[Background and Overview][1][2][3]
Phosphoric acid is the general name for the acid produced after the combination of phosphorus pentoxide and water. Including metaphosphoric acid (HPO3), pyrophosphoric acid (H4O7P2), polyphosphoric acid (H6P4O13), but generally refers to orthophosphoric acid (H3PO4). It is a colorless and transparent syrup-like liquid, odorless, sour, easy to absorb moisture in the air, easily soluble in water and ethanol, and releases a lot of heat when dissolved. 88% of phosphoric acid becomes crystalline after cooling. It loses moisture when heated to 150°C. It becomes pyrophosphoric acid at 200°C and metaphosphoric acid above 300°C. Phosphoric acid is non-toxic, corrosive and can corrode quartz. Used for making phosphates, compound fertilizers, phosphate extractants, dyes, synthetic detergents, medicines, and also used for metal pickling and rust prevention. Industrially, it is produced by reacting apatite Ca3(P antioxidant O4)2 with sulfuric acid, and purer phosphoric acid can be produced by reacting nitric acid with phosphorus. Phosphoric acid is a non-oxidizing, non-volatile acid. When heated to 213°C, it loses water and is converted into pyrophosphoric acid, and further loses water to obtain metaphosphoric acid. It is moderately acidic and corrosive to the skin. When P4O10 is dissolved in water, a mixed solution of metaphosphoric acid, pyrophosphoric acid and orthophosphoric acid can be obtained. The interaction between P4O10 and water is a reversible process. P4O10 releases heat when combined with water. When the temperature rises, the tendency of phosphoric acid to lose water increases, and it can be converted into pyrophosphoric acid, and finally, into colorless glassy metaphosphoric acid. The structure of all phosphoric acids is based on the “PO4” tetrahedron as the basic unit. There is a single PO3-4 ion in orthophosphoric acid; in pyrophosphate, tripolyphosphate, tetrapolyphosphate, etc., there are two, three or four PO4 tetrahedrons connected by oxygen atoms respectively. They are condensation forms of phosphoric acid, containing more than 1 phosphorus atom, and are called polyphosphoric acid. Cyclic polyphosphoric acid is metaphosphoric acid. Phosphoric acid is mainly used in pharmaceutical, food, soap and other industries, and is also used as a chemical reagent. Industrially, concentrated sulfuric acid is used to treat apatite Ca5(PO4)3F to produce phosphoric acid; the high-purity reagent phosphoric acid uses white phosphorus as raw material, and the P4O10 obtained after burning is dissolved in water to obtain phosphoric acid. It can also be prepared by hydrolyzing phosphorus halide, calcium phosphide or other phosphides.
[Synthesis][3]
1. Wet phosphoric acid
Wet phosphoric acid is phosphoric acid produced by decomposing phosphate rock with a highly acidic inorganic acid or acid salt.
Industrial wet phosphoric acid refers to phosphoric acid obtained by decomposing phosphate rock with sulfuric acid, also known as “extracted phosphoric acid”. During the production process, depending on the reaction process slurry temperature, phosphoric acid concentration, sulfuric acid dosage and other process conditions, calcium sulfate hydrate exists in three crystal forms, namely dihydrate (CaSO4·2H2O), semihydrate (CaSO4·1 /2H2O), anhydrous (CaSO4)
2. Thermal phosphoric acid
The thermal phosphoric acid process is more complex than the wet phosphoric acid process, and the technical entry barrier is higher, which is mainly reflected in the manufacturing, operation and maintenance of yellow phosphorus electric furnaces and waste heat by-product steam phosphorus combustion tower equipment.
1) Production principle of thermal phosphoric acid: The production of thermal phosphoric acid includes the combustion of phosphorus and the hydration of acid anhydride.
A: Combustion of yellow phosphorus. In the combustion hydration tower isoparaffin, phosphorus is oxidized with a large amount of air to form phosphoric anhydride P2O5. The phosphoric anhydride sublimates at 359°C. Due to the high temperature, it The steam is polymerized into P4O10. The combustion reaction of phosphorus is a complex multi-stage reaction. The reaction product contains not only P4O10, but also a small amount of lower oxides of phosphorus, P4 O, P4O2, P4O6, etc. After hydration of lower oxides of phosphorus, hypophosphorous acid and phosphorous acid are produced. Generally in industrial production, excess air is used for combustion reactions.
B: Hydration of phosphoric anhydride. As an intermediate reaction oxide, polymerized P4O10 or gaseous P2O5 can strongly absorb pollutants in the air. Moisture, especially under high temperature combustion conditions, generates metaphosphoric acid. As the phosphoric anhydride hydrates, the entire reaction proceeds toward the formation of phosphoric acid. At different temperatures, the hydration reaction is also different. At 700°C, metaphosphoric acid is generated.
2) Thermal phosphoric acid production process
The production methods of thermal phosphoric acid include one-stage method and two-stage method. The one-stage method refers to the early phosphoric acid production that directly used the phosphorus steam coming out of the phosphorus-making electric furnace to burn and hydrate to produce phosphoric acid, which has now been eliminated. The two-stage method refers to a method in which phosphorus vapor produced in an electric furnace is condensed into liquid phosphorus and then burned and hydrated to produce phosphoric acid. It is also the production method used by most thermal phosphoric acid plants today.
A: One-step combustion hydration method: The one-step combustion hydration method is to carry out the combustion oxidation of yellow phosphorus and the hydration absorption of phosphorus pentoxide gas in the same tower. In this process, the combustion hydration tower is the most important part of the process. Yellow phosphorus is atomized in the tower and burned in the air flow to generate phosphorus pentoxide, which is finally hydrated. These two process steps are both carried out in the combustion hydration tower, and the reaction formula is as follows:
B: Two-step combustion hydration method: The two-step combustion hydration method is to conduct the combustion of yellow phosphorus and the hydration process of phosphorus pentoxide in two pieces of equipment respectively. In this process, compressed air is used in a phosphorus nozzle to turn liquid phosphorus into extremely fine particles, which are thoroughly mixed with air and oxidized. The P2O5 gas generated by combustion enters the hydration tower and contacts the circulating cold phosphoric acid in countercurrent, absorbing water to synthesize phosphoric acid. In this process, a large amount of heat in the gas isQuantity was also taken away. After absorbing P2O5 gas, the concentration of circulating phosphoric acid gradually increases, and the temperature also increases. In order for the phosphoric acid to continue to be used, it must be cooled using a heat exchanger. In addition, the unabsorbed P2O5 in the hydration tower enters the exhaust gas treatment system in the form of acid mist for treatment. There are various forms of exhaust gas absorption, and electric demisters, fiber demisters and Venturi demisters are commonly used.
The comparison between the two processes is shown in the table:
3. Kiln-process phosphoric acid
The kiln phosphoric acid process and the electrothermal phosphoric acid process have the same basic principle, that is, they both use coke to reduce the phosphorus in the phosphate rock into gaseous elemental phosphorus, and then oxidize and hydrate it to synthesize phosphoric acid. The kiln phosphoric acid process uses coal gas to heat low-grade phosphate rock powder in a rotary kiln to perform a reduction and oxidation reaction, and the circulating acid absorbs the rotary kiln gas to prepare industrial phosphoric acid. Process:
The reaction process can be divided into the following three steps:
In the first step, phosphate rock undergoes a reduction reaction with the participation of SiO2 and C:
Step 2, the P4 vapor and CO gas produced by the above reaction undergo oxidation reaction
Step 3, P2O5 is hydrated in the hydration tower:
[Application][4]
Phosphoric acid occupies an important position in the national economy and is an intermediate raw material for the production of phosphate fertilizers and phosphates. Phosphoric acid and its downstream products are widely used in chemical, agricultural, pharmaceutical, food, electronics and other industries.
[Main reference materials]
[1] Encyclopedia of Chinese Adult Education·Chemistry·Chemical Engineering
[2] Health Dictionary
[3] Ma Chao, Wu Yuanxin, Jin Fang, et al. Research status and prospects of industrial production of phosphoric acid [J]. Chemical Engineering, 2013, 41(6): 74-78.
[4] Tian Haoyi, Kang Mingxiong, Liu Genyan, et al. Analysis and prospects of phosphoric acid production technology in my country [J]. Chemical Minerals and Processing, 2011, 40(1): 1-5.
