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The current stage of development of the ferrous metallurgy is marked by special attention to decarbonization as a key factor in reducing CO2 emissions. The report of the Intergovernmental Panel on Climate Change (IPCC [Key World Energy Statistics, n.d.]) in 2018 set a goal to limit global warming to 1.5 °C by 2050.
Achieving this goal is largely due to the speedy introduction of the best available technologies (BAT) into metallurgical practice. The level of reduction of specific emissions in the steel industry by the introduction of BAT is estimated at 15–20% (from 1.8 to 1.44 t CO2/t of steel [Holappa, 2020]).
The greatest potential for reducing emissions from full-cycle steel mills comes from replacing sinter in the blast furnace charge with briquettes. The effectiveness of the synergy of briquetting and sintering with partial replacement of sinter with briquettes in a blast furnace charge was studied in detail for the first time in Bizhanov (2022).
An important role in the transformation of modern ferrous metallurgy into a green one is called upon to play cold (non-firing) briquetting. The manufacture of briquettes does not involve the consumption of carbon, in contrast to traditional industrial agglomeration technologies whereas 35–45 kg of coke breeze is consumed per 1 ton of sinter, and 18–25 m3 of natural gas is consumed for the production of pellets. Among the criteria for belonging to BAT, one of the most important is the almost complete absence of harmful emissions in the production of briquettes.
Briquetting will not lose its relevance in the realities of green metallurgy due to the intensive pyrometallurgical nature of the main metallurgical technologies, which inevitably lead to the formation of fine materials that are promising for recycling (dust, sludge, fines, etc.). Recycling of such materials without their preliminary agglomeration is impossible. The largest mining and metallurgical company VALE announced plans for the early commissioning of briquette factories with a capacity of 6 million tons of briquettes from hematite iron ore concentrate both for blast furnaces and DRI reactors in 2024.
The reducibility of the briquette is a key factor in obtaining a product with the desired metallurgical properties. In turn, the reducibility is largely determined by the porosity of the briquette material.
In most studies known from the literature, the porosity of briquettes is investigated depending on the magnitude of the applied pressure. It is clear that the amount of pressure applied in the production of briquettes manifests itself differently in different briquette technologies. Thus, in vibropressing, the briquetted mixture is practically not subjected to compression, and the compaction process is carried out due to fluctuations in the viscosity of the mixture synchronously with the vibration phases due to the properties of thixotropy manifested by the hydrated binder. The values of the applied pressure are insignificant (0.2 MPa) and may not be taken into account in extrusion briquetting, the pressure in the working chamber of the extrusion extruder reaches values of 3-4 MPa, which is an order of magnitude less than the pressure values typical for roll briquetting (from 10 to 100 MPa).
High values of pressure exerted on the mixture briquetted by the roller press can significantly change the structure of the briquette components, the size and shape of their particles, which can cause both a decrease in porosity and its growth due to the appearance of internal cracks.
Speaking in general about the study of the metallurgical properties of briquettes, including porosity, an important remark should be made concerning the incorrectness of projecting the results of the study of laboratory briquettes, in the vast majority of cases manufactured by means available in the laboratory that have little in common with the main types of commercial briquette technologies, to the specific conditions of industrial briquetting process. A significant number of scientific studies of the briquetting process are based on the study of the properties of laboratory briquettes made by lever or piston presses, which obviously does not allow us to identify the features of the metallurgical properties of briquettes based on the studied mixtures during their manufacture by one or another briquetting equipment (roller, press, vibropress or extruder).
The method of manufacturing has a significant effect on porosity and, consequently, on the reducibility and metallization of briquettes.
The main contributions of the paper are as follows.