JP7704996B2 - Method for producing humic acid-containing powder, humic acid-containing water, soil improvement method, and method for promoting plant growth - Google Patents

Description

The present invention relates to a humic acid-containing powder, a method for producing humic acid-containing water, a method for improving soil, and a method for promoting plant growth. More specifically, the present invention relates to a humic acid-containing powder having specific properties, a method for producing humic acid-containing water using the humic acid-containing powder, a method for improving soil using the humic acid-containing water, and a method for promoting plant growth using the humic acid-containing water.

Humic acid is said to have the effect of promoting plant growth, etc. For this reason, the use of humic acid as fertilizer has been proposed.
Humic acid is present in soil and land water. Specifically, humic acid is contained in lignite or peat, or exists as a metabolic product of bacterial groups. While natural humic acid originates from animals and plants, industrially, humic acid that is the oxidative decomposition product of young coal such as lignite, or alkali metal salts or alkaline earth metal salts of the oxidative decomposition product are known.

Patent documents 1 and 2 describe a method for producing a humic acid extract using a specific method, and a specific humic acid extract.

Patent No. 6231059 International Publication No. 2021/117755

Patent Documents 1 and 2 describe a humic acid extract, that is, liquid humic acid.
On the other hand, in consideration of ease of transportation and storage, powdered humic acid can be produced in advance, dissolved in water at the time of use to make an aqueous solution, and the aqueous solution can be used as a fertilizer or soil conditioner. In fact, powdered humic acid is also available on the market.

However, according to the inventor's findings, powdered humic acid that was previously available on the market is difficult to dissolve in water, and it can take a long time to turn it into an aqueous solution. This leads to reduced work efficiency when using powdered humic acid as a fertilizer or soil conditioner.

The present invention has been made in consideration of these circumstances. One of the objects of the present invention is to provide a humic acid-containing powder that is easily soluble in water.

The inventors have completed the invention provided below and solved the above problem.

1.
A humic acid-containing powder having a median diameter _D50 of 10 to 50 μm and a loose bulk density of 0.40 to 0.8 g/ ^cm3 .
2.
1. The humic acid-containing powder according to claim 1,
A humic acid-containing powder having a moisture content of 15% by mass or less.
3.
The humic acid-containing powder according to 1. or 2.
The humic acid-containing powder has a particle circularity of 0.90 to 1.
4.
1. The humic acid-containing powder according to any one of 1. to 3.
The humic acid-containing powder is obtained by drying a humic acid extract obtained by extracting a humic acid crude product obtained by oxidizing young charcoal with nitric acid using an aqueous solution having a pH of 1 to 7.
5.
1. The humic acid-containing powder according to any one of 1. to 4.
A humic acid-containing powder having a cumulative 10% diameter _D10 of 10 μm or more.
6.
1. The humic acid-containing powder according to any one of 1. to 5.
A humic acid-containing powder, in which (D ₅₀ -D ₁₀ )/D ₅₀ is 0.1 to 0.50, where D ₁₀ is the cumulative 10% diameter.
7.
A method for producing humic acid-containing water, comprising dissolving or dispersing the humic acid-containing powder according to any one of 1. to 6. in water to produce humic acid-containing water.
8.
7. A method for improving soil, comprising spraying the humic acid-containing water produced by the method described in 7. above onto soil.
9.
7. A method for promoting plant growth, comprising providing a plant with humic acid-containing water produced by the method described in 7.

The humic acid-containing powder of the present invention has good solubility in water.

Hereinafter, an embodiment of the present invention will be described in detail.
In this specification, unless otherwise specified, the expression "X to Y" in the description of a numerical range means from X to Y. For example, "1 to 5% by mass" means "1% by mass to 5% by mass."

<Humic acid-containing powder>
The humic acid-containing powder of this embodiment has a median diameter _D50 of 10 to 50 μm and a loose bulk density of 0.40 to 0.8 g/ ^cm3 .

The reason why such humic acid-containing powder dissolves well in water can be explained as follows. Just to be clear, the following explanation includes speculation, and the present invention is not limited by the following explanation.

A humic acid-containing powder having a too small median diameter _D50 is liable to form "lumps" on the water surface when added to water, and is therefore difficult to dissolve in water.
In contrast, a humic acid-containing powder with a median diameter _D50 that is too large is less likely to form lumps, but has a smaller surface area relative to the powder volume. In other words, the contact area with water per unit volume of the powder is reduced, which is thought to slow the dissolution rate in water.
The median diameter _D50 is 10 to 50 μm, that is, the median diameter _D50 is neither too large nor too small, so that the powder is less likely to form lumps when put into water, and the contact area with water per unit volume of the powder is sufficient, which is considered to result in good solubility in water.

In addition, since the loose bulk density is 0.8 g/cm3 or ^less , it is guaranteed that there are gaps between the particles that make up the humic acid-containing powder through which water can penetrate, which is thought to increase the solubility in water.
Incidentally, in terms of ensuring gaps for water to enter, the smaller the loose bulk density, the better, but in this embodiment, from the standpoint of manufacturability and reducing the space required for storing the humic acid-containing powder, the lower limit of the loose bulk density is set to 0.40 g/ ^cm3 .

The humic acid-containing powder of the present embodiment can be manufactured by using appropriate raw materials and undergoing an appropriate manufacturing process. As an appropriate manufacturing process, a process of evaporating water from a humic acid extract and powdering it by using an atomizer disk method can be mentioned.
As far as the inventors' investigations in a limited time are concerned, it is difficult to obtain a humic acid-containing powder having a median diameter _D50 of 10 to 50 μm and a loose bulk density of 0.40 to 0.8 g/ ^cm3 by a powdering process other than the atomizer disk method. However, this does not mean that the manufacturing method of the humic acid-containing powder of this embodiment is limited.
The raw materials and manufacturing process will be described in more detail at a later date.

The explanation of the humic acid-containing powder of this embodiment will continue.

(particle size distribution)
As described above, the median diameter _D50 of the humic acid-containing powder of this embodiment is 10 to 50 μm. _D50 is preferably 15 to 45 μm, more preferably 20 to 40 μm.
The fact that the median diameter _D50 is a relatively large value of 10 μm or more also leads to suppression of deterioration of the humic acid in the humic acid-containing powder of this embodiment. This is because, when the humic acid extract is dried to obtain a humic acid-containing powder as described below, if the particle diameter is relatively large, the effect of hot air during drying can be mitigated.

When the cumulative 10% diameter of the humic acid-containing powder of this embodiment is D ₁₀ , D ₁₀ is preferably 10 μm or more, more preferably 10 to 25 μm, and even more preferably 10 to 20 μm. When _{D 10} is 10 μm or more, in other words, when the fine particles of less than 10 μm in the humic acid-containing powder are less than 10%, scattering of the humic acid-containing powder due to static electricity is suppressed, and the handleability of the humic acid-containing powder tends to be improved (fine particles tend to be easily scattered by static electricity).

From the same viewpoint, the value of (D ₅₀ -D ₁₀ )/D ₅₀ is preferably 0.1 to 0.50, more preferably 0.10 to 0.50, even more preferably 0.2 to 0.5, particularly preferably 0.20 to 0.50, particularly preferably 0.3 to 0.5, and most preferably 0.30 to 0.50. (D ₅₀ -D ₁₀ )/D ₅₀ being 0.5 or less means that the particle size distribution of the humic acid-containing powder is sharp on the smaller particle size side than D _50. In other words, (D ₅₀ -D ₁₀ )/D ₅₀ being 0.5 or less means that the ratio of fine particles in the humic acid-containing powder that are easily scattered by static electricity is small.

From another viewpoint, when the cumulative 90% diameter of the humic acid-containing powder of this embodiment is defined as D ₉₀ , D ₉₀ is preferably 30 to 70 μm, more preferably 35 to 65 μm, and even more preferably 35 to 60 μm. When _{D 90} is 70 μm or less, in other words, when the fine particles of more than 70 μm in the humic acid-containing powder are less than 10%, the amount of undissolved coarse particles is reduced, and the solubility of the humic acid-containing powder in water tends to be better.
The value of (D ₉₀ -D ₅₀ )/D ₅₀ is preferably 0.6 to 1.00, more preferably 0.60 to 1.00, even more preferably 0.65 to 0.9, and more preferably 0.65 to 0.90. (D ₉₀ -D ₅₀ )/D ₅₀ being 1.0 or less means that the particle size distribution of the humic acid-containing powder is sharp on the larger particle size side than D _50. In other words, (D ₉₀ -D ₅₀ )/D ₅₀ being 1.0 or less means that the ratio of coarse particles that are likely to remain insoluble in the humic acid-containing powder is small.

The values of _D10 , _D50 and _D90 can be measured in a dry state by a laser diffraction/scattering method. The values of _D10 , _D50 and _D90 obtained by the laser diffraction/scattering method are usually values based on volume. The values of _D10 , _D50 and _D90 described in this specification are also values based on volume unless otherwise specified.
The measurement can be carried out, for example, using an apparatus manufactured by HORIBA.

(Loose bulk density)
As described above, the loose bulk density of the humic acid-containing powder of the present embodiment is 0.40 to 0.80 g/cm ^3. The loose bulk density is preferably 0.40 to 0.70 g/cm ³ , and more preferably 0.40 to 0.60 g/cm ³ .
The loose bulk density can be measured by the method described in the Examples below.

(Moisture percentage)
The moisture content of the humic acid-containing powder of this embodiment is preferably 15% by mass or less, more preferably 1 to 15% by mass, even more preferably 3 to 12% by mass, and particularly preferably 5 to 10% by mass.
By not having too much moisture content, it is possible to expect improved storage properties such as suppression of clumping during storage and suppression of deterioration of humic acid. In addition, by not having too much moisture content, the humic acid-containing powder tends to be less likely to adhere to the container wall. In other words, by not having too much moisture content, the handling properties of the humic acid-containing powder tend to be improved.
The moisture content may be small, but does not necessarily have to be close to zero because there is a balance between the energy cost for removing moisture and the above-mentioned improvement in storage and handling properties.

(Circularity)
It is preferable that the particles in the humic acid-containing powder of this embodiment have an appropriate circularity.
The circularity can be determined by analyzing an electron microscope image of the particles in the humic acid-containing powder. Specifically, it can be determined by the following steps (1) to (3).
(1) Using 100 random particles in an electron microscope image of a humic acid-containing powder as a guide, the area s and perimeter l of each particle are determined. The area s and perimeter l can be determined using image analysis software.
(2) From the above s, l and the circularity constant π, calculate the circularity of each particle using the formula 4πs/ ^l2 .
(3) The arithmetic mean value of the circularity of each particle obtained in (2) above is adopted as the circularity in this specification.

The circularity is preferably 0.90 to 1, more preferably 0.90 to 1.00, even more preferably 0.9 to 0.98, particularly preferably 0.90 to 0.98, particularly preferably 0.9 to 0.97, and most preferably 0.90 to 0.97. A moderately large average circularity reduces the contact area between particles, making the particles smoother and improving handleability, and reducing the tendency for the particles to clump together. Further improvement in solubility in water can also be expected.

(Melanic Index (MI))
The humic acid-containing powder of this embodiment preferably has an appropriate melanic index (MI). The MI is preferably 2 to 5, more preferably 2.0 to 5.0, even more preferably 2 to 3, and particularly preferably 2.0 to 3.0. The fact that the MI is within this range is considered to mean that the humic acid in the humic acid-containing powder has an appropriate amount of hydrophilic groups. It is considered that the presence of an appropriate amount of hydrophilic groups makes the water solubility or water dispersibility of the powder even better.
The method for measuring the MI is described in the section "Production method of humic acid crude product" below as steps (1) to (5). However, if the particle size of the humic acid-containing powder is sufficiently small, the grinding or sieving in step (1) may not be necessary.

(Raw material for humic acid-containing powder)
As an example, the humic acid-containing powder can be produced using a humic acid extract as a raw material.
As an example, a humic acid extract can be produced by first oxidizing young coal such as lignite with nitric acid to obtain a crude humic acid product, and then extracting humic acid from the crude humic acid product.
A method for producing humic acid crude product and a method for extracting humic acid from the humic acid crude product will be described below.

- Method for Producing Humic Acid Crude Product As described above, humic acid crude product can be obtained by oxidizing young coal such as lignite with nitric acid.
Young coal is coal with a low carbon content, and is defined as coal with a carbon content of 83% by mass or less. Preferably, young coal with a carbon content of 78% by mass or less is used. Such young coal is in the process of being coalified and contains a large amount of aliphatic hydrocarbon structures, so it is susceptible to oxidative decomposition by nitric acid. As young coal, one or more of peat, lignite, brown coal, subbituminous coal, etc. can be used.

In the case of oxidation with nitric acid, it is preferable to mix 500 to 1000 kg of concentrated nitric acid in anhydrous equivalent per 1000 kg of young coal. The mixing time can be, for example, 20 to 60 minutes.
Oxidation with nitric acid is a violent chemical reaction that is accompanied by foaming. When foaming is intense, it may be difficult to control the chemical reaction, so a defoamer may be used. As the defoamer, it is preferable to use hydrophobic silica, which is a component similar to that of soil, considering that humic acid is used for soil improvement. Examples of commercially available hydrophobic silica products include the SIPERNAT series from Evonik.

By adjusting the amount of nitric acid added, the melanic index (hereinafter abbreviated as MI) of the resulting humic acid crude product can be adjusted to 2.0 to 3.0. MI is an index used to classify humic acids, and is defined as the ratio of absorbance at wavelengths of 450 nm and 520 nm (A ₄₅₀ /A ₅₂₀ ) in the absorption spectrum of a sodium hydroxide extract.
(Reference: Kumada Kyoichi, Chemistry of Soil Organic Matter, 2nd Edition, Academic Press Center (1981), Japanese Journal of Soil and Plant Nutrition, Vol. 71, No. 1, pp. 82-85 (2000)).

More specifically, the MI can be calculated by the following procedure.
(1) The sample is crushed in a mortar and then sieved through a 250 μm sieve to obtain a 250 μm sieve-passing product.
(2) Approximately 10 g of the above 250 μm sieve fraction is placed in a weighing bottle with a known mass and weighed accurately. The weighing bottle is left in a dryer maintained at 105° C. for approximately 12 hours, and then returned to room temperature in a desiccator and weighed again. The weight loss is regarded as moisture and the moisture content of the sample is calculated.
(3) 0.10 g of the 250 μm sieve product in terms of dry mass equivalent and 45 mL of 0.5 mol/L sodium hydroxide aqueous solution are placed in a 50 mL centrifuge tube, and shaken at room temperature of 20° C. for about 1 hour at a speed of 250 rpm. (4) After shaking, centrifuge at 3,000 x g for about 10 minutes. The resulting supernatant is filtered through Advantec No. 5C filter paper to obtain a filtrate. The absorbance at 450 nm and the absorbance at 520 nm of the filtrate are measured using distilled water as a blank. At this time, if the absorbance at 450 nm is 1.0 or more, add 0.1 mol/L sodium hydroxide aqueous solution to adjust the absorbance to 0.8 or more but less than 1.0, and then measure the absorbance at 520 nm.
(5) Calculate the ratio (absorbance at 450 nm/absorbance at 520 nm) to obtain the MI.

The MI of 2.0 or more measured as described above means that the humic acid in the humic acid crude product has sufficient hydrophilic groups and is therefore well soluble in water. In other words, the extraction rate in water (specifically, water with a pH of 7.0 or less) is improved. Also, if the MI is 3.0 or less, excessive oxidation reactions are suppressed, leading to reduced nitric acid costs.

The MI can be increased or decreased by increasing or decreasing the amount of nitric acid during the production of humic acid crude product. Usually, the MI increases when the amount of nitric acid is increased. From the above viewpoint, it is preferable to increase the MI by increasing the amount of nitric acid, but in terms of the balance with the cost, the MI is preferably 3.0 or less.
From the viewpoint of achieving both an improvement in extraction rate and cost, the MI is more preferably 2.2 to 3.0, and even more preferably 2.2 to 2.4.
By the way, the MI of natural humic acid is generally below 1.7.

Method of extracting humic acid from humic acid crude product Water is added to the humic acid crude product obtained as described above or to a humic acid crude product obtained by another method, and an alkali such as potassium hydroxide is also added appropriately to adjust the pH to, for example, 1 to 7, preferably 5 to 7, or to a pH of up to 3. The liquid thus obtained is typically stirred at 40 to 90°C for 0.5 to 1 hour, and then cooled to 40°C or lower.
The unreacted residue can be separated from the cooled liquid by a method such as centrifugation or a filter press to obtain a humic acid extract.

Incidentally, the descriptions in Patent Documents 1 and 2 can be referred to for information on the method for producing humic acid extract.

(Method for producing humic acid-containing powder)
The humic acid extract obtained as described above or a humic acid extract obtained by another method can be dried to produce a humic acid-containing powder.
In this embodiment, it is preferable to dry and powder the humic acid extract using the atomizer disk method. By adopting this method, it is possible to stably produce a humic acid-containing powder having a median diameter _D50 of 10 to 50 μm and a loose bulk density of 0.40 to 0.8 g/ ^cm3 . Although the details are unknown, it is believed that the characteristics of the humic acid extract and these powdering methods match, making it possible to produce the desired humic acid-containing powder.
Incidentally, according to the knowledge of the present inventors, when the water is evaporated and powdered from the humic acid extract by a method other than those mentioned above (for example, the spray drying method), it is difficult to produce a humic acid-containing powder having a _D50 or loose bulk density within the above-mentioned numerical range.

The atomizer disk method is a method in which the liquid raw material is sprayed into a space where hot air is blowing using a high-speed rotating disk, thereby drying and powdering the liquid raw material.
When the atomizer disk method is adopted, it is particularly preferable to adopt the following conditions in order to obtain a powder having a desired _D50 and loose bulk density.
Hot air temperature (hot air inlet temperature): preferably 150 to 300°C, more preferably 170 to 260°C
Raw material throughput: preferably 0.5 to 10 kg/hr, more preferably 1 to 5 kg/hr
Disk rotation speed: preferably 10,000 to 40,000 rpm, more preferably 15,000 to 30,000 rpm

An example of an apparatus capable of powderization using the atomizer disk method is the L-8i device manufactured by Okawara Kakoki Co., Ltd.

<Method of producing humic acid-containing water>
The above-mentioned humic acid-containing powder can be dissolved or dispersed in water to produce humic acid-containing water, which is preferably spread on soil for soil improvement.
The ratio of humic acid-containing powder to water when producing humic acid-containing water is not particularly limited, but when spraying on soil, it is preferable to dissolve or disperse 1 part by mass of humic acid-containing powder in 100 to 30,000 parts by mass of water.

<Method of improving soil and method of promoting plant growth>
By spraying the above-mentioned humic acid-containing water onto soil, the soil can be improved, specifically, the soil can be improved to be suitable for plant growth.
Similarly, the growth of plants can be promoted by providing the above-mentioned humic acid-containing water to plants. Specifically, the growth of plants can be promoted by spraying the above-mentioned humic acid-containing water on soil in which plants grow or are planted. Examples of plants include various edible plants.

There are no particular limitations on the amount of humic acid water to be applied to the soil, but in order to obtain a sufficient soil improvement effect or plant growth promotion effect, and to balance costs, the amount of humic acid water to be applied may be, for example, 10 to 50 t/ha. In terms of humic acid powder, the amount to be applied may be 0.01 to 0.1 kg/ha.

Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various configurations other than those described above can be adopted. Furthermore, the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope of the present invention that can achieve the object of the present invention are included in the present invention.

The embodiments of the present invention will be described in detail based on examples and comparative examples. It should be noted that the present invention is not limited to the examples.

<Preparation of humic acid extract>
The humic acid extract was prepared according to the following procedure.
(1) 500 g of lignite having a carbon content of 77% by mass was prepared, which was crushed so that the powder passed through a sieve with 250 μm mesh.
(2) Using a 2-liter beaker, 500 g of the above-mentioned lignite and 630 g of nitric acid with a concentration of 48% by mass were heated in a water bath at 70° C. for about 1 hour to carry out an oxidation reaction. At this time, a small amount of a defoaming agent (hydrophobic silica) was used to suppress excessive foaming. A humic acid-containing liquid was obtained by this oxidation reaction.
(3) The obtained humic acid-containing liquid was dried at 105° C. to obtain a crude humic acid product. The MI value of this crude humic acid product was 2.20 (measured by the above-mentioned method).
(4) About 900 mL of 0.5 mol/L potassium hydroxide aqueous solution was added to 100 g of the obtained humic acid crude product, and then 1.0 mol/L potassium hydroxide aqueous solution was added while monitoring with a pH meter to adjust the pH to 6.5. Then, a small amount of water was added to finely adjust the solid-liquid ratio to a solid-liquid ratio of 1:10.
(The solid-liquid ratio is defined as the amount of extraction solvent relative to the humic acid crude product. If 100 g of humic acid crude product is placed in a 1 L beaker and solvent is added to make 1000 mL, the solid-liquid ratio is 1:10.)
(5) The liquid obtained in (4) above was heated at 80° C. for 1 hour. In this way, humic acid was extracted with water to obtain a humic acid extract stock solution.
(6) The above humic acid extract stock solution was centrifuged under the condition of a centrifugal force (xg): 3000. The supernatant thus obtained was used as a humic acid extract solution in the production of the following humic acid-containing powder.

<Production of humic acid-containing powder>
Examples 1 to 3
The humic acid extract obtained in the above <Preparation of humic acid extract> was powdered using the atomizer disk method. Specifically, an apparatus manufactured by Okawara Kakoki Co., Ltd., product number: L-8i, was used as the apparatus, and a humic acid-containing powder was produced under the operating conditions shown in Table 1 below.

Comparative Examples 1 to 6
The humic acid extract obtained in the above <Preparation of humic acid extract> was powdered by spray drying. Specifically, a humic acid-containing powder was produced using an existing spray drying device under the operating conditions shown in Table 1 below.

<Measurement of physical properties of humic acid-containing powder>
Particle size distribution: The particle size distribution was measured in a dry state using a laser diffraction/scattering particle size distribution measuring device LA-960 (manufactured by HORIBA).
In the measurement, the refractive index was set to 1.81. The air pressure was 0.40 MPa. Under these conditions, the particle size distribution (volume basis) of the powders of each Example and Comparative Example was automatically measured under atmospheric conditions. From the particle size distribution obtained, the values of _D10 , _D50 , and _D90 were obtained. Based on these values, the values of ( _D50 - _D10 )/ _D50 and ( _D90 - _D50 )/ _D50 were calculated.

Loose bulk density The powder was gently filled into a 25 mL stainless steel container, and the mass of the powder was measured. The measured mass was divided by the volume (25 mL) to obtain the loose bulk density.
The measurement was carried out three times, and the average value of the three measurements was used as the loose bulk density.

Moisture content The moisture content was measured using a halogen moisture meter HX204 (Mettler Toledo).
The drying temperature was set to 105° C., and the moisture content was automatically measured using 1 g of powder from each Example or Comparative Example. The measurement was carried out three times, and the average value of the three measurements was used as the moisture content.

Circularity Images of particles in the humic acid-containing powder were obtained using a scanning electron microscope (Regulus 8220, Hitachi High-Tech Corporation). Based on these images, the circularity was calculated according to the above-mentioned procedures (1) to (3).
Incidentally, the area s and perimeter l of the particles in the image were determined using image analysis software ImageJ (version 1.53). At this time, the number of measurement points for the area and perimeter was set at 100.

・Melanotic Index (MI)
The melanic index (MI) of the humic acid-containing powder of Example 1 was measured by the above-mentioned method. The MI was 2.2.

<Performance evaluation>
(Solubility in Water)
1 g of the humic acid-containing powder of each Example or Comparative Example was gently placed in a beaker containing 500 mL of water at room temperature, and then slowly stirred using a glass stirring rod. When the humic acid-containing powder dissolved quickly, it was evaluated as having good solubility, and when the humic acid-containing powder became lumpy on the water surface and was difficult to dissolve, it was evaluated as having poor solubility.

(Handling: Evaluation of scattering due to static electricity)
The humic acid-containing powder was sprinkled on a polyethylene film (negatively charged) and the ease with which it adhered to the film was used to evaluate the ease with which it would scatter due to static electricity.
Specifically, the ease of adhesion when the humic acid-containing powder of Example 1 was sprinkled onto a polyethylene film was used as a standard, and if the ease of adhesion of the powder was the same as in Example 1, it was rated as good handleability, and if the powder was clearly more easily adhered than in Example 1, it was rated as poor handleability.

(Whether or not humic acid has changed)
Thermal analysis was used to evaluate whether humic acid had been altered and converted into other components.
Specifically, the humic acid-containing powders of each Example or Comparative Example were thermally analyzed using the following apparatus and conditions to obtain TG curves and DTA curves. The freeze-dried humic acid extract obtained in the above <Preparation of humic acid extract> (which was not heated during drying, so in principle the humic acid was not altered) was also thermally analyzed in the same manner.
Equipment: Differential thermal and thermogravimetric simultaneous measurement equipment TG-DTA2000SR (manufactured by NETZSCH)
Measurement temperature: from room temperature to 500°C
Heating rate: 3°C/min Reference: α-alumina Measurement sample amount: 4.5 mg

The TG curve (curve 1-1) and DTA curve (curve 1-2) obtained for the humic acid-containing powder of each Example or Comparative Example were compared with the TG curve (curve 2-1) and DTA curve (curve 2-2) obtained for the freeze-dried product of the humic acid extract.
When the shapes and peak positions of curves 1-1 and 2-1 are almost the same, and when the shapes and peak positions of curves 1-2 and 2-2 are almost the same, it is evaluated that the deterioration of humic acid has been sufficiently suppressed, and this is recorded as "no deterioration" in Table 1. In contrast, when the shapes and/or peak positions of curves 1-1 and 2-1 are clearly different, and/or the shapes and peak positions of curves 1-2 and 2-2 are clearly different, it is evaluated that the deterioration of humic acid has occurred, and this is recorded as "deterioration" in Table 1.

The powdering conditions (operating conditions), physical properties of the humic acid-containing powder and evaluation results are summarized in Table 1.

As shown in Table 1, the humic acid-containing powders of Examples 1 to 3, which had a median diameter D ₅₀ of 10 to 50 μm and a loose bulk density of 0.40 to 0.80 g/cm ³ , dissolved well in water.
In addition, the humic acid-containing powders of Examples 1 to 3 were easy to handle and were prevented from scattering due to static electricity. This is thought to be related to the small _D10 of the humic acid-containing powders of Examples 1 to 3 or the small value of ( _D50 - _D10 )/ _D50 .
Furthermore, the deterioration of the humic acid in the humic acid-containing powders of Examples 1 to 3 was suppressed. This is considered to be because the particle size was relatively large, which reduced the effect of hot air during drying.

This application claims priority based on Japanese Patent Application No. 2023-037236, filed March 10, 2023, the disclosure of which is incorporated herein in its entirety.

Claims (9)

A humic acid-containing powder having a median diameter _D50 of 10 to 50 μm and a loose bulk density of 0.40 to 0.8 g/ ^cm3 . The humic acid-containing powder according to claim 1,
A humic acid-containing powder having a moisture content of 15% by mass or less. The humic acid-containing powder according to claim 1 or 2,
The humic acid-containing powder has a particle circularity of 0.90 to 1. The humic acid-containing powder according to claim 1 or 2,
A humic acid-containing powder having a melanic index of 2 to 5 . The humic acid-containing powder according to claim 1 or 2,
A humic acid-containing powder having a cumulative 10% diameter _D10 of 10 μm or more. The humic acid-containing powder according to claim 1 or 2,
A humic acid-containing powder, in which (D ₅₀ -D ₁₀ )/D ₅₀ is 0.1 to 0.50, where D ₁₀ is the cumulative 10% diameter. A method for producing humic acid-containing water, comprising dissolving or dispersing the humic acid-containing powder according to claim 1 or 2 in water to produce humic acid-containing water. A method for improving soil, comprising spraying humic acid-containing water produced by the method of claim 7 onto soil. A method for promoting plant growth, comprising providing plants with humic acid-containing water produced by the method of claim 7.