2.2.1 Curing/stabilization treatment
The traditional harmless treatment of EMR can be divided into sorting processingt, curing treatment and so on(Shu et al, 2016;Lan et al, 2021). Sorting processing can sort out the qualified raw materials by using the different physical and chemical properties of EMR and then can be used to produce electrolytic manganese. Solidification treatment is the most primary way of harmless treatment of EMR, including lime curing, cement curing, fly ash curing, self-glue curing, etc(Vinter et al, 2016;Chuwi et al, 2012). By using cement and other solidification materials to fix the harmful elements such as heavy metals in EMR into inert basic materials,the slag can be used rationally.
Mu et al(2020) used composite portland cement (PC32.5R) as curing agent to cure / stabilize EMR and the results showed that manganese-bearing mineral in EMR, MnSO4·2H2O, (NH4) 2Mn (SO4) 2·6H2O and soluble Mn, transformed into more stable insoluble mineral phases such as manganese calcium pyroxene (CaMnSi2O6) and manganese silicate (MnSiO3) after curing by PC32.5R. Meanwhile, hydrated gel can bind all the components in EMR and effectively inhibit the dissolution of Mn in EMR solidified body. Some studies say that cement has poor fixation effect on ammonia nitrogen in electrolytic manganese slag, and the solidification after treatment will produce different degrees of volume increase, and more land resources will be wasted in the subsequent stacking process. EMR-cement gel system has limited curing capacity for NH4+ and the solidified body releases NH3 for a long time, which is easy to cause secondary pollution and is not conducive to the production and utilization of resource products. Zhou et al(2013) found that CaO is safer and more convenient than NaOH by studying the effect of additive types on Mn2+ solidification and ammonia nitrogen removal efficiency. With the analysis of immobilization mechanism. Du et al(2020) discovered that calcium oxide and sodium bicarbonate are beneficial to immobilize the soluble manganese of precipitation and oxidation products. The chemical stability experiment process is shown in Fig. 2. In practice, lime is not used solely for the cementation and solidification of EMR, and is mostly used in combination with fly ash, cement and other materials(Malviya et al, 2006). The curing efficiency of EMR-based cementitious materials ( EMR-P ) prepared by Lan et al(2021) for ammonia and manganese is over 95 %, and its unconfined compressive strength is over 18MPa, This is a new method for filling electrolytic manganese slag in mines. The technical process is shown in Fig. 3 below. The solidification and stabilization of Mn2 + and NH4 + - N in electrolytic manganese slag by Shu et al(2018). using MgO and different phosphate resources showed that Mn2 + was mainly stable in the form of Mn(H2PO4)2·2H2O、Mn3(PO4)2·3H2O、Mn(OH)2、MnOOH, and NH4+-N was stable in the form of NH4MgPO4·6H2O. The experimental principle diagram is shown in Fig. 4 below. Xue et al(2020) synthesized a new composite cementitious material with ground granulated slag, clinker, lime and EMR as raw materials. The analysis shows that gypsum is conducive to the rapid stability of heavy metals (except manganese). At the same time, CSH gel can effectively immobilized all heavy metals by HM-gel reaction, thus ensuring the environmental protection of the new material. Similarly, Zhang et al(2020) used blast furnace slag, red mud and carbide slag as stabilization / solidification ( S / S ) agents to treat electrolytic manganese slag. The principle is shown in Fig. 5. The charge balance effect produced Mn2SiO4 and Ca4Mn4Si8O24, and formed new chemical bonds. Meanwhile, Mn2 + was oxidized to more stable MnO2.
2.2.2 Resource utilization
Because the composition of EMR is becoming more and more complex, its harmless treatment is becoming more and more difficult, and meanwhile, in order to alleviate the great pressure on the environment caused by the slag, some scholars have carried out a large number of EMR resource utilization work. The recycle and utilization of manganese in EMR is an important way to make full use of the slag. Biological method(Lan et al, 2021) acid leaching method(He et al,2020) water washing precipitation method(Baumgartner et al, 2014) are commonly used. Due to its mineral composition, EMR is mostly recycled as cement materials, road base, brick making materials, heavy metal adsorption materials and chemical fertilizer production in agricultural field.
Ⅰ Manganese recovery
In recent years, because of the increasing lack of natural resources and increasing attention to environmental protection. The recycle of manganese from waste residue, by extracting Mn with heated sulfuric acid or hydrochloric acid solution as solvent, has attained more and more attention. Li et al(2015) used it as raw material to produce MnO2 by two-step method. The recycling process is shown in Fig. 5. The XRD spectrum of MnO2 ,synthesized by EMR, shows that the characteristics of γ-MnO2 XRD spectrogram no other phase is detected, indicating that the purity of the final product is high. EDX analysis proved that the purity of the finished product is as high as 90.3%, which can be directly used as chemical raw materials or raw materials to manufacture electrode materials. In the study of ultrasonic-assisted extraction of manganese from EMR, Li et al(2008) found that particle size parameters had the greatest influence on the extraction effect of manganese. Under specific extraction conditions, the efficiency of ultrasonic extraction of manganese could reach about 90%, which proved that ultrasonic-assisted extraction technology was more advantageous than traditional technology.
Ⅱ Cement material
Wang et al(2013) added EMR as an activator to Ground granulated blastfurnace slag (GGBS) to prepare EMR-GGBS cement. Through the research on the activity index and compressive strength, it shows that the cement strength exceeds that of slag silicate cement (P· S) 32.5 levels, even reaching the level of P· S 42.5 and 52.5. The sulfur concrete prepared by Wang et al(2014) from EMR has high compressive strength and good durability. EMR, aggregate and modified sulfur are uniformly dispersed to form a compact packing structure. The detection showed that the leaching content of heavy metal ions was lower than the specified value in the national integrated wastewater discharge standard ( GB8978-1996 ), and it was not harmful to the environment.
Ⅲ Road base material
Zhang et al(2019) used EMR and other solid wastes to prepare road base materials. The preparation process is shown in Fig. 7(Zhang et al, 2019). The experiment proved that EMR-RM-CS had high unconfined compressive strength and durability, and the leaching test results were in line with China 's groundwater standards, indicating that EMR-RM-CS system could solidify heavy metals well, providing a new direction for the resource utilization of EMR. Chen et al(2021) verified the feasibility of the application of EMR in highway subgrade. In order to remove manganese and ammonia nitrogen in EMR, 10 % lime was used for curing treatment. Considering that the slope height in actual is generally 8m ~ 12m, combined with the stability calculation results of numerical simulation, they put forward a section scheme, manganese slag highway slope height of 10m and slope ratio of 1:1, and the calculation results showed that heavy rainfall will also affect the stability of manganese slag slope at the same time.
Ⅳ Autoclaved Brick/Permeable Brick
Du and others(2014) used EMR to produce autoclaved bricks. Under the conditions of EMR mass ratio of 30-40%, cement mass ratio of 10.5-12%, formation pressure of 15-20Mpa, W / C ratio of 0.4 and steam pressure of 1.2-1.5Mpa, the compressive strength of autoclaved bricks is greater than 15Mpa, dry shrinkage is less than 0.11%, compressive strength loss is less than 10% and weight loss is less than 2%. Experiments showed that the compressive strength and leaching toxicity of the autoclaved brick meet the requirements of relevant standards. The C-S-H gel and calcium zirton in the brick sample form the pressure strength mechanism. Wang et al(2019) prepared non-sintered permeable brick with electrolytic manganese slag as raw material. The study found that the compressive strength of non-sintered permeable brick can reach 3.53Mpa after 28 days, meeting the requirements of GB / T25993-2010. Ammonia can also be recycled during the pretreatment of EMR. The leaching experiment of permeable brick showed that these harmful substances in EMR have been effectively solidified.
Ⅴ Heavy metal adsorption material
Li et al(2015) took EMR as raw material to prepare MnO2 by two-step method. The XRD spectrum of MnO2 synthesized by EMR showed the characteristics of γ-MnO2 XRD spectra and no other phase was detected which means that the purity of the final product was high. A kind of EMR-made calcium silicate hydrate has been prepared during the preparation process, which has good adsorption properties for diluted Mn2+ and phosphate ions in water and is a potential large-scale wastewater treatment material. Lan et al(2021) prepared a new material ( A- EMS ) for removing heavy metals by ball milling active electrolytic manganese slag. Adsorption and removal mechanisms of heavy metals are shown in Fig. 9. Powder A-EMS has remarkable effect on removing heavy metals in wastewater. Block A-EMS ( porous brick ) can be used to build barrier walls to prevent heavy metal leaching in tailings ponds. The leaching toxicity test results can meet the national standard ( GB / T3838-2002 ) limit. Therefore, A-EMS can be widely used to remove heavy metals in wastewater, intercept and solidify heavy metals in mine wastewater.
Although the harmless treatments and resource utilizations of EMR provide many directions for future development, most of them are in the laboratory stage. The resource utilization of manganese slag is difficult to mass production, and the efficiency of manganese slag treatment is difficult to achieve expectations. Another reason is that the cost of harmless or resource treatment is too high or the process is too complex to produce economic benefits. A single technology will not meet the huge demand of electrolytic manganese industry. Only by effectively integrating various technologies can the problem of electrolytic manganese slag be fundamentally solved. Among them, the building materials industry has the strongest consumption ability, and can carry out technical research in this field. The research on new environmental protection bricks in this paper is helpful to the large consumption of electrolytic manganese slag and create certain economic value.