Phosphoric acid (phosphate), because of its many aspects of performance has beenexcellent, use and consumption can not be underestimated. Phosphoric acid is widelyused in basic chemical industry and fine chemical industry, involving many fields suchas food, petroleum, material synthesis and so on. It is the highest oxide hydrate ofphosphorus, the fifth main group element. From the periodic table of elements, we canjudge that its acidity is weaker than that of sulfuric acid and perchloric acid, and itbelongs to the strong ternary acid.

　　There are two main industrial processes for the preparation of phosphoric acid: wetprocess and thermal process. Wet process is directly from phosphate rock to phosphoricacid, while thermal process phosphoric acid is obtained from phosphate rock to yellowphosphorus, and then from yellow phosphorus to phosphoric acid.

　　Whether it is wet-process or thermal-process phosphoric acid, there is no difference inits appearance; even according to a few simple indicators in the national standard totest, it will not reflect any difference. Here, wet-process acid and thermal-processacid are introduced in detail.

　　Wet-process phosphoric acid

　　Generally speaking, as long as the phosphoric acid produced by the decomposition ofphosphate rock by liquid leaching can be collectively called wet process phosphoricacid. The development of wet process phosphoric acid is a process of gradualimprovement. From the first industrial production in Germany in 1870 to now, itstechnology and production process are becoming increasingly perfect. At the beginning,batch production with batch reactor was adopted, each batch was about 1-2 tons. Theconcentration of phosphoric acid was only 8%-10%(P2O5). After the adoption of Doyleprocess in 1915, the concentration of phosphoric acid could be increased to 22%-23%(P2O5). After the 1940s, the production technology of wet process phosphoric acidhas made remarkable progress, and new processes have been emerging. The equipment isconstantly updated, which greatly improves the production intensity and strengthens thetheoretical research of wet process phosphoric acid. Now, the production method of wetprocess phosphoric acid and the subsequent purification method have been very mature.

　　The liquids used to prepare wet-process phosphoric acid are generally inorganic acidswith strong acidity, such as nitric acid, hydrochloric acid, sulfuric acid,fluorosilicic acid, etc. The phosphoric acid obtained after liquid-solid separationoften contains many impurities. According to the different acid used, it can be dividedinto sulfuric acid method, hydrochloric acid or nitric acid method and ammoniumbisulfate method, and the most commonly used method is sulfuric acid method.

　　Wet-process phosphoric acid contains many impurities, which greatly limits itsapplication. It is mainly used as raw material for high-concentration fertilizer. Asmall part of purified phosphoric acid also participates in the production of finephosphate products. Some enterprises with excellent production technology have purifiedand refined wet-process phosphoric acid, and their products are also used in someindustries with fine requirements.

　　Thermal-process phosphoric acid

　　Compared with the development of wet-process phosphoric acid, the development ofthermo-process phosphoric acid was later. About the beginning of last century, the USDAcarried out a pilot test on the production of phosphoric acid by electric furnace.During World War I, the USDA built the first production plant of phosphoric acid byelectric furnace near Charlotte. Since 1933, the United States has devoted itself tothe research of thermal production, completed the test and construction from small-scale, pilot-scale to large-scale plant, and first built a two-step production plantfor all stainless steel. However, the development of thermal phosphoric acid in Chinais relatively late. In 1958, primary phosphorus was first used by Shanghai Institute ofChemical Engineering. The intermediate test of acid-cooling process and water-coolingprocess was adopted. After years of research and improvement, the domestic productiontechnology of thermal phosphoric acid has become mature. CDchem was one of the powerfulmanufacturers of thermo-process phosphoric acid at that time.

　　The main production process of thermal phosphoric acid is the combustion of yellowphosphorus and the subsequent hydration process. According to the production process ofcombustion hydration, it can be divided into combustion hydration one-step method andcombustion hydration two-step method. Electric-thermal two-step method is mainly usedin the world. The so-called two-step method, also known as water-cooling method, is tofirst add the mixture of phosphorus ore, silica and coke into the electric furnace, andthen heat it to more than 1300°C by the electric furnace, so that the furnace chargecan be melted to produce decomposition and reduction reaction, resulting in furnace gascontaining a large amount of element phosphorus (later, the enterprise developed aproduction process for recovering yellow phosphorus tail gas). The furnace gas thenenters the condensation system and is condensated. After separation, yellow phosphoruswas refined. Yellow phosphorus is transported to the phosphoric acid manufacturingsection, oxidized by yellow phosphorus and then absorbed to form phosphoric acid. Themain reaction process is as follows:

　　P4+5O2—P4O10

　　P4O10+6H2O—4H3PO4

　　Through the above process and reaction process, we can know that metal ions can not bereduced and volatilized in the process of yellow phosphorus production. Therefore, inaddition to F (fluorine), As (arsenic) and other volatile substances into yellowphosphorus products, other impurities will be removed with slag. Thermal phosphoricacid has high purity and few impurities. It is mainly used in the production of highquality phosphoric acid and phosphate, such as pharmaceutical grade, food grade,electronic grade and special phosphate.

　　From the process of thermal phosphoric acid, we can know that its products have theadvantages of high concentration and less impurities. Therefore, it can often be usedas raw materials for the production of phosphate needed by high-tech, such as finephosphate with high added value, food grade, electronic grade, toothpaste grade, etc.

　　Although the current wet purification technology has been progressing, and according tofood-grade standards, phosphoric acid produced by individual manufacturers meets; butthere is uncertainty and incompleteness in the inspection of industrial chemicals;therefore, the state requires food-grade products to strictly prohibit the use of wetacid as raw materials, while some more demanding electronic products, aviation-gradeproducts can not be used.