Iron Reduction from Concentrates of Hydrometallurgical Dressing

The article presents results of theoretical and experimental studies of the processes of iron solid-phase reduction from an iron-containing concentrate obtained as a result of hydrometallurgical dressing of ferromanganese and polymetallic manganese-containing ores with coals of grades D (long-flame) and 2B (brown). The method of thermodynamic modeling using TERRA software complex was used to study the reducing properties of hydrocarbons by calculating equilibrium compositions in the temperature range of 373–1873 K. The authors obtained the dependences of compositions and volume of the gas phase formed as a result of the release of volatile components during heating on the temperature for the coals of the grades under consideration. As a result of thermodynamic modeling, the optimal temperatures and consumption are determined, which ensure the complete iron reduction from an iron-containing concentrate. The results of experimental studies were obtained by modern research methods using laboratory and analytical equipment, as well as methods of statistical processing. Results of the coals analysis carried out using the Setaram LabSys Evo thermal analyzer showed that the process of thermal decomposition of coals of the studied grades proceeds according to general laws. The process of thermal decomposition of long-flame coal proceeds less intensively than of brown coal. The results of an experimental study of the processes of thermal decomposition of reducing agents have shown that volumes of the gas phases, formed when coals are heated to a temperature of 1173 K in an argon atmosphere, practically coincide with the calculated values. As a result of thermodynamic modeling and experimental study, the optimal consumption of D and 2B grades of coal is determined at a temperature of 1473 K. The best reducing agent with a minimum specific consumption is long-flame coal of D grade. When determining the optimal amount of reducing agent in charge mixtures during the study of metallization processes, it was found that with an excess of reducing agent, it is possible to achieve almost complete extraction (98–99%) of iron from the concentrate.


INTRODUCTION
Presently, the modern state and development of metallurgy require introducing new power-and resource-saving technologies, which will partially allow solving the problem of reduction of power and raw material costs in metallurgic industry [1-10] 1, 2 .
One of such promising branches in metallurgy is direct reduction of metals from oxide materials [11][12][13] of both natural and technogenic origin.
For ferromanganese ores of Kemerovo region -Kuzbass, on the basis of theoretical and experimental studies, a two-stage enrichment scheme was proposed, which allows obtaining high-quality concentrates of manganese (58-60 % Mn) and iron-containing concentrates (48-54% Fe) [14]. The pointed concentrates are applicable in metallurgic production.
Development of technologies of direct metal reduction requires preliminary studies in order to determine the optimal conditions of realization of reduction processes, which are accompanied by a complex of physical-chemical transformations occurring under conditions of solid, liquid and gaseous media. STEEL

RESEARCH METHODS AND MATERIALS
Thermodynamic modeling and laboratory researches of the processes of thermal decomposition and the reducers and metallization of the concentrate were carried out in order to determine the optimal conditions of the processes of reduction of iron from the concentrate obtained during hydrometallurgical enrichment of ferromanganese ores.
Determination of the range of admissible values of thermodynamic functions and parameters of complex multicomponent metallurgical systems, where state changes are accompanied by phase, polymorphous and chemical transformations, is a complicated problem as compared with formulations of classical thermodynamics for elementary systems or separate reactions. Such a problem can be solved only by complete thermodynamic analysis, which allows determining equilibrium composition of the system by the properties of its components at given thermodynamic parameters. For this purpose, it is proposed to use the method of thermodynamic modeling based on searching for an extremum of the thermodynamic potential, and namely the maximum of entropy.
As a tool for carrying out computational experiments using the methods of thermodynamic modeling, we selected software complex "Terra" developed in N.E. Bauman Moscow State Technical University and assigned for computing thermodynamic properties and the composition of equilibrium state phases of arbitrary systems with chemical and phase transformations [15,16].
The considered software complex is currently widely used in thermodynamic researches due to high credibility of the obtained data [17,18].
The processes of thermal decomposition of coals were studied using thermal analyzer Setaram LabSysEvo. The methods of thermogravimetry allow determining the change of mass loss during heating of a sample in the given conditions with a high accuracy. Samples of the reducers were heated to temperature of 1373 K at speed of 10 K/min in conditions of free air access.
The volumes and compositions of gaseous phases, formed during heating of the reducers to temperature of 1173 K, were determined during experimental studying of the processes of thermal decomposition of the reducers. The researches were carried out using experimental assembly presented in Fig. 1.
The experimental methodology was concluded in the following. Samples for the experiments were prepared by refining of the reducer and screening fractions from -0.125 to +0.050. The obtained powder with mass of 4 g was placed into a ceramic boat and slightly compacted. The boat 3 with the hinge was placed into the center of quartz pipe 2, after which the system was sealed. Then, the system was blown by a neutral gas at open atmosphere valve 9 during 15 min. Pressure in the system was maintained within the range of 5-15 mm WC according to the indications of the U-shaped manometer 6. After stopping the supply of the neutral gas, the furnace was turned on and heated to a temperature of 1273 K at speed of 20 K/min. The resulting gases displaced the brine solution from the gas collector 5. When the set temperature was reached, the sample was held for 15 min until the water flow into flask 13 complexly stopped, after which the shut-off valves 8 and 12 were closed. Volume of the formed gas corresponded to that of water in flask 13. Temperature for the collected gas was registered by the thermometer 11 and displacement of gas through the dryer into working chamber of the chromatograph was carried out. Analysis of the sampled gases was carried out on chromatograph LKhM-8D. When studying the metallization process, reduction firing was carried out in Tamman furnace at temperatures of 1173, 1273, 1373, and 1473 K and isothermal holding time of 30-80 min.
To study the effect of temperature and duration of reduction firing on the processes of metallization, briquettes were made from iron-containing concentrate, brown, long-flaming coals and a binder. As a binder, we used liquid glass. The content of carbon in the briquette was calculated for the complete reduction of iron from the iron-containing concentrate: The composition of the briquettes is presented in Table 2.
The resulting briquettes (one at a time) were placed in a graphite crucible, after which the crucible with the briquette was covered with a graphite lid. To release the excess amount of gas, a hole was preliminarily made in the cover.
The crucibles with briquettes were placed into the furnace preliminarily heated to the experimental tem- 2 3 Fe O 3C 2Fe 3CO.
perature. When the required holding time was reached, the crucibles with briquettes were removed from the furnace and cooled in the open air. The obtained materials were subjected to chemical analysis.

RESULTS AND DISCUSSION
At first stage, reductive properties of the coals were researched using software complex "Terra" by computing equilibrium compositions in temperature range 373-1873 K. Figure 2 presents dependences of the compositions and volume of gas phase, which is formed as a result of emission of volatile components during heating, from temperature for coals of grades D and 2B. Temperature range 373-1173 K is characterized by decreasing of the volume concentration of water vapor as well as increasing of the concentrations of hydrogen and carbon monoxide СО. At the same temperatures, carbon dioxide CO 2 is present in slight amount in the system, the maximal concentration of which constitutes 0.16 and 0.08 m 3 /kg at temperature of 873 K for coals of grades D and 2B. At temperature of 1173 K compounds Н 2 О, СН 4 and СО 2 completely disappear, the composition of the system is stabilized, and gas phase consists of only Н 2 and СО. The content    Then, the process of solid-phase reduction of iron from the concentrate, obtained as a result of hydrometallurgical enrichment of ferromanganese ores by coals of grades D and 2B, was researched. The computations were carried out for 1 kg of concentrate, coal consumptions varied in the range 0.1-0.7 kg/kg of concentrate. Since the results of thermodynamic modeling of reduction processes obtained earlier showed that the reduction of iron from iron-containing materials occurs at temperatures above 1073 K [14][15][16][17][18], then the researches were carried out in the range from 1073 to 1773 K. The research results showed that the given temperature range does not influence the reduction process. Also, the amount of reduced iron depends only on consumption of the reducer. Dependences of the degree of iron reduction on the consumption of coal of different grades at temperature of 1473 K are presented in Fig. 3, which implies that the required amount of coal of grades D and 2B for complete reduction of iron from 1 kg of ore constitutes 0.30 and 0.55 kg. At these consumptions of reducers in the system, iron oxides disappear. During the increase of coal consumptions above the pointed values, the mass of reduced iron remains constant and constitutes 0.58 kg/kg of concentrate (Fig. 3).
The results of analysis of coals, carried out using thermal analyzer Setaram LabSys Evo, are shown in Fig. 4, and the graphs were plotted using the values of mass change (TG).
The research results showed that the process of thermal decomposition of the researched grades of coal occurs according to the general regularities and  For brown coal, stage II begins at temperature of 716 K, i.e., 60 K lower than thermal decomposition of long-flaming coal. The amount of gaseous products emitted during thermal decomposition of brown coal is significantly larger than during decomposition of long-flaming coal. Mass loss of brown and long-flaming coals constituted 90.6 and 84.5%, respectively. The process of thermal decomposition of long-flaming coal is less intensive than of brown coal.
The results of experimental studying of the thermal decomposition processes of the reducers showed that the gas phase volumes formed during heating of 1 g of reducers to 1173 K in the atmosphere of argon constitute 1.20 and 0.85 L for brown and long-flaming coals, which almost coincides with the computed data (Fig. 2). The compositions of gas phases are presented in Table 3.
Analysis of the gases showed that the coal phase mainly consists of hydrogen. The largest amount of hydrogen is contained in the gas phase of brown coal and the largest amount of carbon monoxide CO is contained in gas phase of coke breeze. Gases formed during thermal decomposition of coals mainly consist of the mixture of H 2 and CO (over 80%), gas of such a composition has a high reduction potential. Gases of close composition (H 2 + CO = 90-92%) are obtained during catalytic conversion of natural gas by technologies Midrex and HyL III, which allows assuming that along with solid carbon, the formed gases will also participate in the processes of reduction of iron from oxides [19].
The research results on the determination of the optimal amount of the reducer in furnace-charge mixtures during studying the processes of metallization (Table 4) showed that in excess of the reducer almost complete (98-99%) extraction of iron from the concentrate can be reached.
The optimal correlations of the components ironcontaining concentrate : coal : binder in furnacecharge mixture for briquetting using brown or long-   CONCLUSIONS Reduction properties of coals (long-flaming of grade D and brown of grade 2B) were researched using software complex "Terra" by computing equilibrium compositions in temperature range 373-1873 K, dependences of the parameters of equilibrium composition of gas phase on temperature during heating of coals of grades D and 2B were determined. The process of thermal decomposition of coals during heating was studied. It was established that the process of thermal decomposition of coals of the considered grades occurs according to the general regularities. At the same time, the process of thermal decomposition of long-flaming coal is less intensive as compared with brown coal. The optimal consumptions of coals of grades D and 2B at temperature of 1473 K were determined as a result of thermodynamic modeling and experimental studying of the process of reduction of iron from iron concentrate. The best reducer with minimal specific consumption is long-flaming coal of grade D. When determining the optimal amount of a reducer in furnace-charge mixtures when studying metallization processes, it was found that with an excess of a reducer, it is possible to achieve almost complete (98-99%) extraction of iron from the concentrate.