阅读说明：本技术 一种抗结块低吸湿磷酸氢二铵固体结晶粉末的制备方法 (Preparation method of anti-caking low-moisture-absorption diammonium hydrogen phosphate solid crystal powder ) 是由 徐玲 裘雪阳 陈建军 刘君锭 朱自强 于 2021-01-29 设计创作，主要内容包括：本发明公开了一种抗结块低吸湿磷酸氢二铵固体结晶粉末的制备方法,利用氨和磷酸对纳米二氧化硅进行预处理；向盛放磷酸溶液的容器中通入过量氨气持续反应,当反体系的pH为3.0～5.0的时候加入预处理后的纳米二氧化硅,持续搅拌至反应终点后冷却、结晶、过滤、分离,获得磷酸氢二铵无水结晶粉末。本发明通过加入预处理的纳米级亲水性二氧化硅分散液,该分散液经氨水处理后再用磷酸进行酸处理,实现对磷酸氢二铵表面微球结构的改性,改善了磷酸氢二铵因吸湿引起的物料结块问题。(The invention discloses a preparation method of anti-caking low-moisture-absorption diammonium hydrogen phosphate solid crystal powder, which comprises the steps of pretreating nano silicon dioxide by using ammonia and phosphoric acid; and introducing excessive ammonia gas into a container containing phosphoric acid solution for continuous reaction, adding pretreated nano silicon dioxide when the pH value of the reaction system is 3.0-5.0, continuously stirring until the reaction end point, and then cooling, crystallizing, filtering and separating to obtain diammonium hydrogen phosphate anhydrous crystal powder. According to the invention, the pretreated nano-scale hydrophilic silicon dioxide dispersion liquid is added, and the dispersion liquid is treated by ammonia water and then is treated by phosphoric acid, so that the modification of the surface microsphere structure of diammonium hydrogen phosphate is realized, and the problem of material caking of diammonium hydrogen phosphate caused by moisture absorption is solved.)

1. A preparation method of anti-caking low-moisture-absorption diammonium hydrogen phosphate solid crystal powder is characterized by comprising the following steps:

(1) introducing ammonia gas into a phosphoric acid solution with the concentration of 50-85 wt% for continuous reaction, adding the pretreated nano silicon dioxide dispersion liquid when the pH value of a reaction system is 3.0-5.0, and continuously stirring;

(2) stopping the reaction when the pH value is 7.8-8.2, and cooling, crystallizing, filtering and drying to obtain anti-caking and low-moisture-absorption diammonium hydrogen phosphate solid anhydrous crystalline powder with good fluidity;

the pretreated nano silicon dioxide dispersion liquid is obtained by treating silicon dioxide with ammonia water and then gradually dripping phosphoric acid.

2. The method for preparing the anti-caking and low-moisture-absorption diammonium phosphate solid crystalline powder according to claim 1, wherein the method comprises the following steps: in the added pretreated nano silicon dioxide dispersion liquid, the mass of the nano silicon dioxide particles is 0.01-5 wt% of that of the 50-85 wt% phosphoric acid solution.

3. The method for preparing the anti-caking and low-moisture-absorption diammonium phosphate solid crystalline powder according to claim 2, wherein the method comprises the following steps: the content of the nano silicon dioxide particles in the pretreated nano silicon dioxide dispersion liquid is 5-10 wt%.

4. The method for preparing the anti-caking and low-moisture-absorption diammonium phosphate solid crystalline powder according to claim 3, wherein the method comprises the following steps: the average particle diameter of the nano silicon dioxide particles is 15-50 nm, and the specific surface area is 150-300 m²/g。

5. The method for preparing the anti-caking and low-moisture-absorption diammonium phosphate solid crystalline powder according to any one of claims 1 to 4, characterized in that: and during preparation of the pretreated nano silicon dioxide dispersion liquid, after dropwise addition, heating to a temperature not higher than 100 ℃, and continuously stirring for 1-5 hours.

6. The method for preparing the non-caking and low-moisture absorption diammonium phosphate solid crystalline powder according to claim 5, wherein the method for preparing the pretreated nano-silica dispersion comprises the following steps:

adding the nano silicon dioxide into 1-5 wt% of ammonia water in stirring, and stirring at the same or different rotating speeds of 50-500 rpm at the temperature of 10-20 ℃ for 1-5 hours;

in the stirring process, 70-85% of phosphoric acid is dripped according to the mass ratio of 1:1, the dripping time is 0.5-5 hours, and the temperature is kept to be less than or equal to 40 ℃ in the process;

after the dropwise addition is finished, the temperature is raised to 40-100 ℃, the rotation speed is 100-300 rpm, and the stirring is carried out for 0.5-2.0 hours, so as to obtain the pretreated nano silicon dioxide dispersion liquid.

7. The method for preparing the non-caking and low-moisture absorption diammonium phosphate solid crystalline powder according to claim 6, wherein the pretreated nano-silica dispersion is prepared by the following steps:

adding nano silicon dioxide into 3 wt% of ammonia water stirred at the rotating speed of 60rpm, stirring for 10-20 minutes at the temperature of 10-20 ℃ and at the rotating speed of 500rpm, and then continuing stirring for 2 hours at the rotating speed of 60 rpm;

in the stirring process, 85 percent of phosphoric acid is dripped according to the mass ratio of 1:1, the dripping time is 1-2 hours, and the temperature is kept to be less than or equal to 40 ℃ in the process;

after the dropwise addition is finished, the temperature is raised to 50-80 ℃, and the mixture is stirred for 1-1.5 hours at the rotating speed of 200rpm, so that the pretreated nano silicon dioxide dispersion liquid is obtained.

8. Solid crystalline diammonium hydrogen phosphate powder, obtained by preparation according to claim 1, characterised in that it simultaneously satisfies the following characteristics:

a) the content of diammonium hydrogen phosphate is not less than 99.0 wt%;

b) the moisture absorption rate is less than or equal to 0.2 wt%;

c) the caking rate is less than or equal to 0.5wt percent.

9. Solid crystalline diammonium hydrogen phosphate powder according to claim 8, characterized in that it also simultaneously satisfies the following characteristics:

d) the content of diammonium hydrogen phosphate is not less than 99.5 wt%;

e) the angle of repose is more than or equal to 65;

f) the screen residue (less than 2mm) is more than or equal to 99.0 wt%.

10. Solid crystalline diammonium hydrogen phosphate powder according to claim 9, characterized in that it also simultaneously satisfies the following characteristics:

f) colorless, clear and transparent at 300 ℃;

g) the water content is less than or equal to 0.2 wt%.

Technical Field

The invention relates to the field of flame retardants and inorganic chemical synthesis, and particularly relates to a preparation method of diammonium hydrogen phosphate solid crystalline powder with excellent caking resistance, low moisture absorption and crystalline powder fluidity.

Background

Diammonium hydrogen phosphate is an inorganic compound, a colorless transparent monoclinic crystal or white powder, is widely used for printing plate making, medicine, fire prevention, electron tubes and the like, is a high-efficiency fertilizer widely applied to vegetables, fruits, rice and wheat, is industrially used as a feed additive, a flame retardant, a fire extinguishing agent and the like, and is also an important substance for preparing ammonium polyphosphate with high crystallization type II polymerization degree.

The ammonium polyphosphate is a halogen-free flame retardant with high phosphorus and nitrogen contents, good thermal stability, small water solubility and high flame retardant property, and has wide application range. The diammonium hydrogen phosphate is used as a common raw material for preparing the ammonium polyphosphate, particularly the crystal II type ammonium polyphosphate with high polymerization degree, and the quality of the ammonium polyphosphate is directly influenced by the difference of the product content and the performance. Products obtained by the existing diammonium hydrogen phosphate preparation process are mainly used in industries such as chemical fertilizers, feeds, commodity addition and the like, and have no requirements on caking resistance, hygroscopicity, fluidity and chromaticity at high temperature of diammonium hydrogen phosphate in industries such as industrial diammonium hydrogen phosphate industry standard HG/T4132-2010 and industries such as feeds, food additives and chemical reagents and national standards, so that the quality of ammonium polyphosphate products with high polymerization degree of crystal II prepared by diammonium hydrogen phosphate is unstable and has very large difference.

Chinese patent application CN110903108A (published 2020, 3/24) discloses a method for preparing diammonium hydrogen phosphate by neutralization reaction of phosphoric acid and liquid ammonia and granulation in an ammoniation machine, which has complicated process and is a co-production process of monoammonium phosphate and diammonium phosphate. Chinese patent application CN110877901A (published 2020, 3/13) discloses a method for preparing diammonium hydrogen phosphate by mixing monoammonium phosphate, liquid ammonia and wet-process phosphoric acid, which is suitable for mass production of diammonium hydrogen phosphate, and the liquid ammonia is extremely corrosive, and requires very high equipment requirements in the preparation process. Chinese patent applications CN106335887A (published 2017, 1 and 18) and CN107161966A (published 2017, 9, 15) adopt a two-step method of neutralization and concentration of wet phosphoric acid and ammonia gas, and the production process is relatively complicated.

The conventional wet-process phosphoric acid is used as a raw material to produce the industrial-grade diammonium hydrogen phosphate, and the steps of filtering for many times, neutralizing for many times, concentrating and the like are adopted, so that the method is suitable for industrial mass production. In the process of preparing ammonium polyphosphate, industrial-grade diammonium hydrogen phosphate has the problem of easy moisture absorption and agglomeration, and is not beneficial to the stability of the quality of ammonium polyphosphate.

Disclosure of Invention

The purpose of the invention is as follows: the invention aims to provide a preparation method of diammonium hydrogen phosphate solid crystal powder which is anti-caking, low in moisture absorption rate, uniform in particle and good in fluidity and is obtained by introducing ammonia gas to react with phosphate radical; it is a further object of the present invention to provide solid crystalline diammonium hydrogen phosphate powder prepared by the aforementioned process.

The technical scheme is as follows: in order to achieve the above object, the present invention provides a method for preparing anti-caking low-moisture-absorption diammonium phosphate solid crystalline powder, which comprises the following steps:

(1) introducing ammonia gas into a phosphoric acid solution with the concentration of 50-85 wt% for continuous reaction, adding the pretreated nano silicon dioxide dispersion liquid when the pH value of a reaction system is 3.0-5.0, and continuously stirring;

(2) stopping the reaction when the pH value is 7.8-8.2, and cooling, crystallizing, filtering and drying to obtain anti-caking and low-moisture-absorption diammonium hydrogen phosphate solid anhydrous crystalline powder with good fluidity;

the pretreated nano silicon dioxide dispersion liquid is obtained by treating silicon dioxide with ammonia water and then gradually dripping phosphoric acid.

The ammonia gas selected by the invention can be from an ammonia gas source in a conventional laboratory or a factory, and can also be reasonably utilized by combining with ammonia gas discharged by other production processes; preferably dry ammonia gas is used for the reaction.

According to the invention, ammonia gas is introduced, and the reaction is carried out on the ammonium hydrogen phosphate solution in the Venturi absorption device, so that the preparation of the diammonium hydrogen phosphate is completed, the pretreated nano-scale hydrophilic silicon dioxide dispersion liquid is added, and the dispersion liquid is treated by ammonia water and then is treated by phosphoric acid, so that the modification of the surface microsphere structure of the diammonium hydrogen phosphate is realized, and the problem of material caking of the diammonium hydrogen phosphate caused by moisture absorption is solved. In the step (1), the concentration of the phosphoric acid solution is more preferably 60 to 70 wt%.

In the added pretreated nano silicon dioxide dispersion liquid, the mass of the nano silicon dioxide particles is 0.01-5 wt% of that of the 50-85 wt% phosphoric acid solution. More preferably 0.1 to 0.3 wt%.

The content of the nano silicon dioxide particles in the pretreated nano silicon dioxide dispersion liquid is 5-10 wt%.

The average particle diameter of the nano silicon dioxide particles is 15-50 nm, and the specific surface area is 150-300 m²/g。

It is to be noted that the addition of the pretreated nano-silica dispersion liquid of the present invention is actually performed when the pH of the reaction system is 3.0 to 5.0, and the pretreated nano-silica dispersion liquid is preferably added at a pH of 4. With the introduction of ammonia gas, when the pH value is 4, the reaction system gradually generates ammonium dihydrogen phosphate, and the pretreated colloidal silicon dioxide is added at the moment, so that the solidification resistance of the crystalline powder of the product can be obviously improved, and the particles are uniform. In order to ensure the aforementioned effect, the pretreated nano-silica dispersion should be added and then stirred for 2-5 hours, preferably, the ammonia gas is introduced and the stirring is continued until the reaction end point of pH 8.

And (3) after the reaction reaches the end point, continuously stirring, cooling for crystallization, centrifugally separating out the residual water, and drying to obtain the anti-caking low-moisture-absorption diammonium hydrogen phosphate solid anhydrous crystalline powder with good fluidity.

In the preparation process of the nanometer silicon dioxide pretreatment, in order to improve the pretreatment effect of the nanometer silicon dioxide, after the phosphoric acid is dripped, the temperature is raised to be not more than 100 ℃, and the stirring is continued for 1-5 hours.

As a further optimization of the present invention, the preparation method of the pretreated nano silica dispersion comprises: adding the nano silicon dioxide into 1-5 wt% of ammonia water in stirring, and stirring at the same or different rotating speeds of 50-500 rpm at the temperature of 10-20 ℃ for 1-5 hours; in the stirring process, 70-85% of phosphoric acid is dripped according to the mass ratio of 1:1, the dripping time is 0.5-5 hours, and the temperature is kept to be less than or equal to 40 ℃ in the process; after the dropwise addition is finished, the temperature is raised to 40-100 ℃, the rotation speed is 100-300 rpm, and the stirring is carried out for 0.5-2.0 hours, so as to obtain the pretreated nano silicon dioxide dispersion liquid.

In a most preferred embodiment of the present invention, the preparation method of the pretreated nano-silica dispersion comprises: adding nano silicon dioxide into 3 wt% of ammonia water stirred at the rotating speed of 60rpm, stirring for 10-20 minutes at the temperature of 10-20 ℃ and at the rotating speed of 500rpm, and then continuing stirring for 2 hours at the rotating speed of 60 rpm; in the stirring process, 85 percent of phosphoric acid is dripped according to the mass ratio of 1:1, the dripping time is 1-2 hours, and the temperature is kept to be less than or equal to 40 ℃ in the process; after the dropwise addition is finished, the temperature is raised to 50-80 ℃, and the mixture is stirred for 1-1.5 hours at the rotating speed of 200rpm, so that the pretreated nano silicon dioxide dispersion liquid is obtained.

The method for pretreating nano-silica is to perform ammonia water alkali treatment on the nano-silica to help the nano-silica to disperse, and then perform acid treatment on the nano-silica to activate the surface of the nano-silica. On one hand, the method is beneficial to the reaction of phosphate ions and ammonia gas on the surface to form seed crystals, and is beneficial to the crystallization of diammonium hydrogen phosphate, so that the final crystallization of diammonium hydrogen phosphate is complete, the particles are uniform and the fluidity is high; on the other hand, the nano-silica pretreated by phosphoric acid is added in the reaction process, and is adsorbed on the surface of diammonium hydrogen phosphate by utilizing high hydrogen bonds on the surface of the silica, so that the crystal particles of the diammonium hydrogen phosphate are nano-anchored and modified, and the high specific surface area of the nano-silica greatly improves the problem of material caking of the diammonium hydrogen phosphate caused by moisture absorption.

Tests prove that the diammonium hydrogen phosphate solid crystal powder obtained by the preparation method can simultaneously meet the following three characteristics:

a) the content of diammonium hydrogen phosphate is not less than 99.0 wt%;

b) the moisture absorption rate is less than or equal to 0.2 wt%;

c) the caking rate is less than or equal to 0.5wt percent.

On the basis of the above, the preferred embodiment of the method also simultaneously satisfies the following characteristics:

d) the content of diammonium hydrogen phosphate is not less than 99.5 wt%;

e) the angle of repose is more than or equal to 65;

f) the screen residue (less than 2mm) is more than or equal to 99.0 wt%.

The best implementation results achievable by the invention also satisfy the following characteristics simultaneously:

g) colorless, clear and transparent at 300 ℃;

h) the water content is less than or equal to 0.2 wt%.

The detection method of the technical parameters is described in the specific embodiment of the invention.

Compared with the prior art, the preparation process of the diammonium hydrogen phosphate solid crystalline powder can obtain anhydrous crystalline powder with the content of more than or equal to 99.0 wt% by carrying out a specific pretreatment process on a silicon dioxide dispersion liquid and only needing one reaction step and simple crystallization and separation steps, the anhydrous crystalline powder has excellent consolidation resistance and low hygroscopicity, the crystalline powder has excellent fluidity, and is colorless, clear and transparent at 300 ℃, and the problems of unstable high-temperature chromaticity and large fluctuation in the prior art are solved.

Drawings

FIG. 1 is a graph showing a comparison of the state after bulk storage of materials in test examples of the present invention, wherein 1A is a product obtained by the preparation of example 2 of the present invention, and 1B is a commercially available product of comparative example 3.

Detailed Description

The invention is further described with reference to the following figures and specific examples.

Example 1

The present example provides a method for preparing solid crystalline powder of diammonium phosphate, which can obtain anhydrous crystalline powder with high content, high quality and anti-caking property, comprising the following steps:

(1) pretreatment of silicon dioxide dispersion liquid: taking nano silicon dioxide particles, wherein the average particle diameter is 15-50 nm, and the specific surface area is 150-300 m²(ii) in terms of/g. Treating the silicon dioxide with ammonia water, gradually dropwise adding phosphoric acid, heating to a temperature not higher than 100 ℃ after dropwise adding, and continuously stirring for 1-5 hours.

(2) Introducing ammonia gas into a phosphoric acid solution with the concentration of 50-85 wt% for continuous reaction, adding the pretreated nano silicon dioxide dispersion liquid when the pH of a reaction system is 3.0-5.0, continuously stirring for 2-5 hours, releasing heat along with the introduction of ammonia gas for reaction, and keeping the temperature at 85-95 ℃; the content of the nano silicon dioxide particles in the pretreated nano silicon dioxide dispersion liquid is 5-10 wt%.

Wherein, in the added pretreated nano silicon dioxide dispersion liquid, the mass of the nano silicon dioxide particles is 0.01-5 wt% of the phosphoric acid solution with the weight of 50-85 wt%.

(3) Stopping the reaction when the pH value is 7.8-8.2, and cooling, crystallizing, filtering and drying to obtain the anti-caking and low-moisture-absorption diammonium hydrogen phosphate solid anhydrous crystalline powder with good fluidity, wherein the powder has the following characteristics:

a) the content of diammonium hydrogen phosphate is not less than 99.0 wt%;

b) the moisture absorption rate is less than or equal to 0.2 wt%;

c) the caking rate is less than or equal to 0.5wt percent.

More preferably, the content of the diammonium hydrogen phosphate can reach more than 99.5 wt% and the water content is less than or equal to 0.2 wt% through further optimization of process parameters. Meanwhile, the repose angle is more than or equal to 65 percent, and the screen residue (less than 2mm) is more than or equal to 99.0 percent by weight; colorless, clear and transparent at 300 ℃.

Example 2

The particle diameter is 20nm, the specific surface area is 200m²14kg of nano silicon dioxide/g is added into 86kg of 3 wt% ammonia water for dispersion and activation under the stirring of the rotation speed of 60rpm, the mixture is stirred for 10 to 20 minutes under the stirring speed of 500rpm at the temperature of 10 to 20 ℃, and the stirring is continued for 2 hours under the rotation speed of 60 rpm; under stirring, 100kg of 85 percent phosphoric acid is dripped in 1.5 hours, and the temperature is kept to be less than or equal to 40 ℃; after the dropwise addition is finished, the temperature is raised to 70-80 ℃, the mixture is stirred for 1-1.5 hours at the rotating speed of 200rpm, and the nano silicon dioxide dispersion liquid prepared by pretreatment with the nano silicon dioxide content of 7 wt% is obtained.

Adding 3000kg of 60 wt% phosphoric acid solution into a venturi reaction device, opening an ammonia switch, introducing the mixture into the venturi reaction device for reaction, adding 51.43kg of pretreated nano-silica dispersion liquid (the mass of the nano-silica is 0.2 wt% of the 60 wt% phosphoric acid solution) when the pH value of the system is 4.0, continuously stirring for 2.5-3 hours, stopping the reaction when the pH value is 8.0, and cooling, crystallizing, centrifuging and drying to obtain diammonium phosphate solid anhydrous crystal powder.

Example 3

The particle diameter is 40nm, the specific surface area is 150m²14kg of nano silicon dioxide/g is added into 86kg of 3 wt% ammonia water for dispersion and activation under the stirring of the rotation speed of 60rpm, the mixture is stirred for 10 to 20 minutes under the stirring speed of 500rpm at the temperature of 10 to 20 ℃, and the stirring is continued for 2 hours under the rotation speed of 60 rpm; under stirring, 100kg of 85 percent phosphoric acid is dripped in 1.5 hours, and the temperature is kept to be less than or equal to 40 ℃; after the dropwise addition is finished, the temperature is raised to 70-80 ℃, the mixture is stirred for 1-1.5 hours at the rotating speed of 200rpm, and the nano silicon dioxide dispersion liquid prepared by pretreatment with the nano silicon dioxide content of 7 wt% is obtained.

Adding 3000kg of 70% phosphoric acid solution into a venturi reaction device, opening an ammonia switch, introducing the mixture into the venturi reaction device for reaction, adding 90kg of pretreated nano silicon dioxide dispersion liquid (the mass of the nano silicon dioxide is 0.3 wt% of the 70% phosphoric acid solution) when the pH of a reaction system is 4.5, continuously stirring for 2.5-3 hours, stopping the reaction when the pH is 8.0, and cooling, crystallizing, centrifuging and drying to obtain diammonium hydrogen phosphate solid anhydrous crystal powder.

Example 4

The particle diameter is 20nm, the specific surface area is 200m²14kg of nano silicon dioxide/g is added into 86kg of 3 wt% ammonia water for dispersion and activation under the stirring of the rotation speed of 60rpm, the mixture is stirred for 10 to 20 minutes under the stirring speed of 500rpm at the temperature of 10 to 20 ℃, and the stirring is continued for 2 hours under the rotation speed of 60 rpm; under stirring, 100kg of 85 percent phosphoric acid is dripped in 1.5 hours, and the temperature is kept to be less than or equal to 40 ℃; after the dropwise addition is finished, the temperature is raised to 70-80 ℃, the mixture is stirred for 1-1.5 hours at the rotating speed of 200rpm, and the nano silicon dioxide dispersion liquid prepared by pretreatment with the nano silicon dioxide content of 7 wt% is obtained.

Adding 3000kg of 85% phosphoric acid solution into a venturi reaction device, opening an ammonia switch, introducing the mixture into the venturi reaction device for reaction, adding 72.86kg of pretreated nano silicon dioxide dispersion liquid (the mass of the nano silicon dioxide is 0.2 wt% of the 85% phosphoric acid solution) when the pH of a reaction system is 4.0, continuously stirring for 2.5-3 hours, stopping reaction when the pH is 8.0, and cooling, crystallizing, centrifuging and drying to obtain diammonium hydrogen phosphate solid anhydrous crystal powder.

Comparative example 1

For comparison, this example provides a conventional diammonium phosphate preparation method without any addition of nano-silica particles or colloidal silica:

adding 3000kg of 50% phosphoric acid solution into the Venturi reaction device, opening an ammonia switch, introducing into the Venturi reaction device for reaction, controlling the end point pH to 8.0, continuously stirring for 2h, and cooling. And crystallizing, centrifuging and drying to obtain a diammonium hydrogen phosphate sample A.

Comparative example 2

This example is substantially the same as example 2, except that 3.6kg of nano silica particles were added at the end of the reaction at pH 8, stirring was continued for 2h, and cooling was performed. And crystallizing, centrifuging and drying to obtain a diammonium hydrogen phosphate sample B.

Comparative example 3

Commercial first-grade diammonium hydrogen phosphate, performance test: content of diammonium hydrogen phosphate: 99.0%, pH: 8.0; clarity: qualified; the water content is less than or equal to 0.10 percent.

Test examples

The solid anhydrous crystal powder of diammonium hydrogen phosphate obtained in the above examples 2 to 4, the sample a obtained in the comparative example 1, and the sample ratio obtained in the comparative example 2 were subjected to a product detection test, and the detection indexes and the detection method were as follows:

1) the content is as follows: fully dissolving the sample, titrating by using a hydrochloric acid standard titration solution, recording the pH value, and calculating the volume of the used standard solution by using a two-stage microtransom method so as to calculate the content of the sample.

2) pH: preparing 1% water solution, soaking in room temperature water bath for 30min, and testing with pH meter.

3) And (3) color sensory evaluation: weighing 20g of sample in a beaker, stirring the sample uniformly by using a glass rod, heating the sample on an electric furnace to 300 ℃ under stirring, and observing the state of the materials in the beaker after melting.

4) Moisture content: the method is carried out according to the regulation of GB/T6283.

5) Fineness: weigh 10g of sample, place on a 2mm standard sieve, manually sieve for 2 minutes, weigh the portion of material left on the sieve, and calculate the percentage of the portion that does not pass through the sieve.

6) Moisture absorption rate: and (3) placing the materials with the concentration of 75.1% of sodium chloride and 83.6% of potassium chloride solution in a dryer at the lower layer, placing the materials after being weighed at 60 ℃ into a weighing bottle in an open manner, keeping the temperature at 30 ℃ for 8 hours, and testing the moisture absorption change rate of the materials.

7) Blocking rate (stacked storage): the product is 25kg PE inner bag/woven composite bag, each layer of the tray is 5 bags, 8 layers are stacked to form one support, after at least 14 days of stacking, the lowest layer of packaged product is taken, and the caking rate is tested. The test method comprises the following steps: after the weight is determined, the test is carried out on a caking release rate test sieve.

8) Angle of repose: the angle of the mounding formed by the free-falling powder was measured on a flowability tester. The flowability of the crystalline powder is an evaluation index, and the larger the flowability, the better the flowability, and the caking property can be measured from the side.

TABLE 1 Effect of the different examples on the product Properties

Index (I)	Example 2	Example 3	Example 4	Comparative example 1	Comparative example 2
						Content, wt%	99.6	99.5	99.5	99.5	99.5
pH	8.0	7.9	8.1	8.0	8.0
						Color intensity	Colorless, clear and transparent	Colorless, clear and transparent	Colorless, clear and transparent	Colorless, clear and transparent	Colorless, clear and transparent
Water content, wt%	0.03	0.05	0.05	0.09	0.09
						Fineness of%	99.7	99.4	99.2	95.2	96.5
Moisture absorption rate%	0.07	0.08	0.10	0.65	0.53
						Caking rate,%	0.12	0.20	0.21	0.61	0.46
Angle of repose, degree	81	69	65	49	51

As shown in table 1: example 2 is the most preferable example, and example 4 is not as excellent as example 2 even if the phosphoric acid concentration is increased to 85%, and also consumes a large amount of phosphoric acid raw material, and also has problems such as increase in coarse particles and difficulty in filtration due to an excessively high concentration. The data of the sample A of the comparative example 1 relative to the data of the examples 2-4 show that the effect is much inferior without adding the pretreated nano-silica dispersion liquid; the sample B of the comparative example 2 proves that the addition of the pretreated nano-silica dispersion liquid at the time of pH 3-5 is very important, and the consolidation resistance, the moisture absorption and the fluidity are obviously improved.