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With China Plastic Machinery
With China Plastic Machinery
Filling materials generally refer to solid materials added to synthetic resins or rubbers as basic components to change their properties or reduce their costs. There are inorganic and organic ones. There are many types and a wide range of applications. In the rubber industry, they are often called reinforcing agents, such as carbon black, white carbon black, clay, precipitated calcium carbonate, etc., which are mainly used to improve tensile strength, hardness, wear resistance and flexural resistance (see rubber reinforcing agents). In the plastics industry, wood powder, cotton fiber, paper, cloth, asbestos, clay, etc. are often used to improve their mechanical properties; mica, graphite, etc. are used to improve their electrical properties.
In the coatings industry, clay, calcium carbonate, talcum powder, barium sulfate, etc. are often used to improve the physical, chemical or optical properties of the coating. In the dye industry, salt, sodium sulfate, urea, etc. are often used to prepare a certain standard concentration. In the papermaking industry, clay, talcum powder, chalk, titanium dioxide, barium sulfate, precipitated calcium carbonate, etc. are often used to improve its opacity, smoothness and ink absorption. In the leather industry, magnesium sulfate, gypsum, sodium sulfate, ammonium sulfate, glucose, etc. are often used to make the leather full, elastic and slightly lighter in color.
The research status of EDI membrane stack filling materials is introduced. It is believed that ion exchange resins will still be the main research object of filling materials in the future, and the research of ion exchange fibers and other new filling materials is in urgent need of new breakthroughs. At the same time, taking resin as an example, three filling methods of ion exchange materials (mixed filling, layered filling, and split filling) are systematically introduced. Finally, the research and development of EDI membrane stack filling materials and filling methods are prospected.
In the EDI membrane stack, the filling material acts as a carrier of ion conduction, plays the role of ion exchange and conduction, and its performance directly affects the progress of the EDI process. The filling material should have the following properties: high exchange capacity; fast exchange speed; strong conductivity; small water flow resistance; high strength; no dissolution, etc.
In addition to meeting the above conditions, the main reason for choosing ion exchange resin as a filling material is that the resin can be used directly without further processing. Therefore, since the first EDI membrane stack was launched by Millipore in the United States in 1987, granular ion exchange resins have been widely used. There are many types of granular ion exchange resins, and the classification methods are different. Generally, they are classified according to the characteristics of the functional groups on the ion exchange resins, the types of counterions on the functional groups, and the resin morphology.
According to the characteristics of the functional groups on ion exchange resins, they can be divided into cation exchange resins and anion exchange resins. Those with acidic functional groups are called cation exchange resins; those with alkaline functional groups are called anion exchange resins. According to the strength of the acid and base on the functional group, it is roughly divided into strong acid, weak acid or strong base, weak base ion exchange resin. Different types of ion exchange resins have certain differences in performance, so as filling materials, different phenomena will occur in the EDI process. Most EDI membrane stacks outside China use strong acid and strong base ion exchange resins. This type of resin has a strong ion exchange capacity and is relatively easy to regenerate. Although weak acid and weak base resins are easily regenerated by H and OH, the ion exchange capacity of the resin becomes weak after regeneration, so they are rarely used. This is mainly determined by the selective adsorption of weak acid and weak base resins. In neutral aqueous solution, the selective adsorption order of weak acid and weak base resins for various ions is: H>>Fe>Ca>Mg>K>Na>Li; OH->>SO>PO->NO>Cl>HCO. It can be seen that the selectivity coefficients of weak acid and weak base resins for H and OH are significantly higher than those for other ions, making it difficult for H and OH on the resin after regeneration to exchange with other ions in the solution. Therefore, the ion exchange capacity of the regenerated resin becomes weaker, and the ion exchange and regeneration process of the resin cannot be carried out continuously and efficiently, which ultimately affects the desalination rate of the membrane stack.
According to the type of counter ions on the functional groups of ion exchange resins, they can be divided into salt-type resins and regeneration-type resins. The so-called salt-type resin refers to the exchangeable groups on the resin as Na or Cl, and the so-called regeneration-type resin refers to the exchangeable groups on the resin as H or OH. The difference in the type of counter ions on the functional groups of the resins will have a great impact on the EDI process. The membrane stacks filled with salt-type and regeneration-type resins have obvious differences in the change trends of the concentrated water conductivity and the produced water resistivity.
According to the morphology of ion exchange resins, they can be divided into gel-type and macroporous-type. The former has internal micropores only in the swollen state, and the pore size is small, generally 2 to 4 nm, so the ion conduction resistance is large and the speed is slow when particle diffusion occurs. Macroporous resins, whether in dry, wet, shrinking or swelling (in water), have more and larger pores than general gel resins, so they have a larger surface area, and ions can easily migrate and diffuse during ion exchange, and the exchange rate is faster. Although macroporous resins have many advantages, they do not bring good deionization effects as EDI membrane stack filling materials. Compared with membrane stacks filled with gel-type resins, their water quality is poor and the membrane stack resistance is large. It is believed that the reason for this phenomenon is that the control factor of ion exchange in the EDI process is "thin film diffusion control"; at the same time, macroporous resins have larger particle sizes, lower filling density than gel-type resins, and their exchange capacity is 30% lower than that of gel-type resins.
In EDI membrane stacks with ion exchange resins as filling materials, in addition to the different types of resins that will affect the EDI process, the particle size distribution range of the resin is also an important factor. It is proposed to use anion and cation exchange resins with uniform or single particle size ranges as filling materials to improve the working conditions of the deionization chamber of the EDI membrane stack. The uniform particle resin mentioned in the patent is made by material injection method, with a particle size of about 0.5-0.7 mm, and the change from the minimum particle size to the maximum particle size is only 35%. Due to the advantages of uniform filling density and small water flow resistance, uniform particle resin is widely used in foreign membrane stacks. The domestic common 201¡Á7 type strong alkaline and 001¡Á7 type strong acid anion and cation exchange resins are selected. After special treatment, the filling density of the resin can be improved, and the performance of the membrane stack can also reach the level of uniform particle resin used abroad.
Extruded AZ61 magnesium alloy and AZ61 magnesium alloy welding wire containing neodymium (Nd)-rich mixed rare earth elements were used as filling materials to perform TIG wire welding on die-cast AZ91D magnesium alloy. In order to compare the tendency of weld pores, self-melting welding was also performed on die-cast AZ91D magnesium alloy. Scanning electron microscopy (SEM) was used to observe the morphology and distribution of weld pores, and X-ray energy dispersive spectrometer (EDS) was used to analyze the weld composition. The results show that there are obvious microscopic and macroscopic pores near the fusion line, which should be mainly hydrogen pores and nitrogen pores inherited from the base material; the use of extruded AZ61 with low gas content as filler material can significantly reduce the porosity of the welded joint, reduce the size and number of coarse pores, which should be due to the dilution effect of the filler material on the gas in the molten pool; the use of AZ61 containing Nd-rich mixed rare earth as filler material can further reduce the porosity of the welded joint, which should be due to: Nd-rich mixed rare earth effectively reduces hydrogen pores by increasing the solid solubility of H in magnesium alloy, reacting with H to form stable hydrides, and increasing the bubble overflow rate, and reduces nitrogen pores to a certain extent by increasing the bubble overflow rate.
Low-magnification optical photographs of the cross section of welds with autogenous welding and addition of filler material. The pores are diffusely distributed on the entire cross section of the autogenous welding weld, and the pore size is large. Due to the presence of pores, the metal bulge in the weld area is obvious. After adding filler wire with low gas content (extruded AZ61 magnesium alloy), the porosity in the joint is significantly reduced. The pores are mainly concentrated in the area near the fusion line at the junction of the weld and the base material. There are fewer pores in the weld area, and the size of the coarse pores is also reduced. Compared with the addition of extruded AZ61 welding wire, the pores in the weld filled with rare earth AZ61 welding wire are not completely eliminated, and the distribution characteristics are relatively close, but the porosity is significantly reduced.
Pores with a diameter greater than 0.2mm in the weld are macroscopic pores, and pores with a diameter less than 0.2mm that can only be observed by OM or SEM are microscopic pores. Microscopic pores are small in size, flared in shape, with a regular nearly circular cross-sectional profile and a smooth inner wall; macroscopic pores are irregular in shape, the smoothness of the inner wall is reduced, there are obvious traces of metal scouring, and there are small pores around them that are merging into them.
After adding filler wire with less gas content, the pores in the joint are significantly reduced, and the macroscopic pore size is also reduced. Analysis shows that, firstly, due to the high gas content of the die-cast magnesium alloy base material, the original small pores in the base material continue to expand, gather and merge during the welding process, resulting in serious pore problems. The added welding wire has a low gas content, which has a "diluting" effect on the gas in the molten pool, reducing the supersaturation of the gas, thereby reducing the porosity in the weld; secondly, the added filler wire has a low Al content, which reduces the Al12Mg17 intermetallic compound phase formed (the Al12Mg17 phase has a very low amount of dissolved hydrogen), and the alloy's solid solubility in hydrogen increases, which to a certain extent reduces the later precipitation of hydrogen.
Compared with the weld of extruded AZ61 magnesium alloy as the filler material, the pores in the weld of AZ61+1.5% Nd-rich mixed rare earth elements were not completely eliminated, but significantly reduced. Rare earth elements were effectively introduced into the weld. Analysis shows that rare earth elements are beneficial to reducing the porosity of welds mainly based on the following three mechanisms:
Mechanism 1: Rare earth metals can absorb and dissolve hydrogen in large quantities, greatly increasing the solubility of hydrogen in magnesium alloys, and the lower the temperature, the greater the rare earth's ability to dissolve hydrogen, and the higher the amount of solid solubility hydrogen. According to the hydrogen trap theory, there are crystal defects such as grain boundaries, phase boundaries, dislocations, and inclusions in magnesium alloys. These defects can capture or delay hydrogen, that is, hydrogen traps. After introducing an appropriate amount of rare earth into the weld, the weld metal grains are refined, the grain boundaries and phase boundaries increase, and various hydrogen traps are formed and increased, which strengthens the trap's binding of hydrogen, and greatly enhances the ability of various traps to fix hydrogen during the solidification of the alloy. It can be seen that if there are rare earth elements in the weld metal, the amount of dissolved hydrogen in the magnesium alloy will increase, the free hydrogen in the alloy will decrease, and thus the hydrogen content that can form hydrogen bubbles will decrease.
Mechanism 2: The possible chemical reactions between RE, Mg and H, H and the change of standard Gibbs free energy with temperature show that rare earth and hydrogen have a great chemical affinity and can form stable hydrides. During the welding process, rare earth elements react with H and H atoms in the weld to form stable hydrides such as LaH and NdH. These hydrides have high melting points and high densities and are very stable in liquid magnesium alloys. As the temperature decreases, these hydrides will become more stable, so they are dispersed in the magnesium alloy melt in the form of tiny particles. During the solidification and cooling of the molten pool, these compounds do not decompose into hydrogen, nor do they gather to form bubbles, which reduces the tendency of pores in the weld.
Mechanism 3: The right amount of rare earth elements can reduce the viscosity of the liquid magnesium alloy molten pool and improve its fluidity. According to the Stocks formula, the bubble escape velocity can be expressed as: V is the bubble buoyancy velocity; g is the acceleration of gravity; r is the bubble radius; ¦Çm is the viscosity of the liquid metal; ¦Ñm is the density of the liquid metal; ¦Ñg is the bubble density. The bubble escape velocity is inversely proportional to the viscosity of the magnesium alloy liquid. The greater the viscosity, the smaller the bubble escape velocity. After the rare earth elements are introduced into the weld, the viscosity of the liquid magnesium alloy decreases, which is more conducive to the escape of bubbles and reduces the probability of pores forming in the weld; this mechanism is applicable to both hydrogen-induced bubbles and N bubbles.
In the welded joints using rare earth element-containing filler materials, the pores are not completely eliminated, because the original gas contained in the base material is more N in addition to H; the combined action of the three mechanisms should be able to effectively reduce hydrogen-induced pores, but for N pores, only mechanism three has a certain reduction effect, so the effect is not significant enough.
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