As previously described, the leachate pH was adjusted by a diluted alkaline solution to avoid the hydrolysis of metallic ions as insoluble substances (Figures 1A, B). For instance, when the pH was adjusted from 2 to 3, the loss of Cu steadily increased from 0.7% to 6.7%, alongside the removal of 76%–97.1% Fe and 3.6%–12.5% Al. Such an insoluble substance was a polymetallic mixture, in agreement with the loss of valuable metal and the formation of secondary waste (Figure 1C) (Zhu et al., 2020b). In addition, such adjustment was commonly tedious but was also the main step in the hydrometallurgy process. In our pilot-scale experiment, the Ni-rich leachate was adjusted by a diluted sodium hydroxide solution, and then, the aggregated greenish flocs were generated (Figure 1D). Such flocs were suspended in the leachate and cannot be separated by plate-frame pressure filtration. In some industrial processes, the acid leachate was also adjusted by an alkaline solution to maintain a desirable pH range and then extracted by special extracting reagents, e.g., P204 (Yuxin et al., 2023), LIX984, and Cyanex 302 (Soeezi et al., 2020). For instance, the Cu/Ni-rich leachate of the Sarcheshmeh tail was adjusted to pH 2–3 and then extracted by 10% LIX984 to separate Cu, followed by adjusting to pH 4–5 for extracting Ni by 30% Cyanex 302 (Soeezi et al., 2020).

To avoid such drawbacks, some reagents can be considered. For instance, the solid urea was homogeneously dissolved into the leachate at room temperature, whilst leachate pH did not change apparently. After that, the leachate was heated to 80°C and then agitated constantly to decompose urea as ammonia with the capture of free H⁺ (Figure 2A), resulting in an increase in leachate pH (Schaber et al., 2004; Guo et al., 2021). The H⁺ consumption was homogeneous to avoid droplet hydrolysis/flocculation. The urea decomposition was associated with some parameters, e.g., the initial pH value, the reaction temperature, and the urea dosage, so that the corresponding pH could be controlled. Other organics showed a similar decomposition reaction to urea and can also be used in pH adjustment.

Under atmospheric conditions, the redox reaction of nitrite to organics or low-valence metallic ions also consumed the free H⁺ (Figure 2A). Such reactions occurred rapidly, not only to induce the metallic precipitation but also to release yellowish nitrogen dioxide, as shown in the bubbles in Figure 2E. When the nitrite was replaced by oxygen gas, the oxidation of metallic ions also took place at room temperature to consume free H⁺, but that of organics did not occur.

Heating the leachate at high temperatures was often accompanied by thermochemistry reactions, including metallic hydrolysis and crystallization, wet oxidation, and pressure dissolution of gases. Especially, the metallic ions were rapidly hydrolyzed at high temperatures and then almost simultaneously converted into highly crystallized minerals. As previously described, the free Fe was hydrothermally hydrolyzed as a hematite block with a drop in leachate pH (Figure 2B) (Lin et al., 2019; Zhu et al., 2020b). Without hydrothermal treatment, the free Fe was stable in the leachate but hydrolyzed as Fe colloid by pH adjustment (Figure 1A), and a portion of Fe colloids was combined as Fe-bearing flocs to cover the free Cu, resulting in the loss of nearly 30% Cu (Zhu et al., 2020b; Jin et al., 2022). In addition, the Fe-bearing flocs showed more coordination sites to adsorb heavy metals compared with hematite (Jianmin, 1994; Qu et al., 2019). At high temperatures, the conversion of Fe colloids to hematite was instantaneous, and thereby, such adsorption drawbacks were effectively repaired. However, with the hydrolysis of free Fe as hematite, free H⁺ was released and accumulated in the leachate, which then reversely inhibited Fe hydrolysis. Other metallic ions showed similar performance to Fe. Therefore, the generated H⁺ should be consumed synchronously to continue the metallic hydrolysis in the high-temperature reaction.

Given that the high temperature was performed in sealed vessels, the following pH adjustment was difficult and can be considered in the following three ways. First, a small number of organics showed a similar structure to urea and decomposed as ammonia to neutralize free H⁺ at a high temperature of >100°C. Second, the redox reaction of nitrite to organics spontaneously occurred at room temperature, but when the nitrite was replaced by nitrate, the corresponding reaction of nitrate to organics was slow at room temperature and started at a temperature of >120°C (Figure 2A) to effectively consume free H⁺. In general, 1 mol of nitrate was reduced by organics with the consumption of 1 mol of free H⁺ (Wieczorek-Ciurowa and Kozak, 1999). Inevitably, yellowish nitrogen dioxide was also generated after opening the vessel (Figure 2G). With the consumption of free H⁺, the metallic hydrolysis continued (Figure 2F). Third, a number of organic acids were used in the leaching of waste slag/sludge and then in the organic anion form. Such organic anions can be removed via wet oxidation with the injection of oxidizing gases, e.g., O₂ and air, in which the free H⁺ was consumed to steadily increase the leachate pH. When the gas was bubbled into the sealed vessel, the decompression valve was opened to release the generated and unreacted gases so that the leachate and the oxidant were mixed to accelerate the following redox reaction. This operation is similar to the oxygen pressure leaching involved in the selective recycling of valuable metals from ores/minerals (Padilla et al., 2007). It is also found that some impurities of Fe/Al were removed as highly crystallized minerals after hydrothermal treatment, whilst the heavy metals were rarely removed and remained at high concentrations in the leachate (Figures 2B, H).

With such methods, some reagents also entered the leachate and became residues after the reaction. The residues included the generated ammonia, the decomposed organics, and the unreacted nitrate. In our previous research, the rest nitrate was 360 mg/L after reaction with the overdose of methanol (Zhu et al., 2020b) but varied to nearly 30 g/L when the added glucose was inadequate (Bian et al., 2022). It is also found that the rest nitrate and TOC were hundreds of mg/L even at optimal conditions. Such rest impurities should be considered in the following purification of valuable metals from the leachate.