Production process of phosphoric acid

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Production processes of phosphoric acid Tags: thermal phosphoric acid, wet phosphoric acid, phosphoric acid, production processes, miscellaneous discussions Category: filtration Phosphoric acid is the basic raw material for producing various phosphorus-based products used in industry and agriculture. Currently, the main production methods for phosphoric acid both domestically and internationally are the “thermal” and “wet” methods. When compared, the process of wet-process phosphoric acid is characterized by relatively lower production costs, but the quality of the product is inferior; moreover, it requires high purity levels of phosphate ore as well as low levels of impurities. Currently, the wet process is primarily used internationally for the production of industrial phosphoric acid, while in China, wet-process phosphoric acid is mainly used to manufacture fertilizers for agricultural use. The process of thermal phosphoric acid is characterized by high product quality, but it is expensive; moreover, it is a energy-intensive process, with electricity accounting for 60% of the total energy consumption in its production chain. As energy shortages become increasingly severe and electricity prices keep rising, the price of thermal phosphoric acid also increases, causing phosphorus-based chemical products that use it as a raw material to gradually lose their competitiveness in the market. Under such circumstances, the phosphate industry continues to improve its production processes in order to reduce energy consumption and production costs. 1. Thermal phosphoric acid is produced using a two-step combustion and hydration process. The thermal phosphoric acid production method uses yellow phosphorus manufactured by electrothermal methods as raw material, and through combustion and hydration, phosphoric acid with a concentration of 85% is obtained. Regarding the recovery and utilization of thermal energy in the production of thermal phosphoric acid, the United States conducted experimental studies before the 1950s, but made little progress and failed to achieve industrial-scale production. In the late 1980s, a large number of such installations were put into operation in Germany. In recent years, a smaller-scale installation has also been put into trial operation in Yunnan Province of China. 　　The thermal phosphoric acid production process equipped with a heat recovery system typically uses a two-step method, in which the combustion of phosphorus and the hydration of P2O5 take place in separate units. Among them, the P2O5 hydration equipment is similar to conventional hydration towers ； Heat exchange tubes are installed in the phosphorus combustion equipment to recover the heat generated by the combustion of phosphorus and produce steam as a by-product. The technical key to phosphorus combustion equipment lies in how to prevent the heat exchange tubes from being corroded by high-temperature P2O5 gas, while simultaneously ensuring good heat transfer conditions. Patented technologies in various countries protect the surface of heat exchange tubes by controlling process conditions to form a special phosphide layer thereon. The former German Hestia Group upgraded its one-step 70,000 t/a H3PO4 plant by adding a new tower in front of the original combustion and hydration tower, dedicated to phosphorus combustion; the original combustion and hydration tower was then converted into a pure hydration tower. The tops of the two towers are connected by pipes, allowing the phosphorus-containing gases generated in the phosphorus combustion tower to be directed into the hydration tower for hydration. The surface of the steel pipes in the phosphorus combustion tower does not have any anti-corrosion lining; instead, the corrosion of the steel pipes is prevented by controlling the process conditions. 　　The Chemical Industry Research Institute of Yunnan Province in China, in collaboration with the Department of Engineering Mechanics at Tsinghua University, has conducted research on the thermal energy recovery of thermal phosphoric acid. They employ a two-step process, in which the combustion of phosphorus and the hydration of P2O5 take place in separate towers. In fact, the phosphorus combustion tower is also a heat recovery device, functioning as a waste heat boiler; the recovered heat is used to produce steam at 0.8 MPa. The heat recovery unit adopts a membrane heat exchanger structure to improve the efficiency of heat recovery and to meet the space requirements for phosphorus combustion. This process was approved by the Yunnan Provincial Department of Science and Technology in 2001, and the industrial heat recovery unit for the first 15,000 t/a thermal phosphoric acid plant came online in 2006 at Chongqing Chuandong Chemical (Group) Co., Ltd. 2. Large-scale wet-process phosphoric acid enters the industrial-grade phosphoric acid industry. The wet-process involves decomposing phosphate rock using inorganic acids (mainly sulfuric acid or hydrochloric acid) to produce crude phosphoric acid suitable for use as fertilizer; this crude product is then purified through various steps to remove impurities, and finally concentrated to yield industrial-grade phosphoric acid with a purity level comparable to that obtained through the thermal process. The main purification methods currently available include chemical precipitation, ion exchange resin treatment, crystallization, solvent precipitation, and solvent extraction. Solvent extraction has attracted widespread attention due to its advantages, including high purity of the resulting products, relatively simple production processes and equipment, low energy consumption, minimal raw material usage, high production capacity, good separation efficiency, high recovery rates, low environmental pollution, and the ability to automate and streamline the production process; it also facilitates the comprehensive utilization of resources. Currently, solvent extraction has become one of the most effective methods for purifying wet phosphoric acid abroad, and many developed countries have officially adopted this method to produce industrial-grade and food-grade phosphoric acid. 　　Since the vast majority of phosphate rock resources in our country are medium- and low-grade phosphates with high impurity content, this poses difficulties for the purification of wet-process phosphoric acid. For over a decade, many research institutions in our country have carried out studies on the purification of wet phosphoric acid, but large-scale industrialization has not yet been achieved. The main reason for this is the high cost of extractants and the difficulties in their recovery, which result in high production costs. Sichuan University and Guizhou Hongfu Industrial Development Co., Ltd. have jointly developed a wet phosphoric acid purification technology with independent intellectual property rights. This process includes steps such as pretreatment, desulfurization, filtration and separation, extraction, further desulfurization, washing, back-extraction, and concentration. The features of this process are as follows: ① A desulfurization and defluorination buffer tank is installed in the pretreatment stage, while a refined desulfurization and iron removal tank is placed between the extraction tank and the washing tank ； ②The extraction, washing, and back-extraction processes are all carried out in a rotary vibrating sieve tray tower ； ③A demulsification tank is installed between the washing tower and the back-extraction tower. The phosphoric acid purification rate of this process is 70%–80%, the total phosphorus yield is 99%, and the solvent consumption is 6 kg/t. 3. Phosphoric acid produced by the kiln method is now in full industrial operation. The kiln method for producing phosphoric acid involves heating low-grade phosphate ore powder with gas in a rotary kiln to carry out reduction-oxidation reactions, with the resulting acid being used to produce industrial phosphoric acid from the gases emitted by the kiln. Experimental research on phosphate production using the kiln method was initiated in our country in 1988. In March 2005, Hubei Sannxin Phosphate Co., Ltd. conducted pilot and pilot-scale tests on phosphate ores with phosphorus contents of 25%, 20%, 18%, 15%, 12%, and 9%, achieving success with a phosphorus reduction rate of 90%. On this basis, the company built another 10,000 t/a industrial phosphoric acid CDK plant. 　　The main feature of the new phosphoric acid production process using kiln methods is that it enables the use of medium- and low-grade phosphate rocks with high impurity content to produce high-quality, high-concentration phosphoric acid ； When the SiO2 content in phosphate rock is high, the P2O5 content can be as low as 17% ； The quality and concentration of the resulting phosphoric acid can reach or approach those of phosphoric acid produced by thermal methods. Furthermore, this process significantly reduces energy consumption in production by making full use of the heat generated by chemical reactions during the manufacturing process. Moreover, this process can use coal as fuel, resulting in relatively low product costs. It is estimated that the cost of phosphoric acid produced by the kiln method lies between that of heat-process and wet-process phosphoric acid. Compared to wet-process phosphoric acid, it is not restricted by the grade of phosphate ore or the content of impurities, nor is it limited by sulfur resources ； Compared to the thermal method, it **reduces production energy consumption and avoids the use of expensive electricity. Therefore, this process is highly suitable for the characteristics of China’s phosphorus resources and holds great prospects for development. 4 A new process for producing phosphoric acid using hydrochloric acid moves out of the laboratory – The technical challenge related to the utilization of low-grade phosphate ores, which has long been a bottleneck hindering the development of the phosphate ore industry, has finally been solved. The pilot plant for the new process of producing industrial phosphoric acid from low-grade phosphate rocks through hydrochloric acid decomposition – a key project in Hubei Province’s science and technology development program undertaken by the Wuhan Institute of Chemical Engineering – was successfully tested in one go at the end of August 2006. The phosphoric acid produced, both in fertilizer grade and industrial grade, met the **specified standards**. This means that after years of effort, the \"hydrochloric acid method\" has finally moved out of the laboratory, taking a major step toward industrial production. 　　“The \"hydrochloric acid method\" can be used directly to produce industrial phosphoric acid from medium- and low-grade phosphate ores, without the need for ore processing, thereby saving a large amount of electrical energy, coal, and sulfur resources. This method is applicable to phosphorus ores of any grade, with a total recovery rate of P2O5 exceeding 93%. 　　It is recommended that the existing thermal phosphoric acid process adopt a two-step method to recover heat energy and reduce production costs. The refining technology for wet-process phosphoric acid needs further improvement in order to reduce the production costs of industrial-grade wet-process phosphoric acid and phosphates, and to replace some heat-process phosphoric acid with refined wet-process phosphoric acid, especially for food-grade applications.