Investigation of the influence of lime content in the charge and its quality on the sintering process of iron ore raw materials

Main objectives of the work was to study the influence of degree of roasting of lime contained in the charge on the sintering process and sinter quality, as well as comparing the impact of changes in the mass fraction of active CaO in the charge by varying degrees of burning lime, and by varying the content of lime in the charge. It is shown that the intensifying effect of lime on the sintering process occurs as a result of a decrease in the content of small classes in the charge. When the lime content in the charge is equal to 3 %, an increase in the degree of lime firing from 5 to 96 % led to an increase in the specific productivity of the plant from 0.96 to 1.87 t/(m 2 · hour). The independence of sintering parameters from the content of active CaO in lime with its constant content in the sinter charge and the decisive role of the content of active CaO in the sinter charge with its constant content in lime are confirmed. A new approach to the production of lime for agglomeration processes is proposed: it should have the lowest cost of the active CaO contained in it.


Introduction
One of the conditions of the sustainable future is the quality improvement and the increase in production of ferrous metals, the vast majority of which are molten by two-stage scheme "cast iron -steel".Therewith in the worldwide iron making industry, sinter constitutes the major component in the metallic burden of most of the blast furnaces as it has numerous properties which lead to reduced coke rate, increased productivity, better heat transfer and permeability, elimination of raw flux, narrowing of softening and melting zones and better quality of hot metal.The sintering process is greatly influenced by the physico-chemical properties of raw materials used, whose characteristics change from time to time [1].
Lime is an important component of sintering charge.It provides the intensification of sintering and improves the quality of sinter.Therefore, the issue of the efficiency increasing of lime usage in the sintering production demands constant attention from workers and researchers.Thus, in Bhilai Steel Plant, Steel Authority of India Limited, a pilot sintering unit has been established in order to conduct experiments for subsequent application in industrial sinter manufacturing.Studies were taken up to assess the effect of quicklime addition in sinter-mix on sintering process of iron ore fines [1].The results found have been encouraging with significant increase in productivity.
Efficiency of lime's utilization in iron ore materials' sintering depends on number of factors, the most important of which are the composition of sinter charge, the quality and quantity of used 1254 (2023) 012001 IOP Publishing doi:10.1088/1755-1315/1254/1/012001 2 lime.A direct relationship is usually observed between the productivity of sintering machines and the consumption of lime, however, at high consumption of lime, a decrease in the strength of the agglomerate and an increase in the amount of fines in it are noted.In fact, to ensure high productivity of sintering machines and sinter quality, the optimal lime consumption should be determined individually, depending on the component composition of the sinter charge, the properties of charge materials and the features of the processing line for preparing the charge, starting from the averaging warehouse.
The decisive factor that determines the optimal consumption of lime introduced into the sinter batch is the mass fraction of the assimilated charge of lime, that is, slaked and passed the stage of dispersion.The influence of this factor is primarily associated with the technology of moistening the charge and with the design and technological parameters of the used mixing equipment [2].
At the same time, the mass fraction of the lime assimilated by the charge depends on the content of active components in the lime -calcium and magnesium oxides, that is, on the quality of the lime.The required quality of metallurgical sinter is provided by the valid choice of the raw material with optimal chemical and granulometric composition and specific selection of tackle [3].When choosing a kiln and assigning thermal modes, it is necessary to be guided by the results of control calcinations of lime and analyzes of the batching capacity of lime.
The core indicator of a lime's quality is the content of active CaO, which determines the degree of lime burning.It is acknowledged [4], that lime, which is used in sintering for the intensification of sintering thin iron ore concentrates, must contain not less than 80 % of CaO + M gO, and the size of seeds must be 3 -0 mm.
With the aim of achieving required lime size in the conditions of sintering production OJSC "Ural Steel" it has been suggested [5] to upgrade the lime preparation section via closed cycle actualization of lime grinding with an installation of a hammer mill and screening machine (to separate lime into fractions -3 and +3 mm).
The CaO content in lime is sometimes [6] called a purity degree of lime, which was received after the limestone burning.This marker depends not only on the degree of dissociation of Calcium Carbonate but also on the degree of purification of initial limestone.

Literature review
The laboratory sinter burnings in Raw Materials Research Center OJSC "Severstal" with the use of lime showed [7] that the dependence of charge pelletization degree on the content of lime in it is extreme.The dependence of the charge sintering rate on the lime content in it also has an extreme nature.The greatest rate of sinter charge which is equal to 16.65 mm/min is obtained by the lime consumption of 28 kg/ton.
In the laboratory and manufacturing conditions the optimal consumption of lime into PJSC "Novolipetsk Metallurgical Plant" (NLMP) sinter charge was determined with consideration of the process of lime hydration and the difference between the amount of water and supreme molecular moisture capacity [2].It was determined that the exceedance of optimal liming level deteriorates a sinter quality.
The research of the lime mass fraction in JSC "ArcelorMittal Temirtau" sinter charge impact on the process of its pelletization research showed [8] that increasing lime content in charge from 1 to 5 % a percentage of thin fraction in charge 1.6-0 mm which influences negatively on the sintering speed decreases from 18 to 5 %.
Theoretical and technological incentives to use lime in the sinter charge were examined in detail in monographies [9,10].The presented results of the impact of lime's quantity in charge correspond to the above-mentioned data.
A significant lime's qualitative characteristic is its reactivity, in other words -rate of hydration that depends on the lime's method of firing and the content of the environment's gaseous state composition.The work [11] demonstrates that industrial furnace calcining largelimestone pellets leads to low reactivity quicklimes both because the calcination temperatureis too high and the CO 2 pressure is too elevated.
To improve a firing thermal behavior on a conveyor it has been suggested [12,13] to use definite proportions of classified lime fractions and solid fuel.In the conditions of PJSC "Metallurgical Plant "Zaporizhstal" the use of improved technology will improve the technical and economic performance of the limestone burning process.
To evaluate the hydration characteristics of the lime, the Constant-temperature Calorimetric Method (CCM) has been used to test four types of lime from the sintering plant, and the granulation and sintering pot test was conducted to validate the results of CCM [14].It showed that the CCM can precisely test the hydration heat and hydration speed of the lime and avoid the shortage of the tradition ways, and the hydration speed of lime is positively related to granulating effect and sintering productivity.The hydration properties of the four types of lime were remarkably different, the whole hydration heat is positively related with the CaO content of the lime, but the hydration speed is more influenced by the impurities and the microstructure of the lime.In this case, the increasing of the SiO 2 and M gO content can reduce the hydration speed of the lime, and the Al 2 O 3 has two way effects.The lime with fine crystal grain size has a large hydration speed.
As can be seen from the above the analysis demonstrates that the lime's impact on a sintering process depends on the charge conditions and the quality of lime, but the reciprocal proportion's impact of this factors have not been examined yet.

Problem formulation
Based on the above, in this work were set following tasks: • to study the impact of the lime burning degree on a sintering process and sinter quality; • to compare the impact of CaO in charge based on the lime burning degree change and lime content change in charge.

Methods
The impact of the lime burning degree on a sintering process and a sinter quality from ArcelorMittal Kryvyi Rih sinterplant's charge was examined in laboratory settings.The chemical composition of charge components is listed in table 1.The quicklime with the grain-size 3-0 mm, which was obtained from limestone burning in an experimental-industrial cyclone furnace was used.The different lime burning degree was obtained by the mode change of furnace.
Mixing and pelletization of charge with the simultaneous moistening up to 7.8 -8.2 % were proceeded in a drum 0.6×1.0m in size during 8 min.Pelletized charge was burned in a sintering bowl with an area of 0.132 m 2 , in a form of layer of 280 mm in height.
The rarefaction under the grate of the sintering bowl in a sintering process was maintained permanent and equal to 11.28 kPa (∼ 1150 mm water column).The preparation conditions of the sinter charge and the mode of its sintering during the research were the same.Each experiment, which included assembling, preparing and sintering the sinter charge, was repeated 3-5 times.
In order to compare the impact of the active CaO content in lime, at a constant flow rate in the charge, sintering was performed with the same types of lime, setting them in the charge in the same amount -3 % of the charge's weight.In this set of sintering, the content of active CaO in sinter charges was 0.71; 0.93 and 1.58 %, respectively.
To test the assumption of the determining impact on the sinter process of the amount of active CaO which is being added by lime into the charge, regardless of the lime burning degree, sintering was performed at a constant content of active CaO in the sinter charge, which was equal to 1.26 %.Herewith was used a lime with a degree of firing of 40, 50, 75 and 96 %, the content of which in the sinter charge in each case was 5.3; 4.1; 2.4 and 1.6 %, respectively.

Analysis
The main results of sintering with lime content in the sinter charge of 3 % at different burning degrees are shown in figures 1 -5.
The intensifying effect of lime on the sintering process is associated, first of all, with a decrease in the gas-dynamic resistance of the pelletized charge layer due to a decrease in the content of small classes (figure 1) and an increase in the strength of individual pellets.This is due to the viscous properties of lime, hydration hardening of which is carried out according to the scheme of dissolution -colloidization -crystallization.When the content of lime in the sinter charge of 3 % increase in the degree of firing of lime from 5 to 96 %  (corresponding to an increase in the content of CaO active in the charge from 0.08 to 2.30 %) led to a decrease in the content of classes 5 -0 and 3 -0 mm in the lump charge from 70.7 to 62.7 % and from 48.5 to 42.8 %, respectively.The equivalent diameter of the granules of the pelletized charge, calculated as a weighted average by weight, increased from 4.38 to 5.14 mm (figure 2).
A similar result of the effect of the lime content in the charge on the sintering performance was obtained in [15], in which the positive effect of adding up to 2 % quicklime to the sinter charge on its gas permeability was experimentally established.A further increase in the lime content in the charge no longer improves its pelletizing.The lower value of the maximum allowable lime content in the charge in [15] compared to the conditions in this study is explained by differences in the composition of the ore part of the sinter charge.
The decrease of the sinter charge gas-dynamic resistance leads to an increase in the specific air flow rate on sintering, as a result of what the vertical sintering rate and the specific productivity of the sinter plant continuously increase (figure 3).
The sintering rate during the experiments changed more than twice, and the specific productivity -from 0.96 to 1.87 t/(m 2 •hour).That is, for each percent of the lime burning degree increase, the specific productivity of the installation increased by an average of almost 0.96 %.A slightly smaller change in the specific productivity compared to the sintering rate is associated with the influence of variable when using lime of different bulk weight burning degrees of the pelletized charge and the yield of a suitable sinter after sintering.
The yield of the +5 mm class after double dumping of the sinter pie from a height of 2 m while increasing the lime burning degree tends to decrease (figure 4).The dependence of test results of sinter in the drum on the lime burning degree is extreme (figure 5), which is obviously caused by a change with the increase in the degree of calcination of lime, the ratio between the development of processes that have the opposite effect on the strength of the sinter.
Comparison of the results obtained with similar studies of obtaining agglomerate with basicity (CaO/SiO 2 ) equal to 1.7 under the conditions of PJSC NLMK [2] confirms the extreme nature of the dependence of the cold strength of the agglomerate on the mass fraction in the charge of active components of lime (or the content of lime in the charge).The highest strength is obtained when lime is introduced into the charge in an amount of 25 -30 kg/t of sinter.
Factors that increase the strength of the sinter with increasing the lime burning degreeimproving the conditions of mineral formation in the heating zone and improving the conditions of melt formation, reducing the melt viscosity, increasing the sintering temperature due to reduced heat consumption for decomposition of calcium compounds (F eO content in the agglomerate with increasing the lime burning degree increased from 10.0 to 13.1 %) -prevail at the lime burning degree up to about 40 %.The sinter obtained with the use of lime with a burning degree of 35 -45 % has the best indicators of strength, both on impact and on abrasion.A further increase in the lime burning degree causes a decrease in the sinter test performance in the drum due to the predominant effect on the cooling rate of the sinter, which increases simultaneously with the vertical sintering rate, that is accompanied by an increase in vitreous phases and internal stresses in the sinter structure.Thus, increasing the lime burning degree from 40 to 96 % led to a decrease in the resistance of the sinter to impact from 57.6 to 51.9 % and abrasion from 7.9 to 12.2 %.
It should be borne in mind that another reason for the decrease in the strength of the agglomerate when a certain level of lime consumption in the charge is exceeded may be an insufficient degree of assimilation of the active components by the lime charge.This is due to the emergence of a water deficit in the charge not only for hydration of lime at the stage of preparing the charge for pelletizing, but also for pelletizing the charge [2].
The main results of the experiments confirm the independence of sintering indicators from the content of active CaO in lime at a constant content in the sinter charge (figure 6, figure 7) and the decisive impact of the active CaO content in the sinter at a constant content in lime (figure 7, figure 8).
At the same time, with an increase in the active CaO concentration in the sinter charge in the range of 0.71 -1.58 %, the sintering rate and specific productivity continuously increase (figure 8), as well as the drum testindicators decrease (figure 9).This is due to the complex nature of the impact of active CaO in the sinter charge on the strength of the sinter.With the increase of active CaO in the sinter charge simultaneously with the development of processes that increase strength, the development get the processes that reduce it; in particular, the number of vitreous phases and the level of internal stresses in the  structure of the sinter increases as a result of increasing the cooling rate.Obviously, in the studied interval of increasing the active CaO content in the sinter charge at a given composition of the charge, the mode of its preparation and sintering, the processes that reduce the strength of the sinter predominate.

Discussion
In the article of Chinese researchers [16], the results of the study are presented, which complement the above-described mechanism of the intensifying effect of quicklime on the sintering process from the position of changing the structure of the sinter.The effect of quick lime on pore characteristics of the high-temperature zone in the iron ore sinter was studied.The equivalent spherical diameter of the pore and its distribution frequency, volume fraction, sphericity, and surface porosity were statistically analyzed.The results indicate that the addition of quick lime increases the permeability of the green bed and shortens the sintering time.With the increase of the addition of quick lime, the porosity of the sinter decreases gradually.1With the increase of quick lime, the total pore volume and total pore length, the number of pore segments, branching nodes and terminal nodes all decrease.
Comparison of the obtained results with the literature data about the impact of lime content in the sinter charge with a constant burning degree on the sintering process [6,7] indicates a similar nature of changes occurring in these cases, that is, when the content of active CaO in the charge is changes.
At the same time, from the standpoint of lime on the sintering process the burning degree change can be replaced by a corresponding change in its content in the charge and vice versa.
Since common in both cases is the change in the amount of active CaO by lime which is being added into the sinter charge, this particular indicator (or the amount of Ca(OH) 2 in the case of slaked lime) determines the effectiveness of lime under the other equal conditions.It can be noted that with the increasing lime content in the sinter charge, the lime burning degree, which corresponds to the maximum strength of the sinter, will decrease, and with the decreasing lime content -will increase.

Conclusions
Studies have shown the high efficiency of using lime as an intensifier of the sintering process.An increase in the amount of active CaO added by lime to the charge from 0.71 to 1.58 % leads to an increase in the specific productivity of the sinter plant from 0.96 to 1.87 t/(m 2 •hour).The mechanism of the lime intensifying effect is associated with its positive impact on the process of charge pelletisation (reduction of the proportion of small fractions of 5-0 and 3-0 mm in the pelletised charge), which is accompanied by a decrease in the gas-dynamic resistance of the charge and a corresponding increase in the vertical sintering rate and specific productivity of the sinter plant.
On the basis of the conducted researches it is possible to draw a conclusion that the efficiency of the limestone replacement by lime at sintering is defined by the quantity of active CaO brought by lime in a sinter charge, and in this sense practically does not depend on the lime burning degree or its concentration in charge (with a constant mesh-size distribution of lime, permanent degree of quenching in the process of preparing charge to the sintering, constant lime distribution degree in charge and other equal conditions associated with the use of lime).
A similar pattern occurs when using lime as a binder in the production of pellets.This implies a new approach to the production of lime for pelletizing processes, which should be based on the technical and economic indicators of the unit for the production of lime.It is necessary to produce such lime, which has the lowest cost of active CaO contained in it.This means that it does not make sense to produce lime for pelletizing processes with a high burning degree, because the closer to the maximum possible burning degree, the more heat a single particle of calcium carbonate needs for decomposition.The rational degree of burning is usually 60-70 %.
Given the increase in the relative heat consumption for the decarbonization of CaCO 3 with increasing degree of its firing, it should be considered impractical to produce for the process of agglomeration of iron ore materials lime with the highest degree of firing.The optimum degree of lime firing should be about 60-70 %.The validity of this conclusion is confirmed in

Figure 1 .
Figure 1.The influence of the lime burning degree on the indicators of palletization: content of class -3 mm ( ) and -5 mm (•) in the pelletized charge, %.

Figure 2 .
Figure 2. The influence of the lime burning degree on the equivalent diameter of pelletized pellets.

Figure 4 .
Figure 4.The influence of the lime burning degree on the indicators of the strength of the sinter after two resets.

Figure 5 .
Figure 5.The influence of the lime burning degree on the indicators of the strength of the sinter after the drum test: output class +5.0 mm ( ) and −0.5 mm ( ) after the drum, %.

Figure 6 .
Figure 6.The impact of the active CaO content in lime on the sintering process at a constant content of active CaO in the sinter charge (1.26 %): sintering speed, mm/min (•); specific productivity, t/(m 2 •hour) (•).

Figure 7 .
Figure 7.The impact of the active CaO content in lime on the indicators of the strength of the sinter after the drum test at a constant content of active CaO in the sinter charge (1.26 %): output of class +5.0 mm after the drum, % (•); output of class −0.5 mm after the drum, % (•).

Figure 8 .
Figure 8.The influence of the content of active CaO in the sinter charge on the sintering process at a constant content of lime in the charge (3.0 %): sintering speed, mm/min (•); specific productivity, t/(m 2 •hour) (•).

Figure 9 .
Figure 9.The influence of the content of active CaO in the sinter charge on the strength of the sinter after the drum test at a constant content of lime in the charge (3.0 %); output of class +5.0 mm after the drum, % (•); output of class −0.5 mm after the drum, % (•).

Table 1 .
The composition of charge materials.