Soil Dispersion Mixer

Soil Dispersion Mixer:

A Soil Dispersion Mixer is a piece of laboratory equipment used to prepare soil samples for particle-size analysis, specifically the hydrometer method.¹ The mixer ensures that the individual soil particles are thoroughly separated and uniformly suspended in a water solution.² This process is crucial because soil particles in their natural state often stick together in clumps (called aggregates or floccules), which would lead to inaccurate test results.

How it Works of Soil Dispersion Mixer:

The Soil Dispersion Mixer operates on the principle of high-speed mechanical agitation and shear force.³ It consists of a powerful motor, a mixing shaft with a special agitator blade (or paddle), and a baffled dispersion cup.⁴

1. Sample Preparation: A soil sample is first pre-treated, often by removing large particles and then soaking it in a dispersing agent solution (like sodium hexametaphosphate, also known as Calgon).⁵ This chemical agent helps to neutralize the forces that cause particles to clump.⁶
2. Mixing: The pre-treated soil and the dispersing agent solution are placed in the dispersion cup.⁷
3. High-Speed Agitation: The motor rotates the agitator blade at a very high speed, typically over 10,000 RPM.⁸ This creates intense turbulence and shear forces that break down the soil aggregates into their individual particles.⁹
4. Baffled Cup: The internal baffles within the dispersion cup prevent the mixture from simply swirling in a circle.¹⁰ They force the slurry to be continuously directed back toward the high-speed blade, ensuring a complete and uniform dispersion of the particles.

After mixing, the dispersed soil-water slurry is transferred to a sedimentation cylinder for the hydrometer test, which measures the settling rate of particles to determine their size distribution.¹¹

Mathematical Calculations of Soil Dispersion Mixer:

The Soil Dispersion Mixer is a mathematical calculations associated with the soil dispersion mixer itself are related to the shear force and tip speed of the agitator blade. Tip speed (also called peripheral velocity) is a key factor in ensuring effective dispersion.¹² It’s calculated by the following formula:

Tip Speed =π×Diameter of blade×RPM

where:

• π is the mathematical constant pi (~3.14159)¹³
• Diameter of blade is the diameter of the agitator blade¹⁴

• RPM is the revolutions per minute of the motor¹⁵

Soil Dispersion Mixer:

The higher the tip speed, the greater the shear force, which leads to better de-agglomeration of the soil particles.¹⁶

Once the soil is dispersed, the subsequent hydrometer test involves a series of complex calculations based on Stokes’ Law to determine particle size distribution.¹⁷ Stokes’ Law relates the terminal velocity of a settling sphere in a fluid to its diameter, the fluid’s viscosity, and the difference in density between the particle and the fluid.¹⁸ The formula is used to find the particle diameter (D) based on how far and how fast the particles settle in the sedimentation cylinder.¹⁹

The video below demonstrates how a soil dispersion mixer is used for hydrometer testing.

Gilson Soil Dispersion Mixer (SA-14)