Vol 57, No 5 (2016)

The joint consideration of subprocesses of particle capture and detachment, and buoying of aggregates in the periodic nonfrothing flotation conditions shows that the mineral load formed on a separate part for its buoying time (τ_m). This load is a part of the equilibrium mineral load, which can be attained under the infinite mineralization time. It is proposed to characterize the composition and attainment rate of the mineral load by two dimensionless parameters, which depend on intensities of subprocesses. The sort parameter of particles (B) has been uniquely determined by the ratio of the detachment intensity to the capture intensity, while the dimensionless time (D) is determined by the ratio of the particle capture and detachment rate to the buoying velocity of the air bubble. The mineralization kinetic equation by many bubbles is derived in the exponential form similarly to the first-order Beloglazov equation. Intensities of capture and detachment subprocesses in the mineralization rate constant (K_m) determine the magnitude of recovery by a separate bubble (ε_bm) for time τ_m, while the air consumption determines the summary recovery ε.

Alkaline roasting-alkaline leaching process was used to recover vanadium from LD (Linz Donawitz) converter slag. The independent leaching parameters investigated were liquid to solid ratio (L/S) (10–20 mL/g), temperature (40–60°C), NaOH concentration (1.0–3.0 M), and time (60–120 minutes). Response surface methodology (RSM) was utilized to optimize the leaching parameters and as a result, the most influencing parameter was found to be liquid to solid ratio. Based on the results, the optimum recovery condition (approx. 99%) was obtained with L/S ratio of 20, temperature of 40°C, NaOH concentration of 3.0M, and leaching time of 100 minutes, respectively. Furthermore, the kinetics of alkaline leaching process was investigated using shrinking core model (SCM) equations. It was found that the rate of vanadium leaching is controlled by a mixed controlling mechanism which is comprised of chemical reaction and diffusion through the solid product layer.

The sorption of Pd(II), Ag(I), and Cu(II) from nitric acid solutions with silicas chemically modified with γ-(aminopropyl)trietoxysiloxane is investigated. It is assumed based on the investigation of solid phases by IR and X-ray photoelectron spectroscopy and thermogravimetry that the following processes occur during the sorption of palladium from nitric acid solutions: coordination of palladium atoms to nitrogen atoms of the functional group, interaction of palladium ions with the silica matrix, and the formation of polynuclear complexes. It is proposed to use a 5% solution of thiourea in 0.1 M HCl. It is determined that the establishment time of constant values of sorption in static conditions is as follows, min: 10 for Ag(I), 20 for Cu(II), and 30 for Pd(II). A sorbability series of ions from 2 M HNO₃ in identical conditions is as follows: A_Ag(I) > A_Pd(II) ≫ A_Cu(II). Silver ions are not sorbed from solutions with the concentration of HNO₃ < 1 M, which will create prerequisites for the separation of ions.

The influence of treating the melts by electromagnetic acoustic fields on the structure and properties of Al–12% Si and Al–20% Si binary alloys is investigated. In the course of experiments, the frequency of the electromagnetic field induced in the loop antenna varies as 500, 1000, and 2000 kHz. The melts are treated after their degassing and refining. It is established that this treatment method of the melts leads to a reduction of the total preparation time of alloy by 12% on average. The short-term treatment of the melts by electromagnetic acoustic fields promotes the refinement of the main phase components of alloys and an increase in their mechanical properties. When treating the Al–12% Si eutectic alloy with a frequency of 500 kHz, α-Al dendrites are refined from 30 to 22 μm and eutectic Si crystals are refined from 13 to 10 μm. When treating the Al–20%Si eutectic alloy with a frequency of 1000 kHz, eutectic Si crystals diminished from 8 to 5 μm and these of primary Si diminished from 90 to 62 μm. The ultimate tensile strength of the Al–12%Si eutectic alloy increases 13% under the mentioned treatment modes, while the relative elongation increases 17%; as for the Al–20% Si eutectic alloy, the same characteristics increases 9 and 65%, respectively. Based on these investigations, it is concluded that the selection of the treatment parameters of the melts of the Al–Si system by electromagnetic acoustic fields should be determined by the silicon content in the alloy. It is necessary to treat the melt by waves with a higher oscillation frequency with an increase in the silicon concentration. This treatment method makes it possible to form the modified fine-crystalline structure of alloy and, consequently, improves their mechanical properties. It can be successfully used when fabricating fine-crystalline foundry alloys and in the production of alloys of the Al–Si system. To determine the optimal treatment parameters depending on the structure of the initial charge and alloy nature, additional investigations are required.

When producing rod and wire wares, residual stresses are formed. This phenomenon is caused both by plastic deformation and by possible thermoplastic deformation, which appears due to contact heating the metal ware surface due to external friction forces. Undesirable residual stresses in surface layers affect the accuracy of metal wares and increase their failure probability, which is observed in the practice of drawing production. In this study, a procedure is proposed to determine the contact heating conditions from the formation prevention criteria of residual stresses. Based on thermoelasticity equations, temperature modes leading to the appearance of thermoplastic deformations are established. Critical values of the temperature difference between the drawing wire surface and the center, at which the wire surface layers transform into the plastic state with the subsequent formation of residual stresses, are determined. Limiting drawing velocities for a series of nonferrous metals (copper, zirconium, and titanium), which, if exceeded, will lead to undesirable residual stresses in the drawn wire, are determined. To increase the critical velocities, it is recommended to implement the hydrodynamic (fluid) friction mode conditions when producing metal wires.

We investigated the influence of multistep up to 650°C on the magnitude of resistivity and hardness of hot-rolled sheets of low-alloyed aluminum alloys containing up to 0.5 wt % Zr. Experimental samples were formed in conditions approaching those implemented for industrial installations of continuous casting and rolling. Testing procedures, including heat treatment, are described. Metallographic analysis of a cast (initial) structure and the structure of experimental samples passing the deformation was performed. Dependences of resistivity (ρ) and hardness on the temperature of the last annealing step are constructed by the results of physicomechanical tests. It is established using the computational and experimental methods that the magnitude of ρ mainly depends on the zirconium concentration in the aluminum alloy. The optimal ratio between the zirconium concentration in the alloy and annealing temperature, which makes it possible to attain the best combination of characteristics of hardness and resistivity, is determined.

Effect of minor Gd addition on the microstructure, mechanical properties and wear behavior of as-cast Mg–5Sn-based alloy was investigated by means of OM, XRD, SEM, EDS, a super depth-of-field 3D system, standard high-temperature tensile testing and dry sliding wear testing. Minor Gd addition has strong effect on changing the morphology of the Mg–5Sn binary alloy. Gd addition benefits the grain refinement of the primary α-Mg phase, as well as the formation and homogeneous distribution of the secondary Mg₂Sn phase. The mechanical properties of the Mg–5Sn alloys at ambient and elevated temperatures are significantly enhanced by Gd addition. The wear behavior of the Mg–5Sn alloy is also improved with minor Gd addition. The alloy with 0.8% Gd addition exhibits the best ultimate tensile strength and elongation as well as the optimal wear behavior. Additionally, the worn surface of the Mg–5Sn–Gd becomes smoother in higher Gd-containing alloys. The best wear behavior of alloy was exhibited when Gd addition was up to 0.8%, showing a much smoother worn surface than that of control sample. The improvement of tensile properties is mainly attributed to the refinement of microstructure and the increasing amount and uniform distribution of Mg₂Sn phase. The larger amount of Mg₂Sn phase uniformly distributed at the grain boundary of Mg–Sn–Gd alloys act as a lubrication during sliding, and combined with smaller grain size improve wear behavior of the binary alloy.

Gas metal arc welding cold metal transfer (GMAW-CMT) method with AlSi₃Mn filler wire was performed on welding of the 5754 aluminum alloy with thickness of 3 mm to the galvanized steel with thickness of 2 mm aluminum alloy to investigate the effect of pulse correction on structure and mechanical properties of welded samples. In accordance with results, GMAW-CMT provides good tensile performance. It was attributed to the various throat weld size and wetting actions because of the influence of pulse correction on structure of welded joints. It was inferred that on employing +5 pulse correction resulted in better and consistent tensile strength of 209 MPa. Furthermore, the results showed that increasing the pulse correction led to increasing of flow in the filler wire and in fact raising of brazed seam width and throat weld size. In addition, the thickness of intermetallic compound layer which was formed along the interface during the GMAW-CMT was varied by changing of pulse correction. It has been found that by increasing the pulse correction from–5 to +5, the throat weld size increased and consequently led to a change in the tensile strength of the welded joints.

The influence of dispersed different nature microadditives on structure formation and volume changes during sintering, and strength of powder carbon and high-chromium steel is investigated. Mechanisms of the effect of additives on the formation of their structure are described. It is shown that the introduction of sodium bicarbonate provides the largest strengthening of carbon steel, while the introduction of boron nitride provides that of high-chromium steel. The level of increasing the strength depends on the sintering temperature and amount of additive.

The results of the investigation into the compaction (sintering) of silicon carbide nanopowders and micropowders in a DO-138 high-pressure apparatus are presented. Compaction modes for both types of materials are identical (a pressure of 3.5–4.0 GPa, a temperature of 1600—1700°C, and a holding time of 10 s). The influence of cladding of SiC nanopowders and micropowders with titanium and titanium nitride on the properties of compacts (cakes) formed under the same sintering modes is investigated. It is established that, when compacting the silicon carbide nanopowder, cakes differ in regards to higher density, hardness, and lower porosity compared with the samples made of finely dispersed technical silicon carbide. A higher activity of titanium relative to SiC makes it possible to chemically associate the grains of the latter due to the formation of intermediate layers of titanium carbide between them. The resulting ceramics possesses a higher density, hardness, and wear resistance. The wear resistance of synthesized composites based on nano-SiC is higher than for a polycrystalline material based on silicon carbide micropowder by a factor of 4.5.

The experiments on the fabrication of materials based on the Ti–3Al–0.5Ta and 3Ti–2Al–Ta systems by self-propagating high-temperature synthesis (SHS) are performed. The influence of the composition of the initial mixture, dispersity of powders, and preliminary mechanical activation on the phase composition and structure of the SHS product is investigated. The optimal ratio between the mechanically activated and initial powder in a mixture for the synthesis of materials is determined. The dependence of the structure of final products on the structure of initial powders is established. The use of porous tantalum leads to the formation of the intermetallic matrix based on titanium aluminide with the uniform distribution of Ta particles. It is noteworthy that tantalum powders of both studied series (which differ by dispersity and morphology) partially reacted already at the stage of mechanical activation with the formation of the Al₂Ta phase. It is shown that aluminum plays the leading role in processes of mechanical activation in Ti–Al–Ta reaction mixtures. Indeed, a considerable rise of unreacted tantalum particles in the microstructure of sintered samples is observed with a decrease in the amount of aluminum in the reaction mixture.

Kinetic features and the contact interaction mechanism of hot-pressed (residual porosity <3%) of titanium carbonitride samples of various compositions with the Ni–25% Mo melt (t = 1400–1500°C, τ = 0.1–25 h) are investigated by electron-probe microanalysis. It is established that the dissolution rate of the interstitial refractory phase (IRP) in the Ni–Mo melt lowers in a series TiC–TiC_0.7N_0.3–TiC_0.5N_0.5, while the degree of process incongruence rises. The composition of intermediate interaction products varies correspondingly. The peculiarities of formation of the most important phase component of the TiCN cermets—K-phase of the Ti_1–nMo_nC_x composition—are revealed. It is proven by the local mass spectrometry method that the K-phase has a carbide nature. It is also established that it is formed only if the initial titanium carbonitride TiC_1–xN_x is sufficiently enriched with carbon (x ≤ 0.5). It is stated that the K-phase is an actual basis of all cermets with the Ni–Mo binder. Its bulk concentration in alloys exceeds the content of the nominal alloy base by a factor of several times. The chemical substantiation of the selection of titanium carbonitride of the TiC_0.5N_0.5 composition as an optimal “precursor” of the K-phase, which is formed during liquid-phase sintering of TiCN cermets, is given originally.

The composition and thermodynamic characteristics of plasma-forming gases (Ar, N₂, H₂, and mixtures Ar + N₂ and Ar + H₂) depending on temperature are investigated using thermodynamic modeling. The equilibrium composition and thermodynamic properties of the plasma–“particle” system are modeled. Pure metals, oxides, technical powders, and powder mixtures were considered as the particle. The influence of power materials TiO₂, Al₂O₃, and Fe₃O₄ on the variation in the plasma enthalpy is investigated. It is shown that computer modeling of the influence of thermodynamic parameters of the heterogeneous plasma jet makes it possible to optimize the modes of the plasma deposition and formation of plasma coatings. The proposed approach substantially shortens the time necessary to develop the formation technologies of coatings with the specified functional properties such as protective, antiwear, etc.

The electrochemical deposition of coatings of double tungsten and molybdenum carbides from tungstate–molybdate–carbonate melts is investigated. The composition of formed coatings is investigated by X-ray fluorescent and X-ray phase analyses. The crystal size and coating thickness are determined using scanning electron microscopy. Optimal deposition parameters of W₂C · Mo₂C coatings are as follows: the melt composition is Na₂WO₄–(1.0–4.0 mol %) Li₂WO₄–(1.0–4.0 mol %) Li₂MO₄–(1.0–5.0 mol %) Li₂CO₃, the cathode current density is 750–1500 A/m², the process temperature is 1123–1173 K, and the electrolysis duration is 4 h.