Background and overview[1]
Potassium metaphosphate is an ideal chlorine-free phosphate-potassium compound fertilizer. In the existing technology, there are two manufacturing methods for potassium metaphosphate:
(1) Use thermal phosphoric acid and potassium chloride as raw materials to produce potassium metaphosphate. This method is to mix potassium chloride and thermal phosphoric acid, heat the reaction to generate potassium dihydrogen phosphate, and then heat it at high temperature to obtain potassium metaphosphate. .
(2) Use elemental phosphorus and potassium chloride as raw materials to produce potassium metaphosphate. The combustion product of elemental phosphorus reacts with potassium chloride to produce potassium metaphosphate.
Due to the strict process conditions for producing potassium metaphosphate by the above method, the reaction equipment needs to be lined with zircon bricks. In addition, the tail gas recovery system is huge, the equipment investment is high, and the by-product hydrochloric acid lacks ways to effectively utilize it. In addition, heat The prices of French phosphoric acid and elemental phosphorus are relatively high, which restricts the application of potassium metaphosphate, an excellent fertilizer, in agriculture. At present, the domestic production technology of potassium metaphosphate is mainly aimed at agricultural fertilizer products and binders. The products only focus on meeting certain specific industrial indicators. The purity is low and cannot be applied to the field of food additives. Therefore, the price of the products is relatively low. . In addition, potassium dihydrogen phosphate, the raw material for the current production of potassium metaphosphate, is mostly prepared from thermal phosphoric acid and potassium chloride, which has a high cost. In comparison, Cabot uses wet phosphoric acid as raw material to prepare industrial-grade potassium dihydrogen phosphate. On this basis, further development of food-grade potassium metaphosphate products will have greater cost advantages. Internationally, there are strict requirements on the properties of food-grade potassium metaphosphate products, including the content of the main ingredient (KPO3), the content of heavy metal impurities, the viscosity of the product, and the form of potassium metaphosphate aggregates.
To prepare potassium metaphosphate, potassium dihydrogen phosphate is usually melted into a transparent liquid when heated to ℃, and the eva resin solidifies into an opaque glassy substance potassium metaphosphate (KPO3) after cooling. . In the process of producing potassium dihydrogen phosphate through the wet route, due to the arsenic element contained in phosphate rock, the arsenic content in potassium dihydrogen phosphate is often greater than 3ppm, which does not meet the requirements of food-grade potassium dihydrogen phosphate. Therefore, the removal of trace arsenic is a very important step in the process of preparing food-grade potassium metaphosphate using wet route potassium dihydrogen phosphate as raw material. Usually, arsenic removal by potassium dihydrogen phosphate chemical precipitation method is to add certain chemical reagents to the potassium dihydrogen phosphate solution to form a precipitate and separate the arsenic impurities in the solution. The precipitate is separated from the solution through a filtration operation to achieve the reduction of arsenic impurities in the solution. Removal. For example, when removing arsenic from potassium dihydrogen phosphate solution, a certain amount of phosphorus pentasulfide reagent is added to react with the impurity arsenic in the solution to generate arsenic sulfide precipitate and remove arsenic ions from the potassium dihydrogen phosphate solution. However, it is difficult to remove arsenic by chemical precipitation and the arsenic content reaches the ppm level, and the potassium dihydrogen phosphate solution needs to be crystallized, filtered, and dried before it can be used for the production of food-grade potassium metaphosphate. Not only is the process cumbersome, but the crystallization mother liquor is also unfavorable for being discharged as wastewater. Environmental friendly.
Physical and chemical properties
Potassium metaphosphate [KPO3, (KPO3)n) is a high-component phosphorus-potassium compound fertilizer with a theoretical content of Potassium oxide 39.87%, phosphorus pentoxide 60.13%. Pure potassium metaphosphate appears as white fibrous crystals with a melting point of 810°C. Its molecular weight and physical properties are related to the manufacturing method. The solid phase structure of potassium metaphosphate is composed of an unbranched P-O-P chain, with one phosphorus atom surrounded by four oxygen atoms in an equilateral tetrahedron. There are two forms of potassium metaphosphate that remain stable at room temperature, one is the fibrous Kurrol’ssalt crystal, and the other is the vitreous product.
Preparation[1, 3]
Method 1: Mix NH4H2PO4 and KCl thoroughly in a certain proportion and heat to above 340°C to generate KPO3, NH4Cl and H2O. The sublimation temperature of NH4Cl is 337°C. During the reaction process, NH4Cl sublimates and separates from the main product KPO3, and is collected by cooling to obtain NH4Cl with higher purity. Since the melting point of NH4H2PO4 is 188°C, it is in a molten state during the reaction and can be evenly mixed with solid KCl. Compared with a pure solid-phase reaction, it is beneficial to increase the speed and yield of the reaction. The specific method is: accurately weigh the ground ammonium dihydrogen phosphate (NH4H2PO4) and potassium chloride (KCl), mix them evenly in equal moles, put them into a crucible furnace that has been constant-temperatured to the required temperature, and heat them until the reaction reaches a certain level. After a certain time, take it out, cool it to room temperature, accurately weigh the mass of the obtained product, add an appropriate amount of 6 mol·L-1 nitric acid solution to dissolve it, and then use the Forhard method to measure chloride ions.
Method 2: Mix monoammonium phosphate and potassium chloride and add them to the reactor for heating. Monoammonium phosphate and potassium chloride react to generate molten potassium metaphosphate, sublimated ammonium chloride and water vapor. The reaction The formula is as follows:
After the reaction is completed, the molten potassium metaphosphate is released, cooled and then pulverized to obtain the potassium metaphosphate product; the ammonium chloride sublimated during the heating reaction is recovered by cooling and used as agricultural fertilizer. In the above process, the mass fraction of monoammonium phosphate used is 60% to 75%, and the mass fraction of potassium chloride is 40% to 25%; the mass ratio of monoammonium phosphate to potassium chloride is 100: (47-55); The reaction temperature is 600℃~900℃; the residence time is 1~4h.
Apply[4]
Potassium metaphosphate is an ideal chlorine-free phosphate-potassium compound fertilizer. Has the following characteristics:
1. It does not absorb moisture and has anti-caking effect. Adding potassium metaphosphate with a mass fraction of 10-50% to other fertilizers can greatly reduce the caking phenomenon.
2. Can rootIt provides different solubility according to the needs of crops, and can slowly or controlled release of chemical fertilizer efficiency, which is beneficial to environmental protection.
3. It has multiple chelating abilities in the liquid mixture and is an ideal raw material for liquid fertilizer.
4. The salt index is very low and there is no residual soil effect.
5. Due to the nodulation effect of polyphosphate, it can reduce the fixation of phosphorus and the loss of potassium in the soil. At the same time, adding an appropriate amount of trace elements can make a multi-element high-efficiency compound fertilizer.
Main reference materials
[1] CN200910102927.5 Agricultural potassium metaphosphate production method
[2] Overview of the development of foreign phosphate-potassium compound fertilizer potassium metaphosphate
[3] Research on preparation methods of potassium metaphosphate
[4] Potassium metaphosphate fertilizer production process
