What is the movement process of materials in a decanter centrifuge?

I. Feeding Stage: Axial Inlet and Circumferential Acceleration

Material Input Path

Materials to be separated (such as suspensions) enter the interior of the drum along the axial direction (axis of the drum) through the central feed pipe, with a low initial velocity (close to laminar flow).

The drum rotates at high speed (1500–4500 rpm), driving the internal materials to rapidly acquire circumferential velocity and rotate synchronously with the drum (similar to a "rigid rotation" state).

Centrifugal Field Initialization

Upon entering the drum, materials are immediately subjected to centrifugal force (directed radially toward the outer wall of the drum). Denser particles (solid phase or heavy liquid phase) begin to migrate radially toward the inner wall of the drum, while less dense liquid phases (light liquid phase) gather toward the center of the drum, forming a preliminary radial stratification (solid phase in the outer layer, liquid phase in the inner layer).

II. Centrifugal Separation Stage: Coupling of Radial Sedimentation and Axial Flow

1. Radial Sedimentation: Centrifugal Motion of Particles

Driving Force: Centrifugal force \(F_c = m \cdot \omega^2 \cdot r\) (where m is particle mass, \(\omega\) is the drum's angular velocity, and r is the particle's radial position) causes solid-phase particles to move toward the drum's inner wall, overcoming the viscous resistance of the liquid phase (Stokes' resistance).

Sedimentation Velocity: The radial sedimentation velocity of particles \(v_r \propto \frac{(\rho_s - \rho_l) \cdot d^2 \cdot \omega^2 \cdot r}{\mu}\) (proportional to density difference, particle diameter squared, and centrifugal force; inversely proportional to liquid viscosity). Fine particles (e.g., < 10 μm) require higher rotational speeds or longer retention times to settle to the drum's inner wall.

2. Axial Flow: Laminar Motion of the Liquid Phase

Flow Direction: The liquid phase (including entrained unsedimented particles) flows from the feeding end (cylindrical section of the drum) to the overflow end (end of the cylindrical section) under axial pressure difference, forming axial laminar flow (Reynolds number \(Re < 2000\) to avoid turbulence disturbing separation efficiency).

Velocity Distribution: The axial velocity of the liquid phase \(v_z\) exhibits a parabolic distribution along the radial direction (fastest at the center, approaching 0 near the drum's inner wall), closely related to the thickness of the liquid ring in the drum (determined by the overflow weir height). The thinner the liquid ring (lower overflow weir height), the faster the axial flow velocity of the liquid phase and the shorter the retention time.

3. Superposition of Motion Trajectories

The actual motion trajectory of individual particles is the vector synthesis of radial sedimentation velocity \(v_r\) and axial flow velocity \(v_z\), extending spirally toward the drum's end:

If particles complete radial sedimentation before reaching the overflow port (\(v_r \cdot t \geq \Delta r\), where t is the liquid phase retention time and \(\Delta r\) is the radial distance from the particle's initial position to the drum's inner wall), they are retained on the drum's inner wall as the solid phase;

If sedimentation is incomplete, they are discharged with the liquid phase from the overflow port as "overflow residues."

III. Solid Discharge Stage: Axial Transportation Driven by Screw Speed Difference

1. Differential Motion of the Screw and Drum

The screw conveyor rotates in the same direction as the drum, but the screw speed \(n_{\text{screw}}\) is slightly lower than the drum speed \(n_{\text{drum}}\), with a speed difference \(\Delta n = n_{\text{drum}} - n_{\text{screw}} = 5–30 \, \text{rpm}\).

The speed difference creates a relative angular velocity between the screw blades and the sediment layer on the drum's inner wall, generating axial pushing force (similar to the principle of a "screw pump").

2. Axial Movement of the Solid Phase

Solid particles settled on the drum's inner wall aggregate into a sediment layer, which is moved along the drum's axis toward the conical slag discharge port by the screw blades:

Cylindrical Section: Sediment initially accumulates, and the screw begins pushing;

Conical Section: The drum's inner diameter gradually decreases, and the sediment is further squeezed and dewatered (called the "drying section"), finally discharged as a solid with low moisture content.

Retention Time Control: A smaller speed difference results in slower screw pushing speed, longer retention time of the solid phase in the drum, and more thorough dewatering (e.g., in sludge dewatering scenarios); a larger speed difference enables faster slag discharge, suitable for materials with high solid content (e.g., mineral slurry separation).

IV. Liquid Overflow Stage: Axial Discharge of the Light Liquid Phase

1. Formation and Stratification of the Liquid Ring

The separated liquid phase (light liquid phase or clear liquid) forms an inner liquid ring in the central region of the drum, whose thickness is determined by the overflow weir height:

The higher the overflow weir, the thicker the liquid ring (larger liquid phase volume in the drum), longer retention time of the liquid phase, and higher separation precision;

The lower the overflow weir, the thinner the liquid ring, larger throughput but lower separation precision.

2. Axial Flow and Discharge of the Liquid Phase

The liquid phase in the liquid ring flows axially toward the overflow port in laminar state and is continuously discharged through the overflow weir:

In solid-liquid separation, clear liquid (e.g., effluent after wastewater treatment) is discharged;

In liquid-liquid separation, if there are two liquid phases (e.g., oil-water stratification), the light liquid phase (oil) is discharged from the inner overflow port, and the heavy liquid phase (water) is discharged from the outer overflow port (requiring a dual overflow weir design).

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