RU2840660C1 - Method of controlling carbon content in metalized pellets - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgical industry and can be used to produce a metallised product with possibility of controlling carbon content independent of the degree of metallisation. When carbon content in metalized pellets is controlled, oxidised pellets are treated with chalk suspension and their further metal coating in shaft furnace. Wherein coal dust is added to chalk suspension at the following ratio of components, in wt.%: water 93.51–95.02, calcium oxide 3.31–3.36, coal dust 1.62–3.18. Thereafter treatment of oxidized pellets by produced suspension is performed within the range of time up to 20 minutes.

EFFECT: enabling improvement in the metalized pellets quality, increase in the metallization degree, as well as the Fe_met and C weight fraction.

1 cl, 3 tbl

Description

The invention relates to the metallurgical industry and can be used to obtain a metallized product with the ability to regulate the carbon content regardless of the degree of metallization.

The steel industry is a major emitter of _CO2 . This is because steel production is primarily carried out from iron ore by reducing the iron oxides contained in the iron ore with a carbon-containing reducing agent. It is also energy intensive, much of which comes from fossil fuels. Globally, the steel industry accounts for 31% of industrial _CO2 emissions, accounting for 4-5% of anthropogenic greenhouse gas (GHG) emissions.

In the case of direct reduction of iron in a shaft furnace, such as the Midrex® technology, the reducing gas mixture consisting of carbon monoxide and hydrogen is obtained by reforming natural gas. Carbon does not play a leading role in this process, but increasing the hydrogen/carbon monoxide ratio has a significant effect on the heat balance of the process and the rates of reduction reactions. In particular, there are good reasons to believe that carbon monoxide in the technology can be replaced by hydrogen.

Further development of the metallurgical industry involves the use of hydrogen instead of natural gas in the production of direct reduced iron to reduce the negative impact on the environment. This will inevitably lead to a decrease in the mass fraction of carbon in the metallized product, the main source of which is _CH4 and CO. The production of direct reduced iron (DRI) with 0% carbon content will have a negative impact on the technical and economic indicators of the subsequent processing, namely, when smelting steel in an electric steelmaking shop.

Currently, at metallization units No. 1-4 (shaft furnaces) of the Oskol Electrometallurgical Plant (OEMK), designed using Midrex® technology in 1983-1987, the carbon content in metallized pellets is regulated using natural gas (CH ₄ ), which is added to the DRI in three zones:

1. In the reduction zone, where the reduction of iron oxides with hydrogen and carbon monoxide occurs. However, some of the carbon obtained from _CH4 and CO is added to the reduction zone. The reaction of metallic iron with natural gas to form iron carbide is endothermic, the reaction of metallic iron with carbon monoxide to form iron carbide is exothermic.

3Fe+СО+ _Н2 → _Fe3C + _H2O

3Fe+2СО→Fe ₃ C+CO ₂

3Fe+ _СН4 → _Fe3C + _2Н2

Carbon formation from CO is inefficient from the reduction process point of view because it uses the reducing agent CO to form the oxidizing agent _CO2 . Carbon formation from _CH4 is efficient because the reaction product is an additional reducing agent _H2 . Increasing the methane concentration in the reducing gas increases the carburization potential in the reduction zone. This is a practical method of controlling carbon. Experience has shown that a 0.1% increase in the methane content of the reducing gas will increase the carbon content of the metallized pellets by approximately 0.1%.

2. A regulated flow rate of natural gas is supplied to the intermediate zone, which is the main tool for supplying and controlling the carbon content in the Midrex® DRI. Natural gas-based fuel in the Midrex® technology contains hydrocarbons, mainly methane, an increase in the carbon content in metallized pellets is obtained as a result of the following chemical reactions:

3Fe+ _CH4 → _Fe3C + _2Н2

CH ₄ →C+2H ₂

These carbon-generating reactions are endothermic and cool the DRI, which is the objective for cold direct reduced iron (CDRI) plants, but not the objective for hot direct reduced iron (HDRI) and hot briquetted iron (HBI) plants. Injecting natural gas into the intermediate zone is an effective way to control the carbon content of the metallized pellets. Injecting natural gas into the intermediate zone has a significantly greater effect on the carbon content of the metallized pellets than injecting it into the cooling zone, due to the fact that the methane concentration is much higher and it is injected at a higher temperature. Natural gas in the intermediate zone, however, promotes an updraft, which greatly reduces the temperature in the shaft, thereby causing a decrease in the metallization degree or the discharge of metallized pellets.

3. In the cooling zone to reduce the temperature of the discharged DRI with cold discharge. The cooling gas contains hydrocarbons, and carbon is added in a manner similar to that described for the intermediate zone.

Experience has shown that the carbon in the finished product is affected by the amount of hydrocarbons (primarily _CH4 natural gas) in the quench gas. Hydrocarbons react with iron to form iron carbide during quenching. Since the quench gas circulates in a closed loop, the addition of natural gas allows a corresponding amount of the quench gas/natural gas mixture to bypass the quench gas boxes, causing a portion of the quench gas to flow upward into the intermediate and reduction zones. This is called "upflow." Upflow of quench gas entering the intermediate and reduction zones increases the carbon content of the metallized pellets because iron carbide forms more rapidly at the higher temperatures in these areas. Carbon formation is limited, however, by the fact that upflow reduces the temperature in the shaft, thereby causing a reduction in the degree of metallization or discharge of the metallized pellets.

The disadvantages of the specified regulation of carbon content in metallized pellets using _CH4 using Midrex® technology are:

1. Increasing the consumption of natural gas reduces the temperature in the mine, thereby causing a decrease in the degree of metallization.

2. Reduction of the degree of carburization of metallized pellets at temperatures below 500°C.

3. Increasing the temperature of the reducing gas allows for increased productivity, but reduces the carburization potential (and carbon) in the furnace reduction zone.

A method for producing metallized pellets is known, which includes treating oxidized pellets with a liquid and then metallizing them in a shaft furnace, characterized in that, in order to reduce the cost price and improve the quality of the pellets, the treatment is carried out with an aqueous chalk suspension at a flow rate of 0.5-1.0 times the dynamic moisture absorption value of the pellets, and in order to increase the temperature threshold for pellet welding during metallization by 10°C in the temperature range of 700-900°C, the density of the suspension is increased by 2 kg/ ^m3 , and in the range of 900-1050°C - by 10 kg/ ^m3 (author's certificate SU 1383785 A1, published on 15.07.1994, we take as a prototype).

The disadvantages of this method of producing metallized pellets are:

1. Reduction in the gas permeability of pellets and, as a consequence, deterioration in recoverability (reduction in the degree of metallization).

2. Reduction of total iron content and increase of waste rock content.

3. The water chalk suspension, getting on the batch and interacting in countercurrent with the process gases, is then cooled and purified in process scrubbers using water. Accordingly, chalk gets into the water, which subsequently leads to carbonate deposits on the internal walls of pipelines and equipment. Solid carbonate deposits reduce the efficiency of the process equipment.

The technical result of the proposed invention is the creation of a method for regulating the carbon content in metallized pellets.

The specified technical result is achieved by the fact that the method for regulating the carbon content in metallized pellets is implemented by using coal dust as part of a chalk suspension, which is used to process oxidized iron ore pellets before loading them into the metallization unit (shaft furnace) to prevent them from sticking together when they subsequently enter the shaft furnace. Oxidized iron ore pellets that are not processed with such a suspension stick together when they enter the shaft furnace, which prevents the free penetration of the reducing gas between them and worsens the iron reduction process, and also leads to disturbances in gas distribution and the descent of oxidized pellets: hanging of oxidized pellets in the intermediate bin of the shaft furnace; hanging of oxidized pellets in the reduction zone; distortion of the backfill surface level (clogging of one or more distribution pipes); peripheral gas flow. In the technological process without using chalk suspension, to prevent the oxidized pellets from sticking together, it was necessary to reduce the temperature supplied to the shaft furnace to ≈700°C, which disrupted the technological process and entailed a decrease in its productivity. With the use of chalk suspension, the temperature of the reducing gas blown into the furnace is ≈950°C, which corresponds to the parameters of the technological process. As was said above, when switching to hydrogen metallurgy, there will be an inevitable decrease in carbon in the metallized product. This forces us to use a larger amount of reducing gas and heat for carburizing the metallized product. OEMK specialists have studied various approaches to carburizing, namely, blowing carbon into the furnace together with reducing gas, the temperature of which is ≈950°C, will not bring the expected result, since the carbon will settle on the inner surface of the pipelines, practically without contacting the oxidized pellets. Applying carbon dust to oxidized pellets will result in it being carried away with the blast furnace gas and will also not bring a positive effect. As a result, OEMK specialists proposed adding coal dust to the chalk suspension and then treating the iron ore oxidized pellets with it before loading them into the shaft furnace. When loaded into the furnace, the chalk suspension sintered on the surface and in the pores of the oxidized pellets, thereby fixing the carbon on them. As the oxidized pellets passed into the depths of the shaft furnace, under the influence of high temperatures of the reducing gas, the carbon began to actively interact with the iron according to the following chemical reaction:

3Fe+C= _Fe3C >928°C

As a result of using this method, a specified carbon content is obtained in metallized pellets during the production of DRI.

To verify the results of the proposed method for regulating the carbon content in metallized pellets, several experiments were conducted on metallization unit No. 4 of OEMK with oxidized iron ore pellets, the process parameters of which are indicated in Table 1.

Table 2 shows the average values of the main performance indicators of metallization unit No. 4 of OEMK during the testing period.

First experiment. A chalk suspension with a density of 1.050 t/ ^m3 was used for the experiment. A mixture of 50 g of coal dust and 500 ml of chalk suspension was prepared in the first container. A mixture of 20 g of coal dust and 500 ml of chalk suspension was prepared in the second container. The oxidized pellets were in the solution for 20 minutes. The following were loaded into the experimental basket from bottom to top:

- oxidized pellets treated with a mixture of 50 g of coal dust (Sample-1);

- oxidized pellets treated with a mixture of 20 g of coal dust (Sample-2);

- oxidized pellets without coating (Sample-3).

That is, without processing with chalk suspension with the addition of coal dust. These oxidized pellets were processed with chalk suspension directly in the industrial bin, into which chalk suspension was continuously fed in accordance with the established technology. Oxidized pellets pre-treated with chalk suspension with the addition of coal dust were placed in a package for the purity of the experiment and to exclude their re-processing in the industrial bin of the shaft furnace.

The chemical composition of oxidized pellets was determined using the X-ray spectral method in the OEMK chemical laboratory. The results of the analyses are presented in Table 3.

Second experiment. A chalk suspension with a density of 1.038 t/ ^m3 was used for the experiment. A mixture of 20 g of coal dust and 500 ml of chalk suspension was prepared in the first container. A mixture of 10 g of coal dust and 500 ml of chalk suspension was prepared in the second container. The oxidized pellets were in the solution for 20 minutes. Three experimental baskets were prepared:

Basket "A" - loaded from bottom to top:

- oxidized pellets treated with a mixture of 20 g of coal dust (Sample-1);

- oxidized pellets treated with a mixture of 10 g of coal dust (Sample-2);

- oxidized pellets without coating (Sample-3).

That is, without processing with chalk suspension with the addition of coal dust. These oxidized pellets were processed with chalk suspension directly in the industrial bin, into which chalk suspension was continuously fed in accordance with the established technology. Oxidized pellets pre-treated with chalk suspension with the addition of coal dust were placed in a bag for the purity of the experiment and to exclude their re-processing in the industrial bin of the shaft furnace;

Basket "B" - oxidized pellets treated with a mixture of 20 g of coal dust (Sample-4);

Basket "B" - oxidized pellets treated with a mixture of 10 g of coal dust (Sample-5).

The chemical composition of oxidized pellets was determined using the wet chemistry method (atomic emission analysis with inductively coupled plasma) in the OEMK chemical laboratory. The analysis results are presented in Table 3.

The obtained results of the series of experiments confirm that the quality of metallized pellets has improved in relation to the oxidized pellets of the current production of metallization unit No. 4 of OEMK. The degree of metallization has increased, as well as the mass fraction of Fe _met and C.

The method for regulating the carbon content in metallized pellets is as follows.

Oxidized pellets are immersed in a chalk suspension containing carbon (e.g. in the form of coal dust) with the following ratio of components, wt. %: water - 93.51% ÷ 95.02%; calcium oxide - 3.31% ÷ 3.36%, coal dust - 1.62% ÷ 3.18%. Oxidized pellets are then removed from the chalk suspension containing carbon and fed into a shaft furnace for metallization.

Claims (1)

A method for regulating the carbon content in metallized pellets, including the treatment of oxidized pellets with a chalk suspension and their subsequent metallization in a shaft furnace, characterized in that coal dust is added to the chalk suspension in the following ratio of components, wt. %: water 93.51-95.02; calcium oxide 3.31-3.36, coal dust 1.62-3.18, after which the treatment of oxidized pellets with the resulting suspension is carried out in a time range of up to 20 minutes.