In construction, the so-called binders are actively used. These are powdery materials which, after mixing with water, form a homogeneous mass. It gradually hardens in air to a stone-like state. All construction binders can be divided into two main groups - inorganic (or mineral), which include lime, gypsum, Portland cement and its various varieties, and organic (or black binders), which mainly include all coal distillation products and oil - bitumen, tar. Astringents underlie any construction. They are used for the preparation of masonry and plaster mortars, various types of concrete (light and heavy). Separate parts of buildings and entire structures are erected from concrete with the addition of binders, any building structures, including steel reinforced. The first binders that were obtained artificially include building gypsum, which is produced by burning gypsum stone at a relatively low temperature.
Specialists attribute to gypsum binders powdered materials, which consist of semi-aquatic gypsum and are produced by heat treatment of two-gypsum gypsum. Depending on the heat treatment conditions, on which the rate of setting and hardening of the material directly depends, gypsum binders are divided into low- and high-calcining. Low-calcining binders mainly consist of semi-aquatic gypsum, which is obtained by heat treatment of gypsum stone at a temperature of 400-450 degrees Celsius. These materials, which include medical gypsum, building plaster, etc., quickly set and harden. High-fired binders (finishing gypsum cement, anhydrite cement, high-fired gypsum) consist of anhydrous calcium sulfate, which is obtained by firing at a higher temperature (from 850 degrees Celsius). They set and harden more slowly than the materials of the first group.
Gypsum is characterized by the greatest variety of applications among all other binders. Materials and products from it can save on cement and fuel, reduce the complexity and cost of construction. Gypsum is widely used for the manufacture of gypsum concrete rolling partitions and partition plates, ornamental decorations and as a plaster material.
So, the scope of gypsum in construction and decoration, if not unlimited, then at least very wide. Let us consider in more detail the production technology of this material. As we mentioned above, building gypsum is made from gypsum stone, which in turn is a product of grinding rock of sedimentary (i.e. chemical) origin, consisting of a natural mineral. Gypsum stone has the following properties: compressive strength - about 80 MPa, true density - 2200-2400 kg / m3, Mohs hardness - 2. The average density of gypsum gravel in bulk is from 1300 to 1600 kg / sq3.
In nature, gypsum is found, as a rule, in the form of three main varieties that differ from each other in crystal structure: alabaster (dense fine- or coarse-grained mineral with crystals randomly oriented in space), gypsum spar (lamellar mineral of a layered structure with flat translucent crystals) and selenite (a fibrous mineral with filamentary interspersed crystals with a silky sheen).
As a result of processing gypsum stone, binders of a light (closer to white) color are obtained. However, the presence of various impurities can give gypsum other shades - from gray to pink or yellow. As such impurities that change the color of the material, sulfur, quartz, pyrite, borates, carbonates and other additives can be used. The production of any binders is a complex of chemical and physico-mechanical effects on the starting materials, which are carried out in a certain sequence. In particular, the production of building gypsum from dense gypsum rocks includes three main stages: crushing gypsum stone, grinding, drying and firing. Gypsum stone is delivered to the plant in the form of pieces with a diameter of 300-500 mm. These are quite large sizes, so gypsum stone is preliminarily sent on a conveyor belt for crushing using a mill (screw and hammer crushers). First, the pieces are crushed to a size of 30-50 mm, and then crushed into cereals with particle sizes up to 15 mm. In modern factories, gypsum stone is crushed in one step using large hammer crushers. To grind crushed stone to a powder state, various types of mills are used (for example, pendulum mills). Wet gypsum stone is poorly crushed, so this stage is often combined with drying. In this case, flue gases coming from the boilers are fed to the mill. By changing their speed, you can adjust the fineness of gypsum grinding (the higher the flow rate, the coarser the source material will be and vice versa).
From the dust mixture resulting from these manipulations, gypsum powder is emitted into dust cleaning systems. All plants where building materials are produced are necessarily equipped with multi-stage air purification systems. Dust collecting devices can create suitable sanitary conditions for work and prevent (or at least significantly reduce) production losses. Raw gypsum is loaded into the boiler using a screw conveyor. During the cooking process, two-water gypsum becomes semi-water, and water vapor is discharged through pipes. Semi-aquatic gypsum is fired at temperatures up to 160 degrees Celsius for 1.5-3 hours, after which it is released into the hopper. After cooling in the storage bins, the gypsum is sent to the finished goods warehouse.
So briefly looks at the gypsum production process. However, it has certain nuances, which will be discussed below.
Gypsum binders are produced by heat treatment of gypsum raw materials to calcium sulfate hemihydrate. Gypsum is burned in special rotary kilns. Such a furnace is an inclined metal drum, along which gypsum stone previously crushed into small pieces slowly moves. In the drum, gypsum is fired by flue gases, which are formed as a result of the burning of various types of fuel in furnace devices during furnaces.
In most cases, drying drums are also used in production, which are heated by gases. In addition, furnaces heated by flue gases on the outer surface of the drum, as well as furnaces in which flue gases first wash the drum from the outside and then pass through its inside, are often used. When using oven models with direct heating, an additional mixing chamber is often installed between the working part of the drum and the furnace. It helps to reduce the temperature of the gases leaving the furnace, which are mixed in it with cold air.
Since the gases in the drum move at a high speed, due to which small particles of gypsum fly out of it, dust collecting devices and smoke exhausters are usually installed behind the drum. Specialists recommend expanding the part of the drum in which the dehydration process takes place most intensively. Due to this, the movement of the gas stream and the material, which has a particularly high mobility (especially during the "boiling"), slows down in it. In the working cavity of the drum there is a special device for mixing gypsum during the firing process. This ensures uniform dehydration of the feed. In addition, during the mixing process, a large contact surface of the calcined material with the hot gas stream is created, which also favorably affects the final product.
In rotary kilns, gypsum is calcined by two main methods - forward flow and counterflow. In the first case, gypsum stone is exposed to high temperatures at the very beginning of firing, and in the second - at the end. When using the direct-flow method, the material is not burned, the temperature of the gases that enter the furnace is about 1200 degrees Celsius, and the temperature of the gases leaving it is about 440 degrees. At the same time, with countercurrent, the temperature of the incoming gases reaches 1000 degrees, and the outgoing - about 380 degrees. It is obvious that when using the direct-flow method, fuel consumption is significantly increased, since in the zone of maximum temperatures only heating and drying of the material is carried out (preparatory stage of processing raw materials). The process of dehydration is directly carried out in the zone of lower temperatures. For this reason, rotary kilns that operate on the principle of counterflow are considered more economical.
Also, in order to save money, hot material emerging from the furnace is immediately sent to the languishing hopper or is subjected to hot grinding. In the latter case, the properties of gypsum will be much better, since due to dehydration of the remaining hemihydrate and binding of the liberated water with soluble anhydrite, the mineral composition of the finished material is equalized.
To produce gypsum of the highest quality in the rotary drum of the furnace, crushed gypsum stone with a uniform particle size is burned. If the particles in the composition of the mass are of different sizes, then the material will be fired unevenly. Small particles are usually burned, sometimes to the state of insoluble anhydrite, and the inner part of large grains is preserved in the form of undecomposed hemihydrate.
As a rule, material with a grain size of up to 0.035 m is loaded into the furnace. Grains smaller than 0.01 m are screened out. When the material is abraded in the furnace during the dehydration process (primarily for softer gypsum rocks), dusty particles are formed that are carried away by the gas stream, as a result of which they pass faster through the furnace. True, a small number of them still manage to completely dehydrate.
Specialists recommend burning fractions of 0.01-0.2 m and 0.02-0.035 m separately from each other. Screened fractions with a grain size less than the specified can be used after additional grinding for the production of gypsum or for the production of raw gypsum, which is used for gypsum solonetzic soils.
So, the gypsum powder production line, where natural gypsum is used as the feedstock, consists of the following units: preparation of the feedstock (loading unit, grinding device, hopper loader), storage warehouse for the prepared feed (for large volumes of production), grinding raw materials (loading device, crushing device, grinding device with dust collector), firing (loading machine, evaporation furnace with dust collector, shaker, furnace), packaging of finished products, sys EMA is an electronic management and control. For the production of gypsum, you will need rotary kilns, digesters and / or combined grinding and roasting plants. Let's try to compare these different types of equipment. The gypsum boiler looks like a cylinder with a spherical bottom concave inward, made of heat-resistant steel and bricked with brickwork. Under such a boiler there is a firebox, the bottom of the boiler itself acts as its arch. Inside the boiler, four flame metal pipes are mounted one above the other. The fuel combustion products wash the bottom of the boiler, and then heat its side walls, after which the gases are discharged into the heat pipes, heat them and are removed through the chimney. This work scheme allows you to evenly heat the raw materials and increases the efficiency of gas use. A vertical shaft with upper and lower mixers installed inside the boiler uniformly mixes the material.
Cooking boilers are widely used in manufacturing enterprises, as they are quite convenient, easy to use and maintain, easy to adjust and allow you to control firing modes. The contents of such a boiler do not come in direct contact with the flame and flue gases, so the material is not contaminated with soot and ash. However, such equipment also cannot do without disadvantages. First of all, it is the fast wear of the bottom and shells of the boiler, the frequency of operation (such equipment cannot work constantly) and the difficulty in collecting gypsum dust. In addition, although the gypsum boiler grinds raw materials, its subsequent milling in ball mills is used to improve the quality of the building material.
Rotating furnaces can operate continuously, unlike digesters, which provides a more convenient operation scheme. They can burn gypsum stone of larger crushing than in digesters, where it is impossible to provide perfect mixing of raw materials. In addition, subject to preliminary processing (grinding) of raw materials, correctly selected firing conditions and grinding of the fired product, rotary kilns allow the production of high-quality building material. The length of the rotary kilns, which are used for roasting gypsum, reaches 14 meters and a diameter of 2 meters. The productivity of such a furnace can be from 5 to 15 tons of gypsum per hour.
One medium-sized plant can produce up to 100 thousand tons of gypsum powder per year. The total staff of such an enterprise (including personnel serving the lines, administrative staff and managers) is 25-30 people.
Sysoeva Lilia
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