Fine particle separation method

Abstract

To provide a fine particle separation method that facilitates efficient separation of fine particles.SOLUTION: A suspension containing sandy soil with a particle size of less than 2 mm and cohesive soil with a particle size of less than 75 μm, and having a pH of 10.5 or more and less than 12.5 is subjected to microbubble flotation, followed by the recovery of the floated material. This method enables particles contained in the muddy water to be separated into floated material and sedimented material (residue) according to their particle size, enabling their separate recovery.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for separating fine particles. [Background technology]

[0002] Conventionally, as a method for separating fine particles from muddy water containing fine particles, a microbubble flotation technique has been known in which fine bubbles (microbubbles) are supplied to the muddy water and the fine particles are caused to adhere to the microbubbles and float up for separation (see, for example, Patent Document 1).

[0003] Here, the particle size of the fine particles contained in the suspension to be subjected to microbubble flotation is relatively small, for example, less than 20 μm. In other words, microbubble flotation is not performed on suspensions containing particles with relatively large diameters (for example, sandy soil), but on fractions of particles whose particle size has been sufficiently reduced by repeated classification using screens, cyclones, etc. (for example, clayey soil). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-250261 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the case of preliminary classification using a cyclone or the like, the smaller the classification point is, the slower the processing speed becomes, and the lower the classification accuracy becomes, which is an inefficient aspect. Therefore, when there is a need to efficiently separate fine particles with a particle size of less than 20 μm, for example, it is difficult to meet the demand with conventional classification methods. Therefore, the object of the present invention is to provide a fine particle separation method that can efficiently separate fine particles. [Means for solving the problem]

[0006] The present invention provides a method for separating fine particles, which involves performing microbubble flotation on a suspension containing sandy soil with a particle size of less than 2 mm and clayey soil with a particle size of less than 75 μm and having a pH of 10.5 or more and less than 12.5, and recovering the floated matter.

[0007] According to this separation method, fine particles small enough to be attached and entrained by microbubbles can be floated. On the other hand, particles with a large diameter are not entrained by the microbubbles and will sink. As a result, the particles contained in the suspension can be separated into floating matter and sediment (residue) according to their diameters, and each type of particle can be collected separately.

[0008] In this separation method, a pH adjuster may be added to adjust the pH of the suspension to 10.5 or more and less than 12.5 before carrying out the microbubble flotation. That is, if the pH of the suspension is originally within the range of 10.5 to 12.4, there is no need to adjust the pH, but if the pH of the suspension is outside the range of 10.5 to 12.4, a pH adjuster can be added to adjust the pH to within the range. The pH adjuster is preferably a hydroxide containing a divalent metal cation.

[0009] The suspension to be subjected to microbubble flotation may be one to which a saturated or unsaturated fatty acid having 8 or more carbon atoms or a salt thereof has been added, or one to which an alcohol, a ketone or an ether having a solubility in water of 0.1% by weight or more has been added.

[0010] The suspension of interest may contain a 2:1 clay mineral. Effect of the Invention

[0011] According to the present invention, it is possible to provide a method for separating fine particles, which can efficiently separate fine particles. [Brief description of the drawings]

[0012] [Figure 1]Photograph showing particle settling due to too high pH. [Diagram 2] FIG. 2 is a flow chart showing the implementation procedure of the present embodiment. [Diagram 3] 1 is a graph showing particle size distribution before microbubble flotation is performed. [Figure 4] 1 is a graph showing particle size distribution after microbubble flotation. [Diagram 5] These are the results of XRF analysis of the floating material and residue. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present invention provides a method for performing microbubble flotation on a suspension containing sandy soil and clayey soil, and for recovering fine particles having a particle size of, for example, 20 μm or less by floating separation. The particles that settle in the microbubble flotation can be recovered and used separately. A preferred embodiment of the present invention is described in detail below. In this specification, the term "suspension" is synonymous with "muddy water", and "muddy water" will be mainly used hereinafter.

[0014] <Target> In this embodiment, the microbubble flotation is carried out on a muddy water containing sandy soil with a particle size of less than 2 mm and clayey soil with a particle size of less than 75 μm. In other words, the muddy water contains both particles with a particle size of 75 μm or more and particles with a particle size of less than 75 μm. The proportion of particles with a particle size of less than 75 μm may be 95 vol. % or less, 85 vol. % or less, or 75 vol. % or less based on the total particles.

[0015] The frequency of the particle size may be such that the mode diameter is within a range of 10 μm to 100 μm, or within a range of 20 μm to 50 μm.

[0016] The mud may contain 2:1 clay minerals such as the biotite group.

[0017] In this specification, the "particle size" is a value measured by a laser diffraction type particle size analyzer. A specific measuring instrument is the "SALD-2300" (flow cell mode) manufactured by Shimadzu Corporation. This "particle size" is a concept different from the particle size at the classification point in the elutriation classification performed in the examples below. The particle size in elutriation is the Stokes diameter based on the difference in the speed of natural sedimentation in still water, and the sedimentation speed of flat coarse particles with large viscous resistance to water is slow, and therefore it is often far from the true particle size. In this respect, the particle size measured by a laser diffraction type flow particle size distribution analyzer is said to be relatively close to the true particle size.

[0018] <Separation procedure> In the separation method of this embodiment, first, water is added to the target soil, and if necessary, sieved to remove sand and gravel, wood chips, and other impurities with a large particle size (for example, 5 mm or more). Next, the soil particles are crushed. Crushing may be performed in a wet mill or a handheld power mixer. When crushing is performed in a wet mill, the grinding medium used in the wet mill may be metal balls, alloy balls, metal oxide balls, metal composite oxide balls, etc., or may be sand or gravel. The grinding medium may have a particle size of 2 to 18 mm, 3 to 14 mm, or 4 to 12 mm. Since the size of soil aggregates is often within these ranges, if the particle size of the grinding medium is within these ranges, the efficiency of crushing the aggregates is high.

[0019] The muddy water after the crushing process is left to stand, and the supernatant fine muddy water is obtained by decantation. This fine muddy water is the muddy water to be subjected to microbubble flotation. The coarse residue can be discarded or crushed again.

[0020] Next, microbubble flotation is performed on the muddy water. Microbubble flotation is performed on muddy water with a pH of 10.5 or more but less than 12.5. If the pH of the muddy water after the crushing is within the range, the process proceeds directly to the microbubble flotation process. If the pH of the muddy water after the crushing is not within the range, a pH adjuster is added to the muddy water so that the pH falls within the range. If the pH of the muddy water is 12.5 or more, the particles in the muddy water tend to aggregate, making it difficult to float fine particles by microbubble flotation. If the pH of the muddy water is less than 10.5, the adsorption of both calcium chemical species and the chemical species of the collector described later to the surface of the soil particles is inhibited, so there is a risk that almost no particles in the muddy water will float.

[0021] The pH adjuster used is preferably a hydroxide containing a divalent metal cation from the viewpoint of preventing aggregation of muddy water particles. Specifically, slaked lime (calcium hydroxide) or magnesia lime (a complex salt of calcium hydroxide and calcium hydroxide) is preferable.

[0022] If necessary, a collector or foaming agent may be added to the muddy water. As the collector, a material that acts as a surfactant due to its effect of stabilizing microbubbles is preferred, and anionic or nonionic surfactants that do not easily cause particle aggregation are preferred. In particular, organic carboxylic acids or salts thereof are preferred, and among these, saturated or unsaturated fatty acids or salts thereof having 8 or more carbon atoms are preferred. The structure of the organic carboxylic acid portion is, for example, C n H 2n+1 Preferred are caprylic acid, pelargonic acid, capric acid, undecylic acid, lauric acid, palmitic acid, and stearic acid, which are carboxylic acids having a saturated hydrocarbon group represented by the formula: or arachidonic acid, palmitoleic acid, oleic acid, linoleic acid, α-linolenic acid, and γ-linolenic acid, which are carboxylic acids having an unsaturated hydrocarbon group. Examples of the salt include sodium salts of these fatty acids. The amount of the collector to be added may be an amount that provides 0.1 ppm to 100 ppm in the mud water, or an amount that provides 10 ppm to 50 ppm.

[0023] The foaming agent is preferably an alcohol, ketone, or ether having a solubility in water of 0.1% by weight or more, because of the effect of stabilizing microbubbles. Specific examples include isopropyl alcohol, ethyl alcohol, butyl alcohol, pentyl alcohol, methyl isobutyl carbinol, methyl isobutyl ketone, and petroleum ether. The amount of the foaming agent added may be 10 ppm to 2000 ppm. The foaming agent may be an alcohol, ketone, or ether having 6 or less carbon atoms.

[0024] Microbubble flotation is generally a method in which microbubbles (bubbles of micrometer size) are introduced into an aqueous suspension of solids, and hydrophobic solid particles that are not easily wetted by water are caused to adhere to the microbubbles and float up for separation. The structure of the flotation machine and the procedure of flotation can be conventionally known. In this embodiment, a long column is placed vertically, and muddy water is supplied from the middle of the column, while microbubbles are supplied from the bottom of the column. Small particles adhere to the microbubbles and are carried upward, so the overflow is collected. On the other hand, large particles settle, so they are collected from the bottom of the column. Both the floating matter and the sediment (residue) can be filtered, dehydrated, and dried to obtain separated particles.

[0025] When a cylindrical column having a diameter of 600 mm and a height of 2500 mm (effective internal volume of 500 L) is used as the microbubble flotation machine, the flotation can be carried out, for example, under the following conditions. ·Mud water supply rate: 1~20L / min Air flow rate: 1~50L / min Microbubble circulating water flow rate: 1-50L / min

[0026] <Effects> Conventional microbubble flotation was targeted at muddy water that had been classified in advance using a cyclone or the like to reduce particle size sufficiently, so all particles were entrained by the microbubbles and floated up. In contrast, in the separation method of the present embodiment, microbubble flotation is performed on muddy water with a wide particle size distribution, that is, muddy water containing "sandy soil with a particle size of less than 2 mm and clayey soil with a particle size of less than 75 μm", so that some small particles (e.g., particles with a particle size of less than 20 μm) can be selectively floated and collected, and the remaining large particles can be allowed to settle and be collected. In other words, flotation also serves the role of classification.

[0027] Furthermore, according to the separation method of this embodiment, the residue contains a large amount of coarse-grained 2:1 type clay minerals (see the Examples below and Figure 5), so the separation method of this embodiment is suitable for separating coarse-grained 2:1 type clay minerals.

[0028] The separation method of this embodiment is also effective in, for example, volume reduction treatment of soil containing radioactive cesium. Since radioactive cesium is adsorbed in large amounts to soil particles with small particle diameters, by classifying the soil using the separation method of this embodiment, the fraction with a small amount of adsorbed radioactive cesium can be recycled, while the fraction with a large amount of adsorbed radioactive cesium is separately recovered. This results in a large volume reduction of the soil before and after microbubble flotation. It is possible to confirm which fraction the radioactive cesium migrates (concentrates) into when the separation method of this embodiment is carried out by carrying out a preliminary test in which non-radioactive cesium is added to the muddy water to be subjected to microbubble flotation.

[0029] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments. EXAMPLES

[0030] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0031] <Condition review> (procedure) 230 g of a soil sample containing particles less than 2 mm in diameter from contaminated soil in Fukushima Prefecture was mixed with 50 g of water and left to stand for 1 hour. This was then placed in a 215 mm diameter pot mill together with 1100 g of water, 1150 g of pebbles (recycled mountain sand), and 4.6 g of calcium hydroxide (2% of the soil sample) (pH 11.6), and thawed for 50 minutes 24 seconds (=8 passes) at a rotation speed (62.5 rpm) that gave a centrifugal acceleration around the outer periphery of the pot of 0.47 G.

[0032] Then, the muddy water that passed through the 2 mm wet vibrating sieve was classified by elutriation at the classification points of 75 μm and 20 μm, and the resulting 20 μm muddy water was further diluted 5 times to obtain a suspension. A simple flocculation test (jar test) was performed using this. 100 mL of the muddy water that had been stirred and homogenized was dispensed into a 100 mL beaker, and various reagents shown in Experimental Examples 1 to 6 in Table 1 were added to obtain the specified pH or concentration, and then the mixture was stirred at about 240 rpm for 30 seconds. After leaving it to stand for 3 minutes, 90 mL of the supernatant was separated into another 100 mL beaker by the decantation method, and the formazin turbidity (FTU) was measured using OPTEX (Optex Co., Ltd. portable turbidity sensor TDM500). The results are shown in Table 1. In addition, "Acetamine" in the table is the trade name of a cationic surfactant ("Acetamine 24" manufactured by Kao Chemical Co., Ltd.).

[0033] [Table 1]

[0034] (result) From the results of Experimental Examples 1 to 4, it was confirmed that when the pH was increased with calcium hydroxide, rapid aggregation proceeded in the pH range of 12.5 or higher, and most of the particles settled (see the photograph in Figure 1). It is believed that the strong aggregation effect of calcium ions was expressed. Moreover, in Experimental Example 5, in which oleic acid was further added to the suspension of Experimental Example 4 to a concentration of 30 ppm, a tendency to promote rapid aggregation was observed. This result means that in the oleic acid addition concentration range assumed in the microbubble flotation of this embodiment, when oleic acid anions are present at pH ≥ 12.5, almost all particles aggregate regardless of whether they are coarse or fine particles, and it was clear that in this chemical addition system, classification and separation of fine particles, including microbubble flotation, becomes difficult. As shown in Experimental Example 6, when 250 ppm of acetamine, a cationic surfactant, was added, rapid aggregation was further promoted even at a pH of 8.0. It was clear that even in this chemically added system, subsequent separation of coarse particles and fine particles was difficult.

[0035] <Microbubble flotation> (Procedure: See Figure 2) Water was added to the dehydrated weathered granite mining mud (ratio of 20 L of water to 4 kg of dehydrated mud), and the mixture was crushed and uniformly dispersed using a hand-held power mixer. After leaving the mixture to stand, the supernatant fine muddy water was obtained by decantation (hereinafter referred to as the "raw liquid"), and the coarse residue was discarded. The amount of raw liquid was 0.915 m 3 The particle size of the original solution was measured using Shimadzu Corporation's "SALD-2300" (flow cell mode). The results are shown in Figure 3.

[0036] 200 g of non-radioactive cesium was added to the original solution and mixed.

[0037] Next, calcium hydroxide was added while measuring the pH of the stock solution with a pH meter. Calcium hydroxide was added until the pH reached 12.2.

[0038] Next, add 10.0 g (0.915 cm 3 Approximately 11 ppm relative to the total weight of the sample, and 0.5 L of isopropyl alcohol were added and mixed.

[0039] A microbubble flotation machine with four cylindrical columns, each 600 mm in diameter and 2,500 mm in height (effective internal volume 500 L), was prepared and used to carry out microbubble flotation under the following conditions: Muddy water was supplied from the top of the column, below the microbubble layer that filled the column. ·Mud water supply rate: 5L / min Air flow rate: 24L / min Microbubble circulating water flow rate: 36L / min

[0040] The fine particles entrained by the microbubbles were allowed to overflow and were collected from the top of the column as the floating matter, while the other components were collected from the bottom of the column as the residue. The floating matter and the residue were filtered, dehydrated, and dried (referred to as "dried matter A derived from the floating matter" and "dried matter B derived from the residue", respectively). The cesium concentration of each of the dried matters A and B was measured by XRF. The results are shown in Table 2, along with the amount recovered and the distribution of cesium. In addition, the particle size of the dried matters A and B was measured using Shimadzu Corporation's "SALD-2300" (flow cell mode) to obtain the particle size distribution (Figure 4). The mineral content of each of the dried matters A and B was also investigated using XRF (Figure 5).

[0041] [Table 2]

[0042] (result) Separation by particle size Comparing Figures 3 and 4, we can see that microbubble flotation succeeded in separating the particles into two fractions with different particle size distributions. The floated material (dried material A) had a smaller particle size, while the residue (dried material B) had a larger particle size. From Figure 5, it was found that quartz (around 2θ = 26-27) and feldspar (around 2θ = 27-28), which are not 2:1 type clay minerals, were prominent in both the floated matter and the residue, meaning that separation was not achieved regardless of particle size. In addition, there was a significant difference in the peaks of biotite minerals such as phlogopite and iron mica, which are 2:1 type clay minerals with flat shapes (around 2θ = 8-9), between dried matter A and dried matter B. This shows that the residue contains a large amount of biotite minerals, which are 2:1 type clay minerals with large particle sizes. It is also known that the area where vermiculite should be detected (around 2θ = 6) is not quantitative, so it is not possible to read anything into this result.

[0043] About Cesium The results in Table 2 show that there is more cesium in the residue. This is thought to be because the residue contains more biotite minerals, which have larger particle sizes and can adsorb more cesium. [Industrial Applicability]

[0044] The present invention can be used to separate particles contained in muddy water.

Claims

1. A method for separating fine particles, comprising carrying out microbubble flotation on a suspension containing sandy soil with a particle size of less than 2 mm and clayey soil with a particle size of less than 75 μm and having a pH of 10.5 or more and less than 12.5, and recovering the floated matter.

2. 2. The method according to claim 1, further comprising the step of adding a pH adjuster to adjust the pH of the suspension to 10.5 or more and less than 12.5 before carrying out the microbubble flotation.

3. 3. The method of claim 2, wherein the pH adjuster is a hydroxide containing a divalent metal cation.

4. 2. The method according to claim 1, wherein the suspension contains a saturated or unsaturated fatty acid having 8 or more carbon atoms or a salt thereof.

5. 2. The method according to claim 1, wherein the suspension contains an alcohol, a ketone or an ether having a solubility in water of 0.1% by weight or more.

6. The method according to claim 1 , wherein the suspension contains a 2:1 type clay mineral.

Images