1. Adjustment Based on Material Viscosity
Viscosity is the most critical factor affecting rotational speed. Generally, the higher the material viscosity, the lower the speed should be, and vice versa.
Low-viscosity materials (e.g., water-based solutions, dilute acids/bases, low-concentration suspensions): These materials have good fluidity and are easy to stir. A higher rotational speed (typically 500-1500 rpm) is suitable to accelerate mass transfer and ensure uniform mixing. For example, when mixing a dilute salt solution, a speed of 800-1200 rpm can quickly achieve a homogeneous state.
Medium-viscosity materials (e.g., emulsions, pastes with low solid content, viscous solutions like glycerin-water mixtures): Their fluidity is moderate, and excessive speed may cause splashing or air entrainment. A medium speed (300-800 rpm) is recommended. For instance, when preparing an oil-in-water emulsion, a speed of 400-600 rpm helps form stable droplets without introducing too many air bubbles.
High-viscosity materials (e.g., thick pastes, high-concentration slurries, molten polymers): These materials have poor fluidity and high resistance to stirring. A low rotational speed (50-300 rpm) is required to prevent the mixer from overloading or the material from being "stuck" and not flowing. For example, when mixing a high-concentration starch paste, a speed of 80-150 rpm ensures the material is evenly stirred without damaging the mixer's motor.
2. Adjustment Based on Material Particle Size and Hardness
For solid-liquid mixing or solid-solid mixing involving particles, particle size and hardness determine the speed needed to break up agglomerates and achieve uniform dispersion.
Fine and soft particles (e.g., talcum powder, powdered activated carbon, fine organic powders): These particles are easy to disperse and do not require high shear force. A moderate speed (300-800 rpm) is sufficient. For example, when dispersing 200-mesh talcum powder into water, a speed of 500-600 rpm can prevent agglomeration without causing the powder to fly.
Coarse or hard particles (e.g., quartz sand, coarse mineral powders, hard ceramic particles): These particles have strong agglomeration tendencies and require higher shear force to break them up. A higher speed (800-2000 rpm) is needed, but it is necessary to ensure that the mixer's impeller is wear-resistant. For instance, when mixing 50-mesh quartz sand into a resin matrix, a speed of 1200-1800 rpm helps disperse the sand evenly without leaving large agglomerates.
3. Adjustment Based on Material Volatility and Sensitivity
For volatile or heat-sensitive materials, rotational speed must be controlled to avoid excessive heat generation or solvent evaporation.
Volatile materials (e.g., ethanol-based solutions, low-boiling-point solvents): High speed will increase the contact area between the material and air, accelerating solvent evaporation. A lower speed (200-500 rpm) should be used, and it is recommended to use a sealed mixing container if possible. For example, when mixing an ethanol-water solution, a speed of 200-300 rpm minimizes ethanol loss.
Heat-sensitive materials (e.g., biological samples, enzymes, thermally unstable polymers): Friction between the impeller and the material at high speed will generate heat, which may denature or decompose the material. A low to medium speed (100-400 rpm) is suitable, and if necessary, a jacketed mixer can be used for cooling. For example, when stirring a protein solution, a speed of 150-250 rpm ensures uniform mixing without causing protein denaturation due to heat.
4. Adjustment Based on Mixing Purpose
Different mixing objectives (e.g., blending, dissolution, dispersion, emulsification) also require corresponding speed adjustments.
Simple blending (e.g., mixing two miscible liquids): The goal is to achieve uniform composition quickly, so a medium speed (300-800 rpm) is appropriate.
Dissolution promotion (e.g., dissolving solid reagents in solvents): A moderate to high speed (500-1200 rpm) can accelerate the dissolution process by increasing the mass transfer rate between the solid and liquid phases.
Emulsification (e.g., preparing oil-in-water or water-in-oil emulsions): Emulsification requires sufficient shear force to break oil droplets into small particles, so a high speed (1000-3000 rpm) is usually needed, and a special emulsifying impeller is often matched.
Dispersion of nanoparticles (e.g., dispersing nano-silica into a solvent): Nanoparticles are prone to agglomeration, so a high speed (1500-3000 rpm) with strong shear force is required, and sometimes auxiliary measures like ultrasonic assistance are combined.