What are the aspects of comprehensive utilization of phosphogypsum?

1. Phosphogypsum can be used to produce sulfuric acid or cement clinker; however, this process is costly, and it can also be utilized to manufacture building materials

What is discussed most these days is the use of phosphogypsum to produce acid. Back then, there were three companies in China doing this: one in Sichuan, one in Tangshan, Hebei, and one in Lubei, Shandong. However, only the last two are still in production today. The facility in Lubei is relatively larger. The main issue is that after years of modifications and practical production experience, its design has changed significantly from the original plan; as a result, Lubei is the only company that possesses this technology (Lihe has a smaller scale). The key aspect of this technology lies in the control during the calcination process, and the role of those responsible for operating the furnaces is extremely crucial – it requires many years of experience. Moreover, high investments are needed, as well as funds related to circular economy practices. Currently, the price of sulfuric acid is low. If prices were as high as they were for sulfur last year, the return on investment for projects that use phosphogypsum to produce sulfuric acid would be decent. However, under the current circumstances, it’s not feasible; the economic hurdles cannot be overcome. Additionally, depending on the location of your company, phosphoric acid can be filtered, and the filter cake, which is phosphogypsum, can be used to produce gypsum boards; it’s difficult to apply it to other purposes

No matter what kind of project it is, the key is to have good technology and ensure that the quality of the products produced meets actual requirements; otherwise, nothing will work. Take building bricks as an example – if the technology used is poor, the bricks produced will not be suitable for users, and in such cases it’s better not to produce them at all.

Regarding the comprehensive development and utilization of phosphogypsum, based on the current state of technological advancement, there are mainly three ways in which it can be utilized: ㈠ producing sulfuric acid alongside cement, which is primarily used for the treatment of phosphogypsum. Due to the uneven distribution of sulfur resources in the country, with the development of wet-process phosphoric acid, using phosphogypsum to produce sulfuric acid and cement simultaneously in areas lacking sulfur is undoubtedly an effective approach that can not only address the shortage of sulfur resources but also find a use for phosphogypsum, thereby enabling comprehensive utilization of these resources. At present, China has established the “3, 4, 6” projects for producing sulfuric acid and cement from phosphogypsum in various enterprises such as Lubei Group (projects with an annual output of 30,000 tons of ammonium phosphate, 40,000 tons of sulfuric acid, and 60,000 tons of cement). Through years of practice, the technology has become mature. However, the process is lengthy, requires many pieces of equipment, and involves high investment. According to the calculations for the “3, 4, 6” project aimed at producing sulfuric acid and cement from phosphogypsum, the total investment in this processing facility is around 100 million yuan, with a capacity to process approximately 100 kt of phosphogypsum per year. (ii) After being washed and purified, phosphogypsum is used to produce potassium sulfate with ammonium chloride as a by-product. First, phosphogypsum is rinsed to remove some impurities, and then it reacts with ammonium bicarbonate to form a slurry of ammonium sulfate and calcium carbonate. This slurry is washed with water to separate out solid calcium carbonate; the filtrate (ammonium sulfate) then reacts with potassium chloride in a second reaction. Again, washing with water is carried out to isolate potassium sulfate, which is obtained after drying ； The filtrate (ammonium chloride) is evaporated and separated to yield the by-product ammonium chloride; this technology is applied by Qingdao Dongfang Chemical Co., Ltd. The technology for producing ammonium chloride as a by-product in the production of potassium sulfate from phosphogypsum via a two-step process has not been widely adopted due to issues such as a long process route, the difficulty in using solid calcium carbonate, and the need to treat large amounts of washing water. (III) Cement retarders: The use of phosphogypsum as a cement retarder abroad has become one of the main ways of utilizing industrial gypsum. Relevant institutions have conducted research on using phosphogypsum as a retarder for high-grade Portland cement, ordinary Portland cement, and slag Portland cement. The results showed that the properties of the cement produced in this way were comparable to those of cement made with natural gypsum; XRD phase analysis revealed that the hydrates were identical. The amount of phosphogypsum used was slightly lower than that of natural gypsum, averaging 4%. Due to its high moisture content and high apparent density, untreated industrial gypsum often causes blockages during use, which makes it unacceptable to cement manufacturers, especially those of medium and large scale. To meet the requirements of cement manufacturers regarding the storage, transportation, and measurement of industrial gypsum, and to ensure more stable quality for this material, industrial gypsum is processed by granulation before being supplied to cement manufacturers as a commercial retarder. This practice is quite common abroad. In addition, with the increasing use of new building materials such as paper gypsum boards and gypsum blocks in recent years, building gypsum powder is being produced from phosphogypsum. Building on the successful research and development efforts over the years in creating cement retarders from industrial gypsum, the Tongling Chemical Research and Design Institute has, taking into account the characteristics of industrial gypsum such as phosphogypsum, developed a production process for converting industrial gypsum into building gypsum powder. This process features low investment costs, reduced energy consumption, easy operation, and advanced technical capabilities. It relies on patented equipment such as the self-developed single-rotor hammer-type drying and calcining machine (patent number: ZL 00219787.1) and the building gypsum converter (patent number: ZL 01221192.3). This process has achieved satisfactory results in industrial gypsum treatment projects at Chongqing Power Plant and Tongling Lvyang Building Materials Co., Ltd.