JPH0223513B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a soil conditioner coated with calcium phosphate.

The cultivated land area in Japan is approximately 512 including rice paddies and fields.
Approximately 60% of the total land area is said to be poor land, and half of it is said to be poor soil. Various studies have been conducted to improve such soil and improve agricultural production.

Zeolite is an inorganic soil improvement material.
Because it is a porous mineral that exhibits a high cation exchange capacity, it is effective in soil improvement for soils with weak fertilizer component retention capacity, weak cation retention capacity, acidic soil, and poorly drained soil, and is abundant in Japan. It is seen as promising because of the

However, since zeolite itself does not contain fertilizer components, it is necessary to apply fertilizer components at the same time instead of using zeolite alone.

On the other hand, when phosphoric acid, one of the main components of fertilizer, is applied to soil, its utilization rate is extremely low at only 5 to 20%, which means that a large portion of the applied phosphoric acid is fixed. are doing. In places such as Japan, where intense leaching occurs under warm and rainy conditions, iron and aluminum ions leached from the soil combine with phosphoric acid to form phosphoric acid compounds such as aluminum phosphate and iron phosphate. It becomes fixed phosphoric acid that is hardly utilized by plants. To prevent phosphate fixation, lime must be applied to neutralize the soil pH and reduce the solubility of iron and aluminum. However, even if such measures are adopted, in areas with heavy rainfall, the soil will be washed with water containing various organic acids, and the phenomenon of phosphoric acid in the soil being washed away cannot be prevented.

Therefore, it would be effective if a soil conditioner containing zeolite coated with hardly soluble calcium phosphate could be supplied at a low cost.

The present inventors have been conducting research on a crystallization dephosphorization method that effectively removes phosphates contained in secondary treated sewage water and wastewater, but in the course of this research, under certain conditions, discovered a phenomenon in which calcium phosphate crystals precipitate even on the surface of granular materials that do not have any dephosphorizing properties, such as granular zeolite, and led to the present invention.

That is, the present invention is a soil conditioner obtained by bringing phosphate-containing water and zeolite into contact in a fluidized bed at a pH of 6 or higher and in the presence of calcium ions.

The reaction that occurs when water containing phosphate is brought into contact with crystal seeds in the presence of calcium ions varies depending on the reaction conditions, but is usually expressed by the following equation (1).

5Ca ²⁺ +7OH ^- +3H ₂ PO ₄ ^- →Ca ₅ (OH) (PO ₄ ) ₃ +6H ₂ O......(1) As can be seen from equation (1), in order to convert phosphate in water to calcium phosphate It is necessary to allow the reaction to proceed to the right, and in order to crystallize the calcium phosphate produced, it is necessary to keep the surface of the zeolite clean and maintain its activity at a high level. Therefore,
In the present invention, the water flow treatment is carried out at a pH of 6 or higher and in the presence of calcium ions so as to form a fluidized bed.

In equation (1), the concentration of calcium phosphate produced is higher than the solubility, and the supersolubility (concentration at which crystals start to precipitate when no crystal seeds exist in the reaction system)
Calcium ion concentration and
In the pH region, that is, in the metastable region, the calcium phosphate produced precipitates on the zeolite surface, and no fine precipitate is produced.

On the other hand, calcium ion concentration and/or
In an unstable region where the pH is high and the produced calcium phosphate exceeds its solubility, calcium phosphate precipitates as fine precipitates. As described above, the metastable region varies depending on the concentration of phosphate ions contained in raw water, and the lower the phosphate ion concentration, the wider it becomes. Therefore, when the concentration of phosphate ions in raw water is high, increasing the calcium ion concentration and/or pH creates an unstable region and tends to form precipitates, but when the phosphate ion concentration is low, , calcium ion concentration or pH
Crystallization in the metastable region is possible even if the
The reaction rate can be increased without forming a precipitate.

In the present invention, it is necessary that calcium ions exist in the raw water that is passed through the zeolite layer so that the reaction of formula (1) above proceeds, and if there are no calcium ions in the raw water or If insufficient, add calcium supplement from outside. The pH of the reaction system is 6 or more, preferably 6 to 12
An alkaline agent is added to the raw water as necessary.

In addition, the contact between water containing phosphate and zeolite is carried out in a fluidized bed state, and the expansion rate is 20 to 80%.
In particular, it is preferably 40 to 80%. If it is less than 20%, the fluidized bed itself will not operate smoothly;
If it exceeds , the effective filling amount will decrease and efficiency will deteriorate.

Zeolite particles having a particle size of about 10 to 300 mesh are used. As phosphate-containing water,
Phosphate-containing sewage can be used and usually has a low phosphorus concentration.
Use approximately 0.1 to 10 mg/dose. As the calcium agent, slaked lime, caustic soda, calcium chloride, etc. can be used, and the amount of calcium added is usually set within the metastable range as described above.

According to the present invention, since the surface of the zeolite is coated with calcium phosphate, the problems of fixation and runoff of phosphoric acid are solved. Furthermore, since sewage can be used as phosphate-containing water, it is possible to recover and effectively utilize phosphorus resources, which is very economically advantageous. Furthermore, in the present invention, since the crystallinity of the calcium phosphate coated on the zeolite surface is relatively low, a sufficient phosphorus source is supplied even in slightly acidic soil. Note that although it is possible to crystallize calcium phosphate by adsorbing phosphoric acid on zeolite in advance and then contacting it with lime, this method is preferable because the crystallinity of the crystallized calcium phosphate is high and it is difficult to replenish the phosphorus source into the soil. It cannot be called a method.

Experimental example By adding each solution of sodium dihydrogen phosphate, calcium chloride, and soda carbonate to Yokohama city water, PO ₄
- P = 2.5 mg/, Ca ²⁺ = 45 mg/, M- alkalinity = 100 mg/, and further
Synthetic water with pH 9 was obtained by adding NbaOH.

Clinoptilolite sieved to a particle size of 0.25 to 0.42 mm was used as the zeolite, and this zeolite was placed in a transparent acrylic column with a diameter of 30 mm and a length of 100 mm.
g Dry filled. The above-mentioned synthetic water was passed through this packed column at a water flow rate of 10 m/hr such that the expansion rate was about 60%. After 60 minutes had passed, the filled zeolite was taken out and analyzed. The analysis results were as listed below.

) When the total weight of zeolite was measured, it was approx.
100g, and a weight increase of 30g was observed.

) It was also observed with the naked eye that the zeolite particles were coated with white crystals and enlarged.

) The zeolite was stirred in a beaker to cause the particles to collide with each other and the white crystals deposited on the surface layer were exfoliated, and the exfoliated crystals were analyzed for phosphorus components. As a result, the phosphorus content was 13.9% and the Ca/P ratio was 2.06.

Furthermore, when this exfoliated crystal was subjected to X-ray diffraction, the strongest line indicating hydroxyapatite was observed. However, the peak was broad, indicating a low degree of crystallinity.

) When examining the solubility of exfoliated crystals at various pH values, it was found that on the alkaline side, the elution of phosphorus was small;
Moderate elution of phosphorus was observed on the acidic side. This is because the exfoliated crystals are hydroxyapatite with a low crystallinity. In addition, due to this property, it is found that it acts effectively as an insoluble phosphate fertilizer in the soil.

Claims (1)

[Scope of Claims] 1. A soil conditioner obtained by bringing phosphate-containing water and zeolite into contact in a fluidized bed at a pH of 6 or higher and in the presence of calcium ions. 2. The soil conditioner according to claim 1, wherein the fluidized bed has an expansion rate of 20 to 80%. 3. The soil conditioner according to claim 1 or 2, wherein the phosphate-containing water is sewage.