WO2001000542A1

WO2001000542A1 - Ca-N-P SOLUTION FERTILIZER

Info

Publication number: WO2001000542A1
Application number: PCT/FI2000/000575
Authority: WO
Inventors: Anders Weckman; Hanna-Mari Kangaslahti
Original assignee: Kemira Agro Oy
Priority date: 1999-06-28
Filing date: 2000-06-27
Publication date: 2001-01-04
Also published as: AU5687200A; EP1200372A1; FI991460A; FI991460A0; FI106712B

Abstract

The invention relates to an acidic stock solution manufactured from urea calcium phosphate nitrate, Ca(H2PO4)(NO3)•CO(NH2)2, which can be used for the manufacture of a solution fertilizer. By using the double salt according to the invention, a considerably high calcium content can be achieved in the stock solution compared with the ratio of phosphor and nitrogen; however, so that the sparingly soluble salts in the solution are not precipitated. The physical properties of the salt used are good and it remains homogeneous in storage.

Description

Ca-N-P solution fertilizer

The invention is related to an acidic stock solution containing calcium, phosphor, and nitrogen, which is used for manufacturing a solution fertilizer. More accurately, the invention relates to a stock solution that is manufactured from one pure compound, urea calcium phosphate nitrate, Ca(H P0₄)(N0₃) CO(NH )₂, by dissolving it in water.

Solution fertilizers have been used and manufactured in quantities for several decades and further global growth is expected. Adding fertilizers directly to irrigation systems has proved to be a very effective and environmentally friendly way to increase growth. As nutrients are in a dissolved liquid form, an accurate and even distribution of nutrients in the desired target is achieved. When solution fertilizers are used, in hot and dry areas under cultivation in particular, as well as in greenhouses and gardens, plants get exactly the nutrients they need and the efficiency of the nutrients is good, 80-90% for nitrogen, for example. At the same time, the efficiency of the application of water is improved. It is also easier for the farmer to handle liquid fertilizers than solid matter.

Typically, the compounds used in solution fertilization are sold in the form of solid matter. The farmer makes from them a strong stock solution by dissolving the compounds in water in accordance with instructions. Thereafter, he takes a required portion of the stock solution and dilutes it into the final, desired concentration of use by adding it directly to the water of the irrigation system by using a dispenser or an injector.

When using irrigation fertilizers, it is important that the dissolved compounds dissolve well enough in water and that they are not precipitated again from the solution even during a long period of storage. Possible precipitates easily block the openings of the nozzles used in irrigation water systems, as well as the nozzles of the dispenser.

When dissolving the typical granular P raw material used in field fertilizers, solid matter of a large particle size often remains in the water, causing a blockage. If we want the fertilizer also to contain calcium and micronutrients, such as Mg, Fe, Cu, Zn, Mn, B, and Mo, the solutions are especially prone to be precipitated depending on which compounds are used as N and P initial materials. The inorganic compounds formed in the solutions, such as dicalcium phosphate in particular, are sparingly soluble and they have a tendency to be precipitated even at low concentrations, if the pH of the solution increases. When the pH is suitably low, nutrients and micronutrients remain better in the solution.

A solution fertilizer should be completely water-soluble. However, in practice, we can allow small numbers of solid particles insoluble in water, provided that the particle size of these particles is low enough, so that they do not cause a blockage in the irrigation systems. Furthermore, the number of these particles in a solution must be low enough. Very small particles (<50 μm) move easily with water, causing no problems in practice. In accordance with the Finnish patent publication FI 98518, a concentrated aqueous suspension fertilizer can be used as a solution fertilizer, containing at least one plant nutrient that includes solid particles insoluble in water. When the pH of the solution was less than 2, the particle size of these particles was less than 50 μm.

It is a known fact that we can use, as the source of phosphor and nitrogen in calcium-bearing solution fertilizers, for example, ammonium phosphate, potassium phosphate or sodium phosphate, phosphoric acid or urea phosphate and nitrates.

However, the problem is that the use of diammonium phosphate (DAP), for example, increases the pH of the solution, so that the calcium phosphate is easily precipitated. The pH of mono ammonium phosphate solutions (MAP) is within 4-5, where the iron and aluminium phosphates are sparingly soluble, however. Nitrate compounds are good sources of nitrogen, because the roots of plants are able to utilize this type of nitrogen quicker and more efficiently than the nitrogen obtained from urea, for example. Nitrates, chlorides or phosphates can be used as the source of calcium. Micronutrients can either be chelated, for example EDTA complexes, which remain better in solutions, or simple inorganic salts that are easily precipitated.

In order to avoid the precipitation of calcium and micronutrients in concentrated stock solutions, Vetanovetz and Peters (patent publication EP 569 513) have developed a solid composite fertilizer, in which the source of phosphor is urea phosphate and the source of calcium is an inorganic salt, such as a nitrate. When this initial mixture consisting of solid compounds is dissolved in water, a non- precipitating, water-soluble, acid c and stable stock solution is generated. The proportion of urea phosphate in the mixture of the compounds can be within 5-95 weight-%, and the proportion of calcium salt can similarly be within 5-95 weight-%. The total concentration of the stock solution can be within 2-40%. In practice, however, stock solutions stronger than 10% are generally not used, as the preparation of such a solution would require agitation in order for the solid initial solution to really efficiently dissolve in the amount of water used, forming a homogeneous solution. The solid initial mixture according to the Vetanovetz and Peters patent can also contain 0.1-50 weight-% of an inorganic micronutrient, such as Fe, Cu, Mn, Zn or Mg salts.

Although its Ca-content is high, the acidic stock solution obtained by using urea phosphate is completely soluble, which is a considerable advantage compared to a similar solution that would be manufactured from mono ammonium phosphate, for example. However, the initial material mixture contains both urea phosphate and calcium mtrate in the form of single compounds. The compounds have different rates of solution and to obtain a homogeneous, concentrated stock solution would require agitation in connection with dissolution. The physical properties of solid urea phosphate are good but the problem with solid calcium nitrate is its hygro- scopicity, which renders the storage of the material difficult. The entire fertilizer mixture can cake together under the effect of atmospheric humidity and heat, and segregate under the effect of the treatment and shaking, so that the homogeneity of the material suffers in storage. The critical relative humidity (CRH) of urea phosphate is 81.9% and that of hydrous calcium mtrate as low as 49.9%. The CRH of the mixture is lower than that of its components, depending on the composition. The melting point of calcium nitrate, Ca(N0₃)₂-4H₂0, is extremely low, 42.7 °C. It might constitute a problem to use the compound in warm countries where the relative atmospheric humidity is often high. In the urea phosphate-calcium nitrate system, the mutual fixed ratios of the nutrients restrict the simultaneous presence of calcium and phosphor. A frequent practice is to add other nutrient components, such as potassium, which further decrease the proportion of calcium in the solution.

The hygroscopicity of calcium compounds has successfully been decreased by foirning urea adducts with nitrate or phosphate (patent publication FR 1 182 058).

Sullivan et al. (patent publication US 4 507 139) prepared a solid field fertilizer, in which urea was added to calcium-bearing phosphatic ore, hydroxy apatite, treated with nitric acid, and a suspension was obtained, which was granulated, dried, and screened. The pH of this solid N-P field fertilizer was low (1.0-4.0) and its stability properties were superior. In more detailed X-ray diffraction studies, it was observed that the advantageous stability and humidity properties of the fertilizer in question were because of a new crystalline compound, urea calcium phosphate nitrate, Ca (H₂P0₄)(N0₃ CO(NH₂)₂. Multi-salts like the ones described above often have better physical properties than corresponding compound mixtures. This is because all components are evenly situated in one crystal lattice, so that on the surface of the crystal, there are fewer ions that become solvated and easily gather moisture around them, compared with the more hygroscopic component of the mixture. Another important matter is the even dissolution of the components. In the solution, the component that is the easiest to dissolve, dissolves first. When there is no agitation, the more sparingly soluble component is surrounded by an almost saturated solution, limiting the dissolution of the other component. In that case, the local nutrient ratios of the solution become distorted. All components of the multi-component salt are released evenly and in a certain proportion. Therefore, the solution fertilizer also becomes thoroughly homogeneous.

The solubility of calcium phosphate has been improved by simultaneously adding urea nitrate to the solution (Kochemba et al. Zhurnai Prikladnoi Kriirmi, 1982 vol. 55 No. 4, pp 914-915). The amount of dissolved calcium and phosphor was increased by adding calcium phosphate and urea nitrate in proportion to one another, so that a greater solubility was achieved than when directly mixing corresponding amounts of urea phosphate and calcium nitrate together and by dissolving the mixture in water. However, the method has no practical value because of the potential explosion risk of urea nitrate.

By using initial materials that keep the pH of solution fertilizers low enough, we can effectively decrease the risk of precipitation caused by sparingly soluble salts, but we should be able to further improve the physical properties of the single components of the initial material mixtures to make them homogeneous and user- friendly.

By using the method described in this invention, we can prepare an acidic stock solution; the crystalline compound used for the manufacture of the solution has excellent physical properties.

The purpose of the invention is to use one pure compound, urea calcium phosphate nitrate, Ca(H₂P0₄)(N0₃)-CO(NH )₂, for the manufacture of a stock solution, which is easy for a farmer to keep homogeneous for a long time and from which it is easy to prepare the desired stock solution of a solution fertilizer.

Furthermore, we surprisingly noticed that this crystalline double salt used as a field fertilizer has better properties when used for a solution fertilizer than the mixture of urea phosphate and calcium nitrate, for example. This double salt provides a higher solubility of calcium in water.

The main features of the invention are disclosed in the appended claims.

According to the invention, we have surprisingly discovered that it is possible to manufacture a stock solution needed for a solution fertilizer by using one pure compound only, which already contains the required nutrients in suitable proportions, the calcium content of which is exceptionally high compared with corresponding mixed fertilizers, and the stability properties of which are very convenient for the user.

Consequently, the invention relates to the use of a crystalline double salt, urea calcium phosphate nitrate, Ca(H₂P0₄)(N0₃)-CO(NH₂)₂, containing calcium, nitrogen, and phosphor and adducted by urea, for the manufacture of the stock solution of a solution fertilizer, in which the concentration of the solution is 12% maximum and the molar ratio of Ca/N is 1/3, the molar ratio of urea/nitrate is 1, and the molar ratio of Ca/P is 1. From the point of view of the simultaneous dissolution of calcium and phosphate, this ratio is the best and it is not achieved by mixing calcium nitrate and urea phosphate, because if the molar ratio of Ca/P is 1, then the molar ratio of urea/nitrate is Vz.

The compound according to the invention provides a stock solution having an exceptionally high Ca content and, at the same time, CO(NH₂) :Ca:No₃:P is 1: 1: 1 : 1, which is not possible to achieve by using, as source materials, for example, urea phosphate and calcium nitrate, in which the ratios are fixed together in accordance with CO(NH₂)₂:P = 1 and Ca:N0₃ = 1:2.

The urea calcium phosphate nitrate salt used for the manufacture of the solution fertilizer according to the invention can be manufactured by using known processes (patent publication US 4 507 139). Other ways to manufacture this crystalline salt include the use of pure phosphoric acid and urea or urea phosphate as the source of phosphor and nitrogen, and calcium carbonate or calcium nitrate as the source of calcium, with the addition of nitric acid, when needed.

The solubility of the salt needed for the manufacture of the stock solution according to the invention must be sufficiently good and the solids content must not cause problems in irrigation systems. When the compound manufactured by the method described above is dissolved in water, less than 2.5% of the solid matter are left over, as calculated from the compound to be dissolved. Thus the water solubility of the compound is at least 97.5%. The particle size of the solid matter not dissolved in water is 5 μm on an average, and more than 95% are less than 50 μm. Changing the conditions of the solution can decrease the amount of solid matter.

For a solution used in irrigation fertilizing, the stock solution according to the invention is typically diluted to less than 2%, so that no precipitation occurs.

The critical relative humidity (CRH) of the salt used for the manufacture of the stock solution according to the invention is 68.9%, and the melting point more than 360 °C. However, the compound begins to decompose thermally at 150 °C.

The stock solution according to the invention can also contain one or more micro- nutrients, such as magnesium, iron, manganese, copper, boron, zinc or molybdenum. These micronutrients can either be added in the form of inorganic salts or metal chelates.

One advantage of the method according to the invention described above is that the generated stock solution is sufficiently acidic, so that the sparingly soluble Ca salts in the solution keep their concentrations. In addition, phosphor, for example, is not precipitated in the form of sparingly soluble salts with impurities in the water, such as magnesium and iron, foirning NH₄MgP0₄-6H₂0 and

Furthermore, when the urea calcium phosphate mtrate salt according to the invention, Ca(H₂P0₄)(N0₃) CO(NH₂) , is used, exceptionally high calcium content is achieved in the manufacture of stock solution, compared with the ratio of phosphor and nitrogen. The solubility properties of the pure salt are good enough so that it can be used as a solution fertilizer.

The physical properties of the salt are very good and the material stays homogeneous even during a long period of storage. The user-friendliness of the material is a considerable commercial advantage.

In the following, the invention is described in detail with the aid of embodiments; however, the invention is not limited to them.

Example 1

The solubility of calcium is restricted by the phosphor content of the solution and vice versa. It is important how much calcium and phosphor the solution simultaneously contains. This can be illustrated by calculating the solubility product Ca*H₃P0 of the two components. Fig. 1 shows the product of calcium and phosphate as calculated for two different cases.

Graph (a) according to Fig. 1 is obtained, when calcium nitrate is used as the source of Ca and urea phosphate is used as the source of phosphor, so that the amount of calcium mtrate in the mixture to be dissolved ranges within 5-95% and that of urea phosphate within 95-5%. The graph shows the solubility product of calcium phosphate as demonstrated by different calcium/phosphor ratios. Even in the most preferred cases, when low calcium nitrate contents are used, the product of the calcium and phosphate contents remains below 300. A considerably better product, about 600 (Point b), is achieved, if urea calcium phosphate mtrate salt is used as the source of calcium and phosphate.

Correspondingly, we can examine the product of calcium and phosphate by using the calcium/nitrogen ratio (Fig. 2, Graph a). As nitrogen does not only come from nitrate but also from urea, it deviates from Fig. 1. When calcium nitrate and urea phosphate are used, the maximum product of calcium and phosphate remains below 300, when the urea phosphate content is 50 weight-%.

According to this example, by dissolving urea calcium phosphate mtrate in water, a considerably higher water solubility of calcium phosphate is achieved than when a mixture of calcium nitrate and urea phosphate is used.

Example 2

A 10% aqueous solution was prepared at 24 °C from a synthetic product that mainly contained urea calcium phosphate mtrate. The contents of the ions in the solution were analysed. The pH of the solution was 2.4. The results were compared with a corresponding solution that was obtained by mixing calcium nitrate and urea phosphate and by dissolving the mixture in water. The content of calcium nitrate in this reference test varied within 25-75 weight-% and that of urea phosphate within 75-25%. Table 1 shows the results of the analyses. Table 1 The analyses of the salt according to the invention and the 10% solutions of the reference mixtures. The analyses are calculated from the composition of the components.

When converting phosphoric pentoxide into a phosphoric acid content, the conversion coefficient is H₃P0₄-% = 1.38- P₂0₅-%.

In all cases that used the mixture of calcium mtrate and urea phosphate, the content of dissolved calcium was smaller than when the salt according to the invention was used. Similarly, the compound according to the invention has the highest solubility product Ca % * P₂0₅ % that depicts the solubility of calcium phosphate.

By dissolving urea calcium phosphate nitrate in water, according to this example, a considerably higher water solubility of calcium phosphate and higher calcium content are achieved than when using mixtures of calcium nitrate and urea phosphate.

Example 3

A 2% solution was prepared from the urea calcium phosphate nitrate, which was prepared according to Example 3, by adding 4.82 g of the product in 236.7 g of water. When the solution was filtered and the insoluble residue weighed, the residue amounted to 0.11 g (2.3 weight-%). In the analyses, the substance proved to be an inactive initial material, Ca₃(P0₄)₂. A particle size distribution analysis indicated that the residue was very fine-grained, with an average particle size of 5 μm, and slowly settling. Example 4

Crystalline urea-adducted double salt was prepared from calcium phosphate, 23.1 g of Ca₃(P0₄)₂, calcium hydroxide, 1.73 g of Ca(OH)₂, nitric acid, 27.4 ml 65%, and 14 g of urea, by mixing the compounds together in the order mentioned above and by adding 2.8 g of water to the mixture. The mixture was agitated for 10 min., after which 500 ml of acetone was added. The obtained precipitate was filtered and washed twice with acetone.

The purity of the double salt was 85% and, on the basis of an X-ray diffractogram, the main impurity it contained was the urea adduct of calcium nitrate, Ca(N0₃)₂-4CO(NH₂)₂. The water solubility of the salt was 11.5% and the residual amount 2.3%.

The thermal stability and the moisture resistance of the salt were compared with the corresponding properties of calcium nitrate. On the basis of a thermo gravimetric analysis, the salt was completely stable in the air up to 150 °C, while calcium nitrate melted already at 42 °C, forming a solution with its own crystal waters. On the basis of climate chamber measurements at 25 °C at the relative humidity of 80%, the change in weight of the salt after 6 hours was 3.9%. The side component of the synthesis, Ca(N0₃)₂-4CO(NH₂)₂, had as high a critical relative humidity as the actual product salt Ca(H₂P0₄)(N0₃)-CO(NH₂)₂, 67-69% at 25 °C, while that of calcium nitrate was clearly lower, 50-47% at 25-30 °C.

This example indicates that the physical properties of the salt prepared by this method are considerably better than those of calcium mtrate and the salt can be used as the initial material of the stock solution of a solution fertilizer.

Claims

1. A chlorine-free stock solution suitable for the manufacture of a solution fertilizer, containing at least calcium, phosphor, and nitrogen, characterized in that the said stock solution is manufactured by dissolving in water a crystalline salt, which mainly consists of urea calcium phosphate mtrate, Ca(H P0 )(N0₃)-CO(NH₂)₂, so that an acidic solution is generated with a content of 12% maximum.

2. A stock solution according to Claim 1, characterized in that its calcium content is 1.5% or more, whereas, at the same time, its phosphor content, P₂0₅, is 2.4% or more.

3. A stock solution according to Claim 1 or 2, characterized in that the content of the urea calcium phosphate nitrate salt of the said stock solution is preferably 10% maximum.

4. A stock solution according to any of Claims 1 to 3, characterized in that it contains insoluble compounds, the content of which is less than 2.5%.

5. A stock solution according to Claim 4, characterized in that more than 95% of the particles in the insoluble compounds have a particle size of less than 50 μm.

6. A stock solution according to any of Claims 1 to 5, characterized in that a 0.01% dilution is prepared from the said solution to be used in irrigation fertilizing.

7. A stock solution according to any of Claims 1 to 6, characterized in that the said solution contains one or more micronutrients.

8. A stock solution according to Claim 1, characterized in that for the salt used in its manufacture critical relative humidity (CRH) is 65% or more, water solubility is 10% or more, and the compound is thermally stable at less than 150 °C.

9. A stock solution according to Claim 1, characterized in that the content of urea calcium phosphate nitrate, Ca(H₂P0 )(N0₃) CO(NH₂)₂, in the salt used in its manufacture is more than 80%, and the salt also contains the urea adduct of calcium nitrate, Ca(N0₃) -4CO(NH₂) , in the form of a soluble component, and calcium phosphate, Ca₃(P0 )₂, in the form of an insoluble component.