China is a big country of sulfuric acid production and consumption. Titanium dioxide, aromatic nitration, dyes, petroleum processing and steel pickling industries all use sulfuric acid, which produces a large amount of sulfuric acid containing wastewater. With the expansion of production scale, the amount of industrial wastewater containing sulfuric acid is also increasing year by year. The treatment of wastewater containing sulfuric acid has become a big issue for related enterprises.
The common treatment processes of wastewater containing sulfuric acid mainly include concentration, chemical oxidation, extraction, crystallization, chemical fertilizer production and neutralization treatment. The operation cost of most treatment processes is high, which becomes the burden of enterprises.
Therefore, it is necessary to find a new way to treat wastewater containing sulfuric acid and create benefits at the same time. According to the chemical characteristics of sulfuric acid, gypsum can be produced by adding special reagents to the waste water containing sulfuric acid at certain reaction conditions, this kind of gypsum is generally called industrial by-product gypsum. The by-product gypsum produced by waste water can be divided into phosphogypsum, desulfurization gypsum, titanium gypsum, fluorogypsum, citric acid gypsum, etc., which can be used as building materials and gypsum products, such as cement retarder, gypsum board, gypsum block, gypsum mortar, plastering gypsum, etc. The process can turn waste acid into treasure and create value.
The average market demand of gypsum in China from 2016 to 2019 is about 2200 million tons, of which the demand of cement retarder, gypsum mortar, self leveling floor, gypsum board and gypsum wallboard accounts for about 96% of the total demand. In recent years, gypsum powder processing market is very popular. The price of ordinary building gypsum is 400 yuan / ton, and the price of some industrial by-product high-strength gypsum with good whiteness has risen to 2200 yuan / ton.
Phosphogypsum accounts for a large proportion of China's industrial by-product gypsum. In 2018, China's phosphogypsum output was 78 million tons, and the comprehensive utilization rate was only 39.7% [1].
At present, phosphogypsum is mostly used in building materials, agriculture and chemical industry in China.
At abroad,The technology of using phosphogypsum as cement retarder is more reliable and riper from Salzgitter company of Germany and Nissan Chemical Industry Company of Japan. According to statistics, 75% of cement retarders produced in Japan use phosphogypsum [3]. Compared with natural gypsum, using phosphogypsum as cement retarder improves cement strength and reduces production cost at the same time .
Since the 1990s, wet limestone gypsum desulfurization process has been gradually used in power plants in China, and the output of desulfurization gypsum has increased year by year. As early as 2010, the output of desulfurization gypsum in China has ranked first in the world [4], but the utilization of desulfurization gypsum in China is far less than that in some developed countries. In Japan, Germany and other countries, the utilization rate of desulfurization gypsum can reach 100%, while the utilization rate in China is only about 80% [5].
At present, desulfurized gypsum is mainly used in the production of cement retarder, and some are used in gypsum block, gypsum board, building gypsum, plastering gypsum , high strength gypsum and agricultural production. At abroad , the application of desulfurized gypsum in the field of materials mainly includes the production of common wall materials and waterproof wall materials, concrete, cement and asphalt, alcium carbonate and calcium sulfate. In agriculture, it is mainly used as soil amendment, fertilizer, in reducing soil erosion and eutrophication, recycling mineral and so on.
The low utilization rate of industrial by-product gypsum in China is mainly due to the different impurity in the wastewater and the different neutralization reaction conditions, resulting in the uneven quality of gypsum, which can not meet the use requirements. Therefore, it is of great significance for the economic development of ecological environment to improve the quality of by-product gypsum .
There are several economic and technically feasible methods to treat wastewater containing sulfuric acid and produce gypsum. One method is to improve the quality of gypsum by controlling the process conditions of wastewater treatment, such as selecting appropriate chemicals, controlling temperature, pH, residence time, slurry return ratio and other factors to improve the quality of gypsum, so as to reach the corresponding standard of gypsum products and building raw materials.
Another method is to change the crystal structure of gypsum by controlling the transformation conditions of each crystal phase of gypsum to prepare high-quality high-strength gypsum. The advantages of high-strength gypsum are high strength and good purity, which can be used for the preparation of ceramic mold gypsum and the production of plastic absorbing mold.
As the quality of industrial by-product gypsum varies greatly, the market standards for different industrial by-product gypsum are not unified and perfect, which increases the difficulty of comprehensive utilization of industrial by-product gypsum. Therefore, it is urgent to establish and improve the requirements and standards for the application of industrial by-product gypsum .There are some development prospection about by-product gypsum we need to focus on [2]:
1. Promote the development of gypsum building materials technology and equipment. The uniformity and stability of raw materials must be solved in industrial scale production. The technology and equipment are the important links to ensure the quality of gypsum building materials. Guided by the "made in China 2025" action plan, we should promote gypsum building materials and equipment with modernization, high-end and intellectualization .
2. Accelerate the research and application of harmless treatment of industrial by-product gypsum. Industrial by-product gypsum contains alkali metal salts, organic compounds and other harmful impurities, which have seriously affected the popularization and application of gypsum building materials products. It is necessary to accelerate the research and application of harmless treatment of industrial by-product gypsum, especially the harmless treatment at the discharge end.
