Electrochemical methods of deep cleaning inorganic substances

In solution, macro- and microcomponents can be in a wide variety of chemical forms: in the form of ions and their associates of simple and polymeric neutral molecules, colloidal and fine particles, etc. Therefore, the behaviour of the component management systems in an electric field is not equivalent and depends not only on the physicochemical properties of the solvent and chemical sorts of matter, but also on the nature of electrode processes - reactions of oxidation and reduction, adsorption phenomena, polarization effects, overvoltage, etc. [160].

It should be noted that not all phenomena detected by passing an electric current through a solution have been studied sufficiently for use in inorganic deep purification technology substances. In particular, very interesting in practical terms appears to be the phenomenon of electric transport in liquid metals [161]. It was found that impurities of Na, K, Rb, Cs and Bi in dilute amalgams migrate during electrotransfer to the anode, i.e., they have negative effective charge; impurities Li, Ag, Au, Mg, Zn, Cd and Ga migrate to the cathode. Impurities of Hg (0.4 at.%) and Pb (0.6 at.%) in liquid potassium at a temperature of about 100°C migrate to the anode. It is believed that electric transport is associated with the entrainment of intermetallic compounds of impurities by the electron flux [161]. The possibility of ion separation by the electrogravity method [162, 163], based on the use of convection motion of the concentration- polarization layer at the membrane-solution interface, which creates a concentration gradient along the height of the membrane, has been little studied. This phenomenon was first discovered by W. Pauli [164] during the electrodialysis of inorganic colloids and was called electrostratification.

Electrodialysis. Electrodialysis was first proposed as a method of purification of substances by Meigrat and Sabates in 1890, and since then this method has been often used to remove impurities from solutions of non-electric trolites, colloids, and suspensions of sparingly soluble substances [163].

Electrodialysis is a complex physicochemical process occurring in an electrolytic cell separated by semipermeable membranes (diaphragms). The use of membranes causes the emergence of not only new processes, such as dialysis and electroosmosis, but also leads to a change in the number of ion transports in the pores of the membranes along compared with the free volume of the solution, to the appearance of a kind of sorting effect [165]. However, the basis of electrodialysis remains the electrolysis process, the role of which increases with a decrease in the concentration of electrolyte impurities [165, 166]. Using ionite membranes expands the scope of electrodialysis as a method of purifying a substance. There is the possibility of removing trace impurities from electrolytes. In particular, electrolytes containing large ions (e.g. soluble and insoluble polyelectrolytes, salts of large inorganic and organic cations and anions), almost do not pass through ionite membranes, are ‘incapable of electrodialysis’ [163] and therefore are easily cleaned from trace elements of ordinary electrolytes.

Two-chamber electrodialyzer with a bipolar membrane

Fig. 4.2. Two-chamber electrodialyzer with a bipolar membrane: 1 - anode; 2 - cathode; 3 - anion exchange part of the bipolar membrane; 4 - cation exchange resin part of the bipolar membrane,

The simplest design of an electrodialyzer is an electrolytic cell with a bipolar membrane (Fig. 4.2). Such two-chamber electrodialyzers assembled in a cascade can be used to purify aqueous solutions of salts of divalent and trivalent metals from alkali metal impurities.

Various practical inorganic works of purification of th substances widely use three- (Fig. 4.3) and five-chamber electrodialyzers with narrow and high cameras (to increase membrane surfaces). The electrodializable substance in dissolved form (in the form of a suspension or colloidal solution) is placed into the middle chamber (Fig. 4.3, item 6). In the process of electrodialysis, ions microimpurities are transferred from the middle to the side chambers, which are periodically or continuously washed with very clean water.

For concentrating removed impurities and reducing the consumption of especially pure water it is necessary to use five- chamber electrodialyzers. Additional chambers of these devices are a kind of electric traps of microimpurity ions [166, 167], which prevent back diffusion of the latter into the middle chamber. In that the water is used only for washing additional chambers are washed [167].

For deep cleaning of non-electrolytes rhe electrodialyzers chamber are combined into a cascade called a multi-chamber electrodialyzer

Three-chamber electrodialyzer

Fig. 4.3. Three-chamber electrodialyzer: 1 - anode; 2 - cathode; 3 - cathode membrane; 4 - anode membrane; 5 - side (electrode) chambers; 6 - middle camera..

[163]. In a multi-chamber electrodialyzer, cation exchangers and anion exchange membranes are arranged alternately. If the possibility of penetration of H+ and OH- ions through the membranes into the middle chamber is not excluded due to non-ideal membranes, this factor in a multi-chamber electrodialyzer affects only those desalination chambers that are in close proximity to the electrode chambers [163].

The electrodes of the electrodialyzers are made only of platinum or highly pure graphite. However, in this case, the possibility of contamination of solutions with electrochemical products of corrosion of electrodes is not excluded.

The efficiency of the electrodialysis process is largely determined by the originally used semipermeable membranes (diaphragms).

In the process of electrodialysis, different sediments (CaC03, BaC04, Fe (OH)3, A1 (OH)3, H,Si03 and others) build up due to the pH gradient formed near the membrane and due to various reactions of the removed ions with the counterions of the membrane, with the presence of colloidal contaminants in the middle chamber or in the initial non-electrolyte, etc. The Fe3+, Al3+ and Pb2+ cations ‘poison’ most often the cationate membrane. The appearance on the membranes of sediments leads to a decrease in current efficiency and an increase in the resistance of the electrodialyzer. Electrode polarity reversal is used to remove precipitation and flow directions of working solutions and washing water [163].

Therefore, the electrodes must be resistant to the products formed both at the cathode and at the anode. The usual desorption of impurities Fe, Cu, Pb, Cd and others from membranes without reversing the polarity is possible only with prolonged electrolysis with a solution of high acidity.

The negative phenomena observed during electrodialysis include the gradual destruction of the anode membranes due to the release of small amounts (traces) of chlorine, bromine on the anode and oxygen [161].

The method of ionic mobility. Inorganic substance purification using the method of ionic mobility (iontophoresis) is based on using minor differences in ion transport numbers of the main component and ions of microimpurities in the electrochemical field. When a sufficiently high potential gradient is combined with a countercurrent of the solvent, a slowdown in the movement of less mobile ions is observed, while more mobile ions go towards the solvent. The less mobile ions seem to be washed off from the more mobile ones [162, 163]. Ion separation efficiency increases with a decrease in diffusion and various convection flows caused by the thermal motion of ions and

The layout of the separation tube for inorganic cleaning substances by the ionic mobility method

Fig. 4.4. The layout of the separation tube for inorganic cleaning substances by the ionic mobility method: 1 - separation tube; 2, 6 - electrodes 3 - tube for supplying a solvent creating a hydrodynamic countercurrent; 4 - tube for supplying the source in the case of removal of cation impurities; 5 - siphon for removal of excess solvent; 7 - pipe water cooling; 8 - conclusion of the solvent, purified from trace impurities; 9 - large porous membranes made of dialysis paper; 10 - output of part of the solvent enriched with microimpurities.

molecules. Therefore, separation tubes (Fig. 4.4), which are the main element of all laboratory facilities using the ion mobility method, are either filled with a fine-grained inert material (silicon dioxide, finely dispersed nozzle made of fluoroplast-4, agar-agar gel and others), or put on cassettes from parallel large-pore membranes that limit the thermal movement of ions and molecules along the solvent stream. The ion mobility method differs from the electrodialysis method only in the absence of in devices and apparatuses (ionophoresisers) of semipermeable membranes. Large-porous membranes used in separation tubes are easily permeable to both anions and cations.

The reaction of two forces - the hydrodynamic pressure of the solvent and the strength of the electric field - leads to the appearance in the separation tube (Fig. 4.4, item 1) of the zones of individual ions in accordance with the values of their carry numbers. The transfer number (relative velocity) of the cation in a solution of a given concentration and temperature is equal to the ratio n+ = wj (w+ + wj, where w+ and w_ are the cation and anion velocities, respectively. The ratio of the concentrations (c, and c,) of ions in two neighbouring zones is equal to the transport numbers w, and n, of these ions: cl/c2 = njnr Under normal conditions the boundary of the zones due to the mutual diffusion of ions will always blurry.

The width of the blurred area of two neighbouring zones, + c,), represents the relative concentration of one of the ions in the zone of another ion at a distance .r from the expected interface of the zones. The origin of the .r coordinate is taken as the boundary of the zone section (,r = 0). At this boundary, cl = c2 (Fig. 4.5).

Thus, the most effective removal of trace elements by the ion mobility method will be observed only if the substance to be purified is formed by large sedentary ions.

When removing microimpurities similar in physicochemical properties, for the most part, resort to the use of various complexing

Counteraction of hydrodynamic pressure

Fig. 4.5. Counteraction of hydrodynamic pressure (linear flow velocity Vp) of the solvent and electric field strength (linear velocity of ions 1 and 2 under the action of the field V, and K„ respectively) reagents that change the mobility of the ions of macro- and microcomponents due to the formation of the latter ionic associates or neutral chelates [164, 165].

The method of ionic mobility (iontophoresis) is a special case of electrophoresis, which is understood as the movement of dispersed electrically charged particles in a liquid medium in an electric field.

Electrophoretic purification is mainly used to remove from nonelectrolytes colloidal particles of hydroxides of iron and aluminum; sulfides of arsenic, copper, lead and other metals; oil emulsions (e.g. organic solvent residue after extraction of impurities) and fine mechanical suspensions.

The considered methods can be attributed to contact-current. The electrodes of devices operating on the basis of a particular effect are in direct contact with the working medium and through the boundary of the electrode - medium section flows current. Joule warm to a certain extent affects the efficiency of the process, increasing or decreasing it, and the initiation of the process requires additional costs.

HF discharge in elemental and isotopic enrichment

At the end of the 60s, employees of the Sukhumi Physical-Technical Institute discovered the separation of isotopes and gas mixtures in a high-frequency discharge with a running magnetic field. The running a wave, interacting with currents in a gas discharge plasma, compresses the gas in the axial direction. The pressure drop is proportional to the power dissipated in plasma. The sign of the isotope separation effect in the HF discharge is determined by the direction of propagation of the travelling waves. At the end of the chamber where the gas pressure rises, the gas enriched with heavy isotopes. One of the likely mechanism of the isotope separation was calculated by the authors [168] as thermal diffusion. It was understood that the radial thermal diffusion effect in neutrals eR = (Ap/2p) RT In (TJ Г () (Ap is the difference of the mass number of separated isotopes, p is the average atomic weight of the isotope mixture), due to the temperature difference between the wall Tal and discharge axis Ta2, is converted to longitudinal and у is multiplied due to the internal gas circulation in the discharge (gas circulation occurs due to the radial inhomogeneity of the force F_). The value RT characterizes the rigidity of the molecules. Calculations showed that under the data conditions of the experiments at an initial pressure

Diagram of an RF installation with a traveling magnetic wave

Fig. 4.6. Diagram of an RF installation with a traveling magnetic wave: 1 - water- cooled discharge chamber; 2 - delay line; 3 - gauge conversions callers; 4 - filling gas; 5 - gas sampling; 6 - to the pump.

p > 5.0 • 10-2 Torr the contribution thermal diffusion in the separation effect is decisive. The authors did not exclude the possibility of the existence of barodiffusion effect at p < 5.0 10": Torr. This pressure

range was not in operation investigated in detail.

A facility was built that exceeded the power of the previous one. The results of the experiments on it were published in [169-171]. The installation diagram is shown in Fig. 4.6 [172]. RF discharge was excited in a water-cooled quartz chamber 1, located on the axis of the solenoid of the delay line (Fig. 4.6, pos. 2), consisting of 60 cells. In the delay line, seven-turn coils with a diameter of 0.12 m and ceramic capacitors were used. The length of the discharge chambers I = 1.1 m, inner diameter d = 0.065 m; connecting nozzles were designed for pumping and sampling gas, as well as sections of the discharge chamber outside the delay line were formed for ballast volumes: VL= 1.1 / and Vg = 1.8 /. The volume of the region of the discharge chamber enclosed within the delay line was 2.8 liters.

For experiments with cadmium vapour [171], a thermally insulated discharge chamber with an inner diameter of d = 0.05 m was used. The length of the delay line solenoid was L = 0.85 m. The delay line was included in the oscillator circuit of the generator (oscillation frequency 80-460 kHz). The phase velocity Fph in the delay line varied in the interval (0.5-1.5) • 105 m/s. The differential value of pressure Дp = pL- p0 reached 2.5 • 10-1 Torr (Fph = 5.5 • 104 m/s, W = 8 kW, / = 80 kHz). When using a delay line, it is possible to independently change the oscillation frequency and phase velocity of the wave. The discharge is excited in neon, krypton and xenon at initial pressures of p < 2.5 Torr. The lower limit was determined by the conditions of HF breakdown and corresponded (in Torrs): 5 • 10-2 (Ne), 4 • 10~3 (Kr), 1 • 1(T3 (Xe). Discharge was ignited by a preliminary gas ionization using a separate inductor (50 MHz, 100 W). The main results were obtained in experiments with xenon and krypton. A number of experiments were carried out upon excitation of a discharge in a Kr-Xe mixture and also in mixtures of both gases with helium and neon (p < 4 • 10'1 Ton).

At this facility, a significantly larger separation effect was obtained. The coefficient of enrichment of a mixture of isotopes of xenon (Xe129-Xe136) exceeded 24%. It is more convenient to characterize the effect by an enrichment coefficient reduced to a unit difference of the isotope masses: s = [(a - 1)/Ap] • 100%. In this case, it is immediately possible to evaluate the separation of any isotopic mixture of a given element. The maximum reduced value is eXe = 3.5%. The increase of the coefficient of enrichment is associated equally with the increase of the power of discharge W, and with a decrease in the phase velocity of the wave V ; together in addition, both quantities determine the pressure drop Ap and, therefore, ratio pL/p0 (Ap = JV/SV , where S is the cross-sectional area of the discharge camera). This ratio reached 150 when discharge took place in xenon and 40 when in krypton.

When setting up the experiments, it was assumed that the initial pressure p ~ Ap is the boundary pressure: barodiffusion predominates for p < Ap, and thermal diffusion prevails for p > Ap.

At low initial pressures (p < 3.0 ■ 10~2 Ton), the effect of diluted gases (He, Ne, Kr) on the separation of xenon isotopes is weak. These experiments led to the conclusion that, during a discharge in a gas mixture (low initial pressures), the isotopes of the easily ionized component are more effectively separated [169]. In experiments on the separation of cadmium isotopes [170], xenon is a necessary ballast additive. The isotope enrichment coefficient of cadmium is sCd = (2.5-3.5)%.

The separation coefficients of the mixtures Kr-Xe, Ne-Xe, He- Xe, Ne-Kr were measured. At low initial pressures (p < 3 • КГ2 Ton), the separation of mixtures is equally effective and is associated with the predominant entrainment of an easily ionized component by a travelling wave. The value of the separation coefficient a is determined by the ratio of partial pressures of the easily ionized component in ballast volumes.

The experiments showed that only isotopes of a component with a lower ionization potential are substantially separated, and the magnitude of the separation effect is determined by the ratio of the partial pressures of this component. Although experiments with a mixture of Ne - Xe at elevated initial pressures were stimulated by the features of thermal diffusion separation of isotopes [173]; it can be noted that an increase in eXe also correlates with an increase in

(P^Po^Xe'

As for the application of the RF discharge in practice, it seems possible to create a cascade of such plants for the production of some isotopes of cadmium and zinc. But with the achieved level of separation effect, this production will not be profitable, since the energy costs alone are more than 105 kWh/EPP.

 
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