HENGFENG Floc Application Process of Flocculant (PAM) in Heavy Metal-Contaminated Soil Leaching Remediation
1. Pretreatment: Wastewater pH Adjustment and Homogenization
Purpose
Citric acid is acidic (pH ≈ 2-3), while polyacrylamide (PAM), especially HENGFENG Floc non-ionic or HENGFENG Floc anionic types, exhibits optimal flocculation performance under neutral to weakly alkaline conditions. Meanwhile, pH adjustment can disrupt part of the heavy metal-citric acid complexes, releasing free heavy metal ions (e.g., Co²⁺) or forming hydroxide/carbonate micro-precipitates, thereby creating favorable conditions for subsequent flocculation.
Operation
Add quicklime (CaO) or sodium hydroxide (NaOH) into the leachate storage tank to adjust the pH to 7.0-8.5, with real-time monitoring via an online pH meter. Simultaneously, start the agitator at a rotational speed of 150-200 r/min to homogenize the wastewater, preventing local pH fluctuations from affecting flocculation efficiency.
2. PAM Selection and Dissolution Preparation
Selection Basis
The wastewater in this case contains negatively charged soil colloids (due to the negative charge on the surface of soil clay minerals) and heavy metal complexes (mostly negatively charged or neutral). Therefore, anionic PAM (with a molecular weight of 8-12 million Da) is preferred. The carboxyl groups (-COO⁻) on its molecular chains can promote agglomeration through "charge neutralization" (adsorbing negative charges on the surface of soil colloids) and "bridging effect" (connecting multiple micro-particles). Compared with HENGFENG Floc non-ionic PAM, it has higher flocculation efficiency, and its cost is lower than that of HENGFENG Floc cationic PAM.
Dissolution Preparation
Dissolution Water: Use deionized water or clarified tap water (to avoid precipitation caused by the reaction of Ca²⁺ and Mg²⁺ in hard water with PAM).
Concentration Control: Mix HENGFENG Floc PAM powder with water at a mass ratio of 0.1%-0.3%, i.e., add 1-3 g of HENGFENG Floc PAM to 1 L of water.
Dissolution Method: First add water into the stirring tank, then slowly sprinkle PAM powder (to prevent caking). Control the stirring speed at 80-100 r/min and the stirring time at 30-60 minutes until the solution becomes transparent and viscous (with no visible particles to the naked eye). If dissolution is insufficient, undissolved PAM particles will form "fish eyes" in the wastewater, which will instead reduce flocculation efficiency.
3. PAM Dosing and Reaction (Flocculation Reaction Tank)
Dosing Method
Adopt "metering pump dripping" to slowly inject the prepared PAM solution into the flocculation reaction tank at a final dosage of 1-5 mg/L (i.e., 1-5 mg of effective PAM component per liter of leachate). The dosing port is installed in front of the stirring impeller to ensure rapid mixing with the wastewater.
Reaction Control (Two Stages)
Rapid Mixing Stage: Control the stirring speed at 200-300 r/min for 1-2 minutes. The purpose is to achieve instant and uniform contact between the PAM solution and the wastewater, enabling PAM molecules to quickly adsorb onto the surface of colloidal particles.
Slow Flocculation Stage: Reduce the rotational speed to 50-80 r/min and maintain it for 10-15 minutes. This stage is critical for "bridging and agglomeration". Slow stirring can avoid damaging the flocs being formed, allowing small particles to gradually aggregate into large flocs (visible "alum flowers") with a particle size of ≥ 100 μm. Meanwhile, the flocs can adsorb cobalt complexes or free Co²⁺ in the wastewater.
4. Solid-Liquid Separation (Sedimentation Tank / Clarification Tank)
Process Selection
Due to the high density of the flocs (containing heavy metal precipitates), vertical flow sedimentation tanks or inclined tube sedimentation tanks are adopted, as they occupy less area and have higher separation efficiency.
Operating Parameters
Control the hydraulic retention time (HRT) of the sedimentation tank at 1-2 hours and the upward flow velocity at 1.5-2.5 mm/s. This ensures that the flocs have sufficient time to settle to the bottom of the tank, forming "sludge" (containing heavy metal precipitates, soil colloids, and PAM flocs), while the upper layer becomes clarified liquid.
Key Function
Through this step, the removal rate of suspended solids (SS) in the wastewater can reach over 90%. At the same time, heavy metals such as cobalt settle along with the flocs, significantly reducing the heavy metal concentration in the clarified liquid and laying a foundation for subsequent up-to-standard discharge or advanced treatment.
5. Subsequent Treatment: Disposal of Sludge and Clarified Liquid
Sludge Treatment
The heavy metal-containing sludge at the bottom of the sedimentation tank (with a moisture content of approximately 80%-90%) is transported to a plate-and-frame filter press via a sludge pump for dewatering, forming sludge cakes with a moisture content of ≤ 60%. These sludge cakes must be managed as hazardous waste and sent to qualified institutions for solidification/stabilization treatment or cobalt recovery (e.g., through pyrometallurgical smelting or hydrometallurgical leaching).
Clarified Liquid Treatment
The upper clarified liquid needs to be tested for pH, chemical oxygen demand (COD, caused by residual citric acid), and heavy metal concentrations (e.g., Co, As, Cu). If it meets the standards (e.g., cobalt concentration < 0.01 mg/L, in line with the Environmental Quality Standards for Surface Water (GB 3838-2002)), it can be directly discharged. If it fails to meet the standards, further advanced treatment using ion exchange resins or membrane separation technology is required to ensure the complete removal of pollutants.
HENGFENG Floc Application Process of Flocculant (PAM) in Heavy Metal-Contaminated Soil Leaching Remediation
1. Pretreatment: Wastewater pH Adjustment and Homogenization
Purpose
Citric acid is acidic (pH ≈ 2-3), while polyacrylamide (PAM), especially HENGFENG Floc non-ionic or HENGFENG Floc anionic types, exhibits optimal flocculation performance under neutral to weakly alkaline conditions. Meanwhile, pH adjustment can disrupt part of the heavy metal-citric acid complexes, releasing free heavy metal ions (e.g., Co²⁺) or forming hydroxide/carbonate micro-precipitates, thereby creating favorable conditions for subsequent flocculation.
Operation
Add quicklime (CaO) or sodium hydroxide (NaOH) into the leachate storage tank to adjust the pH to 7.0-8.5, with real-time monitoring via an online pH meter. Simultaneously, start the agitator at a rotational speed of 150-200 r/min to homogenize the wastewater, preventing local pH fluctuations from affecting flocculation efficiency.
2. PAM Selection and Dissolution Preparation
Selection Basis
The wastewater in this case contains negatively charged soil colloids (due to the negative charge on the surface of soil clay minerals) and heavy metal complexes (mostly negatively charged or neutral). Therefore, anionic PAM (with a molecular weight of 8-12 million Da) is preferred. The carboxyl groups (-COO⁻) on its molecular chains can promote agglomeration through "charge neutralization" (adsorbing negative charges on the surface of soil colloids) and "bridging effect" (connecting multiple micro-particles). Compared with HENGFENG Floc non-ionic PAM, it has higher flocculation efficiency, and its cost is lower than that of HENGFENG Floc cationic PAM.
Dissolution Preparation
Dissolution Water: Use deionized water or clarified tap water (to avoid precipitation caused by the reaction of Ca²⁺ and Mg²⁺ in hard water with PAM).
Concentration Control: Mix HENGFENG Floc PAM powder with water at a mass ratio of 0.1%-0.3%, i.e., add 1-3 g of HENGFENG Floc PAM to 1 L of water.
Dissolution Method: First add water into the stirring tank, then slowly sprinkle PAM powder (to prevent caking). Control the stirring speed at 80-100 r/min and the stirring time at 30-60 minutes until the solution becomes transparent and viscous (with no visible particles to the naked eye). If dissolution is insufficient, undissolved PAM particles will form "fish eyes" in the wastewater, which will instead reduce flocculation efficiency.
3. PAM Dosing and Reaction (Flocculation Reaction Tank)
Dosing Method
Adopt "metering pump dripping" to slowly inject the prepared PAM solution into the flocculation reaction tank at a final dosage of 1-5 mg/L (i.e., 1-5 mg of effective PAM component per liter of leachate). The dosing port is installed in front of the stirring impeller to ensure rapid mixing with the wastewater.
Reaction Control (Two Stages)
Rapid Mixing Stage: Control the stirring speed at 200-300 r/min for 1-2 minutes. The purpose is to achieve instant and uniform contact between the PAM solution and the wastewater, enabling PAM molecules to quickly adsorb onto the surface of colloidal particles.
Slow Flocculation Stage: Reduce the rotational speed to 50-80 r/min and maintain it for 10-15 minutes. This stage is critical for "bridging and agglomeration". Slow stirring can avoid damaging the flocs being formed, allowing small particles to gradually aggregate into large flocs (visible "alum flowers") with a particle size of ≥ 100 μm. Meanwhile, the flocs can adsorb cobalt complexes or free Co²⁺ in the wastewater.
4. Solid-Liquid Separation (Sedimentation Tank / Clarification Tank)
Process Selection
Due to the high density of the flocs (containing heavy metal precipitates), vertical flow sedimentation tanks or inclined tube sedimentation tanks are adopted, as they occupy less area and have higher separation efficiency.
Operating Parameters
Control the hydraulic retention time (HRT) of the sedimentation tank at 1-2 hours and the upward flow velocity at 1.5-2.5 mm/s. This ensures that the flocs have sufficient time to settle to the bottom of the tank, forming "sludge" (containing heavy metal precipitates, soil colloids, and PAM flocs), while the upper layer becomes clarified liquid.
Key Function
Through this step, the removal rate of suspended solids (SS) in the wastewater can reach over 90%. At the same time, heavy metals such as cobalt settle along with the flocs, significantly reducing the heavy metal concentration in the clarified liquid and laying a foundation for subsequent up-to-standard discharge or advanced treatment.
5. Subsequent Treatment: Disposal of Sludge and Clarified Liquid
Sludge Treatment
The heavy metal-containing sludge at the bottom of the sedimentation tank (with a moisture content of approximately 80%-90%) is transported to a plate-and-frame filter press via a sludge pump for dewatering, forming sludge cakes with a moisture content of ≤ 60%. These sludge cakes must be managed as hazardous waste and sent to qualified institutions for solidification/stabilization treatment or cobalt recovery (e.g., through pyrometallurgical smelting or hydrometallurgical leaching).
Clarified Liquid Treatment
The upper clarified liquid needs to be tested for pH, chemical oxygen demand (COD, caused by residual citric acid), and heavy metal concentrations (e.g., Co, As, Cu). If it meets the standards (e.g., cobalt concentration < 0.01 mg/L, in line with the Environmental Quality Standards for Surface Water (GB 3838-2002)), it can be directly discharged. If it fails to meet the standards, further advanced treatment using ion exchange resins or membrane separation technology is required to ensure the complete removal of pollutants.
