Study of the main metallurgical characteristics of iron ore raw materials (sinter and pellets)

The paper presents the results of studies of the chemical, phase composition and metallurgical characteristics of titanomagnetite sinter. The iron ore used in blast furnaces of JSC ‘EVRAZ NTMK’ is titanomagnetite sinter obtained from ores of the Gusevogorsky deposit. Samples of sinter with different basicities as well as with addition of binding polymers in the amount of 300 and 500 g per ton of sinter were investigated. The results of industrial tests of the production and blast furnace smelting of sinter with different basicities and additives of binding polymers are presented on the example of the operation of blast furnaces no. 5 and 6 of JSC ‘EVRAZ NTMK’. It was shown that an increase in the basicity of the sinter from 2.1 to 2.4 and the introduction of a polymer binder (in the amount of 500 g per ton of sinter) positively affect the complex of sinter metallurgical characteristics – durability after reduction, reducibility, softening and melting temperatures, and also decrease the coke rate in blast furnace smelting by 1.0–1.2%.


Introduction
The consumption of stocks of traditional magnetite raw materials forces us to reconsider our attitude to metal production methods and complex ore processing schemes, taking into account market demands and the use of regional resources. Ores with a low titanium content (Gusevogorsky deposit) are processed according to the metallurgical scheme, including the smelting of pig iron in blast furnaces of JSC 'EVRAZ NTMK'.
The use of Ural titanomagnetites consists in the efficient provision of high-quality raw materials for blast furnace production of JSC 'EVRAZ NTMK'. Raw materials processing technologies continue to develop continuously [1], therefore, those measures that are aimed at producing sinter with improved metallurgical characteristics are of particular interest [2][3][4].

Chemical and phase compositions of titanomagnetite sinter
Samples of titanomagnetite sinter with a basicity of 2.1 were selected for the study. The chemical composition of the studied samples of sinter is given in Table 1. The designation of the samples is as follows: 1sinter of current production (sinter machine no. 1); 2sinter (lower part of the pallet, sinter machine no. 1); 3sinter (middle part of the pallet, sinter machine no. 1); 4sinter (upper part of the pallet, sinter machine no. 1). Figure 1 shows a typical diffraction pattern of samples of sinter. The main phase in sinter no. 1-4 is Fe3O4 (magnetite), Fe2O3 (hematite) is contained in a smaller amount, and Ca-containing silicate of a complex composition -Ca2,3Mg0,8Al1,5Fe8,3Si1,1O20 -is also well manifested (named SFCA in  [5][6][7]). The sinter contains γ-Ca2SiO4 (dicalcium silicate). In samples of sinter with a basicity of 2.1, the increase in the unstabilized γ-Ca2SiO4 phase goes from the top of the layer to the bottom. Since the presence of this phase leads to disintegration of the initial sinter, a similar pattern is expected during further experiments on reduction and durability after reduction.  Diffractogramm of sinter of current production: v -Fe3O4; + -Fe2O3;z -Ca2,3Mg0,8Al1,5Fe8,3Si1,1O20; o -γ-Ca2SiO4.

Sinter with basicity 2.4
Samples of titanomagnetite sinter with increased basicity of 2.4 were selected for the study. The chemical composition of the sinter is shown in Table 1. The designation of the samples is as follows: 5sinter (sinter machine no. 2); 6sinter (lower part of the pallet); 7sinter (middle part of the pallet); 8sinter (upper part of the pallet). Figure 2 shows the characteristic diffractogramm of samples of the sinter. The main phase in sinter no. 5-8 is Fe3O4, Fe2O3 is contained in a smaller amount, and Ca-containing silicate of a complex composition -Ca2,3Mg0,8Al1,5Fe8,3Si1,1O20-is also well manifested. There are small amounts contain γ-Ca2SiO4 (dicalcium silicate). In sinter samples with a basicity of 2.4, the increase in the unstabilized γ-Ca2SiO4 phase is opposite from the bottom of the layer to the top.   Figure 2. Diffractogramm of sinter with basicity 2.4: v -Fe3O4; + -Fe2O3;z -Ca2,3Mg0,8Al1,5Fe8,3Si1,1O20; o -γ-Ca2SiO4.

Sinter with additives of binding polymers 300 and 500 g/ton of sinter
Samples of titanomagnetite sinter with increased basicity of 2.4 and additives of binding polymers in the amount of 300 and 500 g per ton of sinter were selected for the study.
The chemical composition of the sinter is shown in Table 1. The designation of the samples is as follows: 9sinter with the addition of 300 g/ton of sinter (upper part of the pallet); 10sinter with the addition of 300 g/ton of sinter (middle part of the pallet); 11sinter with the addition of 300 g/ton of sinter (lower part of the pallet); 12sinter with the addition of 500 g/ton of sinter (upper part of the pallet); 13sinter with the addition of 500 g/ton of sinter (middle part of the pallet); 14sinter with the addition of 500 g/ton of sinter (lower part of the pallet).
As a polymer additive, a line of additives of the 'Thermoplast SV' series of the company 'Polyplast -UralSib' specialized for mining and processing plants was used [8].

Metallurgical characteristics of sinter
The results of studies of Low Temperature Index (LTD+6.3), also called Durability after reduction, in accordance with ISO 13930 [9], the temperature range of sinter samples in accordance with State Standard of Russian Federation no. 26517-85 [10], and sinter reducibility in accordance with State Standard of Russian Federation no. 17212-84 [11] are shown in Table 2.
On the whole, according to the samples taken from the middle and lower layers, the durability of the sinter (with a polymer additive content of 500 g per ton of sinter) significantly improved. The reduction of the sinter with a polymer additive leads to the formation of metallic iron, wustite (FeO), partially magnetite (Fe3O4). Silicate phases are formed against the background of iron-forming phases: β-2CaO·SiO2 and 2CaO·Al2O3·SiO2. Also, 3CaO·Al2O3 is weakly manifested in the samples. Tables 3-4  The metallurgical characteristics of the sinter changed in stages as follows (Table 5). Attention should be paid to the uneven distribution of the quality indicator along the height of the layer; the sinter from the upper layer has the least value. An increase in the basicity of the sinter also led to an increase in 'hot' durability, which is confirmed by published data. In particular, in paper [13][14][15], an increase in the hot durability of the sinter is explained by an increase in the amount of the SFCA phase with an increase in basicity.

Results of industrial tests of production and blast furnace smelting of sinter with different basicity and additives of binding polymers [12]
The results show a decrease in the position of the cohesion zone down in height and a narrowing of the temperature range, which should generally have a positive effect on the operation of the blast furnace.  The addition of a polymer binder in the amount of 500 g/ton to the sinter burden has a more significant effect on the quality of the sinter than the addition of 300 g/ton. The durability of the sinter during reduction significantly improved in all three indicators. So, LTD+6.3 amounted to 39.9 % against 11.01 % in the base period. Attention should be paid to the uneven distribution of quality indicators along the height of the layer; the sinter from the upper layer has the least value. The reducibility of the sinter with the addition of a polymer binder in an amount of 300 g/ton, and especially in an amount of 500 g/ton, increased to 64.9-69.61 %. This indicator is close to recommended for the blast furnace process. The results of determining the softening and melting temperatures for sinter with polymer additives show a decrease in the cohesion zone down in height and a narrowing of the temperature range, which will generally have a positive effect on the operation of the blast furnace.