Background and overview[1][2]
Phosphorus is one of the essential elements for the human body. As an important food ingredient and functional additive, phosphate has been widely used in processed poultry products, fruit products, vegetable products, dairy products, soy products, baked goods, seafood and convenience foods. At present, the manufacture of phosphates above food grade is usually based on thermal phosphoric acid. Among them, thermal phosphoric acid has higher requirements on the grade of raw materials (ore), and its preparation process is energy-intensive, polluting, and the process is complicated. The cost remains high; wet phosphoric acid contains more impurities, such as sulfates, organic compounds, arsenic, lead and other metal ions, and the content of these impurities must be effectively removed or reduced before it can be applied.
Diammonium hydrogen phosphate is also called second-generation ammonium phosphate. Colorless or white monoclinic crystal or crystalline powder. There is an ammonia smell. It has a refreshing and salty taste. Relative molecular mass 132.06. Relative density 1.619. Melting point 155℃ (decomposition). Refractive index 1.53. Insoluble in ethanol and acetone, soluble in water: 42.9 at 0℃, 62.8 at 10℃, 69.0 at 20℃, 75.2 at 30℃, 81.8 at 40℃, 89.2 at 50℃, 106.0 at 70℃. 5% aqueous solution pH =7.8~8.3. It gradually loses ammonia in the air and becomes ammonium dihydrogen phosphate. It decomposes when it is higher than the melting point (216°C), loses water and ammonia, and turns into a mixture of ammonium metaphosphate (NH4PO3) and phosphoric acid.
Diammonium hydrogen phosphate is a popular nitrogen and phosphorus compound fertilizer. It has a high concentration of active ingredients and is not prone to moisture absorption and caking. It is suitable for almost all soils and crops. The U.S. phosphate fertilizer industry already produces large quantities of it. Coking plants in the western United States use high-purity electric furnace phosphoric acid to absorb ammonia in coke oven gas to produce diammonium hydrogen phosphate compound fertilizer, which contains 21% nitrogen and 53% phosphorus pentoxide. Diammonium hydrogen phosphate is one of the coke oven gas purification products in the United States. This product is not yet available in Chinese coking plants. Diammonium hydrogen phosphate can be used industrially as a flame retardant for fabrics and fibers, a culture solution in the fermentation industry, a food additive and a feed additive, an acidifier in the printing and dyeing industry, and a dry powder fire extinguishing agent ingredient for firefighting.
Quality standards
Purpose[3]
1. Used as feed additive for ruminants.
2. In the food industry, it is used as food leavening agent, dough conditioner, yeast food and fermentation aid for brewing, and can be used as a buffer.
3. Used as analytical reagents, such as pH buffers, precipitants such as magnesium, zinc, and nickel, supporting electrolytes in electrochemical analysis, seed soaking agents, corrosion inhibitors, and preparation of culture medium reagents.
4. Used for flame retardant treatment of fabrics, paper, wood and plant fibers; impregnation treatment of lamp wicks to increase their durability; self-extinguishing treatment after matches are lit. It can be used as dry powder fire extinguishing agent and phosphorus in fluorescent lamps, welding melts of tin, copper, bronze and zinc, and yeast culture nutrients. Used in the refining (purification) process of sugar. Used for chrome dyeing of wool fabrics in the printing and dyeing industry. Used as fertilizer in agriculture. In addition, it is also used in the printing and platemaking industry, pharmaceutical industry, electronic tube manufacturing industry, ceramics and enamel industry, etc.
Preparation[3][4]
The neutralization method of reacting thermal phosphoric acid and liquid ammonia or extracting phosphoric acid and liquid ammonia to produce diammonium hydrogen phosphate is the main method of industrial production at present. The former product has high purity and the process is relatively simple; the latter uses wet method Extract phosphoric acid at low cost.
1. Thermal phosphoric acid neutralization liquid ammonia method
Dilute phosphoric acid with water (water: phosphoric acid = 1.3:1) into dilute phosphoric acid. After metering, add it to an enamel reaction tank with stirring and jacket. Under stirring, ammonia is introduced through a round tube ammonia distributor. Neutralization reaction is carried out with gas, and after neutralization until the pH of the reaction solution is 8 to 9, filter it while it is hot, and then send it to a cooling crystallizer, where the crystals are precipitated by cooling, and the mother liquor is centrifuged and dried to obtain the finished product of diammonium hydrogen phosphate. The reaction equation is:
The centrifuged mother liquor is sent to the iron remover and ammonium sulfide is added to remove iron. After filtration, the filtrate is concentrated and sent to a fine-tuning tank. Ammonia gas is introduced to fine-tune the pH to 8-9 before returning to the cooling crystallizer. The process flow is as shown in the figure:
2. Extraction of phosphoric acid and neutralization of liquid ammonia method
Extract phosphoric acid and add a certain amount of hydrogen peroxide to oxidize the ferrous iron in the phosphoric acid solution. The phosphoric acid is sent to the tubular mixing reactor for neutralization reaction with ammonia. After the second stage reaction, the pH reaches about 8.0. , after being filtered by the plate and frame filter press, the filtrate is sent to a fine-tuning tank to adjust the pH to 7.8-8, sent to the evaporator to evaporate and concentrate to a relative density of 1.3, and placed in a cooling crystallizer. The cooled crystallization liquid is centrifugally separated. Dry to obtain the finished product of diammonium hydrogen phosphate. The reaction equation is:
The residue filtered by the plate and frame filter press is washed with water. The washing liquid and mother liquor are combined and sent to the reaction liquid storage tank for acid adjustment. Impurities are removed by filtration and recycled. The washed residue after drying contains 35% phosphorus pentoxide and 5% ammonia, which is a compound fertilizer with high fertilizer efficiency.
3. Use wet phosphoric acid as raw material to produce hydrogen phosphate for feed�Ammonium
The production process requires two steps of defluorination, three stages of ammoniation, and pyrolysis of triammonium phosphate into diammonium hydrogen phosphate for feed. Its production cost is lower than the method using yellow phosphorus as raw material. In the wet phosphoric acid method, the extracted phosphoric acid containing 20% to 30% P2O5 and 1.2% -2% is sent to the reaction device through the gas purification system. The three-stage ammoniation makes the impurities form an easy-to-filter precipitate, and the precipitate is separated by a filter press. The filtrate contains P2O5<15% and SO42- is a diammonium phosphate solution of 2.5%-3% and about 1% solid suspended matter.
In order to further remove dissolved impurities and part of the solid suspension, the filtrate is sent to another reaction device to be saturated with ammonia, and then placed in a cooling crystallizer for cooling and crystallization to generate triammonium phosphate crystals. The crystals are separated from the mother liquor through a conical settling tank, and then dehydrated by centrifugation. The crystals are put into a boiling furnace for pyrolysis into diammonium hydrogen phosphate. After drying, the finished product of diammonium hydrogen phosphate for feed is obtained. The reaction equation is:
The mixed gas containing diammonium hydrogen phosphate dust and ammonia gas is dusted by the cyclone separator and then sprayed and absorbed with wet phosphoric acid in the scrubber. The process flow is as follows:
Main reference materials
[1] Practical Fine Chemical Dictionary
[2] Encyclopedia of Chinese Metallurgy·Coking Chemical Industry
[3] Non-protein nitrogen feed additive diammonium hydrogen phosphate
[4] Chen Kui; Zhu Jiawen; Wu Bin; Ren Hongrui; Sheng Yong; Li Bing. Method for preparing food grade diammonium hydrogen phosphate using industrial grade wet phosphoric acid. CN201010278694.7, application date 20100910
