1. Introduction to APP
Ammonium polyphosphate (APP, ammonium polyphosphate) is a low-N and high-P polyphosphate with a pH near neutral. Its general chemical formula is (NH4 )n+2PnO3n+1. According to the degree of polymerization, APP can be divided into three types: oligopolymerization, medium polymerization and high polymerization; the higher the degree of polymerization, the less water-soluble phosphorous acid. Usually, when n20, it is water-insoluble (as shown in Table 1).
Table 1 Technical indicators of water-soluble and water-insoluble polyammonium
At present, APP has entered the field of chemical fertilizers as a new type of fertilizer and is often used as the basic phosphate fertilizer for preparing high-concentration liquid compound fertilizers. Solid ammonium polyphosphate products are generally composed of 41% orthophosphate, 54% pyrophosphate (P2O74-), 4% tripolyphosphate (P3O105-) and 1% tetrameric and above tetrameric polyphosphate. Liquid ammonium polyphosphate products generally contain higher amounts of tripolyphosphate and phosphate ions than granular products, but the composition varies. The common range is 30-40% orthophosphate, 50-55% pyrophosphate, and A portion of tripolyphosphate and tetrapolyphosphate. The commonly used liquid APP ratios abroad are: 8-24-0, 10-34-0, 11-37-0, 11-44-0, 8-28-0, etc. (as shown in Table 2), and the solid ratio is 12-57- 0.
Table 2 Properties of various liquid APP ratios
Note: The 11-44-0 formula is unstable at 0℃.
2. A brief history of the development of APP
In 1820, Gay-Lussac laid the theoretical foundation for the application of ammonium phosphate as a flame retardant.
In 1892, Tammann discovered water-insoluble high molecular weight APP.
In the late 1950s, APP was first introduced to the world by the Tennessee Valley Authority (TVA) in the United States.
In 1965, Monsanto Company of the United States first successfully developed and industrialized ammonium oligophosphate.
In 1965, Frazier.A.W. et al. prepared crystalline long-chain APP by amination of polyphosphoric acid.
In 1969, Shen.C.Y. et al. proposed different preparation methods to prepare APP with I~V structures.
In 1976, Waerstad.K.R and others proposed that VI-type structure APP could be obtained by heating different crystal forms at 340~350℃.
In the 1970s, Japan, the former Soviet Union and other countries began to mass-produce ammonium oligophosphate and used it in agricultural fertilizer applications.
In the 1980s, the development and production of ammonium polyphosphate started in my country.
In 1985, Shanghai Research Institute of Chemical Industry began research on urea phosphate and its derivatives – urea ammonium polyphosphate.
3. The development and advantages of APP as fertilizer
In recent years, APP has gradually entered the field of liquid fertilizer production, and has been widely used in developed countries such as the United States and Europe. The United States developed APP for fertilizer use in the 1960s. The thermal/wet method was used in a tubular reactor to react polyphosphoric acid with ammonia at high temperatures to produce ammonium polyphosphate solution. Grade 10-34-0 used the wet method. Production, grade 11-37-0 is produced by thermal method.
Figure 1 Liquid APP (10-34-0, 11-37-0 )
There are currently 130 factories in the United States producing agricultural APP, with an annual output of 2 million tons. Other countries include Belgium, France, Russia, etc. These basic liquid fertilizers can be mixed with nitrogen solutions and potassium fertilizers to produce liquid compound fertilizers. However, at present, there are still few companies specializing in the production of ammonium polyphosphate fertilizers in my country. Its properties, composition and production methods are still controversial, and there are large gaps in product quality, quantity and application scope.
Figure 2 Comparison of domestic and foreign APP production
APP It is an important phosphorus raw material for the development of water-soluble fertilizers. It has the following advantages:
(1) High phosphorus nutrient content, near-neutral pH value, and high safety factor for crop use.
(2) It has high solubility and can be prepared with liquid fertilizer with high phosphorus content; it has a low crystallization temperature and is easy to produce and use.
(3) It has a chelating effect on metal ions, which can prevent metal impurities in the solution from forming precipitation. Adding trace elements to liquid fertilizer is beneficial to making high-concentration and high-quality liquid fertilizer. Phosphorus is not easily fixed by metal ions such as iron and calcium in the soil. Instead, it can form soluble complexes with ineffective trace elements in the soil and be absorbed and utilized by plants.
(4) Although it is not directly absorbed by plants in the soil, it can be slowly hydrolyzed into orthophosphate in the soil and then used by plants. It is a slow-soluble long-acting fertilizer.
(5) It can be made into foliar fertilizer for foliar spraying on crops. As a fertilizer, the application rate of polyphosphate can be 3 times higher than that of orthophosphate, but it will not burn the leaves. The neutral solution can remain on the leaves for several days without being evaporated or salted out.
4. APP hydrolysis problem
Orthophosphate can be directly absorbed and utilized by plants, but pyrophosphate, tripolyphosphate, tetrapolyphosphate, etc. can only be hydrolyzed into orthophosphate It can then be absorbed and utilized by plants. The hydrolysis reaction controls the absorption of phosphorus by plants. The hydrolysis of polyphosphate is mainly affected by factors such as enzymes, temperature, pH value, and metal ions.
(1) Phosphatase in soil is the key enzyme for the hydrolysis of pyrophosphate. Phosphatase can catalyze the hydrolysis of ammonium polyphosphate very quickly, and the hydrolysis rate is 106 times faster than without enzyme catalysis.
(2) Temperature is an important factor affecting the hydrolysis of ammonium polyphosphate. The higher the temperature, the faster the hydrolysis of polyphosphate. Under the same pH conditions, the hydrolysis rate of ammonium tripolyphosphate is 27 times faster than that at normal temperature; in soil, the hydrolysis rate of polyphosphate at different temperatures isRate: 10℃＜20℃＜30℃. There is an activation energy for breaking between P-O-P chains in phosphate. Every time the temperature increases by 5°C, the chemical reaction rate doubles.
(3) The hydrolysis of ammonium polyphosphate is affected by pH value. The lower the pH value, the faster the hydrolysis of polyphosphate. When the temperature is lower than 25°C and the pH is near neutral (pH=6.4), the ammonium polyphosphate solution is very stable. In a low pH environment, the P-O-P chain reacts with (H3O)+ and hydrolysis is accelerated.
(4) Metal ions can catalyze the hydrolysis of ammonium polyphosphate, and the hydrolysis rate decreases as the activity of metal ions decreases (K+＜Na+＜Ca2+＜Mg2+＜Al3+). Metal ions can activate phosphorus atoms in polyphosphate and increase the number of activated molecules in the reaction.
Liquid fertilizer is a time-saving, labor-saving, efficient and fast fertilizer that has been widely used in developed countries abroad. Water-soluble APP is an important raw material in the development of liquid fertilizers. It has the advantages of chelation, compounding, slow release, and resistance to crystallization. It will play an important role in the development of liquid fertilizers in my country. However, in foreign countries, most liquid fertilizers are ready-to-use, mostly clear liquid type, and the pH value is nearly neutral, so there is basically no hydrolysis of ammonium polyphosphate fertilizer before use. However, my country has a vast territory, and the long-distance transportation of liquid fertilizers increases the hydrolysis of ammonium polyphosphate, and many fertilizers in my country are acidic, which also causes the hydrolysis of ammonium polyphosphate. Therefore, in the development process, it is necessary to rationally understand and continuously improve the production technology of APP water-soluble fertilizers, understand the decomposition and nutrient release mechanism of APP in different soil types in my country, and develop different functions and The formulation of APP-containing liquid fertilizer is designed to meet the needs of modern agricultural production.
