Composition and method for removing hardness of acid wastewater

Abstract

The invention belongs to the technical field of industrial wastewater treatment, and particularly relates to a composition and method for removing hardness of acid wastewater, and the composition comprises a composite agent and activated filter residues; the composite agent comprises the following components in parts by mass: 100 parts of oxalic acid; 1-10 parts of disodium hydrogen phosphate, 0.5-5 parts of sodium fluosilicate and 0.1-0.3 part of a surfactant; the mass ratio of the activated filter residues to the composite agent is (0.2-0.5): 1; the activated filter residues are prepared by activating filter residues after solid-liquid separation. Through the synergistic effect of oxalic acid, disodium hydrogen phosphate and sodium fluosilicate, synchronous deep removal of calcium and magnesium ions is realized; meanwhile, through cyclic utilization of activated filter residues, the precipitation efficiency and the solid-liquid separation performance are further improved, and the overall hardness removal rate is increased to 90% or above.

Description

Technical Field

[0001] This invention relates to a composition and method for removing hardness from acidic wastewater, belonging to the field of industrial wastewater treatment technology. Background Technology

[0002] In industrial processes such as hydrometallurgy, metal surface treatment, and mining, large quantities of highly acidic wastewater are often generated. The pH value is typically below 2.0, and it contains high concentrations of calcium ions (Ca). 2+ ), magnesium ions (Mg 2+ This wastewater contains hardness ions such as sulfate, fluoride, silicate, and other heavy metal ions. If this type of wastewater is directly reused or enters a membrane treatment system (such as reverse osmosis or electrodialysis), the calcium and magnesium ions in it readily combine with sulfate, fluoride, and silicate ions to form hard scale such as calcium sulfate, calcium fluoride, and magnesium silicate. These deposits can severely clog pipes, filter media, and membrane modules, reduce heat exchange efficiency, and even lead to system shutdown for cleaning, seriously affecting the continuity and economy of production.

[0003] Currently, chemical precipitation is widely used in industry to remove hardness. This involves adding alkaline agents such as lime, soda ash, or sodium hydroxide to raise the pH of wastewater to 10-11, causing calcium ions to precipitate as calcium carbonate and magnesium ions as magnesium hydroxide. However, this method has significant drawbacks: First, neutralizing strongly acidic wastewater to a strongly alkaline state requires a large amount of alkali, resulting in high costs and completely destroying the original acidic state of the wastewater. If the treated effluent needs to be reused in acidic processes, acid must be added again for pH adjustment, which not only increases acid consumption and operational complexity but also further drives up treatment costs. Second, the calcium carbonate and magnesium hydroxide produced by this method are mostly amorphous, high-moisture-content flocculent sludge with poor settling and dewatering performance, leading to high sludge disposal costs. Furthermore, during co-precipitation, these flocculents easily adsorb or encapsulate valuable metal ions (such as copper, nickel, and cobalt) in the wastewater, resulting in resource loss. Raising the pH can also cause other heavy metal ions (such as iron, aluminum, zinc, and manganese) to form hydroxide precipitates, resulting in the ineffective consumption of alkali agents and potentially causing secondary pollution problems.

[0004] In recent years, to overcome the shortcomings of alkaline precipitation methods, attempts have been made to directly use oxalic acid to remove hardness under acidic conditions. Theoretically, this method does not require pH adjustment and utilizes oxalic acid to form calcium oxalate precipitate with calcium ions. However, practice has shown that this process has significant limitations: firstly, the generated calcium oxalate crystals are small in size, making complete separation through conventional filtration difficult, resulting in still high residual hardness in the effluent; secondly, under acidic conditions, when magnesium ions are present in the wastewater, oxalic acid forms a soluble magnesium oxalate complex, which not only fails to effectively precipitate and remove magnesium but also interferes with the precipitation effect of oxalic acid on calcium ions, leading to a low overall hardness removal rate, typically difficult to consistently reach above 50%, thus failing to meet stringent industrial reuse or discharge standards.

[0005] To address the aforementioned issues, existing technologies have attempted to improve hardening removal efficiency through composite agents, achieving, to some extent, simultaneous removal of calcium and magnesium. However, in practical applications, this method still faces problems such as low precipitate separation efficiency and difficulty in directly recycling filter residue. In particular, the calcium oxalate crystals formed during precipitation are still mainly fine needle-shaped, requiring further improvement in filtration performance. Furthermore, the large amount of filter residue generated increases disposal costs and hinders resource recycling.

[0006] Therefore, existing technologies, whether traditional alkaline precipitation, emerging acidic oxalic acid precipitation, or combined reagent methods, cannot achieve efficient and selective removal of calcium and magnesium hardness ions while maintaining the acidic environment of the wastewater system, and simultaneously solve the problem of difficult precipitate separation. There is an urgent need in this field to develop a novel acidic hardness removal technology that can efficiently and selectively precipitate calcium and magnesium ions without introducing external alkalinity or disrupting the system's acidity, and can induce the formation of easily separable crystalline precipitates, thereby fundamentally improving the efficiency, stability, and economy of hardness removal from acidic wastewater. Summary of the Invention

[0007] In view of the above-mentioned deficiencies of the prior art, the present invention provides a composition and method for removing hardness from acidic wastewater.

[0008] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: One objective of this invention is to provide a composition for removing hardness from acidic wastewater, comprising a composite agent and an activated filter residue, wherein the mass ratio of the activated filter residue to the composite agent is 0.2 to 0.5:1. The compound agent comprises the following components by mass: 100 parts oxalic acid; 1-10 parts disodium hydrogen phosphate; 0.5-5 parts sodium fluorosilicate; and 0.1-0.3 parts surfactant.

[0009] Furthermore, the surfactant is a fatty alcohol polyoxyethylene ether.

[0010] Furthermore, the activated filter residue is prepared by the following method: A1. After drying the filter residue, place it in a high-energy ball mill, add potassium citrate, and ball mill it. A2. Disperse the ball-milled powder in a carboxymethylation reagent solution to carry out the carboxymethylation reaction. After the reaction is complete, filter, wash, and dry.

[0011] Furthermore, in step A1, the amount of potassium citrate added is 1% to 5% of the mass of the filter residue, the ball milling speed is 30 r / min to 38 r / min, the ball milling time is 4 h to 6 h, and the ball-to-material ratio is 5 to 10:1.

[0012] Further, in step A2, the carboxymethylation reagent solution is an ethanol solution of sodium chloroacetate, wherein the mass fraction of sodium chloroacetate is 10% to 20%, and the pH is adjusted to 10 to 11 with sodium hydroxide; the reaction temperature is 50℃ to 70℃, the reaction time is 2h to 3h, and the drying temperature is 50℃ to 70℃.

[0013] Furthermore, the main component of the filter residue is calcium oxalate.

[0014] Compared with the prior art, the beneficial effects of the above-described composition of the present invention are as follows: This invention utilizes the synergistic effect of oxalic acid, disodium hydrogen phosphate, and sodium fluorosilicate to not only efficiently precipitate calcium ions with oxalic acid, but also to provide HPO4 through the synergistic effect of disodium hydrogen phosphate. 2- ) and sodium fluorosilicate (providing F - This method opens up a highly efficient magnesium removal pathway for soluble magnesium oxalate complexes, converting magnesium into magnesium hydrogen phosphate or magnesium fluoride precipitate, thereby achieving simultaneous deep removal of calcium and magnesium ions. The overall hardness removal rate can be stably increased to over 90%, which is far higher than the single oxalate method (<50%).

[0015] This invention further incorporates activated filter residue as a component of the composition. Through the synergistic effect of high-energy ball milling and potassium citrate, stable calcium oxalate crystals undergo lattice distortion and localized amorphization. Simultaneously, potassium citrate, acting as a crystal form inducer, embeds itself into the lattice under mechanical force, promoting the formation of metastable crystalline phases. Furthermore, through a carboxymethylation reaction, the residual hydroxyl groups (-OH) on the calcium oxalate surface undergo an etherification reaction with sodium chloroacetate, grafting a high density of carboxymethyl groups (-CH₂COOH) onto the calcium oxalate surface, significantly enhancing the adsorption capacity and crystallization-inducing ability of the filter residue surface. When used in conjunction with the composite reagent, the activated filter residue acts as a seed crystal, providing crystallization sites for newly formed precipitates, lowering the nucleation energy barrier, accelerating the precipitation reaction rate, and simultaneously inducing crystal growth on its surface to form larger and denser aggregates, thereby improving precipitation efficiency and solid-liquid separation performance.

[0016] A second objective of this invention is to provide a method for removing hardness from acidic wastewater, which employs the composition for removing hardness from acidic wastewater as described above, and includes the following steps: S1. Add the composition to the acidic wastewater to be treated and stir to react; S2. After the reaction is complete, solid-liquid separation is performed to obtain softened dilute acid and filter residue containing hard precipitate. The amount of the composition added, based on the molar ratio of oxalic acid to the total calcium and magnesium ions in the acidic wastewater to be treated, is 1.2 to 1.5:1. A portion of the filter residue obtained in step S2 is used to prepare the activated filter residue in the composition for recycling, while the remainder is returned to the copper pyrometallurgical system for ore blending.

[0017] Furthermore, the acidic wastewater to be treated is a dilute acidic wastewater generated during the hydrometallurgical process, which has undergone sulfidation to remove copper and arsenic; the Ca content of the acidic wastewater to be treated is... 2+ Concentrations range from 100 to 500 mg / L, Mg 2+ The concentration is 50–300 mg / L, and the pH is 0.3–1.5.

[0018] Furthermore, in step S1, the stirring reaction is carried out in two stages: the first stage of the stirring reaction is carried out at a speed of 200-300 r / min for 20-30 min; the second stage of the stirring reaction is carried out at a speed of 30-60 r / min for 30-40 min.

[0019] Compared with the prior art, the beneficial effects of the above-described method of the present invention are as follows: The entire treatment process of this invention is carried out under the original acidic conditions of the wastewater (pH 0.3-1.5), without the need to add alkali for neutralization or subsequent acid reversion, which greatly simplifies the process flow, saves on the consumption of acid and alkali reagents and corresponding operating costs, and maintains the acidity stability of the wastewater system, which is conducive to subsequent direct reuse or concentration for acid production.

[0020] In the composite agent used in this invention, the hydrolysis product of sodium fluorosilicate can serve as a seed crystal or template to induce calcium oxalate crystals to transform from slender needle-like crystals to a denser, more regular short columnar crystal form. At the same time, the addition of surfactants (such as fatty alcohol polyoxyethylene ethers) can adsorb onto the surface of newly formed microcrystals, regulate crystal growth, prevent excessive agglomeration into a colloidal state, and ultimately form porous, dense agglomerates with appropriate particle size.

[0021] This invention combines activated filter residue with composite agents. Experimental data shows that after adding activated filter residue, the hardness removal rate can be further increased from about 84% to over 90%, achieving significant technical results.

[0022] This invention primarily precipitates specific calcium and magnesium compounds under acidic conditions, exhibiting minimal co-precipitation of valuable metal ions in wastewater, thus reducing resource loss. The resulting filter residue mainly consists of calcium oxalate, magnesium hydrogen phosphate, and magnesium fluoride. Part of the filter residue is activated and recycled for the preparation of composites, while the remaining residue can be returned as raw material to processes such as copper pyrometallurgy for ore blending, achieving resource utilization of waste. This invention is carried out at room temperature, with mild reaction conditions, simple operation, and easy implementation in existing wastewater treatment facilities. Detailed Implementation

[0023] The principles and features of the present invention are described below with reference to examples. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0024] Example 1 Take 1 L of acidic wastewater (arsenic-removed liquid) from the sulfidation process after removing copper and arsenic from dilute acid in smelting. Its water quality is: Ca 2+ Concentration 100.21 mg / L, Mg 2+ The concentration was 297.68 mg / L, and the pH was 0.31.

[0025] Preparation of the composition: (1) Compound agent: Oxalic acid: Disodium hydrogen phosphate: Sodium fluorosilicate: Fatty alcohol polyoxyethylene ether = 100: 1: 5: 0.1 by mass ratio.

[0026] (2) Activated filter residue: Weigh out the activated filter residue and the composite agent at a mass ratio of 0.2:1.

[0027] The preparation method of activated filter residue is as follows: After drying the filter residue obtained by solid-liquid separation at 100℃, it is placed in a high-energy ball mill, potassium citrate (1% of the mass of calcium oxalate precipitate) is added, the speed is 30 r / min, the grinding time is 6 h, and the ball-to-material ratio is 5:1; the ground powder is dispersed in a carboxymethylation reagent solution, sodium chloroacetate (10% by mass) + sodium hydroxide (adjusting pH to 10) + ethanol (solvent), and stirred at 50℃ for 3 h, then filtered, washed, and dried (60℃).

[0028] Calculate the total molar number of calcium and magnesium ions in the wastewater, and weigh the required mass of the composition according to the ratio of oxalic acid to the total molar number of calcium and magnesium in the composition being 1.2:1.

[0029] At room temperature (approximately 25°C), the composition was added to the arsenic-removed liquid. The mixture was first rapidly stirred at 200 r / min for 30 min to ensure thorough dispersion and initial reaction. Subsequently, the stirring speed was reduced to 30 r / min, and the mixture was slowly stirred for 40 min to promote crystal growth and flocculation.

[0030] After the reaction, the mixture is filtered to achieve solid-liquid separation, yielding clear dilute acid and filter residue. Part of the filter residue is activated and recycled, while the remaining residue can be returned to the copper pyrometallurgical system for ore blending. The hardness of the dilute acid has been significantly reduced, allowing it to be directly reused in production or further concentrated for sulfuric acid recovery.

[0031] The concentrations of calcium and magnesium ions in the dilute acid after treatment were measured, and the total hardness removal rate was calculated to be 91.14%.

[0032] Example 2 Take 1L of the arsenic-removed solution; its water quality is: Ca 2+ Concentration 497.82 mg / L, Mg 2+ The concentration was 50.46 mg / L, and the pH was 1.48.

[0033] Preparation of the composition: (1) Compound agent: Oxalic acid: Disodium hydrogen phosphate: Sodium fluorosilicate: Fatty alcohol polyoxyethylene ether = 100: 10: 0.5: 0.3 by mass ratio.

[0034] (2) Activated filter residue: Weigh out the activated filter residue and the composite agent at a mass ratio of 0.5:1.

[0035] The preparation method of activated filter residue is as follows: After drying the calcium oxalate precipitate (100℃), place it in a high-energy ball mill, add potassium citrate (5% of the mass of calcium oxalate precipitate), rotate at 38 r / min, grind for 4 hours, and the ball-to-material ratio is 10:1; disperse the ground powder in a carboxymethylation reagent solution, sodium chloroacetate (20% by mass) + sodium hydroxide (adjust pH to 11) + ethanol (solvent), stir and react at 70℃ for 2 hours, filter, wash and dry (60℃).

[0036] The composition is added according to the ratio of oxalic acid to the total molar amount of calcium and magnesium in the composition being 1.5:1.

[0037] At room temperature, stir at 300 r / min for 20 min, then stir at 60 r / min for 30 min.

[0038] After the reaction was completed, the mixture was filtered, and the hardness of the treated dilute acid was measured. The total hardness removal rate was calculated to be 91.63%.

[0039] Example 3 Take 1L of the arsenic-removed solution; its water quality is: Ca 2+ Concentration 305.67 mg / L, Mg 2+ Concentration 163.47 mg / L, pH 1.0.

[0040] Preparation of the composition: (1) Compound agent: Oxalic acid: Disodium hydrogen phosphate: Sodium fluorosilicate: Fatty alcohol polyoxyethylene ether = 100: 5: 2: 0.2 by mass ratio.

[0041] (2) Activated filter residue: Weigh out the activated filter residue and the composite agent at a mass ratio of 0.3:1.

[0042] The preparation method of activated filter residue is as follows: After drying the calcium oxalate precipitate (100℃), place it in a high-energy ball mill, add potassium citrate (3% of the mass of calcium oxalate precipitate), rotate at 35 r / min, grind for 5 h, and the ball-to-material ratio is 8:1; disperse the ground powder in a carboxymethylation reagent solution, sodium chloroacetate (15% by mass) + sodium hydroxide (adjust pH to 10) + ethanol (solvent), stir and react at 60℃ for 2.5 h, filter, wash and dry (60℃).

[0043] The composition is added according to the ratio of oxalic acid to the total molar amount of calcium and magnesium in the composition being 1.3:1.

[0044] At room temperature, stir at 250 r / min for 25 min, then stir at 45 r / min for 35 min.

[0045] After the reaction was completed, the mixture was filtered, and the hardness of the dilute acid after treatment was measured. The total hardness removal rate was calculated to be 92.37%.

[0046] Comparative Example 1 Unlike Example 1, this comparative example does not use the composition of the present invention, but only uses pure oxalic acid in an equal molar amount as in Example 1 for treatment, while other operating conditions are the same as in Example 1.

[0047] After the treatment was completed, the total hardness removal rate was calculated to be only 50.21%.

[0048] Comparative Example 2 Unlike Example 1, this comparative example uses the same composite reagent (oxalic acid + disodium hydrogen phosphate + sodium fluorosilicate + fatty alcohol polyoxyethylene ether) as Example 1, but does not add activated filter residue. That is, only the composite reagent is used and the composition of the present invention is not used as a whole. Other operating conditions are the same as in Example 1.

[0049] After the treatment was completed, the total hardness removal rate was calculated to be 81.64%.

[0050] Comparative Example 3 Unlike Example 1, this comparative example uses the same compound reagent (oxalic acid + disodium hydrogen phosphate + sodium fluorosilicate + fatty alcohol polyoxyethylene ether) as Example 1, but the added filter residue is not activated, and the amount of filter residue added and other operating conditions are the same as in Example 1.

[0051] After the treatment was completed, the total hardness removal rate was calculated to be 86.67%.

[0052] The calculation formulas for the hardness removal rate of the above embodiments and comparative examples are as follows: Hardness removal rate = (initial hardness - post-treatment hardness) / initial hardness × 100%.

[0053] Comparative analysis of effects: The hardness removal rates of Examples 1-3 and Comparative Examples 1-3 are summarized in Table 1 below: Table 1. Hardness removal rate of the liquid after arsenic removal

[0054] As shown in the table above, under the same acidic wastewater and similar process conditions, the hardness removal rate of the composition provided by this invention (Examples 1-3) was consistently above 90%, significantly better than the treatment effect of using oxalic acid alone (Comparative Example 1, removal rate <50%). This fully demonstrates the synergistic effect of the components in the composite agent of this invention, especially its key role in the deep removal of magnesium ions and the improvement of precipitate properties, thereby achieving the technical goal of efficient and stable hardness removal under acidic conditions.

[0055] Further comparison of Example 1 and Comparative Example 2 shows that, compared with using only the composite agent (Comparative Example 2), the hardness removal rate increased from 81.64% to 91.14% when using the composition of the present invention (composite agent + activated filter residue, Example 1). This indicates that the addition of activated filter residue significantly enhances the hardness removal effect of wastewater. The mechanism is as follows: after high-energy ball milling and potassium citrate treatment, the activated filter residue undergoes lattice distortion, and after carboxymethylation modification, carboxymethyl functional groups are grafted onto its surface, giving it stronger adsorption and induced crystallization capabilities. After being added to the reaction system, the activated filter residue acts as a seed crystal, providing crystallization sites for new precipitates, lowering the nucleation energy barrier, accelerating the precipitation reaction rate, and simultaneously inducing crystal growth on its surface to form larger and denser aggregates, thereby improving precipitation efficiency and solid-liquid separation performance.

[0056] Further comparison of Example 1 and Comparative Example 3 shows that, compared with the use of the composite agent + unactivated filter residue (Comparative Example 3), the hardness removal rate increased from 86.67% to 91.14% when using the composition of the present invention (composite agent + activated filter residue, Example 1). This clearly demonstrates that the activation treatment of the filter residue is a key step in improving the hardness removal effect. Although the unactivated filter residue may also act as a seed crystal to some extent, its surface structure remains unchanged, and its adsorption capacity and ability to induce crystallization are weak. It cannot effectively reduce the nucleation energy barrier, promote crystal growth and agglomeration like the activated filter residue. However, the activated filter residue, which has undergone high-energy ball milling to introduce lattice distortion, potassium citrate embedding to induce metastable crystal phase formation, and carboxymethylation grafting of high-density carboxymethyl functional groups, has significantly optimized surface properties, thereby more effectively exerting a synergistic effect and further improving the hardness removal rate.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composition for removing hardness from acidic wastewater, characterized in that, It includes a composite agent and an activated filter residue, wherein the mass ratio of the activated filter residue to the composite agent is 0.2 to 0.5:1; The compound agent comprises the following components by mass: 100 parts oxalic acid; 1-10 parts disodium hydrogen phosphate; 0.5-5 parts sodium fluorosilicate; and 0.1-0.3 parts surfactant.

2. The composition for removing hardness from acidic wastewater according to claim 1, characterized in that, The surfactant is a fatty alcohol polyoxyethylene ether.

3. The composition for removing hardness from acidic wastewater according to claim 1, characterized in that, The activated filter residue is prepared by the following method: A1. After drying the filter residue, place it in a high-energy ball mill, add potassium citrate, and ball mill it. A2. Disperse the ball-milled powder in a carboxymethylation reagent solution to carry out the carboxymethylation reaction. After the reaction is complete, filter, wash, and dry.

4. The composition for removing hardness from acidic wastewater according to claim 3, characterized in that, In step A1, the amount of potassium citrate added is 1% to 5% of the mass of the filter residue, the ball milling speed is 30 r / min to 38 r / min, the ball milling time is 4 h to 6 h, and the ball-to-material ratio is 5 to 10:

1.

5. The composition for removing hardness from acidic wastewater according to claim 3, characterized in that, In step A2, the carboxymethylation reagent solution is an ethanol solution of sodium chloroacetate, wherein the mass fraction of sodium chloroacetate is 10% to 20%, and the pH is adjusted to 10 to 11 with sodium hydroxide; the reaction temperature is 50℃ to 70℃, the reaction time is 2h to 3h, and the drying temperature is 50℃ to 70℃.

6. The composition for removing hardness from acidic wastewater according to claim 3, characterized in that, The main component of the filter residue is calcium oxalate.

7. A method for removing hardness from acidic wastewater, characterized in that, The composition for removing hardness from acidic wastewater as described in any one of claims 1 to 6 is used.

8. The method for removing hardness from acidic wastewater according to claim 7, characterized in that, Includes the following steps: S1. Add the composition to the acidic wastewater to be treated and stir to react; S2. After the reaction is complete, solid-liquid separation is performed to obtain softened dilute acid and filter residue containing hard precipitate. The amount of the composition added, based on the molar ratio of oxalic acid to the total calcium and magnesium ions in the acidic wastewater to be treated, is 1.2 to 1.5:

1. A portion of the filter residue obtained in step S2 is used to prepare the activated filter residue in the composition, and the remainder is returned to the copper pyrometallurgical system for ore blending.

9. The method for removing hardness from acidic wastewater according to claim 8, characterized in that, The acidic wastewater to be treated is dilute acid wastewater from a hydrometallurgical process, generated after copper and arsenic removal via sulfidation; the Ca content of the acidic wastewater to be treated is... 2+ Concentrations range from 100 to 500 mg / L, Mg 2+ The concentration is 50–300 mg / L, and the pH is 0.3–1.

5.

10. The method for removing hardness from acidic wastewater according to claim 9, characterized in that, In step S1, the stirring reaction is carried out in two stages: the first stage of stirring reaction is carried out at a speed of 200-300 r / min for 20-30 min; the second stage of stirring reaction is carried out at a speed of 30-60 r / min for 30-40 min.

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