Vol 58, No 2 (2017)

The results of experimental investigations into the zeta potential of ultrafine sulfides (chalcopyrite, tennantite, galenite, sphalerite, pyrite, and pyrrhotine), the floatability of monomineral fractions of sulfides of the floatation coarseness (–0.1 + 0.05 mm) in a mechanical flotation machine, and the floatability fine particles of sulfide (–0.044 + 0.010 mm) in the Hallimond tube are presented. The preparation procedure of ultrafine powders and sulfhydril collectors to measure the zeta potential is presented. Zeta potentials of the surface of mineral particles and insoluble forms of sulfhydril collectors are investigated in the pH range from 2.0 to 12.5 (an acidic medium was formed by using H₂SO₄ and basic one by using NaOH or Ca(OH)₂), and various values of zeta potentials are found for sulfides in the sodium hydroxide and lime medium. Zeta potentials for all sulfides are negative in the NaOH medium at pH > 9.5, they are positive (1–18 mV) in the Ca(OH)₂ medium at pH > 11, and zeta potentials for chalcopyrite are positive in the studied range \(p{H_{Ca{{\left( {OH} \right)}_2}}}\) 9.0–12.5. The values of the isoelectric point in the medium of sulfuric acid and sodium hydroxide are as follows: for chalcopyrite—pH 6.5 and 8.8, for tennantite—pH 3.0, for sphalerite—pH 5.1 and 6.4, for pyrite—pH 3.1 and 8.9, and for pyrrhotine—pH 7.0. In the lime medium, the value for tennantite and sphalerite pH 12.0, for galenite—pH 11.2, for pyrite—pH 9.5 and 11.2, and for pyrrhotine—pH 9.5 and 12.1. Measurements of zeta potentials of ultrafine sulfide particles give the opportunity to refine the interaction mechanism of sulfhydril collectors with sulfides and associate the nonselective recovery of final tailings of sulfides in the highly alkaline lime medium with the contribution of the electrostatic component during the adhesion of ultrafine sulfide particles on bubbles and their mechanical carryout into the froth product.

The results of physical simulation of the behavior of bubbles formed due to the electrochemical evolution of oxygen on an inert anode during the high-temperature electrolysis of alumina slurry in the fluoride melt are presented. Similarity criteria are calculated, the experiments for a water model with vertically oriented electrodes are performed, and the data on the behavior of bubbles in the slurry are found with the help of video recording. The 20% aqueous solution of sulfuric acid with an alumina content of 30 vol % was used as the model electrolyte. The experiments were performed in a range of current densities from 0.05 to 0.25 A/cm². Video was recorded using a Nikon D3100 camera with a recording frequency of 30 frames/s. The data on the motion dynamics of the bubbles, the quantitative data that characterize coalescence, and the bubble lifting velocity are found. To determine the average lifting velocity, 125 bubbles were analyzed. They were 0.8–2.3 mm thick. The bubble motion is performed in the slug regime with lifting velocity of 1.0–2.3 cm/s. The bubble layer thickness was about 5 mm. Further investigations will be directed to finding new data on the behavior of bubbles for various solid phase contents, current density, electrode slope angle, and granulometric composition.

To produce castings of titanium, nickel, copper, aluminum, and zinc alloys, graphite molds can be used, which makes it possible to provide a high cooling rate. No die coating and lubricant are required in this case. Computer simulation of casting into graphite molds requires knowledge of the thermal properties of the poured alloy and graphite. In addition, in order to attain adequate simulation results, a series of boundary conditions such as heat transfer coefficients should be determined. The most important ones are the interface heat transfer coefficient between the casting and the mold, the heat transfer coefficient between the mold parts, and the interface heat transfer coefficient into the environment. In this study, the interface heat transfer coefficient h between the cylindrical aluminum (99.99%) casting and the mold made of block graphite of the GMZ (low ash graphite) grade was determined. The mold was produced by milling using a CNC milling machine. The interface heat transfer coefficient was found by minimizing the error function reflecting the difference between the experimental and simulated temperatures in a mold and in a casting during pouring, solidification, and cooling of the casting. The dependences of the interface heat transfer coefficient between aluminum and graphite on the casting surface temperature and time passed from the beginning of pouring are obtained. It is established that, at temperatures of the metal surface contacting with a mold of 1000, 660, 619, and 190°C, the h is 1100, 4700, 700, and 100 W/(m² K), respectively; i.e., when cooling the melt from 1000°C (pouring temperature) to 660°C (aluminum melting point), the h rises from 1100 to 4700 W/(m² K), and after forming the metal solid skin on the mold surface and decreasing its temperature, the h decreases. In our opinion, a decrease in the interface heat transfer coefficient at casting surface temperatures lower than 660°C is associated with the air gap formation between the surfaces of the mold and the casting because of the linear shrinkage of the latter. The heat transfer coefficient between mold parts (graphite–graphite) is constant, being 1000 W/(m² K). The heat transfer coefficient of graphite into air is 12 W/(m² K) at a mold surface temperature up to 600°C.

A new cold deformation method of cast magnesium is proposed. The method includes the implementation of the upsetting technique under lateral pressure. The billet is initially placed into a jacket made of ductile metal and then into a container. The billet is affected with a puncheon placed into the container with a gap. Under the press force, the jacket metal flows into the gap and forms the pressure. This results in an increase in the level of compressing stresses, which increases the ductility of magnesium. Deformation tests of cast magnesium samples showed that the relative reduction without destruction can be increased from 12‒18 to 60–70%. Such an increase in ductility makes it possible to produce deformed magnesium billets without heating. A simplified removal procedure of the samples from jackets after the completion of deformation is foreseen. It is revealed that the process can be performed at average upsetting pressures of 820–830 MPa, which is acceptable for modern tool materials.

In this work, the effect of aging period on the characteristic transformation temperatures, thermodynamic parameters and structural variations of CuAlNiMn shape memory alloys were investigated. Aging was performed at above the austenite finish temperature of the un-aged specimen (120°C) for six different retention times, namely 1h, 2h, 3h, 4h, 5h and 6h. The changes in the transformation temperatures were examined by differential scanning calorimetry at different heating/cooling rates. The aging period was found to have an effect on the characteristic austenite and martensite transformation temperatures and thermodynamic parameters such as the enthalpy and entropy of alloys. High-temperature order-disorder phase transitions were determined using a differential thermal analysis, which showed that all the un-aged and aged specimens had an A2 → B2, B2 → L2₁ and an L2₁ → 9R, 18R transition. The structural analysis of the un-aged and aged specimens was performed through X-ray diffraction measurements at room temperature. The intensities of the diffraction peaks varied according to the aging time.

The corrosion rate was determined and corrosion damage features of the samples made of AK4-1 aluminum alloy were investigated in the NACE solution containing hydrogen sulfide (H₂S). The alloy was investigated in the ultra-fine-grain state compared with the coarse-grain state formed after standard treatment T6 (quenching + ageing). The alloy was nanostructured by equichannel angular pressing (ECAP). It is shown that the alloy corrosion rate after ECAP is higher than after T6 treatment by a factor of 1.9. Herewith, the total corrosion takes place in alloy after ECAP, while pitting corrosion is also observed after T6 treatment in addition to the total corrosion. The corrosion effect strongly affects surface-roughness parameters of the samples made of AK4-1 alloy after ECAP compared with the samples after T6 treatment.

The possibility of reinforcing the Al/Al₂O₃ laminated cermet matrix with metal rapidly solidified alloy fibers (steel, titanium, and aluminum), as well as discrete duralumin chips, is shown. The maximal reinforcing effect was attained when using titanium and steel fibers with their content of 20 and 10 vol %, respectively, due to the implementation of several energy-intensive destruction mechanisms. Reinforced composites are characterized by the following properties: ρ = 2.30–2.85 g/cm³, σ_bend = 180–250 MPa, K_1c = 7.5–15 MPa m^1/2, and KCU = (18–35) × 10³ J/m². The Al/Al₂O₃–C_coke.residue has ρ = 2.21–2.23 g/cm³ at a very low sliding friction coefficient of 0.17 (the counterbody is a ball made of steel ShKh-15 under a load of 1 N). The oxide-adhesion bond type, which makes it possible to remove spent grains from the grinding work zone and implement the self-sharpening mode, is formed for the “Al/Al₂O₃–fused alumina grains” composite. The material that contains kaolin fibers is ultra-light-weight ceramic insulation (0.25–0.5 g/cm³), λ = 0.07–0.2 W/(m K) in the 20–1000°C range. The material including alumina spherulites combines rather high hardness (σ_bend = 10–50 MPa) and porosity (42–52%) and has increased thermal stability due to the rapid elimination of the temperature gradient on structural elements having a micrometer-size cross section.

The results of studying the properties of copper galvanic coatings fabricated using an L1-210 v2 galvanic installation (Italy) using the bright copper plating electrolyte produced by 24 Karata (Moscow) and the addition of electroerosion copper nanopowder fabricated by the electroerosion dispersion (EED) method using copper wire scrap in distilled water are given. An original setup developed by the authors (RF Patent 2449859) was used for the EED of conductors. The friction coefficient and wear factor found when testing coatings using a Tribometer automated friction machine (CSM Instruments, Switzerland) indicate the absence of substantial distinctions in the wear resistance of the samples. Surface hardness tests of the sample were performed using a DM-8 automated microhardness tester according to the micro-Vickers method with an indenter load of 25 g by ten imprints with a free selection of the indentation point according to GOST (State Standard) 9450–76. The indenter loading time was 15 s. It is established that the microhardness of a copper coating with the addition of copper nanoparticles is 15% higher than that of steel substrate and the sample with a standard copper coating.

The microstructure and mechanical properties of four matrices of a complex chemical composition—VK15–Cu, VK15–Cu–Sn, VK20–Cu, and VK20–Cu–Sn—which are applied when fabricating diamond drilling tools, are investigated. The samples of matrices were formed by the infiltration of formations of the hard-alloy charge with copper at t = 1100°C and with bronze at t = 1000°C. The slices of diamond-free matrices were fabricated; their metallographic and electron probe microanalysis were performed; and their hardness, microhardness, density, and porosity were determined. Hardness of matrices was established by the HRC scale and microhardness was established using a PMT-3 device under a load of 2 N. It is revealed that (i) hard and strong diamond-tool matrices with open porosity no larger that 3% are formed by the infiltration of formations of the hard-alloy charge of VK15 and VK20 grades by the melts based on copper and bronze, and infiltration passes more completely when using bronze; (ii) matrices formed by the infiltration of formations of the hard-alloy charge of the VK15 grade with bronze are hardest. The application of matrices based on the hard-alloy charge of the VK20 grade when fabricating the drilling diamond tool is unreasonable because of an abrupt decrease in its abrasive ability when drilling hard materials.