Physicochemical Properties and Production Process of Potassium Pyrophosphate

Potassium pyrophosphate exists in various forms, mainly including K4P2O7 (anhydrous, trihydrate, and 3.5hydrate), K3HP2O7 (anhydrous and hemihydrate), K2H2P2O7 (anhydrous and hemihydrate), and KH3P2O7. K4P2O7 is the most widely used in industry. Both potassium phosphite and potassium pyrophosphate can be used as fertilizers and can be mixed. Potassium pyrophosphate salts and sodium pyrophosphate salts have many similarities and can therefore be substituted for each other in industrial applications, but there are also minor differences. Potassium pyrophosphate salts hold a certain market share under specific conditions and are important fine chemical raw materials in the electroplating and food processing industries. In the detergent industry, potassium pyrophosphate must be used in two special cases: one is for heavy-duty liquid detergents to control foam; the other is for hard surface detergents, mainly used for cleaning surfaces such as machines, walls, doors, glass, tiles, and stoves. Phosphorus and potassium are both biological elements and are essential for life. Therefore, it is both a food additive and a nutritional supplement for plant growth.

I. Physicochemical Properties

1. Physical Properties

It is usually a colorless, lumpy (or powdery) crystal, highly hygroscopic and deliquescent. It is readily soluble in water but insoluble in ethanol; at 25°C, its solubility in 100g of water is 187g. A 1% aqueous solution has a pH of 10.2. The anhydrous form has a melting point of 1109°C (1090°C for type I), and a relative density of 2.534. As temperature increases, the hydrated crystals gradually lose water, forming different polymorphs. The transition temperature from type I to type I is 278°C.

2. Chemical Properties

It possesses various chemical properties of potassium ions and pyrophosphate. The following are relevant to production and application:

① Identification of pyrophosphate. When potassium pyrophosphate solution is mixed with silver nitrate solution, a white precipitate of silver pyrophosphate is formed, as shown in the following reaction: K₄P₂O₇ + 4AgNO₃ → Ag₄P₂O₇ + 4KNO₃. This precipitate dissolves in dilute ammonia and dilute nitric acid, but is insoluble in acetic acid. This is a characteristic reaction of the pyrophosphate ion. Phosphate ions react with silver ions to form a yellow precipitate of silver phosphate (Ag₃PO₄), which dissolves in nitric acid, ammonia, and acetic acid.

② Like sodium pyrophosphate, it can chelate with alkaline earth metal ions in water, effectively blocking these ions.

③ It can chelate with iron ions to form a soluble, colorless complex ion, thus finding applications in the food processing industry.

④ Potassium pyrophosphate is generally stable, but it undergoes hydrolysis in boiling water, degrading to dipotassium hydrogen phosphate.

The reaction K4P2O7 + H2O → 2K2HPO4 has a slightly slower reaction rate compared to sodium pyrophosphate. In pure water, the reaction is even slower, or almost nonexistent. Therefore, there are differences when using them as dyeing auxiliaries.

II. Production Process Generally, it is obtained by heating and dehydrating potassium orthophosphate salts to polymerize. Since potassium orthophosphate salts are water-soluble, purification is relatively easy. As long as the purity of the raw materials and the conversion temperature during production are well controlled, an ideal potassium pyrophosphate product can be obtained.

1. Production Method of Acidic Potassium Pyrophosphate Industrially, acidic potassium pyrophosphate is produced using potassium dihydrogen phosphate as raw material, or potassium carbonate or potassium hydroxide and phosphoric acid as raw materials. However, the purity of industrial potassium dihydrogen phosphate is not ideal, often mixed with dipotassium hydrogen phosphate, which will produce potassium pyrophosphate or potassium phosphate during polymerization, thus affecting the purity of the product. Therefore, potassium carbonate (or potassium hydroxide) and phosphoric acid are often used as raw materials to first obtain ideal potassium dihydrogen phosphate, which is then dried and polymerized to obtain the product dipotassium pyrophosphate. The chemical reactions are as follows: K₂CO₃ + 2H₃PO₄ → 2KH₂PO₄ + H₂O + CO₂ or KOH + H₃PO₄ → KHPO₄ + H₂O 2KH₂PO₄ → K₂H₂P₂O₇ + H₂O. Industrial KCO₃ (or KOH) is dissolved (with charcoal added for decolorization if necessary), filtered, and then reacted with filtered industrial phosphoric acid. The final pH is strictly controlled at 4.4-4.7 (ideally 4.6). The product is then filtered again (iron, silicon, arsenic, and other impurities should be removed if necessary), concentrated, and crystallized for later use. The obtained potassium dihydrogen phosphate is dried at 70-80℃ and then transferred to a box-type conversion furnace for slow polymerization. During polymerization, the polymerization temperature must be strictly controlled and must not exceed 230℃, because above 240℃, a multimolecular polymer (KPO3)n will be produced.

2. Production Method of Potassium Pyrophosphate

Potassium pyrophosphate is obtained by dehydration polymerization of dipotassium hydrogen phosphate at 350~400℃, the reaction is as follows:

2KHPO4 (heated to 350~400℃) --- K4P2O7 + H2O

The hydrous crystalline salt of potassium pyrophosphate can be prepared by dissolving anhydrous potassium pyrophosphate in water, the reaction is as follows:

K4P2O7 + 3.5H2O — K4P2 K₄P₂O₇·3.5H₂O is heated from 79℃ to above 155℃, undergoing trihydrate and monohydrate reactions to become anhydrous. The reactions are as follows: K₄P₂O₇·3.5H₂O → K₄P₂O₇·3H₂O + 0.5H₂O; K₄P₂O₇·3H₂O → K₄P₂O₇·H₂O + 2H₂O; K₄P₂O₇·H₂O → K₄P₂O₇ + H₂O

Industrial production currently mainly uses the reaction process of potassium hydroxide and phosphoric acid. The following example uses the two-step neutralization and calcination method. Potassium hydroxide (potassium carbonate) is added to a neutralization tank, dissolved in water, and then neutralized with phosphoric acid under stirring. If potassium carbonate is used as a raw material, it can be directly added to the neutralization tank in the amount theoretically required to produce dipotassium hydrogen phosphate. Phosphoric acid is then added, controlling the pH to approximately 8.8. The reacted solution is heated, and activated carbon is added for decolorization. After passing through a filter and a concentrated crystallizer, it is dehydrated by a centrifuge and dried to obtain dipotassium hydrogen phosphate crystals. These crystals are then calcined and polymerized in a conversion furnace to obtain the finished potassium pyrophosphate product. Pyrophosphate is mainly used in electroplating, where the requirements for certain impurity ions are very strict. Substandard electroplating solutions will directly affect the electroplating effect. Therefore, strictly controlling process conditions is the primary task to ensure the quality of potassium pyrophosphate. Important process conditions include pH control, polymerization conversion temperature, and quality control of the finished product after calcination.

①pH control. During production, if the pH of the neutralization solution is too low, potassium dihydrogen phosphate will be mixed in with the generated dipotassium hydrogen phosphate, resulting in the formation of some (KPO3)n during polymerization. Conversely, if the pH of the neutralization solution is too high, potassium phosphate will be mixed into the dipotassium hydrogen phosphate, preventing polymerization even at high temperatures. The presence of K3PO4 or (KPO3)n will affect the electrochemical properties of potassium pyrophosphate used in electroplating solutions. Therefore, the degree of neutralization should be controlled so that the pH reaches 8.4 at the reaction endpoint.

② Polymerization conversion temperature. Theoretically, the polymerization temperature is 350~400℃. In practice, there is a certain temperature difference between the conversion furnace and the material, which should be corrected during temperature measurement. The actual furnace temperature can reach 500℃, but it should not exceed 600℃. At such high temperatures, although the resulting product is white, the amount of water-insoluble matter increases and cannot be treated; this is called "burning" in production, resulting in substandard product quality. If K3PO4 is present, the product will turn black during calcination.

③ Quality control of the finished product after calcination. The calcined product should be neither black nor pure white. The product is considered(qualified) if no yellow precipitate forms when silver nitrate solution is added to the solution.