Agglomeration (metallurgy)

Dwight and Lloyd Drum Type Agglomeration Machine, 1891

Agglomerate production workshop, Nurberg, Sweden, 1926

Agglomeration as a method of agglomeration was discovered by chance in 1887 by British researchers F. Geberlein and T. Hatington during experiments on desulfurizing roasting of non-ferrous metals on a grate ^[6] . Firing was carried out as follows. A layer of burning pieces of coke or coal was poured onto the grate, on which a layer of sulfide ore was then laid. From below, air was supplied from the blower through the grill. Passing through a layer of fuel, the air provided its intense combustion. Hot combustion products, moving on, heated the layer of ore located above. At temperatures of 400-500 ° C, ignition of sulfides occurred. As a result of their combustion, additional heat was released, which was transferred by a gas stream to the ore layer located even higher. Thus, the sulfide combustion zone moved in the direction of gas movement, passing successively the entire ore layer located on the grate. Ore burning was carried out without supplying heat from the outside — only due to the heat released during the burning of sulfides. “Ignition” fuel (pieces of hot coke or coal), located initially on the grate, served only to ignite the ore sulfides of the lowest layer ^[7] .

In the course of research, it turned out that during the firing of ores with a high sulfur content , so much heat was released and the temperature rose to such a level that the fired pieces of ore were melted to each other. After the end of the process, the ore layer turned into a crystallized porous mass - sintered. Pieces of crushed sinter, which were called “agglomerate”, turned out to be quite suitable in terms of their physicochemical properties for mine smelting ^[7] .

The comparative simplicity of the technology and the high thermal efficiency of the oxidative sintering of sulfide ores have attracted the attention of specialists in the steel industry . There was an idea to develop a thermal method for sintering iron ore materials based on a similar technology. The absence of sulfur in iron ores as a heat source was supposed to be compensated by the addition of small particles of carbon fuel to the ore: coal or coke. Iron ore sinter using this technology in the laboratory was first obtained in Germany in 1902-1905. ^[eight]

The first industrial plant for the production of agglomerate was the Heberlein boiler - a conical steel bowl, at some distance from the bottom of which a grate was fixed, and in the bottom there was a pipe for supplying blast from the blower. The process was distinguished by the fact that burning particles of coal or coke were a source of heat for softening and partial melting of ore grains. A layer of pieces of incandescent solid fuel located on the grate is covered with a thin layer of sinter mixture - a mixture of fine moist ore with particles of coke. After that, the blast was turned on, and the gas heated in the layer of fuel burning on the grate was raised up, igniting and burning the fuel contained in the charge in the lower layer of the sintered material. When the combustion zone reached the surface, the next layer of the sinter charge was loaded. Thus, the process continued until the entire bowl was filled with the finished agglomerate (the boiler with a capacity of 15 tons was filled within 12 hours). After that, the fan was turned off, the boiler was overturned and the resulting block of agglomerate was manually broken into smaller pieces ^[9] .

In Russia, the first 6 Heberlein boilers were commissioned in 1906 at the Taganrog plant , and in 1914, another 5 bowls at the Dnieper Metallurgical Plant . At the same time, work was underway on the creation of alternative sinter plants, devoid of the shortcomings of Heberlein boilers: low productivity, heavy physical labor of workers. Agglomeration bowl designs with significantly better process characteristics have been developed. In the years 1914-1918. the sinter plant with rectangular (stationary) bowls of the Grinevalt system was built at the Dnieper plant, and in 1925 at the Goroblagodatsky mine a factory with 28 round bowls (2.3 m in diameter) was built by the Swedish company AIB. Fundamentally, the agglomeration process in the bowls proceeded in the same way as in the Heberlein boilers. The difference was that the thickness of the sintered layer was reduced to 250-300 mm, and the blast mode was changed to vacuum — air was sucked into the layer from above due to the vacuum created by the fans under the grate. Therefore, ignition (ignition of particles of solid fuel mixture) was also produced from above. In rectangular bowls, ignition was carried out using mobile incendiary furnaces with gas burners ^[10] .

Since each of the aforementioned sinter plants had some significant drawbacks (one of the most serious is low productivity), neither bowls nor tube furnaces were widely used in metallurgy. A breakthrough in the field of ore sintering was made by two American engineers A. Dwight and R. Lloyd, who developed the structure in 1906, and in 1911 commissioned the first continuous conveyor belt sintering machine . The process of sintering ores was carried out according to the same principle as in Heberlein boilers or in bowls - the heat needed to melt the ore grains was released during layer-by-particle burning of solid fuel as a result of air suction through a charge laid on the grate. Success in the rapid and widespread dissemination of agglomeration as the main method of sintering iron ore materials was predetermined by the very successful design of the sintering machine. The sintering area of ​​the first Dwight-Lloyd sintering machine was 8.1 m ² (with a tape width of 1.05 and a length of 7.7 m); daily productivity - 140 tons of sinter during sintering of blast furnace dust ^[11] .

During the 1990s, the size of sintering machines grew immeasurably - the sintering area increased to 600 m ² or more: the daily output reached 15,000-18,000 tons of sinter. The steel grades from which various machine parts are made have changed, but the basic structure of the machines has remained unchanged ^[11] .

Agglomeration of iron ore concentrate (sometimes mixed with ore, metallurgical waste) is the final operation in a package of measures for preparing iron ore for blast furnace smelting. The main objective of this operation is to turn a small ore concentrate into larger pieces - sinter, the use of which in blast furnace smelting ensures the formation of a charge layer of good gas permeability, which is an indispensable condition for high-performance operation of a blast furnace.

High-intensity blast-furnace smelting is possible with a large amount of coke burning in the furnace of a blast furnace, which, on the one hand, leads to the release of a large amount of heat, and, on the other hand, to the formation of free space in the lower part of the furnace (due to gasification of solid coke) where blast furnace charge. Good gas permeability of the charge is necessary so that a large volume of gases generated during coke combustion has time to pass through the intermask channels of the layer at relatively small gas pressure differences between the furnace and the top (150-200 kPa at the height of the charge layer 20-25 m) ^[8] .

The composition of the charge

The general scheme of the sintering process by the method of suction includes the following steps.

A typical mixture used for the production of iron ore sinter consists of the following components:

1. small iron ore material, usually a concentrate;
2. crushed fuel — coke ( fraction 0–3 mm), content in the charge 4–6%;
3. crushed limestone (fraction 0–3 mm), content up to 8–10%;
4. return - substandard agglomerate from previous sintering (fraction 0-8 mm), content 25-30%;
5. iron-containing additives - blast furnace dust from blast furnaces, mill mill scale, pyrite cinders of sulfuric acid production, etc. (fraction 0–3 mm), content up to 5%.

The components dosed in a predetermined ratio are mixed, moistened (to improve pelletizing ) and, after pelletizing without compaction, they are loaded onto the grate with a layer of 300-400 mm. Then turn on the supercharger - the fan working on a suction. A rarefaction is created under the grate, due to which the flow of hot furnace gases is first sucked into the layer, which ensures “ignition” of the charge, that is, the surface layer is heated to about 1200 ° C (within 1.5–2.0 minutes). The atmospheric air that then enters the bed for the rest of the process provides intensive combustion of the charge coke particles. In the zone of maximum temperatures (1400-1450 ° C), the ore grains partially melt, coalesce, and then a porous structure — agglomeration cake — forms during subsequent crystallization.

Zone Mode

At each moment of time, fuel particles heated up to 700-800 ° C ignite in the charge layer adjacent to the lower boundary of the combustion zone. At the same time, the combustion of fuel particles at the upper boundary of the combustion zone ends. As a result of this, the combustion zone, combined with the melting zone, continuously moves downward in the direction of gas flow, as if “penetrating” into the layer of the initial charge and leaving behind a zone of cooling agglomerate.

The determining zone of the process is the horizon with maximum temperature - the melting zone - the agglomerate formation zone. Above this zone is a layer of porous agglomeration sinter. In the zone of intensive heating located below, the sintered material is rapidly heated - at a speed of up to 800 deg / min and the same rapid cooling of the combustion products occurs. Leaving this zone, a gas with a temperature of 300-400 ° C enters the wet mixture - a drying zone is formed. In this zone, the gas is cooled to 50-60 ° C and leaves it saturated with water vapor. In the cold mixture below (15–20 ° С), the gas cools, becomes supersaturated, and part of the water vapor in this condensation zone in the form of droplets is deposited on the lumps of the mixture, increasing their moisture content. Since the speed of movement of the condensation zone is several times greater than the speed of movement along the layer of the drying zone, a layer of waterlogged charge forms over time between these zones. In this case, the thickness of the layer of the initial charge decreases rapidly.

The total agglomeration time can be divided into three periods:

• initial, when the main zones of the sintered layer are formed (during this period, the sinter charge is ignited, approximately the same time overmoistening of the entire charge layer occurs);
• the main period, when the thermal and gas-dynamic regimes are stabilized and there is a movement along the layer of zones of formation of agglomerate, intense heating, drying;
• final, during which all zones of the sintered layer “wedge out” in sequence, only cooling agglomeration sinter remains on the tape.

The process is considered complete when the agglomerate formation zone reaches the grate of the sintering trolleys . At a vertical sintering speed of 20 mm / min, a charge layer 300 mm thick turns into agglomerate in 15 minutes.

Process Features

The modern agglomeration process refers to the type of bedding, when the air passing through the sintered ore material provides two main processes:

1. burning solid fuel charge and
2. carries out heat transfer from one elementary layer to another.

In this regard, high technical and economic indicators of the agglomeration process can be achieved only with intensive air flow into the sintered layer. Meanwhile, sinter blends containing pulverized iron ore concentrates (with a particle size of less than 0.1 mm) have a very high gas-dynamic resistance. Therefore, a mandatory preparatory operation is the pelletizing of the charges - the process of forming granules 2-8 mm in size. A layer of such a pelletized, well-gas-permeable charge allows achieving high velocities of gas flow (up to 0.5-0.6 m / s) at relatively small pressure drops above and below the layer (10-15 kPa).

One of the characteristic features of the agglomeration of iron ore materials is the intense heat and mass transfer in the charge layer due to its high specific surface (30-50 cm ² / cm ³ ). This explains the relatively small height (15–40 mm each) of the zones of melting, intense heating, drying, and condensation. The consequence of this feature of the process is the short residence time of each elementary volume of the sintered material at high temperatures — 1.5-2.0 minutes. Therefore, technologists must ensure such conditions (particle size of the charge components, gas velocity in the bed, etc.) so that the main chemical-mineralogical and physical processes that produce the desired quality sinter: carbon and sulfur burnout, dissociation of carbonates , heating of ore particles to melting temperatures, their adhesion, etc.

The second feature of the agglomeration process is the appearance of an inhomogeneous temperature field in the bulk of the sintered material. Due to the point distribution of fuel particles in the charge, the foci of combustion and melting alternate with areas of the material (charge or sinter) that are in the solid state. As a result of local shrinkage of the molten material, pores of 3-10 mm in size are formed in the combustion zone. Due to this feature, a porous sufficiently gas permeable layer structure is preserved in the zone of existence of the melts. Additional pores arise during the evolution of gases from the combustion of carbon, sulfur, dissociation of carbonates, reduction of iron oxides, etc.

The third feature of the agglomeration is that the combustion of fuel particles in the layer occurs under conditions of double heat recovery: the air entering the combustion zone is preheated to 1000–1100 ° C in the layer of cooling sinter, and the fuel (and the rest of the charge) before ignition Heats up to 700-800 ° C with a stream of hot gases leaving the combustion zone. During about 80% of the sintering time, the gas exiting the layer has a temperature of 50-60 ° C. This means that the bulk of the heat from the ignition and combustion of carbon solid fuel of the charge remains inside the layer and is involved in heat transfer processes.

Another positive feature of the agglomeration of iron ore materials is that, as a result of the partial reduction of iron oxides in the moderate temperature zone, the melting points of such reduced materials are significantly reduced - by 150-200 ° С, which significantly reduces the need for heat per process - this reduces the fuel content in the mixture while maintaining a sufficiently high strength of the sinter. The aforementioned makes agglomeration by the suction method an extremely effective process in terms of thermal performance: when the carbon content in the charge is only 3-5%, it is possible to heat the sintered material to 1400-1450 ° C ^[13] .

Since 1955, in the world metallurgy on an industrial scale, they began to use a new method for sintering thin iron ore concentrates - the production of pellets . When pellets were smelted in US blast furnaces , specific coke consumption was reduced, and furnace productivity was almost doubled. Благодаря активной рекламной кампании, которую развернули фирмы-разработчики технологии и изготовители оборудования фабрик по производству окатышей, у многих металлургов сложилось впечатление, что окатыши обладают неоспоримыми преимуществами перед агломератом. В МЧМ СССР было принято решение, что стратегическим направлением развития подотрасли подготовки железорудного сырья к доменной плавке является интенсивное строительство фабрик по производству окатышей с постепенным сокращением, а в конечном счете с полной ликвидацией агломерационного производства. Любые попытки ученых и производственников в 60-х гг. ХХ века дать объективную оценку новому способу окускования решительно пресекались. Замалчивались результаты работы ряда доменных печей Японии на хорошо подготовленном офлюсованном агломерате по сравнению с проплавкой окатышей (неофлюсованных). Результаты такой тенденциозной технической политики не замедлили сказаться. Вскоре после начала использования в доменной плавке на ММК окатышей ССГОК пришлось аварийно останавливать доменные печи по причине интенсивного износа засыпных аппаратов и огнеупорной кладки, обусловленного значительным повышением содержания пыли в доменном газе из-за сильного разрушения окатышей в ходе доменной плавки.

Последовавший за этими событиями объективный анализ показал, что окатыши не являются «абсолютно» лучшим видом окускованного рудного сырья. Они обладают рядом серьезных недостатков по сравнению с агломератом:

• окатыши невозможно получать из относительно грубых концентратов, а дополнительное измельчение до необходимой крупности (-0,05 мм) значительно удорожает концентрат;
• окатыши сильнее агломерата разрушались в ходе восстановительных процессов;
• чрезвычайно трудно технологически получать окатыши повышенной (до 1,4—1,5) основности ;
• при работе доменных печей только на окатышах возникают определенные затруднения из-за ухудшения газопроницаемости слоя и развития процессов шлакообразования.

Главным достоинством агломерации является универсальность — процесс спекания идет достаточно успешно с использованием рудных материалов в широком диапазоне по крупности (от 0 до 10 мм); допустимы некоторые отклонения от оптимальных параметров по влажности шихты, содержанию в ней твердого топлива и др.

Несомненным преимуществом окатышей перед агломератом является их хорошая транспортабельность: они мало разрушаются в ходе железнодорожных или морских перевозок. Таким образом, целесообразно окускование тонкого рудного концентрата производить путем производства окатышей в том случае, когда горно-рудный комбинат (с обогатительной фабрикой) находится на значительном удалении от металлургического завода.

Среди металлургов существует мнение, что агломерация и производство окатышей — не конкурирующие, а дополняющие друг друга методы окускования ^[14] .

• Sintering machine
• Nourishing
• Окатыши
• Брикет (металлургия)

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