RU2731948C1 - Method of cleaning aluminum and its alloys from intermetallides and other non-metallic inclusions - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to a method of cleaning aluminum and its alloys from intermetallides and non-metallic inclusions. Method involves overheating of molten aluminum or its alloy after melting to temperature of 750–800 °C and pouring it into a rotary mold pre-unwound until gravitation coefficient equal to 170–200 is achieved near the outer wall of the rotating mold. During mold rotation, heat is removed from mold outer wall in direction of its rotation axis with average cooling rate of 0.02–0.1 °C/s for movement of intermetallides and nonmetallic inclusions by pressure of flat crystallization front towards rotation axis of casting mold at cooling of melt in same direction. When the casting temperature is lower than the solidus temperature, the cast is cooled and the size of the central part of the casting occupied by displaced intermetallides and non-metallic inclusions is determined, and this central part of the cast is removed.

EFFECT: provides an increase in the degree of purification.

1 cl, 3 dwg

Description

The invention relates to the field of metallurgy, and more precisely to metallurgists of aluminum and its alloys, but can be used in other fields of metallurgy.

Purification of aluminum and aluminum materials from various non-metallic impurities is an urgent task of modern metallurgy. The demand for pure and highly pure aluminum materials is constantly growing. The existing technological approaches to the problem do not allow to fully obtain the desired results. Briefly, they are reduced to various kinds of lifts of bound inclusions onto the surface of the melt with their subsequent collection. This also includes the processes of bubbling with various gases of melts of aluminum materials, which make it possible to bind unwanted metallic impurities, converting them into intermetallic compounds. Thus, the ultimate goal of obtaining aluminum materials of a given purity is the removal of intermetallic compounds.

Recently, metallurgical science has noted the previously underestimated role of the modulator of the main service properties of metallic materials, in particular aluminum, which are small (hundredths of a mass%) impurities of nanosized particles or lumps of intermetallic compounds of various nature. This explains the scatter in the properties of the castings obtained, it would seem, under the same conditions for their production. The negative role of intermetallic inclusions in aluminum recycling is especially noticeable, where the problem of removing intermetallic compounds and other non-metallic inclusions is the main obstacle to obtaining secondary aluminum of a given quality.

The existing methods of cleaning melts with ceramic filters, chemical treatment, followed by cleaning the mirror of the melt from floating bound inclusions are not efficient enough. The method of cleaning zone melting is quite effective in the end result, but it is not acceptable in terms of the duration of the process (up to several tens of hours), the complexity of the hardware and the high cost of the technological process, which makes its use expedient for obtaining pure and highly pure aluminum in relatively small volumes.

Known patent RU 2312156, C22 B21 / 00, C30B 13/00, C22B 9/02, publ. December 10, 2007 (taken as a prototype), which describes a method for the production of highly pure metals and single crystals from them (including aluminum and aluminum alloys), which consists in the fact that to obtain a single crystal structure and accompanying refining, the crystallization of the melt is carried out in a force field centrifuges with a gravity coefficient ensuring the creation of adequate supercooling in the melt and equal to the difference between the optimal value of the supercooling value corresponding to the maximum linear crystal growth rate and the interval of growth metastability, while this difference is determined by the expression:

Where

θ - gravity coefficient correction;

A, B, L, M - coefficients from which the numerical value of B is determined based on the thermodynamic characteristics of the crystallizer, which determine the rate of heat processes, and the numerical values of A, L, M are determined by the physicochemical characteristics of metals;

K _g is the coefficient of gravity;

T is the crystallization temperature;

ΔT _about - supercooling of the melt, obtained empirically. In this case, when refining the melt, the latter is cooled volumetrically at a rate of 0.02-0.08 ° C / s.

In this method, the process of displacement by the flat crystallization front (FC) of inclusions not required by the crystallization process itself into the central part of the crystallizer is a side, not optimized process. The proposed range of gravitational coefficients Kg is completely inappropriate for organizing the process of displacing PCs of non-metallic impurities, since the opposing force of separation diffusion becomes insurmountable with increasing Kg. Moreover, it ignores the necessary thermodynamic conditions for directional crystallization, and the proclaimed conditions for carrying out the procedure under conditions of uniform volumetric cooling seem to be unattainable due to the effect of latent heat on the process. Also, the necessary overheating of the melt before pouring into the mold is not taken into account by an amount that ensures the presence of the melt in the liquid phase until the mold of the mold is completely filled and the required Kg is created in the mold. The gravitational coefficient is a dimensionless value that shows how many times the acceleration of centrifugal forces in the rotating mold of the mold is greater than the acceleration of gravitational forces on the earth's surface: Kg = ω ² R / g.

The present invention is aimed at achieving the technical result, which consists in increasing the degree of purification of aluminum and its alloys by using the process of displacement by the crystallization front of intermetallic compounds and non-metallic inclusions.

The specified technical result is achieved by the fact that the method for purifying aluminum and its alloys from intermetallic compounds and other nonmetallic inclusions is characterized by the fact that the melt of aluminum or its alloy after melting is overheated to a temperature of 750-800 ° C and poured into the mold of the crystallizer, previously untwisted until reaching the outer the walls of the rotating mold of the mold, the value of the gravitational coefficient equal to 170-200, and during the rotation of the mold of the mold, heat is removed from the outer wall of the mold towards its axis of rotation with an average cooling rate of 0.02-0.1 gC / s to move intermetallic compounds and other non-metallic inclusions by the pressure of the flat crystallization front in the direction to the axis of rotation of the mold of the mold during cooling of the melt in the same direction, and when the temperature of the casting is less than the temperature of the solidus, the casting is cooled and the size of the central part of the casting occupied by the displaced intermetallics and not metal inclusions, and this central part of the casting is removed.

These features are essential and interconnected with the formation of a stable set of essential features, sufficient to obtain the specified technical result.

The new method is illustrated by explanatory illustrative material on which:

fig. 1 is a diagram of the advancement of the crystallization front by its two-phase zone;

fig. 2 - diagram of the action of forces on the grain of the impurity;

fig. 3 is a structural diagram of the mold of the crystallizer.

A new method of purification of aluminum and its alloys is proposed by using the process of displacement by the crystallization front of intermetallic compounds and non-metallic inclusions, in which the size of the inclusions does not play a significant role. The proposed method is devoid of the above-mentioned disadvantages of the prototype and allows you to purify the aluminum melt already in the process of crystallization of this melt.

According to the present invention, a method for purifying aluminum and its alloys from intermetallic compounds and other non-metallic inclusions is based on the following sequence of interrelated actions:

- after melting, the melt of aluminum or its alloy is overheated to a temperature of 750-800 ° C and is poured into the mold of the mold, which was previously unrolled until the gravity coefficient of 170-200 is reached at the outer wall of the rotating mold of the mold;

- during the rotation of the mold of the crystallizer, heat is removed from the outer wall of the mold towards its axis of rotation with an average cooling rate of 0.02-0.1 gC / s;

- this is done to move intermetallic compounds and other nonmetallic inclusions by the pressure of the flat crystallization front in the direction to the axis of rotation of the mold of the crystallizer when the melt cools in the same direction;

- when the temperature of the casting is less than the temperature of the solidus, the casting is cooled and the size of the central part of the casting occupied by the displaced intermetallics and non-metallic inclusions is determined, and this central part of the casting is removed.

The practice of metal and alloy purification existing in modern metallurgy is based on the use of filters through which the purified melt is poured. The degree of purification is determined by the filter mesh size. Modern industrial filters usually retain inclusions with a dimension of 5 microns or more, that is, the larger the size of the undissolved inclusions, the easier it is to filter them. And the range of sizes of inclusions from 5 microns and less, including nano-sized particles and lumps of nanoparticles, is simply not available for modern industrial filters for cleaning. Partially the solution of the problem is approached by the method of purification by zone melting with all its named disadvantages.

The proposed method solves this problem. It seems important that it is the small-sized inclusions that are "filtered" in the first place. The fact is that the smaller the size of the inclusion, the easier it is for the plane crystallization front Front (FC) to advance it with its two-phase zone and the less the resistance of the force of separation diffusion (Fig. 1).

In alloys that solidify in the temperature range, this surface corresponds to the onset of crystallization, which ends at a certain distance from it, namely, on an isothermal surface corresponding to the temperature of a nonequilibrium solidus. The region between the boundaries of the beginning and the end of solidification consists of two phases, and the amount of the solid phase in it changes in accordance with the temperature distribution and the rate of crystallization. The width of the two-phase zone is equal to the quotient of dividing the value of the temperature range of crystallization of the alloy by the average value of the temperature gradient in this zone. In turn, the temperature gradient is steeper, the higher the cooling intensity, the smaller the thickness of the solidified layer of the ingot, and the lower the thermal conductivity of the alloy.

The proposed method for purifying aluminum and its alloys from intermetallic compounds and other non-metallic inclusions is based on the process of displacing non-metallic inclusions, including intermetallites, by a flat front of the FC, moving to the central part of a mold rotating at a given speed ω in a gradient field of centrifugal forces F from the outer wall of the mold to the center in the channel determined by the vector of the thermodynamic preference of the mold. The thermodynamic vector is set by the design of the mold (Fig. 2). In this case, the pressure of the crystallization front is counteracted by the centrifugal force F acting on the mass m of the displaced particle. As a result of the imposition of two opposing forces on the particle, there are restrictions on the mass of the displaced inclusions.

A feature of the claimed method is that the gravity coefficient Kg is the same at any points of the melt, equidistant from the axis of rotation of the mold of the crystallizer, which ensures that absolutely flat FCs are obtained. Exposure to increased gravity leads to a sharp decrease in the two-phase zone, which provides much more efficient cleaning (refining) of the melt. The listed circumstances, analytically calculated and experimentally confirmed, allow us to assert that this method, unlike any existing methods for cleaning aluminum and its alloys from intermetallic compounds and other non-metallic inclusions, is several times more effective.

The crystallization front (or growth front) is the isothermal surface, which is the boundary of the melt-crystal phase transition and moves along the alloy in the casting mold as it crystallizes. In this case, in the presence of a flat PC, impurities move by this front, and if the crystallization front is curved, then a spiral distribution of impurities should also be revealed in the cross section of the ingot. The number of turns of this spiral depends on the curvature of the crystallization front. The effect of displacement of impurities by the motion of the FC in the presence of overheating of the melt is indicated in the dissertation of Cand. Physics and Mathematics Aseeva Danil Leonidovich "Nonlinear dynamics of the two-phase zone in the processes of solidification of melts", Yekaterinburg, 2006, RSL OD, 61: 07-1 / 285, 131 pages. However, this process is unstable under normal conditions of atmospheric pressure leading to their simultaneous distribution along the length of the casting during the movement of impurities.

To exclude the "breakthrough" of impurities into the crystallized zone within the implementation of the claimed method, the melt of aluminum or its alloy / water after melting is overheated to a temperature of 750-800 ° C and poured into the prepared mold of the crystallizer (Fig. 3). The preparation of the mold of the crystallizer consists in that, before pouring the superheated melt, the rotor 1 of the mold together with the lined mold is spun until the gravity coefficient of 170-200 is reached at the outer wall of the rotating mold. The unrolling of the mold of the crystallizer to the value of the gravitational coefficient equal to 170-200 makes it possible to significantly narrow the width of the two-phase layer, at which the effect of the impurity on the solidification process (the appearance of grain) does not affect the change in the temperature of the phase transition (liquidus) of the mixture. For a melt, as is the case in modern crystallization processes, the liquidus temperature is a function of the impurity concentration, and the crystallization process is accompanied by the directional solidification of the melt by the diffusion of impurities (at present, the diffusion of impurities in the solid phase is traditionally neglected). A decrease in the thickness of the two-phase layer leads to inhibition of the process of diffusion of impurities and the simultaneous movement of these impurities in front of the PC in the direction of directed cooling of the melt.

A feature of the execution of the lined mold of the crystallizer is that it has, in one way or another, organized thermodynamic preference for cooling the melt from the outer wall 2 to the inner central part of the mold (to its axis of rotation 3) (Fig. 3). To form a given cooling mode in a given direction, the mold is equipped with a shaped thermodynamic insert 4, which affects the degree of cooling of the mold sections. This shaped insert regulates the heat transfer from the melt and its crystallized part through the side walls. FIG. 3 shows a mold of a crystallizer, which is a disk-shaped (rotary type) bowl on the axis of rotation, inside which there is an annular hollow insert made of graphite (collapsible graphite insert) with inclined walls, the thickness of which increases in the direction from the axis of rotation to the outer (outer wall). This design of the mold makes it possible to obtain the thermodynamic characteristics of the mold, which make it possible to form a preferential extraction of heat from the outer wall of the mold towards the center with an average cooling rate of 0.02-0.1 gC / s.

After the completion of the crystallization processes (the casting temperature is less than the solidus temperature), the casting is cooled and, by taking samples, the size of the central part of the casting occupied by the displaced non-metallic inclusions is determined. Then the contaminated part of the casting is removed in one way or another. As a result, a ring-shaped casting with a refined metal or alloy is obtained.

The obtained values of gravitational coefficients, overheating temperatures and cooling rates were theoretically calculated and further corrected in the mode of experimental melts and represent to a greater extent empirically obtained values at which the implementation of the claimed method is possible and at which it is possible to obtain high degrees of purification of aluminum and its alloys.

To experimentally test the operability of the invented method for purifying aluminum and its alloys from intermetallic compounds and other non-metallic inclusions, a laboratory metallurgical centrifuge was used with a working diameter of the mold of the mold rotor equal to 300 mm and the ability to regulate the revolutions of the centrifuge rotor in the range of 1200-2500 rpm. To create the technologically necessary preferred direction for cooling the melt in the mold, a collapsible graphite insert was made (Fig. 3). The insert was sub-caliber inserted into the rotor mold of a laboratory centrifuge. For the experimental heats, A99 grade aluminum was chosen. The melt was obtained in a graphite ladle of a laboratory resistance furnace. The centrifuge together with the resistance oven was mounted in an oversized metal box, the inner volume of which was filled from above with dried argon. After obtaining a slightly superheated melt with a temperature of 750 ° C, it was poured with a special manipulator of the furnace ladle into a mold preheated to 200 ° C with a thermodynamic insert, previously untwisted to 1800 rpm. Upon completion of the process, the resulting casting was removed and cut into templates. The obtained laboratory results showed a very high efficiency of the process.

The minimum efficiency of aluminum purification was observed when using raw materials with a purity of 99.99%, the output was aluminum with a purity of 99.9992%. The maximum efficiency was observed when obtaining aluminum with a purity of 99.99992% from 99.997% raw materials in one refining cycle.

In addition, the present invention is expediently applied for efficient refining of any metals and their melts.

The present invention is industrially applicable, since, based on the known thermodynamic and metallurgical processes, it allows the use of the flat FC displacement process as a cleaning process from intermetallic compounds and other non-metallic inclusions of aluminum or its alloy.

Claims (1)

A method for purifying aluminum and its alloys from intermetallic compounds and non-metallic inclusions, characterized by the fact that the melt of aluminum or its alloy after melting is overheated to a temperature of 750-800 ° C and poured into a rotating mold, previously untwisted until the gravitational coefficient is reached at the outer wall of the rotating mold, equal to 170-200, and when the mold rotates, heat is removed from the outer wall of the mold towards its axis of rotation with an average cooling rate of 0.02-0.1 ° C / s for moving intermetallic compounds and nonmetallic inclusions by the pressure of the flat crystallization front in the direction of the axis of rotation of the mold during the cooling of the melt in the same direction, and when the temperature of the casting is less than the temperature of the solidus, the casting is cooled, the size of the central part of the casting occupied by the displaced intermetallic compounds and nonmetallic inclusions is determined, and this central part of the casting is removed.

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