Application of layered double-layer hydroxide in synchronous prevention and control of acid production and heavy metal release of mining waste rock

Abstract

The invention discloses application of layered double-layer hydroxide in synchronous prevention and control of acid production of mining waste rocks and release of heavy metals, and the preparation of the layered double-layer hydroxide comprises the following steps: S1, adjusting the pH value of acidic mine wastewater to 4.0-5.0 by using alkali liquor, and carrying out induced reaction for 15-35 minutes to obtain first pretreated wastewater; s2, alkali liquor is continuously added into the reaction system to adjust the pH value to 8.4-9.0, a co-precipitation reaction is carried out for 50-80 min, and second pretreated wastewater is obtained; and S3, carrying out solid-liquid separation on the second pretreated wastewater, washing with deionized water until the supernate is neutral, drying, grinding and sieving to obtain the layered double-layer hydroxide. The layered double-layer hydroxide is used as a blocking agent for treating mining waste rocks, so that the mining waste rocks can be effectively prevented from synchronously producing acid and releasing heavy metals, the technical problem that the mining waste rocks release pollutants to the environment is solved, meanwhile, the treatment cost of mine wastewater is reduced, and AMD is treated from the source.

Description

Technical Field

[0001] The invention relates to the technical field of waste treatment, and in particular to an application of a layered double-layer hydroxide in synchronously controlling acid production and heavy metal release from mining waste rock. Background Art

[0002] Acid mine drainage (AMD) is a typical pollutant formed by the treatment of mining waste rock. Currently, there are many methods for treating AMD, including chemical neutralization, biological treatment, physicochemical method, etc. Among them, chemical neutralization method is widely promoted due to its advantages such as high treatment efficiency and strong applicability.

[0003] The AMD treatment method represented by the chemical neutralization method focuses on the treatment of terminal wastewater. However, the source of AMD is the release of acid and metal ions into the environment by mining waste rock, that is, mining activities will cause the continuous formation of AMD. Therefore, only by effectively treating the corresponding mining waste rock can AMD be reduced from the source. Although the chemical neutralization method is more widely applicable, it still has many problems: first, the method consumes a large amount of chemical reagents, and the various metal sludges formed with AMD also have the risk of secondary pollution, which requires special treatment and disposal, seriously increasing the treatment cost; second, mining waste rock will continue to form AMD under the action of rainwater infiltration, oxygen and microorganisms.

[0004] In order to prevent and control environmental pollution caused by mining waste rock, the commonly used methods for treating mining waste rock include: covering and vegetation restoration, waste rock pile closure, neutralization treatment, microbial treatment, water circulation and recovery, artificial wetlands, chemical precipitation, biological adsorption, thermal treatment and physical isolation, etc. The main purpose is to artificially isolate mining waste rock from contact and penetration with air and rainwater, slow down the oxidation process, use alkaline substances to neutralize the acid water that has been formed, close the acid water formed by the internal circulation of mining waste rock, destroy the sulfide in the waste rock and reduce its oxidation and acid production, etc. Among them, for the covering method, its treatment effect is not obvious; for the waste rock pile closure treatment and water circulation recovery method, although AMD is controlled, the cost is expensive; for chemical precipitation, biological adsorption and other methods, additional reagents need to be added.

[0005] After searching, the current treatment of mining waste rock mainly includes biological method and chemical stabilization method. Among them, the biological method mainly uses the growth process of microorganisms or plants to reduce the release of heavy metals from waste rock, such as document 1 (Organic Matter Amendment and Plant Colonization Drive Mineral Weathering, Organic Carbon Sequestration, and Water-Stable Aggregation in Magnetite Fe Ore Tailings. Environmental Science & Technology 53 (23) (2019) 13720-13731.) and document 2 (Inhibition of cadmium releasing from sulfide tailings into the environment by carbonate-mineralized bacteria. Journal of Hazardous Materials 419 (2021) 126479.). The chemical stabilization principle is to promote the precipitation or transformation of metal elements in waste rock to a stable state by adding materials through synthesis, such as Reference 3 (Modification of cemented paste backfill with calcined layered double hydroxides for lead-containing tailings disposal. Powder Technology 414 (2023) 118089.), Reference 4 (Chlorella sorokiniana FK-montmorillonite interaction enhanced remediation of heavy metals in tailings. Science of The Total Environment 876 (2023) 163208.) and Reference 5 (Immobilization mechanism of As, Mn, Pb and Zn ions in sulfide tailings by the addition of triethylenetetramine-montmorillonite nanocomposite. Chemical Engineering Journal 435 (2022) 134817.).However, the above-mentioned technologies have disadvantages such as slow effect, unstable treatment effect, and the need for synthetic preparation of materials. In particular, there is a large amount of mining waste rock in the mines, which undoubtedly increases the production costs of operators.

[0006] There are also related reports on technologies for preventing and controlling the acid production of waste rock, such as Reference 6 (Utilization of industrial waste products as pozzolanic material in cemented paste backfill of high sulphide mill tailings, Journal of Hazardous Materials 168 (2009) 848-856.) and Reference 7 (Use of sludge stabilization products for remediation of heavy metal (loid) s-contaminated mine tailings: Physicochemical, biochemical and microbial mechanisms, Chemical Engineering Journal 488 (2024) 150640.). However, the above technologies mainly use alkaline substances to cover the surface of waste rock to neutralize the acid water formed from the waste rock.

[0007] Regarding the synthetic layered double hydroxide (LDHs) materials, it has been reported that they are used in the remediation of heavy metal contaminated soil, such as reference 8 (Theoretical study on the mechanism of super-stable mineralization of LDHs in soil remediation, Chemical Engineering Journal 451 (2023) 138500). There are also related reports on obtaining layered double hydroxides (LDHs) by resource-based industrial by-products in mining areas, such as Chinese patent CN202210304940.4, which discloses that layered double hydroxides (LDHs) are obtained by regulating the chemical neutralization reaction process of AMD or other metal wastewater, but the technology of recycling them as a barrier agent to treat mining waste rock has not been reported. Summary of the invention

[0008] In view of the technical problems existing in the prior art, the purpose of the present invention is to provide an application of a layered double-layer hydroxide in the synchronous prevention and control of acid production and heavy metal release of mining waste rock. The preparation of the layered double-layer hydroxide refers to Chinese patent CN202210304940.4. By using the layered double-layer hydroxide to treat mining waste rock, the synchronous acid production and heavy metal release of mining waste rock can be effectively restricted, which not only solves the technical problem of mining waste rock releasing pollutants into the environment, but also reduces the treatment cost of mine wastewater, thereby controlling AMD from the source.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] The invention provides an application of a layered double-layer hydroxide in synchronously controlling acid production and heavy metal release from mining waste rock.

[0011] Preferably, the preparation of the layered double hydroxide comprises the following steps:

[0012] S1, adjusting the pH value of the acidic mine wastewater to 4.0-5.0 with an alkali solution, and performing an induction reaction for 15-35 minutes to obtain a first pretreated wastewater;

[0013] S2, continue to add alkali solution to the reaction system to adjust the pH value to 8.4-9.0, and perform coprecipitation reaction for 50-80 minutes to obtain a second pretreated wastewater;

[0014] S3, separating the solid from the obtained second pretreated wastewater, washing with deionized water until the supernatant is neutral, drying, grinding and sieving to obtain a layered double hydroxide (LDH).

[0015] Preferably, the molar ratio of divalent metal ions to trivalent metal ions in the acidic mine wastewater is 2-4.

[0016] Preferably, the alkali solution is a sodium hydroxide solution or a potassium hydroxide solution; the mass volume concentration of the alkali solution is 10-15%.

[0017] Preferably, the drying temperature is 70-90° C., and the mesh size of the ground sieve is 80-100 meshes.

[0018] Preferably, the layered double hydroxide is used as a barrier agent to treat mining waste rock. The specific treatment methods include: directly sprinkling the double hydroxide on the surface of the mining waste rock, or filling the double hydroxide at the bottom, in the middle of the mining waste rock, and mixing it with the waste rock, but are not limited to the above methods.

[0019] Preferably, before using the layered double-layer hydroxide to treat mining waste rock, the method further includes: crushing the mining waste rock to a particle size of 20 mesh-2 mm, and crushing the layered double-layer hydroxide to a particle size below 80 mesh.

[0020] Preferably, the mass ratio of the double-layer hydroxide to the mining waste rock is 1 to 3:100.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The present invention provides an application of a layered double hydroxide in the simultaneous prevention and control of acid production and heavy metal release from mining waste. Experimental results show that a "buffer-type" material is formed through resource utilization as a barrier to treat mining waste. Due to its effective "wrapping" of mining waste, it can effectively prevent mining waste from releasing acid and heavy metals into the environment, thereby achieving source reduction of acid mine drainage (AMD) and effectively reducing the environmental risk level of mining waste.

[0023] (2) In the present invention, since layered double hydroxide (LDH) has properties such as stable spatial structure, abundant -OH sites and excellent adsorption performance, it is recycled for treating mining waste rock, which not only reduces the environmental risk of acid mine drainage but also saves production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The XRD results of the neutralized sludge obtained by two-step chemical neutralization treatment of AMD;

[0025] Figure 2 Characterization results of the neutralized sludge obtained by two-step chemical neutralization treatment of AMD: (a) SEM; (b) TEM;

[0026] Figure 3 The XRD test results of mining waste rock samples;

[0027] Figure 4 Schematic diagram of the structure of LDHs treating mining waste rock under different filling methods: R1, R2, R3, and R4 represent the blank, LDHs filled in the middle of the column, at the bottom of the column, and mixed with mining waste rock, respectively;

[0028] Figure 5 The effect of LDHs in different filling methods on preventing the release of heavy metals from mining waste rock under simulated rainfall: (ae) are the concentration changes of Fe, Cu, Cd, Mn and Zn in the leaching solution; (f) is the heavy metal removal rate result; where L represents LDHs, D, M and Mix represent materials filled at the bottom, in the middle and mixed with the mining waste rock.

[0029] Figure 6Results of pH and acidity changes of leachate after long-term treatment of mining waste rock: (a) pH; (b) acidity; (c) acidity removal rate; (d) pH change of mining waste rock when LDHs are filled at the bottom, middle and mixed with waste rock (where L represents LDH, D, M and Mix represent LDHs filled at the bottom, middle and mixed with waste rock);

[0030] Figure 7 :(a) Ca in the leachate after long-term mixing 2+ Change results; (b) Mg 2+ Concentration changes; (c) total acidity concentration in the leachate; (d) LDHs contribution to acidity removal;

[0031] Figure 8 The results of the changes in the forms of heavy metals in mining waste rock after long-term treatment of LDHs and mining waste rock mixed filling: (ae) are Fe, Cu, Cd, Mn and Zn elements in mining waste rock; (f) is the toxic leaching result (where L-mix represents the mixed treatment of LDHs and mining waste rock, and Blank represents the blank experimental group);

[0032] Fig. 9 The following are: (a) Al2p orbital analysis results in LDHs; (b) Mg2p orbital analysis results in LDHs;

[0033] Fig.10 XRD results of mining waste rock after LDHs treatment;

[0034] Fig.11 The morphology of mining waste rock: (a) mining waste rock after LDHs treatment; (b) blank experimental group, i.e., untreated mining waste rock;

[0035] Fig.12 TEM test results of LDHs before and after use: (ab) LDHs before use; (cd) LDHs after use;

[0036] Fig.13 :(a) Stability of LDHs after use; (b) Environmental assessment index results after long-term treatment of mining waste rock. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] Terminology and explanation:

[0039] Mining waste rock: refers to the solid waste generated during the mining process, including low-grade ores, surrounding rocks, etc., which are mainly sulfide metal minerals. Since mining waste rock often contains heavy metals, it has a high environmental risk;

[0040] Acid mine drainage (AMD): A type of highly acidic, high-metal ion industrial wastewater generated by mining operations;

[0041] Causes of Acid Mine Drainage (AMD): Mining activities lead to the exposure of sulfide metal minerals to the environment, where they are formed through a series of physical, chemical and biochemical reactions under the combined action of microorganisms, rainwater and oxygen;

[0042] Chemical neutralization method: a method for treating acidic mine wastewater, which refers to adding alkaline reagents (such as NaOH) to AMD to precipitate metal ions in the water by increasing the pH of the water.

[0043] Example 1

[0044] (1) Testing of acid production potential of mining waste rock and determination of its components

[0045] The mining waste rock samples were obtained from a dumping site (GCJ0231.65235495°N118.63393207°E). The acid production potential, composition and element content of the mining waste rock were tested. The results are shown in Tables 1-2 and Figure 3 .

[0046] Table 1. Test results of acid potential of mining waste rock

[0047]

[0048] The results in Table 1 show that the net acid production pH, i.e., NAG-pH, of the mining waste rock sample is 2.2, and the net acid production, i.e., NAG, is 45.864 kg·H2SO4 t -1 Since NAG-pH≤2.5 waste rock is moderately or highly acidogenic, when NAG>40kg·H2SO4 t -1 The waste rock is considered to be severely acid-generating, so the mining waste rock sample belongs to the severe acid-generating type of waste rock.

[0049] Table 2. Element content analysis results of mining waste rock samples

[0050]

[0051] The elemental analysis results in Table 2 show that in addition to O and Si, the mining waste rock samples also include S and heavy metal elements such as Mn, Cu, Fe, Zn, etc.

[0052] Figure 3 The results show that the mining waste rock samples are composed of sulfide metal minerals and metal oxide minerals, including MnS, (Zn, Mn)S, CuS, MgO, (MgO) 0.841 (MnO) 0.159 , Cu2O, MnO2, Fe3O4, MnO2, CaCO3, CdCO3.

[0053] In summary, it can be concluded that mining waste rock samples contain a variety of heavy metals and have strong acid-producing capacity.

[0054] (2) Neutralize AMD to obtain (buffering properties) LDHs

[0055] First, actual acid mine drainage water was collected from a certain location, and its water quality parameters are shown in Table 3.

[0056] AMD was treated by a two-step chemical neutralization method, and the specific operation process was as follows: first, a 10% (w / v) NaOH solution was prepared as a neutralizing alkali solution, and then the prepared neutralizing alkali solution was slowly added to AMD to adjust the pH of AMD from 3.0 to 5.0, and the reaction was maintained at this pH for 20 minutes; then, the neutralizing alkali solution was continued to be added to raise the pH of AMD to 9.0, and the reaction was maintained at this pH for 60 minutes. After the reaction was completed, the obtained mud-water mixture was centrifuged to remove the residual alkali solution in the sludge, and the sludge was washed with deionized water until the supernatant was neutral, and the washing was stopped to obtain neutralized sludge. The obtained neutralized sludge was taken out, dried overnight in a forced air drying oven (75°C), and ground and sieved (80 mesh).

[0057] Table 3. Water quality parameters of acid mine drainage

[0058]

[0059] Note: Molar ratio M 2+ / M 3+ =3.58, (M represents metal ions); (2) wastewater pH = 3.0.

[0060] The neutralized sludge material after grinding and sieving was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and field emission transmission electron microscopy (TEM). Figure 1-2 .

[0061] Figure 1The results show that the main characteristic peaks (003), (006), (009) and (110) of the neutralized sludge material after grinding and sieving match those of layered double hydroxides (LDHs). Although some impurity peaks (such as the characteristic peak of Mn oxide) also appear at the same time, the intensities of the impurity peaks are significantly weaker than the characteristic peaks of LDHs. Therefore, it is believed that LDHs is the main component in the neutralized sludge material.

[0062] Figure 2 The results show that the neutralized sludge material has an obvious layered structure in its microscopic morphology, which is similar to the spatial structure of layered double hydroxides (LDHs).

[0063] The above results indicate that the present invention successfully prepares layered double hydroxides (LDHs), and therefore in subsequent experiments, layered double hydroxides (LDHs) are used to represent the neutralized sludge material.

[0064] (3) To explore the effect of LDHs in blocking acid production and heavy metal release from mining waste rock

[0065] (A) Using the above-obtained LDHs to treat mining waste rock, specifically, a method for synchronously preventing and controlling the acid production and heavy metal release of mining waste rock, comprising the following steps:

[0066] First, the mining waste rock was crushed to a particle size of 20 mesh-2mm, and the LDHs were crushed to a particle size of less than 80 mesh for standby use; the process of mining waste rock producing acid and releasing heavy metals into the environment was reduced by column experiments and simulated rainfall: 1wt% of the total mass of mining waste rock was used as the addition amount of LDHs. Considering that the acidic gas in the air is mainly CO2, CO2 was used to adjust the pH of distilled water to weak acidity (pH=5.0), and the rainfall in the summer flood season of a mine in a certain place was simulated. The 7-day rainfall (112mm) was converted into a reaction column volume of 125mL to ensure that the waste rock was continuously immersed after the rainwater was added to the reaction column. After 7 days of immersion, all the original rainwater was discharged and fresh rainwater was added again. In order to prevent the loss of LDHs and mining waste rock, an ultra-fine quartz sand filter layer was also set at the bottom of the reaction column. The indicator parameters in the leachate were monitored throughout the process, including heavy metals, pH and acidity. Finally, the operation was repeated until the leaching liquid index tended to be stable, then the reactor was disassembled, and then the LDHs and mining waste were characterized and analyzed. In addition, considering that the mixing method of LDHs and mining waste may affect the treatment effect, the treatment effect of LDHs mixed with mining waste (denoted as L-Mix) and LDHs separately filled in the middle (denoted as LM) and the bottom (denoted as LD) of mining waste was further investigated (see Figure 4 ).

[0067] (B) Evaluation of the performance of LDHs in treating mining waste rock:

[0068] (a) Determine the concentration of heavy metals (pollutants) released in the leachate. The results are shown in Figure 5 .

[0069] Figure 5 a- Figure 5 The results show that the heavy metal elements released by mining waste rock into the environment under long-term immersion in rainwater mainly include Fe, Cu, Cd, Mn and Zn. In the first 7 days, the release concentration of Mn exceeded 10 mg / L, Fe exceeded 3 mg / L, and the release concentrations of Zn, Cu and Cd were relatively low. After 7 days, the release of all the above heavy metals decreased significantly, indicating that the release of metal elements in mining waste rock mainly occurred in the first 7 days. The above results prove that the heavy metal elements in mining waste rock will pollute the environment under natural conditions.

[0070] Figure 5 fThe results show that the effect of LDHs in blocking the release of heavy metals from mining waste rock is affected by the filling method. When the material and waste rock are mixed and filled, the blocking rates of LDHs for Cu, Fe, Cd, Mn and Zn released from mining waste rock are approximately 73.08%, 62.77%, 64.59%, 85.56% and 58.48%, respectively; when the material is filled at the bottom of the waste rock, the blocking rates of LDHs for Cu, Fe, Cd, Mn and Zn released from mining waste rock are approximately 69.72%, 55.91%, 60.93%, 77.13% and 55.77%, respectively; when the material is filled in the middle of the waste rock, the blocking rates of LDHs for Cu, Fe, Cd, Mn and Zn released from mining waste rock are approximately 58.74%, 35.47%, 51.18%, 46.55% and 38.89%, respectively. It can be seen that the buffering effect of heavy metals is best when LDHs and mining waste are fully mixed and filled, followed by LDHs filled at the bottom of the waste, and finally LDHs filled in the middle of the mining waste. This is because the contact area between the mining waste and LDHs is increased under the condition of full mixing, so that the heavy metal ions released from the mining waste into the water body can be removed in time. Other filling methods do not have this advantage, especially when LDHs are filled in the middle of the mining waste, which will cause the bottom waste to release metals and cannot be fully combined immediately.

[0071] (b) The effect of LDHs treatment on the buffering capacity of leachate pH was evaluated by measuring the change in leachate pH, which is extremely important for inhibiting the acid production capacity of mining waste rock. The results are shown in Figure 6 .

[0072] Figure 6aThe results show that after 7 days of rainwater soaking, the pH of the mining waste rock leachate is 5.77, which indicates that the mining waste rock has a certain acid production capacity; after LDHs treatment, the pH of the leachate is improved to varying degrees, and the improvement effect is closely related to the filling method. When LDHs are mixed with mining waste rock and LDHs are filled at the bottom and middle of the waste rock, the pH of the leachate on the 28th day of treatment is 6.89, 6.84 and 6.34 respectively. This is because when LDHs are filled at the bottom and middle, LDHs are not in sufficient contact with the mining waste rock and the leachate, resulting in poor treatment effect.

[0073] (c) The inhibitory effect of LDHs treatment on acid production of mining waste rock was evaluated by testing the acidity of the leachate. The results are shown in Figure 6 .

[0074] Figure 6 b The results show that mining waste rock will continue to release acid to the outside world when soaked in rainwater, and the acid release amount will decrease with the extension of reaction time. This is because mining waste rock itself contains carbonate minerals (see Figure 3 ), after long-term immersion, alkalinity will be generated and a certain amount of acidity will be consumed. However, after adding LDHs, the acidity of the leaching solution was significantly reduced, especially after the 28th day of LDHs treatment, the acidity of the leaching solution was less than 20 mg / L.

[0075] The acidity removal effect of LDHs on the acidity in the leachate during the reaction period was calculated based on the acidity of the leachate in the blank group. Figure 6 cThe results show that when LDHs and waste rock are mixed and filled, the removal rate of acidity in the leachate is the best (50%). When LDHs are filled to the bottom and middle of the waste rock, the removal rates of acidity in the leachate are 40.77 and 34.62%, respectively. The pH of the waste rock also changes to varying degrees after long-term treatment. The pH of the blank group of waste rock is 5.79, which shows that even after long-term natural immersion, the waste rock still has the potential to release acidity into the environment. However, the addition of LDHs will increase the pH of the waste rock to varying degrees, but the improvement performance is affected by the filling method. Overall, the pH improvement effect of LDHs and waste rock mixed treatment conditions is the best (pH = 6.88), and the other pHs are 6.34 (filled at the bottom) and 6.57 (filled in the middle). The results show that the LDHs recovered from the neutralization treatment of AMD and the waste rock mixed filling have the best effect on blocking the release of heavy metals and acid production from the waste rock.

[0076] (d) Blocking the acid production mechanism of waste rock

[0077] The acid-generating performance of isolating waste rock is crucial to inhibiting the release of H+ and the formation of acidic water bodies. The relevant mechanisms are analyzed in depth below.

[0078] According to the above Figure 6 a and Figure 6 b The results show that the pH of the leaching solution in all experimental groups gradually increased and the acidity gradually decreased, among which the effect of the LDHs addition experimental group was more obvious, which shows that both waste rock and LDHs provide corresponding alkalinity. Therefore, it is necessary to determine the contribution ratio of mining waste rock and LDHs to acidity removal.

[0079] In addition to metal oxides, mining waste also contains carbonate minerals of Mg, Ca, and Cd ( Figure 3 ), elemental analysis results show that carbonate minerals are mainly composed of MgCO3 and CaCO3 (Table 3). When carbonate minerals are immersed in water for a long time, CO3 in the solid phase 2- Will hydrolyze to produce OH - . Determination of Ca in the leachate 2+ and Mg 2+ The concentration changes, as well as the total acidity concentration in the leachate and the contribution ratio of LDHs to acidity removal are shown in Figure 7 .

[0080] Figure 7 a and Figure 7 b The results show that waste rock immersion will be accompanied by Ca 2+ and Mg 2+ The release of metal ions was the highest in the first 7 days, and the metal ion concentration gradually decreased as the reaction progressed. In order to quantify the contribution of CaCO3, MgCO3 and LDHs to the removal of acidity in the reaction column, the acid removal concentration of the blank experimental group provided a reference for the LDHs addition experimental group. It was approximately assumed that carbonate dissolved to form CO3 2- The degree of hydrolysis was the same, because the waste rock in the blank experimental group released Mg 2+ and Ca 2+ The total concentrations of CaCO3 and MgCO3 were 533.7 and 122.4 mg / L, respectively, so CaCO3 and MgCO3 contributed 18.7% and 81.3% to the acidity removal, respectively, that is, MgCO3 in the blank group made a major contribution to the acidity removal. It was further found that after the addition of LDHs, the overall acidity in the leaching solution decreased significantly, with the total acidity concentrations in the blank group and the LDHs addition experimental group being 390.4 and 190.2, respectively.

[0081] Since the acidity concentration of simulated rainwater is 620.7 mg / L, the acidity removal concentration of LDHs in the LDHs experimental group was further calculated to be 200.2 mg / L ( Figure 7 c), LDHs, CaCO3 and MgCO3 contribute 46.5%, 10.0% and 43.5% to the acidity removal, respectively. It can be seen that waste rock itself has a removal effect on acidity, among which MgCO3 has the highest contribution, and LDHs further enhances the acidity removal effect ( Figure 7 d).

[0082] LDHs were characterized by XPS to prove their acid removal performance. Figure 8 .Depend on Figure 8 The results show that LDHs split Al-O (74.32 eV) and Al(OH)3 (73.45 eV) on the Al2p orbital, and split Mg-O (49.90 eV) and Mg-OH (48.91 eV) on the Mg2p orbital. Therefore, the above results confirm the acidity removal mechanism and the excellent pH buffering properties of LDHs.

[0083] (e) Mechanism of blocking the release of heavy metals from waste rock

[0084] Determining the changes in solid phase morphology and elements before and after waste rock and LDHs treatment can provide intuitive evidence for blocking the release of heavy metals. The results are shown in Table 4.

[0085] Table 4. Changes in the proportion of elements on the surface of materials before and after LDHs treatment of waste rock

[0086]

[0087]

[0088] The results in Table 4 show that before treatment, the proportions of C, Mg, Mn, Fe, Cu, Zn and Cd in LDHs were 5.88%, 21.58%, 3.82%, 0.48%, 0.64%, 0.37% and 0, respectively, and after treatment, the proportions of the elements were 6.80%, 17.55%, 4.52%, 1.52%, 0.73%, 0.49% and 0.03%, respectively. This change indicates that the metal ions in the leachate are fixed and precipitated by carbonate and ion exchange.

[0089] Ion replacement is also an important adsorption pathway for LDHs. Considering that the leachate contains many divalent metal ions, the Mg content in the leachate was further determined. 2+ The results showed that on the 7th day of waste rock leaching, the Mg content in the leaching solution 2+ The concentration was the highest, with the blank group at 533.7 mg / L, which was due to the release of magnesium-containing minerals in the waste rock ( Figure 7 b) Mg content in the experimental group treated with LDH and waste rock mixture 2+ The concentration was as high as 659.1 mg / L, and the concentration gradually decreased as the reaction progressed. Compared with the blank group, the Mg in the leaching solution of the LDHs treatment group was 2+ The increase in concentration was 432.5 mg / L, which indicated that metal cations were removed by LDHs through ion exchange.

[0090] In addition to the ion exchange mechanism, precipitation and complexation are also important barrier mechanisms for LDHs, so it is still necessary to determine the existence form of heavy metals after being fixed. XRD was used to analyze the components of LDHs after use, and the results are shown in Fig. 9 .

[0091] Fig. 9 The results show that the characteristic peaks of the used LDHs are still detected, which indicates that the structure of the recycled LDHs is relatively stable. In addition to the original characteristic peaks, the composition of LDHs also changes, especially the oxides and carbonates of Cd, Cu, Zn, and Mn appear. This indicates that the heavy metals released by the waste rock under long-term immersion are removed by adsorption on the one hand, and on the other hand, they are combined with CO3 in the simulated rainwater. 2- Combine the precipitates.

[0092] (f) Evaluate the long-term treatment effect of waste rock

[0093] Although the concentration of heavy metals released after long-term immersion does not change significantly, considering that mining waste rock is still rich in metal elements, it is necessary to further analyze the metal forms of the treated waste rock to determine its leaching toxicity concentration. Given that the treatment effect is best when LDHs and mining waste rock are mixed, the following blank experimental group is combined with the analysis of the metal forms of the waste rock after mixed filling treatment. The results are shown in Fig.10 .

[0094] Depend on Fig.10 a- Fig.10 The results show that on the 7th day of waste rock treatment, the total acid-extractable and reduced states of Fe, Cu, Cd, Mn and Zn in the blank group accounted for 8.1%, 17.2%, 61.2%, 27.4% and 13.9% respectively. After the 28th day of treatment, the proportion of unstable Fe, Cu, Cd, Mn and Zn in the blank group waste rock (the sum of acid-extractable and reduced states) was 2.8%, 14.0%, 26.3%, 11.1% and 8.0% respectively, which indicates that the metal ions in the leachate are mainly released from the acid-extractable and reduced states.

[0095] Furthermore, the metal forms in the waste rock of each experimental group after 28 days of treatment were analyzed. The results showed that the residual Fe, Cu, Cd, Mn and Zn in the blank group accounted for 57%, 50%, 42%, 31% and 42% respectively, and the metal stability of the waste rock in the LDHs addition group was only slightly improved compared with the blank group, among which the residual Fe, Cu, Cd, Mn and Zn accounted for 70%, 57%, 54%, 38% and 54% respectively, which showed that the addition of LDHs could not directly stabilize the acid-extractable and reduced metals in the waste rock to the residual state. Therefore, it can be concluded that LDHs plays the role of "barrier". When mining waste rock releases heavy metals and acidity into the water environment, the pollutants will be captured and fixed in time due to the full mixing of waste rock and LDHs, thereby reducing the environmental pollution risk of waste rock.

[0096] Fig.10 f is the metal ion concentration in the leachate of the experimental waste rock sample, simulating the leaching of toxic components in sanitary landfills and municipal solid waste. The results are used to further distinguish hazardous waste from ordinary waste. According to China's hazardous waste identification standards, the concentration thresholds of Cu, Zn, and Cd in the leachate are 100, 100, and 1 mg / L, respectively. However, the TCLP leachate concentrations in the waste rock after LDHs treatment are all lower than the hazardous waste identification standard values.

[0097] The bioavailability and biotoxicity of the main toxic metals in the waste rock after LDHs treatment were further evaluated. The results showed that the concentrations of Cu, Zn and Cd in the experimental group were all lower than 1 mg / L, but the concentrations of TCLP-Fe and Mn in the blank experimental group were 20.56 and 28.23 mg / L, respectively. Compared with the LDHs treatment group, the concentrations of TCLP-Fe and Mn in the LDHs treatment group were 12.48 and 14.92 mg / L, respectively, which were 50% lower than those in the blank experimental group. This shows that LDHs plays an important role in reducing the toxic concentration of toxic metals in the waste rock leachate and that the treated waste rock is a common waste.

[0098] The above results show that LDHs have the ability to prevent waste rock from releasing heavy metals and producing acid.

[0099] (4) Stability evaluation of LDHs

[0100] Considering that LDHs are typical metal sludge, and waste rock releases pollutants that are fixed by LDHs, and waste rock still releases pollutants during the subsequent soaking process, it is necessary to analyze the state of waste rock after treatment and evaluate the stability of LDHs after use. The state of waste rock after treatment and the structural morphology of LDHs before and after use were characterized, and the results are shown in Fig.11 .

[0101] Fig.11 a- Fig.11 b is the morphology of waste rock in the experimental group treated with LDHs and the blank experimental group. The results show that the morphological characteristics of waste rock in the blank group disappeared after LDHs treatment, and were replaced by flaky substances wrapped on the surface of waste rock particles. This may be due to the adhesion of LDHs to the surface of mineral particles after long-term immersion in water. It can be seen that the waste rock in the experimental group treated with LDHs was immediately fixed by the surrounding coating materials after releasing pollutants, which means that LDHs have the ability to block the release of heavy metals and acid production from waste rock.

[0102] Fig.12 a- Fig.12 b The transmission results show that LDHs are composed of multiple flake-like materials stacked to form a typical layered structure before use; Fig.12 c- Fig.12 d The results show that the layered structure of LDHs remains intact after use, that is, the spatial structure of LDHs is relatively stable.

[0103] In view of the complexity of the actual treatment environment and the fact that LDHs are metal sludge, it is necessary to identify the environmental stability of the material. The metal leaching concentration is an important criterion for the stability of the reaction material and the stability after metal ion fixation. The used LDHs were leached using a dilute sulfuric acid solution (pH = 3), and the LDHs had good stability. Specifically, the release of Cd, Cu, Fe, Mn and Zn by LDHs per unit material mass was 0.08, 0.20, 13.15, 17.56 and 3.85 mg / Kg, respectively. Therefore, the content of toxic metal ions in the leachate is less than 3% of the total. Referring to my country's "Hazardous Waste Identification Standards", it can be seen that LDHs are in a "safe" state after use as barrier materials (see Fig.13 a).

[0104] Further environmental risk analysis is conducted on the treated waste rock. Generally speaking, RAC≤1 is considered a safe level, 1%<RAC≤10% is considered a low-risk level, and 10%<RAC≤30% is considered a medium-risk level. Fig.13 b The results show that even if the waste rock in the blank group releases heavy metals into the environment, Cd is still at a medium risk level, Cu, Mn, and Zn are at a low risk level, and Fe is at a safe level. By comparison, LDHs reduced Cd from a medium risk to a low risk level, and reduced Cu, Mn, and Zn from a low risk to a safe level. In summary, LDHs are not only stable in nature, but also have the lowest release concentration after fixing metal ions, and the environmental risk of the waste rock treated with LDHs is effectively reduced.

[0105] Parts not involved in the present invention are the same as the prior art or can be implemented by the prior art. Although the embodiments of the present invention have been shown and described, it is understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. Application of a layered double hydroxide in the simultaneous control of acid production and heavy metal release from mining waste rock.

2. The use of the layered double hydroxide according to claim 1 in the simultaneous control of acid production and heavy metal release from mining waste rock, characterized in that: The preparation of the layered double hydroxide comprises the following steps: S1, adjusting the pH value of the acidic mine wastewater to 4.0-5.0 with an alkali solution, and performing an induction reaction for 15-35 minutes to obtain a first pretreated wastewater; S2, continue to add alkali solution to the reaction system to adjust the pH value to 8.4-9.0, and perform coprecipitation reaction for 50-80 minutes to obtain a second pretreated wastewater; S3, separating the solid from the obtained second pretreated wastewater, washing with deionized water until the supernatant is neutral, drying, grinding and sieving to obtain a layered double hydroxide.

3. The use of the layered double hydroxide according to claim 2 in the simultaneous control of acid production and heavy metal release from mining waste rock, characterized in that: The molar ratio of divalent metal ions to trivalent metal ions in the acidic mine wastewater is 2-4.

4. The use of the layered double hydroxide according to claim 2 in the simultaneous control of acid production and heavy metal release from mining waste rock, characterized in that: The alkali solution is a sodium hydroxide solution or a potassium hydroxide solution; the mass volume concentration of the alkali solution is 10-15%.

5. The use of the layered double hydroxide according to claim 2 in the simultaneous control of acid production and heavy metal release from mining waste rock, characterized in that: The drying temperature is 70-90° C., and the mesh number of the ground and sieved is 80-100 meshes.

6. The use of the layered double hydroxide according to claim 2 in the simultaneous control of acid production and heavy metal release from mining waste rock, characterized in that: The application of layered double hydroxide to treat mining waste rock includes the following methods: directly spreading the double hydroxide on the surface of the mining waste rock, or filling the double hydroxide at the bottom, in the middle, or mixed with the mining waste rock.

7. The layered double hydroxide according to claim 2 is used for synchronously controlling acid production from mining waste rock and Application of releasing heavy metals, characterized by: When using layered double hydroxides to treat mining waste rock, The mass ratio of the double-layer hydroxide to the mining waste rock is 1 to 3:100.

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