Type of wet-process phosphoric acid: the most basic method in production: characteristics of decomposing phosphate rock powder with inorganic acid: high product purity, production technology, nitric acid method, hydrochloric acid method and sulfuric acid method. It is concluded that wet-process production is to decompose phosphate rock powder with inorganic acid, separate crude phosphoric acid and make phosphoric acid products after purification. The cost of wet-process phosphoric acid is 20%~30% lower than that of hot-process phosphoric acid. After proper purification, the purity of the product can be equivalent to that of hot-process phosphoric acid. Wet-process phosphoric acid is in a dominant position in phosphoric acid production. According to the different inorganic acids used, wet-process phosphoric acid process can be divided into sulfuric acid method, nitric acid method and hydrochloric acid method. The reaction formula of ore decomposition is as follows: ca5f (po4) 3+10hno3 = = 3h3po4+5ca (NO3) 2+HF? ( 1- 1)Ca5F(PO4)3+ 10 HCl = = 3 h3po 4+5 CaCl 2+HF？ ( 1-2)Ca5F(PO4)3+5h2so 4+nH2O = = 3 h3po 4+5Ca so 4 nH2O+HF？ (1-3) The common feature of these reactions is that they can all produce phosphoric acid. However, the calcium salt produced by calcium in phosphate rock has different forms and characteristics. After the reaction is terminated, how to economically separate calcium salt and produce phosphoric acid is the key to the problem. Accordingly, the production process of wet-process phosphoric acid can be divided into anhydrous method, semi-water method, dihydrate method and semi-water-dihydrate method. Among them, dihydrate process is dominant in wet-process phosphoric acid process because of its mature technology, stable and reliable operation and strong adaptability to ores. More than 80% of phosphoric acid in China is produced by wet-process phosphoric acid dihydrate process. The dihydrate process has the characteristics of simple process, mature technology and strong adaptability to ore types, which is especially suitable for medium and low grade ores and occupies a leading position in wet-process phosphoric acid production. The production process nitric acid method was first developed by Aoda Company, which is called Aoda method. Phosphate rock is decomposed by nitric acid to produce phosphoric acid and water-soluble calcium nitrate, and then calcium nitrate is separated by freezing, solvent extraction and ion exchange. Affected by the price, high energy consumption and long process of nitric acid, it is rarely used in industry. Hydrochloric acid method In the early 1960s, Israel Mining Engineering Company (I.M.I) developed the famous IMI method, which realized the industrialization of phosphoric acid production by hydrochloric acid method for the first time [5]. Phosphate rock reacts with hydrochloric acid to produce aqueous solution of phosphoric acid and calcium chloride, and then the phosphoric acid is extracted and separated by organic solvent (such as fatty alcohol, acetone, trialkyl phosphate, amine or amide). However, there are some problems in this method, such as complex process and difficult economic recovery of by-product calcium chloride. The characteristic of sulfuric acid method is that phosphoric acid, the product of ore decomposition, is in liquid phase, and calcium sulfate, the by-product, is in solid phase, so its solubility is very small. The separation of the two is a simple liquid-solid separation, which has incomparable advantages over other processes. Therefore, the process of producing phosphoric acid from sulfuric acid is in a dominant position in wet-process phosphoric acid production. However, a large amount of phosphogypsum waste residue can not be effectively utilized, and the problem of three wastes is serious. The value of n in the reaction formula (1-3) depends on the form of calcium sulfate crystals and can be 0,1/2,2. Calcium sulfate anhydrous (CaSO4), calcium sulfate hemihydrate (caso 4·0·5h2o) and calcium sulfate dihydrate (CaSO4 2H2O) can be produced at different reaction temperatures and phosphoric acid concentrations. There are still some insurmountable defects in the dihydrate method. Due to extensive processing technology, most harmful impurities in the ore enter phosphoric acid. Especially fluorine, once it enters the liquid phase, it is difficult to separate. More seriously, a large amount of phosphogypsum waste was produced, which caused serious pollution and waste. Therefore, Chen Xuexi et al. [3 1] improved the existing technological process through research and carried out the reaction in two steps. The first step: leaching phosphate rock with phosphoric acid, and reducing the pollution of the chemical reaction itself to the target product by controlling the conditions of the chemical reaction. The main reaction equation of phosphoric acid and phosphate rock is as follows: ca5f (po4) 3+7h3po4+5h2o → 5ca (h2po4) 2h2o+HF? (1-4) Step 2: Provide suitable crystallization conditions, so as to significantly reduce the impurity content, especially the phosphorus content in the by-product phosphogypsum, improve the phosphorus recovery rate, make phosphogypsum meet the requirements of building materials production, and solve the pollution problem of phosphogypsum from the source. The ionization reaction of sulfuric acid can be completed instantly: Ca (H2PO4) 2H2O+H2SO4+(n-1) H2O → caso4 NH2O? +2HPO4 (1-5) step-by-step reaction method is completely different from the traditional idea of "pollution first and then treatment", which accords with the basic principle of green chemistry and the requirements of sustainable development strategy. The second method needs to be tested in practice.