CaO in clinker (less than 1%) and thereby eliminates the need for its storage. In this case, the clinker from the refrigerator is sent directly to grinding.

Before grinding, the clinker is crushed to grains 8-10 mm in size to facilitate the operation of the mills.

Clinker is crushed together with gypsum, hydraulic and other additives, if the latter are used. Joint grinding ensures thorough mixing of all materials among themselves, and high homogeneity of cement is important factor its quality.

Hydraulic additives, being highly porous materials, usually have high humidity (up to 20-60% or more). Therefore, before grinding, they are dried to a moisture content of approximately 1%, having previously been crushed to grains of 8-10 mm in size. Gypsum is only crushed, since it is introduced in small quantities, and the moisture contained in it is easily evaporated by the heat generated when grinding cement as a result of impacts and abrasion of the grinding media in the mill.

Cement leaves the mill at temperatures up to 100° C or more. For cooling, as well as to create a reserve, it is sent to a warehouse. For this purpose, silo warehouses equipped with mechanical (elevators, augers), pneumatic (pneumatic pumps, air chutes) or pneumomechanical transport are used.

Cement is shipped to the consumer in containers - in multilayer paper bags weighing 50 kg or in bulk in containers, automobile or railway cement tankers, in specially equipped vessels. Each batch of cement is supplied with a passport.

In Fig. 2.2. A technological scheme for the production of cement using the wet method is presented.

Rice. 2.2.

Rice. 2.2. Technology system producing cement using the wet method (continued)

Rice. 2.2. Technological scheme for producing cement using the wet method (conclusion)

Portland cement and its varieties are the main binding material in modern construction. In the USSR, its production accounts for about 65% of the production of all cement.

Portland cement- a product of fine grinding of clinker obtained by firing before sintering, i.e. partial melting of the raw material mixture, ensuring the predominance of highly basic calcium silicates in it (70...80%). To regulate setting and some other properties when grinding clinker, add a small amount of gypsum (1.5...3.5%). In accordance with GOST 10178-85, the name Portland cement (PTs-DO) is retained for such non-additive cement. Ш Raw materials and production.

To obtain high-quality Portland cement, the chemical composition of the clinker, and therefore the composition of the raw material mixture, must be stable.

Numerous studies and practical experience show that the elemental chemical composition of clinker should be within the following limits (% by weight): CaO - 63...66; SiO2 - 21...24; A12O3 - 4...8; Pe2Oz - 2...4, their total amount is 95... ...97%. Therefore, for the production of Portland cement, raw materials should be used that contain a lot of calcium carbonate and aluminosilicates (limestones, clays, calcareous marls). More often, artificial raw material mixtures of limestone or chalk and clayey rocks are used with a ratio between them in the raw material mixture of approximately 3:1 (% by weight): CaCO3 - 75...78 and clay substance - 22...25. Instead of clay or to partially replace it, waste from various industries (blast furnace slag, nepheline sludge, etc.) is also used. Nepheline sludge resulting from the production of alumina already contains 25...30% SiOЈ and 50...55% CaO; it is enough to add 15...20% limestone to it to obtain a raw material mixture. At the same time, the productivity of the furnaces will increase by approximately 20%, and fuel consumption will decrease by 20...25%. To ensure the required chemical composition of the raw material mixture, corrective additives containing the missing oxides are used. For example, the amount of S1O2 is increased by adding tripoli and flask to the raw material mixture. The addition of pyrite cinders increases the Fe2O3 content.

Used as fuel natural gas, less often fuel oil and solid fuel in the form of coal dust. The cost of fuel accounts for up to 26% of the cost of finished cement, so cement plants pay a lot of attention to saving it.

Portland cement technology Basically it comes down to preparing a raw material mixture of the proper composition, firing it until sintering (clinker is obtained) and grinding into a fine powder.

The raw mixture is prepared using a dry or wet method (see 5.2). In accordance with this, cement production methods are distinguished - dry and wet. In the USSR, the wet method of cement production predominates, but the dry method is being increasingly introduced. The most important advantage of the dry production method is not only a reduction in heat consumption for firing by 1.5...2 times than with the wet method, but also higher specific removal rates in dry method furnaces.

The firing of the raw material mixture is often carried out in rotary kilns, but sometimes (with the dry method) in shaft kilns.

A rotary kiln (5.2) is a welded steel drum with a length of up to 185 m or more, a diameter of up to 5...7 m, lined on the inside with refractory materials. The drum is laid on rollers at an angle of 3...4° to the horizontal and slowly rotates around its axis. Due to this, the raw material mixture loaded into the upper part of the furnace gradually moves to the lower end, where fuel is injected, the combustion products of which are sucked towards the raw material mixture and burn it. The nature of the processes occurring during firing of the raw material mixture prepared using dry and wet methods is essentially the same and is determined by the temperature and time of heating the material in the furnace. Let's consider these processes.

In the drying zone, the raw material mixture entering the upper end of the furnace meets hot gases and gradually, with an increase in temperature from 70 to 200 ° C (drying zone), is dried, turning into lumps, which, when rolled, disintegrate into smaller granules. As the raw material mixture moves along the furnace, further gradual heating occurs, accompanied by chemical reactions.

In the heating zone at 200...700 °C, organic impurities in the raw materials are burned, chemically bound water is removed from clay minerals and anhydrous kaolinite Al2O3-2SiO2 is formed. Preparatory zones (drying and heating) with the wet production method occupy 50...60% of the length of the furnace, while with the dry method of preparing raw materials, the length of the furnace is reduced due to the drying zone.

In the decarbonization zone at a temperature of 700... s..l 100 °C, the process of dissociation of calcium and magnesium carbonates into CaO, MgO and CO2 occurs, clay aluminosilicates decompose into individual oxides SiO2, A12O3 and Fe2O3 with a highly loosened structure. Thermal dissociation of CaCO3 is an endothermic process that occurs with high heat absorption (1780 kJ per 1 kg of CaCO3), therefore the heat consumption in the third zone of the furnace is greatest. In the same zone, calcium oxide in the solid state reacts with clay decomposition products to form low-basic silicates, aluminates and calcium ferrites (2CaO-SiO2, CaO-Al3, 2CaO-Fe2O3).

In the zone of exothermic reactions, the fired mass, moving, quickly heats up from 1100 to 1300 ° C, and more basic compounds are formed: tri-calcium aluminate 3CaO-Al2O3 (C3A), tetra-calcium aluminate ferrite 4CaO-Al2O3-Fe2O3 (C4AF), but part of the oxide calcium still remains in free form. The fired material is aggregated into granules.

In the sintering zone at 1300...1450 °C, the fired mixture partially melts. C3A, C4AF, MgO and all low-melting impurities of the raw material mixture go into the melt. As the melt appears, C2S and CaO dissolve in it and, interacting with each other, form the main clinker mineral - tricalcium silicate 3CaO-SiO2(C3S), which is poorly soluble in the melt and, as a result, is released from the melt in the form of small crystals, and the fired material is sintered into pieces 4...25 mm in size, called clinker.

In the cooling zone (the final stage of firing), the temperature of the clinker drops from 1300 to 1000 °C, and its structure and composition are finally fixed, including C3S, C2S, C3A, C4AF, the glassy phase and minor components.

Upon leaving the kiln, the clinker must be quickly cooled in special refrigerators to prevent the formation of large crystals in it and to preserve the glassy phase in a non-crystallized form. Without rapid cooling of the clinker, cement with reduced reactivity towards water will be obtained.

After aging in the warehouse (1...2 weeks), the clinker is converted into cement by grinding it into a fine powder, adding a small amount of gypsum dihydrate. The finished Portland cement is sent for storage into silos and then to construction sites.

Dry method of cement production significantly improved. The most energy-intensive process - decarbonization of raw materials - is removed from the rotary kiln to a special device - a decarbonizer, in which it proceeds faster and using the heat of the exhaust gases (5.3). According to this technology, raw flour does not first enter the oven, but into a system of cyclone heat exchangers, where it is heated by exhaust gases and, already hot, is fed into the decarbonizer. Approximately 50% of the fuel is burned in the decarbonizer, which allows almost complete decomposition of CaCO3. The raw meal prepared in this way is fed into the kiln, where the rest of the fuel is burned and clinker is formed. This makes it possible to increase the productivity of technological lines, reduce fuel and energy resources, approximately halve the length of the rotary kiln, and accordingly improve the layout of the plant and the land area it occupies.

The USSR created a low-temperature salt technology for cement production, based on the discovery of Soviet scientists. The essence of the discovery lies in the establishment of a new phenomenon - the formation of a highly basic calcium silicate - alinite, close in composition to alite in the temperature range 9OO...11OO°C, i.e. significantly lower than the crystallization temperatures of tricalcium silicates - alites. Alinite, which is the main binding phase of the new type of Portland cement clinkers, determines their high hydraulic activity. The inclusion of chlorine anions in the structure is prerequisite formation of alinite and clinkers of a new type. The introduction of, for example, 10... 12% CaC12 into the charge is accompanied by the formation of a calcium chloride melt at extremely low temperatures (600...800 C), which shifts all the main reactions of mineral formation to the temperature range of 1000... 1100 "C and makes it possible to obtain clinker at low temperatures.

Implementation new technology will reduce specific fuel consumption and dramatically increase the productivity of furnaces and grinding equipment.

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Introduction

Chapter 1. Technological operations for the preparation of raw materials

1.1 Raw materials for cement production

1.1.1 Carbonate rocks

1.1.2 Clay rocks

1.1.3 Corrective additives

1.2 Basic technological operations for obtaining raw materials

1.2.2 Crushing

1.2.6 Heat treatment raw materials

Chapter 2. Portland cement production technology

2.1 Material composition of Portland cement

2.2 Technological scheme for the production of Portland cement using the dry method

2.3 Special types of Portland cement

Bibliography

Application

Introduction

The word "cement" refers to collective concepts - it unites different kinds binding materials obtained by burning certain rocks and subjected to crushing. They were called binders for their ability to combine (bind) into a single whole both individual particles of small fillers and larger fragments.

The ancient foremen of pyramids, mausoleums and other cyclopean buildings had at their disposal only building gypsum and puffed lime, obtained by burning gypsum stone and limestone. For several thousand years, concrete and mortars based on it were the only known binding materials (not counting clay), and dung and bird eggs were the first modifying additives. The huge dome of the “All-Gods-Temple” (the ancient Roman Pantheon: 43 meters in span); the largest fence in the world, stretching over 5000 km, is the Great Wall of China; concrete gallery of the legendary labyrinth in ancient Egypt; massive religious buildings of the Hindus - all these construction masterpieces were created by using the “great-grandmothers” and “great-grandfathers” of modern cements. Time passed, and other binding materials, obtained artificially and capable of turning into a plastic mass when mixing (kneading) with water, hardening not only in the air, but also in the water element, were created by the inquisitive minds of mankind.

Cement is not a natural material. Its production is an expensive and energy-intensive process, but the result is worth it - the result is one of the most popular building materials, which is used both independently and as a component of other building materials (for example, concrete and reinforced concrete). Cement plants are usually located immediately on the site where raw materials for cement production are extracted.

In Russia, the production of Portland cement was expanded only in late XIX V. A.R. worked a lot on its creation and improvement. Shulyachenko, who is called “the father of Russian cement production.” His merit lies in the fact that high-quality domestic Portland cements replaced foreign-made cements. In Russia, the first plant for the production of Portland cement was built in 1856, and by the beginning of the 1st World War, 60 cement plants were already operating with a total capacity of about 1.6 million tons of cement per year.

Chapter 1. Technological operations for the preparation of raw materials

1.1 Raw materials for cement production

1.1.1 Carbonate rocks

They are widespread in nature, which contributes to the development of cement production based on them. The carbonate rocks used are limestone, chalk, limestone-shell rock, marble, calcareous tuff, marls, etc. All these rocks contain mainly carbon dioxide calcite CaCO 3 . Limestones consist of calcite crystals of various sizes. Chalk is a loose, weakly cemented rock with earthy silt. The quality of carbonate raw materials depends on its structure, the amount of impurities, and the uniformity of their distribution in the mass of raw materials. Carbonate rocks containing 40-43.5% CaO and 3.2-3.7% MgO are suitable for cement production. It is desirable that the total content of Na 2 O and K 2 O does not exceed 1%, and SO 3 - 1.5-1.7%. Rocks with a constant chemical composition and a uniform fine-crystalline structure are more favorable. admixtures of finely dispersed clays and amorphous silica are useful when they are evenly distributed in carbonate rock. A special type of carbonate raw material is marl - a transitional rock from limestone to clay. Marl is a natural finely dispersed mixture of sedimentary origin, clay-sand rocks (20-50%) and calcium carbonate (50-80%). Depending on the CaCO 3 content, marls are divided into sandy, clayey and calcareous. The most valuable raw material is lime marl, containing 75-80% CaCO 3 and 20-25% clay. In terms of its chemical composition, it is close to the Portland cement raw material mixture. This composition of raw materials significantly simplifies the production technology. Marls in which the CaCO 3 content corresponds to the composition of the Portland cement raw material mixture are called natural. The quality of the raw materials determines the firing temperature, furnace productivity and properties. final product. The higher the density of limestone, the more difficult the firing process is. The properties of the raw materials influence the choice of firing unit.

1.1.2 Clay rocks

Clay raw materials (clays, clay marl, shale, loess, etc.) are necessary for the production of Portland cement. Clays have different mineralogical and granulometric compositions, even within the same deposit. The mineralogical composition of clays is represented mainly by hydrous aluminosilicates and quartz, the chemical composition of clays is characterized by the presence of three oxides, %: SiO 2 -60-80, Al 2 O 3 -5-20, Fe 2 O 3 - 3-15.

1.1.3 Corrective additives

With a particularly favorable chemical composition of raw materials, a Portland cement mixture of the required composition can be prepared from only two components - carbonate and clay. But in most cases, it is almost impossible to obtain a given raw material mixture from two components, so third and even fourth components are used - corrective additives containing a significant amount of one of the oxides missing in the raw material mixture. Pyrite cinders from sulfuric acid plants are usually used as an iron-containing additive, and less commonly, blast furnace flue dust. Alumina-rich low-iron clays and bauxites are used as aluminous additives. Silica additives include quartz sand, opoka, and tripoli. The content of oxides in corrective additives should be, %: for ferrous Fe 2 O 3 - at least 40; for siliceous SiO 2 - no less than 70; for aluminous Al 2 O 3 - no less than 30. Ferrous additives are the most widely used. Bauxite is also a corrective additive in the production of Portland cement clinker. Bauxite is aluminum hydroxide with impurities of Fe 2 O 3, SiO 2, CaO, MgO and TiO 2.

1.1.4 Active mineral supplements

These include natural or artificial mineral substances that do not themselves have astringent properties, but when mixed in finely ground form with lime, when mixed with water, they form a dough that, after hardening in air, can continue to harden under water, and when mixed with Portland cement increase its water resistance and anti-corrosion properties. The introduction of active mineral additives somewhat reduces the cost of cement.

1.1.5 Man-made products from other industries

The most widely used materials in the cement industry are blast furnace and electrothermophosphorus slags, fuel slags and ash, nepheline (belite) sludge, and gypsum-containing waste. The use of slag at cement plants helps solve the problem of providing them with raw materials for the depreciation period. Nepheline (belite) sludge is a waste product from the complex processing of apatite-nepheline rocks into alumina, soda, and potash. Since the sludge has passed through partial heat treatment, it consists mainly of dicalcium silicate, a mineral that is part of Portland cement clinker and is capable of hydraulic hardening. Granulated slag and nepheline sludge are similar in composition to Portland cement raw material mixture, therefore they can be used not only as active mineral additives, but also as components of a Portland cement raw material mixture. Since these materials have already undergone heat treatment, do not contain CaCO 3 and include a number of minerals similar in composition to cement clinker minerals, firing of charges containing nepheline sludge and slag requires less fuel consumption. For example, when using nepheline sludge, the productivity of rotary kilns increases by approximately 25%, and the specific consumption of fuel for burning clinker, electricity and grinding media is reduced (by approximately 20%). But ground slags and nepheline sludge cause thickening of raw cement sludge. Increased alkali content in nepheline sludge can reduce the quality of cement.

Fig.1. Raw materials for the production of Portland cement

1. 2 Basic technological processes for obtaining raw materials

1.2.1 Extraction and transportation of raw materials

Operations for the extraction and transportation of raw materials are the most important technological stages of production. In the production of Portland cement, the share of costs for the extraction of raw materials is about 10% of total costs. In each individual case, the method of extracting raw materials must be carefully justified, since the costs of subsequent technological operations depend on this. The choice of extraction method is preceded by an analysis of the chemical composition of the raw material. Raw materials are extracted open method directly from the surface of the earth. The rock layer is usually covered by a layer of waste rock, so the complex of mining operations includes its removal - stripping. The final cost of raw materials largely depends on the cost of stripping operations. They are carried out using bulldozers, excavators, etc. Hard and dense rocks (limestone) are usually mined by explosion. Drilling and blasting operations provide both separation of rock from the massif and crushing of oversized pieces. The peculiarity of such work in the quarries of cement factories is the relatively small volumes of daily production and the limited permissible size of pieces of blasted rock. Drilling machines of percussion-rope or rotary drilling are more often used. Loose and soft rocks (chalk, clay, etc.) are mined without preliminary preparation by direct excavation using single- or multi-bucket (rotary) excavators, which perform two operations at once: separating the rock from the formation and loading the finished raw materials.

To deliver raw materials to the plant, rail and road transport, aerial cableways, belt conveyors, and hydraulic transport are usually used. Railway transport It is most effective to use in shallow quarries with a volume of raw materials transported over 2 million tons/year with a transportation distance of more than 8 km. The advantages of this type of transport: high productivity, reliable operation in any conditions, low energy consumption, long service life of rolling stock; disadvantages: high capital costs for the construction of a railway track and operating costs for its maintenance and repair. Automobile transport It is advisable to use it for transporting materials with complex surface topography, small volumes of transportation and transportation distances of up to 8 km. Soft, loose and small-sized rocks are delivered to the plant at a distance of 1-6 km in favorable climatic conditions by conveyor belts. At cement plants with low productivity, located in very rough terrain, as well as on the plain at the intersection of technological paths from mining workshops highways, railways, etc. use aerial cableways. Their advantages include independence from the terrain, the possibility of full automation production processes, low labor intensity of maintenance; The disadvantages are low productivity and high capital costs.

1.2.2 Crushing

Crushing is the process of mechanically breaking up solids. The purpose of crushing is to reduce the size of pieces of raw materials to such an extent that subsequent grinding is carried out with the least energy consumption. Materials are crushed using the following methods: crushing, splitting, impact, fracture, abrasion. Jaw, cone, roller and hammer crushers are used for crushing materials.

The choice of crushing scheme and type of crushing equipment depends on the properties of the feedstock; soft rocks (chalk, clay) are crushed according to a single-stage scheme in roller crushers to pieces measuring 200 mm. In them, the material is crushed by crushing between rollers rotating towards each other. At different speeds of rotation of the rolls, abrasion of the material also occurs. Depending on the properties of the source material, smooth, corrugated and toothed rollers are used. Hard rocks (limestone, marble) are crushed according to a two-stage scheme (Fig. 2):

1. On jaw crushers up to pieces measuring 75-200 mm. Such crushers use methods of crushing, splitting and partial abrasion of the material. The advantages of this type of crusher are simplicity, reliability, and the ability to process fairly wet materials.

2. On hammer crushers to pieces measuring 8 - 10 mm. On this crusher, grinding is carried out by impact and partially by abrasion.

1.2.3 Fine grinding of materials (grinding)

The main unit for fine grinding and grinding of Portland cement raw mixtures is a ball tube mill, characterized by its simple design, reliability and ease of operation, providing high degree grinding. To protect the drum and the bottom of the mill from premature wear, they are lined with longitudinal and end steel or cast iron plates. The grinding of material in a ball mill is carried out by impacts of freely falling grinding bodies.

A significant disadvantage of ball mills is the low intensity of movement of the grinding media. Also, during dry grinding, the crushed material is heated to a temperature of 100 - 200 0 C, which leads to increased wear of the armor lining, grinding media, and can also cause thermal decomposition of the crushed materials. For successful operation of dry grinding mills, it is necessary to carry out ventilation of the mill space (aspiration). The air flow rate is provided by a fan that draws air through the mill and subsequent cleaning devices. Cold air entering the mill cools the housing lining, the grinding media and the material being ground. Passing through the mill, it carries away the smallest particles, preventing them from sticking to the grinding media. Thanks to aspiration, mill productivity increases by 20-25%, dust emissions are reduced, and sanitary and hygienic working conditions are improved. Dispersion (reduction of strength in the initial stages) of cement clinker is carried out through the use of grinding intensifiers.

1.2.4 Autogenous mills

A promising direction in the development of technology for grinding raw materials is the use of cascade mills, in which grinding of materials is carried out without the use of grinding media - according to the principle of self-grinding. The mill (Fig. 3) is a short hollow rotating drum of large diameter, closed on both sides by end walls with hollow pins. The internal cavity of the drum is lined with armor plates with lifting blades. The material enters the mill through axle 1, is thrown away when the drum rotates to the periphery on the blades, rises last and falls down again, hitting along the way against pieces of material entering the mill and again against the blades. The optimal degree of filling of such mills with material is 20...25%. Grinding in a mill occurs due to the impact of the material on the blades and the collision of the pieces being ground. To enhance the grinding effect, a small number of steel balls (5...6% of the internal volume of the mill) can be loaded into the mill.

Rice. 3. Dry autogenous grinding mill "Aerofol": 1 - loading axle; 2 - transverse beaters; 3 - toothed protrusions; 4 - discharge pipe

The efficiency of the autogenous grinding process is determined by the maximum size of the pieces of the starting material, as well as the ratio of large and small fractions. The optimal size of the material fed into the mill depends on its diameter and rotation speed. Pieces of limestone fed into a mill with a diameter of 7 m should have a size of 350 - 450, chalk - 500 - 800 mm. The main advantages of autogenous mills are simplicity of design and maintenance, low rotation speed of working bodies, low specific energy costs for grinding, absence of grinding media, combination of crushing and grinding processes in one apparatus, high performance(up to 500 t/h). Autogenous grinding mills are designed for dry grinding (Aerofol mill). The creation of such a unit made it possible to process raw materials with a moisture content of 20 - 22% using the dry method. The large diameter of the loading pins allows a significant volume of hot gases to pass through, so gases can be used relatively high temperature(exhaust gases from rotary kilns).

1.2.5 Processing, transportation and storage of powders

Properties of powdered materials

Powdered materials are energy-rich systems capable of self-regulation of their properties and interaction with external environment. Their activity is manifested in autohesion and adhesion. Autohesion is a connection between particles in contact that prevents their separation; adhesion characterizes the interaction of particles with the surface of solid macroscopic bodies (the walls of pipelines, silos - stainless steel containers for storage, and reloading of bulk materials, etc.). Autohesive properties largely determine the behavior of powdered materials during processing. Autohesive interaction of powders entails a number of complications during technological processes. Unloading silos (cement, raw mixtures, etc.) becomes more difficult due to arching and material hanging on the walls. Dust collection equipment becomes clogged with dust, so its design has to be complicated and energy consumption for cleaning increases. The formation of agglomerates makes it difficult to obtain a homogeneous mixture when mixing powders.

Transporting powders

Used for moving dry bulk materials Various types transport systems: mechanical - screw conveyors and elevators and pneumatic - pneumatic chamber and pneumatic screw pumps, air chutes. It is advisable to use mechanical transport systems to move small volumes of materials over short distances. But the complexity of the design and the abundance of moving units complicate the operation of mechanical transport systems and reduce their utilization rate.

Currently, the transportation of powders within the plant is carried out mainly pneumatically using screw and chamber pumps. The main advantages of this method are the ability to move over long distances, the absence of dust, simplicity and reliability of operation. The aeration chute (Fig. 4) is divided in height into two parts by a special airtight partition. The lower tray serves as an air duct into which compressed air is pumped, and powder saturated with air enters the upper outflow (transport). Air chutes are simple in design, installation and operation; wear-resistant; eliminate losses from spraying and provide normal working conditions for maintenance personnel. But they are applicable only for transportation ranges up to 40 m.

Rice. 4. Aeration chute:

1 - fan; 2 - loading hopper; 3 - fabric filter; 4 - top tray; 5 - porous partition; 6 - bottom tray

Homogenization and storage of powdered materials. To obtain homogeneous powders with high mobility, it is necessary to prevent the formation of autohesive contacts and destroy them if they occur. Homogenization of Portland cement raw mixtures is carried out by stirring. The higher the intensity of mixing, the shorter its duration, the smaller the size of the units and the greater their productivity. Mixing of the dry charge is organized in silos with pneumatic mixing. Silos with a flat base are preferred as they distribute air more evenly. The dimensions of the silo depend on the method of homogenization, the capacity of the workshop, as well as the features technological process.

Compressed air supplied to the silos through a breathable bottom saturates the material and transforms it into a pseudo-fluid state. The bottom is laid out with special boxes consisting of a metal body and porous aerial tiles. Aerotiles are made from ceramics, metal-ceramic alloys, textiles, etc. Passing in thin streams through the pores in the tiles, air enters the silo and, when moving upward, carries with it particles of flour. The place of the material raised by the air stream is occupied by a non-arinated charge located next to this zone. Thus, all the powder in the silo begins to move and mix. Mixing powders in a silo consumes a lot of compressed air and, therefore, electricity. The disadvantage of silos of this type is the insufficient degree of homogenization with large quantities of mixture, and a significant need for volumes of compressed air.

More efficient and economical is the use of two-tier silos. Initial raw material mixtures of various compositions enter several silos of the upper tier, and then, after specifying the composition, they are mixed in a given ratio in larger silos of the lower tier. The two-tier arrangement of silos allows not only to reduce production space and construction costs, but also to use the effect of gravitational mixing. When material is discharged from an upper tier silo into a lower tier silo, the speed of movement is higher in the center of the silo and gradually decreases towards the periphery, which causes horizontal layers of material of different levels to move towards the center, where they are simultaneously removed.

The autohesive properties of powders are especially clearly manifested when stored in silos. This is facilitated by the pressure of the overlying layers of material on the underlying ones and the presence of water vapor in the air. To weaken the autohesive interaction of powders, it is recommended that the air supplied for mixing them be preheated to a temperature 15-20 0 C higher than the temperature of the powder. This helps prevent the adsorption of moisture by the material.

Silos are unloaded pneumatically using unloading devices located on the side or under the bottom of the silo, 15-20% of which is laid out with aerial tiles. Dehydrated air is supplied under pressure. Passing through the pores in the aerial tiles, the air loosens the powder and allows it to flow downhill to the unloading mechanisms.

1.2.6 Heat treatment of raw materials in the production of Portland cement

Physico-chemical principles of firing Portland cement clinker. The formation of Portland cement clinker is preceded by a number of physical and chemical processes, as a result of which the clinker acquires a complex mineralogical composition and microcrystalline structure. These processes occur within certain temperature boundaries - technological zones of the furnace. In the main firing unit - a rotary kiln - with the wet method of cement production, zones are distinguished along the movement of the material: I - evaporation, II - heating and dehydration, III - decarbonization , IV- exothermic reactions, V- sintering, VI- cooling. With the dry production method, this zone is absent. Preparatory zones I - II occupy 50...60% of the furnace length, zone decarbonization - 20...25, exothermic reaction zone - 7...10, sintering zone - 10...15 and cooling zone - 2...4% of the furnace length. In Fig. Figure 5 shows the temperature distribution of the material and gas flow across the zones of the rotary kiln.

Rice. 5. Distribution of temperature of the material and gas flow across the zones of the rotary kiln: 1 - material; 2 - gas flow; I-VI - oven zones

In the heating zone at a temperature of 200...650 °C, organic impurities burn out and the processes of dehydration and decomposition of the clay component begin. Dehydration and decomposition of hydrous calcium aluminosilicates into oxides leads to the formation of a number of intermediate compounds, which subsequently significantly affect the rate of CaO binding.

In the decarbonization zone at a temperature of 900... 1200 0 C, dissociation of calcium and magnesium carbonates occurs with the formation of free CaO and MgO. At the same time, the decomposition of clay minerals continues. In the zone of exothermic reactions at a temperature of 1200 - 1300 0 C, the process of solid-phase sintering of the material is completed. As a result, minerals 3CaO*Al 2 O 3 are formed; 4CaO*Al 2 O 3 *Fe 2 O 3 and 2CaO*SiO 2. However, a certain amount of free lime remains in the mixture, which is necessary to saturate the dicalcium silicate to tricalcium silicate (alite).

In the sintering zone at a temperature of 1300 - 1450 0 C, partial melting of the material occurs, starting in the surface layers of the grains, and then gradually spreading to their center. The time for complete absorption of calcium oxide and formation of alite in the sintering zone is 20 - 30 minutes.

In the cooling zone, the temperature of the clinker drops from 1300 to 1100 - 1000 0 C. Part of the liquid phase crystallizes with the release of crystals of clinker minerals, and part hardens in the form of glass. The boundaries of the zones in a rotating kiln are quite arbitrary and unstable. By changing the operating mode of the kiln, you can shift the boundaries and length of the zones and thereby regulate the firing process.

Apparatus for heat treatment. They operate on the principle of both counterflow and cocurrent. From the point of view of heat consumption, co-current flow is more profitable than counter-flow, since in the latter case the temperature of the waste material is higher and there is more heat loss. However, counterflow is more often used, which is associated with a greater temperature difference between the coolant and the material in such devices and, accordingly, a higher heat exchange rate, which makes it possible to reduce the firing time. Thermal units in clinker production are rotary kilns. They are a steel drum that consists of shells (an open cylindrical or conical structural element) connected by welding or riveting, and has an internal lining of refractory material (Fig. 6). The profile of furnaces can be either strictly cylindrical or complex with expanded zones. The expansion of a certain zone is carried out to increase the duration of residence of the fired material in it. The furnace, installed at an angle of 3 - 4 0 to the horizontal, rotates with a frequency of 0.5 - 1.5 min -1. Rotary kilns generally operate on the counter-current principle. Raw materials enter the furnace from the upper (cold) end, and from the lower (hot) end a fuel-air mixture is injected, burning over 20 - 30 m of the furnace length. Hot gases, moving at a speed of 2 - 13 m/s towards the material, heat the latter to the required temperature. The length of time the material remains in the furnace depends on its rotation speed and angle of inclination, amounting, for example, in a furnace measuring 5x185 m, 2 - 4 hours. The cross-section occupied by the material in rotary kilns is only 7 - 15% of the volume, which is a consequence of the high thermal resistance of the moving layer and is explained by both the low thermal conductivity of the particles of the fired material and their weak mixing in the layer.

Rice. 6. Rotary kiln size 5x185 m:

1 - smoke exhauster; 2 - feeder for feeding sludge; 3 - drum; 4 - drive; 5 - fan with a nozzle for injecting fuel; 6 - grate cooler.

dry Portland cement raw material additive

The flame torch and hot gases heat both the surface layer of the material and the lining of the furnace. The lining, in turn, transfers the resulting heat to the material by radiation, as well as by direct contact. With each revolution of the furnace during contact with the gas flow, the temperature of the lining surface increases, and upon contact with the material it decreases. Thus, the material perceives heat only in two cases: either when it comes into contact with the heated surface of the lining, or when it is on the surface of the layer. The productivity of a rotary kiln depends on the volume of the internal part, the angle of inclination of the kiln to the horizon and rotation speed, temperature and speed of gas movement, quality of raw materials and a number of other factors.

An important advantage of rotary kilns is their technological versatility due to the ability to use different types of raw materials.

Heat exchange devices

Effective use of heat in rotary kilns is possible only by installing a system of intra-kiln and oven heat exchange devices. In-furnace heat exchange devices have a developed surface, which is either constantly covered with material in direct contact with gases, or works as a regenerator, receiving heat from gases and transferring it to the material. These devices increase the heat exchange surface between gases and materials also because, by reducing the speed of movement of the material, they increase the filling factor of the furnace. As a result of installing in-furnace heat exchange devices, in addition to the main task - reducing heat consumption - a number of other tasks can be solved: intensify the mixing process, reduce dust removal. This allows you to improve the operation of the furnace and increase its productivity.

In Russia, furnaces with cyclone heat exchangers are mainly used for firing dry raw material mixtures. Their design is based on the principle of heat exchange between exhaust gases and raw meal in suspension (Fig. 7).

Rice. 7. Scheme of cyclone heat exchangers for a rotary kiln:

1 - chimney; 2 - cyclone heat exchangers; 3 - screw feeder; 4 - scraper conveyor; 5 - raw flour supply hopper; 6 - bucket elevator; 7 - heat; 8 - adapter head; 9 - rotary kiln; 10 - dust collectors; 11 - smoke exhauster.

Reducing the particle size of the fired material, significantly increasing its surface and maximizing the use of this surface for contact with the coolant intensify heat transfer. Raw flour in a system of cyclone heat exchangers moves towards the gases leaving the rotary kiln at a temperature of 900 - 1100 0 C. The average speed of gases in the flues is 15 - 20 m/s, which is significantly higher than the speed of movement of raw flour particles. Therefore, the raw flour entering the gas duct between the upper stages I and II of the cyclones is carried away by the flow of gases into the cyclone heat exchanger of the first stage. Since the diameter of the cyclone is much larger than the diameter of the gas duct, the speed of the gas flow decreases sharply and particles fall out of it. The material settled in the cyclone enters the gas duct connecting stages II and III through a shutter - a flasher, and is carried out of it by gases into the cyclone of stage II. Subsequently, the material moves in gas ducts and cyclones of stages III and IV. Thus, the raw flour falls down, passing successively cyclones and gas ducts of all stages, starting from the relatively cold (I) and ending with the hot (IV). In this case, 80% of the heat exchange process is carried out in gas ducts and only 20% occurs in cyclones.

The residence time of raw flour in cyclone heat exchangers does not exceed 25...30 s. Despite this, raw flour not only manages to heat up to a temperature of 700...800°C, but is completely dehydrated and decarbonized by 25...35%.

The disadvantages of furnaces of this type are high energy consumption and relatively low durability of the lining. In addition, they are sensitive to changes in furnace operating mode and fluctuations in the composition of raw materials. After passing through the cyclone heat exchangers, the raw flour temperature 720 - 750°C enters the decarbonizer - an apparatus for removing free carbonic acid from water by blowing air through this water (Fig. 8). Raw meal particles and melted fuel are dispersed and mixed. The heat released during fuel combustion is transferred to raw material particles flour, which heat up to 920 - 970°C. The material in the cyclone heat exchanger - decarbonizer system remains for only 70 - 75 s and during this time is decarbonized by 85 - 95%. Installing a decarbonizer allows you to increase clinker removal from 1 m 3 of the internal volume of the kiln by 2.5 - 3 times. Besides, in decarbonizer You can burn low-quality fuel and household waste. The dimensions of the installation are small, and it can be used not only in the construction of new plants, but also in the modernization of existing furnaces. Furnaces operating in Russia with cyclone heat exchangers and decarbonizers measuring 4.5 x 80 m have a productivity of 3000 tons/day at specific consumption heat 3.46 MJ/kg clinker.

Rice. 8. Rotary kiln with cyclone heat exchanger and calciner:

1 - smoke exhauster; 2 - electric precipitator; 3 - cyclone heat exchanger; 4 - decarbonizer; 5 - rotary kiln 4.5 x 80 m; 6 - installation of case temperature control; 7 - grate refrigerator; 8 - installation for cooling and humidification of furnace exhaust gases.

Lining ovens

To protect the body from high temperatures, the inside of the furnace is lined with refractory materials, which simultaneously act as insulation, preventing excessive heat loss in environment. Lining must have certain properties: chemical resistance to the material being fired, fire resistance, heat resistance, thermal conductivity, mechanical strength, abrasion resistance, elasticity. Since the linings of different zones of the furnace operate under different temperature conditions, they are lined with different refractories. The lining of the sintering zone, the highest temperature zone of the rotary kiln, is subject to particularly difficult conditions. The most advanced type of refractory for such a zone is periclase-chromite bricks with a low chromite content. The average durability in the cement industry of this lining is about 230 days.

The service life of the lining is increased by a number of technological methods: strict adherence to the clinker firing regime; uniform supply of raw materials and fuel; constancy of the chemical composition, grinding fineness and moisture content of the raw materials; constancy of composition, humidity and fineness of grinding of solid fuel. These factors ensure stability of the furnace operating mode, reduce temperature fluctuations in the lining and deformation of the body.

The main condition for reliable operation of the lining is the creation and preservation of a protective layer of coating on its working surface. The clinker melt interacts with the lining material, sticks to it, forming a layer of coating up to 200 mm thick. The process of formation of the coating and its properties depend on the melting temperature, the amount and composition of the liquid phase and the operating mode of the furnace. The coating protects the lining from destruction, reducing the temperature of the surface of the brick and reducing the stresses arising in it, protects the brick from temperature fluctuations inside the furnace, as well as from the chemical and mechanical effects of the fired material.

Intensification of firing processes

Furnace units are the most energy-intensive equipment. In cement production, their share accounts for about 80% of the costs of thermal and electrical energy. In order to reduce these costs, furnace designs are continuously improved and ways to intensify the firing processes are sought. The problem of intensifying the operation of rotary kilns mainly includes two tasks: finding the most rational methods for reducing the specific heat consumption for clinker firing and increasing the thermal power of the kiln. A number of factors affect furnace performance. Firstly, the factors that lead to a change in the specific heat consumption for firing clinker: the composition and structure of the raw material, its humidity and reactivity, etc. Secondly, the productivity of the furnace increases if the surface of contact of gases with the material increases, the speed of movement of the gas increases flow, fuel combustion is carried out with minimal excess air. All measures that help increase the useful heat of combustion of fuel accelerate the process of clinker formation. These include the installation of intra-furnace and oven heat exchange devices, reducing sludge moisture through dewatering in concentrators or by introducing sludge thinners, etc.

The thermal power of a furnace is the most important design characteristic that determines its performance. Increasing the amount of fuel burned in the same volume of combustion space is one of the ways to increase furnace productivity. An effective means of intensifying the process and furnace productivity is to increase the temperature of the heated material.

An effective means of intensifying the firing process is to burn part of the fuel in the decarbonization zone directly in the material layer. The specific heat consumption for clinker firing can be reduced by introducing mineralizers into the raw material mixture. They make it possible to accelerate solid-phase reactions, reduce the temperature at which the liquid phase appears, improve its properties, and improve product quality. An important reserve for intensifying the roasting process is the utilization of dust collected from exhaust gases. Fine, partially calcined dust is similar in composition to the raw material mixture. The return of dust to the furnace helps to increase the productivity of the unit, reducing the consumption of raw materials, fuel, and electricity. Fuel consumption can be reduced by improving the technological scheme, design solutions of decarbonizers, refrigerators and auxiliary equipment.

Cooling of fired materials

The material leaving the rotary kiln has a temperature of about 1000 0 C. Returning the heat of the material to the kiln can significantly reduce fuel consumption. This is achieved by cooling the material with air, which is then supplied to the furnace for combustion of fuel. The cooling mode affects both the further technological process and the properties of the finished product. Grinding hot materials leads to a decrease in mill productivity and an increase in specific energy consumption. Portland cement clinker is especially sensitive to cooling. Rapidly cooled clinkers are easier to grind and to a certain extent improve the quality of cement. Therefore, it is necessary that the clinker cooling process be as complete as possible and proceed quickly, especially in the initial stage. The more complete the clinker cooling, the less heat loss.

Three types of coolers are widely used: drum, recuperator and grate. In the production of Portland cement clinker in modern rotary kilns, grate push coolers are used (Fig. 9). The horizontal grate with movable grates is driven by a crank mechanism. The shape of the grates is such that when moving forward, the clinker is poured onto the next row of grates; when moving in the opposite direction, it slides along the grate. Due to the fact that some grates move and others do not, the clinker is constantly mixed. The cooler chamber is divided into two parts. Clinker from the edge of the rotary kiln in the neck of the cooler is exposed to “sharp blast” (10...12 kPa), which ensures uniform distribution of clinker across the width of the grate and its rapid initial cooling. This hot air with a temperature of 450 0 C is sucked into the furnace, where it is used for combustion of fuel as secondary air. Cold air also enters the second part of the sub-grid space of the cooler, which is exposed to partially already cooled clinker and can be used for drying raw materials. At the discharge end of the cooler, a hammer crusher is installed, designed to crush large pieces of clinker (“weld”).

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Rice. 9. Diagram of a clinker grate cooler of the Volga type:

1 - rotary kiln; 2 - receiving shaft; 3 - grate; 4 - drive; 5 - window for discharging excess exhaust air into the atmosphere; 6 - roar; 7 - hammer crusher; 8 - scraper conveyor; 9 - windows for general blast; 10 - general air fan; 11 - sharp blast fan.

Since air in a grate cooler is sucked through a layer of material, the heat exchange surface increases significantly and the cooling process is intensified. The cooling rate is controlled by changing the speed of movement of the grate, the thickness of the material layer and the amount of air.

Advantages of grate coolers - high speed and degree of cooling (up to 40 - 60 0 C), good efficiency, low specific energy consumption (9 - 11 MJ/t clinker). The main disadvantage is the heat exchange principle, which is unfavorable from the point of view of recovery, since the air does not move countercurrent to the material, but perpendicular to it. A large amount of heat is lost when excess air is released into the atmosphere. The disadvantages of grate coolers also include difficulty in operation and repair, less reliable operation, and large capital investments.

Chapter 2. Portland cement production technology

2.1 Material composition of Portland cement

Portland cement GOST 10178-76 is a hydraulic binder that hardens in water and in air and is a product of fine grinding of clinker obtained by firing before sintering an artificial raw material mixture, the composition of which ensures the predominant content of calcium silicates in the clinker (70-80%).

Ordinary silicate cement, or Portland cement, obtained by finely grinding clinker and gypsum together, is a greenish-gray powder that, when mixed with water, hardens in air (or in water) into a stone-like mass. Gypsum is added to Portland cement to regulate the setting time. It slows down the onset of setting and increases the strength of cement stone in the early stages. Along with ordinary Portland cement (without additives), designated by the index PC D0, two types of Portland cement with mineral additives are produced, designated by the index PC D5 and PC D20. In the first, it is allowed to add an additional 5% of active mineral additives, and in the second, more than 5, but not more than 10% of additives of sedimentary origin (tripol, opoka), or up to 20% of additives of volcanic origin, gliez, granulated blast furnace and electrothermophosphorus slag. The ratio of clinker, gypsum and additives characterizes the material composition of Portland cement. The quality of clinker depends on the chemical and mineralogical composition. The chemical composition is characterized by the content of various oxides, and the mineralogical composition is characterized by the quantitative ratio of minerals formed during the firing process. Portland cement clinker consists mainly of, % by weight: CaO-64...67; SiO 2 - 21...25; A1 2 0 3 - 4...8; Fe 2 0 3 -- 2...4. In addition, the clinker may contain MgO, TiO 2, alkalis, etc.

The most important oxides that make up clinker (CaO, SiO 2, Al 2 0 3 And Fe 2 0 3), interact during the firing process, forming clinker minerals. Portland cement clinker consists of a number of crystalline phases that differ from each other in chemical composition. Main clinker minerals:

alit - 3CaO * SiO 2 (abbreviated notation C 3 S);

belit - 2CaO * SiO 2 (C 2 S);

tricalcium aluminate 3 CaO * A1 2 0 3 (C 3 A);

calcium aluminoferrites of variable composition from 8 CaO

* 3 A1 2 0 3 * Fe 2 O 3 to 2CaO * Fe 2 0 3 (C 8 A 3 F...C2F).

The mineralogical composition of clinker affects the production technology of Portland cement and its properties. Knowledge of the mineralogical composition of clinker allows us to predict the properties of Portland cement: the rate of strength gain under various hardening conditions, durability in fresh and mineralized waters, heat generation during hardening, etc. This makes it possible to select the required cement in accordance with the type of structure and its operating conditions.

Alite is the most important material of clinker, its main carrier astringent properties. It makes it possible for cement to quickly harden and achieve high strength.

Belite interacts with water much more slowly than alite and in the initial stages of hardening has low strength. But over time, whiteite gains strength and is not inferior to alite in terms of strength indicators.

Tricalcium aluminate quickly hydrates and actively participates in setting processes, but its contribution to the final strength of the cement stone is relatively small. With an increase in the content of calcium aluminoferrites, cements harden slowly, but achieve high strength. Regulation of the mineralogical composition ensures the production of cements with desired properties.

2.2 Technological scheme for the production of Portland cement using the dry method

Cement production in its enlarged form consists of the following main stages:

· Extraction, primary grinding of raw materials in quarries and delivery to platform cement plant, warehousing;

· grinding and averaging (homogenization) of the crushed mixture, preparing it for firing;

· thermochemical processing of raw materials to produce clinker - the starting material for processing into cement, clinker cooling;

· grinding clinker with additives for cement (the amount and composition of additives depend on the chemical and mineralogical composition raw materials and clinker, required grade of cement);

· cement supply to warehouse, storage, packaging and shipment.

For the production of cement, wet, dry and combined methods are used.

Dry production method. The basic technological scheme for producing Portland cement using the dry method is shown in Fig. 10.

Rice. 10. Schematic flow diagram for producing Portland cement using the dry method

Grinding of materials in mills can be carried out at a raw material moisture content of no more than 1%. In nature, there are practically no raw materials with such humidity, so a mandatory operation of the dry production method is drying. It is advisable to combine the drying process with grinding of raw materials. This efficient solution has found its way into most new dry production plants. A ball (pipe) mill combines the processes of drying, fine grinding and mixing the components of the raw mixture. The raw mixture comes out of the mill in the form of a fine powder - raw flour.

Increasing requirements for saving fuel consumption force materials with increasingly higher humidity to be processed using the dry method. On the other hand, such materials are characterized by reduced density and, accordingly, strength. It is advisable to carry out preliminary grinding of such materials in Aerofol autogenous mills, which allow processing raw materials with a moisture content of up to 25%. However, the raw material does not have time to dry completely, and in a ball mill, simultaneously with the additional grinding of large particles and obtaining a homogeneous raw material mass, it must be dried.

Raw flour is fed into reinforced concrete silos, where its composition is adjusted to the specified parameters and homogenized by mixing with compressed air. Next, the finished mixture is fired in rotating kilns with baking heat exchangers. The resulting clinker is cooled in a cooler and supplied to a warehouse, where a stock is created to ensure uninterrupted operation of the plant. At the same time, keeping clinker in storage improves the quality of cement. The warehouse also stores gypsum and active mineral additives. These components must first be prepared for grinding. Active mineral additives are dried to a moisture content of no more than 1%, and the gypsum is crushed. Combined fine grinding of clinker, gypsum and active mineral additives in ball (pipe) mills ensures the production of cement High Quality. From the mills, the cement enters silo-type warehouses. Cement is shipped either in bulk (in automobile and railway cement tankers, specialized ships) or in containers - multi-layer paper bags.

The main advantage of the dry production method is reduced fuel consumption. Also, with the dry method, the volume of furnace gases is reduced by 35 - 40%, which accordingly reduces the cost of dust removal and provides great opportunities on the use of heat from exhaust gases for drying raw materials. An important advantage of the dry production method is the higher clinker removal from 1 m 3 of the kiln unit. Another important factor is that when firing using the dry method, the consumption of fresh water is significantly reduced.

In the global cement industry, the dry production method has taken a leading place. Currently, the share of the dry method in Japan, Germany and Spain is 100%, in other developed countries - 70 - 95%. In Russia, the share of the dry production method is only 13%.

Appendix 1 shows a diagram of the equipment layout of a technological line for the production of cement using a dry method with a capacity of 3000 tons/day. Limestone and clay are taken as the starting materials. Limestone undergoes two-stage crushing in jaw crushers and then in hammer crushers. The clay is crushed in roller crushers and dried in drying drums. Each component of the raw material charge coming from the warehouse is sent to hoppers 1, equipped with gates and weighing dispensers 2, and then to conveyors 3, delivering them to the feed hopper of the mill 4.

In the department for grinding raw materials, two raw mills 4 measuring 4.2x10 mm are installed. When the moisture content of the charge does not exceed 8%, the mill operates with a supply of hot drying gas from baking heat exchangers. If the moisture content of the raw materials is higher, a combustion device is installed, from which hot gas is additionally supplied to the mill.

Each mill operates according to a pneumatic unloading scheme with an air-pass separator 5. The grain separated by the separator is returned to the mill for finishing, the finished product through cyclones 14, air chutes and a flow meter enters dry raw flour silos 13, equipped with a mixing aeration system. From silos 13, raw flour is sent through air chutes 15 and then by pneumatic lifts to a cyclone heat exchanger (10, 11), where it is heated by gases leaving the oven to 700... 750°C and partially (up to 20%) decarbonized, after which it is supplied into a rotary kiln 12.

Overview of production methods

The main differences lie in the preparation of clinker - a mixture of limestone and clay, ensuring a predominance of calcium silicates. The higher their proportion in the composition, the better the quality of the binder; for example, in Portland cement it reaches 80%. Grinding and mixing of components is carried out different methods: grinding in water, dry with air, or a combination of these processes. Non-traditional methods for preparing raw materials include the production of clinker-free binder using the so-called cold technology without prolonged firing.

1. Features of the wet method.

The essence of this option is the separate primary processing of raw materials: chalk, clay, converter sludge or other iron-containing additives. They are crushed into fractions up to 10 mm and soaked in water. Each component has its own moisture content: clay – within 20%, chalk – 29, sludge – up to 70. They are combined and mixed in a mill in a state of suspension until maximum homogeneity. The final moisture content of the resulting mass lies in the range of 30-50%, in this form it enters the plumbing basin to control and adjust the composition, after which it is fed into the kiln.

The wet method was used in almost all factories former USSR, its main advantage is the ability to adjust the composition and control the characteristics. The output mixture has a homogeneous structure, which has a positive effect on the quality of cement. The disadvantages include significant costs for the preparation of clinker: the fractions go through the crushing stage 2-3 times, are soaked in mash, mixed in a mill, and evaporated for a long time; all this process requires a lot of energy resources.

2. Advantages of dry manufacturing.

All initial components are crushed without wetting, they are dried in separation drums before entering the mill. After grinding, the dry mixture (humidity does not exceed 1%) is supplied to silo stations, where final mixing is carried out using compressed air and the chemical composition is adjusted. The clinker is formed in screws; due to the introduction of water-saturated clay, its moisture content is not zero, but before firing it does not exceed 13%. This allows you to reduce the heat costs for evaporating water from it by 1.5-2 times. The homogeneity of mixing remains high; the method is ideal for Portland cement and other quality grades.

The dry method involves the use of ovens of any type, but maximum effect observed during firing in rotating and shaft units. The main advantage is the ability to decarbonize components outside the main heating zone, ideally by using the energy of exhaust gases. It involves the inclusion of cyclone heat exchangers in the circuit in front of the decarbonizer; clinker enters the kiln without carbon dioxide and preheated. This makes it possible to reduce the heating zone of shaft furnaces, the size of the equipment and the area it occupies.

Dry technology is the most economical production method; the remaining stages are carried out according to the standard scheme (firing, cooling, grinding, adding gypsum and impurities, packaging). It requires careful preparation of raw materials to achieve uniformity. Water is added solely for the purpose of adjusting the composition and obtaining granules convenient for baking; the entire mixing load falls on the compressed air supply systems.

3. Combined scheme.

There are two options for combining the wet and dry methods: preparing and mixing the sludge in a wet state, followed by evaporation to 18%, or crushing and combining using the second technology with the introduction of a small proportion of water to form granules of about 10-15 mm in size. In the first case, special evaporator filters are included in the circuit; a rotary kiln is most often used for firing. Final stages are no different from the wet and dry methods.

4. Clinker-free production.

In addition to limestone, the raw material used is not clay, but waste from the metallurgical industry: blast furnace slag, fly ash. The expensive clinker production required for Portland cement is skipped and all components are dry mixed and melted. The resulting slag is ground to a powdery state and combined with active and ash additives. The production of clinker-free cement using the dry method (another term is “cold”) can significantly reduce heat treatment costs. Unlike clinker, it is fired quite long time in furnaces at temperatures not lower than 1400°, the slag melts many times faster.

The advantages include a reduction in consumed energy resources and equipment lines, the possibility of recycling metallurgical waste and a simplified scheme for the preparation and processing of raw materials. Clinker-free production is 2-3 times superior to traditional production in terms of environmental friendliness and cost. The resulting binder gives concrete special resistance to wear and aggressive environments. An additional advantage is more low temperature hydration of solutions based on them. The quality of the product is manifested only if the slag is finely ground and the components are carefully dosed.

Feasibility of own production

It is impossible to prepare high-quality cement at home; crushed limestone and ash dissolved in water are only suitable for filling seams or fixing small parts (subject to the addition of liquid glass). To create a binder that meets GOST standards and with a strength grade higher than M200, a line of equipment is required, including crushers, conveyors for feeding raw materials, mills, dispensers, sorting, granulating, clinker and screw machines, a drum kiln, a cooler and baling machines. Maintenance of a cement plant requires at least 40 people and is characterized by high energy consumption.

The main supplier of equipment is China; selling companies offer both individual machines and fully equipped lines. There are no complaints about the reliability of the units and the quality of the Portland cement produced. The initial cost of a mini-factory is at least 1,000,000 rubles; the location will require an area of 30,000 m2. But despite the significant initial investments, this business is considered profitable due to the demand for this building material. The average cost of 1 ton of cement is from 800 to 1000 rubles, and sales – from 3500 to 4000.

Profitability largely depends on the quality and availability of raw materials, the proximity of quarries and the degree of development of the marketing network. With a full cycle, the plants produce from 330 tons/day, which means over 60,000 rubles of net profit per day.