A Novel Method of Hydro-Vacuum Dispersion of Metallurgical Melts: Research and Implementation

Abstract

In parallel with the gradual expansion of the consumption of powder materials and the increasing demands placed on them, the competition between the producers of powders is continually intensifying. There is no doubt that the future belongs to technologies that provide high productivity and low-cost powders. Consequently, the technology and techniques of powder production need constant revision and modernization. For this goal we have developed and proposed a new method and installation for hydro-vacuum dispersion of melts, the essence of innovation and advantage of which lies in sucking and dispersing the melt in the direction opposite to the action of the force of gravity, under gravity overload 150-200g conditions, where the main work is performed by hydraulic rarefaction resulting from a sharp refraction of direction (on 162-degree angle) and rapid expansion (х10) of a high-pressure water annular flow, with the superimposition of spatial shock- pulsating waves generated in the outer shell of the formed cone-shaped vortex. The device is characterized by high production and low energy costs, while powders - by increased specific surface, improved purity and high activity. The enhanced activity of our powders is due to the formation of non-equilibrium mechanoactivation structural-deformation stresses in them, which leads to the accumulation of excess chemical energy in them. It is justified that the application of the method is also highly effective for dispersing slag melts and obtaining amorphous hardened powdery raw materials with high hydraulic activity, suitable for the production of construction cement. Appropriate recommendations for the industrial implementation of the developed innovative technology have been proposed.

1. Introduction

Among the numerous methods of dispersing metallic melts into functional powders, today, the most widespread in industrial practice is the technology of high-pressure water or gas jet spraying (atomization) of the melt [1, 2]. Nevertheless, due to the difficulty of mastering the technological process and the limited functional capabilities of the equipment (periodic work cycle, technical service difficulty, low productivity, harmful effects on the environment [3, 4]), the issue of industrial unification and large-scale implementation of the mentioned technology remains a problem.

A comprehensive overview analysis of the achievements and problems of further development of modern technologies for the production of powder materials is presented in the study [5].

According to the results of this research especially acute problems are to be solved in 3D printing technology for large-size products and—in the direction of obtaining biocompatible powder materials. Particularly problematic is the need to eliminate the phenomenon of non-uniform temperature distribution in the powder bed when machining large workpieces. This makes it difficult to control the stress distribution and leads to effective deformation of the workpiece. In addition, increasing the size of the moulded part also leads to an increase in the amount of unmelted residual powder. This significantly degrades the mechanical properties of the product and reduces its service life. All this leads to the necessity of an updated approach to both the technologies of additive manufacturing and—methods of obtaining and controlling the quality and properties of the obtained powders. We have to search for new, fundamentally different from conventional technologies methods of obtaining powders with specified sizes, shapes and properties.

Also problematic is the solution of the problem of increasing the efficiency of powders used in the processes of hydromallurgical recovery of minerals from their aqueous solutions, which are produced during mining of the latter [6].

Unlike additive manufacturing, powders used in hydrometallurgy, pyrometallurgy, SHS-metallurgy or other related industries do not need to have a clearly defined spheroidal shape. The main requirement for them is a highly developed surface area (high specific surface area) and a high degree of mechanochemical activation.

Provision of guaranteed one-stage cost-effective production of powders with the noted properties is the primary task of our research. Known and currently used powder production installations cannot directly produce powders with the desired properties. All of them require additional physical, mechanical or chemical treatment.

In addition, the gained practice shows, that due to the peculiarities of the technological processes, it is practically impossible to process melts of different types (of different chemical composition and viscosity) with one plant. In each individual case, the equipment needs to be retooled, to change working nodes and their operating modes. In addition, in the large-scale production of powders (for example, the production of iron powder), operational management of the efficiency of the melt sputtering process, including stabilization of the conditions of formation into individual granules, long-term maintenance of operational properties, elimination of the problem of oxidation, gas saturation and reduction of pyroporosity, has remained an unsolved task until now [7, 8, 9]. As a result, the powders obtained by this method are inevitably subjected to additional sorting-mechanical crushing in an inert gas environment, as well as to further thermal, hydrogen-reducing treatment [10, 11].

Similar problems exist in the relatively less common centrifugal dispersion and ultrasonic disintegration processes of atomization metallurgical melts [12, 13, 14]. In all cases, the powders require a finishing thermomechanical (activation) treatment [15, 16]. Without this step, the powders remain in a passive (chemically low-activity) phase. Therefore, their direct use in mining and metallurgical, machine-building, construction and other fields is less effective [17, 18, 19]. The need to optimize the alloy dispersion process and hydrogen reduction treatment of the obtained powders is especially acute when dispersing active metal alloys such as aluminum, magnesium, Armco Iron, low-carbon alloyed steels, modified cast iron, etc. Due to all of the above, the process of dispersing metallurgical melts into functional powders is considered one of the most high-tech, expensive and difficult production processes, which requires further research and optimization.

Based on the above, the main purpose of the study was the development and implementation of a conceptually new, drastically different methodological approach to the process of dispergation of melts, leading to the direct production of powder particles in a mechanically activated state. To achieve this purpose, it is necessary to solve the task of non-uniaxial volume-shift deformation of particles solidifying during dispergation. Practical provision of obtaining the desired effect is possible by forced acceleration of the melt jet leading to simultaneous gradient linear and angular acceleration, fragmentation and solidification in the field of forces directed opposite to the universal gravitational force. Such an approach to the dispersion process will certainly lead to the formation of solidified melt particles in a non-equilibrium, energetically stressed structural-phase state.

The relevance and significance of the shock-wave effect on the dispersed jet is also emphasized in the studies [20, 21], which found that the liquid metal droplet undergoes breakup through the shear-induced entrainment mode for the studied range of Weber number values (We ≈ 400–8000). The prevailing mechanism is explained based on the relative dominance of droplet deformation and Kelvin-Helmholtz wave formation.

We have established that such a special condition necessary for obtaining the desired effect is the initiation of vertical suction and hydromechanical dispersion of the melt, carried out with the help of a specially formed for this purpose lifting force of a high-speed annular water flow with a rarefied (hydro vacuum) core. The new method and device constructed in this case should provide high productivity, technological flexibility and versatility, where the problem of eliminating the need for additional chemical and physical-mechanical treatment of the resulting powders should also be solved.

2. Research methodology and materials

As a rational solution to the above problem, we chose the way of improving the design of the pilot unit [22] and expanding the functional capabilities of the innovative method of hydrovacuum dispersion of alloys [23], specially developed by us earlier for the above purpose. The general arrangement of the new dispersion unit and its main process juncture is illustrated in Figure 1.

Figure 1.

Innovative installation of hydrovacuum dispersion of metallurgical melts: H—hydro-vacuum dispergator; WP—electric drive of water pump; M—manometer; P—pressure regulator; HT—high-pressure water tap; IPC—inverter for smooth pressure control; S—suction nozzle; C—casting ladle; M—mold for casting; L—liquid melt; HC—hydrocyclone; PS—pulp precipitation sector; T—tank for purified water.

The main technological unit (Hydro-vacuum Dispergator) contains: the body/housing 1 with a lateral high-pressure water tangentially delivery elbow pipe 2; the Venturi tube of relatively small dimeter and length 3 concentrically mounted in the body 1, with bottom position of the inlet duct 3′ and the circular collector channel 4 formed between the inner surface of the body/housing 1 formed and shell of Ventura tube 3, coupled with the high-pressure water delivery lateral elbow pipe 2; the semi-toroidal internal (convergent-divergent) confuser 5 formed through the lower end of the Venturi tube 3–3′ and the collector channel 4 in the extreme lower peripheral part of the body/housing 1; the suction nozzle 6 axially placed from its inner surface at a (l) longitudinal and (h) transverse distance with a cylindrical perforation 6′ (d); lower 7 and upper 7′ flat toroidal flanges for sealing the water-injection hydraulic system (body 1, annular duct 4); in the converging duct 3′ of the Venturi tube 3, the dispersion chamber 8, formed on the top of the sucker 6 with the diffuser 8′; the pulp discharger 9 within the diffuser 8′ flexibly connected to the upper end of the pipe 3, and the guiding channel 9′ in its hydrocycline-type collector-settler.

For research and technological improvement of the design developed by us, the simulation modeling method was applied. In particular—the computer software module Solidworks Flow Simulation [24] based on the laws of theoretical hydrodynamics and hydromechanics was used for the simulation modeling of the device. When modeling, the flow rate (velocity) of the water supplied to create a hydro-vacuum in the device was used as a variable parameter, the range of variation of which was within the limits of 0.015–0.029 m³ s⁻¹. During the calculations, variation of the geometrical parameters (l, h) of the semi-toroidal confuser (5) forming a conical vortex from the water supplied at high pressure from the circular collector channel (4) of the dispersion-diffusion chamber (8–8′) of the body (1), from the (initial) values l = 16.69, h = 5 mm, within ±50% deviation limits, obtained when making design solution. At the same time, the pressure of the water supplied in the circular collector channel (4) of the device was varied in the range of 4–23 bar. Through the comparative analysis of the modeling results carried out as a result of the mentioned approach, the optimal values of the construction and technological parameters necessary and sufficient to ensure the desired theoretical value of hydro-vacuum (−1 bar) were determined.

In order to evaluate the accuracy and effectiveness of the data obtained as a result of the modeling, standard methods and tools of hydrometry were used for the experimental research, including a digital water pressure measuring manometer with an electronic transmitter PTL-25A (measurement range 0–25 bar) and a vacuum meter Leybold THERMOVAC TTR 91 (measurement range 5.10⁻⁴ ÷ 1000 microbar).

For recording vibrations of the hydro-vacuum dispersion chamber, a digital gyroscope mounted on the device, a noise meter (Integrating Sound Level Meter) for recording the acoustic spectrum, including the ultrasonic range of acoustic noise, liquid metal temperature measuring contact thermocouples with digital galvanometers, etc. were also used. Accumulation and processing of experimental data was carried out online, directly during the conduct of experimental studies, through the use of a special experimental computer platform Grab ExP.

Experimental studies were conducted, alternately, both using a high-purity aluminum of the 1000 series, aluminum scrap, secondary gray cast iron and its slag melts. An induction furnace Termolit ICMEF-0.06 was used for melting. Structural and metallographic studies were performed using a metallographic microscope Neophot 32 (Carl Zeiss).

To evaluate the quality of the activity of the obtained aluminum powder, a special tool—a FANN-432 model manometric calcimeter [25], with an automated magnetic stirrer and an automatic sodium hydroxide supply function was used, which led to the rapid removal of the oxide film from the surface of the investigated particles and an increase in the rate of hydrogen release from dissociated water molecules.

To evaluate the activity of iron powder obtained from the melt of secondary cast iron, the method of analyzing the cementation (extraction) ability of copper from acid sulfide solutions was used [26]. In turn, to assess the hydraulic activity of the obtained slag granulate, the method of lime (CaO) absorption was applied. For this purpose, an aqueous solution of lime (Ca(OH)₂) with a density of 1.0 g/cm³ was prepared. The change in CaO concentration was monitored at intervals of 24 h, by titrimetric method with 1 H hydrochloric acid solution, according to GOST 22688-2018.

Structural and metallographic studies of the obtained powder particles were carried out using both metallographic optical microscope “Neophot 32 (Carl Zeiss)” and autoemission scanning electron microscope “JSM-7800F” and X-ray diffractometer “DRON-7”.

3. Technological features and advantages of the hydro-vacuum dispersion process

In contrast to the traditional methods of production of functional powders of metallurgical melts for various purposes, the essence of the innovativeness and advantage of our proposed technology lies in the suction-dispersion of metallurgical melts in the opposite direction of the force of gravity, under 150–200 g gravity overload conditions, where the main work performs the rarefaction formed in the vortex core of the conical profile formed as a result of the rapid expansion of the annular flow delivered by high pressure and the sharp 162° change of the flow direction in a closed space (hydraulic vacuum). The shear forces (tensile stresses) vertically, bottom-up created on the surface of the cylindrical jet of melt absorbed in the vacuum action zone, inclined to each other by 36°, and directed upwards by 72° from the horizontal plane, create fundamentally different and hydrodynamically optimized conditions of melt dispersion. At the same time, intense processes of hydraulic friction, hydromechanical adhesion and capillary disintegration take place, where, in contrast to traditional technologies, the impact of hydrodynamic flows of water with increasing angular acceleration on the particles (droplets) taken from the dispersed melt continues until they solidify and enter a special collector. This additionally conditions large-scale 3D twisting-deformation and intense micro-volume cavitation-impulse (ultrasonic acoustic) treatment of the formed particles, thus achieving the irreversible effect of their microstructural segregation, gradient migration and tension (mechanoactivation). Consequently, targeted powders are obtained with improved purity, increased surface area and activity.

The process of vertical suction and dispersion of the metallic melt in Figure 2 is represented as a hydro-mechanical scheme.

Figure 2.

Hydro-mechanical scheme of hydro-vacuum dispersing process: υ—linear velocity of displacement of melt particles, m/s; α—angle of attack of injected water on the suction jet, deg; d₁—initial diameter of the suction jet, mm; d₂—diameter of the jet after vortex-stretching influence, mm; ω—angular velocity of rotation, rad/s; ξ—value of hydraulic resistance, kg/m³.

The vertically sucked liquid metallic jet, extending through the zone of hydraulically rarefied funnel formed in the core of the cone-shaped water jet shell injected from under the semi-toroidal annular confuser (5), in the process of axial displacement and exit from the compression zone, under the influence of force hydro-mechanical impact narrows in cross-section (d₁ > d₂), accelerates, and then, due to elastic forces of acceleration expands again, accelerates, and in the consequence of excited powerful centrifugal-tangential tensile forces destroys, splashing in the dispersion chamber (8) with small shapeless drops. The process proceeds so quickly that the possibility of vapor accumulation and excessive pressure rise is completely excluded. At the same time, vapor shells formed around the surfaces of high-temperature metal droplets (with a critical temperature of 375°C [27]), in addition to intensive heat exchange and cooling effect, increase the potential energy of repulsion, contributing to their quick removal from the zone of rapid hydrodynamic interaction. It is noteworthy that both before entering and after leaving the zone of primary radial compression, force stretching and vortex twisting (Figure 2, Section 1), in the body of the metal jet, drawn vertically directed cone-shaped core of the funnel, under the influence of the formed dynamic vacuum and shock waves of rarefaction, bubble cavitation clusters (caverns) with the effect of pseudo-boiling are formed. These clusters progressively expand (grow) as the absorbed metal is released from the zone of hydrodynamic compression, stretching and twisting. It is in the marked zones of pseudo-boiling, where the density and strength of the liquid metal weakens the most, that centers of disintegration appear, supplied in the dispersion chamber (8) of the liquid metal jet. Some of the gases released after the disintegration of the liquid metal jet (Figure 2, zone FSZ) were carried away with water, and the other part, most likely, together with the shells of water vapor formed on the surfaces of the particles, formed new cavitation bubbles. As the marked vapor-gas mixture cooled and condensed, and, accordingly, the internal pressure of the shell dropped, which occurred in micro- and nanosecond intervals, these cavitation bubbles collapsed and generated cumulative micro explosions coinciding in the direction of its flow vector. Due to the high dispersion speeds (water supply speed 250–400 m/s) and, consequently, the high intensity of cavitation detonations, the frequency of these micro-explosions was in the ultrasonic range. Photographs showing the formation, growth, and collapse of cavitation caverns are presented in Figure 3, obtained as a result of high-speed photofixation (exposure rate 10⁵ frames/s).

Figure 3.

Nucleation (1), growth (2) and collapse (3) of cavitation cavities in the process of hydrovacuum dispersing.

The above-mentioned cumulative impact effect, caused by the phenomenon of cavern collapse, clearly manifested itself in Figure 4, which shows an illustration of the surface of a powder particle with a cavitation ablation crater, and with subsequently formed satellite nano-sized spheroidal particles.

Figure 4.

The surface of a powder particle with a cavitation ablation crater (a), and with subsequently formed satellite spheroidal nanosized particles (b).

From the point of view of explaining the hydromechanical laws of the process of angular twisting of a dispersed jet and shear deformation of individual droplets emitted from it, it is necessary to emphasize that, based on the laws of hydrodynamics and the properties of Newtonian liquids, in the process of hydromechanical capture and acceleration of the melt, in the peripheral layers of the latter, due to differences in displacement velocities across the jet cross-section (the central zone always lags behind the peripheral zone, υ_min < υ_max), the initial conventional rectangular microparticle of the dispersed melt will always deform linearly and turn into a parallelogram. In the diagram (Figure 2, Section 1), this deformation is indicated by a shift dl. The segment dl characterizes the magnitude of the deformation during the time dt, i.e. dl = dυ·dt, then the velocity gradient dυ/dx = dl/dt·dx but dl/dx = tgγ, then dυ/dx = tgγ/dt. Therefore, the transverse velocity gradient is the relative shear/displacement strain rate. Thus, the tangential stresses important from the point of view of the fragmentation of the sucked melt jet depend linearly on the relative strain rate, which means that, unlike the deformation of solids, the magnitudes of the tangential stresses and the degree of associated linear and angular shear, in this case, mainly depend on the hydrodynamic injection power, the angle of attack α, the depth of the associated rarefaction and the area of its effect, rather than on the simple linear velocity of the water flow υ. The formation of a hydraulic structure of a cone-shaped dynamic vortex with an evacuated funnel, adjustable in power and geometric parameters, providing vertical (axial) intake and dispersion of the target melt through the force action of rarefaction shock waves with accompanying cavitation pseudo-boiling fronts, can be considered one of the main distinctive technological features of the process of obtaining metal powders by the method hydro-vacuum dispersion.

The mentioned phenomenon provides obtaining powder particles with fibrous, non-equilibrium structurally tense (i.e. mechanically activated) state, which is a result of flattening and asymmetric twisting of drops of liquid metallic breaking away from the liquid metal sucked in the dispergator under conditions of volumetric impact by shock-pulsating waves of hydraulic rarefaction. A schematic of this process with the formation of the Kelvin-Helmholtz wave effect is depicted in longitudinal Section 1′ in Figure 2. This feature can be considered as an essential distinguishing sign of particles obtained by hydrovacuum dispersion of melts. This feature is clearly illustrated in Figure 5, which shows the characteristic morphology and microstructure of a particle obtained by hydrovacuum dispersion of liquid cast iron.

Figure 5.

Surface morphology (a) and microstructure (b) of iron powder particles obtained by hydrovacuum dispersion of melts.

The technological system implementing the mentioned process (operating plant, water circulation mechanism, pulp separator, etc.) is distinguished by its compactness and mobility. It can be used practically in metallurgical production of any scale for large-tonnage production of activated, functional powder from any type of molten metal.

In order to regulate the granulometric composition of the powder particles, the device also provides for the regulation of the rarefaction formed in the core of the vortex of the melt suction cone profile, both by controlling the speed and pressure of the water supplied to the device, and also by changing the vortex profile (shape and height), which is achieved by a semi-toroidal confusor that vertically sprays water supplied by pressure in the body of the device by adjusting the section (dimensions l, h) of the confuser (convergent-divergent) nozzle (5).

Implementation of such a fundamentally different controlled process provides the possibility of continuous production of absolutely homogenized pulp, and therefore, relatively homogeneous, highly activated functional powders.

The proposed technology is environmentally safe. No smoke, steam or dust is released during the work process, because the water circulating in the technological system itself performs the function of both an air filter and a steam condenser. The device operates quietly and without significant vibrations, is compact and easy to install (installation is not related to significant reconstruction of the smelting plants or foundry area). The technical water used in the working area circulates in its own pipe system and also does not pollute the environment. The technology is energy-efficient, eliminates high-temperature heat treatment of the received powders in a regenerating environment. Therefore, it is highly profitable and cost-effective.

4. Research results and discussion

The results of simulation modeling and parallel experimental research (Figure 6) showed us that in order to operate the device with the maximum possible efficiency, which was expressed in the condition of providing technical −1 bar power vacuum with minimum expenses (water pressure), it is necessary to reduce the parameters h and l of the semi-toroidal confuser (Figure 1, position (5)) to the optimal values obtained by the simulation model. These values are: h_opt = 3.24 mm, l_opt = 8.75 mm.

Figure 6.

Simulation model of hydro-vacuum dispersing unit operation and prototype in industrial conditions.

It was determined that, as a result of the optimization of the mentioned parameters, in order to obtain −1 bar rarefaction in the hydro-vacuum dispersion chamber (Figure 1, position 8), in exchange for the supply of water at a rate of 0.029 m³ s⁻¹ into the circular collector channel (4) with a pressure of 16–20 bar used by us so far it will be enough to develop a speed of 0.022 m³ s⁻¹ and a pressure of 13.2 bar. The effectiveness of the mentioned solution is clearly proven by the results of the report on the acceleration rates of the melt particles absorbed/dispersed under the influence of the pressure drop and created rarefaction in the confuser-diffuser pair (5–8), which is presented in Figure 7 in the form of diagrams.

Figure 7.

Results of the simulation study: Indicators of pressure drop and velocity increase of the water supplied to the dispersion chamber in relation to its vertical location, before optimization ((a) and (c)) and after optimization ((b) and (d)) of the dispersing process.

From the diagrams presented, it can be seen that the reduction of the height (h) of the confuser (5) by 35% and the thickness (l) by 47% increases the pressure drop rate in the mentioned transition zone from 11 to 8 bars and increases acceleration of the water-carried metal particles from 3.0–3.5 to 4.1–4.3 × 10² m s⁻¹, which also leads to a change in the amplitude-frequency characteristic of shock waves generated in the device. This is one of the key factors in terms of improving the intensity of melt dispersion and the kinetics of mechanoactivation of the formed powder particles.

From the diagrams showing the results of the reporting data, it can also be seen that as a result of the optimization of the geometrical parameters of the confuser (5) in the diffuser part of the hydro-vacuum device 8–8′, the amplitude of the shock-wave impact generated on average decreases from 0.35 to 0.065 units. On the other hand, it is observed that the frequency decreases by 40% after reaching the height of 120 mm, and in the optimized case, on the contrary, it increases sharply after reaching the height of 180 mm. In addition, it can be seen from the diagrams that if in the first case the zone of active dispersions is located in the height range of 25–180 mm, as a result of parametric optimization, the height of this zone increases to 250 mm. This also indicates the activation of cavitation processes and the strengthening of shock waves accompanying the rarefaction front.

By checking the adequacy of the obtained results, which was carried out by modernizing the structural elements of the device in accordance with the simulation model and a new trial-experimental investigation, we were convinced of the high (≈95%) accuracy and efficiency of the latter. This is clearly confirmed by the graphic material constructed as a result of the experimental research data presented in Figure 8, where it can be seen that as a result of the parametric optimization of the device and process, similar to the simulation model, the amplitude of the vibrations of the dispersion chamber decreases, and the frequency increases, so that the gravitational overload, that is, the average rate of the vacuum suction-acceleration of the dispersed particles decreases relatively insignificantly, by 0.004 m s⁻².

Figure 8.

Vibration of the dispersion chamber of the hydro-vacuum unit before (indicated by the red arrow) and after the working process: (a) vibration’s sweep, mm and overload dynamics before optimization of operating parameters and water pressure, (b) after optimization.

The above is further supported by the data recorded by the digital audio recorder and the acoustic spectrograms obtained as a result of their processing in Figures 9 and 10. In particular, it can be seen from these spectrograms that after parametric optimization of the hydro-vacuum device, the power of the noise emitted during its operation, i.e. the density of its spectrum, decreases at the expense of reducing the amplitude of acoustic waves, i.e. the kinetic energy. This leads to the ability to transfer the potential energy accumulated in these waves to a greater distance (height), and this is very important in terms of increasing the efficiency of cavitation currents acting on dispersed particles, i.e., secondary mechanoactivators. The said is also quite similar to the results of the above-mentioned simulation modeling (Figure 5(d)) and supports it.

Figure 9.

Amplitude spectrum of the audio signal before and after pumping the melt into the dispersion chamber: (a) spectrum before optimizing the geometrical parameters of the converging duct (5); (b) after optimizing.

Figure 10.

Acoustic spectrum of the hydro-vacuum dispersion process in the established operating mode of the equipment: (a, a′) before optimization and (b, b′) after it.

In order to compare the hydromechanical processes generated in the dispersion chamber of the device, in particular, the model and actual profiles of the force hydrodynamic currents, during the experimental work, the procedure of artificial freezing of the jet sucked into the device was carried out, which was carried out by means of a gentle reduction of the water pressure and braking of the dispersion process.

Based on the imprint (Figure 11) left on the peripheral layers of the cylindrical flow of molten metal absorbed by the hydrodynamic shell of the vortex of water sucked into the disperser and conically ejected from the confuser (5), there is no doubt that the ejected water shell, passing through the 36° inclined skirts of the conical cap of the suction nozzle (6), experiences angular acceleration corresponding to the direction of negative rotation of the water tangentially delivered from the tube (2) in the circular collector channel (4), identical to the forces shown in the graphical simulation model (Figure 2).

Figure 11.

Effect of hydromechanical action on the suction metal flow: (a) suction profile for aluminum and (b) the same for gray cast iron.

It is noteworthy that according to the existing imprints, the actual indicator of the inclination of the vector of the angular movement of the hydrodynamic vortex of water was the value provided by the design α ≈ 18 ± 0.5 deg, which indicates the operation of the device in conditions as close as possible to the optimal ones. Deviation from the optimal values of the technological regimes for the larger value (water pressure 13.2 bar, velocity 0.022 m³ s⁻¹) led to a decrease in the mentioned angle, and to a small value, on the contrary, to an increase.

The study of the granulometric composition of the obtained powders, the results of which, in turn, are presented in the form of histograms illustrated in Figure 9, showed that the dominant fraction of particles obtained after the optimization of the hydro-vacuum dispersion process is 100–150 μm, which is on average 60–80 μm less than the fraction formed before the optimization of the process (Figure 12). In addition, its specific share has increased from 10–15% to 25–34%. This effect is also clearly illustrated by the micro-images shown in Figure 13.

Figure 12.

Granulometric composition of obtained powders before (a) and after (b) optimization of the hydro-vacuum dispersion process.

Figure 13.

Cast iron powder microparticles before (a) and after (b) optimization of the hydro-vacuum dispergation process.

The laboratory analysis performed on the manometric calcimeter determined that the reduction of the dispersity of the obtained aluminum powder particles led to an increase in the content of active particles from 95–96% to 99%, which is explained by an increase in the specific surface of the latter and the degree of mechanoactivation, that is, in the concentration of the hidden physico-chemical energy accumulated in them.

Experimental studies also revealed the need to manage the technological parameters having a significant influence on the granulometric composition of powders, such as: the initial temperature of the melt and the speed of its absorption in the disperser. It is significant that the diameter of the inner cylindrical channel of the suction nozzle is the key factor in regulating the speed of melt absorption, i.e. dispersion cost. It was determined experimentally that in the case of hydro-vacuum dispersion of aluminum, the rational weight ratio “liquid metal: water” is 1:20 ÷ 1:23. Therefore, in the case of a water velocity of 0.022 m³ s⁻¹, the optimum speed of aluminum supply to the dispersion chamber shall be 0.0011–0.00095 m³ s⁻¹, which in mass units corresponds to a speed of 2.5–3 kg s⁻¹.

When dispersing liquid cast iron, the rational weight ratio “liquid metal: water” is 1:50–1:55; the optimal vacuum depth is −0.85 ÷ −0.91 bar, achieved at the pressure of injected water 12–16 bar.

The operational management of the process is carried out through a programmed operating computer/laptop, where the function of the supporting signal is performed by the operationally controlled granulometric composition of the produced powder. In case of deviation from the latter’s target indicators, a control impact signal is generated by means of a special portable express analyzer integrated with a computer with a special controller, based on which the computer program using the MATLAB simulation modeling database automatically makes a decision to manage the dispersion process, in particular, the suction speed of the molten metal, by appropriate adjustment of the pressure and consumption of the water tangentially entering in the circular collector channel (4) and semi-toroidal convergent-divergent confuser (5).

The results of the analysis of the chemical activity of the iron powder we obtain clearly showed the advantage of our method of hydrovacuum dispersion of melts. In contrast to conventional powdered iron (e.g. CMS Magnetics brand, 200 microns, Fe 99.95%), with the help of which copper is extracted from sulfate solutions and quarry wastewater from the mining and enrichment of copper ores of the Kazreti deposit (Georgia) in the form of its divalent hydroxide Cu(OH)₂, which requires subsequent energy-intensive reduction firing in a hydrogen environment, our powder, due to the presence of an amorphous phase, predominantly containing γ modification of iron (Fe_γ) [28], directly ensured the production of particles of pure elemental copper. The sequence of chemical reactions taking place was as follows:

$${\text{Fe}}_{\text{γ}}+{\text{Fe}}_3\text{C}+{\text{4CuSO}}_4+{\text{4H}}_2\text{O}\to {\text{4FeSO}}_4+\text{2Cu}+\text{2Cu}{\left(\text{OH}\right)}_2+{\text{2H}}_2+\text{C}$$E1

$${\text{Fe}}_{\text{γ}}+{\text{Fe}}_3\text{C}+{\text{6H}}_2{\text{SO}}_4\to {\text{2Fe}}_2{\left({\text{SO}}_4\right)}_3+\text{C}+{\text{6H}}_2$$E2

$${\text{Fe}}_2{\left({\text{SO}}_4\right)}_3+\text{3Cu}{\left(\text{OH}\right)}_2\to {\text{3CuSO}}_4+\text{2Fe}{\left(\text{OH}\right)}_3$$E3

$${\text{CuSO}}_4+{\text{Fe}}_{\text{γ}}\to {\text{FeSO}}_4+\text{Cu}$$E4

In contrast to the traditional cementation process [26], here the iron consumption is reduced due to the formation of Fe³⁺ trivalent iron ions (Fe₂(SO₄)₃) active for copper extraction. The need for sulfuric acid is also reduced because, in contrast to the compared process, there is no formation of free water molecules neutralizing the set acidity. As a result, it leads to the possibility of maintaining the acidity at a less aggressive level (pH 2.0–2.5 instead of pH 1.0), gives the opportunity to reduce the molar ratio of copper-iron M(Cu):M(Fe) to 1.0:1.1, which leads to savings of iron powder consumption by 30–35% to 2.5–3 kg/m³.

As a result of experimental studies, it was also found that the use of hydrovacuum dispersion method is also highly effective for dispersing slag melts and obtaining from them amorphous solidified (glassy) powdery raw materials with high hydraulic activity, suitable for the production of construction (Portland) cement [29]. In the case of hydrovacuum dispersion of slag, the rational weight ratio “slag: water” was 1:18.

Figure 14 shows that the surface of the grains is smooth and vitreous, without crystal roughness, the grains do not contain pores and metallic inclusions. Comparatively small granules consist of smoother and transparent, vitreous formations.

Figure 14.

Morphology (1), EDS and XRD spectrograms (2) of granules obtained by hydro-vacuum granulation of gray cast iron slag [29].

XRD spectral analysis (Figure 14(2)) showed that the XRD diffractogram does not contain clearly distinguished diffraction peaks of the crystalline component, which of course indicates the absence of such phases, accordingly, it can be stated that the structure is mainly composed of vitreous, hydrous active, amorphous phase.

The results of the analysis of hydraulic activity of the obtained slag granules showed that on average their activity on CaO absorption is 650–750 mg/g, which is almost twice higher than that of slag granulated by traditional water technologies [21, 30, 31]. The slag granulated in this way, in addition to its use in the production of construction and ceramic materials [32], can also be successfully used as a substitute for mineral abrasives applied for sandblasting of parts for various purposes [33].

The proposed method of hydrovacuum dispersion of metallurgical alloys and the device for its realization on the basis of the above experimental studies is implemented and started up at the production site of “Geo Enterprise” LLC. [34].

5. Conclusions

A systematic analysis of the above results allows us to conclude that:

1. A new method for dispersing metallurgical melts has been developed, the essence of which is to initiate the process of forced vertical vacuum suction of the liquid melt in the form of a thin jet and its droplet fragmentation by gradient (layer-by-layer) acceleration in a force field of hydromechanical action. The source of the suction effect is the rarefied core of the water vortex formed in the central zone of the dispersion chamber;

2. The process is dynamically controlled and can be carried out continuously. The pulp formed with water cools instantly without emitting steam, smoke or dust;

3. The productivity of the proposed method and dispersing unit is on average 10 times higher than that of conventional atomization installations of identical dimensions. This increase in productivity will undoubtedly lead to a significant reduction in the production cost of the resulting powders. In addition, dynamic vacuumization of metal flow and hence additional possibility of its physical purification from harmful non-metallic and gas impurities can lead to the possibility of using cheap secondary metals and alloys as a starting material instead of currently used pure metals;

4. A schematic hydromechanical and computer simulation modeling of the operating features of the dispersion chamber was carried out. A comparative analysis of the data obtained by modeling with experimental data was performed. It is determined that the further improvement of the simulation model and the consideration of parameters of such essential importance as the initial temperature of the melt, its viscosity and the speed of its suction in the dispersion chamber, will give us the opportunity to identify the necessary technical conditions for the optimization of hydro-vacuum dispersion regimes for the free-flowing melt of practically any type and technological complexity without conducting preliminary experimental studies;

5. Morphological and physio-chemical studies of the obtained powders have shown their advantage over analogs in terms of mechanochemical activity, which is achieved by the peculiarity of the proposed method of hydromechanical impact on the surface of the hardenable particles unevenly three-dimensional shock waves of rarefaction and pulsating cavitation detonations;

6. It has been established that the method and unit of hydro-vacuum dispersion are universal and can also be effectively used for the treatment of slag melts. It is shown that the high degree of structural amorphization and glass transition of the obtained slag microgranules gives them high hydraulic activity suitable for the production of Portland cement;

7. From the viewpoint of industrial implementation of the developed unit, the optimal operating modes of the hydro-vacuum device for dispersing metal melts are: water pressure—13.2 bar, water supply rate—0.022 m³ s⁻¹, thickness of the outlet neck of the annular confuser for the reverse ejection (on 162°) of water—3.24 mm, the suction speed of the melt—2.5–3 kg s⁻¹, which, when working on aluminum melts, is provided by the suction cylinder nozzle with the inner channel diameter of 8 mm. In the case of dispersing liquid cast iron, the rational diameter of the suction cylinder nozzle is 4.5 mm, and in the case of granulation of slag melts—15 mm.

Acknowledgments

The authors acknowledge funding support from the Shota Rustaveli National Science Foundation of Georgia, Grant no. AR-22-1495.

For technical support, the authors also acknowledge the directors of the R. Dvali Institute of Mechanics of Machines and “Geo Eenterprise” LLC, Mr. Tamaz Natriashvili, (Academician of the National Academy of Sciences of Georgia) and Mr. Zurab Magradze.

For support during the modeling and technical measurement phase, special thanks to Anzor Kuparadze and Nata Sulavkvelidze, managers of G3D Ltd.

Conflict of interest

The authors declare no conflict of interest.