Thermal-process Acid and Wet-process Acid
Editor：川东化工Time：2019-04-03Hits：
Phosphoric acid (phosphate), because of its many aspects of performance has been excellent, use and consumption can not be underestimated. Phosphoric acid is widely used in basic chemical industry and fine chemical industry, involving many fields such as food, petroleum, material synthesis and so on. It is the highest oxide hydrate of phosphorus, the fifth main group element. From the periodic table of elements, we can judge that its acidity is weaker than that of sulfuric acid and perchloric acid, and it belongs to the strong ternary acid.
There are two main industrial processes for the preparation of phosphoric acid: wet process and thermal process. Wet process is directly from phosphate rock to phosphoric acid, while thermal process phosphoric acid is obtained from phosphate rock to yellow phosphorus, and then from yellow phosphorus to phosphoric acid.
Whether it is wet-process or thermal-process phosphoric acid, there is no difference in its appearance; even according to a few simple indicators in the national standard to test, it will not reflect any difference. Here, wet-process acid and thermal-process acid are introduced in detail.
Wet-process phosphoric acid
Generally speaking, as long as the phosphoric acid produced by the decomposition of phosphate rock by liquid leaching can be collectively called wet process phosphoric acid. The development of wet process phosphoric acid is a process of gradual improvement. From the first industrial production in Germany in 1870 to now, its technology and production process are becoming increasingly perfect. At the beginning, batch production with batch reactor was adopted, each batch was about 1-2 tons. The concentration of phosphoric acid was only 8%-10%(P2O5). After the adoption of Doyle process in 1915, the concentration of phosphoric acid could be increased to 22%-23%(P2O5). After the 1940s, the production technology of wet process phosphoric acid has made remarkable progress, and new processes have been emerging. The equipment is constantly updated, which greatly improves the production intensity and strengthens the theoretical research of wet process phosphoric acid. Now, the production method of wet process phosphoric acid and the subsequent purification method have been very mature.
The liquids used to prepare wet-process phosphoric acid are generally inorganic acids with strong acidity, such as nitric acid, hydrochloric acid, sulfuric acid, fluorosilicic acid, etc. The phosphoric acid obtained after liquid-solid separation often contains many impurities. According to the different acid used, it can be divided into sulfuric acid method, hydrochloric acid or nitric acid method and ammonium bisulfate method, and the most commonly used method is sulfuric acid method.
Wet-process phosphoric acid contains many impurities, which greatly limits its application. It is mainly used as raw material for high-concentration fertilizer. A small part of purified phosphoric acid also participates in the production of fine phosphate products. Some enterprises with excellent production technology have purified and refined wet-process phosphoric acid, and their products are also used in some industries with fine requirements.
Thermal-process phosphoric acid
Compared with the development of wet-process phosphoric acid, the development of thermo-process phosphoric acid was later. About the beginning of last century, the USDA carried 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 by electric furnace near Charlotte. Since 1933, the United States has devoted itself to the 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 plant for all stainless steel. However, the development of thermal phosphoric acid in China is relatively late. In 1958, primary phosphorus was first used by Shanghai Institute of Chemical Engineering. The intermediate test of acid-cooling process and water-cooling process was adopted. After years of research and improvement, the domestic production technology of thermal phosphoric acid has become mature. CDchem was one of the powerful manufacturers of thermo-process phosphoric acid at that time.
The main production process of thermal phosphoric acid is the combustion of yellow phosphorus and the subsequent hydration process. According to the production process of combustion hydration, it can be divided into combustion hydration one-step method and combustion hydration two-step method. Electric-thermal two-step method is mainly used in the world. The so-called two-step method, also known as water-cooling method, is to first add the mixture of phosphorus ore, silica and coke into the electric furnace, and then heat it to more than 1300°C by the electric furnace, so that the furnace charge can be melted to produce decomposition and reduction reaction, resulting in furnace gas containing a large amount of element phosphorus (later, the enterprise developed a production process for recovering yellow phosphorus tail gas). The furnace gas then enters the condensation system and is condensated. After separation, yellow phosphorus was refined. Yellow phosphorus is transported to the phosphoric acid manufacturing section, oxidized by yellow phosphorus and then absorbed to form phosphoric acid. The main 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 be reduced and volatilized in the process of yellow phosphorus production. Therefore, in addition to F (fluorine), As (arsenic) and other volatile substances into yellow phosphorus products, other impurities will be removed with slag. Thermal phosphoric acid has high purity and few impurities. It is mainly used in the production of high quality 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 the advantages of high concentration and less impurities. Therefore, it can often be used as raw materials for the production of phosphate needed by high-tech, such as fine phosphate with high added value, food grade, electronic grade, toothpaste grade, etc.
Although the current wet purification technology has been progressing, and according to food-grade standards, phosphoric acid produced by individual manufacturers meets; but there is uncertainty and incompleteness in the inspection of industrial chemicals; therefore, the state requires food-grade products to strictly prohibit the use of wet acid as raw materials, while some more demanding electronic products, aviation-grade products can not be used.
