JP2008036585A - Apparatus and method for separating suspended matter in liquid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for promoting the surfacing of micro bubbles when suspended matter in a suspension is subjected to flotation by using micro bubbles. <P>SOLUTION: The flotation of the suspended matter is promoted by dispersing micro bubbles having the diameter of 1-100 μm in the suspension by using a micro bubble generator 4 to stick the suspended matter to the surfaces of the dispersed micro bubbles and irradiating the micro bubble-dispersed suspension with the ultrasonic wave, which is emitted from an ultrasonic transmitter 3 and has the frequency of 20 kHz to 1 MHz, to promote the agglomeration among micro bubbles. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  Known technology. Even now, some wastewater sludge (slurry) is concentrated by floating selection. Used in large-scale sewage treatment plants. In addition, it is also used for wastewater containing a lot of fats and fibers in paper mill wastewater, livestock farms and food factories. In sludge flotation, a pressurized flotation method is used as a method for obtaining fine bubbles. Bubble sizes vary from microbubbles to larger. In the microbubble state, the dispersibility is good and the specific surface area is large, so that many fine particles can be adsorbed and floated. However, while its dispersibility is good, it is a problem that the flying speed is remarkably slow.

  In addition, in the treatment of heavy metals in industrial wastewater, compounds that are hardly soluble in water, such as hydroxides, sulfides, or ferrite compounds, are generated and separated by coagulation precipitation (coprecipitation) that is adsorbed thereto. Usually, metal iron hydroxide such as iron hydroxide produced by coprecipitation is amorphous gel-like fine particles and has very poor sedimentation separation. Therefore, the sedimentation separation property is improved by aging at high temperature (up to 80 ° C.) for a long time and in some cases promoting crystallization. For this reason, floating sorting, which has a higher processing speed than sedimentation separation, is considered effective, but in order to float a large amount of fine particles, the bubbles to be introduced must be microbubbles. However, it has not been put into practical use because of the extremely low ascent rate. The following patents have been published as floating sorting technology using microbubbles.

Japanese National Patent Publication No. 8-511472

  Industry: For example, plating industry wastewater. Plating cleans the surface of the iron or other material and then applies a thin film of metal to improve the surface properties of the material. Many chemicals and materials used are harmful, such as heavy metals, cyanide and chromic acid. There are two types of waste water: waste liquid with aging chemicals used for surface treatment and plating, and low-concentration continuous waste water for washing processed materials. The components are divided into pickling, alkali treatment systems, cyan systems, chromium systems, and other systems containing toxic heavy metals. Table 1 shows an example of plating wastewater quality.

  Waste metal heavy metal treatment: Hydroxide generation-coagulation precipitation treatment (coagulation precipitation method). The coagulation sedimentation method is one of unit operations in water treatment, and is a method in which substances existing in a suspended state in water are aggregated with a coagulant and precipitated, and then separated from the liquid. That is, the removal of heavy metal ions in the waste liquid generates a compound that is hardly soluble in water, such as a hydroxide, sulfide, or ferrite compound, and precipitates and separates by a coagulation precipitation method.

  Example of heavy metal processing method. When removing heavy metals in waste liquid, heavy metals can be removed from the liquid by solid-liquid separation if the heavy metals already exist in the form of insoluble compounds, but they are dissolved in the liquid in an ionic state. In some cases, water-insoluble hydroxides, sulfides, etc. must be generated and precipitated and then removed. Usually, a method of precipitating heavy metals as hydroxides is used.

Heavy metal ions present in water precipitate as metal hydroxides with increasing pH (increased OH ^- ion concentration). On the other hand, ferric ion (Fe ³⁺ ) added as a coagulant (coprecipitant) also precipitates as ferric hydroxide (Fe (OH) ₃ ). Precipitate while agglomerating. If this precipitate is removed from the liquid, heavy metal ions will not be present in the liquid.

  Precipitation removal utilizing the production of insoluble compounds of heavy metal ions is based on the principle of solubility products. In other words, the solubility of heavy metal hydroxides in water is often very low, and heavy metals that are acidic and dissolved in an ionic state become hydroxides and precipitate when neutralized with alkali. A necessary condition for the precipitation of the metal hydroxide is that the product of the metal ion concentration and the hydroxyl ion concentration is larger than the solubility product of the hydroxide.

In the formation of a precipitate when many substances coexist, it is rare that only a specific substance precipitates and precipitates, and precipitates with some other substances. Such precipitation with other substances is called coprecipitation. The phenomenon of precipitation induced by some kind of precipitation even under conditions that do not precipitate when present alone is called induced precipitation. For example, when cadmium is removed from waste liquid, it is not less than 0.1 mg / l of the effluent standard value at a pH of 10 or less as calculated from the solubility product. Even below, it may be below the reference value. The coprecipitation phenomenon is said to be due to the metal ions adsorbed on the precipitate or encapsulated in the precipitate when it is formed. Usually, ferric ions (Fe ³⁺ ) are added to the waste liquid to improve the treatment capacity by utilizing the coprecipitation phenomenon. Arsenic cannot be precipitated and removed as a hydroxide, but is adsorbed and removed by iron hydroxide (Fe (OH) ₃ ). Since it is clear that As (V) has better removability than As (III), co-precipitation removal is performed after oxidizing As (III) in the pretreatment for waste liquid containing arsenic. Is done.

  There are many types of flocculants (coprecipitants) used for waste liquid treatment, but inorganic flocculants are generally used as the main flocculants in the flocculant precipitation method. The aggregation ability of the inorganic flocculant varies depending on the valence of the metal ion, and the higher the valence, the stronger the aggregation force. Trivalent iron and aluminum are used as the inorganic flocculant used for the coagulation and precipitation treatment from the viewpoint of treatment cost and coagulation effect. However, when considering the treatment of arsenic, iron salts are more advantageous. is there.

  If flocs (aggregates of precipitated particles) are fine and difficult to settle even when metal hydroxide is produced by the coagulation precipitation method, the precipitates must be enlarged by linking the flocs to promote sedimentation. For this purpose, a polymer flocculant is used as an agglomeration aid. There are cationic, anionic and nonionic polymer flocculants, but anionic or nonionic effects are high for metal hydroxides. Thereafter, the entire solution is transferred to an aging tank and allowed to stand for aging of floc. The precipitate after aging is solid-liquid separated by a filter.

  Usually, metal iron hydroxides such as iron hydroxide produced by the coagulation precipitation method (coprecipitation method) are amorphous gels and have very poor sedimentation separation properties. Therefore, the sedimentation separation property is improved by aging at high temperature (up to 80 ° C.) for a long time and in some cases promoting crystallization. FIG. 1 shows an example of a conventional treatment flow of heavy metal coagulation sedimentation method in wastewater.

  Invention content. Purpose and concept. It is an invention of a novel method and apparatus for high-speed floating sorting of a suspension in a liquid. Suspensions are intended for metal hydroxides with poor sedimentation and separation properties caused by heavy metal coagulation sedimentation treatment as described above, and other fine particles in liquids (for example, magnetic materials prepared in solution, etc.) Functional fine particles). Usually, floating sorting using bubbles is much faster than sedimentation separation. However, since millimeter-order bubbles cannot efficiently disperse in a solution (slurry) containing a large amount of fine particles such as amorphous metal hydroxide and adhere to the surface, such as sedimentation separation. Many other methods have been adopted. Therefore, in the present invention, first, microbubbles having a particle size of several μm to 100 μm are dispersed in the slurry using a microbubble generator. Since microbubbles have a small particle size, they have good dispersibility and are stably dispersed in the slurry for a long time. Since the microbubbles have a small particle size, they have a large surface area and adsorb a large amount of suspension (fine particles) on the surface. However, microbubbles are extremely slow to float due to their high dispersibility, which hinders high-speed floating sorting. Therefore, by irradiating the slurry in which the microbubbles are dispersed with ultrasonic waves whose frequency and output are controlled, the microbubbles are aggregated to promote the floating, and as a result, the floating sorting of the suspended matter is remarkably promoted. By controlling the frequency and output of the ultrasonic wave, the floating sorting speed can be controlled.

  One of the target suspensions is amorphous iron hydroxide fine particles generated by the coagulation-precipitation treatment of heavy metals in wastewater. As described above, the iron hydroxide produced by the coagulation-precipitation treatment is usually used. Fine particles are aged for a long time, and in some cases, aged for a long time at a high temperature (up to 80 ° C.) and separated by sedimentation.

  Theoretical background (behavior of microbubbles under ultrasonic irradiation). Bubbles under ultrasonic irradiation receive a radiation force called Bjerknes force, and they flow in the traveling direction of the ultrasonic waves in the traveling wave, and in the standing wave, depending on the sound pressure and the resonance radius of the bubble, the belly or node of the pressure Be drawn to. In addition, bubbles gathered there may aggregate. As shown in FIG. 2, when the bubble diameter is larger than the resonance bubble diameter determined by the ultrasonic frequency, the bubbles gather at nodes with little pressure fluctuation. On the other hand, when the bubble diameter is smaller than the resonance radius, the bubbles gather in the belly where the pressure fluctuation is large. For example, in our experiments, ultrasonic waves of 38 kHz (resonant bubble diameter of about 170 μm) and 430 kHz (resonant bubble diameter of about 15 μm) are irradiated to an aqueous solution in which microbubbles (bubble diameter of about 10 μm to 60 μm) are dispersed. As a result, in the case of 38 kHz, the microbubbles are smaller than the resonant bubble diameter, and therefore gather in the belly where the pressure fluctuation is severe. For this reason, as shown in FIG. 3, they move at high speed and aggregate each other in a cluster, and rise in a very short time of a few seconds of commas. In this case, in the floating sorting, the fine particles adhering to the surface of the bubbles are separated and the floating sorting does not work well. This has been confirmed in an experiment using iron hydroxide fine particles described later. For floating selection, it is necessary to control the movement of the microbubbles and the flying speed to some extent. Therefore, when 430 kHz ultrasonic waves are irradiated, the microbubbles are larger than the resonant bubble diameter as shown in FIG. Therefore, it moves slowly and agglomerates each other widely. While aggregating, it gradually rises due to sound pressure and buoyancy. In this case, as shown in the experimental results to be described later, the floating selection succeeds because the fine particles float while attached to the bubbles.

  Laboratory level test. In the experiment, using a syringe connected to a glass column (inner diameter: 10 mm, length: 150 mm), microbubbles (bubble diameter: about 10 μm to 60 μm) were added in about 20 ml of distilled water to which 0.01 ppm of surfactant (TritonX100) was added. Dispersed. The glass column in which the microbubbles were dispersed was immersed in a water tank provided with a throw-in type ultrasonic vibrator. When irradiated with ultrasonic waves (frequency: 38 kHz or 430 kHz, output: 0 to 600 W), as described above, the individual microbubbles actively move according to the frequency of the ultrasonic waves and agglomerate with each other, which significantly promotes the ascent. I found out. Fig. 5 shows an outline of the experimental apparatus.

Next, in the same manner, microbubbles were dispersed in a slurry in which gel-like iron hydroxide (2 line-Ferrihydrite, FeOOH · 0.801H ₂ O) fine particles prepared at pH = 7 were dispersed, and ultrasonic waves were applied. The iron hydroxide slurry used in the experiment was prepared by the following method. In 50 ml of distilled water, 2.02 g of iron nitrate nonahydrate (Fe (NO ₃ ) ₃ · 9H ₂ O) was dissolved. A 1 mol / L sodium hydroxide (NaOH) aqueous solution was diluted to 0.125 mol / L. Then, 120 ml of NaOH aqueous solution was titrated into Fe (NO ₃ ) ₃ aqueous solution while stirring with a magnetic stirrer at room temperature of 25 ° C. Titration was stopped when the pH reached 7, and the solution was stored at 25 ° C. In this method, a kind of amorphous iron hydroxide fine particles called Ferrihydrite (FeOOH · 0.801H ₂ O) is formed. Subsequently, a surfactant (TritonX-100) is added to 20 ml of iron hydroxide slurry so as to have a concentration of 0.01 ppm, and is taken into the column type experimental apparatus shown in FIG. Soak the column in an ultrasonic bath and keep the ambient temperature at 25 ° C. Stir the syringe 10 times each to disperse the microbubbles in the slurry in the column. After stirring, the mixture was sonicated 20 seconds later.

  The microbubbles floated and agglomerated with the fine particles attached, and the speed of the fine particle floating selection was remarkably accelerated. For example, FIG. 6 shows photographs taken for comparison at a frequency of 430 kHz and an output of 0 W (no ultrasonic irradiation), 100 W, and 600 W. The levitation separation was remarkably promoted by irradiating 430 kHz ultrasonic waves. After 1 minute, levitation separation hardly progressed without ultrasonic irradiation, but at 100W, levitation separation progresses to about 1/4 of the column, and at 600W, nearly half of the column. It can be seen that although the height of the final ascent after 20 minutes is almost unchanged, the ascent rate is remarkably accelerated by irradiating ultrasonic waves. However, when the same experiment was performed at a frequency of 38 kHz as described above, only the microbubbles aggregated and floated at a high speed, and the iron hydroxide fine particles remained as they were in the solution. Further, when the sedimentation separation was performed without using microbubbles and ultrasonic waves, it took 30 minutes or more to complete the sedimentation of the iron hydroxide fine particles. From this, it can be seen that microbubble floating separation under ultrasonic irradiation is significantly faster than sedimentation separation.

  FIG. 7 shows a photograph of the bottom when the iron hydroxide fine particles float and separate. When the ultrasonic waves are not irradiated, the microbubbles gradually float with their buoyancy while being dispersed in the solution. On the other hand, when 430 kHz ultrasonic waves are irradiated, it can be seen that the microbubbles float while agglomerating. The part where the microbubbles are sparse is considered as the antinode of the sound wave, and the part where the microbubbles are aggregated is considered as the node of the sound wave. FIG. 8 shows an enlarged photograph of microbubble behavior with iron hydroxide fine particles attached. Iron hydroxide fine particles are on the order of nanometers and are considerably smaller than microbubbles. When the photograph is seen, it turns out that the aggregate of iron hydroxide fine particles has floated in the state adhering to the microbubble.

  Intent of the invention. For separation and concentration of various sludges (slurries) produced by wastewater treatment, floating sorting is a fast and effective method. The floating selection is performed by adsorbing fine particles on the surface of bubbles introduced into the slurry and floating in that state. However, with respect to hydrophilic gel-like fine particles, it is difficult to disperse them in the slurry with ordinary millimeter-order bubbles, and the fine particles cannot be efficiently adsorbed on the surface and floated. Thus, microbubbles are used to remarkably improve the dispersibility in the slurry and the adsorption amount of the fine particles, but in that case, the bubble floating speed is remarkably slowed, and the advantages of floating selection are impaired.

When the applicant immerses a glass column in which microbubbles are dispersed as described above in a water tank and irradiates with ultrasonic waves (38 kHz or 430 kHz, 0 to 600 W) using a vibrator, individual microbubbles It was found by observation with a high-speed camera that the waters vigorously move and aggregate each other, and the flying is remarkably promoted. Therefore, this phenomenon was applied to the floating selection of suspension (slurry) using microbubbles. In other words, when microbubbles are similarly dispersed in a slurry in which amorphous iron hydroxide (2 line-Ferrihydrite, FeOOH · 0.801H ₂ O) fine particles are dispersed and irradiated with ultrasonic waves, the microbubbles adhere to the fine particles. In this state, the particles floated and agglomerated, and it was confirmed that the speed of fine particle floating selection was significantly accelerated.

  Product image. Product image diagrams are shown in Figs. FIG. 8 shows a schematic diagram of the floating sorting tank type apparatus. A liquid (slurry) in which suspensions (fine particles) are dispersed is supplied into the tank. A microbubble generator is installed in the floating sorting tank. For example, the microbubble generation method is a swirl flow method or a pressure dissolution method. An ultrasonic transducer is installed at the bottom of the tank. Adjust the frequency (10kHz ~ 2MHz) and output by the ultrasonic transmitter. The suspension selected by floating is collected from the upper part of the tank.

  FIG. 9 shows a schematic diagram of a column type floating sorting apparatus. A liquid (slurry) in which the suspension is dispersed is supplied into the column. A microbubble generator is installed in the column. For example, the microbubble generation method is a swirl flow method or a pressure dissolution method. An ultrasonic transducer is installed at the bottom of the tank. Adjust the frequency (10kHz ~ 2MHz) and output by the ultrasonic transmitter. The suspension selected by floating is collected from the upper part of the tank.

An example of the conventional coagulation-precipitation process flow of heavy metals in wastewater. Relationship between bubble behavior and ultrasonic vibration under ultrasonic irradiation (Bjerknes force). Photo of microbubble behavior under ultrasonic irradiation (38kHz). Photograph of microbubble behavior under ultrasonic irradiation (430kHz). Microbubble levitation behavior of amorphous iron hydroxide fine particles (comparison by ultrasonic output). Microbubble levitation behavior of amorphous iron hydroxide fine particles (enlarged photo of levitation bottom). A photograph of microbubble behavior with iron hydroxide fine particles attached under ultrasonic irradiation. Schematic of a floating sorting tank type device. Schematic of a column type floating sorting apparatus.

Claims (2)

  Use a microbubble generator in the suspension solution to disperse microbubbles with a bubble size of 1 μm to 100 μm in the suspension solution, attach the suspension to the surface of the microbubble, and separate the suspension by floating. In that case, the suspension solution in which the microbubbles are dispersed is irradiated with ultrasonic waves having a frequency of 20 kHz to 1 MHz to promote the aggregation and floating of the microbubbles and promote the floating separation of the suspension. Separation device for suspension in liquid.   Use a microbubble generator in the suspension solution to disperse microbubbles with a bubble size of 1 μm to 100 μm in the suspension solution, attach the suspension to the surface of the microbubble, and separate the suspension by floating. In that case, the suspension solution in which the microbubbles are dispersed is irradiated with ultrasonic waves having a frequency of 20 kHz to 1 MHz to promote the aggregation and floating of the microbubbles and promote the floating separation of the suspension. Separation method of suspension in liquid.