How to produce phosphoric acid

Phosphoric acid is the production of a variety of industrial and agricultural phosphorus products of the basic raw materials, the current domestic and international production process of phosphoric acid are mainly "thermal" and "wet" two. Comparison of the two, the wet process of phosphoric acid is characterized by relatively low product costs, but the quality is poor, and the grade and impurity content of phosphorus ore have higher requirements, the international preparation of industrial phosphoric acid is mainly used in the wet process, China's wet phosphoric acid is mainly used for the production of agricultural fertilizers. Thermal phosphoric acid process is characterized by good product quality, but the price is more expensive, and is a high-energy technology, electric energy in the thermal phosphoric acid manufacturing chain in the total weight of 60%. With the increasingly serious energy shortages, electricity prices are rising, thermal phosphoric acid prices have also risen, resulting in its raw materials for the gradual loss of market competitiveness of phosphorus chemical products. In this situation, the phosphoric acid industry continues to improve the production process, with a view to reducing energy consumption and production costs. Thermal phosphoric acid using two-step combustion hydration technology Thermal phosphoric acid process, that is, electrothermal production of yellow phosphorus as raw material, after combustion hydration and made of phosphoric acid content of 85%. For thermal phosphoric acid production in the recycling of thermal energy, the 1950s before the United States conducted experimental research, but did not make great progress, not to mention industrialized production. late 80s, Germany has a larger scale of this type of device into operation. In recent years, China's Yunnan Province, there is also a smaller set of devices put into trial operation. Thermal phosphoric acid production process with heat recovery device usually adopts a two-step method, that is, the combustion of phosphorus and P2O5 hydration were carried out in two devices. Among them, the P2O5 hydration equipment is similar to the traditional hydration tower; phosphorus combustion equipment is equipped with heat exchanger tubes to recover the heat of phosphorus combustion and by-production of steam. Phosphorus combustion equipment, the key to technology is how to prevent the heat exchanger tube by high temperature P2O5 gas corrosion, but also to provide good heat transfer conditions. Patented technology in various countries are through the control of process conditions, so that the surface of the heat exchanger tube to form a special layer of phosphide to protect. The original German Hoechst Group on its one-step 70,000t/a H3PO4 device for the transformation, that is, in front of the original combustion and hydration tower to add a tower, dedicated to the use of phosphorus combustion, the original combustion and hydration tower is changed to a simple hydration tower, the two towers are connected to the top of the pipeline to phosphorus combustion tower of phosphorus-containing gases into the hydration tower for hydration. There is no anti-corrosion lining on the surface of the steel pipe in the phosphorus combustion tower, but by controlling the process conditions to prevent the steel pipe from being corroded. China's Yunnan Provincial Research Institute of Chemical Industry, in cooperation with the Department of Engineering Mechanics of Tsinghua University, conducted a study on the thermal energy recovery and utilization of thermal phosphoric acid. They used a two-step method, that is, the combustion of phosphorus and the hydration of P2O5 are carried out in two towers. In fact the phosphorus combustion tower is also a heat energy recovery unit, equivalent to a waste heat boiler, and the recovered heat energy is used to produce 0.8 MPa steam. The heat recovery device adopts a membrane heat exchanger structure to improve the heat recovery efficiency and to meet the space required for phosphorus combustion. The process has been in 2001 through the Yunnan Provincial Science and Technology Department acceptance appraisal, the first set of 15,000t / a thermal phosphoric acid plant heat recovery industrialization of the device in 2006 in Chongqing Chuandong Chemical (Group) Co. Large wet phosphoric acid into the industrial grade phosphoric acid industry Wet phosphoric acid process, that is, by the phosphorus ore through the decomposition of inorganic acid (mainly sulfuric acid or hydrochloric acid), the first crude phosphoric acid used for fertilizers, and then purified by a variety of steps in addition to the impurities, and finally concentrated to make the purity of industrial-grade phosphoric acid equivalent to the thermal process. Currently the main purification methods are chemical precipitation, ion exchange resin method, crystallization, solvent precipitation and solvent extraction method. Solvent extraction method has the advantages of high product purity, relatively simple production process and equipment, low energy consumption, low consumption of raw materials, large production capacity, good separation effect, high recovery rate, less environmental pollution, the production process is easy to realize automation and continuity, and is conducive to the comprehensive utilization of resources, thus attracting widespread attention. At present, the solvent extraction method has become one of the most effective methods for purifying wet phosphoric acid abroad, and many developed countries have formally adopted the solvent extraction method for the production of industrial-grade and food-grade phosphoric acid. As the vast majority of China's phosphate resources are high impurity content in the low-grade phosphate, to the wet process of phosphoric acid purification difficulties. 10 years, many research units in China to carry out the wet process of phosphoric acid purification of the research work, but so far has not yet formed a large-scale industrialization, which is mainly due to the extractant price is expensive, difficult to recycle, resulting in high production costs. Sichuan University and Guizhou Hongfu Industrial Development Co., Ltd. have cooperated to develop a wet phosphoric acid purification technology with independent intellectual property rights, which includes pretreatment, desulfurization, filtration and separation, extraction, deep desulfurization, washing, reverse extraction and concentration, etc. The characteristics of the process are as follows: ① Setting up a buffer tank of sulfurization and defluorination at the pretreatment stage, and setting up a refined sulfurization and iron removal tank in the middle of the extraction and washing tanks; ② extraction, washing and reverse extraction processes are all carried out in a rotary vibrating screen, which is the most suitable method for the purification of phosphoric acid. Extraction, washing and reverse extraction processes are carried out in the rotating vibrating plate tower; ③ set up a demulsification tank between the washing tower and reverse extraction tower. The process of phosphoric acid purification rate of 70% to 80%, the total yield of phosphorus 99%, solvent consumption of 6kg / t. Kiln phosphoric acid formally put into industrial operation Kiln phosphoric acid process, that is, in the rotary kiln heated with gas low-grade phosphate rock powder, reduction and oxidation reaction, by the cycle of acid absorption kiln gas preparation of industrial phosphoric acid. China's kiln phosphoric acid from 1988 began experimental research, in March 2005 Hubei Sanxin Phosphoric Acid Co., Ltd. has been containing phosphorus 25%, 20%, 18%, 15%, 12%, 9% and other grades of phosphorus ore for small and medium-sized trials, and achieved success, the phosphorus reduction rate of 90%. On this basis, the company has built 10,000t/a industrial phosphoric acid CDK plant. The main features of the new process of kiln phosphoric acid are: it can use the middle and low-grade phosphorus ore with high impurity content to produce high-quality phosphoric acid with high concentration; when the SiO2 content in the phosphorus ore is high, the P2O5 content can be as low as 17%; and the quality and concentration of the phosphoric acid produced can reach or be close to that of the thermal phosphoric acid. In addition, the process makes full use of the chemical reaction heat of the production process, which significantly reduces the production energy consumption. And the process can use coal as fuel, so that the product cost is relatively low. It is estimated that the product cost of kiln phosphoric acid is between thermal and wet phosphoric acid. Compared with the wet phosphoric acid, it is not subject to the limitations of phosphate grade and impurity content, and not subject to the limitations of sulfur resources; compared with the thermal method, it greatly reduces the production of energy consumption, and can avoid the use of expensive electrical energy. Therefore, the process is very consistent with the characteristics of China's phosphorus resources, has good prospects for development. Hydrochloric acid law phosphoric acid process out of the laboratory has been restricting the development of the phosphorus industry bottleneck - low-grade ore utilization technical problems have finally been cracked. Wuhan Chemical Industry Research Institute of Hubei Province, the key project of scientific and technological research - hydrochloric acid decomposition of low-grade phosphate ore manufacturing industrial phosphoric acid new process (referred to as "hydrochloric acid method") pilot plant, at the end of August 2006, a one-time trial success, the production of fertilizer-grade phosphoric acid and industrial-grade phosphoric acid, the quality of the quality of the phosphoric acid and industrial-grade phosphoric acid, the quality of the phosphoric acid. Fertilizer grade phosphoric acid and industrial grade phosphoric acid, the quality of which meets national standards. This means that after many years of efforts, "hydrochloric acid method" finally out of the laboratory, a big step towards industrialized production. "Hydrochloric acid method" can be directly utilized in the low-grade phosphorus ore manufacturing industrial phosphoric acid, do not need ore dressing, can save a lot of electricity, coal and sulfur resources. The method is applicable to any grade of phosphorus ore, the total recovery rate of P2O5 can reach more than 93%. It is recommended that the existing thermal phosphoric acid process adopt the two-step method to recover heat energy and reduce production costs. The refining technology of wet phosphoric acid needs to be further improved to reduce the production cost of industrial grade wet commercial phosphoric acid and phosphate, and to replace some of the thermal phosphoric acid with refined wet phosphoric acid, especially food grade phosphoric acid. Production methods are many, depending on the production conditions, the local environment and the current stage of the market supply and demand

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