Red mud is a kind of solid wastes produced in the process of alumina industrial production (Kong et al., 2017). About 1–2 tons of red mud will be produced for one ton of alumina. By 2018, the global red mud stockpile was reported to reach 4.6 billion tons and the annual output about 200 million tons (Xue et al., 2019). According to the production process, red mud can be classified as Bayer process red mud, sintering process red mud and combined process red mud. Bayer process red mud accounts for 95% of the total amount in the world (Power et al., 2011). A large amount of piled red mud poses a serious threat to the environment, especially the harm caused by the leakage of pollutants into water༈Jha et al., 2008; Ding et al., 2018; Bombik et al., 2020༉.
During the alumina production process, a large number of NaOH and certain kinds of fluorine-rich raw materials are needed, resulting in the red mud containing alkali, fluoride and other pollutants. Therefore, the red mud yard becomes the source of alkali pollution and fluorine pollution to the surrounding environment. Researches revealed that there are two kinds of alkali in red mud, namely soluble alkali and non-soluble alkali (Zhu et al. 2015; Xue et al. 2016a), also known as free alkali and bound alkali. Soluble alkali mainly includes sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium aluminate dihydrate, sodium silicate, potassium hydroxide and potassium carbonate, etc. (Xue et al. 2017); Non-soluble alkali mainly includes sodalite, nepheline, hydrated garnet, tricalcium aluminate, and calcite, etc. The soluble alkali is soluble in water and thus has significantly negative impact on the environment. Fluorine can be divided into water-soluble fluorine (Ws-F), exchangeable fluorine (Es-F), iron manganese bound fluorine (Fe/Mn-F), organic bound fluorine (Or-F) and residual fluorine (Res-F) (Muravyeva et al., 2014; Yuan et al., 2008). Tessier five-step extraction method was usually used for fluoride extraction (Tessier a et al. 1979; Xie et al., 1999; Wu et al. 2002). Water soluble fluorine (Zhang et al. 2020; Gao, 2013) and Es-F can be dissolved in water and thus have a great impact on the water environment. The influence of fluorine pollution depends not only on the total content of fluoride, but also on the form of fluorine. The greater the content of Ws-F and Es-F, the greater the negative influence on the environment.
When experiencing rainfall, pollutants will be released from the newly stacked red mud by the surface runoff in a short time. The Chinese standard GB/T 51238 − 2018 has regulated the requirements for compaction degree of stacked red mud. However, the actual compactness of the red mud is commonly found to fail to meet the specification due to mechanical compaction operation and other uncertain factors. For this reason, during the rainy seasons in the south area of the nation, rainwater will have different effects on pollutants released from red mud in the vertical runoff direction under different compaction conditions of stacked red mud. Therefore, in this work, constant head dynamic leaching test was employed to study the release and migration characteristics and the change of occurrence content of pollutants in the process of vertical seepage in red mud with different compaction degrees, so as to clarify the influence of different compactness on the release and migration of red mud pollutants. Through the above research, the environmental effects of water infiltration on red mud under different compaction conditions are clarified. Based on which, the scientific basis and technical guidance are provided for the prevention and control of main pollutants in red mud storage yard.