Iron ore is the focal raw material for ferrous metallurgy. Like any other ore, it consists of useful minerals, waste rock, and impurities that can be either beneficial (chromium, manganese, vanadium, etc.) or harmful (sulfur, phosphorus ). The ore may contain varying amounts of valuable minerals. Depending on this, it is classified as high-grade ore and low-grade ore. The high-grade ore is used directly in production. Low-grade ore is sent for concentration with less than 35% iron content.
Methods of fine-grinding have become commonly used to extract a useful component out of ore fully. As a result, a concentrate with a fraction of 0.1 mm or less is obtained, which is impossible to use in producing iron, hot metal, and steel without prior sintering (lumping).
When mining ore, many fines (fractions less than 10 mm) are formed, which negatively influence the gas permeability of the burden and inhibit the reduction process, consequently affecting the blast furnace output.
According to BF smelting conditions, a minimal fraction of ore loaded into the furnace should not exceed 10-12 mm. Coarsening (clotting) is used to obtain pieces of the required size for rational use of natural dusty and small-piece iron-containing materials.
Clotting is the final and very important stage in preparing ores for smelting. There are two main methods for raw material clotting:
- Agglomeration (sinter production): A sinter feed of iron ore with a 5-60 mm particle size is produced during agglomeration;
- Pelletizing (pellet production): During pelletizing, 10-20mm pellets are produced.
Currently, agglomeration (sintering) is the most common method, and it has several significant advantages over pelletizing. One advantage of the iron sintering process is its versatility—the process is quite successful with a wide range of ore fractions (from 0 to 10 mm).
Along with clotting, other physical and chemical processes impact iron ore quality. During sintering, up to 90-98% of sulfide and up to 50-60% of sulfate sulfur burn out, and up to 10-20% of arsenic and a significant amount of zinc are also removed. The alkalis content that may be present in the sinter burden is reduced by approximately 30%.
The global sinter plants market is anticipated to grow at a CAGR of 4.4% over the projected period, owing to the increasing demand for iron and steel products in various industries such as construction, automotive, machinery, and others. Major trends influencing this growth are technological advancements in sintering processes for increased production capacities, investments by major players into new greenfield projects with high-capacity sintering facilities, and the introduction of advanced technologies to improve the efficiency of existing sinter plants.
The iron ore sinter plant market is expected to grow by USD 1.15 billion from 2022 to 2027. However, the market’s growth momentum will progress at a CAGR of 9.77% during the forecast period. The improvement in sinter technology is notably driving the market growth.
Principle of Sintering
Agglomeration is principally a process of heating the burden layer to the temperature of softening and partial melting and then cooling down to the solidification temperature. As a result, a liquid phase is formed, which hardens the entire burden mix and makes it a relatively solid monolith. At the same time, the lumpiness and continuous penetration of air through the burden mix ensure the porous structure of the sinter. The heat required for sintering is obtained from the combustion of carbonaceous fuel, added to the material being sintered.
Iron Ore Sintering Process
Sintering burden mix on the grate of an agglomeration machine with air suction through the burden layer is the most common agglomeration method.
Sinter burden includes:
- iron-containing materials (concentrate, ore, flue dust) – 40 – 50%;
- flux (limestone) that improves the blast furnace performance indicators – 10 – 15%;
- return (fines, off-spec sinter) – 20 – 30%;
- solid fuel (fine coke) – 4 – 6%;
- moisture (is added to improve granulation of fine particles of the burden) – 6 – 9%
The agglomeration process begins with the preparation of the burden mix for iron ore sintering and includes the following stages:
- material sizing,
- burden components dosing,
- mixing,
- moistening and pelletizing of the burden.
Fine iron ore concentrate (<0.074 mm) and sinter ore fines (<10 mm) do not need crushing. Limestone and fuel supplied to the sinter plant have a particle size exceeding that required for successful sintering. Limestone and fuel particle size should be no more than 3 mm. The content of grade > 3 mm should not exceed 3% for limestone and 5% for fuel, provided that the amount of class 0 – 0.5 mm for fuel should be minimal. Therefore, there is a need to grind limestone and fuel.
Special attention is paid to the dosing accuracy of fuel and limestone since the content of these components in the sinter burden determines the thermal level of sintering, basicity value, and mechanical strength of the sinter.
The burden components in a given ratio are mixed in mixing drums with water added to obtain a degree of homogeneity up to 80-85%.
The mixture is then sent to the pelletizing drum for moistening and pelletizing. As a result of pelletization, 5-8 mm granules are formed from burden mix fines. This provides the necessary gas permeability of the layer in the sinter machine, which makes it possible to carry out the iron ore sintering process in a layer with a height of up to 450 mm.
Fig 1. The ignition furnace of the sinter machine
The burden sintering is carried out on a moving grate of the sinter machine. The air intake required for the oxidation of solid carbonaceous fuel is performed by exhausters installed on the sinter gas path. A vacuum is created under the grate, due to which a flow of hot hearth gases is first sucked into the layer for the burden “ignition”. This means the surface layer is heated to approximately 1200 °C within 1.5-2.0 minutes. As the sintered burden moves to the discharge part of the sinter machine, the combustion starting from the surface of the burden layer successively passes through the entire thickness of the material and ends at the grate.
The solid fuel of the sinter mix burns almost completely. Ore fines in the zone of maximal temperatures (1400-1450°C) are melted and sintered, and then, during subsequent crystallization, a porous structure – an agglomeration cake is formed. The burden mix, prepared for sintering, is laid in a layer on the grates of the continuously traveling grate cars of the sintering machine. When the cars pass under the ignition furnace (Figure 1), the upper surface layer of the burden is heated by the flame of gas burners. When the burden temperature reaches 700÷800°C, solid fuel (coke breeze) ignites. The suction of atmospheric air maintains coke breeze combustion through the burden.
As the sintering burden moves to the discharge part of the sinter machine, combustion starts from the surface of the burden layer, successively passes through the entire thickness of the material, and ends at the grate. In the combustion layer, the temperatures reach 1500°C. The solid fuel of the burden burns almost completely, and ore fines are melted and sintered, forming a porous sinter “cake.” The formed and agglomerated material is cooled by sucked-in air and transfers heat to the underlying layers.
Under the grate cars of the sinter machine, there are vacuum chambers connected by a gas duct to a large process fan. This creates underpressure up to 16kPa for pulling air through the burden layer, transporting gaseous combustion products (60÷150°С) to the gas cleaning plant, and further releasing them into the atmosphere through a flue stack.
Figure 2 shows a diagrammatic cross-section of the agglomerated layer with zones indicating where the main processes occur in the burden.
The most important physical and chemical transformations occur in the zone of melting and intense heating. Chemical compounds formed during these processes determine the final agglomerate composition.
Fig. 2. Layout of individual zones in the longitudinal section of the agglomerated layer. 1 – grate; 2 – hearth layer; 3 – over moisturization zone; 4 – burden heating zone; 5 – combustion and sintering zone; 6 – sintered agglomerate (air heating and sinter cooling zone).
Hot Sintered Mass Processing
The sinter cake leaves sinter machines with a temperature of 800-900°C and is crushed in a single-roll crusher and screened to separate fines <5.0 mm from the sinter.
In modern sinter factories, the agglomerate is cooled after crushing to a temperature not exceeding 100°C through linear or ring coolers, using a forced cold air supply into the sinter layer.
Cooled and crushed agglomerate are sorted to separate the “cold” return, “bed,” and effective agglomerate.
Before loading the sinter burden onto the sinter machine’s cars, a “bed” layer—agglomerate with a particle size of 8-13 mm—is laid first. This prevents material from spilling through the gaps of the grate, protects the grate from exposure to high temperatures, and eliminates the burden of “burning on” to the grate.
The returns are sent for re-sintering and then used as an additive to the main burden.
Effective sinter is sent to the blast furnace shop.
Types of Sinter
There are different types of ore agglomerates, which have unique properties depending on the content of ore-enriching components:
- Dolomitized agglomerate is obtained from raw materials fluxed with dolomite. The product contains two or more percent of magnesium. Dolomite increases slag mobility in a blast furnace and reduces the impact of temperature changes and ore composition on physical and chemical properties.
- Sinter with high iron content is called iron ore agglomerate.
- Manganese agglomerate is an iron ore agglomerate with manganese ore added.
- Ferromanganese agglomerate is used in blast and ferroalloy furnaces, where ferromanganese is produced. It has increased meltability reducibility and lower porosity than the iron ore product.
- In a metalized agglomerate, iron oxides are partially reduced to iron.
- A stabilized agglomerate stands out in the group of materials. It is processed mechanically after sintering. The obtained product is calibrated, which increases smelting efficiency and improves hot metal quality.
- Agglomerate phosphorites are dried, dewatered, and decarbonized to produce phosphorous. High temperatures of up to 1600 degrees Celsius melt silicates, bringing them to a glassy binder. It fixes apatite grains.
- Chromite agglomerate is obtained from chromium ores or a mixture of chromite and serpentine.
Quality Requirements for Iron Ore Production
The product of sintering (agglomeration) is agglomerate, a lumpy black porous product.
Fig 3. Agglomerate
The sinter, the main iron ore component of blast furnace smelting, must have properties that maximally satisfy the blast furnace process’s requirements.
The quality of agglomerate is assessed by its chemical composition and mechanical, physical, and chemical properties. The following requirements are imposed on the agglomerate:
- High iron content
- Minimum impurities
- Uniform granulometric composition – minimum content of fines 0-5mm and +40mm fraction
- Basicity, providing high cold strength of agglomerate
- Stable chemical composition
- High hot strength
- High reductibility
- Slight difference in softening and melting temperatures
Requirements for chemical composition are individual for each enterprise.
Sinter Machines
Fig 4. General view of a sintering machine
Features of Sinter Machines
At steelmaking enterprises, sintering is carried out mainly in belt-type sinter machines, a continuous chain of sinter cars with a grid bottom made of grates that move along closed guiding ways.
Table 1. Technical specification of domestic-made sinter machines
| Characteristics m2 | КЗ-50 | АКМ-75 | АКМЗ-85/160 | МАК-240/138 | АКМ-312 |
| Total active area, m2 | 50 | 75 | 160 | 240 | 312 |
| Sintering area, m2 | 50 | 75 | 85 | 138 | 312 |
| Cooling area, m2 | – | – | 75 | 102 | – |
| Number ov vacuum cameras, pcs. | 13 | 15 | 17/15 | – | 26 |
| Pallet width, m | 2,0 | 2,5 | 2,5 | 2,5 | 4,0 |
| Effective length, m | 25 | 30 | 64 | – | 78 |
| Number of pallets, pcs. | 70 | 86 | 151 | – | 130 |
| Drive power, kW | 11 | 13 | 32 | – | 85 |
| Number of exhausters, pcs. | 1 | 1 | 1 | 2 | 2 |
| Exhauster capacity, m3/min | 3500 | 6500/7500 | 6500 | – | 9000 |
Technologies for Reducing Sintering Emissions
Reduction of fuel consumption for the sintering process in iron-making decreases emissions of harmful gaseous substances into the atmosphere:
- An increase in the sintered layer height by 10 mm reduces specific solid fuel consumption by 0.8-1.0% and fines content in the sinter (5-0 mm);
- Dosing and automatic control of burden materials, reducing fluctuations in the mass fraction of iron in sinter burden by 10%, decreases specific consumption of solid fuel by 1.5%;
- Using the heat of heated air from coolers in the ignition hearth decreases gaseous fuel consumption for ignition to 20%;
- Recycling of sinter gases decreases solid fuel consumption by 10-15%, gross emissions of harmful substances into the atmosphere are proportionally reduced to the degree of recycling by 30-35%;
Replacement of ineffective gas cleaning devices with new ones that are highly efficient.
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Conclusion (Summary)
Agglomeration processes save graded fuel resources, increase the productivity of metallurgical units, expand the base of raw materials, and make it possible to utilize various valuable iron-containing wastes from metallurgical production and eliminate their disposal.
