Some aspects of ferrohydrostatic separation of minerals and the recycling of ferrofluid

[摘要] ENGLISH ABSTRACT:Ferrohydrostatic separation (FHS) of materials is a float and sinktechnique which utilizes ferrofluid exposed to a non-homogeneousmagnetic field. The efficiency of material separation depends onnumerous variables. The most important variables, which wereinvestigated individually, are the effects of moisture content, ferrofluidlevel, feedrate, particle size and material density distribution on separationefficiency.It is important to recover and recycle the ferrofluid attached to theproducts of separation so as to reduce the cost of the FHS technology andthe amount of kerosene disposed of to the environment. This promptedresearch into some of the factors affecting ferrofluid recovery. The factorsthat were investigated are the effects ofFHS operation, material moisturecontent, particle size and porosity.The separation efficiency was found to be dependent on all the variablesinvestigated. The effect of material moisture content is less pronouncedfor particles larger than 2.8 mm. This implies that wet feed materialshould be screened before ferrohydrostatic separation and material whichparticle size is less than 2.8 mm should preferably be treated dry. Wetmaterial (less than 2.8 mm) floats, even though its density is greater thanthe cut-point density. This is owing to the immiscibility of the watercoating the particles and the kerosene-based ferrofluid used for separation.It was found that the effect of ferrofluid level on separation efficiency is afunction of both the density difference of the particles to be separated andthe particle size. Separation efficiency as a function of ferrofluid level ispoor for particles larger than 2 mm and is good when the densitydifference of the material to be separated is high, for instance between2700 kg/nr' and 3530 kg/nr'. This shows that for efficient separation ofcoarse particles and near density material (material with density close tothe cut-point density), the ferrofluid level should be controlled, preferablyclose to the maximum possible level.The effect of feedrate on separation efficiency is also a function of thedensities of the particles to be separated. An increase in feedrate leads topoor separation for particles with densities close t~ each other. Thisimplies that separation of near density material requires accurate feedratecontrol. It has been shown from simulation and modelling that the effective cutpointdensity changes as the particle moves through the chamber until iteventually reaches its terminal velocity, given that the chamber is ofsufficient size for this to occur. The effective cut-point density increases tothe maximum as the particle enters the ferrofluid pool but settles down toa relative constant once the particle has reached its terminal velocity. Theeffective cut-point density was shown to decrease with an increase inparticle magnetisation. It was found that this decrease in the cut-pointdensity determines the density difference (difference between twoparticles) achievable when non-magnetic material is treated together withmagnetic material. It is therefore important to magnetically scalp the feedmaterial for efficient separation. When the material is not scalped,magnetic and nonmagnetic material with the same density might report todifferent density fractions, which leads to poor separation. This magneticcontribution to the effective density can be utilised in the separation ofmaterial with same density but different magnetisation.The efficiency offerrofluid recovery was found to be dependent on all thevariables investigated. The amount of ferrofluid drawn from the FHSseparator was found to decrease with an increase in the magnetic field.Furthermore, the amount of ferrofluid that remains attached to theparticles after allowing ferrofluid to drain from the material is the same asthat attached to the FHS products of separation at high magnetic fields.This shows that it is important to operate the ferrohydrostatic separator athigh magnetic fields in order to attract most of the ferrofluid back to theseparator.T-heamount of ferrofluid adsorbed onto and absorbed by the particles wasfound to decrease with an increase in the material moisture content. Thisis due to two factors. The first is that water occupies the vacant pores inthe material. The second is that water forms a layer on the particle surfacewhich is immiscible with kerosene-based ferrofluid. This phenomenonleads to a reduction in cost of the ferrohydrostatic separation technologywhen wet material as opposed to dry material is treated. As alreadydescribed coarse material larger than 2.8 mm can be treated wet withoutdetrimental effects on separation. For -8+4 mm particles, the ferrofluidloss ranges from 0.6 down to 0.14 kg/tonne of feed for 0 to 10 % materialmoisture content respectively.The amount of ferrofluid lost per tonne of feed was found to range from0.73 to 0.56 kg for-O.85+O.5 mm to -12+8 mm particle sizes respectively.The increase in ferrofluid loss in small particles is due to the increase insurface area in small particles for ferrofluid adsorption.The increase in porosity increases the amount of ferrofluid lost due to thedifficulties in recovering ferrofluid embedded in the pores of the particles.Adding water to coarse material lowers the amount of ferrofluid lost byreducing porosity. Modelling the amount of ferrofluid lost, as a functionof particle size and porosity, would assist in determining the amount offerrofluid required to treat a known amount of material.The quality of ferrofluid recovered was found to be the same as thatinitially used for material separation. This implies that the separationefficiency would not be affected by the use of recycled ferrofluid.

[发布机构] Stellenbosch University