Mineral Processing Flocculants for Faster Sedimentation
By adjusting molecular weight, charge type, dosage and process parameters, mineral processing flocculants can greatly accelerate sedimentation and realize highly efficient solid-liquid separation. The detailed analysis is presented as follows:
I. Molecular Weight and Charge Type: Core Factors Determining Flocculant Performance
High-molecular-weight Flocculants
High-molecular-weight flocculants such as polyacrylamide with a molecular weight ranging from 8 million to 20 million link fine particles via adsorption bridging, forming large and dense flocs to speed up sedimentation. In coal washing industry, specially modified polyacrylamide can produce obvious sedimentation within 5 seconds. The formed flocs are large and compact with low sludge volume, delivering a remarkably higher settling rate.
Matching of Charge Types
Anionic flocculants: Suitable for positively charged suspended particles such as bauxite tailings. They form flocs through electrostatic attraction and bridging effects.
Cationic flocculants: Applied to fine negatively charged minerals for kaolin purification. They neutralize surface charges of particles and facilitate flocculation.
Non-ionic flocculants: Ideal for acidic slurry, which agglomerates particles mainly through physical adsorption.
Case: A bauxite concentrator adopted anionic polyacrylamide with a molecular weight of 12 million to 14 million. The tailings sedimentation rate was greatly improved, and overflow water as well as filter press water could be directly recycled for production.
II. Dosage Optimization: Precise Control for Desirable Flocculation
Optimal Dosage Range
The appropriate dosage must be determined through tests. Either excessive or insufficient dosage will slow down sedimentation.
In a tailings sedimentation test at a hematite processing plant, the optimal reagent dosage was 40 to 50 grams per ton of ore. The polymer chains fully adsorbed particles, achieving a sedimentation rate of 0.5 to 0.55 meters per hour.
When the total tailings slurry concentration was 20%, the maximum sedimentation rate reached 187.5 millimeters per minute at a flocculant dosage of 30 grams per ton. Once the dosage exceeded 40 grams per ton, the sedimentation rate dropped by 13.3%. Flocculation failure led to re-stabilization of suspended particles.
Relationship between Slurry Concentration and Dosage
Higher slurry concentration brings stronger electrostatic repulsion between particles and more intense Brownian motion. Accordingly, more flocculants are required to overcome resistance.
When the slurry concentration rose from 10% to 20%, the maximum sedimentation rate decreased by more than 50%. If the concentration further increased to 40%, the flocculant dosage needed to be multiplied to maintain satisfactory sedimentation performance.
III. Process Parameter Optimization: Key to Higher Flocculation Efficiency
pH Value Control
pH value affects the ionization degree of flocculants and surface charge properties of particles.
Polyaluminium Chloride (PAC) achieves the best flocculation effect within the pH range of 6 to 8. Its performance will deteriorate beyond this range.
Iron-based flocculants rapidly form ferric hydroxide flocs at pH 5 to 7, with the optimal pH ranging from 6.0 to 6.4.
Mixing and Stirring Intensity
Rapid mixing stage: Intense stirring is required to disperse flocculants evenly in water and ensure full contact with particles.
Reaction stage: Gradually reduce stirring intensity to avoid breaking the formed flocs. Moderate stirring helps generate large flocs, while overly fast stirring will damage floc structures.
Temperature Impact
Water temperature influences the hydrolysis rate of flocculants and floc formation speed. Low temperature slows down molecular movement and reduces particle collision frequency, thus restraining flocculation.
When the water temperature is below 5 ℃, aluminum-based flocculants hydrolyze extremely slowly with greatly weakened performance. At 10 ℃ to 15 ℃, the generated aluminum hydroxide flocs are loose and hard to settle.
IV. Practical Application Cases of High-efficiency Sedimentation
Tailings Sedimentation
A copper ore concentrator optimized the flocculant dosage at 20 grams per ton and controlled the pH value at 7. The tailings sedimentation rate was tripled, and the underflow concentration exceeded 60%, fully meeting the requirements for water reuse.
Coal Washing Wastewater Treatment
Special polyacrylamide for coal washing makes coal slime particles aggregate into flocs rapidly via adsorption bridging. Sedimentation completes within 5 seconds. The filter press efficiency increases by 40%, and the moisture content of filter cakes drops from 75% to 58%.
Iron Ore Processing
Caustic starch is adopted as a selective flocculant and combined with reverse flotation technology. The iron concentrate grade is increased by 7% to 10%. Meanwhile, flocculation accelerates tailings sedimentation and cuts down wastewater discharge.