RU2634110C2 - Method for producing metal powder - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method for producing metal powder includes selection of initial raw material and its grinding with control of specific surface of the produced powder, at that specific surface of the initial raw material is determined, and selection of raw material and its grinding is carried out according to condition:

, where S_s.i. - specific surface area of the initial material (m²/r), S_s.p. - specific surface area of the produced powder (m²/g).

EFFECT: improvement of powder quality expressed in stabilisation of granulometric properties, reduction of morphological diversity of particles, increase of bulk density and improvement of compressibility, reduction of pyrophoricity and sensitivity to friction.

5 cl, 2 tbl, 4 dwg, 6 ex

Description

The invention relates to the field of powder metallurgy, in particular to the production of metal powders by mechanical grinding of feedstock, and can find application in mechanical engineering, pyrotechnics and chemical technology.

Known methods for producing powders by mechanical grinding of the original relatively large-sized metal raw materials both dry and wet [F. Eisenkolb. Advances in powder metallurgy. Per. from German. - M.: Metallurgy, 1969, p. 25-27 / 1 /]. Mechanical grinding, in particular, includes crushing (particle size 2.0-2.5 mm) and grinding (particle size from 0.1 μm to 1 mm). For crushing, jaw, roller, hammer crushers, roller and finger mills are used, and for grinding - ball rotating and vibration, planetary, vortex, jet and attritor mills [Porous permeable materials. Directory. Ed. S.V. Belova. - M.: Metallurgy, 1987, p. 63, 64/2 /].

In addition, there are physicochemical methods for producing metal powders / 1 /, as well as methods of production in which physicochemical

 methods and mechanical grinding. In particular, in the method for producing zirconium powder [V.M. Orlov, L.A. Fedorova, B.T. Kalinnikov, P.T. Gusev, V.V. Yaroshenko, E.G. Orlikova, A.P. Suslov, C.V. Baranov, S.I. Smooth, S.M.-A. Valeev. The method of producing zirconium powder. RF patent No. 2304488, IPC B22F 9/18, C22B 34/14. Bull. No. 23, 08/20/2007 / 3 /] the methods of sodium thermal reduction of potassium hexafluorozirconate are combined, followed by crushing and grinding. As a result of the physicochemical process of producing metal zirconium by the method / 3 /, a sintered metal zirconium is formed with a highly developed surface and significant internal porosity, which, after removal of by-products and unreacted components of the initial charge, is crushed and ground into powder.

A known method of producing titanium powder [S.V. Baranov, S.M.-A. Valeev, V.V. Ershov, V.I. Kapitonov, O.N. Levchenkova, M.B. Makarov, S.I. Equal, V.V. Rudskikh, V.V. Rybnikov. A method of obtaining a fine titanium powder. RF patent №2240896, IPC B22F 9/16, publ. 11.27.2004 / 4 /], in which metal titanium is subjected to mechanical grinding in the form of a commodity titanium sponge of the TG-100 brand. For preliminary embrittlement of the feedstock, a sponge hydrogenation operation is used. Then, the hydrogen-rich feed is crushed and thermal decomposition of the crushed hydride is carried out in dynamic vacuum, followed by crushing and screening of the desired fraction of the metal powder (in this case, <40 μm).

The disadvantage of the methods / 3, 4 / is that as a result of the use of porous raw materials (sponges), powders with a highly developed particle surface, substantial internal and surface porosity, and a large variety of shapes (habitus (Habitus - from Latin habitus - appearance, appearance) are obtained The appearance of a person, animal, plant, crystal. [Great Encyclopedia of Cyril and Methodius. Electronic version, 2010]. The same synonym applies to the appearance of powder particles)). These circumstances are often signs of insufficient quality of powdered metals, manifested in the instability of technological properties and operational characteristics, can cause increased pyrophoricity, as well as high sensitivity to external influences (friction, electric spark, shock). This can also lead to the initiation of a pyrophoric reaction of both the powders themselves with oxygen and then with air nitrogen, and unauthorized ignition, for example, pyrotechnic compositions based on them or compositions for self-propagating high-temperature synthesis (SHS). In addition, the final product requires an additional labor-intensive batching operation (combining various powder specimens with different properties in order to obtain a mixture of them with predetermined controlled parameters).

The closest analogue to the claimed invention is a method for producing fine titanium powder [I.K. Kremzukov, N.M. Kolobyanin, S.A. Peleskov, A.I. Vedeneev, E.V. Levakov, A.S. Mityashin, A.Yu. Postnikov. A method of obtaining a fine titanium powder. RF patent No. 2301723, IPC B22F 9/16. Bull. No. 18, 06/27/2007 / 5 /]. The method is carried out according to the hydride technology for producing a powder, like / 4 /, but hydride embrittlement of the feedstock is carried out at the maximum saturation with hydrogen (440-468 cm ³ / g / 5 / instead of 340-370 cm ³ / g / 4 /), use at least two-stage grinding as vacuum thermal decomposition of the hydride, with additional grinding is carried out with a residual hydrogen content in the cake from 100 to 380 cm ³ / g). In addition, control of the specific surface of both intermediate decomposition and grinding products and the finished powder is used in / 5 /. It is accepted that the grinding of the hydride formed should be carried out to a specific surface of at least two times the specific surface of the finished powder.

The disadvantage of the prototype method / 5 / is that it does not limit the porosity and specific surface area of the feedstock, which can lead to the same undesirable consequences regarding the quality of the finished product as in the methods / 3, 4 /, in the case of using raw materials with developed surface and internal porosity (for example, spongy raw materials). Powders are obtained with a highly developed particle surface, significant internal and surface porosity, as well as a wide variety of particle habit, which are signs of insufficient quality of powder materials, causing instability of technological properties and operational characteristics. In addition, to obtain a conditioned final product may require additional labor-intensive operation of the charge [V.M. Orlov, L.A. Fedorova, P.G. Berezhko, P.T. Gusev, V.V. Yaroshenko. Sodium zirconium powders. In the collection “Innovative Potential of Kola Science. Apatity: Publishing House of the Kola Science Center RAS. 2005, p. 198-201 / 6 /].

The problem solved by the invention is to improve the quality, to stabilize the technological properties and operational characteristics of the resulting metal powders.

The technical result of the invention is the stabilization of particle size distribution, reduction of morphological diversity of particles, improvement of structural and mechanical properties (e.g., increase in bulk density) and technological characteristics (e.g., improvement of compressibility), decrease in pyrophoricity and sensitivity to friction.

To solve the problem and achieve the specified technical results in the proposed method for producing metal powders, including the selection of the feedstock and its grinding with control of the specific surface of the obtained powder, the specific surface of the feedstock is additionally determined, and the choice of raw materials and its grinding are carried out in accordance with the condition:

where S _beats.s - the specific surface of the feedstock, m ² / g,

S _beats.p - specific surface of the powder based on it, m ² / g

If the raw material does not satisfy condition (1), then it is finalized by thermal compaction:

- remelting;

- zone melting;

- sintering under pressure;

- hot pressing;

or choose a source compact metal feed.

The quality of metal powders obtained by mechanical grinding of large raw materials according to the structural homogeneity of particles (isodiametricity, cleaved surface, Gaussian size distribution, absence or minimality of surface and internal porosity, equality of pycnometric and x-ray density) is directly related to the degree of surface development, surface and internal porosity of the feedstock. The manifestation of the quality of the feedstock is associated with the parameters responsible for the structural and mechanical properties [Adrianov E.I. Methods for determining the structural and mechanical characteristics of powder materials. - M.: Chemistry, 1982, 256 pp. / 7 /] with technological and operational characteristics of powders, such as: bulk density (see Fig. 1, Tables 1 and 2), compressibility (see Fig. 2), pyrophoricity and sensitivity to external influences causing a pyrophoric reaction and unauthorized ignition of both the powders themselves and mixtures based on them.

Existing mechanical grinding technologies, in this case crushing and grinding, cannot fundamentally change self-similarity [E. Feder. Fractals. Per. from English M .: Mir, 1991, 254 p. / 8 /] the structure of the feedstock and its small fragments - particles of the resulting powder. If a porous sponge is crushed and milled, then the structure of individual particles bears the signs of the structure of the progenitor, namely surface and internal porosity, morphological diversity of particles, the presence of sponge blocks and aggregates based on them, the pores of which are filled with a fine fraction of particles. In such powders there are very few individual large (more than 5-10 microns) non-porous particles, they simply have nowhere to come from, because the characteristic distance between the pores in the sponge does not exceed a value of 20-50 microns. And, on the contrary, there are a lot of particles in the powder, the surface of which carries self-similar signs of the inner and outer surfaces of the sponge. Their number is determined by the internal and external porosity of the sponge and in proportion to its specific surface. The ratio of the specific surface of the raw material S _beats to the specific surface of the powder obtained from it S _beats is a parameter:

in physical terms, it is the probability that any randomly selected powder particle carries a part of the surface of the feedstock. The same parameter can be considered as the fraction of particles whose surface contains part of the surface of the feedstock.

The presence of a correlation between the parameter P _S and bulk density (see Fig. 1) - the most important structural and mechanical characteristic / 7 /, reflecting the quality and behavior of the powders in accordance with their functional purpose - allows us to predict the quality of the powders and select raw materials by additional determination specific surface of the feedstock and comparing it with the specific surface of the powder based on it.

Thus, the solution of the problem of the invention and the achievement of technical results is achieved by additionally determining the specific surface area of the feedstock and calculating its ratio to the specific surface of the resulting powder so that this value is within the range of 0.00001-0.2 (see Fig. 1) . This circumstance determines the choice of raw materials in accordance with condition (1).

An increase in the quality of powders, expressed through an increase in their bulk density, provides for a decrease in the specific surface of the raw material, i.e. a decrease in the parameter P _S at a given or achieved specific surface area of the powder (see Fig. 1). If it is not possible to select a suitable sufficiently dense raw material from which powders of proper quality are obtained, for example, by chemical composition, then thermal compaction is carried out relative to the porous raw material (sponge) or raw material with a highly developed surface. For this, the raw materials should be re-melted, then, possibly, subjected to zone melting for additional purification from impurities, sintering under pressure or hot pressing. However, the simplest way to select the feedstock in accordance with relation (1) is to use a finished compact and relatively compact raw material (rods, ingots, sheets, wire, grains, shots, etc.), provided that it has sufficient chemical purity, at least significantly higher than that required for powder based on it. Mechanical grinding operations, as a rule, lead to an increase in the content of impurities, both hardware and gas, related to the atmosphere in which the grinding is carried out, therefore, insufficient purity of the feedstock can cause excess contaminants or limited impurities in the finished powder.

Often, for certain purposes, raw materials with a high content of "gas" impurities localized on the surface of crystalline grains are required. This creates the prerequisites for reducing the ductility of the source metal and the possibility of finer grinding to obtain ultrafine powders. Surface oxide, nitride compounds and diffusion-saturated layers reduce pyrophoricity to a certain acceptable level. Such properties, for example, should have sodium-thermal zirconium powders obtained by technology / 3 /. There is no finished metal raw material for them. In this case, the metal cake formed during the reduction of sodium potassium hexafluorozirconate may have the desired chemical composition, but an overly developed surface and pronounced internal porosity. In this case, to give the resulting powder the required quality, it should be modified by thermal compaction.

In FIG. 1 shows the correlation dependences of the average bulk density of powder materials on the average value of the ratio of the specific surface of the raw material to the specific surface of the resulting powder P _S for titanium and zirconium powders.

In FIG. 2 shows the dependence of the degree of compaction (relative density) of the compacts achieved at pressing pressures: 500; 1000; 2000; 4000 and 6000 kgf / cm ² , from the values of the bulk density of titanium powders.

In FIG. Figure 3 presents electron microscopic images of fragments of particles of titanium powders obtained from a highly porous sponge (a), a low-porous sponge (b), and compact (iodide) raw material (c).

In FIG. 4 shows electron microscopic images of fragments of particles of sodium thermal zirconium powders obtained by grinding dense cake (a-c) and porous cake (gf)

Table 1 shows some properties of the initial titanium raw materials and powders based on it.

Table 2 shows some of the established and predicted properties of the initial zirconium raw materials and powders based on it.

The task and technical result are achieved due to the fact that to obtain a metal powder, raw materials with the smallest specific surface area are selected. To do this, determine the specific surface of the raw material and powder based on it, for example, by the method of BET, Deryagin or Tovarova. Calculate the ratio of the specific surface of the raw material and powder and check the compliance of the obtained values with the condition:

If the raw material is too plastic and does not undergo sufficient mechanical grinding (crushing and grinding), then, if necessary, embrittlement is carried out, for example, using partial or complete hydrogenation. Then the raw materials are ground into powder and degassed (for example, dehydrogenation), fractionated, for example, by screening on sieves. Powder fractions are classified by sieve mesh size (for example, <40 μm), used in accordance with their purpose, or returned to the powder production cycle as an additive to the feedstock, as, for example, this was done in / 5 /.

If it is not possible to select a suitable sufficiently dense raw material from which powders of the proper quality are obtained, for example, by chemical composition, then thermal compaction of the porous raw material with a highly developed surface is carried out. For this, the raw material is remelted, for example, in a vacuum or inert medium. The remelting of raw materials also leads to its purification from some volatile impurities. Then, if necessary, subjected to zone melting for additional purification from impurities. Sometimes it is necessary to keep the grain surface of crystallites gas-saturated, for example, oxygen and nitrogen. In this case, sintering under pressure or hot pressing is carried out. These operations not only reduce the internal and external porosity of the raw material, but also provide smoothing of both the external and internal surfaces. In addition, they help to reduce the number of surface defects of crystalline grains and, thus, reduce not only the surface roughness, but also the diversity of the habit of the particles of the obtained powder. In this case, diffusion of surface impurities (for example, oxygen and nitrogen) deep into the grains also occurs, which creates additional prerequisites for reducing the pyrophoricity and sensitivity of powders to influences initiating the pyrophoric reaction.

If a suitable, for example, ready-made compact raw material in the form of rods, ingots, plates, wire, grains, etc., is available, for example, then you should choose it as it certainly satisfies condition (1).

Example 1. As the feedstock was chosen titanium sponge ΤГ-100 with different porosity and size of the pieces, the properties of which are shown in table 1. The selected feedstock was subjected to hydrogenation to a hydrogen content of 340-370 cm ³ / g, for example, as in the method / 4 / , or 440-468 cm ³ / g, as in the method / 5 /, then the hydrogenated brittle raw material was crushed, dehydrated to some residual hydrogen content and again crushed in ball mills. Then, the resulting powder was sieved through a 40 μm sieve and finally dehydrogenated by heating in vacuo. The specific surface area of the powder was measured and the specific surface area of the feed was previously determined. In addition, to control the quality of the obtained powder, the bulk density, surface development factor K _S were measured, and the parameter P _{S was} calculated. The surface development factor K _S is determined from data on the average particle size established using a particle size analyzer, for example, by the optical method, as the ratio of the measured true specific surface to the specific surface of hypothetical spherical particles with a diameter equal to the average particle size. The smaller this value, the less developed the surface of the particles, the higher the quality of the powders. The values of the parameters characterizing the quality of the obtained powders are given in table 1. It can be seen that the choice of a denser feedstock, in this case from pieces of a TG-100 sponge with different sizes and densities, as well as the correspondence of its specific surface to condition (1) provides an increase in quality powders characterized by values of bulk density, surface development factor (see table. 1). An illustration of a higher quality of powders according to the signs of structural and dimensional uniformity is electron microscopic images of fragments of powder particles from a sponge with a higher density in comparison with a powder obtained from a more porous sponge (see Fig. 3 a, b).

Example 2. As a source of raw materials selected titanium trademark sponge brand TG-90 more dense than the sponge TG-100M and TG-100A. The use of hydrogenation technology and subsequent mechanical grinding, as in example 1, made it possible to obtain a powder with properties significantly exceeding the properties of powders from a less dense sponge (see Table 1 for the values of bulk density, parameter K _S , parameter P _S ), and practically approaching according to the properties of powders from dense raw materials (see table 1, examples 3 and 4), with the exception of the pycnometric density, which characterizes the internal porosity of the particles, which is less than x-ray for powders obtained from sponges (see table 1, when EASURES 1-4).

Example 3. The method of vacuum melting in an electric arc furnace with a cold crucible was used to melt the ΤΓ-100 titanium sponge into compact ingots with the subsequent removal of the skull layer (the outer layer contaminated with instrumental impurities). The ingots were hydrogenated, crushed and milled in the same manner as in examples 1 and 2. After screening fractions <40 μm, a powder was obtained with satisfactory properties that exceeded the values typical for sponge powders (see table 1 in comparison with example 3 with examples 1 and 2).

Example 4. As the source of titanium raw materials, we selected finished (commercial) semi-finished products and rolled products having a compact internal structure (iodide rods, welding wire from an alloy of technically pure titanium VT1-00) and a relatively compact structure (electrolytic grains). The use of hydride embrittlement and grinding technology is similar to examples 1-3 and allowed to obtain powders of high and stable quality, which was estimated by high values of bulk density, pycnometric density, in this case equal to X-ray, small values of the surface development factor (see table 1, example 4 in comparison with examples 1-3). Such high quality indicators are predominantly due to the chipped nature of the particles, which is visible on electron microscopic images (see Fig. 3c). The use of a compact feedstock for the preparation of finely divided powders implies a known correspondence of the specific surface of the selected feedstock to condition (1) (see table 1 for the value of the parameter P _S in comparison with examples 1-3).

Higher quality of metal powders, manifested in higher values, in particular bulk density, leads to an improvement in their compressibility or compactibility, i.e. ability to increase the packing density of particles under the influence of external pressure 111 (see Fig. 2)

The quality of powder materials is also manifested through their behavior in the technological cycle of manufacturing products in accordance with the functional purpose of the powders. For example, high pyrophoricity (the ability to spontaneously ignite and burn when interacting with air) suggests an increased risk of handling finely divided powder metals and mixtures based on them. The characteristics of powders reflecting their pyrophoricity can be indicators of their sensitivity to friction, which determine the hazard class of powders and mixtures based on them. So, for example, in addition to structural and mechanical quality indicators (bulk density) and particle size assessment from electron microscopy images for ultrafine zirconium powders, the degree of sensitivity to friction of both the powders and SHS compositions based on them was also evaluated (see table 2) .

Example 5. Powders were obtained by crushing and grinding a cake of metallic zirconium obtained by reducing potassium hexafluorozirconate with sodium metal in sealed steel apparatus heated to 800–900 ° C [A.N. Zelikman, O.E. Crane, G.V. Samsonov. Metallurgy of rare metals. - M.: Metallurgy, 1964, p. 316/9 /]. The use of the specified "temperature regime and large quantities of loading (up to 40 kg of charge) provides a relatively dense metal cake.

The same principle of obtaining powder, but using small loads in the reaction apparatus (approximately 400 g of the initial charge) and lower temperatures of 400-650 ° C / 3 / leads to the formation of a more loose metallic cake.

Subsequent crushing, deactivation ("quenching") or vacuum distillation of excess sodium, leaching of salts in an aqueous medium and additional grinding makes it possible to obtain fine powders that are stored in a wet state due to high pyrophoricity. Before intended use, the powders are dried in a precautionary manner, for example with the selection of relatively small portions.

The higher density of the obtained cake according to the technology / 9 / allows to achieve a higher consumer quality of the powders in comparison with the powders from the relatively loose cake formed using the technology / 3 /. This can be seen from such established signs as: higher values of bulk density, lower values of the form factor (the ratio of the average values of the larger particle size to the smaller d _b / d _m ), less sensitivity to friction of both the powders themselves and compositions based on them ( see table 2, example 5).

In FIG. Figure 4 presents electron-microscopic images of fragments of powder particles obtained by grinding a relatively dense and porous cake, illustrating the much higher uniformity of particle habit characteristic for powders from denser raw materials, the choice of which is dictated by condition (1).

Example 6. Zirconium powder is obtained from a compact or relatively compact raw material, which is taken as iodide zirconium, electrolytic zirconium, melted or compacted by sintering under pressure (hot pressing) a magnetothermic zirconium sponge [G.L. Miller. Zirconium. - M .: Publishing house of foreign literature, 1955, p. 347-351 / 10 /]. To give the raw material the necessary brittleness, it is hydrogenated at temperatures of 235-800 ° C to a content of 120-242 cm ³ (ns) of hydrogen per 1 g of zirconium. Zirconium hydride is crushed and ground, for example, in ball mills to a particle size not exceeding 37 microns / 10 /. Then zirconium hydride is decomposed in a dynamic vacuum at a temperature of 400-800 ° C and again milled, sieved through a sieve with the desired mesh size (for example, 40 μm), selecting the desired fraction of the finished powder. Some established and predicted properties of the raw materials and obtained powders are presented in table 2. It is seen (see example 6 in comparison with example 5) that the use of compact or relatively compact raw materials leads to the production of higher quality powders, established by the parameters of bulk density, shape factor and friction sensitivity.

Claims (8)

1. A method of producing a metal powder, including the choice of feedstock and its grinding with control of the specific surface area of the obtained powder, characterized in that the specific surface area of the feedstock is determined, and the choice of feedstock and its grinding is carried out in accordance with the condition:

where S _beats.s - the specific surface of the feedstock, m ² / g; S _beats.p — specific surface area of the obtained powder, m ² / g. 2. The method according to p. 1, characterized in that before grinding the feedstock, an operation is performed on its thermal compaction. 3. The method according to p. 2, characterized in that the thermal compaction of the feedstock is carried out by remelting it, or zone melting, or by sintering under pressure, or by hot pressing. 4. The method according to p. 1, characterized in that they use a compact feedstock. 5. The method according to p. 1, characterized in that they produce embrittlement of the feedstock.

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