Abstract

Sedimentation based processes are widely used in industry to separate particles from a liquid phase. Since the advent of the “Nanoworld” the demand for effective separation technologies has rapidly risen, calling for the development of new separation concepts, one of which lies in hybrid separation using the superposition of a magnetic field for magnetic particles. Possible product portfolio of such separation consists of pigment production, nanomagnetics production for electronics and bio separation. A promising step in that direction is magnetic field enhanced cake filtration, which has by now progressed from batch to continuous operation.

In sedimentation processes in a mass force field the settling behaviour of particles strongly depends on physico-chemical properties, concentration and size distribution of the particles. By adjusting the pH, the interparticle forces, in particular the electrostatic repulsion, can be manipulated. For remanent magnetic particles such as magnetite, pre-treatment in a magnetic field could lead to a change of interparticle interactions. By magnetizing the particles apart from van der Waals attraction and electrostatic repulsion, an additional potential is induced, the magnetic attraction, which could easily dominate the other potentials and result in agglomeration in the primary minimum. By sedimentation analysis, a wide spectrum of parameters like pH, magnetic field strength and concentration have been investigated. The results show a strong increase of sedimentation velocity by magnetic flocculation of the raw suspension. This leads to a rise in throughput due to the acceleration of sedimentation kinetics by imparting a non-chemical interaction to the physico-chemical properties in the feed stream of the separation apparatus.

Keywords: Magnetic flocculation; Field enhanced separation; Cake filtration; Magnetic structuring; Magnetic separation

Nomenclature

a

distance (m)

A

Hamaker constant (V × C)

c_F

floc concentration (vol%)

c_V

particle volume concentration (vol%)

d_F

floc size (m)

[dp/dt]_t1

slope of tangent at evaluation point (Pa/s)

g

earth gravity (m/s²)

I

ionic strength

k

Boltzmann constant (J/K)

k

floc building number, 1

M

magnetization (A m²/kg)

p₀

initial differential pressure (Pa)

p₁

differential pressure at evaluation point (Pa)

p_Q1

initial differential pressure at evaluation point (Pa)

q

charge (C)

Q₃

cumulative volumetric distribution, 1

s

relative distance, s = 2(a + x)/x, 1

t₁

time at evaluation point (s)

T

temperature (K)

u_fl,St

Stokes single floc settling velocity (m/s)

u_S

Richardson–Zaki settling velocity (m/s)

u_St

Stokes settling velocity (m/s)

V_el

electrostatic potential (V × C)

V_ges

total potential (V × C)

V_mag

magnetic potential (V × C)

V_vdW

van der Waals potential (V × C)

x

particle size (m)

z

valency of ion, 1

Greek symbols

_r

dielectric number, 1

γ

substitution parameter, 1

η

viscosity (Pa s)

κ

Debye–Hückel distance (m)

ρ_F

density of floc (kg/m³)

ρ_l

density of liquid (kg/m³)

ρ_s

density of solids (kg/m³)

ζ

zeta potential (V)

Constants

e₀

charge of electron, 1.6 × 10⁻¹⁹ C

k

Boltzmann constant, 1.38 × 10⁻²³ J/K

₀

dielectric constant in vacuum, 1.6 × 10⁻¹⁹ A s/(V m)

μ₀

magnetic vacuum permeability, 1.257 × 10⁻⁶ V s/(A m)

Article Outline

Nomenclature

1. Introduction

2. Theory

3. Experimental

3.1. Materials

3.2. Methods

4. Results and discussion

5. Conclusions

Acknowledgements

References