Igor V. Timoshkin

Liberation of valuable inclusions in ores and slags by electrical pulses

Results of comparative liberation tests on electrically disintegrated and mechanically comminuted oxide ores containing hematite and PGM and sulphide ores containing complex Cu sulphides and pentlandite showed that disintegration of ore aggregates by electrical pulses generates a higher percentage of liberated particles and lower percentage of fine material than that obtained by mechanical comminution. Mechanism of electrical disintegration creating a specific liberation process by splitting mineral aggregates along boundaries of minerals with different electrical parameters is described. The main effect of liberation by electrical pulses manifests in the generation of a higher percentage of the coarse monomineral particles and in the general increase of liberated material with dimensions of particles, which can be efficiently treated by the separating equipment. Comparative amount of monomineral particles in the fine ( 106 μm), this difference sometimes exceeds 40%. General improvement in the recovery and grade was achieved on both oxide ores and in the pentlandite ore.

Liberation of minerals by high-voltage electrical pulses

Abstract Liberation of minerals by the high voltage electrical pulses is a new specific liberation technology, capable of producing of the high percentage of the monomineral particles at disintegration of mineral aggregates. The specific liberation effect at the disintegration of rock by electrical pulses is due to the locality of the explosive plasma streamers at the interface of mineral components of the ore aggregates with different permittivities and electrical conductivities. Three models with calculation of the distribution of electrical fields in the solids water-system showed concentration of fields at the interface of solids and liquid and different mineral components of ore aggregates.The comparative liberation tests on magnetite and haematite samples showed the efficiency of the new technology on the basis of substantially smaller recovery of impurities of SiO2 and P into iron oxides concentrates, obtained from electrically disintegrated ores. The energy consumption and the equipment for liberation of minerals by high-voltage electrical pulses is discussed.

High‐Intensity 405 nm Light Inactivation of Listeria monocytogenes

The antimicrobial properties of light is an area of increasing interest. This study investigates the sensitivity of the significant foodborne pathogen Listeria monocytogenes to selected wavelengths of visible light. Results demonstrate that exposure to wavelength region 400–450 nm, at sufficiently high dose levels (750 J cm−2), induced complete inactivation of a 5 log10 population. Exposure to wavelengths longer than 450 nm did not cause significant inactivation. Analysis of 10 nm bandwidths between 400 and 450 nm confirmed 405(±5) nm light to be most effective for the inactivation of L. monocytogenes, with a lesser bactericidal effect also evident at other wavelengths between 400 and 440 nm. Identification of the optimum bactericidal wavelength enabled the comparison of inactivation using 405(±5) nm filtered light and a 405 nm light‐emitting diode (LED) array (14 nm FWHM). Results demonstrate similar inactivation kinetics, indicating that the applied dose of 405 nm light is the important factor. Use of the 405 nm LED array for the inactivation of L. monocytogenes and other Listeria species resulted in similar kinetics, with up to 5 log10 reductions with a dose of 185 J cm−2. Comparative data for the 405 nm light inactivation of L. monocytogenes and other important foodborne pathogens, Escherichia coli, Salmonella enteritidis and Shigella sonnei, are also presented, with L. monocytogenes showing higher susceptibility to inactivation through 405 nm light exposure.

Transient electrical field across cellular membranes - pulsed electric field treatment of microbial cells

The pulsed electric field (PEF) treatment of liquid and pumpable products contaminated with microorganisms has attracted significant interest from the pulsed power and bioscience research communities particularly because the inactivation mechanism is non-thermal, thereby allowing retention of the original nutritional and flavour characteristics of the product. Although the biological effects of PEF have been studied for several decades, the physical mechanisms of the interaction of the fields with microorganisms is still not fully understood. The present work is a study of the dynamics of the electrical field both in a PEF treatment chamber with dielectric barriers and in the plasma (cell) membrane of a microbial cell. It is shown that the transient process can be divided into three physical phases, and models for these phases are proposed and briefly discussed. The complete dynamics of the time development of the electric field in a spherical dielectric shell representing the cellular membrane is then obtained using an analytical solution of the Ohmic conduction problem. It was found that the field in the membrane reaches a maximum value that could be two orders of magnitude higher than the original Laplacian electrical field in the chamber, and this value was attained in a time comparable to the field relaxation time in the chamber. Thus, the optimal duration of the field during PEF treatment should be equal to such a time.

Hydrodynamic modelling of transient cavities in fluids generated by high voltage spark discharges

Application of a voltage pulse having a rise time of tens of nanoseconds to electrodes immersed in water results in streamer development and the formation of a highly conductive plasma channel between the electrodes. The electrical resistance of such channels decreases rapidly from a few ohms to a few tens of milliohms due to Joule heating resulting from the high current which flows through the plasma. The dynamics of the plasma resistance depend on the parameters of the discharge circuit and the medium in which the discharge takes place. The resistance of the channel reaches a minimum value approximately at the moment of the peak current for under-damped current oscillations. During the resistance collapse, the pressure inside the channel rises to several GPa, causing a rapid expansion of the channel which forms a cavity in the liquid resulting in a high power ultrasound pulse. The cavity expands to a maximum size which is dependent on the circuit driving the discharge and the properties of the plasma discharge channel. The cavity then collapses producing a second acoustic pulse. In this paper the dynamic resistance of the spark channel is described using a phenomenological model based on the plasma channel energy balance equation used by Braginskii. The model which links the hydrodynamic characteristics of the channel and the resulting cavity with the parameters of the electric driving circuit allows the development of the plasma channel and cavity to be predicted. The peak high-power ultrasound (HPU) pressures calculated using this approach are compared with the pressure values estimated by an analytical model which uses a constant value of the spark channel resistance derived from experimental data. Comparisons are also made with direct measurements of HPU output made using a Pinducer sensor. Although the model is based on a phenomenological description of the plasma channel dynamics and its resistance and requires the value of the spark constant, the results obtained using this approach provide a reasonable agreement with experimental measurements and could therefore be used for the estimation of HPU pulse characteristics in practical applications of spark discharges in water.

Plasma channel miniature hole drilling technology

With growing economic and environmental pressures, oil companies seek ways to increase the oil recovery from every well. Re-entry drilling of horizontal sidetracks from existing wells and multilateral drilling for new wells are techniques which help to achieve that increase. However, new approaches are needed to reduce the cost and to improve efficiency of rotary drilling methods used for these purposes. A new nonrotary plasma channel drilling (PCD) method for drilling of small diameter holes (3.5-15 cm) in oilfields for sidetrack creation and multilateral drilling is described in this paper. The method relies upon the use of the energy of impulse spark discharges which are formed inside the rock formation ahead of the drill position. Repeated formation of the plasma channel results in an effective and controlled drilling action. Plasma channel drills developed at Strathclyde University are able to cut clearly defined circular holes in medium hard sandstone with speeds of up to 16 cm/min using specific energies as low as 400 J/cm/sup 3/. Plasma channel drilling has the potential to increase the lifetime of oil wells and would significantly reduce the cost of exploration drilling and subsurface data acquisition. PCD technology can also be a cost-effective and practical solution for other applications such as mineral mining, water boring and scale removal where micro-hole drilling is essential.

Photoinactivation of Bacteria Attached to Glass and Acrylic Surfaces by 405 nm Light: Potential Application for Biofilm Decontamination

Attachment of bacteria to surfaces and subsequent biofilm formation remains a major cause of cross‐contamination capable of inducing both food‐related illness and nosocomial infections. Resistance to many current disinfection technologies means facilitating their removal is often difficult. The aim of this study was to investigate the efficacy of 405 nm light for inactivation of bacterial attached as biofilms to glass and acrylic. Escherichia coli biofilms (103–108 CFU mL−1) were generated on glass and acrylic surfaces and exposed for increasing times to 405 nm light (5–60 min) at ca 140 mW cm−2. Successful inactivation of biofilms has been demonstrated, with results highlighting complete/near‐complete inactivation (up to 5 log10 reduction on acrylic and 7 log10 on glass). Results also highlight that inactivation of bacterial biofilms could be achieved whether the biofilm was on the upper “directly exposed” surface or “indirectly exposed” underside surface. Statistically significant inactivation was also shown with a range of other microorganisms associated with biofilm formation (Staphylococcus aureus, Pseudomonas aeruginosa and Listeria monocytogenes). Results from this study have demonstrated significant inactivation of bacteria ranging from monolayers to densely populated biofilms using 405 nm light, highlighting that with further development this technology may have potential applications for biofilm decontamination in food and clinical settings.

Bactericidal Effect of Corona Discharges in Atmospheric Air

This paper explores the possibilities of using impulsive and steady-state corona discharges for biodecontamination operations. A high tension tubular corona electrode was stressed with positive or negative dc voltage with magnitude up to 26 kV, and a grounded mesh was used as an opposite electrode. Different operational regimes of this corona generator were investigated for the production of ozone in air flow and the inactivation of microorganisms. The test microorganisms used in this work were Escherichia coli and Staphylococcus aureus, populations of which were seeded onto agar plates. These bacterial plates were located behind the grounded mesh electrode to assess bactericidal efficacy. The results show that corona discharges have a strong bactericidal effect, for example, positive flashing corona discharges were able to reduce populations of the test microorganism by ~ 94% within a 30-60-s time interval. Negative steady-state corona discharges also produce noticeable bactericidal effect, reducing population of E. coli and S. aureus by more than 97% within a 120-s energization interval. The bactericidal efficiency of different corona discharge modes and its correlation with ozone levels produced by these discharges are discussed. The results obtained in this work will help in the design and development of compact plasma systems for environmental applications.

Energy consumption and liberation of minerals in explosive electrical breakdown of ores

Abstract The disintegration of brittle composite dielectrics—in particular, ores and smelting slags—by explosive electrical breakdown is discussed. Electrical pulses are used to produce intergranular fragmentation and can liberate the constituents of composite solids with higher efficiency than conventional mechanical comminution. The energy consumption of the process can be reduced by the choice of the electrode configuration and by adjustment of the waveform of the pulses according to the spark constant of the disintegrating dielectric. As an example, the liberation of platinum-group metals from a South African ore is analysed from metallurgical and energy consumption perspectives.

Generation of high-power ultrasound by spark discharges in water

It is impractical to achieve the desired combination of power and bandwidth from conventional electromechanical acoustic sources. However, these characteristics can be achieved by the use of pulsed power technology to generate high-power ultrasound (HPU). High-voltage pulses induce the electrical breakdown of water and the resulting bubble formation and collapse produce acoustic waves of high power and frequency. The dynamics of spark generated bubbles are formulated to predict the development of the bubble radius with time and an experimental system to produce a consistent source of spherically symmetric HPU acoustic waves is described. Pressure pulses due to both bubble formation and collapse were detected and, although their relative amplitudes varied, their frequency spectra did not differ significantly. The amplitude of the acoustic output rises sharply for applied pulse energies up to /spl sim/25 J but the effect saturates indicating little gain and poor efficiency by using high-energy pulses. Variation of the source topology in the form of the electrode separation was found to be the most important factor in the acoustic output. The detected HPU increased as the source became larger but as the two-thirds power of the electrode separation, thereby showing progressively diminishing enhancement. The frequency content of the acoustic signal did not appear to vary with either applied pulse energy or the electrode separation.