RU2754862C1 - Method for producing silumins using amorphous microsilica - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to a method for producing silumins using amorphous silica as a source of silicon. The method includes using amorphous silica produced from the dust of the gas purification systems of electrothermal furnaces as a silicon component, introducing a silicon-containing charge directly into the aluminum melt, wherein preliminary preparation of the silicon-containing charge is first conducted, including the transfer of the amorphous silica to the crystalline phase by reactions in solid phases at a temperature of 800°C using finely dispersed aluminium powder as a recovering agent, then the prepared charge is introduced into the aluminium melt under a layer of a low-modulus cryolite subsequently drained for reuse.

EFFECT: production of hypoeutectic, eutectic and hupereutectic silumins complying with the requirements of GOST 1583–93, with a fine grain structure and increased mechanical properties is provided due to the use of micro- and nanodispersed particles of recovered silica as raw materials for producing silumins in an electrolyser or in a casting ladle immediately prior to pouring into a mixer with minimal power consumption.

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Description

The invention relates to the metallurgy of non-ferrous metals, namely to the production of silumins using amorphous silica fume as a source of silicon.

Obtaining silumins by this method makes it possible to increase the economic efficiency of the process of their production due to the use of amorphous silicon-containing raw materials (dust from gas cleaning systems of electrothermal furnaces), which has an extremely low cost, instead of technical silicon.

From the existing state of the art, various methods for producing silumins are known, the use of which depends on the purpose of the alloys, their composition, the possibility of providing the necessary purity for non-metallic inclusions and gases, the type of feedstock, the volume and conditions of production, as well as economic considerations.

1. Carbothermal (ore-thermal) method, in which silumins are obtained by reducing natural aluminosilicon minerals (kaolinite, kyanite, sillimanite, etc.) in electrothermal furnaces (patents RU No. 2010881, RU No. 2484165). The disadvantages of this method are its high energy consumption, the complexity of the preparation of charge materials, as well as a high degree of contamination of the alloy by impurities and non-metallic inclusions (mainly carbon, which is the main reducing agent).

2. Metallothermal method based on the reduction of silicon from its compounds (oxygen, halide) with aluminum [Belyaev AI, Bochvar OS, Bunov NN. Aluminum alloys. Metallurgy of aluminum and its alloys. M .: metallurgy, 1983]. The method consists in introducing a mixture of aluminum and silica powders onto the surface of an aluminum melt, local heating the mixture to a temperature of 1100–1200 ° C and then obtaining an aluminum-silicon alloy. The disadvantage of this method is the need to use aluminum and silica in a powdery state, the need for significant overheating of the melt, the complexity of obtaining an alloy of a given composition, as well as contamination of the resulting alloy with iron impurities.

3. Dissolution of crystalline silicon in an aluminum melt (patent RU No. 2010881). This method is the main industrial method for producing foundry silumins, which is used in aluminum plants. The main advantage of the method is its high productivity and the possibility of obtaining alloys with a given silicon content. However, this method also has a number of significant disadvantages - large irrecoverable losses of metal due to waste, low assimilation of silicon of fine fractions (less than 5–6 mm), high energy costs.

4. A method for producing alloys using a "liquid" master alloy, which consists in pouring molten silicon into a vacuum ladle with an aluminum melt in it (patents RU No. 2432411, RU No. 2215803). Despite the high quality of the alloys obtained, the implementation of this method requires a close territorial location of enterprises producing aluminum and silicon.

5. The electrolytic method, which consists in the joint reduction on the cathode of aluminum and silicon (patent RU No. 2556188, copyright certificate SU No. 1826998, patent RU No. 2030487) [Melting and casting of aluminum alloys. Ref. ed. / M.B. Altman [et al.] - 2nd ed. - M .: Metallurgy, 1983. - 352 p.]. This method is distinguished by the possibility of using relatively cheap compounds as a raw material and obtaining alloys with a low content of non-metallic and gas inclusions. The main disadvantages of the method are the high energy costs for its implementation, as well as the difficulty of maintaining the optimum concentration of silicon in the electrolyte, which results in an increase in the process temperature and a decrease in the current efficiency.

6. Liquid-phase technologies for producing aluminum-silicon alloys, consisting in the direct reduction of silicon from its oxides directly in the aluminum melt.

Among the existing methods for producing silumins, analogs of the claimed technical solution are methods based on the reduction of silicon from its oxide by aluminum, in which amorphous microsilica is used as a source of silicon, as well as methods in which the reduction of silica occurs directly in an electrolyzer for the production of aluminum.

From the existing state of the art known "Method for producing an aluminum-silicon alloy" (patent BY No. 17860 C1 2013.12.30, IPC C22C 1/03, C22C 21/06, publ. 30.12.2013), according to which silumins are obtained by introducing silica into molten aluminum or an alloy based on it. The introduction of silica fume is carried out in the crystallization interval by mechanical mixing of the alloying element. As a result, a master alloy is obtained, which is subsequently used for alloying aluminum or alloys based on it. After introducing the obtained aluminum-silicon master alloy into the melt (t = 800–900 ° С), it is held at a temperature above the liquidus line for 20–60 min, treated with a refining flux and subsequent casting.

The main disadvantage of this method is that the ligature is obtained by mechanically mixing silica into a melt in a solid-liquid state. The process of mechanical mixing is characterized by complexity of automation, and the fact that the kneading takes place in the range of crystallization of aluminum or an alloy thereof indicates poor recovery of silicon for the interaction in the system «Al-SiO _2". Moreover, obtaining silumins using these master alloys is associated with the difficulty of achieving the required chemical composition of the alloy, as well as with possible contamination of the metal with impurities and non-metallic inclusions.

There is also known "A method for producing an aluminum-silicon alloy in an electrolyzer for the production of aluminum" (patent RU No. 2599475, IPC C25C3 / 06, publ. 10.10.2016), according to which silumins are obtained in an electrolyser for the production of aluminum using amorphous silicon-containing oxide raw materials. As an amorphous silicon-containing oxide raw material, microsilica is used, obtained in the process of cleaning process gases in the production of silicon and silicon-containing alloys, which is loaded into the electrolyte melt using automatic power supply units of the electrolyzer.

A common feature of the analogue with the claimed invention is the use as a silicon-containing raw material of amorphous microsilica obtained during the processing of dust from gas cleaning systems of ore-thermal furnaces for silicon production.

The disadvantage of this method is that a prerequisite for its implementation is the need for preliminary mixing of microsilica with alumina and fluoride salts, since otherwise there is the formation of microsilica agglomerates in the electrolyte melt, which reduce the intensity of dissolution of SiO ₂ and can lead to the formation of sediment at the bottom of the electrolyzer. Moreover, the introduction of quartzite (5–40 wt%) into the cell can lead to a decrease in the cell productivity and current efficiency.

The closest in technical essence and the presence of similar features to the claimed method is "Method for producing silumin" (patent RU No. 2683176, IPC C22C1 / 02, C22C21 / 02, publ. 03/26/2019). The method includes preliminary heat treatment of microsilica at a temperature of 200-300 ° C, the introduction of the amount of microsilica required to obtain a hypoeutectic and eutectic alloy into an aluminum melt (t = 670-900 ° C) together with an inert gas flow, magnetohydrodynamic (and / or mechanical) stirring of the melt, which ensures the suction of microsilica particles into a vortex funnel formed in liquid aluminum. To increase the wettability of dispersed particles of amorphous microsilica, alloying of an aluminum melt with magnesium in an amount of up to 1% (wt.) Is used, as well as heat treatment of microsilica.

A common feature of the prototype with the claimed invention is the use of amorphous microsilica as a silicon-containing raw material, as well as the fact that silumin is obtained by introducing a silicon-containing charge directly into the aluminum melt.

The disadvantage of this method is the complexity of the technological design of the process. Moreover, when a large amount of microsilica is introduced into the melt, which is necessary to obtain eutectic and hypereutectic alloys, the temperature of the melt decreases.

The objective of the claimed invention is to obtain hypereutectic, eutectic and hypereutectic silumins that meet the requirements of GOST 1583-93, using as a source of silicon amorphous silica fume, which is a by-product of silicon production.

The technical result of the invention consists in:

- the use of a pre-prepared silicon-containing mixture (60% SiO ₂ , 40% Al + 20% 3NaF · 2AlF ₃ ) as a source of silicon in the process of obtaining silumins, in which silicon was reduced from microsilica during sintering;

- obtaining hypereutectic, eutectic and hypereutectic silumins that meet the requirements of GOST 1583-93 (in an electrolyzer or in a casting ladle immediately before pouring into a mixer) with minimal energy consumption (all processes - both the preparation of the charge and the alloy itself are carried out at temperatures up to 850 ° C), and with minimal losses of the components involved in the process (silicon dioxide almost completely transforms into crystalline silicon and then into a melt, and aluminum oxide from this process, after decomposition in cryolite, passes into a melt in the form of metallic aluminum);

- ensuring effective dissolution of aluminum oxide formed during the conversion of silicon dioxide into crystalline silicon, due to the use of low-modulus cryolite in the process;

- obtaining silumins with a fine-grained structure and increased mechanical properties due to the use of micro- and nanodispersed particles of reduced silica as raw materials for obtaining silumins.

The presence of distinctive features allows us to conclude that the claimed invention meets the “novelty” condition of patentability.

Comparison of the proposed technical solution not only with the prototype, but also with other technical solutions in this and related areas did not allow identifying sources containing information about the popularity of the totality of all the distinguishing features of the proposed technical solution.

A new set of features of the proposed method for producing silumins, namely:

1. Preliminary reduction of amorphous silicon dioxide to crystalline silicon during reductive firing (at T = 800 ° C) of a pressed tablet consisting of silicon dioxide, crushed aluminum and low-modulus cryolite.

2. Grinding the obtained sinter and introducing it into an aluminum melt covered with a layer of low-modulus cryolite.

The method differs in that the reduced silicon in the composition of the crushed sinter is completely assimilated in the melt:

- in cryolite, aluminum oxide dissolves, and aluminum and silicon, fusing with each other, pass into the melt to a predetermined concentration;

- additionally formed cryolite serves as a medium for processing the next portions of the introduced charge.

The method differs in that it can be implemented both in the process of electrolysis of cryolite-alumina melts - by feeding the previously reduced sinter together with alumina into the electrolyzer, and in the casting department - by loading the sinter at the bottom of a casting ladle or mixer, followed by pouring an aluminum melt and coating a layer of low-modulus cryolite.

Based on the foregoing, it can be concluded that the claimed invention meets the patentability condition "inventive step".

In practice, it seems rational to organize a site for the production of a silicon-containing charge from amorphous microsilica - a waste product of enterprises for the production of crystalline silicon. This is economically justified, since emissions (in the form of dust) of amorphous silicon dioxide make up 95% of the output of marketable products, and the charge prepared according to the proposed technology is a full-fledged component for the production of high-quality silumins.

It is rational to locate such a site directly under the roof of the shop for the production of crystalline silicon, in this case, the silicon-containing charge will be the second type of manufactured product. In the future, a certain percentage of silicon fines can be mixed into this mixture, which is also a production waste.

In fact, from a technological point of view, the charge preparation process is not complicated and is implemented in several stages:

- grinding of components;

- dosage and mixing;

- pressing of briquettes;

- firing at temperatures up to 850 ° C.

It should be noted that none of the listed methods for obtaining silumins using microsilica has not found industrial application due to the difficulties of implementation in existing production, the low percentage of assimilation of microsilica by aluminum melt, and, as a consequence, a large formation of waste in the form of slags.

The main difference between the proposed method is that it is implemented in two stages, which include preliminary preparation of the silicon-containing charge and its introduction into the aluminum melt.

The purpose of the first stage is the recovery of silicon from amorphous microsilica. This stage includes a number of sequential operations:

1) preparation of a charge consisting of silicon dioxide (60% (wt.)), PA-2 aluminum powder (or aluminum shavings) (40% (wt.)) And low-modulus cryolite in an amount of 20% by weight of the main components. Cryolite in the composition of the charge is necessary to create an atmosphere of reductive firing and shift the equilibrium of the reaction 3SiO ₂ + 4Al → 2Al ₂ O ₃ + 3Si towards the formation of finished products;

2) pressing the mixture into a tablet;

3) burning the tablet at a temperature of 800 ° C for 30 minutes;

4) grinding the obtained sinter to a fraction of 100 microns, which provides the maximum degree of contact in the Al – Si system.

The ratio in the charge of silicon dioxide and powdered aluminum is due to the stoichiometric calculation:

3SiO ₂ (180 mol) + 4Al (108 mol) → 2Al ₂ O ₃ (204 mol) + 3Si (84 mol)

From this it can be seen that 62.5% of silicon dioxide and 37.5% of aluminum are involved in the reaction.

Cryolite in the composition of the charge is necessary to create a reducing firing atmosphere and shift the equilibrium of the reaction towards the formation of finished products.

In the process of pressing, for the effective course of the reaction, it is necessary to ensure close contact of micron particles of silicon dioxide and aluminum.

The temperature of 800 ° C is quite sufficient for the so-called reaction in solid phases - by analogy with the process of firing cement clinker.

Thus, by the method of active experiment, it was possible to achieve 95% of the transition of amorphous silicon dioxide to crystalline silicon. This is confirmed by the results of X-ray phase analysis (Fig. 1) shown on the diffractogram of a sintered silicon-containing mixture (□ - peaks of Si; ◊ - peaks of Al ₂ O ₃ ; ∆ - peaks of Al; ○ - peaks of Na ₃ AlF ₆ ; ▼ - peaks of Na ₅ Al ₃ F ₁₄ )

Grinding of the resulting sinter helps to increase the area of its contact with the melt. The bulk density of the resulting charge is 1.4-1.6 g / cm ³ , this is slightly less than the specific gravity of molten low-modulus cryolite, which makes it possible to create a mechanical mixture of these components.

The purpose of the second stage is the maximum assimilation of the resulting charge in the aluminum melt. Experimental melting to obtain hypereutectic silumins using a charge prepared according to the above technology was carried out in a foundry laboratory.

The process was organized as follows. Silicon-containing charge was loaded into the crucible from borosilicated graphite. Next, solid aluminum was placed in the crucible. After that, it was backfilled with a five-centimeter layer of cryolite, corrected with aluminum fluoride to a melting temperature of 800 ° C (c.o. = 1.5). Melting was carried out in an induction furnace at a temperature of 850–900 ° С. After the metal was melted, it was intensively mechanically mixed for 5 minutes.

Crystalline silicon, which was formed as a result of reactions in solid phases during tablet firing, is smoothly assimilated in the aluminum melt. Alumina produced by the same reaction dissolves in cryolite, and aluminum and silicon fuse and go into a melt.

Dissolution of alumina in cryolite is associated with the exchange of F ^- and O ² - ions between the AlF ₆ ³ - anions of molten cryolite and the lattice of alumina. The Al ³⁺ cations belonging to cryolite pull the O ^2- anions out of the alumina lattice with their strong field. As a result of this exchange, the integrity of the crystal lattice of alumina is violated and its dissolution occurs. Thus, the "solvents" of aluminum oxide in cryolite are aluminum cations Al ³⁺ , which are included in the cryolite complexes AlF ₆ ^3- and AlF ₄ ^- .

In the course of laboratory studies, 780 g of a charge (consisting of 28% silicon, 50% aluminum oxide, and 20% cryolite), 1000 g of A7 grade aluminum, and 500 g of cryolite were loaded into a crucible made of borosilicated graphite.

As a result of experimental melting, a hypereutectic alloy weighing 1230 g was obtained, containing more than 16% silicon, i.e. almost all crystalline silicon and aluminum from aluminum oxide from the charge passed into the melt, which is confirmed by the results of spectral analysis.

As a result of experimental melting, a hypereutectic alloy (m = 1340 g) was obtained with a silicon content of more than 16 wt%, which is confirmed by the results of optical emission spectroscopy (Table 1).

Table 1

Impurity content,% (mass.) Si Fe Mg Mn Zn Cu Ga Ti V Raw aluminum 0.110 0.112 0.004 0.023 0.011 0.009 0.001 0.008 0.005 The resulting alloy 16.40 3.072 0.004 0.085 0.012 0.009 0.001 0.008 0.005

Cryolite from the smelting was poured separately for recycling.

Thus, empirically proved the possibility of almost complete assimilation of silicon from the composition of amorphous silicon dioxide in an aluminum melt, and the production of both alloys normalized by the silicon content and hypereutectic ligatures for the preparation of various compositions with other alloying additives.

There are a number of factors that complicate the preparation of silumins using amorphous microsilica by simply introducing it into an aluminum melt (similar to the dissolution of crystalline silicon in an aluminum melt):

1) low contact area of the microsilica powder with the melt [Pai B.C., Geetha Ramani, Pillai R.M., Satyanarayana KG. Role of magnesium in cast aluminum alloy matrix composites // Journal of Materials Science. - 1995. - Vol. 30. - P. 1903-1911];

2) the presence of gas films on the surface of microsilica particles, which prevent effective interaction at the interface between the liquid and solid phases [Gowri Shankar MC and Jayashree, PK and Kini, Achutha U and Sharma SS Effect of silicon oxide (SiO ₂ ) reinforced particles on aging behavior of Al – 2024 Alloy // International Journal of Mechanical Engineering and Technology. - 2014. - Vol. 5 (9). - P. 15-21];

3) the presence of 40-50% (vol.) Of air in the powder, which reduces its density, heat capacity and thermal conductivity.

These factors fundamentally complicate the assimilation of microsilica by molten aluminum, making this process difficult and economically inexpedient.

The proposed solution, for the preliminary preparation of a silicon-containing charge, fundamentally solves this problem and makes it possible to use this type of raw material for the production of high-quality silumins.

Compliance with the condition of patentability "industrial applicability" is proved by experimental data.

Example 1.

A silicon-containing charge prepared according to the above described technology was loaded into an industrial induction furnace with a rotary crucible with a volume of 0.8 m ³ .

The mass of the loaded charge is 520 kg.

The content of crystalline silicon in the charge is 28%.

On the surface of the charge, A7 aluminum weighing 650 kg was loaded in the form of finely chopped wire rod.

The "cake" formed in this way was densely covered with a layer of low-modulus cryolite, corrected with aluminum fluoride to a melting point of 800 ° C (c.o. = 1.5).

A load was applied to the inductor, smoothly (within 45 minutes) bringing the temperature inside the crucible to 850–900 ° C.

With the appearance of a liquid phase, a borosilicated graphite impeller was introduced into the melt and intensive mixing was carried out for 5 minutes.

After settling the melt and separating the phases, first cryolite, then metal, was poured from a rotary crucible into separate lined containers.

The weight of the resulting alloy was - 887 kg, which is 93% of the theoretically possible.

The silicon content in the alloy was - 16%, i.e. almost all silicon (95.5%) passed into the melt.

Example 2.

The process conditions are similar to example 1, but the charge was not backfilled with low-modulus cryolite.

Example 3.

The process conditions are similar to example 1, but the operating temperature for the preparation of silumins is 700-750 ° C.

Example 4.

The process conditions are similar to example 1, but the process time was 30 minutes.

The results for each example are shown in Table 2.

table 2

Impurity content,% (mass.) Si Fe Mg Mn Zn Cu Ga Ti V Example 1 Raw aluminum 0.110 0.112 0.004 0.023 0.011 0.009 0.001 0.008 0.005 The resulting alloy 16.40 3.072 0.004 0.085 0.012 0.009 0.001 0.008 0.005 Example 2 Raw aluminum 0.110 0.112 0.004 0.023 0.011 0.009 0.001 0.008 0.005 The resulting alloy 2.30 3,054 0.003 0.086 0.013 0.009 0.001 0.008 0.005 Example 3 Raw aluminum 0.110 0.112 0.004 0.023 0.011 0.009 0.001 0.008 0.005 The resulting alloy 7.50 3.539 0.005 0.071 0.012 0.010 0.001 0.008 0.005 Example 4 Raw aluminum 0.110 0.112 0.004 0.023 0.011 0.009 0.001 0.008 0.005 The resulting alloy 10.20 3.031 0.003 0.092 0.011 0.009 0.001 0.008 0.005

As can be seen from the table, the greatest influence on the process of reducing silicon from amorphous microsilica is exerted by the use in the process of low-modulus cryolite, corrected with aluminum fluoride to a melting point of 800 ° C (c.o. = 1.5), as well as the process temperature (examples 2.3 ). In example 4, a good degree of transition of silicon into an aluminum melt is also observed, however, due to the shortening of the process time, the silicon concentration in the resulting alloy did not reach its maximum value.

Claims (1)

A method for producing silumins, including the use as a silicon component of amorphous microsilica obtained from the dust of gas cleaning systems of electrothermal furnaces, introducing a silicon-containing charge directly into an aluminum melt, characterized in that a preliminary preparation of a silicon-containing charge is carried out, including the transfer of amorphous silica into a crystalline phase solid phase at a temperature of 800 ° C using fine aluminum powder as a reducing agent, providing for the preparation of a charge consisting of 60 wt.% silicon dioxide, 40 wt.% PA-2 aluminum powder or aluminum shavings and low-modulus cryolite in an amount of 20% by weight the main components necessary to create an atmosphere of reductive firing and shift the equilibrium of the reaction 3SiO ₂ + 4Al → 2Al ₂ O ₃ + 3Si towards the formation of finished products, the mixture is pressed into a tablet, the tablets are fired at a temperature of 800 ° C in t After 30 min, the resulting sintered cake is ground to a fraction of 100 μm, which provides the maximum degree of contact in the Al-Si system, and then the prepared charge is introduced into the aluminum melt under a layer of low-modulus cryolite, corrected with aluminum fluoride to a melting temperature of 800 ° C with the possibility of subsequent draining cryolite for reuse.

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