Maximizing the Recycling of Iron Ore Pellets Fines Using Innovative Organic Binders (1)
agglomeration,pellet fines,briquettes,organic binders,reduction
Source: internal company
The iron and steel industry is considered to be the backbone of industrialization. Traditionally, fine ores are treated either by the process of sintering and/or by pelletizing to prepare an agglomerate with suitable metallurgical characteristics for ironmaking processes. The study put forward by  compared sinter to pellets and clearly stated various environmental and technical benefits of using pellets instead of sinters. To increase the gas permeability in the shaft and the furnace efficiency during the direct reduction process, pellets are the preferred feed when in comparison with lump ore.
Inevitable generation of pellet fines mainly occurs during the transportation, handling, or screening of pellets before charging them to the shaft furnace. Previous studies suggest that around 24 million tons of pellet fines are generated all around the world by the year 2020. Parallelly, utilizing these generated pellet fines is of utmost importance to increase resource efficiency, thus ensuring the conservation of natural resources. Generally, the generated fines are less than 8 mm in size and possess more than 65 wt.% of iron content. Aside from saving the generated pellet fines, there is a need to agglomerate these fines into the form of briquettes or pellets before charging them to the furnace. One of the main reasons for agglomeration is to maintain the size and, hence, facilitate the reduction of gas flow through the charge uniformly at a high rate. Fines charge will give rise to a non-permeable bed and is susceptible to getting carried away during high flow rates. Therefore, to increase the permeability and to limit the material blowing out of the furnace as dust, various agglomeration methods have been adopted, of which the most commonly used ones are sintering, pelletizing, and briquetting. Briquetting is the process of compaction of fines into chunks of regular shape with the use of a vibro-press, piston press, extruder, or roller press. By preparing the fine ores by the method of pelletization or briquetting, gaseous emissions are lowered, in comparison with the sintering route. Pelletization process is preferred for the agglomeration of very fine particles, of micron ranges. Hence, briquetting will be the best type of agglomeration for coarser particles. Furthermore, variables such as binder type, binder dose, compaction pressure, mixing, surface characteristics, and size distribution of raw material influence the final quality of the processed briquette.
Nowadays, the iron ore pellet fines are used as a part of the sinter mixture during the sintering process to produce sinter for the blast furnace. In Nordic countries (e.g., Sweden and Finland) where sintering is not available anymore, the pellet fines are recycled into a blast furnace, thereby mixing it with other steel mill residues and briquetting in form of hexagonal shape briquette using a vibro-press. However, the main problem arises as these two uses are becoming obsolete since they are not suitable for the hydrogen-based direct reduction process.
Around 500 kg of waste (solid by-products such as slags) are generated for each ton production of crude steel. Replacement of traditionally used inorganic binders with an organic binder for briquetting of pellet fines will reduce the amount of gangue constituents (such as alumina, silica, and calcium) in the final product, which should be limited to an amount lesser than 2 wt.%. Furthermore, the incorporation of organic binders gives rise to better reducibility even though the strength is compromised. The usage of organic binders contributes to shorter diffusion paths, thereby introducing a greater number of reduction sites in the material simultaneously, whereas the use of organic binder alone is not preferred because of their property of high decomposition rate when subjected to high temperature. Hence, the most plausible way to obtain briquettes that can sustain material attenuation due to attrition is to design the mix with a combination of both organic and inorganic binders.
The potential of agglomerating pellet fines with organic and inorganic binders needs to be analyzed as the first part. Hence, the current work mainly concentrates on the potential of recycling disregarded pellet fines for iron production by agglomerating with an organic binder, without compromising the mechanical and reduction properties of the agglomerates. The effect of reduction with hydrogen is also explored in this study, so as to check whether the produced briquettes fulfill the pre-requisite strength before and after reduction. Furthermore, a way to upscale the briquette production using an extruder is tested and the produced briquettes were subjected to strength and reduction tests.
2. Materials and Methods
2.1. Materials and Sample Preparation
Pellet fines were supplied in powdered form, which required further analysis. The supplied pellet fines were characterized according to their moisture content and the chemical composition of various elements present. By reviewing previous research works and recommendations given by binder developers, six organic binders (see Table 1) along with one inorganic binder (sodium silicate trihydrate, Na2SiO3·3H2O), were selected for further analysis and testing. Primarily, screening of binders should be done to select the most promising binders among the selected ones.
Table 1. Selected binders for briquetting.
|Kempel||Organic||Anionic Polyacrylamide||Kemira, Helsinki, Finland|
|Lignosulfonate||Organic||C20H24Na2O10S2||Borregaard, Sarpsborg, Norway|
|Alcotac CB6||Organic||Polyacrylate (C17H18O6S)||BASF, Heidelberg, Germany|
|Alcotac FE14||Organic||Anionic Polyacrylamide||BASF, Heidelberg, Germany|
|Sodium silicate trihydrate||Inorganic||Na2SiO3·3H2O||Commercial product|
In all experimental research work involving powdered materials, sample preparation is of utmost importance to obtain a consistent mixture. In this work, the supplied pellet fines needed to be prepared to select a representative sample for further analysis. This was achieved by mixing the entire amount of supplied material with the help of an Eirich intensive mixer, which is a compact equipment used to acquire a homogeneous mixture whenever the material is supplied in mass quantity and is difficult to mix the sample manually. Chemical and physical characterization was done for iron ore pellets fines and developed briquettes, as briefly described in the following sub-sections.
2.1.1. Chemical and Phase Analysis
Chemical analysis of the supplied pellet fines was conducted using X-ray Fluorescence (XRF) technique to understand the chemical composition of various elements present in the supplied sample. To check for the emissions from the briquettes, the carbon and sulfur content in the pellet fines was examined using LECO CS230 analysis. Phase analysis was performed using X-ray diffractometer (XRD). A Panalytical Empyrean XRD (Malvern Panalytical B.V., Almelo, The Netherlands) in θ-θ geometry with Cu Kα radiation (λ = 0.154184 nm), a beam current of 40 mA and beam voltage of 45 mV was used to determine the variation in phase composition with respect to reduction extent.
2.1.2. Particle Size Distribution
Particle size largely influences the surface area, compaction, and mechanical properties of the final material. The extent of finer particles influences the degree of densification for the processed briquettes. In the experiments of , it was also proved that the stability and homogeneity of the produced briquettes will be adversely affected while incorporating the use of bigger size fractions (>5 mm) of raw materials during the process of briquetting. Hence, to attain an overview of the size ranges of the sample, a mechanical sieve shaker (Retsch AS200 basic) was incorporated in this work to determine the particle size distribution. To determine the size ranges, sieves of different sizes (0.063, 0.25, 0.5, 1, 2, 2.8, 4, 5.6, 7.1, 10, and 11.2 mm) were stacked upon in increasing order, from bottom to top, and a proper shaking time (5–10 min) was given so as to ensure proper settling down of the powdered sample material.
2.1.3. Moisture Content Analysis
The processability and strength of the developed briquettes largely depend upon the moisture content in them and, hence, it is required to determine the initial moisture content in all of the samples and binders, so as to add up to the total moisture content in the produced briquette. According to the experiment conducted by , the amount of initial moisture present in the sample affects the shrinkage property, which in turn affects the strength of the final product. All of the individual moisture content within each material was determined by precisely weighing and loading it to a Mettler Toledo moisture analyzer (Mettler-Toledo AG Laboratory & Weighing Technologies, Greifensee, Switzerland) with a halogen heating unit.
2.2. Briquetting and Testing
In this study, cylindrical shape briquettes were produced using a hydraulic piston press (Herzog, HERZOG Maschinenfabrik GmbH & Co. KG, Osnabrück, Germany). An equal weight (~20 g) was loaded into the mold (diameter = 20 mm) for each pass. Briquetting was done at different compaction levels (50–200 kN) to investigate the effect of pressure on the briquette strength. Thereafter, each briquette was tested using a hydraulic compression testing machine (ENERPAC Applied Power GmbH, Düsseldorf, Germany) for obtaining their Cold Compressive Strength (CCS) and Splitting Tensile Strength (STS). To measure the compressive and splitting strength of the briquettes, they were placed on a designated metal base and subjected to compression using a mobile probe with a velocity of around 20 mm/min. As the load on the briquette increased, the compressive tester machine automatically recorded the corresponding compression force values in Newtons (N). The briquettes were compressed in the longitude position so as to test for the CCS, while each briquette was compressed in the radial direction during STS measurement. The tests were performed in accordance with ISO 4700:2007 standard. In order to obtain credible strength values, three briquettes were tested for each CCS and STS measurement, which were then averaged. After every compaction of different recipes, disintegrated briquettes were collected and tested for moisture content for further analysis.
2.3. Design of Experiments
Design of Experiments (DoE) is an approach used when conducting an experiment in which several parameters need to be considered and optimized. This experimental study, in turn, deals with many parameters and it is necessary to determine their effects on strength. The program MODDE 13, by Sartorius Stedim Data Analytics AB (Sartorius Lab Instruments GmbH & Co. KG, Goettingen, Germany) was used. In order to set up the DOE, we started by identifying the factors, responses, and limits of the process. The influencing parameters were binder percentage, compaction pressure, and moisture content. Thereafter, parameters of interest were entered, and their respective requirements were entered so as to obtain a sweet spot contour plot. The sweet spot is the area in the plot where all the required criteria are met. DoE analysis, thus, gives the optimum condition for producing a briquette with the maximum compressive and splitting strength. One of the main abbreviations is the use of S.S instead of sodium silicate. Other abbreviations that are being used throughout the workare shown in Table 2.
Table 2. Abbreviations and definition.
|Green Compressive Strength||GCS||Compressive strength just after production|
|Air compressive strength||ACS||Compressive strength after air drying for 7 days|
|Drying compressive strength||DCS||Compressive strength after over-drying for 2 h at 105 °C|
|Green Splitting Strength||GSS||Splitting strength just after production|
|Air splitting strength||ASS||Splitting strength after air drying for 7 days|
|Drying splitting strength||DSS||Splitting strength after over-drying for 2 h at 105 °C|
2.4. Reduction of Developed Briquettes
Reduction progression was monitored using Thermogravimetric analysis (TGA), which was performed using a Netzsch STA 409 instrument (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany). Briquette stability and the amount of volatile component were determined by monitoring the weight change of the samples when heated at a constant rate of 20 °C/min until the temperature reached 950 °C. The experiment was continued until the required reduction extent was achieved (25%, 50%, 75%, or 100%). Hydrogen was made to be the reducing gas and a flow rate of 100 mL/min was selected for the analysis. Interrupted reduction tests were also performed to check the variation in mass loss percentage and strength variation at different reduction extents. Thereafter, strength tests for the samples after the reduction were crosschecked with the required magnitude of compressive strength, that is, 15–20 kg/cm2, as suggested by previous research work.
2.5. Upscaling Trials
Upscaling is of interest when it comes to industrial trials. Upscaling of work is, hence, required so as to confirm that the same promising results can also be obtained during large-scale trials. In this work, large-scale production of briquettes was facilitated with the use of an extruder (Mod. DEX-80, Tallers Felipe Verdés, Barcelona, Spain) using 20 mm steel die for extruded pellets. By making use of an extruder, briquettes with lower compaction pressure can be made at a higher production rate. The recipe made of the binder that possesses the best green compressive strength was selected for the extrusion since the extruded briquettes are highly susceptible to breaking during handling. Recipes were then tested for compression and splitting tensile strength in order to obtain a comparison over the labor-intensive hand press briquetting. Drop test and the reducing behavior for the extruded briquettes were also analyzed at a later stage.
Manu, K., Mousa, E., Ahmed, H., Elsadek, M., & Yang, W. Maximizing the Recycling of Iron Ore Pellets Fines Using Innovative Organic Binders. Materials, 16(10), 3888. https://doi.org/10.3390/ma16103888
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