A method for fixing heavy metal zinc using calcium sulfate composed of different crystal phases

Abstract

This invention discloses a method for immobilizing heavy metal zinc using calcium sulfate with different crystal phase compositions. The method includes: quenching an equimolar mixture of sodium sulfate and calcium chloride solutions in an ethanol-water mixed solvent at 25–35°C, controlling the volume percentage of ethanol in the mixed solution to be 50–66%, and treating the zinc-containing solution so that the proportion of hemihydrate calcium sulfate in the resulting solid phase reaches 16% or more, thereby achieving the immobilization of metallic zinc. This invention effectively removes zinc from contaminated solutions by co-precipitating and immobilizing heavy metal zinc during crystal growth. The method is simple, has a short processing time, and low cost, and can retain substituted ions in the crystal lattice for a relatively long time, making it suitable as an engineering barrier for retaining various ions.

Description

Technical Field

[0001] This invention relates to a method for fixing heavy metal zinc, specifically a method for fixing heavy metal zinc using calcium sulfate with different crystal phase compositions. Background Technology

[0002] Acid mine drainage (AMD) left over from mining operations is characterized by a pH < 3 and contains large amounts of sulfates and toxic heavy metal ions such as cadmium (Cd), copper (Cu), iron (Fe), manganese (Mn), lead (Pb), zinc (Zn), nickel (Ni), chromium (Cr), arsenic (As), and selenium (Se).

[0003] Neutralization precipitation is the most widely used method for treating acidic AMD in engineering. Common neutralizing agents include low-cost lime and limestone (see Research on Prevention and Treatment Technology of Acidic Wastewater from Coal Mines [J]. Environmental Protection Technology, 2022, 28(5): 57-64). Although this method can effectively remove heavy metal ions and sulfates from acidic mine wastewater, it still consumes a lot of chemical reagents, easily generates secondary pollutants, has poor treatment effect on low-concentration wastewater, and is not effective for heavy metal ions (such as Zn) that are difficult to precipitate. 2+ It has drawbacks such as not being able to effectively remove (see A review of recent strategies for acid mine drainage prevention and mine tailings recycling[J]. Chemosphere,2019.219:588-606).

[0004] Zinc typically begins to precipitate at pH above 6, and completely precipitates at pH above 9. This makes it difficult for zinc to precipitate as hydroxide and be removed from the aqueous solution during the neutralization process of AMD. Therefore, when AMD contains highly soluble toxic heavy metals, such as Mn, it is particularly problematic. 2+ Ni 2+ and Zn 2+ A stronger neutralizing agent is needed to achieve a very alkaline pH (see A summary of passive and active treatment technologies for acid and metalliferous drainage (AMD). FIFTH AUSTRALIAN WORKSHOP ON ACID DRAINAGE, 29-31 AUGUST 2005, FREMANTLE, WESTERNAUSTRALIA).

[0005] Zinc is an essential trace element for the human body, but excessive zinc intake can harm the health of plants and animals. Therefore, research on zinc, a heavy metal in AMD (addictive disease), is worthy of attention. Methods for removing heavy metal ions from water bodies include ion exchange, bioremediation, adsorption, and co-precipitation. In the long run, ion exchange and adsorption methods are affected by competing ions, making them difficult to operate and prone to releasing adsorbed ions back into the water. Bioremediation is complex to operate and is significantly affected by the environment.

[0006] Calcium sulfate (CS) is the main component of gypsum. Based on its molecular formula and water of crystallization content, it can be classified into dihydrate calcium sulfate (DH), hemihydrate calcium sulfate (HH), and anhydrous calcium sulfate (AH). Under certain conditions, these three types of calcium sulfate can interconvert. DH is the most common naturally occurring calcium sulfate crystal form and is often used in AMD (Advanced Metallurgical Processing) as a co-precipitation material to remove various heavy metal ions. However, it is less effective for resolving the difficult-to-precipitate Zn. 2+ In contrast, the calcium sulfate dihydrate phase generated by the neutralization precipitation method cannot neutralize Zn. 2+ Reduce to above the national surface water / groundwater Class III standard (see Mechanism of heavy metal removal by limestone coupled with AMD endogenous iron oxide [J]. Environmental Science and Technology, 2021, 44(10): 89-97; Treatment of Mine Drainage at the Sarcheshmeh Porphyry Copper Mine, Iran. Mine Water Environment, 2011, 30: 216-230).

[0007] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0008] The purpose of this invention is to provide a method for immobilizing heavy metal zinc using calcium sulfate with different crystalline phases, solving the problem that existing dihydrate calcium sulfate phases cannot effectively immobilize Zn. 2+ To address the issue of reducing water quality to or above the national Class III standard for surface water / groundwater, this invention addresses the problem by co-precipitating and immobilizing heavy metal zinc during crystal growth, effectively removing zinc from contaminated solutions. The method of this invention is simple, has a short processing time, and is low in cost. Furthermore, it can retain substituted ions in the crystal lattice for a relatively long time, making it suitable as an engineering barrier for retaining various ions.

[0009] To achieve the above objectives, the present invention provides a method for fixing heavy metal zinc using calcium sulfate with different crystalline phase compositions. The method includes: quenching an equimolar mixture of sodium sulfate and calcium chloride solutions at 25–35°C using an ethanol-water mixed solvent, controlling the volume percentage of ethanol in the mixed solution to be 50–66%, and treating the zinc-containing solution so that the proportion of hemihydrate calcium sulfate in the resulting solid phase reaches 16% or more, thereby achieving the fixation of metallic zinc.

[0010] Preferably, the zinc-containing solution contains more than 20 mg / L of Zn.

[0011] Preferably, the concentrations of both the sodium sulfate solution and the calcium chloride solution are 1M.

[0012] This invention utilizes a method for immobilizing heavy metal zinc using calcium sulfate with different crystalline phases, solving the problem that existing dihydrate calcium sulfate phases cannot effectively immobilize Zn. 2+ Lowering the water quality to above the national surface water / groundwater Class III standard has the following advantages:

[0013] (1) The present invention effectively removes zinc from the contaminated solution by co-precipitation and fixing heavy metal zinc during crystal growth. The method of the present invention is simple, has a short processing time and low cost, and can retain substituted ions in the crystal lattice for a long time, which makes it a suitable engineering barrier for retaining Zn.

[0014] (2) The present invention utilizes calcium sulfate with different crystal phases to fix heavy metal zinc. It uses a lower amount of organic matter to improve the effect of neutralization precipitation in removing Zn. By adjusting the alcohol-water ratio of the solution, calcium sulfate phases with different crystal phases are synthesized, and calcium sulfate solid containing hemihydrate calcium sulfate phase is synthesized at room temperature. When the proportion of hemihydrate calcium sulfate reaches a certain level, the calcium sulfate phase can reduce Zn to a certain range above the emission level III standard. Attached Figure Description

[0015] Figure 1 The XRD patterns are of calcium sulfate phases with different crystal phase compositions according to the present invention.

[0016] Figure 2 This is a distribution diagram of calcium sulfate hemihydrate (HH) and calcium sulfate dihydrate (DH) with different crystal phases according to the present invention.

[0017] Figure 3 The diagram shows the performance of calcium sulfate with different crystal phases in fixing Zn according to the present invention.

[0018] Figure 4 The images show the fixation effects of samples 1 and 3 on Zn ions with different contents. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that: for conditions not specifically specified in the examples, standard conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0021] In this invention, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are used for simplicity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible secondary ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0022] The features mentioned in this invention can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification, provided that there is no contradiction in the combination of these features. Each feature disclosed in the specification can be replaced by any alternative feature that provides the same, equivalent, or similar purpose. Therefore, unless otherwise specified, the disclosed features are merely general examples of equivalent or similar features.

[0023] Example 1

[0024] A method for immobilizing heavy metal zinc using calcium sulfate with different crystalline phase compositions, taking an aqueous solution of zinc sulfate heptahydrate containing 1 g / L (0.0035 mol / L, zinc content 230 mg / L) as an example, the method includes:

[0025] Equimolar amounts of 1M sodium sulfate (10 mL) and calcium chloride solution (10 mL) were mixed, and zinc sulfate heptahydrate aqueous solution was added to fix the initial Zn ion content in the solution to 100 mg / L. The mixed solution was poured into ethanol-water mixed solvents with different volume ratios within the range of 25-35℃ to quench the reaction. The mixture was shaken for a few seconds, and the volume percentage of water in the mixed solution was controlled to be 60%, 50%, 40%, 38%, and 34%, respectively, to achieve the fixation of metallic zinc. The resulting solid phases were recorded as sample 2, sample 3, sample 4, sample 5, and sample 6, respectively.

[0026] After the reaction is complete, the remaining Zn content in the solution is measured.

[0027] Comparative Example 1

[0028] The method for immobilizing heavy metal zinc using calcium sulfate dihydrate (DH) phase, taking an aqueous solution containing 1 g / L zinc sulfate heptahydrate as an example, includes:

[0029] Prepare 10 mL of 1 M H2SO4 solution and add it to an Erlenmeyer flask. Weigh out an equimolar amount of calcium hydroxide (0.7412 g), pour the calcium hydroxide into the aforementioned Erlenmeyer flask, add a certain amount of water (UP) to make a total volume of 100 mL, mix to form lime milk, add zinc sulfate heptahydrate aqueous solution, fix the initial Zn ion content to 100 mg / L, stir and react for 3 h, and record the resulting solid phase as sample 1.

[0030] After the reaction is complete, the remaining Zn content in the solution is measured.

[0031] Comparative Example 2

[0032] The method for fixing zinc using anhydrous calcium sulfate (AH), taking an aqueous solution containing 1 g / L zinc sulfate heptahydrate as an example, includes:

[0033] Weigh 0.15g of SDBS and shake it evenly in 3mL of UP water. Then pour in 20mL of cyclohexane and stir for 30min. Then add 10mL of 30wt% sulfuric acid and stir for 3h. Then add 0.3g of calcium carbonate solid and 0.0057g of zinc sulfate heptahydrate and stir for 30min. The resulting solid phase is recorded as sample 0.

[0034] Qualitative Study of Experiment Example 1

[0035] like Figure 1 The figure shows the XRD patterns of calcium sulfate phases with different crystal phase compositions (calcium sulfate hemihydrate HH and calcium sulfate dihydrate DH) obtained in Example 1 of the present invention and the calcium sulfate dihydrate phase obtained in Comparative Example 1. Sample 1 is the pure calcium sulfate dihydrate phase obtained from Comparative Example 1 when the HH content is 0. Samples 2 to 6 are calcium sulfate phases composed of HH and DH with gradually increasing HH content obtained from Example 1. It can be seen that as the ethanol content in the ethanol-water mixed solvent increases, the HH content in the calcium sulfate phase increases.

[0036] Experimental Example 2 Quantitative Study

[0037] For the solid phases obtained in Example 1 and Comparative Example 1, i.e., samples 1-6, the water content of the solid phase was quantified by thermogravimetric analysis and differential scanning calorimetry (TG-DSC). Assuming the solid phase contains x mol of DH (calcium sulfate dihydrate) and y mol of HH (calcium sulfate hemihydrate), and the water content is A, then according to the theory, the water content of DH is 20.93%, and the water content of HH is 6.21% (referring to Role and Fate of the Lead During the Conversion of Calcium Sulfate Dihydrate to Alpha-Hemihydrate Whiskers in Ethylene Glycol-Water Solutions [J]. Chemical Engineering Journal, 2019, 372: 74-81). The molar fraction of HH in the solid was calculated using the following formula:

[0038] (172.16g / mol×x mol)×20.93%+(145.14g / mol×y mol)×6.21%

[0039] =(172.16g / mol×x mol+145.14g / mol×y mol)×A%

[0040] like Figure 2 The figure shows the distribution of calcium sulfate hemihydrate (HH) and calcium sulfate dihydrate (DH) with different crystal phase compositions obtained in Example 1 of the present invention. The proportion of HH gradually increases in samples 2 to 6.

[0041] Experimental Example 3: Performance Study of Fixed Zn

[0042] The zinc fixation effects of the methods in Example 1, Comparative Example 1, and Comparative Example 2 were compared, and q was calculated. e Fixed capacity, calculated as follows:

[0043] q e = (Initial Zn ion content - Remaining Zn content in solution) / Theoretical amount of calcium sulfate precipitate to be formed

[0044] like Figure 3 The above is a performance diagram of calcium sulfate with different crystal phases for fixing Zn according to the present invention. It can be seen that as the proportion of HH in the solid phase gradually increases, the capacity of the solid phase to fix Zn gradually increases. Moreover, the fixing capacity of calcium sulfate phase containing a certain proportion of HH is better than that of pure phase DH and anhydrous AH. After HH gradually increases to a certain proportion, i.e., sample 3, it gradually tends to reach equilibrium.

[0045] Experimental Example 4: The effect of Sample 3 on the fixation of Zn ions with different contents

[0046] Based on the general concentration range of Zn in AMD (Advanced Microorganisms) both domestically and internationally, four common concentration points were selected for comparison. Specifically, zinc sulfate heptahydrate aqueous solutions with Zn concentrations of 20, 25, 30, and 40 mg / L were treated to compare the Zn removal effects of two calcium sulfate phases. The experiments are as follows:

[0047] 1) Following the method in Example 1, the volume percentage of water in the mixed solution was controlled to be 50% to treat the zinc sulfate heptahydrate aqueous solution;

[0048] 2) The zinc sulfate heptahydrate aqueous solution was treated according to the method in Comparative Example 1.

[0049] like Figure 4 As shown, the horizontal axis represents the residual Zn content measured in the solution. The results show that when the proportion of hemihydrate calcium sulfate reaches 16% or more, the calcium sulfate phase composed of HH and DH can reduce the Zn concentration from 40 mg / L to below 1 mg / L, reaching or exceeding the Class III standard.

[0050] In summary, the method for fixing heavy metal zinc using calcium sulfate with different crystalline phases of the present invention improves the effect of Zn removal by neutralization precipitation by using a lower amount of organic matter. By adjusting the alcohol-water ratio of the solution, calcium sulfate phases with different crystalline phases are synthesized, and calcium sulfate solid containing hemihydrate calcium sulfate phase is synthesized at room temperature. When the proportion of hemihydrate calcium sulfate reaches 16% or more, the calcium sulfate phase can reduce Zn from 40 mg / L to below 1 mg / L, reaching or exceeding the Class III standard.

[0051] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method for immobilizing heavy metal zinc using calcium sulfate with different crystalline phase compositions, characterized in that, The method includes: Mix 10 mL of 1 M sodium sulfate solution with 10 mL of calcium chloride solution, and add zinc sulfate heptahydrate aqueous solution to fix the initial Zn ion content in the solution to 100 mg / L. Quench the mixture in ethanol-water mixed solvents with different volume ratios within the range of 25-35 °C, shake for a few seconds, and control the volume percentage of water in the mixed solution to be 40%, 38%, and 34% respectively to fix metallic zinc, so that the proportion of calcium sulfate hemihydrate in the resulting solid phase reaches 16% or more.

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