RU2590742C2 - Method for production of low-carbon ferrochrome - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy and can be used in production of low-carbon varieties of ferrochrome. Method uses charge in form of homogenised mixture of crushed materials at ratio (%): chromium concentrate, lime and ferrosilicon 75 % (45-44):(40-44):(15-12) respectively, ferrochrome is released from electric furnace with carbon content 0.08-0.20 % and its vacuum oxygen decarburisation in ladle to carbon content 0.03-0.01%. With carbon content in range of 0.08-0.20 % ferrochrome is first blown from above with gaseous oxygen at flow rate of 0.3-0.7 m³/(t·min) at residual pressure in range of 6.70-1.33 kN/m² to obtain [C] = 0.05-0.06% and temperature 1,720-1,760°C, then oxygen blowing is terminated, residual pressure is decreased to 0.130-0.067 kN/m² and ferrochrome decaburised without oxygen blowing to obtain 0.03-0.01 % [C].

EFFECT: invention ensures reducing carbon content in obtained ferrochrome up to 0,03 -0,01% without significant oxidation of chromium, and also reduce power consumption and obtaining content of chromium in ferrochrome more resistant to melting.

1 cl, 4 dwg, 2 ex

Description

The invention relates to ferrous metallurgy and can be used in ferroalloy production, specifically in the production of low-carbon ferrochrome.

According to GOST 4757-91 (ISO 5448-81), the carbon content in the low-carbon ferrochrome of the highest grades ФХ001А-ФХ004А should be 0.01-0.04%.

The economics of producing high-carbon ferrochrome have caused numerous work to find cheap ways to decarburize it. A known method of decarburization of high-carbon ferrochrome in the solid state from dried briquettes made of finely divided high-carbon ferrochrome with an oxidizing agent when heated in vacuum [1]. Relatively cheap and affordable oxidizing agents with a low content of non-reducing impurities have been tested: rich hematite ore; nickel oxide; quartz sand; technical chromium oxide; oxidized ferrochrome. The expediency of using oxidized ferrochrome (mixed with the initial high-carbon ferrochrome) is shown.

To obtain sufficient amounts of oxidized ferrochrome, a tubular rotary kiln with electric heating is used. Ferrochrome powder (66.5% Cr, 5% C, 0.6% Si) was in the hot zone for 10-13 minutes at a calcination temperature of 1000-1300 ° C. The best results were obtained at a firing temperature of 1090-1180 ° C.

Powder blend ferrochrome and an oxidizing agent with binder additives were briquetted (on a hydraulic press).

Decarburization was carried out in a vacuum furnace with the possibility of heating the mixture to a temperature of 1350-1400 ° C. From high-carbon ferrochrome, depending on the oxidizing agent used, decarburized briquettes of ferrochrome containing 0.01-0.03% carbon were obtained.

Disadvantages:

- the process is complex and multi-stage;

- low productivity (duration - several tens of hours);

- for industrial application of the method requires complex and bulky equipment.

A known method of decarburization of high-carbon ferrochrome (high-temperature FeCr) in an oxygen converter (CC) [2, p. 224-226].

The purge of the metal with gaseous oxygen is carried out in several periods. In the first, QC is filled in. FeCr at a temperature of 1600 ° C.

After pouring in. To increase the temperature of FeCr to 1725-1825 ° C, aluminum is added (0.5-0.8% of the smelting mass) and oxidized with gaseous oxygen at a flow rate of 150-200 m ³ / h supplied from below. During this period, the oxygen flow rate is 1.5-2.0 m ³ / t. Carbon oxidation is negligible.

In the second period, the blast with a flow rate of 250-400 m ³ / h is fed from above, the temperature is increased to 1825-1875 ° C. The greatest decrease [C] from 5-8% to 1.7-1.0%.

To further reduce carbon, a special vacuum hood is put on the converter and a vacuum of 3999-10664 Pa is created (1 Pa = 1 n / m ² ; 1 atm = 9.81 · 10 ⁴ n / m ² ). Oxygen is supplied to the surface at a low flow rate. During this period, the carbon content can be reduced to 0.2-0.5%.

Disadvantages:

- do not get low carbon content;

- the process is high-temperature (up to 1875 ° C) and long (several hours), which sharply reduces the lining resistance and process productivity;

- the process is economically unprofitable.

Currently available methods of decarburization of high-carbon ferrochrome have not given technology capable of replacing the silicothermic method of its production.

The closest in technical and technological essence and the achieved effect to the proposed method is a method for the production of low-carbon ferrochrome in a silicothermic manner in an electric furnace [3, p. 232-235].

The method consists in the fact that before turning on the electric current, ferrosilicochrome of the first two fillings is loaded onto the hearth of the furnace, then the current is turned on and the load is gained for 10-15 minutes. Ore and lime of the first three fillings (2.1 tons of ore, 1.9 tons of lime and 0.72-0.76 tons of ferrosilicon chromium are used for one filling) are loaded uniformly as the electric arc increases. After the first two blades are melted (after the consumption of 1550-1700 kWh / t of loaded charge is reached), slag is released from the furnace and another 80-90% of ferrosilicochrome of the third and fourth fillings are loaded onto the bottom. After a set of electrical loads load ore and lime of these spikes. 20 minutes before the end of the smelting give the remaining ferrosilicon in portions for 10-15 minutes. With a normal silicon content (0.5-1.0%), the melt is released. The temperature of the slag 1800 ° C, metal 1760 ° C. Next, ferrochrome is vacuumized for 3-5 minutes at a residual pressure of 3999-5332 Pa (1 Pa = 1n / m ² ; 1 atm = 9.81 · 10 ⁴ n / m ² ).

The disadvantages of this method are, as can be seen, the separation of the processes of melting of the ore-calcareous part of the charge from the process of reduction of oxides by silicon ferrosilicochrome. Most of the ferrosilicochrome sits in the second part of the heat. In this case, the heat of exothermic reactions for the reduction of chromium and iron oxides is not fully used. This inevitably leads to an increase in the duration of smelting, an increase in carbon content, and energy consumption.

Further, during the reduction of chromium from chromium ore by ferrosilicochrome, complications arise due to the fact that chromium is reduced from chromium spinels of variable composition. The chemical composition of chromium ores has large fluctuations. The content (Cr ₂ O ₃ ) varies from 54 to 48%, MgO - from 14-16 to 19-22%, which will certainly reduce reproducibility in the chromium content in the resulting ferrochrome (see Fig. 3 row 1), worsens the technical and economic options.

Then, silicon in ferrosilicochrome at melting temperatures is bound to silicides, the dissociation of which requires an additional heat consumption. Carbon in the alloy comes from the charge and from graphitized electrodes, the amount of which increases with increasing melting time under current. Ferrosilicochrome, in which it is in the form of colloidal particles of SiC, is also a carbon source.

Subsequent evacuation of ferrochrome for 3-5 minutes at a residual pressure of 3999-5332 Pa, carried out according to the prototype, is not accompanied by oxidation of carbon.

The task to which the invention is directed is to guarantee a reduction in the carbon content in ferrochrome to ≤0.03-0.04% without chromium oxidation, a reduction in the duration of smelting in an electric furnace and energy consumption (in fact: meeting the requirements for higher grade ferrochrome, type FH001A-FH004A).

This problem is solved by the fact that ferrosilicon is introduced into the charge, a reducing agent having a higher silicon activity and containing less silicides, the decomposition of which requires an additional heat consumption. When using ferrosilicon FS75, its rational share in the composition of the charge is experimentally determined within 12-15%; a decrease below 12% leads to underreduction of chromium, an increased proportion (more than 15%) is accompanied by an increased silicon content in the alloy.

As a part of the mixture, it was proposed to use chromium concentrate (with Cr ₂ O _{3 /} ≥50%), which has a significantly more stable composition compared to chrome ore.

The authors experimentally established a rational ratio of the shares of chromium concentrate, lime and ferrosilicon 75% in a mixture of charge materials, (%): (45-44) :( 40-44) :( 15-12), respectively. An increase in the proportion of lime in the charge in excess of 44% increases the viscosity of the slag, reduces the chromium content in the alloy and contributes to an increase in the carbon content in ferrochrome from the electrodes. A decrease in the proportion of lime below 40% worsens the conditions for the dissolution of chrome spinelide and the reduction of chromium. Deviation away from the ratios of the components of the mixture accepted in the invention leads to a deterioration of the process parameters.

The use of charge mixture in the form of a mixture of finely ground components (for example, 1-3 mm in size) greatly increases the contact surface between the charge particles, more stable and faster reduction of chromium and iron by silicon ferrosilicon.

An important element is the proposed application of filling the entire mixture in a short time (almost immediately through the heat in the roof of the furnace) on a metal melt prepared for ignition of the charge mixture. Metallothermal reactions proceed slowly if they start with a small amount of the initial mixture consisting of crushed metal oxide and a reducing metal, and the remaining part of the mixture is gradually introduced into the reaction zone after ignition. Therefore, when filling the entire homogenized charge onto molten metal, its most complete penetration and reduction occurs. Convective transfer of heat generated to the overlying layers of the mixture is realized by the movement of the formed melt under the action of capillary forces into the adjacent layer of the mixture with repeated repetition. Heat is more fully absorbed by the charge layers located above.

The carbon content achieved during decarburization of the Fe — C — Cr melt depends on the level of [C] equal. The decarburization process is implemented under the condition [C] fact> [C] equal. The equilibrium concentrations of carbon [C] equal in a high-chromium metal depend on the contents of chromium, residual pressure and temperature:

In fig. Figure 1 shows the graphical interpretation obtained by the authors of the dependence [C] for a melt with 60% Cr on temperature and residual pressure (taking into account the determination of the component activities in slag and metal melts). The decarburization of ferrochrome with high [Cr] = 60% to low carbon contents (≤0.03%) without significant fumes (oxidation) of chromium must be carried out at a pressure of Pco≤1 kN / m ² , constantly lowering its value and maintaining the process temperature level such that the condition [C%] fact ≥ [C%] is equal. To obtain very low [C] fact ≤0.03%, the Pso values should be in the range of 0.01-0.001 atm (1 atm = 98.1 kN / m ² ) at a temperature of 2023 ° K (1750 ° C).

The rate of carbon oxidation and its duration in a Fe-C-Cr melt with oxygen purge in vacuum at a content of [C] ≥0.05-0.06% depends on the oxygen supply intensity. Its rational level is 0.3-0.7 m ³ / (t · min). At a lower value than 0.3 m ³ / (t · min), the speed becomes low, and at> 0.7 m ³ / (t · min) it leads to an increase in chromium oxidation.

Continued oxygen purge at [C] <0.05-0.06% leads to the predominant oxidation of chromium, therefore, it stops and further oxidation of carbon is due to oxygen dissolved in the metal and slag. The process is accompanied all the time by continuous mixing of the smelting with argon at a rational intensity in the range of 0.01-0.02 m ³ / (t · min), which ensures the delivery of the underlying metal layers to the zone of lowered external pressure.

Decarburization of the initial high-carbon (from 6.5-8.5% C) ferrochrome to low carbon values (≤0.03%), even using vacuum technology, will require several hours (at a decarburization rate of 0.006-0.010% C / min) according to technological and economic considerations are not rational, and such a process has not been implemented anywhere. It seems rational to combine a fast-flowing process for producing ferrochrome with a low carbon content obtained by a rationally organized process of silicothermal reduction of chromium and iron oxides in an electric furnace, where the carbon content ranges from 0.07 to 0.20% (see Fig. 2, row 2 ) with a short process of its subsequent oxidation to low carbon values of 0.04-0.01% (Fig. 2 row 3) at low pressure by vacuum oxygen decarburization, for example in a ladle.

T.O. with the aim of accelerating the reduction of carbon content in ferrochrome to low (not more than 0.03-0.04% [C]) values without significant oxidation of chromium and reduce energy consumption in the proposed method:

- used new components in the mixture, and experimentally determined their rational ratio;

- increased activity (molar fraction) of silicon in the reducing agent due to the replacement of FeSiCr with ferrosilicon 75%; the composition of the mixture used chromium concentrate (with Cr ₂ About ₃ ≥50%), which has a significantly more stable composition in comparison with chrome ore;

- the mixture is applied in the form of a homogenized mixture of ground charge materials: “chromium concentrate + lime + ferrosilicon” (sits almost once without separation of the components);

- Combined processes by melting the charge mixture due to the heat of exothermic reactions of silicothermic reduction of chromium and iron oxides and electric arcs;

- a combination of silicothermal production of ferrochrome with a low carbon content (0.07-0.20% [C]) in an electric furnace followed by oxidation of carbon to low values (not more than 0.03-0.04%) under reduced pressure without chromium oxidation was applied .

The method is as follows.

Example 1

Metal wastes and recycled slag are placed on the bottom of the furnace, then electric current is turned on. After reaching the specified power, the whole mixture of charge finely ground (up to 0.1-3.0 mm) materials is loaded onto the formed melt, consisting of: chromium concentrate - 45%, lime - 40%, ferrosilicon grade FS 75-15%. The mixture is loaded all of the feed hopper through the arch hole of the furnace on a hot metal melt in an electric furnace. The silicothermal reduction of chromium and iron oxides begins and the charge is resisted by the heat generated from exothermic reactions and electric arcs. The heat generated is almost completely absorbed by the charge layers located above. The recovery process is progressively moving upstream layers of the heaped charge. Then, a mixture of lime and ferrosilicon is added to the slag formed to reduce the content (Cr ₂ O ₃ ), and slag is released. Next, metal is refined from silicon (up to 0.3-0.4%) with additives of a mixture of Cr-concentrate and lime.

At [Si] 0.3-0.4% and a temperature of 1720 ° C, ferrochrome with a carbon content of 0.07-0.20% is released from the furnace into a ladle, which is placed in a vacuum chamber, ferrochrome is purged with argon with an intensity of 0.01 m ³ / (min · t), the pressure above the melt is reduced to 6.70-1.33 kN / m ² and ferrochrome is purged with gaseous oxygen from above with an intensity of 0.35 m ³ / (t · min) to obtain a carbon content of 0.05 -0.06%. The temperature rises to 1750 ° C. Thereafter, purging the oxygen is stopped, the residual pressure is reduced to 0,130-0,067 kN / m ² and ferrochromium argon was stirred with an intensity of 0.02 m ³ / (min · m) to obtain therein [C] = 0,03-0,01% . The decarburization time with gaseous oxygen from 0.2% [C] to 0.05% will be τ = Δ [C] / Vc = 0.15 / 0.006 = 25 min. After oxygen purging, the decarburization time from 0.05 to 0.02% due to oxygen dissolved in ferrochrome will be about 10 minutes. After that, chromium is reduced from slag by additives of a mixture of ferrosilicon with lime in a ratio of 2: 1 in an amount of 20 kg / t with stirring with argon for 3 minutes. Then the pressure is increased and the process is completed.

Example 2. The method is carried out as in example 1, but load the entire mixture of charge finely ground materials in the composition: chromium concentrate - 44%, lime - 44%, ferrosilicon grade FS 75 - 12%; ferrochrome is released from the electric furnace into the ladle at a temperature of 1760 ° C; if in ferrochrome after release from the electric furnace [C] ≤0.06-0.07%, the pressure is immediately reduced to 0.130-0.067 kN / m ² and carbon oxidation is carried out without oxygen purging to 0.03-0.01% [C] with stirring with argon with an intensity of 0.02 m ³ / (t · min).

As a result of the implementation of the proposed method, the carbon content in the resulting ferrochrome decreases from 0.07-0.20% after being released from the electric furnace (Fig. 2, row 2, before evacuation) to 0.03-0.01% after evacuation (Fig. 2 , row 3). Get more stable from smelting to smelting the chromium content in ferrochrome (Fig. 3), reduced energy consumption (Fig. 4).

Literary sources

1. I.D. Kirichenko. Ferroalloy production. Materials of the All-Union Meeting of Ferro-Smelters. - M., 1961, T. XVI, p. 161-180.

2. M.A. Riss. Production of ferroalloys. - M.: Metallurgy, 1985, p. 224-226.

3. M.A. Riss. Production of ferroalloys. - M.: Metallurgy, 1985, p. 232-235.

4. G. Knüppel. Deoxidation and vacuum treatment of steel. - M.: Metallurgy, 1973.

Claims (2)

1. A method for the production of low-carbon ferrochrome, including the filling and melting of metal waste, recycled slag and a mixture of chromium-containing material, lime and a silicon-containing reducing agent in an electric furnace, heating and additional reduction of the resulting slag, the production of slag and ferrochrome, and vacuum-oxygen decarburization, which is different from carbon decarburization, that they use the mixture in the form of a homogenized mixture of crushed materials with the ratio, (%): chromium concentrate, lime and ferrosilicon 75% (45-44) :( 40-44) :( 15-12) respectively Naturally, ferrochrome is released at a carbon content of 0.08-0.20% and its subsequent vacuum-oxygen decarburization to a carbon content of 0.03-0.01%. 2. The method according to p. 1, characterized in that when the carbon content of 0.08-0.20%, first ferrochrome is flushed from above with gaseous oxygen with a flow rate of 0.3-0.7 m ³ / (t · min) at a residual pressure of interval 6,70-1,33 kN / m ² to the carbon content of 0.05-0.06% and the temperature of 1720-1760 ° C, followed by blowing of oxygen was stopped, the residual pressure is reduced to 0,130-0,067 kN / m ² and decarburized to carbon content 0.03-0.01%.

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