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Using Direct Reduction and Coal Process of a Rotary Kiln Without melting the components, iron ore can be converted directly into metallic iron using the coal-based direct reduction rotary kiln process.
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Low capital costs and a lack of a coking coal demand are advantages of the process. By using carbonaceous material found in non-coking coal, the metallic iron in this process is created by reducing iron oxide below the fusion temperature of iron ore (1535 degrees Celsius).
It is frequently referred to as "direct reduced iron" since the reducing agent and iron ore come into direct touch during the reduction process (DRI).
Due to its "honeycomb structure," the reduced product with a high degree of metallization is frequently referred to as sponge iron.
Utilizing a Rotary Kiln's Direct Reduction and Coal Process Iron ore can be transformed into metallic iron utilizing the coal-based direct reduction rotary kiln method without melting the constituent parts.
The technique' benefits include low initial investment costs and a lack of a demand for coking coal. In this method, metallic iron is produced by reducing iron oxide below the fusion temperature of iron ore using carbonaceous material contained in non-coking coal (1535 degrees Celsius).
Since the reducing agent and iron ore come into direct contact during the reduction process, it is frequently referred to as "direct reduced iron" (DRI). The reduced product with a high degree of metallization is usually referred to as sponge iron because of its "honeycomb structure."
components, raw the primary raw materials for the rotary kiln method used to produce DRI are I sized graded iron ore or iron ore pellets, (ii) non-coking coal, and (iii) dolomite, which is used in tiny amounts to scavenge sulfur.
The primary ingredient in the coal-based direct reduction process is iron ore of the proper purity. Lump iron ore is used in most coal-based direct reduction plants.
The primary requirements for quality are I a high Fe content with little gangue and harmful substances like phosphorus and sulfur, (ii) good handling properties, (iii) calibrated to size with little fines, (iv) a high reducibility, (v) little decrepitude during reduction, and (vi) good compatibility with the reductant coal used.
Sized iron ore must be of sufficient grade to satisfy physical, metallurgical, and chemical standards. Strength and granulometry are necessary physical prerequisites.
The ore must be extremely strong and hard. The ore should have a minimum tumbler strength of 90%. Calibrated ore in the size range of 5 mm to 20 mm is typically used, depending on the reducibility.
High reducibility, thermal stability, and a low propensity for sticking and disintegration during heating and reduction are among the ore's metallic requirements.
Since no other chemical change occurs during the direct reduction process other than the elimination of oxygen, the ore's chemical needs are an important property.
The gangue material in DRI, which comes from iron ore and has silica, alumina, sulphur, and phosphorous concentrations, has a negative impact on the economics of the succeeding steelmaking process.
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Therefore, the ore should have a high iron content and a low gangue content. The ore must meet the following physical and chemical requirements. 5 mm to 20 mm in size Less than 5% per size oversize and undersize is acceptable.
More than 90% on the tumbler index less than 5% contamination (laterite, etc.) Fe total: 65% minimum less than 3% silica Maximum 5% of silica and aluminum maximum of 0.01% sulfide No more than 0.05% phosphorus Maximum 1% moisture.
Non-coking coal serves two purposes in rotary kilns. It serves as both a reducing agent and a source of heat for the process.
Non-coking features, low ash content, low sulphur content, good reactivity, high ash fusion temperature, and medium volatile matter are the main non-coking coal quality requirements.
The coal's non-coking nature is essential because the coking reaction causes rings to form inside the kiln. For the rotary kiln operation, the highest practicable caking index is 3.
The low ash level of the coal is essential because high ash causes the kiln volume that can be used for reduction to be underutilized.
As more energy is needed to heat the inert ash material to reaction temperature, it also increases the consumption of fixed carbon. The practical maximum for efficient operation is 20% ash content.
To keep the sulphur in the DRI under control, low sulphur content is necessary. Sulfur content in coal may not exceed 0.01%.
Since the forward reaction, a crucial step in the solid-state reduction process involves coal reacting with carbon dioxide produced by burning coal to replenish carbon monoxide, good coal reactivity is required (Boudouard reaction).
For the Boudouard reaction to proceed at the necessary rate, the coal must be sufficiently reactive. When producing DRI, bituminous and sub-bituminous coals often exhibit good reactivity.
Another crucial property is the temperature of ash fusion. Low-ash-fusion-temperature coals produce sticky mass that makes kiln operation unstable. This is because accretions occur inside the kiln, slowing the reduction rate by depositing slag on the ore's surface.
So, for rotary kiln operation, coals with ash fusion temperatures of more than 1400 deg C are preferred. For a coal-based rotary kiln process, the amount of coal's volatile matter must be optimal.
Low volatile matter coal has very little char reactivity, which leads to inferior metalized products. Even though they produce highly reactive char, very high volatile coals are not desired since they increase the kiln's gas volume.
Normal volatile matter for rotary kiln processes should be between 25% and 30%. In the rotary kiln, direct reduction coals must have low swelling indices. The swelling index should typically be less than 1. The non-coking coal has the following physical and chemical characteristics.
What Equipment, Facilities Are Need
The coal-based direct reduction rotary kiln method requires the following tools and resources. Area for preparing raw materials - This section includes Vibro-feeders, crushers, and screens.
Because sized material (5 mm to 20 mm) is typically purchased, iron ore crushing is typically not required. Iron ore is typically transported to a groundhopper, from where it is transported to a screen with the aid of a Vibro-feeder, where screening separates oversize (+ 20 mm) and undersized (- 5 mm) particles.
To obtain sized material, the oversized material is transported to the oversize crusher. The receiving size for non-coking coal is typically -200 mm, and it is crushed in a closed circuit with primary and secondary screens by primary and secondary crushers.
The screens separate the two fractions that are required for the operation, namely coal lumps for feed coal (5 mm to 20 mm) and coal fines (-5 mm) for injection coal.
Since the sized material (3 mm to 8 mm) is typically purchased and directly supplied to the day bin through a conveyor, dolomite crushing, and screening is typically not required.
Through conveyors, all raw materials are fed into the day bin from their discharge ports. Section for storing raw materials - It consists of boxes for storage (day bins). There are distinct storage bins available for various raw materials.
The bins are typically made to store raw ingredients for no less than one day's worth of consumption.
The system for feeding raw materials into the kiln is made up of weigh feeders that are placed beneath each bin to take various raw materials out of the bins in the proper proportion and transport them to the conveyors.
The reduction reaction takes place in the rotary kiln, which is at the center of the coal-based reduction process.
The kiln's capacity determines its length and diameter. Typically, the length of the kiln is lined with high alumina castable refractories, with dams at the feed end and discharge end.
Depending on the size of the kiln, the gradient is typically 2.5 degrees downward, and the kiln is supported by two to four support stations. By adjusting the kiln's slope and rotational speed, materials can be transported through it at a specific rate.
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The kiln has input and outlet cones at its opposite ends that are cooled by separate fans. There are little sample apertures built inside the kiln casing. Hydraulic systems are used to move the kiln longitudinally on its riding rings.
The high end of the inclined kiln receives metered amounts of coal and iron ore. Additionally, a portion of the coal is pneumatically injected from the kiln's discharge end.
Prior to reduction, the burden first goes through a pre-heating zone where coal is devolatilized, and iron ore is heated to a pre-heating temperature.
Installing an appropriate number of air injection tubes made of heat-resistant steel equally distributed throughout the length of the kiln and flowing countercurrent to the flow of iron ore allows for temperature and process control in the kiln.
Special internal swirlers are used in the air tube tips to enhance combustion uniformity. LDO (light diesel oil) is used in a central burner at the kiln discharge end to heat the cold kiln.
Following initial heating, the fuel supply is cut off, and the air is injected into the burner to facilitate coal combustion. Fixed thermocouples and "quick response thermocouples" (QRT) ports are used to measure the temperatures within the kiln.
Fixed thermocouples are positioned throughout the length of the kiln to enable temperature monitoring at various points.
If they become coated with ash, ore, or accretion, fixed thermocouples may occasionally give erroneous readings. In these circumstances, QRT is utilized to keep track of the kiln temperatures.
Rotating cooler - At a temperature of around 1000 degrees Celsius, the product (DRI), along with coal ash, calcined dolomite, and residual char, is discharged from the kiln.
The hot DRI is transferred to a rotating cooler via an enclosed duct at the kiln discharge end that is fitted with a lump separator and an access door for eliminating lumps. The cooler is a suitable size rotating cylinder that is 2.5 degrees inclined.
Water sprayed on the colder top surface cools the DRI indirectly. The make-up water is piped to the cooling tower together with the cooling water, which is gathered in troughs beneath the cooler.
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Solids are cooled to about 100 °C without meeting the air when they are emptied into the cooler through a closed funnel. Large (greater than 50 mm) accretions that could clog or harm the cooler discharge mechanisms are removed by a grizzly in the chute.
Through a lump gate, these accretion lumps are expelled independently. Through a double flap valve, the remaining material is emptied onto a conveyor. The product is subsequently transported by conveyor to the product separation system.
Magnetic separators and vibrating screens make up the product separation segment. Belt conveyors are used to moving the cooled product to the product separation section.
DRI, char, and coal ash are components of the cooler discharge products. The product is initially screened in a double deck screen with 3 mm and 20 mm screens in the product separation building.
To separate the magnetic and non-magnetic components, the screened product is passed into magnetic separators.
The additional 20 mm DRI is first removed from the product during screening. Screening separates the undersize (a mixture of DRI, char, and coal ash) into fractions of +/- 3 mm. A magnetic separator is used to separate out each fraction.
Most of the non-magnetic material in the + 3 mm fraction is char, which can either be recycled back into the kiln or used as fuel in power plant boilers.
The non-magnetic portion of the -3 mm fraction, which mainly consists of wasted calcined dolomite, ash, and fine char, is discarded. Each fraction's magnetic component is DRI.
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