【Frontiers in HaiChuan Chemical Technology】Qingdao Institute of Energy has developed new technologies for the high-value utilization of gypsum generated as an industrial by-product
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The Qingdao Institute of Energy has developed new technologies for the high-value utilization of industrial by-product gypsum. Industrial by-product gypsum includes phosphogypsum, desulfurization gypsum, titanium gypsum, fluorogypsum, polylactic acid gypsum, citric acid gypsum, mirabilite gypsum, salt gypsum, boron gypsum, and mold gypsum from ceramic industries. The annual domestic output of such gypsum amounts to 258 million tons (with phosphogypsum and desulfurization gypsum accounting for 85% of this amount), representing approximately 6% of the total volume of industrial solid waste. Currently, the historical stockpile has exceeded 1.1 billion tons. Traditional methods of disposal such as storage, landfilling, and dumping not only consume large amounts of land resources but also pose significant environmental and ecological risks as well as safety hazards; they are truly characterized by \"three excesses\" – excessive existing stock, excessive new additions, and excessive hidden dangers. How to achieve the resource-based recycling of large amounts of industrial by-product gypsum has always been a key issue of concern for ** and enterprises. In 2022, eight departments including the Ministry of Industry and Information Technology jointly issued the \"Implementation Plan for Accelerating the Comprehensive Utilization of Industrial Resources,\" aiming to achieve a comprehensive utilization rate of 73% for industrial by-product gypsum by 2025. However, the actual utilization rate is currently only 61.6%. Issues such as low technical level, low added value, and high treatment costs exist, resulting in poor economic viability for resource utilization; the reliance on subsidies makes it difficult to achieve fundamental improvements in this regard. Therefore, there is an urgent need for a step-by-step utilization of industrial by-product gypsum that aims to make it harmless, reusable as a resource, and suitable for high-end applications (the \"three transformations\"), particularly to address issues related to its utilization rate as well as the economic aspects of the relevant processing technologies. To effectively address the challenges posed by the \"three excesses and three transformations\" associated with industrial by-product gypsum, the Green Reaction Separation and Process Intensification Technology Center at Qingdao Institute of Energy developed a technology approach based on using waste to treat other waste. It created new technologies, processes, and equipment for the resourceful, high-value, and large-scale utilization of industrial by-product gypsum. By combining the resource utilization of gypsum waste with the fixation and utilization of CO2 (historical deposits and annual emissions of industrial by-product gypsum can respectively fix more than 500 million tons and 110 million tons of CO2), and by utilizing inexpensive domestic carbon-containing chemical raw materials such as ammonium carbonate and ammonium bicarbonate – which are produced by the reaction of concentrated ammonia with CO2 to capture carbon – high-purity, high-value chlorine-free sulfur-containing potassium fertilizers, namely potassium sulfate, were successfully synthesized. Potassium sulfate is a \"strategic resource\" that is rare among salts; 70% of it in China is produced by converting potassium chloride. This technology provides high-quality fertilizers for large areas of saline-alkali land, as well as for vegetables that prefer potassium over chlorine, such as potatoes, sweet potatoes, tomatoes, red peppers, and beets, as well as for economic crops such as fruits, tobacco, and tea.
This technology has the following notable features: 1. It enables the resourceful and high-value utilization of materials: using traditional KCl and NH4HCO3/(NH4)2CO3 as raw materials, along with low-concentration ammonia water (~5 wt.%) as a modifier, calcium and sulfur are converted into calcium carbonate and high-value potassium sulfate, respectively. The profit from one ton of potassium sulfate products can reach nearly a thousand yuan, and it has broad prospects for industrial application. 2. Development of new processes for producing high-purity products: Through process innovation, such as the use of reusable phase-transfer reagents and continuous flow reaction techniques, this project significantly increased the concentration of gypsum slurry (≥2 mol/L) as well as the reaction conversion rate. It enabled the successful production of three types of high-purity products, including nano- and micro-scale calcium carbonate that can be adjusted in terms of properties, ammonium chloride, and potassium sulfate. In particular, the high-quality agricultural-grade potassium sulfate produced enables a significant improvement in the technical and economic efficiency of this process route, providing a ‘drive’ for the large-scale and high-value utilization of industrial by-product gypsum. 3. Develop key technologies for energy savings, cost reduction, and efficiency improvement: By utilizing mild reaction conditions at normal temperature and pressure, along with a distinctive two-step gypsum conversion technique (as shown in the formula below), the costs associated with reaction processing and the separation of multi-component crystals can be reduced. Furthermore, energy consumption and production costs are significantly reduced through the use of green, reusable phase-transfer reagents and mild distillation using liquid ammonia or alcohols. 4. Development of specialized equipment for the production process: To enhance the corrosion resistance of large reactors under high-chlorine conditions (conventional corrosion-resistant stirred reactors with a capacity of ≤200 m3), improve mixing and mass transfer within the reactors, and control the particle size and distribution of the crystallized product, the team developed a new type of efficient circulation-type reaction-crystallization equipment that is suitable for the production process, integrates reaction and separation functions, and is easy to scale up industrially. This equipment enables stepwise scaling up of industrial corrosion-resistant reactors to capacities of ≥1000 m3, thereby achieving economies of scale.
Using phosphogypsum as the raw material, the research team first selected green and inexpensive phase-transfer agents that could meet the requirements of industrial-scale production in order to facilitate the dissolution of gypsum； Secondly, by using a reusable phase-transfer reagent and a mixed mother liquor containing saturated products as the simulated solution, efficient leaching of the poorly soluble by-product gypsum, effective separation of impurities, and a significant increase in reaction rate were achieved ； Then, under normal temperature and pressure conditions, high-purity products were successfully prepared, with no impurities such as ammonium sulfate, ammonium potassium sulfate, or ammonium potassium chloride formed. Finally, a systematic study was conducted on the recycling efficiency of the phase-transfer reagent, demonstrating its good reusability. This study has for the first time established a synthetic route for producing high-quality, chlorine-free agricultural potassium sulfate from industrial by-product gypsum, overcoming the bottleneck problem of poor quality of potassium sulfate that is common in conventional conversion techniques. It provides important fundamental data and theoretical basis for the large-scale utilization of industrial gypsum waste and the low-cost production of high-value products, holding significant theoretical and industrial application value. The relevant research findings were recently published in the traditional chemical engineering journal The Canadian Journal of Chemical Engineering, with Assistant Researchers Xiao Hang and Zhang Haidong, along with Researcher Huang Qingshan, serving as co-corresponding authors of the paper. A **patent application** has been filed for the technologies related to this research achievement. This work was funded by projects such as the **National Natural Science Foundation**, the Shandong Provincial Natural Science Foundation, and the \"Analytical Technology Development\" program of the Qingdao Institute of Energy. (Writing/Photos by Xiao Hang)
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